Clean Techn Environ Policy DOI 10.1007/s10098-014-0777-9
ORIGINAL PAPER
Catalytic wet oxidation of phenol under mild operating conditions: development of reaction pathway and sludge characterization Kanhaiya Lal • Anurag Garg
Received: 15 February 2014 / Accepted: 2 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Several process industries discharge wastewater with enormous amount of toxic phenolic compounds. Wet oxidation (WO) is considered among the potential cleaner treatment methods for such waste streams. The present study demonstrates the effectiveness of two homogeneous copper salts (nitrate and sulfate) as catalysts during batch WO process conducted under mild temperature and pressures (i.e., 120 °C and oxygen pressure = 0.5 MPa). The catalytic oxidation showed around 90 % reduction of phenol, chemical oxygen demand and total organic carbon from the wastewater within 2 h. The oxidation reaction pathway at mild conditions is also proposed based on the presence of intermediates/by-products. Fourier transform infrared spectroscopy confirmed the formation of polymerized compounds containing alcoholic/ phenolic species. The inductively coupled plasma-atomic emission spectroscopy analysis revealed the presence of *40 % of the total copper in sludge. The copper recovery from the treated wastewater and sludge and its reuse in the oxidation process should be studied in future. Keywords Homogeneous catalyst Phenol Polymerized product Reaction pathway Wet oxidation
Electronic supplementary material The online version of this article (doi:10.1007/s10098-014-0777-9) contains supplementary material, which is available to authorized users. K. Lal A. Garg (&) Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India e-mail:
[email protected]
Introduction Phenol is listed as one of the priority pollutants along with 10 other phenolic compounds (United States Environmental Protection Agency (US EPA) 2012). It can be found in varying concentrations (0.5–24 g/L) in the effluents generated from several process industries such as petrochemicals, refineries, coal processing, pharmaceutical, pulp and paper mills and olive mills (Busca et al. 2008; Chasib 2013). In India, the maximum discharge limit for a phenolic compound in public sewers is 5 mg/L (The Environment (Protection) Rules 1986). Several chemical and biological treatment methods are suggested for the removal of phenol from the wastewater. Generally, the conventional aerobic biological processes have not been found efficient for higher phenol concentrations ([200 mg/L) (Marrot et al. 2006). However, the biological processes employing mixed microbial cultures have been tested for high phenol concentration (up to 1,500 mg/L) in the recent past (Sahariah and Chakraborty 2013; Senthilvelan et al. 2014). Chemical oxidation processes are gaining popularity due to their capability of destroying the toxic and recalcitrant pollutants. Wet oxidation (WO) is considered a potential alternative for the treatment of concentration waste streams with high chemical oxygen demand (COD) ranging from *5,000 to 2,00,000 mg/L (Delgado et al. 2006). The process is performed at moderate-to-high temperature (125–320 °C) and elevated pressures (0.5–20 MPa) in the presence of oxygen or air (Mishra et al. 1995). To achieve high removal of pollutants at mild operating conditions, the reaction is carried out in the presence of a catalyst which is termed as catalytic wet oxidation (CWO). Several studies using homogeneous as well heterogeneous catalysts have been reported on CWO of phenolic and other industrial streams (Kulkarni and Dixit 1991; Wu et al. 2003; Garg et al. 2007, 2010; Garg and
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K. Lal, A. Garg
Mishra 2010; Arena et al. 2010; Garg and Mishra 2013; Arena et al. 2014). In general, the homogeneous systems have shown better performance due to the absence of mass transfer limitations since reactants and catalyst exist in the same phase. But an additional step is required for the removal of dissolved catalyst from the treated wastewater. Despite this drawback, the study on homogeneous WO is required since some commercial catalytic processes use homogeneous Cu2? salts (Luck 1999) so more in-depth information is needed for efficient operation (like reaction pathway and the nature of polymeric compounds). Apart from this, the designing of a homo-catalytic WO system is comparatively easy and the instability (mechanical and thermal both) of heterogeneous catalysts is a major concern. Copper salts have exhibited the promising performance for model pollutants such as phenol (Kulkarni and Dixit 1991; Lin and Ho 1996; Arena et al. 2010; Garg et al. 2010; Garg and Mishra 2013), thiocyanate (Collado et al. 2010) and benzoic acid (Velegraki et al. 2011) as well as other real waste streams like sewage sludge (Bernardi et al. 2010), landfill leachate (Garg and Mishra 2010) and pulping effluent (Garg et al. 2007). Kulkarni and Dixit (1991) carried out a batch CWO study on phenol (concentration = 