Adsorption of Chromate by Subsurface Soil Horizons

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Abstract
Chromate adsorption was measured with and without reactive cosolutes on four subsurface soil horizons differing in pH and mineralogy, and on clay fractions from two of the oxide‐containing subsoils. Chromate adsorption was greatest in lower pH materials enriched in kaolinite and crystalline iron oxides. Over a range in pH, chromate adsorption to subsoil was similar to that observed for pure‐phase oxides. Chromate binding was reversible to pH and was depressed in the presence of SO2‐4 and dissolved inorganic C, which compete for adsorption sites. A surface site density for crystalline Al‐substituted iron oxides in the subsoils was estimated from chromate adsorption on the clay fractions using the model FITEQL and outer sphere surface coordination constants from Al‐goethite. The estimated site density for soil crystalline iron oxides was well below that of clean oxides, suggesting surface saturation by indigenous soil ions. The calculated site density and surface binding constants for Al‐goethite were used in the Triple Layer Model to calculate the effect of ionic strength, cosolutes, solids concentration, and sorbate concentration on CrO2‐4 adsorption. Model calculations were in good qualitative agreement with experimental results.