Zero-valent Iron System Research Articles

Mechanisms of action of chlorella biochar-loaded nano zero-valent iron systems on Escherichia coli: Insights from cell biology and untargeted metabolomics

Surface modification can significantly improve nano zero-valent iron (nZVI) stability. However, different methods and materials significantly affect nZVI toxicity, and the effects on bacterial metabolic processes and corresponding molecular mechanisms require further study. In this study, Chlorella biochar-loaded nano zero-valent iron (nZVI/BC) was prepared, showing optimum removal of Microcystis aeruginosa (M. aeruginosa) at a dosing rate of 12.5 g L−1. The toxicity against Escherichia coli (E. coli) was investigated, revealing time-dependent inactivation efficiency, achieving −3.60 log at 30 min and −5.0 log for a single nZVI. Transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) analyses revealed that nZVI exhibited stronger adsorption to cells compared to nZVI/BC, with both nZVI and nZVI/BC primarily interacting with amine, carboxyl, and ester groups on the cell surface. Physiological, biochemical, ROS quenching, and metabolomics experiments indicated that ROS played a key role in the inactivation of E. coli. This study proposes a strategy to balance the stability of nZVI with its biocompatibility.

The role of magnetite for enhancing reduction of N-nitrosodimethylamine by zero-valent iron

Mixed-valent iron mineral has significant effects in redox reaction and engineering applications. Magnetite (Fe3O4) combined with zero-valent iron (ZVI) were chosen to reduce N-nitrosodimethylamine (NDMA). Enhancement had been shown in ZVI-Fe3O4 system than the sum of removals of ZVI and Fe3O4 systems. Lower solution pH value facilitated the removal of NDMA, the concentration of dissolved oxygen (DO) had little effect. The main reduction products of NDMA by ZVI-Fe3O4 were 1,1-dimethylhydrazine (UDMH) and dimethylamine (DMA). The total nitrogen mass was unbalanced. By capturing the active species in the ZVI-Fe3O4 system, active hydrogen atoms had been determined. Catalytic hydrogenation was proposed to be the mechanism. Electrochemical tests showed that ZVI-Fe3O4 had a lower open circuit potential and a higher corrosion current than ZVI, the presence of Fe3O4 promoted the corrosion of ZVI. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses of solid powders showed that the corrosion was more serious and no other substances were formed on the surface of powders in ZVI-Fe3O4 system after reaction. X-ray photoelectron spectroscopy (XPS) showed that the value of ratio of peak area of Fe(III) to Fe(II) of ZVI-Fe3O4 surfaces was lowest. The Fe(III) on the powders’ surface could be reduced by ZVI and the surface passivation of ZVI would be prevented, which would be benefit for the generation of active hydrogen atoms.

Sulfide mediated zero valent iron for enhanced nickel removal under neutral condition: Reaction kinetic, mechanism, and stability

The application of sodium sulfide (Na2S) as sulfidation reagent of zero valent iron (ZVI) has been well studied, however, the effect of Na2S existence on ZVI reactivity remains obscure. In this study, Ni(II) removal performance, mechanism, and stability by ZVI with Na2S were investigated at neutral condition. The results showed that Ni(II) removal sharply increased from < 5–100% by ZVI within 60 min with the increase of Na2S concentration, but the subsequent Ni(II) release occurred. In addition to the slight Ni(II) removal by Na2S in aqueous phase, Ni(II) was mainly removed by the adsorption and reduction by Na2S mediated ZVI system. Characterization analysis indicated that Na2S could lead to the severe Fe0 corrosion and the main corrosion product on ZVI was lepidocrocite, which played the dominant role for Ni(II) immobilization. Meanwhile, the formation of iron sulfides on ZVI surface would enhance Ni(II) reduction and partially transform the adsorbed Ni(II) to NiS. Moreover, the higher ionic strength could further enhance the initial Ni(II) removal rate but the stability became poor. Regardless of the additive methods, the Ni(II) removal significantly enhanced in the system of ZVI and Na2S combination. Overall, this work reveals the sulfide mediated Fe0 corrosion and provides a potential method for enhancing the reactivity of ZVI in the treatment of Ni(II) pollution.

