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H2O2 process: Effect of temperature
Journal of Hazardous Materials 176 (2010) 1051–1057
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Clofibric acid degradation in UV254 /H2 O2 process: Effect of temperature Wenzhen Li, Shuguang Lu ∗ , Zhaofu Qiu, Kuangfei Lin State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, No. 130, Meilong Road, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 24 July 2009 Received in revised form 27 November 2009 Accepted 29 November 2009 Available online 3 December 2009 Keywords: Clofibric acid (CA) UV254 /H2 O2 Pharmaceutical Temperature Wastewater treatment plant (WWTP)
a b s t r a c t The degradation of clofibric acid (CA) in UV254 /H2 O2 process under three temperature ranges, i.e. T1 (9.0–11.5 ◦ C), T2 (19.0–21.0 ◦ C) and T3 (29.0–30.0 ◦ C) was investigated. The effects of solution constituents including NO3 − and HCO3 − anions, and humic acid (HA) on CA degradation were evaluated in Milli-Q waters. CA degradation behaviors were simulated with the pseudo-first-order kinetic model and the apparent rate constant (kap ) and half-life time (t1/2 ) were calculated. The results showed that higher temperature would favor CA degradation, and CA degradation was taken place mostly by indirect oxidation through the formation of • OH radicals in UV254 /H2 O2 process. In addition, the effects of both NO3 − and HCO3 − anions at two selected concentrations (1.0 × 10−3 and 0.1 mol L−1 ) and HA (20 mg L−1 ) on CA degradation were investigated. The results showed that HA had negative effect on CA degradation, and this effect was much more apparent under low temperature condition. On the other hand, the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. While, at higher nitrate concentration the inhibitive effect on CA degradation under three temperature ranges was observed, and with the temperature increase this negative effect was apparently weakened. However, at lower nitrate concentration a slightly positive effect on CA degradation was found under T2 and T3 conditions. Moreover, when using a real wastewater treatment plant (WWTP) effluent spiked with CA over 99% of CA removal could be achieved under 30 ◦ C within only 15 min compared with 40 and 80 min under 20 and 10 ◦ C respectively, suggesting a significant promotion in CA degradation under higher temperature condition. Therefore, it can be concluded that temperature plays an important role in CA degradation in UV/H2 O2 process. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Human pharmaceuticals and their metabolic forms have emerged as a novel class of pollutants because of their potential adverse impacts on human health and the environment even at trace levels. Of special concerns are those that have been found to be resistant in water and wastewater treatment processes [1–3]. In these concerns blood lipid regulators are paid more attention to environmental scientists due to their large consumption in terms of thousands of tons annually for therapeutic purposes suffering from angiocardiopathy problems, such as coronary heart disease, high blood pressure, arrhythmia, cardiac function failure, etc., therefore leading to clofibric acid (CA), the active metabolite of clofibrate and other lipid regulators, be frequently detected in wastewater treatment plant (WWTP) effluent, surface water, groundwater, drinking water and biosolids [4–7]. Several studies have demonstrated that biodegradation of CA in WWTP was limited. For example, Zorita et
∗ Corresponding author. Tel.: +86 21 64253533; fax: +86 21 64252737. E-mail address: [email protected] (S. Lu). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.11.147
al. [8] reported 55% CA removal in a conventional WWTP in Sweden and it could be improved to 61% in tertiary treatment process by additional chemical treatment following a sand filter. However, Zwiener and Frimmel [9] investigated the biodegradation of CA in short-term tests with a pilot WWTP and biofilm reactor for municipal wastewater treatment, and found only 5% of CA could be eliminated. In addition, it is worthy to note that CA tends to be hardly biodegraded in natural environment, and abiotic losses and adsorption play only a minimal role in the fate of CA in aquatic system [10]. Andreozzi et al. [11] reported that half-life times for the direct CA photolysis were around 40 days in spring and up to 250 days in winter, indicating its high stability towards conventional biodegradation and persistency in aquatic environment. Therefore, significantly irreversible adversity might be induced due to its accumulation in natural environment, for instance, the resistance of bacteria and adverse change of current ecological system, and hence further threatening the human health [12–14]. Hence, improving the CA removal efficiency in WWTP will be favorable for less pharmaceutical dispersion into the natural environment. Several trials have been tested on CA removal using various processes. Sirés et al. [15] investigated CA removal by anodic
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oxidation with Pt and boron-doped diamond (BDD) as anodes, showing that CA was more rapidly destroyed on Pt than on BDD due to its strong absorbance on Pt surface. They [16] also tested CA degradation by electro-Fenton and photoelectron-Fenton processes, and found about 80% of CA mineralization was achieved with the electro-Fenton process and more than 96% of CA removal by photoelectron-Fenton process. Moreover, several studies have demonstrated that UV can also decompose organic compounds including pharmaceuticals by direct photolysis, or by indirect photolysis through an advanced oxidation process (AOP), especially the highly reactive, unselective and short-lived hydroxyl radicals (• OH) can be generated in UV/H2 O2 process and further promote organic compound oxidation. For instance, ketoprofen, diclofenac, ceftiofur, sulfamethoxazole, sulfamonomethoxine and antipyrine were easily degraded by UV254 treatment [17,18]; whereas, Vogna et al. [19] showed that UV/H2 O2 process was able to degrade carbamazepine effectively comparing with UV alone. Andreozzi et al. [20] indicated that more than 90% of CA (C0 = 322 mg L−1 ) could be removed by UV radiation (17 W, = 254 nm) at pH 5.5 with 1.0 × 10−2 mol L−1 H2 O2 addition. Doll and Frimmel [21] achieved 90% of CA (C0 = 0.53 mg L−1 ) degradation under a 1000 W Xe shortarc lamp (1.35 × 104 Einstein m−2 s−1 nm−1 , < 400 nm) in aqueous solutions suspended with 80 mg L−1 TiO2 (P25) at pH 6.5 in 5 min. All above researches showed potential of pharmaceutical removal using UV based AOP. Ultraviolet (UV) disinfection of wastewater is generally applied using low-pressure mercury lamps emitting monochromatic light at 254 nm. It has been paid much attention recently for WWTP effluent disinfection because it does not produce any regulated disinfection byproducts (DBPs) in contrast to chlorine as disinfectant, therefore reduces halogenated DBPs formation. In addition, with the increasing demand of water resource, reclamation and reuse of the treated municipal wastewater are becoming more and more attractive, and less halogenated DBPs