Surface complexation modeling of Eu(III) adsorption on silica in the presence of fulvic acid.

Humic substances (HS) substantially affect heavy metal (M) adsorption on mineral surfaces. However, quantitative descriptions of ternary systems involving M, HS and mineral surfaces remain unclear. This study examines adsorption in a model ternary system including Eu(III), fulvic acid (FA) and silica, and describes the adsorption of Eu(III) and FA by combining a double-layer model (DLM) and the Stockholm humic model (SHM). SHM explains the binding of H and Eu to FA and the DLM for FA and Eu(III) adsorption on silica. Experimental results showed that the presence of FA promotes Eu(III) adsorption at acidic pH values, but decreases it at basic pH values, which indicates the formation of ternary surface complexes. Modeling calculations have shown that two ternary surface complexes are required to describe the experimental results in which Eu3+ acts as a bridge between the surface site and FA. The present study suggests that the discrete-site approach to HS is a promising method for interpreting the adsorption data for M, HS and mineral ternary systems. [ABSTRACT FROM AUTHOR]