Ian Burke | University of Leeds (original) (raw)

Papers by Ian Burke

The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million ... more The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million m3 of
red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically
alkaline pH and contains several potentially toxic elements, including arsenic. Aerobic and anaerobic
batch experiments were prepared using soils from near Ajka in order to investigate the effects of red
mud addition on soil biogeochemistry and arsenic mobility in soil–water experiments representative of
land affected by the red mud spill. XAS analysis showed that As was present in the red mud as As(V) in
the form of arsenate. The remobilisation of red mud associated arsenate was highly pH dependent and
the addition of phosphate to red mud suspensions greatly enhanced As release to solution. In aerobic
batch experiments, where red mud was mixed with soils, As release to solution was highly dependent
on pH. Carbonation of these alkaline solutions by dissolution of atmospheric CO2 reduced pH, which
resulted in a decrease of aqueous As concentrations over time. However, this did not result in complete
removal of aqueous As in any of the experiments. Carbonation did not occur in anaerobic experiments
and pH remained high. Aqueous As concentrations initially increased in all the anaerobic red mud
amended experiments, and then remained relatively constant as the systems became more reducing,
both XANES and HPLC–ICP-MS showed that no As reduction processes occurred and that only As(V)
species were present. These experiments show that there is the potential for increased As mobility in
soil–water systems affected by red mud addition under both aerobic and anaerobic conditions.

The failure of the Ajka red mud depository in October 2010 led to the largest single release of r... more The failure of the Ajka red mud depository in October 2010 led to the largest single release of red mud into
the surface water environment. This study provides a comparative assessment of stream sediment quality in
the Torna–Marcal–R´aba catchment between post-disaster surveys (2010) and follow up surveys at an
identical suite of 21 locations in 2013. The signature of red mud apparent in initial surveys with high Al,
As, Cr, Na, V was only apparent at a small number of sample stations in recent surveys. These constitute
<1 km of stream, compared to the >20 km reach of affected sediments in the immediate aftermath of
the spill. Concentrations of red mud-derived contaminants are predominately associated with fine
fractions of the red mud (<8 mm). This enhances transport out of the system of red mud-derived
contaminants and, along with extensive remedial efforts, has substantially limited the within-channel
inventory of potentially ecotoxic metals and metalloids.

Science of The Total Environment

Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are... more Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are produced annually. We analyzed RM which was released in the course of the Kolonatar disaster in Hungary into the environment in acute and genotoxicity experiments with plants which are widely used for environmental monitoring. We detected induction of micronuclei which reflect
chromosomal damage in tetrads of Tradescantia and in root cells of Allium as well as retardation of root growth with contaminated soils and leachates. Chemical analyses showed that RM contains metals, in particular high concentrations of vandadium. Follow up experiments indicated that vanadate causes the effects in the plants. This compound causes also in humans DNA damage and positive results were obtained in carcinogenicity studies. Since it was found also in RM from other production sites our findings indicate that its release in the environment is a global problem which should be studied in more detail.

Applied Geochemistry

137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 13... more 137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the sorption behaviour of Cs across a wide range of environmental conditions. This work investigates the effect of varying solution composition (pH and competing ions) on the sorption of Cs to micaceous aquifer sediment across a large concentration range (1.0x10-11 – 1.0x10-1 mol L-1 Cs). Experimental results show that Cs exhibits three distinct sorption behaviours at three different concentration ranges. At very low concentrations <1.0x10-6 mol L-1 Cs sorption was unaffected by competition with Na+ or H+ but significantly reduced in high ionic strength K+ solution. Secondly between 1x10-6 and 1.0x10-3 mol L-1 Cs is strongly sorbed in a neutral pH, low ionic strength background but sorption is significantly reduced in solutions with either a high concentration of Na+ or K+ ions or low pH. At high concentrations > 1.0x10-3 mol L-1 Cs sorption is reduced in all systems due to saturation of the sediment’s sorption capacity. A multi-site cation exchange model was used to interpret the sorption behaviour. From this it was determined that at low concentrations Cs sorbs to the illite frayed edge sites only in competition with K+ ions. However once the frayed edge sites are saturated the Cs sorbs to the Type II and Planar sites in competition with K+, Na+ and H+ ions. Therefore sorption of Cs at concentrations > 1.0 x 10-6 mol L-1 is significantly reduced in both high ionic strength and low pH solutions. This is a significant result with regard to predicting the migration of 137Cs in acidic or high ionic strength groundwaters.