Nucleophiles promotes the decomposition of electrophilic functional groups of tetracycline in ZVI/H2O2 system: Efficiency and mechanism

When zero-valent iron (ZVI) is prepared and applied under neutral conditions, it is easy to form oxides or hydroxides on its surface, which hinders the electron release of ZVI. To this end, a nucleophile was introduced into the ZVI system to inhibit the precipitation of iron ions, improve the conductivity of the solution, and promote the removal efficiency of electrophilic functional groups in organic compounds. In this study, the addition of nucleophiles such as ethylenediamine, methylamine and dimethylamine to the ZVI/H2O2 system resulted in an enhanced removal efficiency of tetracycline (TC) under neutral condition, while electrophiles such as EDTA-2Na and oxalic acid dihydrate impeded the removal of TC. Experimental results demonstrated that the presence of nucleophiles could effectively promote the release of iron ions and increase the proportion of ferrous in both aqueous solution and solid surface of ZVI. Experimental and theoretical calculation results revealed that the electrophilic functional group was eliminated in the TC molecule, and the toxicity of the treated solution was reduced significantly. Overall, this work provides a selection of the conditions and pollutants applicable to ZVI under neutral pH conditions.

Promoted iron corrosion and subsequent hexavalent chromium removal in zero-valent iron systems by oxidant activation

Zero-valent iron (ZVI), as an effective medium, is widely used to eliminate heavy metal ions in filter tanks. However, it will react with Cr(VI) to generate Fe–Cr precipitates with low conductivity on its surface, resulting in slow iron corrosion and low Cr(VI) removal efficiency. In this study, three oxidants (KMnO4, NaClO, and Na2S2O8) were employed to promote iron corrosion in ZVI systems for enhanced Cr(VI) removal at a concentration of 5 mg/L through batch tests and column experiments. The ZVI/KMnO4, ZVI/NaClO, and ZVI/Na2S2O8 systems achieved significantly higher Cr(VI) removal rates of 31.5%, 52.8%, and 65.9% than the ZVI system (9.8%). Solid phase characterization confirmed that these improvements were attributed to promoted iron corrosion and secondary mineral formation (e.g., lepidocrocite, ferrihydrite, and magnetite) by oxidants. Those minerals offered more reaction sites for Cr(VI) reduction, adsorption, and sequestration. Cycle experiments indicated that ZVI/oxidant systems could stably remove Cr(VI). In long-term column experiment, the ZVI/NaClO column showed a much longer life-span and exhibited a 34.8 times higher Cr(VI) removal capacity than that of the ZVI column. These findings demonstrated that ZVI in combination with a reasonable amount of oxidants was a promising method for removing Cr(VI) in practical filter tanks and provided a new insight to enhance Cr(VI) removal.

Degradation of 1,2,3-trichloropropane in aqueous solution by ZVI-enhanced Fenton system: Performances and mechanisms

In this research, zero-valent iron (ZVI) was chosen as a reductant in the classic Fenton system to accelerate the generation of Fe(II). The removal of 1,2,3-trichloropropane (TCP) was only 65.3% in classic Fenton system, while with the addition of ZVI its removal could be enhanced to 95.4% in 120 min. Hydroxyl radicals (HO•) was confirmed to be the major radical during TCP degradation by electron paramagnetic resonance (EPR) detection and radical quenching tests. Moreover, the mechanism of TCP degradation in H2O2/Fe(II)/ZVI system was proposed. In the presence of ZVI, Fe(III) was reduced to Fe(II), further reacted with H2O2 and produced more HO• for TCP removal. 2,3-Dichloro-1-propene and 2-chloro-2-propen-1-ol were the main intermediates in TCP degradation and two possible pathways were proposed. Finally, TCP removal in different solution matrixes and for other chlorinated contaminants removal were tested, revealing the broad-spectrum reactivity and excellent performance of ZVI-enhanced Fenton system in the remediation of chlorinated organic compounds.

