Environ. Sci. Technol. 2000, 34, 1286-1291
Odor Control in Wastewater Treatment: The Removal of Thioanisole from WatersA Model Case Study by Pulse Radiolysis and Electron Beam Treatment T H O M A S T O B I E N , * ,† WILLIAM J. COOPER,† MICHAEL G. NICKELSEN,† ENRIQUE PERNAS,‡ KEVIN E. O’SHEA,‡ AND KLAUS-DIETER ASMUS§ Department of Chemistry, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403, Department of Chemistry, Florida International University, University Park, Miami, Florida 33199, and Radiation Laboratory, and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
A novel electron beam irradiation process has proven to be effective in removing thioanisole (methyl phenyl sulfide, CH3S-C6H5) from aqueous solutions. This paper presents substrate destruction data at pH 5 and pH 9. To apply a kinetic model, which predicts removal efficiencies, overall rate constants for reactions of thioanisole with •OH radicals, hydrated electrons, and hydrogen atoms were newly measured or carefully re-determined by pulse radiolysis. The respective values are (9.90 ( 0.13) × 109, (3.1 ( 0.1) × 108, and (3.24 ( 0.08) × 109 (M s)-1. Comparison of the model predictions to the experimental results revealed a very good agreement at pH 5 and an overestimation of the removal at pH 9. The presence of an additional scavenger at pH 9 but not at pH 5 reacting with •OH radicals, hydrated electrons and hydrogen atoms is considered to be responsible for the decrease in removal efficiency.
Introduction Key issues in treating wastewater and biosolids (residuals) management are not only the removal of toxic pollutants but also the removal of odors. Several classes of compounds have been identified that are responsible for causing such odors: sulfides (1), dicyclopentadiene and analogues (2), disinfection byproducts (i.e., chlorinated phenols (3), and short-chain aldehydes (4, 5) to name a few. A typical treatment procedure to remove odors involves several stripping and scrubbing stages with the addition of substantial amounts of conventional oxidizers, e.g., hypochlorous acid and/or hydrogen peroxide. Our novel approach is intended to replace the latter multistep procedure with just one single treatment utilizing an electron beam. The interaction of highly accelerated electrons with water generates a number of very reactive intermediates (i.e., * Corresponding author e-mail:
[email protected]; phone: (910)962-7285; fax: (910)962-3013. † University of North Carolina at Wilmington. ‡ Florida International University. § University of Notre Dame. 1286
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hydrated electrons, •OH radicals, and •H-atoms) that can react with odor-causing substrates. Research on the irradiation of waters containing, for example, benzene, toluene, phenol, chloroform, and carbon tetrachloride as well as several other common organic pollutants has shown efficient removal for low solute concentrations (6-8). To demonstrate the feasibility of our radiation treatment method, we chose a specific odorous aromatic thioether, thioanisole (methyl phenyl sulfide, CH3S-C6H5, TA). While the radiation chemistry of aliphatic thioethers has been extensively studied (9-14), fewer publications deal with their aromatic counterparts. This includes some recent investigations on the oxidation of TA by different pulse radiolytically generated radicals (15-17). To the best of our knowledge, nothing has been reported with respect to the reduction of TA with hydrated electrons and hydrogen atoms. The goal of our study is to assess the feasibility of an electron beam treatment for the degradation of TA in aqueous solutions and to simulate the results on the basis of reaction mechanistic models (18). These models utilize known radiation chemical processes in pure water and account for the interference of further solutes in the sample solution. Each reaction in the model can be described by its integrated kinetic equation. To calculate radiation dose-dependent substrate concentrations, rate constants for each reaction have to be known. Since the documented rate constant (16) for TA reacting with •OH radicals failed to satisfactorily simulate our electron beam treatment results under all of the applied experimental conditions, we re-determined this crucial value. In addition, we report pulse radiolysis results on the reduction of TA by hydrated electrons and hydrogen atoms. With our experimentally obtained rate constants, we are able now to simulate the radiation chemical processes of a large-scale electron beam treatment of TA solutions and to compare the theoretical and experimental results.
