Annabelle F Keene

Arsenic effects and behavior in association with the Fe(II)-catalyzed transformation of schwertmannite.

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(8)O(8)(OH)(8-2x)(SO(4))(x)). When schwertmannite-rich 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 1 mM 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.

Arsenic Mobilization in a Seawater Inundated Acid Sulfate Soil

Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and S0(4)-reducing conditions in previously oxic-acidic 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 (~400 μg/L) occurred in the shallow (<1 m), intertidal, redox transition zone between Fe-oxidizing and S0(4)-reducing conditions. Primary mechanisms of As mobilization include the reduction of solid-phase As(V) to As(lll), reductive dissolution of As(V)-bearing secondary Fe(lll) 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 Fe(2+)-enriched porewater in the intertidal zone, leading to accumulation of As(V)-enriched Fe(lll) (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.

Iron and arsenic cycling in intertidal surface sediments during wetland remediation.

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.

Reactive trace element enrichment in a highly modified, tidally inundated acid sulfate soil wetland: East Trinity, Australia

This study examines the abundance of trace elements in surface sediments of a former acid sulfate soil (ASS) wetland subjected to marine tidal inundation. Sediment properties of this highly modified study site are compared with those of an adjacent unmodified, intertidal mangrove forest. Whilst some trace elements (Al, Cd, Mn, Ni and Zn) were clearly depleted due to mobilisation and leaching in the previous oxic-acidic phase, other trace elements (As and Cr) displayed significant enrichment in the tidally inundated ASS. Many trace elements were strongly associated with the reactive Fe and acid volatile sulfide (AVS) fractions, suggesting that trace elements may be adsorbed to abundant reactive Fe phases or sequestered as sulfide minerals. These findings provide an important understanding of the fate and mobility of reactive iron, AVS and trace elements during tidal remediation of a formerly acidified Great Barrier Reef (GBR) catchment.

Pore water sampling in acid sulfate soils: a new peeper method

This study describes the design, deployment, and application of a modified equilibration dialysis device (peeper) optimized for sampling pore waters in acid sulfate soils (ASS). The modified design overcomes the limitations of traditional-style peepers, when sampling firm ASS materials over relatively large depth intervals. The new peeper device uses removable, individual cells of 25 mL volume housed in a 1.5 m long rigid, high-density polyethylene rod. The rigid housing structure allows the device to be inserted directly into relatively firm soils without requiring a supporting frame. The use of removable cells eliminates the need for a large glove-box after peeper retrieval, thus simplifying physical handling. Removable cells are easily maintained in an inert atmosphere during sample processing and the 25-mL sample volume is sufficient for undertaking multiple analyses. A field evaluation of equilibration times indicates that 32 to 38 d of deployment was necessary. Overall, the modified method is simple and effective and well suited to acquisition and processing of redox-sensitive pore water profiles>1 m deep in acid sulfate soil or any other firm wetland soils.

Longitudinal correlation of late Quaternary terrace sequences of Widden Brook, southeastern Australia

Terrace remnants on Widden Brook, southeastern Australia, were examined and correlated longitudinally to establish their evolutionary history. Three discontinuous terrace sequences, the Baramul, Widden and Kewarra, were identified in a 26 km reach using sedimentology, topography and chronology. Each terrace sequence occurred within a geomorphically distinct valley setting: an upstream constriction, a valley expansion and a highly constricted downstream section. Radiocarbon and optically stimulated luminescence ages indicated that each terrace sequence was formed during the Late Pleistocene and Holocene (16.7–0.5 ka cal BP). However, their sedimentology and topography were shown to differ significantly. We present evidence that both climate and the exceedance of intrinsic geomorphic thresholds were major contributing factors responsible for the formation of these terrace sequences.

Tracing the Outputs from Drained Acid Sulphate Flood Plains to Minimize Threats to Coastal Lakes

Drainage of acid sulphate flood plains for agriculture and urban development has led to the acidification of coastal waterbodies, major fish kills and other environmental effects in the subtropical areas of eastern Australia. These have produced problems for local governments and conflicts in communities. Here, we trace the effects of drainage from an acidified, subtropical flood plain on water quality and sediments in an estuarine lake, Cudgen Lake, in northern New South Wales. This shallow, brackish lake was once a renowned fish and prawn nursery. The local government has monitored water quality in drains and the lake since 1990. This has revealed episodic discharge events with pH as low as 2 and concentrations of dissolved iron and aluminium that are toxic to gilled organisms. Highly acidic waters were found to accumulate in drainage channels after rains following long dry periods. These were then discharged into the lake, causing major fish kills, low benthic organism biodiversity and infestations of acid-tolerant reeds and disease-bearing mosquitoes. High concentrations of arsenic, mercury and lead were found to have accumulated in iron monosulphides in the lake sediments, together with massive amounts of aluminium and iron. Ion ratios show that these metals were mobilized by drainage from the acidified soils within the flood plain. Although the lake sediments at present represent a sink for these metals, disturbance and oxidation of the monosulphides in the sediments could release major contaminants into the lake. The heavy metals sequestered in the lake sediments could partly explain the low benthic biodiversity. These results provide pointers for enhancing the conservation and fishery values of the lake.