The effect of sulfate on aluminum concentrations in natural waters: some stability relations in the system Al 2O 3-SO 3-H 2O at 298 K

Abstract

While gibbsite and kaolinite solubilities usually regulate aluminum concentrations in natural waters, the presence of sulfate can dramatically alter these solubilities under acidic conditions, where other, less soluble minerals can control the aqueous geochemistry of aluminum. The likely candidates include alunogen, Al 2(SO 4) 3 · 17H 2O, alunite, KAl 3(SO 4) 2(OH) 6, jurbanite, Al(SO 4)(OH) · 5H 2O, and basaluminite, Al 4(SO 4)(OH) 10 · 5H 2O. An examination of literature values shows that the log K sp = −85.4 for alunite and log K sp = −117.7 for basaluminite. In this report the log K sp = −7.0 is estimated for alunogen and log K sp = −17.8 is estimated for jurbanite. The solubility and stability relations among these four minerals and gibbsite are plotted as a function of pH and sulfate activity at 298 K. Alunogen is stable only at pH values too low for any natural waters (<0) and probably only forms as efflorescences from capillary films. Jurbanite is stable from pH < 0 up to the range of 3–5 depending on sulfate activity. Alunite is stable at higher pH values than jurbanite, up to 4–7 depending on sulfate activity. Above these pH limits gibbsite is the most stable phase. Basaluminite, although kinetically favored to precipitate, is metastable for all values of pH and sulfate activity. These equilibrium calculations predict that both sulfate and aluminum can be immobilized in acid waters by the precipitation of aluminum hydroxysulfate minerals. Considerable evidence supports the conclusion that the formation of insoluble aluminum hydroxy-sulfate minerals may be the cause of sulfate retention in soils and sediments, as suggested by Adams and Rawajfih (1977), instead of adsorption.