Speciation of prehydrolyzed Al salt coagulants with electrospray ionization time-of-flight mass spectrometry and 27Al NMR spectroscopy

Based on the speciation results of the most two concerned coagulant component (i.e., monomer and Keggin-Al13), Al species in polymeric Al salt coagulants were fully investigated with the combination of electrospray ionization time-of-flight mass spectrometry and 27Al NMR spectroscopy. Keggin-Al137+ could transform into Al13n+ (n = 1–3) by dehydrogen reaction without destroying the Keggin structure in mass spectrometer. There exist differences in the intensity and the observed sequence of the Al13n+ (n = 1–3) species in the mass spectra of polymeric Al coagulants. Several other polymers (i.e., Al193+, Al203+ and Al16n+, n = 1–3) might also be formed by the decomposition and repolymerization of Keggin-Al137+. Like monomeric Al salt coagulant, species in polymeric Al coagulants with low basicity were mainly detected as low polymers with mono-charge in mass spectrometry. With the increase of basicity, the dominant species often transform into high polymers with higher charges and fewer categories. The Al133+ species detected in monomeric Al coagulant should have octahedral structure and be formed by self hydrolysis, which is different with the species detected in purified Al13 coagulant. On the whole, the detected species in mass spectrometry could roughly represent their dissolution status in original solutions and could also be used to explain the difference of their coagulation performance in water treatment process.

Keggin-Al137+ in 27Al NMR spectroscopy was mainly detected as several mid polymers with fewer positive charges in ESI TOF mass spectrometry. As monomeric Al species, Al137+ with Keggin structure might decompose and polymerize in mass spectrometer, but the formed species could still represent their dissolution status.Figure optionsDownload as PowerPoint slideHighlights
► Purified monomers and Keggin-Al137+ coagulants were studied to help explain Al speciation in mass spectrometry.
► The Al133+ species might simultaneously have octahedral and Keggin structure in polymeric Al coagulants.
► The detected Al species in mass spectrometry could roughly represent their dissolution status.