Edward N. Nxumalo

Nitrogen Doped Carbon Nanotubes from Organometallic Compounds: A Review

Nitrogen doped carbon nanotubes (N-CNTs) have become a topic of increased importance in the study of carbonaceous materials. This arises from the physical and chemical properties that are created when N is embedded in a CNT. These properties include modified chemical reactivity and modified conductivity and mechanical properties. A range of methodologies have been devised to synthesize N-CNTs. One of the procedures uses a floating catalyst in which an organometallic complex is decomposed in the gas phase in the presence of a nitrogen containing reactant to give N-CNTs. Most studies have been limited to ferrocene, ring substituted ferrocene and Fe(CO)5. This review covers the synthesis (and properties) of N-CNTs and other shaped carbon nanomaterials (SCNMs) produced using organometallic complexes. It summarizes the effects that physical parameters such as temperature, pressure, gas flow rates, type and concentration of N source etc. have on the N-CNT type, size and yields as well as the nitrogen content incorporated into the tubes that are produced from organometallic complexes. Proposed growth models for N-CNT synthesis are also reported.

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.

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.

The Occurrence and Diversity of Waterborne Fungi in African Aquatic Systems: Their Impact on Water Quality and Human Health

Currently, there is a worldwide growing interest in the occurrence and diversity of fungi and their secondary metabolites in aquatic systems, especially concerning their role in water quality and human health. However, this concern is hampered by the scant information that is available in the literature about aquatic fungi and how they affect water quality. There are only few published reports that link certain species of aquatic fungi to human health. The common aquatic fungal species that have been reported so far in African aquatic systems belong to the hyphomycetes kingdom. This paper thus aims to survey the information about the occurrence and factors that control the distribution of different species of fungi in African aquatic systems, as well as their effect on water quality and the possible metabolic pathways that lead to the formation of toxic secondary metabolites that are responsible for the deterioration of water quality. This review will also investigate the analytical and bioanalytical procedures that have been reported for the identification of different species of waterborne fungi and their secondary metabolites.

Enhanced solar light photodegradation of brilliant black bis-azo dye in aqueous solution by F, Sm3+ codoped TiO2

This research focuses on improving the photocatalytic efficiency of TiO2 during the photo-mineralisation of brilliant black (BN) bis-azo dye pollutant in aqueous solution. This was achieved by improving the visible light activity of TiO2 photocatalyst semiconductor through co-doping of fluorine (F) and trivalent samarium ions (Sm3+) into a TiO2 matrix using a modified sol-gel synthesis method. Structural, morphological, and textural properties were evaluated using ultra-violet /visible spectroscopy (UV-visible), Raman spectroscopy, scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction spectroscopy (XRD). Photocatalytic and degradation efficiencies were assessed by decolourisation of BN dye in aqueous solution. Complete degradation of BN was attained after an irradiation time of 3 h using F, Sm3+-TiO2 (0.6% Sm3+) compared to 73.4% achieved using pristine TiO2. Pseudo first order kinetics rate constants (Ka) were 2.73×10-2 and 6.6×10-3 min-1 for Sm3+-TiO2 (0.6%Sm3+) and pristine TiO2, respectively, which translates to a remarkably high enhancement factor of 4. The results obtained established that doping of TiO2 by F and Sm3+ enhances the photocatalytic performance of TiO2 during solar light radiation which enables the utilisation of freely available and clean solar energy.

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.