Le système turbiditique profond associé au fleuve Ogooué s’étend sur plus de 500 km au large des côtes Gabonaises. Le bassin versant du fleuve est soumis à un climat équatorial propice à la mise en place d’une couverture végétale très dense et les apports fluviatiles en nutriments conduisent à une forte productivité primaire océanique dans le Golfe de Guinée. Ce système est ainsi un objet d’étude privilégié pour analyser les différents modes d’accumulation de matière organique au niveau d’une marge continentale dominée par un fleuve. Une analyse morpho-bathymétrique de la zone, associée à l’interprétation de données de sismique très haute résolution a permis d’appréhender l’architecture et le mode de fonctionnement de ce système. Cette étude a permis de mettre en évidence une zone à forte densité de chenaux turbiditiques méandriformes alimentés par une rampe de plusieurs canyons entaillant le plateau continental mais non connectés au delta actuel du fleuve. La zone distale de lobes est quant à elle située au débouché d’une large vallée qui canalise les écoulements turbiditiques. Des analyses sédimentaires (granulométrie, XRF, DRX) et géochimiques (%Corg, δ13Corg, palynofaciès, n-alcanes et tétraéthers) ont été réalisées sur neuf carottes Küllenberg prélevées dans différents environnements de dépôt du système. L’objectif est d’étudier la teneur en matière organique et son origine en fonction du mode de dépôt des sédiments et de comprendre la dynamique spatio-temporelle de son accumulation sédimentaire. Les analyses réalisées ont permis de révéler une binarité marquée dans les modes d’accumulation au sein de ce système. Les sédiments silto-argileux issus d’une sédimentation hémipélagique apportent peu de carbone organique continental mais une quantité importante de carbone organique marin qui forme des agrégats organo-minéraux avec les minéraux argileux de smectite exportés par le fleuve. Les écoulements turbiditiques sableux permettent quant à eux de faire transiter jusque dans les lobes distaux de fortes concentrations de débris végétaux très bien conservés. Ces séquences turbiditiques peuvent entrainer des enrichissements marqués en carbone organique continental (>5% TOC) au niveau des levées des chenaux turbiditiques et des dépôts sableux de lobes et constituent le principal mode de dépôt de Corg dans ces éléments architecturaux. La teneur en matière organique est également très fortement modulée par les variations glacio eustatiques. Durant les périodes de bas niveau marin relatif, l’accumulation organique est bien plus forte que durant les périodes de haut niveau marin. Ces fluctuations s’expliquent par l’effet combiné d’une plus forte productivité marine enregistrée en période glaciaire et d’un transfert sédimentaire entre le continent et le domaine profond très intensifié grâce au bas niveau marin. L’activité du système turbiditique est en effet dépendante de la connexion entre le système fluviatile et les têtes de canyons qui n’est possible que lorsque le niveau marin est bas. Bien que l’export de carbone continental vers l’océan dans ce système soit fort, les estimations du taux de préservation du carbone continental exporté par le fleuve montrent que la majorité du carbone est reminéralisé dans l’océan. Les taux de sédimentation relativement faibles sur l’ensemble du système du fait de la prédominance de la sédimentation hémipélagique sur la sédimentation turbiditique ne permettent pas une préservation optimale de la matière organique. / The Ogooué deep-sea turbidite system is located on the Gabonese margin and extends up to 500 km off the coastline. A dense vegetation due to the wet equatorial climate prevails on the Ogooue drainage basin and a strong primary productivity is observed in the waters of the Gulf of Guinea. These two features make the Ogooue system a good example to study the characteristics of organic carbon accumulation on a river-dominated ocean margin. A morpho-bathymetric analysis of the area combined with a study of seismic lines allowed us to define the architecture and the dynamic of the system. It is composed of an area where numerous meandering channels are localized and fed in sediments by several aligned canyons. The canyons heads incise the shelf but do not extend to the present-day Ogooué delta. Terminal depositional lobes are present at the exit of a large valley that funnels turbidity currents. Sedimentary (grain size, XRF, XRD) and geochemical (%Corg, δ13Corg, palynofacies, n-alkanes and GDGTs) analysis were performed on nine Küllenberg cores taken in different sedimentary environments of the system. These analyses aim at defining the Corg content of the cores as well as its origin and to link these parameters with the sedimentation type. The temporal and spatial repartition of carbon accumulation was also considered. Results show that hemipelagic sedimentation appears as quite inefficient to transport terrigenous organic carbon but is effective to bury marine organic carbon that forms aggregates with the smectite clay particles supplied by the river. Conversely, the sandy turbidites transport high quantity of terrigenous Corg to the deep sea areas. This Corg appears mostly under the form of well-preserved plant debris. These organic-rich turbidites (>5% TOC) form the main way of deposition of organic carbon in the levees of the turbidite channels and in the terminal lobes. Organic carbon accumulation is also strongly controlled by glacio eustatic variations. Low sea-level periods are characterized by very high organic carbon accumulation compared with high sea-level periods. Theses variations are the results of the combination of a higher marine productivity during glacials and a more intense export of terrigenous particles (both mineral and organic) during sea-level lowstand. The turbidite system activity is indeed governed by the connection between the canyons ‘heads and the riverine delta. This connection occurs only during lowstand. However, even though the export of organic carbon in the system is strong, the estimated preservation rates attest that most of the terrigenous organic carbon is mineralized in the ocean and not buried in the sediments. Globally low sedimentation rate on the whole system and the domination of hemipelagic over turbiditic sedimentation do not allow a better preservation of the organic material.

