Remediation of arsenic-contaminated soil using nanoscale schwertmannite synthesized by persulfate oxidation with carboxymethyl cellulose stabilization.

Schwertmannite (SCH) is a promising material for adsorbing inorganic arsenic (As). We synthesized SCH nanoparticles (nano-SCH) via a modified chemical oxidation method and investigated the application of nano-SCH for the remediation of As-contaminated soils. The production of nano-SCH was successfully prepared using the persulfate oxidation method with carboxymethyl cellulose stabilization. The spherical structure of the nano-SCH particles had an average hydrodynamic diameter of 296 nm with high specific surface areas (108.9 m2/g). Compared with SCH synthesized via the H2O2 oxidation method, the percentage of Fe3+ precipitation in nano-SCH synthesis increased from 63.2% to 84.1%. The inorganic As adsorption capacity of nano-SCH improved by 2.27 times at solution pH = 6. After remediation of heavily As-contaminated soils by using 5% nano-SCH, the leachability of inorganic As rapidly decreased to 0.01% in 30 d. Correspondingly, the immobilization efficiencies of inorganic As in soil reached >99.9%. The inorganic As fractions in treated soil shifted from specifically and nonspecifically bound forms to amorphous and crystalline hydrous oxide-bound fractions. After treatment with 5% nano-SCH for 60 d, soil pH slightly decreased from 5.47 to 4.94; by contrast, soil organic matter content increased by 20.9%. Simultaneously, dehydrogenase concentration in soil decreased by 22.4%-34.7% during the remediation process. These changes in soil properties and As immobilization jointly decreased microbial activity and initiated the re-establishment of bacterial communities in the soil. In summary, this study presents a novel and high-productivity technology for nano-SCH synthesis and confirms the high As immobilization effectiveness of nano-SCH in the remediation of As-contaminated soils.

Chemical Mineralization of AMD into Schwertmannite Fixing Iron and Sulfate Ions by Structure and Adsorption: Paving the Way for Enhanced Mineralization Capacity.

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

Nitrogen-doped titanium dioxide/schwertmannite nanocomposites as heterogeneous photo-Fenton catalysts with enhanced efficiency for the degradation of bisphenol A.

Potential health risks related to environmental endocrine disruptors (EEDs) have aroused research hotspots at the forefront of water treatment technologies. Herein, nitrogen-doped titanium dioxide/schwertmannite nanocomposites (N-TiO2/SCH) have been successfully developed as heterogeneous catalysts for the degradation of typical EEDs via photo-Fenton processes. Due to the sustainable Fe(III)/Fe(II) conversion induced by photoelectrons, as-prepared N-TiO2/SCH nanocomposites exhibit much enhanced efficiency for the degradation of bisphenol A (BPA; ca. 100% within 60 min under visible irradiation) in a wide pH range of 3.0-7.8, which is significantly higher than that of the pristine schwertmannite (ca. 74.5%) or N-TiO2 (ca. 10.8%). In this photo-Fenton system, the efficient degradation of BPA is mainly attributed to the oxidation by hydroxyl radical (•OH) and singlet oxygen (1O2). Moreover, the possible catalytic mechanisms and reaction pathway of BPA degradation are systematically investigated based on analytical and photoelectrochemical analyses. This work not only provides a feasible means for the development of novel heterogeneous photo-Fenton catalysts, but also lays a theoretical foundation for the potential application of mineral-based materials in wastewater treatment.

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.

Iron Isotopes in Acid Mine Drainage: Extreme and Divergent Fractionation between Solid (Schwertmannite, Jarosite, and Ferric Arsenate) and Aqueous Species.