will be favorable. Several studies have reported that CA could be only limitedly reduced during its passage through a WWTP, making it a potential contaminant of potable water supplies, therefore, it is expected that applying UV/H2 O2 process might be able to further degrade CA in WWTP effluent before its being discharged into the environment. However, it is worthy to note that temperature might be an important key factor effecting CA degradation when applying UV/H2 O2 process for CA removal from a real WWTP effluent since most WWTPs encounter various seasonal changes. On the other hand, CA degradation in UV/H2 O2 process is also influenced significantly by solution matrices, such as constituents including nitrate and bicarbonate anions, and other dissolved organic matters (DOMs) like humic acid (HA), because they all can work as photo sensitizer and/or • OH sinker [22–24]. And so far their influences on CA degradation under various temperature conditions are not thoroughly investigated yet. Therefore, the objectives of this study were to investigate the effect of temperature on CA degradation by UV/H2 O2 based AOP in Milli-Q waters. The influences of solution constituents including nitrate and bicarbonate anions, and HA on CA photolysis were evaluated. CA degradation patterns were simulated using the pseudo-first-order kinetic model, and the apparent rate constants and half-life times were calculated under various operational conditions. And finally the process was applied to a real WWTP effluent for demonstrating its practical application under various temperature conditions. 2. Materials and methods 2.1. Chemicals CA (99% purity) was purchased from J&K Chemical Co. Ltd. (Beijing, China). High performance liquid chromatography (HPLC)
Table 1 Characteristics of Longhua WWTP effluent. Parameter
Value
pH Turbidity (NTU) Dissolved organic carbon (DOC, mg L−1 ) Chemical oxygen demand (COD, mg L−1 ) Cl− (mg L−1 ) NO3 − (mg L−1 ) SO4 2− (mg L−1 ) CA
7.1 8.0 16.4 39.0 177.5 41.6 155.2 Not detectable
graded methanol and acetonitrile, and all other reagents (analytical reagent) including H2 O2 (30% (w/w) solution), NaNO3 , NaHCO3 , humic acid (fulvic acid > 90%) and t-butyl alcohol (TBA) were purchased from Shanghai Jingchun Reagent Co. Ltd. (Shanghai, China). Ultra-pure water from a Milli-Q water process (Classic DI, ELGA, UK) was used for preparing aqueous solutions. 0.1 mol L−1 NaOH were used for solution pH adjustment. The secondary effluent (see Table 1) of WWTP employed with conventional activated sludge process was collected from Longhua WWTP, Shanghai, China, and filtered using fiber filters (0.45 m, Waters Corporation, Shanghai, China) before spiked with CA compound. For all the tests, CA was spiked in the solution and controlled at initial concentration of 10 mg L−1 . 2.2. CA degradation experiments Experiments were carried out in a valid volume of 800 mL cylindrical glass reactor (an inner diameter of 8.0 cm and a height of 25 cm) with tap water through jacket for temperature control. Three temperatures, 10 ◦ C (T1), 20 ◦ C (T2) and 30 ◦ C (T3) were investigated, but actually they varied in the ranges of 9.0–11.5, 19.0–21.0 and 29.0–30.0 ◦ C, respectively. A 10 W low pressure mercury lamp (Shanghai, China), emitting at 254 nm monochromatic wavelength, was immersed in solution in the reactor center (light path < 3.0 cm) with a photon flux of 2.09 × 10−5 Einstein cm−2 s−1 entering the reactor estimated from hydrogen peroxide actinometry [25]. Aqueous solutions were stirred by a magnetic bar throughout the experiments in order to remain homogeneous. Twenty seven experiments were conducted under various conditions which were summarized in Table 2. In a typical experiment, 500 mL of aqueous CA solution were put in the reactor and 170 L of H2 O2 (30%, w/w) was added to achieve an initial H2 O2 concentration of 100 mg L−1 based on calculation. During the experiments, each sample of 3.0 mL was taken regularly at given times and immediately analyzed by HPLC. 2.3. Analytical methods All samples for CA concentration measurement were prefiltrated through 0.22 m glass fiber filter before injection into HPLC. Modified CA analysis protocol based on Doll and Frimmel researches [26] was conducted by HPLC (LC-VP, Shimadzu, Japan) equipped with a diode array detector at = 230 nm using Kromasil 100-5C18 column (4.6 mm × 250 mm, 5 m) with a constant temperature of 35 ◦ C. The mobile phase consisted of a 75:25 methanol–acetonitrile mixed solution (1:1, 0.1% acetic acid) and buffered aqueous solution KH2 PO4 (5 mmol L−1 , 0.1% acetic acid). The flow rate was 1.0 mL min−1 , and the injection volume of the samples was 20 L. A series of CA standard solutions (0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10 mg L−1 ) were prepared, and the correlation coefficient (R2 ) between CA concentration and peak area was 0.9994, and the limits of detection (LODs; S/N ≥ 3) and the limits of quantification (LOQs; S/N ≥ 10) were 5 and 18 g L−1 , respectively. The reproducibility of standard solution was within 2% (injection num-
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Table 2 Summary of CA degradation performance under various conditions. Operational conditions
Rate constants, kap (min−1 )
Half-life time, t1/2 (min)
Correlation coefficient, R2
CA removal (%)
C pH 4.5 pH 7.0 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 NO3 − = 0.1 mol L−1 , pH 4.5 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 HCO3 − = 0.1mol L−1 , pH 4.5 HA = 20.0 mg L−1 , pH 4.5 TBA = 0.021 mol L−1 , pH 4.5 WWTP effluent, pH 7.1
0.232 0.172 0.219 0.078 0.150 0.068 0.056 0.033 0.070
2.98 4.04 3.16 8.90 4.61 10.11 12.36 21.07 9.94
0.9898 0.9995 0.9943 0.9865 0.9998 0.9991 0.9976 0.9986 0.9979
96.7 (15 min) 92.4 (15 min) 96.8 (15 min) 68.7 (15 min) 89.6 (15 min) 64.0 (15 min) 56.0 (15 min) 38.2 (15 min) 99.6 (80 min)
T2 = 19.0–21.0 ◦ C 10 pH 4.5 11 pH 7.0 12 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 13 NO3 − = 0.1 mol L−1 , pH 4.5 14 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 15 HCO3 − = 0.1mol L−1 , pH 4.5 16 HA = 20.0 mg L−1 , pH 4.5 17 TBA = 0.021 mol L−1 , pH 4.5 18 WWTP effluent, pH 7.1
0.448 0.399 0.615 0.156 0.459 0.101 0.101 0.067 0.136
1.55 1.73 1.13 4.45 1.51 6.87 6.88 10.39 5.12
0.9965 0.9951 0.9999 0.9724 0.9993 0.9969 0.9983 0.9981 0.9991
99.5 (12 min) 99.7 (15 min) 99.2 (8 min) 90.4 (15 min) 99.6 (12 min) 77.9 (15 min) 77.3 (15 min) 63.0 (15 min) 99.5 (40 min)
T3 = 29.0–30.0 ◦ C 19 pH 4.5 20 pH 7.0 21 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 22 NO3 − = 0.1 mol L−1 , pH 4.5 23 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 24 HCO3 − = 0.1mol L−1 , pH 4.5 25 HA = 20.0 mg L−1 , pH 4.5 26 TBA = 0.021 mol L−1 , pH 4.5 27 WWTP effluent, pH 7.1
0.827 0.924 1.107 0.255 0.876 0.172 0.181 0.107 0.319
0.84 0.75 0.62 2.72 0.79 4.04 3.84 6.50 2.17
0.9882 0.9993 0.9991 0.9944 0.9910 0.9986 0.9930 0.9983 0.9976
99.3 (6 min) 99.6 (6 min) 98.8 (4 min) 97.8 (15 min) 99.4 (6 min) 92.4 (15 min) 93.2 (15 min) 79.4 (15 min) 99.2 (15 min)
Test no. T1 = 9.0–11.5 1 2 3 4 5 6 7 8 9
◦
CA = 10.0 mg L−1 ; H2 O2 = 100 mg L−1 .
ber of sample n = 7), as indicated by the relative standard deviation (RSD).
