Competition between particles and dissolved organic matter for trace metal binding in unpolluted soil solutions: Monitoring and thermodynamic approaches

Abstract

Abstract In this study, monitoring and thermodynamic approaches were combined in order to identify the dominant features, which control the speciation of Cd, Cu, Pb, and Zn in solutions from unpolluted planosolic soil (North Massif Central, France). Three sites were selected according to topography and monitored from January to May 2006 using piezometers. The sandy-loam to clay-loam organic-rich horizon (horizon A) and the gravely and concretion-rich horizon (horizon E) were chosen to assess the potential impact of type of horizon on the chemical speciation of metals in the soil solutions. Binding of metals to organic ligands was studied by differential pulse anodic stripping voltammetry (DPASV) in order to acquire experimental data which could be used for the validation of speciation models. These data, together with data from the literature on properties of natural particulate matter, were used to determine the speciation of Cd, Cu, Pb and Zn with a thermodynamic code (PHREEQC). The observed and predicted speciation were compared to estimate the contributions of natural dissolved organic matter and of particles to the binding of metal ions. There was a good match between observed and predicted results, validating the approach used. Results show that particles (mixed iron and aluminium oxides ± clays ± organic matter) play the main role in the speciation of the trace metals in soil waters rather than complexation with dissolved organic matter. Moreover, particles govern speciation to a greater extent in horizon E than in horizon A, which can be linked to hydrodynamics (reaction time between water and soil due to porosity) and chemical reactivity of the solution circulating.