Ind. Eng. Chem. Res. 2003, 42, 1833-1839
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Electrochemical Degradation of Cresols for Wastewater Treatment D. Rajkumar† and K. Palanivelu* Centre for Environmental Studies, Anna University, Chennai 600 025, India
This paper presents the results of the electrochemical degradation of cresols for wastewater treatment. The experiments were performed in an electrochemical undivided cell reactor using a Ti/TiO2-RuO2-IrO2 anode. Preliminary experiments were conducted using sodium chloride and sodium sulfate as supporting electrolytes to study the effects on the removal of chemical oxygen demand (COD). Operating variables such as the initial pH, chloride concentration, initial cresol concentration, charge input, and current densities were studied using chloride as the supporting electrolyte for maximum degradation. The current efficiency and energy consumption during the degradation were calculated. The present investigation showed the formation of a higher concentration of adsorbable organic halides at the beginning of electrolysis, but this decreased to lower levels during prolonged electrolysis. The total organic carbon removal values were between 50 and 60% though maximum COD removal values were achieved. Introduction The pollutants, especially organic compounds, released from industry to the environment are toxic to humans and other living organisms. The organic pollution is mainly due to the presence of polynuclear aromatic hydrocarbons, phenolic compounds, halogenated hydrocarbons, pesticides, etc. Phenol and phenolic compounds are among the most prevalent forms of chemical pollutants in industrial wastewater. Phenolic compounds in the wastewater stream mainly come from oil refineries, coal conversion plants, petrochemicals, polymeric resins, coal tar distillation, pharmaceuticals, etc. Phenols are the dominant organic contaminants in wastewater from coal conversion and coal coking processes, and they generally comprise 40-80% of the chemical oxygen demand (COD).1 The phenolic compound concentration is higher in coal carbonization wastewater, which contributes about 8000 mg/L.2 Phenolic compounds present in industrial wastewater are toxic and biorefractory in nature. Nowadays, various government agencies prescribe the legal limits and laws for toxic materials in the wastewater released from industries. When such a law is enforced, methods must be available to treat the effluent to meet the discharge standards. Treatment technologies available for phenolic waste are physical, chemical, biological, and electrochemical processes. Thermal decomposition of cresols in supercritical water3,4 at 460 °C in the absence of oxygen showed the rate of reactivity in the order of o-cresol f p-cresol f m-cresol. Photocatalytic oxidation5 of cresols with an ultraviolet light/titanium dioxide (UV/TiO2) system showed complete removal after 2.5 h of reaction * To whom correspondence should be addressed. Tel.: +9144/ 22351723 ext. 3195. Fax: +9144/22354717. E-mail: kpvelu@ hotmail.com. † Present address: Tamilnadu State Pollution Control Board, Tiruppur 641 601, India.
time at pH 7. Microbial degradation of cresols by Pseudomonas sp. was reported by Ahamad and Kunhi.6 The rate of degradation of the three isomers was in the order of o-cresol f p-cresol f m-cresol. Broholm and Arvin7 reported that cresols and other phenolic compounds in the coal carbonization effluent were degraded under aerobic conditions and mixed nitrate and iron-reducing conditions. Electrochemical oxidations8 of cresols on glassy carbon electrodes modified with cobalt(II) phthalocyanine and cobalt(II) octabutoxyphalocyanine were reported for electrochemical analyses of o-, m-, and p-cresols. Electrochemical treatment is one of the treatment methods used for the removal of both organic and inorganic impurities from wastewater. Many researchers are attempting to use electrochemical methods for the treatment of wastewater. A number of review papers are available regarding the application of electrochemical methods for environmental cleanup.9-11 In the electrochemical process, the pollutants are destroyed by either a direct or an indirect oxidation process. In a direct anodic oxidation process, the pollutants are first adsorbed on the anode surface and then destroyed by the anodic electron-transfer reaction. Organic compounds are destroyed by application of the required potential.12 In an indirect oxidation process, strong oxidants such as hypochlorite/chlorine, ozone, and hydrogen peroxide are electrochemically generated. The pollutants are then destroyed in the bulk solution by an oxidation reaction of the generated oxidant. All of the oxidants are generated in situ and are utilized immediately.12,13 Among the oxidants, generation of hypochlorite is cheaper, and most of the effluents have a certain amount of chloride. The electrochemical treatment involves the application of an electrical current to the effluent to convert chloride to chlorine/hypochlorite. The chlorine/hypochlorite oxidizes the pollutants and is then reduced to a chloride ion.
