ABSTRACT
290Chromium and arsenic have been identified as cocontaminants in wastes from wood preservative manufacture, paint and ink manufactures, petroleum refineries, as well as some municipal wastewaters. Inadequate storage and improper disposal practices of chromium and arsenic have caused many incidences of soil and groundwater contamination in industrialized areas. Both chromium and arsenic represent potential threats to the environment, human health, and animal health due to their carcinogenic and toxicological effects. Hexavalent chromium Cr(VI) and arsenic have been considered as important priority pollutants worldwide owing to numerous health problems arising from groundwater contaminated by these two pollutants. Therefore, the World Health Organization (WHO) has established a provisional guideline of 10 μg/L for arsenic and 50 μg/L for Cr(VI) in drinking water. Recent public concern regarding arsenic and Cr(VI) in drinking water has promoted the investigation of treatment technologies with the potential to remove them simultaneously to levels well below the drinking water maximum contaminant level.
In this chapter, a case study of simultaneous removal of chromium and arsenate using Fe(II) is illustrated in detail. The feasibility and mechanisms of simultaneous removal of Cr(VI) and As(V) by Fe(II) were investigated. The influence of various parameters (e.g., pH, Fe(II) dosages and initial Cr(VI)/As(V) ratios) and the individual and combined influences of various geochemical constituents (e.g., calcium, phosphate, silicate, and humic acid) on the simultaneous removal of chromium and arsenate were also studied. The results indicate that Fe(II) is very effective for simultaneous removal of chromium and As(V) under neutral conditions. Chromium removal by Fe(II) is controlled by both the rate of Cr(VI) reduction by Fe(II) and the solubility of Fe0.75Cr0.25(OH)3 at pH 4.0–6.0, but by the extent of Cr(VI) reduction under alkaline conditions under oxic conditions. The presence of As(V) resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the Cr(VI) reduction by Fe(II) and inhibition of the Fe0.75Cr0.25(OH)3 and FeOOH precipitation by HAsO4 2−. As(V) removal by Fe(II) alone was trivial but was improved significantly at pH 4.0–9.0 due to the presence of Cr(VI). It was the oxidative property of Cr(VI) that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of As(V). As(V) was removed by both adsorption and coprecipitation with Fe0.75Cr0.25(OH)3 and FeOOH precipitates. The presence of PO4 3−, humic acid (HA), or SiO3 2− affects 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. They exert influences on arsenate removal via two ways: compete for adsorption sites and depress the precipitation of Fe0.75Cr0.25(OH)3. Singly present Ca2+ ions show negligible effect on chromium removal throughout the pH range 4.0–10.0, yet notably increase arsenate removal at pH > 7.0. The presence of Ca2+ promotes the aggregation of colloidal Cr(III)/Fe(III)–anion complexes, attenuating the detrimental impacts of anions on chromium removal under alkaline conditions. As(V) removal is increased correspondingly, but the degree of enhancement varies with respect to the competitive capability of the respective anion.291
