Synergistic waste valorization: One-step hydrothermal synthesis of magnetic hydrochar from red mud and peanut shells for effective methylene blue adsorption

Abstract

Printing and dyeing wastewater is characterized by a high concentration of organic pollutants, elevated alkalinity, and significant water quality fluctuations, which lead to severe environmental pollution and potential risks to human health. This study presents an innovative approach combining red mud (RM) and peanut shells (PS) to synthesize magnetic hydrochar (MHC) via a one-step hydrothermal method for methylene blue (MB) removal. The physicochemical properties and synthesis mechanism of MHC were characterized through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), and Brunauer-Emmett-Teller (BET) analysis. The hydrothermal carbonization process of PS promoted magnetite formation, and the obtained MHC exhibited a saturation magnetization of 24.3 A·m²·kg⁻¹. Under the conditions of adsorption time at 2 h, pH = 10, starting MB concentration at 160 mg·L⁻¹, and MHC dosage at 1 g·L⁻¹, the maximum adsorption capacity reached 123.26 mg·g⁻¹. The adsorption process conformed to the Freundlich isotherm and the pseudo-second order kinetic models, indicating adsorption predominantly occurred on the heterogeneous surface, with chemisorption as the rate-controlling step. Thermodynamic analysis confirmed that the process was spontaneous, endothermic, and associated with increased entropy. The adsorption mechanism involved complexation, π-π interaction, electrostatic interaction and hydrogen bonding. These findings provide valuable insights into the synthesis and practical utilization of RM and PS-based MHC for wastewater treatment.