An Innovative Method for Oxidizing Hydrogen Sulfide using Hydroxyl Radicals Jigna Patel Carolina Franco Diego J. Díaz Cherie Yestrebsky Department of Chemistry University of Central Florida 4000 Central Florida Blvd. Orlando, Florida 32816

ABSTRACT

A novel method for treating odors in pump stations, wet wells, and other enclosed spaces has been available since 2000. The technology uses an air, water, and ozone mixture to create a fine particle mist that increases the surface area and probability that an odorous compound reacts with the oxidants. With over 225 installations across the United States, the technology needs to be studied in greater detail to verify the claim that it can effectively oxidize hydrogen sulfide thereby removing odors around the treated area. An additional claim was made that the mixture of ozone and nano-sized water particle was used to produce hydroxyl radicals in quantities that can effectively oxidize hydrogen sulfide, mercaptans, and other odorous compounds. At the sites where the technology is being used, effectiveness is primarily measured by the no-odor standard using the nose. The difficulty is that the removal efficiency cannot be measured directly in the treatment area since it is being used as the reactor while most other odor control technologies draw the odorous air away from the wastewater process and treat it in a separate reactor.

This study quantified hydroxyl radical generation using an indirect method. This method consisted of the reaction between benzene and hydroxyl radicals to form phenol as the product. The study also investigated the optimum ratio of ozone to hydrogen sulfide per given area, followed by the verification of the resulting products.

KEYWORDS

Odor control, ozone, hydrogen sulfide, hydroxyl radical, benzene, oxidation

INTRODUCTION

The hydroxyl radical, ·OH, is one of the most reactive species of the reactive oxygen species (ROS) family (Page et al, 2010). Hydroxyl radicals are generated by several methods such as the decomposition of hydrogen peroxides by photolysis or in atmospheric chemistry by the reaction of water with excited atomic oxygen (Takeda et al, 2004). Hydroxyl radicals are widely used in treating odors in wastewater treatment due to their effectiveness in destroying organic chemicals because they are reactive electrophiles that react rapidly and non-selectively with nearly all electron-rich compounds (Stasinakis, 2008). The theory behind the odor control technology, OHxyPhogg, is that it generates a hydroxyl radical fog using air, ozone and water with the aid of a patented nozzle. The fog is used to oxidize hydrogen sulfide and other odorous compounds such as amines and mercaptans. The instability of these hydroxyl radicals makes its direct detection difficult as it reacts rapidly with its surroundings, requiring an indirect determination (Otsuka et al, 1992). A widely used indirect technique includes the reaction of hydroxyl radicals with benzene resulting in the formation of phenol (Blakrishnan et al, 1970). The reaction is stoichiometric, where one molecule of phenol is generated per each hydroxyl radical reacting with one molecule of benzene (Bahidsky et al, 2004). During this study, hydroxyl radicals were measured by reacting them with benzene to form phenol, which was then analyzed using UV-Visible spectrophotometry (UV-Vis) and high performance liquid chromatography (HPLC).This reaction allowed for easy quantification of the radical as the stoichiometric mole ratio of hydroxyl radicals to phenol is 1:1. While the UV-Vis allowed for the detection of the maximum absorption of the hydroxyl radical, HPLC allowed for compound separation of the sample solutions and aiding in the quantification of the generated hydroxyl radicals.

After the verification and the quantification of the hydroxyl radicals, the next step was to study the optimum ratio of ozone needed to oxidize hydrogen sulfide per given area. This was performed through injecting ozone with air into the reaction chamber and measuring its stabilized concentration. Then a specific amount of the hydrogen sulfide gas was added and its degradation was monitored for a period of time.

