Mécanismes d'enlèvement et la stabilité des solides du DNC-As dans les biofiltres passifs en climat froid

« RÉSUMÉ : Le drainage neutre contaminé (DNC) est une problématique environnementale récente, moins étudiée que le drainage minier acide (DMA), toutefois susceptible de causer des impacts considérables sur les milieux naturels. Le DNC est caractérisé par des concentrations faibles à modérées en contaminants dissous, tout en gardant un pH proche de la neutralité (6,5–9). Le traitement du DNC est complexe, pour certains contaminants en particulier (ex. As) et surtout sur les sites miniers fermés et abandonnés. Les résidus associés aux anciennes mines d’or et d’argent sont souvent enrichis en As entrainant la formation du DNC-As. La remobilisation de l'As en provenance des résidus est un risque potentiel qui demeure peu documenté dû au manque de connaissance sur la stabilité à long terme de l'As capturé au sein de ces résidus. Par conséquent, des solutions de traitement efficaces sont nécessaires pour gérer le DNC-As et limiter son impact sur les écosystèmes environnants. Les systèmes de traitement passifs, reconnues comme des technologies développées pour résoudre la problématique du DMA sur les sites miniers restaurés et abandonnés, ont montré une efficacité satisfaisante pour le traitement du DNC. Les biofiltres passifs, en particulier, ont prouvé leur efficacité pour traiter un DNC-As à l’échelle pilote et sur le terrain. Néanmoins, les biofiltres passifs génèrent des quantités considérables de résidus de post-traitement avec une stabilité chimique et environnementale variable. Pour mieux anticiper leur devenir en fin de cycle de vie et après excavation, une caractérisation au cas par cas de ces résidus de post-traitement est nécessaire afin de mieux comprendre leur comportement, sélectionner les voies de gestion adéquates et limiter le potentiel de lixiviation des contaminants. Dans ce contexte, les principaux objectifs de la thèse étaient comme suit : (1) évaluer et quantifier les mécanismes et les processus responsables de l’immobilisation de l’As dans les biofiltres passifs étudiés et identifier les principaux facteurs influençant cette immobilisation; (2) identifier les phases minérales porteuses d’As et étudier la stabilité des résidus de post-traitement passif contaminés en As; (3) évaluer l’influence de l’échelle du biofiltre sur la nature des phases minérales et les mécanismes d’enlèvement prépondérant de l’As. L’approche méthodologique consistait, dans un premier temps, à échantillonner les résidus de post-traitement du biofiltre passif de terrain Wood-Cadillac avant de réaliser des caractérisations physicochimiques et minéralogiques. Ensuite, des essais de lixiviations statiques ont été réalisés pour évaluer la mobilité potentielle des contaminants, en particulier de l’As, et statuer sur la stabilité des résidus de post-traitement. La deuxième étape consistait à échantillonner les résidus de post-traitement d’un biofiltre de laboratoire ainsi que deux biofiltres pilotes de terrain traitant le DNC-As. L’ensemble des échantillons a été soumis à des caractérisations physicochimiques, minéralogiques et environnementales pour déterminer les mécanismes de l’enlèvement de l’As et statuer sur la stabilité chimique des résidus.»