Effective immobilization of Cd(II) in soil by biotic zero-valent iron and coexisting sulfate

In this work, zero-valent iron (ZVI) combined with anaerobic bacteria was used in the remediation of Cd(II)-polluted soil under the mediation of sulfate (SO42−). Owing to hydrogen-autotrophic sulfate reduction, serious corrosion occurred on sulfate-mediated biotic ZVI in terms of solid phase characterization as massive corrosive products (e.g., goethite, magnetite and green rust) were formed, which were crucial in the immobilization of Cd(II). Thus, this integrated system achieved a 4.9-fold increase in aqueous Cd(II) removal and converted more than 53% of easily available Cd(II)-fractions (acid-extractable and reducible) to stable forms (oxidizable and residual) based on the sequential extraction results as compared to the sulfate-mediated ZVI system. Increasing SO42− concentration and ZVI dosage both demonstrated positive correlation to Cd(II) immobilization, which further reflected that hydrogenotrophic desulfuration acted an essential role in improving Cd(II) immobilization. It indicated that hydrogenotrophic desulfuration could accelerate iron corrosion and promote reactive mineral formation through biomineralization, as well as generate cadmium sulfide precipitates (CdS) to achieve excellent immobilization performance for Cd(II). Besides, this reaction was favorable under neutral pH condition. Our results highlighted the promoted effect of hydrogen-autotrophic desulfuration on ZVI corrosion to immobilize Cd(II) and offered a practicable technique in Cd(II)-polluted soil remediation.

Enhancement of cottonseed oil refining wastewater treatment by zero valent iron under sunlight irradiation and O2 bubbling

This study is the first to apply a zero-valent iron (ZVI) system in the treatment of cottonseed oil (CTO) refining wastewater. The results indicated that the ZVI system can effectively degrade and mineralize CTO in the wastewater, whereas sunlight irradiation and O2 bubbling can considerably enhance CTO degradation, removing 93.5% of CTO and 69.0% of chemical oxygen demand within 180 min. In addition, a low concentration (0.1 mM) of SO42- and Cl− in the wastewater improved CTO degradation, whereas a high concentration (>1 mM) of these anions considerably inhibited the degradation process. However, NO3- at all concentrations hindered CTO degradation. Furthermore, OH and O2- were the main active species for CTO degradation in the ZVI system under dark conditions. However, in addition to these two species, photogenerated hole (h+) played a key role in CTO degradation under sunlight irradiation. This observation might be derived from the photocatalytic effect due to photoexcitation of the iron corrosion product, γ-FeOOH. Our findings show that the ZVI system assisted by sunlight irradiation and O2 bubbling is feasible for CTO-refining wastewater treatment and can guide the real wastewater treatment project.

Enhanced Cr (VI) removal with Pb (II) presence by Fe2+-activated persulfate and zero-valent iron system

ABSTRACT Combined heavy metals such as chromium (Cr (VI)) and lead (Pb (II)) in natural water have globally posed severe environmental and public health risk. Here the removal of Cr (VI) and Pb (II) mixed pollutants using Fe2+-activated persulfate (PS) with extra zero-valent iron (ZVI), which was not only a supplementary Fe2+ source, but also a high-efficiency absorbent, was investigated. During removal, pivotal factors of initial pollutant concentration, dosages of ZVI and PS, initial pH and temperatures were examined. Interestingly, generating a lot of H+ in the process of Fe (II) activating persulfate were helpful to the corrosion of ZVI over a large range of pH (1–9). Under the optimum condition, removal efficiency of Pb (II) and Cr (VI) have reached 100% and 94.26% respectively. The removal mechanism was suggested as a three-step reaction that the Pb (II) boosted the removal of Cr (VI) by co-precipitated in wastewater, and the Pb (II) and Cr (VI) were adsorbed and subsequently reduced to Pb0 and Cr3+ as Cr(OH)3 or Cr3+-Fe3+ hydroxides on ZVI surface. Cr (VI) and Pb (II) adsorption kinetics agreed with the pseudo-second-order reaction rate expression. In addition, we were surprised to found that the contribution effect of chromium and lead co-precipitation for their removal by Fe (II) – PS-ZVI has strong dependence on initial pH and concentration ratio of Cr (VI) and Pb (II). The result indicated that Fe (II)-PS-ZVI system should be a favourable removal technology for Cr (VI) and Pb (II).

