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The increase in use and extraction of Rare Earth Element (REE) over the last few decades has raised concerns about their behaviour in the environment. One of the factors controlling their fate is adsorption onto iron (Fe) (oxy)hydroxides. In natural systems, aluminium (Al) substitutions of iron often occur in these (oxy)hydroxides, as Al is also one of the most abundant elements at the Earth's surface. Though it has been shown that these substitutions can have an impact on the adsorption of different metallic contaminants, to date, no study has been carried out on the impact of substitution on the adsorption of REE. To fill this gap, a series of ferrihydrites and goethites with varying levels of Al and gallium (Ga) substitution were synthesized, with Ga used as it is a chemical analogue of Al with an ionic radius comparable to that of Fe, unlike Al whose radius is smaller. The synthetised (oxy)hydroxides were subjected to REE adsorption at acidic to near neutral pH. REE complexation modes were studied using EXAFS collected at the neodymium L 3 -edge. Results show that the amount of adsorbed REE is controlled both by the pH of solution and the structure of the adsorbing (oxy)hydroxide. Adsorbed REE form both bidentate mononuclear (BM) and bidentate binuclear (BB) complexes. Unlike goethite, in the ferrihydrite the substitution affects the size, shape and organisation of the surface sites of the particle. The number of sites allowing BM complexation decrease with increasing substitution, as Al and Ga inhibit the protrusion of metal octahedra at the (1−10) plane. Additionally, Al substitution inhibits the growth of the step wise (1−1−1) plane and its reciprocals, leading to the particle shape changing from spherical to cylindrical and a loss of adsorption sites capable of forming both BM and BB complexes. With this study we demonstrate that the substitution of Fe by Al and Ga in goethite has little to no effect, while in the ferrihydrite it leads to a modification in particle shape, accompanied by a reduction in adsorption sites, resulting in a decrease in total adsorption. Consequently, contrary to goethite, substitution in ferrihydrite strongly reduces its capacity to trap and transport REE. [ABSTRACT FROM AUTHOR]

Goethite (α-FeOOH) contains two structurally-distinct oxygen sites that can potentially be independently interrogated for isotopic composition by analyzing bulk goethite δ18O (δ18O bulk) and the δ18O of water extracted from it (δ18O extracted). The isotopic contrast between these two components may form the basis of a paleothermometer (Miller et al., 2020). We sought to better understand the behavior of goethite undergoing vacuum dehydroxylation, the key step in this potential paleothermometric technique. A refined method for making these measurements applied to a suite of natural and synthetic goethites revealed that δ18O extracted is extremely sensitive to extraction temperature and to details of the vacuum environment in which the goethite is dehydroxylated. This sensitivity likely arises from the fundamental mechanism by which evolved water is removed from goethite in vacuum, via nm-wide pores penetrating nascent hematite surrounding dehydroxylating goethite. These pores may provide favorable conditions for back reaction that scrambles the oxygen isotopic composition, and also for kinetic mass fractionation. The character of these pores is known to vary with goethite composition, crystallinity, and extraction temperature. We identified additional sample-specific factors that affect δ18O extracted. For example, some synthetic goethites undergo dehydroxylation at surprisingly low temperature (<90 °C) in vacuum, under conditions usually used to remove nonstoichiometric water. In addition, some of the synthetic goethites have spectroscopic characteristics suggesting a range of OH bonding environments that complicate the idea that goethite has just two structurally (and potentially isotopically) distinct oxygen sites. Instead, in fine grained or poorly crystalline goethite there may be a continuum of physisorbed water, chemisorbed water, and stoichiometric water that is simultaneously released during vacuum extraction. After investigating a wide range of extraction conditions, we determined that rapid heating to 255 ± 5 °C is optimal for achieving high degrees of water extraction with the least apparent back reaction. When applied to synthetic goethites grown between 5 °C and 70 °C, we found no compelling correlation between synthesis temperature and the contrast between δ18O bulk and δ18O extracted. Analyses of two natural samples, known to have grown at ∼5 °C and ∼ 25 °C, revealed no significant difference in measured isotopic contrast. Together these observations suggest that goethite has no resolvable formation temperature information encoded in the bulk and extracted water oxygen isotope ratios as interrogated by the method we explored. If there is temperature-dependent isotopic contrast between the two oxygen sites in goethite, the use of the dehydroxylation method appears unable to reliably reveal it. [ABSTRACT FROM AUTHOR]

Uncertainty exists over what environmental conditions and mineralogical/chemical properties are required to ensure retention of helium such that (U Th)/He dates record the time of goethite crystallization. We undertook vacuum step heating experiments to determine He diffusion parameters for extrapolation to Earth surface conditions on 10 goethite specimens in which we had created a uniform distribution of 3He. Arrhenius plots of apparent diffusion coefficients on all samples follow the same pattern. At temperatures <200 °C the data define arrays consistent with progressive degassing of increasingly large crystallites. However, above 200 °C the computed diffusivities increase dramatically until about 80% of the helium is extracted, after which they suddenly decline. The sudden increase in diffusivity at 200 °C coincides with the onset of dehydration of the goethite structure, a process which continues throughout the remainder of the step heat. There is a strong correlation between evolved water and 3He amounts. These observations likely reflect previously reported processes of formation, growth, and coalescence of pores as the phase transition to hematite proceeds. While significantly higher dehydration temperatures of ∼270 °C are observed using techniques such as thermogravimetric analysis in air, the long step durations, and vacuum conditions of our experiments destabilize goethite. Orders of magnitude differences in the computed diffusivities among our samples may reflect crystallite-size-distribution control on dehydration kinetics, and the qualitative size distributions we infer from the step heats are consistent with SEM observations of crystallite lengths. Vacuum step-heating experiments in which goethite is decomposing cannot be used to determine He diffusion behavior in nature, but the inferred crystallite size distribution provides some useful indirect evidence. A strong relationship exists between metrics of the crystallite size distribution from step heating results and the degree of He retention in nature inferred from the 4He/3He method. This observation provides evidence supporting the validity of the 4He/3He technique for estimating retention, and further suggests that the dominant control on He retention in nature is the crystallite size distribution. Experiments under hydrothermal conditions in which goethite remains stable may be an alternative approach to address the question of He diffusion behavior in nature. • Goethite He retentivity explored for (U Th)/He geochronology. • Vacuum step heat experiments cause decomposition of goethite. • These experiments cannot be used to directly infer He diffusivity. • Major control on decomposition behavior is crystallite size. • 4He/3He method likely remains viable for estimating He loss in nature. [ABSTRACT FROM AUTHOR]

This study investigated the impacts of γ-radiation at a final dose of 25 kGy on the physicochemical properties of a range of wet iron oxyhydroxides (2-line ferrihydrite, lepidocrocite, and goethite) synthesized in the presence of varying concentrations of the polysaccharide Na-alginate (starting solutions containing C/Fe ratios of 0, 0.5, 1.0, and 1.5). The degree of impact to the minerals was examined by chemical extractions, and various analytical techniques including XRD, FTIR-ATR, Mössbauer spectroscopy, and N 2 adsorption-desorption isotherms, as well as by assessing their bioavailability towards the model Fe(III) reducing bacteria Shewanella putrefaciens CN32. Across all the coprecipitates studied, it was found that γ-irradiation led to a substantial dissolution of Fe and the concomitant release of Fe(II) and alginate into solution. Despite this observation, the bulk mineralogy and crystallinity of the studied iron oxyhydroxides, as determined by XRD and FTIR-ATR, did not appear to change. However, analyses via Mössbauer spectroscopy (77 and 5 K) revealed that the crystallinity of the 2-line ferrihydrites increased post-irradiation. Moreover, among the minerals studied, the specific surface area and porosity decreased for only the post-irradiated 2-line ferrihydrite coprecipitates with a C/Fe ratio of 0.5 or 1.0. The bioreduction rates of the studied minerals and their irradiated counterparts did not significantly differ, whereas the extent of bioreduction of post-irradiated 2-line ferrihydrite coprecipitates (C/Fe 0.5, 1.0, and 1.5) exhibited a significant increase of up to 28%. In contrast, the extent of Fe reduction for select post-irradiated lepidocrocite (C/Fe 0.5) and goethite (C/Fe 1.5) coprecipitates was slightly higher than determined for their corresponding native controls. The observed differences in bioavailability between the native and irradiated coprecipitates were attributed to irradiation induced alteration of particle aggregation and coagulation as determined by particle size analyses and visual observations. In summary, the findings suggest that even a low total dose of 25 kGy, γ-radiation can lead to significant physicochemical changes in coprecipitates with relatively low organic matter content. Therefore, future research designed to investigate organic matter-Fe composite systems in natural samples should be cognizant of the potential effects of sterilization through γ-irradiation. These effects could potentially result in inaccurate over- or under- estimations of the bioavailability of Fe or organic matter, as well as the possible adsorption capacity of organic matter-Fe coprecipitates for contaminants. • γ-radiation to a final dose of 25 kGy led to significant dissolution and reduction of iron oxyhydroxide-alginate coprecipitates • γ-radiation affected the particle size of 2-line ferrihydrite and lepidocrocite coprecipitates • Pure and 2-line ferrihydrite coprecipitates exhibited increased crystallinity post-irradiation • The bioavailability of 2-line ferrihydrite coprecipitates significantly increased post-irradiation [ABSTRACT FROM AUTHOR]

