Composite carbon membranes for the desalination of water

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2007.Electrodialysis is a method of water desalination which involves the separation of TDS through an ion-exchange membrane under a potential gradient. In this study it was attempted to reverse engineer the composite carbon ion-exchange membrane used in a prototype plant and electrochemically evaluate a prototype desalination cell. The influence of applied potential on the capacitance of the various electrode surfaces and possible electrode reactions was investigated. A model was also suggested to describe the conductivity through the membrane. The composition of composite carbon membranes were determined by compositional analysis using various analytical tools. Elemental analysis, done with PIXE and EDS, showed that the membranes contained chloride, fluoride, oxygen, carbon, and possibly hydrogen. With LC-MS and IR it was established that the membranes consisted of two polymers with no carbonyl or aromatic functional groups. After further thermal analysis the following possible compounds remained: hexafluoropropylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene (PCTFE), polyoxyethylene oxide (PEO) and polyethylene glycol (PEG). This assessment is in good agreement with the contents of US patent 4,153, 661, which describes the composite membrane

Spectroscopy, morphology, and electrochemistry of electrospun polyamic acid nanofibers

Polyamic acid (PAA) nanofibers produced by using the electrospinning method were fully characterized in terms of morphology and spectroscopy. A PAA nanofiber–modified screen-printed carbon electrode was applied to the detection of selected sulfonamides by following an electroanalytical protocol. The polyamic acid (PAA) nanofibers were characterized using Fourier transform infrared (FTIR) spectroscopy to study the integrity of polyamic acid functional groups as nanofibers by comparing them to chemically synthesized polyamic acid. A scanning electron microscope (SEM) was used to confirm the morphology of the produced nanofibers and 3D arrangement at the electrode interface. The Brunauer–Emmett–Teller (BET) method was used to determine the surface area of the nanofibers. Atomic force microscopy (AFM) was used to study the porosity and surface roughness of the nanofibers

Electrodeposited CuO thin film for wide linear range photoelectrochemical glucose sensing

Cupric oxide (CuO) has been used as a non-enzymatic glucose sensor for decades. However, there is a paucity of publications on bare CuO based photo electrochemical (PEC) glucose detection. In this study, a photo active CuO thin film was electrodeposited onto conductive glass and its band gap was tuned by etching in NH3 solution. A 6 W light-emitting diode (LED) bulb was used as the light source for PEC glucose oxidation. Various physical and electrochemical characterization techniques were used to study the PEC behavior of the CuO thin film electrode during glucose oxidation. The electrochemical oxidation of glucose was found to be an irreversible electron transfer process controlled by diffusion at the electrode surface under illumination and dark conditions. Electrochemical impedance spectroscopy (EIS) confirmed that the charge transfer resistance in the light decreases by several orders of magnitude. Good amperometric performance was obtained for the CuO film with a 4 s response time and negligible interference from other species present in human blood. The as prepared sensor exhibited a remarkable wide linear range up-to 29 mM

