Comparison study on transformation of iron oxyhydroxides: Based on theoretical and experimental data

We have investigated the catalytic transformation of ferrihydrite, feroxyhyte, and lepidocrocite in the presence of Fe(II). In this paper, the transformation from akaganeite and goethite to hematite in the presence of trace Fe(II) was studied in detail. The result indicates that trace Fe(II) can accelerate the transformation of akaganeite and goethite. Compared with the transformation of other iron oxyhydroxides (e.g., ferrihydrite, feroxyhyte, lepidocrocite, and akaganeite), a complete transformation from goethite to hematite was not observed in the presence of Fe(II). On the basis of our earlier and present experimental results, the transformation of various iron oxyhydroxides was compared based on their thermodynamic stability, crystalline structure, transformation mechanism, and transformation time.     

Sorption of two aromatic acids onto iron oxides: experimental study and modeling

The transport of aromatic carboxylate compounds in the environment can be strongly influenced by adsorption onto certain minerals, such as iron oxides and hydroxides, found in ground water and soils. Batch experiments with five iron oxides were conducted to quantify the contributions to adsorption from different iron mineral surfaces and compare adsorption characteristics of selected organic acids (gentisic acid (GA) and 1-hydroxy-2-naphthoic acid (HNA)). Because of their widespread abundance in soils and sediments, goethite, lepidocrocite, ferrihydrite, hematite, and magnetite were investigated. Sorption of two organic acids onto iron oxides was examined over a wide range of conditions (pH, ionic strength, and sorbate concentration). Specific surface area and mineral surface charge proved be important for the adsorption of these compounds. The sorption isotherm was described well by the Tempkin equation for both organic acids, with the adsorption constant higher for HNA than GA. For modeling the sorption edges of ferrihydrite and hematite, surface reactions involving the formation of mononuclear (1:1) surface species were proposed. These results indicate that the generalized two-layer model, with the assumption of homogeneous surface sites, could predict sorption on iron oxides over a range of pH conditions. The results of this study suggest that the mineralogy of the iron oxides and the pH value should be considered when predicting sorption of aromatic acids onto iron oxides and their fate in the soil and the environment.     

Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces

Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.     

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 0.1 mol/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 100 ppm 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 (∼9 ppm 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.     

Copper and zinc removal from aqueous solution by mixed mineral systems II. The role of solution composition and aging

This study investigates Cu and Zn removal onto binary mixed mineral sorbents from simulated wastewater, relevant to streams impacted by acid mine drainage and effluents. Mixed suspensions of kaolinite/montmorillonite and kaolinite/goethite exhibited different sorption behavior from the single mineral components, reducing Cu and Zn removal (except Cu sorbed on montmorillonite/goethite) over the range of pH investigated. Cu and Zn removal by the electrolyzed systems showed a complex response to increased ionic strength, which increased solid concentration, leading to lower Cu and Zn sorption. Enhanced Cu sorption on the montmorillonite/goethite as age increased may be attributed to increased hydroxylation of the mineral surface resulting in the formation of new reactive sites.     

Fe(II)-induced transformation from ferrihydrite to lepidocrocite and goethite

The transformation of Fe(II)-adsorbed ferrihydrite was studied. Data tracking the formation of products as a function of pH, temperature and time is presented. The results indicate that trace of Fe(II) adsorbed on ferrihydrite can accelerate its transformation obviously. The products are lepidocrocite and/or goethite and/or hematite, which is different from those without Fe(II). That is, Fe(II) not only accelerates the transformation of ferrihydrite but also leads to the formation of lepidocrocite by a new path. The behavior of Fe(II) is shown in two aspects—catalytic dissolution-reprecipitation and catalytic solid-state transformation. The results indicate that a high temperature and a high pH(in the range from 5 to 9) are favorable to solid-state transformation and the formation of hematite, while a low temperature and a low pH are favorable to dissolution-reprecipitation mechanism and the formation of lepidocrocite. Special attentions were given to the formation mechanism of lepidocrocite and goethite.     