The nitrate-reducing Fe(II)-oxidizing (NRFO) process plays a crucial role in geochemical Fe N coupled cycling under anoxic conditions. Both nitrate and nitrite can act as electron acceptors for NRFO bacteria, resulting in the formation of Fe(III) minerals. Nevertheless, the stability of Fe(III) minerals generated during the NRFO process remains unclear. In this study, the Fe(III) minerals were obtained using the model NRFO bacteria strain Acidovorax sp. BoFeN1 in the presence of nitrate or nitrite. The stability of the Fe(III) minerals was examined in the presence of the model Fe(III) reduction bacteria Shewanella oneidensis MR-1 (strain MR-1), the chemical reducing reagent sodium dithionite (Na 2 S 2 O 4), and hydrochloric acid (HCl), respectively. The main Fe(III) minerals were goethite in the nitrate treatments (named M nitrate) and tightly attached to the surface of cells. Meanwhile, the main Fe(III) minerals were lepidocrocite in the nitrite treatments (named M nitrite), showing slight adherence to the cell surface. The reductive dissolution kinetics of M nitrate by strain MR-1 revealed a gradual increase in Fe(II). Little Fe(II) was formed when reduced M nitrite , which was attributed to the residual nitrite acted as a competitive electron acceptor to Fe(III), inhibiting the microbial Fe(III) reduction processes. Furthermore, the Fe(III) minerals were harvested by centrifugation and then reacted with sodium dithionite and hydrochloric acid (HCl), respectively. The incorporation of sodium dithionite and hydrochloric acid facilitated the dissolution of Fe(III) minerals. In the M nitrite treatments, the Fe(III) minerals exhibit enhanced stability due to their inherent higher crystallinity and the protective influence of cell surface and microbial carbon. This study broadens our understanding of the formation and transformation of microbial Fe(III) minerals, thereby improving our knowledge of biogeochemical iron cycling under anoxic conditions. [Display omitted] • Goethite/lepidocrocite were formed by the nitrate/nitrite-reducing Fe(II) oxidation • Dissolution of secondary minerals by MR-1, Na 2 S 2 O 4 and HCl were difference in nitrate/nitrite treatment • Goethite exhibited more stability due to its higher crystallinity and surface cell encrustation [ABSTRACT FROM AUTHOR]

Schwertmannite formed under constant pH maintained by alkali additives exhibits great arsenic (As) adsorption capacities and has been used to remediate As-contaminated soil. However, variable soil moisture contents probably induce the phase transformation of schwertmannite, consequently affecting the environmental fate of contaminants in the mineral. This study investigates the effects of extreme water conditions such as water-flooding and wet-to-dry transition at 40 °C in paddy soil on the phase transformation of schwertmannite formed under constant pH (Sch-2.7) or uncontrolled pH (Sch) and their underlying mechanisms. Sch-2.7 exhibited significant phase transformation under the above conditions due to the lower crystallinity and more structural defects, whereas Sch remained stable. In water-flooding environments, goethite was the final transformation product within 120 h. Especially, Sch-2.7 more rapidly transformed into goethite in aerobic environments than in anaerobic environments. This was because Sch-2.7 catalyzed the oxidation of self-released Fe(II) by dissolved oxygen (DO) even under acidic (pH < 2.5) conditions, generating reactive oxygen species (ROS, such as ·OH radical and H 2 O 2), which significantly expedited goethite formation. During the wet-to-dry transition, Sch-2.7 transformed within 4 h, surpassing the speed under water-flooding conditions. However, the extent of transformation was limited, predominantly on the surface. As water progressively evaporated, Fe and SO 4 2− released from Sch-2.7 enriched on its surface, forming a hydrophobic layer of FeOOH and parabutlerite. The surface layer effectively prevented the residual elements in Sch-2.7 from leaching, resulting in a significant leaching reduction of 92.8% for SO 4 2− and 99.7% for Fe, thus delaying further phase transformation. The transformation pathway variations of schwertmannite formed with alkali additives under different moisture contents have important implications for comprehensively understanding its behavior as an As remediation agent in paddy soil. [ABSTRACT FROM AUTHOR]

Although rare-earth elements (REEs) in authigenic phosphates and carbonates derived from marine sediments are potential proxies (e.g., REE patterns) for reconstructing paleoenvironmental conditions, the REE patterns appear to be inconsistent. Some marine authigenic phosphates show a "bell-shaped" pattern with an enrichment of the middle REEs (MREEs; Nd Ho), while others exhibit a "modern seawater-like" pattern characterized by a strong depletion in cerium and an enrichment in heavy REEs (Er Lu, Y, and Sc). Although the origin of the "bell-shaped" pattern in phosphates has been extensively debated, the evidence is often ambiguous; therefore, the origin of the pattern remains unclear. In this study, we investigate the microscale and nanoscale mineralogical and geochemical characteristics of the REE-enriched Early Cambrian phosphorites. In situ elemental analyses reveal that the codeposited phosphates ("bell-shaped" REE pattern) and carbonates ("modern seawater-like" pattern) have different REE patterns. A microscale investigation shows that phosphate grains contain some dispersed goethite nanophase, and a nanoscale investigation using transmission electron microscopy coupled with electron energy-loss spectroscopy shows that Fe2+ is distributed along the rim of the goethite nanophase, indicating that the goethite were reduced. These findings suggest that the REE pattern and concentration in phosphates could have been influenced by the release of REEs during the reduction of Fe (oxyhydr)oxide in the early diagenetic process before the final deposition. Although additional evidence is required, our results are important for understanding the effects of Fe redox cycling on the REE pattern and enrichment in marine authigenic phosphates. In addition, our results suggest that compared with authigenic phosphates, authigenic carbonates would provide a more reliable reconstruction of paleoenvironmental conditions. [ABSTRACT FROM AUTHOR]

Deep-sea REY (lanthanides and yttrium) can be initially scavenged by nano ferromanganese (oxyhydr)oxides via ion adsorption and lattice substitution, while subsequent aging and partial recrystallization induced by reduction during diagenesis usually cause release and redistribution of REY. Previous studies find positive correlations among REY, goethite (α-FeOOH) and phosphorus/phosphate in some marine (micro)nodules, but the mechanism responsible for these correlations is yet unclear. The interaction between iron oxyhydroxides and metal oxyanions has been studied extensively, but no study focuses on interactions between iron oxyhydroxides, REY, and phosphate. This study investigates the immobilization and release mechanisms of REY by synthetic goethite at macro- and molecular scales by comprehensive usage of ad/desorption experiments, inductively coupled plasma mass spectrometry, X-ray absorption fine structure (XAFS) spectroscopy, and density functional theory calculations. The results indicate that goethite exhibits an adsorption preference for heavy REY, displaying obvious lanthanide tetrad effects. Phosphate (0.02 mM ≤ C P ≤ 0.2 mM) however, greatly enhances REY adsorption by forming ternary complexes, particularly light REY. EXAFS interpretations further confirm the formation of ternary complexes in La-adsorbed samples, and that phosphate can link trivalent REY and goethite on the surface. Moreover, some CeIII is further oxidized to CeIV with the help of dissolved oxygen. These observations illuminate how phosphate contributes to rare earth enrichment during early diagenesis. As goethite recrystallization is common in hypoxic environment, we further monitored REY behavior during recrystallization and find that previously adsorbed La/Ce and phosphate are released and combined to form LaPO 4 /CePO 4 precipitates. Importantly, the CeIV reduced during recrystallization indicates that the positive Ce anomaly is likely to pass down, potentially explaining why a positive Ce anomaly is observed in diagenetic nodules and apatite at lower depth. Our study provides insights into the role of phosphate in REY-adsorption on goethite and possible mechanisms of REY enrichment, and desorption during early diagenesis, corresponding with REY patterns observed in marine iron oxyhydroxides. [Display omitted] • Adsorption of ΣREY on goethite reaches up to ∼3000 ppm with the help of phosphate. • Goethite contributes to positive Ce anomaly by CeIII oxidized on its surface by DO. • Goethite recrystallization causes REY redistribution and Ce reduction in sediment. [ABSTRACT FROM AUTHOR]

