A novel phosphorus (P) adsorbent, ferrihydrite-modified diatomite (FHMD) was developed and characterized in this study. The FHMD was made through surface modification treatments, including NaOH treatment and ferrihydrite deposition on raw diatomite. In the NaOH treatment, surface SiO2 was partially dissolved in the NaOH solution. The dissolved Si contributed to form stable 2-line ferrihydrite, which deposited into the larger mesopores and macropores of the diatomite. The 2-line ferrihydrite not only deposited into the pores of the diatomite but also aggregated on the surface. Filling the larger mesopores and macropores of the diatomite and aggregation on the diatomite surface with 0.24 g Fe/g of 2-line ferrihydrite resulted in a specific surface area of 211.1 m2/g for the FHMD, which is an 8.5-fold increase over the raw diatomite (24.77 m2/g). The surface modification also increased the point of zero charge (pHPZC) values to 10 for the FHMD from 5.8 for the raw diatomite.<p> Effects of the formation process parameters such as concentrations of FeCl2, NaOH, and drying temperature on the formation mechanism and crystalline characteristics of FHMD were studied by using X-ray absorption near-edge structure (XANES) spectroscopy. The spectra were recorded in both the total electron yield (TEY) and the fluorescence yield (FY) modes to investigate the chemical nature of Fe and Si on the surface and in the bulk of ferrihydrite-modified diatomite, respectively. It was found that only the surface SiO2 was partially dissolved in the NaOH solution with stirring and heating, whereas the bulk of diatomite seemed to be preserved. The dissolved Si was incorporated into the structure of ferrihydrite to form the 2-line Si-containing ferrihydrite. The crystalline degree of ferrihydrite increased with the increasing FeCl2 concentration and the Brunauer-Emmett-Teller (BET) specific surface area of FHMD decreased with the increasing FeCl2 concentration. The NaOH solution of higher concentration partially dissolved more surface SiO2 and the crystalline degree of ferrihydrite decreased with the increase in NaOH concentration. The dehydroxylation on the surface of FHMD occurred in the high temperature calcination, causing an energy shift in the Si L-edge spectra to the high energy side and an increase in the crystalline degree of ferrihydrite. In this study, the optimal synthesis conditions for the FHMD with the least crystalline degree and the highest surface area were found to be as the follows: 100 mL of 0.5M FeCl2 solution, 6M NaOH solution and the drying temperature of 50 ºC.<p> Phosphorus adsorption behavior and adsorption mechanism of FHMD were investigated in the research. The Langmuir model best described the P adsorption data for FHMD. Because of increased surface area and surface charge, the maximum adsorption capacity of FHMD at pH 4 and pH 8.5 was increased from 10.2 mg P/g and 1.7 mg P/g of raw diatomite to 37.3 mg P/g and 13.6 mg P/g, respectively. Phosphorus showed the best affinity of adsorption onto FHMD among common anions. K-edge P XANES spectra demonstrate that P is not precipitated with Fe (III) of FHMD, but adsorbed on the surface layer of FHMD.<p> Phosphorus removal from lake water and limiting phosphorus release from sediment by FHMD was examined. Phosphorus removal from lake water proceeded primarily through P adsorption onto the surface of FHMD. When a dose of FHMD of 250 mg/L was applied to lake water, a total phosphorus (TP) removal efficiency of 88% was achieved and a residual TP concentration was 17.0 µg/L which falls within the oligotrophic TP range (3.0-17.7 µg/L). FHMD settled down to the bottom of the 43 cm high cylinder within 6 hours, which suggested that retention time of FHMD in the 5.5 m of Jackfish lake water column was close to the equilibrium time of P adsorption onto FHMD (72 hours). During the 30-day anoxic incubation period, TP concentrations in lake water treated by 400, 500 and 600 mg/L of FHMD showed a slight decrease and maximum TP concentrations remained at levels lower than 15 µg/L. The addition of FHMD resulted in a marked increase in Fe-P fraction, a pronounced decrease in labile-P and organic-P fractions, and stable Al-P, Ca-P and residual-P fractions. The effect of FHMD on limiting P release was comparable with those of the combination of FHMD and alum solutions with logarithmic ratios of Al to mobile P of 0.5 and 0.8. FHMD not only can effectively remove P from lake water but also keep a strong P-binding capacity under anoxic conditions and competition for P with alum at high amounts.<p> The role of gypsum on stabilizing sediment and the optimum dose of gypsum were investigated. The effectiveness of gypsum in stabilizing sediment was proved by the fact that at the same agitation speed, turbidities and soluble reactive P (SRP) concentrations of samples treated with gypsum were much lower than those of sample without gypsum. The optimal thickness of the gypsum layer was found to be 0.8 cm.