Environ. Sci. Techno/. 1995, 29, 3007-3014
Chemical Oxidation by Photolytic Decomposition of Hydrogen Peroxide C H I H - H S I A N G LIAO A N D M I R A T D . GUROL’ Environmental Studies Institute, Drexel University, Philadelphia, Pennsylvania 191 04
well as by developing and testing a kinetic model of the process. The H2O21UVprocess was first investigated in “pure” water, and then the results were applied to the effect of naturally occurring species such as carbonate ions and natural organic matter. A continuous-flow stirred tank reactor (CSTR) was used for experimental observations as well as for modeling purposes. The CSTR was preferred over a batch reactor to obtain steady-state conditions, which generally allow a more effective evaluation of the kinetic systems.
Experimental Approach This paper describes a study of a chemical oxidation process involving simultaneous application of hydrogen peroxide solution and ultraviolet light (H20dUV) for removal of organic pollutants from aqueous solution. The process was investigated experimentally in a continuous-flow stirred tank reactor (CSTR) under various operational conditions, Le., H202 dosage, UV light intensity, and liquid residence time. Synthetic solutions of a model organic compound, n-chlorobutane (BuCI), were oxidized at various pH and in the presence of various amounts of humic material and carbonate/bicarbonate ions in order to examine the effect of water quality on the process efficiency. A kinetic model of the process, which was developed based on HzOdUV-induced radical oxidation of organic compounds, was successfully verified in pure water as well as in synthetic solutions.
Introduction Since the late 1960%numerous researchers have used the H202/W process to oxidize various organic substances in water. Recently, commercial units employing this process have been developed for on-site oxidation of organic contaminants in groundwater. The success of this process has been generally atttributed to the stoichiometric formation of hydroxyl radical (‘OH) by photolytic decomposition of H202. The hydroxyl radical is an extremely reactive and nonselective oxidant and, thus, when produced in sufficient quantities, can lead to complete oxidation of organic compounds to carbon dioxide, water, and inorganic ions. In spite of the commercial success of the H202/UV process, the chemistry, kinetics, and engineering principles associated with the process have not been elucidated adequately. The recent careful studies by Glaze and Lay (11, Yao et al. (21, and Wolfrum (3) provided a better, although partial, understanding of the process fundamentals. The present study attempts to add to the existing literature through a systematic investigation of the effect of the process parameters and water quality on the process performance in terms of the rate of H202 photolysis and the oxidation rate of the organic contaminant(s)of interest. This objective is realized by experimental observations as * Corresponding author e-mail address:
[email protected]. Edu; fax: (215) 895-2267.
0013-936X/95/0929-3007$09.00/0
G 1995 American Chemical Society
The laboratory studies were conducted using a bench-scale circular quartz reactorwith a 10-cm 0.d. and a liquidvolume of 840 mL. The reactor was inserted into a chamber that contained concentrically placed 16 removable low-pressure mercurylamps that emit Wlight primarily at 254 nm (93% of the light emitted in the UV range). It should be noted that the quantum yield for H202 photolysis does not change significantlywith wavelength (6).The reactorwas operated in continuous flow mode and stirred vigorously to mimic a CSTR. The schematic diagram of the experimental setup is shown in Figure 1. In order to avoid stripping of the volatile compounds by mixing, no head space was allowed within the reactor. The temperature of the reactor content increased only slightly during the experiments because of the presence of a continuously operating fan within the chamber. The hydraulic characteristics of the reactor were studied by applying a tracer (methyleneblue) in pulse mode for various liquid residence times (r). The results of these pulse tracer analyses showed than an ideal CSTR has been approximated very closely under all the experimental conditions (4). Hence, the performance of the reactor was governed only by the chemical kinetics and the liquid residence time. After reaching steady-state conditions, Le., after a time period equal to 3r, water samples were collected from the effluent stream for analysis. The oxidation efficiency of the process was determined by monitoring the removal rate of n-chlorobutane (BuC1) under various experimental conditions. Butyl chloride was used in this study as a probe compound for ‘OH; its reaction with H202 and direct photolysis by UV radiation are negligible. Hence, the conclusions drawn from this study would be restricted to the compounds that can react only with the primary oxidant *OHin the H202/UVsystem. Materials. The 30% and 50%certified solutions of H202 were purchased from Fisher, and peroxidase type VI (POD reagent) was purchased from Sigma Chemical. n-Chlorobutane (BuCl, 99.9%) and pentyl chloride (PeC1, 99%) were obtained from Aldrich Chemical Company. Potassium trioxalatoferrateand certifiedACS sodium bicarbonate were received from the Alfa Company and Fisher Scientific, respectively. The stock solution of humic acid was prepared by dissolving solid humic acid obtained from Fluka AG in a 0.1 N NaOH solution and then filtering it through No. 1 Whatman filter paper. The stock solution was diluted at desired proportions to be used in the experiments. Analyses. BuCl in aqueous samples was analyzed by static head space gas chromatography (GC) using pentyl
VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
3007
L
I I
feeding tank
1
photoreactor chamber
FIGURE 1. Experimental setup.
