Effect of Substitution on Irreversible Binding and Transformation of Aromatic Amines with Soils in Aqueous Systems HUI LI, † LINDA S. LEE,* ,† 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 amine- contaminated 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 first- order 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 (k app ) 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 k app values and both Hammett constants and half-wave oxidation potentials (E 1/2 ), which are indicators of the intrinsic solute reactivity. Dimerization was only evident for amines with reactivity greater than methylaniline or with E 1/2 < 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 Mn 2+ 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 amino- group reactivityresult 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 (SOM) and mineral domains, and protonated aromatic amines are associated with negative-charged sites on soils through cation exchange.Both hydrophobicinteractionsand cation exchange are reversible and appear to reach equi- librium 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 beinga relativelyslow mechanism.Liand Lee (1)found that irreversible bindingand/or transformation reactionsare slow relative to reversible sorption processes and appear to occur in parallel such that reversible sorption processes directlyimpact amine concentrationsavailable forirreversible reactions. Environmentalassessment and subsequent remediation ofaromatic 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 ofthe 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 estergroups(9).The reactivity ofSOM functionalgroups varies with respect to the inductive, resonance,and stericeffectsinduced bysurroundinggroups. Numerous studies have demonstrated that bioavailabilityof aromaticaminesisreduced when covalentlybound to humic substances (10-13). Bollag et al. (6, 14, 15) proposed the addition ofenzymesand oxidative mineralsasa remediation strategy to enhance such binding and reduce the toxicity of the parent compounds. The addition ofoxidases or mineral oxidantshave been shown to oxidize the aminesand produce free radicals that can react with SOM functional groups and enhance covalent binding ofaromatic amines (5, 14, 16, 17). Manganese(III/IV)and iron(III)oxides/hydroxidescommonly 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 ofaromaticamine reactionswith individualsoilcomponents (e.g., humic substances, manganese oxides, and mont- morillonite)have been measured,and both covalent binding to humicmaterialsand oxidative couplingat mineralsurfaces have been observed. Different time scales on the reactions, minutesforoxidative couplingand hoursto daysforcovalent 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 reactivityofthe amino group.Electron-donatinggroups(i.e., CH3) facilitate the formation and stabilization of organic amine radicals, leading to increasing transformation and covalent binding of aromatic amines, while electron- withdrawinggroups(i.e., Cl)result in decreasingthe 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 ofthese electroniceffectson reactivity ofsubstituted anilines.The lower the E1/ 2 value and the more negative the Hammett constant, the greater is the intrinsic *Correspondingauthor phone: (765)494-8612;fax: (765)496-2926; e-mail: lslee@purdue.edu. † Department of Agronomy. ‡ School of Civil Engineering. Environ. Sci. Technol. 2000, 34, 3674-3680 3674 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000 10.1021/es000956+ CCC: $19.00  2000 American Chemical Society Published on Web 07/27/2000