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Soil mineral composition affects soil behavior, but field estimation of mineral composition has proved difficult. To investigate the potential of predicting soil mineral composition in situ, 15 soils representing diverse mineral composition from New South Wales, Australia, were scanned with visible near-infrared (VisNIR) and portable X-ray fluorescence (pXRF) spectrometers to a depth of 1 m at 2.5-cm scanning increments. The presence of phyllosilicate and Fe oxides was assessed using a pattern-matching algorithm with VisNIR spectra from mineral end-member libraries. Rule-based iterative partitioning was then applied on pXRF elemental compositions based on mineral stoichiometry to determine the abundance of kaolinite, smectite, illite, hematite, goethite, calcite, gypsum, and quartz. This fusion model correctly identified the most abundant mineral in a sample with 72% accuracy. Mineral predictions were stable under variable moisture and surface conditions, as experienced when scanning samples in situ and under air-dry and ground conditions in the laboratory. Relative changes in mineral composition within a profile and across horizon boundaries were accurately expressed. The fusion model accurately quantified the abundance of quartz (Cohen’s kappa coefficient, 0.67), and CaCO₃ (Cohen’s kappa coefficient, 0.76; Lin’s concordance correlation coefficient, 0.96; RMSE, 20.9 g kg⁻¹). The dominant phyllosilicate mineral was identified correctly with 86% accuracy, although accurate quantification of phyllosilicates and Fe oxides was not achieved. Conjoint use of VisNIR and pXRF spectroscopy as part of a fusion model approach showed great potential to provide comprehensive estimation of soil mineral composition in situ.

Goethite is a sink for antimonate Sb(OH)₆– [Sb(V)] in soil and sediments. The pH- and ionic strength (I)-dependent mass distribution of Sb(V) between adsorbed and solution phases at equilibrium may be used to indirectly infer an adsorption mechanism (inner vs. outer sphere surface complexation) and to develop predictive surface complexation models. The objectives of this study were to characterize the adsorption of Sb(V) by goethite as a function of pH, I, and in the absence or presence of competing ligands, SO₄ or PO₄. Ligand adsorption was at a maximum in strongly acidic suspensions and decreased with increasing pH. Adsorption envelopes of Sb(V) where not influenced by I, but increasing I decreased SO₄ and increased PO₄ retention. In strongly acidic suspensions, SO₄ and PO₄ retention was not influenced by I. Ligand adsorption decreased the isoelectric point (IEP) of goethite from 8.5 to 6.3 [Sb(V)] and 4 (PO₄). The adsorption of SO₄ did not affect the IEP, although SO₄ reduced the zeta potential of goethite when pH < IEP. Antimonate and PO₄ adsorption was hysteretic; SO₄ adsorption was reversible. Sulfate did not impact Sb(V) retention; adsorbed Sb(V) generated greater negative surface charge, decreasing SO₄ adsorption. Both Sb(V) and PO₄ competed for adsorption sites, resulted in reduced retention of both in competitive systems. The triple layer formulation of the charge distribution multisite complexation model, coupled with inner sphere surface complexation, was used to describe the adsorption envelopes for Sb(V), SO₄, and PO₄, and to predict adsorption in competitive systems.

Iron and manganese oxides are common minerals in a wide range of soils, and they largely control the fate of P. It is often implicitly assumed that phosphate does not exchange O atoms with minerals and water during adsorption, desorption, and precipitation reactions, but this has not yet been fully verified. In this study, we applied an ¹⁸O stable isotope labeling technique to individually label O atoms in Fe and Mn oxides, water, and phosphate, and tested the exchange of O among them during phosphate sorption and desorption reactions. Physico-chemical properties of ¹⁸O-labeled goethite, hematite, hausmannite, and birnessite minerals were characterized using X–ray diffraction, Brunauer–Emmett–Teller (BET) surface area and pore size distribution, and scanning electron microscopy. All sorption and desorption results showed fast initial kinetics, with hausmannite having the highest sorption capacity, followed by a relatively slower sorption–desorption reaction before reaching equilibrium. Oxygen isotope ratios in Fe and Mn oxides, phosphate, and water showed no exchange of O between phosphate and mineral or water during either sorption or desorption reactions. These results attest to the integrity of the P–O bond during these reactions and thus enable the phosphate O isotope composition to be used as a reliable tool for tracing phosphate sources in soils and other environments.

