10:45 〜 11:00
[BBG02-06] モンゴル アルカリ塩湖における低温下での重金属濃縮と吸着作用
キーワード：重金属、鉄酸化物吸着、アルカリ塩湖、凍結作用
Closed-basin lakes in the Valley of Gobi Lakes of Mongolia are alkaline-water environments with enrichments of heavy metallic elements (e.g., As, Mo, and U) (Gankhurel et al., 2022). In cold seasons, these lakes are frozen, causing partitioning of elements among surface ice, lake water, and sediments (e.g., Yoda et al., 2021). Freezing of these lakes causes an accumulation of carbonate species (i.e., HCO3-), leading to pH decreases of the lake water from 9 to 8. Thus, these major chemical changes could affect other species, such as heavy metallic elements (e.g., As, Mo, and U).
We conducted a field survey of these lakes in January 2020 and December 2022. We analyzed concentrations of heavy metals in the lake water beneath the ice, and their abundances in the ice. As a result of analyses, heavy metals accumulate in lake water by freezing of lakes, the same as major elements (e.g., Cl). In especially, Mo and U concentrations reach around 0.9 and 0.5 mg/L, respectively. On the other hand, accumulations of As are limited. The heavy metals are contained in ice with abundances proportional to their concentrations in the lake water. Based on X-ray absorption fine structure (XAFS) analyses, As in ice is mainly As(V)-bearing salts (e.g., NaH2AsO4) and could be partly bound to Fe oxide particles.
These behaviors of heavy metals in the lakes would be reproduced by our geochemical model. Our model results suggest that the pH decrease promotes adsorptions of As to suspended particles of Fe oxide (e.g., ferrihydrite), resulting in suppressed accumulation of As in the lake water. However, the decrease in pH is insufficient for adsorptions of Mo and U, which favor pH less than 8. Therefore, the two metals accumulate similarly to major dissolved species.
Our results suggest that adsorptions of heavy metals to Fe oxides could be a significant factor for their behaviors, even under low temperatures in closed-aqueous environments. In addition, the pH dependence of the adsorptions would control their concentrations. The mechanisms observed on the lakes could be applicable to other various frozen-closed aqueous conditions, such as lacustrine environments on the past Earth and Mars, water contained in planetesimals, and ocean and water reservoirs in icy planetary bodies.
We conducted a field survey of these lakes in January 2020 and December 2022. We analyzed concentrations of heavy metals in the lake water beneath the ice, and their abundances in the ice. As a result of analyses, heavy metals accumulate in lake water by freezing of lakes, the same as major elements (e.g., Cl). In especially, Mo and U concentrations reach around 0.9 and 0.5 mg/L, respectively. On the other hand, accumulations of As are limited. The heavy metals are contained in ice with abundances proportional to their concentrations in the lake water. Based on X-ray absorption fine structure (XAFS) analyses, As in ice is mainly As(V)-bearing salts (e.g., NaH2AsO4) and could be partly bound to Fe oxide particles.
These behaviors of heavy metals in the lakes would be reproduced by our geochemical model. Our model results suggest that the pH decrease promotes adsorptions of As to suspended particles of Fe oxide (e.g., ferrihydrite), resulting in suppressed accumulation of As in the lake water. However, the decrease in pH is insufficient for adsorptions of Mo and U, which favor pH less than 8. Therefore, the two metals accumulate similarly to major dissolved species.
Our results suggest that adsorptions of heavy metals to Fe oxides could be a significant factor for their behaviors, even under low temperatures in closed-aqueous environments. In addition, the pH dependence of the adsorptions would control their concentrations. The mechanisms observed on the lakes could be applicable to other various frozen-closed aqueous conditions, such as lacustrine environments on the past Earth and Mars, water contained in planetesimals, and ocean and water reservoirs in icy planetary bodies.