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Oxidation of chlorophenols with hydrogen peroxide in the presence of goethite
Chemosphere 40 (2000) 125±130
Oxidation of chlorophenols with hydrogen peroxide in the presence of goethite Ming-Chun Lu* Department of Environmental Engineering and Health, Chia Nan College of Pharmacy and Science, Tainan 717, Taiwan, ROC Received 10 March 1999; accepted 3 May 1999
Abstract The use of goethite (a-FeOOH) and hydrogen peroxide was recently found that they could eectively oxidize organic compounds. The study was to investigate the eect of goethite particle size, goethite concentration, Fe2 and Fe3 on the 2-chlorophenol oxidation. Results indicated that 2-chlorophenol can be decomposed with hydrogen peroxide catalyzed by goethite and the oxidation rate increased with decreasing goethite particle size. 2-Chlorophenol degradation was almost retarded with 0.8 g/l of goethite because ferrous ions could not be produced at this condition. Addition of Fe2 and Fe3 can enhance the catalytic oxidation rate of 2-chlorophenol very eciently. In conclusion, the main mechanism of goethite catalyzing hydrogen peroxide to oxidize 2-chlorophenol may be due to the catalysis of ferrous ions and goethite surface. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Goethite; Hydrogen peroxide; Fenton reaction; Chlorophenols; AOPs
1. Introduction Among the limited number of alternatives available for remediating contaminated soil, in situ remediation techniques such as soil washing, vapor extraction, a stream stripping are merely separation technologies; it is necessary to treat the pollutants removed from the soil. In situ bioremediation technologies are usually applied in the destruction of certain hazardous organic compounds; however, many contaminants are toxic to microorganisms. In this study, hydrogen peroxide was selected as the chemical oxidant. Hydrogen peroxide is safe, ecient, easy to use and suitable for wide usage on contamination prevention. It was ®rst used to reduce odor in wastewater treatment plants, and from then on, hydrogen peroxide entered the realm of wastewater treatment
*
Fax: +886-6-2666-411. E-mail address: [email protected] (M.-C. Lu)
(Elizardo, 1991). However, hydrogen peroxide itself is not an excellent oxidant for most organic substances of interest. In general, hydrogen peroxide is combined with UV light, iron salts or ozone to produce a higher amount of hydroxyl radicals. In fact, the mixture of ferrous salts and hydrogen peroxide is known as FentonÕs reagent, producing hydroxyl radicals with powerful oxidizing abilities to degrade certain toxic contaminants (Spacek et al., 1995; Lipczynska-Kochany et al., 1995; Miller et al., 1996). Besides, contaminants may be oxidized into biologically degradable matter by just adding hydrogen peroxide, and oxygen produced from hydrogen peroxide enhances the decomposition abilities of microorganisms. It can also be used on soil remediation, because ferrous ions are often present in groundwater (Tryan et al., 1991; Kong et al., 1998). The primary oxidant in this catalytic reaction is believed to be hydroxyl radicals, generated by conversion of hydrogen peroxide. The reaction of hydrogen peroxide with ferric ions, which is referred to as Fenton-like reaction. Recently, the use of goethite (a-FeOOH) and hydrogen peroxide was found that they could eectively
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M.-C. Lu / Chemosphere 40 (2000) 125±130
oxidize organic compounds due to the catalysis of goethite surface and ferrous ion generation. Ferrous ions are produced from the reductive dissolution of goethite shown as below (Zinder et al., 1986): a-FeOOH s 3H eÿ () Fe2 3H2 O
1
As the following equation shows, hydrogen peroxide provides electrons: H2 O2 ! 2H O2 2eÿ
2
Combining Eqs. (1) and (2), the following equation is obtained: 1 a-FeOOH s 2H H2 O2 2 1 2 ! Fe O2 2H2 O 2
3
Hydroxyl radicals are therefore produced by Fenton reaction: Fe2 H2 O2 ! Fe3 HOá OHÿ
4
This process has potential in hazardous waste treatment as goethite exists in soil and can be recycled to use. Gurol and Ravikumar (1994) pointed out that hydrogen peroxide can oxidize pentachlorophenol and trichloroethylene adsorbed on sand particles by interacting with the natural iron content of sand particles. Lin and Gurol (1996) also found that n-butyl chloride was oxidized by hydroxyl radicals generated from the interaction of hydrogen peroxide with goethite particles. In our previous studies, ferrous ions and hydrogen peroxide have been used to oxidize dichlorvos with an attempt to explore the behavior of dichlorvos oxidation and how factors such as pH, [H2 O2 ] and [Fe2 ] may in¯uence the dichlorvos decomposition (Lu et al., 1997,1999). This study was to investigate the oxidation of chlorophenols with hydrogen peroxide in the presence of goethite. 2-Chlorophenol was selected as the model compound to explore the eect of goethite particle size and concentration on the oxidation eciency. Fe2 and Fe3 were also added to study their eect on the 2chlorophenol oxidation. 4-Chlorophenol and 2,4-dichlorophenol were used to compare their oxidation ef®ciencies with 2-chlorophenol by the goethite/H2 O2 process. 2. Material and methods Goethite used in this study was purchased from Aldrich. Goethite particles have a diameter between 0.21 and 0.044 mm. The density of granular goethite were ranged from 3.99 to 4.58 g/cm3 . The concentration of the replaceable surface hydroxyl groups of the goethite was