chloride (PeCl) as an internal standard (5).The feedstocks containing BuCl were prepared by spiking a known amount of BuC1-saturatedMilli-Q water into containers containing predetermined volumes of water at 20 f 1 "C. During the experiments, samples of about 300 mL were taken from the influent and effluent lines and analyzed as follows: 20 mL of the sample was transferred into a 30-mL vial and was spiked with 1mL of PeC1-saturated Milli-Q water. The vial was then capped and thermostated at 78 "C in a water bath for about 1 h. A 500-pL sample of the equilibrium gas mixture was drawn from the head space by a gas-tight syringe and was injected into GC equipped with a flame ionization detector. The coefficient of variation (CV), Le., the standard deviation expressed as the percent of the arithmetic mean was measured as 10% and 11%for BuCl and PeCl concentrations, respectively. However, the CV for the ratio of the peak areas of BuCUPeCI was 0.6% in the absence of humic material and 3%in the presence of humic material. A statistical t-test indicated that the effect of humic acid on the peak areas of BuCl and PeCl was not significant at 95% confidence interval. Hydrogen peroxide concentrations in the water samples collected from the influent and effluent lines of the reactor were measured by the method of horseradish peroxidasecatalyzed oxidation of N,N-diethyl-p-phenylenediamine (PODIDPD) (7). The precision and the accuracy of this method was checked by diluting the H202 solutions at various predetermined proportions and by measuring the concentrations using a prepared calibration curve. The maximum relative bias estimated using this technique was 10%. The concentration of H202 in the stock solution was measured by the potassium permanganate method (8).Both methods were calibrated by the direct spectrophotometric measurement at 254 nm. W intensity was measured by using the ferrioxalate actinometry (9, 10). The incident light intensity measured in this study varied between 2.46 x 10 ~'and6.79x 10-4EinsteinL-'min-'with6-161amps operating simultaneously. The measured intensity over a period of 1year showed no deterioration of the lamps, and the random variation in the measured intensity was within 5% of the average value. The dissolved or total organic carbon content (DOC/ TOC) of water samples was measured by a Dohrman DC80 carbon analyzer, according to the Standard Methods ( I 1 ) . The alkalinity of water was measured by titrating it with a standardized sulfuric acid ( 1 1 ) . The p H was measured by an Accumet pH meter (Fisher Scientific). A Tn'-visible spectrophotometer (Milton Roy) was used to 3008
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ENVIRONMENTAL SCIENCE & T E C H N O L O G Y , VOL. 29 N O 12,1995
measure the absorbance of water samples and H202 solutions at 254 nm. Reaction Mechanism in Pure Water. The reaction steps 1-4 presented below are accepted in the literature to represent the mechanism of hydrogen peroxide photolysis in pure water (12-15). Furthermore, the quantum yield for the process, which can be defined as the number of moles of hydrogen peroxide decomposed per mole of light photon absorbed, has been estimated as 1.0 for the overall quantum yield (@I) (12,14, 15) and as 0.5 for the primary (12-15), Le., for the initiation reaction, quantum yield (ak,) for relatively high light intensity, and for low peroxide concentrations. light absorptionlinitiation
H,O,+ hv
- 2'OH
(1)
propagation
'OH
- HO,'/O,-
+ H,O,/HO,-
+ H,O
k, (2)
termination
HO,'
+ HO,' - H,O, + 0,
k,
(4)
In addition, the disproportionation reaction of of HO2.i 02*-is expected to contribute to H 2 0 ~regeneration (161,as shown below: HO,'
+ O,-* + H,O - H,O, + 0, + OH- k, pK,,,, = 4.8 (17, 18) HO,' O,*- + H+
-
(5) (6)
