Rare earth elements mobility processes in an AMD-affected estuary: Huelva Estuary (SW Spain)

The movement of rare earth elements (REEs) in estuarine systems remains poorly understood. Existing studies have examined REE behavior in freshwater and marine environments separately. However, the transition between these two regimes is less explored. This gap motivated the study of Huelva Estuary, where acidic river water mixes with seawater. The estuary is known for high REE concentrations in acidic riverine inputs. No prior work had resolved how pH changes affect REE partitioning between dissolved and solid phases. Understanding this could clarify how REEs behave in similar estuaries globally. The study aimed to fill this knowledge gap by examining REE speciation and mineral associations. The findings may help predict REE mobility in other anthropogenically impacted estuaries. This research contributes to the broader field of environmental geochemistry.

The study aimed to investigate how rare earth elements behave in the Huelva Estuary, where acidic river water mixes with seawater. The estuary is influenced by acid mine drainage (AMD), leading to high REE concentrations in the Tinto River. The researchers wanted to determine how pH changes affect REE mobility and speciation. They also sought to understand the mineralogical associations of REE in precipitates formed during mixing. The study focused on the transition from acidic to neutral pH conditions. The researchers aimed to identify the dominant REE-binding phases at different pH levels. They used sequential extraction to analyze precipitates from mixed solutions. The goal was to clarify the geochemical processes controlling REE distribution in estuarine environments.

The researchers collected water samples from the Tinto River and seawater. They mixed these to simulate estuarine conditions at various pH levels. Sequential extraction was used to analyze precipitates from the mixed solutions. The method involved four extraction steps to isolate different mineral phases. The first step targeted soluble salts, while the second focused on Fe-bearing minerals. The third step extracted poorly crystallized schwertmannite, and the fourth targeted well-crystallized goethite and hematite. REE concentrations were measured in both dissolved and particulate fractions. The study compared REE-normalized patterns across different pH levels to assess speciation changes.

The study found that dissolved REE concentrations decrease as pH increases. At pH < 6, REE-normalized patterns are parallel and similar. Most REE in precipitates are associated with the residual phase. At pH 3 and 3.5, REE are mainly bound to soluble salts. At pH 4 and 5, REE are distributed among salts, schwertmannite, and goethite-hematite. The highest REE concentrations in precipitates occur at pH 6. At higher pH, REE are released back into solution. The results show that pH strongly influences REE speciation and mineral associations.

The study shows that pH is a key factor in REE mobility within the Huelva Estuary. REE partitioning between dissolved and solid phases changes with pH. The researchers propose that REE bind to different mineral phases depending on pH. At low pH, REE are mainly in soluble salts. At higher pH, they associate with Fe-bearing minerals like schwertmannite and goethite. The study suggests that newly formed minerals at pH 6 retain REE. However, at higher pH, REE are released back into solution. The findings highlight the importance of pH in controlling REE behavior in estuarine systems.