10.1021/ie020759e CCC: $25.00 © 2003 American Chemical Society Published on Web 03/28/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003
Anode: 2Cl- f Cl2 + 2e-
(1)
2H2O + 2e- f H2 + 2OH-
(2)
Cathode:
Bulk solution: Cl2 + H2O f HOCl + H+ + Cl-
(3)
HOCl f H+ + OCl-
(4)
Though a number of papers are available for phenol electrochemical degradation by both direct and indirect electrochemical methods, cresol degradation by this method was not given much importance. Because cresols also are found in higher concentrations in various industrial effluents, studies were attempted to degrade by the electrochemical method. The use of a titanium electrode coated with noble oxides for wastewater treatments is very popular because of its performance, stability, cost, and lifetime.14 Recently, we have reported the degradation of resorcinol15 using a titanium substrate insoluble anode, which is classified under a dimensionally stable anode. Here we report the studies on the degradation of cresols using the same anode, i.e., Ti/TiO2-RuO2-IrO2. Experimental Section Experimental Setup. The electrochemical reactor used was the same one reported previously.15 The material used as an anode was titanium mesh coated with TiO2-RuO2-IrO2 (a mixed triple oxide) supplied by M/s Titanium Equipment and Anode Manufacturing Company Ltd., Chennai, India. Because the electrode used in the study was a mesh, it was more appropriate to report the current density values as the total current over the true electrode surface area, neglecting the contribution of voids in the mesh. The true metal surface of the working electrode (10 × 5 cm) was 27.7 cm2. The cathode material used in the present study was graphite carbon supplied by M/s Carbone Lorraine Ltd., Chennai, India. Cathode was a plate type with a dimension of 10 × 5 cm and a thickness of 3 mm. The effective surface area of the cathode was 50 cm2. A stabilized dc power supply was used as the source of electric current for the experiments. The current and voltage were adjustable between 0 and 6 A and between 0 and 60 V with digital displays. The experiments were conducted by a batch process. An undivided cell of 500 mL capacity (glass beaker) was used throughout the study. The anode and cathode were positioned vertically and parallel to each other with an inter electrode gap of 1 cm. These electrodes were dipped in the electrolyte solution. The reactor was kept in a glass bowl containing water to maintain a constant temperature (∼30 °C) of the electrolytic cell. The solution was constantly stirred with a magnetic stirrer in order to maintain a uniform concentration of the electrolyte solution. Analysis, Instruments, and Procedure. The pH of the solution was measured using Elico pH meter model L1 120. The initial pH of the electrolytic solution was set appropriately using dilute sodium hydroxide and dilute sulfuric acid. The COD values were determined by an open reflux, dichromate tritrimetric method
as described in standard methods. The total organic carbon (TOC) of the initial electrolyzed solution was determined using a TOC analyzer micro N/C model (Analytika jena, Germany). The concentrations of adsorbable organic halides (AOX) at different times of electrolysis were determined according to a DIN EN 1485-IDC AOX analyzing system using a multi X 2000 model (Analytika jena, Germany). The three steps involved in AOX measurement are adsorption of the organochlorine compound onto the activated carbon, mineralization of organically bound halogen (chlorine in this case) through combustion, and determination of the chlorine concentration by microcoulometric titration. The current efficiency (CE) of the electrolysis was calculated using the following relation introduced by Comninellis and Pulgarin:16
CE (%) )
CODt - CODt+∆t FV × 100 8I∆t
(5)
where (COD)t and (COD)t+∆t are the chemical oxygen demands at times t and t + ∆t (in grams of O2 per liter), respectively, and I is the current (amperes), F is the faraday constant (26.8 A‚h), V is the volume of the electrolyte (liters). The energy consumption for the removal of 1 kg of COD was calculated and expressed in kilowatt hours. The average cell voltage during the electrolysis was taken for calculating the energy consumption. The stock solutions of o-cresol (SD Fine Chemicals, India), m-cresol, and p-cresol (CDH, India) were prepared by dissolving the compounds using 4 g of sodium hydroxide/10 g of compound with distilled water. The stock solutions were stored in amber bottles in the refrigerator (4 °C). Design of Experiments. In the present study, the effects of variables were studied using a multivariate method by conducting the experiments based on a BoxBehnken, second-order composite design for four variables.17 Design Expert software (version 2.05)18 was used to study the effect of operating variables on cresol degradation. The operating parameters (variables) for the experimental design were the initial pH (X1), chloride concentration (X2), initial concentration of cresol (X3), and charge (X4). The lower, base, and upper levels of each variable were designated as -1, 0, and +1, and by variation of these levels, 27 experiments were conducted. In addition to the above three levels of experiments, the levels between the lower and base and the upper and base were designated as -0.5 and 0.5, respectively, and by variation of these levels, another eight experiments were conducted. A quadratic polynomial model represents the response Y.
