Ferrous sulfide nanoparticles can be biosynthesized by sulfate-reducing bacteria: Synthesis, characterization and removal of heavy metals from acid mine drainage

Ferrous sulfide nanoparticles (nFeS) have proven to be effective in removing heavy metals (HMs) from wastewater. One such approach, which has garnered much attention as a sustainable technology, is via the in situ microbial synthesis of nFeS. Here, a sulfate-reducing bacteria (SRB) strain, Geobacter sulfurreducens, was used to initially biosynthesize ferrous sulfide nanoparticles (SRB-nFeS) and thereafter remove HMs from acid mine drainage (AMD). SRB-nFeS was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) coupled to an energy dispersive spectrometer (EDS), three-dimensional excitation-emission matrix (3D-EEM) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Such characterization showed that SRB mediated the reduction of SO42- to S2- to form nFeS, where the metabolized substances functioned as complexing agents which coordinated with nFeS to form biofunctional SRB-nFeS with improved stability. One advantage of this synthetic route was that the attachment of nFeS to the bacterial surface protected SRB cells from HM toxicity. Furthermore, due to a synergistic effect between nFeS and SRB, HM removal from both solution and AMD by SRB-nFeS was enhanced relative to the constituent components. Thus, after 5 consecutive cycles of HM removal, SRB-nFeS removed, Pb(Ⅱ) (92.6%), Cd(Ⅱ) (78.7%), Cu(Ⅱ) (76.0%), Ni(Ⅱ) (62.5%), Mn(Ⅱ) (62.2%), and Zn(Ⅱ) (88.5%) from AMD This study thus provides new insights into the biosynthesis of SRB-nFeS and its subsequent practical application in the removal of HMs from AMD.

Autotrophic denitrification based on sulfur-iron minerals: advanced wastewater treatment technology with simultaneous nitrogen and phosphorus removal

Autotrophic denitrification technology has many advantages, including no external carbon source addition, low sludge production, high operating cost efficiency, prevention of secondary sewage pollution, and stable treatment efficiency. At present, the main research on autotrophic denitrification electron donors mainly includes sulfur, iron, and hydrogen. In these autotrophic denitrification systems, pyrite has received attention due to its advantages of easy availability of raw materials, low cost, and pH stability. When pyrite is used as a substrate for autotropic denitrification, sulfide (S2-) and ferrous ion (Fe2+) in the substrate will provide electrons to convert nitrate (NO3-) in sewage first to nitrite (NO2-), then to nitrogen (N2), and finally to discharge the system. At the same time, sulfide (S2-) loses electrons to sulfate (SO42-) and ferrous ion (Fe2+) loses electrons to ferric iron (Fe3+). Phosphates (PO43-) in wastewater are chemically combined with ferric iron (Fe3+) to form ferric phosphate (FePO4) precipitate. This paper aims to provide a detailed and comprehensive overview of the dynamic changes of nitrogen (N), phosphorus (P), and other substances in the process of sulfur autotrophic denitrification using iron sulfide, and to summarize the factors that affect wastewater treatment in the system. This work will provide a relevant research direction and theoretical basis for the field of sulfur autotrophic denitrification, especially for the related experiments of the reaction conversion of various substances in the system.

FeS as excellent co-activator driving nano calcium peroxide oxidation for contaminants degradation: Performance and mechanisms

In this study, ferrous sulfide (FeS) was introduced to nano calcium peroxide (nCP)/Fe(III) system to facilitate the generation of Fe(II), more than 90% of naphthalene (NAP) could be removed at a wide pH range of 3-9. As a heterogeneous reductant, FeS could mitigate competitive reactions with reactive oxygen species (ROS), which favored the NAP degradation. As evidenced by scavenging experiments, HO• was the major ROS contributing to NAP degradation. The role of sulfur species (S2-, SO32-, and S2O32-) in nCP/Fe(III) system was investigated with S2O32- showing the preferable reactivity in Fe(III) reduction. In addition, the surface-bound HO• and surface Fe(II) were detected and the role of them on NAP degradation was revealed and concluded that both dissolved and surface Fe(II) contributed to NAP degradation, whereas surface-bound HO• was not superior to solution HO• in degrading NAP. Furthermore, nCP/Fe(III)/FeS system showed high feasibility to different solution matrixes and various types of water as well as the broad-spectrum reactivity to other toxic organic pollutants, exhibiting promise for practical application to remediate complex contaminants.

