Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

Abstract

Environmental context. Uranium and arsenic, two elements of human health concern, are commonly found at sites of uranium mining, but little is known about processes influencing their environmental behaviour. Here we focus on understanding the chemical and physical processes controlling uranium and arsenic transport at an abandoned uranium mine. We find that the use of sedimentation ponds limits the mobility of uranium; however, pond conditions at our site resulted in arsenic mobilisation. Our findings will help optimise restoration strategies for mine tailings.Abstract. Although As can occur in U ore at concentrations up to 10 wt-%, the fate and transport of both U and As at U mine tailings have not been previously investigated at a watershed scale. The major objective of this study was to determine primary chemical and physical processes contributing to transport of both U and As to a down gradient watershed at an abandoned U mine site in South Dakota. Uranium is primarily transported by erosion at the site, based on decreasing concentrations in sediment with distance from the tailings. Sequential extractions and U X-ray absorption near-edge fine structure (XANES) fitting indicate that U is immobilised in a near-source sedimentation pond both by prevention of sediment transport and by reduction of U VI to U IV . In contrast to U, subsequent release of As to the watershed takes place from the pond partially due to reductive dissolution of Fe oxy(hydr)oxides. However, As is immobilised by adsorption to clays and Fe oxy(hydr)oxides in oxic zones and by formation of As-sulfide mineral phases in anoxic zones down gradient, indicated by sequential extractions and As XANES fitting. This study indicates that As should be considered during restoration of uranium mine sites in order to prevent transport.

Thioarsenic Species Associated with Increased Arsenic Release during Biostimulated Subsurface Sulfate Reduction

“…16,43 However, a recent study 21 investigated arsenic mobilization in columns under abiotic and microbial sulfate reducing conditions and found enhanced arsenic release in the sulfidic columns, supporting our field results. Along with the increased arsenic concentrations, their speciation measurements by a similar IC-ICP-MS method revealed the presence of monothioarsenate (5−10%) and dithioarsenate (36−77%).…”

Reactivity of As and U co-occurring in Mine Wastes in northeastern Arizona

“…For example, sequential extraction studies on fine grained sediments collected from mine wastes in South Dakota reported that U became labile at acidic, reducing and oxidizing conditions; however, As was mostly released under reducing conditions (Kipp et al, 2009). Differences in As and U reactivity were also observed in soils collected at various distances from a mine waste site in South Dakota (Troyer et al, 2014a). Sequential extraction results from this study show that U is released as an acid extractable fraction (suggested to be carbonate related) and the As is released through the reductive dissolution of iron oxy(hydr)oxides (Troyer et al, 2014a).…”

“…Reduced and oxidized forms of As [e.g., As (III) and As(V)] and U [e.g., U(IV) and U(VI)] can adsorb on Fe or manganese (Mn) (oxyhydr)oxides and clay minerals by forming both inner-sphere and/or outer-sphere complexes (Cheng et al, 2009; Latta et al, 2014; Massey et al, 2014; Zhang et al, 2016; Blanchard et al, 2017). The reactivity of As and U, which can co-occur in the environment, depends on pH and redox conditions, and the two elements generally exhibit opposite behavior when present in similar redox conditions (Troyer et al, 2014a). For example, the oxidation of reduced U(IV) minerals can cause the oxidative dissolution and release of labile U(VI) (Ulrich et al, 2009; Cerrato et al, 2013), while the reductive dissolution of Fe(III)-oxides associated with As can cause As to mobilize (Troyer et al, 2014a).…”

“…The reactivity of As and U, which can co-occur in the environment, depends on pH and redox conditions, and the two elements generally exhibit opposite behavior when present in similar redox conditions (Troyer et al, 2014a). For example, the oxidation of reduced U(IV) minerals can cause the oxidative dissolution and release of labile U(VI) (Ulrich et al, 2009; Cerrato et al, 2013), while the reductive dissolution of Fe(III)-oxides associated with As can cause As to mobilize (Troyer et al, 2014a). Carbonate can compete with As oxyanions for sorbent sites on mineral surfaces allowing As oxyanions to stay in solution (Kim et al, 2000; Radu et al, 2005; Blake and Peters, 2015), and it can complex with U and calcium (Ca) to form ternary complexes (Dong and Brooks, 2006; Borch et al, 2010; He et al, 2018).…”

Hard X-ray synchrotron biogeochemistry: piecing together the increasingly detailed puzzle

“…These collaborative and multi-technique approaches are condiciones sine quibus non in the quest towards a mechanistic understanding of biogeochemical cycles (e.g. Troyer et al [22] ) and in linking these to macroscopic phenomena (e.g. Wynn et al [23] ).…”