Process-based modeling of arsenic(III) oxidation by manganese oxides under circumneutral pH conditions.

Numerous experimental studies have identified a multi-step reaction mechanism to control arsenite (As(III)) oxidation by manganese (Mn) oxides. The studies highlighted the importance of edge sites and intermediate processes, e.g., surface passivation by reaction products. However, the identified reaction mechanism and controlling factors have rarely been evaluated in a quantitative context. In this study, a process-based modeling framework was developed to delineate and quantify the relative contributions and rates of the different processes affecting As(III) oxidation by Mn oxides. The model development and parameterization were constrained by experimental observations from literature studies involving environmentally relevant Mn oxides at circumneutral pH using both batch and stirred-flow reactors. Our modeling results highlight the importance of a transitional phase, solely evident in the stirred-flow experiments, where As(III) oxidation gradually shifts from fast reacting Mn(IV) to slowly reacting Mn(III) edge sites. The relative abundance of these edge sites was the most important factor controlling the oxidation rate, whereas surface passivation restricted oxidation only in the stirred-flow experiment. The Mn(III) edge sites were demonstrated to play a crucial role in the oxidation and therefore in controlling the long-term fate of As. This study provided an improved understanding of Mn oxide reactivity and the significance in the cycling of redox-sensitive metal(loid)s in the environment.

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes.

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010-2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5-510 µg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 µg/L for the studied period of 2010-2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations.

Deoxygenation Prevents Arsenic Mobilization during Deepwell Injection into Sulfide-Bearing Aquifers.

Coal seam gas (CSG) extraction generates large volumes of coproduced water. Injection of the excess water into deep aquifers is often the most sustainable management option. However, such injection risks undesired sediment-water interactions that mobilize metal(loid)s in the receiving aquifer. This risk can be mitigated through pretreatment of the injectant. Here, we conducted a sequence of three push-pull tests (PPTs) where the injectant was pretreated using acid amendment and/or deoxygenation to identify the processes controlling the fate of metal(loid)s and to understand the treatment requirements for large-scale CSG water injection. The injection and recovery cycles were closely monitored, followed by analysis of the observations through reactive transport modeling. While arsenic was mobilized in all three PPTs, significantly lower arsenic concentrations were observed in the recovered water when the injectant was deoxygenated, regardless of pH adjustment. The breakthrough of arsenic was commensurate with molybdenum, but distinct from phosphate. This allowed for the observed and modeled arsenic and molybdenum mobilization to be attributed to a stoichiometric codissolution process during pyrite oxidation, whereas phosphate mobility was governed by sorption. Understanding the nature of these hydrochemical processes explained the greater efficiency of pretreatment by deoxygenation on minimizing metal(loid) mobilization compared to the acid amendment.