Effect of Humic Acid Complexation and Iron(II) Reduction on the Fate of Neptunium in Anaerobic Aqueous Environments

Neptunium (Np) is a long-lived, highly radiotoxic transuranic element of critical concern in the nuclear fuel cycle and the final geological disposal of radioactive waste. Owing to the redox sensitivity of Np and its tendency to complex with natural organic matter, the environmental mobility of Np can be significantly affected by a wide variety of geochemical processes. In this study, we investigated the interactions among Np(V), humic acid (HA), and Fe(II) under anaerobic aqueous conditions. Techniques including solvent extraction, ultrafiltration, dynamic light scattering, and UV-vis-NIR absorption spectroscopy were employed to quantitatively characterize the impacts of HA and Fe(II) on the oxidation-state distribution and aqueous speciation of Np. The results demonstrate that HA can effectively complex Np(V) and subsequently reduce Np(V) to Np(IV) to a limited extent. The presence of Fe(II) significantly alters the reaction pathway, markedly promoting the reduction of Np(V) and the formation of colloidal species that enhance Np retention in the natural environment. This work provides the first mechanistic insight into Np redox transformation and speciation evolution in the Np-HA-Fe ternary system, highlighting the synergistic role of Fe(II) and HA in governing the environmental fate of Np. These findings offer critical scientific support for the accurate prediction of Np mobility in groundwater and the reliable long-term safety assessment of geological repositories.