KINETICS OF SULFATE DESORPTION FROM 2 SPODOSOLS OF THE LAURENTIANS, QUEBEC

The kinetics of sulfate desorption from two Spodosols at different levels of antecedent sulfate sorption was investigated using a batch technique. The release of sulfate from the B horizons of these soils was measured for periods ranging from 15 minutes to 96 h at 5, 25, and 40-degrees-C. Of the five simple kinetic equations (Elovich, first-order, second-order, parabolic diffusion, and power function) tested the best representation of the time-dependent desorption of sulfate was obtained with the Elovich equation. Desorption kinetics was also adequately described using the power function equation although standard errors of estimate (SE) were higher than for the Elovich model. In both horizons, sulfate desorption was rapid during the first 2 h of reaction. In fact, about 50% of the sulfate desorbed during the experiment was released in the first 5 minutes. After 4 days of reaction, however, desorption was still proceeding slowly. The slopes of the regression equations (apparent rate coefficient) for the Elovich model increased with increasing reaction temperature. These results evidenced the necessity of considering soil temperature when modeling the seasonal movement of sulfate and of associated cations in soil solution. Of the two horizons studied, adsorbed sulfate reservior depletion occurred proportionally more rapidly from the Laflamme B horizon.