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Abstract
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Phenoxy herbicides are families of chemicals that have been developed as commercially important herbicides, widely used in agriculture. Excessive consumption and increasing use of these pesticides to control pests of agricultural products and the continued entry of these pollutants into the environment due to their lack of biodegradability as well as their toxicity and carcinogenicity, has become one of the most challenging environmental problems today. Hormonal-like herbicides 2-Methyl-4-chlorophenoxyacetic acid (MCPA) and (2,4- Dichlorophenoxy)acetic acid (2.4-DCPA) are among the phenoxy herbicides that in mixed form, provide essential tools for modern farming for control of undesirable vegetation on grazing land, in crop, and non-crop lands. Natural clay minerals, especially layered double hydroxides (LDHs), are among the various materials that are used for pesticide adsorbents. LDHs have received special attention, mainly due to the large specific surface area associated with their layered structure, ease of synthesis, and the possibility of modifying their surfaces. In this work, a magnetized MgAl-LDH@Fe3O4 composite was prepared by coprecipitation method and used for the simultaneous removal of MCPA and 2.4-DCPA herbicides from aqueous solution by adsorption process. It should be noted that after the adsorption process, the magnetized MgAl-LDH@Fe3O4 nanocomposite can be separated and collected from the aqueous solution by an external magnet. Characterizations of the adsorbent were performed by various techniques, including Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and thermal analysis. The partial least square method was used to determine the concentration of each individual herbicide and in the mixture of them in the aqueous solution. Next, simultaneous adsorption of MCPA and 2.4-DCPA by LDH was optimized using Doehlert experimental design. The optimum conditions for adsorption were obtained as, adsorbent dosage 40.20 mg L−1, pH 6.8 and initial concentration 28.35 mg L−1. The equilibrium adsorption data were obtained using the Langmuir, Freundlich, Temkin and Dubinin-Radush-Kevich isotherms models. The results indicated that the experimental adsorption data were controlled by Freundlich models. The maximum adsorption capacity of MCPA and 2.4-DCPA herbicides on the MgAl-LDH@Fe3O4 adsorbent was obtained as, 134.50 and 131.30 mg g-1, respectively. The kinetic data ofadsorption process were evaluated as pseudo-first order, pseudo-second order and intraparticle diffusion; the obtained results were well described by the pseudo-second-order model. Adsorption thermodynamic studies were also investigated. The positive ΔH◦ and negative values of ΔG° at various indicated that adsorption process is endothermic and spontaneous in nature respectively. The positive ΔS° value indicates the increase of disorder at the solid-solution interface during adsorption. K
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