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Keywords:

  • Electrochemistry;
  • Microtox®;
  • Boron-doped diamond;
  • Ecotoxicology;
  • Mixture toxicology

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES

In the present study, the Microtox® test was used to track the toxicity of electrochemical effluents to the marine bacteria Vibrio fischeri as a function of reaction time. When electrochemistry was used to degrade aqueous phenol using different reactor configurations, two reaction pathways were identified, chlorine substitution and oxidation, depending on whether the electrolyte contained chloride. For a boron-doped diamond (BDD) anode, electrochemistry using Na2SO4 electrolyte produced a significantly more toxic effluent than when using NaCl electrolyte with all other conditions remaining the same. This effect is attributed to the reaction pathway, specifically the production of benzoquinone. Benzoquinone was produced only during electrochemistry using Na2SO4 and is the most toxic potential intermediate, having nearly 800 times more toxicity than phenol. Although the use of NaCl produced a lower toxicity effluent than Na2SO4, caution should be observed because of the production of chlorinated phenols, which can be of special environmental concern. When comparing graphite rod and BDD plate anodes in terms of toxicity evolution when using Na2SO4, BDD was found to produce a lower toxicity effluent; this is a result of the increased oxidizing power of BDD, reducing the formation of benzoquinone. In this comparison, the type of anode material/electrode configuration did not seem to affect which intermediates were detected but did affect the quantity of and rate of production of intermediates. Environ. Toxicol. Chem. 2012;31:494–500. © 2011 SETAC


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Toxicity is a major concern when dealing with electrochemical treatment. In the case of phenol treatment, many of the intermediate products produced are significantly more toxic than phenol itself, as summarized in Table 1. Toxicity is a way to judge the effectiveness of a treatment process and a way to quantify the environmental impact of an effluent or individual compound. Published research on toxicity dates back to the late 1800 s, with a physician reporting on a medication overdose 1. A significant amount of research has been devoted to studying the toxicity of individual compounds 2 as well as the toxicity of controlled mixtures of toxic compounds 3. Mixture toxicity is very important to the study of effluents, in which many different compounds can be expressing their toxicities together. Mixtures of toxic compounds may express a very different toxicity than simply the sum of the individual toxicities because of synergistic or antagonistic interactions among the compounds present. Toxicity of effluents is an area that has received little research attention, and much of the current body of research lacks depth and addresses toxicity only in passing.

Table 1. Microtox® 30 min toxicity expressed as effective concentration 50% (EC50), with coefficient of variation (CV) and number of samples (n)
CompoundEC50a (mM)CV (%)n
  • a

    Efffective concentration for 50% inhibition. Lower EC50 indicates higher toxicity.

Benzoquinone0.000310.89
4-Chlorophenol0.00842.64
2,4-Dichlorophenol0.01315.84
2,4,6-Trichlorophenol0.0567.162
2,6-Dichlorophenol0.0625.153
Hydroquinone0.10767.210
2-Chlorophenol0.1361.782
Catechol0.22911.64
Phenol0.23912.719

Phenols are a common type of organic waste found in a variety of industries, including oil refining, dye making, plastic production, and production of pharmaceuticals 4. Although various treatment strategies exist to treat phenols, electrochemistry is a promising new treatment method that numerous researchers have studied with various types of electrodes. Work has been done with titanium 5, 6, platinum 7, glassy carbon 8, graphite 9, and boron-doped diamond (BDD) 10, 11. Interest in BDD electrochemistry has increased since the discovery that BDD has several advantages over other types of electrodes. Specifically, BDD has a large working potential window, high resistance to fouling, and long-term stability in air 12. Boron-doped diamond is also an inert, nonactive electrode by which direct oxidation occurs mainly through highly active hydroxyl radicals 13. In addition to direct oxidation at the electrode, indirect oxidation can also occur in the presence of a chloride electrolyte. Electrochemistry in the presence of chloride has been studied 9, 14, but few authors have attempted to create or verify a reaction pathway for destruction of phenol when chloride is present.

Phenol is a commonly studied model organic compound for different types of waste treatment processes and is studied along with many other compounds in its class, such as chlorophenols and quinones. Phenol and related compounds have also received significant regulatory attention. Chlorinated phenols such as 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), pentachlorophenol (PCP), and phenol itself are listed by the U.S. Environmental Protection Agency as priority pollutants 15. A main criterion for listing is presence on the toxic pollutant list. Inclusion on the toxic pollutant list is determined by toxicity, persistence, degradability, and the types of organisms affected by the compound 16.