Goethite and kaolinite are ubiquitous in natural environments. In soils they are often cemented together as a binary association, which has a significant influence on the structure and properties of soils. In this study, the mineralogy (using X-ray diffraction [XRD], thermal analyses, and infrared analysis), interactions, and stability of a goethite-kaolinite association (GKA) and a goethite-kaolinite mixture (GKM) were investigated. In GKA, goethite almost completely coated the surface of kaolinite, whereas for GKM the coating was partial. Infrared (IR) spectra showed that the vibrational frequencies of Fe-OH in GKM and GKA shifted to higher wave numbers and those of Al-OH, O-Al-O, Al-O-Si, and Fe-O shifted to lower wave numbers. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy demonstrated that interactions between goethite and kaolinite increased the chemical shifts of Si-29 and Al-27. The shifts of both the IR and the MAS NMR parameters were larger for GKA than for GKM. In addition, the IR bands of Al-OH and the XRD peaks of goethite were weaker with GKA than with GKM. As expected, particle size distributions showed that GKA had a higher mechanical stability than GKM. The thermal stability of the kaolinite in GKA was higher than that of GKM, but that of goethite in GKA was lower than that in GKM. These results imply that: (i) kaolinite and goethite interact by electrostatic attraction, anion ligand exchange, surface coordination, and hydrogen bonds; (ii) the interaction strength was much stronger for GKA than for GKM; (iii) a cation exchange reaction and isomorphous substitution of Fe by Al for goethite occurred in GKA.

Agricultural liming materials are used to lower soil acidity and to improve microbial functionality and plant growth. A brownish colored agricultural lime was found to contain up to 180 mg kg ―1 Cu, an amount well above any fertilizing materials code threshold. The dark color of the milled material was due to ample black and brown dendrites consisting of the tectomanganate mineral romanechite (ideally (Ba,H 2 O) 2 (Mn 4+ ,Mn 3+ ) 5 O 10 ) and a mixture of the two nanocrystalline Fe oxides ferrihydrite and goethite. A four-step sequential extraction analysis of the lime powder was not sufficient to argue whether the Cu load was contained in the carbonate matrix or in the oxide dendrites, or, in the latter case, in which of the three oxide phases identified. Electron microprobe analysis confirmed that the manganate dendrites were responsible for elevated Cu concentrations in the limestone. Focused micro-x-ray absorption spectroscopy (μ-XAS) revealed the bonding environment of Cu in the romanechite dendrites to be octahedrally coordinated as the Jahn-Teller cation Cu 2+ bound into lattice sites of the tectomanganate phase. The Mn-Cu, Cu-Mn, and Cu-Cu shell distances indicated a subsritution of the Mn 3+ tectomanganate tunnel edge sites by the Cu 2+ cations. This structural intercalation of Cu is clearly more stable than Cu cations bound as surface adsorbate complexes. The Cu contained in the liming material is therefore not likely to be mobilized and bioavailable, and therefore the lime poses little threat to agricultural soils.

Iron oxide-kaolinite associations are important components of tropical and subtropical soils and have significant influence on the physical and chemical properties of soils. In this study, the formation and transformation of Fe oxide-kaolinite associations as a function of pH, temperature, and time were investigated at different Fe(II)/ Fe(III) molar ratios (R). Results show that the formation of crystalline Fe oxides was significantly inhibited due to the presence of kaolinite, while accelerated by Fe(II). The formation of lepidocrocite- and goethite-kaolinite associations were accelerated by Fe(II) at R = 0.04 to 0.06, an initial pH (pHi) of 5 to 8, and a temperature (T) of 50 to 70°C; the formation of hematite-kaolinite association was accelerated by Fe(II) at R = 0.06, pHi 7 to 8, and T= 60 to 80°C; magnetite-kaolinite association was obtained at R = 0.06, pHi 9, and T = 60°C or at R = 0.1 to 0.5, pHi 7, and T = 60°C. The pH as a function of time (pHt) decreased sharply when crystalline Fe oxides were formed in the presence of Fe(II), Fe(III), and kaolinite. The decrease in pHt was slow, however, in the system with Fe(III) and kaolinite but without Fe(II) and in the system with Fe(II) and kaolinite but without Fe(III). The morphologies of lepidocrocite, goethite, hematite, and magnetite in associations are strip shaped, nanorod like, pseudo-cubic shaped, and nanosphere like, respectively. In a system with Fe(II), Fe(III), and kaolin

