Ultrafast catalytic reduction of toxic nitroaromatics and organic colouring dyes by using Au/ZIF-11: Efficient wastewater treatment

Abstract

An ultrafast catalytic method to reduce various toxic nitrophenols, nitroaniline and organic dyes by using gold nanoparticles decorated-ZIF-11 (Au/ZIF-11) composite for wastewater treatment has been described. Three new Au/ZIF-11 composites (abbreviated as Au0.1Z, Au0.2Z and Au0.3Z) have been synthesized via direct reduction of HAuCl4 (0.1 to 0.3 mM) using sodium borohydride (NaBH4) in the suspension of pre-synthesized ZIF-11 and characterized by various spectroscopic and morphological/surface techniques. TEM images confirmed the formation of spherical and uniformly distributed gold NPs, having average particle size of 7.35 ± 0.95 nm, on the surface of ZIF-11 microcrystals. Au0.2Z displayed exceptionally high catalytic efficiency towards the reduction of nitrophenols (2-NPhenol, 3-NPhenol and 4-NPhenol), 4-nitroaniline (4-NANI) and 2,4,6-triNPhenol at substrate's concentration: 1.58 × 10−4 M in the presence of NaBH4 (0.04 M) at ambient condition. k̅app, (apparent rate constant) was (4.64/3.24/1.90) × 10−2 s−1 having activity coefficient k′ (k′ = k̅app/M) value of 928.83, 647.59 and 380.61 s−1 g−1 for 3-NPhenol, 4-NPhenol and 2-NPhenol, respectively, using Au0.2Z ([catalyst] = 15.87 μg/mL). Further, Au0.2Z can be re-used up to 5 cycles with more than 90% conversion efficiency. Moreover, Au0.2Z also reduced organic colouring dyes (methylene blue (MB), rhodamine B (RhB), congo red (CR) and methyl orange (MO)) in the presence of NaBH4 and the corresponding k̅app was 7.88 × 10−3–9.22 × 10−4 s−1 with k′ value of 157.71–9.22 s−1 g−1. The probable mechanism involving Langmuir-Hinshelwood apparent first-order kinetics model described that Au nanoparticles immobilized onto ZIF-11 may serve as antenna to transfer electrons and hydrogen from borohydride (BH4−) ions to the acceptor (nitro group/organic dye). Therefore, Au0.2Z can effectively be used for environmental remediation application to remove organic pollutants from the industrial wastewater.