Examination of stable Fe isotopes is a powerful tool to explore Fe cycling in a range of environments. However, the isotopic fractionation of Fe in acid mine drainage (AMD) has received little attention and is poorly understood. Here, we analyze Fe isotopes in waters and Fe(III)-rich solids along an AMD flow-path. Aqueous Fe spanned a concentration and δ56Fe range of ∼420 mg L-1 and + 0.04‰ at the AMD source to ∼100 mg L-1 and -0.81‰ at ∼450 m downstream. Aqueous As (up to ∼33 mg L-1) and SO42- (up to ∼2000 mg L-1), like aqueous Fe, decreased in concentration down the flow-path. X-ray absorption spectroscopy indicated that downstream attenuation in aqueous Fe, As, and SO42- was due to the precipitation of amorphous ferric arsenate (AFA), schwertmannite, and jarosite. The Fe(III) in these solids displayed extreme variability in δ56Fe, spanning +3.95‰ in AFA near the AMD source to -1.34‰ in schwertmannite at ∼450 m downstream. Similarly, the isotopic contrast between solid Fe(III) precipitates and aqueous Fe (Δ56Feppt-aq) dropped along the flow-path from about +4.1 to -1.1‰. The shift from positive to negative Δ56Feppt-aq reflects divergence between competing equilibrium versus kinetic fractionation processes.

Efficient utilization of the electron energy of antibiotics to accelerate Fe(III)/Fe(II) cycle in heterogeneous Fenton reaction induced by bamboo biochar/schwertmannite.

The discharge of antibiotics evokes environmental health crisis, and is also a waste of organic energy. Currently, heterogeneous Fenton for antibiotics removal has attracted growing attentions due to wide pH range and no iron sludge production, however, it often suffers from a low formation rate of Fe(II), resulting in difficult application of heterogeneous Fenton technology in sewage treatment. To overcome this drawback, bamboo biochar (BB) is coupled with schwertmannite (Sch) through Acidithiobacillus ferrooxidans-mediated Fe(II) oxidation reaction to obtain a heterogeneous catalyst (Sch/BB) with high adsorption performance and Fenton activity. According to the analysis of experimental results, electrons around C (from BB) can easily transfer to Fe by Fe-O-C bonds to expedite ≡Fe(III)/≡Fe(II) cycle, while electrons of antibiotics adsorbed on Sch/BB surface are effectively utilized to maintain the efficient regeneration of ≡Fe(II) through BB electron shuttle or Fe-O-C bonds between Sch/BB and pollutants, further causing a superior Fenton activity (98% antibiotics removal) of Sch/BB. Moreover, due to its excellent adsorption performance, Sch/BB as filter materials can effectively remove dye pollutants in flow wastewater. These findings provided a high-activity and practical heterogeneous Fenton catalyst for pollutants degradation, while a new perspective for efficient utilization of the electrons of organic pollutants was given.

Photooxidation of Fe(II) to schwertmannite promotes As(III) oxidation and immobilization on pyrite under acidic conditions.

Pollution of arsenic (As) in acid mine drainage (AMD) is a universal environmental problem. The weathering of pyrite (FeS2) and other sulfide minerals leads to the generation of AMD and accelerates the leaching of As from sulfide minerals. Pyrite can undergo adsorption and redox reactions with As, affecting the existing form and biotoxicity of As. However, the interaction process between As and pyrite in AMD under sunlight radiation remains unclear. Here, we found that the oxidation and immobilization of arsenite (As(III)) on pyrite can be obviously promoted by the reactive oxygen species (ROS) in sunlit AMD, particularly by OH. The reactions between hole-electron pairs and water/oxygen adsorbed on excited pyrite resulted in the production of H2O2, OH and O2-, and OH was also generated through the photo-Fenton reaction of Fe2+/FeOH2+. Weakly crystalline schwertmannite formed from the oxidation of Fe2+ ions by OH contributed much to the adsorption and immobilization of As. In the mixed system of pyrite (0.75 g L-1), Fe2+ (56.08 mg L-1) and As(III) (1.0 mg L-1) at initial pH 3.0, the decrease ratio of dissolved total As concentration was 1.6% under dark conditions, while it significantly increased to 69.0% under sunlight radiation. The existence of oxygen or increase in initial pH from 2.0 to 4.0 accelerated As(III) oxidation and immobilization due to the oxidation of more Fe2+ and production of more ROS. The present work shows that sunlight significantly affects the transformation and migration of As in AMD, and provides new insights into the environmental behaviors of As.

Co-adsorption of As(III) and phenanthrene by schwertmannite and Fenton-like regeneration of spent schwertmannite to realize phenanthrene degradation and As(III) oxidation.

