Sorption of U(VI) on goethite: Effects of pH, ionic strength, phosphate, carbonate and fulvic acid

Efficient removal of U(VI) from aqueous solution using poly(amidoxime-hydroxamic acid) functionalized graphene oxide

The efficient development of selective materials for uranium recovery from wastewater and seawater is crucial for the utilization of uranium resources and environmental protection. The potential of graphene oxide (GO) as an effective adsorbent for the removal of environmental contaminants has been extensively investigated. Further modification of the functional groups on the basal surface of GO can significantly enhance its adsorption performance. In this study, a novel poly(amidoxime-hydroxamic acid) functionalized graphene oxide (pAHA-GO) was synthesized via free radical polymerization followed by an oximation reaction, aiming to enhance its adsorption efficiency for U(VI). A variety of characterization techniques, including SEM, Raman spectroscopy, FT-IR, and XPS, were employed to demonstrate the successful decoration of amidoxime and hydroxamic acid functional groups onto GO. Meanwhile, the adsorption of U(VI) on pAHA-GO was studied as a function of contact time, adsorbent dosage, pH, ionic strength, initial U(VI) concentration, and interfering ions by batch-type experiments. The results indicated that the pAHA-GO exhibited excellent reuse capability, high stability, and anti-interference ability. Specially, the U(VI) adsorption reactions were consistent with pseudo-second-order and Langmuir isothermal adsorption models. The maximum U(VI) adsorption capacity was evaluated to be 178.7 mg/g at pH 3.6, displaying a higher U(VI) removal efficiency compared with other GO-based adsorbents in similar conditions. Regeneration of pAHA-GO did not significantly influence the adsorption towards U(VI) for up to four sequential cycles. In addition, pAHA-GO demonstrated good adsorption capacity stability when it was immersed in HNO3 solution at different concentrations (0.1-1.0 mol/L) for 72 h. pAHA-GO was also found to have anti-interference ability for U(VI) adsorption in seawater with high salt content at near-neutral pH condition. In simulated seawater, the adsorption efficiency was above 94% for U(VI) across various initial concentrations. The comprehensive characterization results demonstrated the involvement of oxygen- and nitrogen-containing functional groups in pAHA-GO in the adsorption process of U(VI). Overall, these findings demonstrate the feasibility of the pAHA-GO composite used for the capture of U(VI) from aqueous solutions.

Uranium recovery from weakly acidic wastewater using recyclable γ-Fe2O3@meso-SiO2

U(VI)-containing acidic wastewater produced from uranium mining sites is an environmental hazard. Highly efficient capture of U(VI) from such wastewater is of great significance. In this study, a mesoporous core-shell material (i.e. γ-Fe2O3@meso-SiO2) with magnetically and vertically oriented channels was rationally designed through a surfactant-templating method. Batch experiment results showed that the material had an efficiency level of >99.7% in removing U(VI) and a saturated adsorption capacity of approximately 41.40 mg/g, with its adsorption reaching equilibrium in 15 min. The U(VI) adsorption efficiency of the material remained above 90% in a solution with competing ions and in acidic radioactive wastewater, indicating its ability to selectively adsorb U(VI). The material exhibited high adsorption efficiency and desorption efficiency in five cycles of desorption and regeneration experiments. According to the results, the mechanism through which γ-Fe2O3@meso-SiO2 adsorbs U(VI) was dominated by chemical complexation and electrostatic attraction between these two substances. Therefore, γ-Fe2O3@meso-SiO2 is not only beneficial to control the environmental migration of uranium, but also has good selective adsorption and repeated regeneration performance when used to recover U(VI) from weakly acidic wastewater in uranium mining.

