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The amount of As(III) adsorbed and the interfacial process are closely associated with the phase transformation of Schwertmannite (SCH). At present, studies on the adsorption characteristics of As(III) on SCH and the accompanying phase transformation process, especially the related mechanisms under the mediation of iron-reducing bacteria (FeRB) and sulfate-reducing bacteria (SRB), are limited in existing literature. With the help of continuous characterization, the adsorption behavior of As(III) on SCH was explored, as well as the transformation processes of SCH during these processes. The findings revealed that the SCH, synthesized by the KMnO4 oxidation and ethanol modification methods, exhibited excellent physical adsorption capacity for As(III) due to their increasing specific surface area and porosity. At room temperature (20°C), the saturation adsorption capacities of As(III) by M-SCH and Y-SCH reached 62.69 and 58.62 mg/g, respectively. Moreover, the generation and phase transformation of As(III)-bearing ferrihydrite were observed within a 60-min timeframe. It is the first time this phenomenon has been observed in such a short time, which is presumed to be an intermediate stage in the transformation of SCH into goethite. Furthermore, both FeRB and SRB could enhance the adsorption capacity of SCH for As(III). Comparatively, SRB has a more substantial impact on SCH's phase transformation. These insights are valuable for the practical application of SCH in treating As(III) pollution. [ABSTRACT FROM AUTHOR]

Discharge of acidic wastewater from mining activities (acid mine drainage (AMD)) is a major global environmental and public health issue. Although several approaches, including chemical precipitation and membrane technology, have been developed to treat AMD, adsorption has emerged as the most promising technology due to its cost-effectiveness and efficacy. Despite the wide adoption of adsorption in treating AMD, the evolution of research in this area remains poorly understood. To address this gap, a bibliometric analysis of the most recent literature involving the application of adsorption in AMD remediation was conducted by merging datasets of articles from Scopus (1127) and the Web of Science Core Collection (1422), over the past decade (2013–2022). This analysis revealed a yearly increase of 11% in research publications, primarily contributed by China, the United States, and South Africa. Keyword analysis revealed that natural schwertmannites and their transformations, activated carbon, zeolites, and clay minerals, are the most extensively employed adsorbents for the removal of common metals (arsenic, chromium, iron, manganese, among others). The findings underscore the need for future focuses on recovering rare earth elements, using nanoparticles and modified materials, pursuing low-cost, sustainable solutions, integrating hybrid technologies, pilot-scale studies, exploring circular economic applications of AMD sludges, and inter-continental collaborations. These insights hold significant future implications, serving as a valuable reference to stakeholders in the mining industry. [ABSTRACT FROM AUTHOR]

• Composite photo–Fenton catalysts of N–doped TiO 2 and schwertmannite were prepared. • N–TiO 2 /SCH displayed a superior BPA degradation activity of ca. 100% within 60 min. • Hydroxyl radical and singlet oxygen are active species in this photo–Fenton system. Potential health risks related to environmental endocrine disruptors (EEDs) have aroused research hotspots at the forefront of water treatment technologies. Herein, nitrogen–doped titanium dioxide/schwertmannite nanocomposites (N–TiO 2 /SCH) have been successfully developed as heterogeneous catalysts for the degradation of typical EEDs via photo–Fenton processes. Due to the sustainable Fe(III)/Fe(II) conversion induced by photoelectrons, as–prepared N–TiO 2 /SCH nanocomposites exhibit much enhanced efficiency for the degradation of bisphenol A (BPA; ca. 100% within 60 min under visible irradiation) in a wide pH range of 3.0-7.8, which is significantly higher than that of the pristine schwertmannite (ca. 74.5%) or N–TiO 2 (ca. 10.8%). In this photo–Fenton system, the efficient degradation of BPA is mainly attributed to the oxidation by hydroxyl radical (•OH) and singlet oxygen (1O 2). Moreover, the possible catalytic mechanisms and reaction pathway of BPA degradation are systematically investigated based on analytical and photoelectrochemical analyses. This work not only provides a feasible means for the development of novel heterogeneous photo–Fenton catalysts, but also lays a theoretical foundation for the potential application of mineral–based materials in wastewater treatment. [ABSTRACT FROM AUTHOR]

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This study describes iron (Fe) pipestems formed around and within root channels during the development of a homogeneous unripe sulfidic sediment into a ripe soil where redox‐ or pH‐active elements segregate. Pipestems and adjacent soil material samples were characterised by chemical and mineralogical analyses and scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS). Pipestems occurred at 60–190 cm and at 30–130 cm in a young Finnish soil and a more mature Australian soil, respectively, while sulfidic materials were found below 170–180 cm. The pipestems consisted of mineral grains cemented together by Fe precipitate. There was an up to 24‐fold enrichment of Fe and an up to 13‐fold enrichment of sulfur (S) in the Finnish pipestems compared to the adjacent soil material, whereas the corresponding enrichment in the Australian soil was up to 27‐fold for Fe and up to 8‐fold for S. In the Australian pipestem matrices, the Fe concentration was as high as 40% compared to 14% in the Finnish ones. It was estimated that about 70 ton ha−1 S had been mobilised from the sulfidic material in the Finnish soil at the depth of 50–150 cm. Part of S has leached out but a substantial amount remained in the soil constituting the stock of retained acidity. Almost all pipestems contained a new solid phase precipitated within the former cortex cells of the plant roots. In the Finnish samples, this precipitate consisted of jarosite and schwertmannite. After oxidative exhaustion of sulfidic material in the surrounding soil, these metastable minerals are gradually hydrolysed, associated with leaching of S. In the mature Australian soil, most of these minerals had already been transformed to goethite. Pipestems formed after roots in unripe soil are sites for synthesis and hydrolysis of minerals and serve as routes for atmospheric oxygen into the reduced subsoil and for soluble reaction products to exit the soil. Pipestems have an important role in the ripening of the soil profile. [ABSTRACT FROM AUTHOR]

Schwertmannite (Sch) is an important Fe(III)-oxyhydroxysulfate in acidic mine drainage-polluted areas, and its stability depends on the surrounding environmental factors and previously bound elements. Phosphorus (P) is a ubiquitous element in Sch-rich environments and can adsorb onto Sch, affecting their fate. To better understand the adsorption behavior of different forms of P (inorganic phosphorus, iP; organic phosphorus, oP) on Sch, the adsorption kinetics of iP and oP on synthetic Sch were investigated; additionally, the stability changes of Sch were analyzed. The adsorption rates of iP and oP on Sch were different, and the adsorption rate of iP was higher than that of oP. The adsorption process conformed to a pseudo-second-order adsorption kinetics model, and the rate-controlling step was the rate of the adsorbate interaction with the adsorption site. The adsorption of iP and oP can improve the stability of Sch, even in a more alkaline environment. The adsorption of iP seemed to stabilize the Sch structure through a strong hydrogen bonding effect in addition to the surface passivation effect. In contrast, the adsorption of oP seemed to be stabilized mainly by the surface passivation effect and the weak hydrogen bonding effect. Therefore, the formation of Sch in an acidic environment immobilizes different forms of P and improves its own stability, which can prevent secondary environmental pollution. [ABSTRACT FROM AUTHOR]

The amount of As(III) adsorbed and the interfacial process are closely associated with the phase transformation of Schwertmannite (SCH). At present, studies on the adsorption characteristics of As(III) on SCH and the accompanying phase transformation process, especially the related mechanisms under the mediation of iron-reducing bacteria (FeRB) and sulfate-reducing bacteria (SRB), are limited in existing literature. With the help of continuous characterization, the adsorption behavior of As(III) on SCH was explored, as well as the transformation processes of SCH during these processes. The findings revealed that the SCH, synthesized by the KMnO4 oxidation and ethanol modification methods, exhibited excellent physical adsorption capacity for As(III) due to their increasing specific surface area and porosity. At room temperature (20°C), the saturation adsorption capacities of As(III) by M-SCH and Y-SCH reached 62.69 and 58.62 mg/g, respectively. Moreover, the generation and phase transformation of As(III)-bearing ferrihydrite were observed within a 60-min timeframe. It is the first time this phenomenon has been observed in such a short time, which is presumed to be an intermediate stage in the transformation of SCH into goethite. Furthermore, both FeRB and SRB could enhance the adsorption capacity of SCH for As(III). Comparatively, SRB has a more substantial impact on SCH’s phase transformation. These insights are valuable for the practical application of SCH in treating As(III) pollution.