<p> Combined application of FHMD and gypsum to P control was investigated in the research. It was found in the 30-day incubation of lake water and sediment treated by FHMD and gypsum that no P release seemed to occur regardless of oxic or anoxic conditions. In order to investigate the 120-day effects of FHMD and gypsum on the P control under anoxic and agitation conditions a lab-scale artificial aquarium was established in an environmental chamber. Daily oscillation of a metal grid did not yield the sediment resuspension due to the gypsum stabilization. The combined application of FHMD and gypsum resulted in a 1 g/L increase in the SO42- concentration in the 120-day aquarium compared with that in the control aquarium; however it did not affect the total kjeldahl nitrogen (TKN) concentrations in both the control aquarium and the 120-day aquarium. The addition of FHMD and gypsum enhanced total alkalinity in the 120-day aquarium, thereby improving buffering capacity of lake water. Under anoxic conditions and sediment resuspension conditions, relative to a large increase in total P (TP) concentrations in the control aquarium, TP concentrations in the 120-day aquarium stayed relatively stable, fluctuating within the range of 9.1-13.3 µg/L. Relative to control sediment, Fe-P was significantly enhanced during the 60-day incubation; however, Fe-P did not appear to increase significantly in the second 60-day incubation. Labile-P and organic-P decreased with sediment depths in both control aquarium and test aquariums; however, Al-P, Ca-P and residue-P increased with sediment depth. Lower Al-P is observed in treatment aquariums than in control sediment.<p> As an effective P adsorbent, FHMD showed a high adsorption capacity as well as a significantly higher affinity for P than other anions. A combined application of FHMD and gypsum effectively reduced sediment resuspension and maintained TP levels within the oligotrophic range under anoxic conditions in the laboratory-scale artificial aquarium.

Over 70% of the Earth s economically recoverable nickel (Ni) resides in laterite ore deposits, however they account for less than half of the current global nickel production. During laterization, nickel and other soluble ions are taken into solution before re-precipitating within iron oxide minerals in the limonite zone, or as serpentines and other phyllosilicates in the layers below this zone. It is these laterite deposits that show the greatest potential for low energy, environmentally conscious processing. The major host of nickel in the limonite zone is the iron-oxyhydroxide mineral goethite, α-FeOOH, where up to 4 mol% Ni has been reported in natural specimens, and even higher levels in synthetic samples (5.5 mol%). The Ni is assumed to be incorporated in the crystal structure of the goethite, but previous characterisation work only demonstrated a weak to moderate correlation of mineral structure change with the nickel content in goethite. Mining companies working on the extraction and recovery of nickel from the limonite zone of lateritic deposits have noticed that the ease with which nickel can be extracted varies greatly; goethite rich ores that appear to have similar mineralogies/geologies can display extreme variation in their leachability. It is not clear why the ores behave in this way, but in order for extraction techniques and subsequent recovery of nickel to be improved, the reasons behind this variability need to be understood. The lateritic ore materials from which nickel is extracted are generally made up of a number of different mineral phases. The multiphase nature of the samples means that characterisation of the goethite-type phases from these materials is challenging. To simplify the system and allow the association of Ni into goethite and/or other iron oxyhydroxide phases to be studied in a controlled environment, a synthetic study was carried out. Ni-bearing goethites have been synthesised at a series of different temperatures and characterised by a range of analytical techniques including PXRD, IR, Raman, TGA, ICP-OES, SEM and TEM. It was found that a second phase, ferrihydrite, co-existed with the goethites, the proportion of which increased at lower synthesis temperatures and with increasing amounts of Ni. Ferrihydrite is known to be a precursor phase in the formation of goethite, but its poorly crystalline nature makes it difficult to identify using standard characterisation techniques such as PXRD. The introduction of Ni to the system increases the stability of the ferrihydrite phase, inhibiting its transformation to goethite. It is believed that some of the Ni thought to be incorporated into goethite could actually reside in an undetected ferrihydrite phase, which could account for the differences