Environ. Sci. Technol. 2005, 39, 7964-7969
Modeling Aqueous Ozone/UV Process Using Oxalic Acid as Probe Chemical TEMESGEN GAROMA* AND MIRAT D. GUROL† Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093 and Department of Civil and Environmental Engineering, San Diego State University, San Diego, California 92182
A kinetic model that describes the removal of organic pollutants by an ozone/UV process is described. Oxalic acid, which reacts with a very low rate constant with ozone and relatively high rate constant with hydroxyl radical (OH•), was used as the probe chemical to model the process. The model was verified by experimental data on concentrations of oxalic acid and hydrogen peroxide (H2O2) under various experimental conditions, i.e., ozone gas dosage, UV light intensity, and varying oxalic acid concentrations.
Introduction In the past, the ozone/UV process has been shown to be effective for the removal of the following materials from wastewater or aqueous solutions: cyanides (1), refractory organic compounds, such as aliphatic carboxylic acids (acetic, oxalic, propionic, succinic, and malonic) and methyl and ethyl alcohols (2), p-chloronitrobenzene (3), pentachlorophenol, and other substituted phenols (4, 5), among other compounds. The ozone/UV process was also used to partially oxidize natural organic matter in water to reduce subsequent formation of trihalomethanes (6-9). More recently, the process was shown to be very effective in degrading fuel oxygenates such as MTBE and TBA in dilute aqueous solutions (10, 11). To predict the removal of organic pollutants by an ozone/ UV process, a comprehensive kinetic model describing the interaction of ozone with UV light is needed. Gurol and Akata (12) have successfully modeled the process from a kinetic and mechanistic point of view, however, the model was calibrated and verified in the absence of organic load, and the calibration and verification of the model in the presence of organic compound(s) is warranted. The major objective of this study is to successfully model the ozone/UV process by using oxalic acid as the probe compound under various experimental conditions. Oxalic acid was chosen as the probe compound because it reacts with a very low rate constant with ozone, k ) 0.04 M-1s-1 (13), and a relatively high rate constant with hydroxyl radical (OH•), k ) 5.3 × 106 M-1s-1 (14) at around neutral pH. Furthermore, the oxidation of oxalic acid produces carbon dioxide as the only byproduct (15). The scavenging effect of carbon dioxide can be taken into account by using the * Corresponding author current address: Parsons Water and Infrastructure, 100 West Walnut Street, Pasadena, CA 91124; tel: 626440-3020; fax: 626-440-6337; e-mail:
[email protected]. † San Diego State University. 7964
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 20, 2005
equilibrium relationship between the carbonate species, hence simplifying the modeling process.
Kinetic Model for the Ozone/UV Process The decomposition of aqueous ozone in pure water is initiated by its reaction with hydroxide ion (OH-), and this reaction leads to the production of radicals that propagate the decomposition process by chains of radical-radical or radical-solute reactions and produce OH• (16-18). OH-
O3 98 OH•
(1)
The above process is known to occur very slowly at low pH levels (16) and the decomposition process can be further accelerated with ultraviolet radiation. In aqueous phase, the photolysis of ozone produces hydrogen peroxide (H2O2) as primary product (19), which then can photolyze into OH• directly and can react with aqueous ozone to produce OH• as shown below.
hv
O3 + H2O 98 H2O2
{
+ O3 f OH•
(2)
hv
98 2OH•
The principal reactions involved in an ozone/UV system in natural waters have been summarized in an earlier publication by the authors (Table 2 in ref 10). On the basis of the proposed reaction mechanisms in this table, the overall rate of change in the concentration of aqueous ozone (O3), hydrogen peroxide (H2O2), and hydroxyl radical (OH•) in a completely mixed batch reactor can be represented by the following expressions, in a manner similar to those used by Gurol and Akata (12). As opposed to Gurol and Akata, here non-steady state was employed for hydroxyl radical concentration because in this system concentration of oxalic acid will change causing concentration of OH• to vary with time. The detailed derivations of these equations are presented in the Supporting Information.
