Conducting binary-exchange experiments is a common way to identify cationic preferences of exchangeable phases in soil. Cation exchange reactions and thermodynamic studies of Pb2+/Ca2+, Cd2+/Ca2+, and Zn2+/Ca2+ were carried out on three surface (0–30？cm) soil samples from Adamawa and Niger States in Nigeria using the batch method. The physicochemical properties studies of the soils showed that the soils have neutral pH values, low organic matter contents, low exchangeable bases, and low effective cation exchange capacity (mean: 3.27？cmolc kg？1) but relatively high base saturations (？50%) with an average of 75.9%. The amount of cations sorbed in all cases did not exceed the soils cation exchange capacity (CEC) values, except for Pb sorption in the entisol-AD2 and alfisol-AD3, where the CEC were exceeded at high Pb loading. Calculated selectivity coefficients were greater than unity across a wide range of exchanger phase composition, indicating a preference for these cations over Ca2+. The values obtained in this work were all positive, indicating that the exchange reactions were favoured and equally feasible. These values indicated that the Ca/soil systems were readily converted to the cation/soil system. The thermodynamic parameters calculated for the exchange of these cations were generally low, but values suggest spontaneous reactions. 1. Introduction Baseline concentration of heavy metals in soils depends on the local geology. This has been supplemented to various degrees by anthropogenic inputs from industrial processes and operations that generate wastes containing considerable amounts of these metals. Heavy metals are nonbiodegradable and hence environmentally persistent and may accumulate in biota and the soils where they can affect soil properties. They interact with soils in various ways but sorption from aqueous solution onto solid particles is an important process that influences their accumulation and transport in the environment [1]. Thus, heavy metals pollution in soils has become a significant topic of concern. Nowhere has the situation become more serious than in developing countries due to the recent establishment of industries producing wastes and effluents containing heavy metals. This has led to increase in the heavy metal burden of the developing countries where waste management practices are poor and the final metal sink is the soil. Metals may interact with soils in many ways: they may be sorbed by the soil depending on the soil pH, organic matter content, cation exchange capacity, and so forth; the oxidation state of some metals
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