Jinsik Sohn

Removal of toxic ions (chromate, arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration membranes

Rejection characteristics of chromate, arsenate, and perchlorate were examined for one reverse osmosis (RO, LFC-1), two nanofiltration (NF, ESNA, and MX07), and one ultrafiltration (UF and GM) membranes that are commercially available. A bench-scale cross-flow flat-sheet filtration system was employed to determine the toxic ion rejection and the membrane flux. Both model and natural waters were used to prepare chromate, arsenate, and perchlorate solutions (approximately 100microgL(-1) for each anion) in mixtures in the presence of other salts (KCl, K(2)SO(4), and CaCl(2)); and at varying pH conditions (4, 6, 8, and 10) and solution conductivities (30, 60, and 115mSm(-1)). The rejection of target ions by the membranes increases with increasing solution pH due to the increasingly negative membrane charge with synthetic model waters. Cr(VI), As(V), and ClO(4)(-) rejection follows the order LFC-1 (>90%) > MX07 (25-95%) congruent withESNA (30-90%)>GM (3-47%) at all pH conditions. In contrast, the rejection of target ions by the membranes decreases with increasing solution conductivity due to the decreasingly negative membrane charge. Cr(VI), As(V), and ClO(4)(-) rejection follows the order CaCl(2)<KCl congruent withK(2)SO(4) at constant pH and conductivity conditions for the NF and UF membranes tested. For natural waters the LFC-1 RO membrane with a small pore size (0.34nm) had a significantly greater rejection for those target anions (>90%) excluding NO(3)(-) (71-74%) than the ESNA NF membrane (11-56%) with a relatively large pore size (0.44nm), indicating that size exclusion is at least partially responsible for the rejection. The ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare ion rejection. For all of the ions, the rejection is higher than 70% when the r(i,s)/r(p) ratio is greater than 0.4 for the LFC-1 membrane, while for di-valent ions (CrO(4)(2-), SO(4)(2-), and HAsSO(4)(2-)) the rejection (38-56%) is fairly proportional to the r(i,s)/r(p) ratio (0.32-0.62) for the ESNA membrane.

Monitoring and modeling of disinfection by-products (DBPs)

In the United States, the newly promulgateddisinfectant/disinfection by-product (D/DBP) regulationsforce water treatment utilities to be more concerned withfinished and distributed water qualities. In this study,monitoring of DBP formation was conducted from three Frenchwater treatment plants trying to assess DBP variationsthrough time and space. Compared to the in-plant totaltrihalomethanes (TTHM) levels, TTHM levels in thedistribution system increased from less than 150% to morethan 300%. Significant variations for TTHM and bromate(BrO3-) levels throughout the seasons were alsoobserved; generally higher levels in the summer and lowerlevels in the winter. Combining chemical DBP models(empirical power functional models) and hydraulicsimulations, DBPs including TTHM and BrO3- weresuccessfully simulated from the full-scale monitoring data,indicating that empirical DBP model can be a potential toolto access DBP formation in actual plants. This study alsoprovides the protocols to assess DBP simulations in thewater treatment systems.

Simultaneously photocatalytic treatment of hexavalent chromium (Cr(VI)) and endocrine disrupting compounds (EDCs) using rotating reactor under solar irradiation.

In this study, simultaneous treatments, reduction of hexavalent chromium (Cr(VI)) and oxidation of endocrine disrupting compounds (EDCs), such as bisphenol A (BPA), 17α-ethinyl estradiol (EE2) and 17β-estradiol (E2), were investigated with a rotating photocatalytic reactor including TiO₂ nanotubes formed on titanium mesh substrates under solar UV irradiation. In the laboratory tests with a rotating type I reactor, synergy effects of the simultaneous photocatalytic reduction and oxidation of inorganic (Cr(VI)) and organic (BPA) pollutants were achieved. Particularly, the concurrent photocatalytic reduction of Cr(VI) and oxidation of BPA was higher under acidic conditions. The enhanced reaction efficiency of both pollutants was attributed to a stronger charge interaction between TiO₂ nanotubes (positive charge) and the anionic form of Cr(VI) (negative charge), which are prevented recombination (electron-hole pair) by the hole scavenging effect of BPA. In the extended outdoor tests with a rotating type II reactor under solar irradiation, the experiment was extended to examine the simultaneous reduction of Cr(VI) in the presence of additional EDCs, such as EE2 and E2 as well as BPA. The findings showed that synergic effect of both photocatalytic reduction and oxidation was confirmed with single-component (Cr(VI) only), two-components (Cr(VI)/BPA, Cr(VI)/EE2, and Cr(VI)/E2), and four-components (Cr(VI)/BPA/EE2/E2) under various solar irradiation conditions.

Predictive models and factors affecting natural organic matter (NOM) rejection and flux decline in ultrafiltration (UF) membranes

Prediction equations for NOM rejection were formulated using parameters, including specific UV absorbance [SUVA = UV absorbance at 254 nm/dissolved organic matter (DOC)] and af/kratio [a ratio of water permeability (D to the mass transfer coefficient (k)], which have been found to influence aspects of NOM and operating conditions, respectively. Attempts were made to formulate relationships between adsorption resistance (Ro) and interfacial membrane concentration (Cm), and specifically between the amount of NOM absorbed (mg C/cm 2 ) and the R o. Flux decline was also formulated in two ways: (1) by the (so-called) empirical flux-decline equation with three flux-decline coefficients, and (2) by the adsorption flux-decline model with two NOM adsorption terms between bulk, the NOM and the membrane surface and an existing NOM adsorption layer.

