KINETICS OF ALUMINUM AND SULFATE RELEASE FROM FOREST SOIL BY MONOPROTIC AND DIPROTIC ALIPHATIC-ACIDS

A batch equilibration study evaluated the influence of naturally occurring low-molecular-weight mono- and diprotic aliphatic acids on the rate of Al and SO42−release in a Cecil soil (Typic Hapludult). We adjusted the pH of the organic acids (OAs) and of the soil suspension (3.8% w/w) to pH 4.0 and allowed them to equilibrate thermally before the experiment. After rapid addition of OAs to the soil suspension, we took solution samples at various time intervals and analyzed for Al, SO42−, and OA concentration. The initial concentration of OA in suspension was 1 × 10−5mol liter−1. Both Al and SO42−release followed pseudo-first-order kinetics, whereas OA adsorption obeyed simple first-order kinetics. The rate of Al release (k1) was more rapid for the diprotic OA treatment (20.4 × 10−8mol s−1), as was SO42−release (1.63 × 10−8mol s−1), compared to the monoprotic OA treatment. The rate of Al release varied inversely with OA chain length and the distance between - COOH functional groups. The addition of substituent - OH groups between the - COOH groups further reduced k1. A similar trend was observed for the rate of SO42−release (k1) into solution. Monoprotic OAs were more rapidly adsorbed to the particle surfaces than were diprotic OAs. We postulate that removal of Al and SO42−from solution occurs via selective mineral precipitation. © 1990 Williams & Wilkins.