Study of mineralogical speciation of arsenic in soils using X ray microfluorescence and scanning electronic microscopy

In this paper we studied the As content in natural contaminated soils, classified as Dystric Leptosol, Chromic Luvisol, Eutric Cambisol and Mollic Leptosol. In soil samples, sieved (<2 mm), total As was determined by XRF and chemical speciation by sequential extraction. As-bearing minerals were concentrated from fine sand fraction of soil (200-20 μm) using heavy liquid. In this fraction, mineralogical speciation was studied by X-ray microfluorescence, XRD with Göbbel mirror and SEM-BEI-EDX. Total As contents ranging from 61.00 to 131.00 mg kg⁻¹. The results of the sequential extraction showed that As was, mainly, in the residual fraction (52.51-98.76 mg kg⁻¹) and in the fraction bound to iron oxyhydroxides (0-36.5 mg kg⁻¹). Mapping of As with X-ray microfluorescence show strongly relationship between Fe and As. Iron (III) oxyhydroxides (FeOHs) (lepidocrocite and goethite), scorodite, angelellite, schultenite and dussertite were identified by XRD analysis as most likely mineral phases. The contents of As, Fe, Pb and Ba obtained with EDX-microprobe, confirmed the results of XRD. The results of sequential extraction and X-ray microfluorescence indicate that As is strongly bound to the soils because the identified As-bearing mineral phases are very stable at the pH conditions of studied soils. Consequently, a low mobility of As can be assumed in these soils.     

Adsorption of UO2 2+ on natural composite materials

The prediction of the adsorption behavior of natural composite materials was studied by a single mineral approach. The adsorption of U(VI) on single minerals such as goethite, hematite, kaolinite and quartz was fully modeled using the diffuse-layer model in various experimental conditions. A quasi-thermodynamic database of surface complexation constants for single minerals was established in a consistent manner. In a preliminary work, the adsorption of a synthetic mixture of goethite and kaolinite was simulated using the model established for a single mineral system. The competitive adsorption of U(VI) between goethite and kaolinite can be well explained by the model. The adsorption behavior of natural composite materials taken from the Koongarra uranium deposit (Australia) was predicted in a similar manner. In comparison with the synthetic mixture, the prediction was less successful in the acidic pH range. However, the model predicted well the adsorption behavior in the neutral to alkaline pH range. Furthermore, the model reasonably explained the role of iron oxide minerals in the adsorption of U(VI) on natural composite materials.     

Interferences in uranium oxidation states during dissolution of solid phases

The effects of iron on uranium oxidation states during sample dissolution were studied. A mineral acid mixture in anaerobic conditions was used for the dissolution the sample and the uranium oxidation states were determined by ion exchange. The first experiments were performed with pure iron chloride compounds. In the second stage, study was made of common iron-containing minerals. Uranium oxidation states were affected when the content of iron compound was as low as 10^-5M. In the case of the natural minerals, pyrite caused uranium to change to an increasingly reduced state, whereas goethite caused it to change to an increasingly oxidized state as the amount of mineral was increased. The interferences of the silicates fell between those of pyrite and goethite. The results indicate that a wide range of common bulk rocks with less than 20 wt% of iron-containing minerals can be reliable chemically analyzed for uranium oxidation state.     

Arsenic adsorption onto hematite and goethite

Surface complexation reactions on mineral affect the fate and the transport of arsenic in environmental systems and the global cycle of this element. In this work, the sorption of As(V) on two commercial iron oxides (hematite and goethite) was studied as a function of different physico-chemical parameters such as pH and ionic strength. The main trend observed in the variation of the arsenic sorbed with the pH is a strong retention in acidic pH and the decrease of the sorption on both sorbents at alkaline pH values. The sorption experiments for these iron oxides show that there is no effect of the ionic strength on arsenate adsorption suggesting the formation of an inner sphere surface complex. At pH values corresponding to natural pH water, both hematite and goethite are able to adsorb more than 80% of arsenic, whatever the initial concentration may be. The iron oxides used in this work should be suitable candidates as sorbents for As(V) removal technologies.     

Sorption of uranium on magnetite nanoparticles

Magnetite (Fe₃O₄) nanoparticle was synthesized using a solid state mechanochemical method and used for studying the sorption of uranium(VI) from aqueous solution onto the nanomaterial. The synthesized product is characterized using SEM, XRD and XPS. The particles were found to be largely agglomerated. XPS analysis showed that Fe(II)/Fe(III) ratio of the product is 0.58. Sorption of uranium on the synthesized nanomaterials was studied as a function of various operational parameters such as pH, initial metal ion concentration, ionic strength and contact time. pH studies showed that uranium sorption on magnetite is maximum in neutral solution. Uranium sorption onto magnetite showed two step kinetics, an initial fast sorption completing in 4–6 h followed by a slow uptake extending to several days. XPS analysis of the nanoparticle after sorption of uranium showed presence of the reduced species U(IV) on the nanoparticle surface. Fe(II)/Fe(III) ratio of the nanoparticle after uranium sorption was found to be 0.48, lower than the initial value indicating that some of the ferrous ion might be oxidized in the presence of uranium(VI). Uranium sorption studies were also conducted with effluent from ammonium diuranate precipitation process having a uranium concentration of about 4 ppm. 42% removal was observed during 6 h of equilibration.     