Y ) b0 + b1X1 + b2X2 + b3X3 + b4X4 + b11X12 + b22X22 + b33X32 + b44X42 + b12X1X2 + b13X1X3 + b14X1X4 + b23X2X3 + b24X2X4 + b34X3X4 (6) where Y ) predicted response (percent COD removal for the present experiment), b0 ) constant, b1, b2, b3, and b4 ) linear coefficients, b11, b22, b33, and b44 ) quadratic coefficients, and b12, b13, b14, b23, b24, and b34 ) cross-product coefficients. To test the estimated regression equation for the goodness of fit, the multiple regression coefficient (r2) has been calculated. The actual and coded values of the variables chosen for conducting experiments are given in Table 1. The
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1835 Table 1. Actual and Coded Levels of Variables Chosen for the Experiments actual value initial chloride concn of charge input (Ah/L) coded initial concn cresol value pH (mg/L) (mg/L) o-cresol m-cresol p-cresol -1 -0.5 0 0.5 1
3.0 4.75 6.5 8.25 10.0
500 1500 2500 3500 4500
100 200 300 400 500
2 4.5 7 9.5 12
2 4 6 8 10
4 8 12 16 20
regression equations and the respective r2 values of the model are provided in Table 2. The regression equations are useful to predict the values of the response (percent COD removal) at any point in the experimental design. The results presented in this paper are only for the selected conditions. However, one can obtain the COD removal at any condition within the experimental region by solving the regression equation. All experiments were conducted at a fixed current density (under galvanostatic conditions) of 7.2 A/dm2. The effect of the current density was studied separately by a univariate method. Results and Discussion Preliminary investigations on the electrochemical degradation of cresols were conducted with chloride and sulfate as the supporting electrolytes to discover their effects on the removal of COD. Sulfate is an electrolyte that does not produce any reactive species normally during the electrolysis. In the indirect electrochemical degradation using chloride as the supporting electrolyte, the destruction of organic compounds is mainly by the attack of active chloro species such as chlorine and hypochlorous acid or hypochlorite ion, which are generated at the anode during the electrolysis of chloridecontaining wastewater. The result showed that higher COD removal occurred only in the presence of chloride. However, the removal values were a lot less in the presence of sulfate. At the end of 12 A‚h/L charge input at 7.2 A/dm2 current density and in the presence of 2500 mg/L chloride, there was 71.7, 78.1, and 56.7% removal of COD for o-, m-, and p-cresol, respectively. Under the same conditions, using 2500 mg/L sulfate showed 9.1, 5.9, and 4.3% COD removal. The previous investigators have used a very high sulfate concentration of about 150 g/L and a very high charge input of about 100 A‚h/L to treat phenol16 and 1,4-benzoquinone.19 Electrolysis at extreme pH conditions (12-13) and use of costly platinum anode16 were also reported for wastewater treatment by direct electrolysis. Such conditions are clearly unsuitable for wastewater treatment. Moreover, complete oxidation of larger molecules requires the transfer of many electrons and leads to high energy consumption.20 In view of the above, further efforts were not attempted to increase either the sulfate concentration or the time of electrolysis. The results of the present study by direct electrolysis showed very less CE and
Figure 1. Effect of the initial pH on the electrochemical degradation of cresols. (conditions: chloride concentration ) 2500 mg/L, concentration of cresols ) 300 mg/L, charge input ) 4 A‚h/L, and current density ) 7.2 A/dm2).