Sulfate is a critical component of schwertmannite. However, the sulfate content of schwertmannite decreases with the increase in AMD-polluted river pH value, and its impact on the surface properties and Cr(VI) adsorption of schwertmannite remains unclear. To address this issue, different methods, including batch adsorption, N 2 physisorption, scanning electron microscopy (SEM), potentiometric titration, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) in combination with two-dimensional correlation spectroscopy (2D-COS) analysis, and surface complexation modeling (SCM) were employed. Our results showed that sulfate existed as both non-protonated bidentate-binuclear complexes and non-protonated outer-sphere complexes in schwertmannite. The surface morphology and particle size of schwertmannite were unaffected by the sulfate content. However, a decrease in sulfate content resulted in a drop in the negative charge on the surface of schwertmannite, which is conducive to particle aggregation. An increase in pH, on the other hand, raised the negative charge and encouraged the dispersion of the particles. And then, reducing the sulfate content of schwertmannite led to a decrease in acid-base buffer and Cr(VI) adsorption capacity of schwertmannite. The adsorption process of Cr(VI) by schwertmannite can be divided into two stages. In the first stage, Cr(VI) was adsorbed primarily through anion exchange reactions with the sulfate; however, the contribution of surface complexation gradually increased as the initial sulfate content decreased because of a reduced likelihood of anion exchange and an increase in the specific surface area of schwertmannite. Additionally, the behavior of sulfate and Cr(VI) in this stage could be predicted based on the initial sulfate content of schwertmannite. In the second stage, surface complexation with hydroxyl sites became the predominant mechanism of adsorption, which still exhibited great potential at higher Cr(VI) concentrations. Finally, the aforementioned hypothesis was used to establish a CD-MUSIC model, and the fitting results demonstrated that the density and Cr(VI) adsorption capacity of the sulfate site dropped as the sulfate content decreased, while the hydroxyl site was less impacted. The findings have provided insights into further understanding the properties of schwertmannite and its adsorption of Cr(VI). [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that schwertmannite (Sch) exhibits good adsorption performance for Cr(VI). In order to further enhance the ability to remove Cr(VI), this study prepared a novel composite (Fe(II)@Sch) by embedding ferrous iron (Fe(II)) on Sch. The adsorption performance of Cr(VI) on Fe(II)@Sch was investigated by batch adsorption experiments, and a possible removal mechanism was proposed through characterization analysis. The results showed that the optimal Fe/Sch ratio for Fe(II)@Sch preparation was 120 mmol/g. Fe(II)@Sch enabled efficient and rapid adsorption of Cr(VI). The maximum Cr(VI) adsorption capacity of Fe(II)@Sch was 4.17 mmol/g at pH 6.0, which was 69% higher when compared to Sch, and 81% of the maximum adsorption could be achieved within 1 min. The embedding of Fe(II) led to a decrease in the particle size and an increase in the specific surface area (SSA) of Sch, which could be considered favorable for adsorption. After four repeated cycles 93.3% of the original Cr(VI) adsorption capacity was still maintained. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analysis showed that the interaction between Fe(II)@Sch and Cr(VI) followed an adsorption–reduction mechanism. The results demonstrated that Fe(II)@Sch could be used as an effective material for removing Cr(VI) from wastewater.Highlights: The optimal Fe/Sch ratio for Fe(II)@Sch preparation was 120 mmol/g. Maximum Cr(VI) adsorption capacity of Fe(II)@Sch was 4.17 mmol/g at optimum pH 6. Adsorption of Cr(VI) on Fe(II)@Sch reached 81% of maximum adsorption within 1 min. Interaction between Fe(II)@Sch and Cr(VI) follows adsorption–reduction mechanism. [ABSTRACT FROM AUTHOR]

Abstract Due to continuing mining activities, Cd(II) and As(III) contamination in acid mine drainage (AMD) has become a major environmental challenge. Currently, there is increasing focus on the use of biochar to mitigate AMD pollution. However, the impact of biochar on the process of Fe(II) oxidation by Acidithiobacillus ferrooxidans (A. ferrooxidans) in AMD systems has not been determined. In this study, we investigated the effects of introducing biochar and biochar-leachate on Fe(II) biooxidation by A. ferrooxidans and on the removal of Cd(II) and As(III) from an AMD system. The results showed that the biochar-leachate had a promoting effect on Fe(II) biooxidation by A. ferrooxidans. Conversely, biochar inhibited this process, and the inhibition increased with increasing biochar dose. Under both conditions (c(A. ferrooxidans) = 1.4 × 107 copies mL–1, m(FeSO4·7H2O):m(biochar) = 20:1; c(A. ferrooxidans) = 7.0 × 107 copies mL–1, m(FeSO4·7H2O):m(biochar) = 5:1), the biooxidation capacity of A. ferrooxidans was severely inhibited, with Fe(II) oxidation efficiency reaching a value of only ~ 20% after 84 h. The results confirmed that this inhibition might have occurred because a large fraction of the A. ferrooxidans present in the system adsorbed to the biochar, which weakened bacterial activity. In addition, mineral characterization analysis showed that the introduction of biochar changed the A. ferrooxidans biooxidation products from schwertmannite to jarosite, and the specific surface area increased after the minerals combined with biochar. Coprecipitation experiments of As(III) and Cd(II) showed that Cd(II) was adsorbed by the biochar over the first 12 h of reaction, with a removal efficiency of ~ 26%. As(III) was adsorbed by the generated schwertmannite over 24 h, with a removal efficiency of ~ 100%. These findings have positive implications for the removal of As(III) and Cd(II) from AMD. Graphical Abstract

Abundant iron and sulfate resources are present in acid mine drainage. The synthesis of schwertmannite from AMD rich in iron and sulfate could achieve the dual objectives of resource recovery and wastewater purification. However, schwertmannite cannot emerge spontaneously due to the Gibbs free energy greater than 0. This results in the iron and sulfate in AMD only being able to use the energy generated by oxidation in the coupling reaction to promote the formation of minerals, but this only achieved partial mineralization, which limited the remediation of AMD through mineralization. In order to clarify the mechanism of iron and sulfate removal by the formation of schwertmannite in AMD, kinetic and thermodynamic parameters were crucial. This work used H2O2 oxidation of Fe2+ as a coupling reaction to promote the formation of schwertmannite from 64.4% of iron and 15.7% of sulfate in AMD, and determined that 99.7% of the iron and 89.9% of sulfate were immobilized in the schwertmannite structural, and only a small fraction was immobilized by the adsorption of schwertmannite, both of which were consistent with second-order kinetics models. The thermodynamic data suggested that reducing the concentration of excess sulfate ions or increasing the energy of the system may allow more iron and sulfate to be immobilized by forming schwertmannite. Experimental verification using the reaction of potassium bicarbonate with the acidity in solution to increase the energy in the system showed that the addition of potassium bicarbonate effectively promoted the formation of schwertmannite from Fe3+ and SO42−. It provided a theoretical and research basis for the direct synthesis of schwertmannite from Fe3+ and SO42− rich AMD for the removal of contaminants from water and the recovery of valuable resources. [ABSTRACT FROM AUTHOR]

