Bryne T. Ngwenya

Comparison of the acid-base behaviour and metal adsorption characteristics of a gram-negative bacterium with other strains

Thermodynamic parameters for proton and metal adsorption onto a gram-negative bacterium from the genus Enterobacteriaceae have been determined and compared with parameters for other strains of bacteria. Potentiometric titrations were used to determine the different types of sites present on bacterial cell walls. Stability constants for adsorption of Pb, Cu and Zn to specific sites were determined from batch adsorption experiments at varying pH with constant metal concentration. Titrations revealed 3 distinct acidic surface sites on the bacterial surface, with pK values of 4.3±0.2, 6.9±0.5 and 8.9±0.5, corresponding to carboxyl, phosphate and hydroxyl/amine groups, with surface densities of 5.0±0.7×10−4, 2.2±0.6×10−4 and 5.5±2.2×10−4 mol/g of dry bacteria. Only carboxyl and phosphate sites are involved in metal uptake, yielding the following intrinsic stability constants: Log Kcarboxyl: Zn=3.3±0.1, Pb=3.9±0.8, and Cu=4.4±0.2, Log Kphosphoryl: Zn=5.1±0.1 and Pb=5.0±0.9. The deprotonation constants are similar to those of other strains of bacteria, while site densities are also within an order of magnitude of other strains. The similarities in surface chemistry and metal stability constants suggest that bacteria may be represented by a simple generic thermodynamic model for the purposes of modelling metal transport in natural environments. Comparison with oxide-coated sand shows that bacteria can attenuate some metals to much lower pH values.

Enhanced resistance to nanoparticle toxicity is conferred by overproduction of extracellular polymeric substances.

The increasing production and use of engineered nanoparticles, coupled with their demonstrated toxicity to different organisms, demands the development of a systematic understanding of how nanoparticle toxicity depends on important environmental parameters as well as surface properties of both cells and nanomaterials. We demonstrate that production of the extracellular polymeric substance (EPS), colanic acid by engineered Escherichia coli protects the bacteria against silver nanoparticle toxicity. Moreover, exogenous addition of EPS to a control strain results in an increase in cell viability, as does the addition of commercial EPS polymer analogue xanthan. Furthermore, we have found that an EPS producing strain of Sinorhizobium meliloti shows higher survival upon exposure to silver nanoparticles than the parent strain. Transmission electron microscopy (TEM) observations showed that EPS traps the nanoparticles outside the cells and reduces the exposed surface area of cells to incoming nanoparticles by inducing cell aggregation. Nanoparticle size characterization in the presence of EPS and xanthan indicated a marked tendency towards aggregation. Both are likely effective mechanisms for reducing nanoparticle toxicity in the natural environment.

Manganese concentrations in Scottish groundwater.

Groundwater is increasingly being used for public and private water supplies in Scotland, but there is growing evidence that manganese (Mn) concentrations in many groundwater supplies exceed the national drinking water limit of 0.05 mg l(-1). This study examines the extent and magnitude of high Mn concentrations in groundwater in Scotland and investigates the factors controlling Mn concentrations. A dataset containing 475 high quality groundwater samples was compiled using new data from Baseline Scotland supplemented with additional high quality data where available. Concentrations ranged up to 1.9 mg l(-1); median Mn concentration was 0.013 mg l(-1) with 25th and 75th percentiles 0.0014 and 0.072 mg l(-1) respectively. The Scottish drinking water limit (0.05 mg l(-1)) was exceeded for 30% of samples and the WHO health guideline (0.4 mg l(-1)) by 9%; concentrations were highest in the Carboniferous sedimentary aquifer in central Scotland, the Devonian sedimentary aquifer of Morayshire, and superficial aquifers. Further analysis using 137 samples from the Devonian aquifers indicated strong redox and pH controls (pH, Eh and dissolved oxygen accounted for 58% of variance in Mn concentrations). In addition, an independent relationship between Fe and Mn was observed, suggesting that Fe behaviour in groundwater may affect Mn solubility. Given the redox status and pH of Scottish groundwaters the most likely explanation is sorption of Mn to Fe oxides, which are released into solution when Fe is reduced. Since the occurrence of elevated Mn concentrations is widespread in groundwaters from all aquifer types, consideration should be given to monitoring Mn more widely in both public and private groundwater supplies in Scotland and by implication elsewhere.