Natural organic matter (NOM), ubiquitous in natural water sources, is generated by biogeochemical processes in both the water body and in the surrounding watershed, as well as from the contribution of organic compounds that enter the water as a result of human activity. NOM significantly affects the properties of the water source, including the ability to transport metals, influence the aggregation kinetics of colloidal particles, serve as a food source for microorganisms and act as a precursor in the formation of disinfection by-products (DBPs), as well as imparting a brown colour to the water. The reactivity of NOM is closely tied to its physicochemical properties, such as aromaticity, elemental composition, functional group content and molecular weight (MW) distribution. The MW distribution is an important consideration from a water treatment perspective for several reasons. For example, low MW NOM decreases the efficiency of treatment with activated carbon, and this fraction is thought to be the portion most difficult to remove using coagulation. The efficiency of membranes in the treatment of drinking water is also influenced by the MW distribution of NOM, while some studies have shown that the low MW fraction contributes disproportionately to the formation of bioavailable organic matter, therefore promoting the formation of biofilms in the distribution system. For these reasons, understanding the MW distribution of NOM is important for the treatment of natural waters for use as drinking waters. Optimisation of a high pressure size exclusion chromatography (HPSEC) method for analysis of the MW distribution of NOM in natural waters is described (Chapter 2). Several parameters influencing the performance of HPSEC are tested and an optimised set of conditions illustrated. / These parameters included eluent composition, ionic strength of the sample, flow rate and injection volume. Firstly, it was found that increasing the ionic strength of the HPSEC eluent resulted in less exclusion of NOM from the stationary phase. Stationary phases used in HPSEC contain a residual negative charge that can repel the negatively charged regions of NOM, effectively reducing the accessible pore volume. By increasing the ionic strength, interactions between the stationary phase and eluent enabled a larger effective pore size for the NOM analytes. However, increasing ionic strength of the eluent also resulted in a loss of peak resolution for the NOM portion able to access the pore volume of the stationary phase. Determining the ideal eluent composition required the balancing of these two outcomes. Matching of the ionic strength of the sample with the eluent was also an important consideration. Retention times were slightly lower when the sample ionic strength was not matched with the eluent, especially for the lowest MW material, although the effect on chromatography was minimal. Flow rate had no effect on the resolution of the HPSEC chromatogram for the portion of material able to permeate the pore space of the stationary phase. Changes in the volume of sample injected had a marked effect on the elution profile of the NOM sample. Besides the obvious limitation of detection limit, only minor changes in elution profile were obtained up to an injection volume of 100 µL. Volumes above this value, however, resulted in significant peak broadening issues, as well as an undesirable effect on the low MW portion of detected DOC. / In Chapter 3, high pressure size exclusion chromatography with UV254 [subscript] and on-line detection of organic carbon (HPSEC-UV254[subscript]-OCD) was used to compare the removal of different apparent MW fractions of DOC by two process streams operating in parallel at the local Wanneroo groundwater treatment plant (GWTP). One of these two process streams included alum coagulation (operating in an enhanced coagulation mode (EC) for increased DOC removal) and the other stream included a magnetic ion exchange (MIEX®) process followed by alum coagulation (MIEX®-C). The MIEX® process is based on a micro-sized, macroporous, strong base anion exchange resin with magnetic properties, which has been designed to remove NOM through ion exchange of the anionic sites in NOM. Water was sampled from five key locations within these process streams, and the DOC at each location was characterised in terms of its MW distribution. HPSEC was carried out using three different on-line detector systems, namely OCD, UV absorbance detection at 254 nm, and fluorescence detection (λex[subscript]= 282 nm; λim[subscript] = 353 nm). This approach provided significant information on the chemical nature of the DOC in the various MW fractions. The MIEX®-C process was found to outperform the EC process: these two processes removed similar amounts of high and low MW DOC, but the MIEX®-C process showed greater removal of DOC from the intermediate MW fractions. The two coagulation processes (EC and coagulation following MIEX®) showed good removal of the fractions of highest MW, while the MIEX® process alone was found to remove DOC across all MW fractions. / These results seem to indicate that anionic groups, particularly susceptible to removal with MIEX® treatment, are well distributed across all MW fractions of NOM. In agreement with previous studies, MIEX®-C outperformed EC in the overall removal of DOC (MIEX®-C removed 25 % more DOC than EC). However, 70% of the additional DOC removed by MIEX®-C was comprised of a surprisingly narrow range of medium-high MW fractions. The development of a novel online organic carbon detector (OCD) for use with HPSEC for determining the MW distribution of NOM is