MEASUREMENT OF ELECTROSTATIC AND SITE-SPECIFIC ASSOCIATIONS OF ALKALI-METAL CATIONS WITH HUMIC-ACID

A discontinuous acidimetric titration method incorporating ultrafiltra tion was developed to measure the association of a soil humic acid wit h Li+, Na+ and K+ (pH 3 to 8). In addition, possible site-specific bin ding of these alkali metal cations was investigated using desorption e xperiments at pH 1. Li, Na and K cations behaved equivalently in the t itrations and the amounts of these cations associated with the humic a cid 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 c ation concentration, but rather on the solution alkalinity. In additio n, the net charge of the humate polyanion made a negligible contributi on to the electroneutrality of the bulk solution under all conditions. These results are consistent with a diffuse layer model of hydrated h umic acid in which the alkali metal cations neutralize the humic charg e. The association of Na+ and K+ with humic acid at pH 1 was successfu lly described by a Langmuir adsorption model. The number of sites per g of humic acid was very small, and greater for K+ than for Na+. Lithi um cations exhibited no detectable humic association at pH 1. These di fferences suggest that humic acids may have a small number of specific binding sites for which the size of the hydrated cation is important.