Multiple roles of ferric chloride in preparing efficient magnetic hydrochar for sorption of methylene blue from water solutions

Highly efficient and cheap magnetic materials have application prospects in wastewater treatment. Herein, Fe3O4-loaded hydrochar (HC-Fe3O4) was obtained from hydrothermal carbonization (HTC) of bamboo with FeCl3 and then added with FeCl3 to form a magnetic sorbent via simple precipitation. The HC-Fe3O4 was characterized with various instruments. The characterizations show FeCl3 plays at least two roles as a catalyst and an oxidant in HTC. The specific surface area of hydrochar enlarged from 39.9731 to 60.9887 m2·g-1 after the addition of FeCl3 during HTC, which showed FeCl3 acted as a catalyst in HTC. XRD indicated Fe3O4 was formed by the structure of HC-Fe3O4, which indicated Fe(III) was reduced to Fe(II) during HTC. Sorption of methylene blue (MB) onto HC-Fe3O4 was better fitted by the Langmuir isotherm and pseudo-second-order kinetic models. Sorption is a spontaneous thermodynamic endothermic process and HC-Fe3O4 is easily separated by an applied magnetic field and reused.