Adsorption of atrazine in packed beds

Abstract

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%.