Research Paper Journal of Chemical Engineering of Japan, Vol 44, No 2, pp 123–129, 2011 Catalytic Wet Oxidation of Wastewater from Pulping Industry Using Solid Waste Containing Iron Oxides Pham Minh DOAN1, Ngoc Dung TRAN2, Thi Hau VU1 and The Ha CAO1 The Center for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, T3 Building, Hanoi, Vietnam Faculty of Chemistry, Hanoi University of Science, Vietnam National University, 19, Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam Keywords: Wet Air Oxidation, Wastewater Treatment, Heterogeneous Catalysis, Black Liquor, Pulping Industry The pulping industry generates great quantities of wastewaters (WW ), where a small amount of black liquor accounts for more than 90% of its entire manufacturing process load in organics Treatment of the black liquor from pulping manufacture in ThaiNguyen province (Vietnam) by catalytic wet oxidation (CWO) under mild reaction conditions (150–180°C, 15 bar) using solid wastes containing iron oxide as heterogeneous catalysts is communicated herein These solid wastes have been found to be active in the oxidation of pollutants in the black liquor and show a high application potential in CWO processes for the treatment of this kind of industrial wastewater Introduction Pulp and paper production is a particularly polluting industry as are major material productions The manufacturing process of the pulping industry produces great quantities of wastewaters which contain high concentrations of inorganic compounds (e.g., Na2CO3, Na2SO4, Na2S, NaOH, NaCl) and organic compounds (e.g., lignins, alcohols, polysaccharide fragments, carboxylic acids) (Adam et al., 1989; Pintar et al., 2001a; Chakar and Ragauskas, 2004) A recovery and/or elimination process of these compounds is necessary to minimize the production cost and/or reduce the pollution generated Vietnam is a tropical country with very high biomass production; therefore, the pulp and paper industry (PPI) has a good potential for growth, which is why the United States government invested $3.6 billion in a master plan to develop the Vietnamese PPI in the period of 2000–2010 The target production capacity of the plan is more than million ton/year of paper pulp by the year 2010, which could almost fully cover the domestic market demand (Ministry of Industry of Vietnam, 1997) Besides its undoubted benefits, PPI could be the heaviest pollution producer, particularly concerning the aquatic environment The sketch of material flows and waste Received on August 26, 2010; accepted on October 8, 2010 Correspondence concerning this article should be addressed to D Pham Minh (E-mail address: doanhoa2000@yahoo.fr) Copyright © 2011 The Society of Chemical Engineers, Japan streams of PPI in Vietnam are summarized in Figure Due to the high contents in organic and inorganic compounds in black liquor, chemical recovery stages are established, including a vacuum evaporation system, Tomlinson’s incinerator with a steam recovery boiler for organics combustion and heat utilization, a caustization stage to recover NaOH and Na2S for reuse in the cooking stage This unit is identified by the dotted rectangles in the right of Figure Therefore, organic contaminants are combusted; 93–95% inorganic chemicals (caustic, sulfide) are recovered and reused in cooking; only solid CaCO3 (from caustization) and a small amount of overflow water and sulfide are produced Nevertheless, in Vietnam, there are only two plants, Bapaco and Cogido (situated at Phu Tho and Dong Nai provinces, Vietnam), which possess this recovery unit Other plants have not been equipped with this technology or other treatment stations Therefore, black liquor and other wastewaters are discharged without adequate treatment Several studies have focused on the treatment of wastewaters of PPI in which different methods were proposed Wallberg et al (2003) studied the ultrafiltration of a Kraft black liquor (56 g/L of lignin, 37 g/L of inorganic materials, 16% of total dry substance and pH 13–14), using a KERASEP