Environ. Sci. Technol. 1997, 31, 2642-2649
Oxidation of Substituted Anilines by Aqueous MnO2: Effect of Co-Solutes on Initial and Quasi-Steady-State Kinetics JO ¨ RG KLAUSEN,* STEFAN B. HADERLEIN,* AND R E N EÄ P . S C H W A R Z E N B A C H Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETHZ), CH-8600 Du ¨ bendorf, Switzerland
The effect of pH, Mn(II), and humic acid (HA) concentration as well as aromatic ring substituents on initial and quasi-steady-state rates of oxidation of monosubstituted anilines by MnO2 has been investigated in batch and completely mixed flow-through reactors. The reaction rates were strongly pH-dependent and increased with decreasing pH. Mn(II), Ca(II), HA, and other organic solutes inhibited the oxidation of substituted anilines. Inhibition by adsorbed Mn(II) was due to blocking of reactive Mn(IV) surface sites. The inhibitory effect of HA and some other organic solutes was in part due to reductive dissolution of MnO2 and, thus, due to formation of adsorbed Mn(II). Initial rates of oxidation of a series of substituted anilines were linearly correlated to polarographic half-wave oxidation potentials, E1/2, indicating that electron-transfer kinetics were relevant for the overall rate of oxidation. However, results from flowthrough experiments demonstrate that such correlations may change or disappear, subject to changing solution composition. Quasi-steady-state rates were generally significantly smaller than initial rates, indicating that the ability of MnO2 to efficiently oxidize organic pollutants may be impaired by long-term exposure of the mineral surface to inorganic and organic co-solutes.
Introduction Substituted aromatic amines are serious environmental pollutants because they commonly occur in soils and aquifers and are toxic and mutagenic to humans (1, 2). Important aromatic amines and derivatives include pesticides such as anilide herbicides (e.g., propanil, alachlor), dinitroanilines (e.g., trifluralin), and aminophenols. These compounds enter aquatic environments through a variety of pathways, either directly or as a result of reductive transformations of nitroaromatic compounds (3-5), pesticides in soils (6, 7), and azo dyes (8). Important transformations of aromatic amines in aquatic and soil/sediment systems include microbial or abiotic oxidation and nucleophilic addition (9-15). Nucleophilic addition to natural organic matter (NOM) results in the formation of soil-bound residues of poorly predictable stability and bioavailability. Oxidative transformations of aromatic amines can lead to intermediates that are more amenable to * Address correspondence to either author. (J.K.) e-mail:
[email protected]; phone: +41-1-823 50 76; fax: +41-1-823 50 28. (S.B.H.) e-mail:
[email protected]; phone: +41-1-823 5524; fax +41-1-823 54 71.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 9, 1997
microbial mineralization (16) but also to products of unknown environmental relevance or even to such of known high toxicity, e.g., 3,3′,4,4′-tetrachloroazobenzene (17). Earlier work has shown that manganese (hydr)oxides have a large potential for oxidative transformations of organic pollutants in subsurface environments (18, 19). In soils, the most important manganese minerals are vernadite and birnessite, which are both layered oxide structures of varying composition that are often non-stoichiometric and of low crystallinity (20-24). Laha and Luthy (25), following up on the work of Stone, who had studied phenolic compounds (26, 27), found that a number of aromatic amines were oxidized at appreciable rates in suspensions of a manganese dioxide that was similar to natural MnO2. The initial rates for the oxidation of aromatic hydroxy and amino compounds by MnO2 correlated with E1/2 of the organic compounds (25, 26), indicating the importance of the electron transfer in the overall kinetics. The initial reaction step in the oxidation of primary aromatic amines is generally the formation of the arylamino radical (eq 1).
ArNH2 f ArNH2•+ + e- f ArNH• + e- + H+
(1)
The stability of this primary anilino radical species is correlated to the oxidation potential, E1/2, of the corresponding aniline and depends on the number, position, and e--donating properties of the substituents on the aromatic ring. In the absence of specific radical scavengers, subsequent N-N and N-C coupling reactions can be expected (2, 9). In their study of the oxidation of aromatic amines, Laha and Luthy identified the major products to be the corresponding azobenzenes and aminodiphenylamines (25), which is in agreement with a radical mechanism according to eq 1. Their work also showed that even the initial stage of the process is characterized by non-pseudo-first-order behavior, indicating that changes in the reactivity of the system over the course of the experiments are to be expected. However, these authors did not investigate the reasons for this behavior any further. The focus of the work presented in this paper was to evaluate some of the factors that control the above changes in MnO2 reactivity and, ultimately, the long-term kinetics of the oxidation of organic compounds by MnO2. To this end, experiments were carried out in batch reactors to investigate the kinetics during the initial phase of the reaction (yielding initial rates) and in completely mixed flow-through reactors to investigate reaction kinetics under quasi-steady-state conditions (yielding quasi-steady-state rates) (28, 29). The results of this study should thus provide a better understanding of the importance of MnO2 as an oxidant for organic pollutants in soils and aquifers than one based on batch kinetic studies alone. The specific objectives were to evaluate (a) the influence of environmental factors such as pH, Mn(II), and NOM concentration on the kinetics of substituted aniline oxidation by MnO2, both during the initial phase of the reaction and under quasi-steady-state conditions, and (b) the effect of aromatic ring substituents on the reaction rates under various solution conditions.
Materials and Methods Chemicals. The monosubstituted anilines, e.g., 4-chloro (4Cl), 3-methoxy (3MeO), 2-methyl (2Me), 4-nitro (4NO2), as well as the buffer compounds were analytical grade (Fluka, Switzerland) and were used as received. No impurities were detected by HPLC/UV analysis at wavelengths between 230 and 240 nm. For sodium bicarbonate buffers, 0.5/99.5 and 20/80 v/v CO2/N2 gas (99.99+%, CarbaGas, Switzerland) was
S0013-936X(97)00053-9 CCC: $14.00
1997 American Chemical Society
TABLE 1. Characteristics of MnO2 Used in This Study property
result
stoichiometry specific surface area (m2 g-1) proton exchange capacity (mol g-1) site density pHpznpc
MnO(1.92(0.01)‚0.88H2O 247 ( 6 2.17E-3/1.98E-3 (I ) 0.1) -3.30E-3 (I ) 0.01) (6.0 ( 1.4) sites nm-2 1.5
pKa1s
-0.7 (I ) 0.1) -1.3 (I)0.01) 3.7/3.45 (I ) 0.1)
pKa2s appearance inner capacitance (Fm-2) morphology crystallography
method (instrument)
4.3/4.34 (I ) 0.01) fine, black powder 2.31 (I ) 0.1), 2.54 (I ) 0.01) aggregates of very small platelets (