Applied and Environmental Microbiology

The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce met... more The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium and Alkaliphilus sp. are capable of using iron (Fe3+) as a final electron acceptor under anaerobic conditions. Iron reduction is associated with the production of a freely diffusible species that upon rudimentary purification and subsequent spectroscopic, HPLC and electrochemical analysis has been identified as a flavin species displaying properties indistinguishable from riboflavin. Due to the link between iron reduction and the onset of flavin production, it is likely that riboflavin has an import role in extracellular metal reduction by this alkaliphilic community.

Environmental Technology

Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble... more Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble, highly toxic Cr(VI) to insoluble Cr(III). This study compared the reduction rates of Cr(VI) by ZVI and two carbonate green rust phases in alkaline/hyperalkaline solutions. Batch experiments were carried out with synthetic chromate solutions at pH 7.7–12.3 and a chromite ore processing residue (COPR) leachate (pH ≈ 12.2). Green rust removes chromate from high pH solutions (pH 10–12.5) very rapidly (<400 s). Chromate reduction rates for both green rust phases were consistently higher than for ZVI throughout the pH range studied; the surface area normalized rate constants were two orders of magnitude higher in the COPR leachate solution at pH 12.2. The performances of both green rusts were unaffected by changes in pH. In contrast, ZVI exhibited a marked decline in reduction rate with increasing pH to become almost ineffective above pH12.

Environmental Science and Technology

Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or... more Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or seawater addition. During acid neutralisation >99 % Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24 %) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86 % Al and 81 % As removal from solution, and both are non-exchangeable with 0.1 mol L-1 phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralisation also resulted in carbonate precipitation, with >99 % Al and 74 % As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilised by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16 % V was removed from solution during neutralisation, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.

Environmental Science and Technology

Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young ce... more Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration and secondary mineral precipitation on 90Sr sorption. After reaction with YCW, Sr sorption was found to be >75 % in all samples up to 365 days, and 98 % in a sample reacted for 365 days at 70 °C. SEM analysis of sediment samples reacted at room temperature showed surface alteration and precipitation of a secondary phase, likely a K-rich aluminosilicate gel. The presence of Sr-Si(Al) bond distances in Sr K-edge EXAFS analysis suggested that the Sr was present as an inner-sphere adsorption complex. Sequential extractions, however, found the majority of this Sr was still exchangeable with Mg2+ at pH 7. For the sample reacted for one year at 70 °C, EXAFS analysis revealed clear evidence for ~6 Sr-Si(Al) backscatters at 3.45 Å, consistent with Sr incorporation into the neoformed K-chabazite phase that was detected by XRD and electron microscopy. Once incorporated into chabazite, 90Sr was not exchangeable with Mg2+ and chemical leaching with pH 1.5 HNO3 was required to remobilise 60 % of the 90Sr. These results indicate that in high pH cementitious leachate there is significantly enhanced Sr retention in sediments due to changes in the adsorption mechanism and incorporation into secondary silicate minerals. This suggests that Sr retention may be enhanced in this high pH zone and that the incorporation process may lead to irreversible exchange of the contaminant over extended time periods.

Industrial & Engineering Chemistry Research

"It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can b... more "It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilise Cr leaching from hyper alkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction. Buffering pH downwards produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analysis confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates
also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that although Cr (VI) reduction by ZVI will occur at hyper alkaline pH, other solutes present in COPR leachate will limit the design life of a PRB. "

Environmental Geochemistry and Health

Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several pot... more Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m3 of bauxite residue (red mud) suspension from the Ajka repository, western Hungary, caused large scale contamination of downstream rivers and floodplains. There is now concern about the potential leaching of toxic metal(loid)s from the red mud as some have enhanced solubility at high pH. This study investigated the impact of red mud addition to three different Hungarian soils with respect to trace element solubility and soil geochemistry. The effectiveness of gypsum amendment for the rehabilitation of red mud-contaminated soils was also examined. Red mud addition to soils caused a pH increase, proportional to red mud addition, of up to 4 pH units (e.g. pH 7  11). Increasing red mud addition also led to significant increases in salinity, dissolved organic carbon (DOC) and aqueous trace element concentrations. However, the response was highly soil specific and one of the soils tested buffered pH to around pH 8.5 even with the highest red mud loading tested (33% w/w); experiments using this soil also had much lower aqueous Al, As, and V concentrations. Gypsum addition to soil / red mud mixtures, even at relatively low concentrations (1% w/w) was sufficient to buffer experimental pH to 7.5-8.5. This effect was attributed to the reaction of Ca2+ supplied by the gypsum with OH- and carbonate from the red mud to precipitate calcite. The lowered pH enhanced trace element sorption and largely inhibited the release of Al, As and V. Mo concentrations, however, were largely unaffected by gypsum induced pH buffering due to the greater solubility of Mo (as molybdate) at circumneutral pH. Gypsum addition also leads to significantly higher porewater salinities and column experiments demonstrated that this increase in total dissolved solids persisted even after 25 pore volume replacements. Gypsum addition could therefore provide a cheaper alternative to recovery (dig and dump) for treatment of red mud affected soils. The observed inhibition of trace metal release within red mud affected soils was relatively insensitive to either the percentage of red mud or gypsum present, making the treatment easy to apply. However, there is risk that over-application of gypsum could lead to detrimental long term increases in soil salinity.

