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Wet air oxidation of resorcinol as a model treatment for refractory organics in wastewaters from the wood processing industry
Journal of Environmental Management 161 (2015) 137e143
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Research article
Wet air oxidation of resorcinol as a model treatment for refractory organics in wastewaters from the wood processing industry Bernd Weber a, b, *, Alma Chavez b, Julio Morales-Mejia b, Sabrina Eichenauer c, Ernst A. Stadlbauer c, Rafael Almanza b noma del Estado de M Universidad Auto exicoeFacultad de Ingeniería, Cerro de Coatepec s/n Col. San Buenaventura, C.P. 50130 Toluca, Estado de M exico, M exico b n, M noma de M Instituto de Ingeniería de la Universidad Nacional Auto exico (UNAM), C.P. 04510 Coyoaca exico, D.F., M exico c Competence Center for Energy and Environmental Engineering, University of Applied Sciences THM, Campus Giessen, 35390 Giessen, Germany a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2015 Received in revised form 20 June 2015 Accepted 23 June 2015 Available online xxx
Wastewater treatment systems are important tools to enhance sustainability in terms of reducing environmental impact and complying with sanitary requirements. This work addresses the wet air oxidation (WAO) process for pre-treatment of phenolic wastewater effluents. The aim was to increase biodegradability prior to a subsequent anaerobic stage. In WAO laboratory experiments using a microautoclave, the model compound resorcinol was degraded under different oxygen availability regims within the temperature range 150 Ce270 C. The activation energy was determined to be 51.5 kJ/mol. Analysis of the products revealed that after 3 h of reaction at 230 C, 97.5% degradation of resorcinol was achieved. At 250 C and the same reaction time complete removal of resorcinol was observed. In this case the total organic carbon content was reduced down to 29%, from 118.0 mg/L down to 34.4 mg/L. Under these process conditions, the pollutant was only partially mineralized and the ratio of the biological oxygen demand relative to the chemical oxygen demand, which is 0.07 for resorcinol, was increased to a value exceeding 0.5. The main by-product acetic acid, which is a preferred compound for methanogenic bacteria, was found to account for 33% of the total organic carbon. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Industrial Wastewater Pretreatment Oxidation process Recalcitrant
1. Introduction Treatment of wastewater and potable water has become essential in modern societies. In practice, environmental compatibility and financial feasibility determine the methodology adopted for domestic and industrial water quality management (Balkema et al., 2001). Currently, efforts are being increasingly devoted to improving the energy efficiency of the processes employed. As a rule of thumb, an average input of 1.4 kWh to 3.2 kWh of electrical energy is necessary to remove 1 kg of biological oxygen demand (BOD), as reported in German wastewater treatment plants (Demoulin and Mervyn, 2003; Owen, 1982). Studies have indicated that the potential exists to decrease this energy input up to 50%
List of non standard abbreviations: DO, dissolved oxygen; HTC, hydrothermal carbonization; LTC, low temperature conversion; WAO, wet air oxidation. noma del * Corresponding author. Facultad de Ingeniería, Universidad Auto xico, Cerro de Coatepec s/n, C.P. 50130 Toluca, Edo. Mex., Mexico. Estado de Me E-mail address: [email protected] (B. Weber). http://dx.doi.org/10.1016/j.jenvman.2015.06.046 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
(Haberkern et al., 2008; Olsson, 2012). One strategy is to rely upon scaled-up energy savings. However, small on-site treatment plants have a higher recycling potential (Bieker et al., 2010). Another strategy favours anaerobic treatment instead of energy-intensive aerobic processes (Diamantis et al., 2011; Descoins et al., 2012). For example treating industrial effluents anaerobically allows a saving of around 11.6 GJ in the form of biogas produced per Mg of COD degraded. Additionally it confers a lower electricity consumption of the order 1100 kWh since it removes the need for aeration (Speece, 1983; Svardal and Kroiss, 2011). Despite the progress that has been made in anaerobic digestion, a variety of recalcitrant compounds are still observed under anaerobic conditions (Sierra-Alvarez and Lettinga, 1991; Schonberg et al., 1997; Jonstrup et al., 2011). Anaerobically non-degradable pollutants, even with co-substrates, include o-cresol, benzene, 4-chlorophenol, alkylbenzenesulphonate, di-isopropyl ether (Kindzierski, 1991; Fedorak and Hrudey, 1984; Bali and Sengül, 2003; Johnston et al., 1996). Degradation of such persistent substances can be enhanced by oxidative pre-treatment, giving rise to further
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applications of efficient anaerobic technologies. Various technical processes have been applied: ozone oxidation, fenton oxidation, photo-fenton oxidation, photolysis/photocatalysis and wet air oxidation (WAO) (Suty et al., 2004; Vogelpohl, 2007; Gunten, 2007). The WAO process has a market share of 3e17% (Mantzavinos and Psillakis, 2004), since it is already a common practice in the pulp and paper industry (Maugans and Ellis, 2002; Luck, 1999), in the pre-treatment of landfill leachate and in the treatment of pharmaceutical wastewaters (Mishra et al., 1995). Process performance may be significantly enhanced when homogenous or heterogeneous catalysts are applied (Luck, 1999; Garg et al., 2010). An organic matter content above 10 g/L is characteristic for the application of WAO technology (Debellefontaine et al., 1996). However when these advanced aerobic treatment processes are used a variety of by-products are formed. The composition pattern of these by-products varies with temperature. Limiting the presence of oxygen favours the production of acetic acid (Li et al., 1991). This resembles the first three steps of classic anaerobic digestion. The subsequent biological anaerobic treatment produces biogas that is commonly used for the onsite generation of electricity with the possibility of waste heat recovery in the WAO process. As a consequence this kind of process combination does not need concentrations of organic matter over 4% to run the WAO process thermally autogenously (Baillod et al., 1985). The emerging practice of sequential exposure to wet air oxidation followed by anaerobic treatment is being exploited for the enhanced transformation of recalcitrant organics (Mantzavinos, 1999). Residual byproducts have the potential to increase biogas yield in subsequent anaerobic digestion and show an improved biodegradability (Kacar et al., 2003; Kang et al., 2011). One review in the literature reports the presence of phenolic substances in wastewaters from refineries, textile and resin industries and coal conversion plants (Veeresh et al., 2005). In addition, the emerging technologies of hydrothermal carbonisation (HTC), low-temperature conversion (LTC) and torrefaction of biomass (Stengl et al., 2012) produce complex wastewaters with inhibitory components in anaerobic consortia originating from the thermal degradation of lignocellulosic materials (Speece, 1999). Resorcinol is an aromatic compound that may be obtained either from nature or synthetically. It is a significant by-product in the manufacture of tires, diazo dyes and pharmaceuticals (Tschech and Schink, 1985). For example, in wastewater from a petroleum refinery Tyiagi et al. detected resorcinol as one of the main constituents (Tyagi et al., 1993). One study classified resorcinol as very toxic after conducting a number of microbiotests (Kahru et al., 2000). Thus, these kinds of toxic effluents demand special attention. As a model component resorcinol has been shown to be slowly oxidized by HClO and NH2Cl (Cimetiere et al., 2009) and by periodate (Feifer et al., 1959). Other investigations report that resorcinol has been efficiently removed from water by electrochemical oxidation with boron-doped diamond electrodes requiring consumption of less than 30 Ah/dm3 (Nasr et al., 2005); by ozone oxidation at 871 Pa partial pressure of ozone, 20 C, neutral pH and high levels of turbulence within the reactor (Sotelo et al., 1990); by adsorption/oxidation with MnO2 and KMnO4 (Zhao et al., 2012); and by sono-photo-catalysis with H2O2 and TiO2 at 0.75 g/L (Silva et al., 2007). In the present study, the process used was WAO, which has the advantage of using inexpensive air as oxidant and thermal energy that can provided by subsequent processing steps. In laboratory-scale WAO experiments the degradation of initial resorcinol concentrations of 25 mg/L and 180 mg/L were investigated with respect to reaction time, temperature and the presence of dissolved oxygen. The formation of by-products is evaluated by UV/Vis analysis. Acetic acid concentrations were determined using an enzymatic method. Changes in biodegradability and the
remaining potential for anaerobic post-treatment were evaluated by TOC, COD and BOD analysis.
