Sabelo D. Mhlanga

UV-assisted reduction of in situ electrospun antibacterial chitosan-based nanofibres for removal of bacteria from water

A greener synthesis of low-swelling uniformly-sized chitosan (CTS)-based nanofibres decorated with silver (Ag) and silver/iron (Ag/Fe) nanoparticles is reported. The synthesis was achieved by electrospinning a solution of CTS blended with varying amounts of polyacrylamide (PAA), polyethylene glycol (PEG) and Ag+ or Ag+/Fe3+ ions. These nanofibres were subjected to UV irradiation under ionised water vapour at low temperature (70 °C). The effect of UV irradiation time on the reduction of the NPs was confirmed using UV-Vis spectroscopy. The microstructure and chemical composition of the Ag and Ag/Fe modified nanofibres was studied using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and ultraviolet-visible spectroscopy (UV-Vis). TEM revealed that the average diameter of the CTS-based nanofibres, AgNPs, and Ag/Fe NPs supported on the CTS-based nanofibres were 471 ± 89 nm, 18 ± 2.5 and 32 ± 8.7 nm respectively. XRD and EDS analysis confirmed the presence of Ag and Fe in the nanofibers. The biocidal effect of the Ag and Ag/Fe NPs supported on the CTS-based nanofibres was investigated using Gram positive (Bacillus cereus, Enterococcus faecalis) and Gram negative (Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Pseudomonas aeruginosa, Proteus mirabilis, Shigella boydii, Shigella sonnei, Enterobacter cloacae) bacterial strains. The nanofibres exhibited a strong biocidal effect on the bacteria suggesting that they can be used as efficient antimicrobial materials in water systems that are contaminated by bacteria.

UV-assisted synthesis of indium nitride nano and microstructures

Indium nitride (InN) has been made the first time by a combined thermal/UV photo-assisted process. Indium oxide (In2O3) was reacted with ammonia using two different procedures in which either the ammonia was photolysed or both In2O3 and ammonia were photolysed. A wide range of InN structures were made by these procedures that were determined by the reaction conditions (time, temperature). The reaction of In2O3 with photolysed NH3 gave InN rod-like structures that were made of stacked cones (6 h/750 °C) or discs (6 h/800 °C) and that contained some In2O3 residue. In contrast, photolysis of both In2O3 and NH3 gave InN nanowires and pure InN nanotubes filled with In metal (>90%). The transformation of the 3D In2O3 particles to the tubular 1D InN was monitored as a function of time (1–4 h) and temperature (700–800 °C); the product formed was very sensitive to temperature. The band gap of the In filled InN nanotubes was found to be 1.89 eV.

Kinetics, equilibrium, and thermodynamics of the sorption of bisphenol a onto N-CNTs-β-cyclodextrin and Fe/N-CNTs-β-cyclodextrin nanocomposites

We analysed the adsorptive behaviour of Fe/N-CNTs-β-CD nanocomposite in the removal of bisphenol A (BPA) from aqueous solution and identified the key influencing parameters. The Fe/N-CNTs-β-CD nanocomposite adsorbent was prepared by dispersing Fe uniformly on N-CNTs-β-CD using a microwave polyol method and characterized using Fourier transform infrared spectroscopy (FTIR), focused ion beam scanning electron microscopy (FIB-SEM), and energy-dispersive X-ray spectroscopy (EDS). The solution pH and temperature had minimal effect on sorption of BPA while the initial concentration and adsorbent mass affected the adsorption of BPA. No leaching of Fe into the water was observed; thus the nanocomposites were found suitable for use in water purification. From equilibrium isotherm studies, the Langmuir isotherm model gave the best description of the experimental data. The Langmuir monolayer adsorption capacities of BPA onto N-CNTs-β-CD and Fe/N-CNTs-β-CD are 38.20 mgċg-1 and 80.65 mgċg-1 at 298 K, respectively. Evidently, these adsorption capacity values gave an indication that uniform dispersion of Fe N-CNTs-β-CD prepared by the microwave polyol method enhances the adsorption of BPA. Meanwhile, the sorption kinetics of BPA onto Fe/N-CNTs-β-CD were best described by the pseudo-second-order model.

