Kinetics of chemical weathering in b horizon spodosol fraction

Abstract

Studies on a B horizon soil from Maine have been conducted to determine the weathering rate dependence on hydrogen ion concentration in soil solution. Effects of soil concentration and solution chemistry on chemical weathering rate were also investigated. Studies were conducted using a laboratory pH‐stat semicontinuous reactor at pH 2.7 and 3.5 for two concentrations of B horizon soil. Homogenized B horizon soil in the size range 63–125 μm was washed in deionized water and acetone and used without further treatment. Results obtained from titrations of 10, 30, and 100 g/L of soil at a constant pH of 2.7 suggest that the weathering rate, expressed in μeq H+/hr/g soil, does not depend on the soil concentration in the pH‐stat reactor. In order to test the effect of solute concentration on weathering rate, experiments were performed at two different concentrations of accumulated solutes. Weathering rates were the same at the two solute concentrations, indicating that significant back reactions were not likely occurring. The weathering rate at pH 2.7 was estimated in the range 0.53–0.93 μeq H+/hr/g soil, at pH 3.0 as 0.52 μeq H+/hr/g soil, at pH 3.5 as 0.36 μeq H+/hr/g soil, and at pH 4.0 as 0.04 μeq H+/hr/g soil. Assuming a weathering rate proportional to {H+}m, the fractional order m was determined to be approximately 0.8. Results were consistent with a surface‐reaction‐controlled dissolution mechanism.