Molecular Identification of Cr(VI) Removal Mechanism on Vivianite Surface

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Desulfovibrio-induced gauzy FeS for efficient hexavalent chromium removal: The influence of SRB metabolism regulated by carbon source and electron carriers

Biosynthetic metal sulfides showed great application prospects in the environmental treatment against high-valence metal pollutants. However, the efficiency of biosynthesis, agglomeration during the reaction process, and the formation of the passivation layer during the reduction process were always the important factors restricting its development. This study explored the composition of the culture medium to promote the growth of highly corrosive sulfate-reducing bacteria (SRB) and its metabolism to produce FeS nanoparticles (NPs). The results showed that reducing the carbon source (CS) and adding electron carriers in the culture medium effectively promoted the production of small, dispersed, and loose FeS NPs in cells. At pH = 7, 24 °C and 10 min reaction time, 0.1 g/L FeS NPs produced by SRB under the conditions of 10 % CS with 10 ppm cytochrome c medium could achieve 100 % removal efficiency of 1 mM hexavalent chromium (Cr(VI)). Under this condition, FeS NPs could be produced by intracellular metabolism in SRB cells, and environmental factors such as pH, metal cations, and Cl- had little effect on the removal of Cr(VI) by this FeS NPs. The surface proteins of FeS NPs significantly enhanced their antioxidant properties. After 7 days of natural environment exposure, the Cr(VI) removal efficiency of FeS NPs was only reduced by 16 % compared with the initial sample. This work provided an in-depth understanding of Cr(VI) removal by SRB biosynthesis of FeS and contributes to the widespread application of FeS in the future.

Enhanced Cr(VI) stabilization by terrestrial-derived soil protein: Photoelectrochemical properties and reduction mechanisms

Glomalin-related soil protein (GRSP) is a stable iron-organic carbon mixture that can enhance heavy metal sequestration in soils. However, the roles of GRSP in the transformation and fate of Cr(VI) have been rarely reported. Herein, we investigated the electrochemical and photocatalytic properties of GRSP and its mechanisms in Cr(VI) adsorption and reduction. Results showed that GRSP had a stronger ability for Cr(VI) adsorption and reduction than other biomaterials, with the highest adsorption amount of up to 0.126 mmol/g. The removal efficiency of Cr(VI) by GRSP was enhanced (4-7%) by ultraviolet irradiation due to the hydrated electrons produced by GRSP. Fe(II) ions, persistent free radicals, and oxygen-containing functional groups on the GRSP surface as electron donors participated in the reduction of Cr(VI) under dark condition. Moreover, Cr(III) was mainly adsorbed on the -COOH groups of GRSP via electrostatic interactions. Based on 2D correlation spectroscopy, the preferential adsorption occurred on the GRSP surface for Cr(VI) in the sequential order of CO → COO- → O-H → C-O. This work provides new insights into the Cr(VI) adsorption and reduction mechanism by GRSP. Overall, GRSP can serve as a natural iron-organic carbon for the photo-reduction of Cr(VI) pollution in environments.

Synergistic effect of novel pyrite/N-doped reduced graphene oxide composite with heterojunction structure for enhanced photo-assisted reduction of Cr(VI) in oxic water: Specific role of molecular oxygen

To avoid severe aggregation and synergistically utilize the intrinsic and photocatalytic reducibility, pyrite (FeS2) was loaded onto N-doped reduced graphene oxides (N-rGO) to fabricate a novel FeS2/N-rGO heterojunction catalyst for enhanced chromium (Cr(VI)) reduction in oxic condition to simultaneously investigate the specific effect and role of dissolved oxygen (DO). Characterization results showed that strong interaction and combination of FeS2 and N-rGO not only achieved the uniform distribution of FeS2, but also increased the defects, and exposed more functional groups. Meanwhile, the Type II heterojunction was formed in FeS2/N-rGO, which facilitated the separation efficiency of photo-generated carriers and electrons, endowing FeS2/N-rGO a superior photocatalytic activity. Cr(VI) was almost completely reduced via FeS2/N-rGO within 60 min under irradiation (Cr(VI) = 10 mg/L, dosage = 0.2 g/L), 3 times that of pristine FeS2 (18.7 %). Trapping and Electron Spin Resonance (ESR) experiments indicated that photo-generated e- and derived O2- species from photoactivation of dioxygen (DO) were the key reactive species for the enhancement of photo-assisted Cr(VI) reduction, rather than reductive Fe2+ and S22- species. Although the photocatalysis of FeS2/N-rGO cannot directly generate hydroxyl radicals (OH), the oxidative OH ascribed to superoxide radicals (O2-), photo-induced holes and free DO preferentially consumed by Fe2+ and S22- with stronger reducibility. Hence, as compared to the anoxic condition, the reduction rate of Cr(VI) was slightly decreased, but still could be totally removed within 60 min in the oxic conditions. Due to the excessive amount of FeS2/N-rGO, Cr(III) after reduction would not be influenced by oxidative species and maintain stability under oxic condition. This study provided a facile modification strategy for FeS2 based composites and uncovered its working mechanism for Cr(VI) decontamination.

