Duc Huy Dang

Surges in gold prices coupled with socio-economic factors have increased small- and medium-scale gold mining in Guyana, which drives the emission of large quantities of mercury (Hg) into the atmosphere. Given the distribution of Hg through the atmosphere, reliable and affordable measurement of concentrations in the air is vital. Moss bags and Hg passive air samplers (MerPAS) were deployed to measure atmospheric Hg around a gold mine in Mahdia, Guyana over a 90-day period as well as a separate 2-day period that encompassed a periodic burn of Hg-gold amalgam (typically 2-h). Mercury in moss and MerPAS were positively correlated over both deployment periods, but Hg concentrations measured during the 2-day event were several-fold higher in both moss and MerPAS compared with the 90-day deployment. Using the 2-day deployment as an estimate of atmospheric Hg, Hg air concentrations around the burning station exceeded 100,000 ng m⁻³ averaged over a 48-h period, and moss Hg concentrations were greater than 250,000 ng g⁻¹ around the burning station, although Hg concentrations in both media decreased rapidly with distance. There was no relationship between Hg in moss and soil Hg at the sampling sites; Hg in soil was instead positively associated with soil organic matter content. Overall, our study shows that Hg concentrations in air at artisanal gold mines in Guyana can exceed international health guidelines of 100,000 ng m⁻³ averaged over a 48-h period and that moss provides reliable estimates of relative Hg concentrations in air over both short-term and long-term exposures.

The use of gadolinium (Gd)-based contrast agents for medical imaging analyses has led to extensive enrichment of Gd in aquatic systems, which is often quantified as positive Gd anomalies. This study aims to evaluate the bioavailability and toxicity of Gd3+ and 2 Gd-based contrast agents (Gd dihydrogen salt hydrate [Gd-DTPA] and gadobutrol monohydrate [Gd-BT-DO3A]) on the green alga Chlamydomonas reinhardtii in the presence and absence of dissolved organic matter (DOM). Growth inhibition tests were performed over 72 hr at pH 6. The chemical equilibrium software Visual MINTEQ was used to calculate the speciation of Gd in exposure media and verify the applicability of the biotic ligand model. Limited bioaccumulation of Gd was detected when algae were exposed to Gd-DTPA or Gd-BT-DO3A, both in the presence and absence of DOM. As for inorganic Gd, the presence of DOM decreased Gd toxicity. The 72 hr-EC50 values based on the total concentration of Gd were estimated at 3.7 [2.9–4.7] µM of Gd for the exposure to Gd without DOM and 6.6 [5.9–7.5] µM of Gd in the exposure with DOM. Internalized Gd by cells and estimated free ion concentrations were strongly linked to toxicity, which confirms the applicability of the biotic ligand model. Moreover, the 72 hr-EC50 estimations based on free Gd3+ ion concentrations were statistically indistinguishable at the 95% confidence level for the exposure conditions with and without DOM: 1.0 [0.6–2.0] µM and 2.1 [1.4–3.1] µM, respectively. These results highlight the central role of chemical speciation determination in assessing the ecotoxicity of metals and that the free Gd ion drives its toxicity to the green alga C. reinhardtii.

Les écosystèmes côtiers méditerranéens sont soumis à de multiples sources de contamination (activités agricoles, industrielles et touristiques) impactant à des degrés divers les compartiments environnementaux (colonne d'eau, sédiments, biota, ...). En particulier, des études précédentes ont démontré que les sédiments de la Rade de Toulon présentent un niveau de multi-contamination très important, qui pourrait avoir des conséquences non-négligeables sur l'écosystème côtier, ce qui nécessite la réalisation de différentes études complémentaires visant à mieux comprendre les processus régissant le transfert potentiel des polluants vers le milieu environnant. Dans ce contexte, les objectifs de cette thèse consistent à étudier les processus contrôlant la dynamique des polluants inorganiques dans la colonne sédimentaire et à l'interface eau/sédiment et à évaluer leur remobilisation potentielle vers la colonne d'eau. Trois modes principaux de dispersion peuvent conduire à une remobilisation des contaminants: (1) la diagénèse précoce mobilisant des éléments nutritifs et des éléments traces métalliques et métalloïdes, (2) la remobilisation des contaminants lors de remises en suspension sédimentaire, et (3) le flux diffusif à l'interface eau/sédiment en lien avec la mobilité des contaminants dans la colonne sédimentaire. Pour atteindre ses objectifs, ce projet de thèse combine des approches in-situ (carottage d'interface, extraction de l'eau interstitielle, traitement/analyse des eaux interstitielles et des fractions solides, …), des expériences en laboratoire (suivi cinétique de la remobilisation des contaminants lors des mélanges eau/sédiment, extractions séquentielles…), des outils de modélisation (calcul de la spéciation chimique, simulation de la partition dissous/particulaire, simulation des profils sédimentaires, réactivité de la matière organique, …) ainsi que le traçage des sources de contamination et de leur transfert entre différents compartiments environnements à l'aide des isotopes stables du Pb. Ces travaux ont été réalisés en étroite collaboration avec le LASEM et avec la contribution de différents partenaires (EPOC/Université de Bordeaux, Ruđer Bošković Institut/Croatie, CAM/Université de Lausanne/Suisse, IFREMER, IRSN, …).

Non-targeted liquid-chromatography tandem high-resolution mass-spectrometry (LC-MS/MS) is increasingly applied for the structure-resolved chemical analysis of dissolved organic matter (DOM). With new developments in mass spectrometry instrumentation and analysis software, the approach has gained substantial momentum over the last decade. However, achieving high-quality analytical data that is reproducible and comparable across laboratories can be a bottleneck in non-targeted metabolomics and organic matter chemical analysis, especially for data reuse in repository-scale analyses. Understanding the capabilities as well as challenges of comparing LC-MS/MS data from different laboratories is necessary for inferring global trends from public datasets. To illuminate instrumentation factors that drive differences and variability, we used a standardized data analysis pipeline, including classical (CMN) and feature-based molecular networking (FBMN) to analyze data from a ring-trial by 24 laboratories on identical sample sets of algal and DOM extracts that were mixed in predefined concentrations and spiked with standards. Our results showed that data sets from similar mass spectrometer types with unified instrument parameters were qualitatively comparable, resolving the same general trends and shared mass spectral features. Inter-laboratory comparability was best for high intensity features, while low intensity features showed greater detection variability. Our analysis also highlights challenges when comparing data from instruments with different acquisition rates or operated with less standardized methods. Lastly, we provide recommendations for data integration, public data sharing, standardization, and best practices for standardized LC-MS/MS data acquisition, which will be critical for long-term time series and inter-comparability of DOM chemical analyses.