Microbiome mediating methane and nitrogen transformations in a subterranean estuary.

Subterranean estuaries (STEs) are important coastal biogeochemical reactors facilitating unique niches for microbial communities. A common approach in determining STE greenhouse gas and nutrient fluxes is to use terrestrial endmembers, not accounting for microbially mediated transformations throughout the STE. As such, the microbial ecology and spatial distribution of specialists that cycle compounds in STEs remain largely underexplored. In this study, we applied 16S rRNA amplicon sequencing with paired biogeochemical characterisations to spatially evaluate microbial communities transforming greenhouse gases and nutrients in an STE. We show that methanogens are most prevalent at the terrestrial end (up to 2.81% relative abundance) concomitant to the highest porewater methane, carbon dioxide and dissolved organic carbon concentrations (0.41 ± 0.02 µM, 273.31 ± 6.05 µM and 0.51 ± 0.02 mM, respectively). Lower ammonium concentrations corresponded with abundant nitrifying and ammonia-oxidising prokaryotes in the mixing zone (up to 11.65% relative abundance). Methane, ammonium and dissolved organic carbon concentrations all decreased by >50% from the terrestrial to the oceanic end of the 15 m transect. This study highlights the STE's hidden microbiome zonation, as well as the importance of accounting for microbial transformations mitigating nutrient and greenhouse gas fluxes to the coastal ecosystems.

Rapid bark-mediated tree stem methane transport occurs independently of the transpiration stream in Melaleuca quinquenervia.

Tree stem methane emissions are important components of lowland forest methane budgets. The potential for species-specific behaviour among co-occurring lowland trees with contrasting bark characteristics has not been investigated. We compare bark-mediated methane transport in two common lowland species of contrasting bark characteristics (Melaleuca quinquenervia featuring spongy/layered bark with longitudinally continuous airspaces and Casuarina glauca featuring hard/dense common bark) through several manipulative experiments. First, the progressive cutting through M. quinquenervia bark layers caused exponential increases in methane fluxes (c. 3 orders of magnitude); however, sapwood-only fluxes were lower, suggesting that upward/axial methane transport occurs between bark layers. Second, concentrated methane pulse-injections into exposed M. quinquenervia bark, revealed rapid axial methane transport rates (1.42 mm s-1 ), which were further supported through laboratory-simulated experiments (1.41 mm s-1 ). Laboratory-simulated radial CH4 diffusion rates (through bark) were c. 20-times slower. Finally, girdling M. quinquenervia stems caused a near-instantaneous decrease in methane flux immediately above the cut. By contrast, girdling C. glauca displayed persistent, though diminished, methane fluxes. Overall, the experiments revealed evidence for rapid 'between-bark' methane transport independent from the transpiration stream in M. quinquenervia, which facilitates diffusive axial transport from the rhizosphere and/or sapwood sources. This contrasts with the slower, radial 'through-bark' diffusive-dominated gas transportation in C. glauca.

Mechanisms of Arsenic and Antimony Co-sorption onto Jarosite: An X-ray Absorption Spectroscopic Study.

Jarosite, a common mineral in acidic sulfur-rich environments, can strongly sorb both As(V) and Sb(V). However, little is known regarding the mechanisms that control simultaneous co-sorption of As(V) and Sb(V) to jarosite. We investigated the mechanisms controlling As(V) and Sb(V) sorption to jarosite at pH 3 (in dual and single metalloid treatments). Jarosite was found to sorb Sb(V) to a greater extent than As(V) in both single and dual metalloid treatments. Relative to single metalloid treatments, the dual presence of both As(V) and Sb(V) decreased the sorption of both metalloids by almost 50%. Antimony K-edge EXAFS spectroscopy revealed that surface precipitation of an Sb(V) oxide species was the predominant sorption mechanism for Sb(V). In contrast, As K-edge EXAFS spectroscopy showed that As(V) sorption occurred via bidentate corner-sharing complexes on the jarosite surface when Sb(V) was absent or present at low loadings or by formation of similar complexes on the Sb(V) oxide surface precipitate when Sb(V) was present at high loadings. These results point to a novel mechanism by which Sb(V) impacts the co-sorption of As(V). Overall, these findings highlight a strong contrast in the sorption mechanisms of Sb(V) versus As(V) to jarosite under acidic environmental conditions.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments.

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between SbVO6 and FeIII(O,OH)6 octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests.