Cr(VI) Reduction and Fe(II) Regeneration by Penicillium oxalicum SL2-Enhanced Nanoscale Zero-Valent Iron

Nanoscale zero-valent iron (nZVI) faces significant challenges in Cr(VI) remediation through aggregation and passivation. This study identified a Cr(VI)-resistant filamentous fungus (Penicillium oxalicum SL2) for nZVI activation and elucidated the synergistic mechanism in chromium remediation. P. oxalicum SL2 and nZVI synergistically and effectively removed Cr(VI), mainly by extracellular nonenzymatic reduction (89.1%). P. oxalicum SL2 exhibited marked iron precipitate solubilization and Fe(II) regeneration capabilities. The existence of the Fe(II)-Cr(V)-oxalate complex (HCrFeC₄O₉) indicated that in addition to directly reducing Cr(VI), iron ions generated by nZVI stimulated Cr(VI) reduction by organic acids secreted by P. oxalicum SL2. RNA sequencing and bioinformatics analysis revealed that P. oxalicum SL2 inhibited phosphate transport channels to suppress Cr(VI) transport, facilitated iron and siderophore transport to store Fe, activated the glyoxylate cycle to survive harsh environments, and enhanced organic acid and riboflavin secretion to reduce Cr(VI). Cr(VI) exposure also stimulated the antioxidative system, promoting catalase activity and maintaining the intracellular thiol/disulfide balance. Cr(VI)/Fe(III) reductases played crucial roles in the intracellular reduction of chromium and iron, while nZVI decreased cellular oxidative stress and alleviated Cr(VI) toxicity to P. oxalicum SL2. Overall, the P. oxalicum SL2-nZVI synergistic system is a promising approach for regenerating Fe(II) while reducing Cr(VI).

Enhanced microbial reduction of Cr(VI) in soil with biochar acting as an electron shuttle: Crucial role of redox-active moieties

Biochar has been proven to participate in the biotic reduction of hexavalent chromium (Cr(VI)) in environment since its involvement may accelerate the extracellular electron transfer (EET). However, roles of the redox-active moieties and the conjugated carbon structure of biochar in this EET process remain unclear. In this study, 350 °C and 700 °C were selected to produce biochar with more O-containing moieties (BC350) or more developed conjugated structures (BC700), and their performances in the microbial reduction of soil Cr(VI) were investigated. Our results showed that BC350 presented a 241% increase of Cr(VI) microbial reduction after 7-day incubation, much higher than that of BC700 (39%), suggesting that O-containing moieties might play more important roles in accelerating the EET process. Biochar, especially BC350 could serve as an electron donor for microbial anaerobic respiration, but its contribution (73.2%) as an electron shuttle for EET was dominant to the enhanced Cr(VI) reduction. The positive correlation between electron exchange capacities (EECs) of pristine and modified biochars and the corresponding maximum reduction rates of Cr(VI) evidenced the crucial role of redox-active moieties in electron shuttling. Moreover, EPR analysis suggested the nonnegligible contribution of semiquinone radicals in biochars to the accelerated EET process. This study demonstrates the crucial role of redox-active moieties, i.e., O-containing moieties in mediating the EET process during the microbial reduction of Cr(VI) in soil. Findings obtained will advance the current understanding of biochar as an electron shuttle participating in the biogeochemical processes of Cr(VI).

Vivianite-induced peroxymonosulfate activation for containment removal under dark conditions: Performance, mechanism and regeneration

The performance and intrinsic mechanism of vivianite, a natural mineral containing structural Fe(II), for peroxymonosulfate (PMS) activation and pollutant degradation under dark conditions were comprehensively explored in this study. It was found that vivianite was able to efficiently activate PMS to degrade various pharmaceutical pollutants under dark conditions, in which the corresponding reaction rate constant of ciprofloxacin (CIP) degradation was 47- and 32-fold higher than that of magnetite and siderite, respectively. SO₄^·-, ^·OH, Fe(IV) and electron-transfer processes were found in the vivianite-PMS system, while SO₄^·- was the main contributor to CIP degradation. Moreover, mechanistic explorations revealed that the Fe site on the surface of vivianite could bind PMS in the form of a bridge position, and thus vivianite could rapidly activate absorbed PMS due to its strong electron-donating ability. Additionally, it was illustrated that the used vivianite could be efficiently regenerated by either chemical or biological reduction. This study may provide an alternative application of vivianite in addition to phosphorus recovery from wastewater.

