Effects of pH on coprecipitation of As(III) with biogenic synthesized schwertmannite and jarosite.

Secondary iron minerals play significant roles in the immobilization of As under acidic conditions, such as acid mine drainage. However, previous research works have not clarified the effect of pH on As(III) removal through coprecipitation with secondary minerals. Therefore, in this study, we aimed to investigate the discrepancy in As(III) coprecipitation with biogenic synthesized schwertmannite (Sch) and jarosite (Jar) at different pH values. For this, concentrations of Fe2+, TFe, SO42-, and As(III) in shake flasks were monitored during an overall incubation period of 83 h at initial pH of 1.5, 2.0, and 2.5. In addition, the physicochemical properties of collected minerals after incubation were identified using scanning electron microscopy, X-ray diffraction, pore size distribution, and Brunauer - Emmett - Teller surface area analyses. Our results showed that almost no mineral synthesis and no As(III) removal were detected in coprecipitated schwertmannite (Co-Sch) system and coprecipitated jarosite (Co-Jar) system at an initial pH of 1.5. The TFe precipitation efficiencies and As(III) removal efficiencies increased considerably and morphologies of Co-Sch and Co-Jar improved significantly when the initial pH value increased from 2.0-2.5. The maximum TFe precipitation efficiency and As(III) removal efficiency reached 30.8% and 89.6%, respectively, for the Co-Sch system, and were 47.5% and 37.4%, respectively, for the Co-Jar system. The overall results show that pH significantly affects the formation of Co-Sch and Co-Jar and the behaviour of As(III) coprecipitation.

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.

Interaction between acid-tolerant alga Graesiella sp. MA1 and schwertmannite under long-term acidic condition.

Schwertmannite (Sch), a typical Fe(III)-oxyhydroxysulphate mineral, is the precipitation reservoir of toxic elements in acid mine drainage (AMD). Acid-tolerant microbes in AMD can participate in the microbe-mediated transformation of Sch, while Sch affects the physiological characteristics of these acid-tolerant microbes. Based on our discovery of algae and Sch enrichment in a contaminated acid mine pit lake, we predicted the interaction between algae and Sch when incubated together. The acid-tolerant alga Graesiella sp. MA1 was isolated from the pit-lake surface water of an acidic mine and incubated with different contents of Sch. Sch was detected as the main product at the end of 81 d; however, there was a weak transformation. The presence of dissolved Fe(II) could be largely attributed to the photoreduction dissolution of Sch, which was promoted by Graesiella sp. MA1. The adaptation and growth phases of Graesiella sp. MA1 differed under Sch stress. The photosynthetic and metabolic activities increased and decreased at the adaptation and growth phases, respectively. The MDA contents and antioxidant activity of SOD, APX, and GSH in algal cells gradually enhanced as the Sch treatment content increased, indicating a defense strategy of Graesiella sp. MA1. Metabolomic analysis revealed that Sch affected the expression of significant differential metabolites in Graesiella sp. MA1. Organic carboxylic acid substances were essentially up-regulated in response to Sch stress. They were abundant in the medium-Sch system with the highest Fe(III) reduction, capable of complexing Fe(III), and underwent photochemical reactions via photo-induced charge transfer. The significant up-regulation of reducing sugars revealed the high energy requirement of Graesiella sp. MA1 under Sch stress. And first enriched KEGG pathway demonstrated the importance of sugar metabolism in Graesiella sp. MA1. Data acquired in this study provide novel insights into extreme acid stress adaptation of acid-tolerant algae and Sch, contributing to furthering understanding of AMD environments.

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.

Effects of Fe(II) bio-oxidation rate and alkali control on schwertmannite microstructure and adsorption of oxyanions: Characteristics, performance and mechanism.

Schwertmannite has attracted increasing interest for its excellent sorption of oxyanions such as AsO43-, CrO42-, and Sb(OH)6-. Controlling biomineralization by adjusting the Fe(II) oxidation rate and implementing alkali control can enhance the yield and adsorption performance of schwertmannite. However, the adsorption improvement mechanism is still unclear. The morphology, crystallinity, specific surface area (SSA) and oxyanion adsorption of schwertmannite synthesized with alkali control of solution pH and different Fe(II) oxidation rates were analyzed in this study. The differences in the adsorption mechanisms of As(V), Cr(VI) and Sb(V) on schwertmannite obtained under different synthesis conditions were also studied. Reducing the Fe(II) oxidation rate or maintaining the solution pH through alkali control significantly increased the SSA of schwertmannite and the proportion of outer-sphere sulfate. Alkali-controlled schwertmannite (Sch-C) exhibited superior As(V) and Sb(V) adsorption performance and slightly greater Cr(VI) adsorption than non-alkali-controlled schwertmannite. The As(V) and Sb(V) adsorption capacities of Sch-C greatly improved because the ultra-high SSA increased the surface hydroxyl content and reduced the passivation effect of amorphous precipitates on the mineral surface, allowing continuous sulfate exchange at inner mineral sites. An increased surface hydroxyl content had little effect on Cr(VI) adsorption, but an increased proportion of outer-sphere sulfate caused a slight increase in Cr(VI) adsorption. Sb(V) has a stronger hydroxyl exchange ability than As(V), but due to its octahedral structure, it exchanges only with outer-sphere sulfate on schwertmannite and hardly exchanges with inner-sphere sulfate.

