Diffusion Kinetics of Sorption Processes on Sorbents Based on Pectinic Polysaccharides

The binding kinetics of heavy-metal ions (using Cd²⁺ as an example) by isolated fractions of microgel and pectinic substances from pomelo, beet pulp, and sunflower calathides obtained by combined fractionation in a reaction solution stream were studied. The contributions of external and internal diffusion to sorption processes of Cd²⁺ by the pectinic sorbents were evaluated using the Boyd-Adamson- Myers and Weber-Morris equations. A mixed-diffusion nature of the rate-limiting step of the overall sorption was found. The effects of the sorbent structure and composition on the intensity of sorption processes were established. Their sorption capacity was shown to depend on the content of galacturonic acid residues. In contrast to pectinic polysaccharides obtained in a reaction solution stream but without using combined fractionation, a change of the ratio of reaction rate constants did not reduce the effective diffusion coefficients for isolated fractions of microgel and pectinic substances. This could be indicative of the occurrence of parallel chemical reactions.