observed in the leachability of natural materials. Characterisation techniques were investigated to try and determine a simple way to identify ferrihydrite in these systems, which could ideally be used in the field to identify the presence of ferrihydrite in goethite rich ore materials. Thermal analysis proved to be particularly promising as a technique which could be used to identify ferrihydrite rich deposits before extraction, enabling the most efficient and environmentally conscious metal recovery process for each deposit to be identified. In order to investigate the way in which Ni partitions itself between structural incorporation into goethite and association with a secondary ferrihydrite phase, a new washing technique was developed using EDTA, which is capable of selectively removing the ferrihydrite phase whilst leaving the goethite intact. This investigation suggests that a maximum of ~2.5 mol% of Ni is structurally incorporated into goethite, regardless of how much is added during the synthesis. Any excess nickel, above that which is substituted into the goethite structure, was found to be associated with the poorly crystalline ferrihydrite phase. Despite being considered a metastable phase, the increased stability of ferrihydrite resulting from the presence of Ni suggests that it may persist in laterite deposits within geological systems. If ferrihydrite is indeed present in nickeliferous laterites, it may be a significant host for Ni, and potentially many other critical elements. Based on the methodology developed whilst studying synthetic samples, a characterisation program for materials from lateritic ore deposits was conducted to investigate the presence of ferrihydrite in natural systems. From the research presented and discussed in this thesis, proof of the presence or absence of ferrihydrite in laterite systems, causing variations in the leachability of the ore materials, could not be conclusively established. The thermal analysis technique developed here successfully identified and quantified ferrihydrite in the presence of goethite in synthetic systems, and showed great potential when used to characterise the lateritic goethite samples, certainly suggesting that ferrihydrite could be present in these natural ore materials. With further refinement of the methodology, to enable a larger range of sample types to be accurately analysed, TGA is a technique which could be used as a screening tool for laterite ores.

The Enhanced Actinide Removal Plant (EARP) located at Sellafield, is a key facility for processing nuclear effluents in the UK. The EARP process decontaminates radioactive waste effluent by inducing the coprecipitation of Fe(III) along with any radionuclides in solution. The resulting radioactive solid phase is then separated from the decontaminated aqueous phase by ultrafiltration processes over several weeks. In the future the EARP facility's role will be expanded to treat effluents produced from nuclear decommissioning activities. However there remains a limited understanding around the mechanisms of radionuclide removal from solution and subsequent sequestration within the solid phase under conditions relevant to such industrial effluent treatment facilities. In this work, the fate of U(VI) and Th(IV) were investigated during the EARP process. XRD and TEM analyses revealed that the solid product from the EARP coprecipitation process was 2-line ferrihydrite which transformed over time to form hematite, with some goethite formation observed in the Th containing system. During this coprecipitation process U was initially removed from solution by adsorption to ferrihydrite as a bidentate, edge-sharing surface complex associated with ternary carbonate complexes. As the U containing system was aged, a maximum range of 61 - 75% U became consistently incorporated within the newly formed hematite phase for a wide range of systems containing 6 orders of magnitude total U:Fe molar ratio. Such a constant proportion of incorporated U suggests that this incorporation occurs during a particle mediated mechanism of hematite growth, such as oriented attachment. Interestingly, Th was removed from solution during the EARP coprecipitation process by a combination of both adsorption and occlusion mechanisms. EXAFS analyses revealed that the local coordination environment of Th associated with the solid phase altered considerably with increased aging time, and correspondingly Th became increasingly recalcitrant to remobilisation over time suggesting the formation of further occluded or even incorporated Th species within the transforming iron (oxyhydr)oxide phases. This thesis progresses the fundamental understanding of radionuclide interactions with iron (oxyhydr)oxide phases during coprecipitation and aging processes under conditions relevant to industrial waste treatment processes such as EARP.