Soil constitutes a direct sink for elements mobilized due to mining and smelting activities. One of the desired pathways for reducing the bioavailability and toxicity of the contaminants is their transformation into sparingly soluble solid phases. Here, we report the formation of secondary mineral phases in extremely contaminated (up to 210 g Zn kg−1,102 g Pb kg−1, 5.7 g Cd kg−1, and 6.4 g As kg−1, respectively) organic-rich soil. Combining mineralogical techniques (SEM, XRD), a nonspecific sequential extraction (CISED) with Mössbauer and X-ray photoelectron (XPS) spectroscopies evidenced two poorly crystalline goethite components differing in crystallite size and As, Zn, Pb, Ca, Al, P substitutions and minor magnetite associated with plant roots (mainly Deschampsia caespitosa , Equisetum palustre, and Carex rostrate) directly below a layer of smelter-derived particles deposited into the soil. SEM was the only method that unambiguously documented the occurrence of ZnCd sulfide microsize aggregates incrusting plant roots and located in between the (hydrous) Fe oxides. Sequential extraction confirmed a complete As immobilization by goethite, while Cd forms a solid solution with ZnS and is lacking in the Fe hydroxides. The partitioning of Zn and Pb between the goethite and sulfide phases depends on soil water saturation. It is proposed that the coexistence of hydrous Fe oxides and nonferrous metal sulfides in the soil subsurface is possible because of redox heterogeneity of the rhizosphere and the decoupling of sulfur and iron cycles. Low mobility of biogenic sulfide ions and the protecting role of organic matter limits goethite sulfidation. The system remains active, adapting to the seasonally changeable plant roots ecology and fluctuations in water saturation. The obtained results are of value in remediation and managing strategies for contaminated soils and in reconstructing processes related to the formation and/or transformation of low-temperature sulfide deposits. [Display omitted] • Smelter-impacted organic-rich soils differing in water saturation were studied. • Chemical, mineralogical, and spectroscopic methods reveal metal(oid) forms in soil. • As, Pb, Zn, and Cd are sequestered in poorly crystalline secondary mineral phases. • Arsenic is immobilized in goethite, while Cd is entirely encapsulated in sulfides. • Rhizosphere is crucial for mineral phase formation and retention of metal(oid)s. [ABSTRACT FROM AUTHOR]

Niobium (Nb) is considered as one of the most immobile elements during supergene weathering due to its low solubility and the expected resistance of the pyrochlore mineral group (A 2 B 2 X 6 Y) in which Nb occupies the B site. Although the resistance of pyrochlore is challenged by direct evidence of alteration in lateritic profiles, the geochemical and mineralogical processes controlling its behavior in the critical zone remains elusive. This study uses a multiscale geochemical, mineralogical and spectroscopic approach to monitor Nb solid speciation in a thick weathering profile. The weathering of the pyrochlore-bearing core carbonatite facies from Morro dos Seis Lagos has resulted in a Fe-enriched laterite with exceptional Nb concentrations (2.91 wt% Nb 2 O 5 on average). The preservation of the successive secondary phases (Ti-, Fe-, Ce-oxides) resulting from weathering processes along the horizons offers the opportunity to track Nb during pyrochlore alteration from the fresh parent rock to the final stages of weathering. Intense lateritization processes have led to an enrichment in Fe and Ti in the pyrochlore B-sites, resulting in the weakening of the structure and its ultimate destabilization due to weaker B-O bonds. After the release of Nb from pyrochlore, the fate of Nb is dependent on the secondary phases formed during weathering. In the lower horizons dominated by Fe oxides, goethite is the main host for Nb (1 wt% Nb 2 O 5) due to its capacity to substitute Fe3+ for Nb5+, in contrast to hematite. In the upper horizons, where Ti oxides have formed during the reworking of the laterite, Nb5+ is preferentially incorporated into brookite and rutile (20 wt% Nb 2 O 5), which may have formed owing to slightly acidic pH and high Nb activity during their growth. The presence of Nb in hollandite and cerianite in a vein from the manganiferous horizon involves the transport of Nb with mobile elements at the scale of the laterite. With this study, we describe a new model accounting for the formation of Nb-enriched laterites. The latter is based on the ability of Nb5+ to substitute for Fe3+, Ti4+, Ce4+ in minerals formed by lateritization processes after its release from pyrochlore. The nature of the secondary Nb-bearing oxides is dependent on the chemistry of the parent rock and the conditions of formation of the laterite. This work, evidencing the mobilization of Nb both at the scale of the mineralogical assemblage and of the entire profile questions the pervasive use of Nb as a geochemical invariant in the critical zone. [ABSTRACT FROM AUTHOR]

The redistribution and transformation of arsenic (As) during the Fe(II)-catalyzed transformation of As-adsorbed ferrihydrite remains unclear. This study investigated the re-crystallization process and products of As-adsorbed ferrihydrite accelerated by Fe(II) and the fate of arsenate and arsenite at pH 7 under anaerobic condition. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and synchrotron based X-ray absorption near edge structure (XANES) were utilized for the As and Fe speciation analysis. Results showed that both As(V) and As(III) can retard the transformation of ferrihydrite. The concentration and speciation of As strongly influenced the mineralogy and morphology of produced crystalline Fe oxyhydroxides. The higher As(V) loading (Fe/As = 40–100) resulted in the conversion of most ferrihydrite to lepidocrocite instead of goethite. The adsorbed As(III) showed weak effect on the re-crystallization of ferrihydrite at the early stage. A considerable fraction of As(III) (70–80%) was oxidized to As(V) during the Fe(II) catalyzed transformation of As(III)-adsorbed ferrihydrite. The oxidation of As(III) to As(V) in turn retarded the transformation of the remaining ferrihydrite to goethite and induced the production of a small amount of lepidocrocite. A temporary release of As(V) was observed during the first 5 days and the released As(V) was re-adsorbed back to the crystalline products, whereas no As was released to the aqueous phase in the As(III)-ferrihydrite systems. During the re-crystallization of ferrihydrite, 1 M phosphate-extractable As decreased quickly by up to 90% in the solid phase, indicating that partial As transformed to more stable phase. Our results suggested that Fe(II)-catalytic transformation of As-adsorbed ferrihydrite could help reducing the mobility and toxicity of As in anoxic aquifers. Unlabelled Image • Concentration and speciation of As influence transformation of ferrihydrite. • More than 80% of As(III) is oxidized to As(V) by Fe(II)-activated ferrihydrite. • Considerable fraction of adsorbed As transformed to phosphate-unextractable phase. [ABSTRACT FROM AUTHOR]

Microbial reduction of Fe(III) is an important biogeochemical process in anoxic or acidic environments. However, this process under oxic and pH-neutral conditions is understudied, and the iron reduction capability of the phylum Actinobacteria is poorly known. In this study, we investigated the capacity of diverse actinobacteria to reduce Fe(III) in the presence of oxygen and at pH 7. From 277 actinobacterial strains isolated from deep sea and terrestrial soil, 109 strains (39.40%) belonging to 17 genera showed the capacity of reducing FeCl 3 under oxic and initially neutral pH conditions. Among the 109 strains, 49 spanning 13 genera also reduced ferric iron oxides under the same conditions. Four non-filamentous strains from four genera were selected for further analyses. All the four strains showed the capacity to reduce goethite under oxic and pH 7 and 8 conditions. Microbial fuel cell (MFC) experiments showed a current enhancement when the strains were cultured in the presence of goethite. Living actinobacterial cells and their metabolites played a key role in reducing Fe(III). The strains produced organic acids and siderophores, which likely promoted Fe(III) reduction due to altered reduction potentials of complexed Fe(III) and inhibition of Fe(II) oxidization by oxygen. SEM/EDS and XRD analyses detected vivianite in the culture of Kocuria oceani FXJ8.057 with goethite, where, unexpectedly, K. oceani FXJ8.057 produced nanowire-like structures. These results demonstrate that actinobacteria are able to reduce Fe(III) in both aqueous and solid forms under oxic and pH-neutral conditions. The mechanisms, however, still need further investigation. [ABSTRACT FROM AUTHOR]

The transformation of Al-substituted goethite in oxidative and reductive atmosphere, which models natural fire, was investigated in detail. Various characterization results indicated that Al-substituted goethite transformed into hematite in an oxidative atmosphere and magnetite followed by zero-valent iron in a reductive atmosphere. Interestingly, the substitution of Al for Fe not only changed the crystal morphology affecting crystal surface reactivity, but also restrained the transformation of goethite into hematite, magnetite and zero-valent iron. In addition, the Al in the goethite was embedded into the crystal structure of thermally formed hematite and magnetite, considerably influencing their surface reactivity. The macroscopic adsorption results indicated that the substitution of Al for Fe increased the adsorption capacity of goethite and the corresponding derivatives except zero-valent iron. For the same Al amount, the adsorption capacity followed an order of goethite > hematite > magnetite, implying a loss of phosphate in goethite-rich soil after experiencing natural fire. However, with increasing in temperature in the reductive atmosphere, Al-magnetite transformed into a mixture of Al-magnetite and zero-valent iron which displayed excellent phosphate adsorption capacities increasing to 1.21–5.96 mg/g. The phosphate adsorption behaviors to thermally formed products were fitted well by surface complexation modeling with five complexation sites, which were more obvious for goethite than for hematite and magnetite. These findings presented in this study represent significant progress toward an understanding of the migration, enrichment and transformation of phosphate in Al-substituted goethite-rich soil in cases experiencing natural fire. [ABSTRACT FROM AUTHOR]

