Biomass-Derived Reduced Graphene Oxide from Coconut Shell Waste as a Multifunctional Adsorbent for Heavy Metal and Dye Removal from Water

The development of sustainable adsorbents from renewable biomass has attracted considerable attention for wastewater treatment applications. In this study, reduced graphene oxide (rGO) was successfully synthesized from coconut shell waste through carbonization, modified Hummer’s oxidation, and chemical reduction. The resulting material was evaluated for the removal of Cd²⁺, Ni²⁺, Pb²⁺, and methylene blue (MB) from aqueous solutions. Morphological characterization by SEM and TEM confirmed the successful transformation of the biomass precursor into graphene-like nanosheets and the formation of a more ordered graphitic structure following reduction. The effects of solution pH, contact time, adsorbent dosage, and initial pollutant concentration on adsorption performance were systematically investigated. Optimum adsorption was achieved at pH 7, an adsorbent dosage of 50 mg, and an equilibrium time of 180 min. Under optimized conditions, the maximum adsorption capacities of rGO reached 110, 102, 95, and 108 mg g⁻¹ for Cd²⁺, Ni²⁺, Pb²⁺, and MB, respectively. Compared with graphene oxide-like nanosheets, the reduction process improved adsorption performance by up to 50%, highlighting the beneficial role of restoring graphitic domains and enhancing surface accessibility. Furthermore, the synthesized rGO exhibited excellent regeneration performance, retaining more than 87% of its initial adsorption capacity after five consecutive adsorption–desorption cycles.