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The combination of zero-valent iron (Fe0) and iron oxide-coated sand (IOCS) was used to remove Cr(VI) and As(V) from groundwater in this study. The efficiency and the removal mechanism of Cr(VI) and As(V) by using this combination, with the influence of humic acid (HA), were investigated using batch experiments. Results showed that, compared to using Fe0 or IOCS alone, the Fe0–IOCS can perform better on the removal of both Cr(VI) and As(V). Metal extraction studies showed that As(V) was mainly removed by IOCS and iron corrosion products while Cr(VI) was mainly removed by Fe0 and its corrosion products. Competition was found between Cr(VI) and As(V) for the adsorption sites on the iron corrosion products. HA had shown insignificant effects on Cr(VI) removal but some effects on As(V) removal kinetics. As(V) was adsorbed on IOCS at the earlier stage, but adsorbed/coprecipitated with the iron corrosion products at the later stage. [Copyright &y& Elsevier]

Abstract: The effects of hardness (Ca2+) and alkalinity (HCO3 −) on arsenic(V) removal from humic acid (HA)-deficient and HA-rich groundwater by zero-valent iron (Fe0) were investigated using batch experiments. Arsenic, in general, is removed from groundwater possibly by adsorption and co-precipitation with the iron corrosion products. However, in the co-presence of HCO3 − and Ca2+, the removal rate of arsenic increased with increasing concentrations of either Ca2+ or HCO3 −. It was observed that the removal of arsenic was significantly enhanced by the formation of CaCO3 as a nucleation seed for the growth of large iron (hydr)oxide particles. In the co-existence of Ca2+, HCO3 − and HA, the presence of HA diminished the positive role of Ca2+ due to the formation of Fe-humate complexes in solution and delaying of the formation of CaCO3. As a result, the formation of the large iron (hydr)oxide particles was inhibited in the earlier stage which, in turn, affected the removal of arsenic. However, after the formation of CaCO3 and the subsequent growth of such particles, the presence of large iron (hydr)oxide particles resulted in the rapid removing of arsenic and Fe-humate by adsorption and/or co-precipitation. [Copyright &y& Elsevier]

The interactions of co-present Cr(VI) and As(V), and the influences of humic acid and bicarbonate in the process of Cr(VI) and As(V) removal by Fe0 were investigated in a batch setting using simulated groundwater with 5mM NaCl, 1mM Na2SO4, and 0.8mM CaCl2 as background electrolytes at an initial pH value of 7. Cr(VI) and As(V) were observed to be subject to different impacts induced by co-existing As(V) or Cr(VI), humic acid and bicarbonate, originating from their distinct removal mechanisms by Fe0. Cr(VI) removal is a reduction-dominated process, whereas As(V) removal principally involves adsorption onto iron corrosion products. Experimental results showed that Cr(VI) removal was not affected by the presence of As(V) and humic acid. However, As(V) removal appeared to be inhibited by co-present Cr(VI). When the Cr(VI) concentration was 2, 5, and 10mg/L, in the absence of humic acid and bicarbonate, As(V) removal rate constants were decreased by 27.9%, 49.0%, and 61.2%, respectively, which probably resulted from competition between Cr(VI) and As(V) for adsorption sites of the iron corrosion products. Furthermore, the presence of humic acid significantly varied As(V) removal kinetics by delaying the formation and aggregation of iron hydroxides due to the formation of soluble Fe¿humate complexes and stably dispersed fine iron hydroxides colloids. In the presence of bicarbonate, both Cr(VI) and As(V) removal was increased and the inhibitory effect of Cr(VI) on As(V) removal was suppressed, resulting from the buffering effects and the promoted iron corrosion induced by bicarbonate, and the formation of CaCO3 in solution, which enhanced As(V) adsorption. [Copyright &y& Elsevier]

Abstract: The influences of various geochemical constituents, such as humic acid, HCO3 −, and Ca2+, on Cr(VI) removal by zero-valent iron (Fe0) were investigated in a batch setting. The collective impacts of humic acid, HCO3 −, and Ca2+ on the Cr(VI) reduction process by Fe0 appeared to significantly differ from their individual impacts. Humic acid introduced a marginal influence on Fe0 reactivity toward Cr(VI) reduction, whereas HCO3 − greatly enhanced Cr(VI) removal by maintaining the solution pH near neutral. The Cr(VI) reduction rate constants (k obs ) were increased by 37.8% and 78.3%, respectively, with 2 mM and 6 mM HCO3 − in solutions where humic acid and Ca2+ were absent. Singly present Ca2+ did not show a significant impact to Cr(VI) reduction. However, probably due to the formation of passivating CaCO3, further addition of Ca2+ to HCO3 − containing solutions resulted in a decrease of k obs compared to solutions containing HCO3 − alone. Ca2+ enhanced humic acid adsorption led to a minor decrease of Cr(VI) reduction rates. In Ca2+-free solutions, humic acid increased the amount of total dissolved iron to 25 mg/l due to the formation of soluble Fe-humate complexes and stably dispersed fine Fe (oxy)hydroxide colloids, which appeared to suppress iron precipitation. In contrast, the coexistence of humic acid and Ca2+ significantly promoted the aggregation of Fe (oxy)hydroxides, with which humic acid co-aggregated and co-precipitated. These aggregates would progressively be deposited on Fe0 surfaces and impose long-term impacts on the permeability of PRBs. [Copyright &y& Elsevier]

The effects of humic acid (HA) on As(V) removal by zero-valent iron (Fe0) from groundwater, associated with corrosion products analyses, were investigated using batch experiments. It was found that arsenic was rapidly removed from groundwater possibly due to its adsorption and co-precipitation with the corrosion products of Fe0. The removal rate of arsenic by Fe0 was inhibited in the presence of HA probably because of the formation of soluble Fe-humate in groundwater which hindered the production of iron precipitates. A longer reaction time was then required for arsenic removal. Such an influence of HA on arsenic removal increased with increasing HA concentration from 5 to 25mgL−1. The binding capacity of HA for dissolved Fe was estimated to be about 0.75mg Femg−1 HA. When the complexation of HA with dissolved Fe was saturated, further corrosion of Fe0 would produce precipitates, which significantly accelerated the removal of arsenic from groundwater via adsorption and co-precipitation with the corrosion products. Iron (hydr)oxides such as maghemite, lepidocrocite, and magnetite were characterized by XRD analyses as the corrosion products, while As(V) was found on the surface of these corrosion products as detected by fourier transform infrared spectrometry and X-ray photoelectron spectroscopy. [Copyright &y& Elsevier]