The Enhanced Actinide Removal Plant (EARP), located on the Sellafield site, is one of the UK's most crucial radioactive effluent treatment plants. EARP removes actinides and select fission products from routine reprocessing effluents by association with a ferric iron oxyhydroxide floc, which is precipitated from acidic effluent streams by the addition of NaOH. The effluent compositions that EARP receives will change in character as the Sellafield site transitions from its current routine reprocessing operations to post-operational clean-out and accelerated decommissioning activities over the next few years. An enhanced understanding of the iron oxyhydroxide formation processes occurring in EARP would help underpin optimisation of current plant efficiency and allow better prediction of changes in efficiency as effluent composition varies. In this study, iron oxyhydroxide formation, properties and evolution with time under EARP-relevant conditions were characterized. These processes were investigated in a pure ferric nitrate system and systems with added sulfate, phosphate and boric acid using a range of techniques including SAXS, TEM and FTIR. In all the experimental systems the iron oxyhydroxide floc was composed of nanoparticulate ferrihydrite aggregated into extensive mass fractal structures. In situ SAXS experiments showed that formation proceeded via a precursor cluster pathway whereby Fe(III) clusters ~ 0.45 nm in radius form rapidly at pH 0.12 - pH 1.5 upon dropwise addition of strong NaOH to the acidic effluent simulants. Further analysis indicates these clusters are Fe13 Keggin clusters, which have previously been shown to be an important structural motif in the ferrihydrite structure. With further pH increase, cluster aggregation occurs along with precipitation of low molecular weight Fe(III) species (mostly monomers), leading to formation of ferrihydrite nanoparticles which preserve the Keggin cluster in the core. Phosphate, sulfate and boric acid exhibit varying interactions with the solid phase throughout the formation process, with both inner and outer sphere adsorption observed for different species. Ageing experiments show that the ferrihydrite floc readily undergoes transformation leading to predominantly hematite formation, except in the presence of phosphate (concentrations > 10 ppm) where transformation is entirely inhibited due to phosphate adsorption to the floc. These results progress the fundamental understanding of the iron oxyhydroxide formation and ageing processes occurring in EARP.

Anthropogenic activities and natural processes over time have led to the release of toxic heavy metal contaminants into the environment. As a consequence, there is an increasing number of illnesses caused by the exposure of humans to heavy metals and metalloids. The dissertation work presented here focused on the synthesis, characterization, and understanding of the surface chemistry, as well as the photo-reactivity, of a variety of iron (oxyhydr)oxide nano-materials that have relevance for the remediation of heavy metal contaminants, such as arsenic and chromium in aqueous environments. The research focused on the photo-induced reductive dissolution of a nano-dimensioned iron oxyhydroxide, ferrihydrite, in the presence of oxalate, the photo-induced arsenite oxidation, and the simultaneous redox transformation of arsenite and chromate in the presence of ferrihydrite and another environmentally relevant iron oxyhydroxide, goethite. The photo-reductive dissolution of ferrihydrite (using simulated solar radiation) in the presence of oxalic acid was investigated with surface sensitive in situ and ex situ techniques that included attenuated total reflectance Fourier transform infrared spectroscopy. Ferrihydrite at a solution pH of 4.5 exhibited an induction period where the rate of Fe(II) release was limited by a low concentration of adsorbed oxalate due to the site-blocking of carbonate that was intrinsic to the surface of the ferrihydrite starting material. The photo-induced decarboxylation of adsorbed oxalate also ultimately led to the appearance of carbonate reaction product (distinct from carbonate intrinsic to the starting material) on the surface. Ferrihydrite that was prepared under carbonate free condition showed a rapid release of Fe(II) upon irradiation and no induction period was observed. Arsenite [As(III)] oxidation in the presence of ferrihydrite and goethite was also investigated. Ferrihydrite or goethite when exposed to As(III) in the dark led to no change in the oxidation state of As(III) reactant. However, exposure of As(III) in the presence of ferrihydrite or goethite to simulated solar light resulted in the