Half-life time (t1/2 ) was calculated from the rate constant as Eq. (6):
2.4. Pseudo-first-order kinetic model
t1/2 =
CA degradation in UV/H2 O2 process was mostly due to the contribution of • OH formation and its involvement in the reactions were as below: k1
CA + • OH−→Int. k2
Int. + • OH−→P
(1) (2)
In this mechanism • OH attack may involve the formation of intermediates (Int.) and then intermediates were further degraded and final products (P) produced. The apparent rate constant kap can therefore be written as follows according to Behnajady et al. [27]: k1 2I0 fH2 O2
kap =
k2 [CA]0
(3)
or more simply as: −d[CA] = kap [CA] dt
(4)
In which kap is the apparent rate constant, is the quantum yield of the photochemical dissociation of H2 O2 , I0 is the incident UVlight intensity, and fH2 O2 represents the UV fraction absorbed by hydrogen peroxide. Eq. (4) can be written as: ln
C i
C0
= kap t
(5)
where Ci represents the CA concentration after exposure to UV/H2 O2 irradiation over t time, and C0 the initial CA concentration.
ln 2 kap
(6)
3. Results and discussion 3.1. Effect of temperature on general CA degradation in UV/H2 O2 process Three temperature ranges were investigated in this study, namely T1 (9.0–11.5 ◦ C), T2 (19.0–21.0 ◦ C) and T3 (29.0–30.0 ◦ C). In our preliminary experiment, no CA degradation took place when 100 mg L−1 of H2 O2 was added alone as the oxidant in CA solution without UV irradiation under above three temperature ranges (data not shown), suggesting that CA cannot be oxidized by H2 O2 alone. In addition, three concentrations of H2 O2 (50, 100, 150 mg L−1 ) were investigated to evaluate the effect of H2 O2 amount on reaction rate in UV/H2 O2 process and found 100 mg L−1 of H2 O2 concentration could achieve better CA removal, and therefore this concentration was selected for the further studies in this research. Compared to the complete CA degradation within 2 h using UV254 irradiation alone under T1 condition (Fig. 1a) in our preliminary experiment, the time could be shorten remarkably with the addition of H2 O2 (Fig. 1b), showing the effectiveness of UV/H2 O2 process in CA degradation. Moreover, two initial solution pH values (pH 4.5 and 7.0) were tested because the pH values of the solutions prepared by Milli-Q waters and WWTP effluent were 4.5 and 7.0, respectively. Apparently, temperature showed significantly positive influence on CA degradation under both pH conditions. The higher the temperature was, the more efficient for CA degradation. The increase of the apparent rate constant kap (Table 2) with temperature increase from 10 to 30 ◦ C attributed to the increasing collision frequency of molecules in solution, therefore enhanced the
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Fig. 2. Effect of TBA on CA degradation in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, TBA = 0.021 mol L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
Fig. 1. Effect of initial solution pH values (pH 4.5 and 7.0) on CA degradation in (a) UV alone process and (b) UV/H2 O2 process (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
reaction between CA and • OH. This phenomenon can also be found in UV/TiO2 process. For instance, Mozia et al. [28] focused on the application of UV/TiO2 in degradation of azo-dye Acid Red 18, and found that a linear positive correlation between the apparent rate constant and the reaction temperature in the range of 293–333 K. Cao et al. [29] studied the photocatalytic degradation of chlorfenapyr in aqueous suspension of TiO2 . They found the degradation rate increased with temperature and the rate constants k were 0.0427, 0.0519 and 0.0594 min−1 , respectively, at 25, 40 and 55 ◦ C. However, whether or not increasing temperature will change CA degradation pathway is not clear, and this will be studied in our future research by using GC–MS to investigate CA degradation byproducts. On the other hand, under lower temperature condition (T1), CA degradation was promoted in acid condition compared to neutral pH condition. In contrast, there was no difference when temperature increased to 20 or 30 ◦ C, indicating that temperature played a much more important role than solution pH under higher temperature condition. It is believed that CA degradation was taken place mostly by indirect oxidation when UV/H2 O2 applied due to the production of the highly reactive • OH radicals by adding H2 O2 in contrast to UV alone process and therefore promoted CA degradation. For further elucidating • OH function, TBA, one of the • OH scavengers, was added in UV/H2 O2 process under three tested temperature conditions. The results are shown in Fig. 2. It was apparent that with the TBA addition CA degradation was significantly inhibited in UV/H2 O2 process, suggesting that indirect oxidation of CA by generated • OH was the main CA degradation mechanism. This is because that in UV/H2 O2 process the induced • OH can react with CA, TBA and H2 O2 . As the reaction rate constant between • OH and TBA (6.0 × 108 M−1 s−1 ) is one order higher than that between • OH and H2 O2 (3.0 × 107 M−1 s−1 ) [30], therefore most • OH is captured by TBA and retards target compound combination with • OH, hence inhibited CA degradation in this study. In addition, great inhibitive effect was observed under lower temper-
ature, indirectly suggesting that the reaction rate between CA and • OH was decreased and it was the key factor limiting CA degradation. Since low initial CA concentration was applied in the study it is unlikely to identify CA degradation products in this test. However, Doll and Frimmel [21] proposed CA degradation pathways in a photocatalytic system when treating 200 mg L−1 of CA. It is believed that CA degradation goes through two ways. One way is CA dechlorination firstly into 2-(4-hydroxyphenoxy)-isobutyric acid, then further break down into isobutyric acid, 2- or 3-hydroxyisobutyric acid and hydroquinone. The other way is first break of CA into isobutyric acid, 2- or 3-hydroxyisobutyric acid and 4-chlorophenol, then further dechlorination of 4-chlorophenol into hydroquinone. We also detected the intermediate 4-chlorophenol by GC–MS in our preliminary study, and believed that CA degradation products may vary with experimental condition, because the color of solution after reaction varied under certain conditions. Further studies will be conducted to identify these by-products and therefore to evaluate the toxicity of these intermediates on aquatic eco-system. The pseudo-first-order kinetic model was applied to simulate CA degradation behaviors. The plot of ln(Ci /C0 ) versus time was shown in Fig. 3, and the calculated apparent rate constant (kap ), half-life time (t1/2 ) and CA removal rate under various conditions are summarized in Table 2. From the correlation coefficient R2 listed in Table 2, it can be seen the experimental data well fitted the model. With the temperature increase from 10 to 20 and 30 ◦ C, the rate constants increased from 0.232 to 0.448 and 0.827 min−1 respectively, corresponding to the t1/2 of 2.98, 1.55 and 0.84 min accordingly in Milli-Q waters.
