Environ. Sci. Technol. 2007, 41, 8459–8463
Abatement of the Inhibitory Effect of Chloride Anions on the Photo-Fenton Process A M I L C A R M A C H U L E K , J R . , †,‡,§ JOSÉ E. F. MORAES,| CAROLINA VAUTIER-GIONGO,| CRISTINA A. SILVERIO,⊥ LEIDI C. FRIEDRICH,† C L Á U D I O A . O . N A S C I M E N T O , ‡,# MONICA C. GONZALEZ,¶ AND F R A N K H . Q U I N A * ,†,‡ Instituto de Química, Universidade de São Paulo, CP 26077, São Paulo 05513-970, Brazil, Centro de Capacitação e Pesquisa em Meio Ambiente (CEPEMA-USP), Universidade de São Paulo, 11573-000, Cubatão, Brazil, Departamento de Química, Universidade Federal de Mato Grosso do Sul, Campo Grande-MS 05508-900, Brazil, Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Diadema-SP, 09972-270, Brazil, Departamento de Química da Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto-USP, Av. dos Bandeirantes, 3900 Ribeirão Preto, SP 14040-901, Brazil, Departamento de Engenharia Química, Escola Politécnica, Universidade de São Paulo, São Paulo-SP, 05508-900, Brazil, and Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina
Received July 28, 2007. Revised manuscript received October 16, 2007. Accepted October 16, 2007.
The inhibition of the photo-Fenton (Fe2+/Fe3+, H2O2, UV light) degradation of synthetic phenol wastewater solutions by chloride ions is shown to affect primarily the photochemical step of the process, having only a slight effect on the thermal or Fenton step. Kinetic studies of the reactions of oxoiron (IV) (FeO2+) with phenol indicate that, if FeO2+ is formed in the photo-Fenton degradation, its role is probably minor. Finally, it is shown that, for both a synthetic phenol wastewater and an aqueous extract of Brazilian gasoline, the inhibition of the photoFenton degradation of the organic material in the presence of chloride ion can be circumvented by maintaining the pH of the medium at or slightly above 3 throughout the process, even in the presence of significant amounts of added chloride ion (0.5 M).
* Corresponding author mailing address: Instituto de Química, Universidade de São Paulo, CP 26077, São Paulo 05513-970, Brazil; courier address: Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, São Paulo 05508-900, Brazil; telephone: ++55-11-3091-2162; fax: ++55-11-815-5579; e-mail:
[email protected]. † Instituto de Química, Universidade de São Paulo. ‡ CEPEMA-USP. § Universidade Federal de Mato Grosso do Sul. | Universidade Federal de São Paulo. ⊥ Ciências e Letras de Ribeirão Preto-USP. # Departamento de Engenharia Química, Escola Politécnica, Universidade de São Paulo. ¶ Universidad Nacional de La Plata. 10.1021/es071884q CCC: $37.00
Published on Web 11/14/2007
2007 American Chemical Society
Introduction Advanced oxidation processes (AOPs) have been widely studied for the degradation of various types of industrial wastewaters (1–5). Oxidation of organic pollutants via AOPs involves the generation of powerful oxidizing agents, such as the hydroxyl radical, that are capable of reacting with compounds by hydrogen abstraction, the addition to unsaturated bonds and aromatic rings, or electron transfer (1, 5). The Fenton reaction (Fe2+/Fe3+, H2O2) and, in particular, the photochemically enhanced Fenton (photo-Fenton) reaction (Fe2+/Fe3+, H2O2, UV light) are considered to be among the most promising AOPs for the remediation of wastewaters (1, 2, 5). The Fenton reaction is a redox process in which H2O2 is reduced to hydroxide ion and the hydroxyl radical by Fe2+, which is oxidized to Fe3+ (reaction 1 in Table 1). Both the hydroxyl radical (1, 5, 6) and oxoiron (IV) or ferryl (6–10), denoted here as FeO2+, are oxidizing species present during the Fenton reaction and have also been proposed (6) to participate in the degradation of organic compounds. The thermal Fenton reaction can be accelerated by irradiation with UV light as the result of photochemical reduction of Fe3+ back to Fe2+. The optimum pH range for the photochemical step (reaction 5 in Table 1) of the photoFenton process is ca. 3–3.3 (11). In this pH range, the predominant UV-light-absorbing Fe3+ species present is [Fe(H2O)5OH]2+ (12, 13), represented for convenience as Fe(OH)2+, which undergoes photolysis to generate Fe2+ plus a hydroxyl radical (reaction 5). Wastewaters containing appreciable saline concentrations (>l% salt), such as those from marine environments, or containing residues from organochlorine pesticide production are particularly problematic to treat. In most cases, they cannot be introduced into biological treatment systems without prior dilution because of salt-induced plasmolysis of cells and/or the loss of cell activity (14). In addition, under the usual photo-Fenton reaction conditions, the presence of high concentrations of NaCl results in strong to moderate inhibition of the degradation process (15–17). Recently, we demonstrated that the photocatalytic step of the photo-Fenton process (reaction 5) is strongly inhibited in the presence of added chloride ion (11) because of the conversion of the hydroxyl radical into the much less reactive species Cl2 · -. In this work, we analyze in more detail the origin of the inhibitory effect of added NaCl on the degradation of synthetic wastewater solutions containing either phenol or an extract of Brazilian gasoline.
