Zero-valent Iron Surface Research Articles

Enhanced hydroxyl radical generation by zero-valent iron/ethylenediamine tetramethylene phosphonic acid for Cr(VI) reduction and 2,4-dichlorophenol degradation

Zero-valent iron (ZVI) has emerged as a promising material for water remediation in recent years. However, the formation of a passivation layer on the surface of ZVI significantly reduces its reactivity. Ethylenediamine tetramethylene phosphonic acid (EDTMPA), a chelating agent, aids in the prevention and passivation of ZVI. This chelating agent has been used in combination with ZVI to remove 2,4-dichlorophenol (2,4-DCP) and hexavalent chromium (Cr(VI)) simultaneously. In this study, the addition of EDTMPA significantly enhanced the removal efficiencies of Cr(VI) and 2,4-DCP by 7.7–10 times compared to ZVI, reaching complete removal of Cr(VI) and 80 % removal of 2,4-DCP within 120 min. Quenching and probing experiments have shown that hydroxyl radicals (•OH) serve as the predominant reactive species responsible for the degradation of 2,4-DCP. Meanwhile, pristine ZVI effectively reduced Cr(VI) to the less hazardous Cr(III) and reacted with oxygen in water to generate Fe(II) and hydrogen peroxide (H2O2), both critical for initiating the subsequent Fenton reaction. The addition of EDTMPA effectively mitigated the formation of the passivation layer on the surface of the ZVI, maintaining its reactivity. The ZVI/EDTMPA system offers an effective method for simultaneous remediation of Cr(VI) and 2,4-DCP by optimizing the application of ZVI.

Removal of levofloxacin by H2O2 and PMS co-activation by sulfide-supported oxalate zero-valent iron enhanced with simultaneous catalysis of SO4-• and 1O2: Major free radicals, synergistic effects and mechanism exploration

It is reported in the literature that sulfide and oxalate are promising ways to modify zero-valent iron (ZVI) for the degradation of antibiotics, but the synergistic activation mechanism of H2O2 and PMS is still unclear. In order to explore this effect, a new type of oxalated and sulfided ZVI (OA-S-ZVI) was synthesized in this study, and levofloxacin (LEV) was degraded in H2O2, PMS and H2O2/PMS systems. The batch experiments confirmed that the removal ability of OA-S-ZVI/H2O2/PMS was 1.65–10 times that of other systems. This result was achieved by modifying ZVI surface catalytic sites and cooperating with H2O2/PMS. At the same time, the synergistic factor S (1.74) confirmed that the composite system H2O2/PMS can capture electrons from the catalyst Fe0 to Fe2+ and adsorb active sites of the catalyst, such as ≡Fe-C2O4-, ≡Fe-OH/OOH and ≡Fe-S. Further calculation and analysis revealed that the binding energy of singlet oxygen (1O2) and sulfate radical (SO4-•) in the composite system decreased and multiple molecular orbitals participated in the removal process. The characterization experiments and EPR experiments explained the transfer free radicals to non-free radicals from a single system to a composite system. This study provides a new perspective on modifying ZVI and utilizing multi-system synergy to effectively remove organic micropollutants from water.

Rapid Oxidative Dissolution of Zerovalent Iron Induced by Sulfite for Efficient Removal of Arsenate and Arsenite: Selective Formation of Scorodite.

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO4•-, •OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

Enhancing and sustaining arsenic removal in a zerovalent iron-based magnetic flow-through water treatment system

