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Degradation of 4-Chlorophenol in Wastewater by Organic Oxidants Swati Sharma,†,‡ Mausumi Mukhopadhyay,† and Z. V. P. Murthy*,† Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat 395007, Gujarat, India, and Department of Chemical Engineering, SarVajanik College of Engineering and Technology, Surat 395001, Gujarat, India
The objective of this work was to investigate degradation and mineralization of model compound 4-chlorophenol (4-CP) using advanced oxidation processes (AOPs). This work focused on the degradation of 4-CP by UV and organic oxidants, such as peroxy acetic acid (PAA), p-nitrobenzoic acid (PNBA), and methyl ethyl ketone peroxide (MEKP), in combination. It was observed that maximum degradation of 4-CP by organic oxidants occurred within the first 10 min. PAA was found to be the best oxidant of all the organic oxidants used. Experiments were also conducted with varying concentration of PAA. Experimental results demonstrated that UV/PAA facilitated 98% removal/mineralization of 4-CP. Mineralization studies were taken up by chloride ion determination and chemical oxygen demand (COD) measurement. The chloride ion concentration was observed to decrease progressively which indicated degradation and mineralization of 4-CP. The COD declined gradually when PAA and MEKP were used as oxidants. The reactions were also followed by HPLC and GC-MS analysis for residual concentration and identification of intermediates and degradation products, respectively. The degraded compound was identified as 4-methyl-3-penten-2-one. 1. Introduction Chlorophenols (CPs) are highly toxic and poorly biodegradable and exhibit carcinogenic and recalcitrant properties.1 Toxicity and persistence of CPs have aroused a great deal of public concern to restore contaminated sites to avoid further risks to the environment.2 The intermediate products of these compounds are reported to be more toxic/refractory than the parent compounds. Complex toxicological effects and omnipresence make remediation research an important task in environmental chemistry.3 Chlorophenols are used as wood preservatives, herbicides, insecticides, and fungicides.4 Chlorophenols are also formed as byproducts, during bleaching of pulp with chlorine, chlorination of drinking water, and incomplete incineration. Improper waste disposal practices, leakages, and accidental runoffs from processes also tend to aggravate the problem of contamination of CPs in wastewater.5 Among CPs 4-chlorophenol (4-CP) is selected as a model compound, because 4-CP is found to be the most difficult one to be anaerobically degraded. Conventional and advanced oxidation processes (AOPs) are the two broad categories of treatment methods employed for 4-CP degradation.6 Conventional methods for 4-CP comprise thermal, physicochemical, and biological processes, such as gas-phase chemical reduction, base-catalyzed decomposition, sodium reduction, and supercritical water oxidation. AOPs are known to facilitate efficient removal of 4-CP by offering a highly reactive species, hydroxyl radical, which is capable of destroying a wide range of other organic pollutants.6,7 AOPs have thus emerged and evolved as potentially powerful techniques for wastewater remediation. Among different advanced oxidation processes, the photolytic processes have been reported to be an effective means of 4-CP degradation in wastewater. It has been reported that use of UV coupled with inorganic oxidant gives promising results compared to UV or oxidant alone.5 Literature reveals that a plethora of information * To whom correspondence should be addressed. E-mail: zvpm2000@ yahoo.com,
[email protected]. Tel.: +91 261 2223371; +91 261 2223374. Fax: +91 261 2227334; +91 261 2201641. † Sardar Vallabhbhai National Institute of Technology. ‡ Sarvajanik College of Engineering and Technology.
