Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)-HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)-HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)-HA complexes decreased, and the proportion of free Ni²⁺ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)-HA complexes was slower than that of Ni(II)-FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.     

Complexation of uranium(VI) with protocatechuic acid—application of iterative transformation factor analysis to EXAFS spectroscopy

The speciation of 1 mM uranium(VI) in carbonate-free aqueous solutions of 50 mM protocatechuic acid (PCA, 3,4-dihydroxybenzoic acid) was studied in the pH range of 4.0 to 6.8 using EXAFS spectroscopy. The uranium L_III-edge EXAFS spectra were analyzed using a newly developed computer algorithm for iterative transformation factor analysis (FA). Two structural different uranium(VI) complexes were observed. The speciation in the pH range of 4.0 to 4.8 is dominated by a 1:2 or 1:3 uranium(VI)/PCA complex with bidentate coordination of the carboxyl group to the uranium(VI) moiety. Already at pH 4.6 significant amounts of a second species are formed. This uranium(VI) species contains two PCA ligands that are bound to the uranium via their neighboring phenolic hydroxyl groups under formation of five-member rings.     

Aqueous uranium(VI) hydrolysis species characterized by attenuated total reflection Fourier-transform infrared spectroscopy

The speciation of uranium(VI) in micromolar aqueous solutions at ambient atmosphere was studied by attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy and by speciation modeling applying the updated NEA thermodynamic database. It can be shown that reliable infrared spectra of micromolar U(VI) solutions are obtained abolishing the restrictions of previous spectroscopic investigations to millimolar concentrations and, consequently, to the acidic pH range. A significant change of the U(VI) speciation can be derived from the spectral alterations of the absorption band representing the antisymmetric stretching mode (nu3) of the UO2(2+) ion observed upon lowering the U(VI) concentration from the milli- to the micromolar range at a constant pH 4 value. The acquisition of spectra of diluted U(VI) solutions allows the increase of the pH up to 8.5 without the risk of formation of colloidal or solid phases. The infrared spectra are compared to the results of the computed speciation patterns. Although a complete interpretation of the spectra can not be given at this state of knowledge, the spectral data strongly suggest the presence of monomeric U(VI) hydroxo species already showing up at a pH value > or = 2.5 and dominating the speciation at pH 3. This is in contradiction to the predicted speciation where the fully hydrated UO2(2+) is expected to represent the main species at pH values below 4. At ambient pH, a more complex speciation is suggested compared to the results of the computational modeling technique. The predicted dominance of the UO2(CO3)3(4-) complex at pH > or = 8 was not confirmed by the infrared data. However, the infrared spectra indicate the formation of hydroxo complexes obviously containing carbonate ligands.     

Simultaneous determination of uranium(VI) and thorium(IV) with carminic acid by derivative spectrophotometry

The reactions of uranium(VI) and thorium(IV) ions with carminic acid have been investigated. These ions react with carminic acid in neutral medium, forming colored complexes. The dark purple or red wine complexes show a high absorption in the visible region (597 nm U(VI) and 616 nm Th(IV)). Chemical variables that affect the reaction have been optimized. The spectral overlapping of the color of complexes has been resolved by first-derivative spectrophotometry. The simultaneous determination of uranium(VI) and thorium(IV) mixtures is accomplished by taking the derivative signal (zero crossing) at 597 nm for U(VI) determination and at 616 nm for Th(IV) determination, respectively. The method has been applied to Tyuyamonite ore, containing in the matrix both ions.     

Structural characterization of U(VI) surface complexes on kaolinite in the presence of humic acid using EXAFS spectroscopy

To determine the influence of humic acid (HA), pH, and presence of atmospheric CO2 on the sorption of U(VI) onto kaolinite, the structure of the surface complexes was studied by U L III-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The best fits to the experimental EXAFS data were obtained by including two uranium coordination shells with two axial (O ax) and five equatorial (O eq) oxygen atoms at 1.77+/-0.02 and 2.34+/-0.02 A, respectively, and two coordination shells with one Al/Si atom each at 3.1 and 3.3 A. As in the case of the binary system U(VI)-kaolinite, uranium forms inner-sphere surface complexes by edge sharing with aluminum octahedra and/or silicon tetrahedra. HA and atmospheric CO2 as well as pH had no influence on the EXAFS structural parameters in the pH range of 5-8. Despite the presence of HA, U(VI) prefers to sorb directly onto kaolinite and not to HA that is bound to the clay surface. X-ray photoelectron spectroscopy (XPS) measurements of kaolinite particles that had been exposed to HA suspensions showed that significant parts of the kaolinite surface are not covered by HA.     

