Geochemical Interactions between Leachants from Actinide-Doped Synroc and Three Australian Granites

Abstract

A study has been undertaken on the interactions of leachates from actinide-doped Synroc and three Australian granites. Leach testing was carried out on Synroc containing 237 Np, 239 Pu and 244 Cm under MCC-1 conditions in close proximity to granite specimens at 70 °C for 28 days.The presence of the granites increased the pH and reduced the total activity released from the Synroc. Preferential uptake of actinides occurred onto specific secondary minerals, especially hematite, other Fe-oxides/hydroxides, leucoxene, sericite, chlorite and clays. The controlling mechanisms for fixation of actinides on the granties appeared to be adsorption, ionic exchange and reduction by Fe(II). The high sorptive capacity of the alteration minerals significantly reduced the concentration of actinides in solution indicating that hydrothermally-altered granites, with a significant proportion of secondary minerals present, are capable of removing actinides from groundwaters.

Radioactivity: A Tool to Explore the Past

“…Several of these decay products may be used for residence time studies in the atmosphere, but the most important member in the 222 Rn decay chain is certainly 210 Pb. 210 Pb is deposited as wet or dry fallout onto the Earth's surface and can be used to obtain age information over the last 100 to 150 years.…”

“…In geological regions containing unconsolidated sediments (e.g., glaciofluvial deposits), surface water may infiltrate and become shallow groundwater. Hoehn and von Gunten [69] and Hoehn et al [70] have used 222 Rn to estimate the residence time and the related flow velocity in groundwater reservoirs of Switzerland. In surface waters, the radioactive equilibrium between 226 Ra and 222 Rn is strongly perturbed, because the water is only in contact with the uppermost layers of sediments and outgases to the atmosphere.…”

“…2.1 Radon-222 222 Rn and its decay products are important atmospheric radionuclides because they are responsible for a sizable fraction of the total public exposure to ionizing radiation. Typical exhalation rates of 222 Rn from the soil are on the order of one atom/cm 2 s or 2 · 10"…”

“…These actinides are mainly contained in the lithosphere and decay via several nuclides to different isotopes of lead. In both decay chains one intermediate nuclide is an isotope of the noble gas radon, 222 Rn (called "radon") in case of 238 U and 220 Rb ("thoron") in case of 232 Th which have half-lives of 3.8 days and 56 s, respectively. These gases can escape from the lithosphere into the atmospheres.…”

“…The most important dating methods are discussed. They include radioactive time markers (e.g., 137 Cs), cosmogenic radionuclides (e.g., ,4 C, "'Be), decay products of 226 Ra (e.g., 222 Rn, 210 Pb), uranium-thorium-lead chronometers, rubidium-strontium and potassium-argon nuclide pairs, applications of radioactive disequilibrium, dating with fission tracks, and thermoluminescence. Some important results, for instance the age of the Earth and the Moon, are also presented.…”