A novel hybrid ferrous sulfide impregnated anion exchanger for trace removal of hexavalent chromium from contaminated water

To effectively remove trace concentration of potential cancer-causing Cr(VI) from impaired drinking water, a novel hybrid material was synthesized through an in-situ synthesis process by dispersing ferrous sulfide nanoparticles within an anion exchanger. Characterization studies revealed that the hybrid material, named hybrid ferrous sulfide impregnated anion exchanger (HISIIX), contained uniformly distributed ferrous sulfide nanoparticles of size 10-40 nm within the anion exchanger host. Apart from FeS₂ nanoparticles, it also included nanoparticles of FeO and FeOOH. The incorporation of ferrous sulfide nanoparticles within the anion exchanger contributed to the significant differences in the Cr(VI) uptake capacity of HISIIX. Validation studies using fixed-bed column proved that HISIIX had significantly high Cr(VI) uptake capacity and was able to run for 4200 bed volumes (BVs) before a breakthrough of 50 μg L^-1 when subjected to a synthetic aqueous solution containing 200 μg L^-1 Cr(VI). Cr(VI) uptake capacity of the parent anion exchanger and HISIIX were determined to be 1.39 mg g^-1 and 3.44 mg g^-1, respectively, when the columns were allowed to run until exhaustion. Ferrous sulfide nanoparticles acted as a reducing agent transforming Cr(VI) anions into Cr(III) precipitates. It also produced sites for further removal of Cr(VI) anions through ligand sorption upon oxidation. The anion exchanger substrate attracted anions selectively via the Donnan membrane principle, resulting in a synergy of three different processes - ion exchange, redox reaction, and ligand sorption that gave the HISIIX a high capacity for the selective Cr(VI) removal from contaminated water.

Efficient removal of aqueous Cr(VI) with ferrous sulfide/N-doped biochar composites: Facile, in-situ preparation and Cr(VI) uptake performance and mechanism

FeS nanoparticles loaded on nitrogen-doped biochar (FeS/BNC) were fabricated by pyrolyzing coffee husks pretreated with Mohr's salt. The nitrogen doping and FeS loading of biochar are simultaneously achieved in one-pot pyrolysis. The elemental analysis, SEM, TEM, XRD, XPS, Raman, FTIR and N₂ adsorption-desorption technologies were used to characterize the composition and structure of FeS/NBC. The appraisement for removing aqueous Cr(VI) testified that FeS/NBC offered a synergistic scavenging effect of Cr(VI) by FeS and NBC. The effect of crucial experimental conditions (FeS/NBC dosage, foreign ions, initial pH and concentration of Cr(VI) solution) were investigated. The Cr(VI) removal capacity was as high as 211.3 ± 26 mg g^-1 under the optimized condition. The practicability of FeS/NBC was examined by using simulated actual samples from tap water and lake water. The mechanism examination showed that surface adsorption/reduction and solution reduction were implicated in the removal of Cr(VI). The current work introduces a novel FeS/NBC composite prepared by an in situ pyrolysis method with excellent potential for chromium pollution remediation.

Carboxymethyl cellulose stabilized ferrous sulfide@extracellular polymeric substance for Cr(VI) removal: Characterization, performance, and mechanism

Iron-based materials, especially ferrous sulfide (FeS), effectively remediate chromium pollution. However, the agglomeration of FeS reduces its reactivity to chromium. Herein, carboxymethyl cellulose stabilized ferrous sulfide@extracellular polymeric substance (CMC-FeS@EPS) was developed to remove hexavalent chromium (Cr(VI)) from water. CMC-FeS@EPS (98.00%) exhibited excellent removal efficiency of 40 mg/L Cr(VI) than those of FeS (57.35%) and CMC-FeS (68.60%). CMC-FeS@EPS showed good removal efficiency of Cr(VI) in wide pH range (from 4 to 9) and the co-existence of ions. FTIR and XPS results demonstrated that EPS functional group accelerated the process of adsorption and precipitation. Electrochemical results showed that CMC-FeS@EPS transferred electrons to Cr(VI) faster than CMC-FeS. In total, this study started from a new idea of using EPS to improve the performance of CMC-FeS, and provided a simple and effective way to remediate chromium pollution without secondary pollution.

