Modeling Arsenate Competitive Adsorption on Kaolinite, Montmorillonite and Illite

Abstract

The adsorption of arsenate (As(V)) on kaolinite, montmorillonite and illite was investigated at varying pH and competing anion concentration while holding As(V) concentration (6.7 × 10−7M), clay suspension density (2.5 g L−1) and ionic strength (0.1 M NaCl) constant. The effects of 2 concentrations of phosphate (P) or molybdate (Mo) (6.7 × 10−7 and 6.7 × 10−6M) on As(V) adsorption envelopes (adsorption vs. pH) gave evidence for direct competitive adsorption (in the case of As(V) + P) and possibly site-specific non-competitive adsorption (As(V) + Mo). Distinct As(V) adsorption maxima occurred at approximately pH 5.0 for kaolinite, 6.0 for montmorillonite and 6.5 for illite, and ranged from 0.15 to 0.22 mmol As(V) kg−1. When both As(V) and P were present at equimolar concentrations (6.7 × 10−7M), As(V) adsorption decreased slightly, whereas As(V) adsorption substantially decreased in binary As(V)/P systems when the P concentration was 6.7 × 10−6M, which was 10 times greater than As(V). The presence of Mo at equimolar (6.7 × 10−7 M) and 10 times greater (6.7 × 10−6M) concentrations than As(V) caused only slight decreases in As(V) adsorption because the Mo adsorption maximum occurred at pH < 4. The constant capacitance surface complexation model was applied to As(V) and P adsorption data and was used to predict As(V) adsorption at varying P concentrations. The model gave reasonable descriptions of As(V) adsorption on the 3 clay minerals at varying pH and in the presence of a competing oxyanion (P), indicating that surface complexation modeling may be useful in predicting As(V) adsorption in soils.