https://ir.unisa.ac.za/handle/10500/2738 Tue, 23 Jun 2026 02:39:02 GMT 2026-06-23T02:39:02Z https://ir.unisa.ac.za/handle/10500/32566 Nickel-manganese phosphate/electrochemical exfoliated graphene nanocomposites for hybrid supercapacitors Kgwadibane, Tshupo The global shift toward sustainable energy technologies has increased the demand for advanced energy storage systems that deliver both high energy and power densities. Conventional supercapacitors provide excellent power output, rapid charge-discharge capability, and superior cycling stability; however, their low energy density restricts practical applications. In contrast, rechargeable batteries offer higher energy densities but are limited by slower charge rates and reduced cycling stability. Hybrid configurations that integrate the complementary advantages of both systems have therefore emerged as a promising approach to achieving balanced performance. Nickel-manganese phosphate/electrochemically exfoliated graphene NiMn(PO4)2/EEG) nanocomposite was synthesized via a hydrothermal method. Structural and morphological analyses, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), high-resolution scanning electron microscopy (HRSEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, confirmed the formation of well well-crystallised composite with homogeneous morphology. The NiMn(PO4)2/EEG composite leverages the multiple redox-active sites and structural robustness of bimetallic phosphate combined with the high conductivity of electrochemically exfoliated graphene, resulting in enhanced charge transport efficiency and electrochemical performance. Additionally, this work presents an investigation of heteroatom-doped EEG thin film synthesised via a two-step process involving graphite intercalation and electrochemically exfoliation in sulphuric-phosphoric acid medium, enabling in-situ doping with nitrogen (N), phosphorus (P), and sulphur (S). The exfoliation products were vacuum filtered to form porous films. Raman spectroscopy revealed Fermi-level shifts of approximately 0.5 eV and heterogeneous defect distributions. The doped EEG films exhibited enhanced electrical conductivity (~10,000 S·m-1) and improved interfacial properties, as evidenced by reduced adhesion forces in force-distance measurements. Electrochemical analyses demonstrated that Fermi-level modulation facilitated rapid interfacial charge transfer by lowering the electrode-electrolyte potential barrier. The doped EEG films achieved a specific capacitance of 150.5 F·g-1 at 1.0 A·g-1, confirming their potential as highly conductive supports for hybrid electrode configurations. Furthermore, a NiMn(PO4)2/EEG composite was integrated with activated carbon (AC) derived from wastewater sludge to fabricate energy storage devices. The NiMn(PO4)2/EEG composite was synthesised hydrothermally, while AC was produced via phosphoric acid activation. Characterisation verified the formation of a mixed-metal phosphate phase anchored on few-layer graphene. The optimised NiMn(PO4)2/50 mg EEG electrode achieved a high specific capacity of 822.1 C·g-1 at 1 A·g-1, significantly outperforming pristine NiMn(PO4)2. In an asymmetric configuration (NiMn(PO4)2/EEG//AC), the device delivered an energy density of 40.0 Wh·kg-1, a peak power density of 6538 W·kg-1, and retained 87% of its capacitance after 5000 cycles at 5 A·g-1. These results underscore the synergistic contribution of EEG towards improved electrical conductivity and redox kinetics, while demonstrating the potential of wastewater sludge-derived AC for sustainable electrode development. Overall, this research integrates synthesis, characterisation, and electrochemical evaluation to elucidate the role of heteroatom-doped EEG in enhancing the electrochemical behaviour of NiMn(PO4)2-based electrodes. By uniquely combining NiMn(PO4)2, heteroatom-doped graphene, and waste-derived activated carbon, this study presents an innovative, sustainability-oriented approach to device design. These findings contribute to the advancement of scalable, sustainable energy storage technologies. Fri, 05 Dec 2025 00:00:00 GMT https://ir.unisa.ac.za/handle/10500/32566 2025-12-05T00:00:00Z https://ir.unisa.ac.za/handle/10500/32534 Nano-engineered