Environ. Sci. Technol. 1997, 31, 2198-2203
Photocatalytically Assisted Hydrolysis of Chlorinated Methanes under Anaerobic Conditions PAOLA CALZA, CLAUDIO MINERO, AND EZIO PELIZZETTI* Department of Analytical Chemistry, University of Torino, 10125 Torino, Italy
The photocatalytic degradation of CCl4, CHCl3, and CH2Cl2 over irradiated TiO2 has been investigated at pH 5 and pH 11 under anaerobic conditions. Chloromethanes degrade through combined reductive and oxidative processes. Photocatalytically assisted hydrolysis, given by sequential reactions involving •OH/e-/H+, is 106-108 times faster than the corresponding thermal process. Dechlorination of chloromethanes is achieved with degradation rates in the order CCl4 > CHCl3 > CH2Cl2. Stable intermediates, either chlorinated (chloromethanes, C2Cl6, and C2Cl4) or dechlorinated (formic acid, formaldehyde, and methanol) have been quantified. The average carbon oxidation number nC remains almost unchanged at the end of the degradation process, although for CCl4 in the early stages it is markedly reduced with slow regrowth of nC toward the initial value. Reaction pathways accounting for the observed results are presented based on literature data concerning transient intermediates.
Introduction The widespread use of halogenated hydrocarbons as solvents, degreasing agents and fumigants has led to their presence in the environments. There is a growing concern about their effect on the ozone layer depletion and adverse effect on health (1, 2). Chloromethanes (CCl4, CHCl3, and CH2Cl2) represent an important subset of this class and are of particular concern for their toxicity and persistency (2, 3). The reactions of oxidation, reduction, substitution, and dehydrohalogenation of halogenated aliphatic components have been extensively investigated (1, 4-7). Abiotic transformations can provide a framework for understanding biologically mediated transformations and are of interest for water decontamination. Many processes have been proposed over the years to destroy these refractory compounds. A chemical process for the degradation of trace amounts of chloromethanes should yield innocuous products such as carbon dioxide and hydrochloric acid, as expected from complete mineralization. Among the most recently proposed chemical technologies are noticeable high energy electron beam (8, 9), reductive dehalogenation with iron metal (10), destructive adsorption on metal oxides at high temperatures (11, 12), electrochemical dehalogenation (13), metal coenzyme-mediated reductive dehalogenation in Ti(III) solution (14), and mineral-supported cobalt macrocycles (15). Interestingly, reductive dehalogenation of aliphatic halocarbons is realized by enzymes even under aerobic conditions (16). Related to these investigations * Corresponding author fax: 39-11-670-7615; e-mail: pelizzet@ silver.ch.unito.it.
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are previous studies with γ-radiolysis (17), pulse radiolysis (18), e-beam (8), and electrochemical and photocatalytic methods (19-26). Photocatalytic degradation with irradiated semiconductors (19-26) or covalently linked cobalt macrocycles to the surface of titania (27) has been established to be effective for the mineralization of organohalogenated aliphatic compounds in aqueous and gaseous phases (24, 28-30). Heterogeneous photocatalytic processes involve reactions at the surface/ solution interface (31). The oxidizing species may be holes (more oxidizing than aqueous hydroxyl radical) or trapped holes (less oxidizing than aqueous hydroxyl radical) (32); the reducing species may be conduction band electrons or trapped electrons, both less reducing than aqueous hydrated electrons (18, 29, 30). Whether a process takes place under photocatalytic conditions is related to the redox property of e- and h+ on one side and the halomethanes on the other. The reaction occurs at the surface, and the overlap between the energy levels of the adsorbed organics and those of e- and h+ has also to be considered (18). Considering that oxidative and reductive photocatalytic processes are operating and because the presence of oxygen adds more and also alternative degradation pathways, to clarify the degradation pathways under well-defined conditions, we investigated the photocatalytic transformations of CCl4, CHCl3, and CH2Cl2 in the absence of extraneous oxidant or reductant species.