Enhanced desorption of cationic and anionic metals/metalloids from co-contaminated soil by tetrapolyphosphate washing and followed by ferrous sulfide treatment

In this study, a novel approach was employed for the remediation of cationic and anionic metals/metalloids co-contaminated soil by tetrapolyphosphate enhanced soil washing coupled with ferrous sulfide treatment. Tetrapolyphosphate could simultaneously enhance the desorption of cationic metals (Pb and Zn) and anionic metal/metalloid (Cr and As) from the contaminated soil in the whole tested pH range of 2-10. With addition of 0.15 mol/L tetrapolyphosphate at pH 7.0, the removal ratio of Pb, Zn, As and Cr could achieve 83.1%, 70.4%, 75.7% and 66.4% respectively. The fractionation analysis of heavy metals/metalloids demonstrated the release of exchangeable and Fe/Mn bound forms contributed to most desorption of Pb and Zn. The decreases of non-specifically sorbed form and amorphous and poorly-crystalline hydrous oxides of Fe and Al bound form were responsible for most removal of As. The comparison with other common washing agents (EDTA, oxalate and phosphate) under their respective optimal dosage could confirm that tetrapolyphosphate was superior to simultaneously desorb the cationic and anionic metals/metalloids with higher efficiency. After 12 h, applying 150 mg/L FeS at pH 3.5 could totally remove Pb, Zn, As and Cr from the washing effluent by sulfide precipitation, reduction and adsorption processes. Higher pH would inhibit the removal of As and Cr by FeS. Meanwhile, the residual of tetrapolyphosphate could be totally recovered from the washing effluent by employing anion exchange resin. This study suggests tetrapolyphosphate enhanced soil washing coupled with ferrous sulfide treatment is a promising approach for remediation of cationic and anionic metals/metalloids co-contaminated soil in view of its high efficiency and simple operation.

A collaborative strategy for enhanced anaerobic co-digestion of food waste and waste activated sludge by using zero valent iron and ferrous sulfide

The application of sulfidated zero-valent iron as an alternative used in coupled anaerobic systems to improve methane production is usually restricted by its high production costs and toxic gasses and wastewater generation. In this study, a collaborative strategy for coupling zero-valent iron (ZVI) and ferrous sulfide (FeS) together into anaerobic systems was used to evaluate the enhancement of methanogenesis during the co-digestion of food waste and waste activated sludge, with the microbial evolution and metabolic pathway revealed. Results showed that the enhanced hydrolysis and acidogenesis process of co-digestion in this coupled anaerobic system could be attributed to synergistic interactions among ZVI, FeS, and microorganisms. Furthermore, both acetoclastic and hydrogenotrophic pathways could be promoted by coupling ZVI and FeS. This study demonstrated that coupling ZVI and FeS together into anaerobic systems would be a promising method for improving the methanogenic performance for municipal solid waste treatment.

Visible light promoted the removal of tetrabromobisphenol A from water by humic acid-FeS colloid

Ferrous sulfide (FeS) and humic acid (HA) are typical black substances in black bloom water. Based on the strong reduction ability of FeS and the photosensitivity of HA, the transformation of toxic organic pollutants by the combination of FeS and HA (HA-FeS) is not clear. In order to explore this issue, the stability of HA-FeS was analyzed by measuring the hydrodynamic diameter and zeta potential of HA-FeS, and then the removal mechanism and possible degradation pathway of tetrabromobisphenol A (TBBPA) by HA-FeS under continuous illumination were discussed. The results showed that the hydrodynamic diameter of FeS was reduced and the stability of FeS was improved, and it was easily suspended after FeS combined with the HA in the water. The combination of HA and FeS promoted the removal of TBBPA in water, no matter it was in the presence or absence of light. Besides, compared with the absence of light, the removal efficiency of TBBPA was improved by HA-FeS with continuous light. There were two reasons for the increase in the removal efficiency of TBBPA by HA-FeS. On the one hand, Fe²⁺ and S^2- of HA-FeS had more stable chemical valence and obtained better reducibility under continuous light than that in the dark. On the other hand, light induced the release of active species (O₂^-, ¹O₂, and OH) in the HA-FeS composite colloid and further promoted the degradation of organic pollutants. Therefore, the black substances (FeS) of black blooms may play a beneficial role in the removal of pollutants under sunlight.

