Use of a New Hybrid Sol–Gel Zirconia Matrix in the Removal of the Herbicide MCPA: A Sorption/Degradation Process

Abstract

A class II hybrid sol–gel material was prepared starting from zirconium­(IV) propoxide and 2,4-pentanedione and its catalytic activity in the removal of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) was revealed. The thermal and structural characterization, performed by thermogravimetry, differential thermal analysis, and diffuse reflectance Fourier transform infrared spectroscopy, demonstrated the hybrid nature of the material. The structure of the material can be described as a polymeric network of zirconium oxo clusters, on the surface of which large part of Zr⁴⁺ ions are involved in strong complexation equilibria with acetylacetonate (<i>acac)</i> ligands. The incubation of MCPA in the presence of this material yielded an herbicide removal fraction up to 98%. A two-step mechanism was proposed for the MCPA removal, in which a reversible first-order adsorption of the herbicide is followed by its catalytic degradation. The nature of the products of the MCPA catalytic degradation as well as the reaction conditions adopted do not support typical oxidation pathways involving radicals, suggesting the existence of a different mechanism in which the Zr⁴⁺:<i>acac</i> enol-type complex can act as Lewis acid catalyst