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H2O2 through an electrochemical reaction of oxygen that can be expressed below (4-7),
A Novel Electro-Fenton Process for Water Treatment: Reaction-controlled pH Adjustment and Performance Assessment
1 O + H2O f H2O2 (overall reaction) 2 2
(2)
The freshly electro-generated H2O2 interacts with the added Fe2+ to form a cathodic electro-Fenton (E-Fenton)-based reaction (4, 8). In fact, the overall reaction of eq 2 is made up of two-half reactions. The cathodic half reaction occurs on a graphite cathode (4, 5):
H O N G L I U , * ,† C H U A N W A N G , † X I A N G Z H O N G L I , ‡ X I A O L I X U A N , †,‡ CHENGCHUN JIANG,§ AND HUA’NAN CUI† Institute of Optoelectronic and Functional Composite Materials, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China, Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China, and School of Civil and Environmental Engineering, Shenzhen Polytechnic, Shenzhen, 518055, China
O2 + 2H + + 2e f H2O2 (cathodic half reaction) (3) Obviously, eq 3 consumes H+, or generates OH-. At the same time, the anodic half reaction occurs on a platinum (Pt) anode to generate H+ as below (5):
1 H2O f O2 + 2H + + 2e (anodic half reaction) 2
A novel electro-Fenton (E-Fenton) process was developed, in which the desired pH for an effective E-Fenton reaction and for a neutral treated media could be obtained by utilizing the reaction-released H+ and OH- in stead of chemical addition. In the laboratory-scale process using three chambers, the substrate solution pH > 4.0 was designed to be adjusted in situ through three sequencing steps: (I) pH reduction, (II) pH keeping for the effective E-Fenton reaction, and (III) pH recovery to neutral while the E-Fenton reaction continued. Experimental results demonstrated that such three-step pH adjustment was successfully achieved in this novel E-Fenton process, and that the pH adjustment was controlled by the E-Fenton reaction process. The performance of the novel process was assessed in terms of dimethyl phthalate (DMP) degradation in aqueous solution. The results revealed that the novel process was effective to reduce the DMP concentration and the total organic carbon (TOC) at steps II and III. Also, through experiments, the initial DMP solution pH > 4.0 was selected to be reduced to 3.5 in Step I of the process. This pH adjustment not only allowed the E-Fenton reaction to occur in its favorable pH range, but also benefited any potential subsequent biological treatment process or a final discharge. Moreover, the iron species could be recycled in the process.
Introduction Among the advanced oxidation processes (AOPs) to degrade aqueous recalcitrant organic pollutants, Fenton method (1-3) has attracted a lot of attention due to its formation of nonselective hydroxyl radicals (•OH):
Fe2 + + H2O2fFe3 + + •OH + OH -
(1)
Recently, interest has been focused on the in situ supply of * Corresponding author phone: (86)2084115573; fax: (86)2084110927; e-mail:
[email protected]. † Sun Yat-sen University. ‡ The Hong Kong Polytechnic University. § Shenzhen Polytechnic. 10.1021/es0622195 CCC: $37.00 Published on Web 03/14/2007
2007 American Chemical Society
(4)
Conventionally, the E-Fenton reaction is performed in a common undivided reactor (5) to allow the H+ and OHcounteraction, or in a divided reactor separated by glass frit of porosity (8) or Nafion membrane (4) which also allows the H+ and OH- to pass through the separator and to counteract. The counteraction serves to achieve a constant pH of 2.0∼4.0 that fortunately favors the E-Fenton reaction as has been richly documented (7-9). However, the achievement of such a narrow pH range is not sufficiently practical for water treatment since many waters, such as sewage, have pH beyond 2.0∼4.0. To achieve a best E-Fenton reaction performance and a neutral treated media, any initial pH (pH0) > 4.0 needs to be reduced to 2.0∼4.0 and recovered to neutral after the E-Fenton reaction. Therefore, the pH adjustment in the E-Fenton process is a critical step in practical water treatment. For example, the pH adjustment in the treatment of landfill leachate by the E-Fenton reaction was implemented using sulfuric acid for preadjustment and sodium hydroxide for post-adjustment (9, 10). This apparently simple manner in fact needs a certain amount of chemical addition and is not convenient to operate as well as not environmentally benign. Alternatively, heterogeneous Fenton reaction using immobilized iron (11, 12) or modified homogeneous Fenton reaction using chelated iron (13, 14) has been reported to work at pH beyond the range of 2.0∼4.0 without preadjustment of pH, whereas concerns arise about the iron leaching of the immobilized iron at low pH (12) and about secondary pollution introduced by the chemical addition. This study had an attempt to develop a novel E-Fenton process with an in situ reaction-controlled pH adjustment function. The process was developed to work at pH > 4.0 free of secondary pollution by utilizing the reaction-released OH- in eq 3 and H+ in eq 4 for the pH adjustment instead of chemical addition and by reusing the iron species that was dosed into the reaction system in the form of ferrous iron. The novel E-Fenton process was carried out in two divided chambers and one common undivided chamber. The former utilized the H+ and OH- separately to obtain a varied pH, and the latter utilized their counteraction to obtain a constant pH. Dimethyl phthalate (DMP), a representative of di-alkyl phthalate esters (DPEs), belongs to the endocrine-disrupting chemicals (15). A great amount of DPEs is drained into the VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scheme of the novel E-Fenton process. aquatic environment through the disposal of the manufacturing wastewater, which greatly contributes to their environmental ubiquity (16). Treatment of aqueous media containing DPEs by biological (17, 18) or chemical (19, 20) methods has been proved time-consuming, whereas DMP is relatively susceptible to be degraded effectively, and the DMP degradation by the E-Fenton reaction has not been reported. Thus, DMP was used in this study to provide a rapid method to assess the performance of the novel E-Fenton process prior to further applied evaluation. This study focused on the description of this E-Fenton process by addressing the pH adjustment and the process performance assessment in terms of DMP degradation in aqueous solution.
