Upcycling waste biomass: Alkali-modified watermelon rind as a lignocellulosic bioadsorbent for copper ion removal

The transition towards sustainable development has placed significant emphasis on biorefinery technologies, which are designed to convert renewable biomass into a variety of valuable products, including biofuels, biochemicals, and functional materials. In this context, the utilization of lignocellulosic biomass, such as agricultural residues, offers a promising pathway for the development of environmentally friendly materials. The study explored into the utilization of NaOH-modified watermelon rind bioadsorbent (NWR) derived from discarded lignocellulosic watermelon rind (W R ) for extracting copper (Cu (II)) ions from aqueous solutions. The W R and NWR biosorbents underwent comprehensive characterization via FTIR, BET, SEM coupled with EDX, TEM/SAED, TGA/DTG, XRD to elucidate their functional groups, surface area, morphology, elemental composition, thermal stability, and phase analysis. The adsorbents exhibited a substantial specific surface area and considerable total pore volume. Differential thermal analysis revealed distinct temperature peaks and degradation points for W R and NWR. Cu (II) ions removal was found to be pH-dependent, with optimal extraction observed at pH 8.0. The adsorption process was accurately described by the Langmuir and pseudo-second-order kinetic models, indicating chemisorption mechanism as the rate-limiting step. The biosorbent's maximum sorption capacity (289.85 mg g −1 ), determined through the Langmuir adsorption isotherm, was remarkably high, underscoring its superior efficiency compared to other biosorbents employed for Cu (II) ions removal. Notably, NWR demonstrated promising regeneration potential, retaining its Cu (II) ions adsorption efficacy over six consecutive cycles. The study conclusively established that this synthesized biosorbent presents an economically viable and highly efficient approach for removing Cu (II) ions from aqueous solutions. • A lignocellulosic bioadsorbent was created by alkali-activating Citrullus lanatus rind, resulting in enhanced adsorption of Cu (II) ions. • Alkali treatment increased the material's microporosity, significantly boosting its adsorption capacity. • Adsorption followed the Langmuir isotherm model and pseudo-second-order kinetic model. • Cu (II) ion adsorption was characterized as chemical, spontaneous, endothermic, and thermodynamically favorable. • The bioadsorbent demonstrated reusability for up to five adsorption-desorption cycles with consistent performance.