REACTIONS BETWEEN ALUMINUM AND SULFATE IN DILUTE NUTRIENT SOLUTIONS WITH VARYING DEGREES OF HYDROLYSIS

Abstract

Effects of hydrolysis of AI (300 μM) at OH/AI molar ratios of 0, 1, or 2 on forms of AI were studied in de-ionized water and in a dilute nutrient solution matrix during 20 d of aging. In the dilute nutrient solution series, varying sulfate (SO4) additions were made to attain a SO4/AI molar ratio of 5 or 20. Aluminum was supplied as AIK(SO4)2δ12H2O or AI(NO3)δ9H2O, and additional SO4 was supplied as CaSO4δ2H2O. The solution pH varied from 4.2 to 4.7 with increase in degree of hydrolysis. The trends in pH with time were generally similar in the absence or presence of basal nutrients. In the absence of basal nutrients, increase in OH/AI molar ratio decreased the concentration of total AI in solution with AIK(SO4)2 but not with AI(NO3)3. However, in the presence of basal nutrients, concentrations of total and monomeric AI decreased with an increase in OH/AI molar ratio in solutions with either AIK(SO4)2 or AI(NO3)3. Increase in the SO4/AI ratio resulted in a slight but consistent decrease in concentrations of monomeric and total AI only in hydrolyzed AI solutions. Precipitates found in hydrolyzed AI solutions were amorphous and contained AI and S, with the S/AI ratio ranging from 0.15 to 0.27, depending on the source of AI, the degree of hydrolysis, the presence of basal nutrients, and the SO4/AI molar ratio. Speciation calculations using the MINTEQ program showed super-saturation with respect to AIOHSO4, AI4(OH)10SO4, and alunite in solutions containing a SO4/AI molar ratio of 5 or 20 using either AIK(SO4)2 or AI(NO3)3 and in solutions containing AIK(SO4)2 with no addition of SO4.