Occurrence of Indium in Seawater and some Marine Sediments

Abstract

LITTLE is known about the occurrence of indium in the marine environment. Shaw¹ succeeded in detecting it at concentrations between 0.06 and 0.28 p.p.m. in four of the eleven marine sediments that he examined. No figures seem to have been published yet for the occurrence of the element in seawater, although Baric and Branica² have deduced from polarographic studies of saturated solutions of indium in seawater medium that its concentration should be 10⁻¹⁰ M. We have determined indium at four depths in the water column at Discovery Station 6917 (23° 00′ N, 18° 42′ W, depth 3,100 m). ‘Millipore’ filtration (0.45 µm) and preliminary concentration of the indium (by means of chelating ion exchange³) were carried out at sea immediately after the samples had been collected. The recovered indium was purified by anion exchange using ‘Dowex AG2–X8’ and then irradiated with thermal neutrons at a flux of 5 × 10¹² neutrons cm⁻² s⁻¹ for four weeks. The irradiated sample was taken up in concentrated hydrochloric acid, equilibrated with 15 mg of inert indium as carrier and submitted to a twelve stage radio-chemical separation to free the ¹¹⁴In from other radioactive species. Indium was converted to its oxinate for determination of the chemical yield (∼ 60 per cent) and for β-counting. The identity of the separated ¹¹⁴In activity was confirmed by measurement of the half-life, plotting an aluminium absorption curve, and by γ-spectrometry. The concentrations of indium found at depths of 2, 500, 1,000 and 2,000 m were 0.31 ± 0.02, 0.12 ± 0.01, 0.10 ± 0.01 and 0.11 ± 0.01 ng/l. respectively. These concentrations are of the order of a hundred times less than those predicted by Baric and Branica². Neutron activation analysis was also used to determine indium in three deep sea sediments. The concentration of indium found in the argillaceous sediment is identical with that found by Shaw¹ in a red clay from the Equatorial Pacific. He was, however, unable to detect indium in a siliceous ooze or in a calcareous ooze.