Arsenforurening af grundvandet er et globalt problem, som påvirker millioner af menneskers helbred over hele verden. Omfattende forskning i løbet af de seneste årtier, har gjort det muligt at identificere de vigtigste processer, som driver arsens (As) forekomst og mobilitet i grundvandet. På trods af den omfattende indsats, er forudsigelse og forståelse af tilstedeværelsen af arsen i grundvand en enorm udfordring i forhold til forebyggelse og behandling af forurening. Arsens mobilitet og skæbne i grundvand styres af samspillet mellem flere samtidigt forekommende processer, som afhænger af de lokale hydrokemiske, geokemiske og hydrodynamiske forhold. Proces-baseret karakterisering af arsens mobilitet udføres generelt med fokus på isoleret geokemiske processer og er derfor ofte begrænset til batch-systemer med et begrænset antal vandopløste stoffer og enkle syntetiske mineraler, især Fe- og Al-oxider. På grund af det komplekse samspil mellem fysiske, kemiske og biologiske processer i naturlige systemer er det generelt kompliceret at adskille virkningen fra specifikke mekanismer på arsens mobilitet alene ud fra analyser af grundvandskemien. Til dette formål er det nødvendigt at udarbejde forsøg af mellemliggende kompleksitet for at identificere og karakterisere de forskellige processer, der påvirker arsens mobilitet. Derudover er numerisk reaktive transportmodeller, der integrerer grundlæggende videnskabelig forståelse af de individuelle processer instrumentelle for den kvantitative fortolkning og forudsigelse af arsens forekomst og skæbne i naturlige systemer.Denne afhandling undersøger arsens mobilitet i semi-kontrollerede gennemstrømningssystemer, der inkluderer samtidigt forekommende processer således at de nærmer sig naturlige systemer ud fra et hydrokemisk og mineralsk perspektiv, og gennem kombinationen af eksperimentelle og numeriske metoder. Det specifikke fokus var på undersøgelse af samspillet mellem arsen og faste mineralfaser i multi-komponent systemer og mineralsammensætninger. Et centralt aspekt i denne forskning var brugen af omfattende eksperimentelle datasæt indsamlet under forskellige hydrokemiske forhold samt modelbaseret fortolkninger ved hjælp af reaktivtransportmodellering koblet med fysiske og geokemiske processer under hensynstagen for de lokale vandkemiske forhold. Denne fremgangsmåde gjorde det muligt at belyse samspillet mellem processer og de specifikke effekter af ladede vandopløste stoffer på As mobilitet.Den første del (kapitel 2 i denne afhandling) fokuserer på frigivelsen af arsen fra vel-karakteriseret jernoxider (dvs. goethit og ferrihydrit) under in-situ eksponering af syntetiske mineralfaser for naturlige grundvandsforhold. Navnlig blev de synergiske og konkurrerende effekter af de vandige ladede specier på transient desorption af arsen fra goethit ved hjælp af overfladekomplekseringsmodeller (engelsk: Surface Complexation Models, SCM). Dette indebar en sammenligning af to ofte anvendte SCM inden for reaktivtransportmodellering (dvs. DDL- og CD-MUSIC modellerne). Disse modeller inkluderer forskellige beskrivelser af de elektrostatiske interaktioner, som finder sted ved grænsefladen mellem overflade og opløsning. De resulterende simuleringer demonstrer vigtigheden af nøje at redegøre for både kemiske og elektrostatiske interaktioner ved grænsefladen i multi-komponent systemer for at beskrive arsens makroskopiske sorptionsopførsel. Hvor nogle specier konkurrerer direkte med arsen om sorptionspladser (f.eks fosfat), kan andre, i særdeleshed større kationer (f.eks. calcium), markant påvirke arsens overfladeladningsadfærd gennem elektrostatiske interaktioner og dermed påvirke arsens sorptionsaffinitet. Dette samspil fører til kompleks non-lineær overfladekompleksationsadfærd af arsen, hvilket indikerer, at sorption af arsen i naturlige systemer er stærkt afhængig af den lokale kemiske sammensætning og de hydrodynamiske forhold. Derudover blev en model udviklet for at simulere frigivelsen af As observeret over tid under in-situ reduktiv transformering af ferrihydrit forskellige steder i felten. Modellen tager højde for koblingen mellem abiotisk og biotisk kinetisk reduktiv opløsning/transformering af ferrihydrit, sorption af arsen og vandige specier naturligt forekommende i grundvandet, samt sekvestration af arsen i nydannede sekundære jernmineralfaser. Den modelbaseret analyse viser at reduktiv opløsning af ferrihydrit er den primære drivkraft bag mobilisering af arsen. Sekundære abiotiske mineraltransformationer udløst af opløsningen af ferrihydrit og afhængige af de lokale hydrokemiske forhold og mineralsammensætningen har dog også grundlæggende betydning for arsens mobilitet.Anden del (kapitel 3 i denne afhandling) vedrører transport af arsen i naturlige mineralsammensætninger, hvilket blev undersøgt under velkontrollerede gennemstrømnings- og vandkemiske forhold gennem laboratorieforsøg. Fokus var på naturligt metal oksider belagt kvartssand, da det repræsenterer den dominante solide fase i flere arsenforurenede grundvandsmagasiner. Arsens mobilitet gennem denne type sand er dog sjældent