IDENTIFICATION OF RADIONUCLIDE PARTITIONING IN SOILS AND SEDIMENTS - DETERMINATION OF OPTIMUM CONDITIONS FOR THE EXCHANGEABLE FRACTION OF THE NIST STANDARD SEQUENTIAL EXTRACTION PROTOCOL

We are developing the NIST standard sequential extraction protocol for identifying the fractionation of radioactive elements in soils and se diments. The procedure is designed to partition a soil or sediment sam ple into six operationally-define fractions. A full-factorial experime ntal design/data analysis methodology was used to establish the optimu m conditions for the first of these six fractions, called ''exchangeab les''. Reaction time, reagent concentration and reaction temperature w ere chosen as experimental variables. The criteria for choosing optimu m conditions were the maximum extraction of U-238 combined with a mini mum dissolution of non-targeted geochemical phases. Reagent concentrat ion (i.e. ionic strength) is shown to be the controlling factor in the extraction of exchangeable U-238. The optimum (exchangeable) conditio ns, based on these experiments:are: time = 1 h; temperature = 25 degre es C and concentration = 0.4 M MgCl2. (C) 1998 Published by Elsevier S cience Ltd. All rights reserved.