Cadmium may be added to the soils through tire residues, oils, urban waste, sewage sludge and phosphated fertilizers. It is easily absorbed and translocated by plants and can potentially harm human health. The objectives of this work were to study cadmium adsorption in surface and subsurface horizons of two highly weathered soils with different textures: Anionic "Rhodic" Acrudox (RA) and Anionic "Xanthic" Acrudox (XA), representing extreme conditions in the scale of weathering and exhibiting a predominant balance of positive charge in the deeper soil layers. Results from these soils were compared with the amount of Cd adsorbed in a Rhodic Kandiudalf (RK), whose electrochemical behavior is the opposite, exhibiting a balance of predominant negative charge along the profile. All soils were collected from the Northern State of São Paulo, Brazil. The maximum adsorption (b) and the affinity constant (K) of Langmuir's equation were obtained from the soil pH and correlated with the chemical, physical and mineralogical soil attributes. Cadmium adsorption in the soils revealed strong pH dependence, increasing with pH. Surface horizons adsorbed greater amounts of cadmium in relation to the subsurface horizons, reinforcing the fact that organic matter has an important role in Cd adsorption. RK adsorbed greater amounts of cadmium, probably due to the negative net charge along the profile. Organic carbon, cation exchange capacity, cation retention, specific surface, clay content, variable negative charge and permanent negative charge were positively correlated to Langmuir's K and b parameters. CEC and pH were the most important factors controlling adsorption of cadmium in the soils.
heavy metal; Langmuir; pH