Adsorption of inorganic and organic ligands onto aluminum hydroxide and its effect in water treatment

The inorganic anions fluoride, sulfate, and orthophosphate are ubiquitous substances in aqueous environments. These chemicals are also applied in drinking water treatment processes where fluoridation, coagulation with sulfate-based metal salts, or the use of passivating corrosion inhibitors or sequestering agents is practiced. If these ions are present at elevated levels in raw waters or added in the treatment train at points other than directly before entering a finished water reservoir, they can exert a negative influence on the removal of particles and natural organic matter.; Although the water chemistry of these inorganic ions seems to be well understood, their potential effects on the surface chemistry of coagulated particles and their ability to compete with organic anions for aluminum hydrolysis products in water treatment has not received much attention.; In the present work, bench and pilot-scale studies were conducted that indicated that removal of particles and natural organic matter by coagulation with alum is substantially decreased when fluoride is present in the raw water or added at concentrations typically used in water treatment plants. The application of fluoride in this manner also resulted in a tenfold increase in finished water aluminum concentrations. Batch adsorption experiments and surface complexation modeling showed that fluoride does not directly impact particle charge suggesting that its primary influence is the ability to form soluble complexes with aluminum.; Although the batch adsorption experiments indicated that sulfate can compete with organic acids for adsorption to positively charged aluminum floc, jar tests did not yield conclusive evidence that this ion has a considerable influence on turbidity and natural organic matter removal. Sulfate primarily interacts with aluminum hydroxide by electrostatic attraction which has the beneficial effect of limiting the positive surface charge of aluminum hydroxide particles.; Orthophosphate was found to form negatively charged surface complexes on aluminum hydroxide and can thus affect particle charge and stability. It also competed strongly with organic matter for adsorption sites, where both fluoride and sulfate at similar concentrations did not have any perceptible influence.; A surface complexation model was developed from experimental data that enables prediction of the effects that inorganic and organic anions exert during coagulation with alum. The model simulated the effects of orthophosphate and fluoride well but overpredicted the influence of sulfate on the adsorption of organic acids to aluminum hydroxide.