EFFECT OF SOIL TYPE ON THE EXTRACTABILITY OF 237Np, 239Pu, 241Am, AND 244Cm AS A FUNCTION OF pH

Abstract

Using an equilibrium batch technique, we determined the extractability of 237Np, 239Pu, 241Am, and 244Cm from widely different kinds of soils as a function of pH. Hydrochloric acid or NaOH was used to adjust the pH. The sorption and extractability of the transuranics studied varied considerably with pH and the kind of soil. A general pattern of extractability, common to all the transuranics studied, was exhibited to a varying degree by each mineral soil, except the calcareous soil. Typically, the extractability was relatively high under strongly acidic conditions, particularly below pH 2. Then, as the pH increased, there was a zone of rapid decrease, which was followed by a zone of relatively low extractability. Above the latter zone, there was an increase to a peak with increasing pH and then a decline. The organic soil used differed from this general pattern in that it showed a progressive increase of extractability in the alkaline pH range instead of peaking and declining. Among the mineral soils, the extractability of 237Np was markedly greater than those of 239Pu, 241Am, or 244Cm in the pH range below pH 6. Above this pH, the relative extractability of the radionuclides depended on the soil. The extractability of 244Cm was practically the same as that of 241Am over the entire pH range. The relative magnitudes of the distribution coefficient, Kd, values around the pH levels of the soils were 237Np < 239Pu < 241Am, 244Cm. Soil organic matter and hydrous oxides of Al, Fe, and Mn appeared to influence the sorption and extractability of the transuranics studied.