Predicting nutrient behavior is ever more critical to understanding and management of the environment, particularly in highly weathered and tropical environments. The fate of nutrients in the environment seems heavily influenced by the kinetics of ion-pair formation on the surfaces of variable-charge minerals coupled with transport processes. The objective of this study was to generate the coupled processes in column experiments and estimate the reaction rates using the frequency domain method. Columns were packed with ground natural minerals (gibbsite, goethite, and hematite). The input signals were designed as a sinusoidal change in NaNO 3 concentration within 0.1 and 0.2 mmol L ―1 and constant pH 4.0, and the highest frequency of the input signals was 0.714 min ―1 . The input and output signals of NO 3 ― and H + concentrations were monitored by ultraviolet―visible light and pH detectors, respectively. A mathematical model was derived to describe the diffusion process of a counterion from aqueous solution to β plane, the recombination-dissociation reaction between a charged surface site and the counterion, and the coupled transport process described by the convection-dispersion equation. Results showed that the output signals were dominated by the designed frequency and thus the coupled processes were linear. The aqueous H + concentration changed linearly with that of the aqueous NO 3 ― concentration. The mathematical model fit the measured transfer function of the processes. The estimated rates of recombination of ion pairs were 52.0, 30.5, and 8.0 L mol ―1 min ― 1 , and the estimated rates of dissociation of the ion pair were 0.189, 0.256, and 0.285 min ―1 for the natural gibbsite, goethite, and hematite, respectively.

Low-molecular-mass organic acids play an important role in many soil processes, including mineral dissolution and nutrient mobilization and uptake. One such organic acid is 2-ketogluconate (kG). This study examined the adsorption of kG by goethite, as influenced by temperature, pH, and ionic environment. Adsorption edge studies showed that kG adsorption was a function of pH but independent of ionic strength. Ketogluconate adsorption also resulted in a negative shift in the surface charge characteristics of goethite. The inclusion of PO 4 and AsO 4 decreased kG adsorption throughout the pH 4 to 10 range; however, SO 4 only decreased kG retention in the pH

The distribution, chemistry, and morphology of Fe nodules were studied in a marine terrace soil chronosequence northwest of Santa Cruz, California. The Fe nodules are found at depths < 1 m on all terraces. The nodules consisted of soil mineral grains cemented by Fe oxides. The nodules varied in size from 0.5 to 25 mm in diameter. Nodules did not occur in the underlying regolith. The Fe-oxide mineralogy of the nodules was typically goethite; however, a subset of nodules consisted of maghemite. There was a slight transformation to hematite with time. The abundance of soil Fe nodules increased with terrace age on the five terraces studied (aged 65,000-226,000 yr). Scanning electron microscopy (SEM) revealed Fe-oxide-containing fungal hyphae throughout the nodules, including organic structures incorporating fine-grained Fe oxides. The fine-grained nature of the Fe oxides was substantiated by Mossbauer spectroscopy. Our microscopic observations led to the hypothesis that the nodules in the Santa Cruz terrace soils are precipitated by fungi, perhaps as a strategy to sequester primary mineral grains for nutrient extraction. The fungal structures are fixed by the seasonal wetting and dry cycles and rounded through bioturbation. The organic structures are compacted by the degradation of fungal C with time.

Although mineralogic quantitative phase analysis (QPA) of the soil clay fraction can provide useful information for the improvement of soil management practices, QPA often requires a combination of several analytical techniques, which can be expensive and time consuming. One alternative that involves a single analysis to give accurate QPA of soils is the use of the Rietveld method to analyze powder x-ray diffraction (XRD) data. In this study, we evaluated the accuracy of the XRD–Rietveld approach for mineralogic quantitative analyses of Oxisol clays when observed structure factors [F(hkl)] of pedogenic minerals (i.e., kaolinite, hematite, and goethite) are used in the Rietveld analyses performed using Siroquant software. The results showed that although the structures of disordered clay minerals are especially difficult to incorporate in standard Rietveld refinement, which relies on ordered three-dimensional structure models, Mineralogic quantification can be accurately done for complex matrices having a large number of phases and various degrees of structural and compositional disorders when observed F(hkl) values are used. It is also possible to develop observed F(hkl) values for Al-hematite and Al-goethite from impure samples of such oxides to use as starting structure models for Rietveld analysis. We believe that this approach can be successfully extended to other geologic materials.