Investigation of Potential Drivers of Elevated Uranium Prevalence in Indian Groundwaters with a Unified Speciation Model

Elevated uranium (U) (>WHO limit of 30 μg L^-1) in Indian groundwaters is primarily considered geogenic, but the specific mineralogical sources and mechanisms for U mobilization are poorly understood. In this contribution, statistical and geochemical analyses of well-constrained metadata of Indian groundwater quality (n = 342 of 8543) were performed to identify key parameters and processes that influence U concentrations. For geochemical predictions, a unified speciation model was developed from a carefully compiled and updated thermodynamic database of inorganic, organic (Stockholm Humic model), and surface complexation reactions and associated constants. Critical U contamination was found at shallow depths (<100 m) within the Indo-Gangetic plain, as determined by bivariate nonparametric Kendall's Tau_b and probability-based association tests. Analysis of aquifer redox states, multivariate hierarchical clusters, and principal components indicated that U contamination was predominant not just in oxic but mixed (oxic-anoxic) aquifers under high Fe, Mn, and SO₄ concentrations, presumably due to U release from dissolution of Fe/Mn oxides or Fe sulfides and silicate weathering. Most groundwaters were undersaturated with respect to relevant U-bearing solids despite being supersaturated with respect to atmospheric CO₂ (average pCO₂ of reported dissolved inorganic carbonate (DIC) data = 10^-1.57 atm). Yet, dissolved U did not appear to be mass limited, as predicted solubilities from reported sediment concentrations of U were ∼3 orders of magnitude higher. Integration of surface complexation models of U on typical aquifer adsorbents, ferrihydrite, goethite, and manganese dioxide, was necessary to explain dissolved U concentrations. Uranium contamination probabilities with increasing dissolved Ca and Mn exhibited minima at equilibrium solubilities of calcite [∼50 mg L^-1] and rhodochrosite [∼0.14 mg L^-1], respectively, at an average groundwater pH of ∼7.5. A potential indirect control of such U-free carbonate solids on U mobilization was suggested. For locations (n = 37) where dissolved organic carbon was also reported, organic complexes of U contributed negligibly to dominant U speciation at the groundwater pH. Overall, the unified model suggested competitive dissolution-precipitation and adsorption-desorption controls on U speciation. The model provides a quantitative framework that can be extended to understand dominant mobilization mechanisms of geogenic U in aquifers worldwide after suitable modifications to the relevant aquifer parameters.

Sulfidation and Reoxidation of U(VI)-Incorporated Goethite: Implications for U Retention during Sub-Surface Redox Cycling

Over 60 years of nuclear activity have resulted in a global legacy of contaminated land and radioactive waste. Uranium (U) is a significant component of this legacy and is present in radioactive wastes and at many contaminated sites. U-incorporated iron (oxyhydr)oxides may provide a long-term barrier to U migration in the environment. However, reductive dissolution of iron (oxyhydr)oxides can occur on reaction with aqueous sulfide (sulfidation), a common environmental species, due to the microbial reduction of sulfate. In this work, U(VI)-goethite was initially reacted with aqueous sulfide, followed by a reoxidation reaction, to further understand the long-term fate of U species under fluctuating environmental conditions. Over the first day of sulfidation, a transient release of aqueous U was observed, likely due to intermediate uranyl(VI)-persulfide species. Despite this, overall U was retained in the solid phase, with the formation of nanocrystalline U(IV)O₂ in the sulfidized system along with a persistent U(V) component. On reoxidation, U was associated with an iron (oxyhydr)oxide phase either as an adsorbed uranyl (approximately 65%) or an incorporated U (35%) species. These findings support the overarching concept of iron (oxyhydr)oxides acting as a barrier to U migration in the environment, even under fluctuating redox conditions.

Elucidation of the sorbent role in sorption thermodynamics of uranium(VI) on goethite

