Browsing by Author "Ndwabu, Sinayo."

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Comparison of extraction methods efficiency for the extraction of polycyclic aromatic hydrocarbons and phenolics in water matrices, sludge and sediment: sources of origin and ecological risk assessment.
(2023) Ndwabu, Sinayo.; Mahlambi, Precious Nokwethemba.; Malungana, Mncedisi.
Polycyclic aromatic hydrocarbons (PAHs) and phenolic compounds (PCs) are persistent and environmentally toxic compounds. This study therefore aimed to determine the levels of both PAHs and PCs in river water, wastewater, sludge and sediment samples. The evaluation of their origin source and ecological risk was also determined. The status of both these contaminants in South African environment is still not fully investigated, which is a gap this study intended to fill together with previous studies that have been carried-out. The PAHs and PCs were extracted using different extraction methods which include a solid phase extraction (SPE) and dispersive liquid-liquid micro-extraction (DLLME) in water matrices. The microwave assisted extraction (MAE) and Ultrasonication (UE) coupled with either filtering (F) or F + SPE as a clean-up technique was used for extraction of solid samples. The analytes extracted form water or sediment were determined using GC-MS. The PAH %recoveries obtained under optimum conditions in liquid samples were determined to be 72.1 - 118% for SPE and 70.7 – 88.4% for DLLME while the LOD and LOQ were 5.00 – 18.0 ng/L and 10.0 – 44.0 ng/L for SPE while they were 6.00 – 20.0 ng/L and 11.0 – 63.0 ng/L for DLLME. The recovery test for PAHs in solid samples gave a range of 93.7% - 121% for UE and 79.6% - 122% for MAE while the LOD and LOQ ranged from 0.0250 μg/kg to 1.21 μg/kg & 0.0800 μg/kg to 3.54 μg/kg for MAE and from 0.0840 μg/kg to 0.215 μg/kg & 0.0190 μg/kg to 0.642 μg/kg for UE respectively. The LOD and LOQs for PCs in both water and solid matrices were 0.01 – 2.00 μg/L and 0.02 – 6.07 μg/L for SPE, 0.05 – 1.20 μg/kg and 0.17 – 3.17 μg/kg for MAE and 0.09 – 1.33 μg/kg and 0.26 - 3.54 μg/kg for UE correspondingly, their %recovery test gave ranges of 75.2 – 112% (SPE), 80.9 – 110% (MAE) and 79.3 – 119% (UE).The optimization and validation of these methods indicated that they can be used for the extraction of PAHs or PCs in liquid samples, however, SPE when compared to DLLME showed to be more accurate and sensitive. Moreover, in solid samples the clean-up method was a deciding factor, with F + SPE cleaned samples giving higher concentrations of both PCs and PAHs than the filtered ones in both MAE and UE. The concentrations of PAHs ranged from nd (not detected) to 1046 ng/L in river water and nd to 778 ng/L in wastewater samples with naphthalene showing dominance over all other PAHs in both water matrices. The PC concentrations at 4.12 to 1134 μg/L for wastewater and nd to 98.0 μg/L for river water were high but still within the maximum allowable limit except for 2.4-DCP (2.4 dichlorophenol) at Wdv4. The concentrations obtained from F + SPE cleaned samples were higher for both PAHs and PCs with a range from 95.96 to 926.0 μg/kg and 1.30 to 310 μg/kg compared to concentrations from filtered only samples at 21.61 to 380.6 μg/kg and 0.90 to 266 μg/kg respectively. Pyrene showed dominance over all other PAHs in both sludge and sediments while 2.4-DCP and PCP dominated the sludge and sediment samples respectively. PAHs were determined to be of petrogenic (water matrices) and pyrolytic (solid samples) origin and on average posed low (water matrices) and a medium to high (solid matrices) ecological risk. The ILCRderm values at 4.98 x 10-1 and 2.62 x 10-1 (DahA) and 5.92 x 10-2 and 5.34 x 10-2 (PCP) were highfor adults compared to that of children at 1.92 x 10-1 and 1.01 x 10-1 (DahA) and 1.39 x 10-2 and 1.26 x 10-2 (PCP) for both sediment and sludge samples respectively. The low values of ILCRderm for children indicates that the have a high risk exposure even at low concentrations of the contaminants. The findings of this study showed that both areas (uMsunduzi river and Darvill wastewater works (WWW) of interest are polluted with PAHs and PCs therefore, more regulations such as the National Environmental Management: Waste Act (NEMWA) are needed to ensure environmental, human and animal safety.
Metallic nanoparticle-graphene quantum dot nanocomposites for the electrochemical detection of methyl parathion.
(2017) Ndwabu, Sinayo.; Nombona, Nolwazi.
Metallic nanoparticle-graphene quantum dot nanocomposites for the electrochemical detection of methyl parathion.
(2017) Ndwabu, Sinayo.; Nombona, Nolwazi.
This work reports on the detection of methyl parathion (MP) on electrodes constructed or decorated with graphene based nanomaterials. The sensing nanomaterials used include graphene quantum dots (GQDs), metal oxide [(MO): Cu2O, NiO, Al2O3 and MnO2] and metal hydroxide [(MOH): Cu(OH)2, Ni(OH)2, Al(OH)3 and Mn(OH)2] nanoparticles (NPs). The nanomaterials were synthesized using prescribed procedures. Characterisation of the materials was achieved using various techniques including transmission electron microscope (TEM) and scanning electron microscope (SEM). The MO NPs sizes were determined by TEM to be between 31 nm and 70 nm whereas the MO NPs sizes were found to be smaller with particle sizes ranging between 20 nm and 55 nm. The nanocomposites of GQDs/MO and GQDs/MOH were synthesized and characterised using SEM. SEM revealed that the MO and MOH nanoparticles were well-dispersed on the surface as well as within the graphene quantum dot sheets. The following electrodes were prepared: a graphene quantum dot paste electrode (GQDPE) decorated with either MO or MOH NPs and a glassy carbon electrode (GCE) modified with either GQDs/MO or GQDs/MOH nanocomposites. The electrochemical characterisation of these electrodes revealed that faster electron transfer kinetics occurred at the GQDPE. The ability of the modified electrodes to electrochemically detect MP was evaluated using cyclic voltammetry and the results revealed that the modified GQDPE did not exhibit any electrocatalytic performance. However, the GCE modified with the GQDs/MOH nanocomposite showed the best catalytic activity with lower detection limits compared to GQDs/MO modified electrodes. Electrochemical characterisation further revealed that amongst all of the metal based nanocomposites, GQDs/Cu(OH)2 exhibited the best catalytic activity with the highest sensitivity towards MP. This work demonstrated for the first time that these sensing nanomaterials have a favourable catalytic behaviour, ideal for the detection of organophosphate pesticides.