Effect of Substitution on Irreversible Binding and Transformation of

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Environ. Sci. Technol. 2000, 34, 3674-3680

Effect of Substitution on Irreversible Binding and Transformation of Aromatic Amines with Soils in Aqueous Systems H U I L I , † L I N D A S . L E E , * ,† CHAD T. JAFVERT,‡ AND JOHN G. GRAVEEL† Department of Agronomy, Purdue University, West Lafayette, Indiana 47907-1150, and School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907-1284

Predicting the irreversible interactions between aromatic amines and soil is essential for assessing mobility, bioavailability and subsequent remediation of aromatic aminecontaminated sites. The kinetics of irreversible binding and/ or transformation of a series of para-substituted anilines and R-naphthylamine were studied on several surface soils for a one- to two-month equilibration period. To estimate reaction rates, a heterogeneous reactivity model was developed assuming that irreversible reactions are firstorder with respect to the amine solution concentration; activation energies vary linearly as a function of reacted sites; and available soil reactive sites change over time but remain more numerous than sites consumed. The validity of the latter assumption was demonstrated for the experimental variables in these studies. The observed change in reaction rates with time was best described using a biphasic approach where apparent rate constants (kapp) and the relationship between activation energies and reacted sites (R) were independently estimated for contact times e 20 h and > 20 h. For both operationally defined time frames, inverse log-linear relationships are observed between kapp values and both Hammett constants and half-wave oxidation potentials (E1/2), which are indicators of the intrinsic solute reactivity. Dimerization was only evident for amines with reactivity greater than methylaniline or with E1/2 < 0.54 V. Reaction complexity and site heterogeneity resulted in a lack of correlation with soil properties. However, preliminary results showing an increase in exchangeable Mn2+ from soils after irreversible reactions with amines were allowed to occur demonstrated that manganese oxides in whole soils play a significant role in causing radical amine cation formation and subsequent coupling.

Introduction Aromatic amine hydrophobicity, dissociation, and aminogroup reactivity result in multiple reversible and irreversible sorption and transformation interactions with various soil domains (1-3). Neutral aromatic amines are physically sorbed by hydrophobic interactions to soil organic matter * Corresponding author phone: (765)494-8612; fax: (765)496-2926; e-mail: [email protected]. † Department of Agronomy. ‡ School of Civil Engineering. 3674

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(SOM) and mineral domains, and protonated aromatic amines are associated with negative-charged sites on soils through cation exchange. Both hydrophobic interactions and cation exchange are reversible and appear to reach equilibrium within 1 day with cation exchange being fast relative to diffusion of neutral species into hydrophobic domains (4). Irreversible processes include both mineral-oxidized transformation and covalent binding to SOM (5, 6) with the latter being a relatively slow mechanism. Li and Lee (1) found that irreversible binding and/or transformation reactions are slow relative to reversible sorption processes and appear to occur in parallel such that reversible sorption processes directly impact amine concentrations available for irreversible reactions. Environmental assessment and subsequent remediation of aromatic amine-contaminated sites may be facilitated by a priori prediction of irreversible processes. Information on the irreversible reaction mechanisms of amines and soils has been gleaned from experiments conducted with model chemicals, humic substances, and pure minerals. Results suggest that the amino functional group may react with carbonyl moieties in SOM through nucleophilic addition to produce imine and aminobenzenquinone structures with further incorporation of the latter into SOM through nitrogen heterocyclic linkages (3, 5, 7, 8). Several other functional groups present in humic substances may be reactive for covalent binding including phenolic, carboxyl, hydroxyl, quinone, hydroxyquinone, and ester groups (9). The reactivity of SOM functional groups varies with respect to the inductive, resonance, and steric effects induced by surrounding groups. Numerous studies have demonstrated that bioavailability of aromatic amines is reduced when covalently bound to humic substances (10-13). Bollag et al. (6, 14, 15) proposed the addition of enzymes and oxidative minerals as a remediation strategy to enhance such binding and reduce the toxicity of the parent compounds. The addition of oxidases or mineral oxidants have been shown to oxidize the amines and produce free radicals that can react with SOM functional groups and enhance covalent binding of aromatic amines (5, 14, 16, 17). Manganese(III/IV) and iron(III) oxides/hydroxides commonly existing in soils and sediments as well as montmorillonites have been shown to oxidize phenols and aromatic amines to produce dimers in aqueous solutions (18-22). The oxidation mechanism proposed included adsorption of the organic compound onto the oxide surface followed by electron transfer from the organic solute to the metal and release of a highly reactive organic radical (20, 23). Kinetics of aromatic amine reactions with individual soil components (e.g., humic substances, manganese oxides, and montmorillonite) have been measured, and both covalent binding to humic materials and oxidative coupling at mineral surfaces have been observed. Different time scales on the reactions, minutes for oxidative coupling and hours to days for covalent binding of amines have been reported (20, 21, 24). Changes in inductive, resonance and proximity effects from substituent addition on the aromatic ring will alter the reactivity of the amino group. Electron-donating groups (i.e., CH3) facilitate the formation and stabilization of organic amine radicals, leading to increasing transformation and covalent binding of aromatic amines, while electronwithdrawing groups (i.e., Cl) result in decreasing the reaction rate (8, 18, 20, 25). Hammett constants (δ) and half-wave potentials (E1/2) can be used as parameters to quantitatively evaluate the influence of these electronic effects on reactivity of substituted anilines. The lower the E1/2 value and the more negative the Hammett constant, the greater is the intrinsic 10.1021/es000956+ CCC: $19.00