100–500 mg/L) at mild operating conditions [Temperature (T) = 80–110 °C and oxygen pressure ( PO2 ) = 0.45 MPa] using oxygen and sulfite as auxiliary oxidant in the presence of copper sulfate catalyst. The complete phenol degradation was achieved only after 20 min reaction at the highest operating conditions. In the study, the reaction pathway was proposed and the authors also characterized the solid precipitate formed during the reaction. Wu et al. (2003) carried out homo-catalytic phenol oxidation (phenol concentration = 1 g/L; catalyst (copper nitrate) concentration = 13.2 mg/L as Cu2?) at low temperature (T = 40–60 °C) and moderate total pressure (1.2–2.3 MPa). Up to 98 %, phenol removal was obtained at a reaction temperature of 60 °C and 0.16 MPa oxygen pressure. Garg et al. (2010) studied CuSO4 catalyzed phenol oxidation at 90 and 160 °C temperatures with air as oxidant. At elevated conditions (T = 160 °C and total pressure = 0.8 MPa), around 82 % phenol and 54 % COD removal could be achieved within 3 h. In another study, Arena et al. (2010) performed CWO on phenolic wastewater (phenol concentration = 1,000 mg/L) using homogeneous sulfate salts of Fe3?, Mn2? and Cu2? at moderate operating conditions (T = 150 °C, PO2 = 0.9 MPa). The complete phenol degradation and 50–70 % total organic carbon (TOC) reduction were reported after 6-h reaction. In a recent catalytic phenol degradation study at mild operating conditions (T = 90–150 °C, total pressures = 0.5 MPa), more than 90 % phenol, COD and TOC reduction was reported (Garg and Mishra 2013).
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From the literature, it was realized that there is meagre information available on the oxidation reaction pathway and sludge formation. The studies reporting phenol degradation routes were generally carried out at moderate-tohigh temperature conditions with heterogeneous catalysts (Santos et al. 2002). The sludge formation during homocatalytic reaction is reported, but the detailed information is lacking. Therefore, the present study was planned to carry out CWO of synthetic phenolic wastewater with homogenous copper catalysts (as copper nitrate and copper sulfate) at mild operating conditions. The results were obtained to elucidate the reaction pathway of phenol degradation by identifying various intermediates and products. Besides, the solid residue formed during the reaction was also quantified and characterized. Materials and methods Chemicals The analytical grade Cu(NO3)23H2O, CuSO45H2O and phenol (with purity [99 %) were purchased from Merck Chemicals Private Limited, Mumbai, India. Among other chemicals, hydroquinone, catechol, p-benzoquinone and organic acids (98–100 % pure) were purchased from Amit Enterprises and Arihant Enterprises Mumbai, India. An oxygen gas cylinder was purchased from Alchemie Gases and Chemical Private Limited, Mumbai, India. Characteristics of synthetic wastewater The synthetic wastewater containing the varying concentrations of phenol (=1, 3, 5 and 10 g/L) was prepared by dissolving the predetermined amount of phenol in tap water. The key characteristics of the synthetic wastewater samples were as follows: pH = 5.5–6.0, COD = 2,240–24,000 mg/L and TOC = 765–7,650 mg/L. WO/CWO experimental setup and procedure WO/CWO experiments were performed in a 0.7-L capacity high pressure batch reactor (made of stainless steel 316) which was supplied by Amar Equipments Private Limited, Mumbai, India. The reactor had heating jacket around the cylindrical portion. A 4 bladed stirring rod was provided in the system to agitate the mixture. The reactor was equipped with the ports for air/oxygen supply and periodic liquid sampling. The temperature of reactor vessel was regulated using the proportional integral derivative (PID) temperature controller. A digital display unit showing pressure, temperature and agitation speed was also attached with the
Catalytic wet oxidation of phenol under mild operating conditions
reactor assembly. The schematic diagram of the experimental setup is provided in supporting information (Fig. S1). To perform a typical WO/CWO run, 250 mL of the synthetic wastewater was added to the reactor with or without catalyst and heated to the desired temperature. Once the reactor contents attained the predetermined temperature, a sample was withdrawn and oxygen was purged into the reactor. The stirrer speed was adjusted to 1,000 rpm throughout the study. This speed has been found sufficient to eliminate the mass transfer limitations (Garg and Mishra 2010). The treated wastewater samples were analyzed for pH, phenol concentration, TOC and COD. Besides, HPLC and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) were also carried out for the selected wastewater samples. Phenol and COD of the samples were analyzed in triplicate, and the average values are reported. Apart from this, the waste sludge obtained after the reaction was quantified and characterized to determine the organic fraction and presence of possible functional groups.