Iron hydroxyl sulfate minerals (IHSMs, including schwertmannite and jarosite) are a promising material for environmental applications with excellent adsorption of metal oxygen anions. The acid mine drainage (AMD) abundant in iron and sulfate ions holds potential for the production of valuable IHSMs, thereby achieving resource recycling whilst simultaneously mitigating water contamination, which is important for the sustainable remediation of AMD. Conventional mineralization, which promotes the generation of minerals from Fe3+ and SO42− through the energy provided by chemical or biological oxidation, can only partially mineralize iron in AMD containing substantial quantities of Fe2+. In this study, an improved method for mineralizing AMD containing iron of a different valence into IHSMs under the induction of KHCO3 was proposed. For AMD containing Fe2+, the combination of KHCO3 and H2O2 was used to promote the hydrolysis of iron (92.7%) and sulfate (11.1%) into IHK minerals, which resulted in a significant increase in iron removal of 75.2% and a minor increase in sulfate removal of 4.9%, compared with the formation of schwertmannite from oxidation solely involving H2O2. For the AMD containing Fe3+, the energy generated by the acid–base reaction in water could also directly promote the formation of IK minerals from 97.2% iron and 6.9% sulfate. XRD and FTIR analyses confirmed the identification of the IHK and IK minerals as IHSMs transitioning from schwertmannite to jarosite. SEM and elemental analyses indicated that the mineral exhibited denser aggregate spheres with the incorporation of KHCO3 in mineralization yet displayed enhanced mineralization abilities for the contaminant ions in AMD. Moreover, despite the SSA of the modified minerals being diminished (2.02, 1.83 and 1.83 m2/g for IH, IHK and IK, respectively), the presence of more sulfate in the mineral enhanced the adsorption capacity of Cr(VI). Furthermore, the water quality results also illustrated that the removal ratios of iron and sulfate in AMD notably increased with the involvement of KHCO3 in mineralization. In conclusion, the KHCO3-induced mineralization of iron-containing (either divalent or trivalent) AMD into IHSMs not only improved the mineralization ratios and contaminant removal ratios for better remediation of AMD but also obtained mineral resources with better adsorption of Cr(VI), thereby fostering the sustainable advancement of the remediation of AMD. Therefore, this innovative strategy employing KHCO3-assisted chemical mineralization to form IHSMs holds ample potential and promises to be an efficacious methodology for the sustainable remediation of iron-rich AMD. [ABSTRACT FROM AUTHOR]

Schwertmannite affects the mobility of antimony (Sb) in acid mine drainage, yet the Sb sorption mechanism remains elusive. In this study, the molecular-level mechanism and the driving force of Sb(V) immobilized on schwertmannite were investigated by a combination of wet chemistry, extended X-ray absorption fine structure (EXAFS) analysis, density functional theory (DFT) calculations, and a surface complexation model (SCM). The observed sorption capacity for Sb(V) was as high as 235.63 mg/g. Such remarkable sorption capacity is primarily achieved by the anion exchange reaction and surface complexation reaction of Sb(V) with sulfate and surface/tunnel inner-surface hydroxyl groups, respectively, and the contribution of the anion exchange reaction to Sb(V) immobilization decreases markedly with increasing Sb(V) loading or pH. The loading of Sb(V) appears to be the key parameter controlling its immobilization mechanism on schwertmannite. Low-loaded Sb(V) primarily generates bidentate-mononuclear and bidentate-binuclear surface complexes (adsorption products) and is concurrently partly incorporated into the Fe-O framework of schwertmannite to form a hexadentate-hexanuclear complex (incorporation product). With increasing Sb(V) loading, Sb is primarily adsorbed on the surface and in the tunnel of schwertmannite with the formation of bidentate-mononuclear (dominant) and bidentate-binuclear complexes without incorporation. The inconsistency of the immobilization mechanism is controlled by the amount of released sulfate induced by Sb(V). The substantial system energy reduction caused by the formation of Sb(V) adsorption and incorporation products on the surface and in the tunnel of schwertmannite is the driving force for the stable immobilization of Sb(V) by schwertmannite, and the energy reduction caused by tunnel complexes is far more obvious than that caused by surface complexes. A charge distribution multisite complexation model was established based on bidentate-mononuclear and bidentate-binuclear surface complexes with fitted Log K values of 21.99 ± 0.18 and 12.84 ± 0.92, respectively, which consistently predicted Sb(V) sorption across pH 3.0–9.0 for all involved Sb(V) loadings. In addition, Sb(V) has a strong stabilizing effect on the schwertmannite structure and broadens its pH stability window (pH 3.0–9.0). This study facilitates elucidation of the fate and geochemical cycling of Sb in the mining environment and helps to promote the application of the microstructure of schwertmannite in other studies of microscopic interface chemistry. [ABSTRACT FROM AUTHOR]

Soluble iron and sulfate in acid mine drainage (AMD) can be greatly removed through the formation of minerals facilitated by seed crystals. However, the difference in the effects of jarosite and schwertmannite as endogenous seed crystals to induce AMD mineralization remains unclear. This paper intends to study the effect of Fe2+ oxidation and Fe3+ mineralization in the biosynthesis of minerals using different addition amounts and methods of jarosite or schwertmannite. The results showed that the addition amount and method of different seed crystals had no effect on the Fe2+ bio-oxidation but would change the Fe3+ mineralization efficiency. With the same amount of seed crystals added, jarosite exhibited a higher capacity to promote Fe3+ mineralization than schwertmannite. Adding seed crystals before the initiation of Fe2+ oxidation (0 h) could significantly promote Fe3+ mineralization efficiency. With the increase of seed crystals, jarosite could not only shorten the time required for mineral synthesis but also improve the final mineral yield, whereas schwertmannite could only shorten the time required for mineral synthesis. When Fe2+ was completely oxidized to Fe3+ (48 h), the supplementary of jarosite could still effectively improve Fe3+ mineralization efficiency, but the addition of schwertmannite no longer affected the final mineralization degree. HIGHLIGHTS The addition of jarosite and schwertmannite had no significant effect on the bio-oxidation of Fe2+ by A. ferrooxidans but would change the mineralization efficiency of Fe3+.; The effect of jarosite addition on the mineralization of Fe3+ was much greater than that of schwertmannite addition.; Adding seed crystals before the initiation of Fe2+ oxidation could significantly promote Fe3+ mineralization efficiency.;

Spiramycin (SPM), a widely employed antibiotic in both clinical therapy and the livestock industry, poses significant challenges in terms of safe and efficacious management. A heterogeneous photo-Fenton system, devised using Schwertmannite (Sch), can effectively degrade contaminants. However, it is accompanied by a relatively low conversion efficiency of ≡Fe3+/≡Fe2+ and a significant iron loss. In this study, a catalyst featuring Fe3O4 and ZSM-5 molecular sieve-modified Sch (Fe3O4/ZSM-5/Sch) was devised to enhance the catalytic activity and stability. The findings revealed that Fe3O4/ZSM-5/Sch exhibited exceptional catalytic activity, with the reaction first-order kinetic exceeding that of pure Sch. The active species including ·OH, h+, e−, ·O2− and SO4·− were identified in the UV/Fe3O4/ZSM-5/Sch-H2O2 system. The enhanced catalytic activity of Fe3O4/ZSM-5/Sch could be ascribed to the effective conversion of ≡Fe3+/≡Fe2+. The photogenerated electrons within Fe3O4 were transported to Sch via ZSM-5, which effectually reduced ≡Fe3+/≡Fe2. Moreover, Fe3O4/ZSM-5/Sch demonstrated outstanding stability; even after six cycles, the degradation efficiency of SPM remained above 86.50%, and the leaching quantity of Fe remained below 0.24 mg/L. This research not only develops an excellent catalyst for the safe treatment of SPM but also proffers innovative perspectives for the future design of efficient iron-based catalysts. [ABSTRACT FROM AUTHOR]

