The influence of citric acid,EDTA, and fulvic acid on U(VI) sorption onto kaolinite

Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 × 10^?6–1 × 10^?4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO₃), and pCO₂ (0–5%) in the presence or absence of 1 × 10^?2–1 × 10^?4 M citric acid, 1 × 10^?2–1 × 10^?4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 × 10^?6–1 × 10^?4 M U(VI) in 0.01 M NaNO₃ were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 × 10^?5 and 1 × 10^?4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 × 10^?6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH ～3 to 7 and decreases from pH ～7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO₂ greatly diminishes U(VI) sorption between pH ～5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH.     

Competition between enzymatic and abiotic reduction of uranium(VI) under iron reducing conditions

Reduction of U(VI) under iron reducing conditions was studied in a model system containing the dissimilatory metal-reducing bacterium Shewanella putrefaciens and colloidal hematite. We focused on the competition between direct enzymatic uranium reduction and abiotic reduction of U(VI) by Fe(II), catalyzed by the hematite surface, at relatively low U(VI) concentrations (< 0.5 μM) compared to the concentrations of ferric iron (> 10 mM). Under these conditions surface catalyzed reduction by Fe(II), which was produced by dissimilatory iron reduction, was the dominant pathway for uranium reduction. Reduction kinetics of U(VI) were identical to those in abiotic controls to which soluble Fe(II) was added. Strong adsorption of U(VI) at the hematite surface apparently favored the abiotic pathway by reducing the availability of U(VI) to the bacteria. In control experiments, lacking either hematite or bacteria, the addition of 45 mM dissolved bicarbonate markedly slowed down U(VI) reduction. The inhibition of enzymatic U(VI) reduction and abiotic, surface catalyzed U(VI) reduction by the bicarbonate amendments is consistent with the formation of aqueous uranyl-carbonate complexes. Surprisingly, however, more U(VI) was reduced when dissolved bicarbonate was added to experimental systems containing both bacteria and hematite. The enhanced U(VI) reduction was attributed to the formation of magnetite, which was observed in experiments. Biogenic magnetite produced as a result of dissimilatory iron reduction may be an important agent of uranium immobilization in natural environments.     

Experimental study and modeling of the sorption of uranium(VI) onto olivine-rock

The sorption of U(VI) onto the surface of olivine has been experimentally investigated at 25 °C under an air atmosphere as a function of pH, solid surface to volume ratio and total U concentration. Sorption has been observed to decrease as the extent of carbonate complexation of U(VI) in solution increases, which is attributed to the competition between aqueous and solid ligands for the coordination of U. The experimental results have been interpreted by means of two different approaches: (1)a semi-empirical model, exemplified by the application of a Langmuir isotherm and (2) a non-electrostatic thermodynamic surface complexation model which includes the formation of the surface species: >SO–UO₂⁺ and >SO–UO₂(OH). The following stability constants for these species have been determined from the thermodynamic analysis: K(>SO–UO₂⁺)=289±71 and K(>SO–UO₂(OH))=(3.4±0.4)×10⁻⁶. The comparison of the sorption of U onto olivine with granites of different origin indicate that the use of this mineral as additive to the backfill of deep high level nuclear waste repositories could retard the migration of U from the repository to the geosphere.     

粘土对地下水中U（VI）的吸附作用及其污染控制研究

进行了U（VI）在粘土上吸附的批实验，其中，粘土样采自我国南方某大型铀尾矿库库底。实验结果表明U（VI）在粘土上的吸附与浸泡液的pH值呈强烈的非线性关系，在pH值近中性时，U（VI）在粘土上的吸附达到了一个最大值，而在偏酸性或偏碱性条件下，U（VI）在粘土上的吸附迅速减少；运用表面络合理论建立了U（VI）在粘土上吸附的表面络合模型（DLM），该模型很好地拟合了实验数据。模型检验表明，它可以精确预测U（VI）在不同热力学条件下的吸附行为；此外，模拟结果表明，U（VI）的粘土上吸附在酸性条件下受固液比（M／V）影响明显，而在碱性条件下主要受浸泡液中HCO3^-和CO3^2-的控制。     

Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm⁻¹ for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.     

