Transformation Of Schwertmannite Research Articles

Phase transformation of schwertmannite in paddy soil under different water management regimes and its impact on the migration of arsenic in soil

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.

Sulfidation of Cd-Sch during the microbial sulfate reduction: Nanoscale redistribution of Cd

Schwertmannite (Sch) is found in environments abundant in iron and sulfate. Microorganisms that utilize iron or sulfate can induce the phase transition of Schwertmannite, consequently leading to the redistribution of coexisting pollutants. However, the impact of the molar ratio of sulfate to iron (S/Fe) on the microbial-mediated transformation of Schwertmannite and its implications for the fate of cadmium (Cd) have not been elucidated. In this study, we examined how S/Fe influenced mineral transformation and the fate of Cd during microbial reduction of Cd-loaded Schwertmannite by Desulfovibrio vulgaris. Our findings revealed that an increase in the S/Fe ratio facilitated sulfate-reducing bacteria (SRB) in mitigating the toxicity of Cd, thereby expediting the generation of sulfide (S(-II)) and subsequently triggering mineral phase transformation. As the S/Fe ratio increased, the predominant minerals in the system transitioned from prismatic-cluster vivianite to rose-shaped mackinawite. The Cd phase and distribution underwent corresponding alterations. Cd primarily existed in its oxidizable state, with its distribution being directly linked not only to FeS content but also showing a robust correlation with phosphorus. The coexistence of vivianite and FeS minerals proved to be more favorable for Cd immobilization. These findings have significant implications for understanding the biogeochemistry of iron (oxyhydr)oxides and Cd fate in anaerobic environments.

Sulfate affinity controls phosphate sorption and the proto-transformation of schwertmannite

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 PO43− g−1), which decreased to 1.5 and 1.2 mmol PO43− 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-SO4 to FeO 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.

Ferric Oxyhydroxylsulfate Precipitation Improves Water Quality in an Acid Mining Lake: A Hydrogeochemical Investigation

Hydrogeochemistry of a lignite pit lake in Lusatia, Germany, was investigated. Anoxic groundwater from the dump aquifer rich in FeII (average ~5911 µmol/L) and SO4 (average ~14,479 µmol/L) contents enter the lake as subsurface inflow; oxidation and subsequent precipitation of poorly crystallized Fe-oxyhydroxysulfate (schwertmannite) occurs and causes acidification (pH~2.8). However, the removal of dissolved loads as solid phases significantly improves the groundwater quality of the downgradient as an outflow. The rainwater isotopic values (δD ~−8.88‰ and δ18O ~−65.86‰) closely matched with the groundwater showing very little isotopic modification, which points to a short residence time of groundwater. The displacement of δD and δ18O values (slope = 5.3) from the meteoric water line reflected the evaporative enrichment of the lake water. The isotopic signature also revealed longer residence times of epilimnion than the hypolimnion waters which are dominated by groundwater. The lake is dimictic and showed abrupt changes in physicochemical parameters along the interface (~0.30 m thick) when separating the epilimnion (upper 4 m) from the hypolimnion (bottom 1.5 m). Lake sediments were found to be dominated by clay size fraction occurring as laminations (thickness: 1~0.5 mm) that reflect seasonal sedimentation. Higher schwertmannite formation in the south as compared to the north (recharge side) also serves as a scavenger of potentially toxic elements which is probably a natural solution to man-made problems. Schwertmannite transformation to goethite releases sulfate which is reduced and fixed as secondary sulfide minerals over time. Overall, waters are of a Ca–SO4 to Ca–Mg–SO4 type with distinct inflow (FeII/FeIII > 2.5) and outflow (FeII/FeIII < 0.5) of groundwater.