METHODOLOGY

I. Verification and Quantification of Hydroxyl Radicals Materials and Reagents Phenol (Fisher Scientific, 99%) was used to prepare standard solutions using saturated benzene as the solvent. A saturated aqueous benzene solution with concentration of 2.23 x 10-2 M was prepared using benzene obtained from Sigma Aldrich (99%). All aqueous solutions of phenol and benzene were prepared using triple-distilled water with 18 ΩM resistivity (Barnstead B- Pure). Acetonitrile (Fisher Scientific, HPLC grade) and triple-distilled water from a B-Pure Barnstead purification system giving ~18 ΩM cm water were used as the eluent solutions for the HPLC. Water samples collected from the experimental setup were analyzed as received. All chemicals and solvents used were of reagent grade and were used without further purification unless specified. Experimental Methods The UV-Vis used was a Varian Cary 50 spectrophotometer. The data interval was set under a minimum ratio of spectral band width (SBW) of 3:1. A quartz cuvette of 10 mm path length and 4 mm path width was used for all experiments.

The HPLC experiments were performed using a Perkin-Elmer series 200 HPLC, using a series 200 binary pump, a series 200 UV-Vis detector with deuterium lamp set at maximum wavelength of 273 nm, a series 200 autosampler, and a series 200 vacuum degasser. The analytical column used was a Zorbax (Agilent) SB-C 18 column. The mobile phase was 3 3 CH3CN:H2O = 50:50 (v/v) with a flow rate of 1 cm / min. A 50 μdm portion of the sample was injected with a 50 μdm3 gas-tight syringe.

Determination of Phenol in Standard Solutions Ten concentrations, from 1.00 x 10-7 M to 5.00 x 10-5 M, of aqueous phenol in saturated benzene (2.23 x 10-2 M) solution, were prepared via serial dilution.12 These solutions were used as HPLC standards. The solvent, 2.23 x 10-2 M benzene, was also analyzed through HPLC for its elution time. A calibration curve was constructed using the ten standards solutions of phenol in order to quantify the hydroxyl radicals present in the unknown water samples collected from the system.

Apparatus and Sampling The experimental apparatus is shown in Figure 1. The experimental water collection apparatus consisted of a black thick polyethylene cylinder which was divided into four chambers, C-1, C-2, C-3, C-4, respectively. Each of the chambers was separated by an acrylic disk with an opening in the middle to allow the passage of smaller sized fog particles. Each chamber had a condenser which condensed the fog particles for easier sample collection and a valve which allowed for the collection of these samples. At the beginning of the cylinder, the machine generated a fog which was injected into the space with the patented nozzle, having special atomizing ability. The nozzle generated sub-1-micron-sized to average 5-micron-sized water particles that were dispersed throughout the entire cylinder. Through the same nozzle, a mist of saturated benzene was sprayed and was allowed to react with the fog in the cylinder. The resulting mixture then traveled through the four chambers and a sample from each was collected as shown in Figure 1.

Figure 1: Diagram of the water collection system. The nozzle was connected to a cylindrical collection unit divided in four chambers. Each chamber has a condenser unit and the water samples were collected and analyzed by HPLC.

II. Methodology for the Reaction Ratio of Ozone to Hydrogen Sulfide Materials and Reagents A low density polyethylene plastic bag (U.S. Plastic Corp.) with a diameter of 45.72 cm and total length of 6.10 m. was used as the reaction chamber and triple-distilled water with 18 ΩM resistivity (Barnstead B-Pure) was used as the water source. Ozone concentrations were measured using an InDevR 2B Technologies, Model 202 monitor while a Vapex Sentinel, Model S4 was used to determine the concentration of hydrogen sulfide. A Tedlar gas bag (10 dm3 by Zefron International Inc.) was filled using a 99% pure hydrogen sulfide gas cylinder (Air gas) and was used as needed. All chemicals and solvents used were of reagent grade and were used without further purification unless specified.

Apparatus and Sampling In order to study the gas-to-gas and gas-to-liquid phase reactions, the accurate concentration of ozone had to be confirmed first. This was performed by filling the initial 3.05 m of the reaction bag with ambient air using a smaller 316 stainless steel nozzle that was embedded inside a plastic case. The reaction bag was sealed tightly over the case ensuring that no gas escapes. The clamp was opened letting the air distribute itself throughout the full 6.10 m. Then a mixture of air and ozone was injected via the same nozzle until an inch of water pressure was observed. After 3 minutes to allow for stabilization, the ozone concentration readings were collected.