The present study is unique in its approach to evaluating the impact of reactor operating conditions on effluent toxicity, building on and extending the methodology of Pandey et al. 17 and Yavuz et al. 4. Research shows that the products produced during electrochemistry of phenol are variable and dependent on several factors, including electrode type and electrolyte, among others 6, 9. The work reported herein is part of a larger study to compare effluents from various electrochemical reactor configurations.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Materials and equipment

All chemicals used in this research were purchased from Fisher Scientific or Sigma-Aldrich. The Microtox® Model 500 Analyzer was manufactured by AZUR Environmental, now known as SDIX, and the Microtox acute reagent and solutions were also purchased from SDIX. Electrochemical samples were analyzed wth an Agilent 1100 high-performance liquid chromatograph (HPLC) equipped with a Hypersil ODS column (Thermo Scientific) 150 mm long by 4.6 mm in diameter at a constant temperature of 30°C. The mobile phase consisted of 85% water to 15% solvent mixture by volume at a flow rate of 0.8 ml/min. The solvent mixture was 90:5:5 acetonitrile to methanol to acetic acid by volume. The limit of quantification for the HPLC method was defined as 10 times the method detection limit. An article describing more detailed information on the development of the method is in preparation. MarvinSketch Version 5.3.8 (2010) was used for drawing chemical structures and creating the reaction pathway. MarvinSketch is a product of ChemAxon.

Electrochemistry experiments

All electrochemistry experiments were performed in a bench-scale electrochemical reactor with a working volume of 1 L and mixed by recirculation at a rate of 15 ml/s. Graphite and stainless-steel electrode rods were 0.63 cm in diameter (Alfa Aesar 10135/graphite 13474/stainless steel). The BDD plate electrodes, supplied by Adamant, were 25- × 50-mm monopolar/mono-Si 2 mm. Stainless-steel plate electrodes used with BDD, supplied by Alfa Aesar, were also 25 × 50 mm. The electrode distance was held constant throughout all experiments at 3.4 cm. A diagram describing the reaction system is given in Figure 1. For all experiments, the applied current was held constant at 120 mA to allow for comparisons between experiments with constant coulombs. Current densities for the graphite and BDD anodes were 6.27 and 9.6 mA/cm2. The initial electrode potential range observed during experiments with graphite and BDD anodes was 4.60 to 5.30 V and 4.75 to 5.71 V. The initial phenol concentration in each experiment was 0.5 mM (47.0 mg/L). Electrolytes used were sodium sulfate (Na2SO4) and sodium chloride (NaCl). The electrolyte in each experiment was held at a constant ionic strength of 0.05 M; Na2SO4 and NaCl concentrations were 2.37 g/L and 2.92 g/L. Samples were taken and analyzed in real time using HPLC to quantify phenol and its intermediates. A 36-min sampling interval was necessary to allow for duplicate runs of the HPLC method that had an 18-min run time. To protect the Microtox test bacteria and stop further reactions, free chlorine present in electrochemical samples was neutralized by adding 5.5 mg/L sodium thiosulfate (Na2S2O3) per 1 mg/L free chlorine in the sample, as given in Standard Methods Test Protocol 9060 18. Free chlorine was measured using an eXact® Micro 7+ Photometer from Industrial Test Systems.

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Figure 1. Diagram of reaction system. 1 = Cathode; 2 = anode; 3 = reference electrode (Ag/AgCl).

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Individual compound toxicity experiments

Prior to the study of electrochemical effluents, the toxicities of potential intermediate products were quantified by using the Microtox test. A fresh sample was prepared from reagent-grade chemicals on the test day unless noted otherwise. All acute toxicity experiments were performed on a Microtox Model 500 Analyzer. Test protocols used were taken from Microtox Manual: A Toxicity Testing Handbook19 and the Microtox Omni software published by AZUR Environmental. Phenol was used as a standard toxicant and was tested each time a new vial of bacteria was reconstituted. The acceptable range for phenol toxicity was 13 to 26 mg/L, as stated in the Microtox manual 19. To determine individual compound toxicities, the basic test was used with one control and four serial dilutions. The dilution scheme was changed if necessary to bracket the effective concentration for 50% inhibition (EC50) in the central portion of the curve. The reported values for toxicity were based on the measured 30 min EC50 in all cases. The EC50 is defined as the concentration of a toxicant that causes a 50% decrease in light output from the luminescent test bacteria, Vibrio fischeri.

Electrochemical effluent analysis

Electrochemical effluent toxicity was determined using the Microtox test with one control and nine serial dilutions. Because effluents contain multiple components, the toxicity is measured relative to a dilution ratio rather than a concentration. Toxicity measured as a dilution ratio was expressed in terms of toxicity units (TU). Toxicity units are a way to normalize toxicity into a standard unit, a measure that has been used in previous studies 20, 21. Toxicity units can be used to compare toxic solutions and also summed across the individual components of an effluent. The number of TU of an individual compound A at a concentration of CA is defined in Equation 1.

