Effects of surface chemistry on deposition kinetics of colloidal hematite (alpha-iron oxide) 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., fate of 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 at short distances determine whether a particle will attach to a collector. Deposition rates are reduced by the presence of repulsive colloidal interactions. Van der Waals attraction and electric double layer repulsion are combined in DLVO theory to describe the total interaction energy between two surfaces. The total interaction energy depends on the solution chemistry and the electric charge and potential of the interacting surfaces. To understand the attachment step of particle deposition, an understanding of the role of simple chemical changes in the water altering the electrostatic interaction is critical.; Deposition experiments using hematite particles and a silica sand were conducted to investigate the influence of specific adsorption on deposition kinetics. A variety 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 the deposition experiments, to provide information about the sign and magnitude of the surface charge on hematite particles.; Experimental results show that the deposition rate is influenced primarily by electrostatic interactions, which are determined by the adsorption of potential-determining ions. In the absence of specifically adsorbed species, hematite deposition is transport limited (favorable deposition) at pH 6.5 and 1 mM NaCl. The addition of 100 micromolar total phosphate results in unfavorable deposition in which the deposition rate is reduced by approximately two orders of magnitude. Polymeric organic compounds produce unfavorable deposition at total concentrations around 10−5 g/L.; The experimental collision efficiency for hematite deposition was consistent with deposition under conditions of surface heterogeneity (i.e., the collision efficiency decreased gradually as electrostatic repulsion increased). The natural silica sand used (Ottawa 30) has a high degree of surface roughness and is expected to be chemically heterogeneous.