Fig. 3. Plot of ln(Ci /C0 ) versus time in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
W. Li et al. / Journal of Hazardous Materials 176 (2010) 1051–1057
Fig. 4. Effect of HA on CA degradation in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, HA = 20 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
3.2. Effects of solution constituents on CA degradation behavior under various temperature conditions Some DOMs are always contained in the effluent of WWTPs as non-biodegradable materials as well as metabolism related soluble microbial products produced by microorganisms. In present work, 20 mg L−1 of HA, corresponding to total organic carbon of 8.63 mg L−1 in typical water sources, was added in Milli-Q waters to investigate its effect on CA degradation under three tested temperature ranges. The results are shown in Fig. 4. It was apparent that CA degradation decreased with the HA addition, and the lower the temperature was, the lower the CA degradation rate. After 15 min CA removal rates reached 56.0%, 77.3% and 93.2% respectively with the temperature increase from T1 to T2 and T3 (Table 2). Correspondingly, the apparent rate constants decreased to 0.056, 0.101 and 0.181 min−1 , which were roughly over 4-folds less than those without HA addition. It is reported that HA can work in two opposite functions in water [31]. On the one hand, it can reduce the available energy for the target organic compound present in solution due to its capability to absorb UV irradiation in a broad range of wavelengths, thus acting as an inner filter [32]. On the other hand, during UV irradiation HA can be promoted to a transient excited state (triplet states, 3 HA* ) in which it may react with oxygen in the solution forming reactive species such as singlet oxygen [33], or to react directly with other organic species, thus promoting their photo-transformation [34,35]. The latter occurs only with substances being able to support an energy transfer from molecules in their triplet states [36]. The overall effect of HA on the phototransformation rate of an organic substance will therefore depend on the balance between these two opposite contributions. Obviously, in this study HA acted mainly as inner filter and its addition resulted in a decrease of CA photo-degradation rate compared to the rate measured in Milli-Q waters only. Moreover, WWTP effluent always consists of a wide variety of inorganic anions such as NO3 − , HCO3 − , SO4 2− and Cl− , etc., and in which some have significant influence on organic compound photolysis during UV irradiation. Similar to HA, nitrate can absorb light in the UV range and act as an inner filter [37,38], and at the same time nitrate is also able to form • OH radicals under UV irradiation, and therefore be expected to promote organic degradation. On the other hand, the presence of bicarbonate can strongly decrease the degradation efficiency by scavenging • OH radicals [39]. In this study NO3 − and HCO3 − anions were separately added in CA solution prepared with Milli-Q waters for investigating both anions effects on CA degradation under three controlled temperature ranges. Two concentrations of 1.0 × 10−3 mol L−1 (typical concentration in WWTP effluent) and 0.1 mol L−1 (100-folds of the former value) were selected. The results showed that the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate
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was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. It was reasonable that bicarbonate, which mainly acted as • OH scavenger, reduced • OH reaction with CA compound and therefore hindered CA removal. This result was also consistent with other researchers for the photo-degradation of herbicide ametryn [40] and endocrine disrupting chemical octylphenol [41]. However, it is worthy to note that the inhibition for CA degradation caused by lower bicarbonate concentration under T3 condition could be neglected, indicating that temperature plays a much more important positive role than bicarbonate anion in CA degradation. On the other hand, at higher nitrate concentration the inhibitive effect on CA degradation under three temperature ranges was observed, and with the temperature increase this negative effect was apparently weakened (Table 2 and Fig. 5). However, at lower nitrate concentration a comparable apparent rate constant compared with Milli-Q waters without any anions was observed under T1 condition, and with the temperature increase, a slightly positive effect was found under T2 and T3 conditions. Apparently the formation of • OH radicals from photolysis of nitrate was promoted under higher temperature condition compared to its negative effect as an inner filter under lower nitrate concentration, also suggesting
Fig. 5. Effect of anions NO3 − and HCO3 − on CA degradation under various temperature conditions (a) T1 = 9.0–11.5 ◦ C, (b) T2 = 19.0–21.0 ◦ C, (c) T3 = 29.0–30.0 ◦ C (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, NO3 − = 1.0 × 10−3 and 0.1 mol L−1 , HCO3 − = 1.0 × 10−3 and 0.1 mol L−1 ).
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ranges was observed, and with the temperature increase this negative effect was apparently weakened. However, at lower nitrate concentration a comparable apparent rate constant compared with Milli-Q waters without any anions was observed under T1 condition, and with the temperature increase, a slightly positive effect was found under T2 and T3 conditions. Moreover, over 99% of CA removal in a real WWTP effluent could be achieved under 30 ◦ C within only 15 min compared with 40 and 80 min under 20 and 10 ◦ C respectively, suggesting a significant promotion in CA degradation under higher temperature condition. Therefore, it can be concluded that temperature is an important key factor for CA degradation in UV/H2 O2 process. Fig. 6. CA degradation in UV/H2 O2 process in practical WWTP effluent under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 7.1, T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
that temperature is an important key factor in CA degradation in UV/H2 O2 process.
Acknowledgement This study was sponsored by Chinese Shanghai Leading Academic Discipline Project (B506). References
3.3. CA degradation in a real WWTP effluent under various temperature conditions In order to confirm the behavior of CA degradation in UV/H2 O2 process in treated wastewater under various temperature conditions, CA was spiked in a real WWTP effluent after filtration and the experimental results are shown in Fig. 6. It was apparent that temperature had a great effect on CA degradation performance. It took 80, 40 and 15 min under T1, T2 and T3, respectively, to achieve over 99% CA removal, showing much more efficiency of CA degradation under higher temperature condition. On the other hand, CA degradation also showed significant decrease in photolysis rate in contrast to Milli-Q waters. Under the neutral pH condition, the rate constants calculated from the pseudo-first-order kinetic model were 0.070, 0.136 and 0.319 min−1 for WWTP effluent compared with 0.172, 0.399 and 0.924 min−1 for Milli-Q waters under T1, T2 and T3 conditions respectively, showing a significant improvement under higher temperature condition. Moreover, around 2.5–3.0folds decrease of photo-degradation rate of CA in WWTP effluent compared to that in Milli-Q waters was probably due to the complex constituents (Table 1) and their two opposite contributions (positive or negative effect on CA degradation), and unfortunately, the final negative effect was apparent due to the WWTP effluent constitutes including dissolved organic matters, Cl− , NO3 − and HCO3 − , etc. Our results were consistent with the studies by Kim et al., in which more than 30 kinds of pharmaceuticals and personal care products (PPCPs) were spiked in a real WWTP effluent and further treated by UV/H2 O2 process, and found that the first order rate constants for most tested PPCPs were 1.5–2.0-folds less than those tested in pure water system [42]. 4. Conclusions The results presented in this study confirmed that temperature had a significant influence on CA degradation in UV/H2 O2 process. The higher temperature would favor CA degradation in Milli-Q waters as well as in WWTP effluent. The degradation curve could be well described by pseudo-first-order kinetic model. In addition, under all investigated temperature ranges HA had negative effect on CA degradation, and this effect was much more apparent under low temperature condition. On the other hand, the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. While, the inhibitive effect on CA degradation caused by higher nitrate concentration under three temperature
