Carbone organique dissous | EPFL Graph Search

Le carbone organique dissous ou COD (en anglais, dissolved organic carbon ou DOC) est un paramètre global de la chimie de l'eau utilisé pour caractériser et suivre l’évolution du taux de carbone dissous dans les eaux (douces, saumâtres ou marines), ou la pollution organique des milieux aquatiques. C'est une fraction parfois importante du Carbone organique total (COT). La pollution par certaines matières organiques et le carbone dissous tend à augmenter dans le monde, dont en France, en Bretagne par exemple. Sa teneur dans l'océan a été longtemps sous-estimée et commence à être mieux modélisée dans les années 1990. Avec d'autres éléments tels que les nitrates ou la silice, il peut être utilisé comme indicateur ou traceur de dégradation des écosystèmes, d'un massif forestier (incendié ou ayant subi une coupe rase, ou d'agrosystèmes dégradés par l'érosion dans les bassins versant alimentant un cours d'eau ou une masse d'eau. Son comportement dans les milieux karstiques ou dans les sols

Phototransformation of Organic Contaminants in Aquatic Systems: Characterization of Long-Lived Photooxidants Derived from Dissolved Organic Matter

Stephanie Christa Remke

The increasing interest in the fate of organic contaminants in the aquatic environment requires the elucidation of relevant abiotic transformation processes. These processes often involve the ubiquitous dissolved organic matter (DOM), a complex mixture of organic compounds present in the aquatic environment. Upon light absorption of DOM a range of photochemically produced reactive intermediates (PPRI) form, which may induce the transformation of organic contaminants. PPRI reacting as oxidants have been proposed to consist of two pools, short-lived photooxidants, such as singlet oxygen (1O2), the hydroxyl radical (.OH) and triplet states of chromophoric DOM (3CDOM*), and long-lived photooxidants (LLPO), whose nature is still unclear. LLPO-induced transformations were hypothesized to cause a concentration-dependent enhancement effect observed for 3CDOM*-induced transformations. Thereby, pseudo-first-order phototransformation rate constants for electron-rich phenols increased when their initial concentrations were lowered from 5.0 to 0.1 mikroM. Phenolic moieties within DOM have been proposed as LLPO precursors. This thesis tests the hypothesis that LLPO are relevant PPRI, formed, concomitantly with 3CDOM* upon DOM light absorption, from phenolic DOM precursors. Experimental methods used in this thesis include steady-state irradiations, laser flash photolysis and studies with DOM surrogates and pre-oxidized DOM. The occurrence of LLPO-induced phototransformations was tested by analyzing target compound transformation kinetics for two initial concentrations, namely 5.0 and 0.1 mikroM in steady-state irradiation experiments. The aforementioned enhancement effect for the lower initial concentration was taken as an indicator for the involvement of LLPO in the phototransformation. Firstly, the relevance of LLPO in the phototransformation of various aquatic contaminants was evaluated. Electron-rich phenols, anilines and phenylureas were identified to be susceptible to LLPO-induced transformations. The LLPO reduction potential could be estimated to lie between 1.0 to 1.3 V vs. SHE. Secondly, LLPO-induced target compound transformation was mimicked by photochemically produced electron-poor phenoxyl radicals. Enhanced target compound transformation was observed for 0.1 mikroM initial concentration with any of the tested phenoxyl radicals, which were generated by the photosensitized oxidation of the parent phenols in aqueous solution. Reactivity increased proportionally with the one-electron reduction potential of the electron-poor phenoxyl radical. Finally, characterization of LLPO precursors was attempted by DOM pre-oxidation. A DOM isolate, Suwannee River fulvic acid, was pre-oxidized, by chlorine and ozone (with and without t-BuOH), characterized by measurements of UV absorbance measurements and electron donating capacity and subsequently tested for its photoreactivity. The chemical pre-oxidation lead to changes of DOM, such as decrease in aromaticity and phenol content, in accordance to previous studies. LLPO precursors were concluded to be composed of oxidant susceptible sites and to likely include phenolic DOM moieties. The results presented in this thesis corroborate the hypothesis of LLPO formed from phenolic DOM precursors upon DOM light absorption. One consequence drawn from this thesis is the recommendation to include target compound concentrations < 1 mikroM in laboratory-scale phototransformation experiments.