 2000 American Chemical Society Published on Web 07/27/2000

TABLE 1. Characterization Data Based on Air-Dried Soils ( CH3-AN > ANIL, Cl-AN > NO2AN. With the exception of COOH-AN, which will be discussed later, trends correspond well to what was expected upon addition of the different substituents to the para-position of the aromatic amine. For example, a strong electron-donating methoxy group in the para-position will result in an increase in electron density on the amino nitrogen, which leads to an increase in nucleophilic reactivity and a decrease in redox potential. CH3O-AN was observed to be highly reactive with more than 85% of the applied solute irreversibly lost on all the three soils investigated (Chalmers, Okoboji, and Toronto soils). In contrast, the addition of NO2, an electronwithdrawing group, resulted in a much less reactive structure (NO2-AN) with only 4-12% solute irreversibly lost during the two-month contact period with the three soils investigated.

FIGURE 1. Irreversible binding and/or transformation with time of several amines (D) on (A) Chalmers soil and (B) Okoboji soil. ANIL and NAPH data are from Li and Lee (1). phase column. Mobile phase composition was optimized for the different aromatic amines with a range from 15/85 to 45/55 (v/v) acetonitrile/0.1 M acetate buffer (pH ) 4.7) at a flow rate of 1.0 mL/min. The UV-Vis detector was operated within the range of 240 nm to 290 nm with the optimal wavelength selected for each individual amine and soil extract matrix. External calibration curves were generated to estimate sample concentrations from peak areas.

Results and Discussion Irreversible Binding and Soil-Induced Transformation Reactions. The rate, magnitude, and mechanisms of soilinduced irreversible reactions significantly varied with the different aromatic amines and soil types tested. Representative trends for the amount of amine irreversibly lost, i.e., covalent binding to soil or soil-induced irreversible transformation (Sirr, µmol/g), over time are shown in Figure 1A,B for several amines on Chalmers and Okoboji soils. In all cases, as available solute concentrations in aqueous solution and more readily reactive soil sites (i.e., lower energies of 3676