described in Chapter 4. With UV absorbance detection, the magnitude of the signal is based on the extinction coefficient of the chromophores in the analytes being investigated; whereas the signal from an OCD is proportional to the actual organic carbon concentrations, providing significantly more information. The development of an online OCD involved the separation of analytes using HPSEC, removal of inorganic carbon species which may interfere with organic carbon determination, oxidation of the organic carbon to carbon dioxide, separation of the produced carbon dioxide from the aqueous phase and subsequent detection of the gaseous carbon dioxide. In the new instrument, following separation of components by HPSEC, the sample stream was acidified with orthophosphoric acid to a concentration of 20 mmol L-1[superscript], resulting in a pH of ≤ 2, in order to convert inorganic carbon to carbon dioxide. This acid dose was found to remove greater than 99 % of inorganic carbon once the acidified sample was passed through a hydrophobic polytetrafluoroethylene (PTFE) membrane allowing the passage of dissolved gases (under negative pressure from a vacuum pump) but restricting the flow of the mobile phase. / Several factors influenced the oxidation of the organic carbon in the next step, including the dose of persulfate, the type and intensity of UV radiation and the composition of the capillary through which the sample stream passes. Through optimisation of this process, it was found that a persulfate dose of 0.84 mmol L-1[superscript] in the sample stream was required for optimum oxidation efficiency. A medium pressure UV lamp was compared to a vacuum UV lamp for its efficiency in oxidation of organic carbon to carbon dioxide. While the medium pressure lamp produced a far smaller percentage of its total radiation at the optimum wavelength for oxidation of organic compounds, the greater overall intensity of the medium pressure lamp was shown to be superior for this application. The composition of the capillary was shown to have a considerable effect on the oxidation efficiency. A quartz capillary, internal diameter 0.6 mm, was compared with a PTFE capillary, internal diameter 0.5 mm, for the oxidation of organic carbon by external UV treatment. While peak width, an important consideration in chromatographic resolution, was greater for the larger internal diameter quartz capillary, the lower UV transparency of PTFE combined with the shorter contact time, due to the reduced internal diameter of the capillary, resulted in a less efficient oxidation step using the PTFE capillary. The quartz capillary was therefore chosen for use in the UV/persulfate oxidation step for oxidation of organic carbon to carbon dioxide. Separation of the produced carbon dioxide from the sample stream was achieved by sparging with nitrogen and contacting the gas/liquid mixture with a hydrophobic PTFE membrane, restricting the passage of the liquid while allowing the nitrogen and carbon dioxide gases to pass to the detection system. / The only factor influencing this separation was the flow of the nitrogen sparge gas, with a flow of 2 mL min-1[superscript] found to be optimum. Detection of produced carbon dioxide was via a Fourier transform infrared (FTIR) spectrometer with a Iightpipe accessory. The Iightpipe accessory was designed for use as a detector for gas chromatography and the small size of the detector cell was ideal for use with this application. Using the new system described, concentrations of a single peak could be determined with a detection limit of 31 ng and a determination limit of 68 ng. The development of the new OCD allowed characterisation of NOM in terms of its MW distribution and the UV and fluorescence spectral properties of each MW fraction. Further characterisation of MW fractions of NOM from a local groundwater bore was carried out by separation of the fractions by preparative HPSEC, followed by off-line analysis. Preparative HPSEC involved the injection of a pre-concentrated groundwater sample multiple times, using a large scale HPSEC column, then collecting and combining material of identical MW. This allowed each MW fraction of the sample to be further characterised as described in Chapter 5. Preparative HPSEC has only previously been applied to a small number of samples for the concentration and fractionation of NOM, where the structural features of the various MW fractions were studied. In the current research, more extensive studies of not only the chemical characteristics, but also the disinfection behaviour, of the MW fractions were conducted. Separation of the sample was conducted on a large diameter silica-based HPSEC column, with fraction collection based on semi-resolved peaks of the HPSEC chromatogram. Nine MW fractions were collected by this method. / After concentration and dialysis to remove the buffer salts in the HPSEC mobile phase, each fraction was re-analysed by analytical HPSEC-UV254[subscript] and showed a single Gaussian shaped peak, indicating discrete MW fractions had successfully been collected. Analysis of the collected MW fractions indicated that 57 % of the organic carbon was in Fractions 3 and 4, with 41 % in Fractions 5-9, leaving only 2 % in Fractions 1 (highest MW) and 2. For each of the nine MW fractions, chorine demand and 7 day trihalomethane formation potential (THMFP) were measured on dilute solutions of the same DOC concentration, and solid state 13[superscript]C NMR spectra were recorded on some of the solid isolates obtained after Iyophilisation of the separate or combined dialysis retentates. The larger MW Fractions 3 and 4 were