Doug Stewart | University of Leeds (original) (raw)
Papers by Doug Stewart
Journal of Industrial Microbiology and Biotechnology, Aug 28, 2012
Environmental science & technology, 2013
Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to &... more Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to < pH 10 by HCl, gypsum, or seawater addition. During acid neutralization >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 nonexchangeable 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 neutralization 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 remobilized by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16% V was removed from solution during neutralization, 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.
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.
Environmental geochemistry and health, 2013
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 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 the 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.
Trace Metals and other Contaminants in the Environment, 2005
Long-term Performance of Permeable Reactive Barriers KE Roehl, T. Meggyes, F.-G. Simon, DI Stewar... more Long-term Performance of Permeable Reactive Barriers KE Roehl, T. Meggyes, F.-G. Simon, DI Stewart, editors. 9 2005 Elsevier BV All rights reserved. Chapter 7 New barrier materials: the use of tailored ligand systems for the removal of metals from groundwater DI Stewart, ...
Abstract Radioactive waste contamination of the environment is a global problem. An increasing am... more Abstract Radioactive waste contamination of the environment is a global problem. An increasing amount of processed waste buried in legacy sites from the 1940's to 1970's is escaping into the environment due to aging and failed storage. Due to the current ...
Bitumen is used to embed low and intermediate radioactive waste because of its high impermeabilit... more Bitumen is used to embed low and intermediate radioactive waste because of its high impermeability and its great resistance to chemicals. This study focused on the bituminous wastes used for stabilisation of soluble and insoluble salts, mainly of nitrate and sulphate. Resaturation in the disposal cell after closure will modify the confinement properties and the presence of water will not only have significant physical impact (salt dissolution, pressure increase, organic matter solubilisation) but may also allow the development of ...
The long-term behaviour of nailed walls in overconsolidated clay has been simulated in centrifuge... more The long-term behaviour of nailed walls in overconsolidated clay has been simulated in centrifuge models. Excavation was represented by the drawdown of a heavy fluid from in front of a previously nailed face. Models with different nail lengths and spacings were ...
Backscatter image intensity under the SEM shows that the lead contaminant in the first soil was c... more Backscatter image intensity under the SEM shows that the lead contaminant in the first soil was concentrated on specific particles. Here, EDS analysis using a'thin-window'detector gave elemental compositions that were compatible with operational speciation data from ...
ABSTRACT An investigation of the influence of humic acids on copper mobility in a model soil is r... more ABSTRACT An investigation of the influence of humic acids on copper mobility in a model soil is reported. Humic acids were extracted from peat with alkali and separated by acid precipitation. Electrokinetic column experiments were then carried out on consolidated ...
ABSTRACT The paper describes the performance of a 5.3 m trial embankment constructed on approxima... more ABSTRACT The paper describes the performance of a 5.3 m trial embankment constructed on approximately 45m of hydraulically placed pulverised fuel ash (pfa). It is planned to redevelop the 17 hectare lagoon containing the pfa as a landfill. There is little variation in ...
ABSTRACT: This paper considers the engineering behaviour of bentonite-enhanced soil mixtures (BES... more ABSTRACT: This paper considers the engineering behaviour of bentonite-enhanced soil mixtures (BES) in relation to their performance as environmental barriers. Data on the swelling and hydraulic conductivity of three different soil mixtures over a range of ...
... for groundwater remediation 315 C Economic aspects 316 1 PRB costs 316 2 Reactive materials 3... more ... for groundwater remediation 315 C Economic aspects 316 1 PRB costs 316 2 Reactive materials 318 3 Economic aspects of the ... Contaminated sites resulting from industrial activity, mining, improper waste disposal or accidents involving hazardous substances pose a ...
Journal of Industrial Microbiology and Biotechnology, Aug 28, 2012
Environmental science & technology, 2013
Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to &... more Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to < pH 10 by HCl, gypsum, or seawater addition. During acid neutralization >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 nonexchangeable 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 neutralization 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 remobilized by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16% V was removed from solution during neutralization, 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.
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.
Environmental geochemistry and health, 2013
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 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 the 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.
Trace Metals and other Contaminants in the Environment, 2005
Long-term Performance of Permeable Reactive Barriers KE Roehl, T. Meggyes, F.-G. Simon, DI Stewar... more Long-term Performance of Permeable Reactive Barriers KE Roehl, T. Meggyes, F.-G. Simon, DI Stewart, editors. 9 2005 Elsevier BV All rights reserved. Chapter 7 New barrier materials: the use of tailored ligand systems for the removal of metals from groundwater DI Stewart, ...
Abstract Radioactive waste contamination of the environment is a global problem. An increasing am... more Abstract Radioactive waste contamination of the environment is a global problem. An increasing amount of processed waste buried in legacy sites from the 1940's to 1970's is escaping into the environment due to aging and failed storage. Due to the current ...
Bitumen is used to embed low and intermediate radioactive waste because of its high impermeabilit... more Bitumen is used to embed low and intermediate radioactive waste because of its high impermeability and its great resistance to chemicals. This study focused on the bituminous wastes used for stabilisation of soluble and insoluble salts, mainly of nitrate and sulphate. Resaturation in the disposal cell after closure will modify the confinement properties and the presence of water will not only have significant physical impact (salt dissolution, pressure increase, organic matter solubilisation) but may also allow the development of ...
The long-term behaviour of nailed walls in overconsolidated clay has been simulated in centrifuge... more The long-term behaviour of nailed walls in overconsolidated clay has been simulated in centrifuge models. Excavation was represented by the drawdown of a heavy fluid from in front of a previously nailed face. Models with different nail lengths and spacings were ...
Backscatter image intensity under the SEM shows that the lead contaminant in the first soil was c... more Backscatter image intensity under the SEM shows that the lead contaminant in the first soil was concentrated on specific particles. Here, EDS analysis using a'thin-window'detector gave elemental compositions that were compatible with operational speciation data from ...
ABSTRACT An investigation of the influence of humic acids on copper mobility in a model soil is r... more ABSTRACT An investigation of the influence of humic acids on copper mobility in a model soil is reported. Humic acids were extracted from peat with alkali and separated by acid precipitation. Electrokinetic column experiments were then carried out on consolidated ...
ABSTRACT The paper describes the performance of a 5.3 m trial embankment constructed on approxima... more ABSTRACT The paper describes the performance of a 5.3 m trial embankment constructed on approximately 45m of hydraulically placed pulverised fuel ash (pfa). It is planned to redevelop the 17 hectare lagoon containing the pfa as a landfill. There is little variation in ...
ABSTRACT: This paper considers the engineering behaviour of bentonite-enhanced soil mixtures (BES... more ABSTRACT: This paper considers the engineering behaviour of bentonite-enhanced soil mixtures (BES) in relation to their performance as environmental barriers. Data on the swelling and hydraulic conductivity of three different soil mixtures over a range of ...
... for groundwater remediation 315 C Economic aspects 316 1 PRB costs 316 2 Reactive materials 3... more ... for groundwater remediation 315 C Economic aspects 316 1 PRB costs 316 2 Reactive materials 318 3 Economic aspects of the ... Contaminated sites resulting from industrial activity, mining, improper waste disposal or accidents involving hazardous substances pose a ...