Silver Nanoparticles Synthesised in Natural Rubber Latex Nanofibre for Heavy Metals removal in Water

Abstract

Water treatment methods, including adsorption by nanoparticles, is an efficient method for contaminant removal in wastewater. Nanoparticles used for water treatment incorporate nanoparticles in micro-matrices, which are non-biodegradable. However, removing remnant nanoparticles from treated water, which forms part of the contamination problem, is expensive. This research aims to synthesise silver nanoparticles (AgNP) in green nanofibre matrix using natural rubber latex (NRL) and polyvinyl alcohol (PVA). The efficiency of the nanofibre composites (AgNP-PVA/NRL) to remove Cd2+ and Hg from wastewater was determined. The NRL mixed with PVA at ratios of 40%, 50%, 60% and 70% PVA/NRL (w/v) were electrospun, using water as solvent. Optimum ratio at which the PVA/NRL nanofibre could be applied in aqueous processes without excess dissolution of the nanofibre was determined. Scanning electron microscopy (SEM) analyses revealed that decreasing PVA content produced relatively rougher but thinner fibres with smaller average pore diameters and mattings, and vice versa. Fourier-transform infrared spectroscopy (FT-IR) analysis and weight loss test by dissolution showed that decreasing PVA content decreased the fibre’s solubility. Silver nitrate (AgNO3) was added to PVA/NRL solution and electrospun to form 0.01 M AgNP and 0.015 M AgNP. SEM and energy dispersive x-ray spectroscopy (EDS) analyses revealed that most AgNP in 0.01 M AgNP were formed near or on the fibre surfaces, whilst that of 0.015 M were mainly within the fibre walls. Transmission electron microscopy (TEM) analysis revealed AgNP sizes were relatively smaller in 0.015 M AgNP (2.82 ± 0.04 nm) than in 0.01 M AgNP (8.15 ± 0.09 nm). X-ray diffractometry (XRD) analysis confirmed the presence of AgNP, and FT-IR analysis showed that amine group in NRL and hydroxyl group in PVA reduced Ag+ to Ago , whilst cis-isoprene (in NRL) stabilised AgNP in the solid matrix. Maximum uptake of Hg occurred at pH 7 and around 60 min for both 0.01 M (40.92 mg/g) and 0.015 M AgNP (19.48 mg/g). Maximum uptake of Cd2+ occurred at pH 7 but at different reaction times of 20 min for 0.01 M AgNP (14.97 mg/g) and 40 min for 0.015 M AgNP (30.11 mg/g). It was also revealed that Hg adsorption was relatively better when the AgNPs were larger and near or on the surface (0.01 M AgNP), forming a monolayer (Langmuir isotherm). Mercury adsorption in 0.015 M AgNP followed the Dubinin-Radushkevich (D-R) isotherm and the Elovich kinetic model, indicating chemisorption, with 0.01 M AgNP having a larger boundary layer (C = 9.9354 mg/g) and smaller desorption constant (β = 0.1217 g/mg). It was revealed that Cd2+ adsorption was relatively better when the AgNPs were relatively more (0.015 M AgNP), and the adsorption behaviour of both nanofibre composites followed Freundlich isotherm. Generally, the nanofibre composites adsorbed Hg better than Cd. In the binary system, the nanofibre composites adsorbed Hg better than Cd, except in 0.015 M AgNP. Measured Ag concentrations (after adsorption experiments) were 0.04 mg/L (0.01 M AgNP) to 0.21 mg/L (Mix 2), all below US EPA SMCL and WHO drinking water guidelines of 0.10 mg/L except for Mix 2. This shows that green synthesised AgNP-PVA/NRL nanofibre composites are good enough for wastewater treatment