Stephanie Handley-Sidhu

Uptake of Sr 2+ and Co 2+ into biogenic hydroxyapatite: implications for biomineral ion exchange synthesis

Biomineral hydroxyapatite (Bio-HAp) produced by Serratia sp. has the potential to be a suitable material for the remediation of metal contaminated waters and as a radionuclide waste storage material. Varying the Bio-HAp manufacturing method was found to influence hydroxyapatite (HAp) properties and consequently the uptake of Sr(2+) and Co(2+). All the Bio-HAp tested in this study were more efficient than the commercially available hydroxyapatite (Com-HAp) for Sr(2+) and Co(2+) uptake. For Bio-HAp the uptake for Sr(2+) and Co(2+) ranged from 24 to 39 and 29 to 78 mmol per 100 g, respectively. Whereas, the uptake of Sr(2+) and Co(2+) by Com-HAp ranged from 3 to 11 and 4 to 18 mmol per 100 g, respectively. Properties that increased metal uptake were smaller crystallite size (<40 nm) and higher surface area (>70 m(2) g(-1)). Organic content which influences the structure (e.g., crystallite arrangement, size and surface area) and composition of Bio-HAp was also found to be important in Sr(2+) and Co(2+) uptake. Overall, Bio-HAp shows promise for the remediation of aqueous metal waste especially since Bio-HAp can be synthesized for optimal metal uptake properties.

A field and modeling study of fractured rock permeability reduction using microbially induced calcite precipitation.

Microbially induced calcite precipitation (MICP) offers an attractive alternative to traditional grouting technologies for creating barriers to groundwater flow and containing subsurface contamination, but has only thus far been successfully demonstrated at the laboratory scale and predominantly in porous media. We present results of the first field experiments applying MICP to reduce fractured rock permeability in the subsurface. Initially, the ureolytic bacterium, Sporosarcina pasteurii, was fixed in the fractured rock. Subsequent injection of cementing fluid comprising calcium chloride and urea resulted in precipitation of large quantities (approximately 750 g) of calcite; significant reduction in the transmissivity of a single fracture over an area of several m(2) was achieved in around 17 h of treatment. A novel numerical model is also presented which simulates the field data well by coupling flow and bacterial and solute reactive transport processes including feedback due to aperture reduction via calcite precipitation. The results show that MICP can be successfully manipulated under field conditions to reduce the permeability of fractured rock and suggest that an MICP-based technique, informed by numerical models, may form the basis of viable solutions to aid pollution mitigation.

A review of the environmental corrosion, fate and bioavailability of munitions grade depleted uranium.

Depleted uranium (DU) is a by-product of nuclear fuel enrichment and is used in antitank penetrators due to its high density, self-sharpening, and pyrophoric properties. Military activities have left a legacy of DU waste in terrestrial and marine environments, and there have been only limited attempts to clean up affected environments. Ten years ago, very little information was available on the dispersion of DU as penetrators hit their targets or the fate of DU penetrators left behind in environmental systems. However, the marked increase in research since then has improved our knowledge of the environmental impact of firing DU and the factors that control the corrosion of DU and its subsequent migration through the environment. In this paper, the literature is reviewed and consolidated to provide a detailed overview of the current understanding of the environmental behaviour of DU and to highlight areas that need further consideration.

Bacterially produced calcium phosphate nanobiominerals : sorption capacity, site preferences, and stability of captured radionuclides

A Serratia sp. bacterium manufactures amorphous calcium phosphate nanominerals (BHAP); this material has shown increased sorption capacity for divalent radionuclide capture. When heat-treated (≥450 °C) the cell biomass is removed and the biominerals are transformed to hydroxyapatite (HAP). Using a multimethod approach, we have elucidated both the site preferences and stability of analogue radionuclide incorporation for Sr, Co, Eu, and U. Strontium incorporates within the bulk amorphous inorganic phase of BHAP; however, once temperature modified to crystalline HAP, bonding was consistent with Sr substitution at the Ca(1) and/or Ca(2) sites. Cobalt incorporation occurs within the bulk inorganic amorphous phase of BHAP and within the amorphous grain boundaries of HAP. Europium (an analogue for trivalent actinides) substituted at the Ca(2) and/or the Ca(3) position of tricalcium phosphate, a known component of HAP grain boundaries. Uranium was surface complexed with no secondary minerals detected. With multiple sites for targeted radionuclide incorporation, high loadings, and good stability against remobilization, BHAP is shown to be a potential material for the remediation of aqueous radionuclide in groundwater.

