Ind. Eng. Chem. Res. 2010, 49, 5583–5587
5583
Fenton Pretreatment in the Catalytic Wet Oxidation of Phenol Aurora Santos,* Pedro Yustos, Sergio Rodriguez, Ernesto Simon, and Arturo Romero Ingenierı´a Quı´mica, Facultad CC, Quı´micas, UniVersidad Complutense Madrid, 28040 Madrid, Spain
Catalytic wet oxidation (CWO) of 1000 mg/L of phenol (PhOH) aqueous solutions has been carried out using a commercial activated carbon as catalyst, placed in a continuous three-phase reactor at 16 bar, and temperature was changed in the interval 127-160 °C. Pure oxygen was fed as gaseous phase. Pollutant conversion, mineralization, intermediate distribution, and toxicity were measured at the reactor exit. The catalyst weight (W) to liquid flow rate (QL) ratio was varied from 0.3 to 17.5 gACmin/mL. Fenton reagent (FR) has been applied to the same phenolic aqueous samples using different amounts of H2O2 (between 10 and 100% of the stoichiometric dose) and 10 mg/L of Fe2+, in a batch way at 50 °C. An integrated process has been proposed that combines FR as pretreatment of the CWO process. In the FR step, 10% of H2O2, 10 mg/L of Fe2+, and reaction times shorter than 40 min are used. Efluent from FR step is fed to the CWO reactor at 127 °C. High mineralization (80-90%) and total detoxification of the effluent was obtained at the reactor exit using W/QL values lower than 10 g · min/mL. Therefore, the FR pretreatment enhances remarkably the efficiency of the CWO at moderate temperature conditions. 1. Introduction Many industries generate wastewater containing organic compounds refractory to conventional biological oxidation.1 Among the most commonly found pollutants in effluents from petrochemical, chemical, coal processing, and pharmaceutical industries are phenol and phenolic compounds.2-14 The technology with the greatest potential to eliminate these pollutants is oxidation. The oxidants most frequently used have been the hydroxyl radical, in advanced oxidation processes (AOPs),15,16 and oxygen, in wet air oxidation technologies (WAO).17 One of the most studied processes for generating the OH• radical is the Fenton system, which employs iron salts and H2O2.18-22 A treatment plant using the Fenton processes tends to be less expensive to build and operate than most other AOPs. A major disadvantage of wastewater treatment by homogeneous Fenton processes is the need for high dosages of iron and H2O2 to obtain a high mineralization of the pollutants. Iron must usually be removed from the water after treatment to recover the metal and comply with regulatory limits for aqueous effluent discharge. This can usually be achieved by increasing the pH of the solution. However, higher pH can lead to the production of large volumes of iron-containing sludges. In addition, while phenols are almost completely oxidized by low amounts of H2O2,13 a H2O2 dosage higher than the stoichiometric amount is required to obtain significant mineralization. Irradiation with UV light is often necessary to accelerate the process and increase TOC conversion.23-25 The WAO process, for its part, has demonstrated high effectiveness in the abatement of the organic pollutants but has the drawback of extreme operational conditions, 150-325 °C and 20-200 bar.17,26 The use of heterogeneous catalysts in the wet oxidation process allows operation at lower pressure and temperature, and has been the subject of numerous studies in recent decades.27,28 The main problem to be solved is finding a catalyst that remains active and stable under operating conditions. Recently, activated carbon (AC) has been used with success as a catalyst that does not require the impregnation of metals.29,30 The catalytic activity in the oxidation reactions in * To whom correspondence should be addressed. E-mail: aursan@ quim.ucm.es.
