Molecular-scale insights into differences in the adsorption of cesium and selenium on biogenic and abiogenic ferrihydrite

Abstract

Adsorption reactions on iron oxyhydroxides (e.g., ferrihydrite, goethite) have considerable impacts on the geochemical cycles of trace elements in natural environments. However, there is a growing recognition that the current state of knowledge that was primarily derived from abiotically produced iron oxyhydroxides is not necessarily applicable to biogenic iron oxyhydroxides (BIOS) that are ubiquitous in natural environments such as groundwater discharge areas and hydrothermal vents. To understand the difference in adsorption on BIOS, we studied Cs(I), Se(IV), and Se(VI) adsorption on both BIOS (consisting of 2-line ferrihydrite) collected from a hot spring and abiogenic ferrihydrite combined with zeta-potential measurements and Cs and Se K-edge extended X-ray absorption fine structure (EXAFS) analyses. The BIOS showed larger adsorption of Cs(I) under a wide pH range (pH > 6) and initial Cs(I) concentrations. The EXAFS analyses revealed that Cs(I) formed an outer-sphere complex on BIOS. The isoelectric point (pHiep) of BIOS (pHiep = 4.1) was notably lower than that of abiogenic ferrihydrite (pHiep = 8.7), suggesting that the negatively charged BIOS are more electrostatically favorable for the adsorption of cationic Cs(I) than the positively charged abiogenic ferrihydrite. Therefore, we concluded the larger adsorption of Cs(I) on BIOS than on abiogenic ferrihydrite is mainly caused by the electrostatic effect, probably resulting from the adsorption and/or coprecipitation of organic and inorganic impurities (e.g., stalks, extracellular polymeric substances, and Si) on the ferrihydrite. In contrast, anionic Se(IV) and Se(VI) adsorption on BIOS was lower than that on abiogenic ferrihydrite, which can be explained by electrostatic repulsion. In particular, the decrease of Se(VI) adsorption was approximately 7–80 times more significant than that of Se(IV) at acidic to neutral pH. The EXAFS analysis showed that Se(IV) formed an inner-sphere complex, whereas Se(VI) formed an outer-sphere complex on both BIOS and abiogenic ferrihydrite. Therefore, the formation of inner-sphere Se(IV) complexes seems to suppress electrostatic repulsion on BIOS because of their stronger affinity for chemical bonding. The results revealed that the adsorption chemistry of BIOS is different from that of abiogenic ferrihydrite and can be explained by both electrostatic and chemical effects. The findings will contribute to a better and more systematic understanding of the adsorption behavior of various trace elements and the role of BIOS in natural environments.