Sorption of antimony(V) to naturally formed multicomponent secondary iron minerals: Sorption behavior and a comparison with synthetic analogs

Antimony (Sb) pollution in water has attracted extensive attention due to the biotoxicity of Sb. Secondary iron minerals readily sorb heavy metal(loid)s and critically affect their cycling in terrestrial environments. However, compared with synthetic pure iron mineral phases, little is known about the Sb sorption behavior and mechanism on natural secondary iron minerals (nSIMs) composed of various mineral phases. In this study, sorption experiments were conducted to investigate the Sb(V) sorption properties of nSIMs from an acid mine drainage zone and corresponding single-component synthetic secondary mineral phases and to compare their sorption behaviors and mechanisms. Spectroscopic analyses indicated that the nSIMs structurally resembled a hybrid of schwertmannite, jarosite and goethite. Sb(V) sorption on nSIMs, schwertmannite, goethite and jarosite was controlled by chemisorption, with maximum Sb(V) sorption capacities of 217.39, 233.65, 32.17 and 35.61 mg/g, respectively. nSIMs demonstrated an excellent Sb(V) sorption capacity equivalent to or greater than that of the single-component phases. XRD, FTIR and Raman analyses indicated that Sb(V) was immobilized on nSIMs mainly through ion exchange with structural SO42− and complexation interactions with surface FeO and FeOH; then, an FeOSb surface phase formed during the dissolution and further transformation of schwertmannite and jarosite into goethite. SEM revealed that nSIMs had an advantage in surface microstructure over the single components. These results suggested that despite the similarities in Sb(V) binding mechanism between nSIMs and schwertmannite, nSIMs might be more reactive for Sb(V) sorption since the nSIM components could mutually influence each other and facilitate Sb(V) sorption. This research suggests that nSIMs have potential for Sb(V) removal and helps elucidate the environmental behavior of Sb(V) associated with nSIMs.

Sulfate availability drives the reductive transformation of schwertmannite by co-cultured iron- and sulfate-reducing bacteria

Schwertmannite (Sch) is a highly bioavailable iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment rich in sulfate (SO42−). Microbial-mediated Sch transformation has been well-studied, however, the understanding of how SO42− availability affects the microbial-mediated Sch transformation and the secondary minerals influence microbes is relatively limited. This study examined the effect of SO42− availability on the iron-reducing bacteria (FeRB) and SO42−-reducing bacteria (SRB) consortium-mediated Sch transformation and the resulting secondary minerals in turn on bacteria. Increased SO42− accelerated the onset of microbial SO42− reduction, which significantly accelerated Sch reduction transformation. The extent of intermediate products such as lepidocrocite (22.1 % ~ 76.3 %, all treatments) and goethite (15.3 %, 10 mM SO42−, 5 d) formed by Sch transformation depended on SO42− concentrations. Vivianite, siderite and iron‑sulfur minerals (e.g., FeS and FeS2) were the dominant secondary minerals, in which the relative content of vivianite and siderite decreased while iron‑sulfur minerals increased with increasing SO42− concentration. Correspondingly, the abundance of FeRB and SRB was negatively and positively correlated with SO42− concentration, respectively; 1 mM SO42− promoted the cymA and omcA expression of FeRB, but 10 mM SO42− lowerd the cymA and omcA expression compared to the 1 mM SO42−; the dsr expression of SRB related linearly to the SO42− concentration. These secondary minerals accumulated on the cell surface to form cell encrustations, which limited the growth and gene expression of FeRB and SRB, and even inhibited the activity of SRB in the 10 mM SO42− treatment group. The 10 mM SO42− treatment group with low-intensity ultrasound effectively restored the SRB activity for reducing SO42− by disintegrating the cell-mineral aggregation, further indicating that cell encrustations limited the microbial metabolism. The results highlight the critical role that SO42− availability can play in controlling microbial transformation of mineral, and the influence of secondary minerals on microbial metabolism.

Uptake mechanism of silicic acid by schwertmannite and its stabilization

As part of a series of studies on the interaction between ferric ions and silicic acid in the hydrosphere, the exchange reaction between SO42- in the schwertmannite tunnel structure and silicic acid in solution was investigated. Increasing pH of the solution from 6 to 11.5 resulted in an increase of adsorbed silicic acid. It was found that rapid substitution of SO42- took place with silicic acid which inhibited the transformation of schwertmannite to goethite. This observation implies more effective stabilization of schwertmannite by silicate compared to that of SO42-, which acts as a template ion in the formation of schwertmannite under acidic conditions. The substitution reaction was determined to be as follows: Schwertmannite-{SO42-} + Si(OH)3O- + OH- → Schwertmannite-{Si(OH)2O22-} + SO42-+ H2O, where schwertmannite-{ SO42-} and schwertmannite-{Si(OH)2O22-} indicate that the SO42- and Si(OH)2O22- species are present in the schwertmannite tunnel structure. The uptake mechanism of silicic acid by schwertmannite is significantly different from its uptake mechanisms by ferrihydrite and goethite.