the liquid phase is attributed to the oxygenated groups at the catalyst surface.32 Temperature and oxygen pressure must be controlled to avoid burning the catalyst. Previous works found that aqueous solutions of PhOH33,34 and substituted phenols like cresols and nitrophenols35 were oxidized to short chain acids and CO2 using a commercial AC at 160 °C and 16 bar with pure oxygen as the oxidant. It was observed that PhOH conversion and mineralization levels were more similar than what were obtained with other catalysts based on copper oxides36-38 or with the Fenton reagent.39 This means that the AC quickly oxidized the first organic intermediates produced in the oxidation of phenols. The present work checks the effectiveness of introducing a Fenton pretreatment before wet oxidation with a commercial AC as catalyst to improve the abatement, mineralization, and detoxification of PhOH aqueous solutions. The objective is to avoid a further iron recovery step and decrease the amount of reactant (H2O2) in the Fenton process, enhancing the effectiveness of the CWO at milder temperature conditions. 2. Materials and Methods 2.1. Fentons Runs. Fenton’s reactions, FR, have been carried out in a batch way by adding different amounts of H2O2 to an aqueous solution containing 1000 mg/L of PhOH. Reaction temperature employed was 50 °C. At this temperature negligible thermal degradation of H2O2 was noticed at 3 h. Taking into account the maximum concentration of iron cation permitted for the discharge of industrial wastewater in the region of Madrid (Spain), 10 mg/L of Fe2+ was the amount selected to carry out the Fenton runs. This avoids having to add a further step for iron elimination and/or recovery. The concentration of H2O2 was in the range of 0.5-5 g/L, corresponding to 10-100% of the stoichiometric dose required for the total mineralization of PhOH. The initial pH was set to 3.5 adjusted by sulfuric acid. The pH and oxidation-reduction potential were measured during the runs by means of pH and ORP electrodes, respectively. Commercial FeSO4 from Fluka was used as iron source, and the hydrogen peroxide reagent used was a solution 30% in weight provided by Riedel-de Hae¨n.
10.1021/ie1004948 2010 American Chemical Society Published on Web 05/14/2010
5584
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
2.2. Catalytic Wet Oxidation Runs. CWO has been studied in a three-phase fixed bed reactor (FBR) by feeding either an aqueous solution of 1000 mg L-1 of PhOH or the FR effluent (10% H2O2), concurrently with a gaseous oxygen flow (90 mL/ min at STP conditions). The catalyst employed was a commercial AC (Industrial React FE01606A) supplied by Chemviron Carbon, selected in a previous work.31 Temperature was varied between 127 and 160 °C and pressure at the reactor was set to 16 bar. The catalyst weight to liquid flow rate ratio (W/ QL) was varied from 0.3 to 17.5 gACmin/mL. A scheme of the experimental setup is given elsewhere.33 The FBR reactor is made of a staninless steel tube 0.75 cm in internal diameter and 25 cm long. At the operational conditions employed, a steady state for the outlet composition of the reactor was achieved in the first 40 h of operation when fresh catalyst is placed in the reactor. For consecutive runs the time required to reach the steady state is about 10-20 h. 2.3. Analytical Methods. Liquid samples were periodically drawn and analyzed. The pollutant conversion and organic intermediates were quantified by HPLC (Hewlett-Packard 1100) using a diode array detector HP G1315A (wavelengths of 192, 210, and 244 nm); a chromolith performance column (monolithic silica in rod form, RP-18e 100-4.6 mm) was used as the stationary phase; a mixture of acetonitrile, water, and a solution of 3.6 mM H2SO4 in the ratio (v/v/v) was used as mobile phase, and a flow rate of 1 mL min-1 was used. The remaining organic content (TOC) was measured with a Shimadzu TOC-V CSH analyzer by oxidative combustion at 680 °C, using an infrared detector. Organic acids were analyzed by ionic chromatography (Metrohm 761 Compact IC) using a conductivity detector, and a column of anion suppression Metrosep ASUPP5 (25 cm long, 4 mm diameter) as a stationary phase and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as mobile phase, at a constant flow rate of 0.7 mL min-1. The toxicity of the liquid samples after treatment was determined by means of a bioassay following the standard Microtox test procedure (ISO 11348-3, 1998) using a Microtox M500 analyzer (Azur Environmental) based on the decrease of light emission by Photobacterium phosphoreum resulting from the exposure to a toxicant. More details for the analytical procedure are given elsewhere.34 All the chemicals used in the analytical methods were purchased from Sigma-Aldrich, and the microorganisms were Microtox Acute Reagent supplied by I.O. Analytical.
