Environ. Sci. Technol. 1996, 30, 2755-2763
Covalent Binding of Aniline to Humic Substances. 1. Kinetic Studies ERIC J. WEBER* Ecosystems Research Division, National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30605
DAVID L. SPIDLE Mantech Inc., Athens, Georgia 30605
KEVIN A. THORN U.S. Geological Survey, Water Resources Division, Denver Federal Center, Mail Stop 481, Box 25046, Denver, Colorado 80225
The reaction kinetics for the covalent binding of aniline with reconstituted IHSS humic and fulvic acids, unfractionated DOM isolated from Suwannee River water, and whole samples of Suwannee River water have been investigated. The reaction kinetics in each of these systems can be adequately described by a simple second-order rate expression. The effect of varying the initial concentration of aniline on reaction kinetics suggested that approximately 10% of the covalent binding sites associated with Suwannee River fulvic acid are highly reactive sites that are quickly saturated. Based on the kinetic parameters determined for the binding of aniline with the Suwannee River fulvic and humic acid isolates, it was estimated that 50% of the aniline concentration decrease in a Suwannee River water sample could be attributed to reaction with the fulvic and humic acid components of the whole water sample. Studies with Suwannee River fulvic acid demonstrated that the rate of binding decreased with decreasing pH, which parallels the decrease in the effective concentration of the neutral form, or reactive nucleophilic species of aniline. The covalent binding of aniline with Suwannee River fulvic acid was inhibited by prior treatment of the fulvic acid with hydrogen sulfide, sodium borohydride, or hydroxylamine. These observations are consistent with a reaction pathway involving nucleophilic addition of aniline to carbonyl moieties present in the fulvic acid.
Introduction Aniline is the parent compound of an important family of industrial chemicals used in the preparation of numerous * Corresponding author e-mail address: Weber.Eric@epamail. epa.gov; telephone: (706) 546-3366; fax: (706) 546-3636.
S0013-936X(95)00934-5 CCC: $12.00
1996 American Chemical Society
synthetic organic chemicals such as agrochemicals, dyestuffs, and pharmaceuticals. Concern exists over the loss of aniline and other aromatic amines to the environment during production processes or incomplete treatment of industrial waste streams. In addition, aromatic amines can enter the environment from the reduction of azo dyes, polynitro aromatic munitions (e.g., TNT), and dinitro herbicides and from the hydrolytic degradation of numerous agrochemicals. An important reaction pathway for aromatic amines in aquatic ecosystems is thought to be covalent binding with constituents of natural organic matter (NOM). Covalent binding has been proposed to result from the nucleophilic addition of the amino functional group with electrophilic sites (i.e., carbonyl moieties) and/or oxidative mechanisms resulting in the formation of anilino radicals that couple with radical species thought to be present in NOM (1). Evidence for the importance of covalent binding of aromatic amines to natural organic matter has been gathered from numerous studies of the reaction of aniline and other aromatic amines with soils and sediments (2-8), dissolved organic matter (DOM) (5, 9), and model compounds thought to be representative of monomeric units forming humic and fulvic acid polymers (5, 6, 9-14). A general feature of the reaction kinetics for the covalent binding of aromatic amines with NOM is that biphasic kinetics are typically observed. The biphasic kinetics are attributed to the occurrence of two reaction processes: a rapid, often reversible process and a slower, irreversible process. The rapid sorption step has been attributed to electrostatic interactions, hydrophobic partitioning, and the formation of labile amine-carbonyl adducts (e.g., imines) (7). The slower process has been attributed to irreversible covalent binding. This type of behavior has been observed for the sorption of a number of aromatic amines in soil-water systems including benzidine, R-naphthylamine, p-toluidine (7), 3,3′-dichlorobenzidine (15), chloroanilines (2), and 4,4′-methylenebis(2-chloroaniline) (16). Similar reaction kinetics have been observed for the binding of several ring-substituted anilines (4) and benzidine (9) to soil-humic isolates. The kinetically slower step is attributed to irreversible covalent binding because of the inability to recover the parent compound by harsh extraction methods. Although no direct spectroscopic evidence exists, covalent binding is thought to occur primarily through the nucleophilic addition of the aromatic amine to carbonyl moieties present in the organic matrix of NOM. The reactions of aromatic amines with quinones and catechols, which require oxidation to the quinone, have been used as chemical models for covalent binding in soil systems and natural waters containing DOM (5, 9, 11-13, 17). It has been proposed that 1,2-nucleophilic addition to the carbonyl group on the quinone represents the fast, reversible reaction and that 1,4-nucleophilic addition (Michael-type addition) to the quinone represents the slower, irreversible binding reaction (4). Subsequent oxidation of the anilinohydroquinone results in the formation of the anilinoquinone.