The formation of water-stable, hierarchical aggregate structure is one of the critical processes in eco-engineering iron (Fe) ore tailings into soil-like medium for sustainable rehabilitation of Fe ore mine site. Through systematically comparing physical structure and mineralogical differences between Fe ore tailings' aggregates and Fe-rich native soil aggregates at a magnetite-Fe ore mine, the present study captured the microstructure, mineralogy and organic matter composition of aggregates in aged (4 years old) tailings, in comparison with those in native soils. A suite of micro-spectroscopic methods have been employed to elucidate their physical, mineralogical and morphological characteristics, including synchrotron based Fe K edge X-ray absorption fine structure spectroscopy (XAFS), and backscattered electrons (BSE)-scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The results revealed significant differences in physical structure of aggregates in Fe ore tailings and natural soils rich in Fe, which was attributed to their different mineralogical and organic components and characteristics. Especially, it was found that the secondary Fe (oxy)hydroxides (i.e., goethite, ferrihydrite) and their interactions with Al/Si rich secondary phyllosilicates (or aluminosilicates) were required in the formation of amorphous Fe-Si-Al rich gels that acted as cementing agents for agglomerating Si-rich particles (e.g., quartz) in soil. In contrast, tailing aggregates (rich in more crystalline primary minerals such as magnetite and biotite) lacked these gels, resulting in poor stability. Comparatively, natural soil aggregates contained more recalcitrate organic carbon groups (e.g., aromatic, carboxyl and aliphatic C) than tailing aggregates, which would have also contributed to the improvement of aggregate stability through organo-mineral associations. This study has improved our understanding of key limiting geochemical factor(s) involved in the aggregation of Fe rich soils, which would provide the basis for formulating effective eco-engineering inputs to accelerate the development of soil-like structure in the Fe ore tailings for sustainable rehabilitation. • Microstructures of aggregates within Fe ore tailings and natural Fe-rich soils were distinctly different; • Soil aggregates were bound with Fe-Si-Al rich gels from interactions between Fe (oxy)hydroxides and secondary phyllosilicates; • Tailing aggregates lacked secondary phyllosilicates and functional organic groups, hindering formation of stable aggregates; • Plant root activities facilitated water-stable aggregate formation. [ABSTRACT FROM AUTHOR]

Rock varnish is a widespread Mn-rich rock coating commonly developed in arid environments, but the mechanism for its formation is still under debate. In this study, rock varnish and adjacent soil dust collected from Gobi Desert were analyzed for exploring the possible abiotic oxidation mechanism of Mn oxides. The occurrence of rock varnish shows a direct and close relationship with strong irradiation of sunlight, giving the first evidence of photochemical genesis. Abundant caves and tunnels with average diameter of ~1–5 μm are observed on varnish surface regions, which facilitate the penetration of sunlight and water. Metal (oxyhydr)oxides, including birnessite, hematite, goethite, rutile and anatase, account for the major components of rock varnish, which are all solar light-responsive semiconducting minerals. The trace elements enrichment patterns revealed by LA-ICP-MS provide the indication of an aqueous origin. The positive Ce anomalies in varnish in contrast to the rock substrate, as well as the positive correlation between Ce and Mn suggest a strong oxidizing environment in the genesis of rock varnish. Therefore, the photo-generated holes and reactive oxygen species (ROSs) on metal oxides in varnish can promote Mn(II) oxidation, which are further demonstrated from thermodynamic and kinetic considerations. ROSs including 1O 2 and OH with strong oxidizing capability are detected by EPR in varnish suspension, providing the direct evidence for Mn(II) oxidation. Laboratory experimental simulations show the photocatalysis of metal oxides in rock varnish greatly promote Mn(II) oxidation by 2.10–7.97 times in comparison with homogenous solution oxidation. All these lines of evidence suggest that light-induced abiotic process may play important roles in the formation of rock varnish together with other abiotic and biotic pathways, which can even provide implications for the evolution of Mn oxides on surface of terrestrial planets. [ABSTRACT FROM AUTHOR]

Both cobalt (Co) and nickel (Ni) substitute for Fe in goethite (α-FeOOH) as well as adsorb on goethite. Co and Ni-rich goethite are the dominant ore mineral in oxide-type laterite deposits. A quantitative understanding of the aqueous solubility of Ni- and Co-rich goethite would help in modelling the formation of laterites and developing methods for metal extraction. In this contribution, we determined the aqueous solubility of substituted nickel and cobalt goethite in both the binary and ternary systems as a function of pH < 1 to 5 and varying concentrations found in Ni laterites. We found the dependency of the solubility product (K sp) of goethite to the type of incorporated ion, concentrations, and pH. The solubility of Ni substituted goethite increases with increasing pH and increasing Ni substitution. A similar trend is observed for Co, although its substitution in goethite appears to increase the stability of goethite. Ksp increases with increasing Ni and Co substitution in bi-metal (Ni and Co) goethite, and are generally low compared to those of single metal substituted goethite with the same concentration. These findings have some implication for substitution mechanism and Ni and Co extraction from laterites and soils. The Ni(OH) 2 -Co(OH) 2 -FeOOH solid solution simulation with PHREEQC is inconsistent with our experimental data and reveals "nonideality" of mixing systems with different chemical and thermodynamic properties as end members, but attained equilibrium with goethite at lower pH. [ABSTRACT FROM AUTHOR]

Abstract The mineral ikaite (CaCO 3 ·6H 2 O) precipitates from a mixture of spring water and seawater as tufa columns which grow at a rate of up to 50 cm per year reaching heights of up to 18 m in Ikka Fjord, SW Greenland. In the fjord, column formation occurs only at the base of a nepheline syenite‑carbonatite complex that flanks the fjord and an association has therefore been proposed. The spring water that seeps up at the bottom of the fjord is oversaturated in Na+ and HCO 3 −. In this study, we show that these ions were acquired by alteration reactions in the syenite‑carbonatite complex: Na+ is released during replacement of nepheline by illite and analcime in nepheline-syenite rocks and HCO 3 – is released by oxidation of siderite to goethite in carbonatite rocks. The chemically charged groundwater mixes with seawater and gives rise to the formation of the tufa columns. We performed a mass balance to show that the mass of the carbonatite in the complex is more than sufficient to provide the CO 2 needed to produce the observed mass of tufa columns. We estimated a time frame of ~600 years to produce the necessary CO 2 to form the 700 ikaite columns in the fjord. [ABSTRACT FROM AUTHOR]

Abstract We have measured the sorption of Ni as a function of surface loading and pH with a series of laboratory experiments. We fit the data using a multisite surface complexation model incorporating the basic Stern model to describe the surface electrostatics. At 0.75 wt% Ni and above, however, we cannot model the sorption data in terms of simple surface complexation but must invoke polynuclear complexes (or surface precipitates) to fit the sorption edges. In addition to surface complexation and surface precipitates, Ni is associated with goethite via structural incorporation. After 21 days of sorption, about 90% of the sample with the highest Ni concentration is reversible while 60% of the sample with the lowest concentration (0.03 wt% Ni) is irreversible. In contrast, after 365 days of sorption about 50–60% of the sorbed Ni is irreversible in all the grades (0.003–2.56 wt% Ni) suggesting almost uniform structural incorporation in the lattice of goethite. Highlights • Nickel forms mononuclear, binuclear and polynuclear complexes with goethite. • After 21 days of sorption, 60–90% of sorbed Ni is reversible in all grades. • After 365 days of sorption, 50–60% of the sorbed Ni is irreversible in all grades. • Thermodynamic constants for modelling nickel sorption are predicted. [ABSTRACT FROM AUTHOR]

Microbially nitrate-reducing Fe(II) oxidation (NRFO) is widespread in anoxic environments at circumneutral pH, in which Fe(II) oxidation is considered to be mainly catalyzed by nitrite via chemodenitrification after microbial nitrate reduction. Cadmium (Cd) is toxic to organisms, and its availability is strongly affected by iron (oxyhydr)oxides and/or bacteria. However, it remains unclear how NRFO affects the fate of Cd with environmentally relevant concentrations, and the relative contributions of microbes and secondary minerals to Cd immobilization during NRFO. Here, Acidovorax sp. BoFeN1, a typical NRFO bacterium, was used to investigate the Cd immobilization during NRFO at pH 7.0 with 0.1, 0.5, and 1.0 mg/L Cd, respectively. Higher initial Cd concentration resulted in a higher amount of Cd immobilized in precipitates, but showed stronger inhibition on nitrate reduction, acetate metabolism, and cell growth. The presence of Cd barely influenced the Fe(II) oxidation during NRFO or chemodenitrification, but retarded the mineral transformation to secondary crystalline minerals, i.e., lepidocrocite and goethite. The extracellular polymeric substances deposited in the bacteria-mineral precipitates were rich in tyrosine-like and tryptophan-like proteins with negatively charged amino and carboxyl groups. The STEM-HAADF images with EDX mapping demonstrated that Cd was strongly co-localized with Fe on the cell surface and in the periplasm. Cadmium immobilization experiments with bacterial cells or during chemodenitrification indicated that the bacterial cells and secondary minerals formed during chemodenitrification are comparably important for Cd immobilization in the Cd0.1 treatment, while the secondary minerals played a predominant role in Cd immobilization in the Cd0.5 and Cd1.0 treatments. These results suggested that Cd was likely associated with the secondary iron minerals, which precipitated with the proteins on the cell surface and in the periplasm. Our findings provide a comprehensive understanding of the roles of Cd, bacterial cells, and secondary minerals formed via chemodenitrification, as well as their interactions during the complicated NRFO process at circumneutral pH. • More Cd is immobilized during the NRFO process with the increasing Cd. • Higher Cd contents show stronger inhibition on nitrate reduction and cell growth. • Lepidocrocite and goethite are the main secondary minerals formed during NRFO. • Bacterial cells and Fe minerals contribute to Cd immobilization in Cd0.1 treatment. • Fe minerals are more important than bacterial cells in Cd immobilization in Cd0.5 and Cd1.0 treatments. [ABSTRACT FROM AUTHOR]

