ABSTRACT
Department of Soil Science, NWFP Agric. University, Peshawar, Pakistan and Department of Soil & Environmental Sciences, University of California, Riverside, California
A review of the adsorption of manganese by soils, layer silicates, hydrous oxides of iron, aluminum and manganese, carbonates, fulvic and humic acids, and soil organic matter is presented. In general, the adsorption of manganese by soils increases as the pH, clay, and organic matter content of the soil increase. Adsorption of manganese by hydrous oxides of iron, aluminum, and manganese also increase as the pH of the system increases. Although the mechanism of adsorption by hydrous oxides is not completely understood, it is generally considered to be related to the creation of negative charge through dissociation of protons from surfaces or association of hydroxyl ions with surfaces. At low ionic strength under acidic conditions the free metal ion is considered to be the dominant species adsorbed, while under basic conditions adsorption of MnOH+ is more likely. Studies on the adsorption of Mn2+ by manganese oxide show that surface charge reversal accompanies the adsorption of Mn2+. This suggests that an inner sphere complex is formed between the manganese oxide surface and the
Mn2+ ion. After removal of exchangeable plus soluble Mn2+ from soils by extraction with 1M NH4OAc, extraction with Cu salts removes additional Mn2+. The Mn2+ extracted with Cu following removal of exchangeable plus soluble is considered to be mainly that complexed by soil organic matter. Electron spin resonance spectroscopy evidence suggests that Mn2+ is bound to soil organic matter as both inner and outer sphere complexes. At pH values greater than about 6, bonding of Mn2+ with soil organic matter is apparently as an inner sphere complex, while at pH values less than about pH 5 bonding of Mn2+ with soil organic matter is considered to be as an outer sphere complex.
