Joon-Wun Kang

The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation

Abstract Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals in sufficient quantity to affect water purification. The theoretical and (practical yield of OH from O3 at high pH, 03/H202, O3/UV and H2O2/UV systems is reviewed. New data is presented which illustrates the importance of direct photolysis in the O3/UV process, the effect of the H202:03 ratio in the O3/H2O2 process, and the impact of the low extinction coefficient of H2O2 in the H202/UV process.

Sonolytic degradation of methyl tert-butyl ether: the role of coupled fenton process and persulphate ion

The sonolytic degradation of methyl tert-butyl ether (MTBE) has been investigated at ultrasonic frequency of 20 kHz. The observed pseudo-first-order rate constant decreased from 1.25 x 10(-4) to 5.32 x 10(-5) s(-1) as the concentration of MTBE increased from 2.84 x 10(-2) to 2.84 x 10(-1) mM. The rate of degradation of MTBE increased with the increase of the power density of ultrasonicator and also with the rise in reactor system temperature. In the presence of oxidising agent, potassium persulphate, the sonolytic rate of degradation of MTBE was accelerated substantially. Tert-butyl formate (TBF) and acetone were found to be the major intermediates of the degradation of MTBE. It is found that the ultrasound/Fe2+/H2O2 method is promising process for the degradation of MTBE. More than 95% degradation of MTBE (2.84 x 10(-2) mM) along with its intermediate products has been achieved during the coupled ultrasound/Fe2+/ H2O2 method. Hence, the coupled ultrasound/Fe2+/H2O2 may be a viable method for the degradation MTBE within a short period of time than the ultrasound irradiation process only. A kinetic model, based on the initial rates of degradation of MTBE and TBF, provides a good agreement with the experimental results.

Bromate removal from water by granular ferric hydroxide (GFH)

The feasibility of granular ferric hydroxide (GFH) for bromate removal from water has been studied. Batch experiments were performed to study the influence of various experimental parameters such as effect of contact time, initial bromate concentration, temperature, pH and effect of competing anions on bromate removal by GFH. The adsorption kinetics indicates that uptake rate of bromate was rapid at the beginning and 75% adsorption was completed in 5 min and equilibrium was achieved within 20 min. The sorption process was well described by pseudo-second-order kinetics. The maximum adsorption potential of GFH for bromate removal was 16.5 mg g(-1) at 25 degrees C. The adsorption data fitted well to the Langmuir model. The increase in OH peak and absence of Br-O bonding in FTIR spectra indicate that ion-exchange was the main mechanism during bromate sorption on GFH. The effects of competing anions and solution pHs (3-9) were negligible. Results of the present study suggest that GFH can be effectively utilized for bromate removal from drinking water.

Kinetics and mechanisms of the sonolytic destruction of non-volatile organic compounds: investigation of the sonochemical reaction zone using several OH* monitoring techniques.

This study investigates the sonolytic degradation mechanism of non-volatile organic compounds and reaction sites for its degradation using various tools that allow OH* to be monitored, such as: the spin-trapping method of OH* detection using non-volatile nitrone trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the hydrogen peroxide analytical methods and the p-chlorobenzoic acid (pCBA)-probe method. These methods can successfully monitor OH* produced during sonochemical processes, and identify the major reaction sites involving OH* of the three proposed reaction zones--within the cavity, in the bulk solution, and at the gas-liquid interfacial (shell) region. The patterns of hydrogen peroxide accumulation under the various conditions suggest that peroxides pre-form at the interfacial region, but the self-scavenging reaction by hydrogen peroxide simultaneously takes place in the same region. The simultaneously measured peroxide concentration, in the absence and presence of DMPO, and that of the DMPO-OH adduct indicated the peroxide production and DMPO-OH adduct formation reaction occur at the shell region. The sonolytic destruction efficiency of ultrasound coupled with Fe(II) has been also investigated. The coupled Fe(II)/ultrasound process was found to enhance the OH* production rate by 70% compared to the ultrasound process alone due to the reaction of Fe(II) with sonochemically produced hydrogen peroxide (Fentons reaction). This accelerated reaction was also found to occur at the shell region rather than in the bulk solution. The enhancement effect of Fe(II)/ultrasound was also examined using pCBA as a probe. 2.8-fold and 3.6-fold increases of the pCBA degradation rate were observed at Fe(II) concentrations of 10 and 20 microM, respectively.

An investigation of the formation of chlorate and perchlorate during electrolysis using Pt/Ti electrodes: the effects of pH and reactive oxygen species and the results of kinetic studies.

