Department of Chemistryhttps://hdl.handle.net/10500/27372024-03-19T10:16:35Z2024-03-19T10:16:35ZAn investigation into the biodegradability of natural organic matter (NOM) and the resulting potential of biodegradable NOM fractions to form disinfection by-productsSambo, Sifiso Penuelhttps://hdl.handle.net/10500/309552024-03-18T11:01:37Z2020-02-01T00:00:00ZAn investigation into the biodegradability of natural organic matter (NOM) and the resulting potential of biodegradable NOM fractions to form disinfection by-products
Sambo, Sifiso Penuel
Not only does the presence of excessive levels of natural organic matter (NOM) in surface waters affect the raw water quality, but it also impacts the water treatment and supply processes. Other notable challenges caused by NOM is its contribution to bacterial regrowth and the formation of ‘toxic’ disinfection by-products (DBPs). DBPs are nuisance chemicals in water systems as they lead to the production of inferior water quality which may affect human health, the eruption of toxins and various disease-causing microorganisms. Most conventional water treatment plants (WTPs) insufficiently remove NOM, primarily the biodegradable dissolved organic carbon (BDOC) fraction. In the presence of bio-available fractions of NOM, conditions are created for opportunistic pathogens to regrow. While chlorination is crucial for the control of microbial contaminants, the co-existence and interaction of residual chlorine and residual NOM in the WTP lead to the introduction of DBPs such as trihalomethanes (THMs). To maintain the quality of potable water during conveyance, the system must be optimized with adequate control and monitoring, particularly of disinfection and microbial control. A better understanding of the biodegradability of NOM fractions and their potential to form DBPs due to interactions with chlorine residues is required.
This study investigated the character of NOM and its fractions in water treatment plants as well as their biodegradability and influence of these fractions on the THM formation potential (THMFP). The aim was achieved through a combination of conventional and advanced NOM characterization techniques. Raw and treated water from a conventional WTP was characterized through specific ultraviolet absorbance (SUVA) (L/mg.m) to define the NOM composition in terms of aromaticity. The water was further isolated into 3 NOM fractions (i.e. Hydrophilic [Hpi], transphilic [Tpi] and Hydrophobic [Hpo]) through the application of the modified polarity rapid assessment method (m-PRAM). Then, the biodegradability was assessed through the BDOC method, which measures the change in DOC of a NOM sample attached to biologically activate sand over a given period. The THMFP assessment was also conducted on each NOM fraction. Lastly, due to the significant correlation between BDOC and biomass production, the impact of the biodegradability of each fraction on bacterial regrowth potential (BRP) was investigated. This was concurrently together with the BDOC studies by monitoring the concentrations of heterotrophic plate counts (HPC) and Total coliforms (TC) on the first and last day of the experiment. The BRP of each fraction was calculated as the difference between the initial and the final concentration of HPC or TC, and only a ≥1x103 increase in the bacterial counts was considered positive for the BRP.
The raw water SUVA ranged between 3.88 L/mg.m and 4.11 L/mg.m, with an even distribution of the Hpi and Hpo NOM obtained through the m-PRAM fractionation. In terms of biodegradability, the Hpi and Tpi fractions were the most biodegradable fractions, with BDOC values of >32% and >29%, respectively. The relatively high BDOC on the Hpi and Tpi fractions substantially contributed to BRP, thereby increasing the HPC to ranges between 121.4 x103 cfu/mL to 197.4 x103 cfu/mL, respectively, while their impact was less significant to THMFP. The Hpi fraction can be confirmed as the primary cause of bacterial regrowth. The strong correlation (i.e. R2= >0.9) between BDOC and BRP allows for the prediction of the BRP in a water sample using the BDOC of each of the NOM fractions.
In terms of THMFP, chloroform (CHCl3) was the most abundant, increasing up to 708 µg/L and 611 µg/L for the raw water and treated water, respectively, while bromodichloroform (CHBrCl2) were detected in very low concentrations (<21µg/L) both in raw and treated water. The formation of CHBrCl2 and CHCl3 was mainly ascribed to the Hpo fraction. The high proportion of the NBDOC to the BDOC observed on the HWM Hpo fraction can be attributed to the higher potential of the Hpo fraction to form TTHMs. Significant correlations (R2) ranging from 0.83 to 0.91 were observed between SUVA and TTHM, confirming that SUVA alone can be successfully used to predict TTHM formation. A relationship between the biodegradability of NOM and DBPFP exists, the less biodegradable the NOM fraction, the more influence they have on the formation potential of DBPs.
