Tae-Mun Hwang

Comparative evaluation of magnetite-graphene oxide and magnetite-reduced graphene oxide composite for As(III) and As(V) removal.

Arsenic removal using Fe3O4-graphene oxide composite (M-GO) and Fe3O4-reduced graphene oxide composite (M-rGO) was investigated. The M-GO was more effective to adsorb both As(III) and As(V) than M-rGO, because the more functional groups existing on the M-GO could lead to synthesize more Fe3O4 with M-GO. As(III) was more favorable to be adsorbed than As(V) onto both M-GO and M-rGO. According to the effect of pH on arsenic removal, the electrostatic interaction between the positively charged surface of Fe3O4-graphene based adsorbents and anionic As(V) species was a major factor to adsorb As(V). The adsorption mechanism of As(III), on the other hand, was strongly affected by a surface complexation, rather than electrostatic interactions. Consequently, in terms of the process energy consumption, energy saving could be achieved via omitting the reduction process to fabricate M-rGO from M-GO and the pre-oxidation process to convert As(III) to As(V).

Characterization of raw water for the ozone application measuring ozone consumption rate

This study was conducted to illustrate an ideal method for characterizing natural waters for ozonation processes in drinking water treatment plants. A specific instrument designed with the flow injection analysis (FIA) technique enabled us to measure accurately the ozone decomposition rate, which was found to consist of two stages: the instantaneous ozone consumption stage and the slower ozone decay stage. The ozone consumption rate was measured at the initial and secondary stages by determining certain parameters called the instantaneous ozone demand (ID) and the pseudo first-order decay rate constant (k(c)). Using the OH*-probe, the yield of OH* per consumed ozone was also measured to determine its potential to produce OH* for the oxidation of micropollutants during the ozonation process. The ozone consumption of the ID values was significant in most natural waters, and substantial amounts of OH* were found to generate during the instantaneous ozone consumption stage. This study also investigated the effects of particulates, ozone doses, and sequential ozone injection on ozone decomposition kinetics and OH* formation yield.

Evidence of singlet oxygen and hydroxyl radical formation in aqueous goethite suspension using spin-trapping electron paramagnetic resonance (EPR).

The generation of reactive species in an aqueous goethite suspension, under room light and aeration conditions, was investigated using the electron paramagnetic resonance (EPR) technique employing spin trap agents. The trap reagents, including 5,5-dimethylpyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP), were used for the detection of OH radicals (OH·) and singlet oxygen (1O2), respectively. On the addition of DMPO to the goethite suspended solution, a DMPO-OH adduct was formed, which was not decreased, even in the presence of the OH· scavenger, mannitol. This result implied a false positive interpretation from the DMPO-OH EPR signal. In the presence of TEMP reagent, a TEMP-O signal was detected, which was completely inhibited in the presence of the singlet oxygen scavenger, sodium azide. With both DMPO-OH and TEMP-O radicals in the presence and absence of radical scavengers, singlet oxygen was observed to be the key species formed in the room-light sensitized goethite suspension. In the goethite/H2O2 system; however, both OH· and singlet oxygen were generated, with significant portions of DMPO-OH resulting from both OH· and singlet oxygen. In fact, the DMPO-OH resulting from OH· should be carefully calculated by correcting for the amount of DMPO-OH due to singlet oxygen. This study reports, for the first time, that the goethite suspensions may also act as a natural sensitizer, such as fulvic acids, to form singlet oxygen.

Investigating synergism during sequential inactivation of MS-2 phage and Bacillus subtilis spores with UV/H2O2 followed by free chlorine

A sequential application of UV as a primary disinfectant with and without H(2)O(2) addition followed by free chlorine as secondary, residual disinfectant was performed to evaluate the synergistic inactivation of selected indicator microorganisms, MS-2 bacteriophage and Bacillus subtilis spores. No synergism was observed when the UV irradiation treatment was followed by free chlorine, i.e., the overall level of inactivation was the same as the sum of the inactivation levels achieved by each disinfection step. With the addition of H(2)O(2) in the primary UV disinfection step, however, enhanced microbial inactivation was observed. The synergism was observed in two folds manners: (1) additional inactivation achieved by hydroxyl radicals generated from the photolysis of H(2)O(2) in the primary UV disinfection step, and (2) damage to microorganisms in the primary step which facilitated the subsequent chlorine inactivation. Addition of H(2)O(2) in the primary disinfection step was also found to be beneficial for the degradation of selected model organic pollutants including bisphenol-A (endocrine disruptor), geosmin (taste and odor causing compound) and 2,4-D (herbicide). The results suggest that the efficiency of UV/free chlorine sequential disinfection processes, which are widely employed in drinking water treatment, could be significantly enhanced by adding H(2)O(2) in the primary step and hence converting the UV process to an advanced oxidation process.

