As release under the microbial sulfate reduction during redox oscillations in the upper Mekong delta aquifers, Vietnam: A mechanistic study

Abstract

Highlights: Microbial sulfate reductionwas found to contribute to arsenic (As) sequestration; During anoxic conditions, the conversion of soluble As(V) to As(III) was observed; The formation of pyrite, rather than mackinawite was observed in redox cycles; Absorption/desorption of aqueous As on Fe-(oxyhydr)oxides and pyrite was observed.

Iron mineral transformations and their impact on As (im)mobilization at redox interfaces in As-contaminated aquifers

“…Berg et al, 2001;Smedley and Kinniburgh, 2002;Ravenscroft et al, 2009;Benner and Fendorf, 2010;Winkel et al, 2011). Many studies have revealed that the mobilization of As into groundwater is related to natural processes, of which the microbially driven oxidation of organic matter coupled with the reduction of As-bearing Fe(III) (oxyhydr)oxides in the deltaic sediments is the most dominant and well-studied mechanism (Nickson et al, 2000;McArthur et al, 2001;Dowling et al, 2002;Harvey et al, 2002;Akai et al, 2004;Islam et al, 2004;Swartz et al, 2004;Postma et al, 2007Postma et al, , 2010Postma et al, , 2012Fendorf et al, 2010;Stuckey et al, 2015;Phan et al, 2019;Glodowska et al, 2020a). A characteristic feature of the As contaminated areas is that orange, often Pleistocene aquifer sands mostly show As groundwater concentrations below the WHO limit of 10 g/L, while gray, mostly Holocene sandy aquifers have dissolved As concentrations 10 to 30 times the WHO limit (Berg et al 2001;van Geen et al, 2013;Jung et al 2015;Stopelli et al, 2020).…”

“…Several factors such as seasonal variations or pumping-induced changes in groundwater head and groundwater flow direction, or the type and availability of organic matter are known to influence the prevalent redox state of the aquifer (e.g. Jung et al, 2015;Stuckey et al, 2015;Eiche et al, 2017;Schaefer et al, 2017;Phan et al, 2019). Highly variable sedimentology, including fluviatile channel fillings and the interfingering of marine and fluviatile deposits of different grain size, can also be responsible for highly contrasting redox subsurface conditions on centimeter-to meter-scales (e.g.…”

Effects of redox oscillations on the phosphogypsum waste in an estuarine salt-marsh system

“…In the phosphogypsum experiment, Fe 2+ in solution first decreases in the oxic half cycles as Eh increases, and free Fe 2+ oxidatively precipitates as FeOOH, a phenomenon catalysed by adsorption of Fe 2+ on mineral surfaces (Silvester et al, 2005), or as amorphous Fe 3+ phosphates. The oxidation of Fe 2+ originating from pyrite oxidation and Fe-(oxyhydr)oxides to Fe 3+ phases was, also, described in similar studies (e.g., Phan et al, 2019). The rapid precipitation of poorly crystalline Fe 3+ minerals under oxic conditions has been described by Megonigal et al (2003).…”

“…The experiment was performed using a custom two part Pyrex® glass redox stat bioreactor system designed by Thompson et al (2006) and modified later by Parsons et al (2013); the system was also used in similar, recent studies (Markelova et al, 2018;Phan et al, 2018;2019) (Fig. 2).…”

Environmental processes that control the chemical recovery of arsenic impacted northern landscapes

“…Sediment redox processes are dynamic and the role of oscillating redox condition on the diffusion and retention of As across the sediment-water boundary has been the focus of substantial research (Arsic et al, 2018;Bennett et al, 2012;Johnston et al, 2020;Parsons et al, 2013;Phan et al, 2019). These studies have typically explored the mechanics of As mobility using laboratory experiments where oxygen conditions have been manipulated in soil or sediment incubations.…”