RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The interaction between copper ions (Cu²⁺), Strichen fulvic acid (FA), and goethite has been studied with batch experiments in the pH range of 3–11. Similar systems with Ca²⁺ have been studied previously and are used here for comparison. Depending on the pH and Cu²⁺ loading, the binding of Cu ions to the solid phase is enhanced or reduced by the presence of FA. Cu²⁺ complexation in the ternary systems can be mostly described reasonably well with the assumption of linear additivity neglecting the interaction of FA and Cu²⁺ at the goethite surface, except for low Cu loading and high FA loading. A surface complexation model, the Ligand and Charge Distribution (LCD) model, is used to describe Cu binding to the mixture of goethite and FA. By considering both type A (goethite–Cu–FA) and type B (goethite–FA–Cu) ternary complexes, the LCD model descriptions of Cu adsorption are in general in good agreement with the data. Results show that the ternary complex with cation bridging (type A) is the dominating Cu species at low pH, low Cu loading and high FA loading, whereas Cu bound to goethite only is more important at high pH, high Cu loading and low FA loading. Complexation by only ligands of adsorbed FA (type B ternary complexes) is not important for Cu. In the interface, Ca²⁺ ions attribute more charge than Cu²⁺ to the 1-plane, where a large fraction of the negative charge of the FA resides, which explains the stronger enhancement of FA adsorption in the presence of Ca²⁺ compared to Cu²⁺.