Surface functionalized nanoscale metal oxides for arsenic(v), chromium(vi), and uranium(vi) sorption: considering single- and multi-sorbate dynamics

Abstract

Surface-functionalized Mn–Fe oxide nanocrystals (NCs) were evaluated for single- and multi-sorbate scenarios considering As(V), Cr(VI), and U(VI) in varied water chemistries (deionized (DI), ground, and sea water) at pH 7.0. Multi-sorbate scenarios were further examined for competitive and/or cooperative effects. Precisely synthesized manganese ferrite (MnFe₂O₄) NCs were compared with iron oxide (Fe₃O₄) and manganese oxide (Mn_xO_y) nanocrystal cores in terms of sorption capacities and colloidal stabilities. Positively charged cetyltrimethylammonium bromide (CTAB) and negatively charged oleyl phosphate (OP) were evaluated and compared as organic coatings. MnFe₂O₄ NCs exhibited both enhanced sorption performance and colloidal stability compared with Fe₃O₄ and Mn_xO_y NC cores when functionalized with the same surfactant coating. For MnFe₂O₄ NCs, maximum sorption of As(V), Cr(VI), and U(VI) were observed to be 2.62, 3.43, and 4.27 mmol g⁻¹, respectively (in DI water). Relative sorption capacity enhancement (compared with Fe₃O₄ and Mn_xO_y) is due to increased surface grafting densities of MnFe₂O₄ NCs, providing a larger number of sorption sites (functional group) for target sorbates and higher repulsive energy (osmotic and elastic–steric interaction) for increased stability, and thus maintaining available surface area. For As(V) and Cr(VI) multi-sorbate systems, all materials evaluated preferentially adsorbed As(V) over Cr(VI). This preference was further investigated and observed using a novel quartz crystal microbalance (QCM) technique.