Acid mine drainage (AMD) poses severe social and ecological issues, thus AMD treatment has been considered a major challenge in environmental protection. Heavy metal ions and sulfate ions must be removed from AMD, and acidity must be neutralized. In this study, a two-step approach was proposed to treat the simulated AMD by using H2O2 and Na2O2 to synthesize schwertmannite to remove heavy metal ions and sulfate ions from AMD and increase the water pH. A total of 0.2 mL H2O2 and 0.16 g Na2O2 are added in the two steps. In the first step, 100 mL simulated AMD was first treated by adding composite oxidants (0.2 mL H2O2 and 0.12 g Na2O2) to produce schwertmannite, which was then separated from the solution. In the second step, 0.04 g Na2O2 was added to the filtrate obtained in the first step to remove residual ions and neutralize acidity. The removal rates of TFe (total Fe), Mn2+, and SO42− reached 99.98%, 99.75%, and 31.62%, respectively, and the pH increased to 7.57. More excitingly, this method also worked very well for raw AMD, which was superior to the H2O2 method. The treatment mechanism was further discussed. Meanwhile, the obtained schwertmannite can be reused as heavy metal ion absorbents. This research has innovative aspects in two areas: firstly, synthesizing schwertmannite without decreasing the water's pH value; secondly, utilizing the contaminants in AMD, such as Fe ions and SO42− as raw materials to form schwertmannite, thus allowing for resourceful usage of AMD. The aim is to contribute to the development and exploration of sustainable AMD management. This method provides a new solution for AMD treatment and simultaneous resource recovery. [ABSTRACT FROM AUTHOR]

Iron oxyhydroxides for heavy metal treatment have attracted wide attention. In this work, iron oxyhydroxides of isomeric FeOOH (GpI) and tunnel-structural schwertmannite/akaganéite (GpII) were selected to study chromium (Cr(III)) adsorption removal from acidic aqueous solutions by batch experiments, under various reaction time, adsorbate/adsorbent level, pH and anions. Adsorption processes well fitted to pseudo-second-order kinetics (R2 = 0.992–0.999, except for 0.829 for Lep). Isotherm data could be fitted by Langmuir (R2 = 0.901–0.985), Freundlich (R2 = 0.884–0.985) and Temkin (R2 = 0.845–0.961) models at pH 3.7. Langmuir maximum adsorption capacities (mg/g) were 10.4−18.8 (FeOOH, except for 3.08 for Gth2) in GpI, and 20.60/43.40 (Sch-Chem/Sch-Bio) and 12.80/24.70 (Aka-Chem/Aka-Bio) in GpII. Adsorption capacities would gradually increase as Cr(III) concentrations increased within 0−40 mg/L, and could be markedly affected by the SO42- and H2PO4- anions. There were stable adsorption capacities at about pH 3.7, and then increased at pH 3.7–4.1. The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that adsorption mechanisms were electrostatic interaction and surface complexation. In addition, three optimal bio-/chem-schwertmannite and lepidocrocite adsorbents had good reusable properties and treating abilities of Cr(III)-polluted waters at pH 4.0. These results could provide a theoretical basis for the application of iron oxyhydroxides in removing Cr(III) from acid wastewaters. HIGHLIGHTS Iron oxyhydroxides were used for Cr(III) adsorption removal.; Cr(III) removal efficiencies were analyzed under various impact factors.; Bioschwertmannite had the highest Cr(III) adsorption capacity.; Schwertmannite and lepidocrocite had good reusability.;

Iron hydroxides have received high attention in the treatment of chromium (Cr) polluted wastewater. In this study, the obtained chemical (or biological) pincushion-schwertmannite spheres had a diameter of 2 − 5 μm (0.5–1 μm), and akaganéite rods had a length of 300–500 nm (100–150 nm) at an axial ratio of about 3. The average diameters (μm) of their agglomerated particles in solutions were 20.6–32.5 (only 0.480 for Aka-Chem). Schwertmannites and akaganéites were used to investigate Cr(VI) adsorption behaviors in aqueous solutions by batch experiments, under various reaction times, initial Cr(VI) and adsorbent levels, pH values, temperature and anions of NO3−, Cl−, CO32−, SO42−, and H2PO4−. Adsorption data well fitted to pseudo-second-order rate model (R2 = 0.999), and Langmuir (R2 = 0.954–0.988) and Freundlich (R2 = 0.984–0.996) isothermal models at pH 7.0. Maximum Cr(VI) adsorption capacities were 119/133 for Sch-Chem/Sch-Bio, and 14.6/83.6 for Aka-Chem/Aka-Bio. The H2PO4− than SO42−/CO32− had a stronger effect on Cr(VI) adsorption. Adsorbents with pHZPC of near to 4.0 still had a good Cr(VI) removal ability at pH 3.0–8.0. The possible Cr(VI) adsorption mechanisms by FTIR and XPS results for schwertmannite and akaganéite were electrostatic attractions and ion exchanges between hydroxyl (or sulfate) and chromate ions. The Cr(VI) adsorption of optimal schwertmannite and bioakaganéite was a spontaneous, endothermic and random process at the temperatures of 288–318 K. They had a good regeneration ability for Cr(VI) adsorption, and removal ratios could reach to about 80% of original values (60–70% in aqueous solution with 60 mg/L Cr(VI) and pH7.0, and 35–50% in wastewater with 120 mg/L Cr(VI) and about pH4.0), after three cycles. Herein, schwertmannite/bioakaganéite have a promising application in treatment of acidic/neutral wastewater with chromate. [ABSTRACT FROM AUTHOR]

Schwertmannite is an important Fe(III)-oxyhydroxysulfate in acid mine drainage (AMD) polluted areas and its stability depends on surrounding environmental factors and previously bound elements. The treatment and neutralization of AMD normally involve the use of lime, which leads to the discharge of abundant Ca in the mining area. Such an environmental disturbance brings up an important and less considered problem of how the reductive transformation of schwertmannite associated with coexisting Ca occurred. Here, the Fe(II)-mediated transformation of Ca-adsorbed schwertmannite and subsequent Ca repartitioning behaviors were investigated. Results showed that adsorbed Ca had a weak inhibitory effect on Fe(II)-mediated schwertmannite transformation. Release of SO 4 2− and SEM images both indicated that transformation rates of schwertmannite decreased under the influence of adsorbed Ca. XRD patterns indicated that adsorbed Ca altered schwertmannite transformation pathways and product compositions upon treatment with 0.4 mmol/L Fe(II). The end products of Sch notably contained both goethite and lepidocrocite; however, transformation products of SchCa only contained goethite all along. Approximately 33.5% of the surface adsorbed-Ca was released into solution within 6 hr after Fe(II) injection. Aqueous Ca behaved in a "first release and then im-mobilization" manner, which indicated dissolution and secondary mineralization drove Ca migration during the Fe(II)-mediated transformation of SchCa. Adsorbed Ca blocked the surface sites for subsequent Fe(II) adsorption, limited the Fe(II)-Fe(III) ETAE, and decreased the transformation rates. This work sheds light on the complex geochemical behavior of schwertmannite under the influences of environmental perturbations in AMD environments. [ABSTRACT FROM AUTHOR]