Co-contamination of arsenic and polycyclic aromatic hydrocarbons (PAHs) in groundwater is frequently reported, and it is thus necessary to develop efficient techniques to tackle this problem. Here, we evaluated the feasibility of utilizing schwertmannite to co-adsorb As(III) and phenanthrene from water solution and regenerating spent schwertmannite via a heterogeneous Fenton-like reaction to degrade adsorbed phenanthrene and meanwhile oxidize adsorbed As(III). The results suggested that schwertmannite with a hedgehog-like morphology was superior to that with a smooth surface for the adsorption removal of As(III) or phenanthrene because of the much higher BET surface area and hydroxyl proportion of the former one, and schwertmannite formed at 72 h incubation effectively co-adsorbed As(III) and phenanthrene from water solution. The adsorption of As(III) and phenanthrene on schwertmannite did not interfere with each other, while the acidic initial solution pH delayed the adsorption of As(III) on schwertmannite but enhanced the adsorption capacity for phenanthrene. The adsorption of As(III) on schwertmannite mainly involved its exchange with SO42- (outer-sphere or inner-sphere) and its complexation with iron hydroxyl surface groups, and phenanthrene adsorption mainly occurred through cation-π bonding and OH-π interaction. During the adsorption-regeneration processes, schwertmannite adsorbed As(III) and phenanthrene firstly, and then it can be successfully regenerated via Fenton-like reaction catalyzed by itself to effectively degrade the adsorbed phenanthrene and meanwhile oxidize the adsorbed As(III) to As(V). Therefore, schwertmanite is an outstanding environmental adsorbent to decontaminate As(III) and phenanthrene co-existing in groundwater.

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics, Isotherms and Adsorption Mechanism.

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe3+ hydrolysis (SCHA) and Fe2+ oxidation (SCHB). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g-1 for SCHA and SCHB. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO42- groups.

A novel arsenic immobilization strategy via a two-step process: Arsenic concentration from dilute solution using schwertmannite and immobilization in Ca-Fe-AsO4 compounds.

Acid mine drainage (AMD) with toxic arsenic (As) is commonly generated from the tailings storage facilities (TSFs) of sulfide mines due to the presence of As-bearing sulfide minerals (e.g., arsenopyrite, realgar, orpiment, etc.). To suppress As contamination to the nearby environments, As immobilization by Ca-Fe-AsO4 compounds is considered one of the most promising techniques; however, this technique is only applicable when As concentration is high enough (>1 g/L). To immobilize As from wastewater with low As concentration (~10 mg/L), this study investigated a two-step process consisting of concentration of dilute As solution by sorption/desorption using schwertmannite (Fe8O8(OH)8-2x(SO4)x; where (1 ≤ x ≤ 1.75)) and formation of Ca-Fe-AsO4 compounds. Arsenic sorption tests indicated that As(V) was well adsorbed onto schwertmannite at pH 3 (Qmax = 116.3 mg/g), but its sorption was limited at pH 13 (Qmax = 16.1 mg/g). A dilute As solution (~11.2 mg/L As) could be concentrated by sorption with large volume of dilute As solution at pH 3 followed by desorption with small volume of eluent of which pH was 13. The formation of Ca-Fe-AsO4 compounds from As concentrate solution (2 g/L As(V)) was strongly affected by temperature and pH. At low temperature (25-50 °C), amorphous ferric arsenate was formed, while at high temperature (95 °C), yukonite (Ca2Fe3-5(AsO4)3(OH)4-10·xH2O; where x = 2-11) and johnbaumite (Ca5(AsO4)3OH) were formed at pH 8 and 12, respectively. Among the synthesized products, johnbaumite showed strongest As retention ability even under acidic (pH < 2) and alkaline (pH > 9) conditions.

Adsorption Performance and Mechanism of Synthetic Schwertmannite to Remove Low-Concentration Fluorine in Water.

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

Acidity and metallic elements release from AMD-affected river sediments: Effect of AMD standstill and dilution.