Applied Geochemistry

Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a di... more Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a diverse population of anaerobic alkaliphiles, despite receiving a continuous influx of a Cr(VI) contaminated, hyperalkaline leachate (pH 12.2). Chromium was found to have accumulated in this soil as a result of an abiotic reaction of Cr(VI) with Fe(II) present in the soil. This sediment associated Fe(II) was therefore acting as a natural reactive zone beneath the COPR and thereby preventing the spread of Cr(VI). In anaerobic microcosm experiments soil microorganisms were able to reduce nitrate at pH 11.2 coupled to the oxidation of electron donors derived from the original soil organic matter, but progressive anoxia did not develop to the point of iron reduction over a period of 9 months. It is not clear, therefore, if Fe(II) can be actively replenished by microbial processes occurring within the soil at in situ conditions. Sodium bicarbonate was added to this soil to investigate whether bioreduction of iron in hyperalkaline chromium contaminated soils could be enhanced by reducing the pH to a value optimal for many alkaliphilic bacteria. The addition of sodium bicarbonate produced a well buffered system with a pH of ~9.3 and iron reducing conditions developed within 1 month once complete denitrification had occurred. Iron(III) reduction was associated with an increase in the proportion of genetic clone libraries that were from the phylum Firmicutes, suggesting that these species are responsible for the Fe(III) reduction observed. Amendment of the pH using bicarbonate may provide a suitable strategy for stimulating the bioreduction of Fe(III) in COPR leachate contaminated soils or other environments where microbial reduction is inhibited by elevated pH.

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem co... more Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of 90Sr in sediment-water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (< 0.01 mol L-1), pH variation is the main control on sorption. The sorption edge for 90Sr was observed between pH 4 – 6 with maximum sorption occurring (Kd ~ 103 L kg-1) at pH 6 - 8. At ionic strengths above 10 mmol L-1, and at pH values between 6 – 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower 90Sr sorption (Kd ~ 40 L kg-1) in the presence of artificial Magnox tank liquor (IS = 29 mmol L-1). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr2+ in either HCO3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ~ 9 O atoms at 2.58 - 2.61 Å after 10 days. This is equivalent to the Sr2+ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ~ 80 % of solid associated 90Sr was exchangeable with 1 M MgCl2 in all experiments. These results suggest that over long periods, 90Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of 90Sr is to be expected.

Environmental science & technology, Jan 1, 2012

Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciatio... more Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciation within samples of bauxite ore processing residue (red mud) collected from the spill site at Ajka, Western Hungary. Cr K-edge XANES analysis found that Cr is present as Cr3+ substituted into hematite, consistent with TEM analysis. V K-edge XANES spectra have E½ position and pre-edge features consistent with the presence of V5+ species, possibly associated with Ca-aluminosilicate phases. As K-edge XANES spectra identified As present as As5+. EXAFS analysis reveals arsenate phases in red mud samples. When alkaline leachate from the spill site is neutralised with HCl, 94 % As and 71 % V is removed from solution during the formation of amorphous Al-oxyhydroxide. EXAFS analysis of As in this precipitate reveals the presence of arsenate Al-oxyhydroxide surface complexes. These results suggest that in the circumneutral pH, oxic conditions found in the Torna and Upper Marcal catchments, incorporation and sorption respectively will restrict the environmental mobility of Cr and As. V is inefficiently removed from solution by neutralisation, therefore, the red mud may act as a source of mobile V5+ where the red mud deposits are not removed from affected land.

Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-r... more Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual biogeochemical processes. This paper reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic–rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated a single, unidentified specie within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring.