Spatial Distribution, Migration, and Ecological Risk of Cd in Sediments and Soils Surrounding Sulfide Mines—A Case Study of the Dabaoshan Mine of Guangdong, China

Acid mine drainage (AMD) resulting from metal sulfide mining activities can lead to contamination by potentially toxic elements (PTEs) primarily concentrated around the mining area and gradually spreading outward. However, ecological risks do not correspond directly to PTE concentrations, making it challenging to effectively manage the mining environment and accurately prevent potential ecological impacts. In this paper, we analyzed Cd levels in sediments, soils, and corresponding rice grains sampled from four villages near Dabaoshan Mine of Guangdong, China, in 2017. Our results reveal that Cd is the most prominent pollutant element, exhibiting significant enrichment and spatial heterogeneity in both soil and sediments and higher accumulation levels in rice grains compared to other PTEs. Cd concentrations in soil decrease from the tailings pond to the river terrace, with a slight increase after Taiping River joins and flows into the alluvial plain. However, the concentrations in sediments show the opposite trend. The bioconcentration factor (BCF) for Cd in agricultural soil from the river terrace is lower than that from the alluvial plain and the degree of exceeding the maximum permit level (MPL) of Cd in rice grains increases along the river. Mineral transformation and topography are important factors in controlling the geochemical behavior of PTEs. Remediation efforts alter the physicochemical properties of the river, resulting in the release of PTEs during schwertmannite transformation followed by their adsorption by clay minerals. Furthermore, the random forest (RF) analysis highlights that the bioavailability and potential ecological risk of Cd in soils are governed by the occurrence form of Cd in different topographies, mainly controlled by TFe2O3, Mn, and CaO in the river terrace and CaO, Al2O3/SiO2, and Mn in the alluvial plain. Therefore, considering the impact of topography on mineral compositions, physicochemical properties, and occurrence form of PTEs in soil and sediments is essential for assessing ecological risk in mining areas.

Iron‑sulfur geochemistry and acidity retention in hydrologically active macropores of boreal acid sulfate soils: Effects of mitigation suspensions of fine-grained calcite and peat

Acid sulfate soils discharge large amounts of sulfuric acid along with toxic metals, deteriorating water quality and ecosystem health of recipient waterbodies. There is thus an urgent need to develop cost-effective and sustainable measures to mitigate the negative effects of these soils. In this study, we flushed aseptically-prepared MQ water (reference) or mitigation suspensions containing calcite, peat or a combination of both through 15-cm-thick soil cores from an acid sulfate soil field in western Finland, and investigated the geochemistry of Fe and S on the surfaces of macropores and in the solid columnar blocks (interiors) of the soil columns. The macropore surfaces of all soil columns were strongly enriched in total and HCl-extractable Fe and S relative to the interiors, owing to the existence of abundant Fe oxyhydroxysulfates (schwertmannite and partly jarosite) as yellow-to-brownish surface-coatings. The dissolution/hydrolysis of Fe oxyhydroxysulfates (predominantly jarosite) on the macropore surfaces of the reference columns, although being constantly flushed, effectively buffered the permeates at pH close to 4. These results suggest that Fe oxyhydroxysulfates accumulated on the macropore surfaces of boreal acid sulfate soils can act as long-lasting acidification sources. The treatments with mitigation suspensions led to a (near-)complete conversion of jarosite to Fe hydroxides, causing a substantial loss of S. In contrast, we did not observe any recognizable evidence indicating transformation of schwertmannite. However, sulfate sorbed by this mineral might be partially lost through anion-exchange processes during the treatments with calcite. No Fe sulfides were found in the peat-treated columns. Since Fe sulfides can support renewed acidification events, the moderate mineralogical changes induced by peat are desirable. In addition, peat materials can act as toxic-metal scavengers. Thus, the peat materials used here, which is relatively cheap in the boreal zone, is ideal for remediating boreal acid sulfate soils and other similar jarosite-bearing soils.