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Reaction Rates. The apparent reaction rates of the irreversible processes (kapp, h-1) and the change in activation energies as a function of reacted sites (R ) ∆Ea/∆θ) were estimated using eq 3. Values for θ and dθ/dt were not explicitly known but were calculated assuming that for each amine molecule irreversibly lost to soil (Sirr, µmol/g), one site on the soil would be reacted (Sirr ) θ), which also allowed estimation of dθ/dt. Values for Cw were measured at discrete time steps. However, to minimize errors between discrete points and employ even time steps for calculating dθ/dt as suggested by Fogler (33), power and exponential functions were employed to fit θ versus time and Cw versus time as exemplified in Figure 2 (parts A and B, respectively). Values for dθ/dt, θ, and Cw were then estimated using 1 h differential time steps throughout the 2-month equilibration period. The observed change in reaction rates with time was best described using a biphasic approach where kapp and R constants were estimated separately for contact times e 20 h (represented as subscript 1) and > 20 h (represented as subscript 2), as exemplified in Figure 2 (parts C and D, respectively). Values for kapp and R are summarized in Table 3 for all soil-solute combinations investigated in this study along with values estimated from aniline and R-naphthylamine data previously reported by Li and Lee (1). Curves for the Sirr versus time using the fitted kapp and R constants are shown as solid lines in Figure 1A,B except for COOH-AN. For the latter case, a plateau was reached within the first 20 h with additional irreversible reactions after 20 h being so small that kapp,2 was essentially zero. For all soil-solute combinations, kapp,1 values are over a magnitude greater than kapp,2. Rates are expected to retard with time because sites with lower activation energies are used up, i.e., fast-reacting sites, and the probability of

TABLE 3. Irreversible Binding/Transformation Rate Constants and Range of Activation Energies of Soil Reactive Sites within Designated Time Frames: e 20 h (Subscript 1) and > 20 h (Subscript 2) solute

soil

m/V (g/mL)

kapp,1 (h-1)

br

CH3O-AN

Toronto Chalmers Okoboji Toronto Bloomfield Chalmers Drummer Okoboji Chalmers Okoboji Toronto Bloomfield Chalmers Drummer Okoboji Chalmers Okoboji Chalmers Okoboji Toronto Chalmers Okoboji

0.9/25 1.9/25 1.2/25 0.8/25 2.0/10 1.8/25 1.6/25 1.0/25 6.0/10 2.7/10 2.0/10 4.5/5 7.0/10 4.0/4 4.0/10 6.0/10 1.8/10 7.0/10 6.5/10 2.5/10 2.5/5 1.0/10

0.140 0.328 0.144 0.039 0.136 0.070 0.027 0.095 0.097 0.034 0.016 0.023 0.024 0.050 0.033 0.018 0.077 0.660 0.794 ∼0.01 ∼0.001 ∼0.004

0.96 × 103 1.98 × 103 1.69 × 103 1.25 × 103 6.79 × 103 4.09 × 103 6.15 × 103 1.41 × 103 3.27 × 104 2.35 × 104 1.59 × 104 25.8 × 104 6.96 × 104 6.34 × 104 2.24 × 104 6.81 × 104 5.10 × 104 1.12 × 105 0.71 × 105 ∼0.8 × 105 ∼6.0 × 105 ∼2.7 × 105

NAPH

CH3-AN ANIL

Cl-AN COOH-AN NO2-ANa

1

kapp,2 (h-1)

r2b

9.50 × 10-3 12.9 × 10-3 7.77 × 10-3 6.08 × 10-3 5.87 × 10-3 4.30 × 10-3 3.60 × 10-3 5.54 × 10-3 8.58 × 10-3 3.07 × 10-3 1.40 × 10-3 2.18 × 10-3 1.45 × 10-3 3.21 × 10-3 2.17 × 10-3 2.05 × 10-3 1.04 × 10-3 ∼0c ∼0 ∼7 × 10-4 ∼2 × 10-4 ∼4 × 10-4

2.46 × 102 4.20 × 102 2.81 × 102 0.93 × 102 9.74 × 102 4.55 × 102 1.42 × 102 1.73 × 102 10.4 × 103 2.49 × 103 2.30 × 103 114 × 103 9.96 × 103 16.9 × 103 4.19 × 103 11.4 × 103 4.39 × 103 ∼0c ∼0 ∼1.9 × 104 ∼7.4 × 104 ∼12 × 104

a Approximate k app and R values for NO2-AN are shown for reference; however, measured irreversible binding was small and very scattered resulting in poor fits to eq 3. b Units of R are (J/mol)/(µmol/g soil). c Irreversible reactions after 20 h were so small that kapp,2 and R2 could not be estimated.

