Mechanochemical synthesis of Fe₂O₃/Zn-Al layered double hydroxide based on red mud

In this work, using industrial waste red mud (RM) as main starting material, a simple method of mechanochemical synthesis (MCS) was introduced as a green approach to synthesize heterogeneous Fe₂O₃/Zn-Al layered double hydroxide (F/ZA-LDH), which could be used as a new low-cost catalyst for photo-Fenton reaction. The optimum preparation conditions of F/ZA-LDH were as follows: mass ratio of Zn(NO₃)₂·6H₂O to RM (m_Zn:m_RM) 2:1, dry milling time 6 h, H₂O dosage 2 mL, ball-to-powder mass ratio 50:1, and milling speed 250 rpm. The effects of the synthesis conditions on the crystal structure and catalytic activity of F/ZA-LDH were analyzed. The F/ZA-LDH was characterized by XRD, TG, XPS, SEM, (HR)TEM. The characterization results showed the composite had a crystallized hydrotalcite-like structure, and the crystalline phases in the optimum F/ZA-LDH were Fe₂O₃ and Zn-Al LDH. A hetero-interfaces between Fe₂O₃ and Zn-Al LDH existed in the synthesized Fe₂O₃/Zn-Al LDH composite. Furthermore, the possible mechanism for F/ZA-LDH formation in the MCS process was proposed. Overall, our results provide a systematic understanding of the preparation of LDH composite through MCS using RM as main material, and our findings help to develop green technology for reusing RM.