An overlooked pool of hydrogen stored in humic matter revealed by isotopic exchange: implication for radioactive 3H contamination

Tritium ( 3 H), the radioactive isotope of hydrogen, is a contaminant occurring in most nuclear liquid releases from nuclear industries. The behavior of tritium in the environment is an ongoing societal concern, notably the storage of tritium in organic compounds in waters, soils and sediments. In particular, the fate of tritium depends on the availability of hydrogen for isotopic exchanges in natural organic matter, especially in lignin-like molecules. Therefore, we studied here H exchangeability using natural lignite and leonardite humic matter samples. We compared results from gas–solid isotopic exchanges ( α iso ) with those from widely used methods of functional group characterization by means of deprotonation ( α depro  =  α carboxyl + α phenol + α alcohol ). Since standard methods for titrating total acidity and determining protonated carboxyl groups are limited by the unconstrained slow deprotonation of humic material, we have modified methods to maximize exchangeable hydrogen reactivity by extending the reaction time in reagent solutions out to 3 weeks, while total OH groups are investigated by the standard acetylation method for alcohol group determination. Results show that more than two times less hydrogen remains available for deprotonation than for isotopic exchanges with an average of α iso  = 35.9% versus α depro  = 15.5%. This finding reveals the existence of a neglected pool of hydrogen likely to play a key role in processes based on hydrogen exchange, such as metal complexation by natural ligands. Results also show the reliability of the isotopic exchange method for hydrogen speciation in soil-type matrices.