HYDROTHERMAL RARE EARTH MINERALISATION IN CARBONATITES OF THE TUNDULU COMPLEX, MALAWI, PROCESSES AT THE FLUID/ROCK INTERFACE

The Tundulu carbonatite complex in southeastern Malawi was intruded during the late Jurassic to early Cretaceous over three episodes. During the first and second episodes, the major rock types were calcite carbonatites, ankerite carbonatites, and apatite carbonatites. These rocks experienced hydrothermal alteration at the close of the second episode, during which quartz-baryte veins containing a significant level of rare earth fluorocarbonates were deposited. Veins in calcite carbonatites contain abundant synchysite [(La, Ce, Nd)Ca(CO3)2)F] with subordinate amounts of parisite [(La, Ce, Nd)2Ca(CO3)3F2] and bastnaesite [(La, Ce, Nd)CO3F], crystallising in the order synchysite parisite → bastnaesite. Some of the parisite is retrograde in origin, having formed as an alteration product from synchysite. Only synchysite has been identified in veins hosted by apatite carbonatites. However, bastnaesite predominates the hydrothermal assemblage in ankerite carbonatites. Parisite and synchysite have been found only as a fibrous fringe between wallrock ankerite and bastnaesite. The crystallisation sequence seen in calcite carbonatites represents a progressive depletion of both Ca2+ and CO2−3 in the fluids from which this mineral suite precipitated, with Ca2+- and CO2−3-poor phases precipitating last. Clearly, the predominance of bastnaesite in veins hosted by ankerite carbonatites suggests insufficient Ca2+ and CO2−3 in the fluids for the ubiquitous precipitation of synchysite and parisite. These observations are consistent with a model in which hydrothermal fluids reacted with the various wallrocks, which then released different amounts of Ca2+ and CO2−3, that subsequently reacted with REEs in the fluid to form the various fluorocarbonates. As such, the Tundulu carbonatites provide a natural laboratory in which compositional phase relationships of rare earth fluorocarbonates can be related to variations in the activities of Ca2+ and CO2−3 within the Ln(CO3)F-CaCO3-F2(CO3)−1 system.

Physical and chemical effects of extracellular polymers (EPS) on Zn adsorption to Bacillus licheniformis S-86.

This study investigated Zn adsorption to an extracellular polymeric substance (EPS)-producing bacterial strain, Bacillus licheniformis S-86. Batch metal adsorption experiments and spectroscopic (EXAFS) analysis were conducted using both native (EPS-covered) cells and EPS-free cells in order to assess the contribution made by EPS to metal adsorption by this strain. Thermodynamic modelling of the macroscopic adsorption data indicated that Zn complexation to both native and EPS-free cells was predominantly to carboxyl (pK(a)5.3-5.4) and phosphate (pK(a)7.4-7.5) functional groups, but with some adsorption to phosphodiester (pK(a)3.3-3.4) groups also evident. EXAFS analysis shows Zn-carboxyl complexation, but possibly with a significant contribution from a second, phosphate functional group. Apparently, EPS removal does not affect the metal adsorption capacity at the metal: biomass ratios used here. As the concentration of carboxyl and phosphate functional groups is only slightly affected by EPS extraction, complexation to these functional groups explains why EPS removal does not reduce the amount of Zn adsorbed by the cells. It was also observed that EPS production induces aggregation of cells in suspension. This may reduce the cell surface area available for metal adsorption, thus counteracting any greater availability of metal-complexing ligands in the EPS layer compared to an EPS-free cell surface. Furthermore, the EPS layer appears to be the major source of dissolved organic carbon (DOC) released to solution during the metal adsorption experiments. This DOC may reduce metal binding to the cell surfaces by acting as a competing complexing ligand. These observations have implications for industrial application of biofilms and suggest that over-production of EPS in bio-reactors may reduce the metal removal efficiency of the biomass.

Fault sealing during deformation-band growth in porous sandstone

We present a simple geometric model for the evolution of fluid permeability during the sequential growth of deformation bands in an ideal elastic-brittle porous granular medium. The model is based on recent mechanical and microstuctural results from laboratory experiments on large (10 cm diameter) sandstone samples that reproduce field observation. The model assumes poroelastic compaction of the rock matrix in the prefailure stage, followed either by bulk shear-enhanced dilatancy or compaction in the postfailure stage, depending on confining pressure, and a constant-porosity shear zone that accumulates slip by sequentially increasing the number of discrete bands linearly with the inelastic strain. For large permeability contrasts between matrix and the deformation band, the model quantitatively explains the entire permeability cycle observed in bulk samples, including the apparent paradox of a negative correlation of bulk permeability with porosity during dilatant slip in the postfailure stage, as observed in recent laboratory tests.