Experimental Section Materials. Thioanisole (methyl phenyl sulfide, Aldrich, 99%) and tert-butyl alcohol (2-methyl-2-propanol, Fisher Scientific, certified) were obtained at the highest purity available and used as received for the pulse radiolysis experiments. Deionized, Millipore-quality water (18 MΩ resistance, 120 µg L-1 TOC) was used in all pulse radiolysis experiments. Methods. Pulse Radiolysis. A model TB-8/16-1S linear electron accelerator, providing 5-50 ns pulses of 8 MeV electrons and generating radical concentrations of 1-3 µM per pulse in all investigated systems, was used for the pulse radiolysis experiments. A detailed description of the experimental setup at the Radiation Laboratory, University of Notre Dame (19), as well as the basic details of the equipment and data analysis (20) have been given elsewhere. The dosimetry was based on the oxidation of 0.01 M thiocyanate anions (SCN-) to (SCN)2•- in aqueous, N2O-saturated solutions at pH 7. Absorptions are given in G‚ units, referring to a G‚ value of 46 400 [molecules (100 eV)-1 M-1 cm-1] for (SCN)2•(21). All experiments were carried out in aqueous solutions. In such systems, the electron pulse produces •OH and •H radicals, hydrated electrons eaq-, the molecular products H2 and H2O2, and solvated protons Haq+. Yields are given as G values, which represent the number of species per 100 eV deposited energy (22). The actual figures listed in eq 1 refer to measured yields in dilute substrate solutions: 10.1021/es990692v CCC: $19.00
2000 American Chemical Society Published on Web 02/25/2000
radiolysis
H2O 98 •
•
-
2.7 OH, 0.6 H, 2.6eaq , 0.45H2, 0.7H2O2, 2.6Haq
+
(1)
In an aqueous, nitrous oxide (N2O)-saturated solution, the solvated electrons are selectively scavenged with a bimolecular rate constant of k2 ) 9.1 × 109 (M s)-1 according to (23)
N2O + eaq-/H2O f •OH + OH- + N2
(2)
and, thus, nearly double the amount of •OH radicals available for reaction. Some experiments required defined N2O/O2 gas mixture ratios in the solution. To prepare these solutions, two water samples were separately saturated with each of these gases and mixed afterward. The corresponding concentrations were calculated according to the volume ratio and the gas saturation concentrations ([N2O] ) 2.2 × 10-2 M and [O2] ) 2.2 × 10-3 M) (24). For selective monitoring the reaction of hydrated electrons with the substrate, 0.5 M 2-methyl-2-propanol (tert-butyl alcohol, t-ButOH) was added to the solution in order to scavenge •OH radicals (k3 ) 6 × 108 M-1 s-1) according to eq 3 (25):
(CH3)3COH + •OH f •CH2(CH3)2COH + H2O
(3)
To study the reaction between the substrate and the hydrogen atoms, the solution was acidified to pH 1 with HClO4. Under these conditions, the hydrated electrons are rapidly converted to H-atoms according to eq 4 (k4 ) 2.3 × 1010 M-1 s-1) (26):
eaq- + H+ f •H
(4)
Electron Beam Treatment. All large-scale experiments were carried out at the Electron Beam Research Facility (EBRF) located at the Virginia Key Wastewater Treatment Plant in Miami, FL. It houses a horizontal 1.5 MeV insulated-core transformer (ICT) electron beam, a water delivery system, and ancillary equipment. Aqueous streams of substrate dissolved in drinking water, a treated groundwater, are exposed to a scanning electron beam in a falling stream approximately 114 cm wide and 0.4 cm thick. The beam current has a range from 0 to 50 mA, providing radiation doses of 0-8 kGy (0-800 krad). The electron beam system is instrumented with resistance temperature devices (RTDs) to obtain direct estimates of the absorbed dose. The RTDs are mounted in the influent and effluent streams immediately before and after the section exposed to irradiation, allowing the absorbed dose to be estimated from the observed temperature difference caused by the energy transferred to water. A more detailed description of the Electron Beam Research Facility is given elsewhere (27). In a typical experiment, 17 m3 (4500 gal) of drinking water was pumped into a tank truck, and 340 mL (2.90 mol) of TA was added, which gave an initial concentration of 170 µM (20 ppm). After being mixed, this substrate solution was pumped through the electron beam at a flow rate of 6.3 L s-1 and irradiated at different doses. Three influent and effluent samples with matched transit times between sampling points were taken immediately before and after an irradiation for each of the applied doses as well as for a control dose of 0 kGy. The different doses including the zero-dose control were applied in random order to minimize the chance of systematic errors. The respective TA concentrations were estimated via high-pressure liquid chromatography (HPLC) analysis using a Beckman System gold autosampler 507, a Beckman System gold solvent module 126, a Beckman detector 168, a 150 × 4.6 mm ODS Ultrasphere column, a Beckman System gold
