Sulfidated zero valent iron as a persulfate activator for oxidizing organophosphorus pesticides (OPPs) in aqueous solution and aged contaminated soil columns

Simultaneous remediation of co-contaminated soil by ball-milled zero-valent iron coupled with persulfate oxidation

The problem of co-contaminated soil at e-waste dismantling sites is serious and constitutes a critical threat to human health and the ecological environment. Zero-valent iron (ZVI) has been proven to be effective in the stabilization of heavy metals and the removal of halogenated organic compounds (HOCs) from soils. However, for the remediation of co-contamination of heavy metals with HOCs, ZVI has disadvantages such as high remediation cost and inability to take into account both pollutants, which limits its large-scale application. In this paper, boric acid and commercial zero-valent iron (cZVI) were used as raw materials to prepare boric acid-modified zero-valent iron (B-ZVI^bm) through a high-energy ball milling strategy. B-ZVI^bm coupled with persulfate (PS) to achieve simultaneous remediation of co-contaminated soil. The synergistic treatment of PS and B-ZVI^bm resulted in the removal efficiency of 81.3% for decabromodiphenyl ether (BDE209) and the stabilization efficiencies of 96.5%, 99.8%, and 28.8% for Cu, Pb, and Cd respectively in the co-contaminated soil. A series of physical and chemical characterization methods showed that the oxide coat on the surface of B-ZVI^bm could be replaced by borides during ball milling. The boride coat facilitated the exposure of the Fe⁰ core, promoted the corrosion of ZVI and the orderly release of Fe²⁺. The analysis of the morphological transformation of heavy metals in soils revealed that most of the heavy metals in the exchangeable, carbonate-bound state were transformed into the residue state, which was the key mechanism for the remediation of heavy metal-contaminated soils with B-ZVI^bm. The analysis of BDE209 degradation products showed that BDE209 was degraded to lower brominated products and further mineralized by ZVI reduction and free radical oxidation. In general, B-ZVI^bm coupled with PS is a good recipe for synergistic remediation of co-contaminated soils with heavy metals and HOCs.

Chemical Bond Bridging across Two Domains: Generation of Fe(II) and In Situ Formation of FeSx on Zerovalent Iron

Sulfidation of zerovalent iron (SZVI) can strengthen the decontamination ability by promoting the electron transfer from inner Fe⁰ to external pollutants by iron sulfide (FeS_x). Although FeS_x forms easily, the mechanism for the FeS_x bonding on the ZVI surface through a liquid precipitation method is elusive. In this work, we demonstrate a key pathway for the sulfidation of ZVI, namely, the in situ formation of FeS_x on ZVI surface, which leads to chemical bonding across two domains: the pristine ZVI and the newly formed FeS_x phase. The two chemically bridged heterophases display superior activity in electron transportation compared to the physically coated SZVI, eventually bringing about the better performance in reducing Cr(VI) species. It is revealed that the formation of chemically bonded FeS_x requires balancing the rates for the two processes of Fe(II) release and sulfidation, which can be achieved by tuning the pH and S(-II) concentration. This study elucidates a mechanism for surface generation of FeS_x on ZVI, and it provides new perspectives to design high-quality SZVI for environmental applications.

Mechanochemically synthesized silicotungsten acidified ZVI composite for persulfate activation: Enhancement of the electron transfer and Fe slowly release mechanism

Zero-valent iron (ZVI) is a promising technology for groundwater treatment, and its efficiency primarily depends on the electron transfer. However, there are still some problems such as low electron efficiency of ZVI particles and high yield of iron sludge that limits the performance, which warrant further investigation. In our study, a silicotungsten acidified ZVI composite (m-WZVI) was synthesized by ball milling to activate PS to degrade phenol. m-WZVI has a better performance on phenol degradation (with a removal rate of 91.82%) than ball mill ZVI(m-ZVI) with persulfate (PS) (with a removal rate of 59.37%). Compared with m-ZVI, the first-order kinetic constant (k_obs) of m-WZVI/PS is 2-3 times higher than that of the others. Iron ion was gradually leached in m-WZVI/PS system, being only 2.11 mg/L after 30 min, having to avoid excessive consumption of active substances. The underlying mechanisms of m-WZVI for PS activation mainly include: 1) were elucidated through different characterizations analyses that accounted for silictungstic acid (STA) can be combined with ZVI, and a new electron donor (SiW₁₂^4-) was obtained, which improved the transfer rate performance of electrons for activating PS; 2) singlet oxygen (¹O₂) is the main active substance for phenol degradation, but other radicals also played an important role. Therefore, m-WZVI has good prospects for improving the electron utilization of ZVI.