Materials and Methods Materials. The reagents ferrous sulfate heptahydrate (FeSO4 · 7H2O, Merck), ferric sulfate hydrate [Fe2(SO4)3 · xH2O, Sigma], hydrogen peroxide (H2O2; 30%, Merck), phenol (Sigma), sodium chloride (NaCl, Sigma), sulfuric acid (H2SO4, Merck), potassium iodide (KI, Sigma), sodium sulfite (Na2SO3, Merck), sodium hydroxide (NaOH, Merck), and commercial gasoline, containing ca. 23% ethanol, were used as received. The phenol solutions were prepared by direct dissolution of the desired amount of phenol in aqueous solution. A synthetic wastewater gasoline stock solution (containing ca. 100 mg of C/L) was prepared as described previously (18), by extraction of commercial Brazilian gasoline with water; this produces a hydrocarbon-contaminated aqueous ethanol solution (19). General Procedure for Photodegradation Experiments. The photochemical reactor has been described previously (18). A 400 W Philips medium-pressure mercury vapor lamp, VOL. 41, NO. 24, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Manifold of Important Reactions Taking Place during the Initial Stages of the Fenton and Photo-assisted Fenton Oxidation of Phenol in the Absence (Reactions 1-5 and 14– 16) and Presence of Chloride Ions Fe2+ + H2O2 f Fe3+ + HO· + OH3+
Fe
2+
+ H2O2 a Fe(HO2) 2+
Fe(HO2) Fe
3+
f Fe
+ H2O f Fe(OH)
Fe(OH)
+
+H
3+
+ H fFe
(1)
(2)
+ HO2·
2+
2+
2+
+
(3) +
+H
(4)
+ H2O
(-4)
hν
Fe(OH)2+ 98 Fe2+ + HO·
(5)
Fe3+ + Cl- f FeCl2+
(6)
FeCl2+ f Fe3+ + Cl-
(-6)
hν
Fe(Cl)2+ 98 Fe2+ + Cl·
(7)
Cl· + Cl- f Cl2·-
(8)
Cl2·- + phenol f
f organic products
(9)
Cl2·- + Fe2+ f 2Cl- + Fe3+
(10)
HO· + Cl- a ClOH·- a Cl· + HO-
(11)
ClOH·- + H+ a ClOH2· a Cl· + H2O
(12)
·-
2+
ClOH + Fe Fe
3+
-
-
3+
II
+
+
f Cl + HO + Fe
+ H2cat a [Fe (cat·)] + 2H
II
+
+
2+
[Fe (cat·)] + H a Fe + Hcat· 2Hcat· a H2cat· + o-benzoquinone
(13) (14) (15) (16)
in a water-cooled Pyrex immersion well, was used as the light source (total output, as determined by 0.15 M potassium ferrioxalate actinometry (20), of 1.2 × 10-4 Ein/s). The initial pH of 3.0 L of synthetic wastewater solutions (phenol or extract of Brazilian gasoline) was adjusted to 3.0 by the addition of concentrated sulfuric acid (H2SO4), and the salinity was adjusted to the desired final value by the addition of NaCl. The solution was circulated with the lamp on until the temperature of the tank had stabilized at 30 °C. The photoFenton reaction was then initiated by the addition of 100 mL of a 0.015 M aqueous solution of FeSO4 · 7H2O, followed by the onset of the addition of 6 M H2O2 (100 mL total, at a rate of 0.83 mL/min). Duplicate samples (5 mL each) were withdrawn at appropriate time intervals for analysis during the course of the irradiation (4.5 h overall). Quenching solution (2 mL; consisting of a mixture of 0.1 M KI, 0.1 M Na2SO3, and 0.1 M NaOH) was added to all samples to interrupt the reaction (6), and the samples were filtered (0.22 µm Millipore Durapore membrane) to remove precipitated iron-containing species and analyzed for remaining total organic carbon (Shimadzu TOC-5000A TOC analyzer). Samples (3.0 mL) of 0.0060 M potassium ferrioxalate in 0.10 M H2SO4 (20) with and without added NaCl were irradiated in 13 × 100 Pyrex culture tubes in a darkroom, employing a merry-go-round reactor (21) equipped with four 8 W black light lamps (Sylvania Model BL350 phosphorcoated lamps with maximum emission at 356 nm, total output, as determined by 0.15 M potassium ferrioxalate actinometry (20), of 3.14 × 10-10 Ein/s). Laser flash photolysis was performed as described previously (11), in the presence and absence of added phenol or Fe2+. 8460