Experimental Section Materials and Reagents. All experiments were carried out using TiO2 Degussa P25 as the photocatalyst. In order to avoid possible interference from ions adsorbed on the photocatalyst, the TiO2 powder was irradiated and washed with doubly distilled water until no signal due to chloride, sulfate, or sodium ions could be detected by ion chromatography. Tetrachloromethane, trichloromethane, and dichloromethane (from Merck); tetrachloroethylene and hexachloroethane (from Aldrich); phosgene (Praxair, Oevel, Belgium); formic acid (Carlo Erba); and formaldehyde, methanol, and tetrahydrofuran (from Merck) were used as received. Sodium chloride was used after drying. Irradiation Procedures. The preparation and irradiation of the samples, the irradiance spectrum, and the cells were identical to those described elsewhere (33, 34). The irradiation was carried out on 5 mL of suspension containing 10 mg L-1 chloromethanes and 200 mg L-1 TiO2. The cells were purged with helium for 30 min before addition of the organic substance and subsequent irradiation. Efforts for eliminating the last remains of oxygen adsorbed onto the TiO2 and for reducing oxygen leakage as low as possible into the cell over the long times of irradiation permit us to obtain reliable data. Total photonic flux (340-400 nm) in the cell and temperature during irradiation was 1.35 × 10-5 Einstein min-1 and 50 °C, respectively. Experiments were run at pH 5 and pH 11 after adjustment with HNO3 or NaOH. Analytical Procedures. The entire content of the cells was analyzed by purge-and-trap/GC or filtered through 0.45µm cellulose acetate membranes (Millipore HA) after the established irradiation time. The filtrates were directly analyzed for formaldehyde, methanol, formic acid, and other C2 compounds. The disappearance of the primary compound was followed using a purge-and-trap system (Teckmar LSC 2000) with cryofocusing connected to a gas chromatograph (Varian STAR 3400) equipped with a FID detector and a 60 m DB5 column (Supelco, 0.25 µm coating). The purge-and-trap’s operative
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FIGURE 1. Photocatalytic hydrolysis of CCl4 6.6 × 10-5 M (ca. 10 mg L-1), intermediate evolution, and chloride formation on TiO2 200 mg L-1 under helium atmosphere as a function of the irradiation time at two pH values.
FIGURE 2. Photocatalytic hydrolysis of CHCl3 8.5 × 10-5 M (ca. 10 mg L-1), intermediate evolution, and chloride formation on TiO2 200 mg L-1 under helium atmosphere as a function of the irradiation time at two pH values.
parameters were as follows: standby 35 °C; purge 15 min; dry purge 3 min; desorb preheat 255 °C; desorb at 260 for 3 min; cryo cooldown at -100 °C; inject at 220 °C; bake at 250 °C for 7 min. The analyses were performed using a double gradient; temperature was linearly increased at 12 °C/min from 37 to 157 °C and then was brought to 300 °C at a rate of 30 °C/min. Under these conditions, retention times were 4.45, 5.76, and 6.65 min for CH2Cl2, CHCl3, and CCl4, respectively. The formation of chloride ions was monitored by suppressed ion chromatography, using a Biotronik IC 5000 apparatus equipped with a BT1AN separation column (20 cm length, 4 mm i.d., Biotronik) and an alkaline buffer eluant containing NaHCO3 (3 mM) at the flow rate of 1.5 mL min-1; under these conditions, the retention time of chloride ions was 4.3 min. Formic acid was detected using the ion chromatograph equipped with a Biotronik BT III OS type anion exclusion column and analyzed using H2SO4 5 × 10-4 M as eluant at 1 mL min-1 flow rate. Formaldehyde, glyoxal, and glyoxylic acid were derivatized as previously described (35). Their chromatographic analyses were accomplished by HPLC with a Rheodyne injector, an RP C18 column (Lichrochart, Merck, 12.5 cm × 0.4 cm, 5 µm packing), high pressure two-pumps gradient (Merck Hitachi L-6200) and a UV-Vis detector (Merck Hitachi L-4200). A Carlo Erba gas chromatograph equipped with FID detector (heated at 250 °C) and a packed column (Haye Sep.P 60/80 mesh, 12 ft length, 1/8 in. diameter) was used for detection of methanol at oven constant temperature (130 °C). Helium was used as the carrier gas. The same gas chromatograph was also used for detection of carbon monoxide with TCD detector heated at 150 °C and with the filament at 250 °C. The injector was kept at 130 °C, and the oven was kept at 50 °C. Under such conditions, the retention time of carbon monoxide was 11 min. For the evaluation of carbon dioxide formation and TOC disappearance, the experiments have been carried out at 100 mg L-1 initial chloromethane concentration. Total organic carbon (TOC) and inorganic carbon (IC) were measured on filtered suspensions using a Shimadzu TOC-5000 analyzer
FIGURE 3. Photocatalytic hydrolysis of CH2Cl2 1.2 × 10-4 M (ca. 10 mg L-1), intermediate evolution, and chloride formation on TiO2 200 mg L-1 under helium atmosphere as a function of the irradiation time at two pH values. (catalytic oxidation on Pt at 680 °C). Calibration was achieved by injecting standards of potassium phthalate.