blevet undersøgt tidligere. Denne forskning er udført i to trin. Først undersøges propagation af pH-fronterne og protoneringsadfæren af mineralsammensætningen. Derefter undersøges effekten af silica porøst medie og vandopløste stoffer på arsens mobilitet. Der blev udført serier af søjleforsøg, som bestod i at injicere As-opløsninger med forskellige pH-værdier og baggrundselektrolytkoncentrationer for at udforske interaktionen mellem arsen, de ladede vandopløste stoffer og mineralsammensætningernes overflade. Forsøgene blev gentaget med forskellige typer silica porøst medie for at sammenligne disses effekt på transport af vandopløste stoffer. Gennemstrømningsforsøgene blev kombineret med karakterisering af det naturlige sands egenskaber samt reaktiv transport modellering. Særligt blev samspillet mellem de ladede vandopløste stoffer og overfladen af mineralsammensætningen beskrevet ved hjælp af overfladekomplekseringsmodeller under anvendelse af en komponent additiv metode, således at der blev taget højde for de særskilte bidrag fra naturligt forekommende kvarts og metaloxider i kvartssandets belægning. Kvartsoverfladen er derudover beskrevet med en bimodal syreadfærd ud fra nyligt opnået viden fra molekulære studier af faseovergange. Disse mangfoldige bevisveje blev brugt til at adskille de geokemiske processer som regulerer protoners, større ioners og arsens mobilitet. Resultaterne viste væsentlig interaktion mellem det silica porøse medium og de vandopløste stoffer, og afslørede en interessant, sekventiel sorptionsmekanisme. Denne mekanisme bestod i en stærk frigivelse af protoner fra kvartsoverfladen ved interaktion med baggrundselektrolyterne, som førte til en gennemgående påvirkning af arsens affinitet for den naturlige sandbelægning.Denne ph.d.-afhandling bidrager til en øget forståelsen af arsens mobilitet ved en omfattende beskrivelse af arsentransport i komplekse naturlige gennemstrømningssystemer. Tolkning og forudsigelse af arsens adfærd i undergrunden kræver at koblede fysiske, kemiske, elektrostatiske og biologiske processer samt de specifikke effekter af individuelle vandopløste stoffer og mineralfaser tages i betragtning. Med henblik herpå er den synergiske kombination af forsøgsdrevet undersøgelser med reaktivtransportmodellering instrumentel til at fremme videnskabelig forståelse og tekniske løsninger af arsenforurening af grundvand. Arsenic groundwater contamination is a problem of enormous scale affecting the health of millions of people around the world. Extensive research over the last decades have allowed identifying the main processes controlling the As occurrence and mobility in groundwater. Despite these tremendous efforts, the prediction and interpretation of the occurrence and mobility of arsenic for mitigation and remediation of As contaminated aquifers remains a formidable challenge. The mobility and fate of arsenic in groundwater is governed by the interplay of multiple co-occurring processes that depend on the local hydrochemical, geochemical and hydrodynamic conditions. However, process-based characterization of arsenic mobility has generally been performed focusing on geochemical processes in isolation and have often been limited to batch systems involving a limited number of aqueous species and single synthetic minerals, in particular Fe and Al oxides. Furthermore, due to the complex coupling of physical, chemical and biological processes in natural systems, it is generally difficult to distinguish the effects of specific mechanisms on the mobility of arsenic from the sole analysis of groundwater chemistry. To this end, experiments with an intermediate level of complexity are required in order to identify and characterize the various processes affecting the mobility of arsenic. Moreover, numerical reactive transport models, integrating fundamental scientific understanding of individual processes, are instrumental for the quantitative interpretation and prediction of the occurrence and fate of arsenic in natural systems.This thesis explores the mobility of arsenic in semi-controlled flow-through systems involving co-occurring processes and approaching natural systems from the hydrochemical and mineral perspectives through the combination of experimental and numerical methods. The specific focus was the investigation of the interactions between arsenic and solid mineral phases in multicomponent systems and mineral assemblage. A key aspect of this research was the use of extensive experimental datasets collected under different hydrochemical conditions and their model-based interpretation with reactive transport simulations coupling physical and geochemical processes and accounting for the local aqueous chemical conditions. This approach enabled to illuminate the interplay between processes and the specific effects of aqueous charged species on As mobility.The first part (Chapter 2 of this thesis) focuses on the release of arsenic from well-characterized iron oxides (i.e., goethite and ferrihydrite) during in-situ exposure of synthetic mineral phases to natural groundwater conditions. In