In areas affected by arsenicosis, zerovalent iron (ZVI)/sand filters are extensively used by households to treat groundwater, but ZVI surface passivation and filter clogging limit their arsenic (As) removal performance. Here we present a magnetic confinement-enabled column reactor coupled with periodic ultrasonic depassivation (MCCR-PUD), which efficiently and sustainably removes As by reaction with continuously generated iron (oxyhydr)oxides from ZVI oxidative corrosion. In the MCCR, ZVI microparticles self-assemble into stable millimeter-scale wires in forest-like arrays in a parallel magnetic field (0.42−0.48 T, produced by two parallel permanent magnets), forming a highly porous structure (87 % porosity) with twice the accessible reactive surface area of a ZVI/sand mixture. For a feed concentration of 100 μg/L As(III), the MCCR-PUD, with a short empty bed contact time (1.6 min), treated ca. 7340 empty bed volume (EBV) of water at breakthrough (10 μg/L), 9.4 folds higher than that of a ZVI/sand filter. Due to the large interspace between ZVI wires, the MCCR-PUD effectively prevented column clogging that occurred in the ZVI/sand filter. The high water treatment capacity was attributed to the much enhanced ZVI reactivity in the magnetic field, sustained through rejuvenation by PUD. Furthermore, most of As was structurally incorporated into the produced iron (oxyhydr)oxides (mostly ferrihydrite) in the MCCR-PUD, as revealed by Mössbauer spectroscopy, X-ray absorption spectroscopy, and sequential extraction experiments. This finding evinced a different mechanism from the surface adsorption in the ZVI/sand filter. The structural incorporation of As also resulted in much less As remobilization from the produced corrosion products during aging in water, in total ∼1 % in 28 days. Furthermore, the MCCR-PUD exihibted robust performance when treating complex synthetic groundwater containing natural organic matter and common ions (∼3700 EBV at breakthrough). Taken together, our study demonstrates the potential of the magnetic confinement-enabled ZVI reactor as a promising decentralized As treatment platform.

Resolve the species-specific effects of iron (hydr)oxides on the performance of underlying zerovalent iron for metalloid removal: Identification of their key properties

Despite the importance of surface iron (hydr)oxides (Fe-(hydr)oxides) for the decontamination performance of zerovalent iron (ZVI) -based technologies has been well recognized, controversial understandings of their exact roles still exist due to the complex species distribution of Fe-(hydr)oxides. Herein, we re-structured the surface of ZVI using eight distinct Fe-(hydr)oxides and analyzed their species-specific effects on the performance of ZVI for Se(IV) under well-controlled conditions. The kinetics-relevant performance indicators (Se(IV) removal rates, Fe2+ release rates, and the utilization ratio of ZVI) under the effect of each Fe-(hydr)oxide roughly followed the order: δ-FeOOH > Fe5HO8·4H2O > α-FeOOH > β-FeOOH > γ-FeOOH > γ-Fe2O3 > Fe3O4 > α-Fe2O3. Multiple linear regression analysis shows that the large pore volume and size (instead of specific surface area), low open-circuit potential, and low electrochemical impedance are key positive properties for kinetics-relevant performance. Besides, for electron efficiency of ZVI, only Fe3O4 increased the value to 50.0%, due to the contribution of its ferrous components, while others did not change it (∼20%). Additional experiments with commercial ZVI covered by individual Fe-(hydr)oxides confirmed the observed species-specific trends. All these results not only provide new basis for mechanism explanation but also have practical implications for the production or modification of ZVI.

Effect of pyrite on the treatment of chlorophenolic compounds with zero-valent iron-Fenton process under uncontrolled pH conditions: reaction mechanism and biodegradability

This current study explored the effect of pyrite on the treatment of chlorophenolic compounds (CP) by Fenton process with micron-sized zero-valent iron (ZVI) as the catalyst. The experiments were conducted in batch reactors with 100 mg L−1 CP, 0–0.02 M H2O2, and variable pyrite and ZVI doses (0–1 g L−1). Our findings show that while the reactor with 1 g L−1 ZVI as the only catalyst achieved only 10% CP removal efficiency due to rapid ZVI surface passivation and ZVI particle aggregation, the CP removal efficiency increased with increasing pyrite dose and reached 100% within couple of minutes in reactors with 0.8 g L−1 pyrite and 0.2 g L−1 ZVI. The CP removal was mainly driven by the oxidative treatment of CPs with some strong radicals such as hydroxyl radicals (•OH) while the adsorption onto the catalyst surface was only responsible for 10 to 25% of CP removals, depending on the type of CP studied. The positive impact of pyrite on CP removal by the ZVI/H2O2 system could be attributed to the ability of pyrite to (1) create an acidic environment for optimum Fenton process, (2) provide support material for ZVI to minimize ZVI particle agglomeration, and (3) stimulate iron redox cycling for improved surface site generation. Following oxidative Fenton treatment, the degradation intermediate products of CPs, including some aromatic compounds (benzoquinone, hydroquinone, etc.) and organic acids (e.g., acetic acid), became more biodegradable in comparison to their mother compounds. Overall, the treatment systems with a mixture of ZVI and pyrite as catalyst materials could offer a suitable cost-effective technology for the treatment of wastewater containing biologically non- or low-degradable toxic compounds such as chlorophenols.

