SORPTION OF CD TO COLLOIDAL FERRIC HYDROXIDES - IMPACT OF PH AND ORGANIC-ACIDS

The sorption (both ad- and desorption) of Cd to or from colloidal ferr ic hydroxides as a function of pH, the concentration of organic acids (acetic acid (AA), EDTA, fulvic acid (FA) and malonic acid (MA)), the precipitation procedure for the ferric hydroxide as well as the contac t time were investigated in the present study. The concentrations of C d, colloids and organic acids were chosen to be representative of natu ral surface waters. The ferric hydroxide were either precipitated in p resence of FA or the acid was mixed with the colloids after the purifi cation steps. The presence of FA during the precipitation of ferric hy droxide, had an impact on the size distribution of the solid phase, bu t it did not have any influence on the sorption behaviour of Cd. The a dsorption process was found to be relatively fast and largely pH-depen dent. It was fully reversible when the pH was decreased, however, with somewhat slower kinetics. The addition of up to 1 mmol L-1 AA or MA d id not cause any measurable desorption of Cd, probably due to the rela tively weak complexes formed between Cd(II)-ions and the anions of the acids. Thermodynamic calculations of the distribution between differe nt soluble Cd-species indicated that EDTA and FA would form stronger c omplexes with Cd. This was indicated in the experiments as enhanced ad sorption at low pH-values due to the adsorption of Cd(II)-organic spec ies, and an increase in the soluble fraction at high pH-values caused by competition for Cd(II)-ions between complex forming groups on the s urfaces and the soluble organic ligands.