Synthesis of a sustainable material based on pecan nutshell for the elimination of diclofenac in aqueous solution: Characterization and adsorption studies

Abstract

In the present study, activated carbons were synthesized from pecan nutshells (Carya illinoinensis) using two physicochemical methods: steam (CAVA) and phosphoric acid (CAAF). Their capacity to remove diclofenac from aqueous solution, as well as their textural, morphological, and physicochemical properties, were evaluated. N₂ physisorption analysis revealed the microporous nature of the carbons, with specific surface areas of 1123.5 and 375.6 m² g⁻¹ for CAAF and CAVA, respectively. Scanning electron microscopy showed a heterogeneous morphology, with evident roughness caused by the irregularity of the surfaces resulting from the activation method used. The point of zero charge was 2.9 for CAAF and 8.9 for CAVA. Surface charge distribution analysis indicated that the surface of CAAF is predominantly negative, while that of CAVA is predominantly positive, which reflects the difference in active sites between the two materials. Infrared spectroscopy allowed for the identification of the functional groups present on the surfaces of the materials generated by the activation methods. Thermogravimetric analysis demonstrated that CAAF is more hydrophilic than CAVA, and both carbons exhibited low ash content (7 % for CAVA and 6 % for CAAF). The maximum adsorption capacity for DCF was achieved at a pH of 7, with values of 49.0 mg g⁻¹ for CAVA and 230.8 mg g⁻¹ for CAAF, decreasing as the pH increased from 7 to 10. Increasing the solution temperature from 15 to 35 °C, the adsorption capacity of CAVA and CAAF increased 1.2 and 1.5 times, respectively. Adsorption increased by 1.7 times for CAVA and 1.2 times for CAAF when the ionic strength was raised with a NaCl concentration from 0.001 to 0.01 M. The proposed mechanism for diclofenac adsorption on CAVA involved electrostatic interactions (pH < pH_PZC), whereas the predominant mechanism in CAAF was π-π interactions (pH > pH_PZC).