ABSTRACT
We present an improved approach to evaluating the activity of urease from electrical conductivity (EC) measurements. In this approach, chemical equilibrium modeling via PHREEQC is used in conjunction with empirical equations for computing EC to develop a function that relates the increase in EC during urea hydrolysis in a closed reactor to the concentration of ammonium species present (and concentration of urea remaining) in the reaction solution. By applying this function to data from continuous measurement of EC during urea hydrolysis, we obtain a profile of the concentration of the urea substrate with time, which is then used to determine the urease activity. The activity of commercially available urease extracted from jack beans was determined using this method and compared well to the activity determined using Nessler's reagent, a commonly used colorimetric assay. This EC-based method is inexpensive and can be used for accurate determination of urease activity for a variety of applications.
ABSTRACT
Biocementation is a ground improvement technique that involves precipitating a mineral (commonly calcium carbonate, CaCO3) in the soil pore space to bind soil particles, in turn increasing the strength and reducing the permeability of the soil. Ureolysis (i.e. hydrolysis of urea) is the most researched calcium carbonate precipitation mechanism, which can be induced through either a microbial (MICP) or enzymatic (EICP) process. While laboratory tests and field trials have provided strong evidence of the efficacy of biocementation in strengthening granular materials, the role of the precipitate-grain interface and the surface chemistry of soil grains in biocementation are largely unknown. This study aims to address this gap. To this end, two geotechnically similar sand samples differing considerably in the amount of iron oxide and iron sulfate on grain surface are biocemented via EICP and tested for unconfined compressive strength (UCS). The biocemented sample containing a high concentration of iron oxide and iron sulfate exhibits almost 50% lower UCS than the other sample. To investigate whether surface chemistry can explain this considerable difference, interactions of CaCO3 with quartz (SiO2), hematite (Fe2O3), and marcasite (FeS2) as polymorphs of silicon dioxide, iron oxide, and iron sulfide, respectively, are simulated using molecular dynamics. The influence of water content at the precipitate-grain interface is also considered. Simulation results indicate that in dry conditions, CaCO3 has almost two times stronger affinity for SiO2 than Fe2O3 and FeS2, suggesting that biocementation is most effective for clean sands. It is also shown that water reduces the precipitate-grain adhesion.