Knowledge regarding mechanisms moderating methane (CH4 ) sink/source behaviour along the soil-tree stem-atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ13 C-CH4 ) to gain insights into axial CH4 transport and oxidation in two globally distributed subtropical lowland species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH4 flux (decreasing with height) and δ13 C-CH4 enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem δ13 C-CH4 (0-40 cm) of Melaleuca and Casuarina (-53.96‰ and -65.89‰) were similar to adjacent flooded soil CH4 ebullition (-52.87‰ and -62.98‰), suggesting that stem CH4 is derived mainly by soil sources. Upper stems (81-200 cm) displayed distinct δ13 C-CH4 enrichment (Melaleuca -44.6‰ and Casuarina -46.5‰, respectively). Coupled 3D-photogrammetry with novel 3D-stem measurements revealed distinct hotspots of CH4 flux and isotopic fractionation on Melaleuca, which were likely due to bark anomalies in which preferential pathways of gas efflux were enhanced. Diel experiments revealed greater δ13 C-CH4 enrichment and higher oxidation rates in the afternoon, compared with the morning. Overall, we estimated that c. 33% of the methane was oxidised between lower and upper stems during axial transport, therefore potentially representing a globally significant, yet previously unaccounted for, methane sink.

Antimonate Controls Manganese(II)-Induced Transformation of Birnessite at a Circumneutral pH.

Manganese (Mn) oxides, such as birnessite (δ-MnO2), are ubiquitous mineral phases in soils and sediments that can interact strongly with antimony (Sb). The reaction between birnessite and aqueous Mn(II) can induce the formation of secondary Mn oxides. Here, we studied to what extent different loadings of antimonate (herein termed Sb(V)) sorbed to birnessite determine the products formed during Mn(II)-induced transformation (at pH 7.5) and corresponding changes in Sb behavior. In the presence of 10 mM Mn(II)aq, low Sb(V)aq (10 µmol L-1) triggered the transformation of birnessite to a feitknechtite (ß-Mn(III)OOH) intermediary phase within 1 day, which further transformed into manganite (γ-Mn(III)OOH) over 30 days. Medium and high concentrations of Sb(V)aq (200 and 600 µmol L-1, respectively) led to the formation of manganite, hausmannite (Mn(II)Mn(III)2O4), and groutite (αMn(III)OOH). The reaction of Mn(II) with birnessite enhanced Sb(V)aq removal compared to Mn(II)-free treatments. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that heterovalent substitution of Sb(V) for Mn(III) occurred within the secondary Mn oxides, which formed via the Mn(II)-induced transformation of Sb(V)-sorbed birnessite. Overall, Sb(V) strongly influenced the products of the Mn(II)-induced transformation of birnessite, which in turn attenuated Sb mobility via incorporation of Sb(V) within the secondary Mn oxide phases.

Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality.

Growing evidence indicates that tree-stem methane (CH4 ) emissions may be an important and unaccounted-for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp-forests; however, no data on tree-stem fluxes from estuarine species currently exist. Here we provide the first-ever mangrove tree-stem CH4 flux measurements from  >50 trees (n = 230 measurements), in both standing dead and living forest, from a region suffering a recent large-scale climate-driven dieback event (Gulf of Carpentaria, Australia). Average CH4 emissions from standing dead mangrove tree-stems was 249.2 ± 41.0 µmol m-2  d-1 and was eight-fold higher than from living mangrove tree-stems (37.5 ± 5.8 µmol m-2  d-1 ). The average CH4 flux from tree-stem bases (c. 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 µmol m-2  d-1 from dead and living stands respectively. Sediment CH4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree-stems act as CH4 conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small-scale temporal measurements, we estimated that dead mangrove tree-stem emissions may account for c. 26% of the net ecosystem CH4 flux.

iAMES: An inexpensive, Automated Methane Ebullition Sensor.

Atmospheric concentrations of methane have increased ∼2.4 fold since the industrial revolution with wetlands and inland waters representing the largest source of methane to the atmosphere. Substantial uncertainties remain in global methane budgets, due in part to the lack of adequate techniques and detailed measurements to assess ebullition in aquatic environments. Here, we present details of a low cost (∼$120 US per unit) ebullition sensor that autonomously logs both volumetric ebullition rate and methane concentrations. The sensor combines a traditional funnel bubble trap with an Arduino logger, a pressure sensor, thermal conductivity methane sensor, and a solenoid valve. Powered by three AA batteries, the sensor can measure autonomously for three months when programmed for a sampling frequency of 30 min. For field testing, four sensors were deployed for six weeks in a small lake. While ebullition was spatially and temporally variable, a distinct diurnal trend was observed with the highest rates from mid-morning to early afternoon. Ebullition rates were similar for all four sensors when integrated over the sampling period. The widespread deployment of low cost automated ebullition sensors such as the iAMES described here will help constrain one of the largest uncertainties in the global methane budget.

