oxidation kinetics of pentachlorophenol by manganese dioxide

Environmental Toxicology and Chemistry, Vol. 25, No. 11, pp. 2912–2919, 2006 䉷 2006 SETAC Printed in the USA 0730-7268/06 $12.00 ⫹ .00

OXIDATION KINETICS OF PENTACHLOROPHENOL BY MANGANESE DIOXIDE LING ZHAO,†円円 ZHIQIANG YU,† PING’AN PENG,*† WEILIN HUANG,‡ SHUNQING FENG,§ and HAIYAN ZHOU§ †State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangdong 510640, People’s Republic of China ‡Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, New Jersey 08901-8551, USA §Instrumentation Analysis and Research Center, Sun Yat-sen University, Guangdong 510275, People’s Republic of China 円円Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ( Received 1 January 2006; Accepted 31 May 2006) Abstract—This study examined the abiotic transformation kinetics of pentachlorophenol (PCP) by manganese dioxide (MnO2) at different solution chemistry and initial concentrations of PCP and MnO2. The measured PCP transformation rates were found to be on the order of 1.07 with respect to [PCP] and 0.91 and 0.87 with respect to [MnO2] and [H⫹], respectively. Dissolved Mn2⫹ and Ca2⫹ as background electrolytes considerably decreased the reaction rate because of their adsorption and hence blocking of active sites on MnO2 surfaces. The dechlorination number, 0.59 chloride ions per transformed PCP after a 1-h reaction, suggests that a fraction of the transformed PCP was not dechlorinated and may be coupled directly to dimeric products. Gas chromatography/ mass spectrometry and liquid chromatography/mass spectrometry/mass spectrometry techniques were used to identify two isomeric nonachlorohydroxybiphenylethers as major products and 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol as minor products. Product identification suggested that the reaction may include two parallel reactions to form either dimers or 2,3,5,6tetrachloro-1,4-hydroquinone and tetrachlorocatechol via simultaneous dehydrochlorination and hydroxylation. Keywords—Abiotic transformation

Pentachlorophenol

Manganese dioxide

Kinetics

diols, and their respective quinones, chloranilic acid, and dichloromaleic acids [8,11]. However, more toxic products, such as polychlorinated dibenzo-p-dioxins, polychlorinated diphenylethers, and polychlorinated hydroxydiphenylethers, could be formed during photolytical treatments [12,13]. Biodegradation by bacteria and fungal species [14–16] is a significant PCP elimination process in soils, surface waters, and sediments under both aerobic and anaerobic conditions. Pentachlorophenol can be degraded aerobically to more oxidized residues and less chlorinated derivatives [17] and anaerobically to methane and carbon dioxide [18]. Tetrachloro-p-benzoquinone and 2,6dichlorohydroquinone were proposed as the metabolic intermediates for PCP during aerobic metabolism [19,20]. Hydroxylation usually occurs during reductive dechlorination, which converts PCP into catechol, hydroquinone, or trihydroxylated forms [12,20]. Field-scale bioremediation of PCPcontaminated soil has been implemented [15], but complete removal of PCP from soils and groundwater could be very difficult because of its high toxicity for many microorganisms. Abiotic transformation of PCP is an alternative means of soil cleanup and water treatment. One approach involves oxidation of PCP by naturally occurring or introduced electron acceptors (e.g., manganese dioxide [MnO2]), and oxidation products often undergo further polymerization [21–26]; MnO2 is a common component of soils and sediments and is a highly oxidative mineral constituent in the environment [27]. Previous work has demonstrated that MnO2 can catalyze PCP oxidation under both aqueous [21,22] and dry [28] conditions. The reported products include tetrachloro-1,4-benzoquinone (major) and less chlorinated phenols (minor) [22]. Polymerized products of PCP, such as dimers, which were reported for phenol oxidation by peroxidase [29], were not reported for MnO2-catalyzed reactions. According to Ulrich and Stone [21], adsorption of PCP on MnO2 is an important process, but the