other possible intermediates), an individual compound in known concentration was injected. To quantify the solid residue, the known volume of treated wastewater was filtered through 0.2 lm filter paper. The residue retained at the filter paper was oven-dried at 100–105 °C temperature for 24 h. Organic and inorganic fractions in the sample were distinguished by heating the known mass of oven-dried solid mass in a muffle furnace at 550 °C temperature for 2 h. The mass retained in the crucible is reported as inorganic fraction, while the balance was accounted as organic portion. Fourier transform infrared (FTIR) spectroscopy (3000 Hyperion Microscope with Vertex 80 FTIR System, Bruker, Germany) was employed to identify the functional groups in the dried solid mass. The total copper concentration in treated wastewater was measured with ICP-AES (HORIBA JobinYvonUltima 2000, France).
Results and discussion Efficacy of copper nitrate as catalyst for CWO of phenolic wastewater
Analytical methods To determine phenol concentration in the liquid samples, the standard colorimetric method using 4-aminoantipyrine (4-AAP) was employed (APHA 1998). The developed color was measured at 510 nm wavelength with a UV– visible spectrophotometer (Thermoelectron Corporation, GENESYS 20, USA). To read the phenol concentration in an unknown sample, a calibration curve was plotted using the average absorbance of three independent sets of known phenol concentrations (1, 2, 3, 4 and 5 mg/L). TOC of the liquid samples was measured using a TOC analyzer (Shimadzu, TOC-VCSH, Japan), whereas COD was determined with a COD reactor (Hach, DRB200 COD reactor, USA) according to the standard closed reflux method (APHA 1998). The intermediates formed during phenol oxidation were analyzed by HPLC (Pump: Model PU-2089i, Intelligent HPLC Pump, Jasco, PDA Detector: MD-2,010/2015, Intelligent UV/VIS Multi wavelength Detector, JASCO, set at 210 nm). A polar embedded C18 column (SynergiTM Fusion RP: 250 mm 9 4.6 mm, particle size = 4 lm, pore ˚ ), purchased from Phenomenex, India, was size = 80 A used to identify the compounds. Phosphate buffer and acetonitrile (ratio = 9:1 v/v) were the mobile phases. The treated wastewater was filtered by 0.2 lm nylon filter after appropriate dilutions, and 20 lm of the diluted sample was injected in the column through the injection port. To find the retention time of various compounds (like phenol and
The performance of copper nitrate catalyzed CWO was studied under varying reaction conditions. The effect of major parameters (such as substrate and catalyst concentration, temperature and pressure) was investigated on the CWO efficacy. Effect of catalyst dose and substrate concentration on CWO process The removal of phenol (initial phenol concentration (Phi) = 1–10 g/L) and TOC was observed in the presence of varying doses of copper nitrate catalyst (=1–5 g/L; corresponding Cu2? concentration = 0.26–1.32 g/L) at 120 °C temperature and 0.5 MPa oxygen pressures. The phenol and TOC degradation patterns are presented in Figs. 1 and 2. The significant phenol degradation was achieved during the catalytic runs (*60–96 % in 3 h duration) though it was only 20–40 % in the absence of catalyst (Fig. 1). The percent phenol reduction was increased with decrease in the initial phenol concentration in the synthetic wastewater. Similar trends could also be observed from the TOC curves (Fig. 2). TOC reduction during non-catalytic runs was very low (only *7–12 %) indicating the phenol conversion into other intermediate organic compounds having almost same number carbon atoms. In contrary, the catalytic oxidation demonstrated a TOC reduction around 55–90 %. No induction period (i.e., the time during which almost no phenol degradation takes place after introducing oxygen in
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K. Lal, A. Garg Fig. 1 Effect of Cu(NO3)23H2O catalyst concentration on phenol removal with reaction time for initial phenol concentration (Phi)= a 1 g/L, b 3 g/L, c 5 g/L, and d 10 g/L (T = 120 °C and PO2 = 0.5 MPa)
the reactor) was observed in either of the reactions (catalytic or non-catalytic) though TOC removal in the first 90 min was insignificant during non-catalytic reaction. For the catalytic WO runs with lower strength wastewaters (Phi = 1 and 3 g/L), most of the phenol and TOC removal was observed in first 90 min. Though for the wastewater with initial phenol concentration of 5 g/L, the degradation of phenol and TOC was continued for longer time at the lower catalyst doses (Cu2? = 0.26 and 0.79 g/L) (Figs. 1c, 2c). From the phenol and TOC degradation patterns for the lowest strength wastewater, it can be observed that the catalyst loading had no substantial impact on phenol degradation though a little difference in TOC removals could be observed after 180 min reaction. Hence, a Cu2? dose of 1 g/L was selected for the synthetic wastewater (Phi = 1 g/L) in all subsequent catalytic runs. The duration of all the runs was kept the same (i.e., 3 h) since a little TOC degradation at a very slow rate (though insignificant) was observed. Besides, the longer reaction times may be needed for less severe oxidation conditions (such as T \ 120 °C and oxygen pressures \0.5 MPa) which were used in some of the subsequent runs.
