Distinct effect of humic acid on ferrihydrite colloid-facilitated transport of arsenic in saturated media at different pH.

Both humic acid and arsenic (As) have a strong affinity to ferrihydrite colloids (FHC). However, little is known about effects of humic acid colloid (HAC) and FHC interaction on transport and deposition of As in porous media. Co-transport of HAC-FHC and As at different pH was systematically investigated by monitoring their breakthrough curves (BTCs) in saturated sand columns. Colloid and solute transport models were used to reveal mechanisms of As transport with HAC-FHC in porous media. Results showed that HAC-FHC regulated As transport. Under neutral pH conditions, As transport was facilitated by FHC loading small chain-shaped HAC, while it was slightly retarded by FHC loading granular HAC. Under alkaline conditions, HAC was chain-shaped and coupled with FHC to facilitate As transport. However, mobile colloid-adsorbed As was less evident due to relatively low adsorption on FHC under alkaline pH in comparison with the transport under neutral pH. The presence of HAC occupied adsorption sites on FHC and thus decreased As adsorption. Arsenic adsorption on mobile FHC decreased with increasing HAC concentration. Therefore, HAC had an antagonistic effect on mixed colloid-facilitated As transport at neutral and alkaline conditions. Arsenic was readily co-deposited with FHC, especially at low pH. This study suggested that morphology and concentration of HAC, mobility of FHC, and solution pH would control As transport in porous media. The fate of As changed from co-transport with HAC-FHC to co-deposition when environmental pH decreased.