Reducing conditions in soils can be documented by the voltage measured between Pt and reference electrodes (Eh) or by using dyes like α,α(-dipyridyl for identifying reduced species (such as Fe 2+ ). Indicator of Reduction in Soils (IRIS) tubes have been recently introduced as an alternative approach for documenting reducing conditions in soils, where a synthetic Fe oxyhydroxide painted on polyvinyl chloride tubes can be reduced, dissolved, and removed by respiring soil microorganisms. In order for the Fe oxyhydroxide paint to adhere to the tubes, it must be synthesized as a mixture offerrihydrite and goethite. Based on thermodynamics, ferrihydrite is predicted to be more easily reduced at higher redox potentials than goethite. We hypothesized that IRIS tubes made using paint composed of different proportions of ferrihydrite and goethite would perform differently in wetland soils. This experiment evaluated the performance of IRIS tubes created using nine Fe oxyhydroxide paints with different proportions offerrihydrite and goethite ranging from 15.2 to 70.9 mol % goethite. Tubes were evaluated following placement in mesocosms in the laboratory using two different wetland soil materials for 13 d. An ANOVA indicated that there were highly significant differences (P < 0.0001) in full or intermediate removal of paint due to paint type. In general, the extent of paint removal was inversely related to goethite content. The minimum goethite content needed to successfully manufacture IRIS tubes is approximately 40%, and we recommend that IRIS tubes not be used when the goethite content is beyond the range of 40 to 60 mol %.

The adsorption of desferrioxamine (DFOB), acetyl desferrioxamine (DFOD1), and a catecholate derivative (DFOMTA)] on goethite (α-FeOOH) and hematite (α-Fe 2 O 3 ) and mineral dissolution behavior at pH 3.0 to 6.0 were studied to yield an understanding of the mechanisms of siderophore complexation on α phases and also to study how variations in siderophore molecular structure, as modified by proton activity, affect siderophore effectiveness at releasing Fe from different soil minerals. Wet-chemistry experiments were conducted to react 1 g L -1 goethite or hematite with 240 μmol L -1 siderophores. Iron release values were found to be as high as 350 ± 20 μmol L -1 , with no evidence of mineral transformation as determined by x-ray diffraction analyses. The proton activity strongly influenced Fe release, but no correlations between Fe concentrations and pH were found. The siderophore molecular structure influenced Fe release; the effectiveness for releasing Fe decreased in the order DFOMTA > DFOD1 > DFOB. Kinetics and Fe siderophore stability constants in solution data trends correlated possibly because the formation of Fe-siderophore complexes limits dissolution for both α-phase trihydroxamate and catecholate siderophores. Incongruency between dissolution kinetics and siderophore adsorption data trends was explained by siderophores triggering dissolution mechanisms that may not require surface adsorption. It is plausible then that the mineralogical composition of the soil could, in part, regulate the production of siderophores by microorganisms and plants. Based on Fe release data for hematite, it was concluded that siderophores contribute to the Fe pool in soil solutions with decreasing proton activity. Thus, the role of siderophores in releasing structural Fe becomes increasingly important as mineral solubility becomes limiting. By contrast, based on Fe release for goethite, the contribution of siderophores to the acquisition of Fe in goethitic soil profiles becomes less pronounced with decreases in the proton activity of the soil solution.

It is unclear whether persulfate digestions completely liberate P associated with mineral colloids in soil solutions and aqueous soil extracts. We tested a modified persulfate digestion using aqueous soil extracts and suspensions of goethite, gibbsite, illite, and montmorillonite that were exposed to 24 μmol P L -1 as orthophosphate (ortho-P) or myo-inositol hexaphosphate (IHP). Digestion was performed by autoclaving (121°C, 60 min) 5 mL of extract or suspension after the addition of 1 mL of solution, which contained 150 mmol L -1 K 2 O 8 S 2 and 180 mmol L -1 H 2 SO 4 . Subsequently, 0.7 mL of 188 mmol L -1 ascorbic acid was added and the sample was heated in a water bath at 95°C for 60 min. For the samples where P was sorbed onto goethite and gibbsite, recoveries of total P were 97.2 ±3.7% (ortho-P) and 102 ±3.7% (IHP). For the samples where P was sorbed onto clay minerals, average total P recoveries were 99.2 ± 5.1% (ortho-P) and 106 ± 3.0% (IHP). No differences between total P concentrations measured in persulfate-digested soil extracts or with an additional digestion with HF and HNO 3 were detected.