determined to be 5 ´ 10ÿ4 mol/g. The intrinsic acidity int int constants of the surface, pKa1 and pKa2 , were 6.2 and 9.2, respectively (Lin and Gurol, 1998). All other chemicals were of reagent grade. Solutions containing goethite and chlorophenols were prepared and then were poured into 250 ml ¯asks after the pH adjustment. In homogeneous solutions of hydrogen peroxide and iron salts, the pH was observed to have a signi®cant eect on the decomposition of hydrogen peroxide and the oxidation eciencies of organic chemicals. Based on the observation of our previous investigations (Lu et al., 1997, 1999), the optimum pH of iron-catalytic system occurs at 3. In this study, we therefore selected initial pH of 3 as the reaction condition using solution of HClO4 but did not control a constant pH during the period of oxidation reaction. The pH variation during the reaction was < 0.1 pH unit. A thermal oscillator tank was used to maintain completely mixed at 30°C with a constant speed. The oxidation reaction was initiated after adding hydrogen peroxide. Samples were regularly taken to be ®ltered through Whitman ®lter papers upon withdrawal to separate goethite from the solutions, and the residual quantities of chlorophenols were measured by an HPLC (Waters LC module 1). Ferric and ferrous ions could be generated during the oxidation reaction. Therefore, the total dissolved Fe was measured by an AA (Hitachi, z-8100).
3. Results and discussion 3.1. Eect of goethite particle size and goethite concentration on the 2-chlorophenol oxidation Goethite particle size was a critical factor on the 2chlorophenol degradation. In order to investigate the eect of goethite particle size on the oxidation reaction, four sizes of goethite were selected; they were 70±80 mesh (0.21±0.177 mm), 80±100 mesh (0.177±0.149 mm), 100±200 mesh (0.149±0.074 mm) and 200±325 mesh (0.074±0.044 mm). Their surface areas are shown in Table 1.
Table 1 The density, surface area and particle size of dierent diameter of goethite Diameter (mm)
Density (g/cm3 )
Surface area (m2 /g)
Size (mesh)
0.21±0.177 0.177±0.149 0.149±0.074 0.074±0.044
3.99 4.08 4.37 4.58
194 196 208 215
70±80 80±100 100±200 200±325
M.-C. Lu / Chemosphere 40 (2000) 125±130
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The initial pH for the reaction was 3 and the same concentration of 2.2 ´ 10ÿ3 M H2 O2 , 3.9 ´ 10ÿ4 M 2-chlorophenol and 0.2 g/l goethite were used in these experiments. The experimental observations are shown in Fig. 1. Initially, the rate of 2-chlorophenol decomposition was slow; however, it increased suddenly after a speci®c time point. Fig. 1 also shows that the rate of 2-chlorophenol oxidation increased with decreasing goethite particle size. 2-Chlorophenol can be degraded completely in this reaction using 0.074±0.004 mm goethite but there still remained 45% of 2-chlorophenol after 11 h when 0.21±0.177 mm of goethite was used. The amount of 2-chlorophenol adsorbed onto goethite surface was less than 8% with four sizes of goethite used in this study. It is clear that the removal of 2-chlorophenol from solution is not due to the adsorption. The eect of goethite particle on the oxidation rate was signi®cant in this study. We observed the 2-chlorophenol degradation for 480 min, and found that the in¯uence of particle size on the oxidation rate can be neglected before 120 min using 100±200 mesh and 200± 325 mesh of goethite. A similar result was also observed before 300 min using 70±80 mesh and 80±100 mesh of goethite. However, Lin and Gurol (1998) did not found the dierence of catalytic activity for the hydrogen peroxide degradation using four sizes of goethite particles less than 0.3 mm, because the particle size did not have a signi®cant eect on the surface area. As listed in Table 1, the particle sizes of goethite in the ranges of 0.21±0.177, 0.177±0.149, 0.149±0.074, and 0.074±0.044 mm were 194, 196, 208 and 215 m2 /g, respectively, indicating that particle sizes have an eect on surface area and oxidation rate. Goethite acted as a catalyst in this reaction. In this case, four concentrations of goethite, such as 0.1, 0.2, 0.4 and 0.8 g/l, were selected to study the eect of catalyst loading on the 2-chlorophenol degradation. The rate of
2-chlorophenol increased after a speci®c time point depending on the goethite loading. It was also found that the reaction rate increased with increasing the catalyst concentration when 0.1, 0.2 and 0.4 g/l of goethite were used. However, 2-chlorophenol degradation almost did not occur at the concentration of 0.8 g/l goethite. This was not consistent with our prediction. Theoretically, the oxidation rate should increase with increasing the goethite concentration, and reached a plateau value to be independent of goethite concentration. In general, it is accepted that the mechanism of goethite catalyzing hydrogen peroxide to oxidize organic chemicals are: (1) the surface of goethite catalyzing hydrogen peroxide to produce hydroxyl radicals and (2) the ferrous ions generated from goethite combined with hydrogen peroxide (Zinder et al., 1986; Miller and Valentine, 1995). Based on the proposed mechanisms, it is assumed that the inhibition for the 2-chlorophenol oxidation may come from the reduction of active surface area or Fe2 generation. Therefore, the experiments of goethite dissolution were conducted to explore the reason of oxidation inhibited at higher goethite concentration in the following section.
