Conceptual and numerical model of uranium(VI) reductive immobilization in fractured subsurface sediments

A conceptual model and numerical simulations of bacterial U(VI) reduction in fractured subsurface sediments were developed to assess the potential feasibility of biomineralization at the fracture/matrix interface as a mechanism for immobilization of uranium in structured subsurface media. The model envisions flow of anaerobic groundwater, with or without acetate as an electron donor for stimulation of U(VI) reduction by dissimilatory metal-reducing bacteria (DMRB), within mobile macropores along a one-dimensional flow path. As the groundwater moves along the flow path, U(VI) trapped in the immobile mesopore and micropore domains (the sediment matrix) becomes desorbed and transferred to the mobile macropores (fractures) via a first-order exchange mechanism. By allowing bacterial U(VI) reduction to occur in the mesopore domain (assumed to account for 12% of total sediment pore volume) according to experimentally-determined kinetic parameters and an assumed DMRB abundance of 10(7) cells per cm3 bulk sediment (equivalent to 4 mg of cells per dm3 bulk sediment), the concentration of U(VI) in the macropore domain was reduced ca. 10-fold compared to that predicted in the absence of mesopore DMRB activity after a 6-month simulation period. The results suggest that input of soluble electron donors over a period of years could lead to a major redistribution of uranium in fractured subsurface sediments, converting potentially mobile sorbed U(VI) to an insoluble reduced phase (i.e. uraninite) in the mesopore domain that is expected to be permanently immobile under sustained anaerobic conditions.     

Laboratory and field investigations of particulate and carbon monoxide emissions from traditional and improved cookstoves

We implemented a program in which emission characterization is enabled through collaborations between academic, US and international non-governmental entities that focus on evaluation, dissemination, and in-use testing, of improved cookstoves. This effort resulted in a study of field and laboratory emissions from traditional and improved biofuel cookstoves. We found that field measured particulate emissions of actual cooking average three times those measured during simulated cooking in the laboratory. Emission factors are highly dependent on the care and skill of the operator and the resulting combustion; these do not appear to be accurately reproduced in laboratory settings. The single scattering albedo (SSA) of the emissions was very low in both lab and field measurements, averaging about 0.3 for lab tests and around 0.5 for field tests, indicating that the primary particles are climate warming. Over the course of three summers in Honduras, we measured field emissions from traditional cookstoves, relatively new improved cookstoves, and “broken-in” improved cookstoves. We found that well-designed improved cookstoves can significantly reduce PM and CO emission factors below traditional cookstoves. For improved stoves, the presence of a chimney generally resulted in lower emission factors but left the SSA unaffected. Traditional cookstoves had an average PM emission factor of 8.2 g kg^?1 – significantly larger than previous studies. Particulate emission factors for improved cookstoves without and with chimneys averaged about 6.6 g kg^?1 and 4.5 g kg^?1, respectively. The elemental carbon (EC) fraction of PM varied significantly between individual tests, but averaged about 25% for each of the categories.     

Resource allocation in Mytilus edulis on the shore and in suspended culture

The annual cycle of carbon and nitrogen content of the flesh of wild and cultivated mussels (Mytilus edulis L.) in Killary Harbour, Ireland, was measured over two years, starting in February, 1980 and ending in November, 1981. The carbon and nitrogen contents of mussel gametes were determined and the allometry of growth of wild and cultured mussel shells was examined with respect to length, weight and organic content. The carbon and nitrogen contents of the organic fraction of the shell were determined. These data were combined with those we had previously published on growth rate, gametogenesis and the annual cycle of ash-free dry weight (AFDW) of mussels in the same locality. Estimates were made of fecundity, reproductive effort and the partitioning of carbon and nitrogen between soma, gametes and shell. In suspended culture, cumulative production after eighteen months is equal to cumulative production after six to seven years on the shore. For comparison, partitioning of carbon and nitrogen resources between soma, shell and gonad is estimated when total cumulative production by wild and cultivated mussels is approximately equal. Differences in resource allocation are considerable. Wild mussels allocate some 57% of their carbon budget and 52% of their nitrogen budget to gamete output. In culture, mussels allocate only 22% of their carbon budget and 19% of their nitrogen budget to gamete output. It is concluded that in response to a higher production rate, cultivated mussels increase allocation of resources to somatic growth.     

Chlorophyll blooms and the spring/neap tidal cycle: observations at two stations on the coast of Connemara,Ireland

Water temperature, chlorophyll a and phaeophytin were recorded every 2 to 4 d, in order to describe short-term variability in phytoplankton blooms. Data were collected from May to September 1984 and March to September 1985 at shore stations on the coast of Connemara, western Ireland. Comparisons with offshore data suggest that the shore data is representative of coastal water. Chlorophyll a was highest at neap tides in spring, and at spring tides in early summer. In late summer, transient chlorophyll blooms occurred at neap tides, possibly due to a localized accumulation of flagellates in stable conditions.     

Microbial physiology-based model of ethanol metabolism in subsurface sediments

Dense non-aqueous phase liquids (DNAPLs) present in the subsurface may contain surface active compounds that impact DNAPL migration and distribution. While a number of studies have revealed the role surface active compounds play in altering the wettability of quartz sand, few have considered the implications for other minerals common to contaminated sites. This study extends understanding of DNAPL/surfactant wettability to iron oxide surfaces. Specifically, quartz and iron oxide-coated sands in a tetrachloroethene (PCE)/water system containing the organic base (an organic molecule that acts as a base) dodecylamine (DDA) were compared at a variety of scales. Wettability of the minerals' surfaces, and the impact of wettability on capillary resistance to DNAPL entry, were assessed as a function of pH through: (i) advancing and receding contact angles, (ii) primary drainage capillary pressure-saturation experiments, and (iii) small, two-dimensional, flow cell experiments. The work revealed that, at neutral pH and under identical boundary capillary pressures, DNAPL invaded quartz sand but not iron oxide-coated sand; however, at low pH, DNAPL invaded both sands equally. These differences were demonstrated to be due to wettability alterations associated with the strength of attractive forces between DDA and the mineral surface, dictated by the isolectric point of the minerals and system pH. Observed differences in DNAPL invasion behavior were consistent with measured intrinsic contact angles and P(c)-S relationships, the latter requiring scaling by the operative contact angle inside the porous medium for a meaningful comparison. This study suggests that the distribution of minerals (and, more specifically, their isoelectric points), as well as the aqueous phase pH at a given site, may have a significant impact on the DNAPL source zone architecture.     

Reactive transport of uranium(VI) and phosphate in a goethite-coated sand column: an experimental study

The migration of uranium(VI) in subsurface environments is strongly influenced by its adsorption/desorption reactions at the solid/solution interface. Phosphate is often present in subsurface systems and was shown to significantly affect U(VI) adsorption in previous batch experiments. In this study, column experiments were conducted to investigate the effects of phosphate on U(VI) adsorption and transport under flow conditions. The adsorption of U(VI) and phosphate was very low on pure quartz sand with negligible effects on U(VI) and phosphate transport. However, U(VI) and phosphate transport was retarded in a column packed with goethite-coated sand. The presence of phosphate, either as a co-solute with U(VI) or pre-adsorbed, greatly increased U(VI) adsorption and retardation. U(VI) and phosphate adsorption in our column experiments were rate-limited, and the adsorption of U(VI) and phosphate was not reversible, with kinetic limitations more pronounced for desorption than for adsorption. This study demonstrated the importance of phosphate in controlling U(VI) mobility in subsurface environments and helped illustrate some phenomena potentially applicable to U(VI) adsorption and transport in natural systems, especially where U(VI) adsorption is rate-limited.