software version 7.11, and an IBM PS/270. The disappearance of TA in aqueous solutions was monitored at 250 nm using a water/methanol (25/75) eluent. The HPLC system was calibrated with freshly prepared standard TA solutions. The relative error in the determination of the TA concentration was (1% based on triplicate runs at each concentration. As a note, the actual nonirradiated influent concentrations of approximately 85 µM (10.0 ppm) differed considerably from our initial estimate of 170 µM (20.0 ppm). This concentration decrease is however consistent for all influent samples as well as for the blank effluent samples. We attribute it to a loss of TA due to evaporation, further to an incomplete dissolution and mixing of TA in drinking water, and to adsorption of the thioether substrate by the metal piping system. It is the remarkable reproducibility that ultimately justifies comparison of the data on a quantitative level and all conclusions derived therefrom. Kinetic Model and Simulation Procedure. In dilute substrate solutions, reaction 1 describes the net chemical result at about 10-7 s after the highly accelerated electrons have passed through the aqueous phase. The three reactive radical species (•OH, eaq-, H•) will react among themselves and with suitable substrates. A collection of all relevant reactions with their corresponding bimolecular rate constants is available in the literature (18). The initial concentration of the radicals, at any experimental dose, was obtained from the G values in reaction 1. The concentrations of OH- and H+ ions were evaluated from the pH, while carbonate and bicarbonate concentrations were calculated from the total alkalinity and pH. For each experimental run, pH, total alkalinity, dissolved oxygen, dissolved organic carbon (DOC), and nitrate ion concentrations were measured, and their values were used in the simulation. Typical experimental values were 3.5 mg L-1 for dissolved oxygen (as O2), 7.1 mg L-1 for DOC (as C), 0.36 mg L-1 for nitrate (as N), and 51 mg L-1 as CaCO3 alkalinity. The computer program MAKSIMA-CHEMIST provided by Atomic Energy of Canada, Ltd. was applied to simulate removal efficiencies. Details of the integration algorithm and validation tests can be found elsewhere (28-30). The input to the kinetic model includes a list of all reacting species, their initial concentrations (via experimental measurements), the reaction rate constants (experimental and literature values), and the applied dose rates. The dose rates were assumed to be constant for the duration of 0.091 s, which is the residence time of the substrate solution in the irradiation beam. The predictability of our model greatly depends on the ability to account for all relevant reactions with their proper rate constants and on the accuracy of the experimentally determined concentrations.
Results and Discussion Reaction of TA with •OH Radicals. The pulse radiolysis of aqueous, nitrous oxide-saturated solutions containing TA (CH3S-C6H5) in the concentration range of 1 × 10-4 to 1 × 10-3 M yields a complex spectrum of intermediates with absorption maxima at 310, 365, and 530 nm (Figure 1). Ioele and co-workers (15) as well as Mohan and Mittal (16) assigned these absorptions to a radical cation TA•+ absorbing at 310 and 530 nm (reaction 5)
CH3S-C6H5 + •OH f [CH3S-C6H5]•+ + OH310 and 530 nm
(5)
and an •OH radical addition product TA(OH)• absorbing at 365 nm (reaction 6)
CH3S-C6H5 + •OH f CH3S-C6H5(OH)• 365 nm VOL. 34, NO. 7, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(6)
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FIGURE 1. UV-Vis spectrum of intermediates formed upon pulse radiolysis of a 1 mM N2O-saturated aqueous thioanisole solution at pH 7, 1 µs after the electron pulse. Another possible reaction pathway for •OH radicals attacking TA, suggested by Ioele and co-workers (15) regarding a reaction of SO4•- with TA, is an H-atom abstraction at the methyl site:
CH3S-C6H5 + •OH f •CH2S-C6H5 + H2O 260 nm
(7)
Our absorption spectrum in Figure 1 cannot positively identify such a product because the overall absorption at around 260 nm is very weak. Considering that analogous aliphatic R-alkylthio alkane radicals, e.g., •CH2SCH3, show relatively strong UV absorptions (280 nm ≈ 3000 M cm-1) (31), it can thus be concluded that the contribution of reaction 7 is only of minor importance, if any. To simulate the removal of TA in a large-scale electron beam treatment, an observed or overall rate constant for each radical reaction (TA + •OH, eaq-, H•) has to be known. Since the documented rate constant for •OH reacting with TA by Mohan and Mittal (16) failed to satisfactorily simulate our results (described in a later section), we determined an overall rate constant via a competition with t-ButOH:
CH3S-C6H5 + •OH f TA - products
(8)
(CH3)3COH + •OH f products
(9)
Such a competition can be expressed mathematically (32):
Abs0(TA - products) Abs(TA - products)
)1+
k9 [(CH3)3COH] k8 [TA]
(10)
