Highly Hydroxylated Porous Nanozirconia for Complete Trace Cr(VI) Removal

Hexavalent chromium [Cr(VI)] is carcinogenic and mutagenic even at very low concentrations, yet there are still few effective methods to remove Cr(VI) to an ultralow level (<10 ppb). Herein, highly hydroxylated and porous nanozirconia (nano-ZrO2) is prepared through a unique top-down microetching method, which simulates the formation of “Karst Landform” in nature. Specifically, BaZrO3 is utilized as the precursor to be etched to form porous ZrO2, while the particle shape is still maintained in spite of the removal of Ba atoms. X-ray photoelectron spectroscopy confirms that the as-prepared ZrO2 has a surface hydroxyl groups density of up to 72 nm–2, which can be protonated in an acidic solution and hence chelated with electronegative Cr(VI) (Cr2O72–). Thus, a distribution coefficient Kd of 4.16 × 105 mL g–1 is reached, which is the highest value for Cr(VI) adsorption at present. Therefore, this porous nano-ZrO2 shows fast and efficient adsorption of trace Cr(VI) (Cr2O72–) from 2010 to 4.8–5.5 ppb within 60 s, which is far lower than the recommended concentration for drinking water (50 ppb; World Health Organization). With excellent recycling performance and high selectivity, this material can be applied in the terminal treatment system of drinking water. Moreover, the mechanism of this trace adsorption is also well investigated, which may guide future material designs in water treatment and the enrichment of trace elements.