Adsorption of cationic–anionic surfactant mixtures on activated carbon

This paper reported the adsorption of cationic–anionic surfactant mixtures, such as octyltriethylammonium bromide/sodium dodecylbenzenesulfonate (OTEAB/SDBS) and dodecylpyridinium chloride/sodium octanesulfonate (DPC/SOS), on activated carbon (AC) in deionized water and in mineralized water systems. The AC surface chemistry was characterized by X-ray photoelectron spectroscopy and ζ-potential determinations. It was observed that in deionized water solution, the addition of SOS obviously promoted the adsorption of DPC, while the existence of OTEAB increased the adsorption of SDBS first and then decreased that slightly with increasing SDBS concentration. In mineralized water solution, the addition of cationic (anionic) surfactants reduced the adsorption of anionic (cationic) surfactants. It was shown that the adsorption of the surfactants on the AC was predominated mainly by the hydrophobic interaction between AC surface and surfactants because of the low oxygen content and very low ζ-potential on the AC surface. There might exist synergism between cationic and anionic surfactants when adsorbing on AC in deionized water due to the electrostatic interaction between oppositely charged surface active ions. Such synergism might be greatly weakened when a large number of inorganic salts exist owing to the “screen” effect of the counter ions of the salt on the electrostatic attraction of oppositely charged surface active ions.