The habitability of the hadal zone is not well understood and how microbes survive and adapt to this extreme environment is still largely unknown. Here we report the discovery of microbial biofilm that experience extensive Fe-mineralization on the surface of rocks from the Challenger Deep in the southern Mariana Trench. We found microbial biofilms of abundant globules internally permineralized by acicular goethite. The EDS analyses, combined with SEM and TEM, show that the globules are enriched in carbon and phosphorus, potentially indicating their biological origin. Cross sections of globules reveal a bilayer structure consisting of an acicular nano-crystalline core and a colloidal envelope. Vibrational spectroscopy reveals that this colloidal envelope is composed of EPS (extracellular polymeric substances). This mineralization is completely different from Fe-mineralization induced by all other known Fe-oxidizing bacteria, suggesting previously unknown interactions between Fe and microorganisms in hadal settings. Our findings highlight the significance of Fe-mineralization and its contributions to the ecosystem in hadal environments. • Discovery of microbial biofilm that experience extensive Fe-mineralization on basalts from the Mariana Trench. • Microbial biofilms of abundant globules internally permineralized by acicular goethite. • Study highlights the significance of Fe-mineralization and its contributions to the ecosystem in the deepest part of the world's ocean. [ABSTRACT FROM AUTHOR]

Stable isotope is a potent geochemical tracer unveiling the dynamic sources of cadmium (Cd) in the environment. However, the sorption process of aqueous Cd onto solid surfaces (e.g., soil) fractionates isotopic signatures, which makes it difficult to interpret its sources and migration processes. Understanding how Cd fractionates during sorption to soil minerals, therefore, provides necessary data to open the isotope fractionation black box of liquid-solid phase partition of Cd. In this study, we explored isotopic fractionation characteristics during Cd sorption to typical soil minerals. Distinct Cd isotope fractionation was observed during sorption, including goethite (∆ 114 / 110 Cd solid − solution = − 0.51 ∼ − 0.47 ‰), montmorillonite (− 0.34 ∼ − 0.32 ‰), quartz (− 0.34 ∼ − 0.30 ‰), gibbsite (− 0.29 ∼ − 0.25 ‰), and kaolinite (− 0.09 ∼ − 0.07 ‰), where lighter isotopes tended to be preferentially sorbed to the solid phase in a reversible manner. We've also conducted a field survey in a historical flooding-affected region to explore Cd isotopic compositions of soil samples adjacent to a polluted creek. Distinct isotopic signatures for surficial soils and deep soils were observed, which was depicted by a binary mixing mode. Leaching of Cd from mine tailings to the creek, as well as its re-adsorption to soil during historical flooding events both led to preferential enrichment of light isotopes in the solid phase, which was consistent with findings from lab sorption experiments. Mineralogical analyses including EPMA, TOF-SIMS, Mössbauer spectra, XRD, and chemical extraction tests, were applied to unveil how Cd was bound to soil minerals at field. Possible linkage between lab and field data were discussed on the basis of these findings. Results from this study are encouraging for future application of Cd isotopes for source tracing. [Display omitted] • How Cd isotopes fractionate during sorption to soil minerals were explored. • Lighter isotopes tend to be preferentially sorbed to the solid phase reversibly. • Field survey was conducted in a flooding-affected area. • Lab fractionation data were used to correct field source apportionment. [ABSTRACT FROM AUTHOR]

Iron (Fe) oxyhydroxides are major phases that may control the cerium (Ce) behavior in the Earth's Critical Zone. However, understanding Ce behavior with Fe oxyhydroxides remains uncompleted. Especially, if thermodynamic calculations suggest that Fe(III) is not a sufficiently strong oxidant, several studies reported the presence of Ce(IV) onto Fe oxyhydroxides. In this study, multiple approaches, including modeling and X-ray absorption spectroscopy, deciphered the driving forces of Ce(III) oxidation to Ce(IV) onto goethite. Results showed that oxidized Ce occurred onto goethite with a Ce(III)/Ce(IV) ratio depending on the Ce concentration in the solution ([Ce] tot). The percentage of Ce(IV) onto goethite ranged from 20% to 50%, linearly increasing with [Ce] tot. Comparable observation with a redox-inert Al-hydroxide (gibbsite), allowed to rule out the importance of Fe(III) redox reactivity as the main driver of Ce(III) oxidation. Instead, thermodynamic calculations suggested that surface precipitation of Ce(IV)-hydroxides, whose formation is favored with increasing [Ce] tot , was an important driving force of the redox reaction. Because the goethite surface seemed to stabilize more strongly Ce(IV) than Ce(III) surface species than does gibbsite, differences in binding mechanisms of Ce(III) and Ce(IV) onto different mineral surfaces have been suggested to play a role on Ce redox speciation. [Display omitted] [ABSTRACT FROM AUTHOR]

The adsorption of extracellular polymeric substances (EPS) onto soil minerals is an important process for understanding bacterial adhesion to mineral surfaces and environmental cycling of nutrients and contaminants. To clarify the molecular level mechanisms and processes of EPS adsorption, the interaction mechanisms between EPS and goethite was explored using two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (CoS) assisted by C 1s near edge X-ray absorption fine structure spectroscopy (NEXAFS). Results show that the amide functional groups of EPS play an important role in its adsorption on goethite, and the adsorption of EPS-proteins on goethite is a function of electrolyte concentration, with increasing adsorption at a higher electrolyte concentration. Results also show that the order in which the EPS functional groups interact and bind with goethite is dependent on electrolyte concentration, where carboxyl and phosphoryl functional groups are the first to adsorb at low electrolyte concentration, while amide groups are the first to adsorb at higher electrolyte concentration. Deconvolution and curve fitting of the amide I band at the end of the adsorption process (~300 min) shows that the secondary structure of proteins is converted from a random coil conformation to aggregated strands, α-helices and turns. This conversion leads to increased adsorption of EPS-proteins and explains the overall adsorption increase of EPS on goethite surfaces with an increasing concentration of electrolyte. Furthermore, the adsorption of the carboxyl functional groups of EPS decreases with increasing electrolyte concentration, likely due to more effective screening of the goethite surface charge with increasing concentration of electrolyte. The integrated results from ATR-FTIR and 2D-CoS allow us to construct a comprehensive overview of EPS-goethite interaction processes at the molecular level, which can be used to improve our understanding of EPS-mineral interactions in the natural environment. These results also provide fundamental information for a better understanding of bacterial biofilm formation on soil and sediment minerals, and facilitate research on the subsequent interaction of nutrients and contaminants with the reactive constituents of biofilms in natural and contaminated environments. [ABSTRACT FROM AUTHOR]

Iron and manganese oxides are ubiquitous in soils and sediments, and their formation and conversion processes affect the migration and transformation of heavy metals and organic pollutants. Mn 2+ was found to affect the formation of iron oxides from Fe 2+ oxidation. However, little attention has been paid to the oxidation process at the initial stage and generation of manganese oxides in the reaction system of Fe 2+ aq and Mn 2+ aq . This work investigated the formation process and mechanism of iron and manganese oxides in a mixed solution system of Fe 2+ aq and Mn 2+ aq under oxic condition. The effect of pH (5.0–9.0) on the reaction process and products was further studied. The results indicated that Fe 2+ aq was oxidized to goethite and lepidocrocite, and Mn 2+ adsorbed on the surface of iron oxides was catalytically oxidized to poorly crystalline manganese oxides by dissolved O 2 in a mixed solution of Fe 2+ aq and Mn 2+ aq . Compared with the absence of Mn 2+ aq , the presence of Mn 2+ aq caused no obvious changes in the species of iron oxides, but significantly increased the oxidation rate of Fe 2+ aq . Formation of manganese oxides on the surface decreased the crystallinity of the iron oxides. The increase of pH enhanced the oxidation of Fe 2+ aq and Mn 2+ aq and formation of lepidocrocite. This work expands our understanding of the interactions and geochemical processes of Fe 2+ aq and Mn 2+ aq . [ABSTRACT FROM AUTHOR]

Organo-mineral composites formed by the association of iron (hydr)oxides and organic matter are widespread in natural environments and play an important role as scavengers of bioessential elements and contaminants. To better understand the mobility and fate of Cu in natural soils and sediments we precipitated ferrihydrite and goethite organo-mineral composites using humic acid as an analogue for natural organic matter, with organic carbon content in the composites ranging from 2 to 16 wt% C. We then measured the adsorption of Cu to the end-member mineral and organic phases and the composites as a function of pH and Cu concentration. We determined the molecular mechanisms of Cu adsorption to the end-member phases and the composites, and used this information to develop molecularly constrained thermodynamic surface complexation models to quantify Cu adsorption. By combining our work here with previous work on the adsorption of Cu to ferrihydrite-bacteria composites, we provide insight into the predominance of Cu-carboxyl binding for Cu adsorption to iron (hydr)oxide organo-mineral composites, and the nature of Cu adsorption behaviour across a range of iron (hydr)oxide composites composed of different minerals and different types of organic matter. Taken as a whole our results show that Cu adsorption to the carboxyl group present in organic matter coatings on iron (hydr)oxides is likely common to most iron (hydr)oxide composites, such that Cu-carboxyl binding provides a key control on the fate and mobility of Cu in soils and sediments. Our work also suggests there is a universal adsorption behaviour for Cu adsorption to ferrihydrite organo-mineral composites, in which the mineral:organic mass ratio is a crucial parameter for determining Cu uptake. Overall we show that ferrihydrite composites composed of different types of organic matter and containing a wide range of organic mass ratios, but where the mineral is the dominant composite fraction, possess additive Cu adsorption behaviour which can be predicted using a component additivity surface complexation model. [ABSTRACT FROM AUTHOR]

