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 i s essential for assessing mobility, bioavailability and subsequent remediat ion of aromatic amine-contaminated sites. The kinetics of irreversible bind ing and/or transformation of a series of para-substituted anilines and alph a-naphthylamine were studied on several surface soils for a one- to two-mon th equilibration period. To estimate reaction rates, a heterogeneous reacti vity model was developed assuming that irreversible reactions are first-ord er with respect to the amine solution concentration; activation energies va ry linearly as a function of reacted sites; and available soil reactive sit es change over time but remain more numerous than sites consumed. The valid ity of the latter assumption was demonstrated for the experimental variable s in these studies. The observed change in reaction rates with time was bes t described using a biphasic approach where apparent rate constants (k(app) ) and the relationship between activation energies and reacted sites (a) we re independently estimated for contact times less than or equal to 20 h and > 20 h. For both operationally defined time frames, inverse log-linear rel ationships are observed between k(app) values and both Hammett constants an d half-wave oxidation potentials (E-1/2), which are indicators of the intri nsic solute reactivity. Dimerization was only evident for amines with react ivity greater than methylaniline or with E-1/2 < 0.54 V. Reaction complexit y and site heterogeneity resulted in a lack of correlation with soil proper ties. 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 i n causing radical amine cation formation and subsequent coupling.