  • equation image(1)

In the case of a complex solution, such as an effluent, there are two ways to determine the toxicity in terms of TU. First, the Microtox test can be used to determine the dilution ratio for 50% inhibition (IC50) of a sample, similar to previously used methods to determine effluent toxicity 4, 17, 20, 21. The IC50 is defined as the dilution ratio of an effluent that produces a 50% effect in the Microtox test organisms. (Note: This is named IC rather than EC based on its original use in the inhibition dosage literature from pharmaceutical studies.) Equation 2 shows the method for calculating toxicity units from IC50, also called TUmeas. Figure 2 provides a clarification of IC50 and TU for an effluent. Lower IC50 values are more toxic, as are higher TU values.

  • equation image(2)
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Figure 2. Visual depiction of dilution ratio, 50% inhibition (IC50), and toxicity units (TU). The left example is more toxic than the right.

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The second method for determining the number of TU in an effluent is by summing the number of TU for each separate compound present in the effluent. This is a prediction of the toxicity, because it is not always possible to quantify every compound that is actually present in an effluent. The predicted toxicity (TUpred) also assumes additive toxicity and does not account for synergistic or antagonistic effects. This method is shown in Equation 3. The importance of this estimate is that it can be computed based on a chemical analysis of the effluent and knowledge of the individual toxicities when toxicity testing is not available.

  • equation image(3)

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES

Individual toxicity data

Individual toxicities of the compounds being studied in this experiment can be found in Table 1. It can be seen that benzoquinone (BQ) is the most toxic compound on an individual basis by a large margin. It is also worth noting that the parent compound, phenol, is the least toxic compound being studied. This observation has implications when considering treatment options, because releasing more toxic products into the environment is not desirable; this underscores the need to evaluate toxicity when judging the efficiency of a treatment.

Reaction pathways

Figure 3 shows the proposed reaction pathway for the electrochemical oxidation of phenol for the conditions being studied. Two main pathways are followed during degradation: direct/indirect oxidation, also known as the quinone pathway; and electrophilic aromatic halogenation, also known as the chlorophenol pathway. The quinone pathway is similar to previously published pathways 5, 7, 8, 11. Two main mechanisms are at work to degrade phenol through the oxidative pathway: direct oxidation at the electrode surface and oxidation by hydroxyl radicals (equation image). Equation 4 shows the reaction for the production of hydroxyl radicals.

  • equation image(4)
thumbnail image

Figure 3. Proposed reaction pathway for electrochemical oxidation of phenol. Compounds in boxes are known to be products but have not been confirmed by experimentation. *Present only when using an NaCl electrolyte. [Color figure can be seen in the online version of this article, available at wileyonlinelibrary.com.]

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Hydroxyl radicals are adsorbed to the surface of the electrode or can diffuse into the bulk solution around the electrode; however, they are very short lived because of their reactivity 11. These radicals can directly oxidize the organic pollutants into intermediate products. Hydroxyl radicals can also react with each other to form water and oxygen, shown in Equation 5.

  • equation image(5)

Direct oxidation at the electrode is also known as electrochemical cold combustion 7, 22. Organics that diffuse to the electrode surface can be converted directly to carbon dioxide and water.

Good agreement exists among researchers with respect to what products are produced along the oxidative pathway of phenol degradation, regardless of the type of electrode used. Considering only the oxidative pathway, it seems that the electrode type does not impact the specific products but only their distribution and rates of production and degradation. Because of this observation, it is possible to produce a general reaction pathway for the direct electrochemical oxidation of phenol, and that can be seen in Figure 3.

The chlorophenol pathway is formed by aromatic substitution of chlorine atoms onto the phenol ring. Chlorine atoms that are substituted onto the ring come from free chlorine (Cl2, OCl, HOCl). During electrolysis with a chloride-containing electrolyte, free chlorine as Cl2 is formed directly at the anode (Eqn. 6), which hydrolyzes in the bulk solution depending on the pH (Eqn. 7). Hypochlorite is formed from hypochlorous acid depending on solution pH (pKa 7.53), as can be seen in Equation 8.

  • equation image(6)
  • equation image(7)
  • equation image(8)

The phenol's hydroxyl group activates the ortho and para positions for oxidation by free chlorine 23. As long as free chlorine is present in the bulk solution, the degree of chlorination will continue to increase until eventually the chlorinated phenols are oxidized by the breaking of the phenol ring. The chlorophenol pathway can be seen in Figure 3.