Unlocking the potential of surface modification with phosphate on ball milled zero-valent iron reactivity:Implications for radioactive metal ions removal

Numerous investigations have illuminated the profound impact of phosphate on the adsorption of uranium, however, the effect of phosphate-mediated surface modification on the reactivity of zero-valent iron (ZVI) remained enigmatic. In this study, a phosphate-modified ZVI (P-ZVIbm) was prepared with a facile ball milling strategy, and compared with ZVIbm, the U(VI) removal amount (435.2 mg/g) and efficiency (3.52×10−3 g·mg−1·min−1) of P-ZVIbm were disclosed nearly 2.0 and 54 times larger than those of ZVIbm respectively. The identification of products revealed that the adsorption mechanism dominated the removal process for ZVIbm, while the reactive modified layer strengthened both the adsorption pattern and reduction performance on P-ZVIbm. DFT calculation result demonstrated that the binding configuration shifted from bidentate binuclear to multidentate configuration, further shortening the Fe-U atomic distance. More importantly, the electron transferred is more accessible through the surface phosphate layer, and selectively donated to U(VI), accounting for the elevated reduction performance of P-ZVIbm. This investigation explicitly underscores the critical role of ZVI's surface microenvironment in the domain of radioactive metal ion mitigation and introduces a novel methodology to amplify the sequestration of U(VI) from aqueous environments.

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.

Characteristics of zero-valent iron surface oxide films under the catalytic interface reactions by assisting ligands in nitrate-contaminated groundwater

The surface passivation layer coating on zero-valent iron (ZVI) particles impedes the electron transfer from ZVI to nitrate. To enhance the efficiency of nitrate reduction by Fe(0), we tested the chemical process and the thickness of the iron oxide film on the surface of Fe(0) particles, utilizing Fe2+aq in aqueous solution and wheat straw as ligands. A novel principal surface catalyzing reaction was formulated as follows: NO3−+0.67Fe2++2.2Fe0+2.33H2O→NH4++1.18Fe3O4+0.64OH−. When Fe2+aq concentration increased from 0 - 200 mg·L−1, the NO3- removal rate increased from 6.95% to 82.6% respectively during 12 h and it was 48%, 72%, 79% and 94% respectively in Fe0/WS ratio of 0, 0.25, 0.5 and 1 system. Uniform surface iron oxide films formed around the Fe(0) particles within 12 h after the adding Fe2+aq or wheat straw to the Fe(0) system. The composition and thickness of these films were dependent on the quantity of added materials. X-ray diffraction (XRD) analysis revealed that surface oxide iron mainly consisted of Fe2+ or Fe3+ oxides, with Fe3O4 being predominant. The X-ray photoelectron spectroscopy (XPS) etching indicated that the addition of Fe(0)/straw at mass ratios of 1 or system with 20 mg·L−1 Fe2+aq resulted in the thinnest surface iron oxide layer. The study demonstrated that reducing the oxide layer’s thickness was achieved through partial catalysis and enhanced complexation capacity. This reduction was facilitated by the introduction of Fe2+aq or wheat straw into the Fe(0) system, potentially improving proton dissociation and promoting the ligand-assisted dissolution of Fe3+ oxides.

Reductive Dechlorination of Chlorinated Ethenes at the Sulfidated Zero-Valent Iron Surface: A Mechanistic DFT Study.

