Technical noteMultiple-phase enzyme-induced carbonate precipitation (EICP) method for soil improvement
Introduction
Biocementation is an innovative soil improvement method for geotechnical applications such as ground improvement (DeJong et al., 2010; Cardoso et al., 2020), slope protection (Salifu et al., 2016; Liu et al., 2020), erosion mitigation (Hamdan and Kavazanjian, 2016; Meng et al., 2021), and seepage control (Jiang and Soga, 2017; Wu et al., 2019). In the most investigated biocementation technique, including microbially induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP), urease catalyzes the hydrolysis of urea and thus results in the production of carbonate ion and the rise of pH. Precipitation of CaCO3 occurs in soil when soluble calcium sources are provided. The biocatalyzed CaCO3 crystals can improve the shear strength and stiffness of soil by interparticle binding and pore filling (Almajed et al., 2018). In contrast to the MICP method, the EICP method uses free urease to catalyze the hydrolysis of urea. Thus, it is free from issues related to bio-safety and controlling microbial growth (activity) in soils (Khodadadi et al., 2017). Besides, the relatively small size of free urease enzyme, compared to bacterial cells, facilitates its transport in soil pores (Almajed et al., 2018).
Adsorption of free urease is weaker than that of live bacteria to the soil particle. In the commonly used single-phase method for in-situ EICP treatment, a mixture of purified urease and cementation solution is injected into the soil matrix throughout the biocementation process (Almajed et al., 2018; Nafisi et al., 2019; Cui et al., 2021). However, the widely adopted EICP treatment method appears to have several drawbacks. Firstly, as reported by Almajed et al. (2018), commercial purified urease is the most expensive component of the EICP solution, which is the main challenge of the large-scale application of EICP at this stage. Secondly, as urease is added into the EICP solution throughout EICP treatment, the enzyme is used inefficiently, which further enhances the cost of biocementation. At last, the mixing may cause intensive precipitation of CaCO3 at the injection point after a few cycles of treatment, leading to a severe clogging and the nonuniformity of CaCO3 during the biocementation treatment. The bio-clogging may impede the further progress of EICP treatment, and a lack of uniformity can result in lower UCS levels (Cheng et al., 2019; Hoang et al., 2020). Therefore, it is urgent to find an inexpensive urease resource and seek for an advanced method in terms of making efficient use of urease and improving the uniformity of CaCO3 precipitation.
In this paper, a multiple-phase method using soybean crude urease as an alternative to purified urease enzyme was proposed to improve the utilization efficiency of urease and the uniformity of CaCO3 precipitation in EICP treatment. The method included a percolation of premixed soybean crude urease and cementation solution followed by several percolations of cementation solution in one cycle of biocementation. The properties of the EICP treated samples after various cycles of biocementation were tested with regard to chemical conversion efficiency, CaCO3 distribution, unconfined compressive strength (UCS), secant modulus (E50), permeability, and microstructure. This paper also provided a comparative study of the multiple-phase EICP method and the commonly used one-phase EICP method focusing on the soil engineering properties.
Section snippets
Soybean crude urease (SCU)
Soybean, a urease-abundant species of legume, was used to prepare crude urease in this study. The soybean crude urease was extracted simply by mixing powdered soybeans and tap water. The extraction process was divided into three steps: (1) Grinding and soaking, (2) salting-out of excess protein, and (3) liquid-solid separation. In step (1), the soybean was ground into powdered soybeans with a shredding machine, and 60-mesh sieved powdered soybeans were soaked in tap water at room temperature
Factors affecting chemical conversion efficiency of EICP in one cycle of biocementation
The effect of powdered soybeans concentration (enzyme activity of SCU) and percolation time (x in MPM x-y) on chemical conversion efficiencies of EICP in one cycle of biocementation was evaluated to determine the preferred treatment strategy. Fig. 1 shows the enzyme activity of SCU extracted using different concentrations of powdered soybeans and its effect on conversion efficiencies of EICP in the sand treated by a single premixed percolation (i.e., without following percolation of CS, MPM
Conclusions
The proposed multiple-phase method included a percolation of premixed SCU and CS followed by four percolations of CS in one cycle of biocementation. Considering the current price of soybeans, the material cost of the SCU with a powdered soybeans concentration of 80 g/L was much lower than that of the frequently-used purified urease. In addition, the utilizing rate of urease was increased by at least four times for CaCO3 production compared with the one-phase method. Furthermore, the proposed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The work described herein was supported by the National Natural Science Foundation of China under grant No. 51978244 and 51979088. The authors are grateful for this support. Any opinions, findings, and conclusions or recommendations expressed in this article are the authors only, and do not reflect the view of the National Natural Science Foundation of China.
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