The sorption process of radionuclides, often conducted at ambient temperature, shows significant sensitivity to the surrounding temperature. Prediction of fate and transport in the environment, therefore, requires accurate thermodynamic data of their species defining sorption-desorption onto solid surfaces. Herein, we examined the thermodynamics of uranium(VI), U(VI), sorption onto goethite with particular emphasis on directly calculating the enthalpy of U(VI) surface species formed under slightly acidic pH conditions. To achieve this aim, a sorption study of U(VI) was carried out on goethite in the pH range 3-10 and modelled using a 2-pK single-site diffuse layer surface complexation model. A binuclear bidentate species of U(VI), (FeO)₂UO₂, reproduces the sorption profile at pH 3-5 while the sorption was under-estimated in the pH >5 region. Precipitation of schoeptite at pH 5-8 was attributed to the underestimation of the predicted sorption behaviour. The species complexation constant was employed in the analysis of heat consumed, measured using an isothermal titration calorimeter, in the titration of the goethite suspension with U(VI) at pH 4.5 ± 0.1. Enthalpy for the U(VI) species was found to be 41 ± 7 kJ mol^-1, suggesting that sorption is an entropically driven process. Comparing thermodynamic data with that of similar U(VI)-iron oxide systems, binding energy of U(VI) surface species, surface hydration and hydrogen binding are suggested as main factors in the sorbent role towards the thermodynamics of the sorption process.

Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Adsorption of U(VI) on the natural soil around a very low-level waste repository

The behaviors of U(VI) in environmental media around radioactive waste disposal site are important for safety assessment of geological repositories. However, the estimation of environmental behaviors of U(VI) in natural media was insufficient. This work aimed to determine the adsorption of U(VI) on natural soil surrounding a candidate very low-level radioactive waste (VLLW) disposal site in southwest China. Results showed that the adsorption process of U(VI) on soils could be well supported by pseudo-second-order kinetic and Freundlich model. The adsorption of U(VI) was pH-dependent but temperature-independent. High ionic strength (NaCl) strongly affected the adsorption process at low pH (2.0-5.5). CO₃^2- remarkably inhibited the U(VI) adsorption, while the adsorption of U(VI) was promoted by PO₄^3- and SO₄^2-. Naturally occurred soil organic matters (SOMs) showed high affinity for U(VI), while the presence of additional humic acid (HA) strongly inhibited U(VI) adsorption. The occurrence of ferrous iron could result in the reduction of U(VI) at low pH values (pH < 4), leading to the promotion of immobilization of U(VI). These findings would provide some guidance for the safety assessments of the VLLW disposal as well as the remediation of contaminated soil.

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Geochemical Characteristics and Uranium Neutral Leaching through a CO2 + O2 System—An Example from Uranium Ore of the ELZPA Ore Deposit in Pakistan

Geochemical characterization studies and batch leaching experiments were conducted to explore the effects of a CO2 + O2 leaching system on uranium (U) recovery from ores obtained from an eastern limb of Zinda Pir Anticline ore deposit in Pakistan. The mineralogy of the ore was identified by Electron Probe Micro-analyzer (EPMA) and Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), showing that pitchblende is the main ore mineral. XRD was also used along with EPMA and SEM characterization data. Experimental results indicate that U mobility was readily facilitated in the CO2 + O2 system with Eh 284 mV and pH 6.24, and an 86% recovery rate of U3O8 was obtained. U speciation analysis implied the formation of UO2 (CO3)22− in the pregnant solution. The plausible mechanism may be attributed to the dissolved CO2 gas that forms carbonate/bicarbonate ion releasing oxidized U from the ore mineral. However, U recovery in the liquid phase was shown to decrease by higher U(VI) initial concentration, which may be due to the saturation of Fe adsorption capacity, as suggested by an increase in Fe concentration with increasing initial U(VI) concentration in the solid phase. However, further studies are needed to reveal the influencing mechanism of U(VI) initial concentration on U recovery in the solid phase. This study provides new insights on the feasibility and validity of the site application of U neutral in situ leaching.

Mn-substituted goethite for uranium immobilization: A study of adsorption behavior and mechanisms

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g^-1 at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16-20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52-56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27-29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O₂ and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H⁺ concentration ([H⁺]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O₂ has potential applications in the separation and recycling of aqueous uranium.