pH Effect on Ozonation of Ampicillin: Kinetic Study and Toxicity Assessment

Ampicillin (AP) is a penicillin-type antibiotic and one of the most widely used bacteriostatic antibiotics in human and veterinary medicine. A kinetic study was performed under different pH conditions (5, 7.2, and 9) to determine the degradation efficiency of AP by ozonation. The second-order rate constants for the direct reaction of AP with ozone were measured to be 2.2 ˜5.4×105 M−1s−1 under the pH conditions tested. The rate constants were greater at higher pH. The potential toxicity of the AP intermediates formed after ozonation under the various pH conditions were examined using a bioluminescence assay on Vibrio fischeri species. The biodegradability of the AP degraded products was also determined by measuring the BOD5/COD of the ozonated samples under the different pH conditions. A lower biodegradability and acute toxicity was observed at the lowest pH (pH 5). These results suggest that higher pH conditions are needed for the removal of AP by ozonation in order to mitigate the residual toxicity that can remain even after complete removal of the parent compound by ozonation.

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...

Optimization and Control of Ozonation Plant Using Raw Water Characterization Method

This study was undertaken to devise an innovative method for optimization and control of ozone dosage in drinking water ozonation treatment plants. The method is based upon a specifically-conceived analytical procedure, which can accurately measure the ozone decomposition rate. This was found to consist of two apparent phases: an instantaneous ozone demand (ID) phase and a relatively slower ozone decay (pseudo first-order rate constant, kc) phase. Those parameters, ID and kc were measured in a demonstration plant by the testing procedure in order to characterize raw water and process water, and utilized an Automatic Ozone Control Unit (ACU) to optimize preozonation (with parameter, ID) and postozonation (with parameter, kc).

Theoretical analysis of different membrane distillation modules

AbstractMembrane distillation (MD) is a thermally driven separation process that uses hydrophobic membranes. Although MD has been considered as an alternative desalination technology, relatively little information is available on the design and optimization of MD modules. Accordingly, this study focused on the comparison of various MD modules and the optimization of their operation conditions. Direct contact MD, air gap MD, and vacuum MD were experimentally compared using laboratory-scale systems. Then, the results were theoretically analyzed using a simple transport model. Different mass transfer mechanisms inside the membrane were considered in the model, including molecular diffusion, the Knudsen diffusion, and viscous flow. Experimental results showed that the temperature dependences of MD modules were different. This suggests that the optimum feed temperature should be different for each MD module. The dominant mass transfer mechanisms were also identified using the theoretical model for better under...

Occurrence of disinfectant by-products during the sequential disinfection process

Abstract Sequential disinfection is considered a viable treatment option for the effective inactivation of pathogenic micro-organisms such as oocysts of protozoan parasites (e.g. Cryptosporidium parvum etc.) and spores of vegetative bacteria (e.g. Bacillus subtilis and Bacillus anthracis) that cannot be treated by the single-step application of chlorine-based disinfectants and for viruses (e.g. Adenovirus) that cannot be easily treated by the single-step application of UV irradiation. The aim of this research was to investigate the inactivation of B. subtilis spore and formation of disinfection by-products (DBPs) during sequential disinfection using a combination of chlorine dioxide and chlorine (ClO2/HOCl) or UV and chlorine (UV/HOCl) sequential disinfection. B. subtilis spores have been widely used as an indicator micro-organism for C. parvum oocysts. Trihalomethanes (THMs) and haloacetic acids have been used as indicator for DBPs. This study evaluates the effect of operating parameters (e.g. chemical d...

Organic fouling and osmotic backwashing in PRO

AbstractThe pressure-retarded osmosis process is the next generation seawater desalination technology and is considered as eco-friendly and economic renewable energy. As such, there are active studies of means of efficient cleaning to restore the membrane performance degraded due to the reversible membrane fouling that inevitably occurs after prolonged operation. This study evaluated the fouling rate by organic alginate, humic, and BSA (bovine serum albumin). Also, we focused on the comparison of cleaning methods which are physical flushing and osmotic backwashing (OB). For the comparison of the cleaning efficiency, we used alginate compound as a model substances representative of natural organic matter. Physical cleaning (PC) is the flushing method by flowing the distilled water on the membrane active and support layer in high velocity to remove the accumulated foulants on the membrane surface. OB is the method of backflow generated by osmosis to remove the accumulated foulants on/in the membrane active ...