An interesting note is that only the ortho and para positions on the phenol ring are eligible for substitution. Because of this, tetra- and pentachlorophenol are extremely unlikely to be formed. Published work on chlorination of phenols showed that the tendency for rupture of the phenol ring increases with an increase in chlorination, with 2,4,6-TCP being the highest chlorinated phenol possible 24. The data in the present study support this observation. The reactor was run beyond the point at which all of the lower chlorinated phenols had been removed completely and the free chlorine level started to increase rapidly. No PCP was found in the effluent. More experimentation may be necessary to rule out PCP as a potential product, but, under the conditions of the present study, no production of PCP was observed.

In the case of either pathway of degradation, the ideal end products are carbon dioxide and water. One of the main aspects of the oxidation is the rupture of the phenol ring. Cleavage of the phenol ring converts the intermediates into organic acids, which are easily biodegradable into carbon dioxide and water.

Electrochemistry with sodium sulfate electrolyte

When a graphite anode was used and no chloride was present in the electrolyte matrix, as shown in Figure 4, the measured toxicity increased rapidly once the electrochemistry began. The only compound produced in significant quantities under these conditions was BQ. Benzoquinone is, by a wide margin, the most toxic compound observed in the present study and, according to Maluleke and Linkov 25, is considered to be one of the most toxic xenobiotics. With BQ being much more toxic than the other degradation products, it controls the toxicity of the effluent. If the contribution of an individual compound to the effluent can be determined using Equation 3, BQ alone contributes 98.5 of the 100.4 TU present in the effluent, or 98%, starting at 518 coulombs (72 min). From this time until the end of the experiment, BQ makes up more than 99% of the total TU present. Prior research by Olmstead and LeBlanc 26 in the area of mixture toxicity with Daphnia magna showed a similar effect when one compound (chlorpyrifos) was much more toxic than the others.

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Figure 4. Degradation and toxicity evolution of effluent from treatment with graphite/stainless steel and Na2SO4 electrolyte. Sum represents the millimolar sum of all measured compounds. Limits of quantification for phenol, benzoquinone, hydroquinone, and catechol were 0.006, 0.004, 0.002, and 0.001 mM.

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It is also interesting to note that the predicted toxicity aligns well with the measured toxicity with respect to the data shown in Figure 4. This alignment is due in part to the presence of a toxicity-controlling component such as BQ. This alignment also supports the premise that the quantified compounds, BQ, HQ, phenol, and catechol, are the major representative compounds responsible for effluent toxicity. Although organic acids and other ring cleavage products were not quanitified in the present study, their contribution to toxicity in this organism appears to be minimal.

Figure 5 shows the difference in toxicity and composition change when using a BDD anode as opposed to graphite. All other experimental conditions were the same except for the size and shape of the electrodes. Similarly to the graphite experiment from Figure 4, an increase in toxicity was due mostly to the formation of BQ as an intermediate. However, intermediates were formed at lower concentrations when using a BDD anode than with the graphite anode. Note that the concentrations of intermediates shown in Figure 5 are below the limits of quantification of the HPLC method; therefore, they are not accurate but do show the general trend. This difference is most important from an effluent toxicity standpoint. As stated earlier, BQ controls the toxicity, and reducing BQ production has a significant impact on toxicity reduction. Boron-doped diamond likely produces less BQ than graphite because of the increased oxidation efficiency with BDD. With the greater oxidation efficiency, intermediates are produced in much lower quantities. It should be noted that differing electrode shape and current density between the graphite and BDD experiments might also have an effect on the results, but these effects are believed to be minimal. At the final sampling time of the experiment, the same number of coulombs for the experiments shown in Figures 4 and 5, there were approximately seven times fewer TU with BDD than with graphite. In comparing the two different anode types, it can be seen that BDD is the better choice from the standpoint of toxicity reduction. Boron-doped diamond is already known to be a superior electrode material from a reactivity standpoint 11, 13, and these findings only confirm that these benefits lead to an overall reduction in effluent toxicity compared with graphite.

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Figure 5. Degradation and toxicity evolution of effluent from treatment with boron-doped diamond/stainless steel and Na2SO4 electrolyte. Sum represents the millimolar sum of all measured compounds. Limits of quantification for phenol, benzoquinone, and catechol were 0.004, 0.009, and 0.003 mM.

