Articles March 2010 Vol.55 No.9: 802−808 doi: 10.1007/s11434-010-0058-x
Environmental Chemistry
SPECIAL TOPICS:
Oxidation of estrone by permanganate: Reaction kinetics and estrogenicity removal SHAO XiaoLing*, MA Jun, WEN Gang & YANG JingJing School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China Received January 23, 2009; accepted August 11, 2009
Permanganate was used as an oxidant to control estrone in the present study. Kinetics was determined at pH 2.5−9.4 and temperature 15–40°C for the reaction of estrone with potassium permanganate. It was found that the reaction is second-order overall and first-order with respect to both estrone and permanganate. The second-order rate constant for the reaction at pH 5.8 and 25°C is 44.45 L mol−1 s−1. The reaction rate first decreased with the increase of pH in the range of 2.5−6.6 and then increased greatly with the increase of pH in the range of 6.6−9.4. In addition, the rate constant exponentially increased with the increase of reaction temperature. Removal of estrogenicity was also investigated during the degradation of estrone using yeast estrogen screen (YES). Results show that the estrogenicity increased in the initial 15 min of reaction and then decreased fast, with a removal rate of 73.8% within the 30 min of reaction. Results also demonstrate that the reaction rate between estrone and permanganate is faster in natural water background than in the ultra-pure water system. Permanganate oxidation is therefore a feasible option for removal of estrone in drinking water treatment processes. However, the contact time must be enough in order to remove estrone without causing the increase of estrogenicity. estrone, permanganate, rate constants, estrogenicity Citation:
Shao X L, Ma J, Wen G, et al. Oxidation of estrone by permanganate: Reaction kinetics and estrogenicity removal. Chinese Sci Bull, 2010, 55: 802−808, doi: 10.1007/s11434-010-0058-x
Endocrine disrupting chemicals (EDCs) are a kind of micropollutants that will elicit adverse effects on endocrine systems of humans and wildlife. They have been implicated in a number of reproductive and sexual abnormalities observed in wildlife [1−3] and reduced sperm counts in human males [4]. These compounds may travel along the water path from wastewater treatment plants to the raw water used for drinking water production [1,5−7]. Studies indicate that mixtures of various EDCs are prevalent in most of natural waters [7]. Some of them are very refractory to be removed by conventional physicochemical water treatments (e.g., coagulation/sedimentation, filtration, and chlorination) and may enter into drinking water distribution systems [8−10]. Phenolic EDCs including natural steroid estrogens have been verified as the dominant form of estrogenic activity in *Corresponding author (email:
[email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2010
surface waters [11]. Studies indicate that these chemicals, even at extremely low concentrations, will cause significant health problems for wildlife and/or humans when they experience a long-term exposure to mixtures of them [1,12−14]. Therefore, effective treatment approaches are very desirable for the destruction of these EDCs from source water. Chemical oxidation can convert hazardous contaminants to nonhazardous or less toxic compounds, and may be a good choice to eliminate EDCs in natural waters. Among those commonly used oxidants in waterworks (e.g., chlorine, ozone, UV photolysis, monochloramine, and chlorine dioxide), permanganate has been verified as an inexpensive, easy and effective oxidant for control or decomposition of many kinds of contaminants, also including phenolic EDCs [15−20]. Abe et al. [20] found that permanganate can completely degrade bisphenol A and 4-t-butylphenol into organic acids and inorganic carbon. They found that the percsb.scichina.com
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manganate oxidation rates are comparable to that of hydroxyl radicals. Moreover, previous studies indicate that the formation of disinfection by-products (DBPs) is greatly reduced in the process of permanganate oxidation [21]. Therefore, permanganate oxidation has already been used in drinking water treatment processes in the waterworks of many Chinese cities, such as Beijing, Shanghai, Nanjing and Wuxi. However, most of these studies mainly concentrated on several kinds of compounds, such as cyanotoxins, trichloroethylene, tetrachloroethylene, and methyl-tert-butyl ether, or the changes of regular water quality parameters during permanganate oxidation. Few studies have centered on the oxidation of representative EDCs. In addition, the main concern of these studies was the concentration change of target chemicals. Little attention was paid to the toxicities of degradation products. Moreover, some of the previous results were obtained from experiments conducted in synthetic water samples. Thus, it is difficult to predict the actual situation in real water treatment processes. Therefore, the primary objectives of this study are (1) to determine the rate constant for the reaction of permanganate with estrone, one of the representative EDCs in surface waters, (2) to evaluate the effect of operating variables such as pH and temperature on permanganate oxidation rate constants, (3) to assess the estrogenicity variation during estrone oxidation by permanganate, and (4) to assess the validity of the determined rate constants when permanganate oxidation is applied to natural waters.
