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Ind. Eng. Chem. Res. 2008, 47, 8478–8483
Electrochemical Oxidation of Refractory Organics in the Coking Wastewater and Chemical Oxygen Demand (COD) Removal under Extremely Mild Conditions Bo Wang,* Xin Chang, and Hongzhu Ma Institute of Energy Chemistry, College of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, China
Coking wastewater causes severe environmental pollution because of its high concentration of organics and other dangerous chemicals, which makes it difficult to reuse by conventional technologies. In this article, we report an investigation of the electrochemical oxidation of coking wastewater at 298 K and 1 atm in the presence of ozone, where the reaction was assisted by potassium permanganate as the catalyst and kaolin as the carrier. The results showed that the refractory organics in wastewater can be effectively removed by this process, and a chemical oxygen demand (COD) removal efficiency of 92.5% was obtained in 80 min at pH 3. Operating parameters such as the current density, initial pH, and amount of catalyst were investigated. Catalyst lifetime was also tested, and the results showed that catalyst activity still remained even after the catalyst had been used three times. The impact of the treated wastewater on chlorophyll was also investigated, with a steady amount of chlorophyll indicating that the treated wastewater could be applied to irrigation. 1. Introduction Currently, coke plants and coal gasification plants are available in developing countries, and a large volume of wastewater containing phenolic compounds, cyanide, thiocyanide, and polynuclear aromatic hydrocarbons (PAHs) is generated by such plants. The presence of these pollutants in water sources produces long-term environmental and ecological impacts and makes treatment of the wastewater by conventional technologies difficult. Many studies have been carried out on the treatment of coking wastewater, including biological treatment,1,2 chemical coagulation,3 and wet oxidation.4 However, preliminary physicochemical treatment or biological processes do not always meet the legal limits for waste discharge in a short time, owing to the characteristics of high chemical oxygen demand (COD) and low biodegradability of coking wastewater.5,6 Wet air oxidation has been found to eliminate the refractory and inhibitory organics in wastewater that are not removed by biological treatment.7,8 Despite the substantial effectiveness of this process in removing organics, the energy required and the high-pressure reactors used make the wet air oxidation of large volumes of wastewater unviable and uneconomical.9,10 Other technologies are available for the treatment of coking wastewater, such as ultrasonic irradiation11 and nanofiltration,12 but the cost still remains an important factor for these processes. Recently, electrochemical oxidation has attracted particular attention for the treatment of wastewater, and it has been applied successfully to the treatment of both synthetic solutions containing phenolic compounds13-22 and real wastewater, such as textile effluents,23-26 landfill leachate,27,28 olive-oil wastewaters,29 domestic sewage,30 and tannery waste liquors.31-35 Nevertheless, the effectiveness of the electrochemical oxidation depends strongly on the experimental conditions and, above all, on the nature of the electrode materials. To date, several anodes, such as Ti/Pt, Ti/Ir, Ti/Pt-Ir, Ti/Co/SnO2, Ti alloys, and threedimensional electrodes, have been tested for the electrochemical oxidation of wastewater.36,37 For example, Li et al. used TiO2/ Ti anodes for the photoelectrocatalytic degradation of methylene * To whom correspondence should be addressed. Tel.: +86 29 85308442. Fax: +86 29 85307774. E-mail:
[email protected].
blue and obtained a COD removal efficiency of 87.0%.38 However, the complicated preparation processes of the electrodes make the industrial application of this approach unpractical. In addition, ozone as a powerful oxidant (E° ) 2.07 V) has also been used in the treatment of pure toxic compounds and concentrated waters from textile mills. For example, Shang et al. employed ozone to treat methyl methacrylate from semiconductor wastewater and achieved a high efficiency.39 On the basis of the above studies, the combined electrocatalysis oxidation of coking wastewater assisted by potassium permanganate at 298 K and 1 atm in the presence of ozone was developed. With porous graphite as the electrodes, inexpensive kaolin as the carrier, and no preparation of the catalyst, the method provides a versatile candidate for industrial application. In this context, the effects of operating conditions, including current density, initial pH, and amount of catalyst, as well as the catalyst lifespan and impact of treated wastewater on chlorophyll, were investigated. 2. Experimental Section 2.1. Materials. The chemical reagents used in the experiments were of analytical grade. Potassium permanganate as the catalyst and kaolin as the carrier, composed of Al4[Si4O10](OH)8 (surface area ) 20 m2 g-1 and pore volume ) 0.5 cm3 g-1), were used without further purification. The raw wastewater used in this investigation was obtained from the outlet of a coke plant near Xi’an City with the treatment processes of A1-A2-O biological treatment and chemical treatment to reduce cyanides. Some significant parameters of the raw wastewater are reported in Table 1. 2.2. Electrochemical Catalytic Oxidative Apparatus. A schematic diagram of the experimental setup is shown in Figure 1. The experiments were conducted in an undivided cell of 300mL capacity at 298 K and 1 atm. For each experiment, 10.0 g of kaolin and 1.0 g of potassium permanganate were added to 250 mL of raw wastewater to form a multiphase electrochemical oxidation slurry reactor. Then, two graphite electrodes were fixed vertically with an electrode distance of 0.5 cm and a total geometric area of 28 cm2. The solution was stirred at 200 rpm with a magnetic stirrer. Electric power was supplied by a
10.1021/ie800826v CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8479 Table 1. Characteristics of Coking Wastewater parameter
raw wastewater
pH COD (mg dm-3) BOD (mg dm-3) BOD/COD ratio suspended solids (mg dm-3) fats and oils (mg dm-3) phenol (mg dm-3) NH4sN (mg dm-3) SCN- (mg dm-3) CN- (mg dm-3) conductivity (µS cm-1) temperature (°C)
3.1 1496 360 0.24 58 75 137 205 184
