Nicholas D.M. Evans

Arsenic’s Interaction with Humic Acid

Environmental Context. Arsenic is of significant environmental concern in much of the world because of its contamination of waters, from mining, industry, sewage disposal, and agriculture. The environmental mobility of arsenic is controlled primarily by adsorption onto metal oxides, especially iron. Humic substances (natural organic matter), which are ubiquitous in aquatic and soil environments, may interfere with this adsorption and arsenic mobility may be increased. Thus, even if it is assumed that humic substances sorb arsenic less strongly than hydrous iron oxides, they may, nevertheless, influence arsenic sorption and mobility, particularly when the iron oxide content in the environment is low. Abstract. The environmental mobility of arsenic is primarily controlled by adsorption onto metal oxide surfaces, particularly iron, aluminium, and manganese. Humic acid (HA) may interfere with this adsorption, thereby increasing arsenic mobility. This study has characterized the interaction of arsenic with HA in a system consisting of HA with As(iii), As(v), and dimethylarsinic acid (DMAA). Three sets of batch experiments were performed at varying pH (3–12), ionic strength (0–0.4 mol dm−3), concentration of each arsenic species (0–100 mg dm−3), and HA concentration (0–10 g dm−3). Arsenic species were shown to react with humic acid. The interaction is postulated to involve bridging metals and deprotonated functional groups within the HA. The association is dependent on pH, ionic strength, and arsenic concentration. The extent of the interaction was greater in the pH range 8–10 for As(v) and DMAA, while it extended to pH 12 for As(iii). The strong pH dependency is probably due to the aqueous speciation of arsenic. The logarithmic conditional association constants for the reactions were found to be 1.97 ± 0.02, 1.58 ± 0.07, and 1.50 ± 0.10 for As(v), As(iii), and DMAA respectively. These values indicate the formation of weak complexes with humic acid.

Stability constants of uranium(IV)-α-isosaccharinic acid and gluconic acid complexes

Summary Conditional and pH independent stability constants have been determined for U(IV) α-isosaccharinic acid (ISA) and gluconic acid (Gl) complexes, under anaerobic and carbonate-free conditions, from pH 3 to 14. The constants are needed for nuclear waste repository performance assessment purposes. The constants were obtained by developing an approach based on the solubility product of amorphous UO2·2H2O. The derived pH independent log β values for U(OH)4ISA and U(OH)4Gl were 49±2 and 50±1 respectively.

Complexation parameters for the actinides(IV)-humic acid system: a search for consistency and application to laboratory and field observations

Summary The coherence of actinide(IV) complexation by humic substances (HS) is reviewed and new data are proposed. In a first attempt, the values of independent data from literature on Th(IV), U(IV), and Pu(IV) are collected, selected, and compiled. The data obtained follow the “classical” trend of increasing conditional formation “constants” with pH, led both by the increasing ionisation of HS and by the extensive hydrolysis of the tetravalent actinides. Even though a fair agreement is evident, the experimental uncertainties do not permit a full analogy between the actinides(IV) to be ascertained. In a second attempt, the experiments from which the original data are available were reinterpreted using only one hydrolysis constant set for U(IV) as an example, considering that all actinides(IV) have analogous humic complexation behaviour. Hence, the obtained evolution of conditional formation “constants” is much more coherent and the uncertainties do not permit to distinguish an actinide(IV) from one another. The obtained data are then applied to independent laboratory and in situ experiments in order to delimit the domain of possible applicability. This exercise demonstrates the treatment of data through analogy in the case of actinides(IV) and would permit to limit and orientate the number of necessary, but difficult, experiment with redox sensitive elements like U, Np, or Pu. It also demonstrates that complexation-only mechanisms may not be sufficient to understand field observations.

Complexation of Ni(II) by α-isosaccharinic acid and gluconic acid from pH 7 to pH 13

Summary Nickel reactions with gluconic acid (Gl) and α-isosaccharinic acid (ISA) have been investigated from pH 7 to pH 13.3. Measurement of the stoichiometries of the products of these reactions showed that NiGl and NiISA were formed at pH<7, Ni2Gl(OH)3 and Ni2ISA(OH)3 were formed at pH values between 7 and 10 and Ni2Gl(OH)4 and Ni2ISA(OH)4 were formed at pH>10. The stability constants for the formation of the soluble complexes were measured using an ion exchange resin method (Schubert method) at pH 7 and 13.3, and differential pulse polarography at pH 7. The polarographic measurements were performed in order to check the validity of the Schubert method at pH 7. This was due to the possibility of any cationic complexes that may have been formed sorbing to the resin. The importance of determining the stoichiometry of the reactions before calculating the stability constants from measurements taken using the Schubert method is emphasised.

Carbon nanoparticle surface functionalisation: converting negatively charged sulfonate to positively charged sulfonamide.

The surface functionalities of commercial sulfonate-modified carbon nanoparticles (ca. 9-18 nm diameter, Emperor 2000) have been converted from negatively charged to positively charged via sulfonylchloride formation followed by reaction with amines to give suphonamides. With ethylenediamine, the resulting positively charged carbon nanoparticles exhibit water solubility (in the absence of added electrolyte), a positive zeta-potential, and the ability to assemble into insoluble porous carbon films via layer-by-layer deposition employing alternating positive and negative carbon nanoparticles. Sulfonamide-functionalised carbon nanoparticles are characterised by Raman, AFM, XPS, and voltammetric methods. Stable thin film deposits are formed on 3 mm diameter glassy carbon electrodes and cyclic voltammetry is used to characterise capacitive background currents and the adsorption of the negatively charged redox probe indigo carmine. The Langmuirian binding constant K = 4000 mol(-1)dm(3) is estimated and the number of positively charged binding sites per particle determined as a function of pH.