collisions with sufficient energy to activate additional reactions will decline as solute molecules are depleted. Differences in apparent rate constants for the various amines are quite pronounced with approximately one to two orders-ofmagnitude variation. No consistent or obvious trends were observed between soils for a given solute, and variation in kapp values within a given time frame (e 20 h or > 20 h) across soils was within a factor of 5 except for NO2-AN on the Toronto soil. Therefore, kapp,1 and kapp,2 values were averaged across soils to assess general trends between solutes. Correlation with Intrinsic Solute Reactivity. Average kapp,1 and kapp,2 values and corresponding confidence intervals are shown as a function of solute reactivity, i.e., Hammett constant (δ+) and half-wave oxidation potential (E1/2) in Figure 3 (parts A and B, respectively). With the exception COOHAN, apparent reaction rates generally decrease in a log-linear manner with decreasing solute reactivity similar to what has been well illustrated for reactions with pure oxides and minerals (20, 21). Log-linear fits shown as solid lines in Figure 3A,B do not include kapp values for COOH-AN. COOH-AN with a pKa of 4.87 exists primarily as an anion (electrondonating group) for the two soils investigated (soil solution pH > 6.5), leading to an intensive increase in electron density on the N in -NH2 group as represented by a δ+ value of -0.41 for the anionic species AN-COO- (34), which better correlates to the reaction rates observed (Figure 2). However, even using the more appropriate δ+ value, reaction rates for COOH-AN are still almost an order-of-magnitude faster than expected relative to the other anilinium compounds. The range of site Ea observed within the operationally defined early contact time (R1) was consistently greater than the range in Ea associated with sites reacted in the subsequent contact time, i.e., R1 > R2. Trends reflect the rapid consumption of the more reactive sites at early times with subsequent reactions having higher activation energies and thus more slowly reacted. For a given aromatic amine, trends in R1 values between soils were as follows Toronto < Okoboji < Chalmers ≈ Drummer < Bloomfield. For the various amines on a given soil, R values increase exponentially with solute reactivity index as illustrated in Figure 4 using Hammett constants for data obtained with Chalmers and Okoboji soils. Trends in R