is available for 4-CP degradation by AOPs coupled with inorganic oxidants;3,5 however, scant literature is available regarding use of organic oxidants. Consequently, the present study is undertaken to investigate the degradation and subsequent mineralization efficacy of the model compound 4-CP by UV and organic oxidants, viz., peroxy acetic acid (PAA), p-nitrobenzoic acid (PNBA), and methyl ethyl ketone peroxide (MEKP). Regarding the use of PAA as an oxidant, literature review reveals that it is useful in oxidative degradation of a highly halogenated porphyrin as tetrakis,8 in the degradation of R-methylnaphthalene in lake sediments,9 for the treatment of polycyclic aromatic hydrocarbon (PAH) contaminated soils,10 and in degradation of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and other recalcitrant compounds including tetrahydrofuran (THF) and 1-methylnaphthalene.11 These findings stress upon the fact that PAA is a promising oxidant. An attempt has been made to investigate the degradation of model compound 4-CP by organic oxidants, viz., PAA, PNBA, and MEKP. Special attention is paid to the chloride content and chemical oxygen demand (COD) of the system. The reactions are followed by high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) analysis for determination of concentration and identification of degradation products, respectively. 2. Materials and Method 2.1. Materials. The model compound, p-chlorophenol, was obtained from Kemphasol (Mumbai, India). All oxidants, namely, peroxyacetic acid (60%), p-nitrobenzoic acid, and methyl ethyl ketone peroxide (45%) were obtained from National Laboratories (Vadodara, India). Hydrogen peroxide was obtained from Molychem, Mumbai, India. The solvents used were acetone (Molychem, Mumbai, India) and hexane (ACS Chemicals, Ahmedabad, India). Distilled water was used in cleaning and experimentation. The UV radiation source was a low-pressure mercury vapor lamp. 2.2. Method. A hollow glass cylinder with an effective volume of 250 mL was used. The UV lamp (254 nm, Philips,
10.1021/ie9018066 2010 American Chemical Society Published on Web 03/03/2010
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New Delhi, India), placed inside a fitting glass tube, was axially centered in the cylinder and immersed in the 4-CP solution. The entire volume was well-stirred before being subjected to UV irradiation. However, periodic stirring was also done during the course of all experiments. UV radiation being a complementary mode of organic compound degradation, a few experimental runs on direct photolysis of 4-CP were carried out and used as reference or blank experiments for other systems using H2O2, PAA, PNBA, and MEKP. Since pollutants in wastewater tend to play dual roles as UV radiation absorber and OH radical scavenger, the initial concentration of 4-CP and organic oxidants were kept as low as 0.4 mM. The initial concentrations in parts per million were as follows: 4-CP, 51.4; H2O2, 1357; PAA, 3040; PNBA, 6684; MEKP, 8410. The pH of the solution was controlled by adding 0.1 N/1.0 N NaOH and 0.1 N/1.0 N H2SO4. All the experiments were performed under identical conditions at pH 9.5 (pH meter, CL-46, Toshniwal, Ahmedabad, India) at room temperature. The overall study consisted of use of four oxidants, namely, H2O2, PAA, PNBA, and MEKP, for 4-CP degradation. Initially six samples were drawn at an interval of 10 min each, and immediately HPLC analysis was done. For further investigation, all subsequent samples were drawn at an interval of 1 min each, and immediately HPLC analysis was done. COD and chloride ion measurements were done per standard methods.12 The experiments were repeated with varying PAA concentrations under identical conditions, and the degraded products were identified by GC-MS. 2.3. Analysis. All of the samples were analyzed by HPLC (Shimadzu LC-2010 AHT, Kyoto, Japan) and GC-MS (CL 6890, Hewlett-Packard, Wilmington, DE, USA) for identification of degraded products; COD analysis was performed by conventional dichromate method and chloride ion measurement was done per standard methods.12 The residual concentration was measured under the following experimental conditions in HPLC analysis: column, Kromasil (250 × 4.6 mm, 5 µm); mobile phase, acetonitrile:water (40:60); pH 2.6, adjustment using phosphoric acid; flow, 1.0 mL/min; built-in UV-visible detector, 210 nm. A blank run of oxidants alone was performed and compared with initial experimental results to avoid any interference in HPLC analysis. GC-MS analysis was carried out for identification of degraded products. Qualitative analysis was performed after extraction with hexachlorocyclohexane. Analysis of the sample was carried out under the following experimental conditions: temperature program, 60 (8 °C/min), 220 (2 °C/ min), and 260 °C (5 °C/min); carrier gas, helium; flow rate, 1 mL/min; injection volume, 1 µL split less; solvent delay, 1 min; column, HP5MS 5% diphenyl and 95% dimethyl polysiloxane; column length, 30 m; detector, mass spectrometer; mass range scanned, 50-500 amu, respectively. 3. Results and Discussion The model compound 4-CP and oxidants (H2O2, PAA, PNBA, and MEKP) are subjected to UV irradiation. Initially the experiments are conducted for 1 h at 10 min interval for H2O2, PAA, UV/H2O2, and UV/PAA oxidants. Fast degradation is accomplished during the initial 10 min, and thereafter no considerable change was observed. Taking this cue, all further samples are drawn at an interval of 1 min each under identical conditions of pH and wavelength. The plot of residual concentration Cf/Co versus time for all oxidants is shown in Figure 1. The concentration decrease of 4-CP using PAA, UV/PAA, PNBA, UV/PNBA, and MEKP oxidants are comparable but UV/MEKP is not showing the same trend.
