Environ. Sci. Technol. 1990, 2 4 , 1566-1574
Quinone and Iron Porphyrin Mediated Reduction of Nitroaromatic Compounds in Homogeneous Aqueous Solution Reni P. Schwarzenbach,” Ruth Stlerll, Klaus L a w t and Josef Zeyer
Swiss Federal Institute of Technology Zurich (ETHZ) and Federal Institute for Water Resources and Water Pollution Control (EAWAG), 8600 Dubendorf and 6047 Kastanienbaum, Switzerland The kinetics of reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols have been investigated in aqueous solution containing reduced sulfur species and small concentrations of either a naphthoquinone or an iron porphyrin. Both the two naphthoquinones and the iron porphyrin used in this study mediated the reduction of the nitro group. In all three cases, the rate of reduction of the nitrobenzenes and of the nitrophenols was strongly pH dependent. Dissociated nitrophenols were reduced -3-4 times more slowly as compared to the nondissociated species. For the substituted nitrobenzenes, the effect of substitution on the reaction rate could be described by a linear free ener y relationship (LFER) of the general form log It = a& k‘ + b, where It is the second-orderrate constant for reaction with the hydroquinone monophenolate or the iron porphyrin, respectively, and Eh’’ is the one-electron reduction potential of the nitroaromatic compound. Competition between different nitrobenzenes was observed in the case of the iron porphyrin, while no effects were found for the reaction with the hydroquinones. The results of this study form an important base for the evaluation and interpretation of reductive transformation processes of nitroaromatic compounds in the environment. In several recent studies ( I - 7 ) , it has been demonstrated that, in reducing environments, a variety of organic pollutants undergo reductive transformations. Such processes include, for example, reductive dehalogenations of halogenated hydrocarbons (2-4), reduction of nitroaromatic compounds (5, 6 ) , and reduction of aromatic azo compounds (7). These reactions are of great interest, since they can lead to transformation products that are of similar or even greater environmental concern than the parent compounds ( I , 2). One important question commonly addressed when dealing with reductive transformations of organic pollutants in the environment is whether such reactions may occur abiotically or whether mediation by living (micro)organisms is necessary. From the available experimental data, it can be concluded that probably both abiotic and biological reductions are important in anaerobic soils, aquifers, and sediments (I, 2). However, to date, the nature and the abundances of the natural reductants present in such systems are unknown. The results of the few studies in which the reduction of a specific chemical has been investigated in different anaerobic sediment/water or soil/water systems suggest that a major portion of naturally occurring reductants is associated with the solid matrix (I, 6 , 7). In these studies, no evident relationship between bulk properties of the systems [e.g., apparent redox potential, pH, total organic carbon, total (reduced) iron] and transformation rates could be established. This lack of correlation between reduction rates and bulk medium properties is not surprising since there is certainly f
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ll,E, Germany. 1966
Environ. Sci. Technol., Vol. 24, No. 10, 1990
a great variety of different natural reductants present that may react with a given reducible organic pollutant. Such species may or may not be indicated by a bulk parameter, particularly by a measurement of the apparent redox potential. Consequently, based on the present knowledge, prediction of reduction rates of a specific compound in a given natural system, as well as derivation of generally applicable structure-reactivity relationships, is rather difficult. Aside from biological electron donors, the most abundant natural reductants present in anaerobic soils and sediments include reduced inorganic forms of iron and sulfur, such as iron(I1) sulfides, iron(I1) carbonates, and hydrogen sulfide (8). Although some of these reductants have been found to react with reducible organic pollutants including, for example, the reaction of hydrogen sulfide with substituted nitrobenzenes (this work), the reaction rates are, in general, much too slow (half-lives in the order of days to weeks) to account for the extremely rapid transformation rates often observed in natural systems. For example, for the reduction of the nitro group of parathion (5)or methyl parathion (6)in anaerobic soils and sediments, half-lives of seconds to minutes have been determined. Consequently, there must be other, more reactive reductants available. These species may not be present in large abundances, but may play the role of electron-transfer mediators; that is, after electron transfer to the pollutant, they may be reduced again rapidly by the bulk of reductants present. Hence, in abiotic systems, similar to what is a well-known phenomenon in biological systems, such species may play the role of “electron carriers” as is depicted by eq 1. (“eledron carrief),,