Co-transport of U(VI) and gibbsite colloid in saturated granite particle column: Role of pH, U(VI) concentration and humic acid

Understanding the in-situ transport behavior of U(VI) in granitic formations is of considerable interest for geological disposal of high-level radioactive wastes (HLW). In this context, the co-transport of U(VI) and representative naturally-occurring colloids, i.e., humic acid (HA) and gibbsite colloid (GC), was studied in granite column as a function of pH, U(VI) concentration and HA amount. It was found that, in addition to pH, co-transport of U(VI) and GC was also controlled by U(VI) concentration, the effect of which can be transport-facilitating and transport-impeding for U(VI) at relatively low concentration (2.0 × 10^-6 mol/L) and for U(VI) at high concentration (5.0 × 10^-5 mol/L), respectively. HA can present opposite effects on GC transport depending on HA amount. The transport-impeding effect by small amount of HA (5 mg/L) is due to strong aggregation between GC and HA from electrostatic attraction and complexation, whereas the transport-facilitating effect by big amount of HA (20 mg/L) is because of the complete HA coating which stabilizes associated colloids and alters surface charge from positive to negative. In ternary co-transport systems, a similar HA-dependent effect was also observed for both U(VI) and GC regardless of presence of high concentration U(VI). Besides the application of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the mechanisms behind binary and ternary co-transport of U(VI), GC and HA were also analyzed by assessing the evolutions of zeta potential and particle size in the column effluents. Finally, a two-site non-equilibrium model and a two-site kinetic attachment/detachment model were applied to describe the breakthrough curves of U(VI) and individual/combined colloids, respectively. The findings of this study indicated that combined effects of GC and HA on radionuclides transport is dominated by the amount of HA, and a facilitating transport of radionuclide can be expected in the underground environment rich in humic acid.

Co-transport of U(VI), humic acid and colloidal gibbsite in water-saturated porous media

The release of uranyl from uranium tailing sites is a widely concerned environmental issue, with limited investigations on the effect of coexistence of various colloids. Gibbsite colloids extensively exist, together with ubiquitous humic substances, in uranium polluted waters at tailing sites, due to high concentration of dissolved Al in acid mine drainage. In this context, we investigated the co-transport of U(VI), gibbsite colloids and humic acid (HA) as a function of pH and ionic strength at a U(VI) concentration (5.0 × 10^-5 M) relevant within mine tailings and related waste. It was found that, owing to electrostatic attraction, gibbsite colloids and HA associated with each other and transported simultaneously regardless of U(VI) presence. Besides the impact of pH and ionic strength, whether gibbsite colloids facilitated U(VI) transport depended on HA concentration. Gibbsite colloids impeded U(VI) transport at relatively low HA concentration (≤5 mg L^-1), because associated colloids loaded with U(VI) were positively charged which favored colloid retention on negatively charged quartz sand in the column. U(VI) together with gibbsite colloids and low concentration HA was completely blocked at natural pH and/or high ionic strength. At relatively high HA concentration (20 mg L^-1), however, the associated colloids showed negative zeta potential which facilitated U(VI) transport because of repulsion between negatively charged colloids and quartz sand. Meanwhile, high concentration of HA dramatically accelerated the transport of gibbsite colloids. These results implied that gibbsite colloids might imped U(VI) migration at uranium tailing sites unless the aquifers are enriched with abundant humic substances.

Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U₆₀ concentration (0.5-2 g·L^-1), mineral concentration (100-500 m²·L^-1), and pH (8-10). The resulting rate law describing U₆₀ interactions with goethite at pH 9 was R = - k_rxn[U₆₀]^0.29±0.02[goethite]^1.2±0.1 where k_rxn = (6.7 ± 2.0) × 10^-4 (g·L^-1)^0.71±0.02(m²·L^-1)^-1.2±0.1(day^-1). The largest fraction of U₆₀ removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U₆₀ may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U₆₀ solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U₆₀ remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U₆₀ at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.

Effect of Natural Organic Matter on Plutonium Sorption to Goethite

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mg_C·L^-1 and 50 mg_C·L^-1 natural organic matter (NOM), 10^-9-10^-10 M ²³⁸Pu, and 0.1 g·L^-1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions.

