Circular economy approach: repurposing modified lemon peel waste for cobalt(II) capture from simulated solutions

Biochar from different agrowastes has been extensively used to eliminate various pollutants from wastewater. In the present work, raw lemon peels (RLP) were activated by microwave irradiation then converted into activated biochar (RLP-BC) by pyrolysis. Afterwards, both RLP and RLP-BC have undergone microwave-assisted chemical modification by hydrogen peroxide and nicotinic acid. These biosorbents were denoted as RLP, RLP-BC, OH-LP, OH-LP-BC, N-LP, and N-LP-BC respectively. They were characterized using different analytical techniques including IR, SEM, and EDX to investigate their properties. Surface topography of selected sorbents was examined using AFM, and their BET data were provided. These biosorbents were then tested for their ability to remove Co(II) from aqueous solutions under varying conditions. The adsorption experiments showed that the modified lemon peel materials showed adsorption affinity for Co(II) ions in alkaline medium, and the highest removal efficiency was achieved at pH 9. The adsorption equilibrium of Co(II) onto the biosorbents was described by the Langmuir isotherm, while adsorption kinetics were modeled using a pseudo-second-order approach for RLP-BC, N-LP, and N-LP-BC adsorbents. Thermodynamic parameters were also assessed. Results indicated that chemically modified biochar was the most effective in Co(II) removal. OH-LP-BC exhibited the highest Qmax (28.9 mg g−1), while N-LP-BC showed the lowest (14.5 mg g−1). Reusability tests demonstrated that these sorbents could be reused up to four times with removal efficiencies exceeding 50%. Furthermore, efficacy of the investigated sorbents in removal of Co(II) from a sample of fertilizer wastewater was explored at the optimized conditions. Our finding proves the suitability of the green sorbents as possible candidates for wastewater treatment application. Six lemon peel-based sorbents were investigated for the sorption of Co (II) cations from simulated solution. Characterization of the prepared sorbents by using various analytical techniques. Exploring the efficacy of the activated and produced biochar's ability to adsorb Co (II) under various testing settings. Applying four adsorption isotherms (Freundlich model, Langmuir model, Temkin model, and D-R model) to study the adsorption process. Thermodynamics and kinetics of adsorption were carefully verified. The reusability study showed a high adsorption capacity of the tested sorbents after four cycles of sorption and desorption.