Reaction 3 above as well as other possible termination reactions involving reactions of 'OH with HOz' and 02.were found to have no significant contribution to the overall reaction mechanism (4)according to a kinetic model that is described later in this paper. Reaction Mechanism in the Presence of Carbonate Species. The carbonate species, HC03- and C03?-, are expected to affect the photolysis of hydrogen peroxide in aqueous solution through various pathways. Both species are known as effective 'OH scavengers, and hence their presence may cause a reduction in the oxidation rate of the target contaminants. Furthermore, the reaction of'OH with H C 0 3 - and COS2-generates the carbonate radicals (HC03' and C03'- ) as oxidation transients ( 19). These two forms of carbonate radicals exhibit similar reactivities toward other species (20). It was suggested that these radicals may disappear rapidly by attacking hydrogen peroxide, resulting in the formation of hydroperoxyl radical (H02./02*-) (21). Ultimately,H02*/02*may react to regenerate Hz02 through disproportionation reaction, or they may react with organic substances if their reaction rate is relatively high. For pH values above 8, the radical COS'- becomes dominant (pK,,e of HC03' is 7.9; ref 22) and reacts with 02*to terminate the reaction. These reactions of the carbonate species are summarized below:
'OH
+ HCO,-/COS'HC0,'-
- HCO,'/CO,'-
C0,'-
+ H+
+ OHpK,,, = 7.9
k; ( 7 ) (8)
-
+ H2O,/H0,-
HCO,'/CO,'-
+ HCO,-/CO,'- co,,- + 0, k,,
HO,'/O,'-
c0;- + 0;-
k9 (9) (10)
Oxidation mechanism of an organic compound, M, in this system includes the reaction of M with 'OH and HO2'/ 02'-, as follows: M f 'OH M
-
+ H0,'/02'-
MOA
kM
-... M*,xi
kM*
(12)
(17)
where
'
(11)
+ hv - excited HM - - -
A,,, (cm-') = €,,*DOC
'OH
rH",o,= (2.303@T6b10~) [H2021
1 - 10-tb[HzOzI (18)
= 2.303~b[H,O,]
Note that here the organic compound is assumed to react only with 'OH and H02'/02'-. However, certain compounds can photolyze under UV light, e.g., certain aromatics and olefins, and some compounds can also react with H202 directly,e.g.,phenol. These reactions may further enhance the oxidation of such compounds. Reaction Mechanism in the Presence of Natural Organic Matter. Natural organic matter might affect the H2021UV oxidation process through several mechanisms. First, the process might be inhibited due to absorption of UV light by the organic matter. Since the major source for 'OH formation is the photolytic decomposition of H202, the rate of which is controlled by the incident light intensity available for H202, the fraction of the incident light intensity available for H202 is of major concern when the UVabsorbing species such as humic material (HM)are present in the solution. In addition to this light filtering effect, humic material is also known as an effective'OH scavenger. Finally, humic material might affect the chain reactions by consumingtheradicals H02'/02*-.These possible reactions involving humic material might be incorporated into the reaction scheme as follows: HM
nm, b is the optical path length, and [H20~1is the concentration of H202. The rate of H202 decomposition can also be described as
Note that q, which is a fraction, becomes close to 1 at relatively small concentrations of H202. Furthermore, for a CSTR, the mass balance for H202 through substitution of eq 17 leads to
which implies a linear relationship between the ratio of the influent to effluent concentration of hydrogen peroxide, if r remains rather constant Le., [H~021~/[H~021 and (lot) within a narrow range of [HzOZI;furthermore, the slope of the straight line becomes equal to 2.303Q~cby. Oxidation Reactions. According to the reaction scheme presented above, the oxidation rate of the model compound M (rM) can be expressed by the following equation: rM
= { k ~ r O H+ l kM*([HOZ'l f [O,'-l)}[Ml
(20)
Additionally, the overall decomposition rate of hydrogen peroxide (rH20z)will be 'H,O,
- {k,+ k,['OHl
+ k,([CO,'-l +
[HCO,'l)l [H2O21- k4,,([HO,'1
+ [02'-1)2/2 (21)
where for sufficiently small concentrations of H202 (mg/L) (13)
+ HM - HMO, - - -
k , = 2.303@p~b10
(22)
and
ICHM, s-' (mg/L of DOC)-' (14) HO,'/O,'-
+ HM - HM*,,--kHM*,s-' (mg/L of DOC)-' (15)
Here, A254 is the absorbance of light by humic material and EHM is the absorptivity of humic material at 254 nm, and DOC represents the concentration of the humic material as dissolved organic carbon. The ~ H Mand k H M * are the rate constants for reaction of humic material, in terms of DOC, with'OH and HO2*/O2.-,respectively. These constants are not available in the literature; however, the extent of radical scavenging by humic material is believed to remain constant during the reaction period (23). Equations for the Kinetic Model. Decomposition of HzOz in Pure Water. According to the definition of quantum yield and the Beer-Lambert law, the overall decomposition rate of H202 in pure water, rI+02,can be described as follows: - @ I = @ I ( 1 - 10-mz0211
