The role of surface conformations in the deposition kinetics of humic matter-coated colloids in porous media

Abstract

The effects of humic matter surface conformations on the deposition rates of polystyrene latex colloids in packed beds of quartz sand were investigated. Positively- and negatively-charged latex spheres were coated with two well-characterized humic substances and their attachment efficiencies to quartz collectors were determined for varying concentrations of NaCl between 0.02 and 0.8 M, at pH 7.4. The deposition behavior of colloids was highly dependent upon the surface conformations of the adsorbed humic matter. Latex colloids coated with the larger molecular size peat humic acid possessed smaller attachment efficiencies than the same colloids in the presence of a smaller molecular size but more acidic surface water fulvic acid. The negatively-charged sulfate latex particles were more stable than the positively-charged amidine latex particles when coated with the same humic matter at most solution conditions used here. The stability of peat humic acid-coated particles was highly susceptible to changes in salt concentrations up to about [NaCl] = 0.6 M, which may be attributed to the presence of electrosteric interactions, and/or the reduction in the solvency and the resulting collapse of the adsorbed layer of the humic matter. Our results suggest that the differences in the steric interaction forces brought about by the surface conformations are primarily responsible for the observed differences in the deposition behavior of the coated colloids for the ionic strength conditions used here.