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This finding has an impact on regulating and managing toxic effluents. Although regulating all constituents of an effluent is not possible or practical, more consideration should be given to those compounds that control toxicity. It has been shown that changing electrode type can affect the composition of the effluent, meaning that it is possible to manipulate the reaction conditions to favor a lower-toxicity product. Another point for consideration is the environmental persistence of the compounds present in the effluent. Although a certain compound may be extremely toxic, the length of time it will exert that toxicity is significant. From the results of the present study, in the case of a nonreactive electrolyte such as sodium sulfate, BDD produces a lower-toxicity effluent if the reaction is to be stopped before complete mineralization of the pollutants. With respect to the Microtox test organism, BDD is the better electrode type compared with graphite because of the production of a lower-toxicity effluent.

Electrochemistry with sodium chloride electrolyte

Figure 6 shows the degradation of phenol and the toxicity change of an effluent from a BDD/stainless-steel system using a chloride electrolyte. In this reaction situation, no compounds from the quinone pathway were observed, likely because of the high oxidizing power of BDD combined with free chlorine. The distribution of toxicity in the effluent is relatively balanced. No one compound had direct control over the effluent toxicity in the same way as BQ did when it was present. 4-Chlorophenol and 2,4-dichlorophenol make up a majority of the effluent toxicity from 259 to 777 coulombs. It can be seen in Figure 6 that at 518 coulombs the predicted and measured toxicities are very similar. When the predicted toxicity is the same as the measured toxicity, it is likely that a majority of the compounds significant to the effluent toxicity have been quantified. At 1,296 and 1,815 coulombs, however, there is a discrepancy between the predicted and measured toxicities. This discrepancy indicates that another compound is present and is having a significant effect on the effluent toxicity, one that has not been identified or quantified. It is likely that this unidentified compound is a di- or trichlorinated aromatic compound. Onodera et al. 27 suggested that, after the formation of 2,4,6-TCP, potential exists for the production of 2,6-dichloro-p-benzoquinone; this hypothesis will be investigated in the future. Nonetheless, this unidentified compound is removed by the end of the experiment, and the predicted and measured toxicities reconverge. It should be noted that in Figure 6 the last data point for TUmeas is a worst-case scenario. The maximum concentration that can be tested using the basic Microtox test protocol is 45%, corresponding to an IC50 of 45 and 2.222 TU. The sample actually contains less than 2.222 TU, but the level cannot be accurately determined using the basic test protocol. Therefore, for the Microtox basic test protocol, 2.222 TU acts similarly to a limit of quantification when measuring the toxicity of an effluent as IC50. The limit of quantification is shown in Figure 6 as a dashed line.

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Figure 6. Degradation and toxicity evolution of effluent from treatment with boron-doped diamond/stainless steel and NaCl electrolyte. Sum is the millimolar sum of all measured compounds. Error bars indicate the range of the data. Limits of quantification for phenol, 2-chlorophenol, 4-chlorophenol, 2,6-dichlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol were 0.006, 0.006, 0.002, 0.004, 0.002, and 0.002 mM.

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The comparison between electrochemistry using a sodium chloride electrolyte versus sodium sulfate is important. In terms of composition, the oxidative intermediates such as BQ, HQ, and catechol were not found when using sodium chloride. This absence has a significant effect on toxicity, because the chlorinated phenols are significantly less toxic than BQ. The BDD and sodium chloride system produces an effluent that, at the most toxic point, is one-third as toxic as BDD and sodium sulfate and 1/25 as toxic as graphite and sodium sulfate. From a strict toxicity standpoint, a clear benefit exists with treatment with BDD and sodium chloride. The problem with this conclusion is that chlorophenols are environmentally persistent 28, and, if the reaction is not taken to completion, there is a risk of adding persistent toxicity to the effluent. Electrochemistry with sodium chloride can also produce unwanted byproducts such as chlorate, and the present study shows that significant amounts of chlorate were produced. Chlorate has a very low Microtox toxicity relative to other potential products, with an EC50 determined to be greater than 11.98 mM (1,000 mg/L). However, chlorate has been found to be a potential carcinogen in mice 29, so it should be regarded with some caution; it is also environmentally persistent. These unwanted products are not produced during electrochemistry with sodium sulfate. However, chloride may naturally be present in an effluent requiring treatment. These concerns bring up the importance of using more than just toxicity readings to judge and compare effluents. Consideration must be given to the environmental persistence and degradability of an effluent as well as the potential for production of unwanted byproducts. As important as the effluent's toxicity is the length of time for which toxicity will be present in the environment. Further study is recommended to analyze the toxicity and persistence of compounds once they are released into the environment. Consideration of toxicity should go beyond the point of discharge.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES

This research was supported by a career grant from the National Science Foundation (grant 0747602). The authors also wish to thank the American Water Works Association for their support with a Larson Aquatic Research Support Scholarship.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. Acknowledgements
  7. REFERENCES
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