Effect of sulfate on Cu(II) sorption to polymer-supported nano-iron oxides: behavior and XPS study

Iron oxides tend to be immobilized within nanoporous supports to improve their feasibility for practical environmental remediation including arsenic and heavy metal removal. Contrary to the co-ions, little is known concerning the effect of counter ions on the performance of the resultant composites. In this study, two hybrid sorbents (denoted as HFO-PS(-) and HFO-PS(0), respectively) were prepared by loading hydrous ferric oxide (HFO) nanoparticles onto two polystyrene beads: PS(-), negatively charged with sulfonic acid groups, and PS(0), covalently bonded with neutral chloromethyl groups. Effects of sulfate on their sorption toward Cu ions were investigated. Consistent with the case in bulky HFO particles, the amount of Cu adsorbed on HFO-PS(0) was markedly promoted by introducing sulfate. As for HFO-PS(-), with monovalent cation as background (Na(+)), it exhibited an apparent decrease in Cu sorption as a result of the competing effect of Na ions and the Cu-SO(4) complexation in solution. Contrarily, the adsorbed Cu was increased by introducing sulfate in the divalent cation background (Ca), because sulfate ions were allowed to access to the loaded HFO nanoparticles due to the screening of the sulfonic acid groups caused by Ca ions. XPS spectroscopy further demonstrated that besides the electrostatic effects, the formation of Cu-SO(4) ternary complexes also accounted for the enhanced Cu sorption on both bulky HFO and hybrid HFO sorbents in the presence of sulfate. These results indicated that the effect of counter-ion ligands on metal adsorption to hybrid iron oxides was largely dependent on the surface properties of the host materials.