Organic and inorganic colloids impacting total iodine behavior in groundwater from the Datong Basin, China

•The effects of colloids on groundwater iodine mobilization were unraveled.•Reducing conditions favored colloidal iodine release into groundwater.•The increase of pH enhanced iodine desorption from colloids.•Fe-OM colloids controlled iodine behavior in large grain size fractions.

The geochemical behaviors of colloids in aquifers played an important role in determining the fate of iodine in groundwater system. To decipher the impact of colloids on iodine mobilization in aquifers, three successive pore-sized colloids filtration (0.45 μm, 30 kDa and 5 kDa) were conducted on in-situ during groundwater sampling. The results showed that the distribution ratios (f) of total iodine (IT) and iron in the dissolved solution (i.e., 5 kDa ultrafiltered) were from 0.78 to 0.99 and from 0.56 to 0.94, respectively. Natural organic matter (NOM) in the colloidal fractions obtained the f values ranging from 0.14 to 1.00. The decreased Eh values from recharge area to discharge area indicated redox potential of groundwater system changed from oxidizing to reducing along groundwater flowpath, and interestingly, the corresponding iodine fractions in groundwater were decreasing in dissolved solution and increasing in colloidal fractions. Inverse correlation between Fe and DOC and positive correlation between iodine and DOC suggested the occurrence of reductive dissolution of iron oxyhydroxides and degradation of organic iodine in groundwater system. Iodine distribution increased in dissolved solution and decreased in colloids with pH increase. Moreover, as pH increased, f (Fe) and f (DOC) decreased in dissolved solution and increased in colloids. Relatively weak correlation of f (IT) with f (Fe) and strong relationship between f (IT) and f (DOC) in the large grain size fractions suggested the Fe-OM complexes controlled iodine distribution in groundwater, which depends on the presence of Fe bridges. Negligible association of iodine with Fe and NOM in the small grain size fractions might be attributed to the effects of abundant OH− content in groundwater.

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