The Science of the total environment, Jan 1, 2012

A number of emergency pollution management measures were enacted after the accidental release of ... more A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assesses the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis, sequential extraction and stable isotope analysis. There is a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing are used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process is particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO2. The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO2 emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Environmental science & technology, Jan 1, 2011

This paper identifies the spatial extent of red mud-derived contaminants and modes of transport w... more This paper identifies the spatial extent of red mud-derived contaminants and modes of transport within the Marcal and Rába river systems after the dyke failure at Ajka, western Hungary. The geochemical signature of the red mud is apparent throughout the 3076km2 Marcal system principally with elevated Al, V, As and Mo. Elevated concentrations of Cr, Ga and Ni are also observed within 2km of source areas in aqueous and particulate phases where hyperalkalinity (pH <13.1) is apparent. While the concentrations of some trace elements exceed aquatic life standards in waters (e.g. V, As) and fluvial sediments (As, Cr, Ni, V), the spatial extent of these is limited to the Torna Creek and part of the upper Marcal. Source samples show a bi-modal particle size distribution (peaks at 0.7 and 1.3µm) which lends the material to ready fluvial transport. Where elevated concentrations are found in fluvial sediments, sequential extraction suggests the bulk of the As, Cr, Ni and V are associated with residual (aqua-regia/HF digest) phases and unlikely to be mobile in the environment. However, at some depositional hotspots , association of As, Cr and V with weak acid-extractable phases is observed.

Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Ti... more Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Timfoldgyar Zrt alumina plant on the 4th October 2010. This study highlights the dispersal of key red mudderived contaminants in downstream fluvial sediments from surveys undertaken within two months of the spill. Source samples contain abundant V (> 1000mg/kg), Cr (> 800mg/kg), Ni (> 250mg/kg) and As (> 80mg/kg).

Environmental Science & Technology, Jan 1, 2010

Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimul... more Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimulation of metal-reducing bacteria on the mobility of the radioactive contaminant technetium in sediments. Additions of electron donor (acetate) were used to construct a redox gradient, from oxic to Fe(III)-reducing conditions with increasing depth, in sediment columns. When 99mTc was pumped through the columns, real-time -camera imaging in combination with geochemical analyses showed that the 99mTc was irreversibly bound to regions of elevated Fe(II) formed via microbial reduction of Fe(III). Electron microscopy with EDX mapping confirmed that the Tc was associated with iron phases, while XAS confirmed reduction of Tc(VII) to insoluble Tc(IV). Molecular analyses of the microbial communities in the columns further emphasized a direct link between the accumulation of biogenic Fe(II) and Tc(VII) reduction and precipitation, with Fe(III)-reducing bacteria more abundant in zones of technetium immobilization. This study offers a novel non-invasive approach to monitor radionuclide mobility at trace concentrations in real-time biogeochemical experiments and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilising technetium.

Environmental science & technology, Jan 1, 2010

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and u... more Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O2+ under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

Environmental Science & Technology, Jan 1, 2010

Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying ... more Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence ofpHand nitrate on bioreduction processes. The systems studied include unamended-, carbonate buffered-, low nitrate-, and high nitrate-groundwaters. During anaerobic incubation, terminal electron accepting processes (TEAPs) in the circumneutral pH, carbonate buffered system progressed to sulfate reduction, and Tc(VII) was removed from solution during Fe(III) reduction. In the high-nitrate system, pH increased during denitrification (pH 5.5 to 7.2), then TEAPs progressed to sulfate reduction. Again, Tc(VII) removal was associated with Fe(III) reduction. In both systems, XAS confirmed reduction to hydrous Tc(IV)O2 like phases on Tc removal from solution. In the unamended and low-nitrate systems, the pH remained low, Fe(III) reduction was inhibited, and Tc(VII) remained in solution. Thus, nitrate can have complex influences on the development of the metal reducing conditions required for radionuclide treatment. High nitrate concentrations stimulated denitrification and caused pH neutralization facilitating Fe(III) reduction and Tc(VII) removal; acidic, low nitrate systemsshowed no Fe(III)-reduction. These results have implications for Tc cycling in contaminated environments where nitrate has been considered undesirable, but where it may enhance Fe(III)-reduction via a novel pH “conditioning” step.