Variations in the low-field magnetic susceptibility of the wind-blown Chinese Loess Plateau (CLP) loess–paleosol sequences reflect changes in the global paleoclimate on different time scales. Magnetic enhancement in paleosols has been ascribed to the neoformation of fine-grained maghemite; however, little is known about the pathway through which this mineral was formed in the CLP paleosols, its relationships with the other pedogenic Fe oxides (viz. hematite and goethite), and the pedoclimatic significance of such relationships. In this work, we characterized various magnetic, chemical, and mineralogical properties of the loess–paleosol units at depths from about 23 to 55 m in the Upper Luochuan section, central CLP. The concentration of pedogenic hematite (Hm) and the frequency-dependent magnetic susceptibility (FD), which is used as a proxy for the concentration of fine-grained pedogenic maghemite, were found to be linearly correlated (R2 = 0.825, P < 0.001). This supports the idea that these two minerals were formed concomitantly during pedogenesis, which is consistent with the results of previous in vitro experiments showing that the ferrihydrite maghemite hematite transformation takes place under aerobic conditions. By contrast, the concentration of pedogenic goethite (Gt) was only weakly correlated with either FD or Hm, which suggests that goethite formed through an alternative pathway. The paleosols above 40 m (S4, S5, corresponding to marine isotope stages 9 and 11, respectively) exhibit a higher degree of weathering and higher Hm/(Hm + Gt) ratio than those below such a depth (S6–S8). This was ascribed to differences in paleoclimatic conditions, which are moister and warmer in the former paleosols than in the latter, rather than to differences in pedogenesis duration.

Conventional methods for investigating heavy metal contamination in soil are time consuming and expensive. In this study, we (i) explored reflectance spectroscopy as an alternative method for assessing heavy metals, and (ii) further explored the physicochemical mechanism that allows estimation of heavy metals with the reflectance spectroscopy method. We first investigated the spectral response of changing concentrations of heavy metals in soils. The results indicated that only at very high concentration can transition elements exhibit their inherent absorption features. In spite of this observation, we successfully predicted low levels of heavy metals in agricultural soils. The best prediction accuracies were obtained for the siderophile elements Ni, Cr, and Co. The poorest prediction was for Cd. The order of prediction accuracy for metals was approximately the same as the order of their correlation coefficients with Fe. Complementary to some previous studies that found that the intercorrelation between heavy metals and active soil components (such as Fe oxides, organic matter, and clay) is the major predictive mechanism, in the present study we concluded that the correlation with total Fe (including active and residual Fe) is the major mechanism. This conclusion was further supported by both correlation analysis and chemical sequential extraction. Correlation analysis showed that all metals are negatively correlated with reflectance while positively correlated with the absorption depth at about 500 nm, a feature resulting from goethite. The chemical forms of heavy metals, which showed that besides the crystalline Fe oxide and organic matter fractions, heavy metals have significant amounts in the residual fraction, also strengthened the conclusion.

Sorption to micro- and mesoporous mineral phases can stabilize organic matter (OM) against microbial decay in soil. Formation of strong bonds that reduce desorbability is one plausible explanation for that effect. With time after sorption, sorbed OM may undergo changes in configuration or may migrate into intraparticle spaces. We tested the possible effects of residence time of OM sorbed to ferrihydrite and goethite. The minerals were loaded with different amounts of water-soluble OM from an Oa horizon, then stored moist (10% w/w water) for up to 1080 d at 4°C. We monitored the content of organic C, the desorbability and chemical stability (by extraction with 0.1 MNaOH-0.4 MNaF and treatment with 1 M NaOCl), and, after freeze-drying, the micro- and mesopore volume (by N 2 and CO 2 adsorption-desorption). Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopywas used to characterize the OM on the mineral surfaces at the beginning and end of the experiment. There was no detectable decrease in sorbed organic C during the experiment; also, the micro- and mesoporosity of the samples remained unchanged. The proportion of desorbable organic C, however, decreased by up to 16%. This was paralleled by more pronounced bands indicative of complexed organic functional groups in the DRIFT spectra. We conclude that with increasing residence time, OM sorbed to porous minerals becomes decreasingly desorbable by the formation of additional chemical bonds to the surface via ligand exchange but not by diffusion into small pores. The decrease in desorbability was accompanied by a decrease in chemical destructibility with NaOCl. The stability of sorbed OM against biological degradation may similarly increase with residence time.

