Response surface modeling for optimization heterocatalytic Fenton oxidation of persistence organic pollution in high total dissolved solid containing wastewater G. Sekaran, S. Karthikeyan, R. Boopathy, P. Maharaja, V. K. Gupta & C. Anandan
Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-013-2024-z
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Author's personal copy Environ Sci Pollut Res DOI 10.1007/s11356-013-2024-z
RESEARCH ARTICLE
Response surface modeling for optimization heterocatalytic Fenton oxidation of persistence organic pollution in high total dissolved solid containing wastewater G. Sekaran & S. Karthikeyan & R. Boopathy & P. Maharaja & V. K. Gupta & C. Anandan Received: 15 March 2013 / Accepted: 18 July 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract The rice-husk-based mesoporous activated carbon (MAC) used in this study was precarbonized and activated using phosphoric acid. N2 adsorption/desorption isotherm, X-ray powder diffraction, electron spin resonance, X-ray photoelectron spectroscopy and scanning electron microscopy, transmission electron microscopy, 29Si-NMR spectroscopy, and diffuse reflectance spectroscopy were used to characterize the MAC. The tannery wastewater carrying high total dissolved solids (TDS) discharged from leather industry lacks biodegradability despite the presence of dissolved protein. This paper demonstrates the application of free electron-rich MAC as heterogeneous catalyst along with Fenton reagent for the oxidation of persistence organic compounds in high TDS wastewater. The heterogeneous Fenton oxidation of the pretreated wastewater at optimum pH (3.5), H2O2 (4 mmol/L), FeSO4⋅7H2O (0.2 mmol/L), Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2024-z) contains supplementary material, which is available to authorized users. G. Sekaran (*) : S. Karthikeyan : R. Boopathy : P. Maharaja Environmental Technology Division, Council of Scientific & Industrial Research (CSIR)—Central Leather Research Institute (CLRI), Adyar, Chennai 600 020, Tamil Nadu, India e-mail:
[email protected] S. Karthikeyan e-mail:
[email protected] V. K. Gupta Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India V. K. Gupta Dr. R M L Avadh University Faizabad, Faizabad, Uttar Pradesh 224001, India C. Anandan Surface Engineering Division, National Aerospace Laboratories, Post Box 1779, Bangalore 560017, India
and time (4 h) removed chemical oxygen demand, biochemical oxygen demand, total organic carbon and dissolved protein by 86, 91, 83, and 90 %, respectively. Keywords Mesoporous activated carbon . Heterogeneous Fenton oxidation . High TDS wastewater . RSM
Introduction The conversion of raw animal hides/skins into leather involves the generation of 3 L of wastewater per kilogram of animal hides/skins (3 L/kg of skins). The low biochemical oxygen demand 5 (BOD5)/chemical oxygen demand (COD) ratio implies the nonamenability of wastewater to biological treatment. The high total dissolved solids (TDSs) (>1 %) content of wastewater reduced the treatment's efficiency. There are many reports on the adverse effect of these high salt concentration on microflora during biological treatment (Kargi and Uygur 1996; Dan et al. 2003; Uygur and Kargi 2004), and thus, the high TDS wastewater is segregated from other streams and evaporated in solar evaporation pans (this is accompanied with emission of malodorous gases). The evaporated residue is stored in the industries as they lack reusable characteristics due to the presence of persistence organic impurities. Hence, treatment of saline wastewater discharged from leather industry becomes the priority task not only to the industry but also to the environmental technologists, too. The advanced oxidation processes (AOP) have been developed for the destruction of a wide range of organic compounds in wastewater (Brillas et al. 2009; Umar et al. 2010; Shrivastava and Rao 2011; Eren 2012; Sharma et al. 2012) that involve in situ generation of highly potent chemical oxidant such as hydroxyl radical (OH•) with a high electrochemical oxidation potential (2.8 V versus normal hydrogen electrode) (Karthikeyan et al. 2013). The hydroxyl radicals react with the persistence organic pollution break them down gradually into smaller fragments, which gives a higher
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biodegradation potential. Among the AOPs, Fenton oxidation of organics has been reported for many industrial wastewater (Bautista et al. 2008; Herney-Ramirez et al. 2008; Atmaca 2009) because of its environmental benign nature. AOP can be attempted for the destruction of organic compounds in high TDS wastewater. However, the efficiency of Fenton reagent was decreased in the presence of soluble chloride ions in the high TDS wastewater (Laat et.al. 2004; Lu et al. 2005) The chloride ions tend to scavenge the hydroxyl radicals generated in the Fenton reaction and forming chlorine radicals, which in turn chlorinate the organics present in the wastewater rendering them bioresistant (Nidheesh et al. 2013). Therefore, in the present investigation, the mesoporous activated carbon was used as heterogeneous catalyst to oxidize the dissolved organics in wastewater. Many researchers have reported that mesoporous activated carbon (MAC) favored the production of hydroxyl radicals from aqueous phase containing dissolved oxygen (Mourand et al. 1995; Khalil et al. 2001; Toledo et al. 2003; Sekaran et al. 2004; Kuznetsova et al. 2004). Mesoporous activated carbon serves to adsorb hydroxyl radical and dissolved organic compounds onto its surface and is followed by oxidation. There are no reports on the heterogeneous Fenton oxidation of dissolved organics in wastewater containing high TDS discharged from leather industry. The response surface methodology (RSM) is an efficient way to optimize the variables by analyzing and modeling the effects of such variables and their responses (Korbahti 2007; Granato et al. 2010; Hong et al. 2011; Liu et al. 2010; Singh et al. 2010). The classical method of experimental optimization involves changing one variable at a time, keeping the others constant. The method, however, requires a large number of experiments to illustrate the effect of individual factors. In addition, this method does not consider the effect of interactions among various parameters (Montgomery 2005). The application of statistical methods to select important parameters from a large number of factors and the interactions between important variables can be understood very easily. Hence, the focal theme of the present investigation was on heterogeneous Fenton oxidation of organics in high TDS containing wastewater and application of RSM for the optimization process variables.