Catalytic hydrolysis: A novel role of zero-valent iron in haloacetonitrile degradation and transformation in unbuffered systems

Efforts to remove highly toxic haloacetonitriles (HANs) is an important step to reduce health risks associated with disinfection by product exposure. Zero valent iron (ZVI) is a versatile material, whose reductant, sorbent and coagulant role has been well understood. However, their catalytic role is less known. In this study, the degradation and transformation of HANs in ZVI system were investigated. Significant decreases of the four HANs in ZVI system were observed, and haloacetamides and haloacetic acids (hydrolysis products of HANs) were the dominant transformation products of HANs. However dehalogenated HANs, Fe (II) and Fe (III) were rarely detected after reaction, indicating that the ZVI acted as a catalyst to promote the hydrolysis of HANs, rather than other previously reported causes (dehalogenation or redox reaction). The HAN degradation rates were dramatically affected by the initial pH, ZVI doses and initial HAN concentration. Kinetic analysis indicated that HAN removal was enhanced with the increase of initial pH (5–9), ZVI doses (1–10 g/L), and initial HAN concentration (25–200 μg/L). ZVI induced the transformation of HANs to haloacetamides, haloacetic acids and other de-halogenated compounds, which reduced the cytotoxicity and genotoxicity by 88% and 85%, respectively. This study helped to understand the fate of HAN during the transmission in cast iron pipes, and provided a theoretical foundation for future HAN control and monitoring efforts.

The capacity and mechanisms of various oxidants on regulating the redox function of ZVI

It is well known that zero-valent iron (ZVI) could catalyze the oxidation of various oxidants to realize the rapid oxidation removal of pollutants. However, in this study, we found that the addition of different oxidants could regulate the redox function of ZVI system. In three different co-treatment systems, the effects of different oxidizers (peroxymonosulfate (PMS), persulfate (PDS), hydrogen peroxide (H2O2)) dosages on the ratios of oxidative degradation rate and reductive degradation rate of p-nitrophenol (PNP) were studied. The effect of the H+ released from oxidizers and the generated reactive oxygen species (ROS) in ZVI/PMS, ZVI/PDS, ZVI/H2O2 systems were detailed discussed. Especially, the contribution of generated ROS for reductive degradation of PNP was quantified in the ZVI/H2O2 system. Based on the results of TOC removal, UV–vis absorption spectra, and intermediates concentration curves, it was found that the degradation of PNP changed from reduction to oxidation with the increase of oxidant proportion. When the molar ratio of ZVI to oxidizer equal to 100, PNP was mainly degraded by reduction accompanied by slight oxidation. Combined with the results of SEM-EDS and XPS, it was confirmed that the enhanced degradation of PNP under the addition of oxidant was mainly related to the generated ROS, the additional H+, and the corrosion products of ZVI.

Total to use UOP technology to produce renewable jet fuel and diesel at French biorefinery

The treatment of aqueous solutions containing arsenic has been studied using TiO2, zero valent iron (ZVI), and a combined system (TiO2 + ZVI) under UV-irradiation. The reactions were carried out for 10 mg L−1 of As(III) at two initial pH values (9 and 3). The experimental results evidenced the significant influence of the pH of the aqueous solution on both the oxidation rate of As(III) and the effectiveness for overall As uptake. The kinetics of As(III) oxidation were enhanced at pH 3, especially in the ZVI system. Acid pH was also found to promote the uptake of produced As(V) when metallic iron was used, which was explained by the adsorption of arsenate on the iron oxides/hydroxides generated by the ZVI corrosion. Compared to ZVI, the rate of As(III) oxidation was always higher when TiO2 was present, achieving a complete depletion of arsenite concentration. However, TiO2 showed a limited capacity for As(V) adsorption, which was mostly released to the aqueous solution. With both ZVI and the combined system (TiO2 + ZVI) final As concentrations of 5 and 3 μg L−1 were respectively attained, values below the WHO recommended limit (10 μg L−1) for arsenic in drinking water.

Coupled Effect of Sulfidation and Ferrous Dosing on Selenate Removal by Zerovalent Iron Under Aerobic Conditions.

