PROSPECT OF INTEGRALLY ASYMMETRIC CTA-BASED AND THIN FILM COMPOSITE PA-BASED MEMBRANES FOR OSMOTICALLY-DRIVEN SEPARATION PROCESS

Abstract

Osmotically-driven separation processes have been known as the forward osmosis (FO) and pressure-retarded osmosis (PRO). Among membrane technologies applied to water/wastewater treatment, FO has recently attracted much attention of scientists due to its saving energy, low fouling propensity and high rejection to a wide range of contaminants. Meanwhile, PRO has been considered as a potentially alternative power generation. Major challenges limiting the application of FO are a lack of desirable membranes, lack of proper draw solution and effect of concentration polarization (CP) phenomenon. The first part of this dissertation focuses on studying desirable membrane structures for osmotically-driven separation process, namely fabricating membranes to understand the impact of the polymer composition and preparing condition on the membrane formation and subsequently performance, investigating the effect of structure on performance of diverse membranes, and studying chlorination of the membranes. The later part of the dissertation examined the effect of concentration polarization on performances of various membrane structures for a suggestion of a preferred membrane configuration. A comparison of the structure and performance between integrally asymmetric and thin film composite (TFC) FO membranes were done in this study to gain a better understanding of the FO transport mechanism, and to suggest appropriate characteristics of FO membranes. The effect of structure parameters (S) on the performance of the membranes was also examined. Under various operating conditions, the TFC membranes yielded superior Jw than the integrally asymmetric CTA-based membranes, especially, with larger Jw leading to higher solute resistance at alkaline feed and draw solutions. However, the integrally asymmetric membranes possessed smoother surfaces, resulting in lower fouling propensity than the polyamide TFC membranes. Besides the structure, chemical composition of the skin layer also affected the performance. TFC membranes from Toray Chemical Korea (TCK) showed lower oxygen atomic content on the active layer (AL), higher negatively charged AL and higher fouling propensity compared to the TFC HTI membrane. The TCK membrane with a woven substrate served as a promising membrane, with 2.25 times higher Jw and 1.48 times less Js/Jw than the TFC HTI membrane. This study showed that a preparation of FO membranes with both enhanced antifouling resistance and TCK membrane-like open and thin structure are required to develop desirable FO membranes. Integrally asymmetric cellulose triacetate/cellulose acetate (CTA/CA)-based membranes for FO were prepared via immersion precipitation method. The CTA/CA-based membranes were prepared in this studied using 1,4-dioxane and acetone as a solvent mixture, methanol and maleic acid as additives. The optimization of casting composition - 1,4-dioxane/acetone ratio and CTA/CA ratio - and preparation conditions - substrate type, casting thickness, evaporation time and annealing temperature - was carried out. Membrane properties and performances were investigated against commercially available integrally asymmetric membranes by using various analytical tools and FO test system. The FO membrane prepared under the optimized composition and preparing conditions had a smooth surface, and showed higher water flux (Jw) and salt resistance (in terms of high salt rejection R and low reverse salt flux (RSF) Js) than the commercial CTA-based membranes. Annealing improved the membrane performance by removing residual additives and solvents. The computerized image processing of optical microscopy images was shown to be useful for assessing the membrane substrates. The effect of concentration polarization on performances of integrally asymmetric and TFC FO membranes was systematically investigated in two membrane configurations - active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS). Jw of the membranes using pure water as FS in AL-DS were 1.7 to 2.6 times higher than those in AL-FS. However, with increasing FS concentration, the Jw in AL-DS declined rapidly and became lower than that in AL-FS. A modeling study showed that the reversal of Jw in the two configurations were resulted from concentrative ECP, and theoretical FS concentrations at which Jw in both configurations were equal were inversely proportional to the initial Jw of the membranes. The turnover of the Jw in AL-DS and AL-FS was also observed within 8 hours when seawater and river water were used as DS and FS, respectively. Moreover, the AL-DS faced the challenge of high fouling propensity and difficulty in cleaning more than the AL-FS. This study suggests that AL-FS membrane configuration is proper for both PRO and FO processes employing real natural water in terms of high membrane performance and ease of cleaning. The available integrally asymmetric CTA-based and TFC PA-based FO membranes were also studied chlorination by hypochlorite. Performances and properties of various FO membranes were tested and characterized using ATR-FTIR, XPS, zeta potential, NMR and, particularly, ToF-SIMS. ToF-SIMS data suggested that chlorine first reacted with acetyl/acetate group, and then pyranose ring. Chlorine chemically bound and weakened H-bond of the CTA-based membrane, especially, cleaved the pyranose ring at high chlorine dosage, which resulted in the increase of water flux and reverse salt flux (RSF) of the chlorinated CTA-based membrane upon addition of chlorine exposure time or chlorine concentration. The CTA-based membrane structure after chlorination at high chlorine dosage was eventually fatigued upon filtration time. Besides, oxidation of CTA-based membrane by chlorine maximized at pH 7. The PA-based membrane was more vulnerable to chlorine at the acidic condition, resulting in more selectivity loss compared with the CTA-based membrane. However, alkaline chlorine solution insignificantly affected both FO membrane types. The TFC PA-based membrane yielded high water flux in FO but need improving chlorine tolerance for real application in which pretreatment or membrane cleaning uses chlorine.