Efficient co-stabilization of arsenic and cadmium in farmland soil by schwertmannite under long-term flooding-drying condition.

Simultaneously stabilizing of arsenic (As) and cadmium (Cd) in co-contaminated soil presents substantial challenges due to their contrasting chemical properties. Schwertmannite (Sch) is recognized as a potent adsorbent for As pollution, with alkali modification showing promising results in the simultaneous immobilization of both As and Cd. This study systematically investigated the long-term stabilization efficacy of alkali-modified Sch in Cd-As co-contaminated farmland soil over a 200-day flooding-drying period. The results revealed that As showed significant mobility in flooded conditions, whereas Cd exhibited increased soil availability under drying phases. The addition of Sch did not affect the trends in soil pH and Eh fluctuations; nonetheless, it led to an augmentation in the levels of amorphous iron oxides and SO42- concentration in soil pore water. At a dosage of 0.5% Sch, there was a notable decrease in the mobility and soil availability of As and Cd under both flooding (34.5% and 53.6% at Day 50) and drying conditions (27.0% and 29.4% at Day 130), primarily promoting the transformation of labile metal(loid) fraction into amorphous iron oxide-bound forms. Throughout the flooding-drying treatment period, Sch maintained stable mineral morphology and mineralogical phase, highlighting its long-term stabilization effect. The findings of this study emphasize the promising application of Sch-based soil remediation agents in mitigating the challenges arising from As-Cd co-contamination. Further research is warranted to explore their application in real farmland settings and their impact on the uptake of toxic metal(loid)s by plants.

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.

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.

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.

Schwertmannite-based heterogeneous Fenton for enhancing sludge dewaterability over a wide pH range.

Fe-based Fenton technology is commonly used to enhance sludge dewaterability, but it requires subsequent neutralization, resulting in excessive chemical consumption. In this study, we investigated the feasibility of using schwertmannite-composited Fe3O4 (Sch/Fe3O4) as a heterogeneous Fenton catalyst to enhance sludge dewaterability without the need for pH adjustment. A high reduction efficiency of sludge specific resistance to filtration (94.4%), moisture content (11.4%) and bound water (45.5%) after Sch/Fe3O4 +H2O2 treatment at initial pH 7.5 were obtained, suggesting that Sch/Fe3O4 +H2O2 posed good dehydration performance without any acidification. SO42- and H+ generation in Sch/Fe3O4 system played an important role in sludge pH decrease, which facilitated sludge cell lysis, intracellular water release, and provided a suitable pH for Fenton reaction. Reactive species (•OH, •O2-, and 1O2) from Sch/Fe3O4 +H2O2 could effectively destroy sludge EPS, releasing more bound water. Additionally, the negatively charged compounds were neutralized by dissolved Fe2+/Fe3+. Sch/Fe3O4, as a skeleton builder, rearranged the dissociative sludge flocs to improve the incompressibility and permeability of sludge cake. Finally, sludge treated with Sch/Fe3O4 +H2O2 achieved organic matters reserve, heavy metals reduction, further benefiting the final disposal.

Removal of arsenic from acidic liquors using chemical and autotrophic and mixed heterotrophic bacteria-produced biogenic schwertmannites.