2. Material and methods 2.1. WAO treatment The WAO treatment of resorcinol on the laboratory scale was performed in a tube reactor as shown in Fig. 1. The tube reactor consists of a laboratory grade stainless steel micro-autoclave equipped with a sodium calcium glass reaction tube with a volume of total 12 mL. The reaction tube was pre-treated with 2 mol/L hydrochloric acid solution overnight and washed with distilled water before use. Resorcinol (CAS-No. 108-4-63) was supplied by Sigma Aldrich. A stock solution with a concentration of 180 mg/L was prepared with distilled water and stored at 2 C. The lower concentrations needed for the experiments were obtained by dilution. 7.5 mL of this aqueous solution of resorcinol of a known concentration was added to the reaction tube. The dissolved oxygen concentration in the sample was adjusted in three different ways: a) Refrigeration overnight resulted in a dissolved oxygen concentration of approximately 7 mg/L due to the water solubility equilibrium. b) Bubbling pure oxygen into the bottom of the reaction tube for 5 min resulted in a dissolved oxygen concentration of 35 mg/L. After 5 min, the cap of the autoclave was closed immediately to maintain the oxygen concentration. In this case oxygen was also present in the gas phase. c) Bubbling of subliming CO2 for 5 min at the base of the reaction tube eliminated nearly all of the oxygen from the system (residual oxygen concentration < 1 mg/L). The potential of oxidation capacity, X, referred to as the oxygen regime is expressed by the following equation:
X¼
cO2 cCOD
(1)
Here, cO2 is the concentration of the dissolved oxygen, and cCOD is the theoretical chemical oxygen demand of the sample. Using concentrations of 25 mg/L and 180 mg/L of resorcinol (1 mg resorcinol ¼ 1.89 mg CODtheoretical) in the experiments, ratios of X ¼ 0.75 (25 mg/L resorcinol and oxygen); 0.1 (180 mg/L resorcinol and oxygen); 0.02 (180 mg/L resorcinol and air); and <0.003 (180 mg/L resorcinol and CO2) were investigated. Thus higher concentrations of resorcinol could not be investigated, because X is reduced proportionally. During the thermal treatment, limited oxygen transfer from the gas phase into the liquid is unavoidable. After the autoclave was closed, it was placed into a laboratory
1. Sample Preparation 2. Oxygen Adjustment 3. WAO Treatment 4. Analysis Sample Volume 7.5 mL 25 mg Resorcinol / L or 180 mg Resorcinol / L
< 1mg O2 / L or 7 mg O2 / L or 35 mg O2 / L
T Outside
-
UV/VIS COD, BOD TOC Acetic Acid GC/FID
T Inside
Fig. 1. Sketch of the sample preparation, wet air oxidation and analysis of products.
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furnace (Model: 5194 from Industrias Sola Basic, Lindberg Division, Mexico). As a temperature reference, the signal from a NieNiCr thermocouple situated on the internal side of the autoclave wall was recorded. Because of the thickness of the autoclave wall, the thermocouple exhibited a slow response to the higher temperature in the furnace, and a reaction time correction was established. The optimised heating program was guaranteed to reach 85% of the final temperature within 1 h. The autoclaves remained in the furnace for an additional 3 h. For the kinetic tests, 1, 2, 3, 4, 5 and 6 h of reaction time were chosen. After the reaction time had elapsed, the autoclaves were rapidly cooled in a water basin. After opening the autoclaves, UV/Vis spectra were recorded (Shimadzu UV 1601), and the samples were stored under refrigeration at 2 C for further analysis. 2.2. Analytical methods Dissolved oxygen in the solutions was measured with a membrane electrode connected to a portable instrument (HI 9146 from Hanna) able to analyse concentrations of between 0 and 45 mg O2/ L. The UV/Vis spectrum was generated from 3 mL of undiluted sample in a 1 cm quartz cuvette over the wavelength range of 180e430 nm. Data management was performed with UVPC software, version 3.9. A Shimadzu GC-14A chromatograph equipped with flame ionisation detector (FID) and an H53-Supreme 30 m capillary column (CS-Chromatographie) with a film thickness of 0.25 mm was used for chromatographic analysis. The operating conditions were as follows: carrier gas (nitrogen) (99.998%) at a constant pressure of 10 kPa; 300 C injector temperature; oven temperature of 50 C (5 min) and then increasing at 15 C/min to 320 C (15 min). The sample solution (3 mL) was injected manually. The quantification of the resorcinol concentration was based on linear regression obtained by injecting standard solutions with concentrations ranging from 20 to 200 mg resorcinol/L. The COD was measured with Merck COD-Test solutions No. 1.14539.0495 (range 100e1500 mg/L) and No. 1.14681.0495 (range 4e100 mg/L). A linear calibration correlation of the light absorbance vs. COD was obtained by oxidising potassium terephthalate, as described in ASTM 5520. The TOC was determined using Hach TOC-test kits no. 27603-45 (for concentrations between 0.3 mg/L and 20 mg/L) and 28159-45 (for concentrations from 15 mg/L to 150 mg/L) in combination with a DR 5000 Spectrophotometer from Hach. The BOD was determined in Winkler bottles by directly reading the dissolved oxygen (DO) with the HI 9146 DO meter (described previously) after 5 days according to ASTM Method 5210 (APHA, 2005). Acetic acid was determined by an enzymatic test method from Boehringer Mannheim/R-Biopharm (UV Test Kit No. 10 148 261 035). In this test, the acetate bonds to Acetyl-CoA. Then, Acetyl-CoA reacts with oxalacetate. During the formation of oxalacetate from Lmalate and NADþ, the reduction of NADþ to NADH is quantified at 340 nm using the Shimadzu Photometer. 3. Results 3.1. UV/Vis analysis of resorcinol degradation UV/Vis spectra of both resorcinol and its intermediates that arise from the WAO of synthetic wastewater at different reaction times are shown in Fig. 2. The spectra of untreated resorcinol shows three strong absorption bands in the UV range at 195, 218 (inflection point) and 274 nm. Superposition of the electronic spectra for
Fig. 2. UV/Vis spectral changes for resorcinol during thermal treatment with different reaction times. Initial concentration: 25 mg/L, T ¼ 170 C, X ¼ 0.75.
treatments is evident. Intermediates have strong absorption peaks at 260, 330 and 370 nm. Reduction of the absorbance at 218 nm indicates that degradation of resorcinol has occurred. For longer treatment, absorption rises again and by-products with strong absorbance at 218 nm are also present. The same trend is observed for the absorbance peak at 195 nm (the original main absorbance of resorcinol at 202 nm is shifted to 195 nm), where the lowest value is reported for 1 h of treatment. For the 2 h treatment, absorbance was found to increase and subsequently decrease slowly, as observed in the 4 h treatment, due to subsequent by-product degradation. Thus, the shapes of the spectra are mainly influenced by the by-products. The pattern of by-products varies with time, as evidenced by the sharp decrease at 195 nm after 1 h (absorbance: 1.56) of reaction time. Interestingly the intensity increases again after 2 h (absorbance: 2.52). After longer treatment periods, the absorbance decreases again. The formation of by-products can be evaluated using the absorbance at 250 nm, where resorcinol shows only a low response. The same effect is observed at 195 nm, where the absorbance of the by-products first increases during 1 and 2 h treatment and then decreases after longer treatments. In accordance with DIN 38402 C2 (Spectral Absorption Coefficient), the absorbance was determined at 254 nm. In order to compare experiments with different initial concentrations, the results must be normalized by creating a by-product index (IBy-products) as follows:
IByproducts ¼
AbsorbanceProduct AbsorbanceInitial AbsorbanceInitial
(2)
The graphs of this index (Fig. 3) show that the temperature, where the maximum value of this index is present, depends on the availability of oxygen (expressed as the potential oxygen capacity X). Thus, the availability of oxygen accelerates both the degradation of resorcinol and the degradation of intermediates. When resorcinol is treated at 150 C in the presence of a nearly stoichiometric availability of oxygen, by-products are formed at highest velocity followed by nearly complete degradation at slightly increased temperatures (190 C). This degradation pattern is shifted to higher reaction temperatures when the oxygen availability is lower.
3.2. Resorcinol degradation Resorcinol degradation can be quantitatively monitored by GC/ FID. The GC analysis also revealed that accumulation of ortobenzoquinone and para-benzoquinone does not occur, which is in
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Fig. 3. Effect of temperature on the by-product index (IBy-products) under different oxygen regimens (X ¼ 0.75; X ¼ 0.1; X ¼ 0.02).
activation constant was determined to be E ¼ 51.5 kJ/mol applying a numeric routine for solving the multiple linear regression. The experimental values correlate to the curve “a” with a R2 of 0.996. Taking into account activation energy and k0 ¼ 44.6 mg L1 s1, derived from curve “a”, the curve “b” can be calculated. The experimental values were found to correlate to the curve “b” with a R2 of 0.822. To calculate curves “a” and “b” shown in Fig. 4, the initial oxygen concentration was used. As the reaction proceeds, the oxygen concentration decreases, and the lack of equilibrium between the gas and liquid phases drives an additional oxygen flux governed by molecular diffusion in the experimental setup. A diffusivity of 2.4$109 m2/s (Çengel and Boles, 2011) for the diffusion of oxygen in water at 200 C results in a kla value of 0.38 m/h and a flux rate of 3.5$106 g m2 s1, which can be depreciated in the analysis. However, the experimental findings reported in a previous study (Lee, 2002) showed kla values 25 times higher than the theoretical values of the Stefan's diffusion tube. For this comparison, the temperature correction reported by Debellefontaine was applied (Debellefontaine et al., 1996). The degradation of resorcinol at the moderate temperature of 170 C is presented in Fig. 5, in which the reaction time varies between 1 and 6 h. Under these conditions the reaction rate can be expressed as a function of dissolved oxygen (taking the calculated activation energy into account):
ra ¼ 66:4$Corg $COn 2 ;L for X ¼ 0:1
Fig. 4. Effect of temperature on residual resorcinol concentration quantified by GC/FID after WAO treatment over 3 h, varying temperature between 150 C and 290 C (cre2 2 sorcinol, initial ¼ 180 mg/L). Curves a (R ¼ 0.996) and b (R ¼ 0.73) are based on experimental E ¼ 51.5 kJ/mol.
