Geochemical and Geophysical Changes during Ammonia Gas Treatment of Vadose Zone Sediments for Uranium Remediation

摘要

Low water content sediments were treated with NH3 gas to evaluate changes in U mobility as a potential field remediation method for vadose zone contamination. Injection of NH3 gas created high dissolved NH3 concentrations that followed equilibrium behavior. High NH3 concentration led to an increase in pH from 8.0 to 11 to 13, depending on the water content and NH3 concentration. The increase in pore water pH resulted in a large increase in pore water cations and anions from mineral-phase dissolution. Minerals showing the greatest dissolution included montmorillonite, muscovite, and kaolinite. Pore water ion concentrations then decreased with time. Simulations based on initial pore water ion concentrations indicated that quartz, chrysotile, calcite, diaspore, hematite, and Na-boltwoodite (hydrous U silicate) should precipitate. Electrical resistivity and induced polarization tomography (ERT/IP) was able to nonintrusively track these NH3 partitioning, dissolution, and precipitations processes through changes in conductivity and chargeability. Ammonia treatment significantly decreases the amount of U present as adsorbed and aqueous species in field-contaminated sediments. In contrast, sediments containing a large fraction of U associated with carbonates generally showed little change. Uranium leaching from sediments containing high Na-boltwoodite decreased significantly by NH3 treatment, but x-ray absorption near-edge structure/extended x-ray absorption fine structure showed no change in the Na-boltwoodite concentration. Therefore, NH3 treatment of contaminated sediment acts to decrease the highly mobile aqueous and adsorbed U by incorporation into precipitates and appears to decrease mobility of some existing U precipitates (Na-boltwoodite) as a result of mineral coating.