Results and Discussion The haloaliphatic degradation has been followed through the primary compound disappearance and halide formation. Figure 1 shows the disappearance of CCl4 at 10 mg L-1 together with the corresponding formation of chloride at pH 5 and pH 11. This and similar plots for CHCl3 (Figure 2) and CH2Cl2 (Figure 3) show that chloride is released. Experiments for CCl4, CHCl3, and CH2Cl2 at 100 mg L-1 and pH 5 have shown that after 72 h of irradiation more than 75%, 90%, and 90% of the organic carbon, respectively, has been mineralized and that chloride is formed above 80%, 90%, and 90% of the
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stoichiometric amount, respectively. Thus, CCl4 degradation can be described by
CCl4 + 2H2O f CO2 + 4HCl
(1)
which is formally a hydrolysis reaction. Due to eq 1, dehalogenation and CO2 formation should take place even in the absence of oxygen. The experiments show that chloromethanes can be fully dehalogenated under anaerobic photocatalytic conditions. However, a careful inspection of the organic intermediates (see Figures 1-3) suggests that, particularly at pH 11, the concentration of non-halogenated C1 compounds slowly changes in the last part of the investigated time window. Since complete mineralization of CHCl3 and CH2Cl2 requires oxygen, their degradation in the absence of oxygen could produce chloride but not stoichiometric CO2. In Figures 2 and 3, the evolution of products with the oxidation state of carbon pHPZC at a rate comparable or higher than that of the reaction -OCCl3 f COCl2 + Cl-. This is supported by the analogy with the degradation of CCl3COO-, the rate of which strongly decreases at pH > pHPZC (22). However, at pH < pHPZC -OCCl3 may be adsorbed at the catalyst surface and react with e-, as CCl3COOdoes (44). At pH 5, the fraction of the protonated trichloromethanol is not negligible. In this form, the reaction with e- and the release of chloride are allowed. (c) Phosgene (COCl2), originated by chloride loss from -OCCl . Spontaneous hydrolysis of phosgene has been 3 determined to occur with a rate constant of about 70 s-1 at 50 °C (42). Because of this fast decomposition rate, it is unlikely to be found under the present experimental conditions. As a consequence, it is not possible to distinguish between the two paths that lead to the formation of the final CO2. However, COCl2 reaction with e- should be a possible path for formation of •COCl radical and subsequent degradation products. (d) Dichlorocarbene (:CCl2) and its hydrated product. Evidence of formation of C2Cl4 supports the presence of this intermediate, which is generated from CCl4 through two consecutive reactions with e- and simultaneous loss of 2 Cl(26), from CHCl3 either by e-/•OH or •OH/e- sequences releasing Cl- (and H2O), and from CH2Cl2 by reaction with 2•OH (and release of 2H2O). Dichlorocarbene is also obtainable from the slow reaction between OH- and CHCl3. At pH 11, this reaction could be a minor source of dichlorocarbene. Investigation of the fate of dichlorocarbene concluded that :CCl2 reacts almost exclusively with H2O (rather than with hydroxide ions) forming carbon monoxide, which further reacts at a very slow rate with OH- forming formate ion (45). In addition to the dimerization reaction forming C2Cl4, the possible transformations of :CCl2 under photocatalytic conditions are (i) the back reaction with e-/H+ (forming •CHCl2 radical); (ii) thermal hydrolysis with water as suggested above, with loss of 2Cl-. Since in the present degradation experiments in the absence of oxygen CO was always below the experimental detection limit, the reaction giving formyl chloride should be slow as compared with redox reactions to which CH(OH)Cl2 alternatively should undergo. These lead to the formation of CO2 (by 2•OH oxidation assisted by
Again, assuming that pathways globally depicted by stoichiometric reactions 14-16 have comparable kinetic weight and following the analysis described above for CHCl3 (see Scheme 1), the concentrations of detected intermediates should be in the ratio [HCHO]/[CH3OH]/[HCOOH] ) 2/1/1. This hypothesis is experimentally verified and implies that carbon average oxidation number should remain unchanged (nC ) 0). Figure 4 depicts the nC variation as a function of irradiation time for CH2Cl2 degradation. No significant variation is observed at pH 5, indicating that for this compound both oxidative and reductive pathways have comparable kinetic weight. It is worth noting that nC ) 0 for CH2Cl2, in which the carbon has an oxidation number 0, is exactly between the two limiting possible values. At pH 