Simultaneous removal of uranium and humic acid by cyclodextrin modified graphene oxide nanosheets

Cyclodextrin-modified graphene oxide nanosheets (denoted as CD/GO) were synthesized by an in-situ polymerization method and characterized by as well as Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and potentiometric acid-base titration. The characterization results indicated that CD was successfully grafted onto GO surfaces by forming a chemical bond. Mutual effects on the simultaneous removal of hexavalent uranium and humic acid by CD/GO from aqueous solution were investigated. The results indicated that U(VI) and humic acid (HA) sorption on CD/GO were greatly affected by pH and ionic strength. The presence of HA enhanced U(VI) sorption at low pH and reduced U(VI) sorption at high pH, whereas the presence of U(VI) enhanced HA sorption. The surface adsorbed HA acted as a “bridge” between U(VI) and CD/GO, and formed strong inner-sphere surface complexes with U(VI). Sorption isotherms of U(VI) or HA on CD/GO could be well fitted by the Langmuir model. This work highlights that CD/GO can be used as a promising material in the enrichment of U(VI) and HA from wastewater in U(VI) and humic substances obtained by environmental pollution cleanup.     

Trace metal speciation measurements in waters by the liquid binding phase DGT device

The speciation measurements of trace metals by the diffusive gradients in thin-films technique (DGT) using a poly(4-styrenesulfonate) (PSS) aqueous solution as a binding phase and a cellulose dialysis membrane (CDM) as a diffusive layer, CDM-PSS DGT, were investigated and showed good agreement with computer modelling calculations. The diffusion coefficients of ethylenediaminetetraacetic acid (EDTA) complexes with Cd²⁺ and Cu²⁺ were measured and compared with those of the inorganic metal ions. CDM-PSS DGT device was tested for speciation measurement in sample solutions containing EDTA, tannic acid (TA), glucose (GL), dodecylbenzenesulfonic acid (DBS) and humic acid (HA) as complexing ligands forming organic complexes with varying stability constants. Lower percentages of DGT labile copper concentrations over total filterable copper concentrations obtained from the deployments in freshwater sites indicated that copper complexes with organic matter were basically not measured by the devices.     

Complexation of Ni, Cu, Zn, and Cd by DOC in some metal-impacted freshwater lakes: a comparison of approaches using electrochemical determination of free-metal-ion and labile complexes and a computer speciation model, WHAM V and VI

Complexation of Ni(II), Cu(II), Zn(II), and Cd(II) by dissolved organic carbon (DOC) in some freshwater lakes in Rouyn-Noranda, Québec, Canada, where they were impacted by effluents from a nearby copper smelter, was measured by kinetic and equilibrium methods using cathodic and anodic stripping voltammetry. The measured free-metal-ion and labile metal-complex concentrations were compared with the predictions made by a widely-used computer speciation model, the Windermere Humic Aqueous Model (WHAM): WHAM V and its improved version WHAM VI. If it is assumed that 65% of the DOC is “active”, i.e. behaving as isolated humic substances such as fulvic acid, both versions of WHAM are able to predict the labile and free-metal-ion concentrations of Ni, Zn, and Cd reasonably well; however, both underestimate the free-copper-ion concentration by one to two orders of magnitude. WHAM VI is generally better than or equal to WHAM V for successfully predicting most of the free-metal-ion concentrations. The modelled competition by Al(III) and Fe(III) in the lake surface waters showed that in most cases Cu(II) was most affected by this competition. WHAM VI predicts a larger effect from the Al(III) and Fe(III) competition than does WHAM V.     

