Transport of humic matter-coated colloids in packed beds

The deposition kinetics of humic matter-coated hematite {dollar}(alpha{dollar}-{dollar}rm Fesb2Osb3){dollar} and polystyrene latex particles in packed beds of quartz were investigated. Three well-characterized humic substances were used. In the presence of Na{dollar}sp+{dollar}, column filtration results indicated that particles coated with the larger molecular size, less acidic, and less polar peat humic acid were more stable than those coated with the fulvic acid samples. This behavior was attributed to the larger electrosteric interaction energy barrier brought about by the adsorbed peat humic acid. Despite the presence of an adsorbed humic coating, hematite colloids, especially at pH {dollar}<{dollar} pH{dollar}sb{lcub}rm pzc{rcub},{dollar} exhibited critical deposition concentrations (cdc). The cdc value for peat humic acid-coated hematite was, however, larger than those for hematite coated with the two fulvic acids samples. A microscale mathematical model incorporating expressions for electrostatic, van der Waals, and steric interaction energies into the convective diffusion equation was developed. In corroboration with our experimental observations, model simulations indicated that prior to the onset of purely steric repulsion, thicker adsorbed layers would result in higher cdc values.; When Ca{dollar}sp{lcub}2+{rcub}{dollar} was used as the electrolyte, smaller differences in attachment efficiencies among the three different humic matter coatings were obtained. A simple charge screening/reduction model was not able to account for our observations. A complex combination of various repulsive and attractive forces was proposed.; Deposition experiments conducted using humic matter-coated polystyrene latex particles indicated that positively-charged amidine particles possessed higher attachment efficiency values compared to the negatively-charged sulfate particles. This was despite the fact that humic substances adsorbed more favorably to the surface of amidine latex particles. The greater stability of humic matter-coated sulfate latex particles was attributed to its larger steric energy barrier brought about by a larger fraction of loops and tails on the surface. Humic acid-coated latex particles exhibited a higher susceptibility to changes in ionic strength than the same particles coated with a fulvic acid sample, at NaCl concentration ranges where electrostatic interactions were unable to confer stability as the only repulsion mechanism. Model simulations showed that the combination of electrostatic and steric energies should indeed result in attachment efficiencies well below unity at such salt concentrations. These simulations also revealed the existence of a relatively deep secondary energy minimum, the depth of which was inversely proportional to the adsorbed layer thickness. The presence of such an energy minimum was suggested as a possible reason for the lack of sensitivity of fulvic acid-coated particles to variations in salt concentrations. Differences in the solvency of humic versus fulvic acid were also suspect as a potential cause.