Diffusive Gradients in Thin Films Reveals Differences in Antimony and Arsenic Mobility in a Contaminated Wetland Sediment during an Oxic-Anoxic Transition.

Antimony (Sb) and arsenic (As) are priority environmental contaminants that often co-occur at mining-impacted sites. Despite their chemical similarities, Sb mobility in waterlogged sediments is poorly understood in comparison to As, particularly across the sediment-water interface (SWI) where changes can occur at the millimeter scale. Combined diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques provided a high resolution, in situ comparison between Sb, As, and iron (Fe) speciation and mobility across the SWI in contaminated freshwater wetland sediment mesocosms under an oxic-anoxic-oxic transition. The shift to anoxic conditions released Fe(II), As(III), and As(V) from the sediment to the water column, consistent with As release being coupled to the reductive dissolution of iron(III) (hydr)oxides. Conversely, Sb(III) and Sb(V) effluxed to the water column under oxic conditions and fluxed into the sediment under anoxic conditions. Porewater DGT-DET depth profiles showed apparent decoupling between Fe(II) and Sb release, as Sb was primarily mobilized across the SWI under oxic conditions. Solid-phase X-ray absorption spectroscopy (XAS) revealed the presence of an Sb(III)-S phase in the sediment that increased in proportion with depth and the transition from oxic to anoxic conditions. The results of this study showed that Sb mobilization was decoupled from the Fe cycle and was, therefore, more likely linked to sulfur and/or organic carbon (e.g., most likely authigenic antimony sulfide formation or Sb(III) complexation by reduced organic sulfur functional groups).

Phosphate-Imposed Constraints on Schwertmannite Stability under Reducing Conditions.

Schwertmannite is a ferric oxyhydroxysulfate mineral, which is common in acid sulfate systems. Such systems contain varying concentrations of phosphate (PO43-)-an essential nutrient whose availability may be coupled to schwertmannite formation and fate. This study examines the effect of phosphate on schwertmannite stability under reducing conditions. Phosphate was added at 0, 80, 400, and 800 µmoles g-1 (i.e., zero, low, medium, and high loading) to schwertmannite suspensions which were inoculated with wetland sediment and suspended in N2-purged artificial groundwater. pH remained between 2.7 and 4.3 over the 41 day experiment duration. Fe(II) accumulated in solution due to dissimilatory Fe(III)-reduction, which was most pronounced at intermediate PO43- loadings (i.e., in the low PO43- treatment). Partial transformation of schwertmannite to goethite occurred in the zero and low PO43- treatments, with negligible transformation in higher PO43- treatments. Overall, the results suggest that intermediate PO43- loadings provide conditions which facilitate optimal reductive dissolution of schwertmannite. At zero PO43- loading, reductive dissolution appears to be constrained by the rapid transformation of schwertmannite to goethite, which thereby decreases the bioavailability of solid-phase Fe(III). Conversely, at high loadings, PO43- appears to stabilize the schwertmannite surface against dissolution; probably via the formation of strong surface complexes.

Antimony and Arsenic Behavior during Fe(II)-Induced Transformation of Jarosite.

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.

Arsenic Mobilization Is Enhanced by Thermal Transformation of Schwertmannite.

Fires in iron-rich seasonal wetlands can thermally transform Fe(III) minerals and alter their crystallinity. However, the fate of As associated with thermally transformed Fe(III) minerals is unclear, as are the consequences for As mobilization during subsequent reflooding and reductive cycles. Here, we subject As(V)-coprecipitated schwertmannite to thermal transformation (200, 400, 600 and 800 °C) followed by biotic reductive incubation (150 d) and examine aqueous- and solid-phase speciation of As, Fe and S. Heating to >400 °C caused transformation of schwertmannite to a nanocrystalline hematite with greater surface area and smaller particle size. Higher temperatures also caused the initially structurally incorporated As to become progressively more exchangeable, increasing surface-complexed As (AsEx) by up to 60-fold, thereby triggering enhanced As mobilization during incubation (∼70-fold in the 800 °C treatment). Although more As was mobilized in biotic treatments than controls (∼3-20×), in both cases it was directly proportional to initial AsEx and mainly due to abiotic desorption. Higher transformation temperatures also drove divergent pathways of Fe and S biomineralization and led to more As(V) and SO4 reduction relative to Fe(III) reduction. This study reveals thermal transformation of schwertmannite can greatly increase As mobility and has major consequences for As/Fe/S speciation under reducing conditions. Further research is warranted to unravel the wider implications for water quality in natural wetlands.

