Iron-induced association between selenium and humic substances in groundwater from deep sedimentary formations

To better understand the impact of natural organic matters (NOM) on facilitating Se-79 migration in underground environments, the association mechanism between Se and humic substance (HS) in groundwater was investigated through incubation experiments using groundwater samples under abiotic and biotic conditions and speciation analyses using a DAX-8 resin isolation technique and a size exclusion chromatography (SEC) coupled online to ultraviolet-visible (UV-Vis) and inductively coupled plasma mass spectrometry (ICP-MS) detection (SEC-UV-ICP-MS). Selenite added into the groundwater was reduced to elemental Se only when microbial activity was stimulated by a nutrient. The DAX-8 resin isolation technique showed that Se associated with HS in both the oxidizing and reducing conditions, but the proportion of the associated Se to the total dissolved Se was the highest under the reducing condition. The SEC-UV-ICP-MS analyses of Se, Fe, and HS extracted from the DAX-8 resin showed that there is a fraction of Se that simultaneously elutes with Fe and HS. Logarithmic peak areas of Se and Fe associated with HS under the moderately oxidizing conditions were linearly correlated regardless of the presence of microorganisms and nutrient (r = 0.947, p <= 0.01 with the addition of Se; r = 0.995, p < 0.01 without the addition of Se). The SEC chromatogram of Se associated with HS under the reducing condition was well-matched with those under the moderately oxidizing conditions. Therefore, these suggest that Fe-induced association between Se and HS can preferentially occur in the groundwater conditions.