Adsorption of atrazine in packed beds

The adsorption of the herbicide atrazine in packed beds filled with solids modelling soil components,
(silica and humic acid), was studied at conditions simulating the adsorption of atrazine in cultivated
soils. The choice of only two constituents of soil in the experimental study was based on the
assumption that these two compounds contribute significantly to the adsorption of herbicides in soil
relative to others, like alumina, calcium carbonate, etc. Packed beds were filled with inorganic
material (silica gel grains) and solid organic compounds (humic acid). Adsorption data were fitted to
the Freundlich isotherm. The presence of humic acid, even in small quantities, increases
considerably the amount of adsorbed atrazine. Increasing humic acid content in the packed beds,
the amount of adsorbed atrazine increased. Reversibility tests showed that the adsorption of
atrazine was reversible. Two steps for adsorption and desorption were observed. The first fast step
is attributed to the physical adsorption on the free adsorption sites of the substrates washed by the
solution, while the second slow step is attributed to the diffusion of the molecules of atrazine within
the structure of the silica and of the humic acid. Study of the hydrodynamics of atrazine containing
solutions flow in the packed beds filled with silica and humic acid showed that the morphology of
insoluble humic acid upon hydration changed its shape resulting in its transport through the bed and
clogging of the narrow pores of the medium. As a result the packed bed permeability was reduced
by 99%.