This work examines the transformation of iron-bearing precursors to jarosite-like minerals in the absence of bacteria or other organic compounds. The composition of the aqueous solution determines the transformation, through which crystallinity and long-term stability ofjarosite increase, whereas the temperature of the environment affects the kinetics of the process. Spectroscopic techniques (FTIR and XPS) were used to characterize the chemical species present on the transformed mineral surfaces. Schwertmannite is the first phase to precipitate as a result of homogeneous nucleation and growth in the bulk of the supersaturated solution. This metastable phase transforms into a crystalline Na-rich member of the (Na,H30)Fe3(S04)(0H)6 solid-solution family after aging for either 3 h at 70 °C or 1 day at 20 °C. XRD analyses show that the crystallinity of natrojarosite increases progressively with reaction time, although its cell parameters and crystallite size remain nearly constant during aging, which reveals the stability of the crystal structure of this secondary phase. Interestingly, the mechanisms governing the transformation from aggregates of schwertmannite into natrojarosite crystals consist of interface-coupled dissolution-precipitation reactions that involve an internal structural reorganization within the individual nanoparticles of the secondary phase, in which Fe3+ is transferred from the solid to the solution while S04-, 0H-, and Na+ move in the opposite direction. The spectroscopic study confirms the mineralogical results and suggests that the crystal structure of jarosite-like minerals may offer interesting geochemical information about the aqueous solutions where they were formed. The transformation kinetics and the apparent activation energy (Ea = 52.1 kJ/mol) of the transformation were estimated using the so-called "time to a given fraction" method, and a temperature-transformation-time (TTT) diagram was established in the range 20-70 °C to define the reaction pathways during the process. [ABSTRACT FROM AUTHOR]

Passive treatment systems are a sustainable solution to mine drainage remediation, however, clarity lacks in the detailed understanding of the geochemical processes that govern the remediation processes. A study was carried out at Shojin River in Southwest Hokkaido, Japan, which has an acidic pH and is high in Fe, to represent the principal processes that occur in mine drainages of similar characteristics. At Shojin, Fe and S were mined from the Shojingawa mine, after which, wastewater contaminated by toxic elements such as Fe, As and Pb continue to flow from the underground. This wastewater flows through industrial and fishing areas, hence ensuring that the water meets environmental standards is important. In particular, the role of colloidal materials in transporting the toxic elements was investigated by analyzing the distribution of the elements (Fe, As and Pb) in the drainage as the dissolved and colloidal fractions, obtained from micro‐, and ultrafiltration respectively. Iron, particularly schwertmannite colloid formation was observed in the Shojin River system. Itwas present in the dissolved fraction in the wastewater like As, but after mixing with the uncontaminated river water, Fe in the colloid fraction gradually increased toward the downstream. Arsenic trends are similar to those of iron, indicating efficient sequestration by the schwertmannite colloids. Lead is dominantly in the dissolved fraction throughout the drainage and is comparatively less sequestered due to competitive adsorption with As and its pH dependence to be efficiently adsorbed. A detailed observation of the concentration trends suggests that pH 3.1 may efficiently allow colloid formation and subsequently, toxic element sequestration. The behavior of Fe colloids in Shojin River was compared to those of a circum‐neutral drainage system (Ainai mine drainage). The formation of the colloids in the acidic system was limited by the pH range, while colloids formed faster and more abundantly in the circumneutral system. The acidic and circum‐neutral systems are abundant in schwertmannite and ferrihydrite colloids respectively. The colloids in both systems were transported after their formation, however, the colloids in the Shojin (acidic) system were transported further than those of the circumneutral system. Aggregation is observed in both systems which results in a deposition of the colloid aggregates and hence an efficient removal of Fe and As, and minimally, Pb from the mine drainages. [ABSTRACT FROM AUTHOR]

Secondary minerals, and mixtures of Fe-Al compounds such as oxohydroxy and hydroxosulfates can be naturally generated from mine water in response to oxidation /bio-oxidation, mixing, evaporation and neutralization processes. Deposits observed in significant quantities, with very high iron content, from the mining perimeters of Certej-Coranda, Rosia Montana, and Boita Hateg were sampled and characterized. Schwertmannite and Goethite can function as adsorbents after drying or further processing. In both cases, the chemical analysis of the product is important in order to promote it to potential users. A preliminary evaluation of the adsorption capacity for As(III) was performed for the original material and for products obtained after calcination. Depending on calcination temperature, adsorption capacity decreases, and a leveling effect can be observed, most probably due to pore sinterizing and transformation to hematite, irrespective of the initial mineral.

Selenium, a crucial trace element for many organisms, including prokaryotes and humans, is toxic at high concentrations, necessitating its removal from wastewater. This study investigates the use of jarosite, a naturally occurring iron sulfate mineral with excellent heavy metal attenuation properties, for selenate (Se(VI)) removal for the first time. Biogenic jarosite was synthesized through Fe(II) oxidation by Acidithiobacillus ferrooxidans at an initial pH ranging from 1.5 to 4.0 (J-1.5 to J-4.0). This resulted in the formation of morphologically diverse particles of biogenic jarosite owing to varying Fe(II) oxidation and precipitation rates. For comparative analysis, non-biogenic jarosite (J-90C) and biogenic schwertmannite (S-2.5) were also synthesized. At 0.2 mM initial Se(VI) concentration, J-2.5 demonstrated superior Se(VI) removal compared to J-3.5 and J-90C. At 2.0 mM Se(VI), J-2.5 still outperformed J-3.5 and J-90C although its overall removal efficiency decreased. Notably, at 0.2 mM concentration, Se(VI) removal by J-2.5 was 63 %, which is comparable to 77 % removal by S-2.5. Furthermore, sulfate release from J-2.5 was significantly lower than that from S-2.5 in both Se-free and Se-containing solutions. This study provides critical insights into the synthesis and application of biogenic jarosite as a replacement for metastable schwertmannite, emphasizing its potential as an excellent Se sink for wastewater treatment. [Display omitted] • pH changes impact morphology of jarosites formed by Acidithiobacillus ferrooxidans. • Structure and chemistry of biogenic jarosite influenced Se removal efficacy. • Se removal by jarosite formed at pH 2.5 higher than bio- and non-biogenic jarosites. • Biosynthesized jarosite stable and efficient Se sink for wastewater treatment. [ABSTRACT FROM AUTHOR]

Rice (Oryza sativa L.) is known to accumulate inorganic arsenic (iAs) and dimethylarsenate (DMA) in its grains, which threatens both human health and rice yield. Although schwertmannite, a metastable Fe (Ⅲ)-oxyhydroxysulfate mineral with extremely high adsorption capacity for iAs, has been proposed to remediate paddy soil to decrease As accumulation in rice, it remains unclear whether the phase transformation of schwertmannite would occur in flooded paddy soil and how its phase transformation changes the soil microbial processes that impact the accumulation of iAs and DMA in grains. Here, we found that amending As-contaminated paddy soil with 0.5%–1% (w/w) schwertmannite decreased the accumulation of iAs and DMA in grains by 37.41%–43.29% and 50.60%–73.89%, respectively, even though schwertmannite has transformed to goethite and secondary FeS was formed in both rhizosphere and bulk soils. The phase transformation of schwertmannite released a considerable amount of SO 4 2− into porewater, thereby increasing the abundances of both sulfate-reducing bacteria and the dsrB gene but decreasing the abundance of iron-reducing bacteria. This result suggested that schwertmannite phase transformation has promoted sulfate-reducing process and weakened iron-reducing process in flooded soil. Such promoted sulfate-reducing process and weakened iron-reducing process in paddy soil can decrease the reductive dissolution of As-bearing (oxyhydr)oxides, increase the formation of secondary FeS mineral for decreasing porewater As concentration, and strengthen the role of Fe plaque as a barrier for As absorption by rice. Additionally, the application of schwertmannite has decreased the abundance of arsM gene and weakened As methylation process in soil. Therefore, the effective decrease of iAs and DMA accumulation in rice grains by schwertmannite can not only be ascribed to the adsorption capacity of schwertmannite for As and the adsorption or incorporation of As by transformation products, but also contributed by the promoted sulfate-reducing process and the weakened iron-reducing process in flooded paddy soil. [Display omitted] • Transformation of schwertmannite into goethite occurs in flooded paddy soil. • Phase transformation alters iron-reduction and sulfate-reduction processes. • Schwertmannite decreases arsM abundance and weakens arsenic methylation process. • Schwertmannite simultaneously decreases the accumulation of iAs and DMA in rice grains. [ABSTRACT FROM AUTHOR]