oxidation of As(III) and a reduction of surface Fe(III) leading to an overall increase in the total As removal. At a solution pH of 5, this conversion of As(III) to As(V) on ferrihydrite resulted in the partitioning of a stoichiometric amount of Fe(II) into the aqueous phase and the majority of the As(V) product remained bound to the ferrihydrite surface. In contrast, the As(III)/goethite system showed a different photochemical behavior in the absence or presence of dissolved oxygen. Under oxic conditions, in contrast to ferrihydrite, the majority of the As(V) product was in the aqueous phase and the relative amount of aqueous Fe(II) was significantly less than in the ferrihydrite circumstance. Experimental observations suggested that in the oxic environment, Fe(II) on the goethite surface was heterogeneously oxidized to Fe(III) by dissolved oxygen resulting in the formation of reactive oxygen species that led to the further oxidation of As(III) in solution. Similarly, various experimental investigations were conducted to test the simultaneous removal of As(III) and Cr(VI) from solution. Our results suggested that a surface mediated spontaneous electron transfer between As(III) and Cr(VI) occurred in the presence of Fe- and Al-(oxy)hydroxides. Both infrared and x-ray absorption spectroscopies were conducted to get more insight into the charge transfer reaction and mechanism of electron transfer reaction. In summary, the research discussed here should help to understand the details of oxidation/reduction reactions occurring at mineral-water interfaces. Perhaps more importantly, the methodologies discussed in this dissertation could potentially be novel and eco-friendly approaches for arsenite as well as hexavalent chromium remediation. / Chemistry

On surfaces and within interfaces occur some of the most important reactions in chemistry, from world changing industrial reactions to critical environmental processes. It is even hypothesized that the chiral nature of life arose from reactions occurring on chiral mineral surfaces. In any case adsorption typically plays a key role. Adsorption can occur on rapid time scales, particularly in catalytic systems, and it can be the precursor to highly stable surface interaction mechanisms such as surface precipitation. Surface adsorption can have a dramatic affect on the resulting surface increasing or decreasing the propensity for further reactivity or adsorption. In order to understand the processes occurring on a surface both the surface and the adsorbate must be understood. This includes a surface with any prior adsorbates. This is why many catalytic studies are done in UHV environments where clean surfaces are prepared for each experiment. The same is true with environmental surfaces, but obtaining pristine surfaces can be problematic, and systems are often extremely complicated involving organic, inorganic, and biological components. Often research is focused on just one component. A significant portion of this dissertation is focused on the adsorption of organic and inorganic species on pristine mineral surfaces. While there is significant research done on environmental surfaces, often times the surface used in studies is not well characterized. In essence lesser attention is paid to the substrate then the adsorbate. This is particularly true of infrared studies similar to the type presented in chapter 5 where carbonate is shown to exist in significant quantity on all ferrihydrite surfaces. Furthermore, chapter 4 highlights the potential for ion mobility on calcite surfaces under ambient conditions and the effect the adsorbates in chapter 3 have on the mobility process. The principal of this dissertation is to characterize fundamental surface processes which occur on calcite and ferrihydrite surfaces under ambient conditions. The hope is that this can lay the ground work for future studies where native adsorption and restructuring is taken into account on mineral surfaces during experimental studies. / Chemistry

