Browsing by Author "Silwana, Bongiwe"

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    Graphene supported antimony nanoparticles on carbon electrodes for stripping analysis of environmental samples
    (University of the Western Cape, 2015) Silwana, Bongiwe; Somerset, V.S.; Iwuoha, Emmanuel
    Platinum Group Metals (PGMs), particularly palladium (Pd), platinum (Pt) and rhodium (Rh) have been identified as pollutants in the environment due to their increased use in catalytic converters and mining in South Africa (as well as worldwide). Joining the continuous efforts to alleviate this dilemma, a new electrochemical sensor based on a nanoparticle film transducer has been developed to assess the level of these metals in the environment. The main goal of this study was to exploit the capabilities of nanostructured material for the development and application of an adsorptive stripping voltammetric method for reliable quantification of PGMs in environmental samples. In the study reported in this thesis, glassy carbon electrode (GCE) and screen-printed carbon electrode (SPCE) surfaces were modified with conducting films of nanostructured reduced graphene oxide-antimony nanoparticles (rGO-SbNPs) for application as electrochemical sensors. The rGO-SbNPs nanocomposite was prepared by Hummer`s synthesis of antimony nanoparticles in reaction medium containing reduced graphene oxide. Sensors were constructed by drop coating of the surfaces of the carbon electrodes with rGO-SbNPs films followed by air-drying. The nanocomposite material was characterised by: scanning and transmission electron miscroscopies; FTIR, UV-Vis and Ramanspectrosocopies; dc voltammetry; and electrochemical impedance spectroscopy. The real surface area of both electrodes were studied and estimated to be 1.66 × 10⁶ mol cm⁻² and 4.09 × 10³ mol cm⁻² for SPCE/rGO-SbNPs and GCE/rGO-SbNPs, respectively. The film thickness was also evaluated and estimated to be 0.36 cm and 1.69 × 10⁻⁶ cm for SPCE/rGO-SbNPs and GCE/rGO-SbNPs, respectively. Referring to these results, the SPCE/rGO-SbNPs sensor had a better sensitivity than the GCE/rGO-SbNPs sensor. The electroanalytical properties of the PGMs were first studied by cyclic voltammetry followed by indepth stripping voltammetric analysis. The development of the stripping voltammetry methodology involved the optimisation of experimental conditions such as selection of adequate supporting electrolyte, choice of pH and /or concentration of supporting electrolytes, deposition potential, deposition time, stirring conditions. The detection of Pd(II), Pt(II) and Rh(III) in environmental samples were performed SPCE/rGO-SbNPs and GCE/rGO-SbNPs at the optimised experimental conditions For the GCE/rGO-SbNPs sensor, the detection limit was found to be 0.45, 0.49 and 0.49 pg L⁻¹ (S/N = 3) for Pd(II), Pt(II) and Rh(III), respectively. For the SPCE/rGO-SbNPs sensor, the detection limit was found to be 0.42, 0.26 and 0.34 pg L⁻¹ (S/N = 3) for Pd(II), Pt(II) and Rh(III), respectively. The proposed adsorptive differential pulse cathodic stripping voltammetric (AdDPCSV) method was found to be sensitive, accurate, precise, fast and robust for the determination of PGMs in soil and dust samples. The simultaneous determination of PGMs was also investigated with promising results obtained. The AdDPCSV sensor performance was compared with that of inductive coupled plasma mass spectroscopy (ICP-MS) for the determination of PGM ions in soil and dust samples. It was found that though the metals could be determined by ICP-MS technique, it was limited from the standpoints of sensitivity, ease of operation and versatility compared to the AdDPCSV sensor. This study has show cased the successful construction and application of novel SPCE/rGO-SbNPs and GCE/rGO-SbNPs AdDPCSV sensors forthe determination of PGMs in environmental samples (specifically roadside dust and soil samples). The study provides a promising analytical tool for monitoring PGMs pollutants that are produced by automobiles and transported in the environment.