Experimental Section Chemicals and Reagents. DMP was analytical reagent and purchased from Xilong Chemical Inc., Shantou, Guangdong, China. Acetonitrile (Dima Technology Inc., Richmond Hill, ON, Canada) was HPLC reagent and other chemicals were analytical reagent. In all experiments, double distilled water was used. The water to prepare Fe2+ solution was bubbled with N2 for 30 min before solution preparation. Sulfuric acid and sodium hydroxide both in 0.1 M concentration were used to obtain pH0 of DMP solution. E-Fenton Process and Experimental Procedure. A laboratory-scale E-Fenton process is illustrated in Figure 1, which consists of three chambers: one common chamber (undivided chamber) with both anode and cathode together (5), and two divided chambers (anodic chamber and cathodic chamber) connected by a salt bridge filled with saturated KNO3 solution and agar. Each chamber has an effective volume of 120 mL. A 2 × 2 cm2 Pt flake was used as the anode, a graphite rod (ø 5 mm and L 80 mm, Chenhua Co. Ltd, Shanghai, China) as the cathode, and a saturated calomel electrode as the reference electrode that was placed in the same chamber with the cathode. A PS-1 potentiostat/galvanostat (Zhongfu Corrosion and Protection Co. Ltd, Beijing, China) was employed to apply a cathodic current to the electrode system. Sodium sulfate solution with 0.05 M concentration was used as the electrolyte. First of all, to obtain an optimal condition of H2O2 generation, H2O2 generation was performed in the Na2SO4 electrolyte solution free of Fe2+ in the common chamber by purging oxygen (99.9%) onto the cathode by a glass frit diffuser under a cathodic current. A needle valve was used to ensure the oxygen flow rate. The solution sample of 1.0 mL was taken and analyzed immediately to measure the H2O2 concentration. Comparatively, the H2O2 generation was also performed in the divided chambers. Then, to obtain an optimal Fe2+ dosage for the E-Fenton reaction in the common chamber at pH 3.5 under the optimal condition of H2O2 2938
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generation, a serial of 5.0 mL Fe2+ solutions with different concentrations was prepared and then was added into the reaction chamber by a metric pump (BTOQ-50M, Longer, Baoding, China) at a fixed rate. The procedure of the novel E-Fenton process included three sequencing steps. In Step I, a DMP solution with pH0 > 4.0 was put into the anodic chamber for a pH decrease. In Step II, the DMP solution with the reduced pH entered into the common chamber for a DMP degradation by the E-Fenton reaction at a constant pH. In Step III, the DMP solution entered into the cathodic chamber for a further degradation by the E-Fenton reaction at a varying pH that was increasing to neutral (∼7.0). An one-off E-Fenton reaction was performed to maintain Step I and Step III by applying an optimal cathodic current for the H2O2 generation between the spatially separated anode and cathode. In Step II, the same current was applied between the anode and cathode placed in the same chamber (common chamber). Prior to Step I, a seeding step was performed. Concurrently with putting a DMP solution with pH 7.0 in the anodic chamber, a 0.05 M Na2SO4 electrolyte solution with pH 3.5 was put into cathodic chamber as a seed solution. After the DMP solution pH was reduced to 3.5 in the anodic chamber by the anodic half reaction of H2O2 generation, the DMP solution was transferred into the common chamber. Then after the treatment of DMP solution by the E-Fenton reaction in the common chamber, the DMP solution was transferred into the cathodic chamber to replace the seed solution. Thereafter, Step I started by putting fresh DMP solution in the anodic chamber and using the DMP solution in the cathodic chamber. During the novel E-Fenton process, the pH adjustment was investigated by monitoring the pH on line in each chamber. Also, the performance assessment in terms of DMP degradation was carried out by taking a sample of 0.5 mL at preset time interval for measurement of DMP concentration and 5.0 mL for measurement of total organic carbon (TOC) concentration. Methanol of 10 µL was injected into the DMP sample to quench potential radical reactions. In Step I, no Fe2+ was dosed into the anodic chamber. In Step II, upon the applying of cathodic current to generate H2O2, the Fe2+ was pumped into the common chamber at an optimal Fe2+ dosage as determined beforehand. In Step III, the Fe2+ was transferred to the cathodic chamber with the DMP solution. At the last phase of Step III, iron precipitate was formed with the pH increase to 3.7. Then, the supernatant was decanted and the iron was left in the chamber