vre et al., 2011). Fig. 4 agreement with other investigations (Lefe shows the effect of the temperature on resorcinol degradation according to GC analysis between 150 C and 270 C for a reaction time of 3 h. When X ¼ 0.1 (180 mg resorcinol/L and 35 mg O2/L), a considerable amount of resorcinol can be degraded at moderate temperatures (curve a). Nearly complete degradation of resorcinol occurs at temperatures exceeding 230 C. Decreasing the oxygen availability as represented by the curve “b” results in a lower level of resorcinol degradation. The degradation of organic compounds can be described using the extended Arrhenius equation (Kolaczkowski, 1999):
dC m ra ¼ resorcinol ¼ k0 eE=ðRTÞ $Corg $COn 2 ;L dt
(3a)
Degradation starts at a low rate, which reaches its maximum at approximately 3 h of reaction time. After 6 h of treatment at 170 C, only a residual concentration (less than 20%) of the untreated sample remains. The correlation curve assuming a constant reaction rate calculated by activation energy and initial concentrations is also shown in Fig. 5 and has R2 correlation of 0.797. It is worth mentioning that an exponential function starting with the initial resorcinol concentration can express the correlation with a minimum R2 of 0.893. The literature reports that the degradation of resorcinol is initiated by the formation of hydroquinone and its subsequent oxidation to orto- and para-benzoquinone (Devlin and Harris, 1984). Ring opening proceeds with the formation of muconic acid and 2,5-dioxo-3-hexenedioic acid. Due to further oxidation steps organic acids like maleic, fumaric, propionic, acetic and formic acid are produced. Other work indicates that further oxidation steps produce oxalic acid (C2H2O4), which is ultimately oxidized to
(3)
where ra is the reaction rate, k0 is the pre-exponential factor, E is the activation energy, R is the universal gas constant, T is the reaction temperature, Corg is the concentration of the organic compound, and CO2 ;L is the concentration of dissolved oxygen. The subscripts m and n are the reaction orders and were chosen according to those proposed by Mishra et al. (1.0 for m and 0.4 for n) (1995). With the observed resorcinol degradation for X ¼ 0.1 at temperatures between 150 C and 270 C for 3 h reaction time (curve a) the
Fig. 5. Effect of reaction time on residual resorcinol concentration quantified by GC/ FID after WAO treatment at T ¼ 170 C (cresorcinol, initial ¼ 180 mg/L; X ¼ 0.1). Correlation curve based on experimental E and A values / k ¼ 5.34*1005 s1.
B. Weber et al. / Journal of Environmental Management 161 (2015) 137e143
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carbon dioxide (Pan et al., 2010). The oxidation of resorcinol into its by-products requires the following percentages of total chemical oxygen demand: benzoquinone 8%, muconic acid as well as 2,5-dioxo-3-hexenedioic acid 16%, and acetic acid 54%. Thus, only for the treatment X ¼ 0.75 the complete oxidation into the stable acetic acid can be reached. 3.3. Formation of acetic acid The oxidation of resorcinol to the thermodynamically stable acetic acid is shown in Table 1 for different oxygen regimes and temperatures between 150 C and 250 C. The acetic acid concentration in the product rose with increasing temperature for all oxygen regimes. At the highest applied temperature (270 C), the acetic acid concentration was stable or decreased under different oxygen regimes, most likely as a result of an oxidative attack on the stable acetic acid (Imamura, 1999). The maximum carbon yield relative to the initial TOC concentration that is transformed into acetic acid by treatment at 250 C was only 3% for the oxygen regime X ¼ 0.02, while the carbon yields for the treatments with X ¼ 0.75 and X ¼ 0.1 were approximately 15%. The kinetics of the formation of acetic acid for the treatment at 170 C and X ¼ 0.1 is almost linear, indicating a zero-order reaction, which is characteristic of a limiting factor. Resorcinol undergoes approximately 8 oxidation steps in the course of transformation to acetic acid (Devlin and Harris, 1984). Consequently different temperature-dependent mechanisms may occur. Both linear and exponential degradation are observed without a limiting factor (Devlin and Harris, 1984). This linear correlation does not cross the centre point (zero point) due to the higher initial oxygen concentration (Fig. 6). When a longer treatment period is used, the reaction may be inhibited by the limited oxygen transfer from the gas phase into the liquid phase in this experiment. It is notable that for complete oxidation of resorcinol into acetic acid, the minimum oxygen regime is X ¼ 0.54, a condition that is only achieved under the experimental conditions where X ¼ 0.75. 3.4. COD, TOC and BOD removal COD, TOC and BOD are useful parameters for evaluating thermal degradation. Whereas the total organic carbon indicates a direct relation to the mineralization of organic matter, interpreting the COD is more complex. For stable organic compounds, in particular, incomplete oxidation of organic matter is reported (Baker et al., 1999). Metals can also be oxidized by this method, which may explain why the COD values for the treatment with X < 0.003 rise to nearly 15%, as shown in Table 2. For other oxygen regimes in which WAO treatment occurs between 250 C and 270 C, the highest COD removal was achieved (X ¼ 0.75 with a residual COD of 29%; X ¼ 0.1 Table 1 Effect of temperature and X on the formation of acetic acid by WAO treatment for a period of 3 h. Temperature ( C)
X ¼ 0.75a
X ¼ 0.10b
X ¼ 0.02b
X < 0.003b
1.2 1.6 2.8 3.8 6.5 9.0 1.6
e e 1.6 1.2 3.4 2.9 3.0
Acetic acid (mg/L) 150 170 190 210 230 250 270 a b
1.1 1.8 4.3 4.4 6.0 6.5 e
TOCinitial ¼ 16.4 mg/L. TOCinitial ¼ 118 mg/L.
2.9 5.2 8.3 15.3 e 41.4 40.3
Fig. 6. Formation of acetic acid during WAO treatment for a period of 1e6 h at 170 C; X ¼ 0.1; TOCinitial ¼ 180 mg/L.
with a residual COD of 26%; X ¼ 0.02 with a residual COD of 78%). These results correlate with the yields for acetic acid. In addition the TOC values decrease with increasing temperature. The highest removal rates were observed between 250 C and 270 C and are in the range of 21%e73%. The contribution of the acetic acid to the residual TOC is 40% for the treatment with X ¼ 0.75 at 250 C and 54% for the treatment with X ¼ 0.1 at 270 C. Limiting the oxygen in the treatment at 270 C with X ¼ 0.02 leads to an acetic acid contribution of only 5% to the total residual TOC. The biological treatability of the products obtained during the WAO treatment was evaluated according to the biological oxygen demand. The degradation of the original resorcinol is very low, only approximately 7%, which is in agreement with values reported by Lepik and Tenno (2000). Thus, by-products generally show better biodegradability when the aromaticity of the compounds is lost rez-Ojeda (Schwarzenbach et al., 2003; Rubalcaba et al., 2007; Sua et al., 2008). The highest BOD yield was achieved using an oxygen regime of X ¼ 0.1 with a treatment temperature of 150 C. Due to the limited quantity of sample solution for BOD analysis of the treatments with lower initial resorcinol concentrations (the oxygen regime of X ¼ 0.75 in Table 2), the BOD could not be determined. No change in the BOD/COD ratio was observed under an oxygen regime of X < 0.003. The resorcinol removal efficiency is directly dependent on the value of X, but the BOD/COD ratio is not. Thus, for an oxygen regime of X ¼ 0.02 at lower temperatures, the BOD value is comparable to the initial BOD and starts to rise when the treatment temperature exceeds 210 C, reaching a maximum of 169.9 mg BOD/L (COD/BOD ¼ 0.55) at 270 C. Unlike the oxygen regime at X ¼ 0.1, the BOD/COD ratio reached a constant value of 0.3, indicating that an equilibrium was reached of the degradation of resorcinol into by-products with higher biological degradability, parallel to the advanced oxidation of these by-products into carbon dioxide. If the oxygen availability is limited, the BOD/COD ratio can be increased to over 0.5 by applying reaction temperatures above 250 C. 4. Conclusions The obtained experimental data obtained shows that the oxidation of resorcinol proceeds at relatively low temperatures (150 Ce270 C) during WAO treatment. COD, TOC and BOD as well as acetic acid, which is the main final product, proved to be useful markers for the assessment of thermal degradation. Different oxygen regimes lead to different BOD concentrations for by-products. BOD/COD ratios of 0.55 in the products
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Table 2 COD, BOD, TOC and BOD/COD in the products obtained by WAO treatment under different conditions. Temperature
X ¼ 0.75a COD
X ¼ 0.10 TOC
X ¼ 0.02
COD
BOD
TOC
BOD/COD
X < 0.003
COD
BOD
TOC
BOD COD
COD
BOD
TOC
BOD/COD
( C)
(mg/L)
(mg/L)
(mg/L)
(mg/L)
(mg/L)
(e)
(mg/L)
(mg/L)
(mg/L)
(e)
(mg/L)
(mg/L)
(mg/L)
(-)
Initial 150 170 190 210 230 250 270 290
47.0 30.9 22.0 17.4 25.9 11.8 13.9 e e
16.4 e e 9.4 6.8 8.0 6.7 e e
356.2 299.0 289.9 243.0 233.8 121.2 105.0 94.0 e
20.8 100.6 85.1 79.1 75.7 39.6 34.4 30.1 e
118.0 100.6 85.1 79.1 75.7 39.6 34.4 30.1 e
0.06 0.34 0.28 0.33 0.32 0.33 0.33 0.32
356.2 321.9 328.1 280.3 309.1 301.9 295.7 280.3 e
20.8 4.3 76.0 17.9 17.9 165.2 137.3 169.9 e
118.0 112.7 112.7 86.9 100.6 91.2 90.3 86.0 e
0.06 0.01 0.23 0.06 0.06 0.55 0.46 0.61
356.2 364.8 380.8 367.2 377.5 360.5 380.5 428.6 410.6
20.8 4.3 e e 16.9 35.8 8.7 17.3 e
118.0 110.1 e 110.1 83.4 130.7 117.8 166.0 142.8
0.06 0.01 e e 0.04 0.10 0.02 0.04
a
Sample volume too small for BOD concentration measurement.