EPFL2021

Chlorinated ethene plume evolution after source thermal remediation: Determination of degradation rates and mechanisms

Christof Holliger, Julien Maillard, Alexandra Marie Murray, Jérémy Zimmermann

The extent, mechanism(s), and rate of chlorinated ethene degradation in a large tetrachloroethene (PCE) plume were investigated in an extensive sampling campaign. Multiple lines of evidence for this degradation were explored, including compound-specific isotope analysis (CSIA), dual C-Cl isotope analysis, and quantitative realtime polymerase chain reaction (qPCR) analysis targeting the genera Dehalococcoides and Dehalogenimonas and the genes vcrA, bvcA, and cerA. A decade prior to this sampling campaign, the plume source was thermally remediated by steam injection. This released dissolved organic carbon (DOC) that stimulated microbial activity and created reduced conditions within the plume. Based on an inclusive analysis of minor and major sampling campaigns since the initial site characterization, it was estimated that reduced conditions peaked 4 years after the remediation event. At the time of this study, 11 years after the remediation event, the redox conditions in the aquifer are returning to their original state. However, the DOC released from the remediated source zone matches levels measured 3 years prior and plume conditions are still suitable for biotic reductive dechlorination. Dehalococcoides spp., Dehalogenimonas spp., and vcrA, bvcA, and cerA reductive dehalogenase genes were detected close to the source, and suggest that complete, biotic PCE degradation occurs here. Further downgradient, qPCR analysis and enriched delta C-13 values for cis-dichloroethene (cDCE) suggest that cDCE is biodegraded in a sulfate-reducing zone in the plume. In the most downgradient portion of the plume, lower levels of specific degraders supported by dual C-CI analysis indicate that the biodegradation occurs in combination with abiotic degradation. Additionally, 16S rRNA gene amplicon sequencing shows that organizational taxonomic units known to contain organohalide-respiring bacteria are relatively abundant throughout the plume. Hydraulic conductivity testing was also conducted, and local degradation rates for PCE and cDCE were determined at various locations throughout the plume. PCE degradation rates from sampling campaigns after the thermal remediation event range from 0.11 to 0.35 yr(-1). PCE and cDCE degradation rates from the second to the third sampling campaigns ranged from 0.08 to 0.10 yr(-1) and 0.01 to 0.07 yr(-1), respectively. This is consistent with cDCE as the dominant daughter product in the majority of the plume and cDCE degradation as the time-limiting step. The extensive temporal and spatial analysis allowed for tracking the evolution of the plume and the lasting impact of the source remediation and illustrates that the multiple lines of evidence approach is essential to elucidate the primary degradation mechanisms in a plume of such size and complexity.

ELSEVIER2019

Partitioning soil organic carbon into its centennially stable and active fractions with machine-learning models based on Rock-Eval (R) thermal analysis (PARTY(SOC)v2.0 and PARTY(SOC)v2.0 (EU))

Juan Carlos Quezada Rivera

Partitioning soil organic carbon (SOC) into two kinetically different fractions that are stable or active on a century scale is key for an improved monitoring of soil health and for more accurate models of the carbon cycle. However, all existing SOC fractionation methods isolate SOC fractions that are mixtures of centennially stable and active SOC. If the stable SOC fraction cannot be isolated, it has specific chemical and thermal characteristics that are quickly (ca. 1 h per sample) measurable using Rock-Eval (R) thermal analysis. An alternative would thus be to (1) train a machine-learning model on the Rock-Eval (R) thermal analysis data for soil samples from long-term experiments for which the size of the centennially stable and active SOC fractions can be estimated and (2) apply this model to the Rock-Eval (R) data for unknown soils to partition SOC into its centennially stable and active fractions. Here, we significantly extend the validity range of a previously published machine-learning model (Cecillon et al., 2018) that is built upon this strategy. The second version of this model, which we propose to name PARTYsoc, uses six European long-term agricultural sites including a bare fallow treatment and one South American vegetation change (C4 to C3 plants) site as reference sites. The European version of the model (PARTY(SOC)v2.0(EU)) predicts the proportion of the centennially stable SOC fraction with a root mean square error of 0.15 (relative root mean square error of 0.27) at six independent validation sites. More specifically, our results show that PARTY(SOC)v2.0(EU) reliably partitions SOC kinetic fractions at its northwestern European validation sites on Cambisols and Luvisols, which are the two dominant soil groups in this region. We plan future developments of the PARTY(SOC) global model using additional reference soils developed under diverse pedoclimates and ecosystems to further expand its domain of application while reducing its prediction error.

COPERNICUS GESELLSCHAFT MBH2021