Ex situ modification of activated carbon through hydrothermal oxidation and sulphur@amine doping for highly efficient Pb2+ sorption: experimental and modelling approaches

Abstract

Lead ions present a multitude of dangers to human health and the ecosystem, even at very low concentrations. In this study, an ex situ approach was used to modify activated carbon (AC) derived from Garcinia kola nut shells. The AC was either directly modified by oxidation using KMnO₄, resulting in an oxidized form (AC-KMnO₄), or modified using an acid treatment leading to acid activated carbon (ACH), which was subsequently doped with sulfur and nitrogen using L-cysteine (ACH-L-cyst). The physico-chemical properties of ACH-L-Cyst and AC-KMnO₄ obtained using ex situ approach assisted hydrothermal oxidation, were evaluated by PXRD, FT-IR, SEM/EDX, Raman spectroscopy, and TGA/DTA analysis. The ability of the compounds to remove lead was assessed using batch sorption and mathematical modelling. The influence of four factors (pH, time, Pb²⁺ concentration and adsorbent mass) on the Pb²⁺ removal process was investigated using the central composite design approach. The experiments showed that the adsorption of Pb²⁺ by both adsorbents was favorable under the following optimum conditions: pH (5), equilibrium time (122.5 min), initial concentration (210 mg L⁻¹), and initial mass of adsorbent (60 mg), with a maximum adsorption capacity of 113 and 141 mg g⁻¹ for ACH-L-Cyst and AC-KMnO₄, respectively. Pseudo-first order and pseudo-second order nonlinear kinetic models better describe the adsorption mechanism by highlighting the coexistence between physisorption and chemisorption on heterogeneous material surfaces. Adsorption isotherm data of ACH-L-Cyst and AC-KMnO₄ were best described by the non-linear model of Langmuir–Freundlich. According to the thermodynamic parameters, the adsorption reaction is endothermic (ΔH° > 0) and spontaneous (ΔG° < 0). The adsorbents showed high stability after six adsorption cycles, demonstrating the potential of these new materials for removing Pb²⁺ ions.