A similar procedure was repeated using an ozone and water mixture. Approximately 30 cm3 of hydrogen sulfide was also added to the reaction bag manually using a syringe, until a hydrogen sulfide concentration of 180-200 ppm was reached, and then the bag was sealed immediately. Two hydrogen sulfide probes were inserted in the reaction bag, at 1.52 and 4.57 m. A schematic diagram of the setup is shown in Figure 2.

Figure 2: Schematic diagram of the experimental setup for monitoring the degradation of hydrogen sulfide using ozone and water mist. (a) Deflated chamber before starting filling the first 3.05 m with the hydrogen sulfide gas and air. (b) First 3.05 m of the chamber are filled with air and about 30 cm3 of hydrogen sulfide gas.

RESULTS

UV-Visible Spectrograph

UV-Visible spectrograph was used in the identification of the phenol generated by the reaction of benzene and the hydroxyl radical. For this experiment, standard solutions with different concentrations of phenol in benzene/water were prepared in the range of 50-500 parts-per- million (ppm). The benzene concentration utilized was 5.0x10-3 M which was the concentration of benzene used as solvent for the preparation of the standard mixtures. The maximum absorption wavelengths recorded were 270 nm for phenol and 250 nm for benzene which when compared to literature, were very similar with values of 270.75 nm or 273.75 nm for phenol and 254.75 nm for benzene (Zhang et al, 2006). These wavelengths were also utilized for the initial parameters in the HPLC. Figure 3b shows the absorbance spectra of the collected samples. 1.5 0.10

0.09 150 nM 50 nM 0.08 H O 300 nM 2 0.07 100 nM C-1 200 nM 1.0 0.06 C-2 500 nM C-3 0.05 C-4 0.04

0.03 0.5 0.02 Absorbance (AU) Absorbance (AU) 0.01

0.00

-0.01 0.0 -0.02 250 300 350 400 200 220 240 260 280 300 320 340 Wavelength (nm) Wavelength (nm) (a) (b) Figure 3: (a) Phenol solutions absorbances at varying concentrations. (b) Blank and chamber water samples absorbance for each chamber.

HPLC of the Standard Solutions

The first step in the quantification of the amount of hydroxyl radicals produced was to generate a calibration curve of known standard solutions. Ten different solutions of phenol in saturated benzene were used as the standard solutions and were analyzed via HPLC. Retention times of 2.6 minutes (min) and 4.4 min were observed for phenol and benzene, respectively. Figure 4 shows a chromatogram of the standard solutions used for the calibration curve. The area of the peak associated with phenol was analyzed for each solution and a plot of phenol concentration versus peak area was constructed. A best fit line was plotted through the data points resulting in R2 value of 0.99998 (Figure 5). This equation was then used to determine the phenol concentration in the various aqueous/fog samples. 100

95 a 90 b 85 c d 80 e 75 f g 70 h 65 i j 60 Response (AU) 55

50

45

40 140 160 300 320 Time (sec)

Figure 4: HPLC spectra of the standards. (a) 5.00 x 10-5 M, (b) 3.00 x10-5 M, (c) 1.00 x 10-5 M, (d) 5.00 x 10-6M, (e) 3.00 x 10-6 M, (f) 1.00 x 10-6 M, (g) 7.00 x 10-7 M, (h) 5.00 x 10-7 M, (i) 3.00 x 10-7 M, (j) 1.00 x 10-7 M

Area = 5.32399 x 109 * Concentration – 29.06812 R2 = 0.99998

Figure 5: Calibration Curve of Phenol Standard Solutions

HPLC of Unknowns

The four samples collected, one from each chamber, were examined via HPLC using the same parameters as for the standard solutions. Phenol had a retention time of 2.6 minutes as shown in Figure 6. The retention time of benzene was observed at approximately 4.5 minutes (Figure 6). The area under this peak was calculated and was then substituted into the linear equation from the calibration curve to solve for the phenol concentration in the four samples. Chamber three, C- 3, had the highest concentration of phenol followed by C-4, C-2, and C-1, respectively, as shown in Table 1. All four samples fall within the linear dynamic range of the calibration curve. The calculated concentrations in terms of molarity and parts per million are as also shown in Table 1. Table 1: Calculated Phenol Concentrations Chamber Peak area Peak height Phenol Phenol (μV*sec) (μV) Concentration Concentration (M) (ppm) C-1 1281.83 316.59 2.62 x 10-7 0.0247 C-2 2766.35 698.85 5.59 x 10-7 0.0526 C-3 4085.23 939.48 8.23 x 10-7 0.0774 C-4 3758.3 888.12 7.57 x 10-7 0.0713

