Arsenic and antimony are important trace elements in magmatic-hydrothermal systems, geothermal systems and epithermal deposits, but their partitioning behavior between melt and aqueous fluid is not well understood. The partitioning of arsenic and antimony between aqueous fluid and granitic melt has been studied in the system SiO2-Al2O3-Na2O-K2O-H2O at 800 degree C and 200 MPa. The partition coefficients of As and Sb between aqueous fluid and melt, are 1.4 +- 0.5 and 0.8 +- 0.5, respectively. The partitioning of As is not affected by aluminum saturation index (ASI) or SiO2 content of the melt, or by oxygen fugacity under oxidized conditions (log fO2 > the nickel-nickel oxide buffer, NNO). The partitioning of Sb is independent of and SiO2 content of the melt. However, aluminum saturation index (ASI) does affect Sb partitioning and Sb partition coefficient for peralkaline melt (0.1 +- 0.01) is much smaller than that for metaluminous melts (0.8 +- 0.4) and that for peraluminous melts (1.3 +- 0.7). Thermodynamic calculations show that As(III) is dominant in aqueous fluid at 800 degree C and 200 MPa and XPS analysis of run product glass indicate that only As(III) exists in melt, which confirms the finding that does not affect As partitioning between fluid and melt. XPS analysis of run product glass show that Sb(V) is dominant in melt at oxidized conditions (log fO2 > -10). The peralkaline effect only exhibits on Sb partitioning, not on As partitioning at oxidized conditions, which is consistent with the x-ray photoelectron spectroscopy (XPS) measurements that As(III) and Sb(V) are dominant oxidation states in melt under oxidized conditions, because the peralkaline effect is stronger for pentavalent than trivalent cations. Permeable reactive barriers (PRBs) are an alternative technology to treat mine drainage containing sulfate and heavy metals. Two column experiments were conducted to assess the suitability of an organic carbon (OC) based reactive mixture and an Fe0-bearing organic carbon (FeOC) based reactive mixture, under controlled groundwater flow conditions. The organic carbon (OC) column showed an initial sulfate reduction rate of 0.4 μmol g(oc)-1 d-1 and exhausted its capacity to promote sulfate reduction after 30 pore volumes (PVs), or 9 months of flow. The Fe0-bearing organic carbon (FeOC) column sustained a relative constant sulfate reduction rate of 0.9 μmol g(oc)-1 d-1 for at least 65 PVs (17 months). The microbial enumerations and isotopic measurements indicate that the sulfate reduction was mediated by sulfate reducing bacteria (SRB). The cathodic production of H2 by anaerobic corrosion of Fe probably is the cause of the difference in sulfate reduction rates between the two reactive mixtures. Zero-valent iron can be used to provide an electron donor in sulfate reducing PRBs and Fe0-bearing organic carbon reactive mixture has a potential to improve the performance of organic carbon PRBs. The δ34S values can be used to determine the extent of sulfate reduction, but the fractionation is not consistent between reactive materials. The δ13C values indicate that methanogenesis is occurring in the front part of both columns. Arsenic and antimony in groundwater are great threats to human health. The PRB technology potentially is an efficient and cost-effective approach to remediate organic and inorganic contamination in groundwater. Two column experiments were conducted to assess the rates and capacities of organic carbon (OC) PRB and Fe-bearing organic carbon (FeOC) PRB to remove As and Sb under controlled groundwater flow conditions. The average As removal rate for the OC column was 13 nmole day-1 g-1 (dry weight of organic carbon) and its removal capacity was 11 μmole g-1 (dry weight of organic carbon). The remove rate of the FeOC material was 165 nmole day-1 g-1 (dry weight of organic carbon) and its minimum removal capacity was 105 mole g-1 (dry weight of organic carbon). Antimony removal rate of the OC material decreases from 8.2 to 1.4 nmole day-1 g-1 (dry weight of organic carbon) and its removal capacity is 2.4 μmole g-1 (dry weight of organic carbon). The minimum removal rate of FeOC material is 13 nmole day-1 g-1 (dry weight of organic carbon) and its minimum removal capacity is 8.4 μmole g-1 (dry weight of organic carbon). The As(III) : [As(III)+As(V)] ratio increased from 1% in the influent to 50% at 5.5 cm from the influent end, and to 80% at 15.5 cm from the influent end of the OC column. X-ray absorption near edge spectroscopy (XANES) shows As(III)-sulfide species on solid samples. These results suggest that As(V) is reduced to As(III) both in pore water and precipitate as As sulfides or coprecipitate with iron sulfides. The arsenic reduction rate suggests that As(V) reduction is mediated by bacterial activity in the OC column and that both abiotic reduction and bacterial reduction could be important in FeOC.

