Dissolution Of Goethite Research Articles

Reductive dissolution of goethite and hematite by reduced flavins

The abiotic reductive dissolution of goethite and hematite by the reduced forms of flavin mononucleotide (FMNH2) and riboflavin (RBFH2), electron shuttles secreted by the dissimilatory iron-reducing bacterium Shewanella, was investigated under stringent anaerobic conditions. At pH 7.0, the reductive dissolution rates of goethite were 3.5μmolesm−2h−1 by 50μM FMNH2 and 0.27μmolesm−2h−1 by 50μM RBFH2; the reductive dissolution rates of hematite were 29μmolesm−2h−1 by 50μM FMNH2 and 151μmolesm−2h−1 by 50μM RBFH2. The extent of reaction was limited by the thermodynamic driving force at circumneutral pH. Both the initial reaction rate and reaction extent increased with decreasing pH. On a unit surface area basis, goethite was less reactive than hematite between pH 4.0 and 7.0. AH2DS, the reduced form of the well-studied synthetic electron shuttle anthraquinone-2,6-disulfonate, yielded higher rates than FMNH2 under most reaction conditions. Two additional model compounds, methyl viologen and benzyl viologen, were investigated under similar reaction conditions to explore the relationship between reaction rate and thermodynamic properties. Relevant kinetic data from the literature were also included in the analysis to span a large range in half-cell potentials. Other conditions being equal, the surface area normalized initial reaction rate (ra) increased as the redox potential of the reductant became more negative. A non-linear relationship was observed between logra and the redox potential for eight reductants at pH 7.0. When pH and reductant concentration were fixed, logra was positively correlated to the redox potential of four Fe(III) oxides over a wide pH range, following a non-linear relationship as well.

Mechanisms of goethite dissolution in the presence of desferrioxamine B and Suwannee River fulvic acid at pH 6.5

Siderophores are Fe3+ specific low MW chelating ligands secreted by micro-organisms in response to Fe stress. Low MW organic acids such as oxalate have been shown to enhance siderophore mediated dissolution of Fe3+ oxides. However, the effect of fulvic acid presence on siderophore function remains unknown. We used batch dissolution experiments to investigate Fe release from goethite in the goethite–fulvic acid–desferrioxamine B (goethite–SRFA–DFOB) ternary system. Experiments were conducted at pH 6.5 while varying reagent addition sequence. FTIR and UV–Vis spectroscopy were employed to characterise the Fe–DFOB, Fe–SRFA and DFOB–SRFA complexes. Iron released from goethite in the presence of SRFA alone was below detection limit. In the presence of both SRFA and DFOB, dissolved Fe increased with reaction time, presence of the DFOB–SRFA complex, and where SRFA was introduced prior to DFOB. FTIR data show that in the ternary system, Fe3+ is complexed primarily to oxygen of the DFOB hydroxamate group, whilst the carboxylate CO of SRFA forms an electrostatic association with the terminal NH3+ of DFOB. We propose that SRFA sorbed to goethite lowers the net positive charge of the oxide surface, thus facilitating adsorption of cationic DFOB and subsequent Fe3+ chelation and release. Furthermore, the sorbed SRFA weakens Fe–O bonds at the goethite surface, increasing the population of kinetically labile Fe. This work demonstrates the positive, though indirect role of SRFA in increasing the bioavailability of Fe3+.

Characterizing and quantifying iron oxides in Chinese loess/paleosols: Implications for pedogenesis