INTRODUCTION

Pentachlorophenol (PCP) is a common biocide and a priority pollutant. It has been widely used principally to control the spread of snail-borne schistosomiasis and for wood preservation. It has also been used in aquaculture as a pond-cleaning reagent [1,2]. Since being introduced commercially in the 1930s, worldwide PCP consumption reached 100,000 tons by 1985 [3]. Pentachlorophenol is highly persistent in the environment and is widely detected at varied concentrations in surface water, soils, sediments, aquatic organisms, food, human milk, and urine [1,4]. The ubiquitous presence of PCP in the environment has raised much concern since it is acutely toxic to plants, animals, and humans. Even at low concentrations of 0.1 to 1 ␮g/L in water, PCP can affect sensitive organisms and lead to adverse changes in aquatic ecosystems [5]. The target organs for PCP toxicity include liver, kidneys, the hematopoietic system, the pulmonary system, and the central nerve system. Pentachlorophenol is known to uncouple oxidative phosphorylation, alter the electrical conductivity of membranes, and inhibit cellular enzymes, such as adenosine triphosphatase [6]. Pentachlorophenol is a weak monoprotic organic acid with a pKa of 4.75 at 25⬚C [7] and is negatively charged at neutral pH conditions. It is difficult to remove from surface waters via hydrolysis and oxidation reactions [8]. Adsorption of PCP in soils and sediments is pH dependent, with capacity being low in neutral and basic soils and high in acidic soils [9,10]. Photolysis is believed to be one of the important transformation processes for PCP in aquatic systems [8]. This process can mineralize PCP to CO2 and chloride ions (Cl⫺) with intermediate products including tetrachlorophenols, tetrachloro* To whom correspondence may be addressed ([email protected]). 2912

Oxidation kinetics of PCP by manganese dioxide

effect of adsorption on the MnO2-catalyzed reaction kinetics has not been well documented. We initiated this study to investigate the rates and possible pathways of MnO2-catalyzed oxidation of PCP and to examine the effect of solution chemistry on the reaction rates. We determined the apparent reaction rate orders with respect to PCP, MnO2, and H⫹ and evaluated the inhibitive effect of Ca2⫹ and Mn2⫹ as the background electrolytes on the PCP reaction rate. With both gas chromatography/mass spectrometry (GC-MS) and liquid chromatography/mass spectometry/mass spectometry (LC-MS/MS), we identified tetrachloro-hydroquinone/ catechol as minor products and dimers as major products. On the basis of the products identified, a possible reaction scheme was proposed to describe the overall transformation of PCP by MnO2. MATERIALS AND METHODS

Chemicals Pentachlorophenol (99%), tetrachlorocatechol (96%), and 2,3,5,6-tetrachloro-1,4-hydroquinone (99%) were purchased from Sigma-Aldrich (Milwaukee, WI, USA). High-performance liquid chromatographic (HPLC)–grade methanol, acetonitrile, and glacial acetic acid were obtained from Merck (Darmstadt, Germany). All chemicals were used as received. Unless otherwise stated, all solutions were prepared from distilled-deionized water and filtered through a 0.45-␮m nylon membrane filter (Supelco Scientific, Bellefonte, PA, USA) prior to use. All solutions were autoclaved at 120⬚C for 30 min before use.

Mineral characterization Manganese dioxide used in this study was purchased from Acros Organics (Fair Lawn, NJ, USA) and characterized by X-ray powder diffraction with a Regaku (Tokyo, Japan) D/ MAX-1200 X-ray diffractometer with a Cu-K␣ radiation (40 KV and 30 mA). Crystallographic d spacings determined by ˚ . Its N2 electron diffraction were 2.42, 2.12, 1.64, and 1.43 A gas Brunauer-Emmett-Teller–specific surface area was 73.6 m2/g, which was determined using a Quantasorb instrument (Quantachrome Corporation NOVA 1000, Boynton Beach, FL, USA).

Solution preparation The background aqueous solutions at pH ranging from 3.5 to 6.6 were prepared by dissolving 2 mM HAc-NaAc as the buffer, and the ionic strength was adjusted to 0.01 M with NaNO3. A preliminary test using only MnO2 and pH buffers but without PCP revealed that soluble [Mn2⫹] was not detected with an atomic absorption spectrophotometry. This was consistent with previous reports [24,30] suggesting that the presence of acetate as pH buffer may have no effect on the observed reaction rates of PCP oxidation by MnO2. The initial aqueous PCP solutions at concentrations ([PCP]o) of 0.01, 0.02, and 0.03 mM were prepared by adding an aliquot of 40-mM PCP methanol stock solution to a given background solution. The resulting initial solutions contained ⬍1‰ methanol by volume and had [PCP]o lower than the solubility limit of PCP at a given pH condition.