electrochemical aptasensors for label-free detection of cryptosporidium, cadmium and arsenic in water Nompetsheni, Indiphile Water is an essential resource for human survival, agriculture, and livestock. However, over the past thirty years, the World Health Organization has reported growing concerns about the impact of environmental pollution on water quality. Water quality is deteriorating due to contamination from microbes and heavy metals. Among the major microbial and potential heavy metals in water are Cryptosporidium (Crypto), cadmium (Cd2+) and arsenic. Crypto is an intestinal protozoan parasite that has become a significant cause of cryptosporidiosis, a gastrointestinal disease that can affect healthy adults and may be fatal for children and individuals with weakened immune systems. In contrast, Cd2+ and arsenic are amongst the most toxic and harmful metal ions found in the environment. These metals are highly mobile and can accumulate and spread throughout ecosystems. When ingested, they cause various health issues, including cardiovascular diseases, acute poisoning and cancer. These contaminants pose serious risks to aquatic species and the ecosystem at large. Early diagnostic methods for detecting Crypto, Cd2+, and arsenic were developed using microscopy, molecular, and spectroscopic techniques. However, these methods often yield false-negative results, are time-consuming, lack sensitivity and specificity, and have low detection limits. Crypto, arsenic, and Cd2+ pose a challenge to delivering safe drinking water due to their low concentrations in large volumes. Consequently, there is a need to develop portable, sensitive, and selective methods for detecting Crypto, arsenic, and Cd2+ at trace levels. This work develops a novel carbon quantum dot viii titanium dioxide (CQD-TiO2), Mil101(Fe)-CQD-TiO2 based-aptasensor platform capable of label-free simultaneous detection of Crypto and heavy metals at trace levels in phosphate buffer solutions and real water samples. The electrocatalysts used in this work were synthesized using precipitation and hydrothermal methods. Various characterization techniques, such as High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were employed to confirm the structural and morphological properties of the synthesized materials. The electrochemical properties of the modified electrodes were studied using cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS), square-wave voltammetry (SWV), and Chronopotentiometry (CP), revealing enhanced reaction kinetics and improved stability. The use of various electrocatalysts as aptamer vehicles on the electrode surface has significantly improved the performance of aptasensors, resulting in greater selectivity, higher accuracy, and lower detection limits. As a result, a CQD-TiO2-based aptasensor platform was developed for detecting Crypto, achieving a detection limit of 0.0024 ng L-1, and a sensitivity of 0.27 mA μM-1. In another study, an electrochemical aptasensor platform based on Mil101(Fe)-CQD-TiO2 ternary composite achieved a detection limit of 0.001 ng L-1 for Crypto with a sensitivity of 0.529 mA μM-1. The aptasensor demonstrated excellent performance in detecting Cd2+ and arsenic, achieving low detection limits of 0.073 ng L-1 for Cd2+ and 0.092 ng L-1 for arsenic, with sensitivities of 0.127 mA μM-1 and 0.0065 mA μM-1. All developed aptasensor platforms demonstrated limits of detection within the limits reported in the literature. Thus, GCE-Mil101(Fe)-CQD-TiO2-Apt-BSA platform showed a low limit of detection, demonstrating high sensitivities and selectivity compared to conventional techniques. The aptasensor platforms showed acceptable recovery rates when tested with real water samples and demonstrated ix good stability, reproducibility, and selectivity. These aptasensors have significant potential for integration with microfluidic and on-chip technology, enabling the early detection of pathogens and trace metals. Wed, 22 Oct 2025 00:00:00 GMT https://ir.unisa.ac.za/handle/10500/32534 2025-10-22T00:00:00Z https://ir.unisa.ac.za/handle/10500/31967 Synthesis of magnetic nanoadsorbents derived from maize waste and their application for the adsorptive removal of selected heavy metal ions from