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Clofibric acid degradation in UV254 /H2 O2 process: Effect of temperature Wenzhen Li, Shuguang Lu ∗ , Zhaofu Qiu, Kuangfei Lin State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, No. 130, Meilong Road, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 24 July 2009 Received in revised form 27 November 2009 Accepted 29 November 2009 Available online 3 December 2009 Keywords: Clofibric acid (CA) UV254 /H2 O2 Pharmaceutical Temperature Wastewater treatment plant (WWTP)
a b s t r a c t The degradation of clofibric acid (CA) in UV254 /H2 O2 process under three temperature ranges, i.e. T1 (9.0–11.5 ◦ C), T2 (19.0–21.0 ◦ C) and T3 (29.0–30.0 ◦ C) was investigated. The effects of solution constituents including NO3 − and HCO3 − anions, and humic acid (HA) on CA degradation were evaluated in Milli-Q waters. CA degradation behaviors were simulated with the pseudo-first-order kinetic model and the apparent rate constant (kap ) and half-life time (t1/2 ) were calculated. The results showed that higher temperature would favor CA degradation, and CA degradation was taken place mostly by indirect oxidation through the formation of • OH radicals in UV254 /H2 O2 process. In addition, the effects of both NO3 − and HCO3 − anions at two selected concentrations (1.0 × 10−3 and 0.1 mol L−1 ) and HA (20 mg L−1 ) on CA degradation were investigated. The results showed that HA had negative effect on CA degradation, and this effect was much more apparent under low temperature condition. On the other hand, the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. While, at higher nitrate concentration the inhibitive effect on CA degradation under three temperature ranges was observed, and with the temperature increase this negative effect was apparently weakened. However, at lower nitrate concentration a slightly positive effect on CA degradation was found under T2 and T3 conditions. Moreover, when using a real wastewater treatment plant (WWTP) effluent spiked with CA over 99% of CA removal could be achieved under 30 ◦ C within only 15 min compared with 40 and 80 min under 20 and 10 ◦ C respectively, suggesting a significant promotion in CA degradation under higher temperature condition. Therefore, it can be concluded that temperature plays an important role in CA degradation in UV/H2 O2 process. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Human pharmaceuticals and their metabolic forms have emerged as a novel class of pollutants because of their potential adverse impacts on human health and the environment even at trace levels. Of special concerns are those that have been found to be resistant in water and wastewater treatment processes [1–3]. In these concerns blood lipid regulators are paid more attention to environmental scientists due to their large consumption in terms of thousands of tons annually for therapeutic purposes suffering from angiocardiopathy problems, such as coronary heart disease, high blood pressure, arrhythmia, cardiac function failure, etc., therefore leading to clofibric acid (CA), the active metabolite of clofibrate and other lipid regulators, be frequently detected in wastewater treatment plant (WWTP) effluent, surface water, groundwater, drinking water and biosolids [4–7]. Several studies have demonstrated that biodegradation of CA in WWTP was limited. For example, Zorita et
∗ Corresponding author. Tel.: +86 21 64253533; fax: +86 21 64252737. E-mail address: [email protected] (S. Lu). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.11.147
al. [8] reported 55% CA removal in a conventional WWTP in Sweden and it could be improved to 61% in tertiary treatment process by additional chemical treatment following a sand filter. However, Zwiener and Frimmel [9] investigated the biodegradation of CA in short-term tests with a pilot WWTP and biofilm reactor for municipal wastewater treatment, and found only 5% of CA could be eliminated. In addition, it is worthy to note that CA tends to be hardly biodegraded in natural environment, and abiotic losses and adsorption play only a minimal role in the fate of CA in aquatic system [10]. Andreozzi et al. [11] reported that half-life times for the direct CA photolysis were around 40 days in spring and up to 250 days in winter, indicating its high stability towards conventional biodegradation and persistency in aquatic environment. Therefore, significantly irreversible adversity might be induced due to its accumulation in natural environment, for instance, the resistance of bacteria and adverse change of current ecological system, and hence further threatening the human health [12–14]. Hence, improving the CA removal efficiency in WWTP will be favorable for less pharmaceutical dispersion into the natural environment. Several trials have been tested on CA removal using various processes. Sirés et al. [15] investigated CA removal by anodic
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oxidation with Pt and boron-doped diamond (BDD) as anodes, showing that CA was more rapidly destroyed on Pt than on BDD due to its strong absorbance on Pt surface. They [16] also tested CA degradation by electro-Fenton and photoelectron-Fenton processes, and found about 80% of CA mineralization was achieved with the electro-Fenton process and more than 96% of CA removal by photoelectron-Fenton process. Moreover, several studies have demonstrated that UV can also decompose organic compounds including pharmaceuticals by direct photolysis, or by indirect photolysis through an advanced oxidation process (AOP), especially the highly reactive, unselective and short-lived hydroxyl radicals (• OH) can be generated in UV/H2 O2 process and further promote organic compound oxidation. For instance, ketoprofen, diclofenac, ceftiofur, sulfamethoxazole, sulfamonomethoxine and antipyrine were easily degraded by UV254 treatment [17,18]; whereas, Vogna et al. [19] showed that UV/H2 O2 process was able to degrade carbamazepine effectively comparing with UV alone. Andreozzi et al. [20] indicated that more than 90% of CA (C0 = 322 mg L−1 ) could be removed by UV radiation (17 W, = 254 nm) at pH 5.5 with 1.0 × 10−2 mol L−1 H2 O2 addition. Doll and Frimmel [21] achieved 90% of CA (C0 = 0.53 mg L−1 ) degradation under a 1000 W Xe shortarc lamp (1.35 × 104 Einstein m−2 s−1 nm−1 , < 400 nm) in aqueous solutions suspended with 80 mg L−1 TiO2 (P25) at pH 6.5 in 5 min. All above researches showed potential of pharmaceutical removal using UV based AOP. Ultraviolet (UV) disinfection of wastewater is generally applied using low-pressure