Sulfide-induced repartition of chromium associated with schwertmannite in acid mine drainage: Impacts and mechanisms

The cycling of Fe/S is often related to the formation and fate of schwertmannite which is particularly suitable as a scavenger for heavy metals and metalloids in acid mine drainage (AMD). However, the interactions between reactivity of S(-II) and schwertmannite with structurally incorporated Cr(VI) remain elusive. This work evaluated dissolution experiments in combination with XRD, SEM, FTIR, TEM, and XPS characterization to provide detailed information regarding the transformation of schwertmannite induced by S(-II) following changes in pH, Cr loading, and S(-II) concentration. Our results found that the presence of sulfide significantly decreased the stabilization of schwertmannite under acidic conditions. The reactivity of the three schwertmannite samples depended on the contents of Cr(VI) that were structurally incorporated and followed the order Sch > 0.13Cr-Sch > 0.17Cr-Sch. High S(-II) concentrations and low Cr doping favored the release of Fe and SO42− from schwertmannite. Attenuation of Cr mobility occurred via elevating the S(-II) concentrations and pH values resulting in Cr concentrations spanning ∼1.39 to ∼0.10 mg L−1 and ∼1.58 to ∼0.12 mg L−1 for 0.13Cr-Sch and 0.17Cr-Sch, respectively. Combining the results of characterization, goethite was the dominant end product constituted secondary phase together with sulfide minerals (FeS, FeS2), iron oxides (Fe3O4, Fe2O3), and CrFe minerals on the bulk mineral surface. The substituted Cr significantly inhibited the reductive transformation of schwertmannite by sulfide and led to the formation of lepidocrocite. Thus, we concluded that a three-stage transformation mechanistic pathway governed partial conversion of schwertmannite to goethite. This finding provides new understanding of the biogeochemical processes of iron minerals affected by reducing substances that control the transport and fate of immobilizing contaminants in an AMD-polluted area.

Thermal Transformation of Natural Schwertmannite in the Presence of Chromium

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.

Fe(II)-mediated transformation of schwertmannite associated with calcium from acid mine drainage treatment

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 SO42− 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.

Synergy of oxalic acid and sunlight triggered Cr(III)-bearing Schwertmannite transformation: Reaction mechanism, Cr and C spatial distribution and speciation on the nano scale

Cr(III)-bearing iron minerals are widely present in acid mine drainage (AMD)-contaminated areas. In the light-transmitting layer of natural systems, dissolved organic matter (DOM) can synergize with solar irradiation to transform Fe-bearing minerals, but the behavior of Cr(III) and its interfacial reaction with schwertmannite (Sch) under the combined effect of DOM and solar radiation remain unclear. Oxalic acid (OA) was selected as a form of DOM to investigate its combined effect with solar radiation on substituted-Cr(III) schwertmannite (Cr-Sch) under acidic conditions (pH = 3). Batch photoaging experiments in combination with in situ attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR), aberration-corrected scanning transmission electron microscopy (Cs-STEM), ultra-performance liquid chromatography-quadrupole-orbitrap mass spectrometry (UPLC-Q-Orbitrap MS), were employed to investigate Cr-Sch photoreductive dissolution process and elemental distribution. With increasing OA concentrations, Sch transformed to different second minerals: goethite and humboldtine under 1 mM and 5 mM oxalate condition, respectively. Photoreductive dissolution processes occur both on the surface of the minerals and in solution. Ligand-to-metal charge transfer (LMCT) is the dominant pathway of Sch photoreduction in solution when oxygen is absent, and it also occurs with superoxide mediated iron reduction (SMIR) under the aerobic conditions. With increasing OA concentrations, the conversion rate of Sch increases (the ratio of humboldtine to goethite increases), and the proportion of extractable Cr and solid-associated C increases. The findings reveal that OA plays a significant role in controlling the cycling of Fe/C and heavy metal elements under sunlight in the AMD-contaminated environment.

An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, CrVIO42- can substitute for SO42- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4−8) and the degree of Cr(VI) substitution (0.16–13 mol% CrVIO42--for-SO42- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.