acid at 700, 800, and 900 °C. The MAC after activation was washed with hot water until the free phosphate was washed out completely. The washed sample was then dried at 110 °C to get the final product, and it was labeled as MAC700, MAC800, and MAC900, respectively. Table 1 demonstrates the physical, chemical, and structural characteristics of MAC800.
Materials and methods Rice husk as the precursor material obtained from the agricultural industry was washed with water several times for the removal starchy matter and dried in hot air oven at 110 °C for 6 h. The dried samples were then sieved to about 1,000 to 600 μm in size, and used in the preparation of MAC. Preparation of electron rich activated carbon matrix Electron rich MAC was prepared in sequential steps such as precarbonization at 400 °C and activation using phosphoric
Characterization of the MAC samples N2 adsorption–desorption N2 adsorption–desorption isotherms of the MAC samples were determined using an automatic adsorption instrument (Quantachrome Corp., Nova-1000 gas sorption analyzer) for the determination of the surface area and total pore volume. In addition, the t-plot method (Gregg and Sing 1982) was applied to calculate the micropore volume and external surface area (mesoporous surface area). The total pore volume was estimated to be the liquid volume of adsorbate at a relative pressure of 0.99. All surface area measurements were calculated from the nitrogen adsorption isotherms by assuming the area of the nitrogen molecule to be 0.162 nm2.
CHNS analysis The elemental composition (carbon, hydrogen, and nitrogen content) of MAC samples were determined, using CHNS 1108 model Carlo-Erba analyzer. Table 1 Characteristics of mesoporous activated carbon samples Parameters
Mesoporous activated carbon MAC700
MAC800
MAC900
SBET (m2/g) Smic (m2/g) Smeso (m2/g) Production yield of carbon (%)
345 203 142 40.66
379 215 165 39.19
439 215 224 37.69
Average pore diameter(Å) Carbon (%) Hydrogen (%) Nitrogen (%) Moisture (%) Decolorizing power (mg/g) Point zero charge (PZC) Apparent density (g/cm3) Free electron density (spins/g) Energy gap (eV) Ash content (%)
38.82 42.56 3.14 0.82 12.24 67.23 6.6 0.65 8.51×1020 1.35 41.24
39.36 41.58 2.85 0.75 13.22 68.98 6.9 0.61 16.05×1021 1.55 41.60
35.28 37.96 2.40 0.50 13.56 69.32 7.1 0.56 15.98×1021 1.52 45.68
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Diffuse reflectance spectrum The diffuse reflectance UV–visible spectrum was recorded, using Cary100 UV–visible spectrophotometer using BaSO4 as a reference compound to estimate their energy band gap.
at B0 =7.04 T under the condition of magic angle spinning (MAS) operating at a Larmour frequency of 59.624 MHz at room temperature. The sample spinning speed to external field was 10 kHz. A 5-μs width, 15 s pulse delay, and a pulse of angle 60°° were used. Chemical shift values were expressed in parts per million relative to internal tetramethyl silane.
X-ray diffraction pattern of MAC800 X-ray photoelectron spectroscopy analysis X-ray diffraction experiments were performed with a Philips X'Pert diffractometer for 2theta values from 10 to 80 °, using Cu Kα radiation at a wavelength of 1.54 Å. The other experimental conditions included the 1/2 ° divergence slits, a 5-s residence time at each step, and the intensity was measured in counts.
