Isothermal, kinetic, thermal, and economic characteristics of NaOH-modified charred–desiccated coconut waste as adsorbent for lead (II) in water phase

In this study, charred–desiccated coconut waste was treated with several chemicals, such as NaOH, NaOCl, H₂O₂, and HNO₃, to prepare chemically modified adsorbents. Fourier transform infrared spectroscopy revealed that chemical modification enhanced the surface chemistry of the adsorbent by introducing new functional groups and increasing their concentration. The chemically modified adsorbents had lower surface areas of 0.13–17.36 m²/g, which were substantially lower than that of the pristine adsorbent. Regardless of the surface area, all modified adsorbents exhibited higher Pb(II) adsorption capacities, wherein that of desiccated coconut-waste char (DCWC)-NaOH was the highest, with a removal efficiency of 96% (Q = 29.854 mg/g) for an adsorption time of 1 min. The equilibrium and kinetic data for the DCWC-NaOH adsorbent best fit the Freundlich isotherm and pseudo-second-order kinetic models, respectively, with a maximum monolayer adsorption capacity of 1666.67 mg/g. The adsorption of Pb(II) ions was an exothermic process, with an optimum temperature of 40 °C. Compared to existing commercial adsorbents, the proposed adsorbent was highly regenerable and considerably low-cost, showing promising potential for commercial applications.