Hydrothermal conversion of one-photon-fluorescent poly-(4-vinylpyridine) into two-photon-fluorescent carbon nanodots

A novel two-photon-fluorescent N,O-heteroatom-rich carbon nanomaterial has been synthesized and characterized. The new carbon nanoparticles were produced by hydrothermal conversion from a one-photon-fluorescent poly(4-vinylpyridine) precursor (P4VP). The carbonized particles (cP4VP dots) with nonuniform particle diameter (ranging from sub-6 to 20 nm with some aggregates up to 200 nm) exhibit strong fluorescence properties in different solvents and have also been investigated for applications in cell culture media. The cP4VP dots retain their intrinsic fluorescence in a cellular environment and exhibit an average excited-state lifetime of 2.0 ± 0.9 ns in the cell. The cP4VP dots enter HeLa cells and do not cause significant damage to outer cell membranes. They provide one-photon or two-photon fluorescent synthetic scaffolds for imaging applications and/or drug delivery.

The solubility of technetium(IV) at high pH

Summary Technetium-99 is a β-emitting fission product of 238U. It is important when considering a safety case for nuclear waste disposal because of its high yield in radioactive waste and long half-life. The aqueous chemistry of Tc in a cementitious repository is likely to be dominated by Tc(VII), as TcO4−, in aerobic areas and by Tc(IV), as TcO2 (am), in anaerobic. Some previous studies have shown an increase in Tc(IV) solubility with increasing pH above pH 9.5 whilst others have not observed this increase. This study found that the solubility of Tc(IV) remains independent of pH until around 13.5, when a small increase can be seen which continues to increase linearly with pH. Modelling suggests that this increase occurs as the species TcO(OH)3− is formed. The formation constant was estimated with data from this study and was found to be logK2=−21.6±0.3.

Sorption of radionuclides to a cementitious backfill material under near-field conditions

Abstract The sorption behaviour of I-, Cs+ , Ni2+, Eu3+, Th4+ and UO22+ on NRVB (Nirex reference vault backfill) a possible vault backfill, at pH 12.8 was studied. Sorption isotherms generated were compared to results obtained in the presence of cellulose degradation products (CDP). Whereas Cs was not affected by the presence of the organic compounds, a notable reduction in the sorption of Th and Eu to cement was observed. The results also indicated limited removal of Ni from solution (with or without an organic ligand) by sorption, the concentration in solution seemingly being determined solely by solubility processes. In the case of uranium, the presence of CDP increased the sorption to cement by almost one order of magnitude. Further studies into the uptake of CDP by cement are being undertaken to identify the mechanism(s) responsible.

Studies on some divalent metal α-isosaccharinic acid complexes

Summary The presence of organic complexants, such as α-isosaccharinic acid (ISA) in an intermediate-level radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides by forming organic complexes in solution. To assess this, stability constants are required for the complexes formed. Constants have been determined for the divalent metal ions, Cd2+, Co2+ and UO22+ with α-isosaccharinic acid (ISA) in near-neutral and alkaline conditions. The effect of these complexes on radionuclide solubility in the near-field and alkaline-disturbed zone is shown. The constants were measured by the ion-exchange (Schubert) method [1] with the exception of U(VI) at pH 13.3, for which the solubility product method [2] was used. At pH7 each complex was of the form M1ISA1, with log β values suggestive of salt formation. At high pH, log β values were between 13 and 20. The number of hydroxide ions involved in the complexation reactions was determined using Bjerrum plots, conductometric titrations and spectrophotometric methods [1]. The constants have enabled speciation calculations to be performed showing the effect of ISA on the metals solubility. Solubility is predicted to increase in the presence of ISA from pH 9 to 13.5, suggesting that it may have an impact on radionuclide behaviour. The largest solubility increases are for Cd and Co, the smallest for U(VI).

Retention of chlorine-36 by a cementitious backfill

Abstract Radial diffusion experiments have been carried out to assess the migration of 36Cl, as chloride, through a cementitious backfill material. Further experiments in the presence of cellulose degradation products were performed to assess the effect of organic ligands on the extent and rate of chloride diffusion. Results show that breakthrough of 36Cl is dependent on chloride concentration: as the carrier concentration increases, both breakthrough time and the quantity retained by the cement matrix decreases. Experiments in the presence of cellulose degradation products also show a decrease in time to initial breakthrough. However, uptake at various carrier concentrations in the presence of organic ligands converges at 45% of the initial concentration as equilibrium is reached. The results are consistent with organic ligands blocking sites on the cement that would otherwise be available for chloride binding, though further work is required to confirm that this is the case. Post-experimental digital autoradiographs of the cement cylinders, and elemental mapping showed evidence of increased 36Cl activity associated with black ash-like particles in the matrix, believed to correspond to partially hydrated glassy calcium-silicate-sulfate-rich clinker.