(“bulk”),,,
(pollutant),,
/ \ (“electron carrier),&/ \ (pollutant),d
(“bulk”),d
In biological systems, species acting as electron carriers include quinoid-type compounds and a variety of transition-metal complexes, in particular, iron complexes (e.g., iron porphyrins) (9). Such types of reactive species are also very likely to exist as constituents of natural organic materials (IO, 11). In various earlier studies, it has been well documented that, in homogeneous aqueous and nonaqueous solution, it is possible to reduce polyhalogenated hydrocarbons with iron(I1) porphyrins (2, 12-14). Recently, the reduction of some nitroaromatic pesticides with a hydroquinone has been reported (15). In all these studies, the reductants were present in excess concentrations. In this paper, we report the results of laboratory studies in which the kinetics of the reduction of a series of substituted nitrobenzenes and nitrophenols have been studied in aqueous hydrogen sulfide or cysteine solutions containing small concentrations of a quinone or a water-soluble iron porphyrin, respectively. The major goals of these experiments were to check whether quinones and iron porphyrins may act as electron carriers (eq 1)in the abiotic
0013-936X/90/0924-1566$02.50/0
0 1990 American Chemical Society
Table I. Names, Structural Formulas, Reduction Potentials ( E-d and Acidity Constants (pK,) of the Two Quinones and the Iron PorphyrinUsed as Electron Carriers structural formula oxidized form
Et,", b"'(PH 7),
reduced from
oxidized form
8-hydroxy-1,4-naphthoquinone (juglone)",*
V
V
0.428
0.033
0.351
-0.152
3.98
0.171
0.065
5.21
6H
0
HJUG
JUG 2-hydroxy-1,4-naphthoquinone
8.68
10.71
lawso one)“^'
>w< 0
6H
HpLAW
LAW
meso-tetrakis(N-methylpyridy1)iron porphind
+ e- z
N '
(>lo)
\
Fe(II1)P
Fe(I1)P
"Data from ref 16; Eho' = Eh" + 2.303RT/2F log ([HtI3 + K,,l('d)[Ht]2 + K,,l('d)K,,p("d)[Ht])/([H+] + Ka(0')). bAqueous buffer solution at 20 "C. CAqueousbuffer solution at 25 OC. dData from ref 17 valid for aqueous 1 M NaN03 at 25 C; Eho' = Eho + 2.303RT/F log [Ht]/([Ht] + K p ) ) .
reduction of nitroaromatic compounds in aqueous solution and to evaluate the effect of substituents on the rate of reduction of the nitro group. Experimental Section Chemicals. All chemicals were used as received. Analysis by HPLC and UV/vis spectroscopy gave no evidence of detectable impurities. Nitrobenzene, 2-chloro-, 3-chloro-, and 4-chloronitrobenzene;2-methyl-, 3-methyl-, and 4-methylnitrobenzene, 2-chloro-, 3-chloro-, and 4chloroaniline, 2-methyl-, 3-methyl, and 4-methylaniline, 2-nitrophenol, 4-nitrophenol, 4-chlor0-2-nitropheno1, 8hydroxy-l,4-naphthoquinone, and 2-hydroxy-1,4naphthoquinone were purchased from Fluka AG, Buchs, Switzerland; 2-nitro-, 3-nitro-, and 4-nitroacetophenone, 2-amino, $amino-, and 4-aminoacetophenone,aniline, and L-cysteine from Merck, Darmstadt, Germany; 4-methyl2-nitrophenol were from Ega Chemie, Steinheim, Germany; 5-fluoro-2-nitrophenol was from Aldrich Chemical Co., Steinheim, Germany; and the meso-tetrakis(N-methylpyridy1)porphin was from Sigma Chemical Co., St. Louis, MO. The iron complex of the porphin was provided by Prof. W. Schneider, Federal Institute of Technology, Zurich, Switzerland. Description of the Model Systems. The reduction kinetics of a series of nitroaromatic compounds were studied in three model systems. Two naturally occurring hydroxynaphthoquinones (juglone and lawsone; see Table I) and a water-soluble iron porphyrin [meso-tetrakis(Nmethylpyridy1)ironporphin; Table I] were evaluated. For the experiments with the two quinones, hydrogen sulfide (typically 5 mM) was used as "bulk electron donor". The experiments with the iron porphyrin were conducted in 5 mM aqueous cysteine solution, since the iron porphyrin precipitated in aqueous hydrogen sulfide, probably due to the formation of axial sulfur bridges between the iron centers. In all systems, pH was controlled with sodium
Table 11. Overview of the Conditions Used in the Various Experiments e1ectron carrier (concn range)
redox buffer (concn)
Eh(pH 7) of redox pH buffer, V range temp, OC
juglone hydrogen sulfide -0.192" (10-150 fiM) (5 mM) lawsone hydrogen sulfide -0.192' (5-250 fiM) (5 mM) iron porphyrin cysteine