after WAO treatment under optimum conditions indicate a biological treatability comparable to that of municipal wastewater (Tchobanoglous and Burton, 2003). The production of acetic acid as a stable intermediate in the WAO treatment of resorcinol is an important benefit that should be exploited in a sequential anaerobic biotechnology strategy. These results suggest a useful application of this new methodology to approaching forestry industry wastewaters, even if the wastewater may seem too toxic to be directly anaerobically treated due to large amounts of lignin-derived compounds. Fortunately, to aid the successful scale-up of the WAO process from laboratory to pilot to full scale, pressurised reactors are commercially available in the favourable pressure range of 5e55 bars. Adequate technical precautions must be taken, as volatile degradation products, such as CO, CO2 and H2, may be produced (Stengl et al., 2012). Acknowledgements This work was performed partly during a postdoctoral research fellowship that was funded by the Institute of Science and Technology of Mexico City (ICyTDF) and by the Universidad Nacional noma de Me xico through PAPIIT projects IN107913, and Auto IN102708. References American Public Health Association (APHA), 2005. Water Works Association and Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, twenty-first ed. American Public Health Association, Washington D.C. Baillod, C.R., Lamparter, R.A., Barna, B.A., 1985. Wet air oxidation for industrial waste treatment. Chem. Eng. Prog. 81, 52e56. Baker, J.R., Milke, M.W., Mihelcic, J.R., 1999. Relationship between chemical and theoretical oxygen demand for specific classes of organic chemicals. Water Res. 33, 327e334. Bali, U., Sengül, F., 2003. The fate and effect of 4-chlorophenol in an upflow anaerobic fixed-bed reactor. Proc. Biochem. 38, 1201e1208. Balkema, A.J., Preisig, H.A., Otterpohl, R., Lambert, A.J., Weijers, S.R., 2001. Developing a model based decision support tool for the identification of sustainable treatment options for domestic wastewater. Water Sci. Tech. 43, 265e269. Bieker, S., Cornel, P., Wagner, M., 2010. Semicentralised supply and treatment systems: integrated infrastructure solutions for fast growing urban areas. Water Sci. Tech. 61, 2905e2913. Çengel, Y.A., Boles, M.A., 2011. Thermodynamics. An Engineering Approach, seventh ed. McGraw-Hill, New York. Cimetiere, N., Dossier-Berne, F., De Laat, J., 2009. Monochloramination of resorcinol: mechanism and kinetic modeling. Environ. Sci. Technol. 43, 9380e9385. Debellefontaine, H., Chakchouk, M., Foussard, J.N., Tissot, D., Striolo, P., 1996. Treatment of organic aqueous wastes: wet air oxidation and wet peroxide oxidation. Environ. Pollut. 92, 155e164. Demoulin, G., Mervyn, C.G., 2003. Influence of on-line our control on operation costs of cyclic treatment plants. KA e Abwasser Abfall 50, 161e165. , E., Mare chal, F., 2012. Energy Descoins, N., Deleris, S., Lestienne, R., Trouve
efficiency in waste water treatment plants: optimization of activated sludge process coupled with anaerobic digestion. Energy 41, 153e164. Devlin, H.R., Harris, I.J., 1984. Mechanism of the oxidation of aqueous phenol with dissolved oxygen. Ind. Eng. Chem. Fund. 23, 387e392. Diamantis, V., Melidis, P., Aivasidis, A., Verstrate, W., Vlaeminck, S.E., 2011. Efficiency and sustainability of urban wastewater treatment with maximum separation of the solid and liquid fraction. In: Murray, M.Y. (Ed.), Comprehensive Biotechnology. Elsevier, Amsterdam, pp. 507e515. Fedorak, P.M., Hrudey, S.E., 1984. The effects of phenol and some alkyl phenolics on batch anaerobic methanogenesis. Water Res. 18, 361e367. Feifer, J., Smith, M.A., Willeford, B.R., 1959. Oxidation of phenols by periodate. J. Org. Chem. 24, 90e93. Garg, A., Mishra, I.M., Chand, S., 2010. Oxidative phenol degradation using nonnoble metal based catalysts. Clean e Soil Air Water 38, 27e34. Gunten, U., 2007. The basics of oxidants in water treatment. Part B: ozone reactions. Water Sci. Tech. 55, 25e29. Haberkern, B., Maier, W., Schneider, U., 2008. In: Umweltbundesamt (Ed.), Steigerung der Energieeffizienz auf kommunalen Klaeranlagen 11/08. ISSN 18624804, Berlin. Imamura, S., 1999. Catalytic and noncatalytic wet oxidation. Ind. Eng. Chem. Res. 38, 1743e1753. Johnston, J.J., Borden, R.C., Barlaz, M.A., 1996. Anaerobic biodegradation of alkylbenzenes and trichloroethylene in aquifer sediment down gradient of a sanitary landfill. J. Contam. Hydrol. 23, 263e283. Jonstrup, M., Kumar, N., Murto, M., Mattiasson, B., 2011. Sequential anaerobiceaerobic treatment of azo dyes: decolourisation and amine degradability. Desalination 280, 339e346. Kacar, Y., Alpay, E., Ceylan, V.K., 2003. Pretreatment of afyon alcaloide factory's wastewater by wet air oxidation (WAO). Water Res. 37, 1170e1176. €tsep, A., Liiders, M., Maloveryran, A., 2000. The Kahru, A., Pollumaa, L., Reiman, R., Ra toxicity and biodegradability of eight main phenolic compounds characteristic to the oil-shale industry wastewaters: a test battery approach. Environ. Toxicol. 15, 431e442. Kang, J., Zhan, W., Li, D., Wang, X., Song, J., Liu, D., 2011. Integrated catalytic wet air oxidation and biological treatment of wastewater from vitamin B6 production. Phys. Chem. Earth Parts A/B/C 36, 455e458. Kindzierski, W., 1991. Anaerobic treatability of a phenolic coal conversion wastewater after diisopropyl ether extraction. Water Res. 25, 479e484. Kolaczkowski, S., 1999. Wet air oxidation: a review of process technologies and aspects in reactor design. Chem. Eng. J. 73, 143e160. Lee, M., 2002. Visualization of oxygen transfer across the airewater interface using a fluorescence oxygen visualization method. Water Res. 36, 2140e2146. vre, S., Boutin, O., Ferrasse, J.H., Malleret, L., Faucherand, R., Viand, A., 2011. Lefe Thermodynamic and kinetic study of phenol degradation by a non-catalytic wet air oxidation process. Chemosphere 84, 1208e1215. Lepik, R., Tenno, T., 2000. Biodegradability of phenol, resorcinol and 5-methylresorcinol as single and mixed substrate by activated sludge. Oil Shale 28, 425e446. Li, L., Chen, P., Gloyna, E.F., 1991. Generalized kinetic model for wet oxidation of organic compounds. AIChE J. 37, 1687e1697. Luck, F., 1999. Wet air oxidation: past, present and future. Catal. Today 53, 81e91. Mantzavinos, D., 1999. Wastewater treatment: wet air oxidation as a precursor to biological treatment. Catal. Today 53, 93e106. Mantzavinos, D., Psillakis, E., 2004. Enhancement of biodegradability of industrial wastewaters by chemical oxidation pre-treatment. J. Chem. Technol. Biotechnol. 79, 431e454. Maugans, C.B., Ellis, C., 2002. Wet air oxidation: a review of commercial sub-critical hydrothermal treatment. In: Proceedings of the 3rd IT Conference, New Orleans, Louisiana. Mishra, V.S., Mahajani, V.V., Joshi, J.B., 1995. Wet air oxidation. Ind. Eng. Chem. Res. 34, 2e48. ~ izares, P., Sa ez, C., Lobato, J., Rodrigo, M.A., 2005. ElecNasr, B., Abdellatif, G., Can trochemical oxidation of hydroquinone, resorcinol, and catechol on boron-
B. Weber et al. / Journal of Environmental Management 161 (2015) 137e143 doped diamond anodes. Environ. Sci. Technol. 39, 7234e7239. Olsson, G., 2012. Water and Energy. Threats and Opportunities. IWA Publishing, London. Owen, W.F., 1982. Energy in Wastewater Treatment. Prentice Hall, Inc., Englewood Cliffs. Pan, L.T., Zheng, S., Wu, L., Zhang, Y.L., Zhou, X.F., Jin, F.M., 2010. Hydrothermal production of formic and acetic acids from syringol. Appl. Phys. Eng. 11, 613e618. rez-Ojeda, M.E., Carrera, J., Font, J., Stüber, F., Bengoa, C., Rubalcaba, A., Sua Fortuny, A., Fabregat, A., 2007. Biodegradability enhancement of phenolic compounds by hydrogen peroxide promoted catalytic wet air oxidation. Catal. Today 124, 191e197. Schonberg, J.C., Bhattacharya, S.K., Madura, R.L., Mason, S.H., Conway, R.A., 1997. Evaluation of anaerobic treatment of selected petrochemical wastes. J. Hazard. Mater. 54, 47e63. Schwarzenbach, P., Gschwend, P.M., Imboden, D.M., 2003. Environmental Organic Chemistry. John Wiley & Sons, Inc., Hoboken. Sierra-Alvarez, R., Lettinga, G., 1991. The effect of aromatic structure on the inhibition of acetoclastic methanogenesis in granular sludge. Appl. Microbiol. Biotech. 34, 544e550. Silva, A.M.T., Nouli, E., Carmo-Apolinario, A.C., Xekoukoulotakis, N.P., Mantzavinos, D., 2007. Sonophotocatalytic/H2O2 degradation of phenolic compounds inagro-industrial effluents. Catal. Today 124, 232e239. Sotelo, J.L., Beltran, F.J., Gonzalez, M., 1990. Ozonation of aqueous solutions of resorcinol and phloroglucinol. 1. Stoichiometry and absorption kinetic regime. Ind. Eng. Chem. Res. 29, 2358e2367. Speece, R.E., 1983. Anaerobic biotechnology for industrial wastewater treatment. Environ. Sci. Technol. 17, 416e427.
