(496f) Reactive Extraction of Metals and Organics Using Trioctylphosphine Oxide Impregnated Membranes

Over the past few decades, one of the most significant developments in the realm of solvent extraction is the segregation of the aqueous and the organic phases using semi-permeable membranes. This technique, also known as non-dispersive solvent extraction, facilitates complete recycle and reuse of the non-aqueous partitioning phase, while the downstream phase separation problems associated with conventional extraction are eliminated. The efficacy of membrane-based extraction has also been advanced through the development of other configurations such as supported liquid membranes and solvent encapsulated polymeric microcapsules. Recently, a new technique has also been developed for impregnation of solid organic extractants such as trioctylphosphine oxide (TOPO) into porous membranes. The resulting extractant impregnated membranes (EIM) combine the advantages of extraction, adsorption and membrane separation, which can be harnessed to facilitate solventless extraction of several metals and organic compounds.

So far, the EIMs have only been used in sequential extraction and stripping of phenols. Efforts to carry out simultaneous extraction and stripping could not fructify due to a non-uniform distribution of the extractant within membrane walls. In this research, two approaches were used to obtain a more even distribution of TOPO within the polymer support. In the first approach, the membranes were first soaked in dichloromethane containing high concentrations of TOPO, and dichloromethane was then evaporated in a controlled environment so that TOPO was distributed evenly throughout the membrane walls. In the second approach, membranes were soaked in molten TOPO at 60 °C first and the temperature was brought down quickly to solidify TOPO at the same spot within the membrane walls. Using both the approaches, the EIMs prepared exhibited good TOPO distribution. These EIMs could be used for simultaneous extraction and stripping.

Chromium (VI) and phenol were chosen as the model compounds to demonstrate the extractive performance of the EIMs. In order to understand the factors affecting extraction, and to obtain insight into extraction mechanisms, the effects of ring substitution on phenol extraction was also investigated. It was observed that the most important factor affecting the partition coefficient was hydrophobicity of the molecule. These results suggest that the EIMs can be a promising technique in sustainable and solventless extraction of several metals and organics.