Fig. 1. Eect of particle size on the 2-CP oxidation (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; goethite 0.2 g/l; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 2. Eect of goethite loading on the 2-CP oxidation (exM; perimental conditions: [2-CP] 3.9 ´ 10ÿ4 [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
3.2. Eect of goethite particle size and goethite concentration on the goethite dissolution To investigate the eect of goethite dissolution on the oxidation rate, total amounts of Fe released from goethite crystal were also analyzed during the oxidation reactions. These experiments were carried out with the same experimental conditions as those in Figs. 1 and 2. Total concentrations of Fe in the reaction mixtures were analyzed with time of reaction. As depicted in Fig. 3, ferric ion concentration increased smoothly before 140 min of reaction time, then it was promoted to a higher
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the Fe2 produced from the reductive-dissolution reaction, leading to the occurrence of Fenton reaction as depicted in Eqs. (1) and (2). 3.3. Eect of Fe2 and Fe3 on the oxidation
dissolution rate. The ferric ion concentrations in the reaction mixtures using 70±80 and 80±100 mesh of goethite were around 1.0 ´ 10ÿ6 M after 300 min, but those using 100±200 and 200±235 mesh of goethite were 1.23 ´ 10ÿ5 and 1.66 ´ 10ÿ5 M, respectively. The tendency of particle size aecting the dissolution reaction was similar to those observed in Figs. 1 and 2. Fig. 4 illustrates the goethite dissolution at dierent goethite concentrations. Total Fe concentrations were proportional to the goethite concentrations ranging from 0.1 to 0.4 g/l. However, almost no Fe generated when 0.8 g/l of goethite was used. According to the above ®nding, it is clear that the inhibition for 2-chlorophenol oxidation may be due to the reduction of iron species. By the comparison of Figs. 1±4, it was found that the rate of 2chlorophenol oxidation increased with increasing the amounts of dissolved Fe. This prompts us to tentatively considerate that the catalyzing capacity may come from
According to the above discussion, the main mechanism of goethite catalyzing hydrogen peroxide to oxidize 2-chlorophenol may be due to the production of ferrous ions from goethite dissolution. Therefore, we compared the goethite/H2 O2 process with other systems such as Fe2 /H2 O2 , goethite/Fe3 /H2 O2 and goethite/Fe2 /H2 O2 processes. Fe2 was used in place of goethite in the goethite/H2 O2 process, and its concentration was 8.2 ´ 10ÿ6 M. Total dissolved Fe in the ®ltrates from the experiments with goethite was around 8.2 ´ 10ÿ6 M. Thus, the homogeneous solution contained roughly the same amount of ®nal dissolved Fe as the heterogeneous systems. Other two systems also added the same amount of Fe as the homogeneous system. As shown in Fig. 5, the rate of 2chlorophenol degradation using goethite as the catalyst is lower than that using Fe2 . Besides, adding Fe2 and Fe3 can improve the catalytic eciencies of goethite for the 2-chlorophenol oxidation reaction. In the cases of Fe2 /H2 O2 , Fe2 /H2 O2 /goethite and Fe3 / H2 O2 /goethite, 2-chlorophenol almost degraded linearly with time of reaction. However, the reaction using goethite only as the catalyst can be divided into two parts. In the beginning of the reaction, the rate of 2-chlorophenol oxidation was slow, and then turned to be faster after about 150 min. After that time, the rate of 2-chlorophenol oxidation using goethite as the catalyst only is closing to those using Fe2 , Fe3 /goethite. Additionally, from Figs. 3 and 4, we can see that the amounts of Fe also increased obviously after
Fig. 4. Changes of concentrations of dissolved Fe with time of reaction using dierent concentrations of goethite (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 5. Comparison of goethite, Fe2 , goethite/Fe2 and goethite/Fe3 catalyzing H2 O2 in degrading 2-CP (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [Fe2 ] [Fe3 ] 8.2 ´ 10ÿ6 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 3. Changes of concentrations of dissolved Fe with time of reaction using dierent sizes of goethite (experimental condiM; goethite 0.2 g/l; tions: [2-CP] 3.9 ´ 10ÿ4 [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
M.-C. Lu / Chemosphere 40 (2000) 125±130
