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Environmental Chemistry March 2010 Vol.55 No.9: 802−808

doi: 10.1007/s11434-010-0058-x

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 tem-perature 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 mi-cropollutants that will elicit adverse effects on endocrine systems of humans and wildlife. They have been implicated in a number of reproductive and sexual abnormalities ob-served 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

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 ex-perience 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 diox-ide), 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 com-pletely degrade bisphenol A and 4-t-butylphenol into or-ganic acids and inorganic carbon. They found that the per-

SHAO XiaoLing, et al. Chinese Sci Bull March (2010) Vol.55 No.9 803

manganate oxidation rates are comparable to that of hy-droxyl 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, trichloro-ethylene, tetrachloroethylene, and methyl-tert-butyl ether, or the changes of regular water quality parameters during per-manganate 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 re-sults were obtained from experiments conducted in syn-thetic water samples. Thus, it is difficult to predict the ac-tual 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 de-termined 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 wa-ter, 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 re-move 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; con-ductivity, 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 per-manganate 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 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 detec-tor. 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 quan-tification procedure. The system was calibrated using stan-dard 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 ob-tained 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 ex-periment. 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 stan-dard 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 con-sidered 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 ex-traction (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 ali-quots were combined and concentrated under a gentle ni-trogen 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

804 SHAO XiaoLing, et al. Chinese Sci Bull March (2010) Vol.55 No.9

recoveries of estrone are 81.7%±7.4% in the procedure. The yeast two-hybrid assay was used to estimate the es-

trogenic activity in the process of estrone oxidation by per-manganate. The yeast (Saccharomyces cerevisiae) cell was engineered with a human estrogen receptor gene and a coactivator gene, which binds to an estrogen response ele-ment regulated-expression plasmid (lac-Z) coded to express β-galactosidase [12]. Upon binding an active ligand, the estrogen-occupied receptor interacts with transcription fac-tors 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 en-zyme metabolizes the colorless substrate, o-nitrophenyl β-D-galactopyranoside (ONPG) into o-nitrophenol (ONP). ONP is normally yellow and can be quantified using a spec-trometer by absorbance at 420 nm. The yeast cells were cultured and the bioassay procedure was carried out as de-scribed detailedly by Ma et al. [22]. The results of estro-genic activities of water samples were expressed as estradiol equivalents (EEQs).

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 sam-ples 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 es-trone

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 perman-ganate ([KMnO4]) concentrations. The rate of estrone deg-radation could be expressed as:

−r = −d[E1]/dt = k2 [E1][KMnO4], (1) 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:

−r = −d[E1]/dt = kobs[E1], (2) kobs = k2[KMnO4], (3)

where kobs is the pseudo-first-order kinetic constant. There-fore, 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

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 perman-ganate (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 calcu-lated. Therefore, the rate of estrone degradation is sec-ond-order overall and first-order with respect to permanga-nate and estrone, and can be expressed by eq. (4).

−r = 44.45[E1][KMnO4]. (4)

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 ( ); [KMnO 4]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) [E1]0

(μmol/L) R2

kobs×10−4

(s−1) 6.328 1.593 0.9889 2.35 12.656 1.593 0.9906 5.25 18.984 1.593 0.9931 9.32 31.640 1.593 0.9915 14.45 50.623 1.593 0.9998 22.08 50.623 1.215 0.9988 21.63 50.623 0.885 0.9915 23.55 50.623 0.581 0.9948 24.58

SHAO XiaoLing, et al. Chinese Sci Bull March (2010) Vol.55 No.9 805

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 per-manganate 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 to-gether with permanganate are feasible options for the re-moval of estrone or other phenolic EDCs during water treatment processes. However, bromate formation must be controlled in the ozonation of natural waters with high bro-mide 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 en-hancing 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 solu-tion for pH<5 and buffered with phosphate or carbonate salts (1 mmol/L) for pH=5−10, respectively. To compare the reaction rate constants at different pH values, we as-sumed that the same order of reaction is valid under alkaline and acidic conditions. Therefore, pseudo-first-order rate constants were calculated by applying eq. (2) (R2=0.9355 ±0.0903). As can be observed in Figure 3, the pH influence on the reaction rate is significant. The reaction rate first

Figure 3 Influence of the pH on the pseudo-first-order rate constant at 25°C. [E1]0=1.481 μmol/L, [KMnO4]0=12.656 μmol/L.

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. The minimum degradation rate of estrone takes place at pH 6.6. In addition, when the pH is vicinal to 9, estrone can be diminished to below the limit of detection in less than 1 min.