Sulfidated nano- and microscale zero-valent iron (S-(n)ZVI) has shown enhanced selectivity and reactive lifetime in the degradation of chlorinated ethenes (CEs) compared to pristine (n)ZVI. However, varying effects of sulfidation on the dechlorination rates of structurally similar CEs have been reported, with the underlying mechanisms remaining poorly understood. In this study, we investigated the β-dichloroelimination reactions of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and trans-1,2-dichloroethene (trans-DCE) at the S and Fe sites of several S-(n)ZVI surface models by using density functional theory. Dechlorination reactions were both kinetically and thermodynamically more favorable at Fe sites compared to S sites, indicating that maintaining the accessibility of reactive Fe sites is crucial for achieving high S-(n)ZVI reactivity with contaminants. At Fe sites adjacent to S atoms, the reactivity for CE dechlorination followed the order trans-DCE ≈ TCE > cis-DCE > PCE. PCE degradation was hindered at these sites due to the steric effects of S atoms. At the S sites, the energy barriers correlated with the CEs' energy of the lowest unoccupied molecular orbital in the order PCE < TCE < DCE isomers. Our findings reveal that the experimentally observed selectivity of S-(n)ZVI materials for individual CEs can be explained by an interplay of the varying reactivities of Fe and S sites in CE dechlorination reactions.

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.

Enhancing Commercially Iron Powder Electron Transport by Surface Biosulfuration to Achieve Uranium Extraction from Uranium Ore Wastewater.

The zero-valent iron (ZVI) has attracted increasing attention due to the enhanced reactivity of ZVI to uranium wastewater. However, ZVI practical application is hampered due to its susceptibility to oxidation and the formation of passivation layers during storage and in situ restoration. To address these issues, we used a biosulfuration approach to modify ZVI for application in uranium ore wastewater treatment. A series of physicochemical characterization tools and photoelectronic analyses showed that BS-ZVI considerably increased carrier separation efficiency and visible light absorption capacity, resulting in a significant photoassisted enhancement effect on uranium extraction. Accordingly, the uranium removal efficiency of BS-ZVI reached 91% within 60 min, and its maximum adsorption capacity was 336.3 mg/g. By analyzing the mechanism, the improved U(VI) removal performance was mostly responsible on the dissolution of the passivation layer on the surface of ZVI, the generation of Fe(II) and FeS, and the important role of Shewanella putrefaciens extracellular polymers (EPS). Overall, the BS-ZVI biohybrid merges with the high activity of ZVI, bio-FeS, and self-regeneration ability of bacteria, expanding a promising new approach for sustainable treatment of uranium mine wastewater.

Bisulfite boosts the reduction activity of zero-valent iron towards p-nitrophenol under anoxic conditions

Zero-valent iron (ZVI) is widely used in the field of wastewater treatment and groundwater remediation, but it suffers from low reactivity due to the surface passivation layer. In the present work, bisulfite was used as an additive to enhance the reduction activity of ZVI under anoxic conditions. It was found that the reaction kinetic constant of p-nitrophenol reduction in ZVI/bisulfite process was approximately 132 times higher than that by ZVI alone. Bisulfite could dissolve and reduce some iron oxide on the ZVI surface, resulting in the exposure of more active sites like Fe0 and Fe(II) for p-nitrophenol reduction through direct electron transfer. Common substances in water had no effects on p-nitrophenol degradation by ZVI/bisulfite process, but phosphate suppressed ZVI corrosion seriously. This work provides a simple, cost-effective and environmental-friendly method to improve the reducing activity of ZVI under anoxic conditions.

Oxalate-modified microscale zero-valent iron for trichloroethylene elimination by adsorption enhancement and accelerating electron transfer

Trichloroethylene (TCE) with trace concentrations is often detected in soils and groundwater, posing potential damages to public health. The elimination of TCE can be achieved through reductive dechlorination using zero-valent iron (ZVI). However, ZVI usually suffers from the presence of passive iron (hydro)oxides layer and low electron transfer rate, thus leading to the unsatisfactory reactivity. Herein, we fabricated oxalated ZVI (Ox-ZVIbm) by mechanical ball-milling of micro-scale ZVI and H2C2O4·2H2O to modify the ZVI surface composition. To be specific, the modification of the iron oxide shell by oxalic acid facilitated the generation of unsaturated coordination Fe(II), enhancing TCE adsorption. Furthermore, the formed FeC2O4 on the iron oxide shell improved electron transfer efficiency, contributing to the enhanced TCE reductive dechlorination. Impressively, the rate of TCE degradation by Ox-ZVIbm was 10-fold higher than that of ZVIbm without oxalate modification. Moreover, Ox-ZVIbm samples were filled in a laboratory Permeable Reactive Barriers (PRB) column to treat actual underground wastewater containing TCE pollutants. The effluent concentration of TCE maintained steadily below 0.21 mg/L for over 10 days, complying with the National Groundwater Class IV standard (GBT 14848–2017). This marks a significant step toward practical groundwater treatment.