To accurately decode the paleoclimatic and paleoenvironmental significance of magnetic properties in Chinese loess/paleosols, neoformation of magnetic iron oxides via pedogenesis as well as the relationship between ferrimagnetic (maghemite and magnetite) and antiferromagnetic minerals (hematite and goethite) of both detrital and pedogenic origins should be well determined. To resolve this problem, magnetic iron oxides from a contiguous loess–paleosol sequence (lower part of the loess unit L1, paleosol unit S1, and the upper part of L2) at Luochuan, Center of the Chinese Loess Plateau, were well characterized and quantified by using an integrated approach including chemical dissolution, diffuse reflectance spectroscopy and rock magnetism methods. Results showed that these magnetic minerals have distinct coercivity and dissolution behavior, which is highly grain size-dependent. More specifically, the paleosol is enriched with nano-sized pedogenic hematites with a medium coercivity of ~130mT, which are also preferentially dissolved in a weak reductive environment. In contrast, the coarse-grained micro-sized detrital hematites with a medium coercivity of ~1T are dominant in loess. In addition, these coarse-grained detrital hematites are more stoichiometric than their pedogenic counterparts as evidenced by the higher band position on the diffuse reflectance spectrum. Goethite can be well distinguished from hematite by its higher coercivity (4–7T) at low temperature (10K). The consistent dissolution behavior of goethite for both loess and paleosol samples in citrate–bicarbonate–dithionite solution indicates that goethite could be less relevant to the pedogenic processes. The reductive dissolution of pedogenic maghemite lags the dissolution of pedogenic hematite and fine-grained goethite, which suggests that average grain size of pedogenic maghemite is relatively larger than those of pedogenic hematite and fine-grained goethite. Detrital magnetite is hard to dissolve and ~50% of its initial remanence survived after dissolution. On the basis of these contrasting behaviors, pedogenic hematite, fine grained goethite and maghemite content was further quantified. Our results suggest that single domain pedogenic hematite was produced after initial formation of pedogenic maghemite. This provides direct evidence for the recently proposed pathway in which ferrihydrite is transformed into a transient maghemite-like phase before its final transformation into hematite.

On modeling the potential impacts of CO2 sequestration on shallow groundwater: Transport of organics and co-injected H2S by supercritical CO2 to shallow aquifers

Proper site selection for CO2 geologic storage requires assessing the impact of potential leakage of CO2 from deep subsurface reservoirs to overlying drinking water aquifers. Although recent studies have largely focused on the mobilization of trace elements in response to the intrusion of CO2 into such aquifers, in this paper we investigate two other leakage issues and potential effects on groundwater quality: the transport of organic compounds by supercritical CO2 from deep storage reservoirs and the upward migration of CO2 with co-injected H2S. Numerical simulations show that organic compounds that may be present at depth, such as benzene, could be mobilized by supercritical CO2 and migrate with the leaking CO2. Modeling results also show that upon the transport of CO2+H2S mixtures through a hypothetical leakage pathway, H2S arrival in the shallower aquifer is delayed in comparison with that of CO2 due to the preferential dissolution of H2S into the aqueous phase. The potentially adverse impacts of leakage on shallow groundwater quality may be exacerbated for cases of leaking CO2+H2S, compared to intrusion of pure CO2, possibly leading to the mobilization of thiophilic elements such as arsenic. Geochemical reactions included in the simulations involve adsorption/desorption, reductive dissolution of goethite, precipitation of pyrite, siderite, and arsenic sulfide phases. The models presented are generic in nature, exploring important processes regarding organic compounds and co-injected H2S, and calling attention to the need for more site-specific studies taking into account the variability and uncertainty of key hydrogeologic and geochemical parameters.