very high energy consumption for all of the cresols studied. The COD removal obtained by direct oxidation using a Ti/TiO2-RuO2-IrO2 anode and sulfate as the supporting electrolyte was not satisfactory for real industrial wastewater treatment applications. Hence, it was decided further to study in detail the electrochemical degradation using chloride as an electrolyte. The effect of the initial pH on COD removal was studied by varying the initial pH between 3.0 and 10.0. Though previous investigations have been done at extreme alkaline pH conditions, a reasonable range was chosen to avoid additional chemicals to increase the pH. Figure 1 clearly shows that the initial pH does not have a significant influence on COD removal. The reason is that indirect electrochemical oxidation of organic compounds takes place mainly via the electrolytically generated chlorine/hypochlorite; however, the production rate of chlorine/hypochlorite was not affected by initial pH conditions. Vijayaraghavan et al.21,22 reported that during the treatment of tannery and textile effluents, under fixed current density, the chlorine production was more or less the same, irrespective of the initial pH values (3.5-8.5). The final pH of the solutions after the electrolysis ranged between 8.0 and 8.5, irrespective of the initial pH conditions. This may be due to the formation of a bicarbonate buffer during the electrolytic degradation of organic compounds.23
CO2 + H2O f H+ + HCO3-
Another reason may be due to the decrease of the H+ ion concentration (liberated as hydrogen gas) during the electrochemical reaction at the cathode. This leads to an increase in the pH of the solution. The hydroxyl ion may also react with carbon dioxide (formed during the cresol degradation) and forms a buffering solution. The effect of the chloride concentration on COD removal was studied by varying the chloride concentra-
Table 2. Regression Equations and the Respective r2 Values for the Model of Electrochemical Degradation of Cresols compd
regression equations
o-cresol
% COD removal ) 61.65 + 2.02X1 + 4.18X2 - 9.62X3 + 16.79X4 - 2.63X12 - 2.7X22 + 3.04X32 - 7.96X42 0.15X1X2 + 1.65X1X3 + 0.1X1X4 + 1.75X2X3 - 1.23X2X4 + 5.68X3X4, r2 ) 0.95 % COD removal ) 67.61 + 1.05X1 + 14.98X2 - 13.45X3 + 17.77X4 - 1.61X12 - 8.01X22 - 4.52X32 - 7.72X42 0.85X1X2 - 0.45X1X3 - 0.1X1X4 + 1.55X2X3 + 2.68X2X4 + 0.25X3X4, r2 ) 0.99 % COD removal ) 55.43 + 1.88X1 + 2.72X2 - 11.74X3 + 17.74X4 - 0.64X12 - 0.79X22 + 0.85X32 - 7.98X42 + 0X1X2 + 0.73X1X3 - 1.13X1X4 + 0.7X2X3 - 0.7X2X4 + 0.7X3X4, r2 ) 0.98
m-cresol p-cresol
(7)
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Figure 2. Effect of the chloride concentration on the removal of COD for various cresols (conditions: initial pH ) 9.0, concentration of cresols ) 300 mg/L, current density ) 7.2 A/dm2, and charge ) 4 A‚h/L).
