Model-Based Analysis of Arsenic Immobilization via Iron Mineral Transformation under Advective Flows

Abstract

Recent laboratory studies have demonstrated that coinjection of nitrate and Fe(II) (as ferrous sulfate) to As-bearing sediments can produce an Fe mineral assemblage containing magnetite capable of immobilizing advected As under a relatively wide range of aquifer conditions. This study combined laboratory findings with process-based numerical modeling approaches, to quantify the observed Fe mineral (trans)formation and concomitant As partitioning dynamics and to assess potential nitrate-Fe(II) remediation strategies for field implementation. The model development was guided by detailed solution and sediment data from our well-controlled column experiment. The modeling results demonstrated that the fate of As during the experiment was primarily driven by ferrihydrite formation and reductive transformation and that different site densities were identified for natural and neoformed ferrihydrite to explain the observations both before and after nitrate-Fe(II) injection. Our results also highlighted that when ferrihydrite was nearing depletion, As immobilization ultimately relied on the presence of magnetite. On the basis of the column model, field-scale predictive simulations were conducted to illustrate the feasibility of the nitrate-Fe(II) strategy for intercepting advected As from a plume. The predictive simulations, which suggested that long-term As immobilization was feasible, favored a scenario that maintains high dissolved Fe(II) concentration during injection periods and thereby converts ferrihydrite to magnetite.

Quantifying Geochemical Processes of Arsenic Mobility in Groundwater From an Inland Basin Using a Reactive Transport Model

“…Kinetic precipitation of siderite was also included in the model, as expressed by equation (D. Postma et al, ; Rawson et al, ; Sun et al, ). where k FeCO3 is a reaction rate constant (mol/L/yr) and SR FeCO3 is the saturation index of siderite (FeCO 3 ).…”

“…These can be reflected by kinetic processes of organic matter degradation and Fe (III) oxide reduction (D. Postma et al, 2016). In addition to redox processes, As adsorption has frequently been included in models (D. Postma et al, 2007Postma et al, , 2016Postma et al, , 2017Rawson et al, 2017;Pi et al, 2018;Sun et al, 2018). Surface complexation models (SCMs) are the most logical approach to describing As adsorption and its responses to changes in groundwater chemicals.…”

Assessing and Managing Large‐Scale Geochemical Impacts From Groundwater Replenishment With Highly Treated Reclaimed Wastewater

“…The observations used to constrain the calibration consisted of chloride, pH, sodium, potassium, calcium, magnesium, iron, bicarbonate, silicate, phosphate, sulfate, fluoride, nitrate, and ammonium concentrations. The procedure of observation weight assignment during automatic calibration was adopted from Sun et al (2018), and based on the magnitude of each observation, the number of observations available, and the uncertainty inherent in each observation type. The resulting inverse problem is well determined as most parameters exhibit a significant reduction in variance upon estimation ( Table 3).…”

“…The oxidation of ferrous to ferric iron was simulated following a previously established rate expression (Eckert & Appelo, 2002;Sun et al, 2018). The iron(III) produced from the reaction was allowed to precipitate as Fe(OH) 3 , which was modeled as an equilibrium reaction (Prommer et al, 2018;Sun et al, 2018). Seibert et al (2016) suggested that in response to acidity generation, proton buffering was the major pH buffering process during the GWRT.…”

Model-Based Interpretation of Groundwater Arsenic Mobility during in Situ Reductive Transformation of Ferrihydrite

“…Another experimental approach that has been adopted in pilot scale experiments consists in assessing the change of known mineral phases exposed to natural groundwater conditions (33,(36)(37)(38). Furthermore, important insights and quantitative understanding of arsenic release and transport processes are obtained when experimental observations are interpreted with numerical modelling both at laboratory (39)(40)(41)(42) and at field scales (34,(43)(44)(45).…”