Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples.

A contamination with the ubiquitous radioactive fission product 137Cs cannot be assigned per se to its source. We used environmental samples with varying contamination levels from various parts of the world to establish their characteristic 135Cs/137Cs isotope ratios and thereby allow their distinction. The samples included biological materials from Chernobyl and Fukushima, historic ashed human lung tissue from the 1960s from Austria, and trinitite from the Trinity Test Site, USA. After chemical separation and gas reaction shifts inside a triple quadrupole ICP mass spectrometer, characteristic 135Cs/137Cs isotope signatures (all as per March 11, 2011) were obtained for Fukushima- (∼0.35) and Chernobyl-derived (∼0.50) contaminations, in agreement with the literature for these contamination sources. Both signatures clearly distinguish from the characteristic high ratio (1.9 ± 0.2) for nuclear-weapon-produced radiocesium found in human lung tissue. Trinitite samples exhibited an unexpected, anomalous pattern by displaying a low (<0.4) and nonuniform 135Cs/137Cs ratio. This exemplifies a 137Cs-rich fractionation of the plume in a nuclear explosion, where 137Cs is a predominant species in the fireball. The onset of 135Cs was delayed because of the longer half-life of its parent nuclide 135Xe, causing a spatial separation of gaseous 135Xe from condensed 137Cs, which is the reason for the atypical 135Cs/137Cs fractionation in the fallout at the test site.

Separation of Ultratraces of Radiosilver from Radiocesium for Environmental Nuclear Forensics.

The presence of environmental radiosilver and the investigation of the 108mAg/110mAg isotopic ratio in the aftermath of a nuclear power plant accident provide valuable information on the condition of the control rods of pressurized water reactors. However, the detection of minute amounts of the γ-emitting radiosilver isotopes is often thwarted by the presence of concomitant and dominating γ emitters, primarily 137Cs, which results in increased detection limits in the γ spectra. We developed a rapid and robust separation protocol for trace silver extraction in the presence of overwhelming activities of 137Cs via the autodepostion of silver on a copper plate. This method achieved a quantitative removal of interfering 137Cs in the deposition product and proved to be very efficient (yields >70% for aqueous samples), rapid (results within 4 h), and robust with respect to varying salinities and composition of the water samples. The autodeposition approach is also applicable for organic samples after acid-assisted microwave digestion. By applying the established sequential extraction protocols for soil, the fate of freshly deposited radiosilver and radiocesium in soil was investigated. Silver showed a high affinity to the soil with a pronounced (>90%) accumulation in the residual fraction after the sequential extraction, whereas radiocesium exhibited higher mobility, allowing for the extraction of major fractions in the first extraction steps. The composition of the aqueous contamination matrix (CaCl2 or Ca(NO3)2) had a significant influence on the binding properties of cesium on soil.

Multicomposite Nanostructured Hematite-Titania Photoanodes with Improved Oxygen Evolution: The Role of the Oxygen Evolution Catalyst.

We present a sol-gel processed hematite-titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm2 at 1.23 VRHE under simulated AM 1.5 G illumination (100 mW/cm2) thanks to the addition of an amorphous cocatalyst with the nominal composition Fe20Cr40Ni40O x . To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states (C SS) compared to the bare photoanode, the function of the catalyst-photoanode interface resembles that of a p-n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states (R ct,ss) was unchanged at potentials between 0.8 and 1.1 VRHE after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated.

Commercially Available WO3 Nanopowders for Photoelectrochemical Water Splitting: Photocurrent versus Oxygen Evolution.

WO3 photoanodes with remarkable photocurrent densities are presented. These photoanodes were prepared from three different commercially available WO3 nanopowders. Doctor blading of the nanopowders followed by a short annealing in air led to nanostructured films. The best photoanode showed a photocurrent density of 3.5 mA cm-2 at 1.23 V vs. RHE in 1 m CH3 SO3 H under AM 1.5 G illumination (100 mW cm-2 ), surpassing values reported so far for bare WO3 photoanodes. The study also showed that the photocurrent was strongly dependent on the electrolyte, indicating oxidation of the electrolyte rather than of water. Oxygen evolution measurements performed in different electrolytes revealed that the amounts of oxygen were highly dependent on the electrolyte. By comparing the photocurrent values in the different electrolytes with the amount of evolving oxygen, it was found that the electrolyte producing the highest photocurrent was the electrolyte with the lowest oxygen evolution. Stability measurements showed that the more oxygen is produced, the less stable is the photoanode. These results clearly underline the difficulty to correlate the photocurrent values with oxygen evolution, drawing the attention to one of the major limitations of photoelectrochemical water splitting.