RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Effects of phthalic acid and chelidamic acid on sorption of Cu²⁺ and Ca²⁺ were examined in goethite (α-FeOOH)/water suspensions. Single-sorbate sorption of Cu²⁺ and Ca²⁺ on goethite over a wide range of conditions (pH, I, sorbate/sorbent ratio) was described reasonably well by the Generalized Two Layer Model (GTLM). In the presence of the organic acids, particularly chelidamic acid, sorption of Cu²⁺ was significantly enhanced at low pH values. Sorption of Cu²⁺ in Cu²⁺-phthalic acid and Cu²⁺-chelidamic acid binary systems was quantitatively reproduced over a wide range of conditions (pH, Cu/organic acid concentration ratio) with the GTLM by using a unique set of Cu²⁺ -organic acid ternary surface complexes for each binary system. Sorption of Ca²⁺ remained unaffected in the presence of phthalic acid, but decreased in the presence of chelidamic acid due to the formation of nonsorbing Ca²⁺-chelidamic acid solution complexes. These effects were predicted reasonably well by the GTLM. Although the exact sorption mechanisms are unconfirmed, available data suggest that enhanced sorption of Cu²⁺ resulted from sorption of Cu²⁺-organic acid complexes in a ligand-like manner, i.e., involving oxide surface sites and carboxyl groups of the organic acids not participating in Cu²⁺ complexation. Results from this work suggest that the reported mineral-humate-cation interactions are consistent with the formation of cation-humate ternary surface complexes.