Kaolin-Supported Silver Nanoparticles as an Effective Catalyst for the Removal of Methylene Blue Dye from Aqueous Solutions

Synthesis of Kaolin-Supported Nickel Oxide Composites for the Catalytic Oxidative Degradation of Methylene Blue Dye

Organic dye contamination of water is a contributing factor to environmental pollution and has a negative impact on aquatic ecology. In this study, unsupported NiO and kaolin-supported NiO composites were synthesized by a one-step wet impregnation-precipitation method through the precipitation of nickel hydroxide onto locally accessible, inexpensive, and easily treated kaolin surfaces by using sodium hydroxide as a precipitating agent. The product was calcined at 500 °C and used for the catalytic oxidative degradation of methylene blue (MB) dye in an aqueous solution. The morphology, structure, and interactions of the synthesized materials were explored by SEM, XRD, and FT-IR spectroscopy. The characterization results revealed the fabrication and the growth of NiO on the kaolin surface. To determine the catalytic oxidative degradation performance of the catalyst, many experiments have been performed using the MB dye as a model dye. The catalytic degradation tests confirmed the importance of NiO and the high catalytic activity of the synthesized NiO/kaolin composite toward MB dye degradation. The oxidative degradation results showed that the optimized precursor amount on the kaolin surface could efficiently enhance the removal of MB dye. The kinetic investigation of the catalytic degradation of MB dye fitted the pseudo-first-order kinetic model. High removal efficiency was observed after eight reuse cycles, proving the exceptional stability and reusability of the composite. The catalytic process also proceeded with a low activation energy of 30.5 kJ/mol. In conclusion, the kaolin-supported NiO composite was established to be a favorable catalyst to degrade a model dye (MB) from an aqueous solution in the presence of inexpensive and easily available NaOCl with a catalytic efficiency of the material higher than 99% of the 20.3 mg catalyst within 6 min with an apparent rate constant, kapp, higher than 0.44625 min-1, which is far better than that of the unsupported catalyst with a kapp of 0.0926 min-1 at 10 mg dose in 20 min.

Ophiorrhiza mungos-Mediated Silver Nanoparticles as Effective and Reusable Adsorbents for the Removal of Methylene Blue from Water

Green synthesis of silver nanoparticles (AgNPs) using a plant extract has attracted significant attention in recent years. It is found as an alternative for other physicochemical approaches because of its simplicity, low cost, and eco-friendly rapid steps. In the present study, Ophiorrhiza mungos (Om)-mediated AgNPs have been shown to be effective bioadsorbents for methylene blue (MB) dye removal (88.1 ± 1.74%) just after 1 h at room temperature in the dark from an aqueous medium for the first time. Langmuir and Freundlich isotherms fit the experimental results having the correlation coefficient constants R2 = 0.9956 and R2 = 0.9838, respectively. From the Langmuir fittings, the maximum adsorption capacity and adsorption intensity were found to be 80.451 mg/g and 0.041, respectively, indicating the excellent performance and spontaneity of the process. Taking both models under consideration, interestingly, our findings indicated a fairly cooperative multilayer adsorption that might have been governed by chemisorption and physisorption, whereas the adsorption kinetics followed the pseudo-second-order kinetics mechanism. The positive and low values of enthalpy (ΔH0 = 4.91 kJ/mol) confirmed that adsorption is endothermic and physical in nature; however, the negative free energy and positive entropy value (ΔS0 = 53.69 J/mol K) suggested that the adsorption is spontaneous. The biosynthesized adsorbent was successfully reused up to the fifth cycle. A proposed reaction mechanism for the adsorption process of MB dye onto Om-AgNPs is suggested. The present study may offer a novel finding such as an effective and sustainable approach for the removal of MB dye from water using biosynthesized Om-AgNPs as reusable adsorbents at a comparatively faster rate at a low dose for industrial applications.