Adsorption of CnTABr surfactants on activated carbons

Cationic surfactants of the quaternary ammonium bromide type C16TABr, C14TABr, C12TABr, and C10TABr were adsorbed from aqueous solutions onto three different types of activated carbons and compared with the adsorption of the anionic surfactant sodium dodecyl benzene sulfonate. The concentrations of both cationic and anionic surfactants in aqueous solutions were determined by UV spectroscopy. In the case of anionic surfactants, a linear dependence of UV absorbance on the surfactant concentration at 200 nm can be related to the benzene fragment of the molecule. The UV sensitivity of CnTABr solutions is controlled by the hydration of bromide counterions, the electron dense nature of bromine, and a charge-transfer-to-solvent phenomenon. Changes in the textural properties of the activated carbons after adsorption of surfactants were studied by N2 adsorption and mercury intrusion porosimetry, and could be correlated to surface characteristics and adsorptive capacity. The mechanism of adsorption was found to be complex, but the porous structure of the carbon exerted a critical role and the cationic surfactants were adsorbed step-by-step. At low surfactant concentrations, adsorption occurred in micropores. We conclude that surfactants are adsorbed as individual molecules. Adsorption from more concentrated solutions occurs in meso- and macropores, and resembles surfactant adsorption on flat carbon surfaces. The carbons display somewhat different tendencies to adsorb CnTABr. Activated carbons with small pores of 0.56-0.77 nm appear to be most effective for surfactant removal. A negative electrostatic potential on the interface of the carbons appears to enhance the adsorption of cationic surfactants, as would be expected. Controlling the electrostatic potential and pore size distribution of the activated carbons would be beneficial in the application of activated carbons for removal of surfactants from wastewater.