Cation Exchange Capacity Variations with pH in Soil Clays

Abstract

Abstract Soil clays containing pedogenically chloritized 2:1 layer silicates, after brief contact at 25C with 2% Na 2 CO 3 (pH 11), developed a proportionately large increment of CEC which was retained after thorough washing with neutral salts (KCl, NaCl) to reduce the pH value below 7. A soil clay from Natal, S. Africa showed the largest increase—from 2.4 meq/100 g at pH 5 to 8.0 meq/100 g after alkaline and KCl treatments. This CEC hysteresis or delta value largely disappeared when washing with a NaOAc buffer of pH 5 followed the alkaline treatment. Similar effects were absent from specimen kaolinite, montmorillonite, and vermiculite. The presence of aluminous, pedogenic chlorite was shown, by X‐ray and analytical investigation, to be a common property of the soil clays exhibiting pH dependent CEC. This finding is supported by separate studies on synthetic aluminous chlorite, which showed variable CEC properties essentially identical to those of these soil clays. Release of the initially blocked isomorphous substitutional negative charge by deprotonation of the positive hydroxy alumina in aluminated soils, clays and allophane is the indicated mechanism accounting for pH dependent, hysteretic CEC charge of the mineral portion of soils.