Batch experiments The MnO2-catalyzed reactions were conducted at 25⬚C using 8-ml glass bottles as batch reactors. The reactions were initiated by adding 5 ml of aqueous PCP solution at a fixed

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[PCP]o to each reactor that contained preweighed MnO2 solid. The reactors were immediately capped with Teflon威-lined septa, placed in the dark, and mixed horizontally on a shaker set at 25⬚C and 200 rpm. At the designated time, four reactors were taken from the shaker, and the reactions were quenched immediately with two different methods. In the centrifugation method, the reactors were centrifuged at 3,000 rpm for 5 min. An aliquot (1 ml) of the supernatant was transferred from each reactor to GC vials for chemical analysis using an HPLC method described later. In the ascorbic acid method, excess Lascorbic acid was added to each reactor to reductively dissolve solid MnO2, releasing PCP sorbed on MnO2 surfaces [30]. This method thus allowed detection of the total unreacted PCP concentrations in both solution and solid phases. After completion of MnO2 dissolution, an aliquot (1 ml) of the clear solution was transferred for HPLC analysis. The difference in the PCP concentrations measured with the two methods was the PCP sorbed on the MnO2 solid phase. In all experiments, two sets of control reactors, containing either PCP or MnO2, were prepared similarly and run simultaneously. The set of reactors with PCP was used to assess solute loss due to sorption on reactor compartments and during the reaction quenching procedure. The results showed that the solute loss was negligible. The Cl⫺ concentrations in the aqueous solution of the control reactors were also analyzed for a mass balance calculation of chloride evolved from the chemical reaction of PCP. Additional experiments were set out to evaluate the effect of O2 and other factors on the MnO2-catalyzed reaction rates. Two sets of reactor systems were prepared respectively in an N2-filled glove box (anoxic) and under regular atmospheric (oxic) conditions. The results showed that no difference existed in the measured reaction rates and product evolution within the observation times, suggesting that the presence of O2 did not affect the reaction rates. The subsequent experiments were thus conducted under atmospheric conditions. Similarly, experiments conducted with amber glass bottle reactors and clear bottle reactors with the influence of indoor fluorescent lighting showed no difference in the observed reaction rates.

Analysis of PCP and Cl⫺ Pentachlorophenol was analyzed with an HP Series 1100 HPLC system (Agilent Technologies, Palo Alto, CA, USA) equipped with a reverse-phase C-18 column (Phenomenex, Torrance, CA, USA; 250 ⫻ 4.6 mm, 5 ␮m) and a diode-array ultraviolet detector set at a wavelength of 220 nm. The mobile phase was a mixture of acetonitrile, water, and glacial acetic acid at 85:15:0.1 (v/v) at a flow rate of 0.8 ml/min. Each sample was analyzed twice, and the injection volume was 20 ␮l. Under these conditions, PCP had a retention time of 7 min. The concentration of PCP was calculated from its HPLC peak areas against six-point linear calibration curves established using external methanol solution standards. The minimum quantification limit was 5.6 ␮M based on the lowest concentration of the external standards. The aqueous concentrations of chloride ions (Cl⫺) resulting from the PCP transformation were quantified with ion chromatography (DX-600-type; Dionex, Sunnyvale, CA, USA) using an AS-11 anion-exchange column (250 ⫻ 4 mm) and an ED 50 electrochemical detector. The mobile phase was a mixture of 2-mM Na2B4O7 aqueous solution and MilliQ (Millipore, Billerica, MA, USA) water (35:65, v/v) at a flow rate of 1.0 ml/min. The injection volume was 50 ␮l. The chloride con-

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centrations were determined based on six-point linear standard curve. The minimum quantification limit was 0.01 mg/L.