Remediation of polybrominated diphenyl ethers contaminated soil in the e-waste disposal site by ball milling modified zero valent iron activated persulfate

Hydrophobic organic compounds (HOCs) in e-waste disposal sites are difficult to remove effectively. There is little reported about zero valent iron (ZVI) coupled with persulfate (PS) to achieve the removal of decabromodiphenyl ether (BDE209) from soil. In this work, we have prepared the flake submicron zero valent iron by ball milling with boric acid (B-mZVI^bm) at a low cost. Sacrifice experiments results showed that 56.6% of BDE209 was removed in 72 h with PS/B-mZVI^bm, which was 2.12 times than that of micron zero valent iron (mZVI). The morphology, crystal form, atomic valence, composition, and functional group of B-mZVI^bm were determined by SEM, XRD, XPS, and FTIR, and the results indicated that the oxide layer on the surface of mZVI is replaced by borides. The results of EPR indicated that hydroxyl radical and sulfate radical played the dominant role in the degradation of BDE209. The degradation products of BDE209 were determined by gas chromatography-mass spectrometry (GC-MS), accordingly, the possible degradation pathway was further proposed. The research suggested that ball milling with mZVI and boric acid is a low-cost means of preparing highly active zero valent iron materials. And the mZVI^bm has promising applications in improving the activation efficiency of PS and enhancing the removal of the contaminant.

Activation of sulfite by micron-scale iron-carbon composite for metronidazole degradation: Theoretical and experimental studies

In recent years, sulfite (S(Ⅳ)), as an alternative to persulfates, has played a crucial role in eliminating antibiotics in wastewater, so there is an urgent need to develop a cheap, environmentally friendly, and effective catalyst. Zero-valent iron (ZVI) has great potential for activated S(Ⅳ) removal of organic pollutants, but its reactivity in water is reduced due to passivation. In this study, a micron-scale iron-carbon composite(mZVI@C-800) prepared via high-temperature calcination was coupled with S(Ⅳ) to degrade metronidazole (MNZ). Under the optimized reaction conditions of mZVI@C-800 dosage of 0.2 g/L and S(Ⅳ) concentration of 0.1 g/L, the MNZ removal rate was up to 81.5 % in acidic and neutral environments. The surface chemical properties of the catalysts were characterized by different analytical techniques, and the corresponding catalytic mechanism was analyzed based on these analytical results. As a result, Fe²⁺ is the main active site, and ^·OH and SO₄^·- were the dominant active species. The increase in efficiency was attributed to the introduction of carbon to enhance the corrosion of mZVI further releasing more Fe²⁺. Additionally proposed were the potential response mechanism, the degradation path, and the toxicity change rule. These results demonstrate that the catalytic breakdown of antibiotics in wastewater treatment can be accelerated by the use of the outstanding catalytic material mZVI@C-800.

Mechanochemically sulfidated zero-valent iron as persulfate activation catalyst in permeable reactive barriers for groundwater remediation - A feasibility study