Sorption of uranium(VI) from aqueous solution onto magnesium silicate hollow spheres

The sorption of uranium(VI) from aqueous solutions was investigated using synthesized magnesium silicate hollow spheres as a novel adsorbent. Batch experiments were conducted to study the effects of initial pH, amount of adsorbent, contact time and initial U(VI) concentrations on uranium sorption efficiency. The desorbing of U(VI) and the effect of coexisting ions were also investigated. Kinetic studies showed that the sorption followed a pseudo-second-order kinetic model. The Langmuir sorption isotherm model correlates well with the uranium sorption equilibrium data for the concentration range of 25–400 mg/L. The maximum uranium sorption capacity onto magnesium silicate hollow spheres was estimated to be about 107 mg/g under the experimental conditions. Desorption of uranium was achieved using inorganic acid as the desorbing agent. The practical utility of magnesium silicate hollow spheres for U(VI) uptake was investigated with high salt concentration of intercrystalline brine. This work suggests that magnesium silicate hollow spheres can be used as a highly efficient adsorbent for removal of uranium from aqueous solutions.     

Three Mechanisms in One Material: Uranium Capture by a Polyoxometalate–Organic Framework through Combined Complexation,Chemical Reduction,and Photocatalytic Reduction

The design and synthesis of uranium sorbent materials with high uptake efficiency, capacity and selectivity, as well as excellent hydrolytic stability and radiation resistance remains a challenge. Herein, a polyoxometalate (POM)–organic framework material ( SCU‐19 ) with a rare inclined polycatenation structure was designed, synthesized through a solvothermal method, and tested for uranium separation. Under dark conditions, SCU‐19 can efficiently capture uranium through ligand complexation using its exposed oxo atoms and partial chemical reduction from U^VI to U^IV by the low‐valent Mo atoms in the POM. An additional U^VI photocatalytic reduction mechanism can occur under visible light irradiation, leading to a higher uranium removal without saturation and faster sorption kinetics. SCU‐19 is the only uranium sorbent material with three distinct sorption mechanisms, as further demonstrated by X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near edge structure (XANES) analysis.     

Three Mechanisms in One Material: Uranium Capture by a Polyoxometalate–Organic Framework through Combined Complexation,Chemical Reduction,and Photocatalytic Reduction

The design and synthesis of uranium sorbent materials with high uptake efficiency, capacity and selectivity, as well as excellent hydrolytic stability and radiation resistance remains a challenge. Herein, a polyoxometalate (POM)–organic framework material ( SCU‐19 ) with a rare inclined polycatenation structure was designed, synthesized through a solvothermal method, and tested for uranium separation. Under dark conditions, SCU‐19 can efficiently capture uranium through ligand complexation using its exposed oxo atoms and partial chemical reduction from U^VI to U^IV by the low‐valent Mo atoms in the POM. An additional U^VI photocatalytic reduction mechanism can occur under visible light irradiation, leading to a higher uranium removal without saturation and faster sorption kinetics. SCU‐19 is the only uranium sorbent material with three distinct sorption mechanisms, as further demonstrated by X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near edge structure (XANES) analysis.     

Sorption of Cypermethrin Diastereoisomers to Quartz,Corundum, Goethite,Kaolinite and Montmorillonite

Differential sorption and degradation of different pesticide stereoisomers in soil may result in accumulation of the most strongly sorbed and the slowest degradable isomers. In this work the pyrethroid cypermethrin (8 isomers) has been used for test of stereochemical interactions with surfaces of the minerals quartz, corundum, goethite, kaolinite and montmorillonite. The sorption of three diastereoisomeric fractions denoted Cis A, Trans C and Cis B + Trans D were quantified by use of GC-ECD in batch experiments with initial cypermethrin concentrations in the range 1-9 µg/L. Correction for cypermethrin sorbed to surfaces of the shaking flasks were accomplished to obtain net sorption isotherms for the minerals, all of which were well fitted by the Freundlich equation. Bonding affinities per unit surface area decreased in the order: corundum > quartz > kaolinite > montmorillonite > goethite. The isotherms for sorption of all diastereoisomeric fractions to quartz, corundum and goethite were all linear, whereas non-linear isotherms were found for sorption of Cis A and Trans C fractions to kaolinite and montmorillonite. Corundum, quartz and goethite showed a significantly stronger sorption of Cis A than the other fractions, while kaolinite sorbed Cis B + Trans D most strongly. The observed differences predict less leaching and slower degradation of the Cis A fraction in subsoils low in organic carbon.     