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    Heavy and precious metal toxicity evaluation using a horseradish peroxidase immobilised biosensor
    (University of the Western Cape, 2012) Silwana, Bongiwe; Somerset, V.S.; Iwuoha, Emmanuel
    Environmental pollution is always the hottest topic in public conversation and one of the most concerned aspects of human health. The thin film sputtered microelectrode devices have been developed to improve the quality of human health, by offering better monitoring capabilities. This thesis is divided into three parts and the studies were performed on chemical sensor technology currently available and under development using modified methods. In the first part of this thesis: (i) the studies are related to synthesis, characterization and polymerisation of polyaniline (PANI) and polyaniline-co-poly(2,2´-dithiodianline) (PANI-co-PDTDA). Polyaniline (PANI) and the copolymer of aniline with dithiodianiline, an aniline derivative containing S-S-links were of interest in polymer synthesis. Electrochemical synthesis was carried out in 1 M HCl and different concentrations of H2SO4 (1, 2.5, and 5 M) solutions for PANI and PANI-co-PDTDA respectively. The PANI and PANI-co-PDTDA were grown electrochemically on the surface of a glassy carbon electrode (GCE) by repetitive cyclic voltammetric scanning. Cyclic voltammetry (CV) was used to evaluate the differences between the electrochemical characteristics associated with growth of the copolymer and homopolymer, polyaniline (PANI). The surface concentration of PANI was estimated to be 2.64 × 10-1 mol.cm-2 while the film thickness was estimated to be 7.09 × 10-10cm and 1.49 × 10-9cm for scan rate and aquare root scan rate. In contrast, PANI-co-PDTDA concentrations (1, 2, 5 and 5 M H2SO4 solutions) gained a surface concentration (G) falling in the range 6.1 x 10-2 - 7.9 x 102 mol.cm-2 and a film thickness in the range 8.16 x 10 -9- 2.05x10-8cm. The second section of this thesis focused on the development of two sensors, Pt/PANI/HRP and Pt/PANI-co-PDTDA/HRP biosensors. The biosensor described in this chapter focus on the use of horseradish peroxidise (HRP) with hydrogen peroxide as substrate, was constructed with the aim of further investigation of inhibition by heavy metals (Cd2+, Pb2+ and Hg2+). To achieve this, the enzyme HRP as the catalytic bio-element, was immobilised on the surface of a platinum electrode with PANI as a mediator. Immobilisation of HRP in conducting polymer matrices of PANI and PANI-co-PDTDA were achieved by electrochemical polymerisation. The use of amperometric detection allowed for the coupling of the biosensor with a portable potentiostat system (PalmSens). Differential pulse voltammetry (DPV) as technique was used as a detection method for inhibition determination. Selection of suitable pH values for biosensor performance was evaluated and the system showed optimal performance at pH 6.8 and 7.2 for Pt/PANI/HRP and Pt/PANIco- PDTDA/HRP biosensors, respectively. The biosensors developed in this work showed detection limits (LODs) of 0.32 mM and 0.0483 mM for PANI/HRP and PANI-co- PDTDA/HRP, respectively. For the Pt/PANI/HRP biosensor, the apparent Michaelis-Menten constant (Km app) value and maximum current (Imax) were evaluated from Lineweaver-Burk plots at various H2O2 concentrations. The values were found to be 0.6 mM and 1.7 μA for the Pt/PANI/HRP biosensor, while for the Pt/PANI-co-PDTDA/HRP biosensor the results were 0.7 mM and 0.27 μA, respectively. The third section investigated the adsorptive cathodic differential pulse stripping voltammetric (AdDPSV) determination of platinum group metals (PGMs), using an ex situ bismuth coated screen printed carbon electrode (SPCE/Bi) as the working electrode and ammonium buffer solution (pH = 9.2) as the supporting electrolyte. The cathodic stripping differential pulse method was used for investigating the electrochemical behaviour and the quantitative analysis of platinum group metals (Pt, Pd and Rh) at the SPCE/Bi surface in the presence of dimethylgloxime (DMG) as a complexing agent. In order to determine the metals at improved detection limits ensuring repeatability and sensitivity, a complete optimization study of voltammetric parameters was performed. The proposed method was successfully applied to the determination of the real samples (sediments & water) collected in the platinum mining area in the North-West and Limpopo Provinces, South Africa. The results were compared with those obtained by the glassy carbon bismuth film (GC/BiF) voltammetric and ICP-AES spectrometry techniques. Well-shaped voltammograms with clear peak potentials were obtained in the analysis of the real samples, offering excellent perspectives on the use of the constructed modified electrodes. The calibration curves for all PGMs investigated were linear with the limit of detection (LOD) at approximately 0.008, 0.006, and 0.005 μg.L-1 for Pd, Pt and Rh, respectively.
