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ADSORPTION OF TWO QUINOLINECARBOXYLIC ACID HERBICIDES ON HOMOIONIC MONTMORILLONITES

¹ Dipartimento di Scienze Ambientali Agrarie e Biotecnologie Agro-Alimentari, Università di Sassari, Viale Italia 39, 07100 Sassari, Italy
² Dipartimento di Biotecnologie per il Monitoraggio Agroalimentare ed Ambientale, Università degli Studi Mediterranea di Reggio Calabria, Feo di Vito, 89124 Reggio Calabria, Italy
³ Dipartimento di Scienze et Tecnologie Agro-Ambientali, Università di Bologna, Via Fanin 40, 40127 Bologna, Italy

^* E-mail address of corresponding author: pusino{at}uniss.it

The adsorption of the herbicides quinmerac 7-chloro-3-methylquinoline-8-carboxylic acid (QMe) and quinclorac 3,7-dichloroquinoline-8-carboxylic acid (QCl) on homoionic Fe³⁺-, Al³⁺-, Cu²⁺-, Ca²⁺-, K⁺- and Na⁺-exchanged montmorillonite was studied in aqueous solution. Adsorption data were fitted to the logarithmic form of the Freundlich equation. Ca- and Na-exchanged montmorillonites were ineffective in the adsorption of QMe. On the other hand, the QMe adsorption on Fe-exchanged montmorillonite was rapid and the equilibrium was attained after 15 min. An H-type isotherm was observed for the QMe adsorption on Fe-clay, indicating a high affinity of the solute for the sorption sites and almost complete adsorption from dilute solution. On the other hand, the adsorption isotherm of QMe on Al- and K-clay was of the S-type. This shape suggests that the solvent molecules may compete for the sorption sites. A Fourier transform infrared (FTIR) study suggested that the adsorption mechanism of QMe on Fe-, Al- and K-clay involves the protonation of QMe molecule due to the acidic water surrounding the saturating cations. The greater acidity of Fe-clay compared with Al- and K-clay explains both the lower QMe adsorption observed on Al and K systems and the lack of adsorption on Na and Ca systems. In contrast, the formation of a Cu complex permitted QMe to be adsorbed to a large extent to Cu-clay as shown by FTIR analysis. The QCl was adsorbed only by Fe-clay and the adsorption isotherm of QCl on Fe clay was of the S-type. This finding is consistent with the lower basic character of the QCl molecule nitrogen. In fact, the replacement of the electron-releasing methyl group in QMe with an electron-withdrawing Cl atom to form QCl makes the nitrogen lone-pair electrons of the quinoline ring unavailable for either protonation or complexation.

Key Words: Adsorption • Herbicides • Infrared Spectroscopy • Montmorillonite • Quinclorac • Quinmerac

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