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The mechanisms of aqueous oxidation-reduction interactions between Cr(VI) and substituted phenols (RArOH) were characterized by kinetic analysis and determinations of reaction products and intermediates.A rapid, preoxidative equilibrium between HCrOr and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent ratelimiting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H+] is well accounted for by three parallel redox pathways involving zero, one, and two protons. The two-proton pathway dominates at pH < 2, the singleproton pathway for 2 < pH < 5, and the protonindependent pathway at pH > 5. The parallel reaction rate expression was fitted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimethoxyphenol for pH 1-6. Beside accurately predicting rates for the calibrated conditions, the model predicts a sharp decline in rates at pH > 6. Rates subsequently measured at pH 7 agreed well with those calculated a priori. Such predictions suggestthatthe proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both oneand two-electron pathways.

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Discrete and continuous multiligand models for metal-humate interactions are compared and analyzed by using both synthetic and experimental data. Discrete ligands (typically two or three) are shown to be a simple and accurate means of predicting metal-humate binding within the range of calibrating titrations. Selection of discrete ligand parameters is best achieved via nonlinear regression. The continuous affinity spectrum model is highly sensitive to experimental error, and thus its usefulness as an aid for selection of discrete ligands is limited. As anticipated, only the weakest, most abundant ligands in a ligand mixture can be identified with the continuous stability function model. The continuous normal ligand distribution model is capable of fitting metal-humate binding data with only three parameters, but only the stronger ligands in the assumed distribution are important for fitting observed data. The discrete ligand approach is probably the most useful way to model metal-humate binding because of the ease with which discrete ligands can be incorporated into chemical equilibrium computer programs. IntroductionMany models for metal-humate interactions have been proposed in the literature in recent years. However, there has been little or no comparison of the various models to determine their relative utility or the circumstances for which one model is better suited than others. In part 1

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A discontinuous acidimetric titration method incorporating ultrafiltration was developed to measure the association of a soil humic acid with Lit, Na+ and K+ (pH 3 to 8). In addition, possible site-specific binding of these alkali metal cations was investigated using desorption experiments at pH 1. Li, Na and K cations behavedequivalently in the titrations and the amounts of these cations associated with the humic acid was measurable at all pH values between 3 and 8. Up to 90% of the total alkali metal cation was humate-associated at pH 8. The absolute amount of humic-associated cation did not depend on the alkali metal cation concentration, but rather on the solution alkalinity. In addition, the net charge of the humate polyanion made a negligible contribution to the electroneutrality of the bulk solution under all conditions. These results are consistent with a diffuse layer model of hydrated humic acid in which the alkali metal cations neutralize the humic charge. The association of Na+ and K + with humic acid at pH 1 was successfully described by a Langmuir adsorption model. The number of sites per g of humic acid was very small, and greater for K + than for Nat. Lithium cations exhibited no detectable humic association at pH 1 . These differences suggest that humic acids may have a small number of specific binding sites for which the size of the hydrated cation is important. I N T R O D U C T I O NMost studies of the association between metal cations and natural organic matter focus on the complexation of a few select trace metals. Interactions between humic substances and the major cations such as Na' and K + have often been either ignored or assumed to be negligible. The need to characterize such interactions is evidenced by the sheer number of systems, both environmental and experimental, which contain appreciable quantities of these cations. In highly organic soils, association with humic matter may decrease the K t availability, increasing the need for amendment with K fertilizers (Kononova, 1966). In estuaries, aquatic humic substances are subjected to strong salinity gradients that can influence humic binding of trace metals. In the laboratory, experiments with aqueous humic material often utilize a background electrolyte containing Na+ or K+ to control the ionic strength. The extent to which Na+ or K + affect the humate conformation, the electrostatic charge or the binding of other cations is unknown.Aqueous alkalimetric titration has been used to investigate the association of fulvic and humic acids with Na+ or K + (Gamble, 1973;Frizado, 1979). Gamble (1973) showed that the amount of fulvic-associated Na+ and K + increased as the titration progressed. However, alkalimetric titrations present an ambiguous picture of cation-humate association. Because NaOH and KOH were used as titrants the increase in cation association with rising pH may have been due to the greater charge of the dissociating fulvate, the increasing alkali metal cation concentration, or both. Interestingly, between pH 5 and 10, Na+ ...