Adsorption of lanthanum to goethite in the presence of gluconate

Summary

We have examined the effect of the gluconate anion, an analogue for cellulose degradation products, on the adsorption of trivalent lanthanum (La³⁺)^# to goethite. Lanthanum is investigated as an analogue for the trivalent actinides. Batch pH adsorption edge experiments were used to quantify the adsorption of La³⁺ in the absence of gluconate and in solutions where gluconate was present at a 1:1 mole ratio to lanthanum. Using available thermodynamic data, it is calculated that lanthanum is primarily present in solution as the free La³⁺ ion at pH values up to 8.5 in the absence of gluconate. Above pH 8.5, solid La(OH)₃ precipitates from solution. In the presence of gluconate, complexation decreases the free La³⁺ concentration in solution. The fraction of La³⁺ complexed increases, from 3% to 50%, as the concentrations of La³⁺ and gluconate were increased. Very little effect on the adsorption of La³⁺ to goethite was observed in the presence of gluconate below pH 7. At pH values above 7, however, gluconate doubled the maximum amount of La³⁺ adsorbed when present at concentrations that saturated the goethite adsorption sites. The presence of gluconate did not appear to inhibit the formation of La(OH)₃(s) at pH 8.5 and milli molar lanthanum concentrations. Adsorption to the goethite surface was represented with a surface complexation approach using the diffuse double-layer model. Intrinsic binding constants for the surface complexes were estimated from the pH adsorption edge data using the computer code FITEQL 4.0 and visual curve fitting. Two surface reactions were used to fit the adsorption data in the absence of gluconate: 1) a strong binding site with no proton release and 2) a much higher concentration of weak binding sites with release of two protons per La³⁺ adsorbed. In the presence of gluconate, a third surface complex was needed that involved a ternary complex of two lanthanum atoms with one gluconate molecule.