No description available.

Semiconductor devices are the building blocks of electronics and communication technology in the modern world. The charge, mass and spin of charge carriers in the semiconductor devices lay the foundations of the technology developments in the modern age. But to date only the electronic charge of the semiconductors has been exploited for such applications. The significance of the spin of charge carriers is completely ignored because in a semiconductor the half of the carriers are in spin-up state and the remainder are in spin-down state. A new electronics termed as spintronics, spin-transport based electronics, is focused to utilise the spin degree of freedom of the charge carriers in addition to its electronic charge. The devices based on these have the potential for various technological advancements like non-volatility, increased data processing speed, decreased electronic power consumption and increased integration densities as compared to the conventional semiconductor devices. In this study, the author intended to study the growth and properties of magnetic impurity doped antimony based III-V compounds and compare these results with those of the films grown by MBE. This thesis is organised into seven chapters. The first introductory chapter gives a brief review of the work on spintronics, diluted magnetic semiconductors, Ferromagnetic / paramagnetic semiconductor hybrid structures with special emphasis on the properties of antimonides which have already been reported in the literature. This is followed by the scope of the thesis. The second chapter deals with technical details of various instruments used in the present research work. Third chapter describes the growth and structural properties of bulk crystals grown by Bridgman method and thin films grown by liquid phase epitaxy (LPE). Bulk crystals of InSb and GaSb doped with magnetic elements such as Mn and Fe are grown with different doping concentrations. Thin films of InSb and GaSb doped Mn with different doping concentrations are grown by LPE. The grown crystals are processed by slicing, lapping, polishing and chemical etching methods. X-ray diffraction studies are carried out to confirm alloy formation and to find the change in lattice parameter if any. From the powder diffraction patterns, the lattice parameter is refined with the help of Retvield refinement program. A systematic change of lattice parameter with the incorporation of magnetic impurities into InSb and GaSb is observed. Scanning electron microscopy and energy dispersive x-ray analysis are carried out to identify the secondary phases and their composition respectively. Chapter 4 gives the detailed magnetotransport studies carried out on InSb and GaSb crystals doped with Mn and Fe. Also, the magnetotransport studies carried out on thin films grown by liquid phase epitaxy are presented here. This chapter is divided into three sections of which the first section deals with Mn doped bulk crystals of InSb and GaSb, the second section deals with Fe doped bulk crystals of InSb and GaSb and the third section deals with Mn doped InSb and GaSb films grown on GaAs by Liquid Phase Epitaxy. Temperature dependence of zero field resistivity, magnetoresistance and Hall measurements are carried out from 1.4 to 300K. All the samples show p type conduction throughout the temperature range studied except for Fe doped InSb. Mn doped crystals show negative magnetoresistance and anomalous Hall effect below 10K. Anisotropy in magnetoresistance is also observed at low temperatures in InMnSb crystals. On the other hand, Fe doped samples exhibit positive magnetoresistance throughout the temperature range and no anomalous Hall effect is observed. Chapter 5 describes the magnetic properties of bulk InMnSb, GaMnSb, InFeSb and GaFeSb crystals so also the thin films of InMnSb/GaAs. DC magnetization measurements are carried out in the temperature range 2 - 300K. The Mn doped InSb and GaSb crystals as well as InMnSb/GaAs films, show a magnetic ordering below 10K which could arise from the InMnSb and GaMnSb alloy formation. Also, saturation in magnetization observed even at room temperature suggests the existence of ferromagnetic MnSb clusters in the crystals which has been verified by scanning electron microscopy studies. In Fe doped InSb crystals, the temperature dependent DC magnetization shows irreversibility under field cooled and zero field cooled conditions below 12K, suggesting a spin glass-like behaviour. Also, magnetization measurement shows the coexistence of ferromagnetic and paramagnetic phases throughout the temperature range studied. Existence of ferromagnetic phase could arise from secondary phases Fe1-xInx or FeSb2 present in the crystal as clusters and paramagnetic phase could arise from the randomly distributed Fe atoms in the InSb matrix. Fe doped GaSb crystals show interesting magnetic property that arises from the FeGa alloy (secondary phase) present in it. The EPR studies on Ga0.98Mn0.02Sb cluster-free (?) crystal suggest that the dominant Mn impurity in GaMnSb is Mn2+ (d5), described as ionized acceptor A−. This