The efficiency of a Swoxid prototype and an antimicrobial nanofiber membrane as POU filters

Gaborone, Mamosete Dorothy (2020-03)

Thesis (MSc)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Clean water is a scarce resource that numerous individuals lack access to. Resultantly, these individuals resort to using surface waters which leaves them exposed to waterborne diseases if they do not clean the water source adequately. Furthermore, these individuals are also exposed to chemical pollutants from agricultural, domestic and industrial waste which drains into rivers as well as downstream from waste water treatment plants (WWTPs). Furthermore, WWTPs which do not remove chemical pollutants efficiently. There are several methods to clean water, one of the most popular being filtration. In this study, the efficiency of two potential point-of-use (POU) filters was determined. The first filter, namely the Swoxid prototype, is comprised of a ceramic membrane functionalised with titanium dioxide (TiO2) and requires ultra violet (UV) light for activation. The mechanism of TiO2 is that when it is activated by UV light, hydroxyl (OH.) radicals are generated, which inactivate bacteria and denature chemical pollutants. To determine the efficiency of the Swoxid prototype to inactivate bacteria, water from a highly polluted source, namely the Plankenburg River, was filtered through the filter in both the absence and the presence of sun exposure. The bacteria tested were the typical indicator organisms namely the coliforms, faecal coliforms, heterotrophic bacteria, Enterococcus spp., Salmonella spp. and Shigella spp. For the first Swoxid prototype, there was complete removal of bacteria under exposure to the sun except for the heterotrophic bacteria and the coliforms. In the case of the coliforms, no bacteria were observed for the first few days; however on the fourth or the sixth day, growth was observed. This suggests that for the first few days, the bacteria are in a stationary phase, likely due to the time required for DNA repair. For most of the bacteria tested, the results of the trials where the first Swoxid prototype was exposed to the sun were comparable to the controlled conditions. This suggests that the removal of bacteria by the Swoxid prototype is mainly due to filtration and not the inactivation of bacteria by the radicals that are generated by the TiO2-UV reaction. The second Swoxid prototype was effective at removing most of the bacteria; however, there was regrowth of the heterotrophic bacteria and coliforms. The results of the Swoxid prototype were comparable to the UV control, suggesting that inactivation was mainly due to UV exposure and not radical degradation. Scanning electron microscopy was used to view the effect of radicals on the membrane of Staphylococcus aureus Xen 36. The results showed that the radicals generated by the TiO2-UV reaction may oxidize the lipid bilayer of bacterial membranes leading to the formation of cracks on the surface of the membrane of bacteria. The Swoxid prototype was also effective in removing several micropollutants, i.e. benzotriazole, codeine, diclofenac, efavirenz and sulfamethoxazole from spiked and river water. However, because the results were comparable to a commercial UV-filter for many of the micropollutants that were tested, it could be that the removal was mainly due to UV bombardment and not removal of the micropollutants by the radicals. Future improvements and amendments to the Swoxid prototype should include an increase in the quantity of TiO2 used to functionalise the membrane. The second filter membrane prototype comprised of a poly (D,L-lactic acid) (PLA) nanofiber membrane functionalised with an antimicrobial solution (biocide) containing copper (Cu(II)) and zinc (Zn(II)) ions. The proposed mechanisms of Cu(II) and Zn(II) entail disrupting the membrane of microorganisms, decreasing the membrane potential and binding to sulfur-containing proteins and DNA. Ten varieties were fabricated that included low, medium and high biocide loading and low and high density membranes. As part of the initial experiments, leaching experiments were performed using inductively coupled mass spectrometry (ICPMS) to determine whether or not the quantity of Cu(II) and Zn(II) that leach is below the limit set United States Environmental Protection Agency (USEPA) and South African standards for drinking water. The results showed that the concentration of Cu(II) and Zn(II), which the maximum that leached were 33.6 μg/L and 100.5 μg/L, respectively were indeed below the limits set in the drinking standards of both the EPA and South Africa. The results also showed that membranes with a higher density leached more