d[O3] ) nO3 - k1[O3][OH-] - ΦPIa - 2k /2[O3][H2O2] dt 2k6[O3][OH•] - k /7[H2O2][OH•] - k /9[OH•][HCO3-] -
∑k
di[O3][Mi]
(3)
d[H2O2] ) γ1{k1[O3][OH-] + ΦPIa - Φ′PI ′a dt k /2[O3][H2O2] - k /7[H2O2][OH•] - k /9[OH•][HCO3-]} (4) d[OH•] ) 2Φ′PI ′a + 2k /2[O3][H2O2] dt
∑ k [M ][OH ] ∑ k [S ][OH ] (5) •
bi
i
•
j
j
In these equations ∑kbi[Mi][OH•] represents the reaction of the target organic compounds with OH• and ∑kj[Sj] represents the scavengers of OH• other than the target organic compounds and bicarbonates, and yet is negligible for solutions prepared in purified water. Furthermore, five rate constants in these equations vary with pH, as shown below: 10.1021/es050878w CCC: $30.25
2005 American Chemical Society Published on Web 09/17/2005
k /2 ) k2 +
k3Ka3 +
k /7 ) k7 +
k8Ka3 +
[H ] [H ] + k K ] [H 12 a3 γ1 ) + k /11 ) k11 + + [H ] + Ka3 [H ]
k /9 ) k9 +
k10Ka2 +
[H ]
In eq 3 nO3 represents the rate of ozone absorption per unit volume and is given by the following expression:
nO3 )
NO 3
(6)
V
in which NO3 is the rate of ozone absorption and can be described by using a model developed by Gurol (20) for ozone absorption in bubble reactors as follows:
{
}
{
kLa
H(Qg/V)
}
d[O3] dt
p
(8)
) -ΦPIa ) -ΦPIo(1 - e-2.3b[O3])
(9)
where Io is the incident UV light intensity, is the absorption coefficient for ozone, which is 3600 M-1cm-1 at 254 nm (21), and b is the effective path length of the UV light in the reactor, which needs to be determined experimentally. Similarly, the rate of decomposition of hydrogen peroxide by UV light can be described as follows:
(
)
d[H2O2] dt
p
) -Φ′PI ′a ) -Φ′pIo(1 - e
-2.3′b[H2O2]
P
) -2.3′bΦ′pIO[H2O2] ) -kobs[H2O2] (12)
(
[H2O2]
[H2O2]O
)
) -kobst
(13)
Equation 13 implies a straight line when ln([H2O2]/[H2O2]o) is plotted versus time, with a slope of kobs. The kobs is a function of Φ′P, IO, ′, and b. Thus, for given values of Φ′P, IO, and ′, b can easily be computed by knowing the value of kobs. For the experimental reactor, the effective path length of UV light was determined experimentally as 7.5 ( 0.9 cm and the result is presented in the Supporting Information. Rate of Ozone Mass Transfer. In an ozone/UV system, ozone is transferred from the gas into the liquid phase and the overall rate of change in aqueous ozone concentration in a completely mixed system with respect to both liquid and gas phase can be given as
d[O3] dt
j)m
) kLa([O3]* - [O3]) - ΦPIa -
∑k [C ][O ] j
j
(14)
3
j)1
where kLa is the volumetric mass transfer coefficient of ozone and [O3]* is saturation concentration of ozone. The term ΦPIa refers to the photolysis of ozone by UV light and [Cj] refers to the concentration of all species that react with ozone. In the absence of UV light and reacting species, eq 14 can be simplified as follows:
d[O3] ) kLa([O3]* - [O3]) dt
(15)
) (10)
where Φ′p is the primary quantum yield for hydrogen peroxide, which is reported in the literature as 0.5 at wavelength of 254 nm (22), I ′a is the rate of UV light absorption by H2O2, and ′ is the absorption coefficient for H2O2, which is 19.6 M-1cm-1 at 254 nm (22). The UV light absorbance of oxalic acid in UV-vis spectrum region dependent on the concentrations of oxalic acid and is negligible for concentration below 0.11% by wt (23). In this study the highest concentration of oxalic acid used was equal to 1 mM (∼0.009% by wt). Thus, the reaction of the oxalic acid with hydroxyl radical (OH•) is the only mechanism through which it is removed. The reaction of oxalic acid with OH• in an ozone/UV system can be expressed by the wellestablished second-order kinetics as follows (24):
d[OXA] ) -kOXA[OH•][OXA] dt
)
d[H2O2] dt
(7)
where kLa is the volumetric mass transfer coefficient for ozone, H is the dimensionless Henry’s constant for ozone, which is 2.86 at T ) 20 °C, Qg is ozone gas flow rate, (O3)inf is influent ozone gas concentration at steady state, and V is volume of water sample. In eqs 3-5 ΦPIa represents the primary decomposition rate of ozone by UV radiation. In this term ΦP is the primary quantum yield for ozone, which is reported as 0.48 at wavelength of 254 nm (12), and Ia is the rate of UV light absorption by ozone. On the basis of the Beer-Lambert Law, and assuming uniform distribution of UV light, the rate of decomposition of ozone can be described as follows:
( )
(
ln
(O3)inf NO3 ) QgH[1 - exp(-θO3)] - [O3] H θO3 )
process at neutral pH was considered. At around neutral pH only one oxalate species, namely bi-oxalate, is dominant and the rate constant of bi-oxalate with hydroxyl radical was used in the kinetic model. To predict the residual concentration of oxalic acid in the ozone/UV system, eqs 3-5 are coupled with eq 11 and solved numerically as described below. Effective Path Length of UV Light in the Experimental Reactor. For very small concentrations of H2O2, the rate of decomposition of H2O2 by UV light given in eq 10 can be simplified by using Taylor’s series expansion to eqs 12 and 13.
