Reaction-based model describing competitive sorption and transport of Cd, Zn, and Ni in an acidic soil

Predicting the mobility of heavy metals in soils requires models that accur ately describe metal adsorption in the presence of competing cations. They should also be easily adjustable to specific soil materials and applicable in reactive transport codes. In this study, Cd adsorption to an acidic soil material was investigated over a wide concentration range (10(-8) to 10(-2 ) M CdCl2) in the presence of different background electrolytes (10(-4) to 10(-2) M CaCl2 or MgCl2 or 0.05 to 0.5 M NaCl). The adsorption experiments were conducted at pH values between 4.6 and 6.5. A reaction-based sorption model was developed using a combination of nonspecific cation exchange reac tions and competitive sorption reactions to sites with high affinity for he avy metals. This combined cation exchange/specific sorption (CESS) model ac curately described the entire Cd sorption data set. Coupled to a solute tra nsport code, the model accurately predicted Cd breakthrough curves obtained in column transport experiments. The model was further extended to describ e competitive sorption and transport of Cd, Zn, and Ni. At pH 4.6, both Zn and Ni exhibited similar sorption and transport behavior as observed for Cd . In all transport experiments conducted under acidic conditions, heavy met al adsorption was shown to be reversible and kinetic effects were negligibl e within time periods ranging from hours up to four weeks.