conclusion was derived from EPR experiments, which reveal a strong absorption line with an effective g factor very close to 2.00, the value typical for centre A−. The absence of EPR signal typical for neutral Mn acceptor A0 suggests that this center is absent in the crystal under investigation. The observed behavior is similar to that of Ga1-xMnxAs and In1-xMnxAs epilayers. EPR studies also reveal that the competition between antiferromagnetic and ferromagnetic phases exists in the studied crystal. Chapter 6 describes the optical measurements carried out on bulk Ga1-xMnxSb crystals and their films with different Mn doping concentrations. FTIR studies are carried out in the temperature range 4 - 300 K. From the FTIR studies, it is found that intra valance spin – orbit splitting band absorption is dominant compared to the fundamental absorption in doped crystals. In higher doped crystals (x > 0.01), fundamental band absorption merges with split-off band and could not be resolved. Free carrier absorption studies are also carried out in the energy range below the band gap. FTIR studies on GaMnSb/GaAs films suggest band gap narrowing effect due to Mn doping. Furthermore the Photoluminescence measurements are carried out in the temperature range 10 – 300 K for all the Mn doped GaSb crystals. PL studies also support the band gap narrowing and band filling effects. A comprehensive summary of this research investigation and scope for the further work are presented in the last chapter.

L’application de la biosorption à l’analyse inorganique est un sujet en plein essor. Dans cette étude, un support à base de cellules de boulanger Saccharomyces cerevisiae a été développé pour la spéciation et la préconcentration des espèces inorganiques de l’antimoine Sb(III) et Sb(V). Nous avons tout d’abord montré qu’il était possible de fixer de manière sélective Sb(III) en présence de Sb(V) sur les cellules dans une large gamme de pH (5-9) et de force ionique (0-0,1M). Un prétraitement thermique de la biomasse (80°C, 30min) permet d’augmenter significativement la cinétique de fixation de Sb(III) sans nuire à la séparation. L’élution de Sb(III) par l’acide thioglycolique à pH 10 est rapide et quantitative, permettant d’obtenir un facteur de préconcentration proche de 9. Nous avons mis en évidence que la rétention de Sb(V) observée à bas pH était due à des complexation avec les groupements sulfhydryle de la paroi cellulaire. La modélisation des isotherme de sorption de Sb(III) (qmax = 450µg.g-1) ont permis de définir 3 types de sites d’affinité et de coordination différentes. Parmi les matrices testées pour l’immobilisation des cellules, le polysulfone s’avère être le plus performant, alliant perméabilité, faible affinité vis-à-vis de l’antimoine et préservation des sites de fixation. Des colonnes remplies de levures immobilisées ont été couplées à un spectromètre ICP-AES et appliquées avec succès à la spéciation de l’antimoine dans l’eau minérale dopée à faible concentration. Les limites de quantification de Sb(III) ont été améliorées d’un facteur 5 par préconcentration / The application of biosorption to inorganic analysis is an expanding research area. In this study, an analytical support based on baker’s yeast Saccharomyces cerevisiae was developed for the speciation and the preconcentration of inorganic antimony species Sb(III) and Sb(V). It was shown that Sb(III) can be retained selectively by the cells in the presence of Sb(V) in a wide range of pH (5-9) and ionic strength (0-0,1M). Heat pretreatment of the biomass (80°C, 30min) significantly increased the kinetics of Sb(III) uptake without degrading the separation. The elution of Sb(III) by thioglycolic acid at pH 10 was rapid and quantitative, allowing to achieve a preconcentration factor close to 9. Interactions between Sb(V) and the cells, as observed at lower pH, were found to be purely electrostatic, while Sb(III) retention was attributed to the complexation of the species with sulfhydryl groups of the cell walls. Three kinds of sites with different affinities and coordinations were identified by modeling Sb(III) sorption isotherms (qmax = 450µg.g-1). Among the different materials tested, polysulfone was found to be the most suitable matrix for yeast immobilization, combining a good permeability to a low affinity for antimony species and preservation of the sorption sites. Columns filled with immobilized cells were coupled with ICP-AES and successfully applied to antimony speciation in mineral waters samples spiked at low concentration level. The limit of quantification was improved by a factor of 5 by preconcentration

