Interaction of Ammonia Gas with Sediments and Pore Water and Induced Uranium Immobilization under Vadose Zone Conditions

Abstract:

Preliminary studies have demonstrated the potential of ammonia gas (NH₃) treatment on contaminated sediment as a vadose zone uranium remediation approach. In this work, we conducted batch, column, and flow wedge experiments to study the ammonia gas transport and interaction with sediments and pore water. The uranium immobilization effectiveness of the ammonia gas treatment technology was also evaluated.

Ammonia gas quickly partitions into sediment pore water and significantly increases the pH (up to ~13.2) and the electrical conductivity (EC). The rate and range of the increase in both pH and EC are dependent on the ammonia concentration in the gas and the pore water content and chemistry. The pH and EC changes follow a similar pattern. During an ammonia gas injection into a heterogeneous system, it was observed that the NH₃ front proceeded faster in layers of lower water content compared to the same sediment layers of higher water content. Elevated pH values (11 to 13.2) initially resulted from the NH₃ gas partitioning into the pore water was buffered down to ~ 9 after 7 months of sediment exposure to the air. The rate of NH₃ diffusion in sediment is a function of the water content in the sediment. Higher cation/anion concentrations during the ammonia gas treatment indicated mineral dissolution due to pH increase, while lower ionic concentrations after the pH buffering revealed significant mineral precipitation. This precipitation incorporates uranium into mineral structures or provides a coating to uranium minerals, therefore achieving uranium immobilization. Treatment with 5% v/v NH₃ gas for one week followed by three weeks buffering resulted in a 75% reduction in the mobile uranium mass. After 2 to 12 months of treatment, the immobile phase of uranium mass increased by up to 2.3 times.