Capillary zone electrophoretic (CZE) study of uranium(VI) complexation with humic acids

The interaction of UO₂ ²⁺ with various humic acids (HA's) has been studied by capillary zone electrophoresis (CZE). The experiments were done in 10 mM acetate buffer with pH 3.3 and 4.0, to avoid hydrolysis of uranium. It was found that in slightly acidic media and low HA concentration (<3 mM), two complexes with uranium(VI) are formed by fast kinetics and uranyl migrates as cationic species. Electrophoretic mobilities are decreasing with the increasing HA/uranium ratio and a low soluble neutral compound is also formed. In addition, it was found that at HA concentrations higher than 3 mM negatively charged species are formed. Similar results were obtained for HA's of different origin (soil, peat, coal derived, IHSS standards). Conditional stability constants of the complexes UO₂ ²⁺-HA for Fluka I HA, were estimated to be log ₁ = 4.18±0.06 and log ₂ = 7.28±0.18.     

Speciation of aluminium(III) in natural waters using differential pulse voltammetry with a pyrocatechol violet-modified electrode

A differential pulse voltammetric (DPV) procedure is proposed for the speciation of aluminium in natural waters using Pyrocatechol Violet chemically modified electrodes (PCV-CMEs). This novel speciation idea is based on the selective determination of different AlIII forms under two pH conditions. The labile monomeric Al fraction (mainly inorganic Al) is analysed at pH 4.8 (0.20 mol dm(-3) NaOAc-HOAc) and the total monomeric Al fraction is analysed at pH 8.5 (0.20 mol dm(-3) NH3.H2O-NH4Cl). The difference is thought to be caused by the weak competition ability of PCV to sequester AlIII from AlIII-natural organic matter complexes. This sensitive and simple speciation method has been applied successfully to aluminium speciation in natural waters sampled from different regions of China. Five fractions are measured directly or indirectly: (i) labile monomeric Al; (ii) total monomeric Al; (iii) acid reactive Al; (iv) non-labile monomeric Al; and (v) acid soluble Al. The results are in satisfactory agreement with those obtained by Driscoll's 8-hydroxyquinoline extraction-ion exchange method.     

Complexation of Uranium(VI) by Gluconate in Alkaline Solutions

Gluconate (C₆H₁₁O₇) is a polyhydroxycarboxylic acid that can be assumed as a representative model compound for a wide variety of additives in cement formulations. It can play an important role in the cementitious environments characteristic of radioactive waste disposal sites, as actinides (such as U(VI)) may form stable complexes with gluconate. As a consequence, the presence of the organic ligand can lead to an enhancement of actinide mobility. The results presented in this work show that gluconate increases significantly the uranium solubility at pH_c = 12; the study of U(VI) speciation in alkaline solutions is complex, mainly due to formation of sparingly soluble uranates of varying compositions (e.g. sodium and potassium uranates). UV–Vis measurements in the alkaline pH range have been used to determine the stability constant for the formation of a 1:1 U(VI):gluconate complex. The results obtained with spectroscopic techniques allow explaining the results from solubility experiments, from both over- and under-saturation conditions.     

Electrocapillary Activity and Adsorptive Accumulation of U(VI)‐Cupferron and U(VI)‐Chloranilic Acid Complexes on Mercury Electrode

Interfacial activity of uranium(VI)‐cupferron and uranium(VI)‐chloranilic acid (CAA) complexes (in 0.1 M acetate buffer pH 4.6 or 0.1 M NaClO₄ respectively) on polarized mercury electrode at 110 mV, 10 mV or ?240 mV respectively vs. saturated calomel electrode (SCE), and under conditions of the application of adsorptive stripping voltammetric techniques was studied. It revealed a competitive effect of interfacial activity of the mentioned complexes consisting in a nonmonotonous effect of the bulk concentration of U(VI) on the adsorption of the mentioned complexing reagents at their constant concentrations. At concentrations lower than 5×10^?5 mol L^?1 the complexes U(VI)‐cupferron or U(VI)‐CAA exhibited a relatively strong electrosorption providing the adsorption coefficients β of the order 10⁴ L mol^?1, the maximum surface excess Γ_m ≈ 5 to 10 μmol m^?2 and average Frumkin interaction coefficients reaching their absolute values 2 to 2.6.     