Enhanced Biogenic Sulfidation of Zero-Valent Iron in Columns: Implications for Promoting Dechlorination in Permeable Reactive Barriers.

Biogenic sulfidation of zero-valent iron (ZVI) using sulfate reducing bacteria (SRB) has shown enhanced dechlorination rates comparable to those produced by chemical sulfidation. However, controlling and sustaining biogenic sulfidation to enhance in situ dechlorination are poorly understood. Detailed interactions between SRB and ZVI were examined for 4 months in column experiments under enhanced biogenic sulfidation conditions. SRB proliferation and changes in ZVI surface properties were characterized along the flow paths. The results show that ZVI can stimulate SRB activity by removing excessive free sulfide (S2-), in addition to lowering reduction potential. ZVI also hinders downgradient movement of SRB via electrostatic repulsion, restricting SRB presence near the upgradient interface. Dissolved organic carbon (e.g., >2.2 mM) was essential for intense biogenic sulfidation in ZVI columns. The presence of SRB in the upgradient zone appeared to promote the formation of iron polysulfides. Biogenic FeSx deposition increased the S content on ZVI surfaces ∼3-fold, corresponding to 3-fold and 2-fold improvements in the trichloroethylene degradation rate and electron efficiency in batch tests. Elucidation of SRB and ZVI interactions enhances sustained sulfidation in ZVI permeable reactive barrier.

Hydroxylamine-assisted retarding surface passive layer of zero-valent iron for highly efficient p-nitrophenol removal via direct electron transfer

Zerovalent iron (ZVI) has been widely used in groundwater remediation due to its excellent reducibility. However, the passivation layer on ZVI surface limits the reactivity of ZVI. In this work, hydroxylamine hydrochloride (HA) has been selected to assist to improve the remediation efficiency of ZVI, and the removal rate constant of p-nitrophenol (PNP) increased nearly 90 times than that of ZVI alone. The effect of HA concentration on the degradation efficiency of PNP by ZVI was also discussed. The concentrations of Fe(Ⅱ) and Fe(Ⅲ) in reaction solution and on ZVI surface during the processes were measured. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) of fresh and used ZVI were characterized, which elucidated that HA could facilitate the reduction of Fe(Ⅲ) to Fe(Ⅱ), retard passivation, and extend the pH range of reaction. The mechanism of high degradation efficient in ZVI/HA system was studied through a series of probing experiments. Fe0 directly donated electron to reduce PNP, which was the fundamental reason. The contribution of Fe(Ⅱ) and active hydrogen generated on ZVI surface to the degradation efficiency of PNP could be negligible. It can be concluded that the addition of HA was an effective strategy to improve the reduction efficiency of ZVI.

Alcohothermal synthesis of sulfidated zero-valent iron for enhanced Cr(VI) removal

Sulfidation of zero-valent iron (ZVI) has attracted broad attention in recent years for improving the sequestration of contaminants from water. However, sulfidated ZVI (S-ZVI) is mostly synthesized in the aqueous phase, which usually causes the formation of a thick iron oxide layer on the ZVI surface and hinders the efficient electron transfer to the contaminants. In this study, an alcohothermal strategy was employed for S-ZVI synthesis by the one-step reaction of iron powder with elemental sulfur. It is found that ferrous sulfide (FeS) with high purity and fine crystallization was formed on the ZVI surface, which is extremely favorable for electron transfer. Cr(VI) removal experiments confirm that the rate constant of S-ZVI synthesized by the alcohothermal method was 267.1- and 5.4-fold higher than those of un-sulfidated ZVI and aqueous-phase synthesized S-ZVI, respectively. Systematic characterizations proved that Cr(VI) was reduced and co-precipitated on S-ZVI in the form of a Fe(III)/Cr(III)/Cr(VI) composite, suggesting its environmental benignancy.

