Removal of the mixture of tributyltin and tetrachloroethylene using microemulsion technique couple with neutral buoyancy concept

Abstract

This research studied the feasibility of using microemulsion technique couple with neutral buoyancy concept to remove the mixture of tributyltin (TBT) and tetrachloroethylene (PCE) from subsurface system. TBT is a hazardous organometal and was used as a biocidal paint to prevent barnacle adhesion to marine vessel. TBT was usually used as a mixture so this study used it as a mixture with PCE which was a common organic solvent so it was likely to be found coexisting with other organic contaminant. Both TBT and PCE are dense non aqueous phase liquid (DNAPL), therefore prevention of downward migration had to be considered when carrying out the removal process. Addition of alcohol was selected as a means to adjust the density of the contaminant to be closed to that of the surrounding water. Hence, most of the experiments in this study were carried out in the presence of alcohol. Major finding in this research were as follows. Hydrophobicity of organometallic under study, tributyltin (TBT), was characterized as an equivalent alkane carbon number (EACN) and was determined to be 11.17 by using Salager’s equation and linear mixing rule. The TBT-PCE mixture under focused was the mixture of 60 wt% TBT with 40 wt% PCE and had its EACN calculated to be 6.5 which was closed to the EACN of tetraethylead, another important organometal. The surfactant system selected for detail investigation was the system of 3.6 wt% SDHS + 0.4 wt% C16 DPDS at 3wt% NaCl (4 wt% total surfactant) with two alcohol types, n-butanol (BuOH) and tert-butanol (TBA). The study was conducted in both batch and continuous flow study by focusing on the removal of TBT and PCE and the density modification of mobilized NAPL. Performance of different alcohol and surfactant concentration were studied. In the continuous flow study, the flushing schemes were explored for both single step flushing which compared between the surfactant system with and without alcohol addition and two-step flushing which used only alcohol addition in the first step to adjust the density and added surfactant in the second step to reduce interfacial tension. Results from six column studies showed that the most efficient system for removal of TBT and PCE while having the density of mobilized NAPL lower than surrounding aqueous phase was the system with two- step flushing with saturated BuOH. The system was preflushed with saturated BuOH solution with 3 wt% NaCl and followed by 3.6 wt% SDHS + 0.4 wt% C16 DPDS at 3wt% NaCl saturated with BuOH. The main removal mechanism was mobilization at which 82.09% of TBT and 85.23% of PCE from total removal was removed as light nonaqueous phase liquid (LNAPL). There was some small DNAPL mobilization occurred but the density was already modified to1.063 mg.L-1, and it was accounted for 13.24% for TBT and 5.74% for PCE from their total removal. The total applied flushing solution was 40 times of the volume between pore space of the sand packed in the column.