Influences of low molar mass organic acids on the adsorption of Cd2+ and Pb2+ by goethite and montmorillonite

This study investigated the influence of low molar mass organic acids, acetic acid, tartaric acid and citric acid on Cd2+ and Pb2+ adsorption by goethite and montmorillonite. The added concentrations of Cd2+ in the batch adsorption experiments were 0.04 mmol L−1 and 0.4 mmol L−1 in the goethite dispersion, and 0.4 mmol L−1 and 8.0 mmol L−1 in the montmorillonite dispersion. The added concentration of Pb2＋ was 0.4 mmol L−1 in both mineral dispersions. The adsorption of Cd2+ and Pb2+ by goethite and montmorillonite was accelerated by low acid concentrations (< 0.6–1.0 mmol L−1) but was inhibited at a concentration between 1.0 mmol L−1 and 3.0 mmol L−1. The inhibiting effect was stronger for Pb2+ than Cd2+ adsorption. The effects of the acid concentrations on Cd2+ and Pb2+ adsorption were more obvious on goethite than on montmorillonite. In the goethite system, Cd2+ adsorption was increased more by citric acid and tartaric acid than by acetic acid at low acid concentrations (< 0.6–1.0 mmol L−1). In the montmorillonite dispersions, Cd2+ adsorption was reduced more by citric acid than by acetic acid and tartaric acid at high acid concentration (> 0.6–1.0 mmol L−1). Pb2+ adsorption was influenced almost in the same manner.