Iodine binding to humic acid

The rate of reactions between humic acid (HA) and iodide (I-) and iodate (IO3-) have been investigated in suspensions spiked with I-129 at concentrations of 22, 44 and 88 mu g L-1 and stored at 10 degrees C. Changes in the speciation of I-129(-), (IO3-)-I-129 and mixed (I-129(-) + (IO3-)-I-129) spikes were monitored over 77 days using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). In suspensions spiked with I-129(-) 25% of the added I was transformed into organic iodine (Org-I-129) within 77 days and there was no evidence of (IO3-)-I-129 formation. By contrast, rapid loss of (IO3-)-I-129 and increase in both I-129(-) and Org-I-129 was observed in (IO3-)-I-129-spiked suspensions. However, the rate of Org-I-129 production was greater in mixed systems compared to (IO3-)-I-129-spiked suspensions with the same total I-129 concentration, possibly indicating (IO3-)-I-129-I- redox coupling. Size exclusion chromatography (SEC) demonstrated that Org-I-129 was present in both high and low molecular weight fractions of the HA although a slight preference to bond with the lower molecular weight fractions was observed indicating that, after 77 days, the spiked isotope had not fully mixed with the native I-127 pool. Iodine transformations were modelled using first order rate equations and fitted rate coefficients determined. However, extrapolation of the model to 250 days indicated that a pseudo-steady state would be attained after 200 days but that the proportion of I-129 incorporated into HA was less than that of I-127 indicating the presence of a recalcitrant pool of I-127 that was unavailable for isotopic mixing. (C) 2016 Elsevier Ltd. All rights reserved.