This study focuses on the dating of millimetric pisoliths from ferruginous duricrusts located in central Amazonia (Brazil), by coupling detailed mineralogy and two relevant dating methods: (i) (U-Th)/He analysis of goethites by mass spectrometry; (ii) analysis of radiation-induced defects of kaolinites embedded in duricrusts by electron paramagnetic resonance spectroscopy (EPR). Three samples collected at different elevations in the landscape were selected. The goethite corrected ages range from 1.1–15.2 Ma and consistently increase with elevation. EPR was performed on kaolinites after several cycles of a deferration procedure. Considering extreme geochemical scenarii, the ages of kaolinites are 1.7–7.1 Ma for a closed system and 4.0–16.7 Ma for an open system with 100% Rn loss. Ages of goethites and kaolinites are close, in the limit of the uncertainty of the methods. They show important periods of duricrust formation at Middle Miocene and at Late Miocene/Pliocene. Data allow the estimation of lowering rates between 3 and 8 m/Ma, in agreement with independent data available in the literature for the central Amazonia region or even for erosion of other cratons in the world. The two dating methods can be compared on common duricrust samples. By revealing single or multi-step weathering/erosion episodes related to relictual paleosurfaces, they also bring promising contributions to an advanced knowledge of ancient and complex lateritic geosystems. [ABSTRACT FROM AUTHOR]

The presence of ferrihydrite in sediments/soils is critical to the cycling of iron (Fe) and many other elements but difficult to quantify. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to speciate Fe in the solid phase, but this method is thought to have difficulties in distinguishing ferrihydrite from goethite and other minerals. In this study, both conventional EXAFS linear combination fitting (LCF) and the method of standard-additions are applied to the same samples in attempt to quantify ferrihydrite and goethite more rigorously. Natural aquifer sediments from Bangladesh and the United States were spiked with known quantities of ferrihydrite, goethite and magnetite, and analyzed by EXAFS. Known mineral mixtures were also analyzed. Evaluations of EXAFS spectra of mineral references and EXAFS-LCF fits on various samples indicate that ferrihydrite and microcrystalline goethite can be distinguished and quantified by EXAFS-LCF but that the choice of mineral references is critical to yield consistent results. Conventional EXAFS-LCF and the method of standard-additions both identified appreciable amount of ferrihydrite in Bangladesh sediments that were obtained from a low‑arsenic Pleistocene aquifer. Ferrihydrite was also independently detected by sequential extraction and 57 Fe Mӧssbauer spectroscopy. These observations confirm the accuracy of conventional EXAFS-LCF and demonstrate that combining EXAFS with additions of reference materials provides a more robust means of quantifying short-range-ordered minerals in complex samples. [ABSTRACT FROM AUTHOR]

Although graphene-based materials have been shown to physicochemically interact with biotic or abiotic particles after entering into the environment, little is known about their impacts on biotransformation of inorganic colloids. Accordingly, the effects of different reduced graphene oxide (rGO) materials on ferrihydrite reduction were investigated by using Shewanella oneidensis MR-1 and lactate as Fe(III) reducer and electron donor, respectively. rGO at 0.5 mg L − 1 stimulated the transformation of ferrihydrite to magnetite, possibly by improving electron transfer. In contrast, higher concentrations of rGO (10–100 mg L − 1 ) hindered the exposure, accessibility and therefore reduction of ferrihydrite, resulting in the formation of a mixture of goethite and magnetite. The adsorption of soil humic acid by coexisting ferrihydrite and rGO resulted in further inhibition of Fe(III) reduction. On the other hand, the enhanced Fe(III) bioreduction by the less adsorbable aquatic humic acid or simpler flavin molecules was further improved by 0.5 mg L − 1 rGO but inhibited by 50 mg L − 1 rGO. Additionally, the settling stability of the rGO-mineral aggregates was altered during bioreduction, which could influence the transport and reactivity of iron mineral and the fate of associated rGO. This study improves our understanding of the impact and behavior of rGO in the environment. [ABSTRACT FROM AUTHOR]

Recent literature has documented systematic variations in the Ni stable isotope compositions of several terrestrial and marine reservoirs of this bioessential trace metal. For example, dissolved Ni in the world's major rivers has been shown to be more enriched in heavy isotopes than the continental rocks from which that Ni was derived (Cameron and Vance, 2014, Geochim. Cosmochim. Acta , 128, 195–211). This observation implies that one or more chemical reactions occurring during weathering of Ni-rich rock drives stable isotope fractionation that results in retention of lighter isotopes of Ni associated with solid phases. We present new Ni isotope analyses of samples from various horizons of the Webster-Addie laterite, near Democrat, North Carolina, USA, as well as results from two sets of experiments designed to constrain the fundamental mechanism(s) driving the isotopic variation observed in that weathering profile. Bedrock samples (dunite and talc-rich, altered dunite) have δ 60/58 Ni values of + 0.06 to + 0.20‰, in excellent agreement with previous research. A sample of “yellow laterite” is enriched in lighter isotopes of Ni, with δ 60/58 Ni = − 0.21‰. Mixed-phase samples containing quartz and traces of serpentine, goethite, and smectite range from + 0.13 to + 0.35‰. Experimental leaching of Ni from olivine in dilute acid at ambient conditions results in no resolvable fractionation, indicating that initial release of Ni to solution from bedrock is not responsible for the isotope variation we observe in the natural samples. Sorption of Ni onto montmorillonite, a minor secondary weathering product present in laterites, is associated with a very small fractionation of + 0.11 ± 0.09‰, with lighter isotopes sorbed on the smectitic clay. Such adsorption can thus account for some, but not all of the isotope systematics observed in the weathering profile. Given previous evidence that sorption of Ni to Fe oxyhydroxides does drive a fractionation of appropriate sense and magnitude (Wasylenki et al., 2015, Chem. Geol. , 400, 56–64), we infer that oxidation of Fe 2+ released from bedrock and precipitation result in retention of a light pool of Ni in solid weathering products, thereby enriching dissolved Ni in heavier isotopes. Because much of the continental crust's Ni budget is hosted by mafic and ultramafic rocks, which weather very rapidly at Earth's surface, and for which Fe oxyhydroxides are extremely common weathering products, this Ni fractionation mechanism may be relevant for terrestrial weathering of Ni-bearing rocks in general, even though the overall mineralogy and chemistry of weathering profiles varies with parent lithology and environmental conditions. Secondary, hydrous, Ni-rich Mg-silicates from veins in the saprolite zone are enriched in heavy isotopes, with δ 60/58 Ni = + 0.29 to + 0.86‰. Similar Ni-rich Mg-silicate samples from several similar profiles around the world display a similar range from + 0.36 to + 0.90‰, suggesting that the processes responsible for isotopic fractionation at the North Carolina locality are general phenomena characteristic of laterites worldwide. We hypothesize that, in laterites, continual downward percolation of meteoric waters and progressive, chromatographic exchange with goethite-bound Ni progressively shifts the Ni contents of solids toward lighter and fluids toward heavier isotopic compositions. When fluids reach the Mg-rich, Si-rich saprolite zone, isotopically heavy garnierite minerals precipitate in veins. The phenomena we report and discuss here may long have been the dominant controls on the distribution of Ni isotopes in Earth's near-surface, terrestrial environment, at least since oxygenation of the atmosphere. Due to lack of abundant oxygen in air and water earlier in Earth history, the biogeochemical cycle of Ni isotopes was almost certainly rather different, such that riverine dissolved Ni was not necessarily enriched in heavy isotopes relative to continental crust, and the oceans may not have been so highly enriched in heavy isotopes of Ni as they are presently. [ABSTRACT FROM AUTHOR]

The hydrogen and oxygen stable isotope composition (δ 2 H and δ 18 O values) of mineral hydration waters can give information on the environment of mineral formation. Here we present and validate an approach for the stable isotope analysis of mineral hydration waters based on coupling a thermogravimetric analyzer with a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. TGA-IRIS generates δ 2 H and δ 18 O values of liquid water samples with precision for δ 2 H of ± 1.2‰, and for δ 18 O of ± 0.17‰. For hydration waters in goethite, precision for δ 2 H ranges from ± 0.3‰ to 1.6‰, and for δ 18 O ranges from ± 0.17‰ to 0.27‰. The ability of TGA-IRIS to generate detailed water yield data and δ 2 H and δ 18 O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. TGA-IRIS analyses of hydrogen isotopes in goethite yields δ 2 H values that reflect the hydrogen of the OH − phase in the mineral and are comparable to that made by IRMS and found in the literature. In contrast, δ 18 O values on goethite reflect the oxygen in OH − groups bound to Fe (Fe-OH group), and not the oxygen bound only to Fe (Fe-O group) in the mineral crystal lattice, and may not be comparable to literature δ 18 O values made by IRMS that reflect the total O in the mineral. TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in goethite, which may allow the mineral to be used as a single mineral geothermometer. TGA-IRIS measurements of hydration waters are likely to open new avenues and possibilities for research on hydrated minerals. [ABSTRACT FROM AUTHOR]

