Reverse Osmosis Separation of Polar Organic Compounds in Aqueous Solution Herbert H. P. Fang' and Edward S.
K. Chian'
Environmental Engineering, Civil Engineering Department, University of Illinois, Urbana, 111. 61801 I The reverse osmosis separation of polar organic compounds from water was examined. A total of 1 2 different reverse osmosis membranes were tested with 13 polar lowmolecular-weight organic compounds consisting of various functional groups. Experimental data indicated that the separations of these compounds were not as effective as those of inorganic salts for all the membranes tested. The aromatic-polyamide-(AP)- and NS-type membranes gave respectively an overall of 50 and 75% separation of polar organic compounds, whereas the cellulose acetate base showed an overall separation ranging from 13-27%. The separation of a given organic compound with a given membrane depends on the chemical nature of the molecule as well as the characteristics of the membrane. In addition, the separation of compounds having the same functional group increases with the increase in the size and branching of the molecules.
A number of new reverse osmosis membranes have been developed in the last few years ( 1 , 2). Separation of inorganic salts has been investigated for most of these membranes, but little work has been done on the study of their separation of polar organic chemicals commonly found in the wastewaters. The objective of this study was to examine the effectiveness of various reverse osmosis membranes for the separation of polar organic chemicals in water. The tested membranes included eight different types of flatsheet membranes made of various materials, as well as four types of modules manufactured in either tubular or hollow fiber form. I t has been reported ( I , 3-5) that the separation of a specific polar organic compound by a given reverse osmosis membrane depends largely on the functional group of the compound, whereas the separation of compounds having the same functional group depends on the size and branching of the molecules. Accordingly, in this study only the low-molecular-weight compounds were selected for membrane testing from each functional group, including acid, aldehyde, amide, amine, ester, ether, ketone, and phenol. Three alcoholic compounds-i.e., methanol, ethanol, and i -propanol-were tested by each membrane for the examination of the steric effect of the solute.
Experimental Testing was performed using the system illustrated in Figure 1. The temperature of the test solution was controlled by a thermoregulator. With sufficient circulation of the solution, the temperature was maintained a t 25 f 0.1 "C. A Yarway (Blue Bell, Pa.) Cyclo-Phram metering pump was employed for delivering the pressure of the test solution. The flow rate of the solution was controlled by adjusting the stroke displacement. The pressure, on the other hand, was adjusted through a pressure regulator placed downstream of the test unit. Pressure pulsations were dampened effectively by a bladder-type accumulator. Pressure gauges and flowmeters were installed a t both the upPresent address, Eastern Research Center, Stauffer Chemical Co., Dobbs Ferry, N.Y. 10522. 364
Environmental Science & Technology
stream and downstream of the test unit. All the wetted parts under high pressure were constructed of 316 stainless steel. The membranes tested in-this study, their abbreviations, configurations, and suppliers are shown in Table I. Flatsheet membranes were tested a t 600 lb/in.* (psig) and a flow rate of 0.30 gal/min (gpm) using stainless steel test cells based upon Manjikian's design (6). As suggested by the suppliers, the tubular modules made of CA and NS-100 membranes were tested a t 600 psig and 1.5 gpm while the hollow fiber B-9 and B-10 modules were tested a t 400 psig and 2.5 gpm, and 750 psig and 4.0 gpm, respectively. Each membrane was first characterized with a 5000parts-per-million-(ppm) sodium chloride solution followed by testing with 13 solutions each containing a single organic compound a t a constant concentration of 1000 ppm a t zero product water recovery. However, with the B-9 and B-10 permeators, a product water recovery of 75% was employed. No pH adjustment was made for these test solutions. At the end of each series of testings, the membrane was again tested with a 5000-ppm sodium chloride solution. By comparing both tests against sodium chloride, the deterioration of a membrane, if any, could be identified. The organic compounds selected for membrane testing, their chemical classifications, and toxicities (7) are shown in Table 11. A Yellow Springs Conductivity Bridge, Model 1485 (Yellow Springs, Ohio), was used to measure the concentration of sodium chloride. At dilute concentration (
