A Magnetic Photocatalytic Composite Derived from Waste Rice Noodle and Red Mud.

Abstract

This study is the first to convert two waste materials, waste rice noodles (WRN) and red mud (RM), into a low-cost, high-value magnetic photocatalytic composite. WRN was processed via a hydrothermal method to produce a solution containing carbon quantum dots (CQDs). Simultaneously, RM was dissolved in acid to form a Fe³⁺ ion-rich solution, which was subsequently mixed with the CQDs solution and underwent hydrothermal treatment. During this process, the Fe³⁺ ions in RM were transformed into the maghemite (γ-Fe₂O₃) phase, while CQDs were incorporated onto the γ-Fe₂O₃ surface, resulting in the CQDs/γ-Fe₂O₃ magnetic photocatalytic composite. Experimental results demonstrated that the WRN-derived CQDs not only facilitated the formation of the magnetic γ-Fe₂O₃ phase but also promoted a synergistic interaction between CQDs and γ-Fe₂O₃, enhancing electron-hole pair separation and boosting the production of reactive radicals such as O₂^·- and ·OH. Under optimized conditions (pH = 8, carbon loading: 10 wt%), the CQDs/γ-Fe₂O₃ composite exhibited good photocatalytic performance against methylene blue, achieving a 97.6% degradation rate within 480 min and a degradation rate constant of 5.99 × 10^-3 min^-1, significantly outperforming RM and commercial γ-Fe₂O₃ powder. Beyond methylene blue, this composite also effectively degraded common organic dyes, including malachite green, methyl violet, basic fuchsin, and rhodamine B, with particularly high efficiency against malachite green, reaching a degradation rate constant of 5.465 × 10^-2 min^-1. Additionally, due to its soft magnetic properties (saturation magnetization intensity: 16.7 emu/g, residual magnetization intensity: 2.2 emu/g), the material could be conveniently recovered and reused after photocatalytic cycles. Even after 10 cycles, it retained over 98% recovery and 96% photocatalytic degradation efficiency, underscoring its potential for cost-effective, large-scale photocatalytic water purification.