The fulvic acid molecular weight fraction studied has two general types of carboxyl groups, one of which is ortho to phenolic -OH groups. The number of each type per gram of fulvic acid has been measured. The acid ionization equilibrium of each type has been calculated as a function of its degree of ionization in 0.1 m KC1 at 25.0 "C. In both cases the acid strength decreases with increasing degree of ionization. The sample used shows the potentiometric titration behavior expected of a low molecular weight polyelectrolyte.

No abstract

A novel technique using on-line microfiltration and HPLC analysis has been used to study sorption kinetics and sorption equilibrium of the herbicide atrazine with the clay minerals montmorillonite, kaolinite, and illite and the clay fraction of a soil. Fast and slow labile sorption have been observed for atrazine along with a reversible but kinetically slow sorption/desorption process that is consistent with diffusion of pesticide into the interior of the clay particles. Labile sorption capacity, mole fraction site coverage, labile sorption equilibrium function, and distribution coefficient were determined for the clay minerals in aqueous slurries with atrazine. The identification and quantitative descriptions of key chemical species avoid some of the commonly reported hysteresis problems. Distribution coefficients are derived from the equilibrium constants so that comparisons may be made with the results of other workers.

Gran's functions corrected for buffering and H 2 0 dissociation have been incorporated into the iterative calculation for the two equivalence points and the acid dissociation equilibria. The experimental and mathematical conditions required for accurate, dependable operation of the method have been examined. The resulting equivalence point calculations are based directly on a chemical interpretation of the system, without approximations. The first equivalence point is higher than previously reported, and is displaced upward by KCI. Detailed chemical information has been obtained for some of the acidic functional groups, from plots of KA = (rnH6mA/6mAH) us. mequiv./g. Carboxyl groups at the chelating sites are essentially all ionized for H + concentrations less than 1 x lo-' m.Les fonctions de Gran corrigees pour le tampon et la dissociation de H,O ont ete introduites dans le calcul iteratif des deux points d'kquivalence et d'tquilibre de dissociation acide. Les conditions experimentales et mathematiques requises pour une operation sare et precise de la mbthode ont Cti: examinees. Les calculs du point d'bquivalence qui en resultent, sont fondes directement sur une interpretation chimique du systtme sans approximations. Le premier point d'equivalence est plus &lev& que celui precedemment rapport6 et se trouve dtplad par KCI. Une information chimique detaillee a tte obtenue pour certains des groupes fonctionnels acides, a partir des courbes de KA = mH6mA/6mAH en mequiv./g. Les groupes carboxyles sur les sites de chelation sont essentiellement et entierement ionises pour des concentrations en H+ infkrieures a 1 x lo-' rn.

The distribution of chlorothalonil among the dissolved, labile sorbed, and bound residue states was monitored during an 18 day period in an aqueous slurry of an analyzed quartz sand soil from Simcoe, ON, Canada. The Simcoe soil is 90.−95.% quartz sand. The online HPLC microextraction method was used for this purpose, because it is the only available technique that can resolve the total amount of a pesticide in a soil into its dissolved, labile sorbed, and bound residue components. The processes for which the molecular level kinetics were determined included labile surface sorption and desorption and bound residue formation. At a reaction time of 14 days, the solution concentration of 0.75 × 10-6 M was 43.3% of the total chlorothalonil, 26.2% was in the labile sorbed state, and 30.5% was a bound residue. There were no chemical reactions and no biodegradation during the 18 day period. The kinetics of mass transfer among the three states were determined and are consistent with intraparticle diffusion. Although the amounts are small, it is suspected that the 5.−10.% nonquartz materials in the Simcoe soil contribute most of the sorption and bound residue effects.

CuZ+ reacts with fulvic acid to form a site-bound chelate on the fulvic acid polymer molecules. It is deduced from literature evidence that this chelate is the same type that Cu2+ forms with salicylic acid. This is supported by a Job's continuous variations plot. The mass action quotient for the chelate formation in 0.1 172 KC1 at 25.0 "C has been measured as a function'of the degree of ionization of the chelating carboxyl groups. The strength of the Cu2+ -fulvic acid chelate is comparable to that of other bidentate Cu2+ chelates.

Ultrafiltration and. HPLC are applied to the study of binding of the neutral polar herbicide atrazine by a well-characterized Laurentian soil. This investigation exploits the results of our previous studies on the interaction of atrazine with fulvic acid and humic components extracted from this soil. The purpose of the study is to compare behavior of separate components to that of the soil assembly. These earlier studies established that a specific site complexation model, formally similar to a Langmuir isotherm, was required to describe adsorption. Partition coefficients would not suffice because a binding capacity limit is found at low solution atrazine concentration. The same behavior is found for the soil. This small stoichiometric binding capacity of the whole soil can be approximated by two terms: (i) a strongly pH dependent term with features approximating a superposition of fulvic and humic behavior; (ii) a weakly pH dependent term, which may resemble known clay mineral behavior. The pH-sensitive binding sites are specific; atrazine and hydroxyatrazine do not compete. The aggregation of humic components has an important effect on atrazine binding. Added fulvic components compete with atrazine for binding sites. The dependence on protonation points to H bonding as a major factor in aggregation.

The HPLC–microfiltration method previously described for determining atrazine and hydroxyatrazine species in organic soil has been adapted to a mineral soil. The method was able to monitor atrazine concentrations of less than 1.0 × 10−6 M, which is over 40 times lower than the atrazine concentrations measured in the earlier organic soil experiments. During kinetics.experiments, it also monitored four free and sorbed chemical species instead of only one. Measurements were obtained with much better accuracy and productivity than can be obtained by conventional methods. The atrazine sorption capacity of the mineral soil was also measured. The analytical chemical data produced can support heterogeneous chemical kinetics calculations.