123
Effect of temperature The CWO performance for phenolic wastewater (Phi = 1 g/L) was studied in the temperature range of 90–150 °C, while the initial oxygen pressure and Cu2? (as copper nitrate) were 0.5 MPa and 1 g/L, respectively. As expected, the higher phenol, TOC and COD removals were observed with increase in temperature (Fig. 3a). At a reaction temperature of 150 °C, TOC of the wastewater was reduced to 80 mg/L (*90 %), while the final TOC was 372 and 108 mg/L after the reaction at 90 and 120 °C temperatures, respectively, after 3 h of reaction. Phenol degradation at 90, 120 and 150 °C temperatures was *60, 92 and 96 %, respectively. A significant enhancement in phenol removal was observed when the reaction temperature was raised from 90 to 120 °C. Further increase in temperature to 150 °C, only a slight improvement in the performance of catalytic process could be found probably due to the formation of carboxylic acids resulting from phenol decomposition. Hence, the effect of oxygen partial pressure was studied at a reaction temperature of 120 °C.
Catalytic wet oxidation of phenol under mild operating conditions Fig. 2 Effect of Cu(NO3)23H2O catalyst concentration on TOC removal with reaction time for Phi= a 1 g/L, b 3 g/L, c 5 g/L, d 10 g/L (Temperature = 120 °C and PO2 = 0.5 MPa)
Effect of oxygen partial pressure The phenol degradation was also investigated at varying oxygen pressures (0.2, 0.5 and 0.8 MPa), while the temperature and Cu2? concentration were kept the same (i.e., 120 °C and 1 g/L, respectively) in all these runs. The total pressure in the reactor was the sum of vapor pressure of wastewater at 120 °C temperature (=0.2 MPa for phenolic wastewater) and oxygen pressure. Hence, the total pressure was 0.2, 0.7 and 1.0 MPa, respectively, at different oxygen levels. The corresponding theoretical oxygen levels in the reactor were 2.06, 5.15 and 8.25 g/L, respectively, which indicate the availability of sufficient oxygen in the reactor for the reactions performed at oxygen partial pressures of 0.5 and 0.8 MPa. The results were compared with nonoxidative reaction conducted under similar temperature and pressure conditions. An increase in reaction pressure beyond 0.2 MPa barely caused any considerable enhancement in phenol, COD and TOC reductions (Fig. 3b). However, the addition of external oxygen at 0.2 MPa pressure exhibited the significant improvement in the CWO performance (phenol reduction [90 %, TOC removal =*87 % and COD removal =*90 %). In the absence of
oxygen, only 60 % phenol was decomposed, while TOC and COD reductions were also quite low (only *20–25 %). Determination of the rate constants The reaction rate constants for the non-catalytic and copper nitrate catalyzed oxidation reactions were determined by assuming pseudo–first-order reaction model with respect to phenol and TOC. The first-order reaction can be expressed as: dC=dt ¼ kC
ð1Þ
where C is the phenol concentration or residual TOC after time t and k is first-order reaction constant. By integrating the above equation and applying the initial conditions (i.e., at t = 0, C = C0), the relation shown as Eq. 2 was obtained which was used to calculate rate constant. lnðC=C0 Þ ¼ kt
ð2Þ
The phenol or TOC removal data till 120 min of reaction was used for the determination of rate constant since the degradation rate was very low in the last 60 min
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K. Lal, A. Garg
(Figs. 1, 2). For CWO runs, the phenol and TOC data fit was better in the first-order reaction compared to non-catalytic WO. It was found that the rate constant with respect to phenol and TOC was decreased with increase in phenol concentration (except for the wastewater with initial phenol concentration = 5 g/L) (Table 1). For catalytic reaction, the rate constants with respect to phenol were 1.36–2.05 times to that obtained for TOC degradation. The phenol and TOC degradation rates were found to increase with catalyst concentration. The maximum degradation rates for both the parameters were 0.0044 and 0.0028 min–1, respectively, when the wastewater (Phi = 1 g/L) was treated with 10 g/L of copper nitrate