Effect of fulvic acid on neodymium uptake by goethite

Experimental studies of the interaction of aqueous neodymium (Nd), Suwannee River fulvic acid (FA), and solid phase goethite were conducted. Results from blank systems (individual Nd and FA), binary systems (Nd–goethite, FA–goethite, and Nd–FA), and ternary systems (Nd–FA–goethite) at 0.1mol/kg and 25°C are reported.In the binary Nd–goethite system a classic sorption edge is observed, whereby virtually all Nd is removed from solution above the goethite point of zero charge (PZC). Similarly, the binary FA–goethite system exhibits strong FA sorption; However in this system near complete removal of FA from solution is observed below the goethite PZC. In the binary Nd–FA system, both aqueous Nd and FA feature a sharp decrease in concentration at ca. pH 9.Various experiments in the ternary system were conducted. For all concentrations, FA enhanced Nd sorption below the goethite PZC, attributed to the formation of a Type B ternary surface complex (mineral–ligand–metal ion). Notably, the 100ppm FA ternary system showed anomalously high dissolved Nd in solution above the PZC (i.e., Nd sorption suppression) and a concomitant increase in goethite dissolution (∼9ppm total Fe3+ observed above circa pH 9.5).Our results suggest that Nd–FA complexation plays a key role in Nd uptake by goethite, and that this process is largely governed by pH: Whereas at pHs below the goethite PZC, Nd–FA complexation facilitates Nd sorption, above the PZC, and particularly at elevated FA concentrations, the formation of aqueous Nd–FA complexes suppresses Nd removal. Moreover, under these conditions, goethite dissolution may also play a role in mitigating Nd uptake by goethite.

Ferredox: A biohydrometallurgical processing concept for limonitic nickel laterites

The Ferredox process is a biohydrometallurgical concept flow sheet targeting tropical limonitic laterites. The process components are designed to (a) facilitate simplified and low-intensity processing, (b) be amenable to modularization, (c) reduce technical process implementation risk, and (d) reduce capital costs for tropical limonite projects. The Ferredox process has been enabled by the recently demonstrated reductive dissolution of goethite, the most prominent nickel limonite host mineral, using elemental sulphur as a reductant at ambient conditions. The reductive dissolution process results in a ferrous iron-based pregnant leach solution, facilitating the recovery of nickel and cobalt prior to iron removal. Iron removal is achieved by ferrous iron oxidation resulting in iron precipitation as either jarosite or schwertmannite, without neutralizing agent addition. Soluble ferric iron, resulting from ferrous oxidation, is used in a reductive acid generation step for the oxidation of elemental sulphur. The resulting ferrous-based acid solution is redeployed to the reductive leaching step. A distinguishing feature, compared to other hydrometallurgical leaching options for limonite, is that all key process components occur at ambient pressure and temperature conditions with mild acid concentrations (pH 1.2–2). The Ferredox concept is described in the context of a nickel limonite ore but is intended as a generic concept for biohydrometallurgical processing of iron oxide ores containing base metals, including copper.

Effect of Small Organic Acid Anions on the Adsorption of Phosphate Anions onto Synthetic Goethite from Aqueous Solution

Batch experiments were conducted to study the effect of small organic acids (citric, oxalic and acetic) on the removal of phosphate anions from aqueous systems using synthesized goethite as the adsorbent. The results showed that phosphate anion adsorption was most significantly inhibited by citric among the three organic acids studied. Phosphate anion adsorption was affected at pH values below 6.5. The relationship between citrate anion-promoted goethite dissolution and citrate anion-suppressed phosphate anion adsorption in equilibrated goethite-phosphate anion-citric acid systems with different contact orders was studied. The results indicated that there is an obvious correlation between the suppression of phosphate anion adsorption and the dissolution of goethite. It is proposed that competition for binding sites play a key role in the reduction of phosphate anion adsorption by citric acid.

Reductive dissolution of ferric iron minerals: A new approach for bio-processing nickel laterites