Schwertmannite was proved to be an efficient material for the remediation of arsenic-contaminated soil. Given that arsenic (As) and cadmium (Cd) coexist in soils and have opposite chemical behaviors, stabilizing them simultaneously is challenging. Here, we reported a compound passivating agent (AMSC) which simultaneously passivated As and Cd in contaminated site soil containing 1659 mg/kg As and 32.26 mg/kg Cd. It consisted of alkali-modified schwertmannite (denoted as AMS) capable of preferentially adsorbing As and fine calcium carbonate (denoted as C) for mainly stabilizing Cd. It was found that the addition of AMSC into the contaminated soil at 5 % for 60 days could drastically reduce water-extractable As and TCLP-As concentrations by over 84 %, and water-extractable Cd and TCLP-Cd concentrations by over 94 %, respectively, due to the conversion of the labile fraction of As or Cd into more stable fractions. Additionally, pH in the AMSC-amended soil was increased to 6.74 from initial 4.50. As was mainly adsorbed into schwertmannite of AMSC by ligand exchange of SO 4 2- with As, while Cd was jointly passivated mainly by calcium carbonate and fewly by schwertmannite. To identify the mineral phase changes of AMSC, quartz sands of different grain sizes were selected for Mössbauer analysis. The percentage of schwertmannite in the sediment amended with AMSC decreased slightly from 51.1 % to 48.9 % within 30 days, while Fe(III) oxyhydroxide (48.9 %) was converted to ferrihydrite (30.3 %) and lepidocrocite (20.8 %), respectively. Therefore, it was expected that AMSC could act as a high-performance and environmentally-friendly amendment to remediate As and Cd-contaminated soils. [Display omitted] • Alkali-modified schwertmannite combined CaCO 3 (AMSC) raised soil pH within 6–7. • AMSC achieved effecient co-stabilization of As and Cd in highly contaminated soils. • AMSC reduced soil availability and mobility of As and Cd. • AMSC is an eco-friendly remediation for high As and Cd-contaminated site soils. [ABSTRACT FROM AUTHOR]

Acidic Cr(VI)-containing wastewater has received increasing attention in recent years. Schwertmannite is a suitable adsorbent for its acid resistance and good adsorption ability. However, it shows poor Cr(VI) adsorption performance under acidic conditions. Herein, inspired by the fast neutralization-mineralization process of acid mine drainage (AMD) triggered by alkaline rocks, a novel nano-scaled schwertmannite (Sch-2.7) with high Cr(VI) adsorption capacity was synthesized at constant pH of 2.7 via adding OH−. Compared with common schwertmannite (Sch), appropriate OH− effectively improved mineral yield (the precipitation efficiency of Fe: 96.75% vs. 29.93%), specific surface area (65.1 m2/g vs. 18.9 m2/g), surface group content, and further Cr(VI) adsorption ability of Sch-2.7. The maximum adsorption capacity was 54.17 (pH = 3), 61.59 (pH = 4), and 66.5 mg/g (pH = 5) for Sch-2.7, whereas only 20.35, 24.51, and 27.17 mg/g for Sch. On average, the former was 2.53 times higher than the latter. Temperature and coexisting ions had little influences on the sorption process of Sch-2.7. The mechanism analysis demonstrated that the Cr(VI) removal by Sch-2.7 was a more thermodynamic favorable process due to abundant reactive-active components on Sch-2.7 for adsorption reaction. This work provided new insight into performance optimization and application potential on Cr(VI) removal of schwertmannite. [ABSTRACT FROM AUTHOR]

In this work, mesoporous carbons (MCs) were synthesized by a template method, and the schwertmannites (Schs) were prepared by a calcination method. They were mixed uniformly by ultrasound and obtained the Schs/MCs nanocomposites. The Schs/MCs/GCE can be used to detect paracetamol (PA) and dopamine (DA) simultaneously. Under the best terms, it exhibits wide linear range of 1.0 to 300 µM for PA and 1.0 to 400 µM for DA. The determination limits are 0.33 µM for PA and 0.24 µM for DA (S/N = 3), respectively. Besides, the sensor is expected to be used for detect PA and DA in pharmaceutical samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ferrihydrite, schwertmannite, and green rust distinctive features are presented. • Their transformation paths, among them and to other Fe-minerals, are systematized. • Current and foreseen environmental and technological applications are discussed. Iron-bearing nanominerals such as ferrihydrite, schwertmannite, and green rust behave as metastable precursors leading to the formation of more thermodynamically stable iron mineral phases (e.g., jarosite, goethite, hematite, and magnetite). However, this transformation may last from days to tens or even hundreds of years, making them the most predominant iron-bearing minerals at environmental conditions and at the human time scale. The present review characterizes ferrihydrite, schwertmannite, and green rust nanominerals according to their main physical and chemical properties, and at both nano- and meso -scales. It also presents a comprehensive review of the multiple past and present Earth environments where these nanominerals have played, and still play, a pivotal role in the geochemistry, mineralogy and environmental nanogeosciences of these environments. Finally, the present and future technological applications of these nanominerals as well as their role in the generation of a more sustainable human-Earth relationship is discussed, with a special emphasis on their use in new circular economies and green based technologies. [ABSTRACT FROM AUTHOR]

Due to its sea urchin-like structure, Schwertmannite is commonly applied for heavy metals (HMs) pollutant adsorption. The adsorption influence parameters of Schwertmannite are numerous, the traditional experimental enumeration is powerless. In recent years, machine learning (ML) has been gradually employed for adsorbent materials, but there is no comprehensive research on the Schwertmannite adsorbent. In this paper, 27 features and 814 groups of experimental data were used to systematically analyze the adsorption modeling of Schwertmannite first time. The results indicated that the adsorption capacity of Schwertmannite was better predicted by the Random Forest (RF) model (the R 2 was 0.874). Then, the RF model was used to analyze the features importance that affects the adsorption of HMs by Schwertmannite. And the importance of Schwertmannite synthesis conditions, Schwertmannite characteristics, adsorption environment, and HMs properties were 11.88%, 30.01%, 48.26%, and 8.19% respectively. Moreover, the synthesis and adsorption conditions of Schwertmannite were predicted and optimized based on RF model, it was predicted that the better synthesis method of Schwertmannite was biological oxidation > Fe 2+ oxidation > Fe 3+ hydrolysis. Finally, a predictive Graphical User Interface Web Page for Schwertmannite-HMs was developed. We hope that this paper can promote the integration of machine learning and Schwertmannite., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

This study explored the feasibility of depositing protective coatings with an Sb scavenger function on mine waste rocks derived from the exploitation of stibnite deposits. Encapsulation treatments were performed using ferrous sulfate as the coating precursor. Different Fe/Sb leachable molar ratios (0.1, 1, and 10) were evaluated using hydrothermal and thermal processes at various temperatures (30, 50, 100, and 150 °C). The environmental characterization of the coated mine waste was established using standard leaching tests. The most effective coatings were analyzed for their mineralogy, chemical composition, and Sb attenuation using X-ray powder diffraction, scanning electron microscopy, and digestion/extraction processes. Additionally, stability tests were conducted to assess the effectiveness of encapsulation under changing environmental conditions. The Fe/Sb leachable molar ratio was found to be a critical factor in reverting the toxic and hazardous characteristics of mine wastes, with an optimal value of 10. The coatings were primarily composed of Fe oxyhydroxy sulfates/Fe oxyhydroxides and calcium sulfate minerals with a bulk Sb content of approximately 1 %. The adsorbed Sb content in the coatings was a small fraction of the total Sb content (< 0.5 %), indicating a strong retention. Moreover, such coatings were stable under different pH (3-8) and redox (100 to -100 mV) conditions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