In acid mine drainage (AMD) polluted rivers, considerable fraction of potential toxic elements are temporarily sequestered by sediments. There are two main potential environmental hazards associated with the sediments, acidity liberation and re-mobilization of metallic elements, during environmental conditions change. The effects of AMD standstill and water dilution on metallic elements migration were assessed in an AMD standstill test and a dialysis experiment. Maintaining AMD standstill, often occurring in AMD damming process, could induce the occurrence of iron secondary minerals precipitation along with attenuation of dissolved elements and a decrease in water pH value. Both field sediments and lab precipitates were confirmed as being dominant with schwertmannite which was the most important source and sink for acidity and metallic elements. The mechanism of cation heavy metals scavenging implied by FTIR results mostly depended on the exchanging of H+ from surface hydroxyl groups (-OH) in schwertmannite-rich sediments. For arsenic oxyanion, its adsorption included surface complexation with iron hydroxyl groups at the mineral surface, as well as anion exchange of SO42- present in the structure. The quantities of acidity release differed significantly from 20 to 3714 mol H+/t depending on the iron hydroxyl minerals type and their contents in the corresponding sediments in 35 d dialysis, with the release rate well fitted by the second order model. Slight degree of phase transformation in schwertmannite dominant sediment had resulted in a high risk of metallic element release during the 35 d dilution duration. The significant risk of metallic elements release was ranked in the order of Cd > Mn > Zn > Pb, and with more than 89% of Cd released from FS6 and 82% from LPS1. Relatively, Cu and As in sediments were much more stable. Overall, damming was an effective and low cost pretreatment strategy for AMD pollution control. Knowledge of the characteristics of iron secondary minerals in river sediments is essential premise for both comprehensive assessment of site contamination status and effective remediation strategy decision.

Common and rare iron, sulfur, and zinc minerals in technogenically contaminated hydromorphic soil from Southern Russia.

Soils formed after the desiccation of Lake Atamanskoe, which has served as a reservoir for liquid industrial waste from the city of Kamensk-Shakhtinsky during a long time, were studied. These soils differ from zonal soils by a strong contamination with zinc and sulfur. Preliminary studies showed that Fe compounds fix a significant part of zinc. This requires to study S, Zn, and Fe minerals. In this work, Mössbauer spectroscopy was used for the identification of iron compounds and scanning electron microscopy was used for the microanalysis of these and other minerals. To facilitate the identification of Fe minerals, brown iron ocher was removed from a contaminated soil sample and analyzed. From electron microscopy and Mössbauer spectroscopy data, ocher contained hydrogoethite with a high content of sorption water and schwertmannite (a rare mineral, probably found in Russia for the first time). The chemical composition of this schwertmannite better corresponds to the Cashion-Murad model than to the Bigham model. Particles of partially oxidized magnetite and wustite enriched with zinc were revealed under electron microscope. Siderite with partial substitution of Fe2+ by Zn2+ was detected. Thus, contaminated hydromorphic soil contains both common minerals (illite, goethite, hematite, gypsum) and rare minerals (schwertmannite, Zn siderite, partially oxidized magnetite and wustite enriched with zinc).

Fabricating Fe3O4-schwertmannite as a Z-scheme photocatalyst with excellent photocatalysis-Fenton reaction and recyclability.

Here we reported an effective method to solve the rate-limiting steps, such as the reduction of Fe3+ to Fe2+ and an invalid decomposition of H2O2 in a conventional Fenton-like reaction. A magnetic heterogeneous photocatalyst, Fe3O4-schwertmannite (Fe3O4-sch) was successfully developed by adding Fe3O4 in the formation process of schwertmannite. Fe3O4-sch shows excellent electrons transfer ability and high utilization efficiency of H2O2 (98.5%). The catalytic activity of Fe3O4-sch was studied through the degradation of phenol in the heterogeneous photo-Fenton process. Phenol degradation at a wide pH (3 - 9) was up to 98% within 6 min under visible light illumination with the Fe3O4-sch as heterogeneous Fenton catalyst, which was higher than that using pure schwertmannite or Fe3O4. The excellent photocatalytic performance of Fe3O4-sch is ascribed to the effective recycling between Fe3+ and Fe2+ by the photo-generated electron, and also profit from the formation of the "Z-Scheme" system. According to the relevant data, photocatalytic mechanism of Fe3O4-sch for degrading phenol was proposed. This study not only provides an efficient way of enhancing heterogeneous Fenton reaction, but also gives potential application for iron oxyhydroxysulfate mineral.

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.

TiO2/Schwertmannite nanocomposites as superior co-catalysts in heterogeneous photo-Fenton process.

