Comparison of three techniques to assess surface heterogeneity of solids in soils

Changes of surface adsorption energy distributions in soils can help to diagnose soil degradation. Techniques are needed to quantify the changes in the energy distributions. We used data on water vapor adsorption in three soils before and after simulated degradation caused by organic matter oxidation, cyclic wetting-drying, and silica acid treatment. To describe adsorption energy distributions we applied a technique that assumes lognormal distribution of adsorption energy and two techniques assuming the distribution function to be piecewise linear: (a) direct fit of the set of nonlinear equations to adsorption data and (b) a preliminary singular decomposition of the matrix of this set of equations. Parameters of the distributions and parameters of local adsorption isotherms were estimated together with their standard errors. Estimated parameters of the local adsorption isotherm were close for all three techniques. Piecewise linear energy distributions were bimodal. Singular decomposition technique provided the most detailed insight into adsorption energy distribution changes after simulated soil degradation.