Ba. Bonn et W. Fish, MEASUREMENT OF ELECTROSTATIC AND SITE-SPECIFIC ASSOCIATIONS OF ALKALI-METAL CATIONS WITH HUMIC-ACID, Journal of soil science, 44(2), 1993, pp. 335-345
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.