Product identification and confirmation The final products of the PCP reaction were identified and confirmed with both GC-MS and liquid chromatography/MS/ MS (LC-MS-MS) methods. In order to obtain sufficient mass of all possible products, two sets of 20 identical reactor systems were prepared with the solution pH set at 4.12. After 24 h, the reaction was quenched by the centrifugation method. For GC-MS analysis, the supernatant was transferred from each of the 20 reactors, combined, and extracted three times with dichloromethane. The reactant and the products adsorbed on MnO2 surfaces were extracted three times with methanol, followed by extraction with dichloromethane three times. The three different extracts were passed through sodium sulfate columns to remove trace water. The eluents were collected and concentrated under a gentle stream of N2 gas. The methanol and dichloromethane extracts from MnO2 were then combined. The reactant and products in the concentrated dichloromethane solutions were derivatized with diazomethane for GC-MS analysis. External standards of tetrachlorocatechol and 2,3,5,6-tetrachloro-1,4-hydroquinone were also methylated with diazomethane and analyzed under the same conditions as the samples for identification of the reaction products. The GC-MS used was Finnigan model GC8000TOP Voyager (Thermo Finnigan, San Jose, CA, USA) with a J&W Scientific DB-5 column (J&W Scientific, Folsom, CA, USA, 30 m ⫻ 0.32 mm ⫻ 0.25 ␮m). The GC conditions were set as follows: on-column injection at 40⬚C, oven held at 40⬚C for 5 min, then heated to 280⬚C at a rate of 4⬚C/min and held at that temperature for 10 min. Carrier gas was nitrogen (99.99%) at 1.0 ml/min. Electron ionization at 70 eV with mass scan range 50 to 650 m/z was conducted. For LC-MS-MS analysis, the combined supernatant was extracted with ENVI-18 solid phase extraction tubes (3 ml; Supelco). After extraction, methanol (5 ml) was applied to elute solutes from the cartridge and was subsequently blown down to 1 ml with a gentle N2 gas. The reactant and products adsorbed on MnO2 surfaces were extracted three times with methanol. The methanol extracts were combined and concentrated to 1 ml with N2 gas. Liquid chromatography was performed on the HPLC system described previously with an injection volume of 20 ␮l. The mobile phase was a mixture of acetonitrile/water/glacial acetic acid (80:20:0.1, v/v) at a flow rate of 0.4 ml/min. The MS-MS system was an API 4000 triple quadrupole mass spectrometer (Applied Biosystems, Foster City, CA, USA), equipped with a turbo ion spray interface that was operated in the negative ion mode at 4,000 V and 450⬚C. Nitrogen was used as the nebulizer, drying, curtain, and collision gas. Both ion source gas 1 (nebulizer gas) and gas 2 (drying gas) were set at 20 L/min, and the curtain gas and the collision gas were set at 10 and 6.0 L/min, respectively. The analysis was performed with the multiple reaction monitoring mode of deprotonated precursor ions [M–H]⫺1 and the related product ion for chlorophenols. Multiple reaction monitoring in the negative ion mode was performed using a dwell time of 200 ms per transition to detection ion pairs. RESULTS AND DISCUSSION

Reaction kinetics Figure 1a presents a typical profile for the decrease of PCP concentrations and increase of Cl⫺ concentration as a function

Fig. 1. Time-dependent concentration profiles of [PCP] and [Cl⫺] (a) and percentage of the transformed versus adsorbed PCP (b). Experimental conditions: pH 4.12, [PCP]o ⫽ 0.02 mM, [MnO2]o ⫽ 0.5 g/ L. PCP ⫽ pentachlorophenol; [MnO2]o ⫽ manganese dioxide.

of time. The two quenching methods clearly yielded different concentration profiles for PCP because of adsorption of PCP on MnO2 surfaces. The ascorbic acid method released the adsorbed PCP by rapidly dissolving the remaining MnO2 in solution, resulting in high PCP concentration at a given observation time, t (h). The difference of the PCP concentrations determined with the two methods at a given t represented the amount of the adsorbed PCP. Figure 1b indicates that the percentage of the adsorbed PCP on MnO2 reached an equilibrium level of around 10% after 0.1 h of reaction. After 0.7 h of reaction, the PCP concentrations appeared to approach relatively constant levels, suggesting deceleration of the reaction likely due to approaching reaction equilibrium and/or possible change of MnO2 surfaces (i.e., reductions of reaction sites and/ or surface areas) during reaction. This is consistent with previous investigations on the oxidation of phenols and anilines by MnO2 [30–33]. The production of Cl⫺ was approximately 0.0077 mM along with 0.0131 mM transformed PCP after 1 h reaction (Fig. 1a). This corresponded to a stoichiometric dechlorination number of 0.59 mole of chlorine atom per mole of the transformed PCP. As shown in the following, the intermediate products identified included 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol, which were formed by replacing a chlorine



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[PCP] ⫽ {[PCP]1o⫺n ⫺ k (1 ⫺ n)t}1/(1⫺n)

(3)

where [PCP]o is the initial PCP concentration. Fitting of the rate data into Equation 3 with a nonlinear procedure yielded n ⫽ 0.9997, indicating that the reaction rate is first order with respect to [PCP]. We thus used a pseudo-first-order rate model, (d[PCP]/dt) ⫽ ⫺kobs[PCP]), to obtain the rate constants (kobs). As shown in Figure 2, the kobs values were 2.86/h for the centrifugation method and 2.14/h for the ascorbic acid method. The rate data obtained for all the reaction systems are summarized in Tables 1 to 3, and the detailed interpretation of the data are presented in the following sections.