Recovery of phosphorus from wastewater containing humic substances through vivianite crystallization: Interaction and mechanism analysis

Vivianite crystallization has been regarded as a suitable option for recovering phosphorus (P) from P-containing wastewater. However, the presence of humic substances (HS) would inevitably affect the formation of vivianite crystals. Therefore, the influences of HS on vivianite crystallization and the changes in the harvested vivianite crystals were investigated in this study. The results suggested the inhibition effect of 70 mg/L HS on vivianite crystallization reached 12.24%, while it could be attenuated by increasing the pH and Fe/P ratio of the solution. Meanwhile, the addition of HS altered the size, purity, and morphology of recovered vivianite crystals due to the blockage of the growth sites on the crystal surface. Additionally, the formation of phosphate ester group, hydrogen bonding, and COOH-Fe²⁺ complexes are the potential mechanisms of HS interaction with vivianite crystals. The results obtained herein will help to elucidate the underlying mechanism of HS on vivianite crystallization from P-containing wastewater.

Enhanced phosphorus release from waste activated sludge using ascorbic acid reduction and acid dissolution

Due to the widespread application of various iron (Fe)-derived substances used in phosphorus (P) removal during wastewater treatment, Fe-P species generated in this process constitute an important part of P speciation in non-digested sludge. SEM-EDS and sequential extraction methods were utilized to analyze the speciation, distribution, and spatial variation of P contained in the sludge. Inorganic P accounted for 91.3% of the total P, and Fe(III)-P represented the greatest percentage (68.5%) in the inorganic P fraction. Ascorbic acid, also known as vitamin C (VC), performed well in releasing P from sludge, especially in combination with subsequent pH adjustment to 3.0 using HCl. Fe(III)-P in sludge was first reduced to Fe(II)-P by VC, then dissolved in acidic conditions to release Fe²⁺ and PO₄^3-. Other metal-P compounds were also partially dissolved and released. VC disrupted the sludge floc structure, releasing organic P via organic efflux. There was a positive correlation (R²>0.97, p<0.05) between the amount of released P and the amount of reductant (VC). There was a synergistic effect between 120 mmol/L VC and acidity, producing the greatest P release of 67.1% of total sludge P. The P release efficiency achieved in this study was higher than other reported methods. Additionally, VC provides a more sustainable option due to its natural biodegradability. Released P and Fe²⁺ can be recovered as vivianite with recovery rates of 88% and 99%, respectively. This finding provides a new direction for effective, sustainable sludge P recovery and utilization.

Carbon nanotubes accelerates the bio-induced vivianite formation

Vivianite widely existed in digested sludge and activated sludge as a potential phosphate resource recovered from wastewater treatment plants (WWTPs). As an important product of extracellular electron transfer (EET) and biological iron reduction, the production of vivianite can be enhanced by conductive materials. Carbon nanotubes (CNTs) with excellent electrical conductivity have been reported to promote electron transfer, which was applied in wastewater treatment to accelerate the degradation of the contaminants. However, the impact of CNTs on vivianite formation was barely reported. In this study, the iron reduction, vivianite recovery, and the biotoxicity of CNTs were investigated in order to determine the influence of CNTs towards the vivianite production. The enhancement of vivianite production after CNTs adding reached up to 17 % by promoting the electron transfer between dissimilative iron-reducing bacteria (DIRB) and Fe(III). However, at the initial stage (0-24 h), Fe(III) reduction efficiency decreased by 81 % after inoculating with sewage sludge, which was attributed to CNTs destroying of the cell membrane (as indicated by SEM, CLSM and AFM analysis). The biotoxicity of CNTs stimulated DIRB to secret extracellular polymeric substances (EPS) and form bio-flocs to resist the physical puncture. After 48 h, the proportion of living DIRB in 1000 mg/L CNTs batch increased to 98 %, which was 79 % higher than 12 h. As a result, the vivianite recovery of raw sewage with 1000 mg/L CNTs increased to 44 ± 1 %, which was 33 % higher than that in the CNT-0.