Transformation of Schwertmannite to erdite nanorod via an alkaline dissolution–recrystallization process for the effective adsorption of oxytetracycline

Schwertmannite (schw) is a common Fe-bearing mineral in the precipitation of mine wastewater and/or steel pickling wastewater. It could be easily converted to goethite and hematite via heating or hydrothermal treatment and could be used as adsorbent to remove contaminants from wastewater. Herein, the spherical schw was converted into erdite nanorod by a simple hydrothermal method with the addition of Na2S. Schw was spherical particle with a size of 0.4–1.5 [Formula: see text]m. After treatment, it was converted to erdite nanorod particles with 100 nm diameter and 200 nm length. By adding MnO2 at the [Formula: see text] ratio of 1, erdite nanorod grew radially to 1–1.5 [Formula: see text]m, whereas MnO2 was reductively dissolved and recrystallized to rambergite. In the absence of Fe, MnO2 was directly transformed to octahedral alabandite. The product EN-0, prepared without MnO2, showed the optimal [Formula: see text] of oxytetracycline (OTC, 7479.6 mg/g), which was 12 times that of schw. In OTC-bearing solution, erdite was unstable and automatically hydrolyzed to generate Fe–SH/Fe–OH-bearing flocs, and it exhibited abundant surface functional groups for OTC adsorption. Subsequently, the hydroxyl and amino groups on the side chain of OTC would also be complexed with the Fe–SH group to generate an OTC–Fe–S ligand, in the form of flake-like particles with a smooth surface. The formed Mn-bearing minerals, for example, rambergite and alabandite, also complexed with OTC as OTC–Mn–S ligands to form quadrangular prism with shoulder and length of 10 [Formula: see text]m and 20–100 [Formula: see text]m, respectively. Spherical schw was converted into a well-crystallized erdite nanorod with the addition of MnO2, and the product showed potential applications in OTC-bearing wastewater treatment.

Arsenic-Imposed Effects on Schwertmannite and Jarosite Formation in Acid Mine Drainage and Coupled Impacts on Arsenic Mobility

This study explores interactions between As and Fe(III) minerals, predominantly schwertmannite and jarosite, in acid mine drainage (AMD) via observations at a former mine site combined with mineral formation and transformation experiments. Our objectives were to examine the effect of As on Fe(III) mineralogy in strongly acidic AMD while also considering associated controls on As mobility. AMD at the former mine site was strongly acidic (pH 2.4 to 2.8), with total aqueous Fe and As decreasing down the flow-path from ∼400 to ∼20 mg L–1 and ∼33,000 to ∼150 μg L–1, respectively. This trend was interrupted by a sharp rise in aqueous As(III) and Fe(II) caused by reductive dissolution of As-bearing Fe(III) phases in a sediment retention pond. Attenuation of Fe and As mobility occurred via formation of As(V)-rich schwertmannite, As(V)-rich jarosite, and amorphous ferric arsenate (AFA), resulting in solid-phase As concentrations spanning ∼13 to ∼208 g kg–1. Schwertmannite and jarosite retained As(V) predominantly by structural incorporation involving AsO4-for-SO4 substitution at up to ∼40 and ∼22 mol %, respectively. Arsenic strongly influenced Fe(III) mineral formation, with high As(V) concentrations causing formation of AFA over schwertmannite. Arsenic also strongly influenced Fe(III) mineral evolution over time. In particular, increasing levels of As(V) incorporation within schwertmannite were shown, for the first time, to enhance the transformation of schwertmannite to jarosite. This significant discovery necessitates a re-evaluation of the prevailing paradigm that As(V) retards schwertmannite transformation.

Arsenic Partitioning during Schwertmannite Dissolution and Recrystallization in the Presence of Fe(II) and Oxalic Acid

Fe(II)-driven transformation of schwertmannite can strongly impact the transport and redistribution of coprecipitated As in reducing acid sulfate soil (ASS)/acid mine drainage (AMD)-contaminated en...

Hydroxyl, Fe2+, and Acidithiobacillus ferrooxidans Jointly Determined the Crystal Growth and Morphology of Schwertmannite in a Sulfate-Rich Acidic Environment.

