Investigating Ammonium By-product Removal for Ureolytic Bio-cementation Using Meter-scale Experiments

Microbially Induced Calcite Precipitation (MICP), or bio-cementation, is a promising bio-mediated technology that can improve the engineering properties of soils through the precipitation of calcium carbonate. Despite significant advances in the technology, concerns regarding the fate of produced NH₄⁺ by-products have remained largely unaddressed. In this study, five 3.7-meter long soil columns each containing one of three different soils were improved using ureolytic bio-cementation, and post-treatment NH₄⁺ by-product removal was investigated during the application of 525 L of a high pH and high ionic strength rinse solution. During rinsing, reductions in aqueous NH₄⁺ were observed in all columns from initial concentrations between ≈100 mM to 500 mM to final values between ≈0.3 mM and 20 mM with higher NH₄⁺ concentrations observed at distances furthest from the injection well. In addition, soil V_s measurements completed during rinse injections suggested that no significant changes in cementation integrity occurred during NH₄⁺ removal. After rinsing and a 12 hour stop flow period, all column solutions achieved cumulative NH₄⁺ removals exceeding 97.9%. Soil samples collected following rinsing, however, contained significant sorbed NH₄⁺ masses that appeared to have a near linear relationship with surrounding aqueous NH₄⁺ concentrations. While these results suggest that NH₄⁺ can be successfully removed from bio-cemented soils, acceptable limits for NH₄⁺ aqueous concentrations and sorbed NH₄⁺ masses will likely be governed by site-specific requirements and may require further investigation and refinement of the developed techniques.