catalyst (Cu2? concentration = 1.32 g/L). In our previous study (Garg and Mishra 2013), the rate constant with respect to TOC for the phenol degradation (Phi = 1 g/L) carried out at the same temperature with a CuSO4 catalyst concentration of 3 g/L was 0.013 min–1 which is almost 5.5 times the k value obtained in the present work (=0.00234 min–1) with the same catalyst concentration. In the past study, the rate constant was determined for the fast reaction step only for first 120 min, while in this study, the rate constant was determined based on the overall TOC reduction in 3 h. Fig. 3 Effect of a temperature and b pressure on the CWO of phenolic wastewater (Phi = 1 g/L, Cu2? dose = 1 g/L, Reaction time = 3 h)
Table 1 First-order reaction kinetic constants for various non-catalytic and catalytic WO runs (catalyst = Cu(NO3)23H2O, T = 120 °C, PO2 = 0.5 MPa) Phi (g/L)
1
3
5
10
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[Cu2?] (g/L)
First-order rate constant (10-3 min-1) with respect to Phenol
R2
TOC
R2
0
0.43
0.958
0.004
0.809
0.26
3.95
0.99
2.26
0.99
0.79
4.13
0.994
2.34
0.984
1.32
4.43
0.996
2.82
0.988
0
0.43
0.975
0.004
0.968
0.26
2.23
0.994
1.17
0.99
0.79 1.32
2.61 2.91
0.996 0.984
1.56 1.78
0.968 0.97
0
0.09
0.884
0.003
0.809
0.26
2.91
0.973
1.43
0.935
0.79
3.34
0.983
1.78
0.952
1.32
3.47
0.946
1.68
0.951
0
0.04
0.835
0.004
0.853
0.26
1.13
0.967
0.78
0.983
0.79
1.87
0.986
1.35
0.992
1.32
2.65
0.977
1.95
0.977
Comparison of the catalytic activities of two homogeneous copper salts (sulfate and nitrate) The catalytic activities of two copper salts, namely CuSO45H2O and Cu(NO3)23H2O, were compared for synthetic phenolic wastewater (initial phenol concentration = 1 g/L). The reaction temperature and oxygen partial pressure were maintained at 120 °C and 0.5 MPa, respectively. Cu2? concentration of 1 g/L was added to the reactor. One run with each catalyst was also performed at similar conditions without adding any oxygen (i.e., PO2 = 0). The addition of any catalyst also enhanced the reductions in phenol, TOC and COD from the wastewater even in the absence of oxygen. It can be seen from Fig. 4a that phenol was degraded up to 55–60 % in the presence of catalyst though TOC and COD were removed by *25–30 %. For non-catalytic run, phenol degradation was around 49 %, but TOC and COD removals were only *10 %. These results indicate the occurrence of other reactions like hydrolysis, oxidation (with oxygen in reactor head space), polymerization and carboxylation. By introducing oxygen ( PO2 = 0.5 MPa), the overall phenol removal was *95.4 %, whereas COD and TOC reductions were around 90 % with both the catalysts (Fig. 4b). pH of the CuSO45H2O and Cu(NO3)23H2O treated wastewater (PO2 = 0) was reduced to 2.60 and 2.46, respectively, after 3-h reaction. Further drop in pH to 2.16 and 2.14, respectively, was observed after the reaction performed with
Catalytic wet oxidation of phenol under mild operating conditions
which indicates the presence of substantial carboxylic acid concentration in the solution. For non-catalytic run, the AOSC was slightly increased from -0.75 (initial) to -0.50 which is an indication of the phenol degradation in its immediate derivatives. The lower reductions in TOC and COD during non-catalytic WO also confirm the presence of such compounds in the treated wastewater. HPLC analysis for CuSO4 catalyzed treated wastewater
Fig. 4 Comparison of removal efficiency of homogeneous copper salts (nitrate and sulfate) for WO/CWO conducted at a in the absence of oxygen and b PO2 = 0.5 MPa (Phi = 1 g/L, Cu2? concentration = 1 g/L, T = 120 °C, reaction time = 3 h)
0.5 MPa oxygen partial pressures. It can be suggested that both the copper salts were equally effective for phenol degradation under the present reaction conditions. Identification and quantification of reaction intermediates Determination of the average oxidation state of carbon (AOSC) in treated wastewater AOSC gives an initial notion of the probable compounds present in a wastewater sample, and it ranges from -4 to ?4. The relationship of AOSC and organic matter present in a wastewater sample is represented as follows (Vogel et al. 2000): COD AOSC ¼ 4 1 0:375 ð3Þ TOC The AOSC of the immediate intermediates formed during phenol oxidation (i.e., catechol and hydroquinone) is below zero (-0.67 to -0.33), whereas low molecular weight carboxylic acids have positive value (from 0 to ?3). After CWO process (in the presence of both the copper salts), the AOSC in the treated wastewater was close to ?1