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Speece, R.E., 1999. Anaerobic Biotechnology for Industrial Wastewaters. Archae Press, Nashville. Stengl, S., Koch, C., Scheer, J., Stadlbauer, E.A., Richarts, F., Altensen, R., Richter, H., Weber, B., Bayer, M.P., Albert, K., 2012. Stein- und Braunkohle aus Biomasse als € l, Brennstoff oder als alternative zur CO2-Abscheidung und Speicherung. Erdo Erdgas, Kohle 128, 208e214. rez-Ojeda, M.E., Carrera, J., Metcalfe, I.S., Font, J., 2008. Wet air oxidation (WAO) Sua as a precursor to biological treatment of substituted phenols: refractory nature of the WAO intermediates. Chem. Eng. J. 144, 205e212. Suty, H., de Traversay, C., Cost, M., 2004. Applications of advanced oxidation processes: present and future. Water Sci. Tech. 49, 227e233. Svardal, K., Kroiss, H., 2011. Energy requirements for waste water treatment. Water Sci. Tech. 64, 1355e1361. Tchobanoglous, G., Burton, F.L., 2003. Wastewater Engineering. Treatment and Reuse, fourth ed. Metcalf & Eddy Inc., McGraw-Hill, New York. Tschech, A., Schink, B., 1985. Fermentative degradation of resorcinol and resorcylic acids. Arch. Microbiol. 143, 52e59. Tyagi, R.D., Tran, F.T., Chowdhury, A.K.M.M., 1993. Biodegradation of petroleum refinery wastewater in a modified rotating biological contactor with polyurethane foam attached to the disks. Water Res. 27, 91e99. Veeresh, G.S., Kumar, P., Mehrota, I., 2005. Treatment of phenol and cresols in upflow anaerobic sludge blanket (UASB) process: a review. Water Res. 39, 154e170. Vogelpohl, A., 2007. Applications of AOPs in wastewater treatment. Water Sci. Tech. 55, 207e211. Zhao, F., Li, X., Graham, N., 2012. Treatment of a model HA compound (resorcinol) by potassium manganite. Sep. Purif. Technol. 91, 52e58.
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Research article
Wet air oxidation of resorcinol as a model treatment for refractory organics in wastewaters from the wood processing industry Bernd Weber a, b, *, Alma Chavez b, Julio Morales-Mejia b, Sabrina Eichenauer c, Ernst A. Stadlbauer c, Rafael Almanza b noma del Estado de M Universidad Auto exicoeFacultad de Ingeniería, Cerro de Coatepec s/n Col. San Buenaventura, C.P. 50130 Toluca, Estado de M exico, M exico b n, M noma de M Instituto de Ingeniería de la Universidad Nacional Auto exico (UNAM), C.P. 04510 Coyoaca exico, D.F., M exico c Competence Center for Energy and Environmental Engineering, University of Applied Sciences THM, Campus Giessen, 35390 Giessen, Germany a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2015 Received in revised form 20 June 2015 Accepted 23 June 2015 Available online xxx
Wastewater treatment systems are important tools to enhance sustainability in terms of reducing environmental impact and complying with sanitary requirements. This work addresses the wet air oxidation (WAO) process for pre-treatment of phenolic wastewater effluents. The aim was to increase biodegradability prior to a subsequent anaerobic stage. In WAO laboratory experiments using a microautoclave, the model compound resorcinol was degraded under different oxygen availability regims within the temperature range 150 Ce270 C. The activation energy was determined to be 51.5 kJ/mol. Analysis of the products revealed that after 3 h of reaction at 230 C, 97.5% degradation of resorcinol was achieved. At 250 C and the same reaction time complete removal of resorcinol was observed. In this case the total organic carbon content was reduced down to 29%, from 118.0 mg/L down to 34.4 mg/L. Under these process conditions, the pollutant was only partially mineralized and the ratio of the biological oxygen demand relative to the chemical oxygen demand, which is 0.07 for resorcinol, was increased to a value exceeding 0.5. The main by-product acetic acid, which is a preferred compound for methanogenic bacteria, was found to account for 33% of the total organic carbon. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Industrial Wastewater Pretreatment Oxidation process Recalcitrant
1. Introduction Treatment of wastewater and potable water has become essential in modern societies. In practice, environmental compatibility and financial feasibility determine the methodology adopted for domestic and industrial water quality management (Balkema et al., 2001). Currently, efforts are being increasingly devoted to improving the energy efficiency of the processes employed. As a rule of thumb, an average input of 1.4 kWh to 3.2 kWh of electrical energy is necessary to remove 1 kg of biological oxygen demand (BOD), as reported in German wastewater treatment plants (Demoulin and Mervyn, 2003; Owen, 1982). Studies have indicated that the potential exists to decrease this energy input up to 50%
List of non standard abbreviations: DO, dissolved oxygen; HTC, hydrothermal carbonization; LTC, low temperature conversion; WAO, wet air oxidation. noma del * Corresponding author. Facultad de Ingeniería, Universidad Auto xico, Cerro de Coatepec s/n, C.P. 50130 Toluca, Edo. Mex., Mexico. Estado de Me E-mail address: [email protected] (B. Weber). http://dx.doi.org/10.1016/j.jenvman.2015.06.046 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
(Haberkern et al., 2008; Olsson, 2012). One strategy is to rely upon scaled-up energy savings. However, small on-site treatment plants have a higher recycling potential (Bieker et al., 2010). Another strategy favours anaerobic treatment instead of energy-intensive aerobic processes (Diamantis et al., 2011; Descoins et al., 2012). For example treating industrial effluents anaerobically allows a saving of around 11.6 GJ in the form of biogas produced per Mg of COD degraded. Additionally it confers a lower electricity consumption of the order 1100 kWh since it removes the need for aeration (Speece, 1983; Svardal and Kroiss, 2011). Despite the progress that has been made in anaerobic digestion, a variety of recalcitrant compounds are still observed under anaerobic conditions (Sierra-Alvarez and Lettinga, 1991; Schonberg et al., 1997; Jonstrup et al., 2011). Anaerobically non-degradable pollutants, even with co-substrates, include o-cresol, benzene, 4-chlorophenol, alkylbenzenesulphonate, di-isopropyl ether (Kindzierski, 1991; Fedorak and Hrudey, 1984; Bali and Sengül, 2003; Johnston et al., 1996). Degradation of such persistent substances can be enhanced by oxidative pre-treatment, giving rise to further
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applications of efficient anaerobic technologies. Various technical processes have been applied: ozone oxidation, fenton oxidation, photo-fenton oxidation, photolysis/photocatalysis and wet air oxidation (WAO) (Suty et al., 2004; Vogelpohl, 2007; Gunten, 2007). The WAO process has a market share of 3e17% (Mantzavinos and Psillakis, 2004), since it is already a common practice in the pulp and paper industry (Maugans and Ellis, 2002; Luck, 1999), in the pre-treatment of landfill leachate and in the treatment of pharmaceutical wastewaters (Mishra et al., 1995). Process performance may be significantly enhanced when homogenous or heterogeneous catalysts are applied (Luck, 1999; Garg et al., 2010). An organic matter content above 10 g/L is characteristic for the application of WAO technology (Debellefontaine et al., 1996). However when these advanced aerobic treatment processes are used a variety of by-products are formed. The composition pattern of these by-products varies with temperature. Limiting the presence of oxygen favours the production of acetic acid (Li et al., 1991). This resembles the first three steps of classic anaerobic digestion. The subsequent biological anaerobic treatment produces biogas that is commonly used for the onsite generation of electricity with the possibility of waste heat recovery in the WAO process. As a consequence this kind of process combination does not need concentrations of organic matter over 4% to run the WAO process thermally autogenously (Baillod et al., 1985). The emerging practice of sequential exposure to wet air oxidation followed by anaerobic treatment is being exploited for the enhanced transformation of recalcitrant organics (Mantzavinos, 1999). Residual byproducts have the potential to increase biogas yield in subsequent anaerobic digestion and show an improved biodegradability (Kacar et al., 2003; Kang et al., 2011). One review in the literature reports the presence of phenolic substances in wastewaters from