mercury lamps emitting monochromatic light at 254 nm. It has been paid much attention recently for WWTP effluent disinfection because it does not produce any regulated disinfection byproducts (DBPs) in contrast to chlorine as disinfectant, therefore reduces halogenated DBPs formation. In addition, with the increasing demand of water resource, reclamation and reuse of the treated municipal wastewater are becoming more and more attractive, and less halogenated DBPs will be favorable. Several studies have reported that CA could be only limitedly reduced during its passage through a WWTP, making it a potential contaminant of potable water supplies, therefore, it is expected that applying UV/H2 O2 process might be able to further degrade CA in WWTP effluent before its being discharged into the environment. However, it is worthy to note that temperature might be an important key factor effecting CA degradation when applying UV/H2 O2 process for CA removal from a real WWTP effluent since most WWTPs encounter various seasonal changes. On the other hand, CA degradation in UV/H2 O2 process is also influenced significantly by solution matrices, such as constituents including nitrate and bicarbonate anions, and other dissolved organic matters (DOMs) like humic acid (HA), because they all can work as photo sensitizer and/or • OH sinker [22–24]. And so far their influences on CA degradation under various temperature conditions are not thoroughly investigated yet. Therefore, the objectives of this study were to investigate the effect of temperature on CA degradation by UV/H2 O2 based AOP in Milli-Q waters. The influences of solution constituents including nitrate and bicarbonate anions, and HA on CA photolysis were evaluated. CA degradation patterns were simulated using the pseudo-first-order kinetic model, and the apparent rate constants and half-life times were calculated under various operational conditions. And finally the process was applied to a real WWTP effluent for demonstrating its practical application under various temperature conditions. 2. Materials and methods 2.1. Chemicals CA (99% purity) was purchased from J&K Chemical Co. Ltd. (Beijing, China). High performance liquid chromatography (HPLC)
Table 1 Characteristics of Longhua WWTP effluent. Parameter
Value
pH Turbidity (NTU) Dissolved organic carbon (DOC, mg L−1 ) Chemical oxygen demand (COD, mg L−1 ) Cl− (mg L−1 ) NO3 − (mg L−1 ) SO4 2− (mg L−1 ) CA
7.1 8.0 16.4 39.0 177.5 41.6 155.2 Not detectable
graded methanol and acetonitrile, and all other reagents (analytical reagent) including H2 O2 (30% (w/w) solution), NaNO3 , NaHCO3 , humic acid (fulvic acid > 90%) and t-butyl alcohol (TBA) were purchased from Shanghai Jingchun Reagent Co. Ltd. (Shanghai, China). Ultra-pure water from a Milli-Q water process (Classic DI, ELGA, UK) was used for preparing aqueous solutions. 0.1 mol L−1 NaOH were used for solution pH adjustment. The secondary effluent (see Table 1) of WWTP employed with conventional activated sludge process was collected from Longhua WWTP, Shanghai, China, and filtered using fiber filters (0.45 m, Waters Corporation, Shanghai, China) before spiked with CA compound. For all the tests, CA was spiked in the solution and controlled at initial concentration of 10 mg L−1 . 2.2. CA degradation experiments Experiments were carried out in a valid volume of 800 mL cylindrical glass reactor (an inner diameter of 8.0 cm and a height of 25 cm) with tap water through jacket for temperature control. Three temperatures, 10 ◦ C (T1), 20 ◦ C (T2) and 30 ◦ C (T3) were investigated, but actually they varied in the ranges of 9.0–11.5, 19.0–21.0 and 29.0–30.0 ◦ C, respectively. A 10 W low pressure mercury lamp (Shanghai, China), emitting at 254 nm monochromatic wavelength, was immersed in solution in the reactor center (light path < 3.0 cm) with a photon flux of 2.09 × 10−5 Einstein cm−2 s−1 entering the reactor estimated from hydrogen peroxide actinometry [25]. Aqueous solutions were stirred by a magnetic bar throughout the experiments in order to remain homogeneous. Twenty seven experiments were conducted under various conditions which were summarized in Table 2. In a typical experiment, 500 mL of aqueous CA solution were put in the reactor and 170 L of H2 O2 (30%, w/w) was added to achieve an initial H2 O2 concentration of 100 mg L−1 based on calculation. During the experiments, each sample of 3.0 mL was taken regularly at given times and immediately analyzed by HPLC. 2.3. Analytical methods All samples for CA concentration measurement were prefiltrated through 0.22 m glass fiber filter before injection into HPLC. Modified CA analysis protocol based on Doll and Frimmel researches [26] was conducted by HPLC (LC-VP, Shimadzu, Japan) equipped with a diode array detector at = 230 nm using Kromasil 100-5C18 column (4.6 mm × 250 mm, 5 m) with a constant temperature of 35 ◦ C. The mobile phase consisted of a 75:25 methanol–acetonitrile mixed solution (1:1, 0.1% acetic acid) and buffered aqueous solution KH2 PO4 (5 mmol L−1 , 0.1% acetic acid). The flow rate was 1.0 mL min−1 , and the injection volume of the samples was 20 L. A series of CA standard solutions (0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10 mg L−1 ) were prepared, and the correlation coefficient (R2 ) between CA concentration and peak area was 0.9994, and the limits of detection (LODs; S/N ≥ 3) and the limits of quantification (LOQs; S/N ≥ 10) were 5 and 18 g L−1 , respectively. The reproducibility of standard solution was within 2% (injection num-
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Table 2 Summary of CA degradation performance under various conditions. Operational conditions
Rate constants, kap (min−1 )
Half-life time, t1/2 (min)
Correlation coefficient, R2
CA removal (%)
C pH 4.5 pH 7.0 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 NO3 − = 0.1 mol L−1 , pH 4.5 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 HCO3 − = 0.1mol L−1 , pH 4.5 HA = 20.0 mg L−1 , pH 4.5 TBA = 0.021 mol L−1 , pH 4.5 WWTP effluent, pH 7.1
0.232 0.172 0.219 0.078 0.150 0.068 0.056 0.033 0.070
2.98 4.04 3.16 8.90 4.61 10.11 12.36 21.07 9.94
0.9898 0.9995 0.9943 0.9865 0.9998 0.9991 0.9976 0.9986 0.9979
96.7 (15 min) 92.4 (15 min) 96.8 (15 min) 68.7 (15 min) 89.6 (15 min) 64.0 (15 min) 56.0 (15 min) 38.2 (15 min) 99.6 (80 min)
T2 = 19.0–21.0 ◦ C 10 pH 4.5 11 pH 7.0 12 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 13 NO3 − = 0.1 mol L−1 , pH 4.5 14 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 15 HCO3 − = 0.1mol L−1 , pH 4.5 16 HA = 20.0 mg L−1 , pH 4.5 17 TBA = 0.021 mol L−1 , pH 4.5 18 WWTP effluent, pH 7.1
0.448 0.399 0.615 0.156 0.459 0.101 0.101 0.067 0.136
1.55 1.73 1.13 4.45 1.51 6.87 6.88 10.39 5.12
0.9965 0.9951 0.9999 0.9724 0.9993 0.9969 0.9983 0.9981 0.9991
99.5 (12 min) 99.7 (15 min) 99.2 (8 min) 90.4 (15 min) 99.6 (12 min) 77.9 (15 min) 77.3 (15 min) 63.0 (15 min) 99.5 (40 min)
T3 = 29.0–30.0 ◦ C 19 pH 4.5 20 pH 7.0 21 NO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 22 NO3 − = 0.1 mol L−1 , pH 4.5 23 HCO3 − = 1.0 × 10−3 mol L−1 , pH 4.5 24 HCO3 − = 0.1mol L−1 , pH 4.5 25 HA = 20.0 mg L−1 , pH 4.5 26 TBA = 0.021 mol L−1 , pH 4.5 27 WWTP effluent, pH 7.1
0.827 0.924 1.107 0.255 0.876 0.172 0.181 0.107 0.319
0.84 0.75 0.62 2.72 0.79 4.04 3.84 6.50 2.17
0.9882 0.9993 0.9991 0.9944 0.9910 0.9986 0.9930 0.9983 0.9976
99.3 (6 min) 99.6 (6 min) 98.8 (4 min) 97.8 (15 min) 99.4 (6 min) 92.4 (15 min) 93.2 (15 min) 79.4 (15 min) 99.2 (15 min)
Test no. T1 = 9.0–11.5 1 2 3 4 5 6 7 8 9
◦
CA = 10.0 mg L−1 ; H2 O2 = 100 mg L−1 .
ber of sample n = 7), as indicated by the relative standard deviation (RSD).
