Shaping polyelectrolyte composites for heavy metals adsorption from wastewater: Experimental assessment and equilibrium studies

Abstract

Design of core/shell composite microparticles for loading/release of organic/inorganic pollutants is of great interest in wastewater treatment. As compared to the classic layer-by-layer strategy, the new approach presented in this study introduced higher organic shell amounts in one-pot deposition step, with less material and energy consumption and lack of toxic by-products formation. Herein, one weak polycation (polyethyleneimine) and two weak polyanions were directly deposited onto silica surface through precipitation of an in-situ formed interpolyelectrolyte coacervate, followed by selective crosslinking with glutaraldehyde and extraction of polyanion chains, confirmed by electrokinetic measurements and FTIR spectra of composites. Twelve composite sorbents were synthesized and tested for adsorption of cadmium, as model heavy metal ion. It was demonstrated that the high sorption occurred onto four newly synthesized composites which is correlated to the deposited shell amount, dependent on the deposition method, polyanion nature and crosslinking ratio. The Cd²⁺ sorbed amount increased with the polyelectrolyte deposited amount and with the accessibility toward active sorption site, less cross-linked composite shells sorbing higher amounts as compared to strong cross-linked shells, the molar ratio [active site]:[Cd²⁺] ranging from 16:1 to 26:1. The best fitting of four isotherm (Langmuir, Freundlich, Sips and Toth) and four kinetics (pseudo-first order, pseudo-second order, modified Freundlich and Elovich) models was assessed by the sum of normalized errors, based on different nonlinear regression error functions, and by the Hannah-Quinn information criterion. In general, the best agreement with the experimental data was found for Toth isotherm and the pseudo-second order kinetic model. Efficient regeneration of the sorbents was possible at least three times. The competitive effect of Pb²⁺ and Ni²⁺ ions was also studied in simulated and real systems. Silica composite sorbents with polyethyleneimine chains as major component of the shell could be very promising in wastewater treatment processes.

Keywords: Core/shell composites; Heavy metal adsorption; Polyelectrolyte coacervation; Wastewater treatment.