The enhanced BDOC method has been successfully optimized for NOM biodegradation studies. The various ways in which systems can be retrofitted to effectively deal with biodegradable NOM can be accomplished through this method. The BDOC is an excellent tool for BOM quantification and is thus crucial in the development of an effective NOM removal strategy. Now that the link between BDOC and TTHM formation has been established, there is a need to conduct an assessment for N-nitrosodimethylamine formation potential (NDMAFP), particularly in the chloraminated distribution network where NDMA is more likely to occur. The study also recommends an investigation into the other NOM fractions such as Hpi-Acids, Hpi-Neutral, Hpo-Base etc., with respect to biodegradability and how they can impact the mechanisms for bacterial regrowth and DBPFP in distribution systems.
No keywords provided.
2020-02-01T00:00:00ZThe removal of dissolved organic matter (DOM) from water using photocatalysis coupled with coagulationMoss, Leratohttps://hdl.handle.net/10500/309542024-03-18T09:28:27Z2024-01-01T00:00:00ZThe removal of dissolved organic matter (DOM) from water using photocatalysis coupled with coagulation
Moss, Lerato
Dissolved organic matter (DOM) comprises of both synthetic and natural organic compounds such as pharmaceuticals, pesticides, natural organic matter (NOM), found in the environment. Most of the DOM pollute drinking water sources, which inevitably end up in water distributed to communities for consumption. There are several methods that are currently employed in water treatment plants to eliminate DOM from drinking water, but the removal efficiencies are not of required standard. The existence of NOM in drinking water is undesirable because it decreases the aesthetic merit of water. Moreover, NOM can result in the generation of disinfection by-products (DBPs) when it reacts with chlorine-based disinfectants. Pesticides are also a major concern as they contribute to drinking water pollution. Water pollution resulting from organic materials such as pesticides have been linked to several adversative effects on the environment and human health. This work is divided into two parts, with both aimed to evaluate a photocatalysis-coagulation integrated process for the removal of DOM in water. The first part of the study focussed on the photocatalytic-coagulation of a herbicide, mecoprop using titanium dioxide (TiO2) as a photocatalyst and ferric sulphate (Fe2(SO4)3) as a coagulant, under Ultraviolet-C (UVC) irradiation. The aim was to facilitate simultaneous removal of mecoprop, background organic matter and turbidity, as well as the removal and recovery of TiO2 nanoparticles (NPs) from surface water. Jar tests were performed to optimize the coagulation conditions ([Fe2(SO4)3] and pH). Subsequently, oxidative degradation experiments were conducted with UVC radiation in a bench scale collimated beam system. Control tests were performed, where removal of mecoprop was evaluated under photolysis, catalysis and coagulation, respectively. Furthermore, the combination of UV-coagulation, UV-TiO2, TiO2-coagulation were employed for the removal of mecoprop from surface water samples. Up to 88% removal of mecoprop was achieved by direct photolysis at a maximum UV fluence of 8000 cm2.mJ-1. Comparatively, photocatalysis with TiO2, displayed complete degradation of mecoprop at UV fluence of 4500 cm2.mJ-1and TiO2 concentration of 100 mg/L. However, when photocatalysis (UV-TiO2) and coagulation (Fe3+) were combined, a maximum degradation rate constant of 0.0034 cm2.mJ-1 was obtained. This was followed by the UV-Fe3+ process, with a rate constant of 0.0031 cm2.mJ-1. The improved mecoprop removal in the photocatalysis-coagulation was due to the synergy between a Fenton-like process (UV/Fe3+) and photolysis (UV), which overall lead to an improved production of hydroxyl radicals. However, the addition of TiO2 into the system improved the degradation rate by 0.0003 cm2.mJ-1, which is negligible. Therefore, the degradation of mecoprop could be performed without the photocatalysis, but with the UV/Fe3+ system alone. The second part of the study entailed the photocatalytic-coagulation removal of humic acid as a model NOM pollutant at a concentration of 10 mg/L, which is the concentration that is usually recorded in natural water. Titanium dioxide was modified by co-doping with varying concentrations of nitrogen and sulphur (1 g, 2 g, 4 g of thiourea, denoted as 1NS-TiO2, 2NS-TiO2, 4NS-TiO2) to achieve a visible light active catalyst. Coagulation experiments were performed using ferric chloride (FeCl3) to evaluate the recovery of NS-TiO2 nanoparticles and background organic matter. Subsequently, coagulation and photocatalysis processes were performed individually as controls and to optimize parameters such as coagulant dose, pH and photocatalyst dose. The photocatalysis-coagulation process was conducted under the optimized conditions ([FeCl3]= 30 mg/L, pH= 6, [2NS-TiO2]= 150 mg/L) under visible light irradiation (250 W). Optical differences were observed between the doped and undoped TiO2. Consequently, the pristine TiO2 (3.19 eV) band gap decreased when doped with nitrogen and sulphur and continued to decrease further with an increase in dopant (1NS-TiO2 = 3.18 eV, 2NS-TiO2 = 2.55 eV and 4NS-TiO2 = 2.41 eV). The results demonstrate that the combined photocatalysis-coagulation treatment process has a higher humic acid removal rate than the photocatalysis, coagulation individual processes (photocatalysis-coagulation k1 = 0.0143 min-1, photocatalysis k1 = 0.0066 min-1, coagulation k1 = 0.0074 min-1). In this case, both processes have been conclusively demonstrated to work synergistically to degrade and remove humic acid.