about 150 min. Therefore, it can be considered that the dissolution should be the main mechanism of goethite catalyzing H2 O2 to oxidize 2-chlorophenol in the present study. 3.4. Comparison of chlorophenol oxidation In order to compare the dierence of oxidation for chlorophenols, 2-chlorophenol, 4-chlorophenol and 2,4chlorophenol were selected as the model compounds. The results of the experiments conducted at initial pH 3 in the presence of 0.2 g/l of goethite particles are presented in Fig. 6. In this ®gure, the percentage of degradation, mineralization and chloride production after 4 h reaction are compared. Obviously, 2-chlorophenol can be oxidized easier than the other two. Within 4 h, 2-chlorophenol almost disappeared completely, but only 14.4% of 2,4-chlorophenol were degraded. Concerning the chloride production, even though 15% of 2chlorophenol was destroyed, we still hardly detected the chloride generation in the reaction mixture. Afterward, the concentration of chloride increased suddenly with time of reaction. It is assumed that the amount of ferrous ions resulted from goethite dissolution was enough to facilitate the Fenton reaction. After 4 h reaction, 63.8% of chloride ions was detected in the reaction solution. This result is dierent from that using Fenton's reagent. Bareni et al. (1987) reported that the percentage chloride production in Fenton system was 90% when 2-chlorophenol was completely degraded. The experimental results in this study are dierent from that reported by Bareni et al. (1987) who used Fenton reagent to investigate the degradation of chlorophenols. Therefore, we can con®rm that goethite surface still has a critical eect on the oxidation of chlorophenols; their degradation pathway was dierent from each other.
Fig. 6. Comparison of degradation, mineralization and chloride production for the oxidation of chlorophenols (experimental conditions: [chlorophenols] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
129
4. Conclusions Chemical oxidation of organic pollutants with hydrogen peroxide/goethite is an emerging technology in treating wastewater and contaminated soil containing organic pollutants. It was found that 2-chlorophenol can be oxidized eciently in this study. Particle size and loading of goethite are important factors aecting the oxidation rate because they can in¯uence the concentration of ferrous ions due to goethite dissolution. Fe2 and Fe3 can create a better oxidation eciency in the presence of goethite and hydrogen peroxide than that in Fenton system. The main mechanism of goethite catalyzing hydrogen peroxide to oxidize chlorophenols may be considered to be the catalysis of ferrous ions and goethite surface. However, according to the comparison of chlorophenol oxidation in terms of degradation, mineralization and chloride production, the reaction characteristics of goethite/H2 O2 and Fenton systems were somewhat dierent from each other. The goethite/ H2 O2 process needs to be further developed because there exist many unknown reactions to be explored.
Acknowledgements This research was supported by the National Science Council, Republic of China (Grant NSC 87-2211-E0419-007). References Bareni, M., Minero, C., Pelizzetti, E., Bergerello, E., Serpone, N., 1987. Chemical oxidation of chlorophenol with Fenton's reagent. Chemosphere 16, 2225±2237. Elizardo, K., September 1991. Fighting pollution with hydrogen peroxide. Pollution Eng., 106±109. Gurol, M.D., Ravikumar, J., 1994. Chemical oxidation of chlorinated organics by hydrogen peroxide in the presence of sand. Environ. Sci. Technol. 28, 394±400. Kong, S.H., Watts, R.J., Choi, J.H., 1998. Treatment of petroleum-contaminated soils using mineralization catalyzed hydrogen peroxide. Chemosphere 37, 1473±1482. Lin, S.S., Gurol, M.D., 1996. Heterogeneous catalytic oxidation of organic compounds by hydrogen peroxide. In: Proceedings of the 18th IAWQ Biennial International Conference, vol. 5, pp. 48±55. Lin, S.S., Gurol, M.D., 1998. Catalytic decomposition of hydrogen peroxide on iron oxide: kinetics, mechanism, and implications. Environ. Sci. Technol. 32, 1417±1423. Lipczynska-Kochany, E., Sprah, G., Harms, S., 1995. In¯uence of some groundwater and surface water constitutes on the degradation of 4-chlorophenol by the Fenton reaction. Chemosphere 30, 9±20. Lu, M.C., Chen, J.N., Chang, C.P., 1997. Chemical oxidation of dichlorvos insecticide with Fenton's reagent. Chemosphere 35, 2285±2293.