47000

46500

C-1 C-2 46000 C-3 C-4

45500

45000

44500

44000 150 225 300 375 450

Figure 6: HPLC spectra overlay of the unknown samples from the four chambers showing phenol and benzene retention times.

Monitoring Degradation of Hydrogen Sulfide

Five trials were performed to obtain an accurate concentration of ozone before its reaction with hydrogen sulfide. The ozone concentration readings showed a slight variation as shown in Table 2. Following the same procedure as mentioned earlier, a second set of trials were performed injecting ozone and water mixture through the nozzle instead of only ozone. The results show a minor deviation in the ozone and water reading when compared to just ozone by itself. Table 2: Calculated Average Ozone Concentrations

Trial [O3] [O3] with H2O (ppm) (ppm) 1 85.84 74.05 2 86.56 69.17 3 87.95 66.32 4 92.49 59.98 5 88.89 65.86 Average 88.35 67.08

The third set of experiments were performed using the same procedure as above except this time, H2S gas was added with just ozone and then to the ozone and water mixture. For these experiments, instead of filling the first 3.05 meters of the bag with just air, it was filled with 3 approximately 30 cm of H2S gas and air until the H2S concentration was reached to be in the 180-200 ppm. The stabilized H2S concentration in the first 3.05 m of the reaction bag was recorded. At that moment, the clamp was opened and the mist was added through the nozzle until the entire bag was filled. The H2S concentration readings were taken at two locations in the reaction bag; at 1.52 m and 4.57 m. The results are shown in Table 3. The ratio of H2S degradation was taken from the sensor readings at 4.57 m since it gave the more stable concentration readings versus the sensor at 1.52 m. When water was present, higher and steady H2S degradation was observed due to the appropriate conditions needed for the reaction. On the other hand, when the reaction was performed with only H2S and O3, the degradation was very inconsistent. It can be concluded that the degradation approached 2 ppm of H2S per every 1 ppm of O3 after the reaction occurred in the presence of water.

Table 3: (a) Experimental Degradation Range of H2S, H2O, and O3 Concentrations. (b) Experimental Degradation Range of H2S and O3 Concentrations. (a)

Ozone + H2S + H2O

[H2S] before rxn [H2S] at 5 ft. [H2S] at 15 ft. Initial [O3] Final [O3]O3 Consumption H2S Degradation H2S:O3 Ratio Run (ppm) after rxn (ppm) after rxn (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Temp (˚F) 1 185.94 43.3 67.0 88.35 31.8 56.55 118.94 2.10 91.94 2 175.77 62.1 51.3 88.35 39.4 48.95 124.47 2.54 92.12 3 182.65 72.9 61.7 88.35 40.3 48.05 120.95 2.52 93.92 4 182.12 70.9 59.1 88.35 40.1 48.25 123.02 2.55 95.54 5 175.33 57.5 62.1 88.35 39.6 48.75 113.23 2.32 96.26

6 187.78 81.4 65 88.35 35.3 53.05 122.78 2.31 95.90 7 182.94 84.3 64.2 88.35 17.5 70.85 118.74 1.67 97.98 8 176.29 76.4 48.5 88.35 19.1 69.25 127.79 1.85 100.22 9 191.94 65 71.1 88.35 19.6 68.75 120.84 1.76 101.12 10 173.96 77.4 52.6 88.35 17.3 71.05 121.36 1.71 102.02

11 181.58 85.5 55.1 88.35 20.1 68.25 126.48 1.85 98.60 12 184.31 76.7 71.1 88.35 17.5 70.85 113.21 1.60 99.86 13 183.73 81.5 61.3 88.35 17.1 71.25 122.43 1.72 102.38 14 199.54 82.9 56.9 88.35 16.8 71.55 142.64 1.99 103.10 15 184.53 79.8 70.3 88.35 15.9 72.45 114.23 1.58 103.28