Reaction orders with respect to PCP and MnO2 surface areas

Fig. 2. Fitting of the rate data to a pseudo-first-order rate model. The equations are the best fit. Experimental conditions: pH 4.12, [PCP]o ⫽ 0.02 mM, [MnO2]o ⫽ 0.5 g/L. PCP ⫽ pentachlorophenol; [MnO2]o ⫽ manganese dioxide.

atom with a hydroxyl. Hence, the dechlorination number suggested that approximately 41% of the transformed PCP had not released Cl⫺ likely by direct incorporation of PCP to products (i.e., reaction of PCP with a free radical to form a dimer or polymers as detailed in the following). The rate of reaction can be expressed with the following equation: rate ⫽

d[PCP] ⫽ ⫺k[PCP] n [MnO2 ]␣ [H⫹ ]␤ dt

(1)

where n, ␣, and ␤ are the reaction order with respect to PCP, MnO2, and H⫹ and k is the reaction rate constant. At constant pH and MnO2 concentration, Equation 1 can be reduced to

d[PCP] ⫽ ⫺kobs [PCP] n dt

(2)

If n 苷 1.0, Equation 2 can be integrated, yielding

The reaction rate orders with respect to the PCP and MnO2 surface areas were evaluated further with the rate data listed in Table 1. As shown in Figure 3a and b, the kobs values were plotted against [PCP]o (Fig. 3a) or the concentration of the solid MnO2 (Fig. 3b) on a log-log scale for the reactor systems initiated with the same conditions (e.g., pH 4.12) but varied [PCP]o levels or different dosages of MnO2. The best fits of the data shown in Figure 3a indicated that all the slopes were around 1. The reaction order with respect to [PCP] was at 1.07 when the initial concentration of MnO2 was 0.5 g/L. Similarly, a linear regression procedure of the rate constants in Figure 3b indicated that the reaction order was at 0.91 with respect to [MnO2]. The linear and positive correlations between the kobs values and [PCP]o or [MnO2]o also suggested that the [PCP]o levels employed in this study were below the saturation point of the MnO2 surface sites. It is intuitive that, once the MnO2 surface sites were saturated with PCP, additional PCP should not increase formation of surface complexes, rendering a slowed overall reaction rate. Similarly, the kobs values increased with the mass of MnO2 (Fig. 3b) because the number of active surface sites was proportional to the amount of MnO2 added. A previous study estimated approximately 8 ⫻ 1018 active Mn atoms or sites per m2 on MnO2 [34]. With a specific surface area of 73.6 m2/g, there are approximately 9.78 ⫻ 10⫺4 mmol

Table 1. Summary of the apparent rate constant kobs and half-life ␶1/2 (h) obtained under different initial conditions Run PCP 1 2

3

pH

[MnO2]oa (g/L)

[PCP]ob (mM)

4.12 4.12 4.12 4.12 4.12d 4.12e 4.12 4.12 4.12 4.12

0.2 0.2 0.2 0.5 0.5 0.5 0.5 1.0 1.0 1.0

0.01 0.02 0.03 0.01 0.02 0.02 0.03 0.01 0.02 0.03

4.12 4.12 4.12 4.12

0.2 0.5 1.0 1.74

0.02 0.02 0.02 0.02

kobsc (per hour)

␶1/2 (h)

0.55 1.08 1.48 1.38 2.86 2.14 4.49 1.76 4.17 6.53

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.033 0.083 0.062 0.032 0.154 0.078 0.241 0.013 0.278 0.796

1.26 0.64 0.47 0.50 0.24 0.32 0.15 0.39 0.17 0.11

1.08 2.86 4.17 8.37

⫾ ⫾ ⫾ ⫾

0.083 0.154 0.278 0.876

0.64 0.24 0.17 0.08

MnO2

a

Manganese dioxide. Pentachlorophenol. c Significant at the 95% confidence level. d Obtained using the centrifugation method for quenching the reaction. e Obtained using the ascorbic acid method for quenching the reaction. b