Both the reactivity and the removal capacity of zerovalent iron (ZVI) for the target contaminant are important for applying ZVI in wastewater treatment. In this study, the feasibility of combining sulfidation treatment and Fe2+ dosing (S-ZVI/Fe2+) to enhance the performance of ZVI for Se(VI) removal was comprehensively investigated under aerobic conditions. Se(VI) was first adsorbed on the surface of ZVI particles and then reduced to Se(IV) and Se(0) with Se(0) being the final product in S-ZVI/Fe2+ system. This system bore the advantages of both sulfidation treatment (S-ZVI) and Fe2+ dosing (ZVI/Fe2+) for Se(VI) removal. The amounts and rate constants of Se(VI) removal in S-ZVI/Fe2+ system were increased by 1.8-32.8 times and 11.7-194.0 times, respectively, compared to those in pristine ZVI system. Sulfidation significantly accelerated the corrosion of Fe0 thus improved the removal rate of Se(VI). The promoting effect of Fe2+ on Se(VI) sequestration by S-ZVI should be mainly associated with the following facts: Fe2+ could maintain a relatively low pH level during Se(VI) removal by S-ZVI; Compared to S-ZVI alone, the consumption of Fe0 in S-ZVI/Fe2+ by O2/H+ was slower, and thus the electron efficiency of S-ZVI was elevated; Fe2+ dosing facilitated electron transfer by forming semiconductive Fe3O4.

Effect of O2, Ni0 coatings, and iron oxide phases on pentachlorophenol dechlorination by zero-valent iron.

This study explores the zero-valent iron (ZVI) dechlorination of pentachlorophenol (PCP) and its dependence on the dissolved oxygen (O2), presence/formation of iron oxides, and presence of nickel metal on the ZVI surface. Compared to the anoxic system, PCP dechlorination was slower in the presence of O2, which is a potential competitive electron acceptor. Despite O2 presence, Ni0 deposited on the ZVI surfaces catalyzed the hydrogenation reactions and enhanced the PCP dechlorination by Ni-coated ZVI bimetal (Nic/Fe). The presence of O2 led to the formation of passivating oxides (maghemite, hematite, lepidocrocite, ferrihydrite) on the ZVI and Nic/Fe bimetallic surfaces. These passive oxides resulted in greater PCP incorporation (sorption, co-precipitation, and/or physical entrapment with the oxides) and decreased PCP dechlorination in the oxic systems compared to the anoxic systems. As received ZVI comprised of a wustite film, and in the presence of O2, only ≈ 17% PCP dechlorination observed after 25days of exposure with tetrachlorophenol being detected as the end product. Wustite remained as the predominant oxide on as received ZVI during the 25days of reaction with PCP under oxic and anoxic conditions. ZVI acid-pretreatment resulted in the replacement of wustite with magnetite and enhanced PCP degradation (e.g. ≈ 52% of the initial PCP dechlorinated after 25days under oxic condition) with accumulation of mixtures of tetra-, tri-, and dichlorophenols. When the acid-washed ZVI was rinsed in NiSO4/H2SO4 solution, Ni0 deposited on the ZVI surface and all the wustite were replaced with magnetite. After 25days of exposure to the Nic/Fe, ≈ 78% and 97% PCP dechlorination occurred under oxic and anoxic conditions, respectively, producing predominantly phenol. Wustite and magnetite are respectively electrically insulating and conducting oxides and influenced the dechlorination and H2 production. In conclusion, this study clearly demonstrates that the dissolved oxygen present in the aqueous solution decreases the PCP dechlorination and increases the PCP incorporation when using ZVI and Nic/Fe bimetallic systems. The findings provide novel insights towards deciphering and optimizing the performance of complex ZVI and bimetallic systems for PCP dechlorination in the presence of O2.