bottom. In the further runs, the precipitate was transferred to the common chamber for reuse, and the precipitate was dissolved at pH < 3.7. Chemical Analysis. The DMP concentration was analyzed by HPLC (Techcomp, LC 2130, Shanghai, China) equipped with a reverse phase column (Waters, XT erra MS C-18, 5 µm) and a UV detector. The mobile phase was composed of 50% acetonitrile and 50% water, and the detection wavelength was 276 nm. The TOC concentration was determined using a TOC analyzer (Shimadzu 5000A). The Fe2+ concentration was determined using 1,10-phenanthroline method (21) using a UV-VIS spectrosphotometer (TU1810, Universal Analysis, Beijing, China). The H2O2 concentration was determined by potassium titanium (IV) oxalate method also using the TU1810 UV-VIS spectrosphotometer. In this method, the interference caused by the cosubstrate was precluded (22). Solution pH was measured by a pH meter (PB-10, Sartorius, Shanghai, China).
Results and Discussion Optimization of H2O2 Generation and Fe2+ Dosage. To obtain an optimal condition for H2O2 generation, a current density range from 0 to 0.8 mA cm-2 and an O2 gas flow rate
FIGURE 2. H2O2 accumulative concentrations at 90 min reaction, pH 3.5, different current densities at 300 mL min-1 O2 flow rate, and different O2 flow rates at 0.45 mA cm-2 current density (A), the DMP degradation by the E-Fenton reaction at pH 3.5, 0.45 mA cm-2 current density, 300 mL min-1 O2 flow rate and different Fe2+ dosages (B). range from 100 to 400 mL min-1 were tested, and the experimental results are shown in Figure 2A. Figure 2A showed that the optimal current density and O2 flow rate were 0.45 mA cm-2 and 300 mL min-1 respectively for the H2O2 generation. Any current density higher than 0.45 mA cm-2 could accelerate H2O2 reduction to water, and any O2 gas flow rate faster than 300 mL min-1 would release gas with bigger bubbles in size, which was detrimental to the oxygen adsorption onto the cathode. No obvious difference was detected in the H2O2 accumulative concentrations between the undivided chamber and divided chambers. Also, no distinct difference in the H2O2 accumulative concentrations was found in the pH range of 2.5∼7.0, suggesting that the pH did not influence the H2O2 generation remarkably. The reason might be that the rate-determining step was not the interaction between the H+ and the O2 but between the electron and the O2 as demonstrated in eq 5 (23).
O2 + e f O 2
(5)
Also, to obtain an optimal Fe2+ dosage for the E-Fenton reaction, the DMP degradation experiment was performed with different Fe2+ dosages and the results are shown in Figure 2B. It can be seen that 0.5 mM Fe2+ was the optimal dosage for the E-Fenton reaction. In addition, our experiment confirmed that the Pt anode allowed the half reaction by eq 4, but other anode materials such as sacrificial iron anode (24) did not lead to this reaction. pH Adjustment in the E-Fenton Process. The strategy to develop this E-Fenton process with respect to the pH adjustment is described as follows: the substrate solution pH0 > 4.0 is first reduced to 4.0 was selected to be reduced to 3.5 in Step VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. pH increase during the remaining DMP degradation under the conditions of FIGURE 5B. I, then the pH 3.5 was kept unchanged in Step II, then the pH 3.5 increased to neutral in Step III. This pH adjustment not only favored the E-Fenton reaction in Steps II and III, but allowed any potential subsequent biological treatment or a final media discharge. In Step III, the remaining DMP degradation occurred also effectively at the increasing pH from 3.5 to ∼7.0, because the time for the pH increase from 3.5 to 4.0 was sufficient for the DMP degradation under our experimental conditions. Comparison Between the Novel Process and the Conventional Process. This novel E-Fenton process with the reaction-controlled pH adjustment is developed based on the conventional E-Fenton process with the traditional pH adjustment by chemical addition. The comparison of the two processes is given in the Supporting Information, Section I.
Acknowledgments This work was supported by Natural Science Foundation of China (project no: 20577071, 50678178, and 50478049) and Natural Science Foundation of Guangdong Province, China (project no: 04009709).
Supporting Information Available Section 1, Comparison between the novel process and the conventional process. This material is available free of charge via the Internet at:http://pubs.acs.org.
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Received for review September 18, 2006. Revised manuscript received November 10, 2006. Accepted February 9, 2007. ES0622195
