<論文・報告>表層地盤における砒素の地下浸透抑制効果の検討

Abstract

自然由来の砒素を含む土や岩石の利用や保管に際して, 表層地盤や吸着効果を有する鉱物材料による砒素の吸着特性を適切に評価することは, 地下水中の汚染状況の推定や, 地下水汚染の対策・管理において重要な要素である. 本研究ではバッチ試験とカラム浸透試験により, 浸透水のpH, 無機鉱物系吸着材の添加, 汚染水との接触時間がまさ土による砒酸の吸着特性に及ぼす影響を評価した. バッチ試験においては, 溶液をpH9.0 から水酸化物イオン濃度を50倍(pH10.7)にすると砒酸イオンの吸着が阻害され吸着量が約50%に低下した. 吸着材を土に質量比5%添加したところ, カラム浸透試験においては供試体の間隙体積の50倍の通水量に達するまで砒酸の移動が抑制された. 一方で, バッチ吸着試験においては, 砒素の平衡濃度が低く, 接触時間が1時間程度と短い場合は吸着材添加の効果がほとんど確認できず, 接触時間を48時間とした場合に顕著な吸着材の効果が表れることを確認した. 以上の結果より, 吸着材の添加量や吸着層の厚さを適切に調節することで, 五価の砒素の溶出による地下水汚染を低減できる可能性が示唆された.

When soil and rocks excavated from huge construction project contains non-anthropogenic toxic heavy metals and metalloids, such as arsenic and lead, groundwater contamination due to leaching of these toxic substances should be carefully considered. To evaluate the groundwater contamination risk appropriately, the sorption characteristics of these toxic substances in the surface unsaturated zone is a key issue. This manuscript addresses the results of batch tests and column percolation tests on the arsenate sorption characteristics of decomposed granite soil. The main focus was on the effects of leachate pH, use of an immobilizing agent to enhance arsenate sorption, and contact time on the sorption behavior. Batch sorption test results indicate that the arsenate sorption capacity under a pH of 10.7 is decreased by 50% compared with a pH of 9.0 because of the competitive sorption of hydroxide ions. The addition of 5% calcium-magnesium composite agent to the soil definitively enhances the arsenate sorption in the column percolation tests in which a certain contact period with soil and permeant is maintained. This result is supported by the batch sorption test result wherein the sorption capacity after 48 hours of contact time is five times greater than that at 1 hour. These testing results support the fact that arsenate sorption in surface soil is expected to effectively prevent groundwater contamination due to excavated soils and rocks by optimizing the contact period and sorption capacity by use of an appropriate immobilizing agent.