Selenate removal by Fe0 coupled with ferrous iron, hydrogen peroxide, sulfidation, and weak magnetic field: A comparative study

Dosing ferrous ions (ZVI/Fe2+), combining with oxidants (e.g., H2O2) (ZVI/H2O2), sulfidation treatment (S-ZVI), and introducing a weak magnetic field (ZVI/WMF) have been widely used to enhance the performance of zerovalent iron (ZVI) for reductive removal of contaminants. Taking Se(VI) as a probe contaminant, this study systematically compared the performances of different ZVI systems (i.e., ZVI/Fe2+, ZVI/H2O2, S-ZVI, and ZVI/WMF) for contaminant removal. All the four tested methods could greatly improve the performance of ZVI for Se(VI) removal. Se(VI) was removed by S-ZVI at S/Fe molar ratio of 0.05 with a much greater rate constant than other enhanced-ZVI technologies while the maximum amount of Se(VI) removal was obtained in ZVI/Fe2+ system with Fe2+ applied at 0.5 mM among the four tested enhanced-ZVI technologies at initial pH 6.0. In addition, Se(VI) removal by ZVI/Fe2+ was least influenced by initial pH compared to the other tested enhanced-ZVI systems, implying its good adaptability of pH. The application of these tested methods could significantly increase the electron efficiency from ∼0.5% to 4.06–8.72% and Fe2+ application was much more efficient in enhancing the electron efficiency than the other three methods. Finally, the perspective of these enhanced-ZVI technologies was compared in terms of their reactivity, selectivity, chemical cost, and pH adaptability and some suggestions for their possible application were provided.

Zero-valent iron (ZVI) Activation of Persulfate (PS) for Degradation of Para-Chloronitrobenzene in Soil.

Para-chloronitrobenzene (p-CNB) in soil has posed significant health risks because of its persistence and high toxicity. The efficacy of catalyzed Zero-Valent Iron (ZVI), activated persulfate, and ZVI-persulfate processes for the degradation of p-CNB in soil was investigated. The p-CNB removal rate significantly increased from 10.8 to 90.1% with increased ZVI dosage from 0.1mmolg-1 to 1.0mmolg-1. The p-CNB removal increased with the decrease of initial pH and a removal efficiency of 85.3% was obtained at an initial pH value of 6.8 in combined system. The p-CNB removal rate in the single persulfate system and ZVI system was 36.5% and 60.2%, while the ZVI-persulfate system showed more sufficient p-CNB removal capacity and the removal rate of p-CNB was 88.7%. Scanning electron microscopy (SEM) and Electron paramagnetic resonance (EPR) was adopted in order to explore the degradation mechanism by ZVI-Persulfate system in soil.

Enhanced azo dye removal from wastewater by coupling sulfidated zero-valent iron with a chelator

Recent research on zero-valent iron (ZVI) has evolved from a proof-of-concept stage to an application-focus stage with the goal of enhancing the applicability of ZVI for water decontamination. Sulfidation represents one of the most prominent ways to enhance the reactivity and efficiency of ZVI. In this study, sulfidated ZVI (S-ZVI) coupled with ethylenediamine tetraacetic acid (EDTA) was used to remove acid red 73 (AR 73) from wastewater and the results showed that the coupling of chelator and sulfidation significantly enhanced the removal of AR 73. The first-order kinetic constant (kobs) followed a bimodal pattern with increasing S/Fe ratio, chelator concentration, and mixing rate, but a unimodal pattern with changes in initial AR 73 concentration. Under the optimal conditions, the removal rate of AR 73 obtained in the EDTA + S-ZVI system was about 6.5 times faster than that in the ZVI system. Further analysis by gas chromatography-mass spectrometry (GC-MS) confirmed that reduction played a major role in the removal of AR 73. X-ray photoelectron spectroscopy (XPS) energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR) were used to investigate the surface chemical compositions of the solids before and after reaction. The results showed that the surface of freshly prepared S-ZVI was covered with FeSx, and EDTA on the surface of S-ZVI could complex with the Fe(II) to form an FeII-EDTA complex. The FeII-EDTA complex can inhibit the oxidation of Fe(II) and reduce the redox potential of the solution, which together enhance the removal of AR 73.