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N
O
NH2
+ H2O
1,2
O
+
O
OH O
1,4
NH
NH OH
O
From the previous discussion, it is apparent that the covalent binding of aromatic amines may occur with organic matter that is associated with a natural surface (i.e., bottom sediment or colloidal material suspended in the water column) or with organic matter that is in solution. As part of a larger effort to investigate the reaction pathways of aromatic amines in aquatic ecosystems, our initial studies have focused on the covalent binding of aniline to DOM. The benefit of working with DOM initially was that competing fate pathways that may occur in soil- and sediment-water systems, such as sorption through cation exchange or hydrophobic processes, would be minimized. The objectives of this study were 2-fold: (1) to probe the reaction mechanism for the covalent binding of aniline to natural organic matter by measurement of reaction kinetics and liquid-phase nitrogen-15 NMR analysis of the anilinereacted fulvic and humic acids and (2) to develop kinetic models to describe the reaction of aniline to dissolved organic matter in natural aquatic ecosystems. The results of the 15N NMR studies are reported in the companion paper (18). Here, we report the results of kinetic studies of the covalent binding of aniline to Suwannee River and IHSS Suwannee River humic and fulvic acid isolates, IHSS soil humic and fulvic acid isolates, unfractionated DOM isolated from the Suwannee River, and whole samples of Suwannee River. The reaction of aniline with SRFA was studied in depth by determining the effects of DOM and aniline concentration, pH, and chemical manipulation of SRFA on reaction kinetics.
Experimental Section Materials. Aniline (99%, Aldrich) was purified by vacuum distillation. 2-Anilino-1,4-benzoquinone was prepared according to the method of Suida and Suida (19). Soil fulvic acid (SFA), soil humic acid (SHA), and Suwannee River humic acid (SRHA) were obtained from the International Humic Substances Society (IHSS). Suwannee River fulvic acid (SRFA) was obtained from J. A. Leenheer (USGS), and Suwannee River dissolved organic matter (SRDOM), which was isolated by reverse osmosis, was obtained from E. M. Perdue (Georgia Institute of Technology) (20). The physical characteristics of these materials are summarized in Table 1. Suwannee River water (SRW) was collected in 20-L cubitainers near the sill where the IHSS samples were collected (Okefenokee Swamp, Georgia). The jugs were stored on ice and transported to the laboratory. The river water was filtered initially through a 0.5-µm filter (MacheryNugel Rundfilter NM GF-2) and then filter-sterilized through a 0.22-µm GF membrane filiter (Millipore). Monobasic and dibasic potassium phosphate (reagent grade, Aldrich),
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hydrochloric acid (Baker), and sodium hydroxide (99%, Aldrich) were used as obtained. All solvents used were of high purity (Fisher Scientific). Solutions of DOM isolates and aniline were prepared in autoclaved double-distilled water and glassware. Analytical Methods. The liquid chromatograph (LC) used consisted of a Gilson 305 gradient chromatographic pump equipped with an Applied Biosystems 783 programmable wavelength detector, a Rheodyne 7161 injector (200µL sample loop), an Alcott 728 autosampler, and a Hewlett Packard 3396A integrator. The detection wavelength was 235 nm. The analytical column was a Waters C18 µBondapak (30 cm long × 3.9 mm i.d., 5-µm particle size). The analytical column was protected with an Alltech-Applied Science Adsorbosphere C18 guard column cartridge. A mixture of 30% acetonitrile (ACN) and 70% water was used to ensure an elution time of 6.0-8.0 min for aniline. Kinetic Studies. All kinetic studies were performed under sterile conditions. To 100 mL of a 500 ppm solution of DOM, which was buffered at pH 7 with phosphate buffer (5 × 10-5 M), was added 100 µL of a 1.0 × 10-3 M solution of aniline in ACN. At preselected times, 2-mL aliquots were removed by syringe from the reaction solution using sterile techniques. The syringe was then fitted with a Waters QMA plus Sep-Pak (weak anion exchange resin), and the 2-mL aliquot was slowly forced through the Sep-Pak cartridge with manual pressure. The Sep-Pak cartridge was then rinsed with 1 mL of water. The aqueous rinse was combined with the filtrate from the initial filtration. The combined filtrate was then analyzed by LC as described above. Sampling by this method was performed in duplicate, and each duplicate sample was analyzed twice by LC. The error between duplicate LC analyses was typically