Arsenic mobility during flooding of contaminated soil: the effect of microbial sulfate reduction.

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.

Floodplain morphology influences arsenic and antimony spatial distribution in a seasonal acid sulfate soil wetland.

Arsenic (As) and antimony (Sb) often co-occur in floodplain depositional environments that are contaminated by legacy mining activities. However, the distribution of As and Sb throughout floodplains is not uniform, adding complexity and expense to management or remediation processes. Identifying floodplain morphology predictor variables that help quantify and explain As and Sb spatial distribution on floodplains is useful for management and remediation. We developed As and Sb risk maps estimating concentration and availability at a coastal floodplain wetland impacted by upper-catchment mining. Significant predictors of As and Sb concentrations included i) distance from distributary channel-wetland intersection and ii) elevation. Distance from channel explained 53 % (P < 0.01) and 28 % (P < 0.01), while elevation explained 42 % (P < 0.01) and 47 % (P < 0.01) of the variability in near-total Sb and As respectively. As had a higher extractability than Sb across all tested soil extractions, suggesting that As is more environmentally available. As and Sb dry mass estimates to a depth of 0.1 m scaled to the lower coastal Macleay floodplain ranged from 113-192 tonnes and 14-24 tonnes respectively. Landscape-scale modelling of metalloid distribution, informed by morphology variables, presented here may be a useful framework for the development of risk maps in other regions impacted by contaminated upper-catchment sediments.

Antimony sorption to schwertmannite in acid sulfate environments.

Schwertmannite is a poorly-crystalline Fe(III) oxyhydroxysulfate mineral that may control Sb(V) mobility in acid sulfate environments, including acid mine drainage and acid sulfate soils. However, the mechanisms that govern uptake of aqueous Sb(V) by schwertmannite in such environments are poorly understood. To address this issue, we examined Sb(V) sorption to schwertmannite across a range of environmentally-relevant Sb(V) loadings at pH 3 in sulfate-rich solutions. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Sb(V) sorption (at all loadings) involved edge and double-corner sharing linkages between SbVO6 and FeIIIO6 octahedra. The coordination numbers for these linkages indicate that sorption occurred by Sb(V) incorporation into the schwertmannite structure via heterovalent Sb(V)-for-Fe(III) substitution. As such, Sb(V) sorption to schwertmannite was not limited by the abundance of surface complexation sites and was strongly resistant to desorption when exposed to 0.1 M PO43-. Sorption of Sb(V) also conferred increased stability to schwertmannite, based on changes in the schwertmannite dissolution rate during extraction with an acidic ammonium oxalate solution. This study provides new insights into Sb(V) sorption to schwertmannite in acid sulfate environments, and highlights the role that schwertmannite can play in immobilizing Sb(V) within its crystal structure.

Extreme iron cycling in a coastal lake-lagoon system driven by interactions between climate and entrance management.

Intermittently closed and open coastal lakes and lagoons (ICOLLs) are ecologically important and hydrologically sensitive estuarine systems. We explore how extreme drought and ICOLL entrance management intersect to influence the geochemical cycling of iron. Opening the ICOLL entrance just prior to an extreme drought in 2019 led to prolonged extremely low water levels, thereby exposing intertidal/subtidal sulfidic sediments and causing oxidation of sedimentary pyrite. Subsequent reflooding of exposed sediments for ∼4 months led to extremely elevated Fe2+(aq) (>10 mM) in intertidal hyporheic porewaters, consistent with Fe2+(aq) release via pyrite oxidation and via reductive dissolution of newly-formed Fe(III) phases. Re-opening the ICOLL entrance caused a rapid fall in water levels (∼1.5 m over 7 d), driving the development of effluent groundwater gradients in the intertidal zone, thereby transporting Fe2+-rich porewater into surface sediments and surface waters. This was accompanied by co-mobilisation of some trace metals and nutrients. On contact with oxic, circumneutral-pH estuarine water, the abundant Fe2+(aq) oxidised, forming a spatially extensive accumulation of poorly crystalline Fe(III) oxyhydroxide floc (up to 25 % Fe dry weight) in shallow intertidal zone benthic sediments throughout the ICOLL. Modelling estimates ∼4050 × 103 kg of poorly-crystalline Fe was translocated into surficial sediments. The newly formed Fe(III)-oxyhydroxides serve as a metastable sink encouraging enrichment of both phosphate and various trace metal(loid)s in near-surface sediments, which may have consequences for future cycling of nutrients, metals and ICOLL ecological function. The additional Fe also may enhance ICOLL sensitivity to similar future drought events by encouraging pyrite formation in shallow (<5 cm) benthic sediments. This system-wide translocation of Fe from deeper sediments into surficial benthic sediments represents a form of geochemical hysteresis with an uncertain recovery trajectory. This study demonstrates how climate extremes can interact with anthropogenic management to amplify ICOLL hydrological oscillations and influence biogeochemistry in complex ways.