Schwertmannite, ubiquitously found in iron and sulfate-rich acid mine drainage, is generated via biological oxidation of ferrous ions by Acidithiobacillus ferrooxidans (A. ferrooxidans). However, little information on the mechanisms of biogenic schwertmannite formation and crystal growth is available. This study deliberately investigated the relationships among mineral morphology, solution chemistry, and phase transformation of schwertmannite in A. ferrooxidans-containing ferrous sulfate solutions. The formation of schwertmannite could be divided into three stages. In the first nucleation stage, crystallites are presented as nonaggregative or aggregative forms via a successive polymerization process. In the second stage, ellipsoidal aggregates, which are identified as ferrihydrite and/or schwertmannite, are formed. In the third stage, needles appear on the surface of ellipsoidal aggregates, which is caused by the phase transformation of ferrihydrite or schwertmannite to lepidocrocite and goethite through a Fe2+ (aq) catalysis-driven pathway. After three stages, a typical characteristic “hedgehog” morphology finally appears. In addition, A. ferrooxidans could significantly speed up the mineral transformation. Solution pH affects the morphology of schwertmannite by acid leaching. The experimental results also reveal that the formation of schwertmannite depend on the content of hydroxyl complexes or the transformation of the monomers to polymers, which are greatly affected by the solution pH.

The potential of granulated schwertmannite adsorbents to remove oxyanions (SeO32−, SeO42−, MoO42−, PO43−, Sb(OH)6−) from contaminated water

The mineral schwertmannite forms in acidic sulfate-rich environments (pH 2.9–4) and has a large sorption potential for oxyanions. In this study, we tested the properties of granulated schwertmannite produced from material obtained from a mine water treatment plant to remove SeO32−, SeO42−, MoO42−, PO43−, and Sb(OH)6−. We studied the kinetics of sulfate release from the granulated schwertmannite in suspensions containing various concentrations of HCO3− and its effect on the sorption kinetics of the oxyanions in synthetic solutions and industrial wastewater through batch experiment at acidic and neutral pH. We further tested the suitability of the granulated schwertmannite adsorbents as filter bed materials to remove SeO32−, MoO42−, and PO43− from synthetic solution via column experiments. Using a two-step pseudo-first order model, we were able to distinguish between an initial fast sorption process and a second slow reaction with characteristic reaction times (1/k) of ~10 min and ~10 h, respectively. Ligand exchange with surface bound sulfate is proposed to be responsible for the fast reaction while intraparticle diffusion and exchange with structural sulfate control the slow process. At neutral pH values, sulfate release is complete within one week, suggesting partial transformation of schwertmannite into a new ferric oxyhydroxide phase. The sorption kinetics were reflected in the results from the column experiments where only the fast sorbing fraction of surface sites participated in the retention of oxyanions at a mean residence times τ of 8 min with retention capacities of 149 μmol∙(g SHM)−1 for SeO32− and 242 μmol∙(g SHM)−1 for MoO42−. Hydraulic residence time increased the retention of PO43− with 213 μmol∙(g SHM)−1 at τ = 4.75 min and 508 μmol∙(g SHM)−1 at τ = 47.5 min. In summary, granulated schwertmannite turned out to be a promising adsorbent to remove the tested oxyanions from contaminated waters. Our results demonstrated that its application in filter bed systems needs to consider the trade-off between residence time and sorption kinetics. An increase in residence time clearly increases the retention capacity.

Environmental Stability of Schwertmannite: A Review

Schwertmannite is sensitive to changes in geochemical, thermal, and microbial conditions. Changes in aqueous pH beyond its stability, i.e. pH 2.5–4.5, triggers its transformation to jarosite or goethite in highly acidic environments (pH ≤ 2.5), depending on the availability of jarosite-directing cations (Na+, NH4+, K+, etc.), while goethite is the common stable end product at pH > 7.5. Schwertmannite with degraded morphology can stably exist for years in oxic intermediate pH environments (pH 5.5–6.5), but the presence of trace amounts of Fe(II)aq yields goethite/lepidocrocite within a few hours, especially at pH ≥ 6.5. Hematite is the sole end product at ≥ 600 °C dry heating, with goethite and ferrihydrite as intermediate phases. Siderite, maghemite, and mackinawite form in anoxic microbial conditions due to dissimilatory reduction of Fe(III) and SO42− to Fe(II) and HS−, while orpiment forms from As(V)-rich schwertmannites. Sorbed contaminants enhance schwertmannite stability by restricting Fe(II)–Fe(III) electron transfer and microbial degradation by occupying surface sites. Although Fe(III) and sorbed ion mobilization typically has negligible effects on schwertmannite transformation, complete schwertmannite-SO4 release is likely in extreme conditions, and in microbial Fe(II)aq-rich media. Dissolution–reprecipitation and solid state transformation mechanisms broadly govern schwertmannite transformation.