Few studies have addressed the mobility of tungsten in mine waste, which could act as a point source for metal leaching of this potentially toxic metal. This study examines the behavior of tungsten in acidic floodplain tailings and circumneutral impoundment tailings at the Cantung Mine in the Northwest Territories, Canada. In the Cantung deposit, tungsten is present as scheelite (CaWO 4), copper is hosted in chalcopyrite (CuFeS 2), and gangue mineralogy consists of abundant pyrrhotite (Fe 1-x S), silicate minerals, and carbonate minerals. Samples of surface waters, tailings porewaters, and solid tailings samples were collected from Cantung Mine. For surface waters, both filtered (0.45 μm) and unfiltered aliquots were collected to compare dissolved and particulate element concentrations. Tungsten concentrations range from 5.8 to 30.0 μg/L in water samples with pH values between 7.1 and 8.1, whereas samples below pH 7.1 have concentrations below the detection limit of 0.1 μg/L. Tungsten and iron concentrations are both on average 1.7 times higher in unfiltered aliquots compared to filtered aliquots, suggesting that tungsten is transported as dissolved species but is also adsorbed to suspended Fe-oxyhydroxide minerals at neutral pH. Tailings were analyzed by scanning electron microscopy (SEM) paired with automated mineralogy software (MLA), synchrotron-based μX-ray diffraction and fluorescence (μXRD-XRF), and partial leach digestions with inductively coupled plasma mass spectrometry (cold and hot hydroxylamine hydrochloride, aqua regia , Li-metaborate fusion, and 4-acid digestion with HNO 3 -HClO 4 -HF-HCl). Samples from the impounded tailings and the highly oxidized Flat River Tailings distributed on the floodplain (FRT) have similar scheelite content (0.15 wt% and 0.21 wt%, respectively), and scheelite shows no evidence of alteration. Synchrotron-based μXRD of Fe-oxyhydroxide minerals in the FRT identified goethite (FeOOH) and lepidocrocite (γ-FeOOH) as products of pyrrhotite oxidation. Rare rims on pyrrhotite from the tailings impoundment have μXRD spectra consistent with hematite (Fe 2 O 3) and maghemite (γ-Fe 2 O 3). The μXRF maps of the hematite-maghemite rims have prominent tungsten peaks, which represent included scheelite grains and possibly structurally incorporated tungsten. The hydroxylamine leaches yield higher tungsten concentrations in tailings samples from the circumneutral impoundments than samples from the acidic FRT, suggesting the tailings impoundments have more tungsten that is associated with poorly crystalline and amorphous Fe-oxyhydroxide phases than the FRT. Over time, labile-hosted tungsten in the FRT may have been washed down the Flat River during Fe-oxyhydroxide recrystallization and high energy flooding events. These results indicate that scheelite has limited solubility in neutral or acid mine waters, and the mobility of the small amount of tungsten released is governed by the present of iron oxyhydroxide minerals, the transport of colloidal material in surface water, and the stability of the depositional environment. [ABSTRACT FROM AUTHOR]

Iron and manganese oxides embody a geochemical system of great environmental, biological, and economical relevance. Chemical equilibria and the stability of Fe Mn phases under surface or near-surface conditions can influence the fate of contaminants in the environment, impact biological metabolic processes, or Fe and Mn phase distribution in weathered ores. In the present work, we focus on the textural, mineralogical, and chemical study of Fe Mn ores from the weathered zone of a vein-hosted deposit outcropping in the Iberian Pyrite Belt, SW Portugal, by means of micro-Raman spectroscopy coupled with electron microprobe microanalysis. The aims of our investigation are i) identifying the several Fe and Mn phases occurring in differently enriched ores samples, ii) relating their chemical composition with possible mineralization mechanisms, and iii) defining the mineral paragenetic pathway related to the observed textural features. Our approach enabled both the identification of the coexisting Fe and Mn phases, and unravelling paragenetic pathways leading to supergene enrichment mineralizations. Collected evidence demonstrates that changes in Eh/ p H can lead to goethite dissolution under reductive conditions, promoting the release of Fe2+ into solution, whose electrochemical interaction with Mn4+ results in the formation of several types of Mn oxides, and secondary goethite. Our data shows a clear relationship between the type of Mn oxide crystallized and the ratio of aqueous Mn3+/Mn4+, alongside other prevalent cations, incorporated into tunnel or interlayer structural sites, which may be also desorbed/solubilized from primary goethite. [ABSTRACT FROM AUTHOR]

The behaviours of incorporated cobalt in goethite during desorption and dissolution is fundamental to our understanding of the best extraction techniques for maximum cobalt recovery. Here, Surface Complexation Modelling, desorption and dissolution of aged synthetic Co-goethite was investigated to demonstrate the transition between surface complexion, surface precipitation and structural incorporation in goethite with different amounts of cobalt found in oxic laterites. Surface Complexation Modelling of cobalt sorption on goethite reveals existence of both surface complexation and polynuclear complexes. Surface complexation starts at a low surface loading (≥ 0.004 wt% Co) to form Fe(OH) 2 Co, and as uptake (0.03 to 0.2 wt% Co) increases, two surfaces complexes, Fe(OH) 2 Co and (> FeOH) 2 Co, are formed. Additional increases in concentration (0.45 to 1.7 wt%) leads to the formation of poly-nuclear complexes (e.g., (> FeOH) 3 Co 2 ) co-existing with other complexes. Structural incorporation would be manifest if Co sorption was irreversible. We find that after four weeks of sorption about 55 to 70% of cobalt is reversible and the retention capacity increases with decreasing concentration. Samples formed at low loading show strong evidence of structural incorporation relative to those formed at high loading. In particular, the sample with 0.004 wt% Co needs a 20% dissolution of goethite before 64% of its incorporated cobalt is released while samples with 1.74 wt% Co requires a dissolution of 7.8% of goethite before liberating 66% of adsorbed cobalt. All the grades of Co-goethite dissolves incongruently with iron, however, the mode of formation should be considered when determining the leaching conditions for maximum recovery. In addition, the mixed Co-goethite grades show strong deviation from congruent dissolution and have a negative effect on the amount of cobalt and iron released during extraction. [ABSTRACT FROM AUTHOR]

Oxalic acid is an important, biologically-produced species in the natural environment. The deprotonated form, oxalate, is dominant in aqueous solutions under circumneutral pH conditions and is a strong ligand for Fe(III). The high affinity of oxalate for Fe(III) means that Fe(III)-oxalate surface and aqueous complexes are common and can lead to ligand-enhanced dissolution. Fe(II) adsorption onto goethite (α-FeOOH) has been shown to enhance dissolution-recrystallization reactions. The goethite (010) face is one of the more common and reactive surfaces on this environmentally critical Fe-hydroxide phase. Hence, this study models both separate and coordinated adsorption of oxalate and Fe(II) onto the (010) face of goethite in order to test for synergistic effects of ligand-promoted and reductive dissolution. Periodic and cluster density functional theory (DFT) energy minimizations were performed to determine the structure, vibrational frequencies and energies of various configurations. The adsorption mechanism of oxalate is verified via comparison to observed IR spectra. The potential roles of oxalate and Fe(II) in ligand-enhanced reductive dissolution of goethite are discussed. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is a toxic metal that causes a myriad of health problems and enters the environment as a result of anthropogenic activities and/or natural processes. The toxicity and solubility of chromium is linked to its oxidation state; Cr(III) is poorly soluble and relatively nontoxic, while Cr(VI) is soluble and a known carcinogen. Solid Fe(II) in iron-bearing minerals, such as pyrite, magnetite, and green rusts, reduce the oxidation state of chromium, reducing its toxicity and mobility. However, these minerals are not the only potential sources of solid-associated Fe(II) available for Cr(VI) reduction. For example, ferric (Fe(III)) (hydr)oxides, such as goethite or hematite, can have Fe(II) in the solid without phase transformation; however, the reactivity of Fe(II) within Fe(III) (hydr)oxides with contaminants, has not been previously investigated. Here, we cyclically react goethite with dissolved Fe(II) followed by dissolved O 2 , leading to the formation of reactive Fe(II) associated with goethite. In separate reactors, the reactivity of this Fe(II) is probed under oxic conditions, by exposure to chromate (CrO 4 2 − ) after either one, two, three or four redox cycles. Cr is not present during redox cycling; rather, it is introduced to a subset of the solid after each oxidation half-cycle. Analysis of X-ray absorption near edge structure (XANES) spectra reveals that the extent of Cr(VI) reduction to Cr(III) depends not only on solid Fe(II) content but also surface area and mean size of ordered crystalline domains, determined by BET surface area analysis and X-ray diffraction (XRD), respectively. Shell-by-shell fitting of the extended X-ray absorption fine structure (EXAFS) spectra demonstrates chromium forms both single and double corner sharing complexes on the surface of goethite, in addition to sorbed Cr(III) species. Finally, transmission electron microscope (TEM) imaging and X-ray energy-dispersive spectroscopy (EDS) illustrate that Cr preferentially localizes on the (100) face of goethite, independent of the number of redox cycles goethite undergoes. This work demonstrates that under oxic conditions, solid Fe(II) associated with goethite resulting from rapid redox cycling is reactive and available for electron transfer to Cr(VI), suggesting Fe(III) (hydr)oxides may act as reservoirs of reactive electron density, even in oxygen saturated environments. [ABSTRACT FROM AUTHOR]

