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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.

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