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General Procedure for High-Performance Liquid Chromatography (HPLC) Analyses. Phenol and its principal degradation products were quantified by HPLC on a Shimadzu LC-10 Liquid Chromatograph equipped with a Shimadzu UV–vis diode array detector. Phenol and its hydroxyaromatic derivatives were separated on a C-18 ShimPac reverse-phase column (5 µm; 4.6 × 150 mm) with detection at 270 nm at a mobile phase flow rate of 1 mL/min. The column was eluted with 0.2% aqueous acetic acid (solution A) for 6 min and then with a linear gradient of solution A with 0–80% of 0.2% acetic acid in methanol (solution B) for 6 min, and this latter mixture (20% A/80% B) was maintained for the remainder of the analysis. The acidic intermediates were analyzed on a Hamilton PRP-X300 ion-exchange column at 40 °C with detection at 220 nm, employing aqueous H2SO4 (pH 2.00 ( 0.02) as the mobile phase. Prior to injection, the samples were filtered through a C18 cartridge with methanol to remove the aromatic intermediates. Peaks were identified by their UV absorption spectrum and by a comparison with retention times of known samples of each compound. Kinetic Studies of Oxoiron (IV) with Phenol in the Absence and Presence of Chloride Ion. The effect of chloride ion on the reaction rate of FeO2+ with phenol was investigated by stopped-flow absorption experiments using a Hi-Tech Scientific SFA-20 Rapid Kinetics stopped-flow accessory and a detection system comprised of a PTI monochromator coupled to a Hamamatsu R666 photomultiplier tube. The signal was acquired by a digitizing scope (Hewlett-Packard 54504), where it was averaged and transferred to a PC computer. FeO2+ was generated in situ from the reaction of ozone with Fe2+ in 1 M HClO4. An ozonator of in-house construction was used to obtain gaseous ozone through an electrical discharge in dry oxygen. Stock solutions of ozone were prepared by bubbling ozone into 1 M HClO4 solutions. The ozone concentration was measured spectrophotometrically at 260 nm [260 ) 3300 M-1 cm-1 (22)]. FeO2+ has a characteristic absorption band in the 290–350 nm wavelength range, with 320 ) 420 M-1 cm-1 (9). The buildup and decay of FeO2+ was conveniently monitored at 320 nm, where the contribution from the absorption of Fe2+, Fe3+, and O3 is negligible. For the experiments performed in the presence of phenol and/or Cl-, one of the stopped-flow syringes was filled with 1.5 × 10-4 M Fe2+ solution containing the appropriate amount of Cl- and/or phenol in 1 M HClO4 and the other with a 1.69 × 10-4 M ozone solution in 1 M HClO4. Control experiments showed that the thermal reaction of Fe2+, Fe3+, or O3 and phenol is negligible under these experimental conditions (9).
Results The effect of pH and added chloride ions on the depletion of phenol and TOC in the photo-assisted Fenton process applied to a synthetic wastewater containing phenol as the organic component is shown in Figure 1. The results clearly indicate that a very low initial pH, such as pH 1, severely affects the depletion of both phenol and TOC. On the other hand, at an initial pH of 3, the addition of chloride ion significantly inhibits the TOC mineralization process after only partial decomposition of the organic material present, without significantly affecting the rate of disappearance of phenol. For a comparison, the effect of pH and chloride ions on the thermal Fenton degradation of phenol was also investigated. TOC depletion (Figure 2) is strongly inhibited at low pH but not by added chloride ion, in agreement with the literature (23, 24). Particularly significant is the fact that the rate of TOC depletion by the thermal Fenton oxidation at pH 3 is indistinguishable from that observed in the photo-assisted
FIGURE 1. (A) Effect of chloride ion and pH on the disappearance of phenol during the photo-assisted Fenton oxidation (200 mM H2O2 and 0.5 mM Fe2+) of 12 mM phenol. pHinitial (b) 1.0 and (4) 3.0 in the absence of NaCl or (9) pHinitial 3.0 in the presence of 0.5 M chloride. (B) Effect of added chloride ion on the decrease in TOC during the photo-Fenton degradation (200 mM H2O2 and 0.5 mM Fe2+) of 12 mM phenol with no pH control at pHinitial 1.0 (b) or 3.0 (4) in the absence of chloride ion and at pHinitial 3.0 in the presence of 0.5 M chloride (9).