found to contain a greater proportion of aromatic and carbonyl carbon, and the lower MW Fractions 5 and 6 and Fractions 7-9 contained greater proportions of aliphatic and O-aliphatic carbon, by this technique. Chlorine demand experiments on each individual fraction with a normalised DOC concentration indicated that the largest MW fraction (Fraction 1) had the lowest chlorine demand. It was concluded that material in this fraction may be associated with inorganic colloids and unavailable for reaction with chlorine. Fraction 3 had the highest chlorine demand, just over two times more than the next highest chlorine demand (Fraction 4) and approximately three times the chlorine demand of Fraction 2. The organic material in Fraction 2 was postulated to contain a mixture of the reactive material present in Fraction 3 and the colloidal associated material present in Fraction 1. / NMR analysis indicated that the difference between Fraction 3 and Fraction 4 was a reduction in reactive aromatic carbon and hence the lower chlorine demand in the latter fraction. Fractions 5-8 had similar chlorine demands, lower than Fraction 4, while Fraction 9 had a very low chlorine demand similar to that of Fraction 1. For Fractions 5-9, the lower aromatic carbon content most likely resulted in the lower chlorine demand. The 7 day THMFP experiments showed some clear trends, with Fraction 1 and Fraction 2 producing the least amounts of THMs but having the greatest incorporation of bromine. Fractions 3 and 4 produced the greatest concentration of THMs with the lowest bromine incorporation, perhaps as they contained fast reacting THM precursors and the higher chlorine concentrations resulted in greater amounts of chlorinated THMs. Fraction 5 and Fraction 6 produced similar levels of THMs over 7 days to Fractions 7-9 (approximately 75% of the amount formed by Fractions 3 and 4), however, Fractions 7-9 formed these THMs more quickly than Fractions 5 and 6, with slightly greater amounts of bromine incorporation. It was thought that the increased speed of formation was due to the smaller MW of these fractions and a simpler reaction pathway from starting material to formation of THMs, as well as some structural differences. This research marks the first report of significantly resolved MW fractions being isolated and their behaviour in the presence of a disinfectant being determined. While the high MW fractions had the greatest chlorine demands and THMFPs, these fractions are also the easiest to remove during coagulation water treatment processes, as shown in Chapter 3. The lowest MW material formed significant amounts of THMs, and also formed THMs more quickly than other MW fractions. / This has important implications from a water treatment perspective, as the lowest MW material is also the most difficult to remove during conventional treatment processes. Solid samples of NOM were isolated from water samples taken from four points at the Wanneroo GWTP using ultrafiltration and subsequent Iyophilisation of the retained fractions, as described in Chapter 6. The sampling points were following aeration (Raw), following treatment by MIEX®, following treatment by MIEX®-C and following treatment by EC. Elemental analysis, FTIR spectroscopy, solid state 13[superscript]C NMR spectroscopy and HPSEC-UV254[subscript]-0CD analysis were used to compare the four isolates. Treatment with MIEX®-C was found to remove the greatest amount of NOM. Additionally, treatment with MIEX®-C was able to remove the largest MW range of NOM, with the remaining material being depleted in aromatic species and having a greater proportion of aliphatic and O-aliphatic carbon. EC treatment completely removed the NOM components above 5000 Da, but NOM below this was not well removed. NOM remaining after the EC train had a lower aromatic content and more aliphatic oxygenated organic matter than the RW. The remaining organic matter after MIEX® treatment contained less aromatic material compared to the RW, but had a greater aromatic content than either of the EC or MIEX®-C samples. HPSEC was a significant analytical technique used throughout this research. Initial optimisation of an HPSEC method was an important development which allowed improved resolution of various MW fractions. The application of this technique and comparison of three detection systems for the study of DOC removal showed, for the first time, the performance of MIEX® treatment at a full scale groundwater treatment facility. / The use of various HPSEC detection systems allowed significant characterisation of the MW fractions, more information than had previously been gathered from such a sample set. This work demonstrated the need for OCD when applying HPSEC to the study of NOM. As such, a system was constructed that built on previously developed systems, with the use of a small detector cell enabling detection limits capable of measuring even the most dilute natural and treated water samples. To study the individual MW fractions in detail, preparative HPSEC was applied and, for the first time, the disinfection behaviour of various MW fractions was examined. Interestingly, the lowest MW fractions, acknowledged to be the most recalcitrant to conventional water treatment processes, produced significant quantities of THMs. Also the formation kinetics of THMs from the low MW fractions indicated that THMs were formed as quickly as, or perhaps even at faster rates than from the larger MW fractions. Finally, structural characterisation of NOM at four stages of the Wanneroo GWTP indicated MIEX®-C treatment was superior to EC, of significant interest for the water industry.