Half-life time (t1/2 ) was calculated from the rate constant as Eq. (6):
2.4. Pseudo-first-order kinetic model
t1/2 =
CA degradation in UV/H2 O2 process was mostly due to the contribution of • OH formation and its involvement in the reactions were as below: k1
CA + • OH−→Int. k2
Int. + • OH−→P
(1) (2)
In this mechanism • OH attack may involve the formation of intermediates (Int.) and then intermediates were further degraded and final products (P) produced. The apparent rate constant kap can therefore be written as follows according to Behnajady et al. [27]: k1 2I0 fH2 O2
kap =
k2 [CA]0
(3)
or more simply as: −d[CA] = kap [CA] dt
(4)
In which kap is the apparent rate constant, is the quantum yield of the photochemical dissociation of H2 O2 , I0 is the incident UVlight intensity, and fH2 O2 represents the UV fraction absorbed by hydrogen peroxide. Eq. (4) can be written as: ln
C i
C0
= kap t
(5)
where Ci represents the CA concentration after exposure to UV/H2 O2 irradiation over t time, and C0 the initial CA concentration.
ln 2 kap
(6)
3. Results and discussion 3.1. Effect of temperature on general CA degradation in UV/H2 O2 process Three temperature ranges were investigated in this study, namely T1 (9.0–11.5 ◦ C), T2 (19.0–21.0 ◦ C) and T3 (29.0–30.0 ◦ C). In our preliminary experiment, no CA degradation took place when 100 mg L−1 of H2 O2 was added alone as the oxidant in CA solution without UV irradiation under above three temperature ranges (data not shown), suggesting that CA cannot be oxidized by H2 O2 alone. In addition, three concentrations of H2 O2 (50, 100, 150 mg L−1 ) were investigated to evaluate the effect of H2 O2 amount on reaction rate in UV/H2 O2 process and found 100 mg L−1 of H2 O2 concentration could achieve better CA removal, and therefore this concentration was selected for the further studies in this research. Compared to the complete CA degradation within 2 h using UV254 irradiation alone under T1 condition (Fig. 1a) in our preliminary experiment, the time could be shorten remarkably with the addition of H2 O2 (Fig. 1b), showing the effectiveness of UV/H2 O2 process in CA degradation. Moreover, two initial solution pH values (pH 4.5 and 7.0) were tested because the pH values of the solutions prepared by Milli-Q waters and WWTP effluent were 4.5 and 7.0, respectively. Apparently, temperature showed significantly positive influence on CA degradation under both pH conditions. The higher the temperature was, the more efficient for CA degradation. The increase of the apparent rate constant kap (Table 2) with temperature increase from 10 to 30 ◦ C attributed to the increasing collision frequency of molecules in solution, therefore enhanced the
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Fig. 2. Effect of TBA on CA degradation in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, TBA = 0.021 mol L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
Fig. 1. Effect of initial solution pH values (pH 4.5 and 7.0) on CA degradation in (a) UV alone process and (b) UV/H2 O2 process (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
reaction between CA and • OH. This phenomenon can also be found in UV/TiO2 process. For instance, Mozia et al. [28] focused on the application of UV/TiO2 in degradation of azo-dye Acid Red 18, and found that a linear positive correlation between the apparent rate constant and the reaction temperature in the range of 293–333 K. Cao et al. [29] studied the photocatalytic degradation of chlorfenapyr in aqueous suspension of TiO2 . They found the degradation rate increased with temperature and the rate constants k were 0.0427, 0.0519 and 0.0594 min−1 , respectively, at 25, 40 and 55 ◦ C. However, whether or not increasing temperature will change CA degradation pathway is not clear, and this will be studied in our future research by using GC–MS to investigate CA degradation byproducts. On the other hand, under lower temperature condition (T1), CA degradation was promoted in acid condition compared to neutral pH condition. In contrast, there was no difference when temperature increased to 20 or 30 ◦ C, indicating that temperature played a much more important role than solution pH under higher temperature condition. It is believed that CA degradation was taken place mostly by indirect oxidation when UV/H2 O2 applied due to the production of the highly reactive • OH radicals by adding H2 O2 in contrast to UV alone process and therefore promoted CA degradation. For further elucidating • OH function, TBA, one of the • OH scavengers, was added in UV/H2 O2 process under three tested temperature conditions. The results are shown in Fig. 2. It was apparent that with the TBA addition CA degradation was significantly inhibited in UV/H2 O2 process, suggesting that indirect oxidation of CA by generated • OH was the main CA degradation mechanism. This is because that in UV/H2 O2 process the induced • OH can react with CA, TBA and H2 O2 . As the reaction rate constant between • OH and TBA (6.0 × 108 M−1 s−1 ) is one order higher than that between • OH and H2 O2 (3.0 × 107 M−1 s−1 ) [30], therefore most • OH is captured by TBA and retards target compound combination with • OH, hence inhibited CA degradation in this study. In addition, great inhibitive effect was observed under lower temper-
ature, indirectly suggesting that the reaction rate between CA and • OH was decreased and it was the key factor limiting CA degradation. Since low initial CA concentration was applied in the study it is unlikely to identify CA degradation products in this test. However, Doll and Frimmel [21] proposed CA degradation pathways in a photocatalytic system when treating 200 mg L−1 of CA. It is believed that CA degradation goes through two ways. One way is CA dechlorination firstly into 2-(4-hydroxyphenoxy)-isobutyric acid, then further break down into isobutyric acid, 2- or 3-hydroxyisobutyric acid and hydroquinone. The other way is first break of CA into isobutyric acid, 2- or 3-hydroxyisobutyric acid and 4-chlorophenol, then further dechlorination of 4-chlorophenol into hydroquinone. We also detected the intermediate 4-chlorophenol by GC–MS in our preliminary study, and believed that CA degradation products may vary with experimental condition, because the color of solution after reaction varied under certain conditions. Further studies will be conducted to identify these by-products and therefore to evaluate the toxicity of these intermediates on aquatic eco-system. The pseudo-first-order kinetic model was applied to simulate CA degradation behaviors. The plot of ln(Ci /C0 ) versus time was shown in Fig. 3, and the calculated apparent rate constant (kap ), half-life time (t1/2 ) and CA removal rate under various conditions are summarized in Table 2. From the correlation coefficient R2 listed in Table 2, it can be seen the experimental data well fitted the model. With the temperature increase from 10 to 20 and 30 ◦ C, the rate constants increased from 0.232 to 0.448 and 0.827 min−1 respectively, corresponding to the t1/2 of 2.98, 1.55 and 0.84 min accordingly in Milli-Q waters.
