Cadmium Adsorption.

CADMIUM ADSORPTION

Cadmium (Cd) is regarded as one of the most toxic  metals to plants and animals. High levels of this element are readily translocated to plant shoots even before phytotoxicity is observed. This metal has great potential to diffuse through the food chain. In this study, Cd adsorption was evaluated in two Oxisols, from Paraná State-Brazil (RhodicEutroperox and Anionic Acroperox) and two Alfisols (NatricPalexeralfs and AridicPaleustalfs) and an Entisol (Xeric Torrifluvents) from the United States of America. Part of the samples was treated with sodium hypochlorite to remove the soil organic matter content (SOM). Cd adsorption isotherms were measured using different Cd concentrations (0 - 1.423 mmol L^-1) and NaNO₃ (0.05 M) as background solution at pH 5.5. Cd adsorption envelopes were measured using 0.044 mmol L^-1, three ionic strengths (0.05, 0.1 and 1M) and NaNO₃ as background solution at different pH values (3-9).These soils were characterized in terms of their cation exchange capacity (CEC),specific surface area (SSA), Al and Fe oxides, organic (OC) and inorganic carbon (IC) contents. The Langmuir isotherms and the Triple Layer Models were adjusted to the Cd adsorption data and the obtained parameters were related to the chemical properties by multiple regression equations using SAS stepwise routine. Cd maximum adsorption capacity (MCA_Cd) was higher for the untreated samples when compared the treated ones, for 3 of the 5 soils evaluated. The other soils presented very close values for both determinations. Almost in all soils and treatments Cd adsorption decreased as the ionic strength increased, but the difference between the 0.05 M and 0.1 M is not clear in all cases. The biggest difference occurred between the 0.1 and the 1M ionic strengths, suggesting outer sphere formation. The untreated NatricPalexeralfs sample did not present differences between the different ionic strengths, suggesting inner sphere formation in SOM. MAC_Cdwas related to the chemical properties by the equation: MAC_Cd =14.208 + 0.241 OC - 2.257 Al (R² = 0.76). The complexation constant for the SO-Cd²⁺ outer sphere complex (LogK_Cd) presented the following regression model: LogK_Cd = -3.343 + 0.094 IC - 0.870 Al (R² =0.502).  We tried to adjust SO₂Cd inner sphere and SO-Cd²⁺ outer sphere complex simultaneously, however this condition did not converge for all samples, only for 50% of them. The samples converged belonged to  AnionicAcroperox soil, which has a large gibbsite content, where the cadmium is absorbed by inner sphere formation and in some samples in the highest ionic strength (1M).

Key words: Cadmium adsorption, organic matter and chemical properties.