Les travaux présentés dans ce mémoire sont consacrés à l'étude des processus d'adsorption des anions phosphate par la rouille verte ferrique carbonatée et la ferrihydrite. L'objectif final vise une application au traitement des eaux usées en milieu rural. La synthèse des deux oxydes de fer a été réalisée et leurs propriétés physico-chimiques caractérisées. Un intérêt particulier a été consacré à comparer les propriétés structurales de la rouille verte ferrique en fonction de différents paramètres de synthèse. Une étude approfondie des propriétés physico-chimiques de surface de la ferrihydrite par spectroscopie de photoélectrons X a quant à elle été réalisée. La réactivité de ces deux oxydes a ensuite été abordée en mode discontinu où l'équation cinétique du pseudo-second ordre et le modèle d'isotherme de Freundlich offrent les meilleurs ajustements. L'influence de divers paramètres a été prise en compte comme le pH, la force ionique, etc. Le mode continu a été envisagé sur un matériau de filtration constitué de l'oxyde de fer déposé sur de la pouzzolane. La méthode de fabrication ainsi que les conditions optimales de préparation du matériau de filtration ont été déterminées. Les mécanismes d'adsorption en condition de flux hydrodynamique ont alors mis en évidence des phénomènes advectifs, diffusifs et une régionalisation de l'eau régissant l'adsorption au sein de la colonne. Des informations telles que les capacités d'adsorption ou l'influence du débit sur le processus d'adsorption ont pu être également obtenues. Une expérience préliminaire sur une eau usée prétraitée met finalement en évidence une quantité adsorbée particulièrement intéressante pour une application industrielle potentielle / The Ph.D. work, presented in this manuscript, is devoted to evaluating phosphate adsorption process on carbonate ferric green rust and ferrihydrite. The main objective concerns an application for the treatment of waste water in rural areas. Both iron oxides are first synthesized and their physico-chemical properties characterized. The ferric green rust structural properties differences as a function of synthesis parameters such as aging period and addition of hydrogen peroxide solution is of particular interest. A detailed study of surface physico-chemical properties by X Photoelectron Spectroscopy is carried out in the case of ferrihydrite. The reactivity of these two iron oxides is then evaluated in batch experiments. Adsorption process follows the pseudo-second order kinetic equation and Freundlich isotherm model which give the best adjustments of experimental data. The influence of various parameters such as pH, ionic strength, etc on phosphate adsorption is also taken into account. Column experiments are afterwards carried out by using filtration material constituted of iron oxide deposited onto pozzolana. The optimal conditions to prepare this filtration material are naturally predetermined. Phosphate adsorption in hydrodynamic mode is characterized by advective and diffusive mechanisms and water regionalization which govern the adsorption process in the column. Moreover, phosphate adsorption capacity and flow rate influence on adsorption process are obtained. Finally, a preliminary experiment on a pre-treated waste water finally shows that the filtration material is potentially interesting for an industrial application

La pollution des eaux constitue actuellement une préoccupation majeure aussi bien d’un point de vue sanitaire qu’environnemental. Afin d’y remédier, des procédés de dépollution efficaces, économiques et durables sont constamment développés et l’adsorption reste une méthode largement utilisée dans le traitement des eaux usées. De nombreux travaux sont consacrés à l’adsorption des polluants cationiques, mais le développement de supports adaptés pour interagir avec les contaminants anioniques est moins abordé. L’objectif principal de ce travail de thèse est de mettre en forme deux adsorbants anioniques : l’HDL (hydroxyde double lamellaire) Mg/Al et la ferrihydrite deux-lignes, et les associant à un gel d’alginate pour produire des matériaux composites pouvant être mis en oeuvre dans des réacteurs spécialisés pour l’élimination de contaminants anioniques. Les résultats de l’adsorption en réacteur batch sur les HDL et la ferrihydrite ont montré une bonne efficacité pour l’élimination de certains anions. La ferrihydrite possède une capacité plus importante que les HDL pour l’élimination des anions inorganiques. L’augmentation du rapport Mg/Al dans l’HDL a favorisé l’adsorption du méthyl orange (MO) ; un rapport Mg/Al de 3 (L3) a été séléctionné pour la suite du travail. Deux protocoles ont été évalués pour associer les solides à un gel d’alginate : l’encapsulation et la synthèse interne dans le gel. La nature des cations réticulant et la création de la macroporosité ont été étudiées. L’encapsulation du solide a diminué légèrement la capacité d’adsorption des anions, cependant, la création de la macroporosité permet de compenser cet effet du gel d’alginate. Les études sur l’adsorption du MO avec les billes encapsulées ont montré que l’augmentation de la taille des billes et de la quantité du solide L3 encapsulée a été accompagnée par une diminution de la capacité d’adsorption du MO. Le remplacement du réticulant Ca2+ par Ba2+ a permis de renforcer la structure du gel et améliorer la cinétique