Partial Oxidation of FeS Nanoparticles Enhances Cr(VI) Sequestration

Iron sulfide nanoparticles (nano-FeS) have shown great potential for in situ remediation of Cr(VI) pollution by reducing Cr(VI) to the less soluble and toxic Cr(III). However, material oxidation that inevitably occurs during storage and application alters its reactivity. Herein, we show that partial oxidation of nanoparticulate mackinawite (FeS) significantly enhances its capability in sequestering Cr(VI). Oxidation of nano-FeS increases its binding affinity to Cr(VI), likely due to preferential inner-sphere complexation of Cr(VI) oxyanions to ferric over ferrous iron in mackinawite/lepidocrocite (FeS/γ-FeOOH) nanocomposites. A trade-off is that oxidation mitigates Cr(VI) reduction by lowering the electron-donating potential of the material and the electron transfer at a solution-material interface and consequently hinders the transformation of adsorbed Cr(VI) to Cr(III). Notably, the rate-limiting step of Cr(VI) sequestration transitions from adsorption to reduction during oxidation, as demonstrated with batch experiments coupled with kinetic modeling. Thus, an optimum oxidation degree exists, wherein the gain in the overall performance from enhanced adsorption overcompensates the loss from inhibited reduction, resulting in maximum sequestration of aqueous Cr(VI) as solid-phase Cr(III). Our findings inform better assessment and design of nanomaterials for Cr(VI) remediation and may be extended to interactions of other oxyanions with natural and engineered nanoparticles during oxidative aging.

Novel co-polymerization of polypyrrole/polyaniline on ferrate modified biochar composites for the efficient adsorption of hexavalent chromium in water

It is still a huge challenge to prepare cheap and effective composite materials for removing hexavalent chromium (Cr(VI)) in sewage treatment. In this study, a noval co-polymerization of polypyrrole/polyaniline on ferrate modified biochar (Ppy/PANI/FBC) was fabricated via ferrate-promoted pyrolysis and in-situ oxidative polymerization of pyrrole and aniline molecules to effectively remove Cr(VI) from polluted water. The Ppy/PANI/FBC quickly decreased Cr(VI) concentration from 38.92 to 3.92 mg/L within 400 min, with an efficient removal efficiency (89.92%), which was significantly higher than that of FBC (4.75%), Ppy/FBC (72.30%), and PANI/FBC (42.43%). These results are mainly caused by its conjugated connection and well-dispersion of Ppy and PANI on the surface of a carbon-based material. Meanwhile, the experimental results were in line with the pseudo-second-order kinetic and Freundlich models. The Ppy/PANI/FBC is featured by a high capacity of Cr(VI) adsorption (up to 203.71 mg/g). In addition, it could be adopted for efficiently removing Cr(VI) over a wide pH range (4-9) because of the positively charged nitrogen (-NH^.+- and = N⁺-). The sorption mechanisms of Cr(VI) were identified, including electrostatic interaction with surface protonated nitrogen (N⁺), ion exchange between the doped Cl^- ions and Cr(VI), chemical decrease of the Cr(VI) to Cr(III) by the iron valence cycle and efficient electron transfer of Ppy/PANI/FBC, as well as surface complexation by amine and oxygen-containing groups. More importantly, 97.98% Cr(VI) was efficiently removed in 20 min by coupling a photocatalytic reaction, also providing a novel idea for the practical use of adsorbents in wastewater treatment.

Biosorption of chromium (VI), iron (II), copper (II), and nickel (II) ions onto alkaline modified Chlorella vulgaris and Spirulina platensis in binary systems

The simultaneous biosorption of chromium (VI), copper (II), iron (II), and nickel (II) was investigated by alkaline-modified Chlorella vulgaris and Spirulina platensis in binary systems. The alkaline modified biosorbents were CV-KCl, SP-KCl, CV-Na2CO3, and SP-Na2CO3. The maximum removal efficiency recorded in this study was 99.7% with a biosorbent dosage of 0.3 g within a pH range of 2 to 6. The highest biosorption capacities obtained were 14.1, 13.5, 21.6, and 15.8 mg/g for Cr (VI), Cu (II), Fe (II), and Ni (II), respectively. The pseudo-second-order best described the biosorption rate, while the Langmuir isotherm model best described the biosorption equilibrium interaction. The values for Gibbs free energy (ΔG°) were in the range of 0.5 to 6.5 kJ/mol (Cr-Fe), 1.3 to 8.4 kJ/mol (Cr-Ni), and 3.9 to 11.3 kJ/mol (Cr-Cu) binary systems. This showed that the biosorption processes were characterized by physisorption reactions. The Temkin constant B values were in the range of 0.339 to 1.485 kcal/mol and the biosorption processes were largely exothermic reactions. The values for the Freundlich constant KF were between 1.4 and 10.4 (L/g), which indicated favourable biosorption. The Temkin isotherm model confirmed a strong binding affinity for Fe (II) and Ni (II). The results suggest that potassium chloride and sodium carbonate modification are very suitable for green algae and cyanobacteria for the efficient removal of heavy metals.