(11)
where kOXA is the reaction rate constant of oxalate species with OH• and [OXA] is the concentration of oxalate species. In this study, the oxidation of oxalic acid by an ozone/UV
The value of the volumetric mass transfer coefficient, kLa, can be determined by following the concentration ozone with time until the saturation concentration, [O3]*, is reached, and fitting measured ozone concentration data into the integrated form of eq 15 presented in eq 16. For the experimental reactor, the volumetric mass transfer coefficient for an influent ozone gas concentration of 50 mg/L, at flow rate of 1.5 L/min, was determined experimentally as 8.8 × 10-3 s-1, and the result is presented in the Supporting Information.
(
ln
[O3]* - [O3]
[O3]* - [O3]O
)
) kLat
(16)
where [O3]O is the concentration of ozone at time t ) 0. kLa may depend on ozone scavengers under high organic loading conditions resulting in enhanced mass transfer coefficient. Interested readers are referred to the work of Gurol and Nekouinaini (25) who reported the effect of organic loads on mass transfer in bubble reactors, and to the work of Glaze and Kang (26) in which hydrogen peroxide at various VOL. 39, NO. 20, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Predicted effect of incident UV light intensity on oxalic acid removal. doses was used as scavenger to investigate the mass transfer enhancement for ozone in a semi batch reactor.
Method of Solution of the Model Equations Equations 3-5, coupled with eq 11, were used to predict the residual concentration of oxalic acid for given influent ozone gas concentration, incident UV light intensity, and water quality parameters such as pH and bicarbonate concentration. The equations involve a system of nonlinear ordinary differential equations with known initial conditions. The solution to the system of ODEs was obtained numerically using ode15s, a subroutine available in MATLAB. The ode15s is a variable-order stiff solver based on the numerical differentiation formulas (NDFs). Initial conditions (concentrations of dissolved ozone, hydrogen peroxide, hydroxyl radical, and oxalic acid at time t ) 0), operational parameters (influent ozone gas concentration, UV light intensity, pH, bicarbonate concentration, volumetric mass transfer coefficient, gas flow rate, and effective path length of UV light in the reactor) and the values of rate constants, for reactions listed in Table 2 in ref 10 and that of oxalate species with hydroxyl radical, are provided as inputs into the computer program. The kinetic model is capable of taking into account change in pH during the oxidation process. pH values at the start and end of the oxidation process are provided as an input into the computer program and the model linearly interpolates for pH values at other times. Similarly, change in sample volume as a result of sample withdrawal is taken into account in the kinetic model.