11, the rates of CH2Cl2 degradation and chloride ion formation are decreased by a factor 10. Also the rate of formation of HCOOH and HCHO is decreased, as observed before for CHCl3. These observations agree with the expected decrease of the oxidation potential of the valence band hole (and related generated species). As CHCl3 and CH2Cl2 degrade also via oxidative pathways involving the redox couples CHCl3/ •CCl and CH Cl /•CHCl , respectively, that are pH indepen3 2 2 2 dent, by increasing pH the driving force for their oxidation is reduced and, consequently, their oxidation rate. This effect is more noticeable as the carbon oxidation state is reduced passing from CCl4 to CHCl3 and CH2Cl2 (see Table 1). For CH2Cl2 and CHCl3, for which both oxidative and reductive reactions are possible, in the absence of other electron or hole scavengers, the photocatalytically induced hydrolysis (dechlorination by the two alternative sequences e-/•OH or •OH/e-) plays a considerable role. Homogeneous abiotic thermal hydrolysis of chloromethanes has been actively investigated in the last century. A recent work quotes hydrolysis half-lives of 40 and 1850 years at 25 °C and pH 7 for CCl4 and CHCl3, respectively (3). Photocatalytically
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HOCCl3 f -OCCl3 + H+
(pKa ∼ 3.5-4)
(17)
dechlorination and hydrolysis) through the COOH radical, which in turn again produces HCOOH, or the HCO radical finally yielding both HCOOH and HCHO. The observed amounts of these products obviously depend on the relative rates of the redox pathways. (e) Dichloromethyl (•CHCl2) and monochloromethyl (•CH2Cl) radicals have been reported in pulse radiolysis experiments (30). •CHCl2 can originate reductively from CHCl3 or oxidatively from CH2Cl2. Their possible reaction pathways are reported in Scheme 1. (f) Methanol. Methanol is not detected under aerobic photocatalytic degradation of CH2Cl2, but is formed under the present conditions. Following the sequence depicted for in Scheme 1, methanol can be formed from •CH2Cl (route A) or through CH2(OH)Cl intermediate (route B). Further reduction of •CH2Cl will produce CH3Cl. Since the thermal hydrolysis of methyl chloride is a slow reaction (k ) 1.5 × 10-4 s-1 with water and k ) 2.4 × 10-2 M-1 s-1 with OH- at 100 °C) (46, 47), the low amount of methyl chloride detected suggests that either reductive dechlorination is very fast as compared to reduction followed by protonation or that the alternative pathway through CH2(OH)Cl (route B) is kinetically favored. Route B that would be followed because of the species CH2(OH)Cl, concurrently to methanol formation, could also generate formaldehyde, which is the main product observed (see Figure 3b). (g) Condensation products (21, 22). As evidenced by C2Cl4 and C2Cl6 formation, radical-radical reactions also occur. Other possible reactions include radical to molecule reaction, such as •CHCl2 + CHCl3, followed by a subsequent redox process. In conclusion, in the absence of oxygen under photocatalytic conditions, halomethanes are efficiently hydrolysed, interconverted, or disproportionated through the pathways described in Scheme 1. After this work was submitted, a paper on photocatalytic degradation of CHCl3 and CHBr3 also giving additional information on CCl4 was published (48). The two sets of data are consistent and agree on the proposed photocatalytically enhanced hydrolysis mechanism, although the two experimental works have been carried out under quite different conditions. The effect of light intensity and concentration of substrate, catalyst, and electron scavengers was rationalized (49). Due to the saturative behavior of the rate versus concentration, it is likely that the rate or the photonic efficiency (Table 1) observed under our conditions is slightly less than the rate of chloride evolution given in Figure 2 of ref 48. The different amount of condensation products (C2 products) is consistent with the different initial concentrations in agreement with a previous report (50). The transformation frame here proposed is suitable to describe in a qualitative way all the observed results, highlighting also the effect of hole and electron scavengers recently reported (48).
Acknowledgments Financial support of CNR, MURST, and PNRA Progetto Antartide Contaminazione Ambientale is kindly appreciated.
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Received for review July 30, 1996. Revised manuscript received March 14, 1997. Accepted March 24, 1997.X ES960660X X
Abstract published in Advance ACS Abstracts, May 15, 1997.
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