Environ. Sci. Technol. 2003, 37, 372-378
Electron-Beam Treatment of Aromatic Hydrocarbons That Can Be Air-Stripped from Contaminated Groundwater. 1. Model Studies in Aqueous Solution GERTRAUD MARK, HEINZ-PETER SCHUCHMANN, MAN NIEN SCHUCHMANN, LUTZ PRAGER,† AND CLEMENS VON SONNTAG* Max-Planck-Institut fu ¨ r Strahlenchemie, Stiftstrasse 34-36, P.O.Box 101365, 45413 Mu ¨ lheim, Germany, and Institut fu ¨ r Oberfla¨chenmodifizierung (IOM), Permoserstrasse 15, 04318 Leipzig, Germany
As a model for the electron-beam degradation of volatile aromatics (benzene, toluene, ethylbenzene, xylenes, BTEX) in groundwater strip gas, to be reported in Part 2, the γ-radiolysis of benzene has been studied in aqueous solutions. Addition of •OH to the aromatic ring gives rise to hydroxycyclohexadienyl radicals which either dimerize or disproportionate. The various dimers undergo acidcatalyzed water elimination yielding biphenyl. Phenol is formed upon disproportionation directly, but also via dihydroxycyclohexadiene which subsequently undergoes acid-catalyzed water elimination. Co-radiolysis of benzene with nitrite generates •NO2 in addition to the hydroxycyclohexadienyl radical. These not only interact with one another (product: nitrobenzene via nitro-hydroxycyclohexadienes) but the •NO2 radical is also capable of abstracting cyclohexadienylic hydrogens. This reaction leads to the formation of 2- and 4-nitrophenol and to further nitrated products that were not identified. These are suggested to be formed in an analogous reaction of •NO2 with the hydroxycylohexadienyl dimers. The effect of O2 on these reactions and the relevance for the gas-phase radiolysis of BTEX is discussed.
Introduction Locally severe instances of pollution of soil and groundwater by aromatic hydrocarbons are largely a legacy of the past, caused by war action on or negligent operation of industrial facilities, such as coking plants or refineries. While the lowermolecular-weight compounds, benzene and its alkylated derivatives (toluene, ethylbenzene, xylenes), “BTEX”, in situ suffer bacterial degradation in the presence of oxygen (1) or other biochemically accessible oxidants, such as sulfate (2), nitrate (2-4), or ferric ion (2,5,6) at appreciable rates, polluted groundwater sometimes moves toward and penetrates water wells sunk into a previously clean aquifer, causing the pollution problem to become acute. The cleaning-up of a contaminated site may also be necessary for other reasons, for example, site reclamation and reconstruction, or in preparation for carrying out underground construction work. * Corresponding author phone: +49-208-306-3694; fax: +49-208306-3951; e-mail:
[email protected]. † Institut fu ¨ r Oberfla¨chenmodifizierung. 372
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A case in point is the situation at a location in Du ¨ sseldorf, Germany, where a coking-gas works and its stocks were bombed out more than five decades ago that, in addition to many years of often careless practice, has led to the spillage of many tons of benzene and other aromatics (2,7-9). Concepts for remediation, often subsumed under the label of “advanced oxidation processes” (AOP), have been developed along various lines, such as γ-irradiation of the soil (10,11); γ- or electron-beam (EB) irradiation of the water at various degrees of pollution (11-16); electrical discharge (17,18); UV/H2O2 (19,20); O3/UV (21-25); O3/H2O2 (26-29); light/metal oxide, usually in the form of UV/titanium dioxide (30,31) or ultrasound (32-35), and have been proposed, more or less convincingly, to offer solutions to these problems whenever the biodegradative approach, making use of the existing community of microorganisms that have become adapted to the particular situation, is impractical. In all of these AOP approaches, the reactive OH