d’adsorption du MO, mais a entrainé une diminution de la capacité d’adsorption. La caractérisation des supports préparés par synthèse interne a montré une structure des solides différente des matériaux attendus (L3 ou ferrihydrite) avec par conséquence une réactivité différente. Des mécanismes d’élimination des anions organiques et inorganiques ont été proposés selon les différents adsorbants étudiés. Les billes ferrihydrite/alginate préparées par encapsulation ont présentés globalement une bonne capacité de rétention des anions notamment pour les anions inorganiques (phosphate, chromate et arséniate). Tandis que celles préparées par synthèse interne ne fixent que les arséniates et les phosphates par différents mécanismes essentiellement la précipitation des anions à la surface des oxyde et oxy-hydroxydes de fer. L’adsorption du MO en réacteur filtrant ouvert a montré que l’augmentation du débit d’alimentation a un effet négatif sur l’adsorption alors qu’une concentration initiale plus grande favorise l’adsorption ce qui est en concordance avec les résultats obtenus en réacteur batch. Un taux de régénération de 30% des billes ferrihydrite/alginate macroporeuse (BPENFh-Ba) a été observé en réacteur batch en utilisant une solution de chlorure d’ammonium avec la possibilité de réutiliser l’adsorbant plusieurs fois.Ce travail confirme les performances et la possibilité d’utiliser des matériaux composite d’HDL ou ferrihydrite dans un gel d’alginate pour l’élimination de composés anioniques contenus dans des eaux usées. / Water pollution is currently a major concern from both health and environmental point of view. In order to solve this problem, efficient decontamination processes, economic and sustainable are constantly being developed and adsorption process remains a widely used method in the wastewater treatment. Many studies are devoted to the adsorption of cationic pollutants, but the development of suitable supports able to interact with the anionic contaminants is less discussed. The main objective of this work is to form two anionic adsorbents: the LDH (Layered Double Hydroxide) Mg/Al and two-line ferrihydrite. These adsorbents were associated with an alginate gel in order to produce composite materials that can be implemented in continuous reactors for the removal of anionic contaminants. The adsorption studies in batch reactor on LDH and ferrihydrite confirmed good removal efficiency for some anions. Ferrihydrite showed a higher adsorption capacity than LDH for the removal of inorganic anions. Increasing the Mg/Al ratio was favorable for the methyl orange (MO) adsorption, and an Mg/Al ratio of 3 (L3) was selected for the following work. Two protocols were evaluated for the integration of adsorbent into alginate gel: encapsulation and direct synthesis in the gel structure. The effect of crosslinking cation and the making of macroporosity were studied. The encapsulation of solids decreased slightly the adsorption ability of anions; however, the macroporosity counterbalances this negative effect of alginate gel. The MO adsorption studies with encapsulated beads showed that the increase of the bead size and the amount of solid in the beads were accompanied with a decrease in the adsorption capacity. The replacement of Ca2+ by Ba2+ strengthened the gel structure and improved the adsorption kinetics but decreased the adsorption capacity. The characterization of the support material prepared by direct synthesis in alginate gel showed a different structure from the expected materials (L3 or ferrihydrite), and therefore a different reactivity. The sorption mechanisms of various organic and inorganic anions were proposed for the studied adsorbents. The ferrihydrite/alginate beads prepared by encapsulation showed a good adsorption for all the anions used in this work, especially for inorganic anions (phosphate, chromate and arsenate); while those produced by direct synthesis in alginate gel removed only arsenate and phosphate essentially by a precipitation mechanism of the anions at the surface of the iron oxides and oxy-hydroxides. The sorption of MO in a continuous reactor showed that the increasing of the flow rate had a negative effect on sorption while higher initial concentration presented a favorable effect on the sorption, which is in accordance with results obtained in batch reactor. A desorption ratio of 30% for ferrihydrite/alginate macroporous beads (BPENFh-Ba) was observed in batch reactor with the possibility of reusing the adsorbent.This study confirms the performance and the possibility to use the composite materials of LDH or ferrihydrite in an alginate gel for the removal of anionic compounds contained in wastewater.