A new incorporation mechanism for trivalent actinides into bioapatite : a TRLFS and EXAFS study

One of the most toxic byproducts of nuclear power and weapons production is the transuranics, which have a high radiotoxicity and long biological half-life due to their tendency to accumulate in the skeletal system. This accumulation is inhomogeneous and has been associated with the chemical properties and structure of the bone material rather than its location or function. This suggests a chemical driving force to incorporation and requires an atomic scale mechanistic understanding of the incorporation process. Here we propose a new incorporation mechanism for trivalent actinides and lanthanides into synthetic and biologically produced hydroxyapatite. Time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure have been used to demonstrate that trivalent actinides and lanthanides incorporate into the amorphous grain boundaries of apatite. This incorporation site can be used to explain patterns in uptake and distribution of radionuclides in the mammalian skeletal system.

Influence of pH, competing ions, and salinity on the sorption of strontium and cobalt onto biogenic hydroxyapatite

Anthropogenic radionuclides contaminate a range of environments as a result of nuclear activities, for example, leakage from waste storage tanks/ponds (e.g. Hanford, USA or Sellafield sites, UK) or as a result of large scale nuclear accidents (e.g. Chernobyl, Ukraine or Fukushima, Japan). One of the most widely applied remediation techniques for contaminated waters is the use of sorbent materials (e.g. zeolites and apatites). However, a key problem at nuclear contaminated sites is the remediation of radionuclides from complex chemical environments. In this study, biogenic hydroxyapatite (BHAP) produced by Serratia sp. bacteria was investigated for its potential to remediate surrogate radionuclides (Sr2+ and Co2+) from environmentally relevant waters by varying pH, salinity and the type and concentration of cations present. The sorption capacity of the BHAP for both Sr2+ and Co2+ was higher than for a synthetically produced hydroxyapatite (HAP) in the solutions tested. BHAP also compared favorably against a natural zeolite (as used in industrial decontamination) for Sr2+ and Co2+ uptake from saline waters. Results confirm that hydroxyapatite minerals of high surface area and amorphous calcium phosphate content, typical for biogenic sources, are suitable restoration or reactive barrier materials for the remediation of complex contaminated environments or wastewaters.

Pathways of radioactive substances in the environment

The release and transport of radionuclides in the environment is a subject of great public concern. The primary sources of radionuclides in the environment are nuclear weapons testing and production, and the processes associated with the nuclear fuel cycle. Whilst nuclear weapons tests have been the main source of atmospheric contamination, resulting in global, low-level contamination, sites associated with weapon production and the nuclear fuel cycle can have localised high levels of contamination, and the spread of this contamination via aquatic pathways represents a significant environmental problem. Migration in the atmosphere will depend on the nature of the radioactive material and the prevailing meteorological conditions. Within surface water and groundwater environments, transport will be controlled by physical processes such as advection and the biogeochemical conditions in the system. In systems with significant flow, advection will be the dominant transport process, but as hydraulic conductivity decreases, chemical processes and conditions become increasingly important in controlling radionuclide migration. Factors such as solution phase chemistry (e.g. ionic strength and ligand concentrations), Eh and the nature of mineral phases in the system have a critical effect on radionuclide speciation, controlling partitioning between solution and solid phases and hence migration. Understanding the complex interplay between these parameters is essential for predicting radionuclide behaviour and migration in the environment.