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Figure 1. Residual concentration vs time for organic oxidants (initial concentration, Co (4-CP), 51.4 ppm; pH 9.5; UV, 254 nm).
HPLC analysis results indicate that maximum 4-CP degradation is accomplished by PAA and a trace amount of 4-CP remained at the end of the reaction. From the retention time (HPLC analysis) of the compounds present after the degradation reaction with PNBA and MEKP, it is evident that considerably large amounts of remnant 4-CP are present. This implies that comparatively less degradation of 4-CP by PNBA and MEKP takes place as compared to PAA. The retention time (HPLC analysis) of the degraded reaction compounds of MEKP and PNBA reactions indicates that degradation by MEKP is comparatively more than that of PNBA. Inconclusive results by oxidants PNBA and MEKP with regard to 4-CP degradation can be attributed to an increase in concentration of compounds having retention times similar to that of 4-CP. It is hypothesized that the initial formation rate of hydroxyl radicals originating from the decomposition of oxidant is so high that much of the hydroxyl radicals are presumed to be consumed by the side reactions before they could be utilized effectively for the removal of the compound. This could be attributed to formation of compounds having retention times similar to that of the model compound. In identical test conditions the suitability of organic oxidants for 4-CP degradation is thus in the following order: PAA > MEKP > PNBA. PAA is thus found to be a promising oxidizing agent. To investigate further with oxidant PAA, experiments are conducted with different initial concentrations of PAA under similar conditions of pH and wavelength. The optimum concentration of PAA is observed to be 0.3 mM. It is known that higher concentrations of oxidant tend to act as a free-radical scavenger, thereby decreasing the concentration of hydroxyl radicals and reducing target pollutant elimination efficiency. Thus, an optimum amount of oxidant is to be added to achieve the best degradation. The concentration of chloride ions released from the target aromatic compounds during the electrochemical oxidation is measured to evaluate the degree of dechlorination. It is the chlorine substitute that mainly accounts for the toxicity of aromatic compounds. Therefore, the degree of dechlorination indirectly represents the detoxification degree. During the degradation process of 4-CP the changes in the concentration of chloride ions with time are illustrated in Figure 2. The experiment is performed three times to confirm the consistency of the results. The error percentage between the results of analysis is less than 5%. As shown in Figure 2, experiments conducted with PAA and PNBA as oxidants for chloride removal recorded an initial rise until 1 min and then a steep fall between the 1 and 2 min interval, and subsequently a progressive decline is observed. MEKP results exhibit a steep
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Figure 2. Chloride ion concentration vs time for organic oxidants (Co (4CP), 51.4 ppm; pH 9.5; UV, 254 nm).
Figure 3. Chemical oxygen demand vs time for organic oxidants (Co (4CP), 51.4 ppm; pH 9.5; UV, 254 nm).
decline until 2 min, and thereafter a plateau is observed. This indicates that, under the prescribed test conditions, 4-CP does undergo degradation. As seen in Figure 3, COD analysis shows that the CODs for H2O2, PAA, and PNBA follow a similar pattern of gradual decline after an initial peak which surfaced within the first minute. However, the result with MEKP shows a plateau first, and thereafter it is marked by a progressive decline. Beyond 6 min no noteworthy change was observed; hence, further results are not reported. This observation is significant for industrial wastewater treatment, as the shortened reaction time alleviates the use of large reactors for treatment. The experiments are performed three times to confirm the consistency of results, and the error percentage between the results is less than 5%. The results reported for the degradation study of 4-CP show that UV irradiation alone is capable of degrading 4-CP but cannot mineralize it. UV/oxidant coupled degradation is supposed to be based on the formation of OH* radicals resulting from photolysis of organic oxidants and subsequent propagation reactions, shown as follows: H2O2 /PAA/PNBA/MEKP + hν f 2OH*
(1)
4-CP oxidation reaction takes place by the free radical mechanism. However, the total mineralization of the intermediates may be affected by pH and temperature. The addition of oxidants to the system significantly enhanced the degradation rate. The decline in concentration of 4-CP in these cases may be attributed to the possibility of formation of intermediates. Complete mineralization in shorter reaction periods is observed for organic oxidants. Because PAA is found to be the suitable oxidant for 4-CP degradation, GC-MS is employed for identification of the intermediates/degradation products formed for this oxidant (Figure 4). As shown in Figure 4A-E, the mass spectra of major peaks correspond to 4-methyl-
Figure 4. Mass spectra showing the degradation compound 4-methyl-3pentene-2-one (A), dichlorobenzene (B), 4-CP standard (C), 4-CP presence after photodegradation (D), and hexachlorobenzene (E).
