pH-dependent effect of zinc on arsenic adsorption to magnetite nanoparticles

The effect of Zn 2+ on both the kinetic and equilibrium aspects of arsenic adsorption to magnetite nanoparticles was investigated at pH 4.5–8.0. At pH 8.0, adsorption of both arsenate and arsenite to magnetite nanoparticles was significantly enhanced by the presence of small amount of Zn 2+ in the solution. With less than 3 mg/L of Zn 2+ added to the arsenic solution prior to the addition of magnetite nanoparticles, the percentage of arsenic removal by magnetite nanoparticles increased from 66% to over 99% for arsenate, and from 80% to 95% for arsenite from an initial concentration of ∼100 μg/L As at pH 8.0. Adsorption rate also increased significantly in the presence of Zn 2+. The adsorption-enhancement effect of Zn 2+ was not observed at pH 4.5–6.0, nor with ZnO nanoparticles, nor with surface-coated Zn–magnetite nanoparticles. The enhanced arsenic adsorption in the presence of Zn 2+ cannot be due to reduced negative charge of the magnetite nanoparticles surface by zinc adsorption. Other cations, such as Ca 2+ and Ag +, failed to enhance arsenic adsorption. Several potential mechanisms that could have caused the enhanced adsorption of arsenic have been tested and ruled out. Formation of a ternary surface complex by zinc, arsenic and magnetite nanoparticles is a possible mechanism controlling the observed zinc effect. Zinc-facilitated adsorption provides further advantage for magnetite nanoparticle-enhanced arsenic removal over conventional treatment approaches. Arsenic adsorption to magnetite nanoparticles at neutral or slightly basic pH can be significantly enhanced with trace amount of Zn 2+ due to the formation of a ternary complex.