6 research outputs found

    Effects of Nano Zero-Valent Iron on Oxidation−Reduction Potential

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    Oxidation−reduction potential (ORP) measurements have been widely used to assess the results of injection of nano zerovalent iron (nZVI) for groundwater remediation, but the significance of these measurements has never been established. Using rotating disk electrodes (RDE) in suspensions of nZVI, we found the electrode response to be highly complex but also a very sensitive probe for a range of fundamentally significant processes. The time dependence of the electrode response reflects both a primary effect (attachment of nZVI onto the electrode surface) and several secondary effects (esp., oxidation of iron and variations in dissolved H<sub>2</sub> concentration). At nZVI concentrations above ∼200 mg/L, attachment of nZVI to the electrode is sufficient to give it the electrochemical characteristics of an Fe<sup>0</sup> electrode, making the electrode relatively insensitive to changes in solution chemistry. Lower nZVI concentrations give a proportional response in ORP, but much of this effect is mediated by the secondary effects noted above. Coating the nZVI with natural organic matter (NOM), or the organic polymers used to make stabile suspensions of nZVI, moderates its effect on ORP measurments. Our results provide the basis for interpretating ORP measurements used to characterize the results of injecting nZVI into groundwater

    Kinetics of Heavy Metal Dissociation from Natural Organic Matter: Roles of the Carboxylic and Phenolic Sites

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    We developed a unifying model for the kinetics of heavy metal dissociation from natural organic matter (NOM) in this study. The kinetics model, integrated with the equilibrium model WHAM 7, specifically considered metal ion reactions with various NOM sites formed by the carboxylic and phenolic sites. The association and dissociation rate coefficients for metal reactions with various NOM sites were constrained by WHAM predicted equilibrium distribution coefficients at specific reaction conditions. We developed the relationship for the dissociation rate coefficients among different binding sites for each metal, which was internally constrained by the metal binding constants. The model had only one fitting parameter, the dissociation rate coefficient for the metal complexes formed with two weak carboxylic sites, and all other parameters were derived from WHAM 7. The kinetic data for metal dissociation from NOM were collected from the literature, and the model was able to reproduce most of relevant data analyzed. The bidentate complexes appeared to be the predominated species controlling metal dissociation under most environmental conditions. The model can help to predict the reactivity and bioavailability of heavy metals under the impact of multiple competing ligands including NOM

    A General Model for Kinetics of Heavy Metal Adsorption and Desorption on Soils

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    In this study, we propose a general kinetics model for heavy metal adsorption and desorption reactions in soils when soil organic matter (SOM) is the dominant adsorbent. The kinetics model, integrated with the equilibrium speciation model WHAM VI, specifically considers metal reactions with SOM and dissolved organic matter (DOM) and accounts for the variations of solution chemistry. Metal reactions with SOM are associated with two groups of sites, one from the monodentate sites and another one from the bidentate and tridentate sites. There are three model parameters, desorption rate coefficients of the two groups of SOM sites for each metal and reactive organic carbon (ROC) for each soil. The applicability of the kinetics model was mainly examined with three elements, Cu, Pb, and Zn, which demonstrate different binding ability with organic matter. The kinetic data were collected with a stirred-flow reactor covering a wide range of experimental conditions, including varying SOM, DOM, Ca, and metal concentrations, reaction pHs, and different flow rates. The kinetics model has been successfully applied to describe heavy metal adsorption and desorption on soils under various reaction conditions

    Kinetics of Cation and Oxyanion Adsorption and Desorption on Ferrihydrite: Roles of Ferrihydrite Binding Sites and a Unified Model

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    Quantitative understanding the kinetics of toxic ion reactions with various heterogeneous ferrihydrite binding sites is crucial for accurately predicting the dynamic behavior of contaminants in environment. In this study, kinetics of As­(V), Cr­(VI), Cu­(II), and Pb­(II) adsorption and desorption on ferrihydrite was studied using a stirred-flow method, which showed that metal adsorption/desorption kinetics was highly dependent on the reaction conditions and varied significantly among four metals. High resolution scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy showed that all four metals were distributed within the ferrihydrite aggregates homogeneously after adsorption reactions. Based on the equilibrium model CD-MUSIC, we developed a novel unified kinetics model applicable for both cation and oxyanion adsorption and desorption on ferrihydrite, which is able to account for the heterogeneity of ferrihydrite binding sites, different binding properties of cations and oxyanions, and variations of solution chemistry. The model described the kinetic results well. We quantitatively elucidated how the equilibrium properties of the cation and oxyanion binding to various ferrihydrite sites and the formation of various surface complexes controlled the adsorption and desorption kinetics at different reaction conditions and time scales. Our study provided a unified modeling method for the kinetics of ion adsorption/desorption on ferrihydrite

    Mechanisms of Synergistic Removal of Low Concentration As(V) by nZVI@Mg(OH)<sub>2</sub> Nanocomposite

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    In this work, by using Mg­(OH)<sub>2</sub> nanoplatelets as support material for nanoscale zerovalent iron (nZVI), nZVI@Mg­(OH)<sub>2</sub> composite was prepared and found to have super high adsorption ability toward As­(V) at environmentally relevant concentrations. It was revealed that the variation of corrosion products of nZVI in the presence of Mg­(OH)<sub>2</sub> and Mg<sup>2+</sup> is an important factor for increase in the adsorption ability toward As­(V). X-ray diffraction (XRD) analysis indicated that the weakly basic condition induced by Mg­(OH)<sub>2</sub> decreases the lepidocrocite (γ-FeOOH) and increases the magnetite/maghemite (Fe<sub>3</sub>O<sub>4</sub>/γ-Fe<sub>2</sub>O<sub>3</sub>) content in the corrosion products of nZVI, and the latter has better adsorption affinity to As­(V). Moreover, extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that the coordination between arsenic and iron minerals is influenced by dissolved Mg<sup>2+</sup>, leading to probable formation of magnesium ferrite (MgFe<sub>2</sub>O<sub>4</sub>) which has considerable adsorption affinity to As­(V). This work provides an important reference not only for the design of pollution control materials but also for understanding arsenic immobilization in natural environments with ubiquitous Mg<sup>2+</sup> ion

    Effects of Sulfidation, Magnetization, and Oxygenation on Azo Dye Reduction by Zerovalent Iron

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    Applications of zerovalent iron (ZVI) for water treatment under aerobic conditions include sequestration of metals (e.g., in acid mine drainage) and decolorization of dyes (in wastewaters from textile manufacturing). The processes responsible for contaminant removal can be a complex mixture of reduction, oxidation, sorption, and coprecipitation processes, which are further complicated by the dynamics of oxygen intrusion, mixing, and oxide precipitation. To better understand such systems, the removal of an azo dye (Orange I) by micron-sized granular ZVI at neutral pH was studied in open (aerobic) stirred batch reactors, by measuring the kinetics of Orange I decolorization and changes in “geochemical” properties (DO, Fe­(II), and Eh), with and without two treatments that might improve the long-term performance of this system: sulfidation by pretreatment with sulfide and magnetization by application of a weak magnetic field (WMF). The results show that the changes in solution chemistry are coupled to the dynamics of oxygen intrusion, which was modeled as analogous to dissolved oxygen sag curves. Both sulfidation and magnetization increased Orange I removal rates 2.4–71.8-fold, but there was little synergistic benefit to applying both enhancements together. Respike experiments showed that the enhancement from magnetization carries over from magnetization to sulfidation, but not the reverse
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