The Sorption Processes of U(VI) onto SiO2 in the Presence of Phosphate: from Binary Surface Species to Precipitation

The ternary system containing aqueous U(VI), aqueous phosphate and solid SiO₂ was comprehensively investigated using a batch sorption technique, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy, time-resolved luminescence spectroscopy (TRLS), and surface complexation modeling (SCM). The batch sorption studies on silica gel (10 g/L) in the pH range 2.5 to 5 showed no significant increase in U(VI) uptake in the presence of phosphate at equimolar concentration of 20 μM, but significant increase in U(VI) uptake was observed for higher phosphate concentrations. In situ infrared and luminescence spectroscopic studies evidence the formation of two binary U(VI) surface species in the absence of phosphate, whereas after prolonged sorption in the presence of phosphate, the formation of a surface precipitate, most likely an autunite-like phase, is strongly suggested. From SCM, excellent fitting results were obtained exclusively considering two binary uranyl surface species and the formation of a solid uranyl phosphate phase. Ternary surface complexes were not needed to explain the data. The results of this study indicate that the sorption of U(VI) on SiO₂ in the presence of inorganic phosphate initially involves binary surface-sorption species and evolves toward surface precipitation.

Surface complexation modeling of U(VI) sorption on GMZ bentonite in the presence of fulvic acid

In this work, experiments and modeling for the interactions between uranyl ion and GMZ bentonite in the presence of fulvic acid are presented. The results demonstrated that FA is strongly bound to GMZ bentonite, and these molecules have a very large effect on the U(VI) sorption. The results also demonstrated that U(VI) sorption to GMZ bentonite in the presence and absence of sorbed FA can be well predicted by combining SHM and DLM. According to the model calculations, the nature of the interactions between FA with U(VI) at GMZ bentonite surface is mainly surface complex. The first attempt to simulate clay interaction with humus by the SHM model.

An overview of the role of goethite surfaces in the environment

Goethite, one of the most thermodynamically stable iron oxides, has been extensively researched especially the structure (including surface structure), the adsorption capacity to anions, organic/organic acid (especially for the soil organic carbon) and cations in the natural environment and its potential application in environmental protection. For example, the adsorption of heavy metals by goethite can decrease the concentration of heavy metals in aqueous solution and immobilize; the adsorption to soil organic carbon can decrease the release of carbon and fix carbon. In this present overview, the possible physicochemical properties of the goethite surface contributing to the strong affinity of goethite to nutrients and contaminants in natural environment are reported. Moreover, these chemicals adsorbed by goethite were also summarized and the suggested adsorption mechanism for these adsorbates was elucidated, which will help us understand the role of goethite in natural environment and provide some information about goethite as an absorbent. In addition, the feasibility of goethite used as catalyst carrier and the precursor of NZVI was proposed for removal of environmental pollution.

The adsorption behavior of U(VI) on granite

The effects of pH, counter ions and temperature on the adsorption of U(VI) on Beishan granite (BsG) were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The adsorption edge of U(VI) on BsG suggested that U(VI) adsorption was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complex was the dominant adsorption species in the pH range of 4.0-9.0. Above pH 9.0, Na2U2O7 might play an important role in the rise of U(VI) adsorption again. Counter ions such as Cl(-), SO4(2-) and PO4(3-) can provoke U(VI) adsorption on BsG to some extent, which was directly correlated to the complexing ability of U(VI)-ligand. More noticeably, the large enhancement of U(VI) adsorption in the presence of phosphate can be attributed to the ternary complex formation (BsG-PO4-UO2), precipitation ((UO2)3(PO4)2(s)) and secondary phase (Na-autunite). Both FA and HA can slightly increase U(VI) adsorption at low pH, whereas they strongly inhibited U(VI) adsorption at high pH range. Artificial synthesized granite (AsG) prepared in the laboratory is impossible to use as an analogue of natural granite because of the large difference in the adsorption and surface properties.