No keywords provided.
2024-01-01T00:00:00ZA three step approach to the degradation of natural organic matter (NOM) from water sourcesNdlangamandla, Nqobile Gugulethuhttps://hdl.handle.net/10500/309532024-03-14T11:53:17Z2017-06-01T00:00:00ZA three step approach to the degradation of natural organic matter (NOM) from water sources
Ndlangamandla, Nqobile Gugulethu
Natural Organic Matter (NOM) is a complex blend of organic compounds that forms naturally via the degradation of plant and animal materials into water sources. NOM in water negatively affects water quality (by causing odor, taste and color problems), negatively affects consumers health (through the disinfection by-products formation which are carcinogenic), increases costs in plant operations (by causing membrane fouling and high coagulant dosage demand) and negatively impacts the ecosystem (through bacterial regrowth and deterioration of surface water sources). In addition, the complexity and the size of NOM hinders most of the available water treatment processes that are in place in South Africa and worldwide from effectively and efficiently removing NOM from water sources. The varying character of NOM in various sources makes it difficult to remove NOM as its composition is not uniform; it depends on the climate, topology, industrial and agricultural activities around a particular area. Hence there is a need for methods that can effectively characterize and degrade NOM (such as photodegradation using TiO2) into smaller pieces for easy removal during water treatment processes.
The characterization of NOM in water was done by collecting samples from different water treatment plants located in various South African geographic locations. The purpose was to get a better understanding regarding the type and the composition of NOM occurring in water. The treatment plants of interest were Magalies Water (MP1, MP2 and MP3); Rietvlei Water (RV); Umgeni Water (HL, UM, MT and AM); Lepelle Water (LE, LO and LF); Midvaal Water (MV); Veolia Water (VP and VH) and Plettenberg Bay Water (P). The sampling was done during the period of September 2015 to September 2016 in order to account for seasonal variations. Samples were collected after each treatment stage for each treatment plant in order to study the treatability of NOM by various treatment processes. Conventional characterization methods such as dissolved organic carbon (DOC), ultra-violet at 254 nm (UV254) and specific UV-absorbance. Natural Organic Matter (NOM) is a complex blend of organic compounds that forms naturally via the degradation of plant and animal materials into water sources. NOM in water negatively affects water quality (by causing odor, taste and color problems), negatively affects consumers health (through the disinfection by-products formation which are carcinogenic), increases costs in plant operations (by causing membrane fouling and high coagulant dosage demand) and negatively impacts the ecosystem (through bacterial regrowth and deterioration. Furthermore, N, Pd co-doped TiO2 (NPT) and MWCNTs/N, Pd co-doped TiO2 (CT) were successfully synthesized via sol-gel method and characterized using FTIR (to confirm for the available functional groups), UV-Vis (to study the effect of doping TiO2 with N and Pd and the effect of the presence of MWCNTs on the absorption edge of TiO2), XRD (to verify the presence of the crystalline phases), Raman (to determine the nature of TiO2 and to verify the presence of MWCNTs), SEM (for morphology), EDS (for elemental composition) and TGA (for thermal stability and to evaluate the amount of MWCNTs present on the nanocomposite). NPT and CT were then tested for their photodegradation efficiency on various NOM containing samples collected from selected treatment plants.Conventional NOM characterization methods include both the on-site characterization (pH, turbidity and conductivity); and bulk characterization (DOC, UV254 and SUVA). The pH was used to determine the alkalinity or the acidity of the water; and it was found to be in a range of 2.50-9.13 with Midvaal (MV) raw water being the most alkaline and Preekstoel (VP) being the most acidic water. The turbidity (a measure of the amount of all the clay particles and colloids in water) of all the water samples at their final stages of the treatment process was found to be in the range of 0.00-3.00 NTU, with the Flag Boshielo Water (LF) having the highest turbidity value and the Magalies Water (MP1) having the lowest turbidity. Lastly, the water conductivity was found to be in the