Abstract
Aqueous two-phase extraction (ATPE) is a green separation technique which uses mixtures of water and environmentally benign polymers such as polyethylene glycol (PEG) as solvents. One of the challenges in implementing this extraction on an industrial scale is finding a suitable method for the isolation of target compounds from water-polymer solutions after the extraction, without diminishing ecological benefits of the method. In this paper, we propose using another green separation technique, supercritical fluid extraction (SFE), for the back-extraction of low molecular weight medium polarity compounds from ATPE solutions. Experiments with two model compounds, caffeine and benzoic acid, showed principal applicability of SFE for this task. Pressure (100–300 bar) and temperature (35–75 °C) of supercritical carbon dioxide play a major role in defining extraction capability. Extraction ratios of 35% for caffeine and 42% for benzoic acid were obtained at high fluid pressure and moderate temperature at 1:6 volume phase ratio. That gives an estimation of 10–20 theoretical steps required for complete exhaustive extraction from the ATPE solution, which is readily achievable in standard counter-current column SFE. Combining these two green methods together not only serves as an environmentally friendly method for the isolation of valuable low molecular weight compounds from diluted water solutions, but also allows for simple, energy effective recuperation of ATPE solvents.
Highlights
Polyethylene glycoles (PEG) are one of the most wide-spread polymeric components used in Aqueous two-phase extraction (ATPE) since they are inexpensive, abundantly available, non-toxic, biodegradable and environmentally safe
supercritical fluid extraction (SFE) from ATPE extract would most likely be implemented in a flow regime using a counter-current column
If back-extraction is performed using SFE, once isolation of the target compound is complete, ATPE phase is left clean because SC-CO2 automatically evaporates at pressure release
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. After ATPE, target compounds exist in a water-polymeric solution. Its isolation from such solutions makes for a tricky task. If back-extraction has to be performed using them, it somewhat depreciates the greenness of ATPE method From this point of view, the approach where first one performs a clean isolation using environmentally benign water-PEG solvent and contaminates it with a hazardous back-extraction solvent seems to be illogical. In this communication, we propose using another green separation process, supercritical fluid extraction (SFE), as a method for performing back-extraction after ATPE. We demonstrate the principal validity of this proposal by extracting two model compounds from water-PEG solutions and briefly consider possible features of this combination of extraction techniques
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