EXPERIMENTAL INVESTIGATION AND REVIEW OF THE SOLIDS CONCENTRATION EFFECT IN ADSORPTION STUDIES

Abundant literature references suggest that unit adsorption (i.e., mass of adsorbate sorbed per unit mass of adsorbent) declines as the mass of available adsorbent per unit volume increases. Adsorption of cadmium by iron oxyhydroxide was experimentally quantified at pH 7.00 for equilibrium aqueous cadmium of 0.025-22000 nM and for suspended iron oxyhydroxide concentrations of 0.5-50 mmol/L. For the full range of solids concentrations, over all equilibrium cadmium concentrations, adsorption was approximated by a single Freundlich isotherm. An exception to this was slightly increased unit adsorption at high relative solids or low relative aqueous concentrations, which were inversely related. Examination of published accounts of the "solids concentration" effect-excluding those which are admittedly caused by experimental artifacts-revealed either inappropriate experimental design or errors of data interpretation.