Enhanced chromium(VI) removal by zero-valent iron in the presence of anions and a weak magnetic field: Batch and column tests

In this study, batch and column tests were conducted to evaluate the influence of selected anions (ClO4−, NO3−, Cl− and SO42−) on Cr(VI) removal by zero-valent iron (ZVI) with and without a weak magnetic field (WMF). Batch kinetic experiments demonstrated a synergetic enhancing effect of WMF and each anion on Cr(VI) removal. In addition, the ratio of the Cr(VI) removal rate constant for each tested anion to that of deionized water (i.e., the promotion factor of the combined effect) exhibited a hyperbolic increase as a function of anion concentration. Based on the magnitudes of their enhancing effects, the anions were ranked from most to least enhancing: SO42− > Cl− > ClO4− > NO3−. The synergetic effect of WMF and anions was also obvious in a continuous flow system, which could both dramatically extend the lifespan of ZVI systems and improve the efficiency of Cr(VI) removal. The influence of WMF and the tested anions on ZVI corrosion products was investigated by scanning electron microscopy and X-ray diffraction. These findings could encourage the application of ZVI/WMF technology in the field.

Dissolution of realgar by Acidithiobacillus ferrooxidans in the presence and absence of zerovalent iron: Implications for remediation of iron-deficient realgar tailings

Realgar (As4S4)-rich tailings are iron-deficient arsenical mine wastes. The mechanisms and products of the dissolution of realgar by Acidithiobacillus ferrooxidans (A. ferrooxidans) in the presence (0.2 g and 2 g) and absence of zerovalent iron (ZVI) are investigated for three stages (each of 7 d with fresh A. ferrooxidans medium addition between the stages). SEM-EDX, FTIR, XPS and selective extraction analysis are used to characterize the solid-phase during the experiments. ZVI addition causes the systems to become more acid-generating, although pH increases are observed in the first day due to ZVI dissolution. Arsenic is released to solution due to realgar oxidation (∼30 mg L−1 in the 0 g ZVI system in Stage I), but low concentrations are observed in the ZVI-added systems (<5 mg L−1) and in Stages II and III of the 0 g ZVI system. As(III) dominates the released As(T) at day 1 (83–89% of As(T)), but is largely oxidized to As(V) at day 7 of each stage (53–98% of As(T)). Arsenic attenuation is attributed to the formation of mixed As-Fe oxyhydroxides and oxyhydroxy sulfates that take up released arsenic and are abundant in the 2.0 g ZVI system, and to passivation of the realgar surface. Consequently, a new strategy that combines A. ferrooxidans and exogenous ZVI addition for treating in-situ iron-deficient realgar-rich tailings is proposed, although its long-term effects need to be monitored.

Selenite removal from groundwater by zero-valent iron (ZVI) in combination with oxidants

We recently demonstrated common oxidants simply coupled with ZVI to continuously drive the accelerated Fe0 corrosion and hence achieve fast and very efficient removal of heavy metals and metalloids from groundwater. In this study, we aimed first to answer a basic question of the oxidant dosage theoretically required to sequester a certain amount of selenite. The specific ratio of oxidant dosage to Se(IV) removal, which reflected the theoretically minimal oxidant dosage required to sequester one mole of Se(IV), was almost independent of the initial Se(IV) concentration but significantly affected by the difference in oxidant species. To sequester one mole of Se(IV), the minimum dosage of the required oxidant was calculated to be 3.94–4.09 for NaClO, 3.90–4.33 for H2O2, and 3.29–3.54 for KMnO4, respectively. Simultaneous aeration increased the removal efficiency of Se(IV) and substantially reduced the required dosage of oxidants. To form a strong contrast with very limited Se(IV) removal by ZVI alone, the coupling of NaClO and H2O2 into the ZVI system remarkably enhanced the performance of Se(IV) removal during the long term fixed-bed experiments with a real groundwater background. The ZVI columns coupled by NaClO and H2O2 steadily treated 10,000 volumes (BV) of real groundwater. X-ray adsorption near edge structure (XANES) demonstrated that more than 85% of the selenium was reduced to Se0 and Se2−, with Se0 as the dominant selenium species sequestered in the solid phase. Synchrotron-based scanning transmission soft X-ray microscopy (STXM) explicitly revealed the ubiquitous spatial distribution of selenium in the corrosion products and hence demonstrated a high accessibility of these corrosion products to selenite.