| Chromium and arsenic have been identified as co-contaminants in wastes from wood preservative manufacture, paint and ink manufactures, and petroleum refineries, as well as some municipal wastewaters. Inadequate storage and improper disposal practices of the wastes have caused many incidences of soil and water contamination. Both chromate and arsenate represent potential threats to the environment, human health, and animal health due to their carcinogenic and toxicological effects. Therefore, the World Health Organization (WHO) and the Ministry of Health of P. R. China have established a provisional guideline of 50μg/L for Cr(VI) and 10μg/L for As in drinking water.Only limited research has explored methods to remove As(V) and Cr(VI) simultaneously from water and with various disadvantages. More effective and economical methods for simultaneous removal of chromate and arsenate from contaminated water should be explored. Moreover, resent studies demonstrated Cr(III), previously considered as innoxious, pose potential threats when it exists in soluble species. Therefore, this study proposed to employ Fe(II) to remove chromium(i.e., both Cr(VI) and Cr(III)) and arsenate simultaneously from the contaminated water with the following objectives: to examine the performance of this process as a function of pH, Fe(II) dosages and initial Cr(VI)/As(V) ratios; to investigate mechanisms of the process and to examine influences of co-existing cation and anions on the process.Results implies that Fe(II) is most effective for chromium and arsenic removal under neutral condtion. In the system with initial Cr(VI) and As(V) concentration of 10μmol L-1 each, over 99% of chromium was removed at pH 6.8 when Fe(II) was dosed at 45μmol L-1 or 60μmol L-1and both Cr(VI) and As(V) at pH 5.9 could meet the drinking water standard when Fe(II) was applied at 60μmol L-1. Chromium removal by Fe(II) was controlled by both the rate of Cr(VI)reduction by Fe(II) and the solubility of Fe0.75Cr0.25(OH)3 at pH 4-6, but by the extent of Cr(VI) reduction under alkaline conditions under oxic conditions. The presence of arsenate resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the magnitude of Cr(VI) reduction by Fe(II) and sequestration of the Fe0.75Cr0.25(OH)3 and FeOOH precipitation by HAsO42-. Arsenate removal by Fe(II) alone was trivial but was improvedsignificantly at pH 4-9 due to the presence chromate. It was the oxidative property of chromate that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with Fe0.75Cr0.25(OH)3 and FeOOH precipitates.The influence of a set of anions (PO43-, humic acid (HA), SiO32-) upon the process was studied under anoxic conditions in the pH range of 4 to 10. Results indicated that these anions may affect chromium removal by Fe(II) through the following three routes: increase Cr(VI) reduction by Fe(II) at pH <5.0, inhibit the precipitation of newly formed Cr(III) and decrease the amount of Cr(VI) reduced by Fe(II) under neutral and alkaline conditions. And they exert influences on arsenate removal via two ways: compete for adsorption sites and sequester the precipitation of Fe0.75Cr0.25(OH)3.Singly present calcium ions showed negligible effect on the chromium removal throughout the pH range 4-10, yet notably increased arsenate removal at pH>7. The enhancement in arsenate removal might be ascribed to the decrease of negative surface charge on the precipitates caused by the adsorption of calcium and co-precipitation of calcium carbonate.The presence of calcium promoted the aggregation of colloidal Cr(III)/Fe(III)-anion complexes, attenuating the detrimental impacts of anions on the chromium removal under alkaline conditions. Arsenate removal was increased correspondingly, but the degree of enhancement varied with respect to competitive capability of the respective anion.
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