Catalytic Reduction of Congo Red to Low-Toxicity Forms Using a Low-Cost Catalyst Based on Modified Bentonite Material

The reduction of azo dyes to less toxic and more easily biodegradable amine derivatives is an effective strategy for the treatment of industrial wastewater. The present work aimed to study the reduction reaction of azo dye Congo red (CR) catalyzed by nanoparticles (NPs) of chromium oxides (Cr 2 O 3 NPs) immobilized on bentonite in the presence of NaBH 4 . Cr(III) ions were intercalated using ion exchange reactions to obtain Cr-bentonite, and then the immobilized chromium cations were treated using NaBH 4 leading to the formation of Cr 2 O 3 NPs-bentonite. The physicochemical properties of the samples were investigated using X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS), atomic absorption spectrometry (AAS), UV–Visible diffuse reflectance (UV–Vis DR), and Fourier-transform infrared (FTIR) spectroscopy techniques. The results showed the formation of various chromium species, in which the most dominant were chromium oxide nanoparticles, on the bentonite surface with an average particle size between 20 and 35 nm. Line-scan analysis showed a reactive catalytic surface due to the excellent distribution of Cr on the bentonite surfaces. The best-performing catalyst, Cr 2 O 3 NPs-bentonite, displayed significant catalytic activity compared to the bentonite and Cr-bentonite materials, with a full reduction time of 630 s and a rate constant, k app , equal to 0.034 s –1 . The resulting products (benzidine and sodium 3, 4-diaminonaphthalene-1-sulfonate) from the catalytic reduction exhibited low toxicity compared to the CR dye; these products are easy to use in chemical synthesis. All results collected from this work indicated that this low-cost catalyst can be exploited to eliminate other dyes from the environment.