Nicholas D. Bryan

Colloidal properties of humic substances

Abstract Humic substances are structurally complex large to macromolecules which occur in soils and natural waters as a consequence of the breakdown of plant and animal residues by microbial activity. A major portion (approx. 50%) of the earths carbon is in the form of humic materials (fulvic and humic acids). The characterisation of humic substances is a major problem due to their heterogeneity both in terms of structure and size and to their tendency to associate in solutions as their concentration increases. Methods of characterisation of humic materials are reviewed and their interactions with substances such as clay particles, herbicides, pesticides and metallic ions which occur in the natural environment considered. The experimental methods of investigating the binding of metallic ions are also reviewed. There have been several approaches to the molecular modelling of metal ion binding based on the representation of humic molecules as colloidal particles with an associated electrical double layer. The various theoretical models have been described and the relative merits of the approaches compared.

Reactions of copper and cadmium ions in aqueous solution with goethite, lepidocrocite, mackinawite, and pyrite

Abstract The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on mackinawite and pyrite, Cd on mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Å ; two O atoms at 2.41 Å ) but with evidence for interaction with two further Fe (or Cu) at 2.92 Å . On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Å and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Å . The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Å ) but with evidence for a second shell of Fe atoms at 3.75 Å . On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Å . For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Å and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides-covellite at lower loadings and chalcocite at higher loadings. Cd interaction with mackinawite seemed to produce a CdS phase.

Modelling the effect of humic substances on the transport of europium through porous media : a comparison of equilibrium and equilibrium/kinetic models

A series of column experiments have been performed, which demonstrate the effect of humic substances on the transport of europium through a sand column. Two approaches have been used to model the results: an approach based on a assumption of local chemical equilibrium, and one which accounts for kinetically hindered reactions. A significant improvement in fit is observed upon the inclusion of kinetics. The model explains the observed behaviour of the europium in terms of two distinct humic bound fractions. One which is exchangeably bound, and interacts with the surface of the column packing, and another, kinetically hindered, which does not.

The kinetics and mechanisms of goethite and hematite crystallization under alkaline conditions, and in the presence of phosphate

Abstract The transformations of 2-line ferrihydrite to hematite (pH 10.7) or goethite (pH 13.7), and of phosphate-doped 2-line ferrihydrite to goethite (pH 13.7), were studied at 60.137 °C using synchrotron- based, in-situ energy dispersive powder diffraction (EDPD). The time-resolved data for the growth of the diffraction peaks were fitted with a pseudo first-order kinetic model. As shown in previous studies, the conditional rate constant of goethite formation increases with increasing pH and is significantly lower than that for hematite crystallization. The activation energies of nucleation for hematite (pH 10.7), pure goethite (pH 13.7), and phosphate-doped goethite (pH 13.7) are 24, 7, and 21 kJ/mol, respectively, whereas the activation energies of crystallization are 69, 39, and 26 kJ/mol. The crystallization of phosphate-doped ferrihydrite produced large rectangular goethite crystals with dense ferrihydrite cores on which the goethite grew epitaxially. The rate of goethite formation is greatly reduced in the presence of phosphate due to an increase in the entropic component of the free energy of activation. This increase in entropy arises from adsorption of phosphate on to the (210) crystal faces, with an associated increase in relative growth rate on the (101) faces.

Probing the Biogeochemical Behavior of Technetium Using a Novel Nuclear Imaging Approach

Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.

Metal-humic interactions: A random structural modelling approach

Abstract The binding of the metal cations copper and calcium to peat humics and ground water (aquatic) humic and fulvic acids has been studied using ion selective electrochemistry under a variety of experimental conditions (pH, ionic strength, etc.). Competition between the metals for sites on humic acid molecules was also studied. The Random molecular model developed by Murray and Linder (1983, 1984) has been adapted in an attempt to simulate humate-metal binding behaviour. The Murray and Linder model uses a random structural approach to identify the likely metal binding sites present in each humic sample. The model contains no adjustable parameters, but instead relies on experimentally derived variables, namely, percentage carbon and hydrogen plus carboxylate and phenol concentrations. The model constructs molecules of humic acids which conform to these measurements and then searches the structures for a series of predefined metal binding sites. After generating a large number of similar structures, all of which conform to the experimental data, average concentrations of the binding sites are calculated. In the present work, the model has been extended to include nitrogen donor atoms and to search for nitrogen containing metal binding sites. In order to predict metal speciation, a version of the geochemical speciation code PHREEQE was devised in which the electrostatic effects produced by the large humic charge were accounted for. Tests on the model showed that the binding strengths predicted by the computer were consistently less than those observed by experiment, probably due to underestimation of the binding site complexity.