Nickel laterites represent the major ore reserves of this base metal present in the lithosphere. However, processing these ores by conventional technologies involves considerable energy or reagent expenditure and consequently is less cost-effective than extracting nickel from sulfide ores. Biological options, using metal-complexing organic acids and mineral acids generated by fungi and bacteria, have been investigated but generally found to be ineffective in terms of extraction dynamics or yields. We have examined the possibility of using bacteria that can bring about the reductive dissolution of ferric iron minerals and thereby facilitate the extraction of nickel from a lateritic ore at relatively low (<30–45 °C) temperatures. Four species of iron-reducing acidophilic bacteria were screened for their abilities to solubilise nickel from a limonitic laterite ore in which the major iron mineral present was goethite. One of these ( Acidithiobacillus ferrooxidans) was selected for further study only the basis of it being able to use a cost-effective energy source (elemental sulfur) to mediate the dissolution of goethite at mildly acidic conditions (pH < 2). Cultures were set up in 2 L bioreactors, maintained at pH 1.8 (±0.1) and 30 °C, and initially aerated (to promote growth of the bacteria on sulfur) and then switched to anaerobic conditions when nickel laterite ore (crushed to <6 mm, with a nickel grade of 0.5%) was added. Over 70% of the nickel present in the ore was solubilised within 14 days, and solubilised metals remained in solution due to the low pH of the leachate. In contrast, only 10% of the nickel was solubilised (by non-reductive acid dissolution) when the cultures were continuously aerated. The results suggest that biological processing of limonitic nickel laterites is technically feasible and, more generically, that reductive dissolution can be used to bioprocess ferric oxide mineral ores.

Evidence for ligand hydrolysis and Fe(III) reduction in the dissolution of goethite by desferrioxamine-B

Desferrioxamine-B (DFOB) is a bacterial trihydroxamate siderophore and probably the most studied to date. However, the manner in which DFOB adsorbs at mineral surfaces and promotes dissolution is still under discussion. Here we investigated the adsorption and dissolution reactions in the goethite–DFOB system using both in situ infrared spectroscopic and quantitative analytical methods. Experiments were carried out at a total DFOB concentration of 1 μmol/m 2, at pH 6, and in the absence of visible light. Our infrared spectroscopic results indicated that the adsorption of DFOB was nearly complete after a 4-h reaction time. In an attempt to determine the coordination mode at the goethite surface, we compared the spectrum of adsorbed DFOB after a 4-h reaction time to the spectra of model aqueous species. However, this approach proved too simplistic in the case of such a complex ligand as DFOB, and we suggest that a more detailed investigation (IR in D 2O, EXAFS of adsorbed model complexes) is needed to elucidate the structure of the adsorbed siderophore. Between a 4-h and 4-day reaction time, we observed the growth of carboxylate stretching bands at 1548 and 1404 cm −1, which are indicators of DFOB hydrolysis. Acetate, a product of DFOB hydrolysis at its terminal hydroxamate group, was quantified by ion chromatography. Its rate of formation was linear and nearly the same as the rate of Fe(III) dissolution. The larger hydrolysis product, a hydroxylamine fragment, was not detected by LC–MS. However, a signal due to the oxidized form of this fragment, a nitroso compound, was found to increase linearly with time, which is an indirect indication for Fe(III) reduction. Based on these findings, we propose that DFOB undergoes metal-enhanced hydrolysis at the mineral surface followed by the reduction of surface Fe(III). While Fe(II) was not detected in solution, this is likely because it remains adsorbed at the goethite surface or becomes buried in the goethite crystal by electron conduction. Taking into account the extent and similarity between the rates of hydrolysis and dissolution, we suggest that a reductive mechanism could play an important part in the dissolution of goethite by DFOB. This possibility has not been considered previously in the absence of light and at circumneutral pH.

Aquifer geochemistry at potential aquifer storage and recovery sites in coastal plain aquifers in the New York city area, USA

