The role of particles in the fate and speciation of trace metals in macrotidal estuaries was studied using a surface complexation model (MOCO). Cadmium was selected as the target metal contaminant due to its reactivity in estuaries: cadmium behavior is mainly controlled by heterogeneous processes (sorption/desorption) related to salinity and suspended matter gradients.Various scenarios of suspended matter distribution according to salinity were simulated. The impact of surface properties (specific surface area, density of surface sites, acidity instrinsic properties and complexation constant) was evaluated using data collected on particles from the Gironde, Loire and Seine estuaries.