Porous titania beads for remediation of arsenic contamination from acid mine drainage.

Hierarchically porous titania beads with and without amine functionalisation have been developed and tested as adsorbents for removal of highly toxic As(V) from environments affected by acid mine drainage (AMD). The unique acid stability of the titania framework enables these adsorbents to function in highly acidified environments and their granular form facilitates practical deployment under continuous flow conditions. Herein, both non-functionalised and amine-functionalised titania beads have been demonstrated to selectively remove As(V) from simulated and real AMD solutions at pH 2.6. Novel selectivity for As(V) over Na(I), Mg(II), Al(III), Si(VI), Ca(II), Co(II), Cu(II), Zn(II), Nd(III) and Ho(III) was achieved, with competing element concentrations similar to or up to an order of magnitude greater than that of As(V). Although Fe(III) and some Fe(II) were also adsorbed by the titania beads, Fe adsorption did not inhibit As(V) adsorption, indicating different adsorption mechanisms for these two elements. The As(V) adsorption capacity of the titania beads decreased from ∼20 mg/g from pure As(V) solution to ∼10 mg/g from real AMD solution, demonstrating the importance of adsorbent testing under applied conditions. Amine functionalisation increased the kinetics of adsorption, but the non-functionalised titania beads showed greater selectivity for As(V) over Fe(II) and Fe(III) and hence were considered preferable for As remediation applications. Nevertheless, the functionalisation ability of the porous titania beads makes them a promising, flexible technology for remediation of a wide range of AMD affected environments.

Geochemical signatures of acidic drainage recorded in estuarine sediments after an extreme drought.

Extreme prolonged drought over south-eastern Australia a decade ago (the Millennium Drought, 1996-2010) triggered extensive acid sulfate soil oxidation and associated acidification. Whilst the significant release of metal-enriched drainage has been documented during this event, the fate of these elements in receiving systems is still largely unknown. Here we investigate the spatial distribution, chemical partitioning, and potential bioavailability of S, Fe, Mn, Al, trace metals (V, Cr, Co, Ni, Cu, Pb) and rare earth elements in the surficial sediments of Lake Albert, South Australia; a system that received prolonged (2007-2011) acidic drainage during the Millennium Drought. The highest concentrations of all metals (Enrichment Factors ranging from 1.06-1.97) were observed in the sediments closest to shorelines where sulfidic material were oxidised, generating metal-enriched acidic drainage. Localised enrichment indicates metals have not been completely redistributed across the system following their initial deposition by physical (wind-driven resuspension) or chemical (diagenetic and redox changes) processes. Based on selective extraction data, these metals are likely partitioned as discrete sulfides (Pb) or are with organic material (V, Cr, Cu, Pb, REEs), Fe monosulfides (V, Co, Ni), or pyrite (Co, Ni, Pb). Lake-wide enrichment of trace metals Cr (mean concentration 57 ppm), Ni (39 ppm), Cu (36 ppm), and Pb (24 ppm) relative to other Lower Murray River sites is also evident, suggesting that metal contamination associated with acidic drainage is not restricted to near-source sites. Importantly, the mobilisation, transport, and accumulation of metals is controlled by sediment transport pathways and system hydrology and will thus function differently under varying states of geomorphology, climate, and anthropogenic modification. Our study shows that extreme drought is recorded as a lasting geochemical signature in estuarine sediments, demonstrating that metal and rare earth element geochemistry provides insights into the distribution and behaviour of contaminants mobilised into dynamic, anthropogenically altered estuaries.

From source to sink: Rare-earth elements trace the legacy of sulfuric dredge spoils on estuarine sediments.

Land disposal of dredged sulfide-rich coastal sediments generates secondary coastal acid sulfate soils (CASS), as previously reduced sulfide minerals oxidise to produce acidic drainage rich in Fe, SO42- and rare-earth elements (REEs). Few studies investigate both the source and the sink of REEs in the context of interpreting their mobilisation and potential use in tracing anthropogenic activity. Here we investigate REE signatures in estuarine sediments (and overlying surface waters) that have received acute, long-term (>15 years) acidic drainage from legacy sulfuric dredge spoils. It was found that the dredge spoil continues to act as a source of acidity (pH 3.5-5.5), Fe and REEs during development of CASS, and contains negligible acid volatile sulfide (AVS, a proxy for FeS) and relatively low concentrations of ΣREE (mean 44.5 mg/kg, range 4.1-362 mg/kg). In the receiving sediments, high AVS concentrations (mean 92.2 µmol/g, range 0.38-278 µmol/g) reflect elevated FeS content, likely due to high inputs of Fe and SO42- from the acidic drainage, and correspond with a high concentration of total S (mean 852 µmol/g, range 105-2209 µmol/g) and an accumulation of ΣREE (mean 670 mg/kg, range 19.9-1819 mg/kg). Importantly, where drain sediments that were previously enriched in highly reactive sulfidic minerals and trace elements and have become exposed to the atmosphere (e.g. Site 3) and partially oxidised, they provide a further source of acidification, remobilising the REEs to the downstream sediments. Interestingly, we also found a clear positive correlation between phosphorous and REEs both in the dredge spoil and sediment, suggesting phosphate minerals may act as a sink for REEs in CASS influenced drain sediments. This is further supported by strong positive gadolinium anomalies (1.1-1.6) and high calculated anthropogenic Gd values (12-38%), which may reflect the influence of phosphate fertiliser on this eutrophic system.