tion from 500 mg/L to a maximum of 4500 mg/L. The reason is that most of the electrochemical experiments by previous investigators were conducted in the range of 2000-15 000 mg/L. The addition of chloride to the electrolysis medium is beneficial in two ways; i.e., it provides sufficient conductivity for the electrochemical reaction and acts as a catalyst for indirect electrolysis. Figure 2 shows that very few significant differences were observed between 500 and 3500 mg/L chloride concentration and above 3500 mg/L there was no remarkable removal of COD for o-cresol. The COD removal was 41.3, 45.7, 48.8, 50.5, and 50.9% respectively for 500, 1500, 2500, 3500, and 4500 mg/L chloride concentrations after 4 A‚h/L charge input. The COD removal values for p-cresol were between 27.9 and 34.7% respectively for 500 and 4500 mg/L chloride concentrations. Among the cresols, only m-cresol showed a higher influence with chloride concentrations. Increasing the chloride concentration from 500 to 3500 mg/L resulted in a major increase in the COD removal from 35.7 to 61.3% with a charge input of 4 A‚h/L for m-cresol. A further increase of the chloride concentration had only a slight influence on COD elimination. Hence, the oxidation of cresols and COD removal occur even in the presence of a small amount of chloride, i.e., 500 mg/L. A similar observation was reported while degrading phenol in the presence of chloride using a Ti/ IrO2 anode.24 Figure 3 shows the effect of the chloride concentration on cell voltage, CE, and energy consumption during the electrochemical degradation of o-cresol. Though the increase of CE is between 23.8 and 29.6 (5.8%) with increasing chloride concentration, the energy consumption drastically decreased because of a decrease of the cell voltage. The energy consumptions were 229.3, 136.8, 83.2, 57.1, and 61.3 kW‚h/kg of COD removal respectively for 500, 1500, 2500, 3500, and 4500 mg/L chloride concentrations. The cell voltage decreased from 16.3 to 4.7 while chloride concnetration increased from 500 to 4500 mg/L. The same trend was observed for other cresols also. To reduce the total dissolved solids load in the treated effluent, it was decided that 2500 mg/L chloride was the optimum for the electrochemical treatment. The influence of the initial concentration of cresols on its degradation was studied in the concentration range between 100 and 500 mg/L. Figure 4 shows the
Figure 3. Effect of the chloride concentration on cell voltage, CE, and energy consumption during the electrochemical degradation of o-cresol (conditions: initial pH ) 9.0, o-cresol concentration ) 300 mg/L, current density )7.2 A/dm2, and charge ) 4 A‚h/L).
Figure 4. Effect of the initial concentration of cresols on the removal of COD (conditions: initial pH ) 9.0, chloride concentration ) 2500 mg/L, current density ) 7.2 A/dm2, and charge ) 4 A‚h/L).
effect of the initial concentration of cresols on the removal of COD. Here increasing the initial concentration showed a decrease in the percentage of COD; however, the net amount of COD removal increased. For example, the COD removal values were 63.7, 55.7, 48.8, 43.6, and 40.0% for 100, 200, 300, 400, and 500 mg/L o-cresol concentrations with net amounts of COD removal of 146, 255, 337, 401, and 460 mg/L, respectively. This may be explained by the fact that, under galvanostatic conditions, the production rate of hypochlorite is constant; however, the radicals/ions have a nonselective nature, which results in an attack on new organic molecules rather than on the intermediate compounds produced. (The amount of hypochlorite radical/ion generated per time unit is equal for low as well as for higher initial concentrations of organics.) Therefore, the absolute amount of COD removal (net amount) was more in higher concentrations of the organic substrate. Hence, the CE and energy consumption were lowered in higher initial concentrations. Similar trends in the removal of the percentage of COD and net amount of COD removal were also observed for increasing m- or p-cresol concentrations. The results suggest that the process is effective at higher cresol concentrations and are encouraging from the viewpoint of industrial wastewater treatment applications. An important operating variable of the electrochemical degradation process is the current density, which is the current input divided by the surface area of the electrode. Because the surface area of the anode for the
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1837
Figure 5. Effect of the current density on the removal of COD for various cresols (conditions: initial pH ) 9.0, chloride ) 2500 mg/L, cresol concentration ) 300 mg/L, and charge ) 4 A‚h/L).
