Iron and arsenic cycling in intertidal surface sediments during wetland remediation
- Authors: Johnston, Scott , Keene, Annabelle , Burton, Edward , Bush, Richard , Sullivan, Leigh
- Date: 2011
- Type: Text , Journal article
- Relation: Environmental Science and Technology Vol. 45, no. 6 (2011), p. 2179-2185
- Full Text: false
- Reviewed:
- Description: The accumulation and behavior of arsenic at the redox interface of Fe-rich sediments is strongly influenced by Fe(III) precipitate mineralogy, As speciation, and pH. In this study, we examined the behavior of Fe and As during aeration of natural groundwater from the intertidal fringe of a wetland being remediated by tidal inundation. The groundwater was initially rich in Fe 2+ (32 mmol L -1) and As (1.81 μmol L -1) with a circum-neutral pH (6.05). We explore changes in the solid/solution partitioning, speciation and mineralogy of Fe and As during long-term continuous groundwater aeration using a combination of chemical extractions, SEM, XRD, and synchrotron XAS. Initial rapid Fe 2+ oxidation led to the formation of As(III)-bearing ferrihydrite and sorption of >95% of the As(aq) within the first 4 h of aeration. Ferrihydrite transformed to schwertmannite within 23 days, although sorbed/coprecipitated As(III) remained unoxidized during this period. Schwertmannite subsequently transformed to jarosite at low pH (2-3), accompanied by oxidation of remaining Fe 2+. This coincided with a repartitioning of some sorbed As back into the aqueous phase as well as oxidation of sorbed/coprecipitated As(III) to As(V). Fe(III) precipitates formed via groundwater aeration were highly prone to reductive dissolution, thereby posing a high risk of mobilizing sorbed/coprecipitated As during any future upward migration of redox boundaries. Longer-term investigations are warranted to examine the potential pathways and magnitude of arsenic mobilization into surface waters in tidally reflooded wetlands. © 2011 American Chemical Society.
Arsenic mobilization in a seawater inundated acid sulfate soil
- Authors: Johnston, Scott , Keene, Annabelle , Burton, Edward , Bush, Richard , Sullivan, Leigh , McElnea, Angus , Ahern, Col , Smith, C. Douglas , Powell, Bernard , Hocking, Rosalie
- Date: 2010
- Type: Text , Journal article
- Relation: Environmental Science and Technology Vol. 44, no. 6 (2010), p. 1968-1973
- Full Text:
- Reviewed:
- Description: Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and SO4-reducing conditions in previously oxicacidic sediments, This creates potential for mobilization of As during the redox transition. We explore the consequences for As by investigating the hydrology, porewater geochemistry, solid-phase speciation, and mineralogical partitioning of As across two tidal fringe toposequences. Seawater inundation induced a tidally controlled redox gradient. Maximum porewater As (∼400μg/L) occurred in the shallow (<1 m), intertidal, redox transition zone between Fe-oxidizing and SO 4-reducing conditions. Primary mechanisms of As mobilization include the reduction of solid-phase As(V) to As(III), reductive dissolution of As(V)-bearing secondary Fe(III) minerals and competitive anion desorption. Porewater As concentrations decreased in the zone of contemporary pyrite reformation, Oscillating hydraulic gradients caused by tidal pumping promote upward advection of As and Fe2+-enriched porewater in the intertidal zone, leading to accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface. While this provides a natural reactive-Fe barrier, it does not completely retard the flux of porewater As to overtopping surface waters. Furthermore, the accumulated Fe minerals may be prone to future reductive dissolution, A conceptual model describing As hydro-geochemical coupling across an intertidal fringe is presented. © 2010 American Chemical Society.
Arsenic mobilization during seawater inundation of acid sulfate soils - Hydrogeochemical coupling at the tidal fringe
- Authors: Johnston, Scott , Burton, Edward , Keene, Annabelle , Bush, Richard , Sullivan, Leigh
- Date: 2012
- Type: Text , Conference proceedings
- Full Text: false
- Description: Coastal Acid Sulfate Soils (CASS) are rich in meta-stable iron (Fe - III) minerals that are important sorbents for arsenic (As) under oxic conditions. Tidal seawater inundation to remediate CASS has recently been trialed on a large scale and has potential to mobilize arsenic during the redox transition. Tidal seawater inundation caused reductive dissolution of As(V)-bearing Fe(III) minerals, resulting in elevated concentrations of Fe 2+ (2000 mg L -1) and As (∼400 μg L -1) in upper-intertidal zone groundwater. Oscillating vertical and horizontal hydraulic gradients caused by tidal pumping promoted upward advection of As and Fe 2+-enriched groundwater within the intertidal zone. This led to flux of As aq and Fe 2+ aq to surface waters and the accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface. Fe(III) (hydr)oxides at the sediment-water interface act as a natural reactive-barrier, retarding As flux to overlying surface waters. However, they also represent a highly transient phase that is prone to reductive dissolution during future redox boundary migration. A conceptual model is presented to explain landscape-scale patterns of As and Fe hydro-geochemical zonation. © 2012 Taylor & Francis Group.
Arsenic effects and behavior in association with the fe(II)-catalyzed transformation of schwertmannite
- Authors: Burton, Edward , Johnston, Scott , Watling, Kym , Bush, Richard , Keene, Annabelle , Sullivan, Leigh
- Date: 2010
- Type: Text , Journal article
- Relation: Environmental Science and Technology Vol. 44, no. 6 (2010), p. 2016-2021
- Full Text: false
- Reviewed:
- Description: In acid-mine drainage and acid-sulfate soil environments, the cycling of Fe and As are often linked to the formation and fate of schwertmannite(Fe 8O8(OH)8-2x(SO4)x).When schwertmanniterich material is subjected to near-neutral Fe(III)-reducing conditions (e.g., in reflooded acid-sulfate soils or mining-lake sediments), the resulting Fe(II) can catalyze transformation of schwertmannite to goethite. This work examines the effects of arsenic(V) and arsenic(III) on the Fe(II)-catalyzed transformation of schwertmannite and investigates the associated consequences of this mineral transformation for arsenic mobilization. A series of 9-day anoxic transformation experiments were conducted with synthetic schwertmannite and various additions of Fe(II), As(III), and As(V). X-ray diffraction (XRD) and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy demonstrated that, in the absence of Fe(II), schwertmannite persisted as the dominant mineral phase. Under arsenic-free conditions, 10 mM Fe(II) catalyzed rapid and complete transformation of schwertmannite to goethite. However, the magnitude of Fe(II)-catalyzed transformation decreased to 72% in the presence of 1 mM As(III) and to only 6% in the presence of 1mM As(V). This partial Fe(II)-catalyzed transformation of As(III)-sorbed schwertmannite did not cause considerable As(III) desorption. In contrast, the formation of goethite via partial transformation of As(III)- and As(V)-sorbed schwertmannite significantly decreased arsenic mobilization under Fe(III)-reducing conditions. This implies that the Fe(II)-catalyzed transformation of schwertmannite to goethite may help to stabilize solid-phase arsenic and retard its subsequent release to groundwater. © 2010 American Chemical Society.