The (001) surface of silicon has been the topic of our study in this thesis. The clean surface, an-adatom or submonolayer adsorption on the surface, the monolayer adsorption and its stability conditions as well as growth simulation on the surface were investigated using the state of the art techniques. We have used ab initio density functional calculations based on norm-conserving pseudopotentials to investigate the Mg adsorption on the Si(001) surface for 1/4, 1/2 and 1 monolayer (ML) coverages. For both 1/4 and 1/2 ML coverages it has been found that the most favorable site for the Mg adsorption is the cave site between two dimer rows consistent with recent experiments. For the 1 ML coverage (2 Mg atoms per 2X1 unit cell) we have found that the most preferable configuration is when both Mg atoms on 2X1 reconstruction occupy the two shallow sites. We have found that the minimum energy configurations for 1/4 ML coverage is a 2X2 reconstruction while for the 1/2 and 1 ML coverages they are 2X1. Same method was also used to investigate the surface stress and energetics of the clean-, Sb-adsorbed-, and Sb-interdiffused-Si(001) surface. It is found that interdiffusion of Sb into deeper layers of Si(001) leads to a more isotropic surface stress but corresponds to a higher total energy configuration. As a result of competition between stress relief and energy gain, the surface with all the Sb atoms adsorbed on top of Si(001) surface layer is predicted to have a less ordered geometry and roughness in z-direction. We have repeated the similar calculations on the Ge(001) surface for comparison. Finally using empirical molecular dynamics method, we have investigated the crystalline growth of silicon on Si(001) as a function of substrate temperature and incident particle energy. Our results show that the increase of substrate temperature enhances the crystallinity in the film grown on the Si(001) surface, on the other hand, the crystalline growth can be enhanced at low temperature by using higher incidence energy.

Antimony (Sb) is an element of increasing concern as an environmental contaminant. Geothermal systems are a potential source of Sb in the fresh waters of New Zealand’s North Island, but little is known about the element’s behaviour within geothermal fluids, and even less about the eventual fate of geothermally produced Sb. The purpose of this thesis was to determine the factors controlling geothermal Sb behaviour in a range of environments, in order to begin to develop an understanding of the eventual fate of Sb produced from geothermal systems. Factors controlling Sb precipitation at two New Zealand geothermal power stations, were determined using field measurements and geochemical model predictions. Approximately 75 % of the incoming Sb, which ranged from 960 – 1650 μg/kg in the incoming fluids was removed from solution within the Ngawha and Rotokawa power stations. It was found that changes in pH and temperature were the most important factors controlling stibnite (Sb2S3) precipitation. Thermodynamic databases could be used to model this precipitation process, once updated with recent published Sb2S3 solubility data. The mobility of Sb from Sb2S3 precipitates in geothermal features at Wai-O-Tapu and Waimangu, two New Zealand geothermal fields, were investigated. At Wai-O-Tapu, daytime variations in aqueous Sb concentrations from the discharge of Champagne Pool may be due to to changing sulfide-sulfate equilibria coupled with photosynthetic bacterial processes. While daytime concentrations of Sb approached 200 μg/kg, most of the Sb remobilised by such mechanisms appears to be removed by adsorption onto suspended particulate material (SPM) or reprecipitation (as Sb2S3) in an anoxic, low pH lake feature downstream. Concentrations of Sb in the discharge from Alum Lake were below the analytical detection limit (<0.2 μg/kg). At Waimangu, these daytime fluctuations were not observed in the discharge of Frying Pan Lake, and concentrations of Sb were ~13 μg/kg. In the absence of any downstream acidic waters, no precipitation was observed and only minor adsorption onto SPM was observed. Most of the Sb produced from Frying Pan Lake is therefore transported into Lake Rotomahana, the system’s receiving environment. Natural Sb removal processes in receiving (non-geothermal) environments were also assessed. In the Waikato River, Sb concentrations were low (~1 μg/kg), compared to those observed in geothermal environments studied. The most important process was adsorption to SPM, which is enhanced at low (< 5) pH conditions, or in the anoxic base of stratified lakes. In Lake Ohakuri, which was stratified during the summer of 2007, there was also the potential for the removal of Sb as Sb2S3 in the presence of sulfide species that form in the anoxic layer. There was evidence that the adsorption of Sb changes with changing Fe concentrations in suspended particulate material, and therefore Sb adsorption was higher in winter than in summer. The behaviour of Sb was conservative in the Port Waikato estuary at the mouth of the river. Throughout the research, Sb was compared to arsenic (As), a metalloid previously thought to exhibit behaviour similar to Sb in aquatic environments. It was found that while any removal processes shown to affect Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by geothermal algae and freshwater macrophytes, there was no such evidence for Sb. Therefore, if geothermally-derived Sb and As did ever significantly contaminate a downstream environment, it should not be assumed that the processes mitigating As contamination will necessarily also apply to Sb.