values are consistent with the observed magnitude of irreversible binding and/or transformation for the various aromatic amines on the different soils. Correlation with Soil Properties. Soil-solution pH was expected to play an important role in controlling both nucleophilic addition and mineral oxidation reactions. Nucleophilic reactions accelerate with increasing pH due to the concomitant increase in the fraction of the neutral amine species, which are believed to be the reactive nucleophilic species (24). On the other hand, reaction rate of mineral oxidation has been shown to decrease with increasing pH (20). No significant trend between reaction rates and soil pH was observed. The maximum change in soil-solution pH during the equilibration was less than 0.4 pH units for all amine-soil combinations. For NAPH, CL-AN, AN, and NO2AN changes in pH were ( 0.1 pH units with random scatter about the median. For CH3O-AN, CH3-AN, and COOH-AN, changes in pH appeared to either consistently increase or decrease and then plateau similar to Sirr with time, but the direction of the change was not similar between soils or solutes. For example, the largest changes were observed with CH3O-AN on Toronto soil with a decrease of ≈0.35 pH units over the first 250 h followed by no change. CH3O-AN on Okoboji soil resulted in only a decrease of 0.15 pH units, whereas an increase of 0.25 pH units was observed with the Chalmers soil. Potential trends across soils were also expected as a function mineral and organic matter contents, but no trends were obvious. The apparent absence of trends is most likely a result of the complexity of organic matter-mineral interactions coupled to the counteracting effect of pH on covalent binding and mineral-oxidation rates. Different soilto-water ratios will result in different buffering capacities of the soil suspensions as well as further complicating interpretation of pH changes across solute-soil combinations. Modeling Assumptions. In modeling covalent binding of aniline to soils, Fabrega (4) assumed that essentially all available reaction sites had been consumed during the equilibration period. In the derivation of eq 3 used to estimate rate constants in this study, the total number of reaction sites (θ0) available was assumed to be much greater than the number of reacted sites (θ). To validate if θ0 . θ was a VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) NAPH concentration in aqueous phase as a function of time, solid line is the fitting result; (B) reacted sites on the Chalmers soil for NAPH as a function of time, solid line is the fitting result; (C) initial reaction rate (e 20 h) fitted against θ and Cw; and (D) subsequent reaction rate (> 20 h) fitted against θ and Cw. reasonable assumption, additional experiments were conducted for a 3-week equilibration period with aniline and R-naphthylamine on the Okoboji soil and the Toronto soil at initial amine concentrations up to 5 times higher than reported previously (1). A dramatic increase in Sirr was observed when solutes were applied at much higher concentrations as illustrated in Figure 5, suggesting that θ0 . θ is valid for the systems in our study. Similar conclusions were reported by Szecsody et al. (35) in which application of sequential pulses of R-naphthylamine to a soil column resulted in increases in the solute mass irreversible “sorbed” with every pulse. An apparent plateau in a reaction with time does not infer that all sites have been consumed, but only that abundance of solute-site collisions with sufficient activation energies is approaching zero. Mineral-Catalyzed Polymerization Reactions. In previous long-term studies, the presence of red-color derivatives, identified as amine dimers and related derivatives, in solvent extracts of soils equilibrated with NAPH was reported along with the absence of such derivatives when equilibration was with ANIL (1). In the current study, red-color derivatives were observed in extracts from soils equilibrated with CH3O-AN, CH3-AN, and NAPH. No colorful derivatives were observed for the other aromatic amines including ANIL as previously reported. UV-Vis scanning of the extracts from soil equilibrated with CH3O-AN showed two significant peaks at 421 and 495 nm, while CH3-AN had only one peak at approximately 498 nm similar to what was observed for NAPH (1). These absorption wavelengths are typical for azobenzene dyes and related structures; therefore, absorption at a given wavelength may be in respone to more then a single product. Absorption intensity increased with soil-solute contact period as previously observed (1). 3678

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GC-MS analysis showed several derivatives in the extracts of soil equilibrated with CH3O-AN with one of the primary derivatives identified as 4,4′-dimethoxyazobenzene. The dominant derivative identified in extracts of NAPH-contaminated soil was proposed to be N-(1-aminonaphyl)-1naphthylamine (1). Concentrations of derivatives extracted from the CH3-AN contaminated soil were too low to detect with GC-MS. The evidence of dimer formation for CH3O-AN and NAPH indicates that aromatic amines undergo a radical coupling reaction oxidized by some soil component, such as Fe(III), Mn(IV/III) oxides/hydroxides (20, 22). The formation of radical cations, which requires loss of an electron from the amine, is the critical step in the dimerization reaction. CH3O-AN and CH3-AN with electron-donating groups have a high potential to lose an electron forming a radical, as evidenced by their lower half-wave oxidation potentials relative to aniline (Table 2). For the combination of aromatic amines and surface soils investigated in this study, oxidative radical coupling was only evident for amines with E1/2 < 0.54 V. In studies with pure δ-MnO2, less reactive aromatic amines such as ANIL were dimerized (20), unlike what we observed with whole surface soils. Soils have redox potentials from -0.4 V to +0.7 V, with oxidized soils +0.4 to +0.7 V and seasonally saturated soils from oxidized (+0.4 to +0.7 V) to highly reduced (-0.25 to -0.3 V) (36). Oxidation potentials for Mn(IV)/Mn(III) range between 1.29 and 1.65 V (25). Since redox measurements were not made in either study, it is not clear how reactivity inferred by E1/2 values can be correlated to soil redox potentials for a priori predictions of irreversible transformation. Also in whole soils, Mn will exist in several amorphous forms resulting in a range of reactivities, which may be on average much less than that of pure manganese

FIGURE 3. Average apparent reaction rates (kapp, h-1) of irreversible process for several aromatic amines as a function of intrinsic solute reactivity: (A) Hammett constant and (B) half-wave oxidation potential, during two characteristic time periods of e 20 h and > 20 h.