1 Materials and methods 1.1
Chemicals
Estrone (E1) is Sigma-Aldrich reagent. Other chemicals including potassium permanganate (KMnO4), hydrochloric acid (HCl), sodium hydroxide (NaOH) and ascorbic acid are of analytical grade and used without further purification. Ultra-pure water used in the experiment was Milli-Q water, 18.2 MΩ cm. The natural water used was taken from Songhua River that is situated in the northern part of Harbin, China. Natural water sample was filtered through a glass fibre filter with pore size of 1 μm (Whatman, GF/B) to remove suspended solids and then stored at 4°C. The main water quality parameters of the filtered natural water are as follows: dissolved organic carbon (DOC), 4.5 mg/L; conductivity, 190 μS/cm; pH 8.2; turbidity, 1.0 NTU. Estrone working solution was prepared daily in Milli-Q water or natural water using a magnetic stirrer and stored in amber glass bottle at ambient temperature. Potassium permanganate working solution (6.328 mmol/L), ascorbic acid working solution (5.678 mmol/L) and other reagents were also freshly prepared in Milli-Q water every four to seven days and stored in dark bottles to avoid light exposure.
1.2
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Analytical methods
High performance liquid chromatography (HPLC) was used to determine the concentrations of estrone in water samples. The analytical system employs a Waters 1500 series binary pump, a Waters Symmetry C18 column (I.D=4.6 mm, length=150 mm, 5 μm particle, made in Ireland), a Waters 717 plus auto-injector and a Waters 2487 dual λ UV detector. The mobile phase was run in an isocratic mode, with Milli-Q water used as mobile phase A and methanol (Dikma, USA) as mobile phase B. The proportion of A/B was 25/75 for the detection of estrone. The total flow rate of mobile phase A and B was 1.0 mL/min. The injection volume was 100 μL for each sample. The wavelengths selected for the quantification were 224 and 280 nm. The analytes were quantified by external standard quantification procedure. The system was calibrated using standard solutions prepared in methanol at six concentration levels by serial dilutions from stock solution (0.370 mmol/L). The peak area vs. injected amount chart was obtained as standard curve with a correlation coefficient (R2) over 0.99. The detection limit is 5 μg/L. 1.3 Permanganate oxidation experiment 100 mL of estrone working solution (0.543−1.593 μmol/L) was placed in cylindrical glass reactor, which was immersed in a thermostatic water bath to perform batch oxidation experiment. Magnetic stirrer was used under the water bath. The temperature of reaction systems was maintained at 25°C except for the experiment conducted in natural water background. All pH values were measured by a pHs-3C pH meter with glass electrode that was pre-calibrated with standard buffer solutions (Leici, Shanghai, China). Compared with initial pH values, the changes of pH for all reaction systems did not exceed 0.3. Therefore, pH values were considered constant in the course of permanganate oxidation. The reaction was initiated by injection of 100−800 μL of KMnO4 working solution. Samples were collected at several time intervals, and quenched immediately with ascorbic acid working solution. The residual estrone concentrations were analyzed directly by HPLC. Each experiment was performed in duplicate. 1.4
Estrogenicity measurement
Water samples were first concentrated by solid phase extraction (SPE) using Waters C18 cartridges (3 mL, 500 mg), which were pre-conditioned with 5 mL methanol and 5 mL Milli-Q water. The cartridges were then eluted with 10 mL of methanol/dichloromethane (80/20, v/v) twice. Two aliquots were combined and concentrated under a gentle nitrogen stream to dryness and then the samples were solvent exchanged to dimethyl sulfoxide (DMSO, HPLC grade, Sigma Chemical Co.) and stored at −20°C for bioassay. The
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recoveries of estrone are 81.7%±7.4% in the procedure. The yeast two-hybrid assay was used to estimate the estrogenic activity in the process of estrone oxidation by permanganate. The yeast (Saccharomyces cerevisiae) cell was engineered with a human estrogen receptor gene and a coactivator gene, which binds to an estrogen response element regulated-expression plasmid (lac-Z) coded to express β-galactosidase [12]. Upon binding an active ligand, the estrogen-occupied receptor interacts with transcription factors and other transcriptional components to modulate gene transcription. This causes expression of the reporter gene lac-Z and the production of β-galactosidase. Then the enzyme metabolizes the colorless substrate, o-nitrophenyl β-D-galactopyranoside (ONPG) into o-nitrophenol (ONP). ONP is normally yellow and can be quantified using a spectrometer by absorbance at 420 nm. The yeast cells were cultured and the bioassay procedure was carried out as described detailedly by Ma et al. [22]. The results of estrogenic activities of water samples were expressed as estradiol equivalents (EEQs).
