Formation Of Schwertmannite Research Articles

Efficient Removal of Trivalent Iron and Sulfate from Coal Mine Gushing Water Using Zero-Valent Iron Powder Combined with Hydrogen Peroxide

The high concentration of trivalent iron (Fe3+) and sulfate (SO4 2-) ions in coal mine gushing water is a major ecological hazard and difficult to treat industrially. In this paper, effective purification of coal mine gushing water was achieved by the two-step cyclic process using zero-valent iron (ZVI) powder and hydrogen peroxide. We investigated the effect of different doses of ZVI and hydrogen peroxide on the removal of Fe3+ and SO4 2-. Single-factor experiments indicated that as the dosage of ZVI increased, the removal of Fe3+ increased and then decreased, with the highest Fe3+ removal rate approaching 100% at the ZVI dosage of 100 mg/L, while the maximum removal rate of SO4 2- for sulphate was achieved at 400 ppm of hydrogen peroxide. The precipitate produced in the purification system was characterized and the results demonstrated that it was a typical secondary mineral, schwertmannite, and that it contained considerably more iron and sulfate than the precipitate formed by the natural sedimentation process in coal mine gushing water. Overall, after the two-step cycle process, Fe3+ and SO4 2- can be effectively removed via the formation of schwertmannite, and the ion concentrations all meet the discharge standards for Chinese mining industry wastewater.

Potassium-Bicarbonate-Induced Mineralized Acid Mine Drainage into Iron Hydroxyl Sulfate Minerals for Better Water Remediation and Resource Reuse

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.

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.

The purification of acid mine drainage through the formation of schwertmannite with Fe(0) reduction and alkali-regulated biomineralization prior to lime neutralization

Acid mine drainage (AMD) contains abundant Fe (II), Fe(III), and SO42−, as well as a large amount of dissolved toxic metals and metalloids, posing a serious threat to the environment. In this study, an integrated technique for the treatment of AMD was proposed. The technique started with pre-oxidation followed by Fe(0) reduction and alkali-regulated biomineralization and then ended with lime neutralization. The technique removed toxic metal oxyanions in the pre-oxidation stage and recovered pure schwertmannite during the subsequent alkali-regulated biomineralization. Fe(III), which could not be directly biomineralized, was reduced to Fe(II) by Fe(0). A small amount of alkali was added to regulate the hydrolytic mineralization reaction after Fe(II) oxidation in AMD, which in a single biomineralization could remove in the form of schwertmannite >95 % of soluble Fe in the AMD. In the subsequent lime neutralization process, the amount of lime required and the sludge produced were reduced by 75.4 % and 84.9 %, respectively, compared to the raw AMD. Additionally, the content of non-ferrous metals in the sludge increased 5.6-fold. Compared with non-alkali-regulated biomineralization, the schwertmannite obtained by the alkali-regulated biomineralization had a higher adsorption capacity for oxyanions (e.g., arsenic, chromium, and antimony). The new approach should significantly reduce the treatment cost of AMD and recover Fe and S elements in the form of valuable secondary minerals, such that it is reasonable to expect that it will be widely adopted in practical applications.

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.

Seed-mediated growth of schwertmannite for treatment of acid mine drainage: Improved adsorption of As(V) and Cr(VI) due to structure-related mechanisms

The biomineralization-driven formation of schwertmannite is an environmentally friendly strategy for acid mine drainage (AMD) treatment. In this study, a targeted approach using seed-mediated growth of schwertmannite was proposed. The biomineralization time was successfully shortened by one-third compared to biomineralization without seed mediation. In addition, the seed-mediated schwertmannite (S-Sch) exhibited superior adsorption performance towards As(V) and slightly improved adsorption of Cr(VI) than a non-seed-mediated schwertmannite sample (Sch). Furthermore, the whiskers that create the typical pincushion structure grew directly on the surface of the seed during the growth process with S-Sch, whereas biomineralization of Sch needed to nucleate crystals before growing the whiskers. More whiskers were found on S-Sch than on Sch, and the whiskers were arranged loosely on S-Sch while they were packed tightly on Sch. Following adsorption, As(V) is enriched at the whisker sites, where substantial > Fe–OH/OH2 existed, thereby inhibiting subsequent sulfate exchange at inner sites, the open structure of S-Sch decreases this passivation and allows continuous sulfate exchange. Cr(VI) adsorption is mainly due to sulfate exchange with little > Fe–OH/OH2 involved. Cr(VI) was distributed evenly on the surface of S-Sch, but an open mineral structure allows more inner sites to be reached, explaining the slightly better performance of S-Sch. The results suggest that the method proposed in this paper could be an effective way to shorten the biomineralization time and provide schwertmannite with excellent performance for the removal of As(V) and Cr(VI), which could lead to practical applications of biomineralization in the treatment of AMD.