Efficient preparation of greener N-doped carbon nanotube composites for water treatment by the microwave polyol method

N-doped carbon nanotubes have unique structures and strong interactions with metal nanoparticles due to the presence of nitrogen. There is actually a need for nanoparticles to treat water, without leaching of toxic metals. Here, we synthesized nanocomposites by deposition of Ag and Fe nanoparticles on N-doped carbon nanotubes with a surface area of 52xa0m2/g and 2xa0% N content to form nanocomposites. Transmission electron microscopy (TEM) of the nanocomposites revealed that the best dispersion of the deposited nanoparticles was achieved by the microwave-assisted polyol method. The Ag and Fe nanoparticles were indeed monodispersed and uniformly distributed on the surface of the N-doped carbon nanotubes. Deposition could be achieved in 5xa0min. The wet impregnation and deposition–precipitation methods gave composites with agglomerated nanoparticles. We observed that leaching of Fe and Ag into water was also influenced by the preparation method. No leaching of nanoparticles was observed when the composites were prepared by the microwave polyol method. This synthesis is therefore efficient with less energy and time. The strong metal/N-doped carbon nanotube interactions render these composites suitable for use in water purification.

Facile Synthesis of Nitrogen Doped Graphene Oxide from Graphite Flakes and Powders: A Comparison of Their Surface Chemistry

Nitrogen-doped graphene oxide (NGO) nanosheets were prepared via a facile one-pot modified Hummers approach at low temperatures using graphite powder and flakes as starting materials in the presence of a nitrogen precursor. It was found that the morphology, structure, composition and surface chemistry of the NGO nanosheets depended on the nature of the graphite precursor used. GO nanosheets doped with nitrogen atoms exhibited a unique structure with few thin layers and wrinkled sheets, high porosity and structural defects. NGO sheets made from graphite powder (NGOp) exhibited excellent thermal stability and remarkably high surface area (up to 240.53 m2 ·g-1) compared to NGO sheets made from graphite flakes (NGOf) which degraded at low temperatures and had an average surface area of 24.70 m2 ·g-1. NGOf sheets had a size range of 850 to 2200 nm while NGOp sheets demonstrated obviously small sizes (460-1600 nm) even when exposed to different pH conditions. The NGO nanosheets exhibited negatively charged surfaces in a wide pH range (1 to 12) and were found to be stable above pH 6. In addition, graphite flakes were found to be more suitable for the production of NGO as they produced high N-doping levels (0.65 to 1.29 at.%) compared to graphite powders (0.30 to 0.35 at.%). This study further demonstrates that by adjusting the amount of N source in the host GO, one can tailor its thermal stability, surface morphology, surface chemistry and surface area.

Thermally and mechanically stable β-cyclodextrin/cellulose acetate nanofibers synthesized using an environmentally benign procedure

ABSTRACT Electrospun cyclodextrin (CD)-based nanofibers with capabilities to remove pollutants from water have been synthesized and characterized. The high-quality nanofibers presented here were synthesized in two simple steps that involved in-situ electrospinning of the nanofibers and all nanocomponents, followed by the reduction of silver (Ag+) and iron (Fe3+) ions to nanoparticles using an environmentally benign process that involved irradiation of the electrospun fibers using a tailor-made UV-equipped furnace at low temperatures. In the previously reported study it was observed that Ag and Fe nanoparticles effectively removed a range of different strains of Gram-negative and Gram-positive bacteria from water. As such, this study focused on improving the thermal and mechanical properties of the nanofibers prepared from polymer blends of β-CDs with cellulose acetate (CA) and small additions (2 wt%) of functionalized multiwalled carbon nanotubes (f-MWCNTs). The electrospinning parameters were varied to determine the optimum conditions for preparation of uniform non-beaded nanofibers. Bead-free and uniform nanofibers were obtained at a polymer concentration of 32% at the ratio of 1:1 β-CDs:CA, syringe injection flow rate of 0.7 mL h−1, 15 cm between the tip of the spinneret and the collector, and a voltage of 16 kV. The addition of f-MWCNTs was found to improve the tensile strength of the nanofibers by twofold, relative to nanofibers with no f-MWCNTs. The thermal degradation of the nanofibers was improved by a magnitude of 50°C. The study has shown that adding small amounts of f-MWCNTs improved the thermal stability and mechanical strength of the CD/CA nanofibers significantly.