-
~ H , O ~
T a
T
0
In a CSTR, the application of the law of mass balance with respect to H202 and the model compound M respectively leads to the the following expressions at steady state:
where [MI, and [MI are the influent and effluent concentrations of M. and
(16)
where Q ~ ithe s overall quantum yield, lais the light intensity absorbed by H202, I, is the incident light intensity, E is the absorptivity of H202, which is 19.6 M-l cm-l (14) at 254
Oxidation Reactions in the Presence of Natural Organic Matter. In the presence of humic material, the transmitted light intensity available for absorption by H202 after traveling a distance of x cm can be expressed for 254 nm VOL. 29. NO. 12, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
3009
4
TABLE 1
Reaction Rate Constants literature value (M-l s-l)
rate constant
kz k3 k4 k5a
k6 k-6
k7 kg ki o kM kM * kHM kHM*
+
2.7 x 107 7.5 x 109 I O P H - ~ ~i*19) 3.7 (24) 8.3 x 105 ( 1 6 ) 9.7 x 107 (16) 7.9 105 ( ~ - 1 ) 5 x 1O'O (76) 8.5 x 106 + 3.9 x io8 x 1 0 ( p ~ - 1 0 3 1 ( 7 9 ) 4.3 x 105 + 3 x 107 x I O ( P H - ~ ~ *(PR ) 6.5 x IO8 (25) 3 x 109 (26) not availableC not availableC not availablec
a ks I S calculated from pK, = 1.2 (27). ks IS calculated from pK, = 4.8 ( 77, 18). Model sensitivity to these constants IS described i n this paper pK,(H202) = 11.8; pK,(H2CO3) = 6.3; pKJHCO3-) = 10 3.
as I, = 1
~ ~ - 30j..i2;1 2
(28) Accordingly, the average photon intensity available for absorption by H202 within the reactor can be estimated as
L'I~ cix J610e-2 I,, = ___ b b
cix
-
-
where r]', which represents the fraction of light available for absorption by H202,is given by
Hence, in the presence of humic material, eq 22 needs to be modified as follows to describe ki: k , = (2.3O3@,cb)(7'Ic,) (31) Furthermore, the radical reactions with humic material represented by reactions 28-30 need to be incorporated into the kinetic model. The system of nonlinear ordinary equations (eqs 203 1)were solved numerically by using the MATLAB program developed by Math Works Inc. The steady-state concentrations of the species [HzO~I, [MI, POHI, ([HO;] + [ 0 2 * - 1 ) , and ([C03*-] + [HCO?'])were predicted under different specified conditions. For this, the process parameters t,I,, b, and [H2O2Ioand the water quality conditions [MI,, pH, and Ci (defined as the summation of [C032-l, [HCO3-1,and [H&03*]),concentration of humic material as DOC, and the absorbance of the solution as A2j4 = E ~ I M -DOC were specified as inputs to the program. The reaction rate constants presented in Table 1 were also used as input to the program. The reliability of the kinetic model was tested by statistical comparison of the effluent concentrations of BuCl and H202 predicted by the model to the experimental measurements. For each set of experiments the mean relative error (MRE) defined as I
was computed. Here, Nis the number of samples; Xi is the 3010
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 12, 1995
-
- 3.5
10=(2.46 6.79).IUp Einsteidl m i n . r=2.59 16 7 6 rnm
1
[H10,jo=0.?4-2 22 rnM
-
3
c12.5 2
: 51.5 e" 1 I
0.5 0 0
0.004
0.008
0.012
1-7, Einsteidl
FIGURE 2. [H~Odo/[H202]- 1 versus lor.
value predicted by the model; k; is the value of the experimental data. The MRE for the normalized concentrations of BuCl and H202 was found to be less than 12% and 5%, respectively. These MRE values were then evaluated statistically for model verification. For that purpose, a sensitivity analysis of the kinetic model was performed by assuming an error of 110%in the measurement of each process parameter. The sign of error was selected so that it may result in either lower or upper-bound conditions. Then the measured concentrations of BuCl and HZO2were checked to see whether they remain within the lower and upper bounds of the model predictions ( 4 ) .
