Acid sulfate soils (ASS) contain substantial quantities of iron sulfide minerals or the oxidation reaction products of these sulfidic minerals. Transformation of iron (Fe) and sulfur (S) bearing ...minerals is an important process in ASS wetlands with fluctuating redox conditions. A range of potentially toxic metals and metalloids can either be adsorbed on or incorporated into the structure of Fe and S bearing minerals. Therefore, transformation of these minerals as affected by dynamic redox conditions may regulate the mobility and bioavailability of associated metals/metalloids. Better understanding of the interaction between Fe/S biogeochemistry and trace metal/metalloid mobility under fluctuating redox conditions is important for assessing contaminant risk to the environment. This review paper provides an overview of current knowledge regarding cycling of Fe, S and selected trace metal/metalloids in ASS wetlands under fluctuating redox conditions and outlines future research challenges and directions on this subject.
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•Potential acid sulfate soils (PASS) contain substantial quantities of sulfidic minerals.•Oxidation of sulfidic minerals in ASS environments generates acidity and secondary Fe(III) minerals.•Seasonal redox oscillations can affect Fe and S speciation in ASS environments.•Transformation of Fe and S mineral phases may regulate metalloids mobility in ASS environments.•Dynamic redox can drive complex biogeochemical processes in freshwater re-flooded ASS.
In floodplain soils, As may be released during flooding-induced soil anoxia, with the degree of mobilization being affected by microbial redox processes such as the reduction of As(V), Fe(III), and ...SO4 2–. Microbial SO4 2– reduction may affect both Fe and As cycling, but the processes involved and their ultimate consequences on As mobility are not well understood. Here, we examine the effect of microbial SO4 2 reduction on solution dynamics and solid-phase speciation of As during flooding of an As-contaminated soil. In the absence of significant levels of microbial SO4 2– reduction, flooding caused increased Fe(II) and As(III) concentrations over a 10 week period, which is consistent with microbial Fe(III)- and As(V)-reduction. Microbial SO4 2– reduction leads to lower concentrations of porewater Fe(II) as a result of FeS formation. Scanning electron microscopy with energy dispersive X-ray fluorescence spectroscopy revealed that the newly formed FeS sequestered substantial amounts of As. Bulk and microfocused As K-edge X-ray absorption near-edge structure spectroscopy confirmed that As(V) was reduced to As(III) and showed that in the presence of FeS, solid-phase As was retained partly via the formation of an As2S3-like species. High resolution transmission electron microscopy suggested that this was due to As retention as an As2S3-like complex associated with mackinawite (tetragonal FeS) rather than as a discrete As2S3 phase. This study shows that mackinawite formation in contaminated floodplain soil can help mitigate the extent of arsenic mobilization during prolonged flooding.
Jarosite can be an important scavenger for arsenic (As) and antimony (Sb) in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. When subjected to reducing conditions, jarosite may ...undergo reductive dissolution, thereby releasing As, Sb, and Fe2+ coincident with a rise in pH. These conditions can also trigger the Fe2+-induced transformation of jarosite to more stable Fe(III) minerals, such as goethite. However, the consequences of this transformation process for As and Sb are yet to be methodically examined. We explore the effects of abiotic Fe2+-induced transformation of jarosite on the mobility, speciation, and partitioning of associated As(V) and Sb(V) under anoxic conditions at pH 7. High concentrations of Fe2+ (10 and 20 mM) rapidly (<10 min) transformed jarosite to a green rust intermediary, prior to the subsequent precipitation of goethite within 24 h. In contrast, lower concentrations of Fe2+ (1 and 5 mM) led to the formation of lepidocrocite. As K-edge XANES spectroscopy revealed some reduction of As(V) to As(III) at higher concentrations of Fe2+, while Sb L1-edge XANES spectroscopy indicated no reduction of Sb(V). The transformation processes enhanced Sb mobilization into the aqueous phase, while As was instead repartitioned to a surface-bound exchangeable phase. The results imply that Fe2+-induced transformation of As/Sb-jarosite can increase Sb mobility and exert major influences on As partitioning and speciation.