Antimony (Sb) is an element of increasing concern as an environmental contaminant. Geothermal systems are a potential source of Sb in the fresh waters of New Zealand’s North Island, but little is known about the element’s behaviour within geothermal fluids, and even less about the eventual fate of geothermally produced Sb. The purpose of this thesis was to determine the factors controlling geothermal Sb behaviour in a range of environments, in order to begin to develop an understanding of the eventual fate of Sb produced from geothermal systems. Factors controlling Sb precipitation at two New Zealand geothermal power stations, were determined using field measurements and geochemical model predictions. Approximately 75 % of the incoming Sb, which ranged from 960 – 1650 μg/kg in the incoming fluids was removed from solution within the Ngawha and Rotokawa power stations. It was found that changes in pH and temperature were the most important factors controlling stibnite (Sb2S3) precipitation. Thermodynamic databases could be used to model this precipitation process, once updated with recent published Sb2S3 solubility data. The mobility of Sb from Sb2S3 precipitates in geothermal features at Wai-O-Tapu and Waimangu, two New Zealand geothermal fields, were investigated. At Wai-O-Tapu, daytime variations in aqueous Sb concentrations from the discharge of Champagne Pool may be due to to changing sulfide-sulfate equilibria coupled with photosynthetic bacterial processes. While daytime concentrations of Sb approached 200 μg/kg, most of the Sb remobilised by such mechanisms appears to be removed by adsorption onto suspended particulate material (SPM) or reprecipitation (as Sb2S3) in an anoxic, low pH lake feature downstream. Concentrations of Sb in the discharge from Alum Lake were below the analytical detection limit (<0.2 μg/kg). At Waimangu, these daytime fluctuations were not observed in the discharge of Frying Pan Lake, and concentrations of Sb were ~13 μg/kg. In the absence of any downstream acidic waters, no precipitation was observed and only minor adsorption onto SPM was observed. Most of the Sb produced from Frying Pan Lake is therefore transported into Lake Rotomahana, the system’s receiving environment. Natural Sb removal processes in receiving (non-geothermal) environments were also assessed. In the Waikato River, Sb concentrations were low (~1 μg/kg), compared to those observed in geothermal environments studied. The most important process was adsorption to SPM, which is enhanced at low (< 5) pH conditions, or in the anoxic base of stratified lakes. In Lake Ohakuri, which was stratified during the summer of 2007, there was also the potential for the removal of Sb as Sb2S3 in the presence of sulfide species that form in the anoxic layer. There was evidence that the adsorption of Sb changes with changing Fe concentrations in suspended particulate material, and therefore Sb adsorption was higher in winter than in summer. The behaviour of Sb was conservative in the Port Waikato estuary at the mouth of the river. Throughout the research, Sb was compared to arsenic (As), a metalloid previously thought to exhibit behaviour similar to Sb in aquatic environments. It was found that while any removal processes shown to affect Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by geothermal algae and freshwater macrophytes, there was no such evidence for Sb. Therefore, if geothermally-derived Sb and As did ever significantly contaminate a downstream environment, it should not be assumed that the processes mitigating As contamination will necessarily also apply to Sb.