<p>This thesis presents results from several studies that have focused on the carbon and nutrient dynamics in soils at the forest – tundra ecotone in Fennoscandia. The main objectives of the study were: (i) to investigate the links between the physical environment, above-ground vegetation communities, soil carbon storage, nutrient status and the chemical composition of the soil organic matter (SOM), and (ii) to quantify trace gas fluxes (methane and carbon dioxide) between mesic soils and the atmosphere. Four main field areas spanning an 8 degree latitudinal gradient were established at the ecotone in 1998 and studied for four years. In addition to the natural gradients we also established a warming treatment. Decomposition rates (i.e. carbon dioxide efflux and litter decomposition) were higher at our forest sites. This was linked principally to the more favourable physical environment at the forest sites, rather than to SOM quality, despite some indications of higher SOM quality at forest sites based upon conventional chemical analysis and ¹³C NMR techniques. Tundra soils stored large amounts of potentially labile carbon that could readily be accessed by microorganisms when transferred to a forest environment. The interrelation between increased soil temperature and reduced soil moisture content is likely to moderate the response of decomposition rates to increased temperatures. Generally, these mesic soils showed net methane uptake from the atmosphere, which was enhanced by the warming treatment. No differences between forest or tundra soils could be detected.</p><p>The major conclusions presented here are that (1) soil carbon storage is likely to be reduced if mountain birch forest replaces tundra heath and (2), methane uptake in mesic soils in the Fennoscandian mountains represents a negative feedback to further environmental change in a warmer climate.</p>

This thesis presents results from several studies that have focused on the carbon and nutrient dynamics in soils at the forest – tundra ecotone in Fennoscandia. The main objectives of the study were: (i) to investigate the links between the physical environment, above-ground vegetation communities, soil carbon storage, nutrient status and the chemical composition of the soil organic matter (SOM), and (ii) to quantify trace gas fluxes (methane and carbon dioxide) between mesic soils and the atmosphere. Four main field areas spanning an 8 degree latitudinal gradient were established at the ecotone in 1998 and studied for four years. In addition to the natural gradients we also established a warming treatment. Decomposition rates (i.e. carbon dioxide efflux and litter decomposition) were higher at our forest sites. This was linked principally to the more favourable physical environment at the forest sites, rather than to SOM quality, despite some indications of higher SOM quality at forest sites based upon conventional chemical analysis and 13C NMR techniques. Tundra soils stored large amounts of potentially labile carbon that could readily be accessed by microorganisms when transferred to a forest environment. The interrelation between increased soil temperature and reduced soil moisture content is likely to moderate the response of decomposition rates to increased temperatures. Generally, these mesic soils showed net methane uptake from the atmosphere, which was enhanced by the warming treatment. No differences between forest or tundra soils could be detected. The major conclusions presented here are that (1) soil carbon storage is likely to be reduced if mountain birch forest replaces tundra heath and (2), methane uptake in mesic soils in the Fennoscandian mountains represents a negative feedback to further environmental change in a warmer climate.

Fjords contain a significant quantity of sediments deposited in coastal zones over the last ~100,000 years. Studies of Northern Hemisphere fjords have shown that a large part of the high concentration of sedimentary organic matter (OMsed) is terrestrial in origin (OMterr), composed of a modern detrital fraction and an old mineral-associated fraction (OMfossil). These results suggest that fjords are disproportionately responsible, on a per area basis, for the burial of organic matter in coastal zones. This study, after a rigorous examination of CuO and GDGT biomarker methods used to quantify terrestrial organic matter in coastal environments, demonstrated this hypothesis in a Southern Hemisphere fjord system, Fiordland, New Zealand. CuO analysis of Doubtful Sound surface sediments indicated a large contribution of vascular plant material to fjord sediments. The BIT Index correlated strongly with both delta13C and C/N values in Doubtful Sound surface sediments, indicated that it may accurately trace the relative proportions of marine and soil organic matter (OMsoil) in Fiordland. However, a detailed analysis of the conversion of the BIT Index to quantitative estimates of terrestrial (soil) organic matter revealed that these values are overestimates. Reconstructions of the BIT Index and tetraethers in cores from two locations on the Louisiana continental shelf demonstrated the influence of the crenarchaeol term on BIT Index-based terrestrial organic matter estimates. The differences in the applicability of the BIT Index to these two coastal environments was most likely due to large seasonal changes in productivity on the Louisiana Continental Shelf as well as higher marine relative to terrestrial inputs. Six cores were reconstructed for contributions from marine OM (OMmar), OMfossil, and OMterrestrial representing the last ~500 years of sedimentation. Spatial variations were larger than temporal variations, owing to negligible development and deforestation in the region. OMterr was the dominant fraction in all but one core, and OMfossil inputs were significant. Additionally, source reconstructions from a variety of biomarkers indicated that Landslides deliver large volumes of detrital organic matter to fjord sediments. These results confirm that fjords bury quantitatively significant volumes of organic carbon on a global scale.

