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

In Situ Preparation of Silver Nanoparticles/Organophilic-Clay/Polyethylene Glycol Nanocomposites for the Reduction of Organic Pollutants

This work focuses on the preparation and application of silver nanoparticles/organophilic clay/polyethylene glycol for the catalytic reduction of the contaminants methylene blue (MB) and 4-nitrophenol (4-NP) in a simple and binary system. Algerian clay was subjected to a series of treatments including acid treatment, ion exchange with the surfactant hexadecyltrimethylammonium bromide (HTABr), immobilization of polyethylene glycol polymer, and finally dispersion of AgNPs. The molecular weight of polyethylene glycol was varied (100, 200, and 4000) to study its effect on the stabilization of silver nanoparticles (AgNPs) and the catalytic activity of the resulting samples. The results showed that the catalyst with the highest molecular weight of polyethylene glycol had the highest AgNP content. Catalyst mass, NaBH4 concentration, and type of catalyst were shown to have a significant influence on the conversion and rate constant. The material with the highest silver nanoparticle content was identified as the optimal catalyst for the reduction of both pollutants. The measured rate constants for the reduction of methylene blue (MB) and 4-nitrophenol (4-NP) were 164 × 10-4 s-1 and 25 × 10-4 s-1, respectively. The reduction of MB and 4-NP in the binary system showed high selectivity for MB dye, with rate constants of 64 × 10-4 s-1 and 9 × 10-4 s-1 for MB and 4-NP, respectively. The reuse of the best catalyst via MB dye reduction for four cycles showed good results without loss of performance.

Facile Synthesis of Natural Kaolin-Based CuO Catalyst: An Efficient Heterogeneous Catalyst for the Catalytic Reduction of 4-Nitrophenol

Water contamination by nitro compounds from various industrial processes has significantly contributed to environmental pollution and severely threatened aquatic ecosystems. Inexpensive, efficient, and environmentally benign catalysts are required for the catalytic reduction of such nitro compounds. This study reports the fabrication of various nanocomposites (NCs) of copper oxide nanoparticles (CuO NPs) supported on a kaolin sheet using straightforward and simple one-pot synthesis procedures that control the metal precursor to kaolin ratios. The selected as-synthesized CuO/kaolin NC was characterized using a range of advanced spectroscopic and microscopic methods, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), high-angle annular dark-field scanning TEM (HAADF-STEM), and N2 adsorption/desorption analysis. The characterization results confirmed the successful incorporation of CuO NPs into the kaolin sheets, which had an average size of about 18.7 nm. The fabricated CuO/kaolin NC was used as a heterogeneous catalyst for the efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride (NaBH4) in an aqueous system at room temperature. The catalyst demonstrated superior catalytic performance with high 4-NP conversion into 4-AP (>99%) in the aqueous phase (50 mL, 20 mg L-1) within 6 min. In addition, the reaction kinetics of 4-NP reduction was also investigated, and the reaction followed the pseudo-first-order kinetics equation with the apparent rate constant of 1.76 min-1. Furthermore, the Arrhenius and Eyring parameters for the catalytic hydrogenation reaction of 4-NP were calculated in order to investigate the catalytic reaction process in more detail. Moreover, the catalyst exhibited excellent reusability and stability over seven repeated catalytic test cycles without any noticeable decline in catalytic activity. Therefore, this paper could provide a novel, efficient, and environmentally promising clay-based non-noble metal oxide nanocatalyst to reduce nitro compounds in the aqueous system.

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.