The sorption of iodate and iodide by riverine sediments: its implications to dilution gauging and hydrochemistry of iodine

Abstract

A study of the sorption of iodate and iodide from solution (40 μg 1 −1 - I) onto fourteen natural river sediments, peat and twelve sediment components showed that in rivers where the suspended sediment load is less than 0.1 gl −1 significant sorption (> 0.5 μg 1 −1) of iodide is unlikely to occur. With iodate there is a greater chance of sorption, particularly on iron hydroxide. The results suggest therefore that iodide should be a reliable tracer during measurement of river flow rates by the dilution technique. Nevertheless, two simple tests which establish whether sorption occurs, are described. Experiments conducted at different ionic strengths (NaCl solution and seawater) suggest that sorptive processes will exert some control over the distribution of iodate and iodide in some estuaries. A mechanism by which the freshwater could be enriched with iodate and iodide from seawater, is proposed. The sorptive behaviour of iodate and iodide on peat conflicts with Kharkar, Turekian and Bertine's contention that where a trace element is adsorbed from freshwater it is always released to a greater or lesser extent on contact with seawater because of displacement by other ions. This paradox is resolved.