KINETICS OF SINGLE AND MULTIPLE METAL ION SORPTION PROCESSES ON HUMIC SUBSTANCES

Abstract

Single and multiple metal ion reaction processes were studied on humic acids (HA) from various sources. A sequential sampling method (samples periodically removed from the bulk reaction volume) was used. Experimental parameters studied included pH, temperature, HA and peat sources, metal type and concentration. The Langmuir-Hinshelwood (LH) kinetics approach was used to describe the sorption kinetics of metal ions onto HA. Great differences were found among the metals in the extent of reaction and sorption kinetics. The extent of reaction was Cr(III) > Pb(II) > Cu(II) > Ag(I) > Cd(II) = Co(II) = Li(I) for single metal adsorption and Cr(III) >> Pb(II) > Cu(II) = Ag(I) = Cd(II) = Co(II) = Li(I) for multiple metal adsorption on Humic Acid No. 1. The multiple ion sorption kinetics were similar between HA from different sources. For peat reactions, the extent of multiple metal ion sorption was Cr(III) > Pb(II) > Cu(II) > Ag(I) > Co(II) = Cd(II) = Li(I). The sorption rates of all metal ions on HA were very rapid. Most adsorption occurred within 2 minutes, but sorption did not reach equilibrium in 1 day under competitive conditions. The presence of Cr(III) greatly decreased the adsorption of all other metals to the extent that some were not adsorbed at all. For all metal ions studied, less total sorption of each metal ion occurred in the presence of multiple competing ions than with single metal ions. Pearson's Hard Soft Acid Base principles and Klopman's generalized perturbation theory of chemical reactivity generally predicted the sorption preference between metal ions and humic substances. A nonlinearity concept was used to rationalize the use of the LH kinetic model.