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 (Cr2O3NPs) immobilized on bentonite in the presence of NaBH4. Cr(III) ions were intercalated using ion exchange reactions to obtain Cr-bentonite, and then the immobilized chromium cations were treated using NaBH4 leading to the formation of Cr2O3NPs-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, Cr2O3NPs-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, kapp, 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.

An important application of clay is as a solid adsorbent for industrial dyes. The aim of the present work was to carry out an experimental—theoretical study of the adsorption of dye mixtures, namely malachite green (MG) and Congo red (CR), by bentonite. Adsorption studies were conducted after evaluation of the impact of several parameters, including pH, adsorbate dose, and contact time, on the removal of MG and CR. The pH of the dye solution is strongly affected by the chemistry of both the dye molecules and of the adsorbent in an aqueous solution. Where both dye molecules exist in solution, the optimum pH was found to be 8.2 in order to achieve the maximum adsorption of both MG and CR. Preliminary studies showed that 60 min of contact time is sufficient to reach adsorption equilibrium. The adsorption studies were carried out using 1.0 g samples of bentonite. The amount of dye adsorbed was found by application of classical least squares to the synthetic dye mixtures. Data from equilibrium adsorption on bentonite were analyzed by Freundlich, Langmuir, Redlich-Peterson, and Temkin isotherm equations using regression analysis for non-linear forms of those equations. For binary-mixture analysis, isotherm parameters were determined from single-component adsorption studies and the theoretical amount of dye adsorbed was calculated using an extended Langmuir isotherm. Non-linear error analysis showed that the Temkin and Redlich-Peterson isotherms gave the best fits to the equilibrium data for adsorptive removal of MG and CR by bentonite.

The adsorption of the anionic dye Congo Red (CR) by Na-, Cs-, Mg-, Cu-, Al- and Fe-montmorillonite was investigated by electronic spectroscopy. Spectra of the aqueous suspensions of different clay concentrations and different loadings were recorded. The spectra were curve fitted and were compared to spectra of aqueous solutions of CR at different pH values. Four bands representing anionic varieties and two representing protonated varieties of CR were identified in spectra of CR-treated Na-, Cs-, Mg-, Cu- and Al-montmorillonite. Spectra of CR-treated Femontmorillonite were slightly different, showing three bands representing anionic varieties and four representing protonated CR. The ratio between areas of bands, which represent the protonated varieties, and those representing the anionic varieties were determined showing correlation with the acidity of the metallic cation. The clay in the aqueous suspensions became coloured after the adsorption of CR. The following stainings were obtained, Na- and Mg-clay, red, Cs-clay, violet-red, Cu-clay, reddish-violet, Al-clay, dark violet and Fe-clay, dark blue. It appears that CR can serve as an indicator of the surface acidity of montmorillonite. The effects of aging on the sorption of the dye and on the surface-acidity of the CR-clay complex were investigated.

The use of natural clays for the removal of dyes from wastewater, an important part of environmental remediation, is desirable due, not least, to their low cost. Palygorskite (PAL), a rigid-structure clay, is a good candidate for use in the elimination of industrial effluents, based on its exceptional adsorptive properties. Recently, a new palygorskite deposit has been discovered in Cuba and its use in the adsorption of dyes has not yet been explored in detail. In the present study, the use of unmodified natural Cuban palygorskite as a host for dyes was evaluated. Congo red (CR) and methylene blue (MB) were the anionic and cationic dyes tested, respectively, because of their wide use and toxicity to the environment. Several physical-chemical parameters were studied in order to establish the best experimental conditions under which to achieve the greatest dye load per gram of clay. Natural mixtures with different percentages of montmorillonite were also tested to evaluate their effect on the adsorption of the dyes. The results indicated that at pH values of ~7–9 and an initial dye concentration of 0.1 mg mL–1, the process was efficient. X-ray diffraction (XRD) analysis proved the surface adsorption of both dyes on the clays. The main interactions involved in the clay-dye system were electrostatic forces and H-bonds. Adsorption of CR seemed to be controlled fundamentally by the palygorskite phase. Such results support the use of this natural clay as an efficient host for the removal of MB and CR from wastewater.