The Macleay River in eastern Australia is severely impacted by historic stibnite- and arsenopyrite-rich mine-tailings. We explore the partitioning, speciation, redox-cycling, mineral associations and ...mobility of antimony and arsenic along >70 km reach of the upper Macleay River. Elevated Sb/As occur throughout the active channel-zone and in floodplain pockets up to the regolith margin, indicating broad dispersal during floods. Sb concentrations in bulk-sediments decay exponentially downstream more efficiently than As, likely reflecting sediment dilution, hydraulic sorting and comparatively greater leaching of (more mobile) Sb(V) species. However, Sb in bulk-sediments becomes proportionally more bio-available downstream. Sb(V) and As(V) species dominate stream fine-grained (<180 μm) bulk-sediments, reflecting oxidative weathering downstream. Increasing poorly-crystalline Fe(III) Fe(III)HCl in bulk-sediments also indicates progressive oxidative weathering of Fe(II)-bearing minerals downstream and significant (P < .05) correlations exist between PO4−3-exchangeable As and Sb fractions and Fe(III)HCl. Accumulations of poorly-crystalline Fe(III) precipitates (mainly ferrihydrite/feroxyhyte) occur intermittently in hyporheic-zone seeps and are enriched in As relative to Sb and contain some As(III) and Sb(III) (~30–40%). There is dynamic in-stream redox-cycling of both Sb and As, with localised S-coordinated As and Sb species re-forming in organic-rich, hyporheic sediments subject to contemporary sulfidogenesis. Sb mainly Sb(V) is comparatively more mobile in hyporheic and surface waters under oxic conditions, whereas As mainly As(III) is more mobile in hyporheic porewaters subject to reducing/sulfidogenic conditions. Repeat water-leaching of bulk-sediments confirms that Sb is proportionally more mobile than As. Mean concentrations of Sb in river water 168 km downstream from the mine are significantly (P < .05) higher than As, while Kd data indicate Sb is more strongly partitioned to the aqueous phase than As. Although the (mainly) oxic flow path of this river favours aqueous Sb mobility compared to As, localised redox-driven shifts in speciation of both elements strongly influence their respective mobility and partitioning.
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•We explore the fate and behaviour of Sb and As in a tailings contaminated river.•Sb(V) and As(V) species dominate most sediments.•Fe(III) precipitates in hyporheic-zone seeps are enriched in As relative to Sb.•Sb(V) and As(III) are more mobile in oxic and reducing hyporheic-zones respectively.•Aqueous Sb mobility is favoured compared to As along the (mainly) oxic flow path.
Microbial sulfate reduction is an important terminal electron accepting process in arsenic-contaminated subsurface environments. Humic acids are ubiquitous in such environments, yet their impact on ...arsenic mobility under sulfate-reducing conditions is poorly understood. In this study, we examined the effects of microbial sulfate reduction and humic acid on arsenic mobilization via a series of advective-flow column experiments. The initial solid-phase in these experiments comprised quartz sand that was coated with As(III)-sorbed goethite (α-FeOOH). The effect of humic acid was assessed by comparing columns that received artificial groundwater in which humic acid was either absent or present at 100mgL−1, whilst the effect of microbial sulfate reduction was investigated by comparing columns that were inoculated with the sulfate-reducer Desulfovibrio vulgaris (ATCC strain 7757) versus abiotic control columns. The presence of high concentrations of humic acid alone did not enhance the overall extent of arsenic release from either the abiotic or the inoculated (sulfate reducing) columns. This is consistent with similar arsenic concentrations in porewaters filtered to both <0.45μm and <3kDa, demonstrating that aqueous arsenic did not form mobile colloidal humic acid complexes. In contrast, microbial sulfate reduction was found to mobilize substantial levels of arsenic relative to those observed in the corresponding abiotic control columns. Iron and sulfur K-edge X-ray absorption spectroscopy (XAS) showed that reaction between goethite and microbially-produced sulfide lead to accumulation of mackinawite (FeS) and elemental S. Microbial sulfate reduction also caused important changes in arsenic speciation, especially the formation of aqueous dithioarsenate and monothioarsenate. However, arsenic K-edge XAS showed that arsenic sulfide mineral phases (orpiment and realgar) did not form during the 60day advective-flow experiment. The formation of poorly-sorbing thioarsenate species appeared to contribute to the observed enhancement of arsenic mobilization from the inoculated columns. Dithioarsenate and monothioarsenate were relatively stable, and were found to make up>40% of aqueous arsenic even at very low porewater sulfide concentrations (i.e. <10μmolL−1). Accordingly, the formation, stability and sorption–desorption of thioarsenate species need to be considered when evaluating and predicting subsurface arsenic mobility.
► The impact of microbial sulfate reduction and humic acid on arsenic mobility was examined. ► Humic acid did not form mobile colloids with arsenic and did not enhance arsenic mobility. ► Sulfate reduction enhanced arsenic mobility, partly due to formation of thioarsenate species.