wastewater samples Mahlaule Glory, Louisah Mmabaki Over the past few years, heavy metal ion (HMI) pollution has become a crucial matter due to their threat to human health and ecological systems.,. Furthermore, HMIs have been reported to be hazardous, persistent, and in some of the global health organizations reports, they have been declared as carcinogens. These HMIs include Pb (II) and Cr (VI) and they are very reactive and highly oxidizing in nature. Thus, the need to remediate these HMIs from wastewater using magnetic nano adsorbents. In this study, three nano-adsorbents such as cellulose nano crystals (CNC), magnetite (M), and magnetic cellulose nanocrystal (MCNC) were synthesized for the removal of Pb (II) in wastewater. The magnetic cellulose nanocrystals (MCNCs) were synthesized using a co-precipitation method from the magnetite (Fe3O4) and cellulose nano crystals (CNCs) were used as a base for stability and easy dispersion of iron for the adsorptive removal of Pb (II) ions. Furthermore, to enhance the adsorption capacity and to improve selectivity of the CNC towards -targeting anionic Cr (VI) ions, the surface modification was conducted by crosslinking CNCs with 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), thereby oxidising the material to form a bridge with the grafting of the polyethyleneimine (PEI). The surface of the CNC-TEMPO-PEI was further magnetised by introducing iron on to the surface material via a co-precipitation method. Fourier-transform infrared spectroscopic (FTIR) analysis revealed the presence of C=O, COOH, CH, OH and FeO stretching frequencies in MCNC, while powder X-ray diffraction (P-XRD) confirmed the formation of MCNC and the monoclinic type 1 cellulose with 1β lattice and magnetite cubic spinel phases of the CNC. Ultraviolet-visible spectroscopy (UV-Vis) showed the presence of both CNC and magnetite at 400 nm. The scanning electron microscopy (SEM) indicated a smooth fibroid surface of CNCs while magnetite (M) displayed 2 morphologies, the rod like and spherical morphology, indicating the presence of iron and oxygen. The MCNC were stable after 600 ⁰C as shown on the thermograms generated from the thermogravimetric analyser (TGA). Last, the Brunauer-Emmett-Teller (BET) displayed surface area, pore size and pore volume improvement of 56 m2/g, 98 Å and 0,1465 cm3/g. Å, respectively, for the MCNC. Following the characterization of the MCNCs nanocomposites, the material was used for adsorptive removal of Pb (II). It was discovered that for the Pb (II) removal efficiency was 97 % with an acceptable precision of ≤ 3 %. The highest efficiency was obtained at optimal conditions of 60 mg dosage, 0,1 ppm concentration within a rapid contact time of 5 min at a temperature of 60 ⁰C and at a pH of 6. These parameters were optimised by using multivariate optimization tools (Minitab) and were also validated against the magnetite and the CNC. A maximum adsorption capacity of MCNC was also obtained at 47,70 mg/g for Pb (II) and the material was re-used for up to 4 cycles. The results revealed that the reaction followed Freundlich isotherms and Pseudo First Order kinetic model with a regression coefficient of 0,98 and 0,96 respectively. The adsorption thermodynamics studies indicated a spontaneous process and an exothermic reaction. On the other hand, the MCNC-TEMPO-PEI was characterised with FTIR, P-XRD, TEM and SEM-EDS techniques. The FTIR confirmed a successful formation and the presence of COOH, OH, Fe-O band and NH2 groups on the nanocomposite. The P-XRD confirmed the crystal structure of CNC-TEMPO and the amorphous structure of both the CNC-TEMPO-PEI and the MCNC-TEMPO-PEI. The SEM-EDS results demonstrated the rod-like, oval and irregular cubic morphology for successful preparation of MCNC-TEMPO-PEI nanocomposite. The adsorption performance of MCNC-TEMPO-PEI on Cr (VI) ions was investigated by using univariate optimization tools. The MCNC-TEMPO-PEI was efficient at 5 ppm, using a 30 mg dosage at 25 ⁰C within the acidic conditions at pH 2 within a rapid contact time of 15 min. The optimised parameters were further validated using 5 various adsorbent materials and the results indicated that the MCNC-TEMPO-PEI was the most efficient by exhibiting the highest