FIGURE 2. Effect of added NaCl on TOC degradation by the Fenton oxidation (200 mM H2O2 and 0.5 mM Fe2+) of 12 mM phenol at either pHinitial 1.0 in the absence (b) and presence (O) of 0.5 M NaCl or at pHinitial 3.0 in the absence (0) and presence (9) of 0.5 M NaCl. (4) Experiments performed with iron added as Fe3+ at pHinitial 3.0 in the absence of chloride ion. Fenton experiments in the presence of NaCl under otherwise identical initial conditions (Figure 2). This clearly implies that the photochemical steps are largely inoperative in the presence of high concentrations of chloride ions. Not surprisingly, photolysis of phenol for the same total time under the same conditions in the absence of Fe2+ and hydrogen peroxide resulted in little or no loss of phenol (0.03 M NaCl for aliphatic hydrocarbons (18); > 0.2 M NaCl for phenols) when the pH of the medium falls to about 2 or below. The fact that the thermal Fenton reaction is not affected by added chloride anions suggests that, under our conditions, the major oxidizing species is not the hydroxyl radical (reaction 1). Although a high-valence oxoiron intermediate, such as ferryl (FeO2+), might, in principle, be an alternative oxidant, the relatively long lifetime of ferryl, in the milisecond time range, and its rather inefficient reactivity argue against a major role for the ferryl species in the thermal Fenton degradation of phenol. In this context, an important observation is that there is no difference between starting with Fe2+ or Fe3+ as the initial iron species (Figure 2). This is most readily explained by taking into account the alternative catechol-assisted Fenton reaction (30–34). This process was initially identified by Hamilton et al. (30), who showed that catalytic amounts of dihydroxybenzenes, such as catechol and 1,4-hydroquinone, present as intermediates in phenol degradation (Figure S1 of the Supporting Information), greatly enhance the rate of the Fe3+/H2O2 degradation of substituted benzenes. They attributed this rate acceleration to the presence of a ternary hydroquinone- or catechol-Fe-H2O2 complex, which they postulated to be the active oxidizing agent instead of hydroxyl radicals. At the same time, however, catechol (H2cat) can reduce Fe3+ to Fe2+ rapidly at acidic pH (31–34), giving rise to the complex [FeII(cat · )]+, (reaction 14), which is unstable at pH < 2 and yields a semiquinone radical (Hcat · ) (reaction 15). The disproportionation of Hcat · can regenerate catechol plus o-benzoquinone (reaction 16). The enhanced regeneration of Fe2+ and the consequent production of HO · (reaction 1) can also contribute to the observed rate acceleration. The complexation of Fe3+ by organic acids, such as oxalic acid, interrupts the regeneration of Fe2+ (33). Thus, once complete oxidation of catechol and the other di- and trihydroxybenzene intermediates has occurred, the Fe2+-Fe3+ redox reactions (reactions 14–16) cease to play a major role. From then on, further oxidation of the organic material becomes much slower unless the reaction mixture is irradiated to produce Fe2+ and hydroxyl radicals. However, in the presence of chloride ion and without pH control, the solution will be so acidic (pH ca. 2) at this point in the degradation process that the production of hydroxyl radicals is efficiently inhibited (either directly via preferential photolysis of FeCl2+
or indirectly via chloride ion scavenging of HO · ) and the mineralization slows to a halt. Circumventing the Chloride Ion Inhibition of the PhotoFenton Process. The experimental results presented in Figure 3 indicate that the inhibition of the photo-assisted Fenton oxidation of phenol by chloride ions may be conveniently circumvented by maintaining the pH of the reaction medium at or slightly above 3. Figure 3 also demonstrates that, even for a complex mixture of components, such as a hydrocarboncontaminated aqueous ethanol solution prepared by extraction of Brazilian gasoline with water (19), the simple expediency of controlling the pH at 3 throughout the process also prevents the inhibition of the photo-Fenton process by added chloride ion.
(13)
(14) (15) (16)
(17)
Acknowledgments This work was supported by CAPES-CEPEMA, FAPESP, and the Alcoa Foundation. A.M. Jr., J.E.F.M., and C.V.-G. thank CAPES, via the Project CAPES-CEPEMA, for fellowship support. C.A.O.N. and F.H.Q. thank the CNPq for senior research fellowship support. C.A.S. and L.C.F. thank the CNPq for graduate fellowship support. A.M. Jr. and M.C.G. thank the Secretaria de Ciencia, Tecnologia e Inovación Productiva (SECyT, Argentina) and CAPES for a collaborative project. M.C.G. is a research member of CONICET, Argentina.
(18)
Supporting Information Available
(22)
Figures S1-S7. This material is available free of charge via the Internet at http://pubs.acs.org.
(19) (20) (21)
(23)
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