Surface charge properties of tropical soils and their influence on cadmium and lead sorption

Reactions of heavy metals with soil are important in determining metal fates in the environment. Important factors affecting heavy metal retention by soils are pH, ionic strength (I), and surface charge, as well as chemical characteristics of heavy metals. The objectives of this study were to (1) measure the surface charge and points of zero charge of surface horizons from two highly weathered soils from Puerto Rico, an Oxisol and Ultisol, as well as mineral standard kaolinite and synthetic goethite using three methods—(a) potentiometric titration, (b) ion adsorption, and (c) electroacoustic mobility; (2) determine sorption characteristics of Cd and Pb in the Oxisol, Ultisol, and a Mollisol (also from Puerto Rico) as a function of varying metal concentration, pH, I, and surface charge; (3) evaluate the ability of the three soils to retain Cd and Pb when the metals were added to these soils in a sequential reaction scheme; and to (4) measure cation selectivity and surface heterogeneity of K/Ca and K/Pb systems in the Oxisol and Ultisol using calculated Gaines-Thomas selectivity coefficients (K_GT) coupled with enthalpy change (ΔH) data obtained using a flow calorimeter. The points of zero charge determined in this study for kaolinite and goethite were comparable with those reported in the literature. The points of zero charge obtained for the Oxisol and Ultisol were consistent with these soils' chemical and mineralogical properties. All soils sorbed Pb in preference to Cd regardless of initial metal concentration, pH, I, surface charge, or sequence of metal addition. Sorption appeared to be more dependent on metal type than soil composition and Pb was much less exchangeable than Cd. The K_GT and surface heterogeneity data demonstrated the importance of soil components in the exchange reactions involving K/Ca and K/Pb systems in the Oxisol and Ultisol. The Oxisol was composed of homogeneous adsorption sites while the Ultisol was made up of sites exhibiting surface heterogeneity. Therefore, retention of Cd and Pb by tropical soils is dependent on both metal type and soil composition. Lead poses less of a threat to groundwater systems than Cd due to Pb's lower mobility and bioavailability.