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Studies of Aged OH-Al Solutions using Kinetics of Al-Ferron Reactions and Sulfate Precipitation¹

ABSTRACT

A series of OH-Al solutions (0.02M in Al, NaOH/Al molar ratio = 1, slow neutralization) aged from 10 d to 55 months were all clear and had similar pH values and monomeric Al concentrations but reacted differently with sodium sulfate and with ferron. The addition of sodium sulfate yielded three kinds of basic aluminum sulfates: tetrahedral crystals of composition Na_0.08 Al(OH)_2.46 (SO₄)_0.31 · xH₂O, x-ray amorphous spherical particles of composition Al(OH)_2.34 (SO₄)_0.33 · xH₂O, and irregular-shaped crystals of composition Al(OH)_2.38 (SO₄)_0.31 · xH₂O. The distribution of these sulfates is related to the aging of the parent OH-Al solutions. The kinetics of Al-ferron color development indicated that the OH-Al polymers in the fresh solutions reacted with ferron rapidly, following a pseudo first-order reaction (rate constant = 0.096 min^–1). With increased aging, the rapidly reacting polymers slowly decreased, whereas another polymeric species that reacted with ferron slowly (rate constant = 0.00038 min^–1) gradually developed. The results suggest that the polymers that reacted rapidly with ferron accounted for the formation of tetrahedral crystals and spherical particles, whereas those polymers that reacted slowly accounted for the formation of irregular-shaped crystals.

¹ New Jersey Agric. Exp. Stn., Publication no. D-15420-1-83 supported by state funds and by the Nat. Sci. Foundation (ENG 78-16497-Hsu).

² Former Graduate Research Assistant and Professor of Soil Chemistry, respectively, Dep. of Soils and Crops, Cook College, New Jersey Agric. Exp. Stn., Rutgers Univ., New Brunswick, NJ 08903. The senior author is now Lecturer, Dep. of Chemistry, Rutgers Univ., New Brunswick, NJ 08903.

Received for publication February 4, 1983. Accepted for publication August 26, 1983.