Figure 1. Phenol conversion and TOC percent removal obtained in the catalytic wet oxidation of PhOH with AC.35 P ) 16 bar, T ) (a)127 °C, (b) 140 °C, (c) 160 °C. Co ) 1000 mg/L phenol.
3. Results and Discussion Catalytic wet oxidation (CWO) of PhOH was carried out at temperatures of 127, 140, and 160 °C. Data at 160 °C were obtained elsewhere.35 Phenol and TOC conversion, the distribution of organic intermediates, and the toxicity units of the reactor effluent were determined at each value of W/QL, and the results obtained are shown in Figures 1, 2, and 3, respectively. As can be seen higher pollutant and TOC conversions vs W/QL are obtained if temperature increases (Figure 1). Cyclic organic intermediates (Figure 2) detected were catechol (CTL), p-hydroxybenzoic acid (4-HBZO), hydroquinone (HQ), and benzoquinone (BQ). These compounds are produced in a serial reaction scheme given elsewhere35 and are further oxidized to the short chain acids (acetic, formic, maleic, and oxalic acid) and to CO2, explaining the maximum noticed for the concentration of these cyclic compounds vs. W/QL. The concentration values of the cyclic compounds at the maximum are lower when temperature increases and are obtained at lower W/QL values. The toxicity units of the reactor effluent (also shown in Figure 3) are in agreement with the organic intermediate profiles
Figure 2. Intermediate distribution obtained in the catalytic wet oxidation of PhOH with AC.35 P ) 16 bar, T ) (a) 127 °C, (b) 140 °C, (c) 160 °C. Co ) 1000 mg/L phenol.
detected and their corresponding EC50. TOC and pollutant conversions values are closer using AC as catalyst than by employing other catalyst based on copper oxides.33,34,37 This means that the first oxidation intermediates, which are cyclic compounds, are quickly oxidized. Moreover, these cyclic compounds are the most toxic in the oxidation route.
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
5585
Figure 3. Results of the TUs evolution obtained in the catalytic wet oxidation of PhOH with AC.35 P ) 16 bar, T ) 127, 140, and 160 °C. Co ) 1000 mg/L phenol.
Figure 5. Influence of the H2O2 concentration in the Fenton runs using phenol as pollutant. Co ) 1000 mg/L phenol, Fe2+ ) 10 mg/L, T ) 50 °C, t ) 180 min.
Figure 4. Results obtained with Fenton reagent at 10% H2O2, Fe2+ ) 10 mg/L, 1000 mg/L phenol. (a) oxidation-reduction potential and pH, (b) TOC and phenol conversion.