radical plays a decisive role. To be effective, irradiation must be carried out under oxidative conditions, in the case of drinking-water purification, not only to oxidize the pollutants but also in order to avoid the formation of nitrite from the trace constituent nitrate (36). Air-stripping has proved to be an efficient method for the remediation of groundwater contaminated by volatile organics. While the destruction of aromatics without their prior expulsion from the aqueous medium has been proposed (3740), it is difficult to carry to a sufficient degree of degradation (41,42). Hence, electron-beam treatment of the strip-gas (4346), like off-gases in general (47-49), suggests itself as a method of choice for the destruction of these compounds, because it induces free-radical oxidation reactions under noncombustive conditions whereby the action of the radiative energy is confined to a relatively small amount of mass. It has already shown promise in the abatement of chloroaromatic pollutants in flue gases (50) and chlorinated ethylenes (44,45,51). While UV irradiation unaided by an adjuvant, for example, H2O2, may be useful in the case of the chlorinated ethylenes (52,53) [for the subsequent degradation of unwanted byproducts see refs 53-55], it is at present less practical in the case of alkylated benzenes, which begin to absorb at wavelengths considerably below 254 nm, the wavelength currently easily available from industrial-scale radiation sources, and the quantum yield of decomposition, mostly into nonradical products, approaches unity only at irradiation wavelengths below 200 nm (56). Free-radical processes set in as the primary products themselves undergo photolysis. The present study deals with the radiolytic decomposition of BTEX, particularly benzene, in the (humid) gas phase (see Part 2) and in aqueous solution. In this context, it is emphasized that the reaction-mechanistic and kinetic information that can be obtained from aqueous-solution studies often complements that sought in atmospheric chemical or other gas-phase studies. In both cases, the main reactive agent is the •OH radical (see also part 2) that readily adds to the aromatic ring (Scheme 1, reaction 1). In the presence of O2, these hydroxycyclohexadienyl radicals 1 are converted into the corresponding peroxyl radicals 2 and 3 (reactions 3 and 4); these reactions are reversible, both in aqueous solution (57-59) and in the gas phase (60-63). As shown in Part 2, this reversibility is of considerable importance in that it may inhibit the oxidation process via reactions 5 and 6, and in the gas phase, via reactions 2, 7, and 8 contribute to the formation of aerosols. It will also be shown in part 2 that •NO2, which is generated in the radiolysis 10.1021/es020580v CCC: $25.00
2003 American Chemical Society Published on Web 12/05/2002
SCHEME 1
FIGURE 1. Gas chromatogram of the products from the γ-radiolysis of N2O-saturated aqueous solutions of benzene after rotary evaporation and trimethylsilylation. of air (64), has an impact on the prospects of the EB process, since it is incorporated in a variety of the products. Nitrated compounds may represent an enhanced environmental hazard, since as a class, they are usually refractory toward biological degradation. The present paper deals with a detailed model study on the •OH-induced reactions of benzene in aqueous solution in the absence of O2, the reactions in its presence having been reported before (57). In the presence of •NO2, the latter interferes with these reactions, and the resulting products have also been studied. These aqueous phase data will be used for the interpretation of the gas-phase radiolyses to be reported in Part 2.