Photoelectrochemical detection of inorganic mercury in aqueous solutions

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010.ENGLISH ABSTRACT: Due to the adverse human health effects associated with mercury (Hg) toxicity, an innovative method for inorganic mercury (Hg2+) determination in aqueous solutions was investigated. The method of Hg2+ determination involved the use of a Hg2+ selective chemosensor immobilised onto an indium tin oxide (ITO) coated electrode in a photoelectrochemical cell. Upon light activation and in the presence of Hg2+ the fluorescent chemosensor absorbed a photon, and charge separation would occur according to the nature of the semiconductive material coated onto the ITO substrate. The charge separators and semiconductors investigated were an n-type carrier TiO2, polyaniline (PANI) and copolymers of polyaniline and 2-bromoaniline (P[A-co-2- BrA]s) with different monomer ratios. The polymer and copolymer coated ITO working electrodes used in the Hg2+ sensitive photoelectrochemical cell were electrochemically evaluated. The results demonstrated that the ITO coated with PANI had superior conductive and semiconductive properties compared to ITO coated with P[A-co-2-BrA]s. The ITO glass substrates were coated with TiO2, PANI or P[A-co-2-BrA]s, followed by the fluorescent chemosensor, a rhodamine 6G derivative (RS). The electrodes were subsequently photoelectrochemically evaluated in a photoelectrochemical cell in the presence of Hg2+. The PANI-RS coated electrode behaved as a photocathode in the presence of Hg2+ under illumination. The PANI-RS photoresponse increased with increasing Hg2+ concentration in the range 10 to 150 μg L-1, with a limit of detection of 6 μg L-1. ITO coated with TiO2 (ITO/TiO2) followed by a composite of PANI and RS had a linear photoanodic response in the Hg2+ concentration range of 10 to 200 μg L-1 and a limit of detection of 5 μg L-1. ITO and ITO/TiO2 coated with the P[A-co-2-BrA]s and RS had considerably lower photoresponses towards Hg2+ in aqueous solutions compared to PANI-RS. The photoresponses decreased with increasing 2-bromoaniline content. The PANI and P[A-co-2-BrA]s coated ITO and ITO/TiO2 electrodes were then also coated with another rhodamine 6G derivative with a thiolactone moiety (RT). The PANI coated electrode yielded a photocathodic response in the Hg2+ concentration range 0.2 to 5 μg L-1. ITO coated with TiO2 had no photoresponse towards Hg2+ due to repulsive forces between TiO2 and the RT molecules. The photoresponses of the working photoelectrodes towards Hg2+ were further evaluated in a custom photoelectrochemical Hg2+ detector. The photoresponses of PANI-RS and PANI-RT gave qualitative results for the presence of Hg2+ in aqueous solutions in concentrations as low as 2 μg L-1.AFRIKAANSE OPSOMMING: Weens die negatiewe gesondheids aspekte toegeskryf aan Hg2+ vergiftiging is ‘n nuwe innoveerende metode van Hg2+ deteksie in wateroplossings ondersoek. Die voorgestelde metode van Hg2+ deteksie behels die gebruik van ‘n Hg2+ selektiewe chemosensor geimmobiliseer op ITO elektrodes in ‘n fotoelektrochemiese sel. Met lig aktivering en in die teenwoordigheid van Hg2+, absorbeer die fluoreseerende chemosensor ‘n foton, gevolg deur lading skeiding, soos bepaal deur die aard van die halfgeleidende material wat op die ITO platjies bedek is. Die halfgeleiers wat ondersoek was, was TiO2, PANI en ko-polimere van PANI met 2-bromoanilien. Die polimeer en kopolimeer bedekte ITO werkende elektrodes wat in die Hg2+ sensitiewe fotoelektrochemiese sel gebruik is, is elektrochemies geevalueer. Daaruit is gevind dat ITO bedek met PANI superieure geleier en halfgeleier eienskappe het in vergelyking met die ITO wat bedek is met P[A-co-2-BrA] kopolimere. Die ITO glas plaatjies is bedek met TiO2, PANI en die P[A-co-2-BrA] kopolimere gevolg deur die fluoresseerende chemosensor, RS. Die elektrodes is onderskeidelik fotoelektrochemies getoets in ‘n fotoelektrochemiese sel in die teenwoordigheid van Hg2+. Die PANI-RS elektrode het soos ‘n fotokatode opgetree in die teewoordigheid van Hg2+ onder beligting. Die foto-reaksie van die PANI-RS elektrode teeonoor Hg2+ het liniêr toegeneem van 10 μg L-1 tot 150 μg.L-1 met ‘n limiet van deteksie op 6 μg L-1. ITO/TiO2 wat bedek is met ‘n samestelling van PANI en RS het ‘n fotoanodiese reaksie gehad teenoor Hg2+ in die liniêre reeks van 10 tot 200 μg L-1 met ‘n deteksie limiet van 5 μg L-1. ITO en ITO/TiO2 bedek met die P[A-co-2-BrA] kopolimere het heelwat laer fotoreaksies getoon teenoor Hg2+ in waterige oplossings Die fotoreaksie het afgeneem met toeneemende 2-bromoanilien inhoud van die kopolimeer. Die PANI en P[A-co-2-BrA] kopolimere bedekte ITO en ITO/TiO2 elektrodes is verder bedek met ‘n tweede rhodamine 6G afgeleide, RT. Die ITO/PANI bedek met RT het weer ‘n fotokatodiese reaksie gehad teenoor Hg2+ in die konsentrasie interval 0.2 tot 5 μg L-1. Die ITO/TiO2 elektrode bedek met die PANI-RT samestelling het geen fotoreaksie gehad teenoor Hg2+ weens afstotende kragte tussen die TiO2 and RT molekules. Die fotoreaksies van die verskeie werkende elektrodes teenoor Hg2+ is verder getoets in ‘n tuisgemaakte fotoelektrochemiese Hg2+ detektor. Die fotoreaksies van die PANIRS en PANI-RT samestellings op ITO het kwalitatiewe resultate gelewer vir die teenwoordigheid van Hg2+ in waterige oplossings in konsentrasies tot so laag soos 2 μg L-1