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performed at pH 5.8 and 25°C and different initial estrone and permanganate concentrations. The values of kobs in these experiments were calculated by linear regression analysis and are summarized in Table 1. From the similar kobs values in experiments performed with different initial estrone concentrations, it can be deduced that the rate of estrone degradation is independent on the initial estrone concentration while an increase in the initial permanganate dose leads to the faster estrone degradation. The plot of kobs values as a function of the initial concentrations of permanganate (eq. (3)) is presented in Figure 2. A straight line was obtained with a correlation coefficient higher than 0.99, confirming the rate of estrone degradation is also first-order with respect to permanganate concentration. After linear analysis of data in Figure 2, a second-order rate constant at pH 5.8 and 25°C of 44.45±0.94 L mol−1 s−1 can be calculated. Therefore, the rate of estrone degradation is second-order overall and first-order with respect to permanganate and estrone, and can be expressed by eq. (4). (4) −r = 44.45[E1][KMnO4].
1.5 Manganese ion measurement The concentration of manganese ion in water samples was determined by a PerkinElmer Optima 5300 DV ICP-AES instrument (made in USA) after the filtration of water samples with cellulose acetate membranes with a pore size of 0.45 μm.
2
Results and discussion
2.1 Kinetics of the reaction of permanganate with estrone The reactions between estrone and permanganate occurred in ultra-pure water background with natural pH 5.8. The reaction was considered as a second-order reaction overall and first-order with respect to estrone ([E1]) and permanganate ([KMnO4]) concentrations. The rate of estrone degradation could be expressed as: (1) −r = −d[E1]/dt = k2 [E1][KMnO4], where k2 is the second-order kinetic constant. In the present study, permanganate was in large excess and its decrease in concentration was smaller than 20% of its initial concentration. Hence, permanganate concentration could be considered constant during reaction and the rate of estrone degradation can be expressed as: (2) −r = −d[E1]/dt = kobs[E1], (3) kobs = k2[KMnO4], where kobs is the pseudo-first-order kinetic constant. Therefore, a plot of ln[E1] as a function of the reaction time leads to a straight line, the slope of which is kobs. The first-order reaction rate with respect to estrone is confirmed in Figure 1, where ln([E1]0/E1]) versus time is shown for experiments
Figure 1 Pseudo-first-order kinetic plot for the oxidation of estrone with permanganate at 25°C and pH 5.8. The solid line is a linear least-squares regression of the data. Initial concentration: [KMnO4]0=6.328 μmol/L, [E1]0=1.593 μmol/L (■); [KMnO4]0= 12.656 μmol/L, [E1]0=1.593 μmol/L (□); [KMnO4]0=18.984 μmol/L, [E1]0=1.593 μmol/L (●); [KMnO4]0 = 31.640 μmol/L, [E1]0=1.593 μmol/L (○); [KMnO4]0=50.623 μmol/L, [E1]0=1.215 μmol/L (▲); [KMnO4]0=50.623 μmol/L, [E1]0= 1.593 μmol/L (△); [KMnO4]0=50.623 μmol/L, [E1]0=0.885 μmol/L (★); [KMnO4]0= 50.623 μmol/L, [E1]0=0.581 μmol/L (☆).
Table 1 Results normalized to plots of ln([E1]0/[E1])-t for degradation of estrone under varied permanganate and estrone initial concentrations [KMnO4]0 (μmol/L) 6.328 12.656 18.984 31.640 50.623 50.623 50.623 50.623
[E1]0 (μmol/L) 1.593 1.593 1.593 1.593 1.593 1.215 0.885 0.581
R2 0.9889 0.9906 0.9931 0.9915 0.9998 0.9988 0.9915 0.9948
kobs×10−4 (s−1) 2.35 5.25 9.32 14.45 22.08 21.63 23.55 24.58
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Figure 2 Pseudo-first-order rate constant as a function of initial KMnO4 concentration for use in calculating the second-order rate constant between estrone and KMnO4.
Due to the effects of pH and background composition, the rate constant will be further enhanced during the permanganate oxidation of estrone in natural water background, which will be discussed in following sections. In comparison with phenolic EDCs’ reactivity with other oxidants, permanganate is a moderate oxidant. The reaction between phenolic EDCs and hydroxyl radicals is fast with an overall rate constant of 9.8×109−1.41×1010 L mol−1 s−1 [23,24]. In the case of ozone, the rate constants are 1.68×104−107 L mol−1 s−1 [23,25,26], while for ferrate (Fe(VI)), it was found to be 6.4×102−7.7×102 L mol−1 s−1 [27]. The rate constants between phenolic EDCs and chlorine are 12.6−131.1 L mol−1 s−1, which is comparative to the case of permanganate [28]. It seems that ozone and chlorine together with permanganate are feasible options for the removal of estrone or other phenolic EDCs during water treatment processes. However, bromate formation must be controlled in the ozonation of natural waters with high bromide level. Similarly, the formation of DBPs in the treated water may be an issue that limits the chlorine dose. On the contrary, permanganate preoxidation may be useful in enhancing the following coagulation and filtration processes [29] and controlling the formation of trihalomethanes and other DBPs [21]. 2.2
Effect of pH
Investigations on the effect of pH on the oxidation rate of estrone were conducted at pH 2.5−9.4. The pH of the tested aqueous solutions was adjusted with 0.1 mol/L of HCl solution for pH