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Lu, M.C., Chen, J.N., Chang, C.P., 1999. Oxidation of dichlorvos with hydrogen peroxide using ferrous ion as catalyst. J. Hazard. Mater., in press. Miller, C.M., Valentine, R.L., 1995. Hydrogen peroxide decomposition and quinoline degradation in the presence of aquifer material. Water Res. 30, 2579±2586. Miller, C.M., Valentine, R.L., Roehl, M.E., Alvarez, P.J.J., 1996. Chemical and microbiological assessment of pendimethalin-contaminated soil after treatment with Fenton's reagent. Water Res. 30, 2579±2586.
Spacek, W., Bauer, R., Heisler, G., 1995. Heterogeneous and homogeneous wastewater treatment ± comparison between photodegradation with TiO2 and the photo-Fenton reaction. Chemosphere 30, 477±484. Tryan, B.W., Watts, R.J., Miller, G.C., 1991. Treatment of four biorefractory contaminants in soils using catalyzed hydrogen peroxide. J. Environ. Qual. 20, 832±838. Zinder, B., Furrer, G., Stumm, W., 1986. The coordination chemistry of weathering: II. Dissociation of Fe(III) oxides. Geochimica Cosmochimica Acta 50, 1861±1869.
Oxidation of chlorophenols with hydrogen peroxide in the presence of goethite Ming-Chun Lu* Department of Environmental Engineering and Health, Chia Nan College of Pharmacy and Science, Tainan 717, Taiwan, ROC Received 10 March 1999; accepted 3 May 1999
Abstract The use of goethite (a-FeOOH) and hydrogen peroxide was recently found that they could eectively oxidize organic compounds. The study was to investigate the eect of goethite particle size, goethite concentration, Fe2 and Fe3 on the 2-chlorophenol oxidation. Results indicated that 2-chlorophenol can be decomposed with hydrogen peroxide catalyzed by goethite and the oxidation rate increased with decreasing goethite particle size. 2-Chlorophenol degradation was almost retarded with 0.8 g/l of goethite because ferrous ions could not be produced at this condition. Addition of Fe2 and Fe3 can enhance the catalytic oxidation rate of 2-chlorophenol very eciently. In conclusion, the main mechanism of goethite catalyzing hydrogen peroxide to oxidize 2-chlorophenol may be due to the catalysis of ferrous ions and goethite surface. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Goethite; Hydrogen peroxide; Fenton reaction; Chlorophenols; AOPs
1. Introduction Among the limited number of alternatives available for remediating contaminated soil, in situ remediation techniques such as soil washing, vapor extraction, a stream stripping are merely separation technologies; it is necessary to treat the pollutants removed from the soil. In situ bioremediation technologies are usually applied in the destruction of certain hazardous organic compounds; however, many contaminants are toxic to microorganisms. In this study, hydrogen peroxide was selected as the chemical oxidant. Hydrogen peroxide is safe, ecient, easy to use and suitable for wide usage on contamination prevention. It was ®rst used to reduce odor in wastewater treatment plants, and from then on, hydrogen peroxide entered the realm of wastewater treatment
*
Fax: +886-6-2666-411. E-mail address: [email protected] (M.-C. Lu)
(Elizardo, 1991). However, hydrogen peroxide itself is not an excellent oxidant for most organic substances of interest. In general, hydrogen peroxide is combined with UV light, iron salts or ozone to produce a higher amount of hydroxyl radicals. In fact, the mixture of ferrous salts and hydrogen peroxide is known as FentonÕs reagent, producing hydroxyl radicals with powerful oxidizing abilities to degrade certain toxic contaminants (Spacek et al., 1995; Lipczynska-Kochany et al., 1995; Miller et al., 1996). Besides, contaminants may be oxidized into biologically degradable matter by just adding hydrogen peroxide, and oxygen produced from hydrogen peroxide enhances the decomposition abilities of microorganisms. It can also be used on soil remediation, because ferrous ions are often present in groundwater (Tryan et al., 1991; Kong et al., 1998). The primary oxidant in this catalytic reaction is believed to be hydroxyl radicals, generated by conversion of hydrogen peroxide. The reaction of hydrogen peroxide with ferric ions, which is referred to as Fenton-like reaction. Recently, the use of goethite (a-FeOOH) and hydrogen peroxide was found that they could eectively