Environmentally benign chitosan-based nanofibres for potential use in water treatment

Abstract Chitosan (CS)-based nanocomposite materials are highly prone to swelling when in contact with water. It is therefore essential to modify them to enhance their resistance to swelling, in order to be applicable in water treatment. In this study, the CS-based nanofibres were prepared using the electrospinning technique. The nanofibres were prepared from a polymer blend of CS, and other polymers (polyacrylamide (PAA) and polyethylene glycol (PEG)) added in small optimized quantities to enhance the ability to electrospun CS. Elastic polyisoprene (PIP) and functionalized multi-walled carbon nanotubes (f-MWCNTs) were incorporated in the electrospinnable solution blend of CS, PAA and PEG to reduce the swelling behaviour of the CS-based nanofibres and to improve their mechanical strength and thermal properties. PIP did not only improve the morphology of the resulting nanofibres but also reduced their swelling behaviour by twofold. The addition of f-MWCNTs was found to improve the tensile strength of the nanofibres by twofold, relative to nanofibres with no f-MWCNTs. The thermal degradation of the nanofibres was improved by a magnitude of 50°C. Antibacterial silver (Ag) and iron (Fe) nanoparticles (NPs) were embedded on the nanofibres for their possible use in disinfection processes. These NPs have demonstrated a potential to kill bacteria in water and, therefore, the prepared nanofibres can be used in disinfection water treatment processes with reduced swelling capacity.

Antimicrobial Properties of Chitosan-Alumina/f-MWCNT Nanocomposites

Antimicrobial chitosan-alumina/functionalized-multiwalled carbon nanotube (f-MWCNT) nanocomposites were prepared by a simple phase inversion method. Scanning electron microscopy (SEM) analyses showed the change in the internal morphology of the composites and energy dispersive spectroscopy (EDS) confirmed the presence of alumina and f-MWCNTs in the chitosan polymer matrix. Fourier transform infrared (FTIR) spectroscopy showed the appearance of new functional groups from both alumina and f-MWCNTs, and thermogravimetric analysis (TGA) revealed that the addition of alumina and f-MWCNTs improved the thermal stability of the chitosan polymer. The presence of alumina and f-MWCNTs in the polymer matrix was found to improve the thermal stability and reduced the solubility of chitosan polymer. The prepared chitosan-alumina/f-MWCNT nanocomposites showed inhibition of twelve strains of bacterial strains that were tested. Thus, the nanocomposites show a potential for use as a biocide in water treatment for the removal of bacteria at different environmental conditions.

Synthesis of PVDF ultrafiltration membranes supported on polyester fabrics for separation of organic matter from water

Polyvinylidene flouride (PVDF) membranes supported on non-woven fabrics (NWF) of polyester are reported. The PVDF membranes were fabricated using the phase inversion method followed by modification of the active top layer of the PVDF thin film by adding polyvinylpyrolidone (PVP) into the cast solution. A PVDF resin was used with N- methyl-2-pyrrolidone (NMP) as a solvent. Sessile drop contact angle measurements and scanning electron microscopy (SEM) were used to study the physical properties of the membranes. Membrane rejection of humic acid was studied using a cross-flow membrane testing unit. The contact angle results revealed that the hydrophilicity of PVDF membranes increased as the PVP concentration was increased from 3 to 10 wt%. SEM analysis of the membranes revealed that the membrane pore sizes increased when PVP was added. AFM analysis also showed that membrane roughness changed when PVP was added. Total organic carbon (TOC) analysis of water samples spiked with humic acid was performed to test the rejection capacity of the membranes. Rejections of up to 97% were achieved for PVDF membranes supported on polyester NWF1, which had smaller thickness and higher permeability compared to polyester NWF2. The NWFs provided the high strength required for the membranes despite the modifications done on the PDVF surface and microstructure.

Chitosan-Based Nanocomposite Beads for Drinking Water Production

Potable drinking water is essential for the good health of humans and it is a critical feedstock in a variety of industries such as food and pharmaceutical industries. For the first time, chitosan-alumina/functionalised multiwalled carbon nanotube (f-MWCNT) nanocomposite beads were developed and investigated for the reduction of various physico-chemical parameters from water samples collected from open wells used for drinking purposes by a rural community in South Africa. The water samples were analysed before and after the reduction of the identified contaminants by the nanocomposite beads. The nanocomposite beads were effective in the removal of nitrate, chromium and other physico-chemical parameters. Although, the water samples contained these contaminants within the WHO and SANS241 limits for no risk, the long-term exposure and accumulation is an environmental and health concern. The reduction of these contaminants was dependent on pH levels. At lower pH, the reduction was significantly higher, up to 99.2% (SPC), 91.0% (DOC), 92.2% (DO), 92.2% (turbidity), 96.5% (nitrate) and 97.7% (chromium). Generally, the chitosan-alumina/f-MWCNT nanocomposite beads offer a promising alternative material for reduction and removal of various physico-chemical parameters for production portable water.