Arsenic (As) contamination in groundwater has been recognized as a global environmental problem, severely threatening the ecological environment and human health. As is mainly appeared in groundwater as inorganic As(III) and As(V) forms. Nano zero-valent iron (nZVI) can effectively remove the heavy metals in groundwater,and it is also one of the potential materials to control As contamination in groundwater. This study evaluated the effects of nZVI on removing As(III) and As(V), where in the influences of reaction time, initial concentration of arsenic and nZVI dosage on the As removal were investigated. Pseudo-first/second order kinetic, Langmuir/Freundlich isotherm and Werber-Morris intra-particle diffusion model were applied to simulate the adsorption kinetic process of As(III)/As(V). As regard to anaerobic and aerobic fluctuation in groundwater with the change of water level and the change of
groundwater oxygen content during ex-situ treatment, comparative analysis was carried out to evaluate the effects of Arsenic removal by nZVI under different oxygen content conditions (anaerobic, low, medium and high oxygen). Additionally, the mechanism of oxygen promoting the removal of arsenic by nZVI was analyzed based on the characterization results (SEM, XRD and XPS). The findings in present study were concluded as follows:
(1) In the process of As(III) removal by nZVI, the removal efficiency of As(III)gradually increased and stabilized with the increase of reaction time, but decreased with the increase of initial concentration of As(III). The results of model fitting showed that the adsorption of As(III) on nZVI conformed to the pseudo-second order kinetic model (R2>0.990), and the maximum adsorption rate constant k2 was 0.304 g·mg-1·min-1. The fitting result of Werber-Morris intra-particle diffusion model suggested that the adsorption process included external diffusion, internal diffusion and adsorption equilibrium, controlling by external and internal diffusion processes, where in the maximum external diffusion rate constant reached to 36.041 mg·g-1·min-0.5. Langmuir isotherm model well fitted the adsorption process of As(III) on nZVI (R2>0.900), and the saturation adsorption capacity was 148.81 mg·g-1. The As(III) removal efficiency elevated with the increase of nZVI dosage, and reached 100% when the dosage was higher than 0.4 g·L-1 (As(III) initial concentration was 50 mg·L-1). It was observed that the presence of oxygen significantly promoted the removal of As(III) by nZVI, whereas the removal efficiencies was different with changing oxygen contents. The As(III) removal efficiency was increased initially, followed by a decrease, but then raised again with increasing of oxygen content. The maximum removal efficiency of As(III) was 96.47% when the molar ratio of O2/nZVI was 0.5, 35.04% higher than that of under anaerobic condition.
(2) In the process of As(V) removal by nZVI, the influence trend of various factors on the removal efficiency of As(V) and the results of model fitting were approximately the same as that of As(III) system. However, the adsorption rate constant, diffusion coefficient (kid) and saturation adsorption capacity (68.97 mg·g-1) of Langmuir isotherm model were all less than that of in As(III) system, indicating that the adsorption of As(V) by nZVI was much slower and As(V) was hard to diffuse on the surface of nZVI.Compared with As(III), it was not completely removed when the dosage was 1.0 g·L-1 (As(V) initial concentration was 50 mg·L-1), indicating that more nZVI was needed to achieve effective removal. The maximum removal efficiency of As(V) was 51.75% when the molar ratio of O2/nZVI was 0.5, 27.14% higher than that of under anaerobic condition. The removal efficiency of As(III) by nZVI was higher than that of As(V) under the same experimental conditions, and nZVI can directly and efficiently remove As(III). Therefore, applying nZVI to remedy As(III) contaminated groundwater was more effecient.
(3) Solid phase characterizations were carried out to study the mechanism of removing As(III)/As(V) by nZVI. The results indicated that mechanism for removing As included adsorption, reduction, oxidation and so on. Adsorption as the main removal mechanism took place in two reaction system. In addition, it was observed that oxygen can significantly affect the oxidation extent of nZVI and the performance of arsenic removal by nZVI. Under low oxygen condition, nZVI was slightly oxidized to amorphous iron oxides which could enhance the arsenic removal efficiency. Under medium oxygen condition, nZVI was oxidized to a large amount of dissolved iron, resulting in a decline of arsenic removal efficiency. In terms of high oxygen condition, dissolved iron were further oxidized to form new amorphous iron minerals which could further enhance the arsenic removal efficiency.
The above results verified that nZVI had certain removal effect on both As(III) and As(V), and removal effect of As(III) was significantly better than that of As(V). Adequate oxygen can significantly enhance the effect of arsenic removal by nZVI, and high removal efficiency of As(III) and As(V) can be maintained after massive oxidation of nZVI. The results can afford data support for evaluating the effect of removing As(III)/As(V) by nZVI with different oxygen content conditions, and provide a theoretical support for the effect of removing arsenic artificial enhancement by nZVI.