membrane (Novasep Corp.) with a cut-off of 15 kDa The temperature was found to have a significant influence on the flux of black liquor, which was 90, 110 and 130 L/m2 h at 60, 75 and 90°C and 100 kPa, respectively Coagulation of a birch pulp filtrate taken from the oxygen bleaching stage, which 123 Fig Sketch of material flows and waste streams of PPI in Vietnam: WW Wastewaters from raw material washing: This kind of WW contains mainly solid dregs, mud, bamboo or wood husks and chips, etc They can be easily removed by settling techniques with or without coagulation Treated wastewater can be reused or discharged WW Black liquor: Black liquor (BL) is the effluent of the cooking of raw materials from the Kraft process to free the cellulose fibres (Smook, 1992) Although this wastewater contributes to only a small amount of total wastewater volume, it contains about 90% organic load of the entire PPI wastewaters or more than half of the energy content of raw materials fed into the digester A high quantity of inorganic compounds was also present as mentioned above A condensate could be formed from two stages of cooking and BL evaporation The main composition of cooking condensate is methanol, volatile organic compounds, and a significant quantity of sulfur compounds, main odor source in PPI WW Bleaching wastewaters (BWW ): The bleaching technology used in Vietnam dates from the 1970’s BWW is formed after a chemical process to improve the brightness and whiteness of pulp, using different bleaching reagents (Cl2, ClO2, NaClO, H2O2, O3, etc.) This effluent is the principal source of organochlorines, which are persistent to conventional biological treatment WW Paper making wastewaters: This wastewater comes from a paper making stage; therefore, it contains mostly suspended solids and is easily treated contains less than 400 mg/L of wood extractives and lignans, was carried out by Leiviska and Ramo (2008), with or without the use of a cationic polyelectrolyte The best result was obtained with a copolymer of acrylamide and methacrylate of medium molecular weight and medium charge density at 72°C and pH 5–6 with extractives removal up to 92% However, the pollutants separated with these techniques must be treated, and thermal treatments are usually applied Font et al (2003) studied the incineration and pyrolysis of lignin separated by 124 the precipitation of a black liquor using sulfuric acid (weight percentage: 63.9% C, 25.8% O, 6.2% H, 0.8% N, 1.7% S) High emissions of CO were found between 25000–90000 mg/kg for incineration and 30–3000 mg/kg for pyrolysis The main by-products formed in the combustion were methane, ethylene, acetylene, benzene, toluene, indene, naphthalene, acenaphthylene, phenantrene, fluorantene and pyrene Therefore, these by-products limit the efficiency of thermal processes Recently, an advanced oxidation process JOURNAL OF CHEMICAL ENGINEERING OF JAPAN was thoroughly investigated for the treatment of different wastewaters Photocatalysis of an alkaline bleaching effluent (initial TOC 980 mg/L, pH 10.3 and COD 2255 mg/L) was performed in a batch reactor using TiO2 and ZnO as photocatalysts (Cristina Yeber et al., 2000) The decolorization was completed and the mineralization was at about 50% after 120 of treatment in the presence of the catalysts Ko et al (2009) studied the ozonation of diluted black liquor (initial COD 165 g/L and COD of diluted effluent between 50 and 600 mg/L) COD conversion reached 75% for diluted effluent of 150 mg/L of initial COD, but it was only 20% when the initial COD was 600 mg/L, after 30 of treatment More recently, wet air oxidation (WAO), which is based on the oxidation in liquid phase of pollutants under high temperature and high pressure, has been demonstrated to be an effective technology for the treatment of different kinds of wastewater (National Academy Press, 1993; Luck, 1999) This process was demonstrated to be efficient for effluent containing a high concentration of organic compounds, up to 100 g/L of COD (Mishra