Kinetics of Chemical Weathering: A Comparison of Laboratory and Field Weathering Rates

J L Schnoor

Aluminosilicate minerals undergo surface-controlled dissolution reactions that are fractional order with respect to bulk H(+) ion concentration in solution. Kinetics of chemical weathering can be explained by surface coordination chemistry, where the detachment of an activated complex is the rate-determining step in dissolution. Dissolution reactions may occur incongruently at first, but eventually the build-up of a cation-depleted solid layer will provide for stoichiometric mineral weathering. Protons and organic ligands may accelerate the rate of mineral weathering. Results of a field study at Bear Brook Watershed, Maine, were compared with laboratory kinetic experiments using size-fractionated soils from the same location. The watershed is a forested, glaciated region with thin podzolic soils and granitic gneiss bedrock. Fractional order dependence of weathering rates on H(+) ion-activity were demonstrated for batch pH-stat experiments of B-horizon soils in the pH range 2.7-4.0. The soils tested were rich in aluminum, and B-horizon samples yielded high concentrations of dissolved aluminum (0.2-1.0 millimoles/L) during the first 50 hr of reaction. This was attributed to ion exchange and a rapid dissolution of amorphous aluminum hydroxide. As a result of the initial rapid reaction, H(+) ion consumption and release rates of other solutes (Si, Al, Ca, Mg, Na) were asymptotic with time. Release rates of ions became constant after 200 hr, and weathering rates were determined in the range 200-400 hr. Flow-through column experiments were also run on the same soils. Soil column results provided the best simulation of field conditions with pH 4 influent (sulfuric acid), and the effluent resulted in a pH of 5.8, similar to Bear Brook. A comparison of laboratory weathering rates and estimates from the field indicated that laboratory rates were one to two orders of magnitude greater than field estimates of chemical weathering. Based on dissolved silica as a conservative tracer of weathering, laboratory rates were on the order of 1 to 10 picomoles/sq m/sec, while field weathering rates were .01 to 1 picomoles/sq m/sec. The discrepancy is likely due to the difficulty of estimating a suitable wetted surface area of weatherable (reacting) minerals in the field, and the possibility of hydrologic control, due to macropore flow through soils. (See also W91-10309) (Geiger-PTT)

Kinetics of Chemical Weathering: A Comparison of Laboratory and Field Weathering Rates

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