Composite carbon membranes for the desalination of water

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2007.Electrodialysis is a method of water desalination which involves the separation of TDS through an ion-exchange membrane under a potential gradient. In this study it was attempted to reverse engineer the composite carbon ion-exchange membrane used in a prototype plant and electrochemically evaluate a prototype desalination cell. The influence of applied potential on the capacitance of the various electrode surfaces and possible electrode reactions was investigated. A model was also suggested to describe the conductivity through the membrane. The composition of composite carbon membranes were determined by compositional analysis using various analytical tools. Elemental analysis, done with PIXE and EDS, showed that the membranes contained chloride, fluoride, oxygen, carbon, and possibly hydrogen. With LC-MS and IR it was established that the membranes consisted of two polymers with no carbonyl or aromatic functional groups. After further thermal analysis the following possible compounds remained: hexafluoropropylene tetrafluoroethylene copolymer, polychlorotrifluoroethylene (PCTFE), polyoxyethylene oxide (PEO) and polyethylene glycol (PEG). This assessment is in good agreement with the contents of US patent 4,153, 661, which describes the composite membrane

Phytostabilization of metals by indigenous riparian vegetation

Given the increasing pressure of man-made activities on riparian zones, the capacity of the riparian vegetation along the Upper Olifants River, South Africa, to phytoextract and phytostabilize aluminium (Al), manganese (Mn) and iron (Fe) from the soil was investigated. The aim of the study was to gain better understanding of the capacity of indigenous vegetation in riparian zones to immobilize metals in the soil, thereby improving river water quality and ecosystem services. Seven commonly-occurring pollution-tolerant riparian plant species were evaluated to establish their potential as bioaccumulators for Fe, Al and Mn. Species included: Cyperus haspan, Schoenoplectus corymbosus, Typha capensis, Phragmites australis, Cynodon dactylon, Cyperus marginatus and Juncus effusus, which were sampled in five riparian areas in the Upper Olifants catchment. The bioconcentration factor (BCF) for Mn was > 1 for all species investigated with a maximum of 5 for Typha capensis, which also showed the highest accumulation of Al (10.26) and Fe (7.03). The remaining species presented with Al and Fe BCF between 0.11 and 2.00, with minimal transfer from root to shoot. When measured against an ideal hypothetical buffer zone, the buffer zones under investigation varied between intact and severely compromised. Intact riparian zones showed elevated metal concentrations in the soil, yet significantly lower concentrations in the river water compared to areas with insufficient vegetative cover. A polluted riparian area overgrown by P. australis effectively phytoextracted 204 960 g/m2 Al, 204 400 g/m2 Fe and 27 887 g/m2 Mn. The two indigenous Cyperus spp. were not ideal for metal immobilization with low bioaccumulation and transfer factors as well as low biomass. High biomass and Al, Fe and Mn phytostabilizing species: P. australis, T. capensis, S. corymbosus and J. effusus, should be considered in the rehabilitation of South African buffer areas