Sulfate availability drives divergent evolution of arsenic speciation during microbially mediated reductive transformation of schwertmannite.

The effect of SO4(2-) availability on the microbially mediated reductive transformation of As(V)-coprecipitated schwertmannite (Fe8O8(OH)3.2(SO4)2.4(AsO4)0.004) was examined in long-term (up to 400 days) incubation experiments. Iron EXAFS spectroscopy showed siderite (FeCO3) and mackinawite (FeS) were the dominant secondary Fe(II) minerals produced via reductive schwertmannite transformation. In addition, ∼ 25% to ∼ 65% of the initial schwertmannite was also transformed relatively rapidly to goethite (αFeOOH), with the extent of this transformation being dependent on SO4(2-) concentrations. More specifically, the presence of high SO4(2-) concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO4(2-) also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO4(2-) reduction. In contrast, where SO4(2-) was less available, there was near-complete reduction of schwertmannite- and goethite-derived Fe(III) as well as solid-phase As(V). As a result, under low SO4(2-) conditions, almost no Fe(III) or As(V) remained toward the end of the experiment and arsenic solid-phase partitioning was controlled mainly by sorptive interactions between As(III) and mackinawite. These As(III)-mackinawite interactions led to the formation of an orpiment (As2S3)-like species. Interestingly, this orpiment-like arsenic species did not form under SO4(2-)-rich conditions, despite the prevalence of dissimilatory SO4(2-) reduction. The absence of an arsenic sulfide species under SO4(2-)-rich conditions appears to have been a consequence of schwertmannite persistence, combined with the preferential retention of arsenic oxyanions by schwertmannite. The results highlight the critical role that SO4(2-) availability can play in controlling solid-phase arsenic speciation, particularly arsenic-sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems.

Drought, megafires and flood - climate extreme impacts on catchment-scale river water quality on Australia's east coast.

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.

Remediation of Pb-contaminated soil using modified bauxite refinery residue.

This study examines amendment of Pb-contaminated soil with modified bauxite refinery residue (MBRR) to decrease soil Pb mobility and bioaccessibility. Amendment experiments were conducted using four soils contaminated with Pb from various sources, including smelting, shooting-range activities and Pb-based paint waste. Lead L3-edge X-ray absorption spectroscopy (XAS) indicated that Pb speciation in these soils was a mixture of Pb sorbed to Fe (hydr)oxide and clay minerals, along with Pb bound to organic matter. Amendment with MBRR decreased water-soluble Pb and/or Toxicity Characteristic Leachate Procedure (TCLP) Pb concentrations. Lead L3-edge XAS and X-ray diffraction (XRD) indicated that Pb retention by MBRR occurred via sorption to Fe- and Al-(hydr)oxides at low Pb loadings, in addition to formation of hydrocerussite (Pb3(CO3)2(OH)2) at high loadings. Soil amendment with MBRR had relatively little effect on gastric-phase Pb bioaccessibility; as quantified via the Solubility/Bioavailability Research Consortium, SBRC, in vitro assay. In contrast, amendment with MBRR caused substantial decreases in relative intestinal-phase Pb bioaccessibility (Rel-SBRC-I) due to increased Pb sorption by MBRR's Fe- and Al-hydr(oxide) minerals as simulated GI tract conditions shifted from the gastric- to the intestinal-phase. These decreases in Rel-SBRC-I point to the potential efficacy of using amendment with MBRR to decrease soil Pb bioavailability.

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.