Goethite is a common iron oxide present in the environment. It is used as a model system to represent the mineral fraction of the soil, whereas humic acid is used as a model system to represent natural organic matter (NOM). In this study, we tried to understand the behaviour of anionic and cationic pollutants in a model system, which could easily represent the organic-mineral interaction in the environment. Specifically, the retention of arsenate and copper over different organo-mineral systems was studied. The obtained results show an increase in the adsorption of copper as the pH and the C content in the organo-mineral composites was raised, whereas in the case of the arsenate, the opposite behaviour was observed. The adsorption behaviour was compared with that of the end-members goethite and humic acid. Furthermore, the NOM-CD model (natural organic matter-charge distribution model) was used to describe the reactivity of the pollutants in the solid/solution interphase and to predict the surface and aqueous speciation. To simulate the behaviour of the organo-mineral composites, a surface component was included to describe the chemical and electrostatic interactions of the NOM with the goethite in the modelling of the solid/solution interphase. This approximation allows the quantification of the effective organic matter, which can be used to describe the copper and arsenate reactivity in organo-mineral systems. [ABSTRACT FROM AUTHOR]

(U–Th)/He geochronology of 33 goethite grains from in situ ferruginous duricrusts overlying the Paraná flood basalt in the Guarapuava region, Paraná, Brazil, reveals ages ranging from 3.6 ± 0.4 to 0.4 ± 0.1 Ma. Thirty-one grains from detrital fragments of ferruginous duricrust yield ages in the 6.2 ± 0.6 to 0.7 ± 0.1 Ma range. The results show that goethites from detrital blocks are generally older than those from the in situ ferruginous layers and that all the goethites from the Guarapuava sites are relatively young (≤ 6.2 ± 0.6 Ma). Goethites show variable U and Th contents depending on the mode of precipitation. Goethite that forms by progressive ferrugization and pseudomorphic replacement of basalts are rich in Th, U-poor, contain various mineral inclusions ( e.g. , kaolinite, rutile, ilmenite) and display a distinct “checkered” appearance. Colloform goethite precipitated within cavities and desiccation cracks contain higher U, show low Th/U values, and are free of mineral contaminants, suggesting direct precipitation from solution. The geochronological results reveal a dynamic evolution for the soils overlying the Paraná basalt, where recurrent duricrust formation and dismantling lead to the evolution of regional colluvial soil profiles while simultaneously preventing the evolution of chemically stratified lateritic weathering profiles. The modes of formation and U, Th, and He contents of goethite at Guarapuava provide useful constraints on the expected compositions and modes of formation of goethite formed on basaltic soils on Mars. [ABSTRACT FROM AUTHOR]

Iron oxyhydroxide polytype minerals, such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ-FeOOH) and akaganeite (β-FeOOH), widely exist in terrestrial environments, and play important roles in mediating heavy metal mobility. Previous studies mainly focus on goethite, while the other three polytypes have been rarely explored. In the present work, Zn retention contents and mechanisms in Zn-coprecipitated γ-, δ- and β-FeOOH minerals were thoroughly investigated. Additionally, Zn mobility during anoxic incubation of Zn-containing lepidocrocite and feroxyhyte samples was monitored. It demonstrates that, feroxyhyte and lepidocrocite can accommodate relatively higher amounts of Zn (7.71 ± 0.20 mol% and 8.56 ± 0.22 mol% respectively) than akaganeite that retains almost no Zn (0.054 ± 0.002 mol%). In contrast, Zn has the most significant effect on akaganeite morphology. Part of total Zn is incorporated into the lepidocrocite or feroxyhyte structure by substitution for lattice Fe while the remaining Zn is adsorbed on the mineral surfaces. The different Zn substitution levels in these FeOOH minerals are greatly related with the mineral structures, that is, the ratio of edge-sharing to corner-sharing [FeO 6 ] and thus the spatial freedom, and the pHs of the mineral synthetic solutions. During anoxic incubation in the presence of 1 mM Fe2+ at pH 6 ± 0.05 for 60 d, both Zn-containing lepidocrocite and feroxyhyte keep stable, while the dissolved Zn concentrations rapidly increase at initial stage, then decrease after reaction for 3 d, and increase again from 10 d till to the end. The final Zn release from feroxyhyte (∼98%) is much higher than that from lepidocrocite (∼30%), probably owing to the higher Fe2+-induced reductive dissolution and desorption rates, and larger amounts of Zn adsorbed on the surfaces of the former. These results help to understand the effects of Zn on the physicochemical properties of FeOOH polymorphs and the geochemical behaviors of Zn associated with Fe oxyhydroxide minerals. • Zn retention amounts decrease in order of lepidocrocite > feroxyhyte > akaganeite. • Zn both incorporates into and is adsorbed onto these FeOOH mineral surfaces. • Zn release from these FeOOH minerals occurs in anoxic acid conditions. • Fe(II) promotes more Zn release from Zn-containing feroxyhyte than lepidocrocite. • Anoxic incubation at pH 6 with 1.0 mM Fe(II) induces no mineral transformation. [ABSTRACT FROM AUTHOR]

In natural environments, competitive interactions of ions with charged particles often control bioavailability and mobility of ions. In these systems, chemical reactions are often in a semi-equilibrium state; thus, along with equilibrium data, kinetic data are of great importance in predicting ion bioavailability and mobility. Therefore, in this research, kinetics and equilibrium adsorption interactions of magnesium, calcium and phosphate on goethite are investigated as a function of time, pH, ion loading and ionic strength in single and binary ion systems. The thermodynamically valid model parameters, obtained from single ion systems by applying the CD-MUSIC model, are used to predict ion interactions in more complex systems, i.e. binary ion systems. Results indicate that the kinetics of phosphate and magnesium adsorption is pH-dependent and electrostatic attractive and repulsive forces dominantly control the reaction. Thus, adsorption of magnesium and calcium influences significantly both the amount of adsorbed phosphate and the reaction time, and vice versa. Therefore, the equilibration time for phosphate and magnesium adsorption on goethite is approximately ≤ 1 h when electrostatic forces are attractive and approximately 24 h when electrostatic forces are repulsive. Variation in ionic strength (0.01–0.3 M NaCl) has no significant effect on the magnesium and calcium adsorption, suggesting a large difference in the adsorption energy of Ca and Mg compared to Na, which is very likely due to the difference in their adsorption mechanism. Based on the model calculations, formation of two inner sphere surface complexes, as (FeOH) 2 Mg and (FeOH) 2 MgOH, is the dominant mechanism for magnesium adsorption. For calcium, however, a monodentate outer sphere surface complex dominates its adsorption at acid pH range and a monodentate inner sphere complex at basic pH range ( FeOHCa). This difference results in a different amount of charge on electrostatic planes that influences strongly phosphate adsorption such that phosphate adsorption is significantly higher in Ca–PO 4 systems than in Mg–PO 4 systems. [ABSTRACT FROM AUTHOR]

The fate of phosphate in the environment is governed by reactions at particle surfaces. These adsorption and desorption reactions display biphasic kinetics involving an initial rapid reaction followed by a substantially slower one extending over long time periods. In this study we have investigated the molecular mechanisms of desorption kinetics of phosphate from ferrihydrite and goethite nanoparticles in the absence of competing ligands. Desorption was studied by means of in-situ infrared (IR) spectroscopy over a wide pH range and a time period of 24 h. The spectroscopic data sets were subjected to multivariate curve resolution alternating least squares (MCR-ALS), which enabled the resolution of surface species characterized by unique IR spectra together with their corresponding kinetic profiles. The desorption results showed the typical biphasic behavior and that increasing positive surface charge of ferrihydrite and goethite slowed down desorption of the negatively charged phosphate ions. Moreover, diprotonated phosphate desorbed faster than monoprotonated phosphate at a given pH. At circumneutral pH values desorption from ferrihydrite was substantially faster as compared to goethite, and this could be ascribed to electrostatic effects and differences in charging between ferrihydrite and goethite. The collective desorption results were explained by a model, consisting of a series monodentate phosphate surface complexes in different protonation states, in conjunction with a description that accounts for the electrostatic effects on desorption kinetics at charged mineral-water interfaces. The fast and slow desorption followed directly from this model and indicated that biphasic kinetics can be caused by a single phosphate surface complex as a result of decreasing surface coverage along with the lateral repulsive interactions between adsorbed phosphate groups. Hence, in contrast to previous models our study has shown that biphasic desorption kinetics do not have to involve several different structural complexes related to either weak and strong sites or a distribution of phosphate between external surfaces and mineral pores. [ABSTRACT FROM AUTHOR]