Influence of pH on copper, lead and cadmium binding by an ombrotrophic peat

The effect of pH on the adsorption of copper (Cu), lead (Pb) and cadmium (Cd) by a peat soil was studied, and the results compared with those corresponding to cation binding by a dissolved peat humic acid (HA), and interpreted with a NICA–Donnan model. A potentiometric titration technique was used to determine the adsorption isotherms for H⁺, at different ionic strengths, and for Cu²⁺, Pb²⁺ and Cd²⁺ at different pH values, in a peat soil. The effect of the ionic strength on proton binding was similar for the soil (solid) organic matter and for dissolved HA. The adsorption isotherms for cation–peat and the binding curves cation–dissolved HA are almost parallel, although more cation was adsorbed per kg of C in the dissolved HA. The effect of pH on cation binding is similar for dissolved organic matter and for the organic soil. At low metal concentration the amount of adsorbed metal followed the order Cu²⁺ > Pb²⁺ > Cd²⁺. The cation-binding parameters obtained with the NICA–Donnan model allow excellent simulation of the effect of pH on the adsorption of Cu, Pb and Cd ions in the studied peat soil. The binding constants for the peat suspension were greater than the corresponding generic parameters for dissolved HA. Speciation calculations showed that for Cu and Pb, the most abundant fraction was the metal adsorbed on peat, whereas for Cd the most abundant fraction was dissolved metal.