Results and Discussion Experiments in Pure Water. These experiments were conducted by irradiating in the CSTR a solution of H202 that contained no additional organic or inorganic matter at various light intensities and liquid residence times. The concentration of H202 in the influent and effluent was measured. The results showed that increasing the light intensity or the residence time significantly increased the decomposition rate and, consequently, reduced the effluent concentration of H202. These results were plotted as [H202],/[H202]versus lotin Figure 2. A linear relationship was observed as expected, according to eq 19. The optical pathlength, b, was then determined as 7.0 cm from the slope of the straight line (2.303@3eby)of 318 Einstein L - I by substituting the literature values of @T = 1 and = 19.6 cm-I M-' at 254 nm and assuming that r] is 1.0. It should be noted that when corrections were made for 7 for higher concentrations of H202, the slope yielded a slightly different b value of 6.8 cm, which was adopted for further modeling purposes even though the model was not sensitive to such a small variation in the b value. The path length of 6.8 cm is a reasonable value for our system, which consists of a 10 cm diameter quartz reactor surrounded with equally distanced 6-16 U V lamps. Effect of Carbonate Ions. Several experiments were conducted in the CSTR by using solutions containing various concentrations of BuC1, H202,and carbonate ions. In these experiments, the UV light intensity and the liquid residence time were kept constant to be able to interpret the effect of carbonate ions on the oxidation rate of BuCl and the photolysis rate of H202. In Figure 3 , the data points for normalized concentrations, Le., the ratio of the effluent to influent concentrations, are presented in squares for BuCl and in circles for H202. Each pair of data represent an experimental run under steady-state conditions for the selected CT. It is obvious that carbonate ions have a detrimental effect on the oxidation rate of BuC1, whereas their effect on the decomposition rate of H202is relatively negligible. According to Figure 4, the effect of carbonate
[H,O,l0=284 pM;
uI
0.8
-
0.6
.-rn
[BUCI]~=~ pM; pH=7.6; C,=4 mM; r=7.16 min: 10=2.46*10"Einstein4-min
r=1.16 min; pH=7.6; 10=2.46*l o 4 Einsteidl-min
- 'k&O
0.4
t 0.2
... .. . .. ..%*=io3 - . , k..'=104
-
0
10
20
m
M.',.]
M.'s''
-
30
40
0
C,, mM
4 a 12 Fluka Humic Acid as DOC, mgll
FIGURE 5. Effect of humic acid.
FIGURE 3. Model sensitivity to k ~ " . 1
0
of carbonate ions on H202 photolysis on the other hand indicates that the overall quantum yield (@TI remains g 0.8 0 unchanged in the presence of carbonate ions. For example, U the observed rate constant (kobs) and the primary rate 0.6 constant ( k J were determined to be 12.7 x loT4s-l and 6.3 .-B 0.4 x s-l, respectively, for the experimental conditions L presented in Figure 4. Hence, the ratio kl / k o b s , which equals @'p/@Pabstis 0.5 (note that kl = 2.303@.,~bl,and that kobs = o.2 0 2.303@0b,~bl,).This calculation indicates that the quantum yield observed in the presence of carbonate ions is about 0 40 a0 C,. mM 1. This conclusion is understandable in view of the FIGURE 4. Model verification: the effect of G. proposed reaction mechanism in which the carbonate radicals generated by reaction of 'OH with HC03-/C032ions on BuCl removal becomes even more pronounced (reaction 7) react further with H202 (reaction 9) and thus under conditions of lower concentration of BuCl. On the regenerate the chain-propagating radicals, H02'/02*-. Acother hand, the decomposition rate of H202 remains rather cordingly, we conclude that the carbonate ions behave constant for CT values up to 90 mM. Similar effects of differently than organic radical scavengers, e.g., acetic acid carbonate on the oxidation rate of 4-chloronitrobenzene or allyl alcohol, which can reduce the quantum yield to 0.5 was reported earlier (28)without a mechanistic explanation. by quenching the chain reactions of hydrogen peroxide The kinetic model was used to predict the