One-step synthesis of magnetic tea waste biochar for efficient hexavalent chromium adsorption: process optimization, characterization, and adsorption mechanism

The booming global market for tea and new-style tea beverages has inevitably generated enormous quantities of waste tea residues, posing an urgent waste management challenge. Herein, spent tea leaves generated from tea beverage factory were employed as the precursor to synthesis N-doped magnetic biochar (NMTB) via one-step hydrothermal carbonization method with the modification by urea and FeCl 3 . The adsorption performance of hexavalent chromium (Cr(VI)) of NMTB was better than pristine tea waste biochar. Furthermore, response surface methodology indicated that the best preparation temperature and time of NMTB were 200 °C and 4 h, respectively. The prepared NMTBs were characterized by SEM, BET, XRD, FTIR and XPS, showing the successful doping of N and Fe. Additionally, NMTB exhibited excellent Cr(VI) removal of 96.90%, with a maximum adsorption capacity of 63.03 mg/g. The adsorption process followed the Langmuir and pseudo-second-order models, suggesting that monolayer sorption and chemisorption dominated the adsorption process. Reusability performance demonstrated that the removal efficiency of NMTB remained at 84.49% after five times reuse, while the removal efficiency in real water sample test reached 96.55%. Moreover, mechanistic studies including FTIR and XPS investigation, zeta potential analysis, quantitative chromium speciation and density functional theory calculations demonstrated that the adsorption is dominated by electrostatic attraction, surface reduction and subsequent surface complexation. The study demonstrated that tea waste-derived magnetic biochar is an effective and regenerable material for Cr(VI) remediation. Moreover, it provides a sustainable strategy for upcycling waste tea residues from the rapidly expanding tea beverage industry, exemplifying a promising value-added approach for environmental remediation.