Removal of 2,4-dichlorophenoxyacetic acid by calcined Zn–Al–Zr layered double hydroxide

The adsorption equilibrium, kinetics, and thermodynamics of removal of 2,4-dichlorophenoxy-acetic acid (2,4-D) from aqueous solutions by a calcined Zn–Al layered double hydroxide incorporated with Zr4+ were studied with respect to time, temperature, pH, and initial 2,4-D concentration. Zr4+ incorporation into the LDH was used to enhance 2,4-D uptake by creating higher positive charges and surface/layer modification of the adsorbent. The LDH was capable of removing up to 98% of 2,4-D from 5 to 400 ppm aqueous at adsorbent dosages of 500 and 5000 mg L−1. The adsorption was described by a Langmuir-type isotherm. The percentage 2,4-D removed was directly proportional to the adsorbent dosage and was optimized with 8% Zr4+ ion content, relative to the total metals (Zr4+ + Al3+ + Zn2+). Selected mass transfer and kinetic models were applied to the experimental data to examine uptake mechanism. The boundary layer and intra-particle diffusion played important roles in the adsorption mechanisms of 2,4-D, and the kinetics followed a pseudo-second order kinetic model with an enthalpy, ΔHads of −27.7 ± 0.9 kJ mol−1. Regeneration studies showed a 6% reduction in 2,4-D uptake capacity over six adsorption–desorption cycles when exposed to an analyte concentration of 100 ppm.

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► Zn–Al–Zr ternary layered double hydroxides synthesized by sol–gel method.
► Zr4+ into the Zn–Al layered double hydroxides improves its adsorption capacity for 2,4-dichlorophenoxyacetic acid.
► Calcination after exposure decomposes 2,4-D completely and allows for LDHs reuse.