The technology of permeable reactive barriers is reliable and economically effective to prevent the spread of pollutants in groundwaters. Yet, it is efficacious only with easily reducible chemicals such as heavy metals and halogenated organics. In the present study, sulfidated zero-valent iron solventless synthesized by ball-milling is proposed as a possible barrier filling for activation of persulfate to achieve sound removal of reduction-resistant organic pollutants (the herbicide atrazine was used as a model pollutant). Preliminary batch experiments demonstrated rapid degradation of atrazine. Continuous experiments executed in columns proved the superior efficiency of sulfidated iron as a persulfate activator, compared to zero-valent iron, in terms of removal of both atrazine and byproducts. Optimal atrazine removal in the column was achieved with 10% sulfidated iron packing, and 9 mM persulfate at a hydraulic residence time of 6.02 h. Under such conditions, the estimated bed length of the reactive barrier for 99% atrazine removal was 8.69 cm. The morphology and surface-active species in the column demonstrated that activation of persulfate mainly occurred at the inlet of the column until the complete usage of the active species. Batch experiments with coexisting ions suggested that they have a minor influence on atrazine removal percentage, while Mg2+, Ca2+, CO2- and HCO- had a significant impact on the kinetics of the process. However, analogous column experiments demonstrated that the coexisting ions have a negative influence on both atrazine and its byproducts. The results obtained in this study corroborate the potential application of persulfate-enhanced permeable reactive barriers for in situ removal of atrazine from underground water.

Peroxymonosulfate Activation by BaTiO3 Piezocatalyst

Peroxymonosulfate (PMS) plays an important role in the advanced oxidation process for environmental remediation. In this study, barium titanate (BTO) piezocatalyst was selected for the activation of PMS driven by ultrasonic power. The degradation of Rhodamine B (RhB) by BTO single component, PMS single component, and BTO/PMS double components were investigated. The results indicated that PMS can be efficiently activated by BTO under an ultrasound with an RhB degradation rate of 98% within 20 min. The ultrasound not only promoted the activation of the PMS itself, but the surface charge carriers of BTO induced by the ultrasound also contributed to the activation of PMS. ·O2−, ·OH, and ·SO4− radicals were found to be the main active species that participated in the reaction. In order to verify the reaction’s environmental applicability, amoxicillin (AMX) as a typical environmental pollutant was studied. BTO/PMS displayed 80% removal efficiency of AMX, and the products generated were less toxic as demonstrated by eco-toxicity comparison. This work provides a promising strategy to improve the utilization of ultrasonic energy and apply it to the field of environmental pollutants treatment.

Metal(loid)s removal by zeolite-supported iron particles from mine contaminated groundwater: Performance and mechanistic insights

Iron-based materials have been widely investigated because of their high surface reactivity, which has shown potential for the remediation of metal(loid)s in groundwater. However, the disadvantages of structural stability and economic feasibility always limit their application in permeable reactive barrier (PRB) technology. In this study, zeolite-supported iron particles (Zeo-Fe) were synthesized by an innovative low-cost physical preparation method that is suitable for mass production. The removal efficiency and mechanism of typical metal(loid)s (Pb²⁺, Cd²⁺, Cr⁶⁺ and As³⁺) were subsequently investigated using various kinetic and equilibrium models and characterization methods. The results of scanning electron microscopy and energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) confirmed that zero valent iron (Fe⁰) and oxidation product (Fe₃O₄) were successfully loaded and efficiently dispersed on zeolite. The synthesized Zeo-Fe exhibited excellent adsorption and redox capacities for the cations Pb²⁺, Cd²⁺ and anions Cr⁶⁺, As³⁺. The increase in the pH resulting from Fe⁰ corrosion also enhanced the precipitation of Fe-metal(loid)s. The maximum removal capacity for Pb²⁺, Cd²⁺, Cr⁶⁺ and As³⁺ was up to 70.00, 9.12, 2.35 and 0.36 mg/g, respectively. The removal processes were well described by the pseudo-second-order kinetic model for Pb²⁺ and Cd²⁺, Lagergren pseudo first-order kinetics model for As³⁺ and double phase first order kinetics model l for Cr⁶⁺. Cr⁶⁺ was rapidly reduced to Cr³⁺ by the Fe⁰ stabilized on Zeo-Fe, and the oxidation of As³⁺ to As⁵⁺ was attributed to the Fe^0/Fe²⁺ oxidation process at the interface over time, which was further demonstrated by the mineral phase and element valence analyses of reacted Zeo-Fe. The removal mechanism for metal(loid)s was a combination of physical and chemical processes, including adsorption, co-precipitation and reduction-oxidation. Conclusively, Zeo-Fe has been shown to have potential as an effective and economical material for removing various metal(loid)s used in PRB.