particular, the synergic and competitive effects of aqueous charged species on transient desorption of arsenic from goethite were investigated using surface complexation models (SCMs). The capabilities of two SCMs broadly applied in the field of reactive transport models (i.e., DDL and CD-MUSIC models) were compared as these surface complexation descriptions include different descriptions of the electrostatic interactions taking place at the surface-solution interface. The simulation outcomes demonstrate the importance to rigorously account for both the chemical and electrostatic interactions taking place at the surface interface in multicomponent systems in order to describe the macroscopic sorption behavior of arsenic. Whereas some species can directly compete with arsenic for sorption sites (e.g., phosphate), others, in particular major cations (e.g., calcium), can significantly affect the surface charge behavior of iron oxides through electrostatic interactions and therefore impact the sorption affinity of arsenic. This interplay leads to non-linear surface complexation behavior of arsenic which indicates that the sorption of arsenic in natural systems strongly depends on the local chemical composition and the hydrodynamic conditions. Furthermore, a model was developed to simulate the observed As temporal release during in-situ reductive transformation of ferrihydrite observed at different spatial locations in the field. The model considered the coupling between abiotic and biotic kinetic reductive dissolution/transformation of ferrihydrite, the sorption of arsenic and aqueous species naturally present in the groundwater as well as the arsenic sequestration into newly-formed secondary iron mineral phases. The model-based interpretation shows that ferrihydrite reductive dissolution is the primary driver for the mobilization of arsenic. However, secondary abiotic mineral transformations triggered by the dissolution of ferrihydrite and dependent on the local hydrochemical conditions and on the mineral composition have also fundamental implications for the mobility of arsenic.The second part (Chapter 3 of this thesis) addresses the transport of arsenic in natural mineral assemblage that was studied under well-controlled aqueous chemical conditions in flow-through laboratory experiments. This work focused on naturally-coated quartz sand since it represents the dominant solid phase in various aquifers contaminated by arsenic. However, detailed investigation of As mobility in naturally-coated quartz sand has hardly been studied. This research was conducted in two successive parts. First, the propagation of pH fronts and the protonation behavior of the mineral assemblage were characterized. Second, the effects of the silica porous media and aqueous charged species on arsenic mobility were investigated. A series of column experiments was performed and consisted in injecting As solutions with different pH and background electrolyte concentrations in order to explore the interactions between arsenic, the aqueous charged species and the surface of the mineral assemblages. The experiments were repeated using different types of silica porous media in order to compare their effects on the transport of aqueous solutes. The flow-through experiments were combined with the characterization of the natural sand properties and with reactive transport modeling. In particular, the interactions between the aqueous charged species and the surface of the mineral assemblage were described with surface complexation models using a component additive approach in order to account for the distinct contribution of quartz and metal oxides present in the natural sand coatings. Moreover, the quartz surface was described with a bimodal acidity behavior based on recent insights gained from interfacial molecular studies. These multiple lines of evidences were used to distinguish the geochemical processes governing the mobility of the protons, major ions and arsenic. The outcomes showed that the silica porous media substantially interacted with the aqueous solutes and revealed an interesting sequential sorption mechanism. This mechanism consisted in a strong release of protons from the quartz surface upon interaction with the background electrolytes which, subsequently, led to a profound impact on the arsenic affinity for the natural sand coatings.In conclusion, this PhD thesis has contributed to advance the knowledge on arsenic mobility towards a comprehensive description of arsenic transport in complex natural flow-through systems. It has been shown that interpretation and prediction of arsenic behavior in the subsurface requires consideration of coupled physical, chemical, electrostatic and biological processes and of the specific effects of individual aqueous solutes and mineral phases. In this thesis, the synergic combination of experimental investigation and reactive transport modeling allowed the quantitative understanding of the controlling effects of natural hydrochemical conditions and mineral assemblages on the fate of arsenic in groundwater.