Removal and Release of the 2,4,5-Trichlorophenoxyacetic Acid Herbicide from Wastewater by Layered Double Hydroxides

Abstract

The Zn2–Al layered double hydroxides (LDHs) have been used to remove the 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) herbicide, which is used as a model for pesticides from the carboxylate family. The adsorption isotherms are governed by Langmuir model (L type). The characterization by various techniques (XRD, FTIR, SEM) indicates that the 2,4,5-T retention is governed by adsorption on the sites of the LDH surface when the molar ratio 2,4,5-T/Cl ≤ 0.1. For greater ratio values, the herbicide undergoes, in addition, intercalation in the LDH interlayer domain leading to an increase of the interlayer distance from 0.778 to 1.855 nm. The retention capacity reaches 718 mg/g with a removal rate of 96% for an optimal 2,4,5-T/Cl molar ratio of 0.5. This capacity is influenced by the solution pH, the charge density of the LDH sheets, the adsorbent/adsorbate molar ratio and the nature of the interlayer anion of the LDH precursor. The partial or total release of the herbicide was observed to depend on the composition of the desorbing solution. This suggests the possibility of LDH recycling and confirms thus its effectiveness in eliminating this type of pollutant from water. In addition, a well-structured [Zn2–Al-2,4,5-T] hybrid material was obtained after herbicide retention and its characterization by chemical analyses and SEM as well as a proposed structural model are presented.