Enzyme-induced carbonate precipitation utilizing fresh urine and calcium-rich zeolites

Abstract

Enzyme-induced carbonate precipitation (EICP) is a biocementation technique that produces comparatively fewer carbon dioxide emissions than traditional cementation. However, the use of synthetic reagents for EICP is costly, and the process produces an ammonium byproduct that is a water pollutant. This study utilized fresh human urine as a source of urea and calcium-rich zeolites as an ammonium adsorbent and a source of calcium ions for the EICP cementation technique. Batch hydrolysis, adsorption, and biocementation experiments were conducted to determine the effects of zeolite type, zeolite form (i.e., exchangeable ion), ammonium adsorption, and calcium release. The results showed that calcium-rich chabazite adsorbed comparable ammonium ions and released 2.7-times more calcium ions on average than calcium-rich clinoptilolite; calcium-rich chabazite also released calcium ions in excess without the addition of calcium chloride in solution. Additionally, synthetic-fresh urine and real-fresh urine had comparable ammonium adsorption and calcium release trends. Finally, inclusion of a pre-hydrolysis step reduced the ammonium adsorption by 3.9-times; additionally, longer adsorption times led to CaCO3 precipitation outside of the sand column, which is an undesirable outcome for soil biocementation. Even with this limitation, the CaCO3 content of sand columns ranged from 0.48% to 0.92%, which signifies the potential of the proposed process for cementation, given a higher initial concentration of urea. The results from this work show the ability to use both a waste stream, human urine, and a naturally occurring material, zeolites, to greatly reduce the economic and environmental stressors of conventional soil binders for applications such as liquefaction mitigation (e.g., Portland cement) and dust suppression (e.g., synthetic polymers).