The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million ... more The tailings dam breach at the Ajka alumina plant, western Hungary in 2010 introduced 1 million m3 of
red mud suspension into the surrounding area. Red mud (fine fraction bauxite residue) has a characteristically
alkaline pH and contains several potentially toxic elements, including arsenic. Aerobic and anaerobic
batch experiments were prepared using soils from near Ajka in order to investigate the effects of red
mud addition on soil biogeochemistry and arsenic mobility in soil–water experiments representative of
land affected by the red mud spill. XAS analysis showed that As was present in the red mud as As(V) in
the form of arsenate. The remobilisation of red mud associated arsenate was highly pH dependent and
the addition of phosphate to red mud suspensions greatly enhanced As release to solution. In aerobic
batch experiments, where red mud was mixed with soils, As release to solution was highly dependent
on pH. Carbonation of these alkaline solutions by dissolution of atmospheric CO2 reduced pH, which
resulted in a decrease of aqueous As concentrations over time. However, this did not result in complete
removal of aqueous As in any of the experiments. Carbonation did not occur in anaerobic experiments
and pH remained high. Aqueous As concentrations initially increased in all the anaerobic red mud
amended experiments, and then remained relatively constant as the systems became more reducing,
both XANES and HPLC–ICP-MS showed that no As reduction processes occurred and that only As(V)
species were present. These experiments show that there is the potential for increased As mobility in
soil–water systems affected by red mud addition under both aerobic and anaerobic conditions.

The failure of the Ajka red mud depository in October 2010 led to the largest single release of r... more The failure of the Ajka red mud depository in October 2010 led to the largest single release of red mud into
the surface water environment. This study provides a comparative assessment of stream sediment quality in
the Torna–Marcal–R´aba catchment between post-disaster surveys (2010) and follow up surveys at an
identical suite of 21 locations in 2013. The signature of red mud apparent in initial surveys with high Al,
As, Cr, Na, V was only apparent at a small number of sample stations in recent surveys. These constitute
<1 km of stream, compared to the >20 km reach of affected sediments in the immediate aftermath of
the spill. Concentrations of red mud-derived contaminants are predominately associated with fine
fractions of the red mud (<8 mm). This enhances transport out of the system of red mud-derived
contaminants and, along with extensive remedial efforts, has substantially limited the within-channel
inventory of potentially ecotoxic metals and metalloids.

Science of The Total Environment

Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are... more Red mud (RM) is a byproduct of aluminum production, worldwide between 70 and 120 million tons are produced annually. We analyzed RM which was released in the course of the Kolonatar disaster in Hungary into the environment in acute and genotoxicity experiments with plants which are widely used for environmental monitoring. We detected induction of micronuclei which reflect
chromosomal damage in tetrads of Tradescantia and in root cells of Allium as well as retardation of root growth with contaminated soils and leachates. Chemical analyses showed that RM contains metals, in particular high concentrations of vandadium. Follow up experiments indicated that vanadate causes the effects in the plants. This compound causes also in humans DNA damage and positive results were obtained in carcinogenicity studies. Since it was found also in RM from other production sites our findings indicate that its release in the environment is a global problem which should be studied in more detail.

Applied Geochemistry

137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 13... more 137Cs (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the sorption behaviour of Cs across a wide range of environmental conditions. This work investigates the effect of varying solution composition (pH and competing ions) on the sorption of Cs to micaceous aquifer sediment across a large concentration range (1.0x10-11 – 1.0x10-1 mol L-1 Cs). Experimental results show that Cs exhibits three distinct sorption behaviours at three different concentration ranges. At very low concentrations <1.0x10-6 mol L-1 Cs sorption was unaffected by competition with Na+ or H+ but significantly reduced in high ionic strength K+ solution. Secondly between 1x10-6 and 1.0x10-3 mol L-1 Cs is strongly sorbed in a neutral pH, low ionic strength background but sorption is significantly reduced in solutions with either a high concentration of Na+ or K+ ions or low pH. At high concentrations > 1.0x10-3 mol L-1 Cs sorption is reduced in all systems due to saturation of the sediment’s sorption capacity. A multi-site cation exchange model was used to interpret the sorption behaviour. From this it was determined that at low concentrations Cs sorbs to the illite frayed edge sites only in competition with K+ ions. However once the frayed edge sites are saturated the Cs sorbs to the Type II and Planar sites in competition with K+, Na+ and H+ ions. Therefore sorption of Cs at concentrations > 1.0 x 10-6 mol L-1 is significantly reduced in both high ionic strength and low pH solutions. This is a significant result with regard to predicting the migration of 137Cs in acidic or high ionic strength groundwaters.

Applied and Environmental Microbiology

The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce met... more The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium and Alkaliphilus sp. are capable of using iron (Fe3+) as a final electron acceptor under anaerobic conditions. Iron reduction is associated with the production of a freely diffusible species that upon rudimentary purification and subsequent spectroscopic, HPLC and electrochemical analysis has been identified as a flavin species displaying properties indistinguishable from riboflavin. Due to the link between iron reduction and the onset of flavin production, it is likely that riboflavin has an import role in extracellular metal reduction by this alkaliphilic community.