refineries, textile and resin industries and coal conversion plants (Veeresh et al., 2005). In addition, the emerging technologies of hydrothermal carbonisation (HTC), low-temperature conversion (LTC) and torrefaction of biomass (Stengl et al., 2012) produce complex wastewaters with inhibitory components in anaerobic consortia originating from the thermal degradation of lignocellulosic materials (Speece, 1999). Resorcinol is an aromatic compound that may be obtained either from nature or synthetically. It is a significant by-product in the manufacture of tires, diazo dyes and pharmaceuticals (Tschech and Schink, 1985). For example, in wastewater from a petroleum refinery Tyiagi et al. detected resorcinol as one of the main constituents (Tyagi et al., 1993). One study classified resorcinol as very toxic after conducting a number of microbiotests (Kahru et al., 2000). Thus, these kinds of toxic effluents demand special attention. As a model component resorcinol has been shown to be slowly oxidized by HClO and NH2Cl (Cimetiere et al., 2009) and by periodate (Feifer et al., 1959). Other investigations report that resorcinol has been efficiently removed from water by electrochemical oxidation with boron-doped diamond electrodes requiring consumption of less than 30 Ah/dm3 (Nasr et al., 2005); by ozone oxidation at 871 Pa partial pressure of ozone, 20 C, neutral pH and high levels of turbulence within the reactor (Sotelo et al., 1990); by adsorption/oxidation with MnO2 and KMnO4 (Zhao et al., 2012); and by sono-photo-catalysis with H2O2 and TiO2 at 0.75 g/L (Silva et al., 2007). In the present study, the process used was WAO, which has the advantage of using inexpensive air as oxidant and thermal energy that can provided by subsequent processing steps. In laboratory-scale WAO experiments the degradation of initial resorcinol concentrations of 25 mg/L and 180 mg/L were investigated with respect to reaction time, temperature and the presence of dissolved oxygen. The formation of by-products is evaluated by UV/Vis analysis. Acetic acid concentrations were determined using an enzymatic method. Changes in biodegradability and the
remaining potential for anaerobic post-treatment were evaluated by TOC, COD and BOD analysis.
2. Material and methods 2.1. WAO treatment The WAO treatment of resorcinol on the laboratory scale was performed in a tube reactor as shown in Fig. 1. The tube reactor consists of a laboratory grade stainless steel micro-autoclave equipped with a sodium calcium glass reaction tube with a volume of total 12 mL. The reaction tube was pre-treated with 2 mol/L hydrochloric acid solution overnight and washed with distilled water before use. Resorcinol (CAS-No. 108-4-63) was supplied by Sigma Aldrich. A stock solution with a concentration of 180 mg/L was prepared with distilled water and stored at 2 C. The lower concentrations needed for the experiments were obtained by dilution. 7.5 mL of this aqueous solution of resorcinol of a known concentration was added to the reaction tube. The dissolved oxygen concentration in the sample was adjusted in three different ways: a) Refrigeration overnight resulted in a dissolved oxygen concentration of approximately 7 mg/L due to the water solubility equilibrium. b) Bubbling pure oxygen into the bottom of the reaction tube for 5 min resulted in a dissolved oxygen concentration of 35 mg/L. After 5 min, the cap of the autoclave was closed immediately to maintain the oxygen concentration. In this case oxygen was also present in the gas phase. c) Bubbling of subliming CO2 for 5 min at the base of the reaction tube eliminated nearly all of the oxygen from the system (residual oxygen concentration < 1 mg/L). The potential of oxidation capacity, X, referred to as the oxygen regime is expressed by the following equation:
X¼
cO2 cCOD
(1)
Here, cO2 is the concentration of the dissolved oxygen, and cCOD is the theoretical chemical oxygen demand of the sample. Using concentrations of 25 mg/L and 180 mg/L of resorcinol (1 mg resorcinol ¼ 1.89 mg CODtheoretical) in the experiments, ratios of X ¼ 0.75 (25 mg/L resorcinol and oxygen); 0.1 (180 mg/L resorcinol and oxygen); 0.02 (180 mg/L resorcinol and air); and <0.003 (180 mg/L resorcinol and CO2) were investigated. Thus higher concentrations of resorcinol could not be investigated, because X is reduced proportionally. During the thermal treatment, limited oxygen transfer from the gas phase into the liquid is unavoidable. After the autoclave was closed, it was placed into a laboratory
1. Sample Preparation 2. Oxygen Adjustment 3. WAO Treatment 4. Analysis Sample Volume 7.5 mL 25 mg Resorcinol / L or 180 mg Resorcinol / L
< 1mg O2 / L or 7 mg O2 / L or 35 mg O2 / L
T Outside
-
UV/VIS COD, BOD TOC Acetic Acid GC/FID
T Inside
Fig. 1. Sketch of the sample preparation, wet air oxidation and analysis of products.
B. Weber et al. / Journal of Environmental Management 161 (2015) 137e143
139
furnace (Model: 5194 from Industrias Sola Basic, Lindberg Division, Mexico). As a temperature reference, the signal from a NieNiCr thermocouple situated on the internal side of the autoclave wall was recorded. Because of the thickness of the autoclave wall, the thermocouple exhibited a slow response to the higher temperature in the furnace, and a reaction time correction was established. The optimised heating program was guaranteed to reach 85% of the final temperature within 1 h. The autoclaves remained in the furnace for an additional 3 h. For the kinetic tests, 1, 2, 3, 4, 5 and 6 h of reaction time were chosen. After the reaction time had elapsed, the autoclaves were rapidly cooled in a water basin. After opening the autoclaves, UV/Vis spectra were recorded (Shimadzu UV 1601), and the samples were stored under refrigeration at 2 C for further analysis. 2.2. Analytical methods Dissolved oxygen in the solutions was measured with a membrane electrode connected to a portable instrument (HI 9146 from Hanna) able to analyse concentrations of between 0 and 45 mg O2/ L. The UV/Vis spectrum was generated from 3 mL of undiluted sample in a 1 cm quartz cuvette over the wavelength range of 180e430 nm. Data management was performed with UVPC software, version 3.9. A Shimadzu GC-14A chromatograph equipped with flame ionisation detector (FID) and an H53-Supreme 30 m capillary column (CS-Chromatographie) with a film thickness of 0.25 mm was used for chromatographic analysis. The operating conditions were as follows: carrier gas (nitrogen) (99.998%) at a constant pressure of 10 kPa; 300 C injector temperature; oven temperature of 50 C (5 min) and then increasing at 15 C/min to 320 C (15 min). The sample solution (3 mL) was injected manually. The quantification of the resorcinol concentration was based on linear regression obtained by injecting standard solutions with concentrations ranging from 20 to 200 mg resorcinol/L. The COD was measured with Merck COD-Test solutions No. 1.14539.0495 (range 100e1500 mg/L) and No. 1.14681.0495 (range 4e100 mg/L). A linear calibration correlation of the light absorbance vs. COD was obtained by oxidising potassium terephthalate, as described in ASTM 5520. The TOC was determined using Hach TOC-test kits no. 27603-45 (for concentrations between 0.3 mg/L and 20 mg/L) and 28159-45 (for concentrations from 15 mg/L to 150 mg/L) in combination with a DR 5000 Spectrophotometer from Hach. The BOD was determined in Winkler bottles by directly reading the dissolved oxygen (DO) with the HI 9146 DO meter (described previously) after 5 days according to ASTM Method 5210 (APHA, 2005). Acetic acid was determined by an enzymatic test method from Boehringer Mannheim/R-Biopharm (UV Test Kit No. 10 148 261 035). In this test, the acetate bonds to Acetyl-CoA. Then, Acetyl-CoA reacts with oxalacetate. During the formation of oxalacetate from Lmalate and NADþ, the reduction of NADþ to NADH is quantified at 340 nm using the Shimadzu Photometer. 3. Results 3.1. UV/Vis analysis of resorcinol degradation UV/Vis spectra of both resorcinol and its intermediates that arise from the WAO of synthetic wastewater at different reaction times are shown in Fig. 2. The spectra of untreated resorcinol shows three strong absorption bands in the UV range at 195, 218 (inflection point) and 274 nm. Superposition of the electronic spectra for
Fig. 2. UV/Vis spectral changes for resorcinol during thermal treatment with different reaction times. Initial concentration: 25 mg/L, T ¼ 170 C, X ¼ 0.75.
