ZVI Surface Research Articles

Zero valent iron-mediated rapid removal of bis-azo dye in solution amended with dyebath additives: Biphasic kinetics and modelling

The effect of widely used dye-bath additives, namely sodium chloride, ammonium sulfate, urea, acetic acid and citric acid, on the reductive removal of azo dye AR73 by zero valent iron was investigated. Na+ induced ‘salting out’ effect on the dye molecules complemented with Cl− induced pitting corrosion of ZVI surface led to improved dye removal rate with increasing NaCl concentration. ‘Salting out’ effect of (NH4)2SO4 together with enhanced iron corrosion by aggressive SO 4 2− and reducing effect of ‘sulfate green rust’ benefitted the reaction rates. However, beyond 1,000mg/L (NH4)2SO4 concentration, complex formation of NH 4 + and SO 4 2− with iron oxides compromised ZVI reactivity. Urea inhibited the reaction by its chaotropic effect on the dye solution and also by wrapping the ZVI surface. Enhanced iron corrosion by organic acids improved the reaction rates. The dye removal followed biphasic kinetics with initial rapid phase, when more than 95% dye removal was observed in all the studies, followed by a slow phase. The experimental data could be well evaluated using biphasic rate equation (R2>0.995 in all the cases). Highest dye removal rate of 0.900 min−1 was achieved at pH 2 with all the additives amended. AR73 removal could be modelled using biphasic model considering the individual effect of each additive. Rapid dye reduction capability at varied solution composition makes ZVI more advantageous and promising for wastewater treatment.

Effect of Cr(VI) concentration on gas and particle production during iron oxidation in aqueous solutions containing Cl− ions

ABSTRACTZero-valent iron (ZVI) is commonly used as a medium in permeable reactive barriers (PRBs) because of its high reducing ability. The generation of H2 gas in PRBs, however, can decrease the permeability of PRBs and reduce the contact area between the PRB and contaminated groundwater. This study investigated the effect of the initial Cr(VI) concentration ([Cr(VI)init]) in aqueous solutions containing Cl− ions on the generation of H2 gas. ZVI chips were reacted in reactors with 0.5-M NaCl solutions with [Cr(VI)init] ranging between 51 and 303 mg/L. The initial pH was set at 3. The oxidation of ZVI chips by Cr(VI) in aqueous solutions containing Cl− ions produced H2 gas and particles (Fe(III)–Cr(III)(oxy)hydroxides). The Cr(VI) removal from aqueous solutions increased as the [Cr(VI)init] increased, as did H2 gas generation. The positive effect of [Cr(VI)init] on H2 gas generation might be due to an increase in the redox potential gradient as [Cr(VI)init] increases. This increased gradient would enhance H+ ion penetration through the passive film (Fe(III)–Cr(III)(oxy)hydroxides), which formed on the ZVI surface, by diffusion from the solution to pits beneath the passive film.

Phosphate removal from aqueous solution using ZVI/sand bed reactor: Behavior and mechanism

This research reports on phosphate removal from aqueous solution using ZVI/sand packed columns. The influence of column preconditioning, consisting of ZVI pre-oxidation before feeding the columns with phosphate solution, revealed that a column aged for 1 day was more efficient than un-conditioned column, 5-days and 10-days preconditioned columns. The distribution of phosphate trapped inside the columns was evaluated by measuring phosphate concentration in the solids at different levels (P1, P2 and P3) along the depth of the columns. The distribution of phosphate inside the columns was determined for a time period up to 46 days, corresponding to column saturation. Results showed heterogeneous trapping along the column before saturation and homogeneous distribution upon saturation. The maximum cumulative trapped phosphate after column dismantling was determined before saturation (after 17 days running) at 130, 68 and 31 mgP/gFe at the inlet-P1, P1-P2 and P2-P3 layers, respectively, whereas the homogeneous distribution of phosphate upon saturation was determined at 132 mgP/gFe throughout the column. Solid supports were characterized using SEM, XRD and XPS. Lepidocrocite and maghemite/magnetite were the only iron oxidation products identified at the different layers inside the columns. XPS results confirmed the sorption of phosphate at the surface of ZVI and its oxidation products and highlighted the formation of an iron phosphate complex.