Antimony redox processes in the environment: A critical review of associated oxidants and reductants

The environmental behavior of antimony (Sb) has recently received greater attention due to the increasing global use of Sb in a range of industrial applications. Although present at trace levels in most natural systems, elevated Sb concentrations in aquatic and terrestrial environments may result from anthropogenic activities. The mobility and toxicity of Sb largely depend on its speciation, which is dependent to a large extent on its oxidation state. To a certain extent, our understanding of the environmental behavior of Sb has been informed by studies of the environmental behavior of arsenic (As), as Sb and As have somewhat similar chemical properties. However, recently it has become evident that the speciation of Sb and As, especially in the context of redox reactions, may be fundamentally different. Therefore, it is crucial to study the biogeochemical processes impacting Sb redox transformations to understand the behavior of Sb in natural and engineered environments. Currently, there is a growing body of literature involving the speciation, mobility, toxicity, and remediation of Sb, and several reviews on these general topics are available; however, a comprehensive review focused on Sb environmental redox chemistry is lacking. This paper provides a review of research conducted within the past two decades examining the redox chemistry of Sb in aquatic and terrestrial environments and identifies knowledge gaps that need to be addressed to develop a better understanding of Sb biogeochemistry for improved management of Sb in natural and engineered systems.

In-situ constructing nanostructured magnesium ferrite on steel slag for Cr(VI) photoreduction

An innovative method is created for transforming iron-rich RO phase (MgO_0.239FeO_0.761) on steel slag surface into nanostructured Mg_0.04Fe_2.96O₄ layer. The phase change process is investigated, and it is found that salicylic acid modification and alkaline roasting procedures remarkably increase the specific surface area from 0.46 m²/g (raw steel slag) to 69.5 m²/g (Mg_0.04Fe_2.96O₄), and the generation of Mg_0.04Fe_2.96O₄ enhances the absorption of visible light and Cr(VI) conversion with 2-times increasement than raw steel slag. Surface complexation between H₂C₂O₄ ligands and Fe metal moiety on Mg_0.04Fe_2.96O₄ induces the intramolecular electron transfer under visible light irradiation based on a ligand-to-metal charge transfer mechanism, thus resulting in Cr(VI) photoreduction, and the catalytic efficiency is above 90% for Cr(VI) (40 mg/L) under inherent pH= 5.5 conditions. Moreover, recyclability tests based on magnetic separation show that the photoreactivity is closely related to Mg content of Mg_0.04Fe_2.96O₄ layer where Mg leaching occurs and finally generates cubic spinel configuration Fe₃O₄. This work highlights the importance of surface functionalization in post-use phases of steel slag in which surface reactivity and application potential can be greatly altered by chemical exposure history and surface transformations. It also provides valuable references for studying the metastable state mechanism of magnesium ferrite photocatalysts.

A facile approach to synthesize CdS-attapulgite as a photocatalyst for reduction reactions in water

At room temperature, a facile approach has been utilized for preparing novel CdS–attapulgite (CdS–ATP) composites and the composites were applied in photocatalytic reduction of p-nitrophenol and Cr(vi). The effect of ATP on the photocatalytic activity of the CdS–ATP composites were studied by controlling the mass ratio of attapulgite. The results showed that the CdS–20%ATP composite has an excellent photocatalytic activity. In order to figure out the key to improve the photocatalytic efficiency, the prepared composites were characterized by Brunauer–Emmett–Teller (BET) specific surface area, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). The superior photocatalytic performance of the CdS–20%ATP composite can be ascribed to the existence of the ATP which can fix the CdS and prevent agglomeration. The interaction between ATP and CdS in the composites facilitates the electron transfer and also promoted their photocatalytic performance. This work provides us with some significant guidance in the development of CdS–ATP composite photocatalysts.

The application of CdS–attapulgite composites in photocatalytic reduction of p-nitrophenol and Cr(vi) demonstrated that the attapulgite could overcome the limitations of CdS.

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr₂O₇^2- and CrO₄^2-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K⁺ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K⁺ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K⁺ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO₄^2- rather than Cr₂O₇^2- is dominant at clay interfaces, and HCrO₄^- can co-exist under acidic conditions. Cr₂O₇^2- transformation to CrO₄^2- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Development of a novel pyrite/biochar composite (BM-FeS2@BC) by ball milling for aqueous Cr(VI) removal and its mechanisms