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Table 2. Effect of solution pH on the apparent reaction rate, half-life ␶1/2 (h), and pentachlorophenol (PCP) speciation pH

[MnO2]oa (g/L)

[PCP]o (mM)

3.50 4.12 4.49 4.79 5.12 5.59 6.62

0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.02 0.02 0.02 0.02 0.02 0.02 0.02

a b

kobsb (per hour)

␶1/2 (h)

Neutral PCP (%)

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.10 0.24 0.41 0.87 2.77 6.93 34.66

94.7 81.0 64.5 47.7 29.9 12.6 1.33

7.23 2.86 1.68 0.80 0.25 0.10 0.02

0.134 0.154 0.333 0.097 0.016 0.004 0.001

Manganese dioxide. Significant at the 95% confidence level.

of active sites per mg of MnO2 used in our study. At the maximum [PCP]o of 0.03 mM and minimum 0.2 g/L of MnO2, the 5-ml reaction solution contained approximately 1.5 ⫻ 10⫺4 mmol of PCP and 9.78 ⫻10⫺4 mmol of active Mn sites. This suggests that there are theoretically sufficient active surface sites on MnO2 for PCP reaction in any of the tested reaction systems. Because the oxidative products of PCP and Mn2⫹ may be adsorbed on or react with the MnO2 surfaces, the actual availability of the active Mn sites may decrease as the reaction proceeded, decelerating the reaction rates, as mentioned previously.

Effect of pH The kobs data obtained at different solution pH conditions are summarized in Table 2. The influence of the solution pH on the reaction rate is expected because PCP is a weak monoprotic organic acid with a pKa value of 4.75. The distribution of the negatively charged PCP and the neutral PCP strictly follows the deprotonation equilibrium relationship and can be calculated using following equation for a weak acid: [PCP⫺ ] [PCP] 1 (%) ⫽ 1 ⫺ (%) ⫽ [PCP]total [PCP]total 1 ⫹ 10 pK a ⫺ pH

(4)

The percentages of the neutral PCP species under the experimental pH conditions were calculated, and the results are listed in Table 2. The neutral PCP species markedly decreased as pH increased from 3.5 to 6.6 shown in Table 2. The log kobs values were plotted in Figure 4 against the solution pH, indicating a linear decrease of log kobs values as a function of pH. A linear regression procedure of the rate constants yielded a slope of 0.87. On the other hand, the trend of the log kobs decrease with pH was similar to that of the neutral PCP species. This result indicated that the reaction rate was dependent on

the concentrations of the neutral PCP species rather than the deprotonated PCP species. The solution pH also determines the charge and charge density of the MnO2 surface. The ␦-MnO2 has a pHzpc (zero proton condition) value of 2.4 [35] and is expected to be negatively charged under all tested pH conditions. Meanwhile, the charge density increases as a function of solution pH. Because of repulsive forces, the negatively charged PCP species is less likely to be adsorbed on MnO2 surfaces, preventing it from any possible reaction with MnO2 surface sites. On the other hand, the neutral PCP could be adsorbed at the MnO2 surface via the van der Waals interactions due to entropic effect in the solution phase. According to the previous analysis, the overall reaction rate of PCP oxidation by MnO2 can be described with the following equation: rate ⫽

d[PCP] ⫽ ⫺k[PCP]1.07 [MnO2 ] 0.91 [H⫹ ] 0.87 dt

(5)

where k is the rate constant with units of (mM)⫺1.78/h. The calculated k value is 1.18 ⫾ 0.06 ⫻ 105 (mM)⫺1.78/h.