Permanganate usually undergoes a three-electron ex-change under pH 3.5−12 (eqs. (5) −(6)) and a five-electron exchange under lower pH conditions (pH<3.5, eq. (7)).

MnO4− + 4H+ + 3e− = MnO2 + 2H2O (5)

MnO4− + 2H2O + 3eˉ = MnO2 + 4OHˉ (6)

MnO4− + 8H+ + 5eˉ = Mn2+ + 4H2O (7)

The redox potential of permanganate is higher in acidic conditions (E0=1.70 V) than that in alkaline conditions (E0=0.59 V) [17], which might induce the higher reaction rate in acidic conditions. However, as to the fast decay of estrone under alkaline conditions, it might be due to the deprotonation of estrone which facilitates the attack by per-manganate [30]. Ladbury and Cullis [31] reported that the ionized organic compounds, especially phenoxy groups, were easily attacked by MnO4

−. Considering the pKa of es-trone 10.3−10.8 [12], the pH adopted is more vicinal to the pKa, where higher percentage of estrone will be dissociated. As a result, the oxidation rate is fast under the alkaline con-ditions. The result agrees with that obtained by Bastos et al. [18] who found that brominated phenols with lower pKa values reacted faster with KMnO4 at pH 7.6.

2.3 Effect of temperature

The effect of temperature on the reaction between perman-ganate and estrone was determined in experiments per-formed at six different temperatures (15, 20, 25, 30, 35 and 40°C) in ultra-pure water background. Pseudo-first-order constants of (2.63–12.07)×10−4 s−1 were also calculated by linear regression analysis (R2

>0.99). The relationship between the rate constant and temperature is showed in Figure 4. It can be seen that the rate constant increased slowly with tem-

806 SHAO XiaoLing, et al. Chinese Sci Bull March (2010) Vol.55 No.9

Figure 4 Pseudo-first-order kinetic plot for the oxidation of estrone by permanganate at different reaction temperatures (a), and the effect of reac-tion temperature on the pseudo-first-order rate constant (b). [E1]0=1.463 μmol/L, [KMnO4]0=12.656 μmol/L, pH 5.8.

perature ranging from 15°C to 30°C, whereas the rate con-stant increased significantly at higher temperature, which results in the exponential growth of the rate constant in the whole studied temperature range. The second-order rate constants (20.8−95.3 L mol−1 s−1) at these temperatures were calculated according to eq.(3) and were used to calculate activation energy (Ea) with the linearized Arrhenius equa-tion (eq. (8)).

lnk2=lnA−Ea/RT. (8) After linear regression analysis (R2=0.9550), a value of

43.07 kJ/mol (10.25 kcal/mol) was calculated for Ea. This activation energy value is a little higher than that measured for other organic compounds during permanganate oxida-tion, such as cyanotoxins MC-RR (28.8 kJ/mol) [17] and chloroethylene (24.36−39.1 kJ/mol) [32,33].

2.4 Estrogenicity removal

The estrogenic activities of estrone solutions were measured after the reaction started for 0, 5, 10, 15, 20 and 30 min in ultra-pure water background. It can be seen from Figure 5 that the estrogenicities of water samples increased in the initial 15 min of reaction. The maximum estrogenicity takes place at 15 min of reaction, which increased by 41.8%

Figure 5 Estrogenicity of estrone solution during the course of perman-ganate oxidation at 25°C. [E1]0=0.543 μmol/L, [KMnO4]0=12.656 μmol/L, pH 5.8.

compared with the initial estrogenic activity of water sam-ple (t = 0). It can only be explained by the formation of in-termediates with higher estrogenicities in the initial stages of the reaction. Because compounds with higher estrogenic-ities always have similar molecular structures as natural estrogens [34]. Therefore, we supposed that these interme-diates probably have similar molecular structures as estrone. However, Abe et al. [20] supposed that KMnO4 directly cleaves the aromatic rings of phenolic compounds to form organic acids and inorganic carbon, since few decomposi-tion products with aromatic rings were observed in the per-manganate oxidation of phenol, bisphenol A and 4-t-butyl- phenol. Thus, further study is needed to determine the by-products of estrone by GC/MS or GC/MS/MS. Then, the estrogenicity decreased after 15 min of reaction. Estro-genicity was removed by 73.8% within 30 min of reaction, which is higher than the removal of estrone (about 60%) within the same time. The phenomena might be induced by the anti-estrogen by-products produced in the later stages of the reaction. These anti-estrogens can interrupt the combi-nation of estrone with the estrogen acceptors contained in the yeast strain, which might lead to the higher removal of estrogenicity in water samples.

