Mg/Zr modified nanobiochar from spent coffee grounds for phosphate recovery and its application as a phosphorous release fertilizer

Abstract

Excessive phosphorous discharge causes eutrophication of the aquatic system. Hence, from the viewpoints of eutrophication control and sustainable use of phosphorous, its recovery from wastewater is essential. Herein, Mg/Zr modified nanobiochar from spent coffee grounds (Mg/Zr/CNBC) is reported as an efficient adsorbent for phosphate recovery, and phosphorous release fertilizer. SEM, XRD, and FTIR characterizations of the adsorbent before and after adsorption suggested that phosphate is involved in both physisorption and chemisorption. Adsorption of phosphate on Mg/Zr/CNBC and factors affecting the process viz pH, contact time, initial phosphate concentrations, and dose of adsorbent were investigated in batch mode experiments. Mg/Zr/CNBC showed a 23.36 and 11.84 % improvement in the adsorption efficiency than the pristine and Mg-modified nanobiochar respectively. Acidic conditions favored the adsorption of phosphate on Mg/Zr/CNBC with maximum adsorption (98.5 %) at pH 1. Adsorption of phosphate followed pseudo-second-order kinetics. The corresponding equilibrium constant (K2) and adsorption capacity were calculated to be 0.316 g/mg. min and 39.4 mg/g respectively. The equilibrium data obtained at different phosphate concentrations better fitted the Freundlich adsorption isotherm model (R2 = 0.998) suggesting multilayer adsorption of phosphate on a heterogeneous surface. Further, the utility of phosphate-laden Mg/Zr/CNBC as a phosphorous-release fertilizer was demonstrated in a pot test. Results of the pot test showed that phosphate-laden Mg/Zr/CNBC improved the growth of Wheat and Chickpeas by 31.16 and 29.63 % respectively. The result highlights a circular economy approach for spent coffee grounds.