Darbā pirmo reizi veikts salīdzinošs reģionāls pētījums par organisko vielu satura izmaiņām Latvijas un Zviedrijas upēs mainīgās antropogēnās slodzes apstākļos, kompleksi izvērtēti vides kvalitātes organisko vielu satura raksturojošo parametru nacionālo monitoringu dati un to izmantojamība ilgtermiņa izmaiņu analīzē; veikts plašs pētījums par organiskā slāpekļa un oglekļa attiecību reģionālajām izmaiņām Zviedrijas upēs; pirmo reizi veikts komplekss pētījums par augsnes, virszemes ūdeņu un ezeru nogulumu humusvielām Latvijā, pierādīta virszemes ūdeņu un ezeru nogulumu humusvielu īpašību veidošanās atkarība no ūdeņos noritošo bioloģisko un ķīmisko procesu rakstura ūdenstilpnēs un tecēs; pētīta augsnes organisko vielu humifikācijas pakāpe un tās izmaiņu raksturs dažāda zemes lietojuma augsnēs Latvijā. / The flow of organic matter in furface waters of Latvia and Sweden under variable antropogenic impact

Polymer flocculants, traditionally used with primary coagulant to enhance flocculation and sedimentation, are used in the coagulation-/microfiltration process as well assuming they can improve membrane performance similarly. However, there are several uncertainties concerning the use of polymer flocculants in the coagulation-microfiltration process. First, polymer flocculants may not have measurable effect on turbidity removal, because microfiltration membranes can remove significantly smaller particles than those removed by the conventional treatment process. Second, the effect of using polymer flocculants on NOM removal has been controversial. Although a number of studies reported improved NOM removal when polymers were used, others reported no or negative impact of polymers on NOM removal. Third, polymer flocculants are high molecular weight organic compounds. When carried over to membrane residual polymers can potentially foul the membranes. Finally, the use of polymer flocculants will change floc properties (i.e. size, fractal dimension, and stickiness) and subsequently bring uncertain effect on cake layer resistance. Therefore, the role of polymer flocculants in coagulation-microfiltration system needs to be carefully assessed for system optimization. In the reported research, three types of polymer flocculants with different charge and molecular weights were tested for comprehensively evaluating the impact of polymer flocculants on the performance of coagulation-microfiltration of surface water. Operation conditions such as inline filtration, direct filtration, and filtration with sedimentation were included. Two membrane reactors were designed to study the mechanism through which polymer flocculants affect the performance of coagulation-microfiltration systems. The result demonstrated that the use of polymer flocculants provides little to no benefit to turbidity and NOM removal in most cases, but pDADMACs can enhance NOM removal if applied properly; All polymer flocculants significantly increased membrane fouling except for pDADMACs when sedimentation proceeds MF; Polymer flocculants increase deposition/attachment of floc particles on the membrane surface through both adsorption of residual polymer on the membrane surface and polymer molecules on the floc particle surface; Even though polymers form larger and more fractal floc particles, they did not have notable impact on cake layer structure.

