Adsorption of Phosphate Dianions by Hybrid Inorganic–Biopolymer Polyelectrolyte Complexes: Experimental and Computational Studies

Hybrid inorganic–biopolymer polyelectrolyte complexes (HIB-PECs) consisting of chitosan, alginate, and an aluminum-based cross-linking agent were prepared at variable chitosan and alginate mass ratios. IR spectral results indicate that chitosan and alginate undergo PEC formation according to the spectral shifts of the −NH 3 + and COO – groups for chitosan and alginate, respectively. Evidence of Al–O–R (where R = alkyl group) and Al–OH bonds are supported by XPS, PXRD, and NMR spectra. Computational studies provide additional support that aluminum undergoes cross-linking with the carboxylate groups of alginate of the PEC. SEM and pH pzc results provide support of variable product morphology and surface charge for the HIB-PECs over the pristine biopolymers. Equilibrium adsorption studies of the hydrogen phosphate dianion at pH 8.5 reveal that chitosan with alginate at a 1:1 (w/w) ratio afforded an HIB-PEC with an optimal adsorption capacity ( Q m = 152 ± 44 mg/g). The Sips isotherm model accounts for the adsorption data, where the uptake parameters indicate a heterogeneous adsorbent surface with physisorption uptake properties. Increased temperatures led to reduced uptake capacity of the HIB-PECs, where competitor anion species had no apparent effects. Computational studies revealed that the −OH sites of the aluminum complex in the HIB-PECs serve as the preferred active site for phosphate anion uptake. Regeneration of the HIB-PECs over four adsorption–desorption cycles did not show any attenuation of the adsorption capacity. This study presents a first example of the application of HIB-PECs for the adsorption of phosphate species in aqueous solution, where the computational and experimental studies provided complementary support that the −OH sites of the aluminum complex serve as the key active adsorption site for the phosphate anion species.