Optimization of induced denitrification strategies in water resources contaminated by agricultural and urban sources

Abstract

[eng] Groundwater pollution poses a significant global environmental threat, mainly attributed to the oxidation of excess nitrogen during fertilization in aerobic conditions. The widespread occurrence of elevated nitrate levels in groundwater causes health hazards such as methemoglobinaemia in children and stomach cancer in adults and deteriorates aquatic life. Addressing this issue necessitates effective remediation strategies to preserve water quality and ensure sustainable access to clean water resources. The primary objective of this thesis is to develop cost-efficient, eco-friendly treatment methods for groundwater nitrate removal, advancing the understanding of nitrate decontamination pathways and quantifying the efficiency of nitrate removal. Chemical, isotopic, and numerical modeling tools are used to characterize nitrate attenuation to achieve this objective. Denitrification, driven by denitrifying bacteria, is the primary mechanism for groundwater nitrate attenuation. The activity of these bacteria, distributed widely in both surface and subsurface water bodies, hinges on the availability of electron donors, a crucial limiting factor for denitrification. This research explores potential low-cost electron donors like two types of wine industry residues to induce or enhance nitrate removal efficiency. Laboratory experiments, including batch and column studies, are conducted to assess their efficiency in inducing denitrification through chemical and isotopic tools. Experimental investigations revealed promising results in nitrate removal with one of the residues, showcasing the potential for field-scale applications. Isotope analysis suggested denitrification as the dominant mechanism for nitrate attenuation, with isotopic fractionation values for quantification of denitrification efficiency in future field studies. To further elucidate the complex interplay of nitrate reduction mechanisms, a conceptual and geochemical numerical model that integrates nitrate and nitrite degradation kinetics, isotopic fractionation and biomass dynamics is developed to simulate N compounds dynamics. The successful simulation of the experimental data by the models sets the stage for its potential application in real-world scenarios. Chemodenitrification can coexist with biological denitrification in iron-rich environments; consequently, this thesis also focuses on the performance of nitrite reduction by ferrous iron in laboratory batch experiments. The research examines the influence of aqueous Fe(II) and Fe(II) minerals on nitrite removal efficiency and the production of nitrous oxide (N2O), a potent greenhouse gas. Chemical and isotopic tools are used to characterize nitrite attenuation and N2O production and quantify the efficiency of nitrite reduction and N2O production. A geochemical model is developed to describe the coupled nitrite reduction-Fe(II) oxidation linked to nitrogen-nitrite isotopic fractionation. The model successfully replicates the experimental data and holds promise for application in other laboratory and field-scale chemodenitrification studies. Partially treated wastewater effluents are recognized for having elevated levels of inorganic nitrogen, primarily ammonium, easily oxidized to nitrate and other nitrogen compounds as groundwater contaminant sources. Safely using these effluents for domestic purposes and groundwater aquifer recharge necessitates ammonium nitrogen remediation. Permeable reactive barriers emerge as promising solutions to address the problem of the high levels of inorganic nitrogen in untreated and partially wastewater effluents. This research evaluates the effectiveness of permeable reactive barriers in subsurface infiltration systems, comparing scenarios with and without reactive barriers. Chemical and isotopic analyses reveal spatial and temporal dynamics of nitrogen compounds, offering valuable insights into the efficiency of nitrogen-ammonium transformation and attenuation pathways. This thesis advances our understanding of pollution remediation of nitrate and other nitrogen compounds by investigating cost-effective and eco-friendly electron donors through innovative removal methods. The research outcomes hold significant implications for environmental sustainability, offering practical solutions to mitigate nitrate pollution in water resources. The research outcomes have been disseminated through three peer-reviewed publications and presented at seven international and national conferences, fostering knowledge exchange and collaboration in the field of groundwater contaminant remediation.

[cat] Aquesta tesi examina la contaminació global de les aigües superficials i subterrànies per nitrats derivada de les activitats agrícoles i urbanes, que és perjudicial per a la salut humana i els ecosistemes. L'estudi aprofundeix en estratègies innovadores i respectuoses amb el medi ambient per a la remoció de nitrats dels aqüífers, centrant-se principalment en la desnitrificació, el principal procés d'atenuació dels nitrats. S'ha investigat el potencial de dos tipus de residus vitivinícoles per millorar la remoció de nitrats dels aqüífers. Les investigacions experimentals revelen resultats prometedors per bé que diferents en eficiència, sent un dels residus capaç d’eliminar nitrats fins a nivell acceptables, en les condicions experimentals utilitzades i per tant, aquest residu podria ser idoni per a aplicacions a escala de camp. Les anàlisis químiques i isotòpiques han confirmat la desnitrificació biològica i els valors de fraccionament obtinguts podrien ser aplicats en futurs estudis de camp per quantificar l'eficiència de la desnitrificació. També s'ha investigat el potencial del Fe(II) com a inductor de la quimiodenitrificació durant el tractament a llarg termini de nitrats. L'estudi destaca la influència de l’oxidació del Fe(II) en la reducción del nitrit a òxid nitrós i els resultats experimentals han permès obtener una llei cinètica del procés acoblat que depèn de la concentració del nitrit i de Fe(II) del sistema. Ambdós experiments s’han reproduit amb un model geoquímic, el qual permetrà siimulacions en escenaris més enllà del rang dels estudis de laboratori i de futures aplicacions a escala de camp. A més, en aquesta tesi també s’explora l'aplicació de barreres reactives permeables en la remediació de compostos nitrogenats durant la recàrrega dels aqüífers. Les anàlisis químiques i isotòpiques han permès identificar com es transformen els compostos nitrogenats en dos sistemes d'infiltració, oferint idees sobre quines son les vies d'atenuació del nitrogen que actuen i els factors que influeixen en l'eficiència de remoció del nitrogen, incloent-hi la temperatura i l'activitat microbiana. La tesi ofereix doncs coneixements i solucions pràctiques per a fer front a la contaminació per nitrats en els recursos hídrics, que cal provar al camp.