Acidity of proton saturated and autotransformed smectites characterized with proton affinity distribution

Potentiometric titration of proton saturated fine fractions of three benton ites were used to characterize the acid centre at the smectite-water interf ace in dispersions. The H-forms were prepared by H+ --> OH- --> H+ ion exch ange with resins. The obtained titration data were used in a thermodynamic calculation of proton affinity distribution. Numerical solving of an integr al adsorption equation revealed a continuous distribution bf proton interac tion sites. Proton affinity distributions clearly detected up to 5 differen t proton interaction sites in all the smectite-water systems, within access ible experimental range of pH's between 2 and 12. The amount of the stronge st acid sites, i.e. those with the lowest pK values, decreased on aging, wh ile the amounts of all weaker acid sites increased with the progress of aut otransformation. The strongest acid sites are connected with free protons p resent in the dispersion, while the weaker acid sites are connected with th e titration of released structural Al3+, Fe3+, Mg2+ cations and/or their hy drolyzed species, and deprotonation of SiOH groups. These results indicate the sources of acidity in acid activated bentonites.