Fig. 3. Plot of ln(Ci /C0 ) versus time in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
W. Li et al. / Journal of Hazardous Materials 176 (2010) 1051–1057
Fig. 4. Effect of HA on CA degradation in UV/H2 O2 process under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, HA = 20 mg L−1 , T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
3.2. Effects of solution constituents on CA degradation behavior under various temperature conditions Some DOMs are always contained in the effluent of WWTPs as non-biodegradable materials as well as metabolism related soluble microbial products produced by microorganisms. In present work, 20 mg L−1 of HA, corresponding to total organic carbon of 8.63 mg L−1 in typical water sources, was added in Milli-Q waters to investigate its effect on CA degradation under three tested temperature ranges. The results are shown in Fig. 4. It was apparent that CA degradation decreased with the HA addition, and the lower the temperature was, the lower the CA degradation rate. After 15 min CA removal rates reached 56.0%, 77.3% and 93.2% respectively with the temperature increase from T1 to T2 and T3 (Table 2). Correspondingly, the apparent rate constants decreased to 0.056, 0.101 and 0.181 min−1 , which were roughly over 4-folds less than those without HA addition. It is reported that HA can work in two opposite functions in water [31]. On the one hand, it can reduce the available energy for the target organic compound present in solution due to its capability to absorb UV irradiation in a broad range of wavelengths, thus acting as an inner filter [32]. On the other hand, during UV irradiation HA can be promoted to a transient excited state (triplet states, 3 HA* ) in which it may react with oxygen in the solution forming reactive species such as singlet oxygen [33], or to react directly with other organic species, thus promoting their photo-transformation [34,35]. The latter occurs only with substances being able to support an energy transfer from molecules in their triplet states [36]. The overall effect of HA on the phototransformation rate of an organic substance will therefore depend on the balance between these two opposite contributions. Obviously, in this study HA acted mainly as inner filter and its addition resulted in a decrease of CA photo-degradation rate compared to the rate measured in Milli-Q waters only. Moreover, WWTP effluent always consists of a wide variety of inorganic anions such as NO3 − , HCO3 − , SO4 2− and Cl− , etc., and in which some have significant influence on organic compound photolysis during UV irradiation. Similar to HA, nitrate can absorb light in the UV range and act as an inner filter [37,38], and at the same time nitrate is also able to form • OH radicals under UV irradiation, and therefore be expected to promote organic degradation. On the other hand, the presence of bicarbonate can strongly decrease the degradation efficiency by scavenging • OH radicals [39]. In this study NO3 − and HCO3 − anions were separately added in CA solution prepared with Milli-Q waters for investigating both anions effects on CA degradation under three controlled temperature ranges. Two concentrations of 1.0 × 10−3 mol L−1 (typical concentration in WWTP effluent) and 0.1 mol L−1 (100-folds of the former value) were selected. The results showed that the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate
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was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. It was reasonable that bicarbonate, which mainly acted as • OH scavenger, reduced • OH reaction with CA compound and therefore hindered CA removal. This result was also consistent with other researchers for the photo-degradation of herbicide ametryn [40] and endocrine disrupting chemical octylphenol [41]. However, it is worthy to note that the inhibition for CA degradation caused by lower bicarbonate concentration under T3 condition could be neglected, indicating that temperature plays a much more important positive role than bicarbonate anion in CA degradation. On the other hand, at higher nitrate concentration the inhibitive effect on CA degradation under three temperature ranges was observed, and with the temperature increase this negative effect was apparently weakened (Table 2 and Fig. 5). However, at lower nitrate concentration a comparable apparent rate constant compared with Milli-Q waters without any anions was observed under T1 condition, and with the temperature increase, a slightly positive effect was found under T2 and T3 conditions. Apparently the formation of • OH radicals from photolysis of nitrate was promoted under higher temperature condition compared to its negative effect as an inner filter under lower nitrate concentration, also suggesting
Fig. 5. Effect of anions NO3 − and HCO3 − on CA degradation under various temperature conditions (a) T1 = 9.0–11.5 ◦ C, (b) T2 = 19.0–21.0 ◦ C, (c) T3 = 29.0–30.0 ◦ C (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 4.5, NO3 − = 1.0 × 10−3 and 0.1 mol L−1 , HCO3 − = 1.0 × 10−3 and 0.1 mol L−1 ).
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ranges was observed, and with the temperature increase this negative effect was apparently weakened. However, at lower nitrate concentration a comparable apparent rate constant compared with Milli-Q waters without any anions was observed under T1 condition, and with the temperature increase, a slightly positive effect was found under T2 and T3 conditions. Moreover, over 99% of CA removal in a real WWTP effluent could be achieved under 30 ◦ C within only 15 min compared with 40 and 80 min under 20 and 10 ◦ C respectively, suggesting a significant promotion in CA degradation under higher temperature condition. Therefore, it can be concluded that temperature is an important key factor for CA degradation in UV/H2 O2 process. Fig. 6. CA degradation in UV/H2 O2 process in practical WWTP effluent under various temperature conditions (C0 = 10.0 mg L−1 , H2 O2 = 100 mg L−1 , pH 7.1, T1 = 9.0–11.5 ◦ C, T2 = 19.0–21.0 ◦ C, T3 = 29.0–30.0 ◦ C).
that temperature is an important key factor in CA degradation in UV/H2 O2 process.
Acknowledgement This study was sponsored by Chinese Shanghai Leading Academic Discipline Project (B506). References
3.3. CA degradation in a real WWTP effluent under various temperature conditions In order to confirm the behavior of CA degradation in UV/H2 O2 process in treated wastewater under various temperature conditions, CA was spiked in a real WWTP effluent after filtration and the experimental results are shown in Fig. 6. It was apparent that temperature had a great effect on CA degradation performance. It took 80, 40 and 15 min under T1, T2 and T3, respectively, to achieve over 99% CA removal, showing much more efficiency of CA degradation under higher temperature condition. On the other hand, CA degradation also showed significant decrease in photolysis rate in contrast to Milli-Q waters. Under the neutral pH condition, the rate constants calculated from the pseudo-first-order kinetic model were 0.070, 0.136 and 0.319 min−1 for WWTP effluent compared with 0.172, 0.399 and 0.924 min−1 for Milli-Q waters under T1, T2 and T3 conditions respectively, showing a significant improvement under higher temperature condition. Moreover, around 2.5–3.0folds decrease of photo-degradation rate of CA in WWTP effluent compared to that in Milli-Q waters was probably due to the complex constituents (Table 1) and their two opposite contributions (positive or negative effect on CA degradation), and unfortunately, the final negative effect was apparent due to the WWTP effluent constitutes including dissolved organic matters, Cl− , NO3 − and HCO3 − , etc. Our results were consistent with the studies by Kim et al., in which more than 30 kinds of pharmaceuticals and personal care products (PPCPs) were spiked in a real WWTP effluent and further treated by UV/H2 O2 process, and found that the first order rate constants for most tested PPCPs were 1.5–2.0-folds less than those tested in pure water system [42]. 4. Conclusions The results presented in this study confirmed that temperature had a significant influence on CA degradation in UV/H2 O2 process. The higher temperature would favor CA degradation in Milli-Q waters as well as in WWTP effluent. The degradation curve could be well described by pseudo-first-order kinetic model. In addition, under all investigated temperature ranges HA had negative effect on CA degradation, and this effect was much more apparent under low temperature condition. On the other hand, the inhibitive effect on CA degradation at both lower and higher concentrations of bicarbonate was observed, and this inhibitive effect was much more apparent at higher bicarbonate concentration and lower temperature condition. While, the inhibitive effect on CA degradation caused by higher nitrate concentration under three temperature
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