Experimental Section Aqueous solutions of benzene that was added with a syringe through a serum cap to N2O- or N2O/O2 (4:1)-saturated MilliQ-filtered (Millipore) water to a benzene concentration of 2 mM were 60Co-γ-irradiated at a dose rate of 0.13 Gy s-1 to a dose of 120 Gy. Irradiated samples were analyzed by HPLC (precolumn 40 mm, column 12.5 cm Nucleosil 5 C18, optical detection by photodiode array), eluent: methanol/water/acetic acid 75:25:1 for biphenyl, 30:70:1 for phenol, and 7:30:1 for the nitrated products. Acids were determined by HPIC (Dionex DX-100, precolumn AG9-SC 4 × 50 mm column AS9-SC, 4 × 250 mm; eluent 1.8 mM CO32-/1.7 mM HCO3- or 3.5 mM CO32-/1.0 mM HCO3-) for the irradiation products. The less-volatile products, in the presence of oxygen largely aliphatic mono- and dicarbonic acids besides other unidentified products, were also analyzed by GC/MS (Hewlett-Packard) after evaporation of the aqueous samples to dryness and trimethylsilylation of the residue, dissolved in 100 µL of dry (over KOH pellets) pyridine, 300 µL of bis(trimethylsilyl)trifluoroacetamide (BSTFA) and ∼20 µL of chlorotrimethylsilane. For the analysis of nondehydrated dimers that predominate under anoxic conditions, the irradiated aqueous solutions were extracted with ether using a Ludwig extractor. After evaporation of the ether, the residue was dissolved in 100 µL of pyridine, with 300 µL of BSTFA and ∼20 µL of chlorotrimethylsilane added for trimethylsilylation. Analysis was by GC/MS (see Figure 1). The product peaks in Figure 1 can be divided into two groups with retention times (a) from 12 to 15 min and (b) from 17 to 20 min. The MS of the products in group (a) are characterized by a prominent m/z 244 (M), which is assigned to structures C6H5-C6H6OSi(CH3)3. The MS of the products in group (b) permit the assignment to structures (CH3)3SiOC6H6-C6H6OSi(CH3)3. Characteristic m/z’s are 151 C6H5OdSi(CH3)2, 167 C6H5-OH-Si(CH3)3, produced by easy cleav-
age of the inter-ring linkage as a result of the absence of aromaticity; and 332 M - 2H, produced by the aromatization of one of the rings. The presence of the hydroxycyclohexadienylbenzene derivatives, group (a), and biphenyl suggests partial aromatization by water elimination during workup.
Results and Discussion Generation of Radicals. The initiating radicals, prominent among them the hydroxyl radical, were generated by the radiolysis of water (reaction 9). The radiation-chemical yields (G values) of the primary radicals are G(•OH) ∼ G(eaq-) ) 2.9 × 10-7, G(H•) ) 0.6 × 10-7, and G(H2O2) ∼ 0.7 × 10-7 mol J-1. The cosolute N2O converts the hydrated electron into •OH (reaction 10); thus, ∼90% of the primary radicals appear as •OH. They react rapidly with benzene (reaction 1, k ) 7.8 × 109 dm3 mol-1 s-1) (65). Product material balance considerations are based on the G and rate constant values and allow conclusions to be drawn regarding the reaction mechanism. ionizing
H 2O 9 8 eaq-, •OH, H•, H+, H2O2, H2 radiation
(9)
eaq- + N2O + H2O f •OH + OH- + N2
(10)
In the absence of O2, H atoms give rise to the formation of cyclohexadienyl radicals (reaction 11, k ) 9.1 × 108 dm3 mol-1 s-1) (65). The presence of cyclohexadienyl (∼10% of benzene adduct radicals) complicates to a minor degree the kinetics of the disproportionation and recombination of the hydroxycyclohexadienyl radicals (∼90%), the reactions of which are of special interest here.
H• + benzene f cyclohexadienyl radical
(11)
In the presence of O2, this problem does not arise, because H• is scavenged by O2 (reaction 12) with the formation of superoxide (equilibrium 13, pKa ) 4.8) (66).
H• + O2 f HO2•
(12)
HO2• a H+ + O2•-
(13)
To study the influence of the reactions of the hydroxycyclohexadienyl radicals with •NO2, the latter is produced from NO2- (reaction 14, k ) 1.1 × 1010 dm3 mol-1 s-1) (65) together with the former in a solution that contains both substrates in comparable concentrations.