concentrations photolysis (14, 15). of BuCl and H202presented in Figure 3. The experimental Effect of Natural Organic Matter. The experiments conditions (Le., z = 7.76 min, Io= 2.46 x Einstein L-I conducted in the presence of humic acid clearly showed min-', b = 6.8 cm, [H20210= 270 pM, [MI, = 8 pM, pH = that the rate of H202 photolysis and the oxidation rate of 7.6, and CT)as well as all the reaction rate constants were BuCl reduced significantly with increasing concentration fed to the program as input. However, since the reaction of humic acid (Figure 5 ) . In these experiments, all the rate constant kh,*, which is the rate constant for the reaction experimental conditions (Le., I,, [H2O2Io,[MI,, t,and pH) of radicals H02*/02'- with BuCl, is not available in the were maintained constant, varying only the quantity of the literature, some arbitrary values were assigned to the k ~ * humic acid as measured by DOC. (0, lo3, and lo4 M-' s-l), and the concentrations of H202 To be able to test the kinetic model in the presence of and BuCl predicted based on these arbitrary values were humic material, the rate constants for the reaction ofhumic compared to the experimentally measured concentrations. material with'OH ( ~ H M )and with H02'/02'- ( ~ H M * )needed The effluent concentrations of H202 were found insensitive to be estimated. The kinetic model itself was used for this to the value of kM*; however, the concentration of BuCl was purpose by first performing a sensitivity analysis with insensitive only when the k ~ was * less than about lo3 M-' respect to the rate constant kHM*. The results showed that SKI. This indicates no appreciable reaction of H02*/02*the model was insensitive to the value of ~ H M in * the range with BuCl, which is reasonable since the rate constant of of 0-lo4 s-l (mg/L)-', and therefore the reaction of humic HO2./O2.-with aliphatic compounds is generally low (16). material with H02'/02*- was ignored for further modeling Consequently, this reaction was ignored for modeling purposes (4). The rate constant ~ H Mfor the humic acid purposes for the rest of the study. was estimated by force fitting the experimental data For verification of the kinetic model, the model equations obtained in the presence humic acid to model predictions were solved for the experimental conditions presented in by the least squares method, Le., by minimizing the Figure 4. The value for k ~ was * assumed to be zero, as summation of the squared differences between the preimplied by the sensitivity analysis. The data points for BuCl dicted and measured concentrations ( 4 ) . As shown in and Hz02 concentrations are presented in Figure 4 by the M tested, that is 1 x Figure 6, within the range of ~ H values squares and circles, respectively, and the solid lines 103-3 x lo4 s-l (mglL of DOC)-', the model was found to represent the predicted values by the model. It was be relatively insensitive; however, the best fit of the model concluded that the experimental data were predicted prediction to the experimental data was still obtained for remarkably well by the kinetic model. a ICHM value of 1.6 x lo4 s-] (mg/L of DOC)-', as shown in Figure 6. This value for k H M is remarkably close to 1.7 x In view of the kinetic model, the effect of the carbonate IO4 s-l (mg/L of DOC)-', which was estimated by an ions on BuCl concentration profiles can be explained by ozonation technique as an average value for the natural the reactions of HC03- and C032- ions with 'OH in organic matter in several Swiss lakes and rivers (29)and to competition with BuCl. The lack of any significant effect kl=6.3.10-4 s.'
-
[H,0,10=454 pM; [B~Cl]~=1.2 pM; T =1.76 min; p H=8.2; 10=2.46*104Einsteidl-min
u
VOL. 29. NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
301 1
k,,=l
6.10's ' ( m a
DOC) ' [H,0,10=284 pM, (BUCI]~=X pM. r=l 76 min.
E
DOC=]X m u 1
-0 0
400 800 [ H,O,la, PM
FIGURE 6. Estimation of
1200
~HM.
Q
0
a
4
12
PH
2.3 x lo4 s-' (mg/L of DOC)-' estimated by Haag by the ozonation technique for 11North American natural waters
FIGURE 7. Effect of pH.
(30).
