COMPETITIVE LEAD AND ZINC SORPTION MECHANISM IN A SANDY LOAM NATURAL SOIL, MINERAL DE POZOS MINING DISTRICT, NORTH-CENTRAL MÉXICO

Heavy metals sorption and desorption by soils constitute important processes that affect their concentration, toxicity and bioavailability in natural environments. Heavy metal adsorption is usually described in terms of two basic mechanisms, specific adsorption (surface complexation) and nonspecific adsorption (ion exchange). The Pb and Zn sorption/ desorption mechanisms onto a natural sandy loam soil were studied by batch experiments, at different pHxs (3.0, 4.5 and 6.0), at different ionic strength (I = 0; 0.02 and 0.1M) and at different background electrolytes solutions (NaCl, NaAcO and NaNO3) and in single and two component system. The experimental results indicated that Pb was more strongly adsorbed onto the soil than Zn. Pb adsorption was probably due to the formation of a mix of inner-sphere complexes onto soil oxides and other soil components and outer-sphere complexes onto soil clay minerals, whereas Zn adsorption was mainly due to non-specific cation-exchange reactions. Freundlich and Langmuir isotherms were good models to reproduce Pb and Zn adsorption in soils. The latter isotherm indicated that the soil under study presented a limited capacity for Pb and Zn sorption. The desorption results with 0.1 M Mg(NO3)2 and 0.1 M NaAcO solutions corroborated the mechanisms proposed. Desorption hysteresis was more evident for Pb than for Zn, being the principal causes of the observed irreversibility the stability of the surface complexes formed and the mechanism through which adsorption occurs. In spite of this limited sorption capacity, the sandy loam soil seems to act as a natural control of Pb contamination. Pb was adsorbed more strongly than Zn and therefore poses less of a threat to ground water systems due to its lower mobility and availability.