present study was fixed at 27.7 cm2, the 2.0 A current input employed in the previous studies was equivalent to 7.2 A/dm2. To study the effect of the current density, experiments have been conducted by varying the current density in the range of 1.8, 3.6, 5.4, and 7.2 A/dm2. The current densities are equal to the current inputs of 0.5, 1.0, 1.5, and 2.0 A, respectively. The applied current is readily converted to the current density. Figure 5 shows the COD reduction during the electrochemical degradation of cresols with current densities between 1.8 and 7.2 A/dm2 at a fixed value of other parameters. Among the cresols, m-cresol showed a higher influence with increasing current density followed by p-cresol. The results of o-cresol showed that the current density does not have a significant influence on COD removal. The results of m-cresol revealed that enhanced removal of COD occurred between 1.8 and 5.4 A/dm2 current densities, and beyond that the improvement was very small. A remarkable increase in the degradation of p-cresol was observed for current densities between 1.8 and 3.6 A/dm2, and there was no remarkable removal of COD above 3.6 A/dm2. Another observation is that, although the current density exerted little influence for o-cresol, it was found that the energy consumption for COD removal increased with an increase in the current density, which in turn was due to an increase in the cell voltage. For example, electrochemical degradation of o-cresol showed an energy consumption of 33.1 kW‚h/kg of COD if operated at 1.8 A/dm2 current density to achieve ∼48% removal of COD. At the same time, the energy consumption was increased to 57.7, 67.7, and 83.2 kW‚h/kg of COD removal as the current density was increased to 3.6, 5.4, and 7.2 A/dm2, respectively, to obtain the more or less same amount of COD removal. Operating at lower current density always consumes less energy but, at the same time, increases the duration of electrolysis. Therefore, it is necessary to choose a current density that is suitable for real industrial application (containing a mixture of cresols) by way of maximum degradation, shorter reaction time, and less energy consumption. Hence, the optimum current density is considered to be 5.4 A/dm2 for cresol degradation. Figure 6 compares the removal of COD at different charge inputs for various cresols. The removal of COD was 73.2 and 73.9% at 8 and 12 A‚h/L, respectively, for m- and o-cresol. However, the removal of COD was 64.9% for p-cresol after passing 20 A‚h/L. Hence, the order of decreasing COD removal for various cresols is as follows: m-cresol > o-cresol > p-cresol. The increase
Figure 6. Effect of the charge input on the removal of COD for various cresols (conditions: initial pH ) 9.0, chloride concentration ) 2500 mg/L, cresol concentration ) 300 mg/L, and current density ) 5.4 A/dm2).
in the rate of degradation of m-cresol in comparison to those of p- and o-cresol is perhaps due to less stability of the resulting radical cations produced from the former by an electronic effect during the degradation.25 Moreover, m-cresol is more electron-withdrawing (destabilizing) in nature than o- and p-cresol. In addition, in m-cresol both the ortho and the para positions are vulnerable to attack by hypochlorite, whereas in o-cresol one ortho position and one para position alone are available for hypochlorite attack, and in p-cresol only the two ortho positions with respect to the phenolic OH group alone are available for the same attack. The electrochemical degradation of all cresols produced foaming during the electrolysis. However, at lower concentrations (below 300 mg/L), no foaming occurred. Foaming appeared only at the middle periods of electrolysis, then gradually decreased, and finally disappeared. The reason may be due to the interaction between the gaseous evolution from the electrode surface and the intermediate products formed during the electrolysis. In the indirect electrochemical oxidation process, the COD removal rate is proportional to the concentration of the organic compound (pollutant) and to the chlorine/ hypochlorite concentration because the indirect oxidation is mediated by chlorine/hypochlorite. Therefore, the kinetics for COD removal is
-d[COD]/dt ) k[COD][Cl2]
(8)
Electrochemical treatment involves the application of an electrical current to the effluent to convert chloride to chlorine and hypochlorite. The chlorine and hypochlorite will oxidize the organic compound and then get reduced to a chloride ion. The process is then repeated in a catalytic fashion. Therefore, the concentration of chlorine/hypochlorite during the electrolysis is assumed to be a constant, and so eq 8 can be written as a pseudofirst-order kinetic equation.
-d[COD]/dt ) k[COD]
(9)
The log plots of the COD concentration curves show the plot of the rate expression.
log
[COD]t [COD]0
) -kt
or
log
Ct ) -kt C0
(10)
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Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003
Figure 7. First-order kinetic model fit of the electrochemical degradation of cresols.