Inhibitive effect of dissolved metal ions Coupled with the oxidation of PCP, MnO2 was reduced to Mn2⫹, which can be adsorbed on negatively charged MnO2 surfaces, slowing the reaction rates. Such an inhibitive effect has been documented for MnO2-catalyzed reactions for hydroquinone, phenols, and anilines [30–33]. To quantify such an effect in our reaction systems, a series of experiments was performed by adding varied concentrations of dissolved Mn2⫹ and Ca2⫹ to the reaction systems at pH 4.12. The resulting kobs values are summarized in Table 3. The table clearly shows that addition of both metal ions considerably decreased the

Table 3. Effect of background electrolyte chemistry on the apparent reaction rate constant kobs and half-life ␶1/2 (h) of pentachlorophenol (PCP) [MnO2]oa (g/L)

[PCP]o (mM)

[M2⫹]o (␮M)

Addition of Mn2⫹ 4.12 4.12 4.12 4.12 4.12

0.5 0.5 0.5 0.5 0.5

0.02 0.02 0.02 0.02 0.02

0.0 5.75 28.75 115.00 575.00

Addition of Ca2⫹ 4.12 4.12 4.12 4.12

0.5 0.5 0.5 0.5

0.02 0.02 0.02 0.02

0.0 57.50 287.50 1150.00

pH

a b

Manganese dioxide. Significant at the 95% confidence level.

kobsb (per hour)

␶1/2 (h)

2.86 2.73 2.60 0.36 0.18

⫾ ⫾ ⫾ ⫾ ⫾

0.154 0.176 0.177 0.018 0.006

0.24 0.25 0.27 1.93 3.85

2.86 2.66 2.48 1.80

⫾ ⫾ ⫾ ⫾

0.154 0.114 0.173 0.079

0.24 0.26 0.28 0.39



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Fig. 4. Effect of solution pH on the apparent rate constant. Experimental conditions: [PCP]o ⫽ 0.02 mM, [MnO2]o ⫽ 0.5 g/L. PCP ⫽ pentachlorophenol; [MnO2]o ⫽ manganese dioxide.

tected in the extracts of supernatants, and the two major products were detected in the methanol extracts of MnO2 surfaces. The mass spectra of the minor products exhibited intense molecular ion clusters at m/z 274, 276, and 278 and strong intensity of [M–37]⫹ ion clusters at m/z 239 and 241, which were the same as the mass spectra of the external standards of tetrachlorocatechol and 2,3,5,6-tetrachloro-1,4-hydroquinone. The retention times of minor products are also the same as standards tetrachlorocatechol and 2,3,5,6-tetrachloro-1,4hydroquinone. These two products were further identified and confirmed by the multiple reaction monitoring model of LCMS-MS using the external standards.

Fig. 3. Dependence of the apparent reaction rate constant on [PCP]o (a) and [MnO2]o (b). The solution pH is at 4.12. PCP ⫽ pentachlorophenol; [MnO2]o ⫽ manganese dioxide.

reaction rate of PCP oxidation by MnO2. At a background Ca2⫹ concentration of 1,150 ␮M and a background Mn2⫹ concentration of 575 ␮M, the kobs value decreased by 37 and 94%, respectively. The inhibition effect became more pronounced as the added background metal ion concentration increased, but such a correlation was not linear. The greater decrease of the rate by addition of Mn2⫹ may be due to its greater adsorptive affinity on MnO2 than Ca2⫹. Moreover, the adsorbed Mn2⫹ may be oxidized at the MnO2 surfaces, thus blocking the active surface sites [36,37]. The observed inhibitive effect of background Ca2⫹ and Mn2⫹ on the reaction rate can be used to interpret the decelerating rates over extended observation time periods described previously. As a product of PCP oxidation by MnO2, Mn2⫹ adsorbed on MnO2 surfaces, deactivating oxide surface and resulting in the slower rate as the reaction proceeded. In natural environments, besides background cations, the presence of other inhibitors, such as humic substances and highly sorptive hydrophobic organic pollutants, may further slow the reaction rate via this inhibitive effect [30,33].

Dechlorination and polymerization products The GC-MS analysis showed two minor (Fig. 5a) and two major (Fig. 5b and c) products. The minor products were de-

Fig. 5. Electron impact mass spectra of methylated tetrachlorocatechol and 2,3,5,6-tetrachloro-1,4-hydroquinone (a) and 2,3,4,5-tetrachloro-6-pentachlorophenoxy-1-methoxybenzene (b) and 2,3,5,6tetrachloro-4-pentachlorophenoxy-1-methoxybenzene (c).