The heterogeneous photo-Fenton reaction is an effective technique in combating organic contaminants for both soil and water remediation, and extensive studies have focused on enhancing its efficiency and reducing its costs. In this work, we developed novel photo-Fenton catalysts by simply milling commercially available TiO2 (P25) with Schwertmannite (Sh), a natural iron-oxyhydroxysulfate nanomineral. We expect that the photo-generated electrons from TiO2 could continuously migrate to Sh, which then could enhance the separation of electron-hole pairs on TiO2 and accelerate the reduction of Fe(III) to Fe(II) on Sh, leading to high degradation efficiency of the target organic contaminants. SEM and TEM results showed the distribution of TiO2 on Sh surface for the nanocomposites (TiO2/Sh). Under simulated sunlight irradiation, the much higher content of Fe(II) was determined on TiO2/Sh than on Sh via a common method in the iron ore, and the consumption of H2O2 and the production of •OH were more significant in the TiO2/Sh system than those in the TiO2 and Sh systems. These results well support our hypothesis that the photo-generated electrons could migrate from TiO2 to Sh on the composites, and can also explain the much higher degradation efficiency of Rhodamine B (RhB) in the TiO2/Sh system. Besides, TiO2/Sh had lower Fe dissolution as compared with Sh, and retained high catalytic stability after four repeated cycles. Above merits of the TiO2/Sh composites, in combining with their simple synthesis method and low-cost property, indicated that they should have promising applications as heterogeneous photo-Fenton catalysts.

Thiocyanate-induced labilization of schwertmannite: Impacts and mechanisms.

Schwertmannite is an amorphous iron(III)-oxyhydroxysulfate that forms in acid mine drainage (AMD) environments. The characteristic of high heavy metal adsorption capability makes schwertmannite a potentially useful, environmentally friendly material in wastewater treatment. Unstable schwertmannite is prone to recrystallization. Understanding the mechanisms that induce schwertmannite labilization and affect its capacity to remove heavy metals are of great environmental and geochemical significance. Thiocyanate (SCN¯) is a hazardous pseudohalide that is also normally found in AMD. However, little is known about the impact of Fe(III)-binding ligand SCN¯ on schwertmannite stability and its subsequent capacity to bind trace elements. Here, we investigated the adsorption of SCN¯ on schwertmannite and subsequent mineral transformation to characterize this little-known process. The appearance of Fe2+ indicated that the interactions between schwertmannite and SCN¯ may involve complexation and reduction reactions. Results showed that the majority of the adsorbed-SCN¯ was immobilized on schwertmannite during the 60-days transformation. The transformation rates of schwertmannite increased with increasing concentrations of SCN¯. Goethite was detected as the dominant transformation product with or without SCN¯. The mechanisms of SCN¯-promoted dissolution of schwertmannite can be described as follows: (1) formation of Fe(III)-NCS complexes on the schwertmannite surface and in solution, a process which increases the reactivity of solid phase Fe(III); (2) the extraction of Fe(III) from schwertmannite by SCN¯ and subsequent schwertmannite dissolution; and (3) the formation of secondary minerals from extracted Fe(III). These findings may improve AMD treatment strategies and provide insight into the use and potential reuse of schwertmannite as a trace element sorbent.

Isolation, identification and arsenic-resistance of Acidithiobacillus ferrooxidans HX3 producing schwertmannite.

Schwertmannite, a ubiquitous mineral present in iron oxyhydroxides formed in iron- and sulfate-rich acid media, favors incorporation of some toxic anions in its structure. We reported an iron-oxidizing bacterial strain HX3 from a municipal sludge that facilitates the formation of pure schwertmannite in cultures. Ferrous iron oxidation by the isolated strain HX3 was optimum at an initial pH of 2.0-3.3 and temperature of 28-35°C. Pure schwertmannite was found through bacterial oxidation of ferrous iron at an initial pH2.8 and temperature 28°C. Following 16S rDNA gene sequence analysis the bacterial strain HX3 was identified as Acidithiobacillus ferrooxidans. The arsenic-resistance A. ferrooxidans HX3 showed the potential of environmental application in arsenic removal from the As(III)- and iron-rich acid sulfate waters directly by As(III) adsorption or the formation of schwertmannite in the environment.

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.

The influence of Mn(II) on transformation of Cr-absorbed Schwertmannite: Mineral phase transition and elemental fate.

Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO42- and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination.