Determination of 231Pa in coastal sediments and sea water and disequilibrium studies in 231Pa/235U activity ratios

Coastal sediment samples are collected from the top surface layers of the sediments from Bombay to Kottilppad along the West coast of India in order to understand the geochemical behaviour of 231Pa and the activity ratios of 231Pa/235U in this region of the Indian sub-continent. These coastal regions offer a unique advantage for studying the geochemistry and transport of natural radioactivity due to the placer deposits of monazite in the southern part of India. The top layers of the surface sediments are leached with 1.0 M oxalic acid for the extraction of protactinium activities without attacking the mineral core of the particles. 233Pa tracer is added to the samples. The chemical recovery obtained by counting 233Pa is found to vary from 40 to 90%. Total uranium leached from the sediment surface is determined by a fluorimetric method and the concentrations of 235U in the leachates are calculated. The activity ratios of 231Pa/235U lie in the range of 11.0 to 21.0. The data of 231Pa activity indicate that there is a deficiency of 231Pa in coastal waters of the ocean and that 231Pa is removed by precipitation as soon as it is formed by radioactive decay of 235U. 231Pa is observed to be bound mostly to the organic humous material on the surface of the sediment. (author)