<p>Mercury contamination in aquatic ecosystems is a concern as anaerobic aquatic sediments are the primary regions of methylmercury production in freshwater and coastal regions. Methlymercury is a bioaccumulative neurotoxin, and human exposure to methylmercury can result in impaired functioning of the central nervous system and developmental disabilities in children. To minimize the risk of human exposure to methylmercury, it is important to be knowledgeable of the various sources which can supply mercury to aquatic ecosystems as well as have a complete understanding of the biogeochemical processes which are involved in methylmercury production in aquatic systems. In this dissertation work, both mercury biogeochemical speciation in anaerobic aquatic sediments and sources of mercury to aquatic systems were addressed. </p><p>The biogeochemical speciation of mercury is a critical factor which influences the fate and transformation of mercury in aquatic environments. In anaerobic sediments, mercury chemical speciation is controlled by reduced sulfur groups, such as inorganic sulfide and reduced sulfur moieties in dissolved organic matter (DOM). The formation of mercury sulfide nanoparticles through stabilization by dissolved organic matter (DOM) was investigated in precipitation studies using dynamic light scattering. Mercury sulfide nanoparticles (particle diameter < 100 nm) were stabilized through precipitation reactions that were kinetically hindered by DOM. To further investigate the interaction between DOM and metal sulfides, similar precipitation studies were performed using zinc sulfide and a number of DOM isolates (humic and fulvic acids) representing a range of DOM properties. The results of these experiments suggest that the mechanism of metal sulfide particle stabilization may be electrostatic or electrosteric, depending on the nature of the DOM molecule.</p><p>The mercury that is methylated in aquatic systems enters these environments via a number of sources, including atmospheric deposition, landscape runoff and other industrial and municipal activities. In two separate field studies, two potential sources of mercury to aquatic systems were investigated: landscape runoff and coal combustion products. The mercury loading to aquatic environments from these sources and their potential for transformation to methylmercury were investigated.</p><p>Landscape runoff from a Duke University campus catchment (Durham, NC) was identified as a source of mercury to a stream-wetland. The source of mercury to the runoff was likely from a `legacy' source of mercury; the historic application of mercury fungicide compounds to turf grass during the 20th century. Downstream of the point where the runoff was discharged to the stream-wetland, methylmercury concentrations were detected in stream sediments (up to 11% of total mercury), suggesting that this legacy mercury could be transformed to methylmercury. </p><p>The environmental impact of coal combustion products (CCPs) with respect to mercury and methylmercury was also investigated in a river system (Roane County, TN) that was inundated with fly ash and bottom ash from the Tennessee Valley Authority Kingston coal ash spill in 2008. Elevated total mercury and methylmercury sediment concentrations (relative to upstream sediments) were detected in regions impacted by the ash spill, and our biogeochemical data suggested that the ash may have stimulated methylmercury production in river sediments.</p><p>The results of this dissertation work address the formation of mercury sulfide (along with zinc sulfide) nanoparticles in anaerobic aquatic sediments. In the current mercury methylation paradigm, dissolved mercury species such as Hg(SH)02(aq) and HgS0(aq) are assumed to be the only mercury species that are available for methylation. The results of this dissertation work suggests that in previous studies, HgS0(aq) may have been mistaken as mercury sulfide nanoparticles which may be formed in under supersaturated conditions (with respect to HgS(s)) where DOM is present. Mercury sulfide nanoparticles are a mercury biogeochemical species that has been largely ignored in the research literature and whose role in the mercury biogeochemical cycle and in mercury methylation remains to be investigated.</p><p> This dissertation work also identifies potential sources of mercury to aquatic systems, namely, landscape runoff and CCPs. Atmospheric deposition is currently considered to be the major source of mercury to inland aquatic water bodies compared to sources such as landscape runoff and CCPs. However, in the watershed studied in this dissertation, landscape runoff was identified as a larger source of mercury than atmospheric deposition, suggesting that these so-called `minor' sources may actually be major sources of mercury to watersheds depending on land usage, and should be considered in watershed models. Furthermore, the environmental hazards of mercury-associated with CCPs has typically been determined through leaching experiments, such as the Toxicity Characteristic Leaching Procedure (TCLP), which are not representative of environmental conditions and do not predict that CCPs may influence mercury methylation in aquatic sediments. Thus, in this dissertation work, we suggest that leaching protocols such as the TCLP should be re-evaluated. </p><p>Overall, this dissertation work will be useful in future studies examining mercury speciation and bioavailability to methylating bacteria in aquatic sediments, and the formation of metal sulfide nanoparticles in aquatic systems. Additionally, data on sources of mercury will be useful in developing policies for the regulation of these sources and in assessing the risk to human health from mercury methylation.</p> / Dissertation

