Monolayers of an organosilane on magnetite nanoparticles for the fast removal of Cr(VI) from water

Monolayers of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane have been established on magnetite nanoparticles to develop a novel magnetic adsorbent for fast decontamination of hexavalent chromium (Cr(VI)) from water. Results indicated that monolayer adsorption of the silane from water took place at low concentrations (<300 mg/L) and around 100% surface coverage was obtained at temperatures >= 90 degrees C. The hydrolysed silane was anchored to the magnetite surface through condensation reactions between its silanol groups and the surface hydroxyl groups of magnetite. The functional amine groups were protonated by acid treatment for adsorbing Cr(VI). The monolayer of the silane on magnetite (MSM) with approximately 100% surface coverage showed extremely rapid adsorption kinetics for Cr(VI), such that the process was complete within 1 min. This enables the treatment of large amounts of sewage per unit time. The adsorption capacity for Cr(VI) was 8.0 mg/g, as estimated from the Langmuir isotherm model. The saturation magnetization of the MSM reached 64.16 emu/g, allowing easy magnetic recovery from water. In the presence of up to 50-fold molar excesses of chloride and nitrate anions, little effect on Cr(VI) removal was seen, but moderate and large impacts were observed with sulphate and hydroxyl anions, respectively. Desorption of adsorbed Cr(VI) and regeneration of the MSM were successfully achieved by NaOH and HCl treatments to deprotonate and protonate the amine groups, respectively. By selecting a silane with suitable functional groups, the surface properties may be tailored for a particular pollutant.