Through the above-mentioned study, it can be concluded that enough contact time must be ensured in order to re-move estrone and its estrogenicity during permanganate oxidation.

2.5 Estrone removal by permanganate in natural wa-ter background

The oxidation of estrone in the natural water background mentioned above was conducted with permanganate dose of 12.656 μmol/L at pH 8.2. A comparison study was con-ducted in Milli-Q water background at natural pH 5.8. To emphasize the effect of pH, experiments were also per-formed in the natural water at pH 5.8 and Milli-Q water at pH 8.2. The former and the latter were adjusted by 0.1

SHAO XiaoLing, et al. Chinese Sci Bull March (2010) Vol.55 No.9 807

mol/L of HCl solution and 0.1 mol/L of NaOH solution, respectively. Since the background concentration of estrone in the natural water was in lower ng/L level, it was therefore negligible in calculation. As shown in Figure 6, the reaction rate in the filtered natural water background is significantly higher than that in ultra-pure water system under the same pH conditions within the studied reaction time. Moreover, the oxidation rate of estrone increased greatly with the pH increased from 5.8 to 8.2 both in natural water and in ul-tra-pure water background. However, the enhancement of removal rates is higher in natural water system than that in ultra-pure water system. The removal of estrone increased by 21%−45% for the former, whereas it increased by 9%−36% for the latter. Additionally, the reactivities be-tween estrone and permanganate in natural water system at pH 5.8 and in ultra-pure water system at pH 8.2 are almost equal. It means that the effect of background components is comparative to that of pH in the fast removal of estrone in the natural water system. Thus, the effect of background components, including cations, anions, organics etc., en-hanced the oxidation of estrone by permanganate, which will be presented in a separate paper in due course.

Permanganate is always applied as a preoxidant in actual drinking water treatment processes and the contact time is often more than 30 min. This means that the removal of the produced MnO2 can be carried out easily in the following traditional treatment units, e.g., coagulation/sedimentation, filtration, etc. However, particular care must be taken to the residual manganese as metallic ions that can cause many problems in drinking water. It means that the permanganate dose has to be optimized during permanganate oxidation. The Chinese government has set a guideline value of 0.1 mg/L for manganese ion in Standards for Drinking Water Quality (GB5749-2006). Therefore, the concentrations of the residual manganese ion were measured by ICP-AES after 30 min of reaction mentioned above between estrone and permanganate in the natural water at pH 8.2. The back

Figure 6 Oxidation of estrone by permanganate in a filtered natural water background at pH 8.2 ( ) and pH 5.8 ( ). Comparison e xperiments were also conducted in the ultra-pure water system at pH 5.8 ( ) and pH 8.2 ( ). [E1] 0=1.248 μmol/L, [KMnO4]0=12.656 μmol/L, ambient tempera-ture, 28±1°C.

ground concentration of manganese ion was 0.005 mg/L in the natural water. Results show that a level of 0.13 mg/L of manganese ion is left in the reaction system. The value can be easily decreased to below 0.1 mg/L by other traditional treatment processes. Thus, it can be assumed that the con-trol of estrone by permanganate oxidation is feasible.

It should also be mentioned that the concentrations of es-trone and other phenolic EDCs are in ng/L or low μg/L range in actual environmental samples [7]. Therefore, fur-ther work is needed to understand the real application of permanganate in potable water treatment and the possible formation of by-products associated with estrone or other phenolic compounds.

3 Conclusions

The reaction of the investigated typical EDCs, estrone with permanganate fits overall second-order kinetics, and first- order with respect to both estrone and permanganate. The second-order rate constant for the oxidation of estrone by permanganate at 25°C is 44.45 L mol−1 s−1. It was found that pH has an important effect on the oxidation rate of estrone. The reactivity between estrone and permanganate decreased with the increase of pH in the range of 2.5−6.6 and in-creased with the increase of pH in the range of 6.6−9.4. The minimum reaction rate takes place at pH 6.6. An activation energy of 43.07 kJ/mol was obtained from experiments conducted at temperatures of 15−40°C. From the compari-son of the results obtained in previous reports with those obtained in the present research, it seems that permanganate is an optional oxidant for controlling estrone in natural wa-ters. However, enough contact time must be carefully en-sured in order to remove estrone to avoid the increase of estrogenicity during permanganate oxidation.

This work was supported by the Ministry of Education of China under the scheme of Key Project of Innovation (Grant No. 705013) and the National Natural Science Foundation of China under the Scheme of National Crea-tive Research Groups (Grant No. 50821002). We are grateful to Prof. M Ma for kindly providing two-hybrid yeast strains.

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