Abstract
Enzyme-induced carbonate precipitation (EICP) is a relatively new bio-cementation technique for ground improvement. In EICP, calcium carbonate (CaCO3) precipitation occurs via urea hydrolysis catalysed by the urease enzyme sourced from plants. EICP offers significant potential for innovative and sustainable engineering applications, including strengthening of soils, remediation of contaminants, enhancement of oil recovery through bio-plugging and other in situ field applications. Given the numerous potential applications of EICP, theoretical understanding of the rate and quantity of CaCO3 precipitation via the ureolytic chemical reaction is vital for optimising the process. For instance, in a typical EICP process, the rate and quantity of CaCO3 precipitation can depend significantly on the concentration, activity and kinetic properties of the enzyme used along with the reaction environment such as pH and temperature. This paper reviews the research and development of enzyme-catalysed reactions and its applications for enhancing CaCO3 precipitation in EICP. The paper also presents the assessment and estimation of kinetic parameters, such as the maximal reaction velocity (Vmax) and the Michaelis constant (Km), that are associated with applications in civil and geotechnical engineering. Various models for evaluating the kinetic reactions in EICP are presented and discussed, taking into account the influence of pH, temperature and inhibitors. It is shown that a good understanding of the kinetic properties of the urease enzyme can be useful in the development, optimisation and prediction of the rate of CaCO3 precipitation in EICP.
Highlights
Enzyme induced carbonate precipitation (EICP) is an innovative ground improvement technique that involves calcium carbonate (CaCO3) precipitation via the hydrolysis of urea (CO(NH2)2) into ammonium (NH4+) and carbonate (CO23−) ions catalysed by the urease enzyme
Other drawbacks of the EICP process can be the lack of nucleation sites, meaning that a portion of CaCO3 is precipitated in the pore spaces, which may remain ineffective in binding soil particles
There may be a reduction in alkalinity with high CaCl2 concentration and there may be an excess of Ca2+ available in the system with a limited amount of carbonate ions [5]. It has been reported in the literature that the presence of the urease enzyme accelerates the urea hydrolysis and reaction speed up to 1014 times compared to the rate of the uncatalyzed reaction [5,41]
Summary
Enzyme induced carbonate precipitation (EICP) is an innovative ground improvement technique that involves calcium carbonate (CaCO3) precipitation via the hydrolysis of urea (CO(NH2)2) into ammonium (NH4+) and carbonate (CO23−) ions catalysed by the urease enzyme. The urea hydrolysis process is often constrained by several factors including the concentration of the substrate, temperature, pH and the presence of inhibitors [1,13,14] This results in a limited operational lifetime of enzymes and difficulty in understanding the mechanisms of an enzyme-catalysed reaction [15,16]. A comprehensive understanding of enzyme kinetics may be useful for accurate monitoring, prediction and controlling of CaCO3 precipitation rates This may result in a significant reduction in costs, yield less undesirable by-products and a shorter treatment duration for practical engineering applications [1], neither theoretical nor empirical approaches to controlling the curing time have been reported in the literature. The functional unit of ureases from plants is made of six identical subunits, called α subunits, each of which are reported to have a molecular weight of around 90 kDa, making the total molecular weight of a subunit approximately 540 kDa [38,39]
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