Mg modified nanobiochar from spent coffee grounds: Evaluation of the phosphate removal efficiency and its application as a phosphorous release fertilizer

Abstract Phosphate enrichment of the aquatic system often impacts their aesthetic and ecological values. Hence, phosphate recovery is essential to maintain the quality of aquatic systems and ensure sustainable use of phosphorous. Herein, Mg-modified nanobiochar from spent coffee grounds (Mg/NBC) is proposed as an efficient adsorbent for phosphate recovery, and as a phosphorous-release fertilizer. The nanobiochar was obtained by acid digestion of the biochar resulting from pyrolysis. The effect of parameters viz pH, contact time, the dose of adsorbent, and initial concentration on the adsorption was investigated in batch experiments. Results indicated that phosphate adsorption on Mg/NBC is favored in acidic conditions with maximum adsorption (95%) at pH 1. Mg/NBC showed a 12.84% improvement in the adsorption efficiency than the pristine nanobiochar. Characterizations of the adsorbent before and after adsorption suggested that phosphate is involved in both physisorption and chemisorption. Adsorption of phosphate followed pseudo-second-order kinetics with an equilibrium constant (K2) of 0.029 g/mg.min. 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. Results of the pot test showed that phosphate-laden Mg/NBC improved the growth of garlic and beans by 10 and 5% respectively. The result highlights the utility of phosphate-laden Mg/NBC as a phosphorous-release fertilizer and illustrates a circular economy approach for spent coffee grounds.