et al., 1995) The main disadvantage of WAO process is the severe conditions required to achieve sufficient oxygen activation, typically being in the range of 180–315°C and 20–150 bar (Luck, 1999) Under such extreme conditions, the selection of reactor materials and safety requirements become critical The use of catalyst, heterogeneous or homogeneous (so CWAO), was usually the best choice for oxygen activation in much milder conditions In other works, CWAO was generally more efficient than WAO under the same reaction conditions (Pintar et al., 2001b; Pham Minh et al., 2005; Barbati et al., 2008; Chaliha et al., 2008) WAO and CWAO of wastewaters from the Kraft bleach plant were carried out by Pintar et al (2001a, 2001b, 2004) WAO was efficient for the oxidation of organic compounds present in these wastewaters (total organic carbon-TOC between 665 and 1331 mg/L) at 190°C and 5.5 MPa in a batch reactor TOC abatement was up to 87% after h of reaction Adding a TiO2 support or supported ruthenium (3 wt%) catalyst enhanced the efficiency of the treatment with TOC removal being nearly total Acetic acid was found to be the final organic product after oxidation Titanium dioxide and supported ruthenium catalysts were shown to be stable over a long reaction time (more than 150 h) in a fixed-bed reactor No leaching of metals was observed after the reaction Other work on the CWAO of effluents from a bleaching plant was carried out by Zhang and Chuang (1998) using different catalysts A supported palladium catalyst was found to be more effective than supported manganese, iron, or platinum catalysts for the oxidation of wastewaters with initial TOC of 720–1500 mg/L at 190°C and 1.5 MPa oxygen pressure WAO and CWAO of diluted black liquor (initial COD after dilution 2700 mg/L, pH 8) were also realized in a batch reactor in the absence and in the presence of different homogeneous and VOL 44 NO 2011 Table Some parameters of the black liquor used in this study Parameter pH d [g/mL] COD [g/L] ABS [%] Solid in suspension Ϫ SSa [g/L] Total solid Ϫ TSb [g/L] Ash residuec [wt%] Value 11.8 1.008 44.4 72.9 1.2 46.5 40.0 ABS: UV adsorption at 390 nm a : mass of substances in black liquor remaining on filter paper in blue band and drying at 103°C b : mass of solid substances in L of black liquor after drying at 103°C c : residue after calcination of dried TS at 700°C with respect to the mass of dried solid substances heterogeneous catalysts (CuSO4, 5%CuO/C, 60%CuO– 40%MnO2 and 60%CuO–40%CeO2) (Garg et al., 2007) The best results were obtained with 5%CuO/C and 60%CuO–40%CeO2, yielding 78%COD conversion after h of reaction at 150°C and 0.85 MPa One important factor in selection of catalysts for wastewater treatment is the cost The present paper shows the results on the use of solid wastes from different production process in Vietnam in WAO, using pure oxygen as an oxidation agent Experimental The wastewater used in this study was taken from HoangVanThu alkaline pulping manufacturer (ThaiNguyen province, Vietnam) It was black in color and had an unpleasant odor It was stored in a deepfreezer and was defrosted just before use Some parameters of this effluent are shown in Table The catalyst precursors were solid wastes taken from four manufacturers in Vietnam: 1—GiaLam water plant (named Cat-1; sludge of ground water processing); 2—LamThao superphosphate plant (named Cat-2; solid waste of SO2 production by incineration of pyrite-FeS2); 3—Vietnam Ford Co (named Cat-3; solid waste from the treatment of wastewater of metal processing shop, facility by FeCl3 and lime coagulation); 4—TanBinh Chem Co (named Cat-4; solid waste of alumina production from bauxite) The choice of these solid wastes was based on their possible richness in iron oxides, which could be used as a heterogeneous catalyst in CWAO (Quintanilla et al., 2008), and their very low costs The original catalysts were obtained by simple a preparation procedure: drying, grinding and sieving to yield particles smaller than 0.5 mm, then thermal