Synergy between autotrophic denitrification and Anammox driven by FeS in a fluidized bed bioreactor for advanced nitrogen removal

On the basis of the metabolic synergy between autotrophic denitrification (AuDen) and anaerobic ammonium oxidation (Anammox), the feasibility of a novel ferrous sulfide (FeS)-driven AuDen and Anammox coupled system (FS-DADAS) was investigated. The nitrogen removal performance of FS-DADAS was investigated in a lab-scale fluidized bed bioreactor fed with synthetic wastewater containing NH₄⁺-N and NO₃^--N. The results of long-term operation (120 days) demonstrated the promising performance of the system with 100% NO₃^--N removal and NH₄⁺-N concentrations lower than 8.11 mg L^-1 in the effluent at a nitrogen loading rate of 0.20 g-N·(L·d)^-1. Sufficient NO₂^--N was provided by the AuDen for Anammox where a high removal rate of total nitrogen (TN) was achieved. The contribution of Anammox to TN removal was at >80%. The reactor could maintain a stable pH with less SO₄^2- production owing to the fact that Fe(II) and S acted as electron donors. FeS gradually transformed into a sheet-like secondary mineral, FeOOH. AuDen (Thiobacillus) and Anammox bacteria (Candidatus Kuenenia) were successfully retained in the bioreactor, with relative abundance values of 18.82%-23.64% and 3.52%-8.67%, respectively. FS-DADAS is a promising technology for the complete removal of TN from wastewaters with low C/N ratios at low energy consumption.

Preparation of a novel iron-based biochar composite for removal of hexavalent chromium in water

The chitosan-stabilized ferrous sulfide nanoparticles were loaded on biochar to prepare a composite material FeS-CS-BC for effective removal of hexavalent chromium in water. BC and FeS-CS-BC were characterized by Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. Batch experiments were employed to evaluate the Cr(VI) removal performance. The experimental results showed that the removal rate of Cr(VI) by FeS-CS-BC(FeS:CS:BC = 2:2:1) reached 98.34%, which was significantly higher than that of BC (44.58%) and FeS (79.91%). In the pH range of 2-10, the removal of Cr(VI) by FeS-CS-BC was almost independent of pH. The limitation of coexisting anions (Cl^-、SO₄^2-、NO₃^-) on Cr(VI) removal was not too obvious. The removal of Cr(VI) by FeS-CS-BC was fitted with the pseudo-second-order dynamics, which was a hybrid chemical-adsorption reaction. The X-ray photoelectron spectroscopy (XPS) analysis result showed that Cr(VI) was reduced, and the reduced Cr(VI) was fixed on the surface of the material in the form of Cr(VI)-Fe(III). Graphical abstract Removal of hexavalent chromium from wastewater by FeS-CS-BC composite synthesized by impregnation.

Factors influencing degradation of trichloroethylene by sulfide-modified nanoscale zero-valent iron in aqueous solution

Sulfide-modified nanoscale zero-valent iron (S/NZVI) has been considered as an efficient material to degrade trichloroethylene (TCE) in groundwater. However, some critical factors influencing the dechlorination of TCE by S/NZVI have not been investigated clearly. In this study, the effects of Fe/S molar ratio, initial pH, dissolved oxygen and particle aging on TCE dechlorination by S/NZVI (using dithionite as sulfidation reagent) were studied. Besides, the feasibility of reactivation of the aged-NZVI by sulfidation treatment was looked into. The results show that the Fe/S molar ratio and initial pH significantly influenced the TCE dechlorination, and a higher TCE dechlorination was observed at Fe/S molar ratio of ∼60 under alkaline condition. Spectroscopic analyses demonstrate that the enhanced TCE dechlorination was associated with the presence of FeS on the surface of S/NZVI. Dissolved oxygen had little effect on TCE dechlorination by S/NZVI, revealing that the FeS layer could be able to alleviate the surface passivation of NZVI caused by oxidation. Aging of S/NZVI up to 10-20 d only slightly decreased the dechlorination efficiency of TCE. Although an obvious drop in dechorination efficiency was observed for the S/NZVI aged for 30 d, it still exhibited a higher reactivity than the bare NZVI. This indicates that sulfidation of NZVI did prolong its lifetime. Additionally, sulfidation treatment was used to reactivate the aged NZVI, and the results show that the reactivated NZVI even had higher reactivity than the fresh NZVI, suggesting that sulfidation treatment would be a promising method to reactivate the aged NZVI.