ARTICLE pubs.acs.org/IECR
Wastewater Ozonation Catalyzed by Iron Anaid Cano Quiroz, Carlos Barrera-Díaz,* Gabriela Roa-Morales, Patricia Balderas Hernandez, Rubí Romero, and Reyna Natividad Centro Conjunto de Investigacion en Química Sustentable UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Toluca, Estado de Mexico, C.P. 50200 Mexico ABSTRACT: The effectiveness of ozone and integrated ozone catalyzed processes with iron plates on industrial wastewater treatment was evaluated. Both processes were tested at different pH values of the industrial wastewater. Ozonation, under optimal conditions of pH 7 and treatment time of 60 min, reduced the chemical oxygen demand (COD) by 45%, color by 82%, and the turbidity by 55%. The addition of iron plates inside the reactor greatly improved the reduction of COD (80%), color (90%) and turbidity (99%). Thus, it was concluded that the ozonation process catalyzed with iron plates noticeably improves wastewater quality. Additional advantages of the process are that there is no sludge generation and it occurs readily at pH 7.
1. INTRODUCTION Water availability is a problem that people face in different regions of the world since many industries generate effluents that contain pollutants which should be eliminated before being discharged.1 Previous studies show that conventional treatment methods (i.e., biological, physical, and chemical) applied to the industrial effluents treatment have limited success since wastewater contains very stable refractory and toxic compounds.2 Another main drawback of these processes is that they are associated with the production of a large amount of sludge, which requires treatment before final disposal. Recently, studies have paid more attention to the treatment of refractory or toxic polluting agents contained in the effluents. The new way of approaching the elimination of these contaminants is through advanced oxidation processes (AOPs).3 AOPs are defined as the processes in which the hydroxyl radical is the main implied oxidant. It has been reported that these technologies are capable of degrading a wide variety of refractory compounds from stabilized lixiviation, and they are an excellent alternative for treating residual waters of high resistance. Indeed, AOPs exhibit a high efficiency, especially when they are compared to processes involving conventional chemical oxidation.4-7 Two of the most simple and most representative advanced oxidation processes are the oxidation by ozone and by the use of the Fenton reagent. Although ozone is a powerful oxidant, a total organic compound mineralization is not likely to be achieved by ozonation only.8 Therefore, the combination of ozone with other technologies, such as UV, ultrasound, electrocoagulation, and flotation, allows the increase of the pollutants removal yield.9-12 On the other hand, Fenton oxidation combines hydrogen peroxide with iron in acidic conditions. These reagents promote the organic pollutants oxidation by generating hydroxyl radicals. Albeit this AOP is very useful for the removal of aromatic compounds due to their fast reaction with hydroxyl radicals, there are occasions in which complete mineralization is not attained. This is mainly due to the formation of Fe (III) compounds, which contribute to the generation of carboxyl acids that are very difficult r 2010 American Chemical Society
to destroy by means of this oxidant.13,14 Even the Fenton based processes (i.e., Electro-Fenton and Foto-Fenton) require one to maintain an acidic pH which makes the addition of chemicals into reacting solution unavoidable.15-17 Another attempt to improve ozonation removal efficiency is the use of metals or their oxides to catalyze ozonation.18-21In this context, the advantages of the use of Fe (III) based catalysts have been reported for the oxalic acid degradation.21 In such a study, a complete mineralization was attained and an interesting mechanism was proposed. It was suggested that mineralization occurs via metal-oxalate complexation rather than via hydroxyl radicals. The present work aims to compare ozonation chemical oxygen demand (COD), color, and turbidity removal efficiency with that of ozonation when Feo plates are used. The effect of pH on both processes has also been established. It is worth mentioning that the above-mentioned process efficiency was evaluated using a real effluent rather than synthetic water. Such effluent has a complex composition since it is the mixture of effluents from chemical, food, beverage, textile, and pharmaceutical industries.