Scanning electron microscopy, including energy dispersive scan analyses and amplitude sweep tests with controlled shear deformation, were conducted on four kaolinitic, Fe-oxide-rich Brazilian Hapludox soils and one montmorillonitic Calciudert soil to elucidate results obtained from a parallel-plate rheometer. In the Brazilian Hapludox soils, the occurrence of pseudosand (an aggregation of fine particles with a grain size of sand), due to Fe (hydr)oxide cementation, had a significant effect on aggregation and microstructural stability. We could observe the presence of hematite, goethite, and, in rare cases, halloysite, as the major stabilizing minerals in such soils. The influences of soil organic matter (SOM), Fe oxides, and clay minerals on micromechanical shear behavior under oscillation were tested under saturated and predrained (at -60 hPa) conditions. Collected data included G (shear modulus), G' (storage modulus), G'' (loss modulus), linear viscoelastic deformation range, deformation limit (gamma L), and yield stress (tau y). From the data, the dissipation of elasticity was derived, and the values ranged from 200 to 1000 Pa in all untreated samples, both Hapludox and Calciudert soils, and to 50 to 500 Pa in SOM-leached Hapludox samples. Minimum tau y values of 10 to 50 Pa occurred after a sodium dithionite treatment (Fe(d) leached) of the four Hapludox samples. These findings show that Fe (hydr)oxides have a more stabilizing effect on the microstructural stability than SOM.

Methodological difficulties of colloid sampling in the vadose zone are limiting our knowledge regarding the relevance of colloid transport for groundwater contamination. We compared the colloid sampling efficiency of five different lysimeters in a column experiment (9 cm length, 8 cm diameter) using 59 Fe-labeled goethite: polyester membranes with a pore diameter of (i) 1.2 μm and (ii) 10 μm, (iii) porous glass plates with 16-μm pore diameter, (iv) wick samplers, and (v) zero-tension lysimeters. Four replications of each lysimeter type were tested with concentrations of 0.1 and 10 mg L -1 goethite. The irrigation rate was 58 mm h -1 , which caused an average transport velocity of 240 mm h -1 . Compared with NO 3 -, a tendency of accelerated transport of goethite in the sand columns was observed. The mean recovery of 59 Fe for all lysimeters was 30.7 ± 6.7% for the small and 3.4 ± 3.5% for the large colloid input concentration. For the small goethite concentration, no differences between lysimeter systems were detected. In contrast, the lysimeters performed differently at large concentrations: zero-tension and 10-μm membrane lysimeters showed the largest (9.1 and 6.8%), wick lysimeters the smallest colloid recovery (0.7%), which was related to trapping of colloids in the wick. We conclude that membranes of 10-um pore size and zero-tension lysimeters are superior for colloid sampling, but the results of the latter may be biased toward an overestimation of colloid transport because of water saturation at the lysimeter-soil interface.

Sorption and desorption processes control the mobility, toxicity, and availability of As in natural environments. Surface coverage and residence time may affect the kinetics of As sorption-desorption from soil components and the transformation of As from desorbable into resistant or undesorbable forms. We performed kinetic studies on the sorption of As(V) onto crystalline or poorly crystalline metal oxides (noncrystalline Al(OH) x , gibbsite, ferrihydrite, and goethite) and its desorption by PO 4 at pH 6.0 as affected by the residence time and the surface coverage (50 or 100%) of As(V). Significant amounts of As(V) were sorbed during the initial period of 0.167 h, ranging from 37.9 to 7L8% when the surface coverage was about 100%. The kinetic data, explained best by the Elovich kinetic model, indicated the following order in As(V) sorption: gibbsite < Al(OH) x < goethite < ferrihydrite. By adding PO 4 immediately after complete sorption of As(V) onto the oxides (50% surface coverage; PO 4 added/As(V) sorbed molar ratio of 4), a much higher proportion of As(V) was desorbed after 24 h of reaction from Al oxides (48-56%) than from Fe oxides (18-23%). The amount of As(V) desorbed decreased with increasing residence time. The kinetics of As(V) desorption by PO 4 as a function of residence time was explained best by the Elovich kinetic model. The kinetics described the rate of rearrangement of As(V) from desorbable into resistant or undesorbable forms, which occurred more rapidly in Al than Fe oxides. After a residence time of 360 h, the percentage of As(V) desorbed from the oxides was reduced significantly (