A coupled electrochemical process for schwertmannite recovery from acid mine drainage: Important roles of anodic reactive oxygen species and cathodic alkaline

The increasing need for sustainable acid mine drainage (AMD) treatment has spurred much attention to strategic development of resource recovery. Along this line, we envisage that a coupled electrochemical system involving anodic Fe(II) oxidation and cathodic alkaline production will facilitate in situ synthesis of schwertmannite from AMD. Multiple physicochemical studies showed the successful formation of electrochemistry-induced schwertmannite, with its surface structure and chemical composition closely related to the applied current. A low current (e.g., 50 mA) led to the formation of schwertmannite having a small specific surface area (SSA) of 122.8 m2 g−1 and containing small amounts of –OH groups (formula Fe8O8(OH)4.49(SO4)1.76), whereas a large current (e.g., 200 mA) led to schwertmannite high in SSA (169.5 m2 g−1) and amounts of –OH groups (formula Fe8O8(OH)5.16(SO4)1.42). Mechanistic studies revealed that the reactive oxygen species (ROS)-mediated pathway, rather than the direct oxidation pathway, plays a dominant role in accelerating Fe(II) oxidation, especially at high currents. The abundance of •OH in the bulk solution, along with the cathodic production of OH−, were the key to obtaining schwertmannite with desirable properties. It was also found to function as a powerful sorbent in removal of arsenic species from the aqueous phase.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments.

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between SbVO6 and FeIII(O,OH)6 octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

The evolution of dissolved inorganic carbon (DIC) and δ13CDIC in acid mine drainage polluted karst rivers: Preliminary findings from laboratory experiments

Acid mine drainage (AMD) has caused serious pollution in the surface water environments of karst areas, but the effect of AMD on the dissolved inorganic carbon (DIC) dynamics in the receiving karst rivers remains poorly understood. The AMD and karst river water were sampled from the Yangliujie River watershed in Southwest China, and mixed different proportions of AMD and karst river water in a laboratory experiment to simulate seasonal hydrological conditions in the river, in an attempt to explore the impact of AMD on the loss rate and evolution of DIC in karst river water. The results showed that AMD can cause a rapid pH drop in karst river water, resulting in approximately 41.0 ± 11.8 % DIC loss in the early stage of mixing as CO2(g) release to the atmosphere. With the biological or chemical oxidation of Fe2+, and followed by Fe3+ biogenic formation of schwertmannite or hydrolysis, the pH decreased and the DIC concentration further decreased in the mixed samples, the degree of decrease was related to the oxidation rate of Fe2+. With decreasing DIC concentration, δ13CDIC enrichment gradually increased. When the mixing ratio of AMD was large, the total H+ in AMD and H+ generated by the oxidation of Fe2+ exceeded the HCO3− content in karst river water, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was small, and the DIC transformation was mainly dominated by the loss of CO2(g). When a small proportion of AMD was mixed in, the HCO3− concentration in the mixed water sample was in excess of the H+ produced by Fe2+ and the total H+ in AMD, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was larger, and the DIC conversion was mainly affected by HCO3− dehydration. These preliminary findings support evidence showing DIC conversion and loss in AMD-affected karst rivers increase the transfer of carbon dioxide to the atmosphere, which diminishes net carbon sink fluxes in karst river basins, so reasonable AMD treatment methods should be applied to minimize these negative impacts.

