Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Abstract

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr2O72- and CrO42-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K+ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K+ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K+ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO42- rather than Cr2O72- is dominant at clay interfaces, and HCrO4- can co-exist under acidic conditions. Cr2O72- transformation to CrO42- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.