Insight into the effects of low-molecular-weight aromatic acids on biochar colloid-assisted transport of Cd2+ through saturated porous media

Abstract

Biochar colloids are likely to control the environmental fate of heavy metals because of the widespread application of this carbon-based material in environmental remediation. Low-molecular-weight aromatic acids (LWMAAs), a class of common organic substances in the environment, are critical in affecting biochar colloid-mediated transport of heavy metals in aquifer media. Nevertheless, thus far, little is known about the effects of LWMAAs on their co-mobility behaviors. Herein, the influences of two aromatic acids, including cinnamic acid and caffeic acid, on the biochar colloid-assisted mobility of Cd2+ in quartz sand were examined. Generally, LMWAAs promoted biochar colloid mobility because of the enhanced electrostatic repulsion, competitive retention between LMWAAs and colloids, and steric hindrance. More importantly, biochar colloids could act as carriers for contaminants to considerably facilitate Cd2+ transport, mainly due to the strong binding affinities of colloids toward Cd2+ and the high mobility of colloids. Intriguingly, although the presence of LMWAAs inhibited Cd2+ adsorption onto biochar colloids, the two aromatic acids enhanced the contaminant-mobilizing transport capacity of colloids, owing to the increased colloid mobility and the formation of stable Cd-LMWAA complexes. Furthermore, the promotion effects of caffeic acid on colloid mobility or colloid-mediated transport of Cd2+ were greater than those of cinnamic acid. This pattern was related to the different chemical properties (e.g., molecular size and/or hydrophobicity) of the two LMWAAs. The observations of this paper provide useful information for understating the effects of coexisting LMWAAs on the co-mobility behaviors of carbonaceous materials and heavy metals in the groundwater environment.