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 PACI 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 27 Al NMR spectroscopy. By using the modified kinetic resolution procedure to fit the scanning absorbance data, the reactive polymeric Al species (Al-b) could be differentiated further into two fractions: the faster reacting polymers Al-b1, and slower reacting polymers Al-b2. They were distinguished by their distinct rate constants of reactions with Ferron. It was identified simultaneously by the 27Al NMR spectroscopy that the Al-b2 fraction closely corresponded to Al-13 of Keggin-structure in most samples, while the Al-b1 at the moderate OH/Al ratios of B < 2 would be assigned to the species of Al-6-type series which were undetectable in current Al-27 NMR spectrum (Al-un). Conventional Al-27 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. (c) 2007 Elsevier B.V. All rights reserved.