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The objective of this study was to develop, optimize, and apply a novel approach to concentrate and isolate organic carbon using reverse osmosis (RO) followed by electrodialysis (ED). Electrodialysis is a membrane separation process employing ion-exchange membranes which allows separating charged from neutral constituents. Due to an electric potential and a concentration gradient as the driving forces of separation, organic molecules can be isolated from inorganic salts without altering characteristics of dissolved organic carbon (DOC). Issues associated with ED separation are related to potential migration of charged low-molecular weight organics through the membrane or losses due to adsorption onto the membrane surface. A 1-stage lab-scale RO-unit, an ED membrane test unit, and an ED-stack (Electrosynthesis, Inc.) were employed to perform experiments with single organic compound solutions varying in molecular weight, surface water samples, and treated wastewater samples. Five different ion-selective membrane pairs were investigated in the membrane test unit to determine the best-suited ED membrane for laboratory-scale operation. To investigate the fate of bulk organics and specific target compounds, cations, anions, dissolved organic carbon (DOC) and UV absorbance were measured. Conductivity, pH, and current were measured online during each ED-experiment. Lessons learned from this study are that RO separation to concentrate organics from NOM and EfOM samples can achieve DOC recoveries larger than 90 percent. Losses of DOC into the RO permeate varied for NOM samples between 7 and 8 percent and for EfOM samples between 1 and 2 percent, respectively. During the ED-membrane selection process, a monovalent-selective membrane combination (Asahi Glass, Inc.) showed the best DOC rejection of all membranes investigated. The DOC rejection of NOM and EfOM concentrates during ED-treatment varied between 96 and 97 percent. It was found that charged low-molecular weight compounds (molecular weight less than 200 Dalton) can only be partly rejected during ED and therefore contributed to the DOC loss. Fouling of humic substances onto the ED membrane was reversible during a subsequent rinsing procedure with 0.1 N HCl-solution. With overall DOC rejection rates of more than 90 percent the RO/ED-approach can be a valuable alternative to the conventional XAD-8/-4 resin approach to concentrate and isolate organics from water samples. This approach recovers more and different fractions as compared to XAD-resin methods and avoids sample contamination and DOC alteration. The approach is faster and capable of processing higher volumes of samples low in initial DOC (such as groundwater samples). Includes 19 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2002 Number of Pages: 18File Size: 1 file , 580 KB