Adsorption of atrazine from aqueous electrolyte solutions on humic acid and silica

The adsorption of, the still widely used, herbicide atrazine on model soil components, such as humic acid and humic acid–silica gel mixtures, was investigated in a series of batch experiments, under different experimental conditions (ionic strength, temperature, and pH). The investigation aimed at obtaining an estimate of the contribution of each of the soil components on the adsorption of atrazine from aqueous solutions. The kinetics of atrazine adsorption on humic acid showed two steps: a fast step, of a few hours duration, and a second slow step, which continued for weeks. The kinetics of adsorption data gave a satisfactory fit to the Elovich equation. Τhe adsorption of atrazine on the test substrates was found to be reversible in all cases. The atrazine uptake data on the test substrates were fitted best with the Freundlich adsorption isotherm. The ionic strength of the atrazine aqueous solutions did affect the amount of the atrazine adsorbed on the test substrates, suggesting that electrostatic forces between atrazine molecules and soil play a significant role in the adsorption process. An increase of temperature resulted in a decrease of atrazine adsorption on humic acid at low atrazine equilibrium concentrations. However, for higher levels of equilibrium concentrations (⩾3 mg/L) the amount of atrazine adsorbed onto the test substrate increased as temperature increased. The calculated isosteric enthalpies of adsorption ranged between slightly exothermic at low atrazine uptake and slightly endothermic at high atrazine uptake, all values being in the range of physisorption.

Adsorption isotherms of atrazine in different mixtures of humic acid and silica at pH 5.5 and θ = 25 °C. Data points fit according to the Freundlich model.Figure optionsDownload high-quality image (56 K)Download as PowerPoint slideResearch highlights
► The paper deals with the adsorption of atrazine, on main components of soils.
► The kinetics of adsorption were represented satisfactorily with the Elovich model.
► Atrazine is expected to be adsorbed on silica gel after 6 h and on HA after 30 days.
► Adsorption data were fitted satisfactorily to the Freundlich model.
► Experiments showed that atrazine was sorbed 28,000 times more on HA than on silica.