Effect of fulvic acids on sorption of U(VI), Zn,Yb, I and Se(IV) onto oxides of aluminum,iron and silicon

The sorption of Yb³⁺, UO²⁺₂, Zn²⁺, I⁻ and SeO²⁻₃ onto Al₂O₃, Fe₂O₃ and SiO₂ were determined by a batch technique in the presence and absence of fulvic acids. The effects of fulvic acid on sorption were compared. The existing general consensus, that humic substances tend to enhance metal cation sorption at low pH, reduce metal cation sorption at high pH and reduce inorganic anion sorption between pH values 3 to 10, was generally shown to be true. However, in this work many exceptions to the general consensus were found. The study indicated that the effect of humic substances on sorption of inorganic cations or anions depends not only on pH, but also on the nature of the oxide, the nature of humic substance, fractionation of the humic substance by sorption, the relative strength of complexes of both soluble and sorbed humic substances, the extent of surface coverage by humic substance, the initial concentration of humic substance and the inorganic electrolyte composition.     

The effect of Si and Al concentrations on the removal of U(VI) in the alkaline conditions created by NH3 gas

Remediation of uranium in the deep unsaturated zone is a challenging task, especially in the presence of oxygenated, high-carbonate alkalinity soil and pore water composition typical for arid and semi-arid environments of the western regions of the U.S. This study evaluates the effect of various pore water constituencies on changes of uranium concentrations in alkaline conditions, created in the presence of reactive gases such as NH₃ to effectively mitigate uranium contamination in the vadose zone sediments. This contaminant is a potential source for groundwater pollution through slow infiltration of soluble and highly mobile uranium species towards the water table. The objective of this research was to evaluate uranium sequestration efficiencies in the alkaline synthetic pore water solutions prepared in a broad range of Si, Al, and bicarbonate concentrations typically present in field systems of the western U.S. regions and identify solid uranium-bearing phases that result from ammonia gas treatment. In previous studies (Szecsody et al. 2012; Zhong et al. 2015), although uranium mobility was greatly decreased, solid phases could not be identified at the low uranium concentrations in field-contaminated sediments. The chemical composition of the synthetic pore water used in the experiments varied for silica (5–250 mM), Al³⁺ (2.8 or 5 mM), HCO₃⁻ (0–100 mM) and U(VI) (0.0021–0.0084 mM) in the solution mixture. Experiment results suggested that solutions with Si concentrations higher than 50 mM exhibited greater removal efficiencies of U(VI). Solutions with higher concentrations of bicarbonate also exhibited greater removal efficiencies for Si, Al, and U(VI). Overall, the silica polymerization reaction leading to the formation of Si gel correlated with the removal of U(VI), Si, and Al from the solution. If no Si polymerization was observed, there was no U removal from the supernatant solution. Speciation modeling indicated that the dominant uranium species in the presence of bicarbonate were anionic uranyl carbonate complexes (UO₂(CO₃)₂⁻² and UO₂(CO₃)₃⁻⁴) and in the absence of bicarbonate in the solution, U(VI) major species appeared as uranyl-hydroxide (UO₂(OH)₃⁻ and UO₂(OH)₄⁻²) species. The model also predicted the formation of uranium solid phases. Uranyl carbonates as rutherfordine [UO₂CO₃], cejkaite [Na₄(UO2)(CO₃)₃] and hydrated uranyl silicate phases as Na-boltwoodite [Na(UO₂)(SiO₄)·1.5H₂O] were anticipated for most of the synthetic pore water compositions amended from medium (2.9 mM) to high (100 mM) bicarbonate concentrations.     