Numerical modeling of humic colloid borne americium (III) migration in column experiments using the transport/speciation code K1D and the KICAM model

The humic colloid borne Am(III) transport was investigated in column experiments for Gorleben groundwater/sand systems. It was found that the interaction of Am with humic colloids is kinetically controlled, which strongly influences the migration behavior of Am(III). These kinetic effects have to be taken into account for transport/speciation modeling. The kinetically controlled availability model (KICAM) was developed to describe actinide sorption and transport in laboratory batch and column experiments. Application of the KICAM requires a chemical transport/speciation code, which simultaneously models both kinetically controlled processes and equilibrium reactions. Therefore, the code K1D was developed as a flexible research code that allows the inclusion of kinetic data in addition to transport features and chemical equilibrium. This paper presents the verification of K1D and its application to model column experiments investigating unimpeded humic colloid borne Am migration. Parmeters for reactive transport simulations were determined for a Gorleben groundwater system of high humic colloid concentration (GoHy 2227). A single set of parameters was used to model a series of column experiments. Model results correspond well to experimental data for the unretarded humic borne Am breakthrough.

Humic acid sorption onto a quartz sand surface: A kinetic study and insight into fractionation

A kinetic study of Aldrich humic acid sorption onto a quartz sand surface has revealed an initial rapid uptake of humic acid molecules followed by a much slower sorption. The humic acid molecular weight and chemical fractionation resulting from adsorption onto the simple quartz sand surface were investigated for the two kinetic steps by coupled asymmetric flow-field flow fractionation-UV/visible absorption spectrophotometry. The molecular weight distribution of residual humic acid in solution after adsorption deviated from the original molecular weight distribution, showing preferential adsorption of certain molecular weight components. This fractionation is different after the two kinetic steps. Humic acid molecules characterised by a molecular weight below 4800 Da and with a weight-average molecular weight (M(w)) of 1450 Da were adsorbed after the fast kinetic step, whereas humic acid molecules in the molecular weight range 1400-9200 Da and of M(w) 3700 Da were adsorbed after the slower uptake. Therefore, the adsorption of low molecular weight humic components takes place initially, and is then followed by the adsorption of higher molecular weight components. Chemical adsorptive fractionation, investigated by studying the 253 nm/203 nm absorbance ratio over time, shows that aromatic components are preferentially adsorbed during the fast kinetic step. The fractionation pattern may be explained by the physicochemical characteristics of the Aldrich humic acid and the underlying sorption processes. The trend for the sorption kinetics of europium onto the quartz sand surface in the presence of humic acid is similar to that of the humic acid itself.

Selectivity in the complexation of actinides by humic substances

The interactions of a range of actinide elements (Th, U, Np, Pu, Am) with humic substances from the Needles Eye natural analogue site were studied by gel permeation chromatography. Bulk humic substances were isolated by ammonia extraction, followed by dialysis against distilled water and freeze-drying. The gel permeation results suggest that Needles Eye humic substances can be fractionated into three incompletely resolved fractions with average molecular weights determined by analytical ultracentrifugation around 49 000 for Fraction 1, around 14 700 for Fraction 2 and around 8000 for Fraction 3. Although there are significant differences between the organic matter elution patterns in individual gel permeation experiments, presumably due to differences in column packing, these are much smaller than the differences between metal ions. The uranium that is naturally present in these humic substances is largely bound in the late-eluting fraction. Spikes of the early actinides, including Np and Pu in controlled valency states, have been added to the humic substances, and gel permeation of the spiked humic substances shows that the three humic fractions vary greatly in their effectiveness and selectivity as ligands for early actinides.

Microbial degradation of isosaccharinic acid at high pH

Intermediate-level radioactive waste (ILW), which dominates the radioactive waste inventory in the United Kingdom on a volumetric basis, is proposed to be disposed of via a multibarrier deep geological disposal facility (GDF). ILW is a heterogeneous wasteform that contains substantial amounts of cellulosic material encased in concrete. Upon resaturation of the facility with groundwater, alkali conditions will dominate and will lead to the chemical degradation of cellulose, producing a substantial amount of organic co-contaminants, particularly isosaccharinic acid (ISA). ISA can form soluble complexes with radionuclides, thereby mobilising them and posing a potential threat to the surrounding environment or ‘far field’. Alkaliphilic microorganisms sampled from a legacy lime working site, which is an analogue for an ILW-GDF, were able to degrade ISA and couple this degradation to the reduction of electron acceptors that will dominate as the GDF progresses from an aerobic ‘open phase’ through nitrate- and Fe(III)-reducing conditions post closure. Furthermore, pyrosequencing analyses showed that bacterial diversity declined as the reduction potential of the electron acceptor decreased and that more specialised organisms dominated under anaerobic conditions. These results imply that the microbial attenuation of ISA and comparable organic complexants, initially present or formed in situ, may play a role in reducing the mobility of radionuclides from an ILW-GDF, facilitating the reduction of undue pessimism in the long-term performance assessment of such facilities.