Cu(II) and Zn(II) than their lower density counterparts. Moreover, the results showed that more Zn(II) leached from the membranes than the Cu(II) which suggests that the Cu(II) is more tightly retained by the PLA fibres. The membranes were then exposed to Escherichia coli Xen 14 and S. aureus Xen 36 and the decrease in bioluminescence was observed over time using the XENOGEN VIVO VISION In Vivo Imaging Lumina System (IVIS). The experiment showed that the membranes with the highest biocide and higher density were most effective at inactivating the metabolism of bacteria. Therefore subsequent experiments entailed filtering reverse osmosis (RO) water spiked with 107 Escherichia coli Xen 14 and S. aureus Xen 36 through the antimicrobial nanofiber membranes to determine the log reductions of the bacteria. However, the membrane with the highest biocide, density and fibre diameter only led to a 1.5 log reduction. The design of the nanofiber membrane was modified to consist of one layer of PLA nanofibers and a second layer containing PLA and the biocide. Three varieties were fabricated consisting of three different densities and controls containing no biocide. Bacteria were only removed when the high planar density membranes were stacked on top of one another with the highest log reduction being 5 when three membranes were used. The nanofiber membranes were also characterized before and after filtration using scanning electron microscopy (SEM). It was determined that filtration does not have a significant effect on the structure of the membrane as the nanofibers were still intact. The pores were, however, larger than bacteria in size and that explains the poor filtration results when just one membrane is used. Moreover, it was discovered through electron dispersive spectroscopy (EDS) that the copper binds more tightly to the membrane than the Zn(II) as the difference in the content of copper before and after filtration was very little. In addition to the structure of the antimicrobial nanofiber membrane, the structure of the bacteria after filtration was also viewed. There were several bacteria with damaged outer surfaces, particularly those that were located near what appeared to be the biocide. The design of the antimicrobial nanofiber membrane can be improved by increasing the density of the membrane and ensuring smaller pore sizes. Additionally, the membranes should be designed in such a way that the Cu(II) leaches more readily at concentrations that are still below the limits set in the EPA and South African Drinking Water Standards.

AFRIKAANSE OPSOMMING: Skoon water is ‘n skaars bron en is ontoeganklik vir vele mense. Gevolglik is hierdie mense oorgelaat om oppervlakwater te gebruik wat hul blootstel aan waterverwante siektes, indien die water nie doeltreffend gesuiwer word nie. Daarbenewens is hierdie individue ook blootgestel aan chemiese besoelstowwe afkomend van landbou-, huislike- en industriële afloop. Afvalwater behandelingsaanlegte verwyder ook nie al die chemiese besoedelstowwe doeltreffend nie. Verskeie metodes word gebruik om water skoon te maak, een van die mees gewildste is filtrasie. In hierdie studie is die doeltreffenheid van twee gebruikspunt-filters bepaal. Die eerste filter, naamlik die Swoxid prototipe, bestaan uit keramiek gefunksionaliseerde titanium dioksied (TiO2) wat geaktiveer word deur ultraviolet (UV) lig. Tydens hierdie aktivering word hidroksiel radikale gegenereer wat bakterieë inaktiveer en chemiese besoedelstowwe kan afbreek. Die doeltreffenheid van die Swoxid prototipe om water te suiwer, wat besmet is met patogene, is bepaal. Water van die Plankenburg rivier is gefiltreer deur die prototipe, met en sonder blootstelling aan sonlig. Die bakterieë wat getoets is, was koliforme, fekale koliforme, heterotrofiese bakterieë, Enterococcus spp., Salmonella spp. en Shigella spp. In meeste van die toetslopies waar die Swoxid blootgestel was aan sonlig, was daar totale verwydering van bakterieë, behalwe koliforme. In hierdie spesifieke geval, is geen bakterieë waargeneem vir die eerste paar dae nie, alhoewel op die vierde en sesde dag, is her-groei waargeneem. Hierdie resultate stel voor dat die bakterieë in stasionêre fase was en eers na vier tot ses dae weer lewensvatbaar was. In meeste van die toetslopies, was die resultate met son- of sonder son blootgestelling, vergelykbaar. Hierdie resultate stel voor dat die verwydering van bakterieë hoofsaaklik deur filtrasie plaasgevind het en dat die radikale wat gegenereer is nie die bakterieë geinaktiveer het nie. Die tweede Swoxid prototipe was doeltreffend om meeste van die bakterieë te verwyder, egter, daar was groei van die heterotrofiese bakterieë en die koliforme. Die