The reach of human health risks associated with metals/metalloids in water and vegetables along a contaminated river catchment : South Africa and Mozambique

BACKGROUND : Anthropogenic pollution was identified as an environmental problem of concern when, in 2008, dozens of crocodiles died in the Olifants River catchment near the border of South Africa and Mozambique. Given the close proximity of households to the river and their making use of river water, we aimed to determine to what extent water pollution has an impact on health of indigent communities in South Africa and Mozambique in the catchment area. METHODS : Water and vegetable samples were collected from the study areas. Biota samples were washed with double de-ionized Milli-Q water and freeze-dried. Heavy metal analyses in water and vegetables were done by means of Inductively Coupled Plasma Optical Emission Spectroscopy. Metal concentrations were applied in a human health risk assessment to estimate health risks. RESULTS : Mean concentrations of antimony, arsenic, cadmium, chromium, mercury, molybdenum, nickel and selenium in water samples from South Africa exceeded the World Health Organization guidelines for safe levels of intake. Only iron exceeded the recommended guidelines in water samples from Mozambique. Metals/metalloids were found in lower concentrations at Mozambique sites downstream of South African sites. In vegetables, uranium was between 10 and 20 times above safe guidelines in South Africa and between 3 and 6 times in Mozambique. Arsenic in water samples posed the highest cancer risk. CONCLUSIONS : Even with a reduction in the metal concentrations in river water from South Africa to Mozambique, the potential to cause adverse human health impacts from direct use of polluted river water is evident in both countries.A United States Agency for International Development grant through the South African Department of Science and Technology (DST/CON 0008/2011).http://www.elsevier.com/locate/chemosphere2019-05-30hj2018Geography, Geoinformatics and Meteorolog

Crystalline Disorder, Surface Chemistry, and Their Effects on the Oxygen Evolution Reaction (OER) Activity of Mass-Produced Nanostructured Iridium Oxides

In the present study, three mass-produced commercial IrOx samples from different suppliers were studied to establish correlations between various properties and their OER activities. The structures of the electrocatalysts at different scales were explored through laboratory instrumentation, powder X-ray diffraction, and synchrotron-based X-ray total scattering experiments combined with pair distribution function analysis. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy using a transmission electron microscope were used to determine respectively the surface and the bulk elemental compositions of the samples. The coherent domain size (CDS) values of IrO$_x$ phases within the catalyst particles were estimated to be ∼10, ∼ 19, and ∼ 54 Å for the three IrO$_x$ samples. Surprisingly, the sample with a CDS of ∼19 Å turned out as the best OER electrocatalyst among the three in terms of mass-specific activity, I$_{OER(m)}$, followed by the 10 and 54 Å species. The amount of surface native compound oxygen was found to be a key parameter for the interface electrochemical accessibility. The intrinsic OER activity, evaluated using area-specific activity, I$_{OER(a)}$, suggests that the oxide with lattice disorder presenting a mixture of tetragonal and orthorhombic phases (70:20 w/w) is of superior intrinsic OER activity; however, the oxide with the presence of a monoclinic-like phase is of inferior intrinsic OER activity, which may also be due to the surface presence of Ir$^{3+}$ along with Ir$^{4+}$. The classic belief that the pure tetragonal phase is the best crystalline structure as the OER catalyst is challenged. Iridium oxides with disordered crystallinities may offer a class of highly active oxygen evolution electrocatalysts. The knowledge thus obtained should have a significant impact on the understanding, selection, and processing of IrO$_x$-based OER electrocatalysts