Organic coatings on Fe oxides can decrease the accessibility of intraparticle pores for oxyanions like phosphate. We hypothesized that the slow sorption of phosphate to goethite coated with polygalacturonate (PGA) is controlled by the accessibility of external goethite surfaces to phosphate rather than by diffusion of phosphate into micropores (0 < 2 nm). We studied the phosphate sorption kinetics of pure and PGA-coated goethites that differed in their microporosity (N 2 at 77 K, 46 vs. 31 mm 3 g -1 ). Because drying may affect the structure or surface coverage of PGA, we also tested the effect of freeze-drying on the slow phosphate sorption. The samples were examined by gas adsorption (N 2 , CO 2 ) and electrophoretic mobility measurements. Phosphate sorption and PGA-C desorption were studied in batch experiments for 3 wk at pH 5. In PGA-coated samples, the slow phosphate sorption was independent of micropore volume. Phosphate displaced on average 57% of PGA-C within 3 wk. Similar to phosphate sorption, the PGA-C desorption comprised a rapid initial desorption, which was followed by a slow C desorption. Sorption competition between phosphate and presorbed PGA depended on the

Uronates are important constituents of maize mucilage and polyuronates are used as a simplified model of the soil-root interface. We tested whether galacturonate (GA) and polygalacturonate (PGA) impair the diffusion of phosphate (PO 4 ) into and out of pores of a synthetic goethite (147 m 2 g -1 ) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the PO 4 desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before PO 4 addition, another part after addition of PO 4 . Before the desorption experiments, the porosity of our samples was analyzed by N 2 gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of PO 4 out of mineral pores. No consistent relation between the micro- and mesoporosity and the rate of the slow PO 4 desorption was observed. Compared with the C-free control, only PGA and MU affected the fraction of PO 4 mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before PO 4 , twice as much PO 4 was mobilized via the fast reaction than in the treatment where PO 4 was sorbed before PGA, suggesting a decreased accessibility of goethite pores to PO 4 . Mucigel, however, showed reversed effects, which is ascribed to its differing chemical composition. In conclusion, PGA seems inappropriate as a model substance for maize MU collected from non-axenic sand cultures. Under the experimental conditions chosen, the efficacy of all organic substances to increase PO 4 solution concentrations by pore clogging and sorption competition is small.

Various circumstances in wetland delineation or mitigation strategies require the documentation of reducing conditions in soils. An innovative approach for assessing reduction in soils known as Indicator of Reduction in Soils (IRIS) has been recently introduced. IRIS tubes are manufactured by coating polyvinylchloride (PVC) pipe with a paint prepared from an Fe oxide (mainly ferrihydrite) suspension. When the iron oxide is synthesized by adding KOH to a FeCl 3 solution, the mineral phase is essentially pure ferrihydrite. Over time, and under particular conditions, the ferrihydrite is transformed to more crystalline forms such as goethite and hematite. The objectives of this study were (i) to assess the mineralogical composition of iron oxide paint synthesized for the manufacture of IRIS tubes and (ii) to assess the significance of mineralogical composition of the Fe oxide paint on desirable qualities for the construction of IRIS tubes. The effects of pH and storage time on the mineralogical alteration of ferrihydrite are examined and compared with performance of the paint by a scale for adhesion and durability. Paint composed of nearly pure ferrihydrite shows poor adhesion and durability. Data suggest that for paint to adhere well to the PVC tubing, the Fe oxide suspension must contain a minimum of 30 to 40% goethite. This desired paint composition can be facilitated during synthesis by titrating the Fe oxide suspension to pH 11 or 12 rather than pH 7.5, which is typically prescribed for the formation of synthetic ferrihydrite.