Differentiation of hydroxyl-aluminum species at lower OH/Al ratios by combination of 27Al NMR and Ferron assay improved with kinetic resolution

The hydroxyl-Al polymers are the most effective components in PACl coagulants, but their chemical speciation is often indistinct especially at lower molar OH/Al ratios (B). Two sets of hydroxyl-Al solutions with different B values and preparation temperatures were studied with both Ferron assay and 27Al NMR spectroscopy. By using the modified kinetic resolution procedure to fit the scanning absorbance data, the reactive polymeric Al species (Alb) could be differentiated further into two fractions: the faster reacting polymers Alb1 and slower reacting polymers Alb2. They were distinguished by their distinct rate constants of reactions with Ferron. It was identified simultaneously by the 27Al NMR spectroscopy that the Alb2 fraction closely corresponded to Al13 of Keggin-structure in most samples, while the Alb1 at the moderate OH/Al ratios of B < 2 would be assigned to the species of Al6-type series which were undetectable in current 27Al NMR spectrum (Alun). Conventional 27Al NMR spectroscopy in combination with Ferron assay improved with kinetic resolution can better explore the chemical speciation for hydroxyl-Al polymers in partially neutralized Al solutions and thus give a better explanation of coagulation mechanisms of these species when they were applied in water treatment.