Redox-induced mobilization of rare earth elements in sediments of the northwestern part of the Persian Gulf

Petrochemical plants constitute the main sources of rare earth elements (REEs) in the Musa estuary. Under different redox conditions, the accumulated REEs in sedimentary beds might be released into the water column. An automated biogeochemical microcosm system was utilized to simulate reducing conditions. The effect of redox potential (Eh) on the release of REEs in an estuarine sediment sample was investigated. REEs were released from sediments by reducing Eh, and the average amount of REEs mobilization was 31.03%. When Eh was decreased from 120 to -150 mV, a decrease in pH (from 8.08 to 6.66), an increase in dissolved organic carbon (DOC) (from 18.93 to 704.84 mg/L), and an increase in the dissolution of Fe-Mn oxy-hydroxides occurred. Lu, Sm, and Pr were the most mobile elements with over 40% release rate, while Ce showed the lowest amount of release. The cluster analysis and principal component analysis exhibited that the indirect effect of Eh via the linked changes of DOC and chemistry of Fe-Mn influenced the solubility and release of REEs. Enrichment factors (EFs) revealed that the Musa estuary is threatened by Ce, Dy, La, Nd, Pr, and Sm. Since the ecotoxicological effects of dissolved REEs cause environmental risk and health issues, serious consideration needs to be given to the high potential mobility of REEs.

Automated summary

Petrochemical plants are the main sources of rare earth elements (REEs) in the Musa estuary, and the accumulated REEs in sedimentary beds can be released into the water column under different redox conditions. A study using a biogeochemical microcosm system found that the release of REEs in an estuarine sediment sample was influenced by the redox potential (Eh). The average mobilization rate of REEs was 31.03% under reducing Eh. Cluster analysis and principal component analysis showed that Eh indirectly affected the solubility and release of REEs through the linked changes of dissolved organic carbon (DOC) and chemistry of Fe-Mn. Enrichment factors (EFs) revealed that the Musa estuary is threatened by Ce, Dy, La, Nd, Pr, and Sm. Serious consideration should be given to the high potential mobility of REEs due to their ecotoxicological effects causing environmental risk and health issues.