where Abs0(TA - products) is the absorption values of the intermediates at 310, 365, and 530 nm without scavenger t-ButOH, whereas Abs(TA - products) represents the decreased absorptions of these intermediates in the presence of competing •OH scavengers. A plot of absorption ratios Abs0(TA - products)/Abs(TA - products) versus corresponding concentration ratios [(CH3)3COH]/[TA] yields a straight line with an intercept of 1 and the rate constant ratio k9/k8 as the slope (see Figure 2). Several corrections had to be made for the 365 nm absorption. Since the absorptions at 310 and 365 nm are close to each other, an overlap can be assumed. We corrected for this by evaluating the contribution of the strong radical cation absorption at 310 nm to the overall absorption at 365 nm. From the work of Ioele and co-workers (15), the ratio for the radical cation absorption at 310 nm versus that at 365 nm was estimated to 9200:500. Our 310 nm absorption value was therefore divided by this factor and then subtracted from the 365 nm absorption. Furthermore, an H-atom adduct to 1288
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FIGURE 2. Competition kinetics: plot of the absorbance ratio Abs0(TA - product)/Abs(TA - product) at 310 (9), 365 (2), and 530 nm (b) versus the concentration ratio [(CH3)3COH]/[TA] upon pulse radiolysis of N2O-saturated aqueous solutions at pH 7. TA, which will be described in a later section, also absorbs at 365 nm. H-atoms are present at a 10% yield relative to •OH in N2O-saturated neutral solutions. To account for this contribution, a residue value was obtained at 365 nm in N2Osaturated solutions containing an excess of t-ButOH (0.5 M tButOH vs 1 mM TA). Under these conditions ≈97% of all •OH radicals have reacted with tButOH while ≈93% of all •H have reacted with TA. The resulting residue absorption can therefore be assigned to the hydrogen addition product. This value was also subtracted from the overall 365 nm absorption. With k9 ) 6.00 × 108 M-1 s-1 (25), we determined k8 ) (9.90 ( 0.13) × 109 M-1 s-1 at 310 and 530 nm and (7.85 ( 0.18) × 109 M-1 s-1 at 365 nm. For a simple mechanism of competing reactions, the rate constant k8 should be an overall rate constant with k8 ) Σkn ) c5k5 + c6k6 + c7k7 and observable at each absorption (33). The fact of different growth rate constants at 310/530 nm, when compared to 365 nm, may be attributed to one or more preceding reaction steps at 365 nm, which could delay the growth. But the absorbance versus time curves at low [(CH3)3COH]/[TA] concentration ratios did not show a different growth or decay behavior when compared to higher concentration ratios, thus ruling out errors due to mechanistic changes. Clearly, the 365 nm results have to be associated with the higher uncertainty due to the overlap of different absorptions at this wavelength. The higher value of (9.90 ( 0.13) × 109 M-1 s-1 at 310/530 nm, indicating a practically diffusion controlled reaction of •OH radicals with TA, is therefore assumed to be the more accurate overall rate constant kobs(TA + •OH) ) Σkn, and this value will be used in all subsequent calculations. To justify our assumption, we fitted absorption-time traces at 530 nm by utilizing the above growth rate constant and accounting for a slow signal decay (see Figure 3). We found a very good agreement between the fitted curve and the experimental data. Reaction of TA with Hydrated Electrons. To study hydrated electron reactions, pulse radiolytic experiments were carried out in neutral, N2-saturated, oxygen-free TA solutions with an addition of 0.5 M tButOH to scavenge •OH radicals. In the UV-Vis range, only the very strong absorption of hydrated electrons is observed. It decays exponentially and is dependent on the TA concentration. A plot of the pseudo-first-order decay rate constants versus TA concentrations, ranging from 0.1 to 1 mM, yields a straight line with a slope of k(TA + eaq-) ) (3.1 ( 0.3) × 108 M-1 s-1 as shown in Figure 4. This rate constant is significantly higher than those for other benzene derivatives in their reaction with hydrated
FIGURE 3. Growth kinetics at 530 nm of 1.022 mM N2O-saturated aqueous thioanisole at pH 7 with a best fit utilizing a growth rate constant of 9.9 × 109 (M s)-1 and accounting for a slow decay. Insert, same parameter fit at a longer time scale.
FIGURE 5. UV-Vis spectrum of intermediates formed upon pulse radiolysis of a 1 mM N2-saturated aqueous, oxygen-free thioanisole solution at pH 1 with an addition of 0.5 M tert-butyl alcohol, 3.5 µs after the electron pulse. Insert, competition kinetics: plot of the absorbance ratio Abs0(365 nm)/Abs(365 nm) versus the concentration ratio [(CH3)2CHOH]/[TA] under similar experimental conditions. the applied conditions (0.5 M tButOH vs 1mM TA), approximately 3.2% of all •OH radicals are reacting with TA. This minor reaction pathway cannot however account for the observed strong absorption at 365 nm. We attribute the observed absorption at 365 nm accordingly to an adduct of hydrogen atoms to the benzene ring:
CH3S-C6H5 + •H f CH3S-C6H5(H)• 365 nm
FIGURE 4. Plot of pseudo-first-order decay rate constants of hydrated electrons at 720 nm versus different thioanisole concentrations in N2-saturated, aqueous 0.5 M tert-butyl alcohol solutions at pH 7. electrons, e.g., benzene 1.2 × (34), toluene 1.2 × (34), phenol 1.8 × 107 (34), anisole
