Optimal removal of diclofenac and amoxicillin by activated carbon prepared from coconut shell through response surface methodology

Abstract

Agricultural residues like coconut shell are widely available in Benin Republic and can be used as adsorbent. In this study, we determine the limits of pharmaceutical substances adsorption by activated carbon from coconut shells. The synthesis of this adsorbent was optimized by the Response Surface Methodology (RSM) with two factors: the impregnation ratio IR and activation temperature. The iodine value was considered as the performance (response) parameter of this synthesis. After characterizing the prepared activated carbon, adsorption tests were performed on diclofenac sodium (DCF) and amoxicillin (AMX) by varying the contact time and the adsorbate-adsorbent ratio Rads. The synthesis results showed that the optimal physicochemical properties of the activated carbon were observed at 740 °C with phosphoric acid (IR = 1.66). Under these optimal conditions, the activated carbon from the coconut shells presented a large microporous specific surface (SBET = 437 m2/g and Vmicro = 0.21 cm3/g), optimal iodine adsorption (930.28 mg/g), amorphous and low heterogeneous chemical composition. In addition, the prepared activated carbon was an excellent adsorbent for the removal of the pharmaceutical substances studied. The experimental adsorption data followed the Langmuir isotherm and the pseudo first-order kinetic model. However, the efficiency varied depending on the nature of the adsorbate and the adsorbate-adsorbent ratio was the main limiting factor in the adsorption process. Optimal elimination greater than 98% was noticed with Rads = 0.10 and a contact time of 15 min (90 min) for DCF (AMX). However, we noticed the complete elimination of AMX (DCF) for Rads ≤ 0.075 (Rads ≤ 0.040). It was observed that the removal efficiency of pollutant was not defined by the adsorption rate constant but the reactivity with the adsorbent.