Adsorption/Desorption Characteristics of Metal Ions on Ferric Oxyhydroxide During a Freeze–Thaw Cycle

Abstract

Iron is a key element that affects bioactivity in the global environment as a nutrient and also as an adsorbent that controls the circulation of other elements in the hydrosphere. This element is abundant in sea ice and is released into the aquatic environment during the thawing process. Due to the low solubility of iron compounds, they exist as solid materials, such as oxides and oxyhydroxides, which effectively adsorb transition metal ions and affect their circulation in the hydrosphere. This process occurs not only in the hydrosphere but also in the cryosphere. In this study, adsorption of first-row transition metal cations on ferric oxyhydroxide (FeOxH) under frozen conditions is evaluated by ex-situ measurements with sample thawing and also by in situ X-ray fluorescence (XRF) measurements without thawing. The adsorption and desorption characteristics of transition metal cations on FeOxH are discussed from the difference in the adsorption ratio between these two measurements. For in situ XRF measurements, linear regression analysis assuming two states of analytes, i.e., adsorbed on FeOxH and dissolved in the freeze-concentrated solution (FCS) is efficient in estimating adsorption ratios. While complete adsorption is found for all metal cations studied here (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) at pH > 8, the adsorption ratio determined by in situ XRF is larger than the corresponding value obtained with the ex situ method with thawing at lower pH. This strongly suggests that metal cations are well adsorbed on FeOxH when concentrated in the FCS under frozen conditions but are desorbed upon thawing. The comparison of the adsorption ratios obtained by in situ and ex situ methods reveals specific adsorption of Mn²⁺ and Co²⁺. Interestingly, the specific adsorption of Mn²⁺ is irreversible, and desorption does not occur upon thawing. In contrast, thawing causes the desorption of specifically adsorbed Co²⁺, suggesting that different mechanisms are responsible for the specific adsorption.