Reaction Mechanism Leading to the Formation of Molecular Hydrogen in the Radiation Chemistry of Water

X-ray-Powered Micromotors

Light-powered wireless manipulation of small objects in fluids has been of interest for biomedical and environmental applications. Although many techniques employing UV-vis-NIR light sources have been devised, new methods that hold greater penetrating power deep into medium are still in demand. Here, we develop a method to exploit X-rays to propel half-metal-coated Janus microparticles in aqueous solution. The Janus particles are simultaneously propelled and visualized in real-time by using a full-field transmission X-ray microscope. Our real-time observation discovers that the propulsive motion follows the bubble growth enhanced by water radiolysis near the particle surface under X-ray irradiation. We also show that the propulsion speed is remotely controlled by varying the radiation dose. We expect this work to open opportunities to employ light-powered micro/nanomotors in opaque environments, potentially by combining with medical imaging or nondestructive testing.

Evolution of heavy ions (He2+, H+) radiolytic yield of molecular hydrogen vs. “Track-Segment” LET values

Ionizing radiation’s effects onto water molecules lead to the ionization and/or the excitation of them. Then, these phenomena are followed by the formation of radicals and molecular products. The linear energy transfer (LET), which defines the energy deposition density along the radiation length, is different according to the nature of ionizing particles. Thus, the values of radiolytic yields, defined as the number of radical and molecular products formed or consumed by unit of deposited energy, evolve according to this parameter. This work consists in following the evolution of radiolytic yield of molecular hydrogen and ferric ions according to the “Track-Segment” LET of ionizing particles (protons, helions). Concerning G(Fe3+) values, it seems that the energy deposited into the Bragg peak does not play the main role for the Fe3+ radiolytic formation, whereas for the G(H2) it is the case with a component around 40% of the Bragg peak in the dihydrogen production. Therefore, as main results of this work, for high energetic Helion and Proton beams, the G(Fe3+) values, which can be used for further dosimetry studies for example during the α radiolysis experiments, and the primary g(H2) values for the Track-Segment LET, which can be used to determine the dihydrogen production by α-emitters, are published.

Reduction of Chromium(VI) mediated by zero-valent magnesium under neutral pH conditions

In an effort to assess the potential use of ZVMg in contaminant treatments, we examined Cr(VI) reduction mediated by ZVMg particles under neutral pH conditions. The reduction of Cr(VI) was tested with batch experiments by varying [Cr(VI)](0) (4.9, 9.6, 49.9 or 96.9 μM) in the presence of 50 mg/L ZVMg particles ([Mg(0)](0) = 2.06 mM) at pH 7 buffered with 50 mM Na-MOPS. When [Cr(VI)](0) = 4.9 or 9.6 μM, Cr(VI) was completely reduced within 60 min. At higher [Cr(VI)](0) (49.9 or 96.9 μM), by contrast, the reduction became retarded at >120 min likely due to rapid ZVMg dissolution in water and surface precipitation of Cr(III) on ZVMg particles. Surface precipitation was observed only when [Cr(VI)](0) = 49.9 or 96.9 μM and increased with increasing [Cr(VI)](0). The effect of dissolved oxygen was negligible on the rate and extent of Cr(VI) reduction. Experimental results indicated that Cr(VI) was reduced not directly by ZVMg but by reactive intermediates produced from ZVMg-water reaction under the experimental conditions employed in this study. In addition, the observed rates of Cr(VI) reduction appeared to follow an order below unity (0.19) with respect to [Cr(VI)](0). These results imply that ZVMg-mediated Cr(VI) reduction likely occurred via an alternative mechanism to the direct surface-mediated reduction typically observed for other zero-valent metals. Rapid and complete Cr(VI) reduction was achieved when a mass ratio of [ZVMg](0):[Cr(VI)](0) ≥ 100 at neutral pH under both oxic and anoxic conditions. Our results highlights the potential for ZVMg to be used in Cr(VI) treatments especially under neutral pH conditions in the presence of dissolved oxygen.

Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate

The production of molecular hydrogen in the radiolysis of dried or hydrated nanoporous controlled-pore glasses (CPG) has been carefully studied using 10 MeV electron irradiation at high dose rate. In all cases, the H2 yield increases when the pore size decreases. Moreover, the yields measured in dried materials are two orders of magnitude smaller than those obtained in hydrated glasses. This proves that the part of the H2 coming from the surface of the material is negligible in the hydrated case. Thus, the measured yields correspond to those of nanoconfined water. Moreover, these yields are not modified by the presence of potassium bromide, which is a hydroxyl radical scavenger. This experimental observation shows that the back reaction between H2 and HO* does not take place in such confined environments. These porous materials have been characterized before and after irradiation by means of Fourier-transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) techniques, which helps to understand the elementary processes taking place in this type of environment, especially the protective effect of water on the surface in the case of hydrated glasses.

Radiolysis of Confined Water: Molecular Hydrogen Formation

The formation of molecular hydrogen in the radiolysis of water confined in nanoscale pores of well-characterised porous silica glasses and mesoporous molecular sieves (MCM-41) is examined. The comparison of dihydrogen formation by irradiation of both materials, dry and hydrated, shows that a large part of the H2 comes from the surface of the material. The radiolytic yields, G(H2)=(3+/-0.5)x10(-7) mol J(-1), calculated using the total energy deposited in the material and the water, are only slightly affected by the degree of hydration of the material and by the pore size. These yields are also not modified by the presence of hydroxyl radical scavengers. This observation proves that the back reaction between H2 and HO(.) is inoperative in such confined environments. Furthermore, the large amount of H2 produced in the presence of different concentrated scavengers of the hydrated electron and its precursor suggests that these two species are far from being the only species responsible for the H2 formation. Our results show that the radiolytic phenomena that occur in water confined in nanoporous silica are dramatically different to those in bulk water, suggesting the need to investigate further the chemical reactivity in this type of environment.