Reduction of U(VI) to U(IV) on the surface of magnetite

To increase the understanding of uranium transport in the environment and in the presence of steel corrosion products, the interaction of U(VI) with natural magnetite has been studied. Sorption studies have been carried out using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The XPS results clearly indicate the reduction of U(VI) to U(IV) on the surface of magnetite facilitated by electron transfer between the Fe and U, leading to a coupled oxidation of Fe(II) to Fe(III).     

Uranium(VI) sorption to hematite in the presence of humic acid

A long-standing problem in aquatic geochemistry has been the incorporation of natural organic matter (NOM) into speciation models. The general effect of NOM on metal ion sorption by particles has been understood for some time, and significant progress has been made in elucidating some of the details of the role of NOM through the use of surrogate organic acids such as citric acid. However, a gap exists between the general observations that have been made of NOM behavior and the inclusion of NOM in surface chemical models for metal ion sorption. In this paper, we report on the results of a study on the sorption of U(VI) by hematite in the absence and presence of Suwannee river humic acid (HA) and over a range of other system conditions (e.g., pH, I). Essential HA characteristics (e.g., its acid/base, metal binding, and surface chemical properties) were “captured” by representing the HA as an assembly of monoprotic acids with assumed pK values and without explicit correction for electrostatic effects. The ternary system (hematite/HA/U(VI)) was simulated through the combination of the binary submodels (i.e., CO₃²⁻/hematite, U(VI)/HA, U(VI)/hematite, and HA/hematite) with model constants fixed at the values determined from simulations of the respective experimental systems. However, the “summed-binary” approach undersimulated experimental results, and the ternary system model required the postulation of two ternary surface (Type A) complexes composed of the uranyl ion, hematite surface sites, and the model ligands comprising the HA. Consideration of the HA in this manner permitted the simulation of HA effects on U(VI) sorption by hematite over a range of solution conditions using a general speciation model.     

The effect of U(VI) bioreduction kinetics on subsequent reoxidation of biogenic U(IV)

Microbially mediated in situ reduction of soluble U(VI) to insoluble U(IV) (as UO₂) has been proposed as a means of preventing the migration of that radionuclide with groundwater, but preventing the oxidative resolubilization of U has proven difficult. We hypothesized that relatively slow rates of U(VI) bioreduction would yield larger UO₂ precipitates that would be more resistant to oxidation than those produced by rapid U(VI) bioreduction. We manipulated U(VI) bioreduction rates by varying the density of Shewanella putrefaciens CN32 added to U(VI) containing solutions with lactate as an electron donor. Characterization of biogenic UO₂ particles by extended X-ray absorption fine-structure spectroscopy and transmission electron microscopy revealed that UO₂ nanoparticles formed by relatively slow rates of U(VI) reduction were larger and more highly aggregated than those formed by relatively rapid U(VI) reduction. UO₂ particles formed at various rates were incubated under a variety of abiotically and biologically oxidizing conditions. In all cases, UO₂ that was formed by relatively slow U(VI) reduction was oxidized at a slower rate and to a lesser extent than UO₂ formed by relatively rapid U(VI) bioreduction, suggesting that the stability of UO₂ in situ may be enhanced by stimulation of relatively slow rates of U(VI) reduction.     

Enhanced microbial reduction of Cr(VI) and U(VI) by different natural organic matter fractions