This study exposes U(VI)-sorbed schwertmannite and jarosite to biotic reductive incubations under field-relevant conditions and examines the changes in aqueous and solid-phase speciation of U, Fe, and S as well as associated microbial communities over 180 days. The chemical, X-ray absorption spectroscopy, X-ray diffraction, and microscopic data demonstrated that the U(VI)-sorbed schwertmannite underwent a rapid reductive dissolution and solid-phase transformation to goethite, during which the surface-sorbed U(VI) was partly reduced and mostly repartitioned to monomeric U(VI)/U(IV) complexes by carboxyl and phosphoryl ligands on biomass or organic substances. Furthermore, the microbial data suggest that these processes were likely driven by the consecutive developments of fermentative and sulfate- and iron- reducing microbial communities. In contrast, the U(VI)-sorbed jarosite only stimulated the growth of some fermentative communities and underwent very limited reductive dissolution and thus, remaining in its initial state with no detectable mineralogical transformation and solid-phase U reduction/repartitioning. Accordingly, these two biotic incubations did not induce increased risk of U reliberation to the aqueous phase. These findings have important implications for understanding the interactions of schwertmannite/jarosite with microbial communities and colinked behavior and fate of U following the establishment of reducing conditions in various acidic and U-rich settings.

The Fragnè mine, located in the Lanzo valley in the municipality of Chialamberto (Turin, Piedmont Region), represented the most important regional site for Fe–Cu sulfide exploitation over a period of more than eighty years (1884–1965). The entire mining area is part of a structural complex in the Lower Piedmont Unit of the Western Alps, characterized by the presence of amphibolite, metabasite ("prasinite"), and metagabbroic rocks. In particular, the pyrite ore deposit occurs as massive mineralizations within interlayered metabasites and amphibolites. In this work, we describe secondary minerals and morphologies of minothems from the Fragnè mine that are found only in abandoned underground works, such as soda straws, normal and jelly stalactites and stalagmites, jellystones, columns, crusts, blisters, war-clubs, and hair, characterized by different mineralogical associations. All minothems were characterized by minerals formed during acid mine drainage (AMD) processes. Blisters are composed only of schwertmannite, war-clubs by schwertmannite, and goethite with low crystallinity and hair by epsomite and hexahydrite minerals. Jelly stalactites and stalagmites are characterized by schwertmannite often in association with bacteria, while solid stalactites and stalagmites are characterized by jarosite and goethite. The results indicate that the mineralogical transformation from schwertmannite to goethite observed in some minothems is probably due to aging processes of schwertmannite or local pH variations due to bacterial activity. On the basis of these results, we hypothesize that all the jelly samples, in association with strong bacterial activity, are slowly transformed into more solid goethite, and are thus precursors of goethite stalactites. [ABSTRACT FROM AUTHOR]

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Schwertmannite (Sch) holds a great promise as an iron material for remediating Arsenic (As)-contaminated paddy soils, due to its extremely high immobilization capacities for both arsenate [As(V)] and arsenite [As(III)]. However, there is still limited knowledge on the mineral phase transformation of this metastable iron-oxyhydroxysulfate mineral in paddy soils, particularly under different water management regimes including aerobic, intermittent flooding, and continuous flooding, and how its phase transformation impacts the migration of As in paddy soils. In this study, a membrane coated with schwertmannite was first developed to directly reflect the phase transformation of bulk schwertmannite applied to paddy soils. A soil incubation experiment was then conducted to investigate the mineral phase transformation of schwertmannite in paddy soils under different water management regimes and its impact on the migration of As in paddy soil. Our findings revealed that schwertmannite can persist in the paddy soil for 90 days in the aerobic group, whereas in the continuous flooding and intermittent flooding groups, schwertmannite transformed into goethite, with the degree or rate of mineral phase transformation being 5% Sch >1% Sch > control. These results indicated that water management practices and the amount of schwertmannite applied were the primary factors determining the occurrence and degree of mineral transformation of schwertmannite in paddy soil. Moreover, despite undergoing phase transformation, schwertmannite still significantly reduced the porewater As (As(III) and As(V)), and facilitated the transfer of non-specifically adsorbed As (F1) and specifically adsorbed As (F2) to amorphous iron oxide-bound As (F3), effectively reducing the bioavailability of soil As. These findings contribute to a better understanding of the mineralogical transformation of schwertmannite in paddy soils and the impact of mineral phase transformation on the retention of As in soil, which carry important implications for the application of schwertmannite in remediating As-contaminated paddy soils. [Display omitted] • A novel method was developed to reflect schwertmannite transformation in soil. • Water management controlled schwertmannite transformation in paddy soil. • Schwertmannite exhibited a strong stability under aerobic conditions. • Schwertmannite transformed to goethite under continuous or intermittent flooding. • Schwertmannite reduced As mobility despite undergoing phase transformation. [ABSTRACT FROM AUTHOR]

Schwertmannite is a metastable mineral playing a crucial role in the immobilization of metal(oid)s in acid mine drainage (AMD) systems. High temperatures associated with wildfires could lead to a sudden schwertmannite transformation, changing the mobility of metal(oid)s. The objective of the present study was to examine the thermal transformation from schwertmannite to hematite, and the subsequent effect on the chromium partitioning. The immobilization of arsenate after thermal transformation and its implications on chromium mobility was also evaluated. Natural schwertmannite, with increasing contents of chromium, was thermally treated between 200 to 800 °C. Transformation products were characterized by solid-phase techniques and selective chemical extractions. Results indicated a transformation to hematite at temperatures above 400 °C. The presence of chromium barely affected the temperature at which the transformation occurred, although partitioning of chromium in the mineral changed with temperature. As the temperature increased from 25 °C to 400 °C, chromium was less mobile and less outcompeted by arsenic adsorption, suggesting a larger contribution of inner-sphere complexes with increasing temperature. At temperatures above 600 °C, non-mobile forms strongly associated with neo-formed hematite were found. Finally, neo-formation of hematite led to a decrease in arsenic adsorption, implying a potentially enhanced arsenic mobility in AMD systems upon wildfires. [ABSTRACT FROM AUTHOR]

Secondary iron minerals play significant roles in the immobilization of As under acidic conditions, such as acid mine drainage. However, previous research works have not clarified the effect of pH on As(III) removal through coprecipitation with secondary minerals. Therefore, in this study, we aimed to investigate the discrepancy in As(III) coprecipitation with biogenic synthesized schwertmannite (Sch) and jarosite (Jar) at different pH values. For this, concentrations of Fe 2+ , TFe, S O 4 2 - , and As(III) in shake flasks were monitored during an overall incubation period of 83 h at initial pH of 1.5, 2.0, and 2.5. In addition, the physicochemical properties of collected minerals after incubation were identified using scanning electron microscopy, X-ray diffraction, pore size distribution, and Brunauer - Emmett - Teller surface area analyses. Our results showed that almost no mineral synthesis and no As(III) removal were detected in coprecipitated schwertmannite (Co-Sch) system and coprecipitated jarosite (Co-Jar) system at an initial pH of 1.5. The TFe precipitation efficiencies and As(III) removal efficiencies increased considerably and morphologies of Co-Sch and Co-Jar improved significantly when the initial pH value increased from 2.0-2.5. The maximum TFe precipitation efficiency and As(III) removal efficiency reached 30.8% and 89.6%, respectively, for the Co-Sch system, and were 47.5% and 37.4%, respectively, for the Co-Jar system. The overall results show that pH significantly affects the formation of Co-Sch and Co-Jar and the behaviour of As(III) coprecipitation.