Impact of environmental factors on the removal of chloramphenicol by zero-valent iron and pyrite mixture

Chloramphenicol (CAP) is an effective bacteriostatic pharmaceutical that has been widely applied in clinical prescriptions, animal husbandry and aquaculture, but the residues excreted by human and animals have resulted in environmental pollution. Here, we investigated the mechanism of enhanced removal of CAP by zero-valent iron (ZVI) and pyrite mixture, the effect of environmental factors (i.e., initial pH, and co-solutes (anions, cations and organic matter)) as well as the potential applications and environmental safety of ZVI/pyrite mixture. Results show that the mixing of pyrite with ZVI enhanced the removal efficiency of CAP from 14.2% to 80–100% with a range of pyrite/ZVI mass ratios from 1.0 to 4.0. The environmental factors can change the surface reactivity and composition of pyrite/ZVI mixture, consequently altering the removal of CAP. Mechanism study revealed that pyrite could suppress the pH increase, mitigate the passivation of ZVI surface, and accelerate the production of reactive Fe(II). The X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization results of the solids after reacting at different pH conditions indicated formation of highly reactive secondary minerals (such as green rust and iron sulfide (FeS)). In addition, in different co-solutes systems, the secondary minerals are differed, leading to the change of ZVI surface reactivity. This study extends the knowledge of the effect of environmental condition on ZVI technology in practical engineering systems.

Polyphenol-modified zero-valent iron prepared using ball milling technology for hexavalent chromium removal: Kinetics and mechanisms

We used polyphenols to modify zero-valent iron (ZVI) by ball milling technology to obtain new catalysts TA-ZVI, EGCG-ZVI and pyGA-ZVI. The polyphenols included tannis acid (TA), epigallocatechin gallate (EGCG) and pyrogallic acid (pyGA). Scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, etc. were used to characterize the ZVIs. The results indicated that polyphenols were successfully coated on the surface of ZVI. TA-ZVI removed 98.2% of 20 mg·L−1 hexavalent chromium (Cr(VI)) within 2 min. In contrast, ZVI achieved only 10.5% removal of Cr(VI) within 2 min. High Cr(VI) removal was obtained by TA-ZVI at pH of 3–9 or with the presence of several ions including NO3−, Mg2+, SO42− and Cl−. Polyphenol-modified ZVI promoted the electron transfer by reducing the resistance, thus increasing the rate of electron transfer of the materials. The number of ortho-phenolic hydroxyl groups was crucial for modifying ZVI to remove Cr(VI). This work presents a new approach to synthesize modified ZVI using polyphenols for the removal of Cr(VI).

Selective removal of Cr(VI) by tannic acid and polyethyleneimine modified zero-valent iron particles with air stability

In this study, a new composite adsorbent for Cr(VI) removal was developed by immobilizing polyethyleneimine (PEI) on the surface of zero-valent iron (ZVI) particles with tannic acid (TA) as a stabilizer. The adsorbent (denoted as Fe-TA-PEI-10) was easy to prepare and regenerate, requiring no conditions for storage. It was found to be particularly effective for Cr(VI) removal from wastewater via reduction and adsorption. Electrochemical analysis revealed that TA significantly reduced the electron transfer resistance of Fe-TA-PEI-10 and reduced the highly toxic Cr(VI)to the less toxic Cr(III). In addition, PEI endowed amino groups to Fe-TA-PEI-10, raising the zero charge point (pHpzc) of Fe-TA-PEI-10 (pHpzc= 7.80), allowing it to adsorb Cr(VI) from the solution rapidly under electrostatic forces and chelating effects. The adsorption process was consistent with the pseudo-first-order model (R2 >0.99) and the Langmuir isotherm model (R2 >0.99), and the maximum adsorption capacity could reach 161.6 mg/g. In particular, this study presented for the first time that TA-modified Fe(0) had excellent stability in the air, and the adsorbent showed no decrease in performance for Cr(VI) removal even after exposure to the air for 30 days. When tested with a simulated electroplating rinsing wastewater, the Fe-TA-PEI-10 showed very high selectivity for Cr(VI) removal. The mechanism of Cr(VI) removal with Fe-TA-PEI-10 was found to be based on adsorption and reduction. This work provided a new scheme for developing efficient and long-lasting reactive adsorbent for Cr(VI) removal.