Elevated levels of arsenic (As) in soils and groundwaters remain a pressing global challenge due to its widespread occurrence and distribution, high toxicity and mobility. In oxygen-limited subsurface conditions, redox-active mineral phases can be important substrates in controlling the fate and mobility of As in the environment. Among these redox-active minerals, green rust (GR) phases, an Fe(II)-Fe(III)-bearing layered double hydroxide, have been shown to be able to sequester a wide range of toxic metals and metalloids, including As. However, very little is known regarding how GR phases interact with As species and what is the fate of the immobilized As under dynamic geochemical conditions. GR phases are suggested to form through the transformation of metastable iron mineral phases in non-sulfidic, reducing environments. However, the exact mechanism and pathway of this transformation, as well as the fate of mineral-associated As (i.e. whether it is re-released back into the groundwater by desorption, dissolution or redox transformation) is not yet known but critically needed for modelling As cycling in contaminated environments. To address these knowledge gaps, I conducted a series of experimental geochemical studies and combined them with various laboratory- and synchrotron-based solid and liquid phase characterization methods to examine the interaction between GR sulfate (GRSO4) and As species [As(III) and As(V)]. Specifically, I performed several batch experiments under anoxic and near-neutral pH conditions to determine As-GR interaction mechanisms during GR formation and transformation. Moreover, I also quantified how these transformation reactions affect the toxicity and mobility of As species in contaminated environments. From the batch adsorption experiments, I showed that synthetic GRSO4 can adsorb up to 160 and 105 mg of As(III) and As(V) per g of solid, respectively. These adsorption capacities are among the highest reported for iron (oxyhydr)oxides that form in soils and groundwaters. Results from this study also show that As removal by GRSO4 can be inhibited by several geochemical parameters such as pH, high ionic strength, presence of co-existing inorganic ions (e.g., Mg2+, PO43-, Si) and low temperature. I also employed an integrated nano-scale solid-state characterization approach to elucidate As-GRSO4 interactions. Specifically, I combined scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray (EDX) spectroscopy together with bulk synchrotron-based X-ray techniques including high energy X-ray total scattering, pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS). With these, I was able to directly visualize and pinpoint As binding sites at the GR surface sites and to identify the binding mechanism for both As(III) and As(V). In the case of As(III)-reacted GR, STEM-EDX maps showed that As(III) were preferentially adsorbed at the GR crystal edges, primarily as bidentate binuclear (2C) inner-sphere surface complexes based from the differential PDF and As K-edge XAS data. For the As(V)-reacted GR, As(V) was sequestered as a newly-formed As-bearing mineral phase parasymplesite and as adsorbed As(V) species at the GR edges (in 2C geometry). To assess the fate of As in subsurface environments, I studied As during GR formation and transformation to quantify As uptake and/or its potential release back into solution and the stability of GR and other Fe (oxhydr)oxide phases in this process. During the Fe2+-induced transformation of As(V)-bearing ferrihydrite, I followed the changes in aqueous behavior and speciation of As, as well as the changes in composition of the Fe mineral phases, as a function of varying Fe2+(aq)/Fe(III)solid ratios (0.5, 1 ,2). In all the ratios tested, GRSO4, goethite and lepidocrocite formed in the early stages of transformation (≤ 2h). However, at low ratios (<2), the initially formed GRSo4 and/or lepidocrocite disappeared as the reaction progressed, leaving goethite and unreacted ferrihydrite after 24 h. At an Fe2+(aq)/Fe(III)solid ratio of 2, GRSO4 was formed and remained in the solids until the end of the 24-h transformation, with goethite and unreacted ferrihydrite. The initial As(V) was partially reduced to As(III) by the surface-associated Fe2+-GT redox couple, and extent of reduction increased from 34 to 44% as Fe2+(aq)/Fe(III)solid ratios increased. Despite this reduction to the more mobile and more toxic As(III) species, no significant As release was observed during the mineral transformation reactions. Finally, I tested the long-term stability and reactivity of GR by aging synthetic GRSO4 in pristine and As-spiked natural groundwater at ambient (25 °C) and low (4 °C) temperatures. The spiked As in the groundwater was completely removed after 120 days at 25 °C while the removal rate was ~2 times slower at 4 °C with only ~66% As removal after 120 days. On the other hand, the stability of synthetic GRSO4 in groundwater was strongly affected by the presence of adsorbed As species and temperature. At ambient temperature, the initial GRSO4 aged in As-free groundwater was converted to GRCO3 by ion exchange within a few days and both GR phases eventually transformed to magnetite after 120 days. Remarkably, both the addition of As species in groundwater and lowering the temperature increased long-term GRSO4 stability through the inhibition of (a) ion exchange in the GRSO4 interlayer (i.e., slower conversion to GRCO3) and (b) transformation of GR to magnetite. Moreover, a synergistic stabilization effect was observed with both As addition and low temperature, significantly enhancing GR stability up to a year. Overall, the work presented in this thesis clearly emphasizes the potential role of GR phases in controlling the mobility and toxicity of As species in subsurface environments. Specifically, I contributed to the fundamental understanding of the reactions involving As(III) and As(V) at GR surfaces, elucidating the relevant binding mechanisms and visualizing specific binding sites of immobilized As species. This work also identified critical geochemical factors that affect As removal and GR formation and transformation under anoxic and circum-neutral pH conditions. More importantly, I was able to show the enhanced long-term stability of GR in natural groundwaters and its prolonged reactivity for As sequestration.