Mass action expressions for bidentate adsorption in surface complexation modeling: theory and practice

The inclusion of multidentate adsorption reactions has improved the ability of surface complexation models (SCM) to predict adsorption to mineral surfaces, but variation in the mass action expression for these reactions has caused persistent ambiguity and occasional mishandling. The principal differences are the exponent (α) for the activity of available surface sites and the inclusion of surface site activity on a molar concentration versus fraction basis. Exemplified by bidentate surface complexation, setting α at two within the molar-based framework will cause critical errors in developing a self-consistent model. Despite the publication of several theoretical discussions regarding appropriate approaches, mishandling and confusion has persisted in the model applications involving multidentate surface complexes. This review synthesizes the theory of modeling multidentate surface complexes in a style designed to enable improvements in SCM practice. The implications of selecting an approach for multidentate SCM are illustrated with a previously published data set on U(VI) adsorption to goethite. To improve the translation of theory into improved practice, the review concludes with suggestions for handling multidentate reactions and publishing results that can avoid ambiguity or confusion. Although most discussion is exemplified by the generic bidentate case, the general issues discussed are relevant to higher denticity adsorption.

The removal of U(VI) from aqueous solution by oxidized multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) have exhibited high sorption capacity for radionuclides due to the unique hollow structure and large surface area. In this study, surface properties of oxidized MWCNTs were characterized by using XRD, SEM, FTIR and potentiometric acid-base titration. The sorption of U(VI) on oxidized MWCNTs as a function of contact time, U(VI) concentration, pH, ionic strength, humic acid/fulvic acid (HA/FA) and carbonate was investigated by using batch technique. The removal of U(VI) by oxidized MWCNTs was strongly dependent on pH and ionic strength. The presence of HA/FA enhanced U(VI) removal on oxidized MWCNTs at low pH while inhibited U(VI) sorption at high pH. The mechanism of U(VI) sorption on oxidized MWCNTs was assumed to be cation exchange/outer-sphere surface complexation in acidic pH and to form precipitation under circum neutral conditions. The oxidized MWCNTs exhibit higher sorption capacity and stronger chemical affinity than pristine MWCNTs.

Impact of water quality parameters on the sorption of U(VI) onto hematite

In this study, the sorption of U(VI) from aqueous solution on hematite was studied as a function of various water quality parameters such as contact time, pH, ionic strength, soil humic acid (HA) or fulvic acid (FA), solid content and temperature by using a batch technique. The results demonstrated that the sorption of U(VI) was strongly dependent on ionic strength at pH<6.0, and outer-sphere surface complexation may be the main sorption mechanism. The sorption was independent of ionic strength at pH>6.0 and the sorption was mainly dominated by inner-sphere surface complexation. The presence of HA/FA increases U(VI) sorption at low pH, whereas decreases U(VI) sorption at high pH. The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) were calculated from the temperature dependent sorption isotherms, and the results suggested that U(VI) sorption was a spontaneous and endothermic process. The results might be important for the application of hematite in U(VI) pollution management.

Temperature effect on U(VI) sorption onto Na-bentonite

U(VI) sorption on a purified Na-bentonite was investigated from 298±2 to 353±2 K by using a batch experimental method as a function of pH, U(VI) concentration, carbonate concentration and solid-to-liquid ratio (m/V). The data at 298±2 K could be well described by a surface complexation model (SCM) with a complex located on layer sites (X2UO2) and three complexes located on edge sites (≡SOUO2 +, ≡SO(UO2)3(OH)5, and ≡SO(UO2)3(OH)7 2−). The intrinsic equilibrium constants (K int) of the surface reactions at 333 ± 2 and 353±2 K were obtained by fitting U(VI) sorption curves vs. pH on the Na-bentonite. The model enables U(VI) sorption in the presence of carbonate (p CO2 =10−3.58 atm) to be described without considering any ternary surface complexes involving carbonate, except for underestimation around pH 7 (6<pH<7.5). The standard enthalpy changes (Δr H m θ) of the surface reactions were evaluated from the K int values obtained at three temperatures (298±2, 333±2 and 353±2 K) via the van´t Hoff equation. The proposed SCM and Δr H m θ of the surface reactions enable U(VI) sorption on the Na-bentonite at other temperatures to be predicted.