treatments is evident. Intermediates have strong absorption peaks at 260, 330 and 370 nm. Reduction of the absorbance at 218 nm indicates that degradation of resorcinol has occurred. For longer treatment, absorption rises again and by-products with strong absorbance at 218 nm are also present. The same trend is observed for the absorbance peak at 195 nm (the original main absorbance of resorcinol at 202 nm is shifted to 195 nm), where the lowest value is reported for 1 h of treatment. For the 2 h treatment, absorbance was found to increase and subsequently decrease slowly, as observed in the 4 h treatment, due to subsequent by-product degradation. Thus, the shapes of the spectra are mainly influenced by the by-products. The pattern of by-products varies with time, as evidenced by the sharp decrease at 195 nm after 1 h (absorbance: 1.56) of reaction time. Interestingly the intensity increases again after 2 h (absorbance: 2.52). After longer treatment periods, the absorbance decreases again. The formation of by-products can be evaluated using the absorbance at 250 nm, where resorcinol shows only a low response. The same effect is observed at 195 nm, where the absorbance of the by-products first increases during 1 and 2 h treatment and then decreases after longer treatments. In accordance with DIN 38402 C2 (Spectral Absorption Coefficient), the absorbance was determined at 254 nm. In order to compare experiments with different initial concentrations, the results must be normalized by creating a by-product index (IBy-products) as follows:
IByproducts ¼
AbsorbanceProduct AbsorbanceInitial AbsorbanceInitial
(2)
The graphs of this index (Fig. 3) show that the temperature, where the maximum value of this index is present, depends on the availability of oxygen (expressed as the potential oxygen capacity X). Thus, the availability of oxygen accelerates both the degradation of resorcinol and the degradation of intermediates. When resorcinol is treated at 150 C in the presence of a nearly stoichiometric availability of oxygen, by-products are formed at highest velocity followed by nearly complete degradation at slightly increased temperatures (190 C). This degradation pattern is shifted to higher reaction temperatures when the oxygen availability is lower.
3.2. Resorcinol degradation Resorcinol degradation can be quantitatively monitored by GC/ FID. The GC analysis also revealed that accumulation of ortobenzoquinone and para-benzoquinone does not occur, which is in
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Fig. 3. Effect of temperature on the by-product index (IBy-products) under different oxygen regimens (X ¼ 0.75; X ¼ 0.1; X ¼ 0.02).
activation constant was determined to be E ¼ 51.5 kJ/mol applying a numeric routine for solving the multiple linear regression. The experimental values correlate to the curve “a” with a R2 of 0.996. Taking into account activation energy and k0 ¼ 44.6 mg L1 s1, derived from curve “a”, the curve “b” can be calculated. The experimental values were found to correlate to the curve “b” with a R2 of 0.822. To calculate curves “a” and “b” shown in Fig. 4, the initial oxygen concentration was used. As the reaction proceeds, the oxygen concentration decreases, and the lack of equilibrium between the gas and liquid phases drives an additional oxygen flux governed by molecular diffusion in the experimental setup. A diffusivity of 2.4$109 m2/s (Çengel and Boles, 2011) for the diffusion of oxygen in water at 200 C results in a kla value of 0.38 m/h and a flux rate of 3.5$106 g m2 s1, which can be depreciated in the analysis. However, the experimental findings reported in a previous study (Lee, 2002) showed kla values 25 times higher than the theoretical values of the Stefan's diffusion tube. For this comparison, the temperature correction reported by Debellefontaine was applied (Debellefontaine et al., 1996). The degradation of resorcinol at the moderate temperature of 170 C is presented in Fig. 5, in which the reaction time varies between 1 and 6 h. Under these conditions the reaction rate can be expressed as a function of dissolved oxygen (taking the calculated activation energy into account):
ra ¼ 66:4$Corg $COn 2 ;L for X ¼ 0:1
Fig. 4. Effect of temperature on residual resorcinol concentration quantified by GC/FID after WAO treatment over 3 h, varying temperature between 150 C and 290 C (cre2 2 sorcinol, initial ¼ 180 mg/L). Curves a (R ¼ 0.996) and b (R ¼ 0.73) are based on experimental E ¼ 51.5 kJ/mol.
vre et al., 2011). Fig. 4 agreement with other investigations (Lefe shows the effect of the temperature on resorcinol degradation according to GC analysis between 150 C and 270 C for a reaction time of 3 h. When X ¼ 0.1 (180 mg resorcinol/L and 35 mg O2/L), a considerable amount of resorcinol can be degraded at moderate temperatures (curve a). Nearly complete degradation of resorcinol occurs at temperatures exceeding 230 C. Decreasing the oxygen availability as represented by the curve “b” results in a lower level of resorcinol degradation. The degradation of organic compounds can be described using the extended Arrhenius equation (Kolaczkowski, 1999):
dC m ra ¼ resorcinol ¼ k0 eE=ðRTÞ $Corg $COn 2 ;L dt
(3a)
Degradation starts at a low rate, which reaches its maximum at approximately 3 h of reaction time. After 6 h of treatment at 170 C, only a residual concentration (less than 20%) of the untreated sample remains. The correlation curve assuming a constant reaction rate calculated by activation energy and initial concentrations is also shown in Fig. 5 and has R2 correlation of 0.797. It is worth mentioning that an exponential function starting with the initial resorcinol concentration can express the correlation with a minimum R2 of 0.893. The literature reports that the degradation of resorcinol is initiated by the formation of hydroquinone and its subsequent oxidation to orto- and para-benzoquinone (Devlin and Harris, 1984). Ring opening proceeds with the formation of muconic acid and 2,5-dioxo-3-hexenedioic acid. Due to further oxidation steps organic acids like maleic, fumaric, propionic, acetic and formic acid are produced. Other work indicates that further oxidation steps produce oxalic acid (C2H2O4), which is ultimately oxidized to
(3)
where ra is the reaction rate, k0 is the pre-exponential factor, E is the activation energy, R is the universal gas constant, T is the reaction temperature, Corg is the concentration of the organic compound, and CO2 ;L is the concentration of dissolved oxygen. The subscripts m and n are the reaction orders and were chosen according to those proposed by Mishra et al. (1.0 for m and 0.4 for n) (1995). With the observed resorcinol degradation for X ¼ 0.1 at temperatures between 150 C and 270 C for 3 h reaction time (curve a) the
Fig. 5. Effect of reaction time on residual resorcinol concentration quantified by GC/ FID after WAO treatment at T ¼ 170 C (cresorcinol, initial ¼ 180 mg/L; X ¼ 0.1). Correlation curve based on experimental E and A values / k ¼ 5.34*1005 s1.
B. Weber et al. / Journal of Environmental Management 161 (2015) 137e143
141
carbon dioxide (Pan et al., 2010). The oxidation of resorcinol into its by-products requires the following percentages of total chemical oxygen demand: benzoquinone 8%, muconic acid as well as 2,5-dioxo-3-hexenedioic acid 16%, and acetic acid 54%. Thus, only for the treatment X ¼ 0.75 the complete oxidation into the stable acetic acid can be reached. 3.3. Formation of acetic acid The oxidation of resorcinol to the thermodynamically stable acetic acid is shown in Table 1 for different oxygen regimes and temperatures between 150 C and 250 C. The acetic acid concentration in the product rose with increasing temperature for all oxygen regimes. At the highest applied temperature (270 C), the acetic acid concentration was stable or decreased under different oxygen regimes, most likely as a result of an oxidative attack on the stable acetic acid (Imamura, 1999). The maximum carbon yield relative to the initial TOC concentration that is transformed into acetic acid by treatment at 250 C was only 3% for the oxygen regime X ¼ 0.02, while the carbon yields for the treatments with X ¼ 0.75 and X ¼ 0.1 were approximately 15%. The kinetics of the formation of acetic acid for the treatment at 170 C and X ¼ 0.1 is almost linear, indicating a zero-order reaction, which is characteristic of a limiting factor. Resorcinol undergoes approximately 8 oxidation steps in the course of transformation to acetic acid (Devlin and Harris, 1984). Consequently different temperature-dependent mechanisms may occur. Both linear and exponential degradation are observed without a limiting factor (Devlin and Harris, 1984). This linear correlation does not cross the centre point (zero point) due to the higher initial oxygen concentration (Fig. 6). When a longer treatment period is used, the reaction may be inhibited by the limited oxygen transfer from the gas phase into the liquid phase in this experiment. It is notable that for complete oxidation of resorcinol into acetic acid, the minimum oxygen regime is X ¼ 0.54, a condition that is only achieved under the experimental conditions where X ¼ 0.75. 3.4. COD, TOC and BOD removal COD, TOC and BOD are useful parameters for evaluating thermal degradation. Whereas the total organic carbon indicates a direct relation to the mineralization of organic matter, interpreting the COD is more complex. For stable organic compounds, in particular, incomplete oxidation of organic matter is reported (Baker et al., 1999). Metals can also be oxidized by this method, which may explain why the COD values for the treatment with X < 0.003 rise to nearly 15%, as shown in Table 2. For other oxygen regimes in which WAO treatment occurs between 250 C and 270 C, the highest COD removal was achieved (X ¼ 0.75 with a residual COD of 29%; X ¼ 0.1 Table 1 Effect of temperature and X on the formation of acetic acid by WAO treatment for a period of 3 h. Temperature ( C)
X ¼ 0.75a
X ¼ 0.10b
X ¼ 0.02b
X < 0.003b
1.2 1.6 2.8 3.8 6.5 9.0 1.6
e e 1.6 1.2 3.4 2.9 3.0
Acetic acid (mg/L) 150 170 190 210 230 250 270 a b
1.1 1.8 4.3 4.4 6.0 6.5 e
TOCinitial ¼ 16.4 mg/L. TOCinitial ¼ 118 mg/L.