Abstract

« ABSTRACT : Contaminated neutral mine drainage (CND) represent an emerging environmental issue that has received less attention compared to acid mine drainage (AMD), yet it can have significant impacts on natural ecosystems. CND is characterized by low to moderate concentrations of dissolved contaminants while maintaining a pseudo-neutral pH (6.5–9). CND treatment remains challenging for certain contaminants (e.g., As) particularly at closed and abandoned mining sites. Tailings from old gold and silver mines are often rich in arsenic (As) leading to the formation of As-CND. The remobilization of As from tailings is a potential risk that remains poorly understood due to a lack of knowledge about the long-term stability of As trapped in tailings. Therefore, efficient treatment solutions are essential to manage As-CND and mitigate its impact on surrounding ecosystems. Passive treatment systems, widely recognized as efficient technologies for addressing AMD on closed and abandoned mining sites, have shown promising results in CND treatment. Among these, passive biofilters have demonstrated their efficiency in treating As-CND at both pilot and field scales. However, passive biofilters generate significant quantities of post-treatment residues with variable chemical and environmental stability. At their end-of-life cycle, to better anticipate their fate after excavation, a case-by-case characterization of these residues is necessary to better understand their behavior, select appropriate management strategies and minimize potential risks of contaminant leaching. In this context, the main objectives of the thesis were the following: (1) evaluate and characterize the mechanisms governing As immobilization within the studied passive biofilters while identifying the most dominant processes; (2) Identify the As-bearing mineral phases and assess the stability of As-contaminated post-treatment residues; (3) Assess the influence of the biofilter scale on the nature of mineral phases and the predominant As removal mechanisms. The methodological approach initially involved sampling the post-treatment residues from the Wood-Cadillac field passive biofilter before conducting physicochemical and mineralogical characterizations. Subsequently, static leaching tests were performed to assess the potential mobility of contaminants, particularly As, and to determine the stability of the post-treatment residues. The second step involved sampling the post-treatment residues from a laboratory biofilter as well as two field pilot-scale biofilters treating As-CND. All samples were subjected to physicochemical, mineralogical, and environmental characterizations to determine the mechanisms of As removal and access the chemical stability of the residues.Contaminated neutral mine drainage (CND) represent an emerging environmental issue that has received less attention compared to acid mine drainage (AMD), yet it can have significant impacts on natural ecosystems. CND is characterized by low to moderate concentrations of dissolved contaminants while maintaining a pseudo-neutral pH (6.5–9). CND treatment remains challenging for certain contaminants (e.g., As) particularly at closed and abandoned mining sites. Tailings from old gold and silver mines are often rich in arsenic (As) leading to the formation of As-CND. The remobilization of As from tailings is a potential risk that remains poorly understood due to a lack of knowledge about the long-term stability of As trapped in tailings. Therefore, efficient treatment solutions are essential to manage As-CND and mitigate its impact on surrounding ecosystems. Passive treatment systems, widely recognized as efficient technologies for addressing AMD on closed and abandoned mining sites, have shown promising results in CND treatment. Among these, passive biofilters have demonstrated their efficiency in treating As-CND at both pilot and field scales. However, passive biofilters generate significant quantities of post-treatment residues with variable chemical and environmental stability. At their end-of-life cycle, to better anticipate their fate after excavation, a case-by-case characterization of these residues is necessary to better understand their behavior, select appropriate management strategies and minimize potential risks of contaminant leaching. In this context, the main objectives of the thesis were the following: (1) evaluate and characterize the mechanisms governing As immobilization within the studied passive biofilters while identifying the most dominant processes; (2) Identify the As-bearing mineral phases and assess the stability of As-contaminated post-treatment residues; (3) Assess the influence of the biofilter scale on the nature of mineral phases and the predominant As removal mechanisms. The methodological approach initially involved sampling the post-treatment residues from the Wood-Cadillac field passive biofilter before conducting physicochemical and mineralogical characterizations. Subsequently, static leaching tests were performed to assess the potential mobility of contaminants, particularly As, and to determine the stability of the post-treatment residues. The second step involved sampling the post-treatment residues from a laboratory biofilter as well as two field pilot-scale biofilters treating As-CND. All samples were subjected to physicochemical, mineralogical, and environmental characterizations to determine the mechanisms of As removal and access the chemical stability of the residues.Contaminated neutral mine drainage (CND) represent an emerging environmental issue that has received less attention compared to acid mine drainage (AMD), yet it can have significant impacts on natural ecosystems. CND is characterized by low to moderate concentrations of dissolved contaminants while maintaining a pseudo-neutral pH (6.5–9). CND treatment remains challenging for certain contaminants (e.g., As) particularly at closed and abandoned mining sites. Tailings from old gold and silver mines are often rich in arsenic (As) leading to the formation of As-CND. The remobilization of As from tailings is a potential risk that remains poorly understood due to a lack of knowledge about the long-term stability of As trapped in tailings. Therefore, efficient treatment solutions are essential to manage As-CND and mitigate its impact on surrounding ecosystems. Passive treatment systems, widely recognized as efficient technologies for addressing AMD on closed and abandoned mining sites, have shown promising results in CND treatment. Among these, passive biofilters have demonstrated their efficiency in treating As-CND at both pilot and field scales. However, passive biofilters generate significant quantities of post-treatment residues with variable chemical and environmental stability. At their end-of-life cycle, to better anticipate their fate after excavation, a case-by-case characterization of these residues is necessary to better understand their behavior, select appropriate management strategies and minimize potential risks of contaminant leaching. In this context, the main objectives of the thesis were the following: (1) evaluate and characterize the mechanisms governing As immobilization within the studied passive biofilters while identifying the most dominant processes; (2) Identify the As-bearing mineral phases and assess the stability of As-contaminated post-treatment residues; (3) Assess the influence of the biofilter scale on the nature of mineral phases and the predominant As removal mechanisms. The methodological approach initially involved sampling the post-treatment residues from the Wood-Cadillac field passive biofilter before conducting physicochemical and mineralogical characterizations. Subsequently, static leaching tests were performed to assess the potential mobility of contaminants, particularly As, and to determine the stability of the post-treatment residues. The second step involved sampling the post-treatment residues from a laboratory biofilter as well as two field pilot-scale biofilters treating As-CND. All samples were subjected to physicochemical, mineralogical, and environmental characterizations to determine the mechanisms of As removal and access the chemical stability of the residues.»