<p>Methylmercury is a bioaccumulative neurotoxin that severely endangers human health. Humans are exposed to methylmercury through consumption of contaminated aquatic fish. To date, effective strategies for preventing and remediating methylmercury contamination have remained elusive, mainly due to the lack of knowledge in regard to how methylmercury is generated and degraded in the aquatic environment. The goal of this dissertation was to study the mechanisms of two transformation processes that govern the fate of methylmercury in natural settings: microbial mercury methylation and methylmercury photodegradation. The role of mercury speciation (influenced by environmental conditions) in determining the reactivity of mercury in these biological and photochemical reactions was the focus of this research.</p><p>Methylmercury production in the aquatic environment is primarily mediated by anaerobic bacteria in surface sediments, particularly sulfate reducing bacteria (SRB). The efficiency of this process is dependent on the activity of the methylating bacteria and the availability of inorganic divalent mercury (Hg(II)). In sediment pore waters, Hg(II) associates with sulfides and dissolved organic matter (DOM) to form a continuum of chemical species that include dissolved molecules, polynuclear clusters, amorphous nanoparticles and after long term aging, bulk-scale crystalline particles. The methylation potential of these mercury species were examined using both pure cultures of SRB and sediment slurry microcosms. The results of these experiments indicated that the activity of SRB was largely determined by the supply of sulfate and labile carbon, which significantly influenced the net methylmercury production in sediment slurries. The availability of mercury for methylation decreased during aging. Dissolved Hg-sulfide (added as Hg(NO3)2 and Na2S) resulted in the highest methylmercury production. Although the methylation potential of humic-coated HgS nanoparticles decreased with an increase in the age of nanoparticle stock solutions, nano-HgS was substantially more available for microbial methylation relative to microparticulate HgS, possibly due to the smaller size, larger specific surface area and more disordered structure of the nanoparticles. Moreover, the methylation of mercury derived from nanoparticles cannot be explained by equilibrium speciation of mercury in the aqueous phase (<0.2 <em>f</em>Ým, the currently-accepted approach for assessing mercury bioavailability for methylation). Instead, the methylation potential of mercury sulfides appeared to correlate with the extent of dissolution and their reactivity in thiol ligand exchange. Additionally, partitioning of mercury to a diverse group of bulk-scale mineral particles and colloids (especially FeS) may be an important process controlling the mercury speciation and subsequent methylmercury production in natural sediments.</p><p>In surface waters, sunlight degradation is believed to be the predominant pathway for the decomposition of methylmercury. The mechanism of this process was investigated in a series of photodegradation experiments under natural sunlight and UV-A radiation, and in the presence of DOM and selective quenchers for photo-generated reactive intermediates. The results suggested that singlet oxygen generated from photosensitization of DOM drove the photodecomposition of methylmercury. The rate of methylmercury degradation depended on the type of methylmercury (CH3Hg+) binding ligand present in the water. CH3Hg -thiol (e.g., glutathione, mercaptoacetate, DOM) complexes were significantly more reactive in photodegradation compared to other methylmercury complexes (CH3HgCl or CH3HgOH), which may be because thiol-binding can effectively decrease the activation energy and thus enhance the reactivity of methylmercury molecules toward the Hg-C bond breaking process. These findings challenge the long-accepted view that water chemistry characteristics do not affect the kinetics of methylmercury sunlight degradation, and help explain recent field observation that methylmercury photodegradation occurred rapidly in freshwater lakes (where CH3Hg-DOM dominate methylmercury speciation) but relatively slowly in sea water (where CH3Hg-Cl control methylmercury speciation).</p><p>Overall, this dissertation has demonstrated that chemical speciation of inorganic mercury and methylmercury determines their availability for microbial methylation and sunlight degradation, respectively. The abundance of these available mercury species is influenced by a variety of environmental parameters (e.g., DOM). This dissertation work contributes mechanistic knowledge toward understanding the occurrence of methylmercury in the aquatic environment. This information will ultimately help construct quantitative models for accurately predicting and assessing the risks of mercury contamination.</p> / Dissertation

The repercussions of climate change are felt worldwide, but Arctic and subarctic regions, where climate warming is expected to be amplified, are especially vulnerable. An episode of mass fish mortality in the Sutton River in the Hudson Bay Lowlands (HBL) of Northern Ontario has elicited the interest of the scientific community. Several lakes were sampled over three years in an effort to better understand and document the changes that may be occurring in these lakes. This study uses sediment cores to assess the history of mercury (Hg) deposition and to assess changes occurring in autochthonous productivity in these lakes. Sediments deposited after the onset of the industrial revolution contained significantly higher concentrations of Hg, with the highest concentrations found in the most recently deposited sediments. Hg concentrations in these pristine lakes rival those of lakes in heavily urbanized areas, indicating that they are in fact subjected to atmospheric deposition of Hg. There was a large variation in [Hg] of the surface sediments of 13 lakes; underscoring the importance of in situ processes in the fate of atmospherically deposited Hg. Methylmercury (MeHg) concentrations were not correlated with total mercury concentrations (THg), demonstrating how THg is a poor predictor of MeHg; the bioaccumulative neurotoxic form of mercury. The S2 fraction of Rock-Eval® Pyrolysis, C:N ratios and ∂13C signatures were used as proxies of autochthonous carbon and all indicated that the lakes have become increasingly productive, presumably due to warmer water temperatures and longer ice-free seasons. Additionally, I use molecular techniques to detect and quantify the merA gene in the sediment; a proxy of bacterial mercury resistance involved in redox transformations. In Aquatuk, Hawley and North Raft Lakes, I observed a subsurface increase in merA genes in the sediment core, independently of a control gene and the [THg]. While I have not been able to explain the driving variables of this subsurface increase, I believe that the role of merA within remote lake sediments deserves further work. Lastly, microcosms were used to measure the production of volatile elemental mercury (Hg(0)) from surface sediments of Aquatuk Lake. I used a combination of analytical and molecular techniques to show that the production of Hg(0) is biogenic and tested the effect of nutrients, pH and ionic strength on the Hg(0) production rates. Ionic strength alone had the greatest impact on Hg(0) production rates, with increased Hg(0) production as ionic strength increases.