NO2- + •OH f •NO2 + OHVOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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373
SCHEME 2
FIGURE 2. γ-Radiolysis of benzene in N2O-saturated aqueous solutions. Yield of phenol immediately after irradiation (O) and after standing for 15 min at pH 1 (b). Products in the Absence of O2. The radiolysis of benzene in deoxygenated N2O-saturated aqueous solution (67) was resumed in order to reexamine the termination reaction of the hydroxycyclohexadienyl radical and to determine the relative importance of the formation and the reactivity of the recombination products under these conditions (Scheme 2). Recombination gives rise to several bi(hydroxycyclohexadienyl) isomers, 8a-8c (reaction 17). Disproportionation produces phenol 4 and benzene, apparently via two different pathways, that is, H-atom transfer and ionic (reactions 15 and 16). Evidence for the latter is the delayed production of phenol 4 (cf. reaction 20) on top of a prompt process (Figure 2). A low pH is required for rapid water elimination. The bi(hydroxycyclohexadienyl) isomers 8a-8c, like cyclohexadienediol 6 from the ionic disproportionation reaction 18 that turns into phenol 4, are slowly (Figure 3) transformed into biphenyl 10 by water elimination under these conditions (Scheme 3). These reactions are acid-catalyzed, and also proceed to a certain extent during the workup and the trimethylsilylation for their identification by GC/MS (cf. Figure 1). A similar behavior has been observed in the case of chlorobenzene radiolysis (68). Figure 3 shows that this dehydration process is multiphasic, which suggests that the rate of aromatization depends on structural detail that varies in the isomeric dimers 8a-8c and that may exist in meso as well as DL forms. The ortho dehydration step (reactions 2123) is probably faster than the para one (reactions 24-26). 374
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FIGURE 3. Re-aromatization by dehydration of hydroxycyclohexadienyl dimers 8a-8c formed in the γ-irradiation of benzene in aqueous solution (20 °C, pH 4). The formation of biphenyl 10 from the various bi(hydroxycyclohexadienyl) isomers 8a-8c provides, in principle, a convenient way for the quantitative estimation of their total, and together with the yield of phenol 4, allows an estimate of the recombination/disproportionation ratio k17/ k15,16 of their termination. The present data, G(biphenyl) ) 1.7-1.8 × 10-7 mol J-1, G(phenol) ) 0.51-0.54 × 10-7 mol J-1, give a value of 3-3.5 for k17/k15,16 (determined after keeping the irradiated solution at pH 1 for 15 min). From the data reported in ref 67 [G(biphenyl) ≈ 2.1 × 10-7 mol J-1, G(phenol) ≈ 1 × 10-7 mol J-1], one estimates a value of ∼2 for this ratio. Thus, it is clear that the formation of these dimeric species is an important process. This and the longevity (on the time scale of free-radical reactions) of the bi(hydroxycyclohexadienyl) species 8a-8c, which are much better scavengers of C-centered free radicals than the aromatics themselves, indicate that they could contribute decisively to aerosol production in the strip-gas situation (see Part 2). The sum of 2 G(biphenyl) + 2 G(phenol) at ∼4.5 × 10-7 mol J-1 falls short of the expected total of 5.8 × 10-7 mol J-1. A shortfall remains even after taking into account a G value of 0.6 × 10-7 mol J-1 of OH-adduct radical lost by its recombination with the H-adduct radical. This product cannot give rise to biphenyl 10 by water elimination. Further,
SCHEME 3
SCHEME 4
one might suspect that in the ortho-ortho dimers 8a, dehydration can also give rise to tetrahydrodibenzofuran, although there is no indication in Figure 1 of any formation of this compound, which is expected to emerge in the vicinity of biphenyl. On the other hand, the quantitative determination of biphenyl in aqueous solution is complicated by its hydrophobicity; that is, even though its vapor pressure is low, its rate of loss from the surface of the solution, where it is enriched, is significant relative to its total content in the solution. Alkylated benzenes resemble the prototype in that in these cases also, •OH reacts predominantly (97% for toluene) (69) by addition to the ring. In aqueous solutions, these •OH adducts eliminate H2O in an acid-catalyzed reaction (k ) 1 × 106 dm3 mol-1 s-1) (69) to yield benzyl-type radicals. Under neutral conditions, this pathway is effectively blocked.