Antimony, as the Sb(V) species, often occurs in oxic soils and sediments as coprecipitates with poorly-crystalline Fe(III)-bearing minerals. It is common for these Sb(V)-Fe(III) coprecipitates to ...also contain varying quantities of co-occurring humic acid (HA). When exposed to reducing conditions, the production of Fe(II) may cause the initial metastable HA-Sb(V)-Fe(III) phases to undergo rapid transformations to more stable phases, thereby potentially influencing the geochemical behavior of coprecipitated Sb(V). However, little is known about the impacts of this transformation on the mobility and speciation of Sb. In this study, we reacted synthetic HA-Sb(V)-Fe(III) coprecipitates (Fe:Sb ratio = 4, and C:Fe molar ratios = 0, 0.3, 0.8 and 1.3) with 0, 1 or 10 mM Fe(II) under O2-free conditions at pH 7.0 for 15 days. Fe K-edge EXAFS spectroscopy revealed that solid-phase Fe(III) in the initial coprecipitates contained a mixture of ∼4/5 ferrihydrite (Fe10O14(OH)2) and ∼1/5 tripuhyite (FeSbO4), regardless of the corresponding amount of coprecipitated HA. Tripuhyite persisted throughout the full experiment duration, while ferrihydrite was partially replaced by goethite (FeOOH) when either 1 or 10 mM Fe(II)aq was added to the coprecipitates. The greatest level of goethite formation (∼55% of solid-phase Fe) was observed in the HA-free/10 mM Fe(II)aq treatment, with ferrihydrite transformation being partially attenuated at higher levels of HA. Mobilisation of aqueous Sb was the greatest for 1 mM Fe(II) treatments at high HA:Fe ratios. Sb K-edge XANES spectroscopy showed that the largest reduction of Sb(V) to Sb(III) (∼37%) and the greatest repartitioning of Sb to the mineral surface (∼7.9–9.8%) occurred in the coprecipitates with the highest HA contents in the presence of 10 mM Fe(II). The results indicate that the amount of HA in HA-Sb(V)-Fe(III) coprecipitates can greatly influence mobility and speciation of Sb in Fe(II)-rich conditions. The results of this study provide new insights into alterations in Sb mobility and retention in response to Fe cycling under organic matter-rich reducing conditions.
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•The mineralogy of the initial coprecipitates comprised ferrihydrite and tripuhyite.•Addition of Fe(II) caused partial transformation of ferrihydrite to goethite.•Tripuhyite persisted throughout the full experiment duration in all treatments.•Mobilisation of aqueous Sb was greatest for low Fe(II)/high HA treatments.•Antimony(V) reduction was greatest in coprecipitates with the highest HA/Fe(II) contents.
Coprecipitated HA is an important factor affecting the mobility and speciation of Sb during Fe(II)-induced transformation of HA-Sb(V)-Fe(III) coprecipitates.
Jarosite KFe3(SO4)2(OH)6 is considered a potent scavenger for arsenic (As) and antimony (Sb) under oxidizing conditions. Fluctuations in water levels in re-flooded acid sulfate soils (ASS) can lead ...to high Fe2+(aq) concentrations (∼10–20 mM) in the soil solution under acidic to circumneutral pH conditions. This may create favorable conditions for the Fe2+-induced transformation of jarosite. In this study, synthetic arsenate As(V)/antimonate Sb(V)-bearing jarosite was subjected to Fe2+(aq) (20 mM) at pH 4.0 and 5.5 for 24 h to simulate the pH and Fe2+(aq) conditions of re-flooded freshwater ASS/acid mine drainage (AMD)-affected environments at early and mid-stages of remediation, respectively. The addition of Fe2+ at pH 5.5 resulted in the formation of a metastable green rust sulfate (GR- SO4) phase within ∼60 min, which was replaced by goethite within 24 h. In contrast, at pH 4.0, jarosite underwent no significant mineralogical transformation. Although the addition of Fe2+(aq) induced the dissolution/transformation of jarosite at pH 5.5 and increased the mobility of Sb during the initial stages of the experiment (Sb(aq) = ∼0.05 μmol L−1), formation of metastable green rust (GR-SO4) and subsequent transformation to goethite effectively sequestered dissolved Sb. Aqueous concentrations of As remained negligible in both pH treatments, with As being mostly repartitioned to the labile (∼10%) and poorly crystalline Fe(III)-associated phases (∼10–30%). The results imply that, under moderately acidic conditions (i.e. pH 5.5), reaction of Fe2+(aq) with jarosite can drive the dissolution of jarosite and increase Sb mobility prior to the formation of GR-SO4 and goethite. In addition, repartitioning of As to the labile fractions at pH 5.5 may enhance the risk of its mobilisation during future mineral transformation processes in Fe2+-rich systems.
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•At pH 4.0, the addition of Fe2+(aq) resulted in no significant transformation in jarosite.•At pH 5.5, 60 min was sufficient for partial transformation of jarosite to green rust.•Formation of green rust and goethite under pH 5.5, sequestered dissolved antimony.•Arsenic was mostly repartitioned to the surface of green rust and goethite.