Electrochemistry of uranium in sodium chloroaluminate melts

The electrochemical behaviour of uranium has been studied in basic, NaCl-saturated NaAlCl₄ melts at 175°C. Solutions of UO₃ exhibit two oxidation/reduction waves (cyclic voltammetry). The first wave corresponds to the U(VI)/U(IV) redox couple and is irreversible (slow electron transfer). The second wave corresponds to the deposition and stripping of an insoluble U(III) compound (U(IV)/U(III)). Solutions of UO₂ or UCl₄ and U(IV) solutions prepared by exhaustive electrolysis of UO₃ behave identically. The cyclic voltammograms of U(IV) solutions are the same as those of UO₃, but they show additional anodic peaks. Analysis of the peak currents (cyclic voltammetry), the limiting currents (pulse polarography) and the non-linear log i-t curves (anodic controlled potential coulometry) leads to the conclusion that uranium (IV) in the basic chloroaluminate melt exists as two different species in slow equilibrium with one another, of which only one species can be oxidized to U(VI). E.m.f. measurements of U(VI)-U(IV) mixtures indicate that the electron transfer process involves the formation of an intermediate U(V) species in a disproportionation equilibrium.     

Extraction of uranium(IV) from phosphoric acid solutions with 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (PMBP)

The extraction of uranium(IV) from phosphoric acid solutions with PMBP and PMBP-TOPO mixtures has been studied. The synergic extraction with PMBP-TOPO is more effective than the simple chelate extraction with PMBP and both systems are more effective than the synergic extraction of uranium(VI) with DEHPA-TOPO. It is established that the complexes extracted are U(PMBP)(4) and U(PMBP)(4).TOPO for the chelate and synergic extraction respectively. The most probable uranium(VI) species in the aqueous phase (2.9-6.33M H(3)PO(4)) is the neutral complex U(H(5)P(2)O(8))(4). Analytical methods suitable for determination of uranium in phosphoric acid solutions have been developed. The highest sensitivity is achieved by combining the synergic extraction with the uranium(IV)-arsenazo III colour reaction.     

Biosorption of uranium(VI) by free and entrapped Chlamydomonas reinhardtii: kinetic,equilibrium and thermodynamic studies

Biosorption of uranium from aqueous solution onto the free and entrapped algae, “Chlamydomonas reinhardtii” in carboxymethyl cellulose (CMC) beads was investigated in a batch system using bare CMC beads as a control system. CMC can be a potential natural biosorbent for radionuclide removal as it contains carboxyl groups. However, limited information is available with the biosorption of uranium by CMC, when adsorption isotherm, kinetics and thermodynamics parameters are concerned. The biosorbent preparations were characterized by swelling tests, FTIR, and surface area studies. The effects of pH, temperature, ionic strength, biosorbent dosage, and initial uranium concentrations on uranium biosorption were investigated. Freely suspended algae exhibited the highest uranium uptake capacity with an initial uranium ion concentration of 1,000 mg/L at pH of 4.5 and at 25 °C. The removal of U(VI) ion from the aqueous solution with all the tested biosorbents increased as the initial concentration of U(VI) ion increased in the medium. Maximum biosorption capacities for free algal cells, entrapped algal cells, and bare CMC beads were found to be 337.2, 196.8, and 153.4 mg U(VI)/g, respectively. The kinetic studies indicated that the biosorption of U(VI) ion was well described by the pseudo-second order kinetic model. The variations in enthalpy and entropy for the tested biosorbent were calculated from the experimental data. The algal cells entrapped beads were regenerated using 10 mM HNO₃, with up to 94 % recovery. Algal cells entrapped CMC beads is a low cost and a potential composite biosorbent with high biosorption capacity for the removal of U(VI) from waters.     

