Magnetic Porous Carbonaceous Material Produced from Tea Waste for Efficient Removal of As(V), Cr(VI), Humic Acid, and Dyes

Magnetic porous carbonaceous (MPC) materials derived from tea waste were synthesized by an integrated biosorption-pyrolysis process and were applied as adsorbents for wastewater cleanup. On the basis of various characterizations, we demonstrated that the formation mechanism of γ-Fe₂O₃ anchored on the porous carbonaceous material surface consisted of the adsorption of iron ions and then the γ-Fe₂O₃ nucleation and growth through pyrolysis at alternative peak temperatures (300-500 °C). The sample pyrolyzed at 300 °C (MPC-300) showed good capacities for As(V) (38.03 mg g^-1) and Cr(VI) (21.23 mg g^-1) adsorption, outperforming that of commercial bulk Fe₂O₃ and many other materials. Moreover, the large available positive charge density can facilitate the effective adsorption of anionic dye (MO) and humic acid (HA) on the γ-Fe₂O₃ surface while the adsorption performance is sluggish for cationic dyes (MB and RhB). Relatively, the adsorption isotherms could significantly conform to the Langmuir model, and the pseudo-second-order dynamic equation was the optimal model to describe the kinetics for the adsorption of As(V), Cr(VI), humic acid, and dye pollutants on MPC-300. Kinetic studies show that MPC-300 can efficiently remove these pollutants in aqueous solution within 3 h. The presence of HA reduced Cr(VI) and As(V) adsorption on MPC-300 at pH < 6.0. The XPS and FTIR analysis further demonstrated that ion exchange between surface hydroxyl groups and Cr(VI)/As(V) dominated the adsorption while the adsorption mechanism of MO and HA was attributed to electrostatic attraction on protonated-OH on the γ-Fe₂O₃ surface. The results suggested that the MPC material was a potential material to remove heavy metal ions, HA, and organic contaminants simultaneously with remarkable adsorption capacity, fast uptake rate, and easy magnetic separation.