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    Spectroscopic and voltammetric analysis of platinum group metals in road dust and roadside soil
    (MDPI, 2018) van der Horst, Charlton; Silwana, Bongiwe; Iwuoha, Emmanuel; Somerset, Vernon
    The emission of toxic compounds by increasing anthropogenic activities affects human health and the environment. Heavy road traffic and mining activities are the major anthropogenic activities contributing to the presence of metals in the environment. The release of palladium (Pd), platinum (Pt), and rhodium (Rh) into the environment increases the levels of contamination in soils, road sediments, airborne particles, and plants. These Pd, Pt, and Rh in road dusts can be soluble and enter aquatic environment posing a risk to environment and human health. The aim of this study is to determine the levels of Pd, Pt, and Rh with spectroscopy and voltammetric methods. Potential interferences by other metal ions (Na(I), Fe(III), Ni(II), Co(II)) in voltammetric methods have also been investigated in this study. At all the sampling sites very low concentrations of Pd, Pt, and Rh were found at levels that range from 0.48 ± 0.05 to 5.44 ± 0.11 ng/g (dry weight (d.wt)) for Pd(II), with 17.28 ± 3.12 to 81.44 ± 3.07 pg/g (d.wt) for Pt(II), and 14.34 ± 3.08 to 53.35 ± 4.07 pg/g (d.wt) for Rh(III). The instrumental limit of detection for Pd, Pt, and Rh for Inductively Coupled Plasma Quadrupole-based Mass Spectrometry (ICP-QMS) analysis was found to be 3 × 10−6 µg/g, 3 × 10−6 µg/g and 1 × 10−6 µg/g, respectively. In the case of voltammetric analysis the instrumental limit of detection for Pd(II), Pt(II), and Rh(III) for differential pulse adsorptive stripping voltammetry was found to be 7 × 10−8 µg/g, 6 × 10−8 µg/g, and 2 × 10−7 µg/g, respectively. For the sensor application, good precision was obtained due to consistently reproduced the measurements with a reproducibility of 6.31% for Pt(II), 7.58% for Pd(II), and 5.37% for Rh(III) (n = 10). The reproducibility for ICP-QMS analysis were 1.58% for Pd(II), 1.12% for Pt(II), and 1.37% for Rh(III) (n = 5). In the case of repeatability for differential pulse adsorptive stripping voltammetry (DPAdSV) and ICP-QMS, good standard deviations of 0.01 for Pd(II); 0.02 for Pt(II), 0.009 for Rh(III) and 0.011 for Pd, 0.019 for Pt and 0.013 for Rh, respectively.
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    Voltammetric and spectroscopic determination of rare earth elements in fresh and surface water samples
    (MDPI, 2018) Makombe, Martin; van der Horst, Charlton; Silwana, Bongiwe; Iwuoha, Emmanuel; Somerset, Vernon
    The increasing demand for rare earth elements in green technology, electronic components, petroleum refining, and agricultural activities has resulted in their scattering and accumulation in the environment. This study determined cerium, lanthanum and praseodymium in environmental water samples with the help of adsorptive differential pulse stripping voltammetry (AdDPSV) and inductive coupled plasma-optical emission spectroscopy (ICP-OES). A comparison of the results of these two analytical techniques was also made. The accuracy and precision of the methods were evaluated by spiking water samples with a known amount of REEs. The detection limit obtained for the stripping analysis was 0.10 g/L for Ce(III), and 2.10 g/L for combined La(III) and Pr(III). The spectroscopic method of determination by ICP-OES was applied to the same samples to evaluate the effectiveness of the voltammetry procedure. The ICP-OES detection limit obtained was 2.45, 3.12 and 3.90 g/L for Ce(III), La(III) and Pr(III), respectively. The results obtained from the two techniques showed low detection limits in voltammetry; the ICP-OES method achieved better simultaneous analysis. This sensor has been successfully applied for the determination of cerium, lanthanum, and praseodymium in environmental water samples, offering good results.