Antimony (Sb) is an element of increasing concern as an environmental contaminant. Geothermal systems are a potential source of Sb in the fresh waters of New Zealand’s North Island, but little is known about the element’s behaviour within geothermal fluids, and even less about the eventual fate of geothermally produced Sb. The purpose of this thesis was to determine the factors controlling geothermal Sb behaviour in a range of environments, in order to begin to develop an understanding of the eventual fate of Sb produced from geothermal systems. Factors controlling Sb precipitation at two New Zealand geothermal power stations, were determined using field measurements and geochemical model predictions. Approximately 75 % of the incoming Sb, which ranged from 960 – 1650 μg/kg in the incoming fluids was removed from solution within the Ngawha and Rotokawa power stations. It was found that changes in pH and temperature were the most important factors controlling stibnite (Sb2S3) precipitation. Thermodynamic databases could be used to model this precipitation process, once updated with recent published Sb2S3 solubility data. The mobility of Sb from Sb2S3 precipitates in geothermal features at Wai-O-Tapu and Waimangu, two New Zealand geothermal fields, were investigated. At Wai-O-Tapu, daytime variations in aqueous Sb concentrations from the discharge of Champagne Pool may be due to to changing sulfide-sulfate equilibria coupled with photosynthetic bacterial processes. While daytime concentrations of Sb approached 200 μg/kg, most of the Sb remobilised by such mechanisms appears to be removed by adsorption onto suspended particulate material (SPM) or reprecipitation (as Sb2S3) in an anoxic, low pH lake feature downstream. Concentrations of Sb in the discharge from Alum Lake were below the analytical detection limit (<0.2 μg/kg). At Waimangu, these daytime fluctuations were not observed in the discharge of Frying Pan Lake, and concentrations of Sb were ~13 μg/kg. In the absence of any downstream acidic waters, no precipitation was observed and only minor adsorption onto SPM was observed. Most of the Sb produced from Frying Pan Lake is therefore transported into Lake Rotomahana, the system’s receiving environment. Natural Sb removal processes in receiving (non-geothermal) environments were also assessed. In the Waikato River, Sb concentrations were low (~1 μg/kg), compared to those observed in geothermal environments studied. The most important process was adsorption to SPM, which is enhanced at low (< 5) pH conditions, or in the anoxic base of stratified lakes. In Lake Ohakuri, which was stratified during the summer of 2007, there was also the potential for the removal of Sb as Sb2S3 in the presence of sulfide species that form in the anoxic layer. There was evidence that the adsorption of Sb changes with changing Fe concentrations in suspended particulate material, and therefore Sb adsorption was higher in winter than in summer. The behaviour of Sb was conservative in the Port Waikato estuary at the mouth of the river. Throughout the research, Sb was compared to arsenic (As), a metalloid previously thought to exhibit behaviour similar to Sb in aquatic environments. It was found that while any removal processes shown to affect Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by geothermal algae and freshwater macrophytes, there was no such evidence for Sb. Therefore, if geothermally-derived Sb and As did ever significantly contaminate a downstream environment, it should not be assumed that the processes mitigating As contamination will necessarily also apply to Sb.

Magister Scientiae - MSc (Biotechnology) / The yearly increase of global population will result in a greater demand for crop production, but with the climates changes and a lack of available agricultural land it will become increasingly more difficult to provide sufficient crops to feed everyone adequately. Application of the PGPE has proven over the past researches to be able enhance growth of plants via various growth promoting mechanisms. To identify suitable growth promoting bacteria candidate, E. plantagineum plant was used to isolate endophytes from the root after surface sterilization. The isolates bacteria were used to inoculate Brassica napus L seeds. The effects of isolate's ability to promote growth were evaluated based on the certain growth parameters after 42 days in the green house. Isolate CP5 produced highest results in all growth parameter. Isolates CP5 was selected as potential candidate as significant improvement was shown by this isolate. This isolate was tested for the ability to produce ACC deaminase, solubilize phosphate, synthesize IAA and siderophore production. Furthermore isolate CP5 growth promotion abilities was tested on Brassica napus L under antimony stress. / 2021-08-31

