INFLUENCE OF IRON-OXIDES ON THE NON-SPECIFIC ANION (CHLORIDE) ADSORPTION BY SOIL

Soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite-EDTA (DE) to dissolve crystalline iron oxides. The amounts of chloride adsorbed by the extracted soils from 1 M NaCl at pH 5 and pH 7 were determined. The differences (.DELTA.Cl) between chloride adsorption at pH 5 and pH 7, attributed to variably charged groups, decreased when iron oxides were removed by EDTA and DE extraction. Close correlations (P < 0.001), with negligible intercepts, were found between EDTA-extractable iron (amorphous iron oxides) and the decrease in .DELTA.CJ following EDTA extraction, and between the difference between DE-extracted iron and EDTA-extractable iron (crystalline iron oxides) and the further decrease in .DELTA.Cl following DE extraction. The difference between .DELTA.Cl for acetate-extracted and DE-extracted samples was calculated from the contents and specific surfaces of amorphous and crystalline iron oxides, together with .DELTA.Cl per m2 for synthetic iron oxides. Calculated and measured values were in good agreement, indicating that soil iron oxides, in relation to chloride adsorption, may be treated as if they consist of only 2 fractions.