Kinetic speciation of nickel in mining and municipal effluents

This study presents the results of kinetic speciation of nickel in undiluted mining and municipal effluents and effluents diluted with receiving freshwaters from the surrounding environment. The dilution ratios used for the dilution of the effluents were arbitrarily chosen, but were representative of the prevailing mining practices. The purpose of the this dilution was to mimic dilution with natural waters that result from dilution of the mining and municipal effluents with receiving freshwaters, so that this study would reveal environmental realities that are of concern to the managers and regulators of water resources. Ligand exchange kinetics using the competing ligand exchange method (CLEM) was studied using two independent techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime (DMG) as the competing ligand to determine the percentage of Ni metal released from Ni(II)–DOC complexes and the rate of dissociation of Ni(II)–DOC complexes. Using a sample containing a mixture of 30% Copper Cliff Mine effluent, 40% Sudbury municipal effluent and 30% Vermillion River water, both techniques gave results showing that the dilution of the effluent samples increased the percentage of nickel released from Ni(II)–DOC complexes. This increase in the release of nickel from the Ni(II)–DOC complexes may be of concern to managers and regulators of water resources. Agreement between the results of these two techniques has enhanced the validity of the competing ligand exchange method used by both techniques.     

Seasonal variation, chemical behavior and kinetics of uranium in an unconfined groundwater system

The seasonal changes in the concentration of uranium in an unconfined groundwater system in Cyprus have been investigated and compared to corresponding changes of boron and salinity, to better understand the chemical behavior of uranium in the respective system. Uranium concentration measurements were performed by alpha spectroscopy after selective pre-concentration, whereas boron concentration analysis and electrical conductivity measurements were carried out by photometry using azomethine-H and an electrical conductivity electrode, respectively. The experimental data show that seasonal variations are mainly related to rainwater infiltration and the specific chemical behavior of a species. Increased levels of uranium and boron in natural water systems are attributed to the increased stability of the uranium(VI)-carbonato complexes and the boric acid, which are in natural waters the predominant chemical species for uranium and boron, respectively. Dilution/dissolution processes govern the seasonal concentration changes of uranium and boron in a groundwater system, however redox-reactions resulting in the reduction of U(VI) to U(IV) affect significantly the concentration of uranium in the respective system, particularly under suboxic conditions.     

Uranium aqueous speciation in the vicinity of the former uranium mining sites using the diffusive gradients in thin films and ultrafiltration techniques

The performance of the Diffusive Gradients in Thin films (DGT) technique with Chelex^®-100, Metsorb™ and Diphonix^® as binding phases was evaluated in the vicinity of the former uranium mining sites of Chardon and L'Ecarpière (Loire-Atlantique department in western France). This is the first time that the DGT technique with three different binding agents was employed for the aqueous U determination in the context of uranium mining environments. The fractionation and speciation of uranium were investigated using a multi-methodological approach using filtration (0.45 μm, 0.2 μm), ultrafiltration (500 kDa, 100 kDa and 10 kDa) coupled to geochemical speciation modelling (PhreeQC) and the DGT technique. The ultrafiltration data showed that at each sampling point uranium was present mostly in the 10 kDa truly dissolved fraction and the geochemical modelling speciation calculations indicated that U speciation was markedly predominated by CaUO₂(CO₃)₃²⁻. In natural waters, no significant difference was observed in terms of U uptake between Chelex^®-100 and Metsorb™, while similar or inferior U uptake was observed on Diphonix^® resin. In turn, at mining influenced sampling spots, the U accumulation on DGT-Diphonix^® was higher than on DGT-Chelex^®-100 and DGT-Metsorb™, probably because their performance was disturbed by the extreme composition of the mining waters. The use of Diphonix^® resin leads to a significant advance in the application and development of the DGT technique for determination of U in mining influenced environments. This investigation demonstrated that such multi-technique approach provides a better picture of U speciation and enables to assess more accurately the potentially bioavailable U pool.     

Quantum mechanical calculation of aqueuous uranium complexes: carbonate, phosphate, organic and biomolecular species

Background  

Quantum mechanical calculations were performed on a variety of uranium species representing U(VI), U(V), U(IV), U-carbonates, U-phosphates, U-oxalates, U-catecholates, U-phosphodiesters, U-phosphorylated N-acetyl-glucosamine (NAG), and U-2-Keto-3-doxyoctanoate (KDO) with explicit solvation by H₂O molecules. These models represent major U species in natural waters and complexes on bacterial surfaces. The model results are compared to observed EXAFS, IR, Raman and NMR spectra.