Tree stems are an important and unconstrained source of methane, yet it is uncertain whether internal microbial controls (i.e. methanotrophy) within tree bark may reduce methane emissions. Here we ...demonstrate that unique microbial communities dominated by methane-oxidising bacteria (MOB) dwell within bark of Melaleuca quinquenervia, a common, invasive and globally distributed lowland species. In laboratory incubations, methane-inoculated M. quinquenervia bark mediated methane consumption (up to 96.3 µmol m
bark d
) and reveal distinct isotopic δ
C-CH
enrichment characteristic of MOB. Molecular analysis indicates unique microbial communities reside within the bark, with MOB primarily from the genus Methylomonas comprising up to 25 % of the total microbial community. Methanotroph abundance was linearly correlated to methane uptake rates (R
= 0.76, p = 0.006). Finally, field-based methane oxidation inhibition experiments demonstrate that bark-dwelling MOB reduce methane emissions by 36 ± 5 %. These multiple complementary lines of evidence indicate that bark-dwelling MOB represent a potentially significant methane sink, and an important frontier for further research.
Tree stem mediated methane emissions represent a potentially important yet poorly constrained source of atmospheric methane. Here we present the first ever quantification of tree stem methane ...emissions from Melaleuca quinquenervia, a widespread iconic Australian lowland tree and globally invasive species. Under two distinct hydrological conditions (wet and dry) we captured 431 tree stem flux measurements encompassing six different vertical stem heights along a 50 m topo-gradient transect, separated into three distinct hydrological zones (upper, transitional and lower). The tree stem methane fluxes closely reflected local topography/hydrology and ranged from - 30.0 to 123,227 lmol m⁻² day⁻¹, with the maximum values amongst the highest values reported to date. The highest methane emissions were observed during wet conditions, within the inundated lower zone and from near the tree stem bases and water table. The average methane flux per tree (scaled to 1 m of stem) for the transitional and lower zones was 52-fold and 46-fold higher during wet conditions compared to dry, whereas the upper zone emissions changed little between seasons. Adjacent soil fluxes followed similar trends along the hydrology gradient with the upper zone tree stem emissions offsetting the adjacent soil methane sink capacity. A clear trend of sharply decreasing methane emissions with stemheight suggests a soil methane origin. A 45-h timeseries of two trees within the lower zone revealed three to fourfold diel variability, with elevated morningtime fluxes. Overall, the study revealed that seasonal hydrological conditions and topo-gradient substantially regulated the methane emissions from M. quinquenervia and that this previously overlooked pathway should be accounted for within wetland methane budgets, especially during inundated conditions.
•Daily sampling of aquatic chemistry after megafires in a large catchment.•Significant increases in major parameters and highly distorted CQ relationships.•Extreme dynamism and transient behaviour ...with monotonic recovery in ∼3–12 months.•Greatest sensitivity to erosion during initial flow events following fire.•Initial dominance of NH4 and rapid biogeochemical cycling of N species.
Increased frequency and intensity of drought, wildfires and flooding due to climate change has major implications for river water quality, yet there are limited high-temporal resolution data capturing the combined transient impacts of these extreme events at large catchment scales. We present flow-stratified water quality data from a large coastal catchment (Macleay River, Australia) spanning severe drought and extensive fires followed by flooding. We examine concentrations (C), discharge (Q) and flux of suspended sediment, major ions, dissolved organic carbon (DOC) and key nutrients (NO3 and PO4), with a focus on the critical first-flush period after the fires. Highly elevated suspended sediment (∼5500 mg L−1; >100x median pre-fire levels) during the initial post-fire period reflected enhanced erosion from fire-impacted, high-relief landscapes, with peak monthly suspended sediment loads of ∼1.1–3.7 t ha−1. The greatest sensitivity to erosion was during initial flow events following fire, highlighting the compounding effect of sequential extreme events on sediment transport. Maximum solute concentrations typically occurred during the first hydrograph peak post-fire with significantly (P = 0.01) elevated major ions following the order of K>Ca>SO4>HCO3≈Mg>Cl>Na, broadly reflecting the composition of ash materials. Distorted CQ relationships for major ions, DOC and nutrients indicated mobilisation behaviour and enhanced surface runoff during initial hydrograph peaks post-fire, with mean concentrations and CQ relationships progressively shifting to those approximating pre-fire within ∼3–12 months. Elevated DOC (∼7x; P = 0.01) displays distinct changes in fluorescence excitation-emission matrix spectral characteristics, attributable to both fire and drought. Both NO3N (160 μM) and PO4 (7.5 μM) were significantly elevated after the fires (∼15–22x; P = 0.01), with maximum monthly loads of 0.82 and 0.14 kg ha−1 respectively. Fast biogeochemical cycling of dissolved inorganic nitrogen (DIN) species occurred during initial flow events following fire, with NH4N initially dominant (>80% of DIN) and exceeding ecosystem guideline threshold values (>100 μM NH4N), followed by rapid (∼1 week) nitrification. The extreme dynamism and transience of water quality parameters highlights the critical importance of high-frequency sampling to adequately capture the compound impacts drought, fires and floods on aquatic systems.
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