Biomass-derived activated carbons from Ziziphus jujube and Dodonaea viscosa for the efficient removal of malachite green: a comparative adsorption performance and mechanisms

Conversion of discarded agricultural leaves into activated carbons offers a low-cost abatement of toxic dye pollution together with value-added use of biomass. Herein, we report the utilisation of agricultural biomass as a precursor of the activated carbon (AC), which is used as an adsorbent for the removal of malachite green (MG). The leaves of Ziziphus jujube and Dodonaea viscosa were pyrolysed at 1073 K and thereafter chemically activated, using NaHCO₃. The resultant AC, having a sponge-like microporous morphology, showed a well-defined porosity and high adsorption capacity towards MG (Z. jujube = 226.3 mg/g, D. viscosa = 208.05 mg/g). Increasing the dosage of various salts exerted only minor interference, indicating that the adsorbent maintains its stability even in the presence of ions. The mechanistic modelling of the adsorption process suggested that the adsorption was a spontaneous endothermic process, which was predominantly governed by the physical interactions. Moreover, electrostatic interaction, π–π interactions, and pore filling mechanism were considered likely interactions governing the adsorption of MG onto the surface of AC. This study highlights the potential of agricultural leaves as a pollutant-removing agent, which underscores its promise for a scalable, circular-economy water-treatment solution.