This thesis investigated the impact of aluminum (Al) substitution in ferrihydrite (FH) on both its chemical and physical properties. Al was coprecipitated with FH under controlled hydrolysis conditions to yield various % mol substitutions. Sulfate adsorption was measured across pHs to examine any changes in surface reactivity. The samples’ morphology, specific surface areas (SSAs), crystallinity, and phase transformation upon heating were evaluated and parameterized to allow conclusions on the role of Al on the physical properties. Sulfate sorption diminished across pHs for all Al saturation levels. X-ray Diffraction revealed goethite (GT) presence was negatively influenced by Al. The SSAs of samples increased non-linearly with increasing % mol Al, indicating a decreasing particle size with more Al content. Transmission Electron Microscopy micrographs showed the FH nanoparticles transformed to acicular/blocky laths of GT crystals and lenticular/platy hematite (HM) crystals with occasional grainy appearance at both room temperature and upon active heating. The phase transformation alongside the derived aspect ratios (length/width) of the GT crystals were retarded by the Al substitution.

Human exposure to arsenic in groundwater is a global concern, and arsenic mobility in groundwater is often controlled by Fe mineral dissolution and precipitation. Additionally, Fe(II)-catalyzed recrystallization of Fe oxides has been shown to enable trace element release from and incorporation into Fe oxides. However, the effect of As(V) on the Fe(II)-catalyzed recrystallization of Fe oxides such as goethite, magnetite, and ferrihydrite remains unclear. Here, we measured the extent of Fe atom exchange between aqueous Fe(II) and magnetite, goethite, or ferrihydrite in the presence of As(V) by reacting isotopically "normal" Fe oxides with 57Fe-enriched aqueous Fe(II). At lower levels of adsorption (≤13.3 μM), As(V) had little influence on the rate or extent of Fe(II)-catalyzed Fe atom exchange in goethite or magnetite. However, Fe atom exchange was increasingly inhibited as As(V) concentration increased above 100 μM. Additionally, adsorbed As(V) may be incorporated into magnetite over time in the presence and absence of added aqueous Fe(II) as indicated by X-ray absorption spectroscopy (XAS) and chemical extraction data, with more rapid incorporation in the absence of added Fe(II). XAS and chemical extraction data are also consistent with the incorporation of As(V) during goethite and magnetite precipitation. Additionally, atom exchange data indicated that low levels of As(V) coprecipitation (As:Fe = 0.0005-0.0155) had little influence on the rate or extent of Fe(II)-catalyzed Fe atom exchange in goethite or magnetite. Atom exchange data indicated that ferrihydrite likely transforms via a dissolution-reprecipitation mechanism both to lepidocrocite at 0.2 mM Fe(II) and to magnetite at 5 mM Fe(II). The presence of 206 μM As(V) slowed the transformation of ferrihydrite to more crystalline iron minerals and slowed the rate of atom exchange between aqueous Fe(II) and ferrihydrite. However, the degree of atom exchange did not directly correlate with the amount of ferrihydrite transformed. In summary, Fe oxide recrystallization processes may affect As(V) uptake and release in the environment, and As(V) may inhibit Fe(II)-catalyzed Fe oxide recrystallization.