One-pot solvothermal synthesis of magnetic biochar from waste biomass: Formation mechanism and efficient adsorption of Cr(VI) in an aqueous solution

Highlights: • Magnetic biochar (MB) was synthesized by a facile solvothermal method. • The formation mechanism of MB was proposed. • The load ratio and particle size of Fe₃O₄ nanoparticles on MB were optimized. • The adsorption of Cr(VI) ions by MB was an adsorption-coupled reduction process. • The MB showed a stable structure with a low Fe leakage at pH 2.0. -- Abstract: A facile one-pot solvothermal method was applied to synthesize a magnetic biochar composite (MB) using phoenix tree leaves-derived biochar as the carbon matrix. The structure of MB was optimized by varying the load ratio and particle size of Fe₃O₄ nanoparticles on biochar. Time-dependent structure and composition evolution of solid and liquid phases during heterogeneous solvothermal process were investigated to understand the formation mechanism of MB. Firstly, Fe²⁺/Fe³⁺ ions were coordinated by oxygen-containing groups on biochar and part of them were hydrolyzed to form iron hydroxides. Then, those iron-containing precursors were thermally decomposed and reduced to iron oxides; and finally Fe₃O₄ nanoparticles were generated. The MB had an adsorption capacity for Cr(VI) of 55.0 mg/g in an aqueous solution, which exceeds those of biochar (39.8 mg/g) and Fe₃O₄ nanoparticles (26.5 mg/g). The adsorption mechanism study reveals that biochar as a carbon skeleton mainly provided binding sites for Cr(VI) and electron-donor groups for reduction of Cr(VI), while Fe₃O₄ nanoparticles mainly involved in the immobilization of newly formed Cr(III) through formation of Fe(III)-Cr(III) hydroxide. MB exhibited a stable structure with a lower Fe leakage at pH 2.0 than that of a comparable magnetic biochar sample prepared by conventional co-precipitation method. Recycling experiments suggested that MB could keep 84% of its initial removal capability for Cr(VI) even after seven cycles. The results indicate that solvothermal method is a promising alternative to prepare magnetic biochar for adsorption of heavy metal-containing wastewater.