resultate van die tweede Swoxid prototipe is vergelykbaar met die UV kontrol wat voorstel dat die verwydering van die bakterieë is hoofsaklik weens aan die blootstelling van die bakterieë aan UV en nie aan die radikale nie. Skandeer elektron mikroskopie is gebruik om die effek van radikale op die membrane van Staphylococcus aureus Xen 36 waar te neem. Die resultate wys dat die lipied bi-laag van bakteriële membrane geoksideer was wat gelei het tot die formasie van krake op die oppervlak van die sel-oppervlak. Die Swoxid prototipe was ook effektief om sekere mikro-besoedelstowwe te verwyder, beide uit steriele gedistilleerde water, voorberei met spesifieke mikro-besoedelstowwe, asook uit rivierwater. Die resultate is vergelyk met dié van ‘n kommersiële UV-filter en met heelwat van die mikro-besoedelstowwe was die resultate vergelykbaar. Dit kan dus wees dat die verwydering hoofsaaklik as gevolg was van UV bombardering, en nie deur die werking van die radikale nie. Toekomstige verbeterings en veranderings aan die Swoxid prototipe wat voorgestel word sluit in die verhoging in die kwantiteit van die TiO2 wat gebruik word om die membraan te funksionaliseer. Die tweede filter membraan prototipe het bestaan uit poly(D,L-melksuur) (PLA) nanovesel membrane gefunksionaliseerd met ‘n antimikrobiese oplossing (biosied) met hoofsaaklik koper (Cu(II)) en sink (Zn(II)) ione. Die voorgestelde meganismes van Cu(II) and Zn(II) sluit in die versteuring van die membraan van mikroorganismes, verlaging in die membraanpotensiaal en binding aan swael-bevattende proteïne en DNA. Tien variasies van die membraan is vervaardig wat ingesluit het lae, medium en hoë biosied lading en lae en hoë digtheid membrane. Eerstens is uitloging eksperimente uitgevoer deur induktiewe-gekoppeld massa spektrometrie (ICPMS) te gebruik om te bepaal of die kwantititeit van Cu (II) en Zn(II) ione wat uitloog laer is as die regulasies gestipuleer deur die Verenigde State Omgewings Beskermings Agentskap (USEPA) en Suid-Afrikaanse standaarde vir drinkwater. Die resultate het getoon dat die konsentrasies van beide Cu(II) en Zn(II) ione laer was as die bepaalde regulasies van USEPA en Suid-Afrika. Die resultate het ook gewys dat daar, uit die membrane met ‘n hoër digtheid, meer Cu(II) en Zn(II) ione uitgeloog het as die laer digtheid membrane. Daar het ook meer Zn(II) ione uitgeloog as Cu(II) ione, wat voorstel dat die Cu(II) ione sterker gebind was aan die PLA vesels. Die membrane is blootgestel aan Escherichia coli Xen 14 en Staphylococcus aureus Xen 36 en die afname in bioluminesensie is waargeneem oor tyd deur die XENOGEN VIVO VISION ‘In Vivo Imaging Lumina System’ (IVIS) te gebruik. Die membrane met die hoogste biosied lading en hoë membraan digtheid was die meeste effektief om die metabolisme van die bakterieë te inaktiveer. Daarom is daar in die opvolgende eksperimente steriele gedistilleerde water, geïnokuleer met 108 selle/mL E. coli Xen 14 en S. aureus Xen 36 deur die antimikrobiese nanovesel membrane gefiltreer om die verlaging van die bakterieë te bepaal. Alhoewel, die membraan met die hoogste biosied lading, dightheid en vesel diameter het slegs gelei tot ‘n 1.5 log verlaging. Die ontwerp van die nanoveselmembraan is verander na een laag PLA nanovesels, bedek met ‘n tweede laag wat die PLA en biosied ingesluit het. Drie variasies is vervaardig wat membrane met drie verskillende digthede ingesluit het en kontroles met dieselfde digthede, maar sonder funksionalisering met die biosied. Bakterieë is slegs verwyder deur twee hoë digtheid membrane op mekaar te plaas, met die hoogste verlaging van 5 log met die insluiting van drie hoë digtheid membrane. Die nanoveselmembrane is ook gevisualiseer voor en na filtrasie deur skandeer elektron mikroskopie (SEM) te gebruik. Dit is bepaal dat die filtrasie geen noemenswaardige effek op die struktuur van die nanoveselmembrane gehad het nie. Daar is ook gevind dat die membraan porie-grootte groter was as bakterieë, wat die onvoldoende filtrasie resultate verduidelik. Dit is ook deur elektron dispersiewe spectroskopie (EDS) bepaal dat die Cu(II) ione moontlik sterkter gebind het aan die membraan as die Zn(II) aangesien die konsentrasie van die Cu(II) ione hoër was op die membraan as die Zn(II) ione na filtrasie. Die strukture van die bakteriële oppervlak was ook beskadig, veral dié wat in kontak was met die geimmobiliseerde metaalione op die membraan. Die ontwerp van die antimikrobiese nanoveselmembraan kan verbeter word deur die membraandigtheid te verhoog om kleiner porie-grootte te verseker. Daarbenewens, kan die membrane op so ‘n manier vervaardig word dat die Cu(II) ione meer doeltreffend uitloog.

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