Release and uptake of protons by allophanic soils in relation to their CEC and AEC

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Abstract
The CEC and AEC, and the net proton release during their measurement, were determined for two allophanic soils, by a CEC and AEC procedure that eliminates washing for the removal of excess saturating salt and by alkali titration of washings of salt solutions used for saturation and equilibration. The CEC and net proton release change parallels the changes in pH, temperature and the cation species, but the following stoichiometric relationships hold in all the measurement: in which the R and U in parentheses stand for release and uptake, respectively. The dependence of the measured CEC on the pH and cation species is primarily due to their effects on proton release from weak acid radicals. The large CEC increase at a higher temperature, which is largely irreversible, is more complicated and possibly includes reactions by which some “bound” functional groups are set free for ionization and/or by whfch an Al-OH-Al bonding is formed between two terminal Al-H2O groups. The summation of exchangeable hydrogen, aluminum and bases can not be equated with the CEC for allophanic soils, when they develop both positive and negative charges.

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