Green synthesis of silver nanoparticles from waste leaves of tea (Camellia sinensis) and their catalytic potential for degradation of azo dyes

Abstract

Tea leaves are discarded after use as waste material. An attempt was made to explore the potential of tea waste as an alternative source of extensive reducing agent for green and biocompatible synthesis of silver nanoparticles (WT-AgNPs) from silver nitrate salt, and prospective utilization for degradation of toxic azo dyes such as methyl orange and Congo red occurring in effluents. Green tea extract was also used as reducing agent to synthesize AgNPs (GT-AgNPs) to compare their physicochemical properties and efficiency vis-à-vis WT-AgNPs. Both WT-AgNPs and GT-AgNPs were characterized by Ultraviolet-Visible (UV) and Fourier Transform Infrared (FTIR) Spectroscopy, particles size and zeta potential measured by Dynamic Light Scattering (DLS), surface morphology and elemental composition determined by Field Emission Scanning Electron Microscopy (FESEM). The average particle size, zeta potential and surface plasmon resonance (SPR) were 74.85 nm, -22.62 mV, 422 nm and 69.53 nm, -21.28 mV, 420 nm for WT-AgNPs and GT-AgNPs, respectively. Catalytic degradation reaction of methyl orange and Congo red using the biogenic AgNPs followed the pseudo-first order kinetics in both cases. The rate constants of methyl orange dye (100 ppm) degradation using the catalysts (WT-AgNPs and GT-AgNPs) were 0.087 s−1(R2=0.922) and 0.099 s−1 (R2=0.963) respectively, showing 93 % and 95.5 % degradation of dye in 40 min in presence of NaBH4. For Congo red (100 ppm) degradation, the rate constants were 0.112 s−1 (R2=0.917) and 0.113 s−1(R2=0.943). Both WT-AgNPs and GT-AgNPs exhibited bacteriostatic effect inhibiting growth of E. coli. The study offers potential use of waste tea leaves as an effective reducing agent for large-scale green synthesis of silver nanoparticles having multifarious environmental applications in general, dye degradation and antibacterial action in particular.