A new method using laser induced fluorescence quenching for studying metal complexes in the solid state applied to CrIII, CuII and PbII in sandy soils

We achieved the first laser induced fluorescence (LIF) quenching experiment in solid soil samples, excited by the 457 nm Ar laser emission line. The natural fluorescence of soil, due to Natural Organic Matter (NOM) was found to be effectively quenched due to the presence of CrIII, CuII and PbII complexed to the soil. The presence of these quenching agents and their concentrations were determined independently. Batch sorption isotherms for CuII, CrIII, PbII were used at fixed pH with increasing amount of metal ions for monitoring the quenching of the fluorescence signal. A relationship was found, based on the model developed in solution by Ryan and Weber [Ryan, D.K., Weber, J.H., 1982. Fluorescence quenching titration for determination of complexing capacities and stability constants of fulvic acid. Anal. Chem., 54: 986–990] that enabled this solid-state LIF technique to rapidly determine complexing parameters in a non-destructive way. Modelling of the fluorescent response upon metal addition on solid samples led to the conditional stability constant of the soil–metal complexes and binding site concentration values of the soil. It allowed, in our context, to predict the relative mobility of these three metal ions in the environment, when they are interacting with the solid phase of soil. We believe this technique to be applicable to other metals binding with the NOM occurring in soil (Cd, Co, Ni, Fe, Hg, Mn, V, etc) and find use in other areas such as environmental metal contamination monitoring and geo-chemical prospecting.