(182g) Silver Nanoparticles Immobilized on Heat Treated Candle Soot for the Efficient Reduction of Aqueous Nitroarenes and Azo Dyes

The imprudent use and disposal of toxic chemicals into the environment in the name of global industrialization is creating a massive ecological crisis. Toxic nitroaromatic compounds like para-nitrophenol (4-NP), para-nitroaniline (4-NA) and meta-dinitrophenol (2,4-DNP) are used as precursors across various industries for the production of pharmaceuticals, pesticides and fungicidal agents, dyes and rubber, explosives and industrial solvents. Synthetic azo dyes like Methylene Blue (MB) and Congo Red (CR) are found in effluents of industries like textiles, paper and pulp and paints and varnish. Dyes and nitro-substituted organic compounds are carcinogenic and cytotoxic to humans and can cause dermal irritation, kidney inflammation and liver cirrhosis. Due to the presence of a nitro group, nitroaromatics are highly water soluble, recalcitrant and stable towards biodegradation and other conventional wastewater treatment methods. Noble metal-catalysed reduction of nitroaromatic compounds to their corresponding environmentally benign aniline derivatives is tone of the only useful and viable routes for environmental remediation of nitroarenes.

A facile in-situ method for preparing stable Ag nanoparticles (average diameter ≈28 nm), decorated on heat-treated candle soot (cs-HT) is hereby reported. The synthesised Ag@cs-HT nano-catalyst with ~2% wt./wt. Ag loading was thereafter investigated for its effectiveness towards catalytic reduction of aqueous nitroarenes like 4-NP, 4-NA and 2,4-DNP and azo dyes like MB and CR using sodium borohydride (NaBH₄) as the reducing agent under ambient conditions. The effect of various process parameters like NaBH₄ dosage, pH and temperature on the efficiency of reduction were investigated. The catalytic reduction followed pseudo first-order kinetics for all nitroarenes and dyes, with 4-nitrophenol achieving the highest degree of reduction (~99.96%) within 5 minutes at 35 °C, solution pH 3 and an optimized nano-catalyst dose of 0.08 g/L and NaBH₄ dosage of 1.8 mM. The detection of H^• radical adducts indicates that Ag nanoparticles adsorb BH₄^− ions from solution to form Ag-H species. The enhanced removal capability of Ag@cs-HT was attributable to the high specific surface area and hydrophilicity of cs-HT (S_BET=152.3 m²/g, average contact angle=23.4°) which facilitates rapid adsorption of BH₄^− ions and reactant molecules, followed by electron transfer from the active Ag-H complex and subsequent reduction of the adsorbed reactant species. The presence of anions like SO₄^2-, CO₃^2- and NO₃^− offered marginal resistance to the reduction process, except for Cl^- which anomalously enhanced it, and PO₄^3- which drastically reduced the reduction efficiency. The synthesized Ag@cs-HT nano-catalyst was found to reusable for at least 8 consecutive cycles for reduction of 4-nitrophenol, without significant loss of catalytic activity and minimal leaching of Ag. The aforementioned outcomes clearly establish Ag@cs-HT as an effective catalyst for removal of highly toxic nitroarenes and azo dyes from aqueous solutions.