The change in concentration of phenol degradation products (during the CWO process using CuSO4 catalyst) with time is shown in Fig. 5a. The physical color change during the process is illustrated in Fig. 5b. Phenol was almost completely degraded within first 30 min. Hydroquinone, pbenzoquinone and short-chain carboxylic acids (maleic, fumaric, oxalic, formic and malonic) were detected as the main intermediates in the starting phase and reached to the maximum concentration after *15 min. A large fraction of these compounds was degraded in 60 min. Catechol was detected in very small concentration compared to the above compounds though acetic acid formation was observed only after 30 min reaction and achieved the peak concentration of *100 mg/L within next 30 min. Acetic acid could be reduced by around 50 % in next 2-h reaction due to its persistent nature in the temperature and pressure conditions adopted for the present study. The theoretical TOC with time was calculated using the mass concentrations of the detected compounds which were compared with the corresponding experimental values (Table 2). The error between experimental and theoretical TOC during the reaction was found to be between 0.22 and 13 %. The slightly lower theoretical TOC values suggest the possibility of some other unidentified compounds in the wastewater. The continuous decrease in pH with reaction time can also be used as an indicator of the formation of acidic species during the reaction. However, the reduction in pH was insignificant after 60 min reaction since most of the higher molecular weight intermediates were converted into low molecular weight acids (particularly in acetic acid) which are not easily degradable under these conditions. Quantification and characterization of the sludge The sludge formed during the catalytic reactions (with CuSO4 and Cu(NO3)2) was quantified and analyzed to determine the contribution of organic fraction. Apart from this, FTIR analysis was performed to obtain information on the functional groups present in the sludge. The sludge produced during copper nitrate catalyzed reaction was lesser in mass (Fig. 6). The contribution of the organic and inorganic compounds (containing copper) was only around
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K. Lal, A. Garg Fig. 5 a Evolution of reaction intermediates during CWO of phenol using CuSO4 as catalyst and b image of samples withdrawn at different time intervals catalyst (Phi = 1 g/L, Cu2? concentration = 1 g/L, T = 120 °C, PO2 = 0.5 MPa)
20 and 80 % by weight. From the treated water analysis by ICP-AES, it was found that approximately 60 % of the total copper was dissolved in the wastewater solution. Generally, the major fraction of copper will remain in the solution as cupric ion (Cu2?) under acidic conditions (pH \ 5) (Castro et al. 2013). However, the presence of other compounds and reaction conditions may affect the solubility of a metal in the aqueous solution. To see the effect of pH on copper solubility, copper sulfate was dissolved in distilled water and the pH was adjusted to 3.0 (similar to the reaction pH) and 8.5–9.0. No copper precipitate could be observed at acidic pH though copper precipitation was noticed at alkaline pH. These observations suggest the polymerized product of phenol forming at elevated temperature and pressure conditions during CWO process can eliminate copper from aqueous solution by virtue of adsorption. The hydrolysis reactions may be expected to produce small inorganic sludge. FTIR pattern of the solid residue revealed the presence of organics with C=C, O–H and C–H bond vibrations (Fig. 7). The organic part of solids residue seems to contain
123
