Effects of raw water conditions on solution-state aluminum speciation during coagulant dilution

The speciation of aluminum in solutions of alum and various prehydrolyzed, aluminum-based water treatment coagulants was investigated by 27Al NMR at 5°C and 25°C. Alum solutions were seen to contain only mononuclear species including an AlSO 4 + complex, while the prehydrolyzed coagulant solutions contained polynuclear aluminum species, as well. The relative proportions of both polynuclear species and AlSO 4 + complex decreased in cold water. The presence of organic matter had little effect on the speciation of aluminum in diluted alum, but markedly reduced the relative proportion of the tridecamer species in the prehydrolyzed coagulant solutions. The relationship between Al speciation in the coagulants and organic matter removal during jar tests was considered, and the possible role of sulfate in both the formation of the tridecamer species and the effect of organic matter on coagulation efficiency was discussed.