0045-6535/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 2 1 3 - 1
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M.-C. Lu / Chemosphere 40 (2000) 125±130
oxidize organic compounds due to the catalysis of goethite surface and ferrous ion generation. Ferrous ions are produced from the reductive dissolution of goethite shown as below (Zinder et al., 1986): a-FeOOH s 3H eÿ () Fe2 3H2 O
1
As the following equation shows, hydrogen peroxide provides electrons: H2 O2 ! 2H O2 2eÿ
2
Combining Eqs. (1) and (2), the following equation is obtained: 1 a-FeOOH s 2H H2 O2 2 1 2 ! Fe O2 2H2 O 2
3
Hydroxyl radicals are therefore produced by Fenton reaction: Fe2 H2 O2 ! Fe3 HOá OHÿ
4
This process has potential in hazardous waste treatment as goethite exists in soil and can be recycled to use. Gurol and Ravikumar (1994) pointed out that hydrogen peroxide can oxidize pentachlorophenol and trichloroethylene adsorbed on sand particles by interacting with the natural iron content of sand particles. Lin and Gurol (1996) also found that n-butyl chloride was oxidized by hydroxyl radicals generated from the interaction of hydrogen peroxide with goethite particles. In our previous studies, ferrous ions and hydrogen peroxide have been used to oxidize dichlorvos with an attempt to explore the behavior of dichlorvos oxidation and how factors such as pH, [H2 O2 ] and [Fe2 ] may in¯uence the dichlorvos decomposition (Lu et al., 1997,1999). This study was to investigate the oxidation of chlorophenols with hydrogen peroxide in the presence of goethite. 2-Chlorophenol was selected as the model compound to explore the eect of goethite particle size and concentration on the oxidation eciency. Fe2 and Fe3 were also added to study their eect on the 2chlorophenol oxidation. 4-Chlorophenol and 2,4-dichlorophenol were used to compare their oxidation ef®ciencies with 2-chlorophenol by the goethite/H2 O2 process. 2. Material and methods Goethite used in this study was purchased from Aldrich. Goethite particles have a diameter between 0.21 and 0.044 mm. The density of granular goethite were ranged from 3.99 to 4.58 g/cm3 . The concentration of the replaceable surface hydroxyl groups of the goethite was
determined to be 5 ´ 10ÿ4 mol/g. The intrinsic acidity int int constants of the surface, pKa1 and pKa2 , were 6.2 and 9.2, respectively (Lin and Gurol, 1998). All other chemicals were of reagent grade. Solutions containing goethite and chlorophenols were prepared and then were poured into 250 ml ¯asks after the pH adjustment. In homogeneous solutions of hydrogen peroxide and iron salts, the pH was observed to have a signi®cant eect on the decomposition of hydrogen peroxide and the oxidation eciencies of organic chemicals. Based on the observation of our previous investigations (Lu et al., 1997, 1999), the optimum pH of iron-catalytic system occurs at 3. In this study, we therefore selected initial pH of 3 as the reaction condition using solution of HClO4 but did not control a constant pH during the period of oxidation reaction. The pH variation during the reaction was < 0.1 pH unit. A thermal oscillator tank was used to maintain completely mixed at 30°C with a constant speed. The oxidation reaction was initiated after adding hydrogen peroxide. Samples were regularly taken to be ®ltered through Whitman ®lter papers upon withdrawal to separate goethite from the solutions, and the residual quantities of chlorophenols were measured by an HPLC (Waters LC module 1). Ferric and ferrous ions could be generated during the oxidation reaction. Therefore, the total dissolved Fe was measured by an AA (Hitachi, z-8100).
3. Results and discussion 3.1. Eect of goethite particle size and goethite concentration on the 2-chlorophenol oxidation Goethite particle size was a critical factor on the 2chlorophenol degradation. In order to investigate the eect of goethite particle size on the oxidation reaction, four sizes of goethite were selected; they were 70±80 mesh (0.21±0.177 mm), 80±100 mesh (0.177±0.149 mm), 100±200 mesh (0.149±0.074 mm) and 200±325 mesh (0.074±0.044 mm). Their surface areas are shown in Table 1.