the substantial amount of aromatic alcohol or phenolic compounds due to the existence of a strong peak of OH bond at wave number of 3,445–3,470 cm–1. Proposed reaction pathway Based on the concentration profiles of the detected intermediates, color evolution and FTIR of solid residue, a reaction pathway was developed (Fig. 8). Though phenol was completely decomposed in 30 min (Fig. 5a), the emergence of hydroquinone and catechol showed the occurrence of parallel reaction pathways. The presence of p-benzoquinone (ascertained by the intense yellow color of the samples collected between 15 and 30 min reaction shown in Fig. 5b) may be attributed to the degradation of hydroquinone via p-benzoquinone formation. Though, obenzoquinone could not be detected in the reaction mixture, but it has been reported that the oxidation of catechol might occur via o-benzoquinone (Kulkarni and Dixit 1991; Devlin and Harris 1984; Collado et al. 2010). Due to the two adjacent C=O groups, o-benzoquinone is unstable and
Catalytic wet oxidation of phenol under mild operating conditions Table 2 Comparison of theoretical and measured TOC with time for a CWO run conducted using copper sulfate catalyst (Phi = 1 g/L, Cu2? concentration = 1 g/L, T = 120 °C, PO2 = 0.5 MPa) Time (min)
TOC (mg/L) 0
15
30
60
120
180
Phenol
765.0
59.2
6.54
–
–
–
Hydroquinone
0
24.8
6.52
–
–
–
Catechol
0
6.63
6.36
–
–
–
p-Benzoquinone
0
42.8
10.2
–
–
–
Oxalic acid
0
38.9
24.3
7.52
9.43
8.56
Formic acid
0
76.4
27.9
5.73
6.64
6.29
Malonic acid
0
112.5
24.1
20.9
22.7
18.5
Acetic acid
0
0
25.2
39.1
26.5
29.1
Maleic acid
0
21.4
11.6
2.72
2.68
0.63
Fumaric acid Th. TOC (mg/L)a
0 765.0
26.6 409.2
18.3 161.0
3.26 79.2
5.6 73.6
9.22 72.3
Exp. TOC (mg/L)b
768.2
415.1
168.2
90.8
84.6
80.2
% Error
0.42
1.42
4.3
12.8
13
9.8
a
Theoretical TOC
b
Experimental TOC
concentrations of maleic and fumaric acids were also observed which could further be oxidized into malonic acid. A significant concentration of oxalic acid was found in the reaction mixture after 3 h run. Eventually, formic acid is decarboxylated by hydroxyl radical to form end product (i.e., H2O and CO2) (Wine et al. 1985; Yu and Savage 1998). Kulkarni and Dixit (1991) have also suggested the formation of two additional compounds 2, 5-dioxo-3-hexenedioic acid and muconic acid which could not be observed in the present study. The formation and detection of various species also depends upon the initial concentration of the substrate and reaction conditions. The proposed pathway is quite similar to the reported ones for heterogeneous catalytic reaction (Santos et al. 2002; Quintanilla et al. 2006). Quintanilla et al. (2006) observed the presence of p-hydroxybenzoic acid which may have resulted due to the transfer of an extra carbon present in activated carbon. The compound was not detected in our study. ˙ ), super oxide Various species like oxygen radical (O radical anion (O˙2), hydroperoxy radical (HO˙2), hydroxyl ˙ ), hydrogen peroxide (H2O2) and ozone (O3) radical (OH may form during the oxidation process depending upon the conditions (Devlin and Harris 1984; Rivas et al. 1998). Hydroxyl radical can react with the organics present in a solution through three different mechanisms: (a) hydroxyl addition, (b) hydrogen abstraction and (c) electron transfer (Oppenla¨nder 2003). In the presence of a transition metal (like Cu2? and 2? Fe ), CWO reaction can follow auto-oxidation mechanism (Atkinson 1986; Larson and Weber 1994). The simplified phenol auto-oxidation mechanism is presented as follows: Cu2þ
C6 H5 OH ! C6 H5 O þ H Fig. 6 Estimation of the solid residue formed during CWO reaction (Phi = 1 g/L, Cu2? concentration = 1 g/L, T = 120 °C, PO2 = 0.5 MPa)
C6 H5 O þ O2 ! C6 H5 OOO
C6 H5 OOO þ H2 O ! C6 H5 OOOH þ HO C6 H5 OOOH ! C6 H5 O þ HO2
could have been converted into p-benzoquinone. Except acetic acid, all the organic acids (i.e., oxalic, formic, malonic, maleic and fumaric) were observed in the sample withdrawn after 15 min which indicates the cleavage of benzene ring. Acetic acid was first appeared in the sample collected after 30 min and reached to the maximum concentration within next 30 min. Among other acids, formic and malonic acids were achieved the peak concentrations within 15 min after the start of the reaction. The decarboxylation of malonic acid produces acetic acid (Wine et al. 1985; Atkinson 1986; Morrison et al. 2011) and the other major acidic compound (i.e., oxalic acid) may form formic acid by decarboxylation. Relatively smaller