adsorption capacity of 4,4 mg/g with a 98% removal. The interaction between the MCNC-TEMPO-PEI and the Cr (VI) ions indicated a chemisorption of the electrostatic forces governing the magnetic and ionic exchange interaction between of the adsorbate and the analyte. The Langmuir adsorption isotherm displayed a correlation coefficient of 0,94 following the PSO kinetic model against the adsorptive removal of Cr (VI) ions. The thermodynamic interaction indicated a non-spontaneous endothermic reaction with a favourable reaction. The adsorbent could be reused at least 8 times with a removal efficiency above 75 %. The results revealed that the real wastewater samples analysed from this study did not contain Cr (VI) ion. Text in English Wed, 01 May 2024 00:00:00 GMT https://ir.unisa.ac.za/handle/10500/31967 2024-05-01T00:00:00Z https://ir.unisa.ac.za/handle/10500/31883 Development of novel mesoporous magnetic adsorbents from industrial waste and their application for removal of lead and efavirenz in aqueous solutions Kgoedi, Thabang Industrial waste materials have garnered increased attention as viable adsorbents that could be used for the extraction of heavy metals and organic pollutants from wastewater. This is primarily due to their abundant availability in large quantities and economical cost-effectiveness. Coal fly ash, bottom ash, and fly ash are examples of industrial waste generated from coal combustion in power plants, while petroleum coke is derived from oil refineries. These waste materials contain diverse functional groups, including carbon, calcium oxide, silicon dioxide, aluminium oxide, and iron oxide, which makes them ideal for the remediation of wastewater. Previous research studies have indicated that modified industrial waste materials possess greater adsorption capabilities. As a result, this study sought to modify coal fly ash (RCFA), bottom ash (RBA), fly ash (RFA), and petroleum coke (RPC) by adding iron oxide (Fe3O4) nanoparticles. This modification enables easy separation with an external magnet and enhances their effectiveness in adsorbing lead and efavirenz. The following adsorbents Fe3O4@APC, Fe3O4@ACFA, Fe3O4@AFA, and Fe3O4@ABA were prepared in a two-step method. The first step was activation of the RPC, RBA, RFA, and RCFA with NaOH then followed by incorporating Fe3O4 nanoparticle. These mesoporous magnetic materials were successfully prepared and characterized using various techniques such as thermogravimetric analysis (TGA), scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDS), ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller analysis (BET). The Langmuir, Temkin, and Freundlich isotherm models were applied to analyse the equilibrium data. The maximum adsorption capacities for obtained for lead were 48.8, 15.63, 12.16, and 270.27 mg/g for Fe3O4@ACFA, Fe3O4@AFA, Fe3O4@ABA, and Fe3O4@APC, respectively. The maximum adsorption capacities for efavirenz obtained were 25.38, 37.64, 13.07 and 76.54 mg/g Fe3O4@ACFA, Fe3O4@APC, Fe3O4@ABA, Fe3O4@AFA respectively. Based on the adsorption isotherms for lead ions, both Fe3O4@ACFA and Fe3O4@AFA, are best described by the Temkin isotherm while Fe3O4@ABA and Fe3O4@APC were best described by the Langmuir and Freundlich isotherm, respectively. Additionally, adsorption of efavirenz was best described by the Langmuir isotherm for all prepared adsorbents. viii The kinetic data were also evaluated for the lead and efavirenz which revealed that the pseudo-second-order equation provided the best correlation for both lead and efavirenz. Thermodynamic parameters suggest that the adsorption process is endothermic and spontaneous for lead. However, for efavirenz it behaved differently on various adsorbents, revealing non-spontaneous adsorption. The adsorption process for lead was endothermic for all adsorbents, whereas for efavirenz it was found to be endothermic for Fe3O4@APC and Fe3O4@ACFA adsorbents, while exothermic for Fe3O4@ABA and Fe3O4@AFA adsorbents. The findings demonstrate that Fe3O4@ACFA, Fe3O4@APC, Fe3O4@ABA, and Fe3O4@AFA possesses the potential to effectively remove lead ions and efavirenz from aqueous solutions. Thu, 01 Feb 2024 00:00:00 GMT https://ir.unisa.ac.za/handle/10500/31883 2024-02-01T00:00:00Z