A Novel Approach using Reverse Osmosis/Electrodialysis (RO/ED) to Concentrate and Isolate Organic Carbon from Water Samples

摘要

The objective of this study was to develop, optimize, and apply a novel approach toconcentrate and isolate organic carbon using reverse osmosis (RO) followed byelectrodialysis (ED). Electrodialysis is a membrane separation process employing ionexchangemembranes which allows separating charged from neutral constituents. Due toan electric potential and a concentration gradient as the driving forces of separation,organic molecules can be isolated from inorganic salts without altering characteristics ofdissolved organic carbon (DOC). Issues associated with ED separation are related topotential migration of charged low-molecular weight organics through the membrane orlosses due to adsorption onto the membrane surface. A 1-stage lab-scale RO-unit, an EDmembrane test unit, and an ED-stack (Electrosynthesis, Inc.) were employed to performexperiments with single organic compound solutions varying in molecular weight,surface water samples, and treated wastewater samples. Five different ion-selectivemembrane pairs were investigated in the membrane test unit to determine the best-suitedED membrane for laboratory-scale operation. To investigate the fate of bulk organics andspecific target compounds, cations, anions, dissolved organic carbon (DOC) and UVabsorbance were measured. Conductivity, pH, and current were measured online duringeach ED-experiment.Lessons learned from this study are that RO separation to concentrate organicsfrom 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 percentand for EfOM samples between 1 and 2 percent, respectively. During the ED-membraneselection process, a monovalent-selective membrane combination (Asahi Glass, Inc.)showed the best DOC rejection of all membranes investigated. The DOC rejection ofNOM and EfOM concentrates during ED-treatment varied between 96 and 97 percent. Itwas found that charged low-molecular weight compounds (molecular weight less than200 Dalton) can only be partly rejected during ED and therefore contributed to the DOCloss. Fouling of humic substances onto the ED membrane was reversible during asubsequent rinsing procedure with 0.1 N HCl-solution. With overall DOC rejection ratesof more than 90 percent the RO/ED-approach can be a valuable alternative to theconventional XAD-8/-4 resin approach to concentrate and isolate organics from watersamples. This approach recovers more and different fractions as compared to XAD-resinmethods and avoids sample contamination and DOC alteration. The approach is fasterand capable of processing higher volumes of samples low in initial DOC (such asgroundwater samples).