range of 135.3–781.3 mS/cm, with the Olifantspoort plant (LO) having the highest conductivity and Plettenberg Bay plant (P) plant having the lowest conductivity. Bulk characterization results showed that the VP raw water had the highest SUVA value (i.e. 7.24 ℓ·mg-1 m) thus high content of high molecular weight and hydrophobic NOM compared to other raw water sources. Regardless of the observed high SUVA in VP raw water; the P plant showed the highest DOC removal efficiency of 90.03% and Hazelmere (HL) plant showed the highest UV254 removal of 88.07%. DOC and UV254 were also used to study the effect of seasonal variations on NOM quantity, quality and treatability. It was shown that the DOC and UV254 was high in autumn (R2) compared to other seasons due to the aromatic nature of the soluble compounds found in leaves, which end up deposited into water sources during the autumn season. Advanced NOM characterization technique, FEEM, gave more and deeper understanding about the composition of NOM in water. FEEM showed that all the raw water samples contain, amongst others, the aromatic protein fraction. NOM fractions (humic and fulvic) were also observed albeit in different quantities in raw waters of VP, HL and P treatment plants. FEEM also proved that the observed high UV254 removal efficiency for VP, HL and P treatment plants was because of the presence of high content of humic substances in the raw waters of these treatment plants. FEEM was also used to link the treatability of NOM to various treatment processes (i.e coagulation and filtration) of P treatment plant. Water after the coagulation showed little traces of humic and fluvic components compared to the raw water samples. Whereas, water after filtration showed very little or no traces of humic fractions.
The N, Pd co-doped TiO2 (0.0-1.0%) was evaluated for its photodegradation efficiency towards NOM containing water samples under visible-light irradiation. The highest photodegradation of 58.8% was achieved with NPT (0.5% Pd) on MV raw water samples. The results were in close approximation to those of conventional processes applied at MV treatment plant (60.0%). NPT (0.5% Pd) was also used to conduct the treatability studies with NOM containing samples obtained from various raw water samples. The results showed different UV254 (aromatic content of NOM) removal efficiencies thus proving the varying character of NOM from various water sources. On the other hand, MWCNTs/N, Pd co-doped TiO2 (CT) (0.5 - 5.0%) nanocomposites were evaluated for their photocatalytic efficiency towards P raw water samples. It was observed that the highest photocatalytic activity was with 1.0% MWCNTs. About 91.2% (UV254) reduction was achieved with CT (1.0% MWCNTs), which is much higher compared to 68.2% achieved with NPT (1.0% Pd). The observed enhanced UV254 reduction is attributable to the large surface area of TiO2 which allows bigger amounts of NOM to be adsorbed onto the surface of the TiO2. Adsorption of high amounts of NOM on the surface of the TiO2 permits the photogenerated radicals to have enough time to interact with NOM.
No keywords provided.
2017-06-01T00:00:00ZFabrication and chararcterization of spectrally selective solar absorber copper oxide (CuO) nanocoatings for photothermal applicationWelegers, Giday Gebregziabherhttps://hdl.handle.net/10500/305822023-10-23T09:05:44Z2022-11-01T00:00:00ZFabrication and chararcterization of spectrally selective solar absorber copper oxide (CuO) nanocoatings for photothermal application
Welegers, Giday Gebregziabher
Solar-to-thermal energy is considered to be the most direct way of converting solar radiation
into usable forms of energy for a wide range of applications, including seawater desalination,
heating water, photocatalysis, space heating and cooling, thermophotovoltaics etc. Spectrally
solar selective absorber (SSSA) surfaces are the major components in photothermal energy
conversions and ideally exhibits a high solar absorptance (α ≥ 0.90) in the wavelength rage
(300 ≤ λ ≤ 2500nm) and low emissivity (ε ≤ 0.10) in the IR wavelength range (λ > 2500nm).
Copper oxide (CuO, tenorite) is a transitional metal oxide from two elements copper (Cu,
([Ar]4s1
3d10)), and oxygen (O, [He]2s2
2p4
). The Cu ions are coordinated by four oxygen ions
in a monoclinic phase of CuO crystals. Basically, CuO is a p-type semiconductor due to Cu
vacancies, and interstitial oxygen within the structure, and it has narrow band gap values of
1.2-1.9 eV that allow it to have a high solar absorptivity in the solar region.