Remove mechanisms of sulfamethazine by goethite: the contributions of pH and ionic strength

Sulfamethazine (SMT) as an ionic compound can enter the soil environment via the application of livestock wastes in agricultural fields and via the abuse of pharmaceuticals. Goethite as one of the most important iron oxides in soil might interact with SMT and influence its environmental behavior and bioavailability in soil. In this study, the sorption properties of SMT on goethite under different solution conditions were investigated. The sorption isotherm exhibited a significant nonlinear shape and desorption hysteresis, while the data could be fitted well by the Freundlich model with the correlation coefficient R ² = 0.991. Sorption capacity initially increased and then decreased as pH values increased from 3 to 13, while the strongest uptake occurred in neutral conditions. The sorption increased slightly and then kept relatively constant as the ionic strength of the solution increased. The results indicated that the sorption mechanism would be altered in different solution conditions. In acidic and neutral conditions, the π–π electron donor–acceptor interactions and outer- and inner-sphere complexions might be the dominating sorption mechanisms. The sorption in alkali conditions might be dominated by electrostatic interactions between SMT and goethite. It should be noted that the heterogeneous sorption affinity of SMT on goethite under various solution conditions will influence its environmental fate.     

The application of X-ray and thermoanalytical techniques to the mineralogical analysis of an interbasaltic horizon

The use of XRD as a diagnostic technique for deeply weathered samples (8) suggested the occurrence of gibbsite and geothite, the presence of cristobalite and lepidocrocite was not recognized and the presence of chlorite was highly questionable. The XRD traces in this study give sparse observations and background interference from the high iron concentration of the samples let to difficulties in interpretation of the minerals.The use of thermoanalytical techniques produced the mineral assembly: gibbsite, lepidocrocite, goethite, kaolinite, cristobalite, and possibly chlorite. It has brought to light the factors such as pretreatment, texture of the sample, sample:diluent ratio and heating rate, must be carefully considered when mineralogical identification of these type of deeply weathered samples is undertaken by the methods studied.     

Sorption study of uranium on carbon spheres hydrothermal synthesized with glucose from aqueous solution

The ability of oxygen-rich carbon spheres (CSs) produced by hydrothermal carbonization with the glucose has been explored for the removal and recovery of uranium from aqueous solutions. The micro-morphology and structure of CSs were characterized by FT-IR and SEM. The influences of different experimental parameters such as solution pH, initial concentration, contact time, ionic strength and temperature on adsorption were investigated. The CSs showed the highest uranium sorption capacity at initial pH of 6.0 and contact time of 25 min. Adsorption kinetics was better described by the pseudo-second-order model and adsorption process could be well defined by the Langmuir isotherm. The thermodynamic parameters, △G°(298 K), △H° and △S° were determined to be ?16.88, 12.09 kJ mol^?1 and 197.87 J mol^?1 K^?1, respectively, which demonstrated the sorption process of CSs towards U(VI) was feasible, spontaneous and endothermic in nature. The adsorbed CSs could be effectively regenerated by 0.05 mol/L HCl solution for the removal and recovery of U(VI). Complete removal (99.9 %) of U(VI) from 1.0 L industry wastewater containing 15.0 mg U(VI) ions was possible with 3.0 g CSs.     

Adsorption of uranium from aqueous solution using biochar produced by hydrothermal carbonization

The ability of biochar produced by hydrothermal carbonization (HTC) has been explored for the removal and recovery of uranium from aqueous solutions. The micro-morphology and structure of HTC were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. The influences of different experimental parameters such as solution pH, initial concentration, contact time, ionic strength and temperature on adsorption were investigated. The HTC showed the highest uranium sorption capacity at initial pH of 6.0 and contact time of 50 min. Adsorption kinetics was better described by the pseudo-second-order model and adsorption process could be well defined by the Langmuir isotherm. The thermodynamic parameters, △G°(298 K), △H° and △S° were determined to be ?14.4, 36.1 kJ mol^?1 and 169.7 J mol^?1 K^?1, respectively, which demonstrated the sorption process of HTC towards U(VI) was feasible, spontaneous and endothermic in nature. The adsorbed HTC could be effectively regenerated by 0.05 mol/L HCl solution for the removal and recovery of U(VI). Complete removal (99.9 %) of U(VI) from 1.0 L industry wastewater containing 15.0 mg U(VI) ions was possible with 2.0 g HTC.