Efficient adsorption and reduction of Cr(VI) in water using one-step H3PO4-assisted prepared Leersia hexandra Swartz hydrochar

Abstract

A novel nitrogen-phosphorus co-doped hydrochar was prepared by a one-step hydrothermal carbonization method using Leersia hexandra Swartz biomass as the feedstock and its inherent nitrogenous nature. The hydrothermal carbonization catalyzed by an appropriate dosage of phosphoric acid increased the removal amounts of Cr(VI) by 43.54 mg/g compared with the original hydrochar, not only retaining the N atoms of the raw material but also endowing nitrogen-phosphorus co-doped hydrochar with well-developed surface morphology and abundant acidic functional groups (such as carboxyl, hydroxyl, –NH–, O–P, etc.). The removal process matched well with the Freundlich (R2 = 0.9384) and the pseudo-second-order (PSO) model (R2 = 1.0000). The model analysis and thermodynamic study indicated that Cr(VI) removal was a spontaneous process in which chemisorption played a dominant role. Combined with the systematic characterization, it was elucidated that the efficient removal of Cr(VI) may be a synergistic effect of electrostatic attraction, in-situ reduction and complexation.