The adsorption behavior and mechanism of Cr(VI) on facile synthesized mesoporous NH-SiO2.

An efficient Cr(VI) adsorbent, mesoporous amine-functionalized silica (NH-SiO2), was successfully synthesized within 2 h by a facile one-step route under room temperature and aqueous solution. The structure properties of the obtained materials were characterized by N2 adsorption-desorption isotherm, XRD, TEM, and FT-IR. The Cr(VI) removal performance was investigated by batch experiment. It was found that Cr(VI) removal performance was dependent on solution pH, and the removal efficiency is above 90% for initial pH in the range of 1.0-4.0. The adsorption process was obeyed by pseudo-second-order model, and the equilibrium adsorption data were fitted well by Langmuir model. The maximum monolayer adsorption capacity was 205.76 mg/g at pH 2.0, which was larger than that of traditional two-step tri-amine-functionalized MCM-41. Additionally, high selectivity was exhibited in NH-SiO2 for removal Cr(VI) from co-presence anions Cl-, NO3-, PO43-, SO42-, and SiO32-. Furthermore, the spent NH-SiO2 could be regenerated by 0.005 M NaOH, and Cr(VI) removal is above 92% after NH-SiO2 recycled four. From the analyzed results of adsorption energy, FT-IR, and XPS, the electrostatic attraction between protonated amine group and HCrO4- was the mainly adsorption mechanism. And then some adsorbed Cr(VI) was reduced to low toxicity Cr(III) on the adsorbent surface by electron transfer from nitrogen in -NBr group to Cr(VI).