On the acid–base stability of Keggin Al13 and Al30 polymers in polyaluminum coagulants
- 380 Downloads
The acid–base stabilities of Al13 and Al30 in polyaluminum coagulants during aging and after dosing into water were studied systematically using batch and flow-through acid–base titration experiments. The acid decomposition rates of both Al13 and Al30 increase rapidly with the decrease in solution pH. The acid decompositions of Al13 and Al30 with respect to H+ concentration are composed of two parallel first-order and second-order reactions, and the reaction orders are 1.169 and 1.005, respectively. The acid decomposition rates of Al13 and Al30 increase slightly when the temperature increases from 20 to ca. 35 °C, but decrease when the temperature increases further. Al30 is more stable than Al13 in acidic solution, and the stability difference increases as the pH decreases. Al30 is more possible to become the dominant species in polyaluminum coagulants than Al13. The acid catalyzed decomposition and followed by recrystallization to form bayerite is one of the main processes that are responsible for the decrease of Al13 and Al30 in polyaluminum coagulants during storage. The deprotonation and polymerization of Al13 and Al30 depend on solution pH. The hydrolysis products are positively charged, and consist mainly of repeated Al13 and Al30 units rather than amorphous Al(OH)3 precipitates. Al30 is less stable than Al13 upon alkaline hydrolysis. Al13 is stable at pH < 5.9, while Al30 lose one proton at the pH 4.6–5.75. Al13 and Al30 lose respective 5 and 10 protons and form [Al13]n and [Al30]n clusters within the pH region of 5.9–6.25 and 5.75–6.65, respectively. This indicates that Al30 is easier to aggregate than Al13 at the acidic side, but [Al13]n is much easier to convert to Alsol–gel than [Al30]n. Al30 possesses better characteristics than Al13 when used as coagulant because the hydrolysis products of Al30 possess higher charges than that of Al13, and [Al30]n clusters exist within a wider pH range.
KeywordsGibbsite Apparent Rate Constant Base Titration Bayerite Acid Decomposition
The work was financially supported by the National High Technology Research and Development Key Program of China (863 Program) (No. 2002AA601290) and the National Natural Science Foundation of China (No. 50874098 and No. 40673003).
- 1.Bertsch PM, Parker DR (1996) In: Sposito G (ed) The environmental chemistry of aluminum. CRC Press, New York, p 117Google Scholar
- 2.Jolivet JP, Henry M, Livage J (2000) Metal oxide chemistry and synthesis from solution to solid state. Wiley, Chichester, p 53Google Scholar
- 3.Casey WH, Phillips BL, Furrer G (2001) In: Banfield JF, Navrotsky A (eds) Nanoparticles and the environment, Reviews in Mineralogy & Geochemistry, vol 44. Mineralogical Society of America and Geochemical Society, Washington, DC, p 167Google Scholar
- 5.Pernitsky DJ (2001) Drinking water coagulation with polyaluminum coagulants—mechanism and selection guidelines. PhD thesis, University of MassachusettsGoogle Scholar
- 10.Fitzgerald JJ (1988) In: Laden K, Felger C (eds) Antiperspirants and deodorants. Dekker, New York, p P119Google Scholar
- 16.Tang HX (1998) Acta Sci Circumst 18:1 in ChineseGoogle Scholar
- 26.Shafran KL, Perry CC (2005) Dalton Trans 2098Google Scholar
- 37.Chen ZY, Luan ZK, Fan B, Zhang ZG, Li YZ, Jia ZP (2006) Chin J Anal Chem 34:38 in ChineseGoogle Scholar