Measurement of electrostatic and site-specific associations of alkali metal cations with humic acid

A discontinuous acidimetric titration method incorporating ultrafiltration was developed to measure the association of a soil humic acid with Li⁺ t, 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 behaved equivalently 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+ andK+ 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.