The mobility and stability of iodine-humic and iodine-fulvic complexes through sand

Abstract

Natural organic carbon is believed to enhance the transport of associated contaminants through the geosphere and it is important to understand the migration behaviour of natural organic compounds. The stability and mobility of 125I labelled humic and fulvic material in a glacial sand-groundwater system was, therefore, studied in columns of glacial sand using natural groundwater from the same aquifer as eluent. Aldrich humic acid and fulvic acid extracted from Drigg groundwater were labelled with 125I in the presence of an oxidising agent. The complexed organics were separated from free I and from oxidising agent using size-exclusion gel chromatography. About 55% of a pulse of 125I-Fu complex which had been prepared from fulvic acid which had been extracted from Drigg groundwater emerged from a 55 cm column unchanged. Such material could conceivably be used as a tracer in field tests but the difficulty of extracting a sufficient quantity of fulvic acid makes this impractical. Aldrich humic was much more strongly sorbed in the columns of sand and would be of little use as an analogue for the natural material. It is well known that dissolved organic carbons in groundwaters can be sorbed onto geological materials by a variety of mechanisms and it appears from this work that the mobility of a humic or fulvic material depends on the extent to which the available sites are already occupied.