FIGURE 4. The change in activation energies as a function of reacted sites (r ) ∆Ea/∆θ) for several amines on the Chalmers and Okoboji soils as a function of intrinsic solute reactivity using Hammett constants (δ+). oxide (e.g., δ-MnO2, standard potential of half-reaction is 1.29 V, much greater than that value of soil). Even if mineral sites can catalyze the oxidation of amines to produce amine radicals, the radicals may react more favorable with SOM functional groups, which are not present in a pure mineral system. Also whole soils will have less available reactive Mn sites compared to pure δ-MnO2 reducing the probability that two or more amine radicals will be formed in close enough proximity to allow polymerization before some other reactions take place (e.g., covalent binding to SOM). Reactions with pure humic substances are characteristically slower than

FIGURE 5. Comparison of aromatic amine irreversible binding and/ or transformation on soils at different initial concentrations: (A) aniline on Okoboji soil at m/V ) 4/10; (B) r-naphthylamine on Toronto soil at m/V ) 0.75/25; and (C) r-naphthylamine on Okoboji soil at m/V ) 1/25. oxidation rates (20, 24), and oxidation rates on whole soils may be retarded due to coating of the mineral oxidant with humic substances (21). As a first step toward identifying the potential role metal components in the soil, high concentrations of ANIL, NAPH, and CH3O-AN (4 and 7 mmol/L amine) in an a 0.005 M CaCl2 electrolyte solution were applied to Toronto and Chalmers soils for one to 2 days followed by extraction of soil with 1.0 M NH4AcO. Extracts were analyzed for Mn, Cr, Cd, Fe, Cu, Al using inductively coupled plasma (ICP) spectroscopy. For both NAPH and CH3O-AN on both soils significantly more Mn2+ was extracted relative to control samples that were equilibrated identically except in the absence of either amine. Higher amounts of extractable Mn2+ were observed for the solute-soil combinations yielding higher Sirr values; however, µmol of amine irreversibly sorbed was greater than the moles of Mn2+ released. Irreversible loss includes covalent binding as well as mineral-catalyzed reactions, and only reduction of Mn(III) would yield extractable Mn2+ whereas reduction of Mn(IV) to Mn(III) would not. No statistical differences relative to the control were observed for the other metals, suggesting that manganese oxides served as the oxidant in the formations of amine radicals. For NAPH, no increase in extractable Mn2+ was observed. Additional work relating mineralogical factors and availability of potentially reactive sites is underway using micro-Xanes spectroscopy. In summary, the magnitude and rate of soil-induced irreversible reactions of aromatic amines were measured in aqueous suspensions with several soils. Both the magnitude of the reaction and reaction rates were positively correlated to intrinsic solute reactivities. Results clearly give support to (1) the catalytic role of Mn-minerals in whole soils to cause the formation of amine radicals; (2) the subsequent reaction of amine radicals with either other amine radicals to form VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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polymers or covalently bond to SOM; and (3) the solute dependent preference for either polymerization or covalent binding to SOM. Aromatic amines incorporated within SOM through covalent binding have been reported to be very resistant to release and biologic uptake, thereby reducing toxicity and mobility. Mineral-catalyzed oxidation of some highly reactive amines produces polymers that are reversibly sorbed by soil; however, their high affinity for surfaces would minimize their release. These findings are useful in making reasonable a priori estimations of reduced mobility and bioavailability with aging of aromatic amines and related compounds in the environment or, likewise, in designing in-situ remediation strategies.

Acknowledgments This research was funded in part by the U.S. Environmental Protection Agency Research Laboratory under Cooperative Agreement 8823581-01-0. Thanks are extended to Dr. Albert Cox for his providing the Bloomfield and Okoboji soil samples.

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Received for review February 2, 2000. Revised manuscript received May 23, 2000. Accepted June 15, 2000. ES000956+