Adsorption of picloram herbicide on montmorillonite: Kinetic and equilibrium studies

• The kinetics of the adsorption of picloram onto montmorillonite was studied in situ by ATR-FTIR.
• A mixed surface reaction intra-particle diffusion-control was the kinetic model that best represented the results.
• The adsorption equilibrium isotherms were obtained at pH values of 3, 5 and 7.
• Two independent adsorption processes on different surface sites were found.
• The interaction of picloram with montmorillonite surface was confirmed by ATR-FTIR.

The kinetics of the adsorption of picloram (PCM) onto montmorillonite (Mt) was studied in situ by attenuated total reflectance Fourier-transform infrared spectra (ATR-FTIR). Pseudo-first-order, pseudo-second-order, intra-particle diffusion and mixed surface reaction intra-particle diffusion-controlled kinetic models were applied to the experimental results. The latter was the model that best represented the experimental results. The adsorption equilibrium isotherms revealed the presence of two independent adsorption processes on different surface sites. The adsorption isotherms normalized by the surface area were fitted with the Langmuir and Freundlich models to obtain the respective parameters.Due to the anionic form of PCM, the adsorption onto the external clay mineral surface sites followed by the entrance of the PCM molecule into the Mt basal space in a less energetically favorable pathway was proposed. The interaction of PCM with montmorillonite surface sites through nitrogen of the pyridine ring and carboxylic group by forming an inner sphere complex was confirmed by ATR-FTIR.

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