Rapid removal of selenate in a zero-valent iron/Fe3O4/Fe2+ synergetic system

Batch experiments were conducted to investigate selenate reduction in a hybridized zero-valent iron (ZVI or Fe0) system that was easily created by mixing magnetite and Fe2+ with ZVI particle (20mesh) and preconditioning for 24h prior to selenate addition. The performance of the hybrid ZVI/Fe3O4/Fe2+ system (hZVI) were compared with those of non-hybrid (ZVI, Fe2+ and Fe3O4 alone) or partial-hybrid systems (ZVI/Fe2+, Fe3O4/Fe2+ and ZVI/Fe3O4). The results showed that precondition of 24h significantly increased the reactivity of hZVI towards selenate reduction possible due to the formation of reactive interface between ZVI and magnetite. hZVI achieved the most effective selenate removal than any of other systems. ZVI/Fe3O4/Fe2+ was a synergetic system, in which any constituent was indispensable for rapid removal of selenate. ZVI was the primary electron donor for selenate reduction. Fe2+ instead of acidic pH (H+) participated in selenate reduction together with ZVI. Moreover, Fe2+ rejuvenated the passivaed surface of ZVI and magnetite, and thus sustained the reactivity of hZVI for rapid removal of selenate. Magnetite served as the primary reaction sites for selenate removal. Se speciation extraction and X-ray photoelectron spectroscopy evidences indicated stepwise reduction of SeVI to SeIV and then to Se0 or Se−II in the hZVI system. Findings of this study could help develop an hZVI technology suitable for treating various selenium-contaminated wastewaters.

Equilibrium, kinetics, and mechanism of lead adsorption using zero-valent iron coated on diatomite

The lead adsorption performances of zero-valent iron coated on diatomite (ZVI-D) were investigated. ZVI-D was prepared by impregnation of diatomite with iron sulfate solution, and followed by reducing with sodium borohydride. The zero-valent iron of ZVI-D was confirmed by XPS. The adsorption performances were tested in a batch reactor with an initial lead concentration in the range of 100–1,250 mg/L. It was found that a lead adsorption isotherm could be described by Langmuir equations with a maximum adsorption capacity of 158.73 mg/g at 298 K. The mean adsorption energy (E) of 54.78 kJ/mol was determined by using Dubinin–Radushkevich isotherm. The kinetics of Pb2+ adsorption was fitted with a pseudo-second-order model. From the thermodynamic study, it was found that the adsorption process was endothermic and spontaneous. An adsorption mechanism was proposed by using XPS data. Three steps of the adsorption were suggested, (i) Pb2+ ions were oxidized to Pb0 on ZVI active sites, (ii)ferrous ions reacted with the Pb2+ to form FeOPbOH, PbO–Fe, PbO2-–Fe2O3, and PbO–FeOOH species on ZVI surface, and (iii) Pb2+ turned to PbO2–Si, PbO–Si, Pb(OH)2–Al, and PbO2–Al2O3 on diatomite surface.

Reductive transformation and enhancement in biodegradability of mono-azo dye by high carbon iron filings (HCIF)

In the present study, reductive degradation of commercially used mono-azo dye, C.I. Acid Orange 7 (AO7), has been investigated using high carbon iron filings (HCIF) in batch reactors. Extent of dye degradation was monitored on the basis of removal efficiencies of various parameters like color and chemical oxygen demand (COD). Enhancement in biodegradability of AO7 dye was evaluated on the basis of an increase in BOD5 and BOD5/COD ratio. The effect of different operating conditions, namely, initial pH, HCIF dosage, and initial dye concentration, were evaluated at mixing intensity of 30 rpm for 120 min. More than 90% decolorization was achieved at all experimental conditions studied. Optimum decolorization efficiency of 99.55% was achieved at pH 3 using HCIF dosage of 28.56 g/l and 100 mg/l initial AO7 concentration. In the adsorption experiments carried out at different initial dye concentrations, a significant amount of unreduced dye remained adsorbed onto the HCIF even after 120 min of reaction time, thus, decreasing the actual reduction efficiency. Under the optimal reaction conditions, COD removal efficiency of 64.71% was observed. The biological oxygen demand (BOD) for 100 mg/l AO7 dye solution increased from 2.32 mg/l of the untreated dye solution to 27.84 mg/l after 120 min. Biodegradability measured as BOD5/COD ratio was increased significantly from 0.0136 of the original solution to 0.464 after 120 min. First-order dye degradation kinetics was evaluated at different operating conditions. In experiments conducted at different pH, highest degradation constant (kobs) of 1.40 min−1 was recorded at pH 3. kobs increased linearly with increase in HCIF dosage whereas highest surface area normalized rate constant (kSA) of 0.0495 l/m/ min was achieved at 28.56 g/l HCIF dosage. The kobs decreased as the initial AO7 concentration increased, suggesting saturation of ZVI surface reactive sites. The results suggest that HCIF can be used as a promising pre-treatment method, for recalcitrant azo dyes during conventional treatment methods.