Table 1 The density, surface area and particle size of dierent diameter of goethite Diameter (mm)
Density (g/cm3 )
Surface area (m2 /g)
Size (mesh)
0.21±0.177 0.177±0.149 0.149±0.074 0.074±0.044
3.99 4.08 4.37 4.58
194 196 208 215
70±80 80±100 100±200 200±325
M.-C. Lu / Chemosphere 40 (2000) 125±130
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The initial pH for the reaction was 3 and the same concentration of 2.2 ´ 10ÿ3 M H2 O2 , 3.9 ´ 10ÿ4 M 2-chlorophenol and 0.2 g/l goethite were used in these experiments. The experimental observations are shown in Fig. 1. Initially, the rate of 2-chlorophenol decomposition was slow; however, it increased suddenly after a speci®c time point. Fig. 1 also shows that the rate of 2-chlorophenol oxidation increased with decreasing goethite particle size. 2-Chlorophenol can be degraded completely in this reaction using 0.074±0.004 mm goethite but there still remained 45% of 2-chlorophenol after 11 h when 0.21±0.177 mm of goethite was used. The amount of 2-chlorophenol adsorbed onto goethite surface was less than 8% with four sizes of goethite used in this study. It is clear that the removal of 2-chlorophenol from solution is not due to the adsorption. The eect of goethite particle on the oxidation rate was signi®cant in this study. We observed the 2-chlorophenol degradation for 480 min, and found that the in¯uence of particle size on the oxidation rate can be neglected before 120 min using 100±200 mesh and 200± 325 mesh of goethite. A similar result was also observed before 300 min using 70±80 mesh and 80±100 mesh of goethite. However, Lin and Gurol (1998) did not found the dierence of catalytic activity for the hydrogen peroxide degradation using four sizes of goethite particles less than 0.3 mm, because the particle size did not have a signi®cant eect on the surface area. As listed in Table 1, the particle sizes of goethite in the ranges of 0.21±0.177, 0.177±0.149, 0.149±0.074, and 0.074±0.044 mm were 194, 196, 208 and 215 m2 /g, respectively, indicating that particle sizes have an eect on surface area and oxidation rate. Goethite acted as a catalyst in this reaction. In this case, four concentrations of goethite, such as 0.1, 0.2, 0.4 and 0.8 g/l, were selected to study the eect of catalyst loading on the 2-chlorophenol degradation. The rate of
2-chlorophenol increased after a speci®c time point depending on the goethite loading. It was also found that the reaction rate increased with increasing the catalyst concentration when 0.1, 0.2 and 0.4 g/l of goethite were used. However, 2-chlorophenol degradation almost did not occur at the concentration of 0.8 g/l goethite. This was not consistent with our prediction. Theoretically, the oxidation rate should increase with increasing the goethite concentration, and reached a plateau value to be independent of goethite concentration. In general, it is accepted that the mechanism of goethite catalyzing hydrogen peroxide to oxidize organic chemicals are: (1) the surface of goethite catalyzing hydrogen peroxide to produce hydroxyl radicals and (2) the ferrous ions generated from goethite combined with hydrogen peroxide (Zinder et al., 1986; Miller and Valentine, 1995). Based on the proposed mechanisms, it is assumed that the inhibition for the 2-chlorophenol oxidation may come from the reduction of active surface area or Fe2 generation. Therefore, the experiments of goethite dissolution were conducted to explore the reason of oxidation inhibited at higher goethite concentration in the following section.
Fig. 1. Eect of particle size on the 2-CP oxidation (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; goethite 0.2 g/l; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 2. Eect of goethite loading on the 2-CP oxidation (exM; perimental conditions: [2-CP] 3.9 ´ 10ÿ4 [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
3.2. Eect of goethite particle size and goethite concentration on the goethite dissolution To investigate the eect of goethite dissolution on the oxidation rate, total amounts of Fe released from goethite crystal were also analyzed during the oxidation reactions. These experiments were carried out with the same experimental conditions as those in Figs. 1 and 2. Total concentrations of Fe in the reaction mixtures were analyzed with time of reaction. As depicted in Fig. 3, ferric ion concentration increased smoothly before 140 min of reaction time, then it was promoted to a higher
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the Fe2 produced from the reductive-dissolution reaction, leading to the occurrence of Fenton reaction as depicted in Eqs. (1) and (2). 3.3. Eect of Fe2 and Fe3 on the oxidation
dissolution rate. The ferric ion concentrations in the reaction mixtures using 70±80 and 80±100 mesh of goethite were around 1.0 ´ 10ÿ6 M after 300 min, but those using 100±200 and 200±235 mesh of goethite were 1.23 ´ 10ÿ5 and 1.66 ´ 10ÿ5 M, respectively. The tendency of particle size aecting the dissolution reaction was similar to those observed in Figs. 1 and 2. Fig. 4 illustrates the goethite dissolution at dierent goethite concentrations. Total Fe concentrations were proportional to the goethite concentrations ranging from 0.1 to 0.4 g/l. However, almost no Fe generated when 0.8 g/l of goethite was used. According to the above ®nding, it is clear that the inhibition for 2-chlorophenol oxidation may be due to the reduction of iron species. By the comparison of Figs. 1±4, it was found that the rate of 2chlorophenol oxidation increased with increasing the amounts of dissolved Fe. This prompts us to tentatively considerate that the catalyzing capacity may come from