Challenges and effectiveness of nanotechnology-based photocatalysis for pesticides-contaminated water: A review

Pesticides have been frequently used in agricultural fields. Due to the expeditious utilization of pesticides, their excessive usage has negative impacts on the natural environment and human health. This review discusses the successful implications of nanotechnology-based photocatalysis for the removal of environmental pesticide contaminants. Notably, various nanomaterials, including TiO₂, ZnO, Fe₂O₃, nanoscale zero-valent iron, nanocomposite-based materials, have been proposed and have played a progressively essential role in wastewater treatment. In addition, a detailed review of the crucial reaction condition factors, including water matrix, pH, light source, temperature, flow rate (retention time), initial concentration of pesticides, a dosage of photocatalyst, and radical scavengers, is also highlighted. Additionally, the degradation pathway of pesticide mineralization is also elucidated. Finally, the challenges of technologies and the future of nanotechnology-based photocatalysis toward the photo-degradation of pesticides are thoroughly discussed. It is expected that those innovative extraordinary photocatalysts will significantly enhance the performance of pesticides degradation in the coming years.

Enhanced elimination of V5+ in wastewater using zero-valent iron activated by ball milling: The overlooked crucial roles of energy input and sodium chloride

The ball-milling technology, a highly efficient and cost-effective method, had excellent application prospects for overcoming passivation issues of normal zero-valent iron (ZVI) to enhance the decontamination efficiency. In this work, we investigated the effects and mechanisms of pH, process control agents (PCA), and main process parameters on the removal of V⁵⁺ using ball-milled zero-valent iron (ZVI^bm). The results showed that ZVI was successfully activated due to mechanochemical action. The enhanced proton conductivity of ZVI^bm leaded to the rapid production of more Fe²⁺, thereby resulting in an order of magnitude higher elimination of V⁵⁺ by ZVI^bm than by ZVI under near-neutral conditions. In addition, the introduction of NaCl in the ball milling process could not only effectively alleviate the agglomeration phenomenon of ZVI^bm, but also effectively enhance its activity. Unexpectedly, due to over-compaction and small size effects, excessive energy input weakened the reactivity of ZVI^bm on V⁵⁺ elimination. Various characterization results confirmed that the removal of V⁵⁺ by ZVI^bm was dominated by reduction and supplemented by adsorption. This work updated the basic understanding of the critical effects of process parameters and NaCl on ZVI^bm in the remediation of vanadium-containing wastewater.

Tailored design of three-dimensional rGOA-nZVI catalyst as an activator of persulfate for degradation of organophosphorus pesticides

In this study, three-dimensional reduced graphene oxide aerogel (rGOA)-supported nanozero-valent iron (rGOA-nZVI) was successfully synthesized via tailored design and applied to activate persulfate (PS) to degrade three organophosphorus pesticides (OPPs; phorate, terbufos and parathion) in water and a historically contaminated soil. The results showed that loading nZVI nanoparticles on rGOA could prevent the aggregation of nZVI. rGOA-nZVI presented a better catalytic performance for PS activation to degrade the three OPPs than bare nZVI and rGOA, with degradation efficiencies of greater than 99.5% within 5 min. pH had negligible effects on the PS activated by rGOA-nZVI (rGOA-nZVI/PS). EPR measurements and radical quenching experiments showed that ·SO₄^- and ·OH were the main radicals responsible for OPP removal in the rGOA-nZVI/PS system. Furthermore, nine intermediates were identified, and the oxidation and scission of C-S-C, P-S/O and PS were the dominant degradation pathways of the three OPPs in aqueous solutions treated with rGOA-nZVI/PS. Additionally, rGOA-nZVI/PS achieved degradation efficiencies of 95.1% for phorate, 79.9% for terbufos and 89.1% for parathion in the contaminated soil, and the detected intermediates could be further degraded except triethylphosphate. Overall, this study provides practical knowledge for OPP removal by rGOA-nZVI/PS in wastewater and actual contaminated soil.