Abstract The effects of injecting oxic water from the New York city (NYC) drinking-water supply and distribution system into a nearby anoxic coastal plain aquifer for later recovery during periods of water shortage (aquifer storage and recovery, or ASR) were simulated by a 3-dimensional, reactive-solute transport model. The Cretaceous aquifer system in the NYC area of New York and New Jersey, USA contains pyrite, goethite, locally occurring siderite, lignite, and locally varying amounts of dissolved Fe and salinity. Sediment from cores drilled on Staten Island and western Long Island had high extractable concentrations of Fe, Mn, and acid volatile sulfides (AVS) plus chromium-reducible sulfides (CRS) and low concentrations of As, Pb, Cd, Cr, Cu and U. Similarly, water samples from the Lloyd aquifer (Cretaceous) in western Long Island generally contained high concentrations of Fe and Mn and low concentrations of other trace elements such as As, Pb, Cd, Cr, Cu and U, all of which were below US Environmental Protection Agency (USEPA) and NY maximum contaminant levels (MCLs). In such aquifer settings, ASR operations can be complicated by the oxidative dissolution of pyrite, low pH, and high concentrations of dissolved Fe in extracted water. The simulated injection of buffered, oxic city water into a hypothetical ASR well increased the hydraulic head at the well, displaced the ambient groundwater, and formed a spheroid of injected water with lower concentrations of Fe, Mn and major ions in water surrounding the ASR well, than in ambient water. Both the dissolved O 2 concentrations and the pH of water near the well generally increased in magnitude during the simulated 5-a injection phase. The resultant oxidation of Fe 2+ and attendant precipitation of goethite during injection provided a substrate for sorption of dissolved Fe during the 8-a extraction phase. The baseline scenario with a low (0.001 M) concentration of pyrite in aquifer sediments, indicated that nearly 190% more water with acceptably low concentrations of dissolved Fe could be extracted than was injected. Scenarios with larger amounts of pyrite in aquifer sediments generally resulted in less goethite precipitation, increased acidity, and increased concentrations of dissolved Fe in extracted water. In these pyritic scenarios, the lower amounts of goethite precipitated and the lower pH during the extraction phase resulted in decreased sorption of Fe 2+ and a decreased amount of extractable water with acceptably low concentrations of dissolved Fe (5.4 × 10 −6 M). A linear decrease in recovery efficiency with respect to dissolved Fe concentrations is caused by pyrite dissolution and the associated depletion of dissolved O 2 (DO) and increase in acidity. Simulations with more than 0.0037 M of pyrite, which is the maximum amount dissolved in the baseline scenario, had just over a 50% recovery efficiency. The precipitation of ferric hydroxide minerals (goethite) at the well screen, and a possible associated decrease in specific capacity of the ASR well, was not apparent during the extraction phase of ASR simulations, but the model does not incorporate the microbial effects and biofouling associated with ferric hydroxide precipitation. The host groundwater chemistry in calcite-poor Cretaceous aquifers of the NYC area consists of low alkalinity and moderate to low pH. The dissolution of goethite in scenarios with unbuffered injectate indicates that corrosion of the well could occur if the injectate is not buffered. Simulations with buffered injectate resulted in greater precipitation of goethite, and lower concentrations of dissolved Fe, in the extracted water. Dissolved Fe concentrations in extracted water were highest in simulations of aquifers (1) in which pyrite and siderite in the aquifer were in equilibrium, and (2) in coastal areas affected by saltwater intrusion, where high dissolved-cation concentrations provide a greater exchange of Fe 2+ (FeX 2 ). Results indicate that ASR in pyrite-bearing, Cretaceous aquifers of the NYC area can be a cost effective way to both store drinking water supplies and significantly reduce ambient concentrations of dissolved Fe.

Rapid Dehydroxylation of Nickeliferous Goethite in Lateritic Nickel Ore: X-Ray Diffraction and TEM Investigation