Environmental Technology

Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble... more Zero-valent iron (ZVI) and green rusts can be used as reductants to convert chromium from soluble, highly toxic Cr(VI) to insoluble Cr(III). This study compared the reduction rates of Cr(VI) by ZVI and two carbonate green rust phases in alkaline/hyperalkaline solutions. Batch experiments were carried out with synthetic chromate solutions at pH 7.7–12.3 and a chromite ore processing residue (COPR) leachate (pH ≈ 12.2). Green rust removes chromate from high pH solutions (pH 10–12.5) very rapidly (<400 s). Chromate reduction rates for both green rust phases were consistently higher than for ZVI throughout the pH range studied; the surface area normalized rate constants were two orders of magnitude higher in the COPR leachate solution at pH 12.2. The performances of both green rusts were unaffected by changes in pH. In contrast, ZVI exhibited a marked decline in reduction rate with increasing pH to become almost ineffective above pH12.

Environmental Science and Technology

Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or... more Red mud leachate (pH 13) collected from Ajka, Hungary is neutralised to < pH 10 by HCl, gypsum or seawater addition. During acid neutralisation >99 % Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24 %) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86 % Al and 81 % As removal from solution, and both are non-exchangeable with 0.1 mol L-1 phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralisation also resulted in carbonate precipitation, with >99 % Al and 74 % As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilised by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16 % V was removed from solution during neutralisation, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.

Environmental Science and Technology

Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young ce... more Results are presented from one year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration and secondary mineral precipitation on 90Sr sorption. After reaction with YCW, Sr sorption was found to be >75 % in all samples up to 365 days, and 98 % in a sample reacted for 365 days at 70 °C. SEM analysis of sediment samples reacted at room temperature showed surface alteration and precipitation of a secondary phase, likely a K-rich aluminosilicate gel. The presence of Sr-Si(Al) bond distances in Sr K-edge EXAFS analysis suggested that the Sr was present as an inner-sphere adsorption complex. Sequential extractions, however, found the majority of this Sr was still exchangeable with Mg2+ at pH 7. For the sample reacted for one year at 70 °C, EXAFS analysis revealed clear evidence for ~6 Sr-Si(Al) backscatters at 3.45 Å, consistent with Sr incorporation into the neoformed K-chabazite phase that was detected by XRD and electron microscopy. Once incorporated into chabazite, 90Sr was not exchangeable with Mg2+ and chemical leaching with pH 1.5 HNO3 was required to remobilise 60 % of the 90Sr. These results indicate that in high pH cementitious leachate there is significantly enhanced Sr retention in sediments due to changes in the adsorption mechanism and incorporation into secondary silicate minerals. This suggests that Sr retention may be enhanced in this high pH zone and that the incorporation process may lead to irreversible exchange of the contaminant over extended time periods.

Industrial & Engineering Chemistry Research

"It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can b... more "It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilise Cr leaching from hyper alkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction. Buffering pH downwards produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analysis confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates
also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that although Cr (VI) reduction by ZVI will occur at hyper alkaline pH, other solutes present in COPR leachate will limit the design life of a PRB. "

Environmental Geochemistry and Health

Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several pot... more Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m3 of bauxite residue (red mud) suspension from the Ajka repository, western Hungary, caused large scale contamination of downstream rivers and floodplains. There is now concern about the potential leaching of toxic metal(loid)s from the red mud as some have enhanced solubility at high pH. This study investigated the impact of red mud addition to three different Hungarian soils with respect to trace element solubility and soil geochemistry. The effectiveness of gypsum amendment for the rehabilitation of red mud-contaminated soils was also examined. Red mud addition to soils caused a pH increase, proportional to red mud addition, of up to 4 pH units (e.g. pH 7  11). Increasing red mud addition also led to significant increases in salinity, dissolved organic carbon (DOC) and aqueous trace element concentrations. However, the response was highly soil specific and one of the soils tested buffered pH to around pH 8.5 even with the highest red mud loading tested (33% w/w); experiments using this soil also had much lower aqueous Al, As, and V concentrations. Gypsum addition to soil / red mud mixtures, even at relatively low concentrations (1% w/w) was sufficient to buffer experimental pH to 7.5-8.5. This effect was attributed to the reaction of Ca2+ supplied by the gypsum with OH- and carbonate from the red mud to precipitate calcite. The lowered pH enhanced trace element sorption and largely inhibited the release of Al, As and V. Mo concentrations, however, were largely unaffected by gypsum induced pH buffering due to the greater solubility of Mo (as molybdate) at circumneutral pH. Gypsum addition also leads to significantly higher porewater salinities and column experiments demonstrated that this increase in total dissolved solids persisted even after 25 pore volume replacements. Gypsum addition could therefore provide a cheaper alternative to recovery (dig and dump) for treatment of red mud affected soils. The observed inhibition of trace metal release within red mud affected soils was relatively insensitive to either the percentage of red mud or gypsum present, making the treatment easy to apply. However, there is risk that over-application of gypsum could lead to detrimental long term increases in soil salinity.