Figure 8. TOC removal during the electrochemical degradation of cresols (conditions: initial pH ) 9.0, chloride concentration ) 2500 mg/L, cresol concentration ) 300 mg/L, and current density ) 5.4 A/dm2). Table 3. Electrochemical Degradation Rate for Different Cresols (Conditions: Initial pH ) 9.0, Chloride Concentration ) 2500 mg/L, Cresol Concentration ) 300 mg/L, and Current Density ) 7.2 A/dm2) compd
COD removal (%)
rate constant k (min-1)
T1/2 (min)
r2
o-cresol m-cresol p-cresol
64.8 67.5 65.3
4.1 × 10-3 5.8 × 10-3 1.8 × 10-3
73.4 51.9 167.3
0.92 0.96 0.91
The slope of the plot of log(Ct/C0) versus time (Figure 7) gives the value of rate constant k (reciprocal minutes). Here, C0 is the initial COD of cresol in milligrams per liter, and Ct is the COD value at time t. Table 3 provides the rate constant (k) and respective r2 values for ∼65% COD removal. The rate constant data and half-life time reveal that m- and o-cresol are easily degraded, whereas p-cresol is relatively difficult to degrade. Apart from the removal of COD, the removal of TOC is also of great importance during the electrochemical degradation of wastewater containing organic pollutants because it indicates the extent of mineralization. The removal values of TOC were 54.9, 48.4, and 26.2% at 8, 12, and 20 A‚h/L charge inputs for m-, o-, and p-cresol, respectively, with 300 mg/L initial concentration (Figure 8). To study the maximum TOC removal, electrolysis experiments were extended up to 24 A‚h/L. After passing 24 A‚h/L, the TOC removal values were 67.4, 60.9, and 49.3%. A further increase in the electrolysis period up to 40 or 50 A‚h/L does not show a significant effect in the removal of TOC. This may be due to the formation of aliphatic compounds, which are stable and immune to further electrochemical degradation.
Figure 9. AOX concentration during the electrochemical degradation of cresols (conditions: initial pH ) 9.0, chloride concentration ) 2500 mg/L, cresol concentration ) 300 mg/L, and current density ) 5.4 A/dm2).
Because the electrochemical oxidation of organic compounds in the presence of chloride mainly takes place by the attack of hypochlorite/chlorine, chlorinated organic compounds may be formed as byproducts during the oxidation reactions. Because halogenated compounds are important issues in wastewater regulations, it is necessary to check the AOX concentration of the water after the electrochemical treatment. The present study showed (Figure 9) the formation of a high concentration of AOX at the beginning of electrolysis, but prolonged electrolysis led to a decrease of the AOX content to lower levels. The formation of a chlorinated organic compound was comparable with the oxidation of phenol in the presence of sodium chloride reported by Comninellis and Nerni.24 However, the present study with cresols showed that the complete removal of chlorinated organic compounds requires a high charge input. A maximum concentration of AOX was found in the electrolysis solution at 4 A‚h/L charge, and it was reduced to 33, 40, and 54 mg/L for m-, o-, and p-cresol. Hence, at the end of the electrochemical treatment, it is recommended that the treated water be given an activated carbon polishing treatment to remove chlorinated organic compounds before the discharge. Conclusion The results of the present study showed the feasibility of application of electrochemical treatment for cresol containing wastewater by the generation of chlorine/ hypochlorite. The optimum operating conditions arrived at by considering maximum degradation, shorter electrolysis time, and less energy consumption were 2500 mg/L chloride concentration and 5.4 A/dm2 current density with higher concentrations of cresols. The variation in the initial pH of the electrolytic medium does not show significant removal of COD. The order of decreasing COD removal for various cresols was as follows: m-cresol > o-cresol > p-cresol. The TOC removal values were only between 50 and 60% though the electrolysis periods were increased to 40 or 50 A‚h/ L. Finally it was recommended to use an activated carbon polishing treatment to remove halogenated organic compounds that were produced during the electrolysis. Literature Cited (1) Giabbai, M. F.; Cross, W. H.; Chain, E. S. K.; DeWalle, E. B. Characterization of major and minor organic pollutants in wastewater from coal gasification process. Int. J. Environ. Anal. Chem. 1985, 20, 11.
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Received for review September 24, 2002 Revised manuscript received February 19, 2003 Accepted February 20, 2003 IE020759E