Products in the Presence of O2. Several reaction pathways are open to the hydroxycyclohexadienylperoxyl radicals 2, 3 that are produced under these conditions (Scheme 4) (5759,70-73). The main oxidation product from benzene is phenol 4 with a G value of 3.1 × 10-7 mol J-1 (reactions 3 and 28), but a multitude of fragmentation products are formed when the hydroxycyclohexadienylperoxyl radicals 3 rearrange into endoperoxide radicals 11 followed by further oxidation and cleavage (reactions 4, 30, and 33). There is a peroxyl-radicaltermination pathway (reaction 31), the importance of which is relatively small under γ-radiolysis conditions but which increases with the dose rate, and along which the aromatic characteristics of the substrate, they are to some extent restored (re-aromatization). Overly high dose rates are thus undesirable, not only because they also may make it harder VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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375
SCHEME 5
TABLE 1. γ-Radiolysis of N2O/O2 (4:1 v/v) Saturated Aqueous Solution of Benzenea product phenol cyclohexa-2,5-diene-1,4-diol hydroquinone catechol mucondialdehyde 5,6-epoxy-4-hydroxycyclohex-2-enone CHO-CHdCH-CO-CO-CH2OH CHO-CHOH-CHdCH-CHOH-CHO 5,6-dihydroxycyclohex-2-ene-1,4-dione CHO-CH2-CHdCH-CO-CHO CHO-CHdCH-CH2-CHOH-CHO CHO-(CHOH)3-CHO CHO-CO-CHdCH-COOH CHO-CHdCH-CO-CH2OH CHO-CH2-CH2-CO-CHO CHO-CO-CHdCH-CHO CHO-CHOH-CO-CH2OH CH3-CHOH-COOH CHO-CO-CHO CH2OH-COOH glyoxal acetaldehyde formaldehyde formic acid oxygen consumption hydrogen peroxide organic hydroperoxides
G, 10-7 mol J-1 3.1 0.1 0.1 0.05 absent 0.02 0.15 0.5 0.01 0.03 0.1 0.2 0.04 0.5 0.1 0.2 0.1 0.2 0.3 0.2 0.2 0.3 0.7 0.9 5.6 1.7 none formed
a 2 × 10-3 mol dm-3 at pH 6.5. Products with G values (unit, 10-7 mol J-1) (57). Dose rate, 0.14 Gy s-1.
to avoid oxygen depletion if the rate of oxygen delivery is insufficient. Table 1 lists the products that have been determined from the γ-radiation-induced oxidation of benzene in oxygenated aqueous solution (57). The G value of benzene degradation, estimated following Table 2, exceeds a total of 5 × 10-7 mol J-1, which indicates that any formation of dimeric products should be unimportant under these conditions. Phenol 4 alone accounts for one-half of the total, and its formation must be explained essentially from the elimination of superoxide from hydroxycyclohexadienylperoxyl 2 (reaction 32). The degradation G value of ∼6 × 10-7 mol J-1 and the low yield of hydroquinone 15 all but rule out disproportionation reactions of hydroxycyclohexadienyl 1 or hydroxycyclohexadienylperoxyl 2, 3 (Russell reaction) as viable sources of phenol 4 under these conditions. 376
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TABLE 2. γ-Radiolysis of Benzenea in the Presence of NO2- in Aqueous Solution, Saturated with N2O or N2O/O2 (4:1 v/v)b
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 2, 2003
N2 O -],
[NO2
mol
dm-3
G(C6H6OH•) G(•NO2) G(NO3-) G(nitrobenzene) G(phenol) G(2-nitrophenol) G(4-nitrophenol)
2×
10-3
2.5 3.3 0.1 0.43 0.28 0.06 0.05
N2O/O2 4×
10-3
1.5 4.3 0.5 0.37 0.05 0.12 0.09
2×
10-3
2.5 3.3 0.7 n.d. n.d. n.d. n.d.
4 × 10-3 1.5 4.3 1.3 absent 0.75 absent absent
a 2 × 10-3 mol dm-3. b Some products with G values (unit, 10-7 mol J-1). Dose rate, 0.125 Gy s-1.
The absence of organic hydroperoxide and the enhanced (as compared with the ever-present “molecular” yield of 0.7 × 10-7 mol J-1) (74) value of 1.7 × 10-7 mol J-1 for G(H2O2) (Table 2) indicates that any hydroperoxides that are formed by the reaction of peroxyl radicals with O2•- (75,76) release H2O2; moreover, O2•-/HO2• may also disappear in the selftermination reaction 35 (66).