Sensitivity of the Kinetic Model Sensitivity of the kinetic model to operational conditions, such as incident UV light intensity (Io), influent ozone gas concentration ((O3)inf), and water quality in terms of bicarbonate and pH, as well as the process parameter effective path length for UV light was investigated. The effects of incident UV light intensity and influent ozone gas concentration on the accumulation of hydrogen peroxide (H2O2) and hydroxyl radical (OH•) in the ozone/UV system was also examined. The following values were used as base values: Io ) 6 × 10-6 E/L-s, (O3)inf ) 48 mg/L, [OXA] ) 1.0 mM, [HCO3-] ) 2 mM, pH ) 7.0, kLa ) 0.0088 s-1, Qg ) 1.5 L/min, b ) 7.5 cm, and kOXA ) 5.3 × 106 M-1s-1. The sensitivity of the kinetic model in relation to variation of selected parameters was tested while keeping constant other parameters at base values. The effect of incident UV light intensity (Io) on the removal of the oxalic acid is presented in Figure 1. The result indicates 7966
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FIGURE 2. Predicted effect of influent ozone gas concentration on oxalic acid removal. that the removal of the oxalic acid increased significantly as the incident UV light increased from 1 × 10-6 to 3 × 10-6 E/L-s, but the increase in the rate of removal of oxalic acid became moderate to further increase in Io. This limitation could be due to the fixed value of influent ozone gas in the ozone/UV system. In addition, at such low concentration (1 mM) oxalic acid does not absorb UV light (23) and hence does not photolyze when exposed to UV light, and the reaction of the model compound with hydroxyl radical (OH•) is the only mechanism through which it is removed. Therefore, for given influent ozone gas concentration, continuously increasing incident UV light intensity does not necessarily increase the removal efficiency of organic pollutants by an ozone/UV process. In Figure 2, the effect of influent ozone gas concentration is presented, and according to the result the removal of the oxalic acid increased continuously as the influent ozone gas concentration increased. From this result it can be inferred that once the decomposition of ozone is initiated with UV light, increasing the influent ozone gas concentration accelerates the decomposition process by the reaction of dissolved ozone with hydroxyl and other radicals as indicated in Table 2 of ref 10. Thus, in the design of an ozone/UV process if minimum UV light intensity that initiates the decomposition of ozone is maintained, increasing the influent ozone gas concentration results in increased removal efficiency of organic pollutants. The effect of varying bicarbonate concentration, pH, and effective path length of UV light on the removal efficiency of oxalic acid has been evaluated and the results are presented in the Supporting Information. Similarly, the effects of incident UV light intensity and influent ozone gas concentration on the accumulation of hydrogen peroxide (H2O2) and hydroxyl radical (OH•) in the ozone/UV system was examined and are also presented in the Supporting Information. In the presence of aqueous oxygen (O2), many target compounds become oxidized to an intermediate radical which may transfer an H-atom or electron to O2 to produce O2-•/HO2-•. In the case of oxalic acid, its oxidation produces oxaylyl radical (C2O4-•) which then under goes a decarbonation process, elimination of CO2. The remaining specious (CO2-•) becomes further oxidized by transferring an electron to aqueous O2 (27). This additional source of O2-•/HO2-• from oxalic acid is not included in Table 2 of ref 10, which lists the principal reactions involved in an ozone/UV system. Therefore, testing the kinetic model for its sensitivity in relation to variation of O2-•/HO2-• is necessary. In Figure 3, the sensitivity of the kinetic model in relation to variation of superoxide radical (O2-•) is presented. The result indicates
TABLE 1. Experimental Conditions
FIGURE 3. Predicted effect of superoxide radical on oxalic acid removal.
FIGURE 4. Predicted effect of superoxide radical on H2O2 accumulation and decay. that as the concentration of O2-• increased from 10-10 to 10-6 M, the removal efficiency of oxalic acid increased as expected. However, further increasing the concentration of O2-• resulted in decreased removal efficiency. This may be due to the scavenging effect of O2-• on OH• (Table 2 in ref 10), with k ) 9.4 × 109 M-1s-1 (28) which is significantly higher than that of oxalic acid at neutral pH: k ) 5.3 × 106 M-1s-1 (14). The sensitivity of hydrogen peroxide (H2O2) accumulation and decay in relation to the variation of superoxide radical (O2-•) is presented in Figure 4. According to the result, H2O2 accumulation and decay is very sensitive to the variation of O2-•, and as the concentration of O2-• increases the concentration of H2O2 accumulated decreases.
Experimental Section In the previous section, the sensitivity of the kinetic model to operational conditions and process parameters was examined. In the following sections, the kinetic model for the ozone/UV process will be verified by using experimental data on concentrations of oxalic acid and hydrogen peroxide (H2O2). The experimental studies were conducted by using the same reactor described in ref 10. In the course of this study, influent ozone gas concentration in the range of 2458 mg/L at constant flow rate of 1.5 L/min was used. The alkalinity of the aqueous solution was adjusted to 2 mM by sodium bicarbonate (NaHCO3). Dilute solutions of sulfuric acid (H2SO4) and sodium hydroxide (NaOH) were used to adjust the pH of the aqueous solution to a value of 7.0 at the start of each experiment, and during the oxidation process
exp. run
oxalic acid (mM)
Io (E/L-s)
(O3)inf (mg/L)
(O3)eff (mg/L)
[O3]water (µg/L)
1 2 3 4 5 6 7 8
0.87 0.88 0.85 0.97 0.14 0.80 0.91 0.96
9.0 × 10-6 9.0 × 10-6 5.8 × 10-6 5.8 × 10-6 2.9 × 10-6 2.9 × 10-6 5.8 × 10-6 5.8 × 10-6
40 58 36 47 24 24 39 53
12 31 15 21 12 12 15 28