Schwertmannite: A review of its occurrence, formation, structure, stability and interactions with oxyanions

In this article, we provide a critical review on the occurrence, formation, structure and stability of schwertmannite – an enigmatic Fe(III) oxyhydroxy-sulfate mineral that is of widespread interest in the soil science, environmental geochemistry , hydrometallurgy and wastewater fields. We also review interactions between schwertmannite and selected, environmentally relevant oxyanions (arsenate, phosphate, antimonate, selenate , chromate , and molybdate). Schwertmannite is one of the most common minerals to form via direct precipitation of iron in acid sulfate waters at pH 2 – 4. As such, it has been found to occur in systems that are affected by acid mine drainage , acid sulfate soils , natural acid rock drainage, and hydrometallurgical settings involving oxidation of iron-sulfide minerals. Despite its official recognition as a new mineral almost 30 years ago, schwertmannite’s poor crystallinity , fine particle size, variable composition and metastability have constrained our ability to conclusively resolve its crystal structure, possibly polyphasic nature, and solubility/stability. Nevertheless, since the early 1990’s, there has been a steadily-growing body of literature showing that, in acid-sulfate systems, schwertmannite strongly impacts iron and sulfur cycling, acidity dynamics, and electron flow. In addition, interactions with schwertmannite also strongly influence the mobility and fate of co-associated oxyanionic species. In parallel, many oxyanions have themselves been found to also exert a powerful control on schwertmannite’s geochemical behavior and mineralogical stability. Schwertmannite-oxyanion interactions occur via adsorption to the schwertmannite surface, anionic exchange with sulfate in the schwertmannite tunnel structure, and co-precipitation by substitution for either structural sulfate or Fe(III) during schwertmannite formation. This array of distinct sorption mechanisms results in oxyanions being able to both stabilize and conversely destabilize schwertmannite. This distinction appears to reflect an interplay of kinetic and thermodynamic constraints, which depend heavily on the specific oxyanion, its concentration (i.e. level of loading on/in schwertmannite), and other background geochemical conditions (e.g. pH and presence of other ions). Resolving these complex interactions and their importance in controlling schwertmannite occurrence and stability represents a challenge for future research.

Seasonal effects of natural attenuation on drainage contamination from artisanal gold mining, Cambodia: Implication for passive treatment

In Mondulkiri province, Cambodia, artisanal gold miners dump tailings and wastewater from gold processing into a tributary of the Prek Te River. In the rainy season, heavy metal concentrations in the tributary decrease below the WHO drinking water standard levels through natural attenuation; however, this does not occur in the dry season. To further understand the natural attenuation mechanism, detailed analyses of the wastewater from tailing and tributary water, tributary sediments, waste rock, and ore minerals were undertaken in both seasons. The high concentration of dissolved Fe in the contaminated tributary plays a significant role in As removal during the rainy season, whereas other elements such as Ni, Se, and Cu concentration decrease due to dilution. Schwertmannite formation, controlled by iron-oxidizing bacteria, was only found at the bottom of the tributary during the rainy season. In the dry season, As, Ni, Se, and Cu concentrations remained at their original levels because there was no formation of schwertmannite or dilution by rainwater. The existing schwertmannite also starts to dissolve as the pH decreases. Seasonal dynamics cause the failure of natural attenuation; thus, methods for maintaining its effectiveness in the dry season are needed. In addition, geochemical modeling was conducted to determine the significant roles of schwertmannite formation and dilution of rainwater in the tributary. Schwertmannite is a potential adsorbent for As removal from drainage. However, dilution provided indirect and direct impacts on the tributary, such as increasing the pH and diluting the concentration of toxic elements.

Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate.