Sonocatalytical degradation enhancement for ibuprofen and sulfamethoxazole in the presence of glass beads and single-walled carbon nanotubes

Sonocatalytic degradation experiments were carried out to determine the effects of glass beads (GBs) and single-walled carbon nanotubes (SWNTs) on ibuprofen (IBP) and sulfamethoxazole (SMX) removal using low and high ultrasonic frequencies (28 and 1000kHz). In the absence of catalysts, the sonochemical degradation at pH 7, optimum power of 0.18WmL(-1), and a temperature of 15°C was higher (79% and 72%) at 1000kHz than at 28kHz (45% and 33%) for IBP and SMX, respectively. At the low frequency (28kHz) H2O2 production increased significantly, from 10μM (no GBs) to 86μM in the presence of GBs (0.1mm, 10gL(-1)); however, no enhancement was achieved at 1000kHz. In contrast, the H2O2 production increased from 10μM (no SWNTs) to 31μM at 28kHz and from 82μM (no SWNTs) to 111μM at 1000kHz in the presence of SWNTs (45mgL(-1)). Thus, maximum removals of IBP and SMX were obtained in the presence of a combination of GBs and SWNTs at the low frequency (94% and 88%) for 60min contact time; however, >99% and 97% removals were achieved for 40 and 60min contact times at the high frequency for IBP and SMX, respectively. The results indicate that both IBP and SMX degradation followed pseudo-first-order kinetics. Additionally, the enhanced removal of IBP and SMX in the presence of catalysts was because GBs and SWNTs increased the number of free OH radicals due to ultrasonic irradiation and the adsorption capacity increase with SWNT dispersion.

Pressure assisted forward osmosis for shale gas wastewater treatment

AbstractThis study investigated the feasibility of using pressure assisted forward osmosis (PAFO) for shale gas wastewater treatment. PAFO combines osmotic gradient across a membrane with external pressure together, which was expected to obtain higher flux than forward osmosis (FO). Experiments were performed in a laboratory-scale PAFO system, which allows the application of external pressure up to 10 bar on the feed solution side. Deionized water and three kind of synthetic shale gas wastewater, including low range, medium range, and high range wastewaters, were used as feed solutions and NaCl was used as a draw solution. The water flux was improved up to 10–15% by applying external pressure to FO when low range and medium range wastewaters were treated. However, the effect of the external pressure was significantly reduced when the high-range wastewater was treated. After FO treatment, air gap membrane distillation was successfully applied to re-concentrate the draw solutes.

Experimental comparison of direct contact membrane distillation (DCMD) with vacuum membrane distillation (VMD)

Abstract Membrane distillation (MD) is a thermally driven membrane process using porous hydrophobic membranes. MD has been investigated as an alternative desalination technology due to its advantages over multi-stage flash (MSF) and reverse osmosis (RO). Nevertheless, it is difficult to design and optimize the MD systems under various conditions, because both thermal and hydrodynamic effects play an important role. Therefore, this study focused on performance analysis of MD systems in different configurations. Direct contact MD (DCMD) and vacuum MD (VMD) were experimentally compared using laboratory-scale systems. A simple model was also applied to analyze the difference between two configurations theoretically. Experimental results indicated that permeate flux in DCMD and VMD were sensitive to the operating conditions. Using same membranes, two MD systems showed different flux behaviors. The influences of operating parameters for DCMD and VMD on overall efficiency were also investigated. The model result...

Application of response surface methodology (RSM) in the optimization of dewetting conditions for flat sheet membrane distillation (MD) membranes

AbstractThis study focuses on the exploration of the optimum dewetting conditions to remove water in pores in membrane distillation (MD) process. Response surface methodology (RSM) was applied to build statistical models for the analysis of flux and liquid entry pressure (LEP) as a function of dewetting temperature and time. A set of MD experiments based on central composite design of experiments method are carried out. Using these experimental results, two response surface (RS) models were developed to predict flux and LEP. The RS models were further used to optimize the dewetting conditions for desired membrane properties. Moreover, the optimum dewetting condition derived from RSM was experimentally verified.

Optimization of dewetting conditions for hollow fiber membranes in vacuum membrane distillation

AbstractMembrane distillation (MD) is a thermal separation process that uses a hydrophobic membrane as a barrier between a liquid phase and a gas phase. Accordingly, MD can only be applied under the conditions where the membrane is not wetted by the feed solution. In this study, a technique to remove water inside the pores of the wetted membranes, or “dewetting,” was developed to mitigate the problems of membrane wetting in MD process. High-temperature air was applied to the wetted membranes using a specially designed device. The dewetting efficiency was analyzed by measuring the liquid entry pressure, water flux, and salt rejection. The response surface methodology (RSM) was applied to explore the optimum conditions for dewetting of MD membranes. Results indicated that dewetting should be done under proper conditions. If the temperature and dewetting time were insufficient, the dewetting was incomplete. On the other hand, the membrane was partially deformed if the temperature was too high and the dewetti...

Hydrophobic surface modification of membrane distillation (MD) membranes using water-repelling polymer based on urethane rubber

AbstractMembrane distillation (MD) is a unit process that uses hydrophobic membranes to separate vapor from saline water. The performance of the MD process is largely affected by the properties of the membranes, which should be porous, hydrophobic, and stable under high temperature conditions. Accordingly, it is essential to develop highly efficient membranes for practical implementation of MD technology. In this study, we applied a water repellent chemical (WRC) made of urethane rubber onto hydrophilic membranes to develop a novel approach for MD membrane preparation. A spin coating method was adopted to introduce hydrophobic coating layers on polyamide membranes. Experiments were carried out in the direct contact membrane distillation mode. Contact angle and liquid entry pressure (LEP) were measured before and after the surface coating. In addition, scanning electronic microscope, FT-IR, and atomic force microscope analysis were conducted to confirm a coating layer of the membrane. The optimum condition...