Applied Geochemistry

Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a di... more Soil collected from beneath a chromite ore processing residue (COPR) disposal site contained a diverse population of anaerobic alkaliphiles, despite receiving a continuous influx of a Cr(VI) contaminated, hyperalkaline leachate (pH 12.2). Chromium was found to have accumulated in this soil as a result of an abiotic reaction of Cr(VI) with Fe(II) present in the soil. This sediment associated Fe(II) was therefore acting as a natural reactive zone beneath the COPR and thereby preventing the spread of Cr(VI). In anaerobic microcosm experiments soil microorganisms were able to reduce nitrate at pH 11.2 coupled to the oxidation of electron donors derived from the original soil organic matter, but progressive anoxia did not develop to the point of iron reduction over a period of 9 months. It is not clear, therefore, if Fe(II) can be actively replenished by microbial processes occurring within the soil at in situ conditions. Sodium bicarbonate was added to this soil to investigate whether bioreduction of iron in hyperalkaline chromium contaminated soils could be enhanced by reducing the pH to a value optimal for many alkaliphilic bacteria. The addition of sodium bicarbonate produced a well buffered system with a pH of ~9.3 and iron reducing conditions developed within 1 month once complete denitrification had occurred. Iron(III) reduction was associated with an increase in the proportion of genetic clone libraries that were from the phylum Firmicutes, suggesting that these species are responsible for the Fe(III) reduction observed. Amendment of the pH using bicarbonate may provide a suitable strategy for stimulating the bioreduction of Fe(III) in COPR leachate contaminated soils or other environments where microbial reduction is inhibited by elevated pH.

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem co... more Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of 90Sr in sediment-water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (< 0.01 mol L-1), pH variation is the main control on sorption. The sorption edge for 90Sr was observed between pH 4 – 6 with maximum sorption occurring (Kd ~ 103 L kg-1) at pH 6 - 8. At ionic strengths above 10 mmol L-1, and at pH values between 6 – 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower 90Sr sorption (Kd ~ 40 L kg-1) in the presence of artificial Magnox tank liquor (IS = 29 mmol L-1). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr2+ in either HCO3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ~ 9 O atoms at 2.58 - 2.61 Å after 10 days. This is equivalent to the Sr2+ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ~ 80 % of solid associated 90Sr was exchangeable with 1 M MgCl2 in all experiments. These results suggest that over long periods, 90Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of 90Sr is to be expected.

Environmental science & technology, Jan 1, 2012

Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciatio... more Results are presented from X-ray absorption spectroscopy based analysis of As, Cr and V speciation within samples of bauxite ore processing residue (red mud) collected from the spill site at Ajka, Western Hungary. Cr K-edge XANES analysis found that Cr is present as Cr3+ substituted into hematite, consistent with TEM analysis. V K-edge XANES spectra have E½ position and pre-edge features consistent with the presence of V5+ species, possibly associated with Ca-aluminosilicate phases. As K-edge XANES spectra identified As present as As5+. EXAFS analysis reveals arsenate phases in red mud samples. When alkaline leachate from the spill site is neutralised with HCl, 94 % As and 71 % V is removed from solution during the formation of amorphous Al-oxyhydroxide. EXAFS analysis of As in this precipitate reveals the presence of arsenate Al-oxyhydroxide surface complexes. These results suggest that in the circumneutral pH, oxic conditions found in the Torna and Upper Marcal catchments, incorporation and sorption respectively will restrict the environmental mobility of Cr and As. V is inefficiently removed from solution by neutralisation, therefore, the red mud may act as a source of mobile V5+ where the red mud deposits are not removed from affected land.

Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-r... more Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual biogeochemical processes. This paper reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic–rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated a single, unidentified specie within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring.

