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DIVISION S-2—SOIL CHEMISTRY

Evaluation of Soil Surface Charge Using the Back-Titration Technique

Dep. of Natural Resource Sciences, Macdonald Campus of McGill Univ., 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9

^* Corresponding author (William.Hendershot{at}mcgill.ca).

Variable surface charge (Q_v) is one of the most important soil properties controlling ion adsorption on the soil solid phase. In this study, the back-titration technique was used to determine the Q_v of soils with a wide range of properties. The procedure defines the Q_v as the OH^– consumption by surface reactions corrected for dissolution of the solid phase and other solution reactions (e.g., metal hydrolysis). The Q_v, varying from 0 to 80 cmol_c kg^–1, was dependent on the pH of the soil suspension and the amount of soil organic matter. We used the non-ideal competitive adsorption (NICA)–Donnan model to simulate the surface charge, assuming a bimodal distribution of H⁺ affinity on the soil solid phase. With the charge data and Microsoft Excel, the NICA-Donnan model parameters were optimized. The model provided an excellent fit to the experimental data. When the pH was below 8, the surface charge was dominantly distributed to the Type 1 sites; the Type 2 sites started to contribute to the total surface charge at pH > 8. Multiple linear regressions showed that the charge maxima (Q_max) of the two sites were related to soil cation-exchange capacity (CEC) and organic C (Org. C); these significant statistical relationships may be used to estimate the surface charge of soils using values of commonly measured soil parameters.

Abbreviations: CEC, cation-exchange capacity • EGME, ethylene glycol monoetheyl ether • HS, humic substances • K, median affinity constant • m, intrinsic site heterogeneity • NICA, Non-Ideal Competitive Adsorption • Org. C, organic carbon • Q_max, maximum charge • Q_p, permanent charge • Q_v, variable surface charge • SSA, specific surface area

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