(b)

Ozone + H2S only

[H2S] before rxn [H2S] at 5 ft. [H2S] at 15 ft. Initial [O3]Final [O3]O3 Consumption H2S Degradation H2S:O3 Ratio Run (ppm) after rxn (ppm) after rxn (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Temp (˚F) 1 183.52 44 85.0 88.35 46.9 41.45 98.5 2.38 90.68 2 194.86 37.8 100.8 88.35 36.3 52.05 94.1 1.81 91.22 3 186.77 44.1 88.8 88.35 44.8 43.55 98.0 2.25 91.58 4 183.98 95.4 55.7 88.35 59.4 28.95 128.3 4.43 89.24 5 178.86 32.3 100.8 88.35 37.9 50.45 78.1 1.55 90.86

6 191.45 29.9 118.9 88.35 65.6 22.75 72.6 3.19 94.10 7 195.09 64.7 81.8 88.35 44.1 44.25 113.3 2.56 95.18 8 189.23 104.9 70.7 88.35 29.3 59.05 118.5 2.01 96.62 9 184.71 72.6 59.7 88.35 45.7 42.65 125.0 2.93 98.42 10 190.56 33.4 92.3 88.35 67.4 20.95 98.3 4.69 96.44

DISCUSSION

The hydroxyl radicals generated from the odor control system were verified using UV-Vis and quantified by HPLC. The generated hydroxyl radicals reacted with benzene to form phenol as the major product as shown in Reaction (1). The 1:1 stoichiometry of hydroxyl radical: phenol allows for quantitation of hydroxyl radicals formed. OH

+ HO (1) Although ozone is also known to react with benzene, the reaction kinetics are considerably slower than the reaction of benzene with the hydroxyl radicals (Beltran, 2004).The reactions are reported to have reaction rates of 3 M-1 sec-1 and 41.2 M-1 sec-1, respectively (Beltran, 2004; Tulley et al, 1981). Moreover, the monitoring of the ozone concentration in all chambers during the collection of samples and the generation of the fog remained constant. In that regard, we are confident that the phenol concentration determined was mostly attributed to the reaction with hydroxyl radicals and not with ozone. In general, the phenol concentrations were observed to be in the low parts per million. As expected, C-3 and C-4 showed the highest phenol concentrations since the smallest fog particles were present in those chambers since the larger particulates cannot get through the holes in the separation disks. For set of experiments performed to study the optimum reaction ratio of hydrogen sulfide to ozone, the presence of water yielded a decrease in the concentration of gaseous ozone due to its interactions with the aqueous phase. Based on the experimental data, it can also be concluded that water is needed to achieve the most efficient and steady oxidation of hydrogen sulfide in the presence of ozone. The experimental ratio suggested that using this technology, significantly less ozone is needed to oxidize a specific amount of hydrogen sulfide when compared to the literature value.

CONCLUSIONS

This study investigated an odor control system that has recently been developed and is currently being used in the United States. The experimental results showed that the system does generate a hydroxyl radical fog via a patented atomizing nozzle. A successful indirect detection and quantification of the hydroxyl radicals was performed through HPLC and UV-Vis spectrophotometer. The detection limit based on the calibration curve was determined to be 1.00 x 10-7 M, which was adequate for the detection range of the collected samples. The optimum reaction ratio of ozone to hydrogen sulfide with and without the presence of water was established. A detailed test of the reaction conditions at different pH levels to examine the optimum generation and efficiency of the hydroxyl radical fog, as well as its reaction with hydrogen sulfide gas into the system to investigate the mechanism and kinetics of the reaction is currently being performed.

Acknowledgements The authors thank Darrel Resch (Vapex Inc.), Robert Jeyaseelan (Parkson), Dr. Matthew Rex and Dr. Christian Clausen III (UCF Department of Chemistry) and the University of Central Florida Department of Chemistry for their intellectual and financial support.

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