AbstractA method for extracting Ni and other metals from lateritic ores by means of shock heating has been investigated. Shock heating releases some of the metal from its goethitic host. Even though the transformation of pure goethite to hematite is known to occur via intermediate hydroxylated phases, the effect of other metals such as Ni substituting for Fe in goethites on this thermal transformation to hematite is unknown. The purpose of this study was to fill this gap, with the hope that the results will lead to more energy-efficient extraction methods and/or a better understanding of Fe geochemistry in thermally activated soils. X-ray diffraction, transmission electron microscopy with EDS, and thermal analysis were used to investigate mineralogical changes in nickeliferous goethites from five oxide-type lateritic nickel ore deposits that had been subjected to shock heating at temperatures in the range 220–800°C. Acicular, nano-sized goethite was the main constituent of the samples with minor to trace amounts of quartz, talc, kaolinite, chromite, maghemite, and Mn oxides. Goethite was partially dehydroxylated to OH-hematite at 340–400°C and had completely altered to well ordered hematite at 800°C. The OH-hematite was characterized by broad XRD peaks for reflections associated with the Fe sublattice. The goethite unit-cell a and b lengths remained almost constant with increasing preheating temperature up to 300°C, while the size of the c axis dimension contracted. The neoformed hematite crystals were larger than the precursor goethite crystals due to development, by sintering and surface diffusion, of regularly ordered hematite domains. The increase (1.5–2.6 fold) in surface area with increasing heating temperature (up to 340–400°C) reflected the development of slit-shaped micropores (∼300°C), which further developed into elliptically shaped micropores (∼400°C) in OH-hematite. With increased heating temperature, well ordered hematite formed with only a few micropores remaining. Such results may contribute to the development of more efficient procedures for extracting Ni from lateritic nickel ores, as the rate of dissolution of goethite in acid in ‘heap and pressure’ leach facilities will be enhanced by the increases in surface area and microporosity. The results may also provide valuable information on the probable effects of natural heating on pedogenic Fe oxides.

Assessment of the BCR sequential extraction procedure for thallium fractionation using synthetic mineral mixtures

This work focused on the specific behavior of Tl-bearing phases in the BCR (Community Bureau of Reference) sequential extraction (SE) scheme, namely Tl-bearing ferrihydrite, goethite, birnessite, calcite, illite, sphalerite and feldspar in their simple model mixtures with quartz. Several significant discrepancies between the obtained and expected behaviors of these phases in the BCR SE were observed. The amount of Tl released as the exchangeable/acid-extractable fraction (55–82% of the total Tl content) showed a substantial H +-promoted dissolution of all Fe(III) and Mn(III, IV) oxides (corresponding to up to 61% of solid Fe dissolved) and incongruent (increased) extraction of Tl from ferrihydrite and goethite. Reductive conditions of the second SE step were insufficient to complete goethite dissolution with corresponding Tl amount retained in the solid phase. Similarly, insufficient oxidation of sphalerite and lower Tl recovery of the oxidisable fraction was identified. In contrast, the BCR SE seems to produce well predictable results of Tl leaching from Tl-bearing calcite and feldspar. Only 70% of total Tl content was extracted from Tl-modified illite in the exchangeable/acid-extractable step, while 30% was associated with the reducible and residual fractions, i.e., Tl was strongly fixed to the illite matrix.

Effectiveness of iron reducing bacteria for the removal of iron from bauxite ores

Current study investigates the possibility of obtaining the removal of iron from bauxite ores, by exploiting the metabolism of iron reducing microorganisms. The experiments were carried out using the microorganism Desulfuromonas palmitatis. The effectiveness of the treatment was evaluated on six different bauxite samples, containing the alumina oxide in the form of diaspore or boehmite. The Fe 2O 3 content of the samples varied between 16.3% and 22.3% and occurred mainly in the form of goethite or hematite, with one sample containing also chamosite. For comparison reasons the same treatment was also applied on three samples of synthetic Fe(III)-oxides, namely ferrihydrite, goethite and hematite. The removal of iron from the six bauxite samples was found to vary between 7% and 29% and the highest extraction corresponded to the sample containing the Fe(II) rich compound, chamosite. The bioavailability of Fe in the three synthetic oxides was highly dependant on their crystallinity. The amorphous ferrihydrite was almost completely dissolved, up to 95%, whereas dissolution of crystalline goethite and hematite did not exceed 9% and 1.2% respectively.