6. The concentration of BuCl decreased dramatically with
For verification of the kinetic model, the results of the experiments presented in Figure 5 were used. The equations of the kinetic model were solved numerically by using the independently determined parameters of b = 6.8 cm, FHM = 0.0867 cm-' (mg/L of DOC)-', and k H M = 1.6 x lo3 s-' (mg/L of DOC)-'. In Figure 5, the open squares and circles represent the experimental data for BuCl and H202, and the solid lines trace the values predicted by the kinetic model. According to the statistical analysis, the mean relative error (MRE) between the model predictions and experimental data was found to be less than 4% for both H202 and BuCl concentrations. Furthermore, the data points remained within the the dashed lines, which trace the model predictions for the lower- and upper-bound conditions (based on f 1 0 % error). The remarkable success of the model in predicting the effect of humic acid (the model is equally effective in predicting the effect of two other humic materials as well ( 4 ) )indicates that the assumptions made while developing the model, Le., that humic material primarily functions as a competitor to the target contaminants through the dual effects of 'OH scavenging and light absorption, are acceptable. However, humic material might also promote the radical chain reactions to generate H202, as reported for sunlight-induced reactions in natural waters (31,32). Hence, to test the possibility of generating any significant amount of H202 by direct UV photolysis under our experimental conditions, a solution of BuCl and humic acid (DOC= 4.66 mg/L) was irradiated by UVlight as anintensity of 2.46 x Einstein L-' min-' in the absence of any H202for up to 17 min. The maximum concentration of H202detected in the solution was 2pM. During that time, the BuCl concentration was reduced by about lo%, indicating the presence of 'OH in the reaction mixture. However, compared to the experimental results obtained in the presence of H202, these numbers are still too small, implying that the role of the humic material as a promoter of the chain reaction is only minor, and humic material primarily functions as an effective inhibitor in this process. The slight difference between the measured and the predicted values of H202 in our experiments might be due to this chain-promoting effect of the human substances. This effect has not been incorporated into the kinetic model because of the difficulty in defining the interaction of the UV light with the ambiguous chemical structure of the humic material. Effect of [HzOz],,.The experimental results regarding the effect of influent H202 concentration on the normalized concentrations of H202 and BuCl are presented in Figure
increasing [H2O2Iobut then stabilized for [HL0210 greater than about 500pM. This phenomenon could be explained by two opposing effects of H202 on the steady-state concentration of 'OH, Le., 'OH is generated through photolysis of H202but is also consumed by H202. This finding implies that there should be an optimal dosage of the chemical oxidant H202 to provide the maximum removal of the contaminant per unit of H202. The normalized concentration for H202was independent of [H202Iounder the experimental conditions. This observation implied that the kobs according to eq 27 remained unchanged as [H202Io was varied. This observation is understandable since the light intensity was relatively high and the contribution of the primary photolysis to the overall decomposition rate of H202 was dominant over the contributions of the subsequent radical reactions. Hunt and Taube (12) also reported that the observed quantum yield was independent of H202concentration at high light intensities. Effect of pH. The effect of pH on concentrations of BuCl and H202 in the presence of humic acid is presented in Figure 7. The highest oxidation efficiency, Le., the lowest BuCl concentration, was obtained at acidic pH values, and it was independent of pH for pH values below 5 where carbonic acid constitutes the major fraction of Cl. However, the oxidation efficiency was reduced drastically with increasing pH above pH 5. At this pH, the equilibrium shifts toward the bicarbonate ion, which is expected to successfully scavenge 'OH. Increasing the pH beyond 7 transforms the bicarbonate to the carbonate ion, which has an even higher reactivity toward 'OH (the rate constant for carbonate and bicarbonate ions are respectively 3.9 x lo8 and 8.5 x lo6 M-' s-l) and as a result is more successful in reducing the oxidation rate of BuC1. Furthermore, the reaction of 'OH with HOz- is much faster than its reaction with H202. For example, the value of k2 is 2.7 x 10- at pH 7 and 1.5 x lo8 M-ls-' at pH 10. Hence, at higher pH less 'OH is available for oxidation of n-chlorobutane. These results indicate that acidification ofwater prior to treatment by H202/UV may prove to be quite beneficial in practical applications. The H202 concentration was basically independent of pH, except that [H202] decreased slightly at the high end of the pH range, probably because of more pronounced attack of carbonate radicals on H ~ O ~ I H O, J as depicted by reaction 9. The trends for both H202 and BuCl were predicted well by the kinetic model. In Figure 7, the solid lines present the vaues predicted based on the average values of the measured process parameters, and the dashed lines trace
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the model predictions for the lower- and upper-bound conditions (based on f 1 0 % error). It should be noted that these solutions contain humic acid, which might change its structure and behavior as a function of pH. However, in the model, the rate constant ICHM and the light absorbance of humic acid estimated and measured respectively at pH 7.6 were applied throughout the pH range of 2-10. Furthermore, the reaction products of BuCl and humic acid might differ with varying pH. In fact, the model seems to fit the data better for the pH values above 6. However, even under these uncertainties, the experimental data still remained within the lower and the upper bounds of the predicted values, which indicates satisfactory performance of the model, especially for the purpose of process engineering design.
