Hydrothermal Synthesis of Mg modified Nanobiochar from Spent Coffee Grounds for Effective Phosphate Removal: Kinetic and Isotherm Studies

Abstract Phosphate removal/recovery is essential to maintain the quality of aquatic systems and ensure sustainable use of phosphorous. Herein Mg‐nano biochar (Mg/NBC) is prepared as an effective phosphate adsorbent through the valorization of spent coffee grounds into nanobiochar by hydrothermal acid digestion of its biochar resulting from a slow pyrolysis. Characterization of the adsorbent was done by SEM, FTIR, BET, and XRD. Mg/NBC showed a 12.84 % higher adsorption capacity than the unmodified nanobiochar. Adsorption studies confirmed that phosphate adsorption on Mg/NBC is favored in acidic conditions removing 95 % of the initial concentrations at pH 1. The Langmuir adsorption isotherm model is better fitted to the equilibrium data with a maximum adsorption capacity of 100 mg/g suggesting uniform monolayer adsorption. Phosphate adsorption on Mg/NBC was initially rapid and followed a pseudo‐second‐order kinetic with an equilibrium constant (K 2 ) of 0.029 g/mg.min. Thermodynamic investigation at different temperatures revealed negative ΔG°, positive ΔH° and ΔS° values suggesting a spontaneous endothermic process involving the chemisorption of phosphate at the later stage of the adsorption. Further, the application of the prepared adsorbent in real wastewater resulted in 90 % phosphate removal suggesting the promising potential of the prepared adsorbent for practical use in phosphate removal/recovery.