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-Fe<sub>2</sub>O<sub>3</sub> anchored on the porous carbonaceous material surface consisted of the adsorption of iron ions and then the γ-Fe<sub>2</sub>O<sub>3</sub> 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<sup>–1</sup>) and Cr­(VI) (21.23 mg g<sup>–1</sup>) adsorption, outperforming that of commercial bulk Fe<sub>2</sub>O<sub>3</sub> 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-Fe<sub>2</sub>O<sub>3</sub> 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-Fe<sub>2</sub>O<sub>3</sub> 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.