| Aluminum salts are used throughout the water treatment industry. Alum is most widely used, but other aluminum based coagulants include sodium aluminate, aluminum chloride, polyaluminum chloride (PACl), and mixtures of an aluminum salt and organic polyelectrolytes. Despite the widespread use of aluminum coagulants, little is known about the speciation and fate of aluminum in water treatment. Aluminum measurements are not routinely made, and the aqueous and analytical chemistry of aluminum is complex.;The objectives of the research were to measure and model the speciation and fate of aluminum in water treatment. Water treatment simulations were conducted using a chemical equilibrium model. An aluminum speciation methodology was tested in laboratory studies and applied to samples collected at two municipal water treatment plants. Aluminum speciation was also measured in jar test experiments where alum and PACl were used to coagulate fulvic acid (FA). Independent variables in the experimental design included pH, DOC, and temperature. Additional experiments were conducted to assess aluminum complexation by FA.;Treatment plant sampling results showed that aluminum in treated waters consists primarily of simple hydrolysis products and not organically complexed species. Dissolved aluminum concentrations were generally in accord with theoretical solubility prior to the final pH adjustment. Shifts in solubility with temperature and the final pH adjustment appear to be important factors influencing the extent of aluminum saturation in treated waters.;Compared to PACl, jar test studies showed that alum is somewhat more effective in removing FA, but produces both higher residual aluminum concentrations and higher turbidity. For both coagulants, slightly better FA removal was observed at pH 7 than at pH 5.5.;Based on the results of complexation studies, a stoichiometric relationship between aluminum and FA was developed. Complexation of aluminum by organic matter is predicted to occur for pH values and DOC concentrations typically found in water treatment. Partitioning diagrams for aluminum are used to identify coagulation mechanisms.;Experimental results are discussed in tems of a competition for aluminum by anions, particularly organic anions and hydroxyl ions. The method of coagulant and base addition influences this competition, and is more important than previously thought. |
