Sensors based on proteins (GST-SmtA and MerR) with distinct binding sites for heavy metal ions were developed and characterized. A capacitive signal transducer was used to measure the conformational change following binding. The proteins were overexpressed in Escherichia coli, purified, and immobilized in different ways to a self-assembled thiol layer on a gold electrode placed as the working electrode in a potentiostatic arrangement in a flow analysis system. The selectivity and the sensitivity of the two protein-based biosensors were measured and compared for copper, cadmium, mercury, and zinc ions. The GST-SmtA electrodes displayed a broader selectivity (sensing all four heavy metal ions) compared with the MerR-based ones, which showed an accentuated selectivity for mercury ions. Metal ions could be detected with both electrode types down to femtomolar concentration. The upper measuring limits, presumably due to near saturation of the proteins' binding sites, were around 10(-10) M. Control electrodes similarly constructed but based on bovine serum albumin or urease did not yield any signals. The electrodes could be regenerated with EDTA and used for more than 2 weeks with about 40% reduction in sensitivity.
Geological
disposal is the globally preferred long-term solution
for higher activity radioactive wastes (HAW) including intermediate
level waste (ILW). In a cementitious disposal system, cellulosic waste
items present in ILW may undergo alkaline hydrolysis, producing significant
quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides.
Although microbial degradation of ISA has been demonstrated, its impact
upon the fate of radionuclides in a geological disposal facility (GDF)
is a topic of ongoing research. This study investigates the fate of
U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching
conditions similar to the far field of a GDF, containing ISA as the
sole carbon source, and elevated phosphate concentrations, incubated
both (i) under fermentation and (ii) Fe(III)-reducing conditions.
In the ISA-fermentation experiment, U(VI) was precipitated as insoluble
U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority
of the uranium was precipitated as reduced U(IV)-phosphates, presumably
formed via enzymatic reduction mediated by metal-reducing bacteria,
including Geobacter species. Overall, this suggests the establishment
of a microbially mediated “bio-barrier” extending into
the far field geosphere surrounding a GDF is possible and this biobarrier
has the potential to evolve in response to GDF evolution and can have
a controlling impact on the fate of radionuclides.
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