Les réactions d'oxydoréduction de l'antimoine (V) et (III) avec la mackinawite (FeS) et du plutonium (III) et (V) avec plusieurs minéraux à fer(II) et des oxydes de fer(III) ont été étudiées dans des conditions d'anoxie. La spectroscopie d'absorption des rayons X fut utilisée pour l'analyse de l'état d'oxydation et de la structure locale du Sb et Pu associés à la phase solide. Après réaction avec la mackinawite, la chukanovite et la magnétite, PuO2, Pu(III) ou des mixtures des deux états d'oxydation ont été observé. A la surface de la magnétite un complexe tridenté du Pu(III) a pu être mis en évidence à l'aide des spectres EXAFS couplé à une simulation de Monte-Carlo utilisant le code de calcul Feff. La quantité relative de Pu(III) est fonction de l'espèce minérale, du ratio solide/liquide, des valeurs pe et pH du système et, potentiellement, de la taille de particule et de la cristallinité de la phase solide de PuO2 en présence de laquelle le Pu(III) existe. Avec la mackinawite à pH 6,2 et à une occupation de surface de 67 nmol/m2 et avec la magnétite jusqu'à pH 8.4 et 56 nmol/m2 de Pu, uniquement du Pu(III) fut trouvé associé à la phase solide. Avec la maghémite contenant du fer(II) résiduel à pH6 Pu(III) et Pu(IV) était, probablement, présents dans des complexes de surfaces similaire à celui formé par le Pu(III) sur la magnétite. Dans les conditions expérimentales (couverture de surface ≤ 77 nmol/m2), aucune formation de PuO2 ne fut observé. Après réaction avec l'hématite et la goethite Pu(IV) était l'état d'oxydation prédominant associé à la phase solide. La sorption et la réduction du Sb(V) avec la mackinawite était fortement fonction du pH. A pH acide la sorption était rapide et Sb(V) fut complètement réduit en Sb(III), formant un complexe Sb(III)-S3 probablement associé à la surface de la mackinawite. La réduction du Sb(V) était couplée à l'oxydation de la mackinawite et la greigite (Fe3S4) fut détectée par XRD. A pH basique, la sorption du Sb(V) est lente et la réduction vers le Sb(III) n'était complète qu'à des ratios de Sb/FeS très bas. Pour des valeurs plus élevé de Sb/FeS la sorption de Sb se faisait en partie par la réduction envers le complexe de Sb(III)-S3 et en partie par une co-précipitation avec le Fe(III). Il a pu être démontré que les minéraux à fer(II) peuvent effectivement contribuer à la réduction et à l'immobilisation de l'antimoine et du plutonium qui sont des contaminants environnementaux d'importance croissante. / Redox reactions of Sb(V) and Sb(III) with mackinawite (FeS) and of aqueous Pu(III) and Pu(V) with various Fe(II)-bearing minerals and Fe(III)-oxides have been investigated under anoxic conditions. X-ray absorption spectroscopy was used to analyze oxidation state and local coordination environment of Sb and Pu associated with the solid phase. After reaction with mackinawite, chukanovite and magnetite, PuO2, Pu(III) or mixtures of the two oxidation states were observed. On magnetite, a tridentate Pu(III) surface complex could be identified from EXAFS combined with Feff-Monte-Carlo simulation. The relative amount of Pu(III) depends on the type of mineral, the solid/solution ratio, the system pe and pH, and, potentially, the particle size and crystallinity of the formed PuO2 solid phase. With mackinawite at pH 6.2 and a surface loading of 67 nmol/m2 and with magnetite up to pH 8.4 and a surface loading of 56 nmol/m2, only Pu(III) was identified associated with the solid phase. With maghemite containing residual Fe(II) at pH6, Pu(III) and Pu(IV) were present in, probably, inner-sphere surface complexes similar to the one formed by Pu(III) on magnetite. Under the given conditions (surface loadings ≤ 77 nmol/m2) formation of PuO2 was not observed. After reaction with hematite and goethite, Pu(IV) was the predominant oxidation state associated with the solid phase. Sorption and reduction of Sb(V) on mackinawite were strongly pH dependent. At acidic pH, sorption was fast and Sb(V) was completely reduced to an Sb(III)-sulfide complex associated with the solid phase. Reduction of Sb(V) was coupled to oxidation of mackinawite and formation of a greigite (Fe3S4) phase could be observed by XRD. At basic pH, Sb(V) was slowly removed from solution and reduction to Sb(III) was complete only at very small Sb/FeS ratios. At higher Sb/FeS, Sb(V) removal occurred partly through reduction to solid phase associated Sb(III)-S3 and partly through co-precipitation with Fe(III). In conclusion, it could be shown that Fe(II) bearing minerals can effectively contribute to the reduction and immobilization of antimony and plutonium, two contaminants of growing environmental importance.