According to the above discussion, the main mechanism of goethite catalyzing hydrogen peroxide to oxidize 2-chlorophenol may be due to the production of ferrous ions from goethite dissolution. Therefore, we compared the goethite/H2 O2 process with other systems such as Fe2 /H2 O2 , goethite/Fe3 /H2 O2 and goethite/Fe2 /H2 O2 processes. Fe2 was used in place of goethite in the goethite/H2 O2 process, and its concentration was 8.2 ´ 10ÿ6 M. Total dissolved Fe in the ®ltrates from the experiments with goethite was around 8.2 ´ 10ÿ6 M. Thus, the homogeneous solution contained roughly the same amount of ®nal dissolved Fe as the heterogeneous systems. Other two systems also added the same amount of Fe as the homogeneous system. As shown in Fig. 5, the rate of 2chlorophenol degradation using goethite as the catalyst is lower than that using Fe2 . Besides, adding Fe2 and Fe3 can improve the catalytic eciencies of goethite for the 2-chlorophenol oxidation reaction. In the cases of Fe2 /H2 O2 , Fe2 /H2 O2 /goethite and Fe3 / H2 O2 /goethite, 2-chlorophenol almost degraded linearly with time of reaction. However, the reaction using goethite only as the catalyst can be divided into two parts. In the beginning of the reaction, the rate of 2-chlorophenol oxidation was slow, and then turned to be faster after about 150 min. After that time, the rate of 2-chlorophenol oxidation using goethite as the catalyst only is closing to those using Fe2 , Fe3 /goethite. Additionally, from Figs. 3 and 4, we can see that the amounts of Fe also increased obviously after
Fig. 4. Changes of concentrations of dissolved Fe with time of reaction using dierent concentrations of goethite (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 5. Comparison of goethite, Fe2 , goethite/Fe2 and goethite/Fe3 catalyzing H2 O2 in degrading 2-CP (experimental conditions: [2-CP] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [Fe2 ] [Fe3 ] 8.2 ´ 10ÿ6 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
Fig. 3. Changes of concentrations of dissolved Fe with time of reaction using dierent sizes of goethite (experimental condiM; goethite 0.2 g/l; tions: [2-CP] 3.9 ´ 10ÿ4 [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
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about 150 min. Therefore, it can be considered that the dissolution should be the main mechanism of goethite catalyzing H2 O2 to oxidize 2-chlorophenol in the present study. 3.4. Comparison of chlorophenol oxidation In order to compare the dierence of oxidation for chlorophenols, 2-chlorophenol, 4-chlorophenol and 2,4chlorophenol were selected as the model compounds. The results of the experiments conducted at initial pH 3 in the presence of 0.2 g/l of goethite particles are presented in Fig. 6. In this ®gure, the percentage of degradation, mineralization and chloride production after 4 h reaction are compared. Obviously, 2-chlorophenol can be oxidized easier than the other two. Within 4 h, 2-chlorophenol almost disappeared completely, but only 14.4% of 2,4-chlorophenol were degraded. Concerning the chloride production, even though 15% of 2chlorophenol was destroyed, we still hardly detected the chloride generation in the reaction mixture. Afterward, the concentration of chloride increased suddenly with time of reaction. It is assumed that the amount of ferrous ions resulted from goethite dissolution was enough to facilitate the Fenton reaction. After 4 h reaction, 63.8% of chloride ions was detected in the reaction solution. This result is dierent from that using Fenton's reagent. Bareni et al. (1987) reported that the percentage chloride production in Fenton system was 90% when 2-chlorophenol was completely degraded. The experimental results in this study are dierent from that reported by Bareni et al. (1987) who used Fenton reagent to investigate the degradation of chlorophenols. Therefore, we can con®rm that goethite surface still has a critical eect on the oxidation of chlorophenols; their degradation pathway was dierent from each other.
Fig. 6. Comparison of degradation, mineralization and chloride production for the oxidation of chlorophenols (experimental conditions: [chlorophenols] 3.9 ´ 10ÿ4 M; [H2 O2 ] 2.2 ´ 10ÿ3 M; [NaClO4 ] 0.1 M; initial pH 3; temperature 30°C).
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4. Conclusions Chemical oxidation of organic pollutants with hydrogen peroxide/goethite is an emerging technology in treating wastewater and contaminated soil containing organic pollutants. It was found that 2-chlorophenol can be oxidized eciently in this study. Particle size and loading of goethite are important factors aecting the oxidation rate because they can in¯uence the concentration of ferrous ions due to goethite dissolution. Fe2 and Fe3 can create a better oxidation eciency in the presence of goethite and hydrogen peroxide than that in Fenton system. The main mechanism of goethite catalyzing hydrogen peroxide to oxidize chlorophenols may be considered to be the catalysis of ferrous ions and goethite surface. However, according to the comparison of chlorophenol oxidation in terms of degradation, mineralization and chloride production, the reaction characteristics of goethite/H2 O2 and Fenton systems were somewhat dierent from each other. The goethite/ H2 O2 process needs to be further developed because there exist many unknown reactions to be explored.
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