Bruce D. Honeyman

Metals in aquatic systems

In this article we address the general question of whether we can yet predict metal-scavenging residence times in natural systems based on current knowledge about metal behavior as derived from laboratory studies. Although we will show that our ability to predict metal behavior is still relatively limited, such a conclusion should not be construed as a criticism of the reductionist approach. Rather, it should be seen that a number of challenges remain for surface chemists interested in the natural environment.

Isotopic evidence for the contemporary origin of high-molecular weight organic matter in oceanic environments

Abstract Previous work has suggested that apparent old 14C ages for oceanic DOC are the result of mixing of different organic carbon fractions. This report provides direct evidence for a contemporary 14C age of a high-molecular-weight (HMW) fraction of colloidal organic carbon (≥10 kD). Colloidal organic matter, COM10 (from 10 kDaltons (kD) to 0.2 μm), isolated from the upper water column of the Gulf of Mexico and the Middle Atlantic Bight (MAB) region, generally has a contemporary age (i.e., younger than a few decades), while COM1 (from 1 kD to 0.2 μm), is apparently old: 380–4500 y b P. Thus, BMW COM10 (3–5% of DOC) from the upper water column is derived from living particulate organic matter (POM) and cycles rapidly, while a significant fraction of low-molecular-weight (≤1 kD) DOM is likely more refractory, and cycles on much longer time scales. The presence of pigment biomarker compounds in COM1 from the upper water column points to selected phytoplankton species as one of the sources of COM. Terrestrial carbon as another source of COM is suggested from the inverse correlation between Δ14C and δ13C values, as well as the increasing δ13C values with increasing salinity. 234Th-derived turnover times of COM10 and COM1 from both the Gulf of Mexico and MAB are consistently short, 1–20 and 3–30 days, respectively. These short residence times support the hypothesis that 14C ages of colloidal fractions of DOC are the result of COM fractions being a mixture of several endmembers with fast and slow turnover rates.

The sorption of thorium (IV) and uranium (VI) to hematite in the presence of natural organic matter

Abstract In most aquatic systems, natural organic acids constitute an important pool of trace-metal binding ligands. The effect of natural organic matter (NOM) on metal ion sorption to mineral surfaces is a complex function of the environment in which the mineral and ions reside, as well as the source, molecular weight distribution and composition of the NOM. In spite of the well-acknowledged importance of NOM in affecting metal-ion speciation and the widely-established use of surface complexation models to predict the behavior of metal ions in heterogeneous systems, little work has been reported on efforts to incorporate NOM into surface complexation models. In this paper we present experimental results for two such ternary (metal ion/metal oxide/NOM) systems, detailing: (1) U(VI)/Suwannee river humic acid (HA)/hematite; and (2) Th(IV)/marine colloidal material (COM)/hematite. Our objective is to describe overall ternary system behavior through the construction of a model composed of ‘binary’ subsystems (e.g. HA/hematite). In all cases, however, the postulation of ternary surface complexes is required to satisfactorily simulate ternary system behavior. We also describe the simulation of HA and COM as a suite of monoprotic acids with fixed pKa values. This approach allows for the simultaneous simulation of both solution and surface reactions of NOM within a consistent chemical framework.

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.

Heterogeneous processes affecting trace contaminant distribution in estuaries: The role of natural organic matter

Abstract Our objective in writing this paper is to frame the issues pertaining to the role of natural organic matter in affecting the speciation and transport (fate) of trace contaminants, which include hydrophilic trace metals and hydrophobic trace organics, within estuaries. In particular, we focus on one aspect of the problem: the partitioning of trace contaminants between organic and inorganic particles, colloids and solution. This paper is developed along three main lines. First, we review the literature with respect to trace metal sorption by metal oxides in the presence of organic ligands. Second, we examine the role of colloidal organic matter in regulating the estuarine behavior of trace contaminants. The focus of this portion of the paper is on the trapping of trace metals within colloidal and particulate organic matter and a re-examination of the ‘particle concentration effect’. Third, we propose a new conceptual model (the ‘percolator’ model) which links several processes (sorption, diffusion, coagulation) and which serves as a framework for evaluating the trapping of trace metals and organic matter within estuarine sediments. One conclusion derived from simulations of the diffusive flux of DOC from sediments is that the magnitude of sediment/water interfacial shear stress does not affect DOC flux until the shear stress is sufficient for the onset of bed erosion. The consequence is that interactions between DOC and sediment materials become the controlling factors in regulating the diffusive flux of DOC.

Scavenging of thorium isotopes by colloids in seawater of the Gulf of Mexico

A suite of surface-water samples from the Gulf of Mexico was analyzed to ascertain the extent of association of Th isotopes (232Th, 234Th) with colloids and the role of colloidal material in scavenging Th from the water column. These are the first measurements of naturally occurring colloidal Th. The fraction of 232Th, 234Th associated with colloids (i.e., 10,000 Dalton < colloids < 0.4 μm) ranged from 10 to 78% of the Th passing 0.4 μm Nucleopore cartridge filters. Colloid mass concentrations were much larger than the corresponding 0.4 μm filter-retained particle concentrations. The conditional partitioning constants for 234Th with colloids, Kc, and filter-retained particles, Kf, are comparable in magnitude. Thorium scavenging rate constants decreased in value with increasing distance from the coast (0.164 to 0.007 d−1), and this is attributed to the decreasing particulate-matter concentration from coastal to deeper Gulf waters. In addition, there exists a highly significant, positive correlation between values of the Th scavenging rate constant and the fraction of 0.4 μm filter-passing Th associated with colloids. An average residence time of 6 days, with a range of 4–26 days, was calculated for the characteristic time scale of colloid transfer through the particle size spectrum, including sedimentation. The large fraction of 234Th which was found to be associated with colloids suggests that Th isotopes can be used as in-situ “coagulometers,” tracing the aggregation of colloidal material with, or into, large aggregates of filter-retained sizes.