As a natural reduction mineral, pyrite (FeS₂), was combined with biochar by simply ball milling technology to synthesize FeS₂@biochar composite (BM-FeS₂@BC) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. SEM, XRD, FTIR, and XPS characterization results showed that the FeS₂ and biochar were successfully combined and biochar suppressed the agglomeration of FeS₂. Batch sorption experiments showed that the BM-FeS₂@BC700 composite (mass ratio of FeS₂-to-biochar = 3:1) had enhanced Cr(VI) removal capacity of 134 mg·g^-1, which were 3-25 times higher than those of the corresponding pristine and ball-milled biochar and FeS₂. The removal of Cr(VI) by BM-FeS₂@BC700 was dosage and pH dependent. The addition of oxalic acid (OA) exhibited a promotion effect on the removal of Cr(VI) by increasing the removal rate of Cr(VI) from 56% to 100%. Reduction, adsorption, and surface complexation were the dominate mechanisms for Cr(VI) removal by BM-FeS₂@BC700. At the equilibrium Cr(VI) concentration of 15.7 mg·L^-1, 92.25% of Cr(VI) was removed through reduction/precipitation and 8.75% was removed by adsorption/surface complexation. The fitting results of the Langmuir model proved that the removal of Cr(VI) by BM-FeS₂@BC700 composite was chemical surface monolayer adsorption. This work demonstrates the potential of ball milling for the preparation of FeS₂@BC composite to remove Cr(VI) from water and wastewater.

Biosynthesis of vivianite from microbial extracellular electron transfer and environmental application

Vivianite (Fe₃(PO₄)₂·8H₂O) is a common hydrous ferrous phosphate mineral which often occurs in reductive conditions, especially anoxic non-sulfide environment containing high concentrations of ferrous iron (Fe²⁺) and orthophosphate (PO₄^3-). Vivianite is an important product of dissimilatory iron reduction and a promising route for phosphorus recovery from wastewater. Its formation is closely related to the extracellular electron transfer (EET), a key mechanism for microbial respiration and a crucial explanation for the reduction of metal oxides in soil and sediments. Despite of the natural ubiquity, easy accessibility and attractive economic value, the application value of vivianite has not received much attention. This review introduces the characteristics, occurrence and biosynthesis of vivianite from microbial EET, and systematically analyzes the application value of vivianite in the environmental field, including immobilization of heavy metals (HMs), dechlorination of carbon tetrachloride (CT), sedimentary phosphorus sequestration and eutrophication alleviation. Additionally, its potential functions as a slow-release fertilizer are discussed as well. In general, vivianite is expected to make more contributions to the future scientific research, especially the solution of environmental problems. Overcoming the lack of understanding and some technical limitations will be beneficial to the further application of vivianite in environmental field.

Reduction of Vanadium(V) by Iron(II)-Bearing Minerals

Fe(II)-bearing minerals (magnetite, siderite, green rust, etc.) are common products of microbial Fe(III) reduction, and they provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active elements such as vanadium; which can exist as V(V), V(IV), and V(III) under conditions typical of near-surface aquatic and terrestrial environments. To better understand the redox behavior of V under ferrugenic/sulfidogenic conditions, we examined the interactions of V(V) (1 mM) in aqueous suspensions containing 50 mM Fe(II) as magnetite, siderite, vivianite, green rust, or mackinawite, using X-ray absorption spectroscopy at the V K-edge to determine the valence state of V. Two additional systems of increased complexity were also examined, containing either 60 mM Fe(II) as biogenic green rust (BioGR) or 40 mM Fe(II) as a mixture of biogenic siderite, mackinawite, and magnetite (BioSMM). Within 48 h, total solution-phase V concentrations decreased to <20 µM in all but the vivianite and the biogenic BiSMM systems; however, >99.5% of V was removed from solution in the BioSMM and vivianite systems within 7 and 20 months, respectively. The most rapid reduction was observed in the mackinawite system, where V(V) was reduced to V(III) within 48 h. Complete reduction of V(V) to V(III) occurred within 4 months in the green rust system, 7 months in the siderite system, and 20 months in the BioGR system. Vanadium(V) was only partially reduced in the magnetite, vivianite, and BioSMM systems, where within 7 months the average V valence state stabilized at 3.7, 3.7, and 3.4, respectively. The reduction of V(V) in soils and sediments has been largely attributed to microbial activity, presumably involving direct enzymatic reduction of V(V); however the reduction of V(V) by Fe(II)-bearing minerals suggests that abiotic or coupled biotic–abiotic processes may also play a critical role in V redox chemistry, and thus need to be considered in modeling the global biogeochemical cycling of V.

Mercury Reduction by Nanoparticulate Vivianite

Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg⁰) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. Fe^II-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized Hg^II reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (Fe^II₃(PO₄)₂·8H₂O), but its reaction with Hg^II has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; Fe^II content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce Hg^II was investigated at circumneutral pH under anoxic conditions over a range of Fe^II/Hg^II ratios (0.1-1000). For Fe^II/Hg^II ratios ≥1, which are representative of natural environments, Hg^II was very quickly and efficiently reduced to Hg⁰. The ability of vivianite to reduce Hg^II was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic Hg^II reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems.

Does soluble starch improve the removal of Cr(VI) by nZVI loaded on biochar?

