Adsorption of bisphenol A from water by surfactant-modified zeolite

Zeolite synthesized from coal fly ash (ZFA) was modified with hexadecyltrimethylammonium (HDTMA) and was examined for the adsorption of bisphenol A (BPA) from water. Two ZFAs were prepared in our laboratory and were characterized to obtain chemical and mineralogical composition, surface area, and total and external cation-exchange capacity among other parameters. HDTMA was confirmed to form bilayer micelles on external surfaces of zeolites. Results indicate that, while ZFA had no affinity for BPA, the surfactant-modified ZFA (SMZFA) showed greatly enhanced adsorption capacity. Uptake of BPA was greatly influenced by pH, increasing at alkaline pH conditions which enable the deprotonation of BPA to form organic anions. The SMZFA with higher BET area and higher amount of loaded HDTMA showed greater retention for BPA. Uptake of BPA by SMZFA was improved slightly in the presence of NaCl, and was enhanced at a low temperature. We propose that BPA anions interact strongly with the positively charged heads of HDTMA, with the two hydrophobic benzene rings of BPA pointing to the inside of HDTMA bilayers. The adsorption of uncharged BPA probably involved hydrophobic partitioning into HDTMA bilayers and the coordination of the oxygen atoms of BPA with positively charged heads of HDTMA.