A constitutive law for low‐temperature creep of water‐saturated sandstones

An accurate predictive model for the long-term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems. While laboratory experiments are consistent with Mohr-Coulomb brittle faulting in the cold, upper continental crust, there is increasing evidence that time-dependent processes may also be important in these rocks, even at low temperature. However, there is some ambiguity as to the exact form of the constitutive law for describing time-dependent behavior of upper crustal rocks. Here we present results of room temperature creep experiments on a suite of water-saturated sandstones spanning a range of petrophysical and rheological properties and underlying deformation mechanisms. On physical and microstructural grounds our analysis suggests that a modified power law creep, of the form ˙ A(d f) , where d is the differential stress and f is the long-term failure (fundamental) strength, provides a more complete description of the experimental data. In particular, the parameters can be used to differentiate between sandstone types, with A, f, and varying systematically with cementation state, rock rheology, and confining pressure. The fundamental strength (f) for time-dependent deformation varies much more than the other parameters of the distribution, making it a potentially sensitive indicator of underlying creep mechanisms. Further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three-parameter model is statistically better in the general case.

Experimental constraints on the diagenetic self-sealing capacity of faults in high porosity rocks

Abstract A thorough understanding of fault seal processes is important in many practical and geological applications, which depend on subsurface flow of fluids. While the mechanisms involved in fault sealing are well known, the microscale processes involved and their relative contribution to sealing remain debatable. In particular, the extent to which diagenetic processes overprint cataclastic fault sealing has not been resolved, mainly due to the long time scales required to measure these effects. Here, we report results from a novel suite of room temperature experiments that combined continuous analysis of dissolved silica using on-line high performance liquid chromatography, with low strain rate creep loading on sandstone cores. This technique allowed changes in silica concentration during different phases of deformation to be resolved, and revealed a 7-fold increase in overall silica concentration immediately after dynamic faulting by localised cataclasis. Calculations based on these results show that the mass of dissolved silica from the resultant fault gouge increased by up to two orders of magnitude relative to that from the intact rock over the same time scale. This increase represents the first stage of the inherent diagenetic sealing capacity of the fault, presumably through localised diffusive mass transfer. Post-test microstructural studies suggest that the magnitude of diagenetic self-sealing depends on lithological and mechanical attributes of the host rock, which control fault gouge microstructure. Our experiments suggest that diagenetic processes may account for permeability reduction of up to two orders of magnitude, comparable to reductions due to cataclasis alone. Together, these two processes account for the 5–6 orders of magnitude reduction of permeability observed in natural faults and deformation bands.

Faults, fluid flow, and petroleum traps

We conducted triaxial deformation experiments on large (0.1-m; 0.33-ft)-diameter cores of four sandstones from the Moab area to investigate the effect of total axial strain and effective confining pressure on the evolution of bulk permeability of faulted samples. Sandstones with low bulk porosities (Dewey Bridge and Slickrock Subkha) exhibited an increase in permeability with increasing inelastic axial strain at low effective confining pressures, whereas those with high porosity (Navajo and Slickrock Aeolian) showed a decline in permeability. However, all samples showed permeability decline with increasing inelastic axial strain at high effective confining pressures. Meanwhile, microstructural observations revealed no systematic dependence of the width of the shear zone and the number of deformation bands on either strain or effective confining pressure, although grain-size reduction was more intense at high effective confining pressures. A new geometric model has been developed based on these observations for a constant effective confining pressure and is shown to provide excellent agreement with the experimental data at all effective confining pressures. However, the parameters of the model depend only weakly on effective confining pressure for low-porosity sandstones, suggesting that cataclastic fault seals in low-porosity rocks have a low sensitivity to burial depth in the range studied here.

Correlation of microseismic and chemical properties of brittle deformation in Locharbriggs sandstone

constant stress rate loading. The exit pore fluid was analyzed after the tests for the concentration of dissolved ions and acoustic emission was monitored in real time throughout the tests. The exit pore fluid silica concentration and microcrack damage derived from the acoustic emission (AE) data both exhibited an exponential increase during the strain hardening phase of deformation. Damage parameters inferred from the AE data predict the stress-strain curves adequately, or at least up to the point of strong microcrack coalescence. The damage parameters and silica signal were strongly correlated by a power law relationship. The observed environment and strain rate dependence of mechanical properties can hence be attributed uniquely to time-dependent crack growth by the stress corrosion mechanism. INDEX TERMS: 1045 Geochemistry: Low-temperature geochemistry; 5102 Physical Properties of Rocks: Acoustic properties; 5104 Physical Properties of Rocks: Fracture and flow; KEYWORDS: stress corrosion, acoustic emission, silica dissolution, subcritical crack growth, deformation rate, brittle fracture Citation: Ojala, I. O., B. T. Ngwenya, I. G. Main, and S. C. Elphick, Correlation of microseismic and chemical properties of brittle deformation in Locharbriggs sandstone, J. Geophys. Res., 108(B5), 2268, doi:10.1029/2002JB002277, 2003.