2. METHODS AND MATERIALS 2.1. Wastewater Samples. Wastewater samples were collected from a wastewater treatment plant in the vicinity, which treats industrial wastewater from the industrial corridor via biological processes. The wastewater was collected in plastic containers at a temperature of 4 °C. Samples were subsequently transported to the laboratory for analysis and treatment. 2.2. Oxidation Reaction. Wastewater samples were treated by two methods, ozonation alone and ozonation added with Special Issue: IMCCRE 2010 Received: March 19, 2010 Accepted: August 25, 2010 Revised: August 10, 2010 Published: September 23, 2010 2488
dx.doi.org/10.1021/ie1006849 | Ind. Eng. Chem. Res. 2011, 50, 2488–2494
Industrial & Engineering Chemistry Research metallic iron. For both methods, ozone was generated from dry air by the use of an ozone generator (Pacific Ozone Technology). All experiments were conducted in a 0.5 dm3 up-flow bubble column, at room temperature (19 ( 2 °C). The inlet gas flow rate was 0.23 dm3 3 min-1, and the average production of ozone was around 0.005 g 3 dm-3. This was continuously fed and dispersed into the reacting solution through a gas diffuser that allowed the production of about 2 mm bubble size. Iron was introduced into the reactor as a plate (240 cm2). pH was continuously monitored and controlled by H2SO4 addition. Its effect on COD, color, and turbidity was established in the range of pH 3-7 for both processes. 2.3. Analytical Procedure. The characterization of samples before and after treatment consisted of determining COD, color, pH, and turbidity (Pt/Co scale), by following the American Public Health Association (APHA) standard procedures. 22 The color of samples (apparent color) was determined by comparing it with a standardized Pt/Co scale (true color) in a Hach spectrophotometer.22 Thus, the units of this parameter are Pt/Co. The turbidity analysis is a method based on the optical effect that is caused when light rays are passed through a water sample and they are scattered due to mineral or organic particles that may contain liquid. The apparatus (Hach spectrophotometer) measures the intensity of scattered light at 90 degrees when a ray of light passes through a water sample and compares it with that of a reference suspension. Thus, the units for this parameter are nefelometrics turbidity units (NTU). 1NTU is equivalent to 1 ppm of formazin.23 In order to quantify the total iron (Fe2þ and Fe3þ) content in solution during the catalyzed treatment at pH 7, the standardized technique of ortophenentroline22 was utilized. 2.4. Cyclic Voltammetric Measurements. Cyclic voltammetry was performed before and after treatment using a cell coupled with a three electrode system. The reference electrode was Ag/AgCl, the auxiliary electrode was a platinum wire, and the working electrode was carbon paste (CPE) that was constituted by a 1:1 graphite and nujol mixture. The waveforms were generated by a potentiostat BAS model Epsilon-2. 2.5. UV-Vis Spectrometry. UV-vis spectra were obtained for raw samples and samples after treatment, using a double beam spectrophotometer Perkin-Elmer 25. The scan rate was 960 nms-1. The samples were scanned in a quartz cell with 1 cm optical path.
3. RESULTS AND DISCUSSION 3.1. Treatment with Ozone. In order to establish the efficiency of ozonation without catalyst, the effect of two variables (pH and time) on COD, color, and turbidity removal was investigated. The results are shown through Figures 1-4. Figure 1 shows the COD as function of treatment time at different pH conditions. There can be distinguished three kinetic stages through the profiles depicted in Figure 1. The first one is associated to the first 5 min of treatment where a clearly more rapid oxidation rate (given by COD diminishment rate) than the one observed in the range of 5-15 and 15-60 min of treatment occurs. Since the reacting solution is a mixture of effluents of several industries (pharmaceutical, automotive, painting, textile, food, and chemical), the presence of several compounds and,
ARTICLE
Figure 1. Effect of treatment time and pH on COD when only ozonation is applied to oxidize the pollutants in the mixture of industrial effluents. Error