Effect of Silica Polymerization on the Oxalate-Promoted Dissolution of Goethite

AbstractNumerous studies have investigated the ligand-promoted dissolution of Fe (oxyhydr)oxides. In natural environments, inorganic ligands can compete with organic ligands for surface sites on (oxyhydr)oxides which may influence dissolution rates. Published research of this interaction and its effect on the dissolution of (oxyhydr)oxides is rare. The objective of the present study was to examine the extent to which silica, as a naturally occurring competitive ligand added in the form of silicic acid, impacts the oxalate-promoted dissolution of the common soil Fe (oxyhydr)oxide goethite. Sorbed silica reduced the oxalate-promoted dissolution rate of goethite at all surface coverages investigated. As initial silica solution concentrations increased from 0.50 mM to 5.0 mM, relatively little change in the dissolution rate was observed. Fourier-transform infrared (FTIR) spectra indicated that, as silica-surface coverages increased, the silica underwent polymerization on the goethite surface. Initially, silicate was associated with surface functional groups, but as polymerization occurred some of the silica appeared to desorb from the goethite surface without being released into the bulk solution, suggesting that silica polymers formed discrete islands or surface clusters that grew away from the goethite surface rather than expanding epitaxially across the surface. Minimal changes were observed in the quantity of reactive goethite surface, which is responsible for the observed dissolution rates, as silica-surface coverages increased.

Temperature Dependence of the Oxidation of 2-Chlorophenol by Hydrogen Peroxide in the Presence of Goethite

The oxidation of 2-chlorophenol by hydrogen peroxide in the presence of goethite has been investigated over the 299–323 K temperature range. Kinetic studies suggest that two different reaction stages exist, and that under the experimental conditions used in this work, each may be modeled as a pseudo-first-order reaction. The first stage involves the dissolution of goethite, based on experiments conducted on the effect of prior stirring time. However, comparison of the apparent activation energy for the first stage with those found in the literature indicates the concomitant oxidation of 2-chlorophenol likely takes places from hydroxyl radicals produced from reaction at the goethite surface, rather than those from a Fenton-based mechanism. The apparent activation energy for the second stage is closer to that for a Fenton-based reaction, suggesting that the second stage mainly involves oxidation of 2-chlorophenol by hydroxyl radicals formed from hydrogen peroxide reacting with the dissolved iron species. Taken together, these results provide further insight towards a molecular-level understanding of the reactivity of chlorinated phenols in the presence of minerals during remediation processes.

Interactions of EDDS with Fe- and Al-(hydr)oxides

The efficiency of EDDS ([S,S]-ethylenediaminedisuccinate) in metal (phyto) extraction has been discussed in many recent papers. This study demonstrated that the presence of Fe- and Al-(hydr)oxides in soils influences the speciation of EDDS and thus can decrease the extraction of the targeted metallic contaminants (e.g., Pb, Cu, Zn). Above all, amorphous and poorly crystalline oxides (e.g., ferrihydrite) seem to significantly control dissolved Fe and Al concentrations in soils in the presence of metal–EDDS complexes and especially uncomplexed EDDS. Metals released from these minerals compete for the chelating agent and the extraction efficiency of the targeted metals is lowered. The formation of stable Cu–EDDS complexes, which are preferentially formed in soils with high Cu concentrations, results into a lower dissolution of ferrihydrite and goethite compared to free EDDS and Al–EDDS. Information about the contents of amorphous and poorly crystalline oxides in the treated soils would thus be beneficial for choosing efficient EDDS dosages.

Adsorption Mechanisms of EDTA at the Water−Iron Oxide Interface: Implications for Dissolution