Removal of 2,4-dichlorophenol from contaminated soil by a heterogeneous ZVI/EDTA/Air Fenton-like system

The removal of 2,4-dichlorophenol (2,4-DCP) from contaminated soil by an amino carboxylic acid-enhanced zero-valent iron/Air (ZVI/Air) Fenton-like system has been performed in this study. The effects of ethylenediaminetetraacetic acid (EDTA) and ethylenediamine-N,N′-disuccinic acid (EDDS) on the catalytic degradation of 2,4-DCP were primarily studied in the presence of ZVI catalyst and O2 oxidant. The result indicated that the degradation of 2,4-DCP in ZVI/Air system was hindered by EDDS but enhanced by EDTA. Complete destruction of 2,4-DCP in contaminated soil was achieved in the heterogeneous ZVI/EDTA/Air (ZEA) Fenton-like system under ambient air and room temperature. ZVI and EDTA were the predominant factors influencing the degradation of 2,4-DCP. The degradation of 2,4-DCP was ascribed to heterogeneous catalytic reaction over the ZVI surface in situ modified by EDTA via the formation of monodentate inner-sphere complexes. In this system, two main reactive oxygen species of O2−/HO2 and Fe(IV) were generated, in which O2−/HO2 was predominant. The novel ZEA oxidation system provides a promising eco-friendly alternative for the remediation of contaminated soils.

Mitigation of Chromium Contamination by Copper-ZVI Bimetallic Particles

Soil and water pollution by chromium is a major environmental concern. While existence of chromium in + 3 state is considered benign, it's presence in the + 6 state poses an environmental concern. Hence reduction of Cr(VI) to Cr(III) is considered as a satisfactory environmental solution to mitigate chromium contamination. The oxidation of a metal substrate can be enhanced by depositing small amount of nobler metal on its surface. The present study hence examines the efficiency of Cr (VI) reduction upon deposition of copper (oxidation potential: −0.34 V) on the surface of (zero valent iron) ZVI (oxidation potential: 0.04 V) particles. Batch experiments and pH and Eh measurements revealed that presence of copper loading on ZVI particles increases the efficiency of Cr(VI) reduction by 11 to 233 % in relation to the uncoated ZVI particles owing to enhanced electron activity and release of hydroxyl ions that converted Cr (VI) to mixed Fe-Cr oxide. The Cr (VI) reduction is accomplished in periods ranging from 60 to 240 min in the batch experiments and obeyed the pseudo first or second order kinetics.

Removing molybdate from water using a hybridized zero-valent iron/magnetite/Fe(II) treatment system

Batch tests were conducted to investigate molybdate removal in a hybridized zero-valent iron/Fe3O4/Fe(II) system (hZVI) in comparison with a conventional ZVI system. The hybrid system was created by employing a nitrate-Fe2+ pretreatment method to coat the ZVI grains with a magnetite (Fe3O4) surface and produce some discrete magnetite particles through a rapid ZVI-nitrate reaction. The co-presence of ZVI, Fe3O4, and Fe(II) species created a highly reactive system. Molybdate, a corrosion inhibitor, could not be effectively removed in a ZVI-only system. With the addition of Feaq2+, molybdate removal was moderately accelerated in a ZVI/Fe(II) system, possibly through the formation of Fe(II)–molybdate–iron oxide complex. In an hZVI system with magnetite and Feaq2+, molybdate was rapidly and sustainably removed via rapid reduction of Mo(VI) to lower valences, during which 1.1mol Feaq2+ was consumed per 1mol molybdate removal. While ZVI is the primary electron source for the redox reaction, magnetite may serve as an e−-conducting medium and host molybdate reduction reaction. Externally-supplied Fe2+ is essential to maintaining magnetite as the iron corrosion product and thus the high reactivity of ZVI surface. This study suggests that the hZVI system could overcome ZVI surface passivation problem and thus provide an effective chemical platform to harness the reactivity of ZVI for molybdate and potentially other water contaminants removal.