Evaluation of phosphate adsorption by porous strong base anion exchangers having hydroxyethyl substituents: kinetics, equilibrium, and thermodynamics.

Phosphate anions are recognized as the main responsible for the eutrophication of surface waters. In this work, two strong base anion exchangers having either N,N-dimethyl 2-hydroxyethylammonium (SBAEx.2M) or N,N-diethyl 2-hydroxyethylammonium (SBAEx.2E) functional groups, as highly efficient sorbents in the removal of phosphate anions, are presented. The influence of the main parameters (pH, contact time, initial concentration of phosphate, temperature) on the adsorption performances was investigated in batch mode. Modeling the kinetics data by Lagergren, Ho and McKay, and Elovich kinetic models indicated chemisorption as the main mechanism of sorption. The sorption at equilibrium was modeled with Langmuir, Freundlich, Sips, Dubinin-Radushkevich, and Temkin isotherm models. The experimental isotherms were the best fitted by Langmuir and Sips isotherms, the maximum sorption capacity for phosphate anions being 233.88 mg g-1 SBAEx.2M and 223.5 mg g-1 SBAEx.2E, at pH 3, and 23 °C. Adsorption of phosphate anions in competitive conditions showed that the interference with co-existing anions was low in the case of Cl- ions and much higher with SO42- ions, the ion exchange having an important contribution in the adsorption process. The adsorption was spontaneous and endothermic, the degree of spontaneity increasing with the increase of temperature. The high level of reusability, the adsorption capacity decreasing with only ~ 7% in the case of SBAEx.2E and with ~ 9% in the case of SBAEx.2M, after five sorption/desorption cycles, recommends these SBAEx as promising adsorbents for phosphate removal.