The catalyst was stable in the interval studied (500 h). After a time-on-stream (tos) of about 40 h, depending mainly if the reactor is filled with fresh catalyst or not, respectively, the catalyst surface achieved a steady state, with the pollutant conversion and mineralization values keeping stable in the time range studied. The decrease in the surface area was about 2/3 of the original value, diminishing from 745 m2/g of the fresh AC to approximately 250 m2/g of the used one. Fenton runs were carried out at 50 °C by using different stoichiometric H2O2 dosage. The stoichiometric amount was calculated according to the following reaction: C6H6O + 14H2O2 f 6CO2 + 17H2O The initial pollutant concentration was always set to 1000 mg/L and the concentration of Fe2+ added was 10 mg/L. Initial pH was 3.5 (adjusted by sulfuric acid) and this variable was not controlled for during the reaction progress. The measurement of the end point of the runs was established from the ORP and pH variation with time. As can be seen in Figure 4a, already at early stage, after addition of the oxidizing agent, H2O2, there is a sudden decrease of ORP, probably due to the fast reaction of the species with a high oxidation potential such as the hydroxyl radicals with phenol. After that, the pH and the ORP values stabilized at reaction times of 10 and 40 min, respectively, indicating that the oxidation run had stopped. It was found that the asymptotic ORP and pH values correspond to an asymptotic value of the TOC over time, as can be seen in Figure 4. Results obtained for pollutant and TOC conversions, intermediates distribution, toxicity units, and pH after 40 min are plotted in Figure 5. As can be seen PhOH is quickly oxidized
and disappears almost completely at low H2O2 dosage (between 10 and 20% of stoichiometric amount). In contrast, the percentage of mineralization after the FR treatment of aqueous solutions of PhOH shows first a linear increase as the amount of H2O2 employed is increased, reaching an asymptotic value of 40% of TOC percent removal for H2O2 doses of approximately 40% of the stoichiometric. Dosages of H2O2 over 40% of the stoichiometric did not improve the percent mineralization achieved (40% TOC percent removal). Thus, PhOH is oxidized faster than these organic intermediates.40 The asymptotic mineralization reached indicates that the excess of H2O2 (over 40%) is not effective in the oxidation of these organic intermediates. This could be due to the complexation of iron cation with the intermediates,41 decreasing the availability of iron and stopping radical OH• generation by H2O2. Again, the profile obtained for the cyclic intermediate shows a maximum. From the results in Figure 5, higher amounts of CTL and HQ are obtained by using the Fenton reagent than those found in the CWO process and BQ is not detected in the FR. The nonidentified TOC increases when the H2O2 amount increases, being about 44% if 100% of H2O2 is used. Zazo et al.,39 who studied the oxidation route of PhOH using the FR, have also found an important amount of nonidentified TOC if stoichiometric amounts of H2O2 were added for values of the Fe2+ to H2O2 ratio between 0.002 and 0.02 mg/mg. The value of the Fe2+/H2O2 ratio used in this work employing a stoichiometric H2O2 concentration is 0.01, so the results obtained here are in agreement with those reported in the literature. Kavitha and Palanivelu13 assume that ring-cleaved products compounds are the main components contributing to the organic residual matter in the treated solution. These authors obtain a COD removal of about 80% and mineralization grades of about 40% using a Fe2+/H2O2 weight ratio of about 0.05, which is higher than the one used in this paper (0.01). These ring-cleaved products could explain the remaining toxicity units measured at higher values of H2O2 concentration that remain higher than the initial, as deduced from Figure 5. Thus, the detoxification
5586
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
Figure 6. Comparison of the CWO efficiency in mineralization and detoxification of the phenolic solution without Fenton pretreatment (CWO) or with Fenton pretreatment (FR + CWO). Phenol as pollutant (Co ) 1000 mg/L, TOCO ) 760 mg C/L). Conditions for FR: 10% H2O2, T ) 50 °C, TOC1 ≈ 0.9 TOCo). Conditions for CWO: (T ) 127 °C, P ) 16 bar). TOC2 ) TOC at the exit of the CWO reactor.
of these phenolic effluents by FR would require stricter conditions or alternative treatment methods such as photoFenton. Taking these facts into consideration, an integrated treatment using the FR and CWO in serial steps can be proposed as a way of optimizing the process. The main objectives are to reduce the amount of H2O2 added in the Fenton step and decrease temperature and W/QL values in the catalytic wet oxidation reactor, obtaining a nontoxic effluent, and achieving a high grade of mineralization. To study the integrated Fenton and catalytic wet oxidation treatment, the samples obtained after the FR treatment of aqueous solutions of PhOH at reaction times of 40 min, using 10% of the stoichiometric H2O2 amount were fed to the CWO reactor. This time, 40 min, is higher than that required for the hydrogen peroxide consumption as can be deduced from Figure 4b. TOC and phenol conversion after the Fenton pretreatment at the conditions above are 10% and 70%, respectively. Temperature and total pressure in the CWO process was set to 127 °C and 16 bar, respectively, and different W/QL values were used. Total conversion of phenol, high mineralization, and a total detoxified effluent is obtained at the reactor exit at low W/QL values (
