Pencil graphite synergistic improvement of zero-valent iron composite for the removal of diclofenac sodium in aqueous solutions: Kinetics and comparative study

Pencil graphite (PG) was used as a carrier material to increase the stability, dispersity, and removal capacity of zero-valent iron (Fe⁰) in an aqueous solution. Freshly synthesized composite adsorbent from Fe⁰ and pencil graphite (Fe⁰-PG) was established. The morphology of Fe⁰ and Fe⁰-PG was characterized using FTIR, SEM, TEM, XRD, and BET techniques. The adsorption mechanism and potential removal of Diclofenac sodium (DCF) onto the surface of Fe⁰ and Fe⁰-PG composite in an aqueous solution were investigated in terms of pH, contact time, initial DCF concentration, adsorption dose, and temperature. The adsorption data fit better the pseudo-second-order kinetic and Langmuir isotherm model via both adsorbents (R² > 0.992). The experimental results indicated that the maximum adsorption capacity q_max of DCF via Fe⁰-PG, obtained from Langmuir isotherm was 48.1 mg.L⁻¹, and it is an extraordinary value in comparison with that of Fe⁰ (23.1 mg.L⁻¹). The maximum removal efficiency was achieved at pH = 6 for both adsorbents (Fe⁰: 51% and Fe⁰-PG: 92%). The calculated value of the activation energy E_a for adsorption of DCF via Fe⁰-PG composite is 112 kJ.mol⁻¹, which signifies the chemisorption nature of the adsorption mechanism. The data suggest that the process for both sorbates is endothermic and spontaneous in nature at high temperatures. Higher environmental temperature is more favorable for uptake of DCF onto the Fe⁰-PG surface. PG was found to be a promising synergistic material, and Fe⁰-PG shows enhancement for DCF removal in aqueous solutions compared with the bare Fe⁰ NPs.