Although direct microbial reduction of Cr(VI) and U(VI) is known, few studies have examined the kinetics and the underlying mechanisms of the reduction of these contaminants by different natural organic matter (NOM) fractions in the presence or absence of microorganisms. In this study, NOM was found to chemically reduce Cr(VI) at pH 3, but the reduction rates were negligible at pH ∼7. The abiotic reduction of U(VI) by NOM was not observed, possibly because of the presence of small amounts of nitrate in the reactant solution. However, all NOM fractions, particularly the soil humic acid (HA), enhanced the bioreduction of Cr(VI) or U(VI) in the presence of Shewanella putrefaciens CN32. The reduction rates varied greatly among NOM fractions with different chemical and structural properties: the polyphenolic-rich NOM-PP fraction appeared to be the most reactive in abiotically reducing Cr(VI) at a low pH, but soil HA was more effective in mediating the microbial reduction of Cr(VI) and U(VI) under anaerobic, circumneutral pH conditions. These observations are attributed to an increased solubility and conformational changes of the soil HA with pH and, more importantly, its relatively high contents of polycondensed and conjugated aromatic organic moieties. An important implication of this study is that, depending on chemical and structural properties, different NOM components may play different roles in enhancing the bioreduction of Cr(VI) and U(VI) by microorganisms. Polycondensed aromatic humic materials may be particularly useful in mediating the bioreduction and rapid immobilization of these contaminant metals in soil.     

Sorption of selenium(VI) onto anatase: Macroscopic and microscopic characterization

The sorption of selenium(VI) onto pure anatase, a polymorph of titanium dioxide, was investigated. At the macroscopic level, batch experiments and electrophoretic mobility measurements were performed. Selenium(VI) retention was found to be pH-dependent, i.e. sorption of selenium(VI) decreases with increasing pH (pH range 3.5-11). Selenium(VI) sorption dependence on the ionic strength was also evidenced, i.e. sorption increases while the ionic strength decreases. Electrophoretic mobility measurements showed that selenium(VI) sorption had no effect on the isoelectric point of anatase. At the microscopic level, XPS (X-ray Photoelectron Spectroscopy) measurements evidenced the absence of reduction of selenium(VI) during the sorption process. Furthermore, the nature of the sorbed surface species at the anatase/liquid interface was elucidated using Attenuated Total Reflection Fourier-Transform Infrared (ATR FT-IR) spectroscopy. The spectroscopic results strongly suggested the formation of outer-sphere complexes on the whole pH range, in agreement with batch sorption experiments and electrophoretic mobility findings.     

Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.     

CTR diffraction and grazing-incidence EXAFS study of U(VI) adsorption onto α-Al2O3 and α-Fe2O3 (11?02) surfaces

Evaluation of the long-term health risks of uranium contamination in soils, sediments, and groundwater requires a fundamental understanding of the various processes affecting subsurface transport of uranium, including adsorption processes at mineral/water interfaces. In this study, the sites of binding and surface complexation of U(VI) adsorbed on the (11?02) surfaces of α-Al₂O₃ and α-Fe₂O₃ have been determined using crystal truncation rod (CTR) diffraction and grazing incidence extended X-ray absorption fine structure (GI-EXAFS) spectroscopy. The available binding sites on the (11?02) surfaces were constrained through bond valence and steric analyses. On both surfaces, U(VI) forms uranyl-carbonato ternary complexes to surface oxygens that are singly coordinated to aluminum or iron. On the α-Al₂O₃ (11?02) surface, a monodentate complex results, whereas on the α-Fe₂O₃ (11?02) surface, the binding is bidentate to adjacent singly coordinated oxygen sites (i.e., binuclear). Differences in protonation of the singly coordinated oxygen atoms, surface charging, U(VI) aqueous speciation, substrate structure, or the electronic structure of surface functional groups may be the cause of these differences in adsorption geometry. Both XPS and CTR diffraction reveal higher U(VI) surface coverages on the α-Fe₂O₃ (11?02) surface than on the α-Al₂O₃ (11?02) surface. This difference cannot be the result of differences in defect concentration alone as CTR diffraction is not sensitive to U(VI) sorbed to defect sites, implying that the α-Fe₂O₃ (11?02) surface has an intrinsically higher affinity for U(VI). The surface complexes observed in this study are different from the bidentate, mononuclear complexes typically derived for U(VI) on powdered aluminum- and iron-(oxyhydr)oxides and clay minerals using U L_III-edge EXAFS spectroscopy. However, the presence of monodentate, mononuclear and bidentate, binuclear complexes may have been overlooked in past EXAFS studies on such substrates, as these complexes have U-Al or U-Fe interatomic distances that are too large to be easily detected by EXAFS spectroscopy.     

Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids

A mixing of metal-loaded acid mine drainage with shallow groundwater or surface waters usually initiates oxidation and/or hydrolysis of dissolved metals such as iron (Fe) and aluminum (Al). Colloidal particles may appear and agglomerate with increasing pH. Likewise chemical conditions may occur while flooding abandoned uranium mines. Here, the risk assessment of hazards requires reliable knowledge on the mobility of uranium (U). A flooding process was simulated at mesocosm scale by mixing U-contaminated acid mine water with near-neutral groundwater under oxic conditions. The mechanism of U-uptake by fresh precipitates and the molecular structure of U bonding were determined to estimate the mobility of U(VI). Analytical and spectroscopic methods such as Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy at the Fe K-edge and the U L_III-edge, and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy were employed. The freshly formed precipitate was identified as colloidal two-line ferrihydrite. It removed U(VI) from solution by sorption processes, while surface precipitation or structural incorporation of U was not observed. EXAFS data suggest a mononuclear inner-sphere, edge-sharing complex of U(VI) with ferrihydrite in the absence of dissolved carbonate. By employing a novel EXAFS analysis method, Monte Carlo Target Transformation Factor Analysis, we could for the first time ascertain a 3-D configuration of this sorption complex without the necessity to invoke formation of a ternary complex. The configuration suggests a slightly tilted position of the adsorbed unit relative to the edge-sharing Fe(O, OH)₆ octahedra. In the presence of dissolved carbonate and at pH ∼8.0, a distal carbonate O-atom at ∼4.3 Å supports formation of ternary U(VI)-carbonato surface complexes. The occurrence of these complexes was also confirmed by ATR-FTIR. However, in slightly acidic conditions (pH 5-6) in equilibrium with atmospheric CO₂, the U(VI) sorption on ferrihydrite was dominated by the binary complex species Fe(O)₂UO₂, whereas ternary U(VI)-carbonato surface complexes were of minor relevance. While sulfate and silicate were also present in the mine water, they had no detectable influence on U(VI) surface complexation. Our experiments demonstrate that U(VI) forms stable inner-sphere sorption complexes even in the presence of carbonate and at slightly alkaline pH, conditions which previously have been assumed to greatly accelerate the mobility of U(VI) in aqueous environments. Depending on the concentrations of U(VI) and carbonate, the type of surface complexes may change from binary uranyl-ferrihydrite to ternary carbonato-uranyl-ferrihydrite complexes. These different binding mechanisms are likely to influence the binding stability and retention of U(VI) at the macroscopic level.     

Scaling of adsorption reactions: U(VI) experiments and modeling

Iron-coated sands were prepared via two common protocols, a precipitation method, where Fe was precipitated directly onto the sand in a single step, and an adsorption method, where pure goethite was prepared in the first step and then adsorbed onto the sand in a second step. The coated sands from both the systems were characterized using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and selective Fe extraction. Although neither of the methods produced a completely crystalline Fe coating, the precipitation method produced sands with larger portions of amorphous Fe than the adsorption method, with the fraction of amorphous Fe decreasing with increasing Fe content. Uranium(VI) adsorption isotherms and pH adsorption edges were measured on three coated sands with Fe contents ranging from 0.04% to 0.3%. Experimentally, the adsorption of U(VI) onto the three sands was more comparable when normalized to surface area than when normalized to Fe content. A surface complexation model, although originally developed for U(VI) adsorption onto amorphous Fe oxide, captured the differences in adsorption when adjusted for the surface area of the coated sand. The findings indicate that surface area is a better scaling parameter than Fe content in predicting U(VI) adsorption to Fe-dominated media. These findings are significant because many common surface complexation models are parameterized on the basis of Fe content rather than specific surface area. Although the interactions of U(VI) and Fe-coated sands were used as representative adsorbate and adsorbent, the general principles may be applicable to other adsorbate–adsorbent systems as well.     