X-ray photoelectron spectroscopy (XPS) was carried out in a SPECS XPS system using 150 W Al Kα radiation. High resolution spectra of C 1s, O 1s, N 1s, and Fe 2p core levels were obtained at pass energy of 25 eV. The spectra were fitted with Gaussian–Lorentzian components to determine different oxidation states.
Free electron density determination using ESR analysis The free electron density of MAC was determined, using electron spin resonance spectroscopy (Bruker X-band CW ESP (EMX 102.7). The spectra were recorded at instrumental parameters: time constant, 2.56 ms; sweep time, 20.9 s; modulation amplitude, 3 G; microwave power, 1 mW to minimize signal distortion and saturation effects. Preliminary experiments were carried out with a 4-hydroxy TEMPO (TEM POL) as reference spin probe compound. TEMPOL was mixed homogeneously with sodium chloride so that 1 g of the mixture contained 5 mg of TEMPOL. A known amount of mixture (containing 2, 2.25, 2.5, 2.75, and 3 μmol of TEMPOL) was transferred into each electron spin resonance (ESR) tube, and the intensity recorded was made equivalent to the theoretically calculated spins. Exactly 50 g of each MAC sample was loaded in the quartz tube (similar dimension to the one used for spin probe method), vacuum was applied to remove air from the carbon surface and the spectrum was recorded. Each sample was analyzed thrice to obtain reproducibility. Scanning electron microscopy Surface morphology of MAC800 was captured using a LeoJeol scanning electron microscope. The MAC800 was coated with gold by a gold sputtering device for clear visibility of the surface morphology. Transmission electron spectroscopy The transmission electron micrographs were captured using a Philips-TEM (CM20) microscope. 29
Si-NMR spectrum of MAC800
29
Si solid state nuclear magnetic resonance (NMR) spectrum was recorded on a Bruker-DSX 300 spectrometer (Germany)
Collection and pretreatment of wastewater for Fenton—oxidation process The high TDS wastewater used in the present study was obtained from commercial tannery located in Chennai processing raw skins into finished leather. The coarse solids such as fleshing, trimmings, and animal hair were removed by passing the wastewater through a nylon screen of aperture of 5 mm. The screened effluent was collected and characterized for pollution parameters. The high TDS wastewater was coagulated and flocculated for the removal of suspended solids by flash mixing at 170 rpm for 5 min with alum (aluminum sulfate) at a dose of 700 mg/L (at strength of 3 %w/v). The wastewater was coagulated at 50 rpm for 20 min. It was allowed to stand for 3 h in a quiescent condition. The supernatant liquor was siphoned off without disturbing the sludge interface and filtered through a sand filter to remove the suspended solids. Fenton activated carbon catalytic oxidation process Preliminary experiments on optimization of process variables such as pH (from 2.5 to 4.5 with incremental increase pH of 1), concentration of hydrogen peroxide (3, 4, and 5 mmol/L) and concentration of FeSO4⋅7H2O (0.1, 0.2, and 0.3 mmol/L) to initiate the oxidation process are presented in Table 2. The solution pH was adjusted using sulfuric acid at proper dilution (strength, 36 N; specific gravity, 1.81; and purity, 98 %). The heterogeneous Fenton oxidation of high TDS wastewater was carried out with H2O2 (30 %w/v), FeSO4⋅7H2O and MAC800 (10 g/L). Compressed air at flow rate of 1.2 L/h and at a pressure of 0.6 kg/cm2 was passed through the wastewater for the generation of hydroxyl radical and there after for the destruction of organics as illustrated under mechanistic view. The Fenton-activated carbon catalytic oxidation (FACCO) reaction was continued at the ambient pressure and at constant temperature for 24 h. Aliquots of samples were withdrawn for
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every 1 h and analyzed for pH, COD, total organic carbon (TOC), and dissolved protein according to standard procedures mentioned (APHA 1992). The homogeneous Fenton oxidation was carried out by dosing the high TDS wastewater with H2O2 and FeSO4⋅7H2O. The other conditions were similar to heterogeneous Fenton reaction. The COD measured in the wastewater samples collected from the Fenton reactor was corrected by determining the residual hydrogen peroxide in the treated sample using iodometric titration, and the same was subtracted from the measured COD for following the method of (Talini and Anderson 1992), in accordance with the equation shown below to prevent the interference of H2O2 in COD analysis. CODA ¼ CODM −Rp 0:25
ð2Þ
where Y is the response, Xi and Xj are variables (i=1–4), β0 is the constant coefficient, and βj, βjj, and βij (i and j=1–4) are interaction coefficients of linear, quadratic, and the secondorder terms, respectively. The RSM including ANOVA was used to obtain the interaction between the process variables for the response on COD/COD0 ratio by hetero-Fenton process. The empirical polynomial model for the selected process was arrived based on the coefficient of determination R2 and Radj2, and the statistical significance was checked by the F test (
