Influence of head group structure of cationic surfactants on the desorption of cesium from clays and clay minerals

The role of cationic surfactants head group for cesium (Cs) desorption from clays and clay minerals was investigated in this study by using two surfactants having similar hydrocarbon chain lengths but different head group types. Two surfactants used were dodecyltrimethylammonium bromide (DTAB) with linear structure and benzyldodecyldimethylammonium bromide (BDAB) containing bulkier head group. The results indicated that Cs desorption from bentonites using both surfactants was accompanied by interlayer expansion. In the case of Na-bentonite, BDAB showed higher Cs desorption efficiency than DTAB due to larger interlayer expansion after intercalation of BDAB. In contrast, the amount of desorbed Cs from Ca-bentonite was comparable, corresponding to the almost identical interlayer expansion after intercalation of both surfactants. It was because Ca ions present in Ca-bentonite must associate with two negatively charged interlayers, hence limiting its expansion. Cs desorption from kaolinite was more governed by surfactant micelles. When the surfactant micelles were formed earlier at low concentration, considerable desorption of Cs was achieved. In this regard, BDAB more efficiently desorbed Cs compared to DTAB owing to rapid formation of BDAB micelles at lower concentrations caused by the increase in head group size. For Cs desorption from illite, both surfactants even exhibited similar insignificant results with HCl used as a control of desorbent. Frayed edge sites in illite probably provoked strong Cs retention causing the difficulty in desorbing Cs. Furthermore, desorption kinetic data of Cs from all samples using both surfactants fitted well with pseudo-second order model, suggesting the occurrence of chemical desorption by involving ion-exchange mechanism.