3-penten-2-one, 1,2-dichlorobenzene, p-chlorophenol, and hexachlorocyclohexane (solvent). As shown in Figure 4, the 4-CP peak appears at a retention time of 9.44 min, 4-methyl3-penten-2-one at a retention time of 2.6 min, 1,2-dichlorobenzene at a retention time of 6.04 min, and hexachlorocyclohexane
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Figure 5. Proposed reaction mechanism for PAA mediated degradation of 4-CP. Table 1. Comparison of Different Oxidants Reported for Chlorophenols (CP) Degradation sr no.
compound
1
4-CP
2
o-CP
3
4-CP
4
oxidant
process conditions
COD
chloride ion
max COD removal within 6 min nearly 100% COD removal (150 min) -
chloride ions formed immediately
UV, 254 nm; H2O2 /PAA/ PNBA /MEKP catalyst, FeSO4 (catalytic wet air oxidation) UV/H2O2; UV, 254 nm
temp, ambient; pH 9.5; 4-CP, 51.4 mg/L
4-CP
electro-Fenton method; 34.56 mg/L dissolved O2; Na2SO4 electrolyte
4-CP, 25 mg/L; pH 2.5; Na2SO4, 19860 mg/L
-
5
4-CP
pH: 4,7,10
-
6
4-CP
TiO2, WO3, SnO2, TiO2/WO3 and TiO2/SnO2 systems OH* released by Ti/IrO2/RuO2 anode (DSA), a self-made C/PTFE O2-fed cathode.
inorganic chloride ions formed slowly chloride ions release by hydroxylation and cathode reductive dechlorination -
pH 12.8
90% COD removed (120 min)
90% chloride ion removal (80 min)
temp, 150 °C; pH 6.0; o-CP,1250 mg/L; FeSO4, 100 mg/L temp, ambient; H2O2, 0.4 mM; pH 2.5 and 9.5
-
degradation compounds
ref
1,4-benzoquinone (intermediate); 4-methyl-3-penten-2-one -
this work
quinones
13
benzoquinone, 1,4-catechol, 4-chloro-1,2-catechol, phenol
14
2,3-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, catechol 4-chlorocatechol, benzoquinone, maleic, fumaric, oxalic, and formic acids.
15
4
16
at a retention time of 18.4 min, respectively. As shown in Figure 4D, GC-MS results confirm the presence of only the remnant parent compound 4-CP (98% degradation) and absolutely no trace of oxidant, PAA. In all of the runs conducted, dechlorination takes place rapidly, thus reiterating the suitability of the method used. This is corroborated by the fact that OH* radicals tend to exhibit nonselective attack which may have targeted the degradation of the intermediates as soon as they are formed. It implies that the intermediates formed during degradation of organic compounds are metastable and are susceptible to oxidation. The reproducibility study is evaluated for several tests, and results are found to be within 5% error. On the basis of the above findings, the mechanism of the reaction is proposed in Figure 5. Oxidants used for chlorophenol degradation under varied experimental conditions of pH, catalyst, temperature with COD reduction, chloride ions release, and the degradation products formed thereof are compared and reported in Table 1.
to compound removal. In the present work, for 4-CP degradation, the performance of H2O2 and organic oxidants (PAA, PNBA, and MEKP) reveals that PAA, PNBA, and MEKP are good organic oxidizing agents but PAA emerges as the best one. In identical test conditions the suitability of organic oxidants for 4-CP degradation is found to be in the following order PAA > MEKP > PNBA. The concentration of 4-CP is found to be reduced, and 98% degradation of 4-CP with no trace of PAA is observed. PAA-mediated reactions in basic media thus facilitated faster degradation and efficient mineralization of 4-CP, proving the efficacy of PAA as a competent and promising organic oxidant for 4-CP degradation in wastewater. Use of organic oxidants is found to be competently good. Chloride content is observed to fall gradually, which suggests that the organic oxidants are capable of successfully mineralizing 4-CP. The chemical oxygen demands for H2O2 and PAA are found to decrease after 4 min with an initial increase.
4. Conclusions
Literature Cited
The extent of mineralization achieved by the UV process is found to be insignificantly small because the target pollutant compound undergoes photochemical reaction as a consequence of light absorption and these transformations rarely contribute
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ReceiVed for reView November 14, 2009 ReVised manuscript receiVed February 17, 2010 Accepted February 19, 2010 IE9018066