HO2• + O2•- + H+ f H2O2 + O2
(35)
Reactions in the Presence of •NO2. Nitrogen dioxide is formed in the radiolysis of air (see Part 2) and may participate in the free-radical reactions under such conditions. For a better mechanistic understanding of the reactions that may have to be considered, some experiments containing •NO2 have been carried out also in the aqueous solution. The coradiolysis of benzene and NO2- at equal concentrations (2 × 10-3 mol dm-3) in N2O-saturated solution yields hydroxycyclohexadienyl radical 1 and •NO2 in the ratio of k1/k14, that is, 0.78 (65), or G(hydroxycyclohexadienyl) ≈ 2.5 × 10-7 mol J-1 and G(•NO2) ≈ 3.3 × 10-7 mol J-1. Several nitrated products have been determined in addition to phenol and the nitrate ion (Table 2, columns 2 and 3). In the absence of any other radicals or substrates, •NO2 reacts by disproportionation, with N2O4 as an intermediate (reactions 36 and 37). The reactions that may proceed in the presence of hydroxycyclohexadienyl radicals and the reaction products are shown in Scheme 5.
2•NO2 / N2O4
(36)
N2O4 + H2O f NO2- + NO3- + 2 H+
(37)
Some mechanistic conclusions may be drawn, for instance, from the results shown in column 2 of Table 2. The initial termination reactions give rise to phenol 4 and bi(hydroxycyclohexadienyl) 8 in the ratio 1:3 (reactions 1517) to N2O4 (reaction 36) and to hydroxynitrocyclohexadiene 16 (reaction 38). The recombination of the OH-adduct radical 1 and •NO2 is a fast reaction (77). Although this may subsequently lead to the formation of the nitrobenzene 17 with the expulsion of a water molecule (reaction 39) (7880), it is conceivable that the hydroxynitrocyclohexadiene 16 reacts also in other ways (80). Its oxidation by, for example, • NO2 (reaction 40), might explain the formation of nitrophenol 19 (reaction 41), traces of which were observed earlier (78) in gas-phase work. It can be shown that if these termination reactions have similar rate constants, G(phenol) by reactions 15 and 16 should be about equal to the value observed (Table 2). Other reactions contributing to the consumption of •NO2 are indicated in Scheme 5, based on the assumption that • NO2 adds to or oxidizes (reaction 42) the olefinic dimeric primary products 8, in competition with the hydrolysis (81) of its dimer N2O4 to nitrate and nitrite. The rate constants for the addition of •NO2 to 1,3-cyclohexadiene structures are on the order of 103 dm3 mol-1 s-1 (82). Although such addition may be reversible (83), these adduct radicals could be stabilized by peroxyl radical formation. The existence of these additional pathways would explain the low G(nitrate) and the apparent deficit with respect to G(•NO2) generally. Yield vs dose plots of the nitrophenols, for example, 19, and of nitrobenzene 17 give the impression that these are primary products (data not shown, but for similar data from a γ-radiolysis experiment that was carried out in humidified air, see part 2), which implies that the hydroxynitrocyclohexadiene 16 is quite reactive and as an intermediate reaches its steady-state already at a low concentration. The absence of nitrobenzene 17 and nitrophenol 19 upon radiolysis in oxygenated aqueous solution (Table 2, column 5) is explained by the removal of hydroxycyclohexadienyl 1 via its peroxyl radical 2 (and 3) through O2•- elimination (reaction 28), which is also reflected in the higher G(phenol). Relevance for the Gas-Phase Radiolysis. In the gas-phase radiolysis of benzene and the other BTEX aromatics, hydroxycylohexadienyl-type and •NO2 radicals are formed, and the reactions of these radicals are an essential factor that determines the nature of the products. The data presented here show the importance of the involvement of •NO2 in the product distribution of •OH-induced reactions of benzene. For example, their reactions with the dimers of the •OHadduct radicals (cf. reactions 42 and 44) explains the observation, reported in Part 2, that the aerosols formed in the gas-phase radiolysis contain a considerable amount of nitrogen. The observation of low-molecular-weight nitrated products in the gas-phase study may also be explained on the basis of the above mechanistic considerations.
(5) (6) (7) (8)
(9)
(10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
(20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33)
Acknowledgments This project has been supported by the German Federal Ministry of Research (Project Nos. 02WT9655/56/57/58).
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Received for review February 7, 2002. Revised manuscript received September 9, 2002. Accepted October 4, 2002. ES020580V