Kinetic and equilibrium study of Ni(II) sorption from aqueous solutions onto Peganum harmala-L

In this study, the adsorption behavior of Ni(II) in an aqueous solution system using natural adsorbent Peganum harmala-L was measured via batch mode. The prepared sorbent was characterized by scanning electron microscope, Fourier transform infrared spectroscopy, N₂ adsorption–desorption and pH_zpc. Adsorption experiments were carried out by varying several conditions such as contact time, metal ion concentration and pH to assess kinetic and equilibrium parameters. The equilibrium data were analyzed based on the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms. Kinetic data were analyzed using the pseudo-first-order, pseudo-second-order and intra-particular diffusion models. Experimental data showed that at contact time 60 min, metal ion concentration 50 mg/L and pH 6, a maximum amount of Ni(II) ions can be removed. The experimental data were best described by the Langmuir isotherm model as is evident from the high R ² value of 0.988. The adsorption capacity (q _m) obtained was 68.02 mg/g at an initial pH of 6 and a temperature of 25 °C. Kinetic studies of the adsorption showed that equilibrium was reached within 60 min of contact and the adsorption process followed the pseudo-first-order model. The obtained results show that P. harmala-L can be used as an effective and a natural low-cost adsorbent for the removal of Ni(II) from aqueous solutions.     

洞穴石笋初始234U/238U值变化的古气候记录意义

通过对我国西南地区3个洞穴9根大型石笋272个初始234U/238U数据的分析研究,发现石笋初始234U/238U值长尺度变化与海洋沉积SPECMAP曲线δ18O记录有一定的正相关关系,与北纬25°夏季太阳辐射能量变化曲线呈一定的负相关关系,石笋初始234U/238U值的变化在冰期时波动强烈,而在间冰期波动相对平缓,在间冰期和冰期(间冰阶和冰阶)转化阶段该值呈跳跃状态变化。末次冰期及全新世阶段石笋初始234U/238U变化记录了该时段内的BA暖期和YD突变冷事件;全新世8200 a BP、7200 a BP、5200 a BP、4200 a BP、2800 a BP、1400 a BP 发生的几次较强冷事件在石笋初始234U/238U值都有相应记录,并且和长尺度的变化规律一致,冷事件发生时石笋初始234U/238U值偏重,暖期偏轻。封闭系统形成的洞穴石笋初始234U/238U变化类似于同地区洞穴石笋的δ18O对气候变化的记录特征,在我国西南地区两者与夏季风变化的强弱呈现一种负相关的关系,洞穴石笋初始234U/238U值可以作为一个有用的古气候替代指标来研究古降水的变化。     

Radioelement (U,Th, Rn) concentrations in Norwegian bedrock groundwaters

Twenty-eight samples of groundwater from bedrock boreholes in three distinct Norwegian geological provinces have been taken and analyzed for content of Rn, U, and Th, together with a wide variety of minor and major species. Median values of 290 Bq/1, 7.6 g/1, and 0.02 g/1 were obtained for Rn, U, and Th respectively, while maximum values were 8500 Bq/1, 170 g/1, and 2.2 g/1. Commonly suggested drinking water limits range from 8 to 1000 Bq/1 for radon and 14 to 160 g/1 for uranium. Radioelement content was closely related to lithology, the lowest concentrations being derived from the largely Caledonian rocks of the Trøndelag area, and the highest from the Precambrian Iddefjord Granite of southeast Norway (11 boreholes) where median values of 2500 Bq/1, 15 g/1 and 0.38 g/1, respectively, were obtained. The Iddefjord Granite is not believed to be unique in Norway in yielding high dissolved radionuclide contents in groundwaters, and several other granitic aquifers warrant further investigation in this respect.