Arsenic penetrates human society through a variety of geological and anthropogenic processes, posing significant health hazards. Acid mine drainage, which contains high concentrations of heavy metals and sulfate, is formed by the biological oxidation of pyrite and other metal-containing sulfidic minerals and is a significant environmental hazard. Adsorption is a simple and effective method for removing arsenic from water. In this study, co-precipitation and adsorption of arsenic with biogenic and chemically produced iron-containing settleable precipitates, i.e. schwertmannites were studied. Autotrophic Leptospirillum ferrooxidans and heterotrophic mixed culture of Alicyclobacillus tolerans and Acidiphilium cryptum oxidized iron at rates from 18 to 23 mg/(L.h) in the presence of 5 and 10 mg/L As3+, and both cultures tolerated up to 100 mg As3+/L although Fe2+ oxidation rates decreased to 3-4 mg/(L.h). At Fe/As ratios of ≥20, As removal efficiencies of ≥95% were obtained by co-precipitation with Fe3+ at pH 3.5-4.5. Because schwertmannite precipitates produced by the heterotrophic culture formed crystals, it was studied for adsorptive removals of As3+ and As5+ and compared with chemically synthesized schwertmannites. As3+ (100 mg/L) adsorption onto biogenic and chemical schwertmannite were 25 and 44%, respectively, at pH 4. At 100 mg As5+/L, adsorption capacity and efficiency onto biogenic schwertmannite were 47 mg/g and 50%, respectively. At 300 mg As5+/L, adsorption capacity and efficiency onto chemical schwertmannite were 169 mg/g and 56%, respectively. In summary, biogenic schwertmannite has potential for As removal via co-precipitation with Fe3+ at pH 3.5-4.5 and Fe/As ratios of ≥20 due to low production cost from acidic mine drainage. In contrast to the schwertmannite generation methods, which are usually performed with autotrophic acidophilic bacteria in the literature, this efficient and modular schwertmannite production process and its evaluation on arsenic adsorption is an important potential in acidic mine drainage treatment containing arsenic.

Arsenic removal and stabilization behavior of schwertmannite@BC (Sch@BC) in contaminated dual media (water/soil): Via sulfate exchange and chemical complexation.

Arsenic (As) is extremely harmful to the ecological environment and human health owing to its high toxicity. The composite that biochar (BC) modified by Schwertmannite (Sch), marked as Sch@BC, were prepared to remediate As-contaminated water and soil with a high efficiency. The characterization results showed that the Sch particles were successfully loaded on the BC, providing more active sites for As(V) adsorption. Compared with the pristine BC, the adsorption capacity of Sch@BC-1 was significantly improved (50.00 mg/g), of which the adsorption capacity kept stable over a wide pH range (pH = 2-8). The adsorption process conformed to pseudo-second-order kinetics and Langmuir isotherm model, which indicated that chemical adsorption was the dominant mechanism and the adsorption rate was controlled by intraparticle diffusion. Sch@BC could adsorb As(V) through electrostatic interaction and ion exchange, forming a FeAsO4 complex and removing As(V). The 5-week soil incubation experiment showed that 3% Sch@BC showed the optimal stabilization effect, while the proportion of stable crystalline Fe/Mn-bound fractionation (F4) increased. Moreover, the results of microbial community diversity showed that Sch@BC interacted with As-resistant dominant microorganisms such as Proteobacteria in soil, promoted their growth and reproduction, and improved the stability of As in soil. In summary, Sch@BC is an excellent agent with broad application prospects for remediating As-contaminated water and soil.

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.

Remediating flooding paddy soils with schwertmannite greatly reduced arsenic accumulation in rice (Oryza sativa L.) but did not decrease the utilization efficiency of P fertilizer.

Planting rice (Oryza sativa L.) in As-contaminated paddy soils can lead to accumulation of arsenic (As) in rice grains, while the application of phosphorus (P) fertilizers during rice growth may aggravate the accumulation effect. However, remediating flooding As-contaminated paddy soils with conventional Fe(III) oxides/hydroxides can hardly achieve the goals of effectively reducing grain As and maintaining the utilization efficiency of phosphate (Pi) fertilizers simultaneously. In the present study, schwertmannite was proposed to remediate flooding As-contaminated paddy soil because of its strong sorption capacity for soil As, and its effect on the utilization efficiency of Pi fertilizer was investigated. Results of a pot experiment showed that Pi fertilization along with schwertmannite amendment was effective to reduce the mobility of As in the contaminated paddy soil and meanwhile increase soil P availability. The schwertmannite amendment along with Pi fertilization reduced the content of P in Fe plaque on rice roots, compared with the corresponding amount of Pi fertilizer alone, which can be attributed to the change in mineral composition of Fe plaque mainly induced by schwertmannite amendment. Such reduction in P retention on Fe plaque was beneficial for improving the utilization efficiency of Pi fertilizer. In particular, amending flooding As-contaminated paddy soil with schwertmannite and Pi fertilizer together has reduced the content of As in rice grains from 1.06 to 1.47 mg/kg to only 0.38-0.63 mg/kg and significantly increased the shoot biomass of rice plants. Therefore, using schwertmannite to remediate flooding As-contaminated paddy soils can achieve the dual goals of effectively reducing grain As and maintaining the utilization efficiency of P fertilizers.

Schwertmannite and akaganéite for adsorption removals of Cr(VI) from aqueous solutions.