A novel material that nano zero valent iron (nZVI) loaded on biochar with stable starch stabilization (nZVI/SS/BC) was synthesized and used for the removal of hexavalent chromium [Cr(VI)] in simulated wastewater. It was indicated that as the pyrolysis temperature of rice straw increased, the removal rate of Cr(VI) by nZVI/SS/BC first increased and then decreased. nZVI/SS/BC made from biochar pyrolyzed at 600 °C (nZVI/SS/BC600) had the highest removal efficiency and was suitable for a wide pH range (pH 2.1-10.0). The results showed that 99.67% of Cr(VI) was removed by nZVI/SS/BC600, an increase of 45.93% compared to the control group, which did not add soluble starch during synthesis. The pseudo-second-order model and the Langmuir model were more in line with reaction. The maximum adsorption capacity for Cr(VI) by nZVI/SS/BC600 was 122.86 mg·g^-1. The properties of the material were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, Brunauer-Emmett-Teller (BET), Fourier-transform infrared (FTIR), and X-ray diffraction (XRD). The results showed that the nZVI particles were uniformly supported on the biochar, and the BET surface areas of nZVI/SS/BC was 40.4837 m²·g^-1, an increase of 8.79 times compared with the control group. Mechanism studies showed that soluble starch reduced the formation of metal oxides, thereby improving the reducibility of the material, and co-precipitates were formed during the reaction. All results indicated that nZVI/SS/BC was a potential repair material that can effectively overcome the limitations of nZVI and achieve efficient and rapid repair of Cr(VI).

Insight into efficient phosphorus removal/recovery from enhanced methane production of waste activated sludge with chitosan-Fe supplementation

Fe(III)-loaded chitosan (CTS-Fe) composite was used for the first time to remove and recover phosphorus (P) from waste activated sludge (WAS) via anaerobic digestion (AD). The P transformation pathway and the effect of CTS-Fe addition on the AD process were investigated using batch experiments. The P fractionation results indicate that non-apatite inorganic phosphorus (NAIP) reduction in the solid phase of sludge at 20 g/L of CTS-Fe addition (6.72 mg/g-SS) was 2.4 times higher than that in the control (2.77 mg/g-SS, no CTS-Fe addition). This is probably brought about by the added CTS-Fe enhanced the reduction of Fe(III)-P compounds in the sludge with phosphate released into the liquid phase. CTS-Fe can efficiently recover 95% of P from the liquid digestate of WAS. Notably, partial Fe(III) on the CTS-Fe was reduced and effectively combined with P to form vivianite crystals on the CTS-Fe surface during the AD process. Characterization analysis demonstrated that ligand exchange and chemical precipitation were the dominant mechanisms for P removal/recovery. Furthermore, the addition of CTS-Fe increased methane production by 11.9 - 32.2% under the tested conditions, likely attributable to the enhanced hydrolysis of WAS under CTS-Fe supplementation. As the P-loaded CTS-Fe particles can be easily separated and recovered from the AD system and further reutilized in agriculture, this study could provide a new approach for simultaneous P removal/recovery and enhanced methane production from AD of WAS.

Effect of groundwater ions (Ca2+, Na+, and HCO3-) on removal of hexavalent chromium by Fe(II)-phosphate mineral

A systematic study was conducted to investigate the effect of major groundwater ions (i.e., Ca²⁺, Na⁺, and HCO₃^-) on removal of hexavalent chromium (Cr(VI)) by an Fe(II)-phosphate mineral (i.e., vivianite). The batch experiments revealed that the second-order rate constant for Cr(VI) removal by vivianite with Ca²⁺ + CO₃^2- (0.076-1.90 mM) and Na⁺ + HCO₃^- (0.26-6.50 mM) was 1.5-5.2 times lower than that without these ions. The removal kinetics of Cr(VI) by vivianite was abruptly slowed down with the increased ion concentration, which showed their inhibitory effect on the reaction. The results of the geochemical modeling and density functional theory calculations showed that the presence of Ca²⁺ + HCO₃^- and Na⁺ + HCO₃^- can form less favorable Cr(VI) species (i.e., CaCrO_4(aq) and NaCrO₄^-) on the Fe-B site of vivianite surface, leading to the inhibitory effect observed in this study. Finally, the X-ray absorption spectroscopy results showed that reductive immobilization of Cr(VI) to Cr(III) occurred by structural Fe(II) oxidation of vivianite to amorphous mixed-valence Fe-phosphate via an inner-sphere complexation. The results suggest that the presence of Ca²⁺, Na⁺, and HCO₃^- in phosphorous-enriched iron-reducing environments may lower the remedial efficiency of Cr(VI) removal.