The interactions between chelating agents and metal oxide particles play important roles for the distribution and availability of metal ions in aquatic environments. In this work, the adsorption of ethylenediaminetetraacetate (EDTA) onto goethite (α-FeOOH) was studied as a function of pH, time, and background electrolyte concentration at 25.0 °C, and the molecular structures of the surface complexes formed were analyzed by means of infrared spectroscopy using the attenuated total reflectance sampling technique. The collective infrared spectroscopic results of this study show that two surface complexes consisting of HEDTA3− and H2EDTA2− predominate at the water−goethite interface within the pH range of 3−9. No direct interactions of these complexes with surface Fe(III) ions were detected; hence, most likely the surface complexes are stabilized at the interface by electrostatic and hydrogen-bonding forces. The formation of the EDTA surface complexes is fast (time scale of minutes), but a slower (time scale of hours to days) dissolution reaction also occurs. The dissolved iron in solution is in the form of the highly stable FeEDTA− solution complex, and the experimental evidence presented indicates that this complex can readsorb to the mineral surface. As dissolution proceeds, the concentration of FeEDTA− in the solution phase increases, and this in turn leads to a buildup of readsorbed FeEDTA− onto goethite. In the pH range of 4−7, this dissolution and readsorption process increases the total EDTA concentration at the surface. Under the experimental conditions in the present study, it is primarily the presence of uncomplexed EDTA in solution that drives the dissolution of goethite resulting in the subsequent readsorption of FeEDTA−, while the HEDTA3− and H2EDTA2− surface complexes are stable during this process.

Surface Chemistry and Dissolution of α-FeOOH Nanorods and Microrods: Environmental Implications of Size-Dependent Interactions with Oxalate

Although recent evidence suggests that particle size plays an important role in the dissolution of iron from mineral dust aerosol, a fundamental understanding of how particle size influences the rate and extent of iron oxide dissolution processes remains unclear. In this study, surface spectroscopic methods are combined with solution phase measurements to explore ligand-promoted dissolution and photochemical reductive dissolution of goethite (α-FeOOH) of different particle sizes in the presence of oxalate at pH 3 and 298 K. Both X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed differences between α-FeOOH particles in the nanometer-size range as compared to α-FeOOH particles in the micrometer-size range (nanorods and microrods, respectively). ATR-FTIR spectra showed a significant presence of surface hydroxyl groups as well as differences in surface complexes formed on nanorod surfaces. Furthermore, the saturation coverage of oxala...

Mechanism of arsenate mobilization from goethite by aliphatic carboxylic acid

Arsenic is highly toxic and therefore represents a potential threat to the environment and human health. The mobility and fixation of arsenic in natural environment is significantly affected by co-occurring dissolved natural organic acids, which are widely present in soils and sediments due to the decomposition of natural organic matters and the metabolism of plant roots and microorganisms. It was reported recently that at lower pH, citrate decreased arsenate adsorption on metal oxides as the result of competitive adsorption. This study examined the relationship between citrate-promoted goethite dissolution and citrate-suppressed arsenate adsorption in the equilibrated goethite-arsenate-citrate systems with different contact orders. The results indicated that there is obvious correlation between the suppression of arsenate adsorption and dissolution of goethite. Arsenate adsorption and goethite dissolution in the presence of citrate (1.0mM), oxalate (1.5mM) and acetate (3.0mM) were compared. The results showed that arsenate adsorption was most significantly inhibited by citrate among the three organic acids. Although oxalate dissolved goethite more significantly, it inhibited arsenate adsorption much less than citrate. Hence it is proposed that the competition for binding sites rather than citrate-promoted goethite dissolution played a key role in the reduction of arsenate adsorption by citrate at lower pH.

In-situ synchrotron diffraction study of the hydrothermal transformation of goethite to hematite in sodium aluminate solutions

The hydrothermal crystallization of hematite (α-Fe 2O 3) from goethite (α-FeOOH), in sodium aluminate solutions has been studied for the first time using in-situ synchrotron time-resolved energy dispersive diffraction. This technique has allowed the transformation to be followed in real time and under the high temperature and pressure conditions (200–250 °C, 42 bar) required to allow (or induce) crystallization. To promote crystallization from goethite, hematite seed was added to accelerate the otherwise slow kinetics of crystallization. The reaction proceeds via goethite dissolution and subsequent hematite growth and the kinetic data was fitted using Avrami–Erofe'ev expression and Sharp–Hancock method. The rate of transformation of goethite to hematite increases at higher caustic concentrations, hematite surface area, and temperature. Crystallization is inhibited by the presence of anatase (TiO 2).