treating in air at 400°C Modified catalysts could be obtained by the addition of copper, manganese and magnesium 125 oxides on the original catalysts using impregnation technique The original catalyst was impregnated with a solution of CuSO4 or MnSO4 After drying, the solid was re-impregnated with a solution of NaOH to transform copper and manganese sulfates into hydroxides The solid was dried again and finally calcinated in air at 400°C for h The modified catalysts containing magnesium oxide were prepared with MgO in powder form Original catalysts were characterized by X-ray diffraction (XRD) using a Siemens D5005 diffractometer with Cu Ka1 ϩ radiation at 0.154184 nm The oxidation of black liquor was performed in a 450-mL batch reactor, equipped with a magnetic stirrer and an electric heating jacket The internal face of the reactor was coated with a Teflon layer to avoid the possible influence from the metal wall For the reaction, 100–150 mL of black liquor, with or without dilution, and 2–3 g of catalyst were introduced into the reactor After purging and heating to the working temperature (150–180°C), the reactor was pressurized and maintained at 15 bar with O2 The oxidation was started by setting of magnetic stirrer speed (1000 rpm) Samples withdrawn from the reactor were analyzed in terms of COD, color and pH The analysis of COD was carried out using a classical dichromate method (APHA, 1995) The color was measured as absorption value (ABS) at 390 nm using a cuvette with a thickness of cm on a UV-VIS 1201 (Shimadzu Corp.) The pH was measured with a pH-meter installed in our laboratory COD and color reductions were calculated with the following Eqs (1) and (2) XCOD [%] ϭ 100(CODt Ϫ COD0) / COD0 (1) XABS [%] ϭ 100(ABSt Ϫ ABS0) / ABS0 (2) (COD0, CODt, ABS0, ABSt: COD and UV absorption values of effluents before and after CWO treatment, respectively) Results and Discussion 2.1 XRD Three original catalysts including Cat-1, Cat-3, Cat-4, pretreated at 400°C in air, were found to be amorphous materials As an example, Figure shows a XRD pattern of Cat-1, which originated from ground water processing On the other hand, Cat-2 seems to have higher crystallinity (Figure 3) We found the presence of magnetite in this sample Other peaks are presently not identified 2.2 Catalytic activity of original and modified catalysts at mild temperature Table shows the results obtained in CWO of the black liquor in the absence of catalyst and in the presence of the heterogeneous catalysts after h of reaction at 150°C and 15 bar oxygen pressure These conditions are typical for CWO because no activity is known at am126 Fig XRD pattern of Cat-1 Fig XRD pattern of Cat-2; *: diffraction of magnetite phase Fe3O4 bient temperature and pressure Comparative activity of catalysts can be evaluated according to the conversion of COD under the same reaction conditions Obviously, CWO is much better than simple WAO Non-catalytic (WAO) can remove only 13% COD of black liquor, while all CWAO experiments reached between 28 and 38% after h of reaction, which is as much as 2–3 times more It should be noted that 150°C and 15 bar are very mild conditions for WAO and CWO process Among four original catalysts, Cat-1 and Cat-3 (reactions No and 11) were slightly better than Cat-2 and Cat-4 (reactions No and 19), with 34% of COD removal To make the catalysts more active, some active components such as CuO, MnO2 and MgO were added Copper and manganese oxides are currently used as the active phase of catalysts in the total oxidation of organic pollutants (Akyurtlu et al., 1998; Hocevar et al., 2000; Hu et al., 2001; Santos et al., 2001; Yoon et al., 2001; Akolekar et al., 2002) Particularly, in the case of black liquor, MgO was added to confirm the results of Robert (Tutorski, 1998), in that Mg salt is active in oxygen JOURNAL OF CHEMICAL ENGINEERING OF JAPAN Table Results on the oxidation of black liquor; temperature: 150°C, pressure: 15 bar with oxygen, stirrer speed: 1000 rpm, reaction time: h, catalyst mass: g, volume of black liquor: 100 mL, initial COD: 44.4 g/L, initial ABS at 390 nm: 72.9%; initial pH: 11.8 No 10 11 12 13 14 15 16 17 18 19 Catalyst Non-catalytic Cat-1 Cat-2 12%CuO/Cat-2 12%MnO2/Cat-2 12%MgO/Cat-2 4%CuO–4%MgO–4%MnO2/Cat-2 6%CuO–6%MgO/Cat-2 6%MgO–6%MnO2/Cat-2 6%CuO–6%MnO2/Cat-2 Cat-3 12%CuO/Cat-3 12%MnO2/Cat-3 12%MgO/Cat-3 6%CuO–6%MnO2/Cat-3 6%CuO–6%MgO/Cat-3 6%MgO–6%MnO2/Cat-3 4%CuO–4%MgO–4%MnO2/Cat-3 Cat-4 delignification Therefore, the modification of original catalysts was carried out on Cat-2 and Cat-3 This modification enhanced the catalytic activity of Cat-2, but decreased the catalytic activity of Cat-3 We presently have no explanation for the last case Further study on the characterization of these catalysts will be necessary to explain these results For the modified catalysts with the addition of one metal oxide (CuO or MnO2 or MgO), we observed an order of activity as follows: MgO Ϸ CuO Ͼ MnO2 This means that MgO was also active in the oxidation of organic pollutants For modified catalysts with the addition of two metal oxides, catalysts containing copper and magnesium oxides showed the highest activity Finally, the highest COD conversion was obtained at 38% with 6%CuO–6%MgO/Cat-2 after h of reaction (No 8) In parallel with the COD reduction, color removal was also observed In all cases, color reduction was more important than COD reduction This observation could be explained by the fact that color removal needs only destructive oxidation, while COD removal requires complete oxidation In fact, under CWO conditions, polyphenolic compounds (lignin) were firstly decomposed into smaller colorless molecules such as acetic acid (Pintar et al., 2001a, 2001b, 2004) Heterogeneous catalysts are known to promote the formation of O-radical species which effectively decompose colored organic compounds (Arena et al., 2010) CWO also oxidizes sulfur based compounds into sulfates having no particular odor The pH of treated effluent was also decreased in VOL 44 NO 2011 CODt [g/L] XCOD [%] ABSt [%] XABS [%] pHt 38.6 29.4 31.6 28.9 30.7 28.7 28.2 27.7 28.2 28.6 29.4 30.2 31.9 30.1 30.5 29.2 29.6 30.4 30.2 13 34 29 35 31 35 36 38 36 35 34 32 28 32 31 34 33 32 32 43.2 37.9 34.1 44.0 40.5 36.3 36.8 44.0 36.8 44.4 36.3 40.5 41.6 38.7 38.2 37.2 39.1 36.8 40.5 41 48 53 40 44 50 50 40 50 39 50 44 43 47 48 49 46 50 44 9.3 8.3 8.3 8.3 8.7 8.4 8.3 8.4 8.8 8.2 8.3 8.4 8.6 8.3 8.4 8.3 8.5 8.6 8.4 Fig Titration of 50 mL of black liquor with sulfuric acid (9.8 wt%) at ambient temperature comparison with that of crude effluent by the formation of shorter acidic molecules in oxidation conditions of WAO and CWO Despite a high COD reduction of up to 38%, the pH of treated effluent was, in most cases, found to be about 8.2–8.4 This may be due to the presence of a buffer system, for example, Na2CO3/NaHCO3, in the black liquor, as illustrated in Figure for the titration of the black liquor with a solution of H2SO4 (9.8 wt%), where we observed a pH buffer domain above 127 Table Influence of ratio of catalyst mass to effluent volume in the oxidation of diluted black liquor; temperature: 150°C, pressure: 15 bar with oxygen, stirrer speed: 1000 rpm, reaction time: h, catalyst: 6%CuO–6%MgO/Cat-3, volume of diluted black liquor: 100 mL, COD and ABS at 390 nm after dilution: 23.5 g/L and 51%, respectively Fig Influence of the temperature in the oxidation of diluted black liquor; pressure: 15 bar with oxygen, stirrer speed: 1000 rpm, catalyst mass: g, volume of black liquor: 150 mL, COD and ABS at 390 nm after dilution: 23.5 g/L and 51%, respectively the value of pH 9.5 This observation was important in confirming that transition metals present in the catalysts not dissolve Deactivation of catalysts is known to occur under lower pH conditions when large amounts of organic acid intermediates are produced (Besson and Gallezot, 2003) 2.3 Influence of temperature The influence of the temperature on COD and color removal was investigated between 150 and 180°C using g of 6%CuO–6%MnO2/Cat-3 and 150 