Acidic drainage drives anomalous rare earth element signatures in intertidal mangrove sediments.

Sedimentary rare earth element (REE) signatures can provide powerful insights into nearshore biogeochemical processes and anthropogenic influences. Despite this, there is limited research investigating REE behaviour in sediments influenced by coastal acid sulfate soils (CASS). Here, we explore REE abundance and fractionation in intertidal mangrove sediments that received CASS drainage for ~15-20y within the Hastings Catchment in NSW, Australia. Sediments close to the CASS discharge point (<200m) were compared with those further downstream (1300-1600m), and at a nearby control site. Average ΣREE concentrations were highest near the CASS discharge point (148-186mg/kg), and decreased with distance downstream (111-146mg/kg) and in control sediments (70mg/kg). Reactive Fe concentrations (defined by 1M HCl extractability) were also significantly higher in surface sediments (0-6cm) near the CASS discharge point. Middle-REE (MREE) enrichments dominated fractionation patterns at all sites (>1.5), with a high proportion (63-100%) of REEs residing in the reactive (1M HCl extractable) sediment fraction. Interestingly, the degree of MREE enrichment was significantly correlated with Ce anomalies (r2=0.72, P<0.001) and the heavy-REE (HREE) to light-REE (LREE) ratios (HREE/LREE, r2=0.74, P<0.001) in the reactive sediment fraction, only in those sites situated closest to the CASS drainage. The observed high MREE enrichments, positive Ce anomalies (>1) and HREE/LREE ratios (>1) are consistent with reactive Fe(III) oxides/oxyhydroxides driving REE retention in these sediments. This study indicates that CASS drainage alters REE signatures in receiving sediments by (1) providing a source of REEs, thereby enhancing sedimentary REE concentrations, and (2) causing accumulation of reactive Fe(III) phases with a high affinity for REEs. Together, these two factors drive the development of distinctive REE signatures in CASS-impacted sediments. The recognition of such signatures may provide a promising tool for identifying coastal sediments receiving anthropogenic CASS drainage inputs.

Trace element reactivity in FeS-rich estuarine sediments: influence of formation environment and acid sulfate soil drainage.

Iron monosulfides (FeS) precipitate during benthic mineralisation of organic C and are well known to have a strong influence on trace element bioavailability in sediments. In this study we investigate the reactivity of trace elements (As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Zn) in sediments containing abundant and persistent FeS stores, collected from a south-western Australian estuarine system. Our objective was to explore the influence of sediment formation conditions on trace element reactivity by investigating sediments collected from different environments, including estuarine, riverine and acid sulfate soil influenced sites, within a single estuarine system. In general, we found a higher degree of reactivity (defined by 1 mol/L HCl extractions) for Cd, Mn, Pb and Zn, compared with a lower reactivity of As, Co, Cr, Cu, Mo and Ni. Moderate to strong correlations (R(2)>0.4, P<0.05) were observed between AVS and reactive Cd, Co, Mn, Mo, Ni, Pb and Zn within many of the formation environments. In contrast, correlations between AVS and As, Cr and Cu were generally poor (not significant, R(2)<0.4, P>0.05). Based on their reactivity and correlations with AVS, it appears that interactions (sorption, co-precipitation) between FeS and Cd, Mn, Pb and Zn in many of the sediments from this study are probable. Our data also demonstrate that drainage from acid sulfate soils (ASS) can be a source of trace elements at specific sites. A principal components analysis of our reactive (1 mol/L HCl extractable) trace element data clearly distinguished sites receiving ASS drainage from the other non-impacted sites, by a high contribution from Fe-Co-Mn-Ni along the first principal axis, and contributions from higher S-As/lower reactive Pb along the second axis. This demonstrates that trace element reactivity in sediments may provide a geochemical signature for sites receiving ASS drainage.

Water chemistry and nutrient release during the resuspension of FeS-rich sediments in a eutrophic estuarine system.

The objective of this study was to investigate the impact of resuspending FeS-rich benthic sediment on estuarine water chemistry. To address this objective, we conducted (1) a series of laboratory-based sediment resuspension experiments and (2) also monitored changes in surface water composition during field-based sediment resuspension events that were caused by dredging activities in the Peel-Harvey Estuary, Western Australia. Our laboratory resuspension experiments showed that the resuspension of FeS-rich sediments rapidly deoxygenated estuarine water. In contrast, dredging activities in the field did not noticeably lower O(2) concentrations in adjacent surface water. Additionally, while FeS oxidation in the laboratory resuspensions caused measurable decreases in pH, the field pH was unaffected by the dredging event and dissolved trace metal concentrations remained very low throughout the monitoring period. Dissolved ammonium (NH(4)(+)) and inorganic phosphorus (PO(4)-P) were released into the water column during the resuspension of sediments in both the field and laboratory. Following its initial release, PO(4)-P was rapidly removed from solution in the laboratory-based (<1h) and field-based (<100 m from sediment disposal point) investigations. In comparison to PO(4)-P, NH(4)(+) release was observed to be more prolonged over the 2-week period of the laboratory resuspension experiments. However, our field-based observations revealed that elevated NH(4)(+) concentrations were localised to <100 m from the sediment disposal point. This study demonstrates that alongside the emphasis on acidification, deoxygenation and metal release during FeS resuspension, it is important to consider the possibility of nutrient release from disturbed sediments in eutrophic estuaries.