The Science of the total environment, Jan 1, 2012

A number of emergency pollution management measures were enacted after the accidental release of ... more A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assesses the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis, sequential extraction and stable isotope analysis. There is a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing are used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process is particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO2. The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO2 emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Environmental science & technology, Jan 1, 2011

This paper identifies the spatial extent of red mud-derived contaminants and modes of transport w... more This paper identifies the spatial extent of red mud-derived contaminants and modes of transport within the Marcal and Rába river systems after the dyke failure at Ajka, western Hungary. The geochemical signature of the red mud is apparent throughout the 3076km2 Marcal system principally with elevated Al, V, As and Mo. Elevated concentrations of Cr, Ga and Ni are also observed within 2km of source areas in aqueous and particulate phases where hyperalkalinity (pH <13.1) is apparent. While the concentrations of some trace elements exceed aquatic life standards in waters (e.g. V, As) and fluvial sediments (As, Cr, Ni, V), the spatial extent of these is limited to the Torna Creek and part of the upper Marcal. Source samples show a bi-modal particle size distribution (peaks at 0.7 and 1.3µm) which lends the material to ready fluvial transport. Where elevated concentrations are found in fluvial sediments, sequential extraction suggests the bulk of the As, Cr, Ni and V are associated with residual (aqua-regia/HF digest) phases and unlikely to be mobile in the environment. However, at some depositional hotspots , association of As, Cr and V with weak acid-extractable phases is observed.

Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Ti... more Between 600,000—700,000 m³ of caustic (pH> 13) red mud suspension were released from the Ajkai Timfoldgyar Zrt alumina plant on the 4th October 2010. This study highlights the dispersal of key red mudderived contaminants in downstream fluvial sediments from surveys undertaken within two months of the spill. Source samples contain abundant V (> 1000mg/kg), Cr (> 800mg/kg), Ni (> 250mg/kg) and As (> 80mg/kg).

Environmental Science & Technology, Jan 1, 2010

Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimul... more Dynamic -camera imaging of the radiotracer 99mTc(VII) was used to assess the impact of biostimulation of metal-reducing bacteria on the mobility of the radioactive contaminant technetium in sediments. Additions of electron donor (acetate) were used to construct a redox gradient, from oxic to Fe(III)-reducing conditions with increasing depth, in sediment columns. When 99mTc was pumped through the columns, real-time -camera imaging in combination with geochemical analyses showed that the 99mTc was irreversibly bound to regions of elevated Fe(II) formed via microbial reduction of Fe(III). Electron microscopy with EDX mapping confirmed that the Tc was associated with iron phases, while XAS confirmed reduction of Tc(VII) to insoluble Tc(IV). Molecular analyses of the microbial communities in the columns further emphasized a direct link between the accumulation of biogenic Fe(II) and Tc(VII) reduction and precipitation, with Fe(III)-reducing bacteria more abundant in zones of technetium immobilization. This study offers a novel non-invasive approach to monitor radionuclide mobility at trace concentrations in real-time biogeochemical experiments and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilising technetium.

Environmental science & technology, Jan 1, 2010

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and u... more Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O2+ under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

Environmental Science & Technology, Jan 1, 2010

Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying ... more Here we examine the bioreduction of technetium-99 in sediment microcosm experiments with varying nitrate and carbonate concentrations added to synthetic groundwater to assess the influence ofpHand nitrate on bioreduction processes. The systems studied include unamended-, carbonate buffered-, low nitrate-, and high nitrate-groundwaters. During anaerobic incubation, terminal electron accepting processes (TEAPs) in the circumneutral pH, carbonate buffered system progressed to sulfate reduction, and Tc(VII) was removed from solution during Fe(III) reduction. In the high-nitrate system, pH increased during denitrification (pH 5.5 to 7.2), then TEAPs progressed to sulfate reduction. Again, Tc(VII) removal was associated with Fe(III) reduction. In both systems, XAS confirmed reduction to hydrous Tc(IV)O2 like phases on Tc removal from solution. In the unamended and low-nitrate systems, the pH remained low, Fe(III) reduction was inhibited, and Tc(VII) remained in solution. Thus, nitrate can have complex influences on the development of the metal reducing conditions required for radionuclide treatment. High nitrate concentrations stimulated denitrification and caused pH neutralization facilitating Fe(III) reduction and Tc(VII) removal; acidic, low nitrate systemsshowed no Fe(III)-reduction. These results have implications for Tc cycling in contaminated environments where nitrate has been considered undesirable, but where it may enhance Fe(III)-reduction via a novel pH “conditioning” step.