Immobilization of Cr(VI) on engineered silicate nanoparticles: Microscopic mechanisms and site energy distribution

Engineered nanoparticles-mediated contaminant transport has been recognized as a significant process governing the mobility of metals and radionuclides in groundwater. Engineered silicate nanoparticles (ESNPs) are attractive materials for the sequestration or extraction of Cr(VI) and other metals and radionuclides from groundwater. While great efforts have been devoted toward the application of these materials for Cr(VI) sequestration, the underlying interface adsorption mechanism is not thoroughly elucidated. This study investigates the immobilization mechanisms of Cr(VI) on a representative ESNPs, NH₂-MCM-41, over a range of water chemistry conditions. By combining batch adsorption experiments with an array of complementary characterizations, we provided spectroscopic and microscopic evidence that the electrostatic interactions between the positively charged NH₂-MCM-41 surface derived from amino functionality and the negatively charged Cr(VI) species was the dominant mechanism responsible for Cr(VI) immobilization. In addition, the weak hydrogen bonding interactions may also contribute to adsorption to a degree. Furthermore, thermodynamic studies suggested a favorable, spontaneous, and exothermic adsorption process. Site energy analysis illustrated that the distribution of energy binding sites on NH₂-MCM-41 is Cr(VI) loading dependent. The new insights provided here can advance understanding of the transport of Cr(VI) associated NH₂-MCM-41 that benefits the application of ESNPs-based technologies for metals immobilization in groundwater.