Aluminum speciation of coagulants with low concentration: Analysis by electrospray ionization mass spectrometry

Aluminum speciation of coagulants with low concentration was investigated in detail by electrospray ionization mass spectrometry (ESI-MS). In particular, comparison with 27Al nuclear magnetic resonance (27Al NMR) and Ferron colorimetric method for aluminum speciation of coagulants was discussed. The results indicated that various hydrolyzed aluminum species (monomeric, dimeric, trimeric and tetrameric aluminum species), which could not be identified by other methods, appeared in ESI-MS spectra. Thus, ESI-MS method was much better for hydrolyzed aluminum species in low concentration. In this paper, sample cone voltage was an important operating parameter during ESI-MS detection, and optimum voltage was 70 V. Too low (40 V) or high (150 V) sample cone voltage was not suitable for detecting all aluminum species. This study also evaluated the effects of coagulant concentration, aging, pH value and organic matter on aluminum species distribution. The results indicated that the distribution of hydrolyzed aluminum species at low concentration was largely dependent on the solution pH. At higher pH conditions, aging (8-12 h) may influence aluminum hydrolysis. Additionally, after adding salicylic acid into coagulant solutions, speciation and stability for both Al13 species and aluminum-salicylate complexes varied significantly at different pH values.