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

Impact of Fulvic Acid and Acidithiobacillus ferrooxidan Inoculum Amount on the Formation of Secondary Iron Minerals.

The catalytic oxidation of Fe2+ by Acidithiobacillus ferrooxidan (A. ferrooxidans) and the synthesis of iron sulfate-based secondary minerals is considered to be of great significance to the treatment of acid mine drainage (AMD). Along these lines, in this work, the shaker experiment was carried out to study the underlying mechanism of the inoculation amount of fulvic acid (FA) and A. ferrooxidans on the synthesis process of secondary minerals. From the acquired results, it was demonstrated that the oxidation rate of Fe2+ increased with the increase in the concentration of fulvic acid in the range of 0.1-0.2 g/L. On top of that, the concentration of fulvic acid in the range of 0.3-0.5 g/L inhibited the activity of A. ferrooxidans. However, A. ferrooxidans retained its activity, and the complete oxidation time of Fe2+ was delayed. When the concentration of fulvic acid was 0.3 g/L, the TFe (total iron) precipitation efficiency was 30.2%. Interestingly, when 0.2 g/L fulvic acid was added to different inoculum systems, the incorporation of a higher inoculum amount of A. ferrooxidans led to an increased oxidation rate. On the contrary, the lower inoculum amount yielded a more obvious effect of the fulvic acid. From the mineralogical characteristics, it was also revealed that a fulvic acid concentration of 0.2 g/L and different inoculation amounts of A. ferrooxidans did not change the mineral facies, whereas pure schwertmannite was obtained.

Effects of Fe(II) concentration on the biosynthesis of schwertmannite by Acidithiobacillus ferrooxidans and the As(III) removal capacity of schwertmannite.

The effect of Fe(II) concentrations on schwertmannite bio-synthesis and the As(III) removal capacity of schwertmannite were investigated in this study. Acidithiobalillus ferrooxidans (A. ferrooxidans) were inoculated into five FeSO4 systems with initial concentrations of 50, 100, 200, 300, and 400 mmol/L, respectively, to bio-synthesize schwertmannite. The Fe(II) of the systems were almost completely oxidised at 48, 72, 120, 168, and 192 h, respectively, and the bio-schwertmannite yield was 1.99, 3.81, 9.36, 12.42, and 21.60 g/L. The results of this study indicate that all minerals harvested from the different systems are schwertmannite. As the initial Fe(II) concentration increases, the effect of the minerals removing As(III) decreases; moreover, the structure and extracellular polymeric substance (EPS) of schwertmannite may regulate the As(III) removal process. The EPS generated by the A. ferrooxidans can absorb As(III). The outcomes of this study provide fresh insights into the bio-synthetic regulation of schwertmannite and play a significant role in treating As-containing groundwater.

Iron­sulfur 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.

Microbial reduction of schwertmannite by co-cultured iron- and sulfate-reducing bacteria.

Schwertmannite (Sch) is an iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment. The transformation mechanism of Sch mediated by pure cultured iron-reducing bacteria (FeRB) or sulfate-reducing bacteria (SRB) has been studied. However, FeRB and SRB widely coexist in the environment, the mechanism of Sch transformation by the consortia of FeRB and SRB is still unclear. This study investigated the Sch reduction by co-cultured Shewanella oneidensis (FeRB) and Desulfosporosinus meridiei (SRB). The results showed that co-culture of FeRB and SRB could accelerate the reductive dissolution of Sch, but not synergistically, and there were two distinct phases in the reduction of Sch mediated by FeRB and SRB: an initial phase in which FeRB predominated and Fe3+ in Sch was reduced, accompanied with the release of SO42-, and the detected secondary minerals were mainly vivianite; the second phase in which SRB predominated and mediated the reduction of SO42-, producing minerals including mackinawite and siderite in addition to vivianite. Compared to pure culture, the abundance of FeRB and SRB in the consortia decreased, and more minerals aggregated inside and outside the cell; correspondingly, the transcription levels of genes (cymA, omcA, and mtrCBA) related to Fe3+ reduction in co-culture was down-regulated, while the transcription levels of SO42--reducing genes (sat, aprAB, dsr(C)) was generally up-regulated. These phenomena suggested that secondary minerals produced in co-culture limited but did not inhibit bacterial growth, and the presence of SRB was detrimental to dissimilatory Fe3+ reduction, while existed FeRB was in favor of dissimilatory SO42- reduction. SRB mediated SO42- reduction by up-regulating the expression of SO42- reduction-related genes when its abundance was limited, which may be a strategy to cope with external coercion. These findings allow for a better understanding of the process and mechanism of microbial mediated reduction of Sch in the environment.