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

Effects of hydrolysis of Al (300 μM) at OH/Al molar ratios of 0, 1, or 2 on forms of Al 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/Al molar ratio of 5 or 20. Aluminum was supplied as AlK(SO4)2•12H2O or Al(NO3)•9H2O, and additional SO4 was supplied as CaSO4•2H2O. 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/Al molar ratio decreased the concentration of total Al in solution with AlK(SO4)2 but not with Al(NO3)3. However, in the presence of basal nutrients, concentrations of total and monomeric Al decreased with an increase in OH/Al molar ratio in solutions with either AlK(SO4)2 or Al(NO3)3. Increase in the SO4/Al ratio resulted in a slight but consistent decrease in concentrations of monomeric and total Al only in hydrolyzed Al solutions. Precipitates found in hydrolyzed Al solutions were amorphous and contained Al and S, with the S/Al ratio ranging from 0.15 to 0.27, depending on the source of Al, the degree of hydrolysis, the presence of basal nutrients, and the SO4/Al molar ratio. Speciation calculations using the MINTEQ program showed super-saturation with respect to AlOHSO4, Al4(OH)10SO4, and alunite in solutions containing a SO4/Al molar ratio of 5 or 20 using either AlK(SO4)2 or Al(NO3)3 and in solutions containing AlK(SO4)2 with no addition of SO4. © 1990 Williams and Wilkins.