Schwertmannite effectively sorbs chromate (Cr(VI)), yet the sorption mechanisms remain elusive. We determined the Cr(VI) sorption mechanisms on schwertmannite at pH 3.2 and 5 using combined macroscopic sorption experiments with molecular-scale characterization and by comparing them to arsenate (As(V)) sorption. Cr(VI) adsorbs as bidentate-binuclear (BB) inner-sphere complexes through exchanging more sulfate and less >Fe-OH/OH2, with 0.59-0.71 sulfate released per Cr(VI) sorbed. While As(V) also forms BB complexes, it exchanges sulfate and >Fe-OH/OH2 equally with 0.49-0.52 sulfate released per As(V) sorbed. At high As(V) loadings, As(V) precipitates as amorphous FeAsO4, particularly at low pH. The abovementioned differences between Cr(VI) and As(V) can be related to their different ionic radii and binding strength. Moreover, Cr(VI) and As(V) preferentially exchange sulfate inner-sphere complexes, increasing the proportion of sulfate outer-sphere complexes in schwertmannite. In turn, the concentration of sulfate outer-sphere complexes increases and then decreases with increasing Cr(VI) loading. Results suggest that an oxyanion, which would form inner-sphere complexes on a mineral surface, preferentially exchanges inner-spherically bound oxyanions than outer-spherically bound ones on the surface, even though both are exchanged. This study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.

Mechanism and formation process of schwertmannite under electrochemical deposition

Schwertmannite has recently attracted increasing attention due to its important geochemical role influencing the fate of toxic elements in acidic waterbodies, as well as its material properties for adsorbing or catalytic degrading environmental pollutants. However, the existing methods are either time-consuming (>30 d) or the obtained products possess small special surface areas (4−14 m2/g). The properties of products synthesized even by the same method are often different due to deviation of mineral forming conditions. In this study, a new method was developed by providing electrochemical energy to promote schwertmannite formation in electrochemical system. The method designed was focused on relatively precise control in pH and the oxidation rate of Fe2+, that is, pH buffering and multi-potential cycling redox for controlling Fe2+ oxidation rate. Adding CH3COOH to FeSO4 electrolyte reduced the pH drop and produced a more homogeneous schwertmannite mineral phase. Precipitative efficiency of 27.0 % for total Fe and 5.4 % for SO42− were obtained under multi-potential step (MPS) mode in 35 °C electrolyte. A sea urchin-like morphology schwertmnanite mineral with relatively bigger Fe/S ratio (5.3−6.2) and higher special surface area (BET of 104−178 m2/g) was synthesized in a relatively shorter time (electrolysis 6 h + aging 24 h) by the electrodeposition method.

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.

Biological materials formed by Acidithiobacillus ferrooxidans and their potential applications.

A variety of biological materials including schwertmannite, jarosite, iron-sulfur cluster (ISC) and magnetosomes can be produced by Acidithiobacillus ferrooxidans (A. ferrooxidans). Their possible formation mechanisms involved in iron transformation, iron transport, and electron transfer were proposed. The schwertmannite formation usually occurs under the pH of 2.0-3.51, and a lower or higher pH will promote jarosite to be produced. Available Fe2+ in the environment and the carrier proteins that can transport Fe2+ to the intracellular membranes of A. ferrooxidans play a critical role in the synthesis of magnetosomes and ISC. The potential applications of these biological materials were reviewed, including removal of heavy metal by schwertmannite, detoxification of toxic species by jarosite, the transference of electron and ripening the iron sulfur protein by ISC, and biomedical application of magnetosomes. Additionally, some perspectives for the molecular mechanisms of synthesis and regulation of these biomaterials were briefly described.

Formation and transformation of schwertmannite in the classic Fenton process

The massive amount of sludge generated by the classic Fenton process, which has often been hypothesized to consist of ferric hydroxide, remains a major obstacle to its large-scale application. Therefore, reutilization of Fenton sludge has recently gained more attention. Understanding the formation, transformation, and properties of Fenton sludge combined with the stages of the Fenton reaction is pivotal, but not well illustrated yet. In this study, SEM-EDS, FT-IR, XRD, and XPS were applied to study the morphology, crystallinity, elemental composition, and valence state of Fenton sludge. The authors report that schwertmannite and 2-line ferrihydrite were generated and transformed in the oxidation phase and the neutralization phase of the Fenton process. SO42− in the solution decreased by 8.7%–26.0% at different molar ratios of Fe(II) to H2O2; meanwhile, iron ion precipitated completely at pH 3.70 with the formation of schwertmannite containing sulfate groups in the Fenton sludge. The structural sulfate (Fe-SO4) in schwertmannite was released from the precipitate with the addition of OH−, and the production of Fenton sludge decreased with increasing pH when pH > 3.70. Goethite was found to form when the final pH was adjusted to 12 or at a reaction temperature of 80°C. Moreover, the possible thermal transformation to goethite and hematite indicated that Fenton sludge can be reused as a raw material for synthesizing more stable iron (hydro)oxides. The results provide useful insights into the formation and transformation of Fenton sludge, with implications for regulating the crystal type of Fenton sludge for further reuse.