The characteristics of chlorate (ClO(3)(-)) and perchlorate (ClO(4)(-)) formation were studied during the electrolysis of water containing chloride ions (Cl(-)). The experiments were performed using an undivided Pt/Ti plate electrode under different pH conditions (pH 3.6, 5.5, 7.2, 8.0 and 9.0). ClO(3)(-) and ClO(4)(-) were formed during electrolysis in proportion to the Cl(-) concentration. The generation rates of ClO(3)(-) and ClO(4)(-) under acidic conditions (pH 3.6 and 5.5) were lower than in basic pH conditions (pH 7.2, 8.0 and 9.0). However, the pH of the solution did not influence the conversion of ClO(3)(-) to ClO(4)(-). The effects of intermediately formed oxidants on the production of ClO(3)(-) and ClO(4)(-) were observed using sodium thiosulfate (Na(2)S(2)O(3)) as the active chlorine scavenger and tertiary butyl alcohol (t-BuOH) as the hydroxyl radical (OH) scavenger. The results revealed that electrolysis reactions that involved active chlorine contributed dominantly to ClO(3)(-) production. The direct oxidation reaction rate of Cl(-) to ClO(3)(-) was 13%. The OH species that were intermediately formed during electrolysis were also found to significantly affect ClO(3)(-) and ClO(4)(-) production. The key formation pathways of ClO(3)(-) and ClO(4)(-) were studied using kinetic model development.

Transport characteristics of gas phase ozone in unsaturated porous media for in-situ chemical oxidation.

Laboratory column experiments were conducted by employing various porous media to delineate the characteristics of gaseous ozone transport in the unsaturated zone under various conditions. Water content, soil organic matter (SOM), and metal oxides (MOs) were found to be the factors most influential in the fate and transport of gaseous ozone in unsaturated porous media. The migration velocity of the gaseous ozone front was inversely proportional to the MO content of the porous media. Increased water content at fixed gas flux decreased the ozone breakthrough time proportionally as a result of reduced gas pore volume (PV) in the column, and increased pore water interfered with reactions of gaseous ozone with SOM and MOs on the surface of porous media. The feasibility of in-situ ozone injection for the remediation of unsaturated soils contaminated with either phenanthrene or diesel-range organics (DROs) was investigated under various conditions. The maximum removal after 1 h of ozone injection was achieved in columns packed with baked sand, followed, in descending order, by glass beads and by sand, indicating that catalytic ozone dccomposition with MOs in columns packed with baked sand enhanced hydroxyl radical formation and resulted in increased contaminant removal. Overall removal efficiency of multicomponent C10-C24 DROs after 14 h of ozonation was 78.7%. Ozone transport was retarded considerably because of the high ozone demand of DROs, requiring more than 6 h for the gaseous ozone to initially break through the soil column under the experimental conditions tested in this study. Overall, gaseous ozone was readily delivered and transported to remediate unsaturated soils contaminated with phenanthrene and DROs.

Characterization of ozone decomposition in a soil slurry: kinetics and mechanism

A series of soil slurry experiments were performed in a carefully conceived reactor set-up to investigate the characteristics of the catalytic decomposition of ozone on a sand and iron surface. Real time on-line monitoring of ozone in the reaction module was possible using flow injection analysis coupled with a computer-controlled UV detector and data acquisition system. The effects of the soil media and size, ozone dosage, pH and p-CBA as a probe compound were examined at the given experimental conditions. Two apparent phases existed, and ozone instantaneously decomposed within one second in the first phase. These were defined as the instantaneous ozone demand (ID) phase, and the relatively slow decay stage. The interactions of ozone with the soil organic matter (SOM) and metal oxides were attributed mostly to the instantaneous decomposition of ozone. From the probe (p-CBA) experiments, 60-68% of total p-CBA removal occurred during the ID phase. The generation of hydroxyl radicals (OH.) was demonstrated and was closely related with metal oxides as well as SOM. Metal oxides in soil surface were considered to have relatively faster reaction rate with ozone and provide more favorable reactive sites to generate higher amount of OH. than SOM. Even at one-tenth concentration of the sands, a goethite-induced catalytic reaction outfitted the removal rate ofp-CBA among all the soils tested. More than 40% of total p-CBA removal occurred on the soil surface. It was inferred that the radical reaction with the probe compound seemed to take place not only on the soil surface but also in the solid-liquid interface. Ozone decomposition and the reaction between OH. and p-CBA appeared to be independent of any change in pH.

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.