Microwave catalytic treatment using magnetically separable CoFe2O4 spinel catalyst for high-rate degradation of malachite green dye

Abstract

The release of toxic chemical dyes from the industrial effluent poses huge challenges for the environmental engineers to treat it. Azo dyes encompass the huge part of textile discharges which are difficult to degrade due to their complex chemical aromatic structures and due to the presence of strong bonds (-N=N-). Thus, the removal of a carcinogenic azo dye (i.e., malachite green (MG)), via microwave (MW) - assisted technology in the presence of spinel cobalt ferrite (CoFe₂O₄) catalyst was investigated. The synthesized CoFe₂O₄ nanoparticles were characterized via XRD, FTIR, TGA, VSM, and SEM-EDAX analytical techniques. The nanoparticles were found to be spherical (17.42 nm), and crystalline in nature. The impact of MW power (300-700W), MW temperature (60°- 90 °C), CoFe₂O₄ dosage (0-1.2 g/L), and initial MG dye concentrations (15-100 mg/L) on MG removal were studied. The maximum MG decolorization (>90%) was observed within 2 min, and the remaining decolorization was completed within next 3 min under the optimized condition. The reaction rates were significantly boosted by MW irradiation, resulting in faster MG degradation with pseudo second-order kinetics rate (R² ∼ 0.99). The MW irradiation would induce the localised hotspot zones over the catalytic surface, thus promoting the reactive radical species generation, which targeted the organic pollutant achieving the higher degree of mineralization (TOC∼90%). The toxicity reduction after the MW treatment suggests that the bulky toxic aromatic chains of the dye compound might have fragmented into simpler, smaller, and less toxic compounds. The ⦁OH played a major role in the degradation of MG dye through demethylation, elimination of benzene rings, and subsequent mineralization to CO₂, and H₂O. No detectable leaching of cobalt (Co) metal was observed from the catalyst, which ensured the stability of Co in the catalyst. Moreover, CoFe₂O₄ was recovered easily after the MW treatment via external magnetic separation, due to its high saturation magnetization value (i.e., 59.5 emu/g). Additionally, cost incurred for dye removal via MW irradiation was compared ($ 308 per kg of the dye pollutant removed) with several other processes, and found on economic side in comparison. According to the findings, the microwave irradiation process assisted by CoFe₂O₄ treatment offers a practical, and potential approach for treating dye laden wastewater, along with the easier catalyst retrieval mechanism using magnetism.

Keywords: Advanced oxidation process; Dye degradation; Malachite green; Microwave-assisted catalytic degradation; Spinel cobalt ferrite.