Effect of pH on adsorption of sulfonated surfactants to a sandy soil

Sorption of two commercial diphenyl oxide disulfonate (DPDS) anionic surfactants to a sandy soil were tested as a function of pH. Surfactant sorption increased at pH lower than the pH(zpc) of the soil, but decreased at pH lower than the pK(a) of the surfactants. The later effect is due to the protonation of anionic surfactant head groups at low pH. Acid/base titrations were used to examine proton interactions with the DPDS surfactants, as well as an acetate- and phosphate-based surfactant. Somewhat surprisingly, titration data showed that the head groups of anionic surfactants exhibit an acid/base chemistry similar to their soluble ligand counterparts, with no change in apparent pK(a) with degree of protonation. Soil titrations were conducted to determine soil surface protonation reactions and the pH(zpc) of the soil. Semiempirical modeling approaches that included the acid/base chemistry of the soil and surfactant, as well as a surface complexation model for surfactant sorption, describe experimental data well, and illustrate the effects of soil pH(zpc) and surfactant acidity on adsorption of the surfactant. Although surfactant sorption to variable charge surfaces such as soils is mechanistically complex, the results of this study suggest that relatively simple semiempirical modeling approaches may be useful to evaluate surfactant behavior in field settings.