Adaptation of the Ion Exchange Method for the Determination of the Free Ionic Fraction of Cadmium in Solution

The understanding of the availability of a metal in soil necessitates a minimum knowledge about its speciation in the soil solution. Here, we evaluated an alternative to the use of ion exchangers for estimating the free ionic fraction of cadmium ( F Cd ) in solution. It is based on the exchange selectivity coefficient ( V K ) rather than the distribution coefficient ( D K ) to estimate F Cd Because V K for the Cd–Ca exchange for the used Amberlite resin was independent of the solution Ca concentration (0.5–7.5 m M ) and pH (range: 4.5–6), the experiment on a solution mimicking the analyzed solution to estimate V K was not necessary. The influence of variable Ca and Mg concentrations in solution on F Cd was assessed in synthetic solutions containing either citrate or malate. The best way to estimate F Cd seemed to treat the exchange data as if Ca was solely present. However, neither the proposed approach nor those applying D K prevent the overestimation of F Cd when Ca is partly complexed in the analyzed solution. A method intending to estimate two replicates of F Cd for a given, unique solution was also studied on solutions issued from sorption–desorption experiments performed on a humic podzol. It consists of two successive supplies of a known resin mass to a unique sample. Both estimates were close and not significantly different.