Adsorption of MCPA pesticide by MgAl-layered double hydroxides

In the present study, the adsorption characteristics of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) on layered double hydroxides (LDHs) were evaluated under laboratory conditions with particular attention to the effect of layer charge, original interlayer anion and morphology. The final objective is the use of LDHs and modified LDH materials as recyclable adsorbents and heterogeneous catalysts for the treatment of contaminated waste waters. The anionic clays tested were [Mg _1−xAl _x(OH) ₂] ^x+[X _x/m ^m−· nH ₂ O] materials. The MCPA adsorption capacity was determined from adsorption isotherms and a kinetic study. We looked at the influence of the pH, the Mg ²⁺/Al ³⁺ ratio, i.e. the anion exchange capacity, the nature of the intercalated anion X (CO ₃ ²⁻, NO ₃ ⁻, Cl ⁻ ) and the morphology of the adsorbent on the extent of adsorption. The adsorption isotherms, described by Freundlich model, are of S-type with tendency to L-type for high MCPA equilibrium concentration. Furthermore, the adsorption capacity increases with the layer charge density. Hence, MCPA adsorption on LDHs occurs by anion exchange in two steps, an external exchange followed by an interlayer exchange, which explain these changes of type within the same isotherm. Besides, the adsorption capacity depends on the nature of the starting anions, following the affinity order (NO ₃ ⁻<Cl ⁻<CO ₃ ²⁻ ) proposed by Miyata and increases with the specific surface area.