Fluorine (F) in water has a negative effect on the environment and human health. Schwertmannite has potential remediation to contamination in solution. In this study, the adsorption mechanism and influencing factors of synthetic schwertmannite for low-concentration F were studied through batch experiments. The results suggested that the adsorption of F by schwertmannite reached equilibrium after about 60 min, and the adsorption efficiency exceeded 94%. The experimental data can be best-fit by the pseudo-second-order kinetic and Langmuir models well. Schwertmannite showed effective adsorption at pH 4, dosage 1.5 g L−1, low temperature, and low concentration of co-existing anion. The adsorption process was a spontaneous and exothermic reaction, which was dominated by chemical adsorption. FT-IR and XPS spectra analysis revealed that F adsorption on schwertmannite through the surface complexation and anion exchange reaction between SO42− and OH− with F−, especially the primary role of OH−. The results can provide theoretical support for the schwertmannite application in the treatment of F-containing wastewater. [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]

• The effect of Mn(II) on Cr-Sch transformation was significantly dependent on pH. • Mn(II) promoted the transformation of Cr-Sch from hematite to goethite at pH 10.0. • Mn(II) was oxidized to Mn(III)/Mn(IV) and uniformly distributed on minerals. • The existence of Mn(II), especially at high concentration, reduced the migration of Cr. • Mn(II) inhibited the release of the outer-sphere coordinated SO 4 2− at pH 7.0. Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO 4 2− and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination. [Display omitted] [ABSTRACT FROM AUTHOR]

Simultaneously stabilizing of arsenic (As) and cadmium (Cd) in co-contaminated soil presents substantial challenges due to their contrasting chemical properties. Schwertmannite (Sch) is recognized as a potent adsorbent for As pollution, with alkali modification showing promising results in the simultaneous immobilization of both As and Cd. This study systematically investigated the long-term stabilization efficacy of alkali-modified Sch in Cd–As co-contaminated farmland soil over a 200-day flooding-drying period. The results revealed that As showed significant mobility in flooded conditions, whereas Cd exhibited increased soil availability under drying phases. The addition of Sch did not affect the trends in soil pH and Eh fluctuations; nonetheless, it led to an augmentation in the levels of amorphous iron oxides and SO 4 2− concentration in soil pore water. At a dosage of 0.5% Sch, there was a notable decrease in the mobility and soil availability of As and Cd under both flooding (34.5% and 53.6% at Day 50) and drying conditions (27.0% and 29.4% at Day 130), primarily promoting the transformation of labile metal(loid) fraction into amorphous iron oxide-bound forms. Throughout the flooding-drying treatment period, Sch maintained stable mineral morphology and mineralogical phase, highlighting its long-term stabilization effect. The findings of this study emphasize the promising application of Sch-based soil remediation agents in mitigating the challenges arising from As–Cd co-contamination. Further research is warranted to explore their application in real farmland settings and their impact on the uptake of toxic metal(loid)s by plants. [Display omitted] • Schwertmannite (Sch) exhibited efficient co-stabilization capabilities for As and Cd. • Sch did not cause significant changes in soil pH/Eh during 200-day flooding-drying period. • Sch reduced soil availability and mobility of As and Cd in both flooding and drying conditions. • Sch exhibited remarkable stability in mineral morphology and mineralogical phase. [ABSTRACT FROM AUTHOR]

Adsorption and reduction are important ways to remediate hexavalent chrome [Cr(VI)] which is extremely hazardous to human health. Previous studies have shown that schwertmannite (Sch) and nano zero-valent iron (nZVI) are potential materials with high adsorption and strong reduction for Cr(VI), respectively. However, it has been well known that bare nZVI has limitations in remediating Cr(VI) due to its susceptibility to self-aggregation and oxidation in practical applications. Hence to address the above limitations, nZVI was added into the internal pore structure of Sch (nZVI@Sch) in this study and Cr(VI) removal capacity by nZVI@Sch in aqueous solution were systematically investigated. Results showed that under the conditions that the concentration ratio of Fe2+ and Sch is 0.25 mol/L, Cr(VI) initial concentration of 10 mmol/L, initial pH of 3, Cr(VI) remediation ability of nZVI@Sch reached up to 114.03 mg/g (with freeze-drying) and 165.69 mg/g (without freeze-drying) in 4 hours, respectively. Based on the characterization analysis of SEM-EDS, XPS and Zeta potential, the multifunctional synergistic effects of capture-reduction-adsorption-coprecipitation as a mechanism for the removal of Cr(VI) by nZVI@Sch were proposed in this study. This study provided a new remediation material and suggestions for chromium pollution remediation. [Display omitted] • A new multifunctional composite material nZVI@Sch for Cr(VI) removal was prepared. • The maximum remediation capacity of nZVI@Sch was up to 114.03 mg/g. • Proposed mechanism for removing Cr(VI) by capture-reduce-adsorption-coprecipitate. [ABSTRACT FROM AUTHOR]

Schwertmannite is a metastable sulfate-rich ferric iron Fe(III) (oxyhydr)oxide and a common mineral in acid mine drainage sites and acid sulfate soils. Schwertmannite is also used as a sorbent in various industrial applications, including phosphate removal in water treatment and environmental remediation. Phosphate sorption to schwertmannite, however, is complex and likely involves ligand exchange for inner- and outer-spherically coordinated sulfate groups, both on the surface and in the tunnel structure of the mineral. Here, we investigated phosphate sorption, concomitant sulfate release and their impact on the structure of schwertmannite as a function of pH and phosphate concentration. Kinetic and equilibrium batch experiments with synthetic schwertmannite were carried out at pH 3, 6, and 8, and the solid-phase was analyzed using a combination of microscopic, spectroscopic, and X-ray diffraction techniques. We found a strong correlation between phosphate sorption and sulfate release, with both following a two-step sorption model. The kinetics of phosphate sorption and sulfate release were faster at higher pH levels. Maximum phosphate sorption was found at pH 6 (1.7 mmol PO 4 3− g−1), which decreased to 1.5 and 1.2 mmol PO 4 3− g−1 at pH 3 and 8, respectively. Fourier transform infrared spectroscopy revealed a shift from inner- to outer-spherical coordination of sulfate with increasing pH. 57Fe-Mössbauer analyses of schwertmannite demonstrated an initial transformation of schwertmannite at neutral to alkaline pH, characterized by a rise in a partially ordered sextet area. This change was interpreted as an increase in crystallinity resulting from the transition from Fe-SO 4 to Fe O domains. The emerging phase differed from the original schwertmannite, but did not represent a complete change to a new crystalline phase, thus indicating a proto-transformation of schwertmannite. This pH-induced proto-transformation was inhibited in the presence of phosphate. We concluded that the phosphate sorption rate and maximum as well as the proto-transformation of schwertmannite were strongly affected by the mineral's affinity for sulfate. Sorption to schwertmannite should primarily be regarded as a competitive exchange reaction between the sorbing oxyanion and the bound sulfate. As a result, the highest phosphate sorption occurred at circumneutral pH, in stark contrast to non-sulfate-containing Fe(III) (oxyhydr)oxides, where phosphate sorption is highest at acidic pH. Our findings are important for a fundamental understanding of the sorption properties of schwertmannite in phosphate-rich environments as they point towards the central role of sulfate coordination for phosphate immobilization. • Phosphate sorption rates and maxima peaked at pH 6. • pH altered phosphate sorption by changing schwertmannite's sulfate affinity. • Sulfate loss at high pH increased schwertmannite crystallinity during equilibration. • Sorbed phosphate stabilized schwertmannite, preventing structural transformation. [ABSTRACT FROM AUTHOR]