Modeling Reduction of Uranium U(VI) under Variable Sulfate Concentrations by Sulfate-Reducing Bacteria

ABSTRACT The kinetics for the reduction of sulfate alone and for concurrent uranium [U(VI)] and sulfate reduction, by mixed and pure cultures of sulfate-reducing bacteria (SRB) at 21 ± 3°C were studied. The mixed culture contained the SRB Desulfovibrio vulgarisalong with a Clostridium sp. determined via 16S ribosomal DNA analysis. The pure culture was Desulfovibrio desulfuricans (ATCC 7757). A zero-order model best fit the data for the reduction of sulfate from 0.1 to 10 mM. A lag time occurred below cell concentrations of 0.1 mg (dry weight) of cells/ml. For the mixed culture, average values for the maximum specific reaction rate,Vmax, ranged from 2.4 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h−1) at 0.25 mM sulfate to 5.0 ± 1.1 μmol of sulfate/mg (dry weight) of SRB · h−1 at 10 mM sulfate (average cell concentration, 0.52 mg [dry weight]/ml). For the pure culture,Vmax was 1.6 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h−1 at 1 mM sulfate (0.29 mg [dry weight] of cells/ml). When both electron acceptors were present, sulfate reduction remained zero order for both cultures, while uranium reduction was first order, with rate constants of 0.071 ± 0.003 mg (dry weight) of cells/ml · min−1 for the mixed culture and 0.137 ± 0.016 mg (dry weight) of cells/ml · min−1 (U0 = 1 mM) for the D. desulfuricans culture. Both cultures exhibited a faster rate of uranium reduction in the presence of sulfate and no lag time until the onset of U reduction in contrast to U alone. This kinetics information can be used to design an SRB-dominated biotreatment scheme for the removal of U(VI) from an aqueous source.

Sorption irreversibility and coagulation behavior of 234Th with marine organic matter

Abstract The partitioning of 234 Th to natural organic matter (NOM) in the colloidal size range (1 kDa–0.1 μm) was evaluated in order to examine the sorption and coagulation behavior of marine colloidal organic matter. Colloids were isolated using large volume cross-flow ultrafiltration and the partitioning of 234 Th was quantified using stirred cell ultrafiltration and radioactive assay. The uptake of 234 Th by NOM is irreversible over a period of 5 days, implying that over the mean life of 234 Th, very little release of 234 Th would occur after binding to NOM. Furthermore, the Th–NOM complex is stronger than the Th–EDTA complex, as EDTA was unable to displace the 234 Th from its association with NOM. The extent of the initial partitioning of 234 Th to suspended matter and colloids is similar and independent of pH in the range from 3 to 9. Coagulation experiments show that 234 Th complexed with low molecular weight (1–10 kDa) colloids is transferred to larger (>0.1 μm) filter retained fractions. However, 234 Th is transferred to a greater extent than is organic matter and this results in greater partitioning coefficients for 234 Th onto particle phases with time. The final equilibrium between 0.1 μm filter-retained and filter-passing 234 Th activity is the same regardless of whether the Th was tagged initially to colloidal or suspended matter fractions. The coagulation of colloidal organic matter, COM, consists of both fast and slow steps, with kinetic rate constants on the order of 0.02–0.03 and 0.003–0.007 h −1 , respectively. The stickiness (or collision efficiency) factor, α , for COM was experimentally determined to be 0.7(−) for seawater conditions. Using the colloidal pumping model of Honeyman and Santschi [J. Mar. Res. 47 (1989): 951], the ‘predicted’ “fast-phase” coagulation rate coefficient is 0.03 h −1 in our coagulation experiments when the measured α value and the experimental conditions are used for model inputs. These experiments demonstrate that coagulation is the dominant step in the transport of 234 Th to the particulate phase.

Assessing conceptual models for subsurface reactive transport of inorganic contaminants

In many subsurface situations where human health and environmental quality are at risk (e.g., contaminant hydrogeology petroleum extraction, carbon sequestration, etc.),scientists and engineers are being asked by federal agency decision-makers to predict the fate of chemical species under conditions where both reactions and transport are processes of first-order importance. In 2002, a working group (WG) was formed by representatives of the U.S. Geological Survey, Environmental Protection Agency, Department of Energy Nuclear Regulatory Commission, Department of Agriculture, and Army Engineer Research and Development Center to assess the role of reactive transport modeling (RTM) in addressing these situations. Specifically the goals of the WG are to (1) evaluate the state of the art in conceptual model development and parameterization for RTM, as applied to soil,vadose zone, and groundwater systems, and (2) prioritize research directions that would enhance the practical utility of RTM.

Thorium sorption in the marine environment: Equilibrium partitioning at the hematite/water interface, sorption/desorption kinetics and particle tracing

AbstractThorium(IV) sorption onto hematite (α-Fe2O3) was examined as a function of pH and ionic strength. Sorption behaved Langmuirian over an eleven order of magnitude range in adsorption densities, Γ: 10−12 to 10−1 moles Th sorbed per mole hematite sites, indicating that the overall free energy of Th adsorption is independent of adsorption density. Modeling of Th sorption was conducted with the Triple Layer Model of Davis and Leckie; reactions considered included solution-phase hydroxy and carbonato complexes of thorium, and carbonate/hematite surface complexes. The entire Th sorption isotherm can be modeled with a single surface complex formation reaction