Biosynthesised bentonite clay-based Cu nanocomposite using Punica granatum L. peel extract for the effective catalytic reduction of hazardous aqueous contaminants

Abstract

ABSTRACT This article describes a facile and green one-pot synthetic process of Cu nanoparticles loaded on bentonite composite (5Cu wt.%/bentonite) using the peel extract of pomegranate (Punica granatum L.) as a reductant. The 5Cu wt.%/bentonite composite was adopted as a heterogeneous catalyst for the NaBH4-assisted reductive degradation of Methylene Blue (MB) dye. The prepared 5Cu wt.%/bentonite composite demonstrated high catalytic efficiency in reducing MB, achieving almost complete reduction within a reaction duration as short as 60 seconds. Indeed, Cu nanoparticles act as electron relays, facilitating the electron transfer from BH4 − anions to MB dye, leading to its conversion into the colourless Leuco-Methylene Blue (LMB). The reduction rate corresponded nicely with the pseudo-first-order kinetic model, with the corresponding normalised rate constant (Knor) value computed as 1.48 min−1 mg−1, outperforming numerous previously documented metal-containing catalysts. Remarkably, the catalyst can straightforwardly be reused, with almost constant reduction efficiency observed across 5 successive catalytic cycles. Furthermore, 5Cu wt.%/bentonite was used in another catalytic reduction reaction, involving the reduction of the toxic Cr(VI) to environmentally benign Cr(III) adopting formic acid as a reducing agent under ambient conditions. The 5Cu wt.%/bentonite catalyst not only showed a high Cr(VI) reduction efficiency of 95.5% within 30 min but also outstanding reusability of 93% removal of Cr(VI) following 5 cycles, demonstrating its catalytic versatility. The Cr(VI) reduction mechanism involves the dehydrogenation of formic acid on the catalyst surface, followed by hydrogen transfer converting Cr(VI) into Cr(III). Based on the straightforward and green synthetic strategy, use of cost-effective and readily available raw materials, excellent catalytic activity, catalytic versatility, and outstanding reusability, the 5Cu wt.%/bentonite composite seemed to be an ideal catalyst for the real-world reduction of harmful aqueous pollutants to achieve environmental sustainability.