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Studies on the Hydrolysis and Precipitation of Aluminum(III)

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Title: Studies on the Hydrolysis and Precipitation of Aluminum(III)
Creators: Rubin, Alan J.; Hayden, Phillip L.
Contributors: United States. Office of Water Resources Research; Ohio State University. Water Resources Center
Keywords: aluminum hydroxide
solubility and colloidal stability
precipitation limits
anions
aluminum
Issue Date: 1973-04
Publisher: Ohio State University. Water Resources Center
Series/Report no.: Project completion report (Ohio State University. Water Resources Center) ; no. 364X
Abstract: A systematic investigation of the aqueous chemistry of aluminum(III) is presented. Aluminum hydroxide precipitates were defined by several experimental techniques and the principal ionic species determined. The pH-concentration limits of aluminum hydroxide precipitation and dissolution were defined by light scattering over a broad range of aluminum concentrations at various times after mixing the reagents. Aluminum hydroxide solubilities were strongly a function of solution age and activity of the condensed phase. Crystalline structures identified by X-ray diffraction measurements were gibbsite, pseudoboehmite, bayerite, nordstrandite and amorphous Al(OH)3(c). A colloidal aluminum hydroxide hydrosol formed in acid solutions up to hydroxide/aluminum ratios of 3.0 and a settleable precipitate formed between 3.0 and 4.0. The principal aluminum species detected in acid solutions by computer analysis of potentiometric data were AlOH^2+ and Al8(OH)20^4+ . Identification of the octameric ion correlated very well with analysis of the boundaries of precipitation in the acid pH range. The practical formation constants refined by the computer program were log *K1 = -5.55 and log B8,20 = -68. 7 (Ionic Strength = 0.15 M , 25°C). Equilibrium constants for aluminum hydroxide and the aluminate ion were also calculated for various solution ages. Using these constants, the distribution of the various fractions of soluble and insoluble aluminum species were calculated along with pH limits of precipitation and dissolution at various concentrations. The calculated limits were almost identical to those determined experimentally at 24 hours. Precipitation experiments conducted in nitrate, chloride, sulfate and phosphate media indicated the solubility and colloidal stability of the precipitate is strongly affected by the type and concentration of anion present. The order of effectiveness was nitrate < chloride < sulfate < phosphate. Various structures of the soluble species and insoluble mixed salts are postulated and discussed. Comparison of pH-concentration limits of solubility and colloidal stability of aluminum hydroxide precipitates with published coagulation domains using similar salts indicates a very close agreement. Apparently, precipitation of insoluble hydroxide is an important factor in the coagulation process when using hydrolyzing metals such as aluminum.
URI: http://hdl.handle.net/1811/36328
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