Effect of substitution on irreversible binding and transformation of aromatic amines with soils in aqueous systems

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 alpha-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 (a) were independently estimated for contact times less than or equal to 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 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.