Comparison of the Biological and Chemical Synthesis of Schwertmannite at a Consistent Fe2+ Oxidation Efficiency and the Effect of Extracellular Polymeric Substances of Acidithiobacillus ferrooxidans on Biomineralization.

Schwertmannite is an environmental mineral material that can promote the natural passivation of heavy metal elements, thereby reducing environmental pollution from toxic elements. However, the fundamental reason for the difference between the chemically (H2O2-FeSO4) and biologically (Acidithiobacillus ferrooxidans-FeSO4) synthesized schwertmannite is still unclear. In this study, X-ray diffraction, scanning electron microscopy, the Brunauer–Emmett–Teller method, and X-ray fluorescence spectrometry were used to compare the structure, specific surface area, and elemental composition of schwertmannite synthesized by biological and chemical methods. The removal capacity of As(III) by the two kinds of schwertmannite and the effects of extracellular polymeric substances (EPS) on biogenetic schwertmannite were also investigated. At a consistent Fe2+ oxidation efficiency, the chemical method synthesized more schwertmannite than the biological method over a 60-h period. The biosynthesized schwertmannite had a “chestnut shell” shape, with a larger particle size and specific surface than the chemically synthesized schwertmannite, which was relatively smooth. The saturated adsorption capacities of the biologically and chemically synthesized schwertmannite were 117.0 and 87.0 mg·g−1, respectively. After exfoliation of the EPS from A. ferrooxidans, the biosynthesized schwertmannite displayed a “wool ball” shape, with rough particle surfaces, many microporous structures, and a larger specific surface area. The schwertmannite yield also increased by about 45% compared with that before exfoliation, suggesting that the secretion of EPS by A. ferrooxidans can inhibit the formation of schwertmannite.

Influence of Ni(II), Cu(II) and Cr(III) on the formation, morphology and molecular adsorption properties of α-FeOOH rust particles prepared by aerial oxidation of neutral Fe(II) solutions

To elucidate the role of alloying metals such as Ni, Cu and Cr in weathering steels on the formation of α-FeOOH rust by atmospheric corrosion of the steels in industrial and urban districts, influence of alloying metal ions on the formation, morphology and adsorption properties of α-FeOOH particles synthesized by aerial oxidation of neutral FeSO4 solution was examined. The addition of Cu(II) and Cr(III) dramatically suppresses the crystal growth of α-FeOOH and turns the rod-shaped α-FeOOH particles into nano-sized irregular ones. Besides, increase of amount of added Cr(III) forms the iron oxyhydroxysulfate named as Schwertmannite (Fe8O8(SO4)(OH)6). Whereas, the Ni(II) addition shows no noticeable influence on the growth of α-FeOOH crystal and particles. These results imply that the inhibitory effect of crystal and particle growth of α-FeOOH is in order of Cr(III) > Cu(II) >> Ni(II). The adsorption of CO2 gas on the α-FeOOH particles is impeded by adding Cu(II) and Cr(III). Whereas, the formation of Schwertmannite by adding Cr(III) enhances the CO2 and SO2 adsorption. These results suggest that alloying Cu and Cr in weathering steels inhibit the crystal and particle growth of α-FeOOH to accelerate the formation of protective rust layer composed of fine α-FeOOH and/or Schwertmannite rust particles on the steels in urban and industrial atmosphere.

