The large size and dynamics of the ocean dissolved organic matter (DOM) pool have made it an important focus of many studies of global elemental cycles. These studies are motivated by the direct and indirect influences of DOM cycling on processes such as global warming and marine productivity. Over the last three decades, numerous compositional studies of various size fractions and chemical extracts have significantly increased our understanding of the origin and cycling of marine DOM. A large fraction of marine DOM, however, still remains compositionally uncharacterized. A major obstacle in the study of marine DOM has been isolating from seawater sufficient quantities for analysis of this highly dilute and chemically complex material.
In this project researchers at the Georgia Institute of Technology and Kansas State University will develop, test, and apply a process using reverse osmosis (RO) in combination with electrodialysis (ED) for the concentration and practical retrieval of significant quantities of essentially unaltered DOM from seawater. RO methods have been shown to recover a significant fraction (90%) of DOM from fresh waters with little physical or chemical alteration. Unfortunately, the extent to which a DOM sample can be concentrated by RO in saline waters is very limited because RO membranes co-concentrate inorganic salts with DOM. Salt accumulation during RO leads to solutions requiring very high pressures during processing to overcome osmotic forces and to formation of membrane-fouling inorganic precipitates. To achieve the same high DOM recoveries from saline waters that are achieved when RO is used to process fresh waters, an independent method for removal of inorganic salts is required. Electrodialysis, a well-established process for removal of inorganic salts from aqueous solutions, is such a method. Preliminary experiments by the co-investigators indicate that ED can significantly reduce salt concentrations in natural waters with little or no loss of DOM. Thus, the proposed process will use ED to reduce the salt concentrations, so that RO techniques can be used to recover DOM.
A series of laboratory experiments using artificial and natural seawater solutions will be used to evaluate and optimize the coupled RO/ED process. During the final stage of this research, the combined technique will be applied to seawater samples. The PIs already have two laboratory-scale ED systems and a portable RO unit capable of processing large volumes of water (hundreds of liters), which represents a significant cost savings to NSF for this work.
Broader Impacts: The expected result from this work is an enabling tool for environmental research that is far superior to the best current techniques (i.e., ultrafiltration) for recovery of marine DOM. Research involving the recovery of complex organic molecules in natural water sources (rivers, lakes, seawater) will be greatly facilitated by a field-deployable method for pre-concentration. In terms of broader educational impacts, this investigation will allow the PIs to train and mentor a diverse group of students and researchers, including several minorities and women.
