Effects of Anions on the Capacity and Affinity of Copper Adsorption in Two Variable Charge Soils

Effects of nitrate,(NO₃⁻) chloride (Cl⁻), sulfate (SO₄^2-, 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⁻ > SO₄^2- 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 log₁₀Kd value was smaller for the TTD soil than for the KTP soil and decreased in the order of Cl⁻ > NO₃⁻ > SO₄^2- > Ac⁻ at low initial Cu²⁺ concentrations (≤40 mg Cu²⁺L⁻¹), whereas at 80 mg Cu²⁺L⁻¹, the log₁₀Kd value was similar for NO₃⁻, SO₄^2-, and Ac⁻, but was slightly higher for Cl⁻. Complete extraction of Cu²⁺ adsorbed in the presence of Ac⁻ was achieved. Influence of NO₃⁻ and SO₄^2- 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 SO₄^2- increased with increasing Cu²⁺ adsorption saturation. The presence of Cl⁻, NO₃⁻, or SO₄^2- 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.