Enhanced Microbial Chromate Reduction Using Hydrogen and Methane as Joint Electron Donors

Hydrogen and methane commonly co-exist in aquifer. Either hydrogen or methane has been individually utilized as electron donor for bio-reducing chromate. However, little is known whether microbial chromate reduction would be suppressed or promoted when both hydrogen and methane are simultaneously supplied as joint electron donors. This study for the first time demonstrated microbial chromate reduction rate could be accelerated by both hydrogen and methane donating electrons. The maximum chromate reduction rate (4.70 ± 0.03 mg/L·d) with a volume ratio of hydrogen to methane at 1:1 was significantly higher than that with pure hydrogen (2.53 ± 0.02 mg/L·d) or pure methane (2.01 ± 0.02 mg/L·d) as the sole electron donor (p < 0.01). High-throughput 16S rRNA gene amplicon sequencing detected potential chromate reducers (e.g., Spirochaetaceae, Delftia and Azonexus) and hydrogenotrophic bacteria (e.g., Acetoanaerobium) and methane-metabolizing microorganisms (e.g., Methanobacterium), indicating that these microorganisms might play important roles on microbial chromate reduction using both hydrogen and methane as electron donors. Abundant hupL and mcrA genes responsible for hydrogen oxidation and methane conversion were harbored, together with chrA gene for chromate reduction. More abundant extracellular cytochrome c and intracellular NADH were detected with joint electron donors, suggesting more active electron transfers.

Synergistic effect of Cu loading on Fe sites of fly ash for enhanced catalytic reduction of nitrophenol

A novel Cu catalyst was developed using water-washed coal fly ash (WFA) as a support material for catalytic reduction of p-nitrophenol (p-NP) in the presence of NaBH₄. Cu/WFA showed ~ × 10⁵ times higher estimated rate constant k_obs-p-NP/C_Cu (L min^-1 g_Cu^-1) compared with Cu/SiO₂, Cu/Al₂O₃, and other Cu catalysts previously reported. Surprisingly, we obtained a significant lower price value (Price'/K) (0.027-0.068 USD/L min^-1) for Cu/WFA in comparison with other Cu catalysts and precious metallic catalysts (Pd, Au, Ag, and Pt). Various surface analyses and additional experiments using Fe/SiO₂, Cu/Fe₂O₃/SiO₂, and Cu/HCl-treated WFA demonstrated that Cu(0) nanoparticles were well loaded on the surface of WFA, where Fe elements were abundant, resulting in a dramatic enhancement of the Cu/WFA catalytic activity. Particularly, X-ray photoelectron spectroscopy revealed the abundance of Cu(0)/Fe(III) and Cu(0)/Fe(II) in the WFA surface. This indicates that Cu(0) was the main driving force for the activation of H_ad molecule, and that the reduction of Fe(III) to Fe(II) by NaBH₄ can accelerate the reduction of Cu(II) to Cu(0). Recycling and phytotoxicity tests showed that Cu/WFA can be applied as a reusable catalyst with low environmental impact, revealing the remarkable potential of non-precious metal/WFA catalyst in the field of environmental remediation.

Structure, dissolution, and plant uptake of ferrous/zinc phosphates

Development of slow release fertilizers by tuning dissolution kinetics can reduce the environmental impact of (micro) nutrients added to crops. Mixed metal compounds may have different dissolution kinetics and plant uptake than single metal compounds. In this study, mixed Fe(II)/Zn(II) phosphates (0-100 at% Zn) were prepared by aqueous precipitation and their structural characteristics and dissolution kinetics in a sand column were measured as model for divalent metal and phosphate release in soil. Three minerals were identified, namely vivianite (Fe₃(PO₄)₂·8H₂O) at 0-20 at% Zn, phosphophyllite (Zn₂Fe(PO₄)₂·4H₂O) at 20-79 at% Zn, and hopeite (Zn₃(PO₄)₂·4H₂O) at 79-100 at% Zn. The Fe-rich materials had high SSA of 42-64 m² g^-1, which decreased to ≤4 m² g^-1 for ≥79 at% Zn. The Fe K-edge and Zn K-edge XANES spectroscopy measurements show that the samples had comparable local structure and contained 13-72% of Fe as Fe(III) due to partial oxidation. In the sand column, Zn(II) and Fe(II) phosphates dissolved near-congruently at steady state (>7 h), whereas mixed Fe(II)/Zn(II) phosphates showed preferential release of Zn over P and Fe, likely due to reprecipitation of Fe. Pot experiments demonstrate that Zn from Fe(II)/Zn(II) phosphates is absorbed by bird's eye chili plants (C. annuum), in agreement with the preferential dissolution of Zn(II). These results may provide insight into the dissolution of other divalent metals, which not only aids in the growth of plants and resulting foodstuff but ultimately leads to reductions in environmental contamination.