ð4Þ
C6 H5 O þ C6 H5 O ! C6 H5 OOC6 H5
ð5Þ
ð6Þ ð7Þ ð8Þ
Hydroxyl radical is neutral electron deficient transient species (Marusawa et al. 2002) and attacks at the highelectron density area available in a molecule. Phenol has high-electron density at ortho- and parapositions due to the resonance effect. The hydroxyl radical attack at these positions leads to the abstraction of hydrogen or addition of oxygen which may lead to the formation of catechol and hydroquinone (Getoff 1996). The formation of p-benzoquinone from hydroquinone can be explained by the stability of free radicals. The attack of hydroxyl radicals on hydroquinone leads to the stability of free radicals (Cary
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K. Lal, A. Garg Fig. 7 FTIR spectra of solid residues formed during CWO of phenol with a copper nitrate and b copper sulfate catalyst
and Sundberg 2007). Hydroxyl radical is highly potent and non-selective. But due to its electrophilic nature, it will attack on electron dense carbon (Marusawa et al. 2002). Out of six carbons in benzoquinone, two (which are linked with OH) have relatively lower electron density due to the electronegativity of oxygen. Oxygen can increase the electron density on other four carbons by resonance, and ˙ on the ring cleavage can be expected upon the attack of OH these carbons. As a result, the two major ring cleavage acidic products (maleic/fumaric acid and oxalic acid) may form (Santos et al. 2002; Quintanilla et al. 2006). The addition of hydroxyl radical to maleic acid leads to the formation malonic acid as a major product. The mechanism of the formation of monocarboxylic acids (acetic and formic) has been explained earlier by the decarboxylation of malonic and oxalic acid in thermal conditions.
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Apart from the above reactions, a solid residue was also formed. The organic fraction may be formed as a result of the combination of phenyl radical (stabilized small life species obtained from phenol) with hydroquinone and pbenzoquinone through a series of chain reactions (Li et al. 2005). The polymerized products observed by FTIR analysis showed the presence of same bonds (C=C (aromatic), C=C (olefinic) and OH (alcohol/phenol) which can be expected after the complexation reactions.
Conclusions From the present study, it can be suggested that CWO can achieve significant phenol degradation in the presence of copper-based homogeneous catalysts. However, the
Catalytic wet oxidation of phenol under mild operating conditions Fig. 8 Proposed reaction pathway of phenol oxidation in the presence of CuSO4 as catalyst (Phi = 1 g/L, Cu2? concentration = 1 g/L, T = 120 °C, PO2 = 0.5 MPa)
reaction conditions were not sufficient for acetic acid degradation which was found in substantial amounts in the treated wastewater. A combination of parallel and consecutive reactions such as hydrogen abstraction, decarboxylation, complexation, etc., contributed to the phenol degradation into various intermediates (phenol derivatives) and end products (organic acids). The absence of the previously reported phenol derivatives like p-hydroxybenzoic acid, 2, 5-dioxo-3-hexenedioic acid and muconic acid was possibly due to the nature and concentration of catalyst, substrate concentration and operating conditions. The presence of acetic acid in the treated wastewater if subjected to the biological treatment can reduce the need for the addition of carbon source which is utilized by microbes. For future work, a process for copper recovery should be developed so that it could be recycled back in the
process. Besides, the economics of homo-catalytic process should be determined. Acknowledgments The authors are thankful to the Sophisticated Analytical Instruments Facility (SAIF), IIT Bombay, for analyzing the wastewater samples.
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