In this investigation, spectrally selective single-layered CuO and Ag@CuO nanocermet
coatings deposited on stainless steel (SS) substrate are introduced. The SS has been widely
used as a substrate for various range applications due to its thermal and chemical stability,
environmental friendliness, and good optical properties. CuO and its plasmonic nanocermet
coatings were successfully demonstrated using facile and reproducible green synthesis,
electrodeposition, and sputtering methods aimed at high absorptance(α), and low emissivity
(ε) values for solar-to-thermal conversion application. In green synthesis, spectrally selective
single-layered CuO nanocoatings and Ag@CuO nanocermet coatings were synthesized from
copper nitrate trihydrate (Cu(NO3)2.3H2O) and silver nitrate (AgNO3) salt precursors using
plant extract (cactus pear) as stabilizing and reducing agent and then deposited on SS
substrates using spin coater at 700, 800, 900, and 1000 rpm. In electrodeposition, the Cu thin
films were reduced on the electrode or SS substrate surface from Cu(NO3)2.3H2O electrolyte at 15, 20 and 25 min deposition time at room temperature and then annealed in a furnace,
results in the growth of nanostructured CuO. Conversely, the Cu films were deposited using
RF sputtering on SS substrate at different thicknesses and then oxidized in alkaline solution at
room temperature. The morphological, structural, compositional, chemical states and
thickness of the coatings were analysed using scanning electron microscopy (SEM), Atomic
microscopy (AFM), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS),
X-ray photometer spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS).
The SEM images confirmed the growth of CuO nanorods, nanowalls, and nanoplates (NPs) from green synthesis, electrodeposition, and sputtering methods, respectively. The Ag@CuO
nanocermet coatings also showed a better dispersibility of white plasmonic Ag NPs in the
nanorods of CuO matrix. The XRD patterns revealed a well-crystalline nature of the
monoclinic phase of CuO, and face centered cubic of Ag metal, the incorporated Ag NPs did
not affect the monoclinic phase of CuO. The EDS clearly confirms compositional purity of
the coatings. The grain size, surface roughness and crystalline size of the coatings depend on
the thickness of the coatings, and were found to increase with coating thickness. The content
of the elements in the coatings and the thicknesses of the coatings were determined by RBS.
The thickness of the coatings is calculated to be 1416×1015 atoms/cm2 (298.2 nm), 1296×1015
atoms/cm2 (272.8 nm), 1153×1015 atoms/cm2 (242.7 nm) and 998×1015 atoms/cm2
(210.2 nm)
at 700, 800, 900, and 1000 rpm, respectively. Raman spectra showed peaks attributed to
Raman active (Ag+2Bg) modes which are characteristics of Cu-O stretching vibrations and
XPS spectra revealed peaks of Cu2p, O1s, and Ag3d core levels; These peaks are typical
characteristics of Cu (II), O(II) and Ag(I), respectively.
The optical properties of CuO nanocoatings, and Ag@CuO nanocermet coatings was
characterized as spectrally selective absorbers using UV-Vis-NIR, and IR spectrometers. The
vital solar selectivity parameters of solar absorptivity (α) and emittance (ε) were evaluated,
respectively from UV-Vis-NIR and IR spectral reflectance in a wavelength range of 300-
2500, and 2500-20000 nm. The optimized coatings exhibit a solar absorptance (α = 0.93, 0.92
and 0.97), and thermal emissivity of (ε = 0.23, 0.28, and 0.40) from green synthesis,
electrodeposition, and sputtering methods, respectively. The incorporated Ag NPs improved
the intrinsic absorption and reflectivity properties of green synthesized CuO nanocoatings
from (α/ε = 0.90/0.31) to (α/ε = 0.93/0.23) at 700 rpm. This is due to the concentrated free
electrons which contribute a plasma resonance frequency and its particle sizes are comparable to or smaller than the wavelength of incident light. The optical bandgap energy (Eg) of CuO
coatings was estimated from reflectance spectra using Kubelka-Munk (K-M) function and
found in the range of 1.65-1.27 eV. The lower band gap values are attributed to higher solar
absorption above the band gap energy. Hence, the CuO nanocoatings and Ag@CuO
nanocermet coatings are capable of a potential candidate(s) for SSSA surfaces in solar to
thermal energy conversion systems.
2022-11-01T00:00:00Z