Removal of linear alkylbenzene sulfonate (LAS) by a cetyltrimethylammonium bromide (CTAB)-aided...

ABSTRACTThis paper reports on the removal of linear alkylbenzene sulfonate (LAS) by conventional and cetyltrimethylammonium bromide (CTAB)-aided coagulation-filtration processes. The addition of CTAB led to the formation of CTAB-LAS aggregates, whose surface charge was negative at a CTAB-LAS molar ratio < 1 and positive at a molar ratio > 1. The efficacy of the coagulation-filtration process strongly depended on the electrostatic interactions between hydroxide floc and both free LAS and CTAB-LAS aggregate. Since free LAS is anionic, its adsorption onto the hydroxide floc was enhanced when the floc was positively charged. Ferric hydroxide floc (pH < 6.0) and aluminum hydroxide floc (pH < 7.4) showed positive surface charges and were thereby effective in removing free LAS. The optimal coagulation pH for removing the CTAB-LAS aggregate depended on the CTAB/LAS molar ratio. A higher coagulation pH was effective in removing LAS under the CTAB/LAS molar ratio > 1. However, in this case, excessive free CTAB remained after coagulation-filtration. Therefore, the lower coagulation pH with a CTAB/LAS molar ratio < 1, with the hydroxide floc and CTAB-LAS aggregate being positively and negatively charged, respectively, was recommended for LAS removal. In chemical industry wastewater, the optimal CTAB dose was higher than that in synthetic wastewater due to coexistent chemicals scavenging CTAB. Consequently, coagulation-filtration using a ferric chloride coagulant under the CTAB/LAS molar ratio of 0.7-2.0 and pH 4.5 was recommended for LAS removal from chemical industry wastewater, where the observed LAS removal efficiency reached 91 ± 3% without any deterioration of COD removal.