mL of diluted black liquor (COD and ABS at 390 nm after dilution with distilled water: 23.5 g/L and 51%, respectively) In these experiments, samples were withdrawn periodically, and thus we used a higher volume of effluent Then, to assure the quantity of oxygen introduced in gas phase of reactor would be sufficient for possible total oxidation, dilution of the effluent with the diluted water was necessary The results are shown in Figure At each temperature, the COD and color of effluent continuously reduced over the reaction time The temperature had an evident influence on COD abatement COD abatement improved with higher temperatures The best result for COD conversion of 58% was obtained at 180°C after h of reaction From these results, the activation energy for COD reduction under experimental conditions was calculated to be a value of 97 kJ/mol On the other hand, the effect of the temperature on the color 128 mcat / Veffluent [g/mL] CODt [g/L] XCOD [%] ABSt [%] XABS [%] / 100 / 100 / 100 / 100 19.8 19.1 17.4 16.4 16 19 26 30 29.5 28.2 26.6 25.0 42 45 48 51 removal was less evident The best result for color removal was 74% at 180°C after h of reaction 2.4 Influence of the ratio of catalyst mass to effluent volume The influence of the ratio of catalyst mass to effluent volume was also investigated Oxidation of 100 mL of diluted black liquor was carried out in the presence of 1–4 g of 6%CuO–6%MgO/Cat-3 at 150°C and 15 bar oxygen pressure The results after h of reaction are presented in Table As expected, the COD and color removal increased with an increase in the catalyst mass to effluent volume ratio 2.5 Discussion Black liquor of the pulping industry is well-known as an important source of industrial wastewater, characterized by high contents of organic and inorganic pollutants In this study, we chose the CWO process as a method for the treatment of black liquor containing 44.4 g of COD The objective of this work was to confirm the efficiency of different solid wastes in the CWO treatment of this effluent The most important result of this work is demonstrating the technical feasibility of using available waste material for the preparation of CWO catalysts Conclusions The present study is the first to prove that industrial solid wastes containing iron oxides are active in CWO of black liquor from PPI Furthermore, MgO was also found to be an active component In the case of the solid wastes from the production of SO2 (Cat-2), the catalytic activity can be enhanced by the addition of a well-known catalytic component such as CuO, MnO2 or MgO For practical application, further research on the composition and nature of the catalysts should be performed to better understand the catalytic behavior of the original and modified catalysts The stability of these catalysts also must be confirmed The reduction in toxicity and biotoxicity of treated wastewaters could be verified prior JOURNAL OF CHEMICAL ENGINEERING OF JAPAN to subsequent final biological treatment Furthermore, it can be expected that the efficiency of the process will be improved by optimizing reactor engineering aspects of the process Acknowledgments We would like to express our deep gratitude to Prof Nguyen Huu Phu, NCST, Vietnam, for the use of Parr Instrument We are also grateful to Nafosted for financial support and our colleagues at CETASD for their technical assistance Literature Cited Adams, T., B Cowan, D Clayton, D Easty, D Einspahr, D E Fletcher and T M Malcolm; 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3—Vietnam Ford Co (named Cat-3; solid waste from the treatment of wastewater of metal processing... 19 Catalyst Non -catalytic Cat-1 Cat-2 12%CuO/Cat-2 12%MnO2/Cat-2 12%MgO/Cat-2 4%CuO–4%MgO–4%MnO2/Cat-2 6%CuO–6%MgO/Cat-2 6%MgO–6%MnO2/Cat-2 6%CuO–6%MnO2/Cat-2 Cat-3 12%CuO/Cat-3 12%MnO2/Cat-3... this wastewater contributes to only a small amount of total wastewater volume, it contains about 90% organic load of the entire PPI wastewaters or more than half of the energy content of raw materials