Schwertmannite formation and properties in acidic drain environments following exposure and oxidation of acid sulfate soils in irrigation areas during extreme drought

This paper describes the occurrences, mineralogical assemblages and environmental relevance of iron-rich precipitates derived from acidic (pH<4) waters containing dominantly schwertmannite from a diverse range of six physical settings across the Lower Murray Reclaimed Irrigation Area (LMRIA) in Australia, comprising: (1) suspended flocculated precipitates in ponded drain water, (2) moist coatings or pastes in drying ponds and drains, (3) hard cemented crusts and aggregates amongst Phragmites roots and stems, (4) dry coatings on concrete and wooden structures, (5) dry coatings on surface soils and vegetation and (6) suspended flocculated precipitates in the mixing zone of drain discharge into the River Murray. Schwertmannite formed in these acid drain environments following exposure and oxidation of deep (~0.5→3.5m) clayey hypersulfidic material (pH>4) that dried, cracked and acidified to form deep sulfuric materials (pH<4) due to river and groundwater levels falling by nearly 2m during the latter part of the Millennium Drought (2007 to 2010). Reflooding events occurred between 2011 and 2015. All samples displayed X-ray diffraction (XRD) patterns typical for schwertmannite. In some samples, additional weak reflections from small amounts of jarosite, natrojarosite, gypsum, hexahydrite, konyaite and halite indicated deposition under variable pH conditions and sulfate concentrations due to different flow or evaporation stages. SEM images indicated that morphological and compositional features of schwertmannite were dominated by: (1) framboid-like spheroidal clusters with Fe/S ratio>5 that were preserved after crystallization and likely formed by dissolution of pyrite and microbial oxidation of Fe2+ by acidophilic bacteria and (2) fibrous spheres (0.3–3μm) with filamentous morphology and a high degree of porosity. Speciation calculations (PHREEQC) using the dissolved metal and major ion concentrations in drain waters supported the XRD results as the saturation index (SI) exceeded zero for schwertmannite in many drains. The precipitates contained high concentrations of metals (Al>Cu>As>Zn>Pb>Co) and nutrients (e.g. P) due to co-precipitation/scavenging of these elements during the formation of schwertmannite. There was also spatial variability in concentrations of metal(loids) in precipitates between drains. A conceptual model explains and summarizes the morphological properties, mineralogy, geochemistry and environmental processes influencing the formation and relative stabilities of schwertmannite-rich precipitates from six diverse physical settings. The environmental relevance, which has significant implications for rehabilitation options is shown in three perspectives: (1) the conditions for schwertmannite formation have persisted in irrigation drains for over 7years, (2) the ability for schwertmannite-rich precipitates to reveal acid sulfate conditions and therefore act as a mineralogical indicator in irrigation systems and (3) the pollution potential of metals and metalloids scavenged by schwertmannite-rich precipitates.

Phosphate-Imposed Constraints on Schwertmannite Stability under Reducing Conditions.

Schwertmannite is a ferric oxyhydroxysulfate mineral, which is common in acid sulfate systems. Such systems contain varying concentrations of phosphate (PO43-)-an essential nutrient whose availability may be coupled to schwertmannite formation and fate. This study examines the effect of phosphate on schwertmannite stability under reducing conditions. Phosphate was added at 0, 80, 400, and 800 μmoles g-1 (i.e., zero, low, medium, and high loading) to schwertmannite suspensions which were inoculated with wetland sediment and suspended in N2-purged artificial groundwater. pH remained between 2.7 and 4.3 over the 41 day experiment duration. Fe(II) accumulated in solution due to dissimilatory Fe(III)-reduction, which was most pronounced at intermediate PO43- loadings (i.e., in the low PO43- treatment). Partial transformation of schwertmannite to goethite occurred in the zero and low PO43- treatments, with negligible transformation in higher PO43- treatments. Overall, the results suggest that intermediate PO43- loadings provide conditions which facilitate optimal reductive dissolution of schwertmannite. At zero PO43- loading, reductive dissolution appears to be constrained by the rapid transformation of schwertmannite to goethite, which thereby decreases the bioavailability of solid-phase Fe(III). Conversely, at high loadings, PO43- appears to stabilize the schwertmannite surface against dissolution; probably via the formation of strong surface complexes.