Summary and Conclusions This study was conducted to investigate the HzO~/UV process for oxidation of a model organic compound (BuCl) as well as for photolytic decomposition of H202 under various types of water quality and operational conditions. The experiments were conducted in a continuous-flow stirred tank reactor (CSTR) in the presence of carbonate ions and humic acid at various pH values, residence times, light intensities, and hydrogen peroxide dosages. It was observed that the oxidation efficiencyfor the target pollutant BuC1, i.e., ([MI, - [MI)/[Ml,, was determined by the rate of photolytic decomposition of H202 as well as the quality of the water to be treated. The higher the rate of H202 photolysis, the higher was the efficiency of oxidation of the pollutant; the rate of photolytic decomposition of H202 increased with increasing incident light intensity and H202 dosage but decreased with increasing W absorbance (A254) of the aqueous solution. The oxidation efficiency of BuCl was hindered with increasing pH, the total inorganic carbon concentration (CT),and the concentration of humic material in water. The extent of the dependency of the oxidation rate of the organic compound on the presence of humic acid is so strong that avoidance of this process for treatment of highly colored waters might be recommended. The adverse effects of carbonate ions and high pH can be alleviated by acidification of water prior to application of the process. A kinetic model based on a proposed reaction mechanism was tested by comparing the model predictions to the experimental data for various operational conditions and water quality. The major inputs to the model are the operational parameters (Le., [H202Io,I,, and t)and the water quality parameters described by pH, CT, DOC, and A2s4. The model exhibited a great capability for predicting the oxidation efficiency for the model compound and the rate of decomposition of H202 by photolysis under all the conditions tested. Carbonate/bicarbonate ions were found to compete effectively with BuCl for consumption of 'OH, causing a dramatic reduction in the oxidation rate of BuCl. However, it was observed that carbonate ions did not affect the overall quantum yield for hydrogen peroxide decomposition, which was predicted to be 1.0 in the presence as well as the absence of carbonate ions. On the other hand, humic material not only competed with BuCl for *OHbut also effectively blocked the light available for photolysis of H202. The rate constant for the reaction of'OH with humic acid was estimated to be 1.6 x lo4 s-l (mg/L of DOC)-'. This model is expected to work reasonably well for predicting the oxidation rate of organic compounds that
are susceptible to 'OH, especially aliphatic compounds. However, the validity of the model may need to be checked further for compounds that might react appreciably with H02'/02'- as well. The model is capable of accommodating mixtures of organic compounds as long as the rate constant of each individual compound with 'OH is available. The model needs to be be modified to accommodate organic compounds that might photolyze or react directlywith H202. The successful verification of the model on various field samples is going to be the subject of an upcoming publication.
Acknowledgments This work was supported entirely by the US. National Science Foundation, Environmental Engineering Program directed by Dr. Edward H. Bryan. C.H.L. was a graduate student at Drexel University at the time of this study. This workis based on his Ph.D. Dissertation, which received the 1994 Doctoral Thesis Award of the Association of Environmental Engineering Professors sponsored by Engineering Science.
Abbreviations and Symbols CSTR BuCl *OH
cv
continuous-flow stirred tank reactor n-chlorobutane hydroxyl radical coefficient of variation
POD/DPD
N,N-diethyl-p-phenylenediamine
5
residence time dissolved/total organic carbon overall quantum yield primary quantum yield light intensity absorbed by H202 incident light intensity absorptivity of H202 at 254 nm optical path length absorbance of light by humic material absorptivity of humic material at 254 nm a fraction defined by eq 3 fraction of light available for absorption by H202 mean relative error [C032-l [HC03-] + [HzC03*]
DOC/TOC @T @!J
IO E
b A254
EHM
11 1'
MRE
CT
+
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Received for review April 25, 1994. Revised manuscript received June 28, 1995. Accepted July 31, 1995.@ ES940254I Abstract published in AdvanceACSAbstracts, November 1 , 1995.
