| Summary: | Hydrous aluminum oxide colloids of 0.5 Am diameter were used to study the coagulation of submicron particles under water-treatment conditions. The research was aimed at understanding the effects of pH and ligands (organic and inorganic) on precipitation of the added coagulant and their influences on incorporation of the colloids into larger flocs. The reduction in the concentration of submicron particles as a result of alum coagulation was monitored by conventional jar-test experiments. Scanning electron microscopy was used for submicron particle counting. Up to three orders of magnitude reduction in submicron particle concentrations were observed in jar-test experiments. Higher pH (i.e., 7.5) and alum dose (i.e., 1.0 mg L⁻¹) favored homogeneous precipitation of aluminum hydroxide, whereas heterogeneous precipitation occurred at lower pH (i.e., 6.5) and alum dose (i.e., 0.5 mg L⁻¹). Homogeneous precipitation, involving formation of Al(OH)₃(s) from aqueous species, formed large masses of light-weight flocs that can effectively remove submicron particles by subsequent coagulation. Heterogeneous precipitation, which involves precipitation on the surfaces of the seed particles, resulted in destabilized particles that can efficiently coagulate with each other. The presence of ligands, inorganic (e.g., HCO₃⁻) and organic (e.g., functional groups of humic substances) inhibited the coagulation process, reducing particle removal up-to 250 fold. While these ligands inhibit coagulation by modifying particle surfaces, they may enhance the precipitation reactions of aluminum hydroxide. The presence SO₄²⁻ enhanced precipitation relative to NO₃⁻. Electrophoretic mobility values were used to derive equilibrium constants for aluminum speciation and precipitation reactions, both on the surface of particles and in solution. The adsorption of ligands lowered the pHiep, by almost 2 pH units in the presence of HCO₃⁻, and to a pH of less than 3 in the presence of organics. Aluminum species elevated the pHiep by 1 pH unit. Stoichiometric ratios of aluminum hydroxide precipitation were determined using a pH stat. This ratio (1.9 to 3.7) is a function of pH, and concentrations of particles and organics. These results were modeled as spherical precipitates (OH/A1 =3) with adsorbed aluminum species (OH/A1 = 1 to 4). The results of this research suggest that the aluminum precipitation pathway dictates the removal of submicron particles. Submicron particles provide most of the surfaces from particulate matter, thus suggesting the importance of surface precipitation for their removal. Samples from water treatment plants indicated 1.5 to 2.0 log removal of submicron particles. These plants were operating at higher pH values (above 7.5) relative to that of maximum removal experiments in laboratory. Plant operations can be optimized by careful control parameters affecting supersaturation ratio, thus improving removal of submicron particles.Such optimization should include efficient rapid mixing to achieve uniform upersaturation ratios, proper coagulant dose, and possibly better pH control.
|
|---|
