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We have tested to see if the generic set of NICA-Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back-titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non-ideal, competitive-adsorption NICA-Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H þ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V D , varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K i int ) andthe heterogeneity of the site (m i ), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by 13 C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.

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