Combined effects of Ca2+ and humic acid on colloid transport through porous media

We report the separate and combined effects of humic acid and Ca2+ ions on the transport of colloidal particles through a sand-packed column. Polystyrene latex particles with a sulfate functional group were used as model colloids. The concentrations of both the inlet solution and the effluent solutions were measured during each experimental run. Breakthrough curves were obtained by taking the ratios of each effluent sample concentration to the inlet solution concentration. In the absence of humic acid, the results indicate that increasing the concentration of Ca2+ increases particle attachment to the sand, thus causing decreased transport rates of latex particles through the porous bed matrix. Once 4 mg/l humic acid was added to the system, changes were observed in the effect that Ca2+ has on latex particle breakthrough. In a system containing calcium, increasing the humic acid concentration was shown to reduce particle attachment and increase transport rates. In the absence of calcium, the ratios for the outlet-to-inlet concentrations were similar for each concentration of humic acid. The electrophoretic mobility was also measured in order to determine the role of electrostatic repulsion in the latex particle transport. The electrophoretic mobility of the latex particles was found to be dependent on humic acid concentration in the absence of Ca2+ but not in its presence.