Metal-induced sulfate adsorption by soils

Sulfate is present in soils as salts of various metals, and the different metals associated with sulfate may significantly affect its reactions on colloidal surfaces and consequently its mobility in soils. Four analytical methods, based on different principles, were evaluated for determination of sulfate in the presence of different counter-ions (metals) and in soil extracts obtained with 0.1 M LiCl, 0.15% CaCl[subscript]2, or 500 mg P L[superscript]-1 as Ca(H[subscript]2PO[subscript]4)[subscript]2. The recovery of sulfate associated with various mono-, di-, and tri-valent metals was quantitative by the methylene blue (MB) method. But, tri-valent metals, such as Al, In, La, and Sc, decreased the recovery of sulfate by the ion chromatographic (IC), turbidimetric (TD), and indirect Ba atomic absorption spectrophotometric (SP) methods. Among the four methods, the MB and IC methods were the most accurate and precise. Studies on the effect of the sulfate counter-ions (metals) on sulfate adsorption by four acid soils (two from Iowa, one from Chile, and one from Costa Rica) at various equilibrium solution pH (3.5-7.0) and two background electrolyte concentrations (1 and 10 mM NaCl) showed that sulfate adsorption was affected by point zero of charge (PZC) and was the greatest at the lowest pH values (ca. 3.5) and decreased with increasing the equilibrium solution pH and background electrolyte. Sulfate adsorption envelopes and isotherms for different sulfate forms were greatly affected by the type and charge of the sulfate counter-ion and, in general, followed the order: In[subscript]2(SO[subscript]4)[subscript]3 \u3e Al[subscript]2(SO[subscript]4)[subscript]3 \u3e CaSO[subscript]4 \u3e MgSO[subscript]4 \u3e Cs[subscript]2SO[subscript]4 \u3e K[subscript]2SO[subscript]4 \u3e (NH[subscript]4)[subscript]2SO[subscript]4. For the same valence of the sulfate counter-ion, sulfate adsorption increased with increasing ionic radius of the sulfate counter-ion. When its counter-ion was maintained at a constant concentration (12.0 mmol[subscript] c L[superscript]-1), sulfate adsorption was greater than when sulfate and its counter-ion were added at equivalent concentrations. Based on solubility diagrams and distribution coefficients (K[subscript] d) vs sulfate surface density ([sigma]) at maximum adsorption (X[subscript] m) relationships, precipitation reactions of metal sulfates were ruled out. The effects of the type and valence of the sulfate counter-ion on sulfate/metal adsorption ratios at various pH values and on the slopes of the linear relationships between sulfate and its counter-ion adsorbed were greater in soils with greater concentrations of hydrous Al and Fe oxides. The K[subscript] d values and the adsorption parameters of the one-surface and two-surface Langmuir equations for sulfate adsorption were greatly affected by the type, valence, and concentration of the sulfate counter-ion. Application of the three transformations of the Langmuir, a modified Langmuir, and the BET equations to the calculation of the sulfate adsorption parameters were compared