Effects of anions on the capacity and affinity of copper adsorption in two variable charge soils

Effects of nitrate (NO3-), chloride (Cl-), sulfate (SO42-), and acetate (Ac-) on Cu2+ adsorption and affinity of the adsorbed Cu2+ were evaluated in two Fe and Al enriched variable charge soils from Southern China. The maximum adsorption of Cu2+ (M, a parameter from the Langmuir isotherm model) in the presence of different anions decreased in the order Cl-> Ac- > NO3- > SO42- 4 for both soils. The clayey loamy soil ( mixed siliceous thermic Typic Dystrochrept, TTD), developed on the Arenaceous rock, adsorbed less Cu2+ than the clayey soil (kaolinitic thermic Plinthudults, KTP), derived from the Quaternary red earths, regardless of anion type present in the medium. The affinity of adsorbed Cu2+ to both soils could be characterized by the Kd ( distribution coefficient) values and successive extraction of the adsorbed Cu2+ with 1-mol NH4Ac L-1. The log(10)Kd value was smaller for the TTD soil than for the KTP soil and decreased in the order of Cl- > NO3- > SO42- > Ac- at low initial Cu2+ concentrations (<= 40 mg Cu2+ L-1), whereas at 80 mg Cu2+ L-1, the log(10)Kd value was similar for NO3-, SO42-, and Ac-, but was slightly higher for Cl-. Complete extraction of Cu2+ adsorbed in the presence of Ac- was achieved. Influence of NO3- and SO42- on the affinity of adsorbed Cu2+ was similar, but the effects of Cl- depended on the initial Cu2+ concentrations. The extracted percentage of the adsorbed Cu2+ in the presence of NO3- or SO42- increased with increasing Cu2+ adsorption saturation. The presence of Cl-, NO3-, or SO42- markedly decreased the equilibrium solution pH for both soils with increasing initial Cu2+ concentrations, and the delta pH values at the highest Cu2+ level were 0.5, 0.63, and 0.55 U for the TTD soil and 0.79, 0.84, and 0.93 U for the KTP soil, respectively for the three anions. The presence of Ac- had a minimal influence on the equilibrium solution pH because of the buffering nature of the NaAc/HAc medium which buffered the released protons. The effects of anions on Cu2+ adsorption and affinity of the adsorbed Cu2+ were dependent on anion types and were apparently related to the altered surface properties caused by anion adsorption and/or the formation of anion - Cu2+ complexes.