2.9 5.2 8.3 15.3 e 41.4 40.3
Fig. 6. Formation of acetic acid during WAO treatment for a period of 1e6 h at 170 C; X ¼ 0.1; TOCinitial ¼ 180 mg/L.
with a residual COD of 26%; X ¼ 0.02 with a residual COD of 78%). These results correlate with the yields for acetic acid. In addition the TOC values decrease with increasing temperature. The highest removal rates were observed between 250 C and 270 C and are in the range of 21%e73%. The contribution of the acetic acid to the residual TOC is 40% for the treatment with X ¼ 0.75 at 250 C and 54% for the treatment with X ¼ 0.1 at 270 C. Limiting the oxygen in the treatment at 270 C with X ¼ 0.02 leads to an acetic acid contribution of only 5% to the total residual TOC. The biological treatability of the products obtained during the WAO treatment was evaluated according to the biological oxygen demand. The degradation of the original resorcinol is very low, only approximately 7%, which is in agreement with values reported by Lepik and Tenno (2000). Thus, by-products generally show better biodegradability when the aromaticity of the compounds is lost rez-Ojeda (Schwarzenbach et al., 2003; Rubalcaba et al., 2007; Sua et al., 2008). The highest BOD yield was achieved using an oxygen regime of X ¼ 0.1 with a treatment temperature of 150 C. Due to the limited quantity of sample solution for BOD analysis of the treatments with lower initial resorcinol concentrations (the oxygen regime of X ¼ 0.75 in Table 2), the BOD could not be determined. No change in the BOD/COD ratio was observed under an oxygen regime of X < 0.003. The resorcinol removal efficiency is directly dependent on the value of X, but the BOD/COD ratio is not. Thus, for an oxygen regime of X ¼ 0.02 at lower temperatures, the BOD value is comparable to the initial BOD and starts to rise when the treatment temperature exceeds 210 C, reaching a maximum of 169.9 mg BOD/L (COD/BOD ¼ 0.55) at 270 C. Unlike the oxygen regime at X ¼ 0.1, the BOD/COD ratio reached a constant value of 0.3, indicating that an equilibrium was reached of the degradation of resorcinol into by-products with higher biological degradability, parallel to the advanced oxidation of these by-products into carbon dioxide. If the oxygen availability is limited, the BOD/COD ratio can be increased to over 0.5 by applying reaction temperatures above 250 C. 4. Conclusions The obtained experimental data obtained shows that the oxidation of resorcinol proceeds at relatively low temperatures (150 Ce270 C) during WAO treatment. COD, TOC and BOD as well as acetic acid, which is the main final product, proved to be useful markers for the assessment of thermal degradation. Different oxygen regimes lead to different BOD concentrations for by-products. BOD/COD ratios of 0.55 in the products
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B. Weber et al. / Journal of Environmental Management 161 (2015) 137e143
Table 2 COD, BOD, TOC and BOD/COD in the products obtained by WAO treatment under different conditions. Temperature
X ¼ 0.75a COD
X ¼ 0.10 TOC
X ¼ 0.02
COD
BOD
TOC
BOD/COD
X < 0.003
COD
BOD
TOC
BOD COD
COD
BOD
TOC
BOD/COD
( C)
(mg/L)
(mg/L)
(mg/L)
(mg/L)
(mg/L)
(e)
(mg/L)
(mg/L)
(mg/L)
(e)
(mg/L)
(mg/L)
(mg/L)
(-)
Initial 150 170 190 210 230 250 270 290
47.0 30.9 22.0 17.4 25.9 11.8 13.9 e e
16.4 e e 9.4 6.8 8.0 6.7 e e
356.2 299.0 289.9 243.0 233.8 121.2 105.0 94.0 e
20.8 100.6 85.1 79.1 75.7 39.6 34.4 30.1 e
118.0 100.6 85.1 79.1 75.7 39.6 34.4 30.1 e
0.06 0.34 0.28 0.33 0.32 0.33 0.33 0.32
356.2 321.9 328.1 280.3 309.1 301.9 295.7 280.3 e
20.8 4.3 76.0 17.9 17.9 165.2 137.3 169.9 e
118.0 112.7 112.7 86.9 100.6 91.2 90.3 86.0 e
0.06 0.01 0.23 0.06 0.06 0.55 0.46 0.61
356.2 364.8 380.8 367.2 377.5 360.5 380.5 428.6 410.6
20.8 4.3 e e 16.9 35.8 8.7 17.3 e
118.0 110.1 e 110.1 83.4 130.7 117.8 166.0 142.8
0.06 0.01 e e 0.04 0.10 0.02 0.04
a
Sample volume too small for BOD concentration measurement.
after WAO treatment under optimum conditions indicate a biological treatability comparable to that of municipal wastewater (Tchobanoglous and Burton, 2003). The production of acetic acid as a stable intermediate in the WAO treatment of resorcinol is an important benefit that should be exploited in a sequential anaerobic biotechnology strategy. These results suggest a useful application of this new methodology to approaching forestry industry wastewaters, even if the wastewater may seem too toxic to be directly anaerobically treated due to large amounts of lignin-derived compounds. Fortunately, to aid the successful scale-up of the WAO process from laboratory to pilot to full scale, pressurised reactors are commercially available in the favourable pressure range of 5e55 bars. Adequate technical precautions must be taken, as volatile degradation products, such as CO, CO2 and H2, may be produced (Stengl et al., 2012). Acknowledgements This work was performed partly during a postdoctoral research fellowship that was funded by the Institute of Science and Technology of Mexico City (ICyTDF) and by the Universidad Nacional noma de Me xico through PAPIIT projects IN107913, and Auto IN102708. References American Public Health Association (APHA), 2005. Water Works Association and Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, twenty-first ed. American Public Health Association, Washington D.C. Baillod, C.R., Lamparter, R.A., Barna, B.A., 1985. Wet air oxidation for industrial waste treatment. Chem. Eng. Prog. 81, 52e56. Baker, J.R., Milke, M.W., Mihelcic, J.R., 1999. Relationship between chemical and theoretical oxygen demand for specific classes of organic chemicals. Water Res. 33, 327e334. Bali, U., Sengül, F., 2003. The fate and effect of 4-chlorophenol in an upflow anaerobic fixed-bed reactor. Proc. Biochem. 38, 1201e1208. Balkema, A.J., Preisig, H.A., Otterpohl, R., Lambert, A.J., Weijers, S.R., 2001. Developing a model based decision support tool for the identification of sustainable treatment options for domestic wastewater. Water Sci. Tech. 43, 265e269. Bieker, S., Cornel, P., Wagner, M., 2010. Semicentralised supply and treatment systems: integrated infrastructure solutions for fast growing urban areas. Water Sci. Tech. 61, 2905e2913. Çengel, Y.A., Boles, M.A., 2011. Thermodynamics. An Engineering Approach, seventh ed. McGraw-Hill, New York. Cimetiere, N., Dossier-Berne, F., De Laat, J., 2009. Monochloramination of resorcinol: mechanism and kinetic modeling. Environ. Sci. Technol. 43, 9380e9385. Debellefontaine, H., Chakchouk, M., Foussard, J.N., Tissot, D., Striolo, P., 1996. Treatment of organic aqueous wastes: wet air oxidation and wet peroxide oxidation. Environ. Pollut. 92, 155e164. Demoulin, G., Mervyn, C.G., 2003. Influence of on-line our control on operation costs of cyclic treatment plants. KA e Abwasser Abfall 50, 161e165. , E., Mare chal, F., 2012. Energy Descoins, N., Deleris, S., Lestienne, R., Trouve
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