The removal of Pb²⁺ ion by MnFe₂O₄/waste tea leaves biochar and mechanism of adsorption

Abstract In order to transform waste tea leaves into a useful/valuable material for removal of Pb2+ ions from wastewater, MnFe2O4/biochar was synthesized. The tea waste was pyrolyzed at 500 °C to obtain the biochar. Effects of the composition of tea leaves on the physicochemical properties of biochar were evaluated. Biochar and MnFe2O4/biochar were mainly organic matter. Regarding inorganic components, aside from Fe and Mn there were considerable albeit small amounts of the mineral elements K and Ca in the MnFe2O4/biochar. The MnFe2O4/biochar is porous with a specific surface area of 24.38 m2 g−1, and the surface is loaded with MnFe2O4 and amorphous MnO2 particles. Also carboxylic acid, hydroxyl, and carbonyl functional groups were formed on the MnFe2O4/biochar surfaces. The surface area and pore volume characteristics of the MnFe2O4/biochar were also increased compared with the baseline biochar, and the prepared MnFe2O4/biochar had mesostructure. The modification of biochar into MnFe2O4/biochar improved adsorption of Pb2+ ions with the removal increased to ∼98%. The Freundlich isotherm and the pseudo-second order kinetic models matched well Pb2+ adsorption onto the MnFe2O4/biochar.