Effects of Surface Chemistry on Deposition Kinetics of Colloidal Hematite (α-Fe2O3) in Packed Beds of Silica Sand

摘要

The removal of colloidal (sub-micrometer) particles from flowing suspensions by deposition on surfaces is important in many natural and industrial processes (e.g., the fateudof colloids and associated pollutants in groundwater systems and water treatment involving separation processes). In deposition, colloidal particles are transported to the vicinity of the collector surface by advection and diffusion. Colloidal interactions atudshort distances determine whether a particle will attach to a collector. Deposition rates are reduced by the presence of repuls4e colloidal interactions. Van der Waals attractionudand electric double layer repulsion are combined in DLVO theory to describe the total interaction energy between two surfaces. The total interaction energy depends on theudsolution chemistry and the electric charge and potential of the interacting surfaces. To understand the attachment step of particle deposition, an understanding of the role ofudsimple chemical changes in the water altering the electrostatic interaction is critical.ududDeposition experiments using hematite particles and a silica sand were conducted to investigate the influence of specific adsorption to hematite on deposition kinetics. Audvariety of electrolytes, both inorganic and organic, were studied (e.g., phosphate, small organic acids, and polymeric organic compounds including fulvic and humic acid). Electrokinetic measurements were carried out, under chemical conditions similar to those in the deposition experiments, to provide information about the sign and magnitude of the surface charge on hematite particles.ududExperimental results show that the deposition rate is influenced primarily by electrostatic interactions, which are determined by the adsorption of potentialuddetennining ions. In the absence of specifically adsorbed species, hematite deposition is transport limited (favorable deposition) at pH 6.5 and 1 millimolar NaCl. The addition of 100 micromolar total phosphate results in unfavorable deposition in which the depositionudrate is reduced by approximately two orders of magnitude. Polymeric organic compounds produce unfavorable deposition at total concentrations around 10^(-5) g/L. It was observed that electrokinetic measurements in the presence of polymeric organic compounds are influenced by the particle concentration when hematite mobility is measured as a function of the total solute concentration. Experimental results indicated that adjusting the total polyelectrolyte concentration by the same factor relating the particle concentrations in the mobility measurements and deposition experiments resulted in matching the hematite surface properties in the mobility measurements to theuddeposition experiments.ududThe experimental collision efficiency for hematite deposition was consistent with deposition under conditions of surface heterogeneity (i.e., the collision efficiencyuddecreased gradually as electrostatic repulsion increased). The natural silica sand used (Ottawa 30) has a high degree of surface roughness and is expected to be chemicallyudheterogeneous.ud