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The two major products had GC-MS spectra with strong intensity of molecular ion clusters at m/z at 508, 510, 512, and 514 with an isotope model of nine chlorine atoms (Fig. 5b and c). The characteristic ions of a major product compound (Fig. 5b, compound B) showed a low [M–Cl]⫹ ion at m/z 473, 475, and 477 and a very intense neutral loss [M–Cl–CH3]⫹ ion at m/z 456, 458, 460, and 462. The characteristic ions of the other major product (Fig. 5c, compound C) exhibited an intense [M–CH3]⫹ ion at m/z 493, 495, and 497 and a less intense neutral loss [M–Cl2]⫹ at m/z 436, 438, 440, and 442. Compound C has a higher concentration than compound B. Careful inspection of the mass spectra of the two major products indicates that they are likely structural isomers with the same chemical formula but different positions of the nine chlorine atoms on the aromatic ring. Considering steric hindrance during their formation, compounds B and C are likely 2,3,4,5tetrachloro-6-pentachlorophenoxy-1-methoxybenzene and 2,3,5,6-tetrachloro-4-pentachlorophenoxy-1-methoxybenzene, respectively. Because the standards of compound B and C are not commercially available, they were not confirmed further with the LC-MS-MS method. The proposed dimer as the major product is consistent with a prior study of oxidative coupling reactions involving phenols as substrates [29]. The phase distributions of the minor and major products and the reactant PCP between solution and MnO2 surfaces should be very different. Compared to PCP, the minor products tetrachlorocatechol and 2,3,5,6-tetrachloro-1,4-hydroquinone are more polar with higher solubilities, whereas the two major products are likely less polar with lower solubilities. It is thus expected that, compared to PCP, the two minor products are less sorptive and should be enriched in the supernatant, whereas the two major products are more sorptive and should be distributed in the solid phase. The formation of the major products is thus more inhibitive to the ensuing oxidation reactions. Such an expectation is consistent with our observations that the reaction rates were decelerated and that the majority of the major products were recovered from the solid phase.

L. Zhao et al.

Fig. 6. Possible reaction scheme for oxidation of pentachlorophenol by MnO2 (manganese dioxide). Tetrachloro-1,4-hydroquinone (A) and tetrachlorocatechol (B) were identified by liquid chromatography/ mass spectrometry/mass spectrometry with synthesized standards, respectively. Methylated tetrachloro-1,4-hydroquinone (C) and tetrachlorocatechol (D) were identified by gas chromatography/mass spectrometry with synthesized standards, respectively. 2,3,4,5-tetrachloro6-pentachlorophenoxy-1-methoxybenzene (E) and 2,3,5,6tetrachloro-4-pentachlorophenoxy-1-methoxybenzene (F) were tentatively identified by gas chromatography/mass spectrometry.

Implications Although the overall abundance of MnO2 in sediments is low (⬃7 ␮M on average), enrichment can occur in the vicinity of oxic/anoxic interfaces [21]. A prior study reported MnO2 abundance as high as 2.5 weight % of the dry weight of sediment [41]. With our rate data listed in Table 2 for pH 6.6 and by assuming that natural manganese oxides have the same specific surface areas as the oxide used in this study and that no background cation or organic compound inhibits the reaction rates, the estimated half-life for PCP oxidation is 35 h. Under field conditions, inhibitive effects of background chemicals may variously slow the reaction rates. As discussed previously, formation of larger dimers with greater hydrophobicity could decrease mobility of PCP and its oxidative products.

Surface reaction scheme According to the observed surface reaction kinetics, identified products, and the published information on the oxidation of phenolic and substituted phenolic compounds mentioned previously [30,38–40], we propose a tentative reaction scheme for the PCP oxidation by MnO2 (Fig. 6). This scheme requires further structural confirmation of the dimer and possible polymers and detailed mass distribution of reactant and products. Pathway I of Figure 6 is the addition of a water molecule to a pentachlorophenol molecule followed by subtraction of a hydrogen chloride (HCl) molecule, that is, simultaneous dehydrochlorination and hydroxylation to form 2,3,5,6-tetrachloro-1,4-hydroquinone or tetrachlorocatechol as the minor products identified. Pathway II of Figure 6 is the coupling of PCP itself to release HCl and yield dimers as the major products. The coupling reactions most likely occur via C–C or C–O coupling at the ortho or para positions [38,39]. In this study, only the dimeric product via C–O, not C–C, was detected. Overall, the results show that PCP oxidation by MnO2 takes place at phenol moiety via mechanisms similar to those of phenol or other substituted phenol oxidation. Further polymerization is likely, but polymeric products beyond dimers were not detected in our reaction systems, suggesting that the trimers or polymers, if formed, are at very low concentrations.

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