Process and mechanism of microbial induced carbonate precipitation

At present, the global ecological environment and geological environment situation is very serious. For environmental heavy metal remediation, traditional physical and chemical remediation methods have been widely used, but the traditional remediation methods have disadvantages such as high cost, high energy consumption, and the use of a large number of chemicals. As a kind of bionic engineering, the principle of biological method is that microorganism fix or degrade pollutants in the environment by microbial metabolic activity.Compared with the traditional physical and chemical methods, biological repair method has lower cost, higher efficiency, and the advantages of green environmental protection,which has been repeatedly proved that can effectively reduce the concentration of heavy metals in the soil and water. Biological mineralization is a kind of biological method, and its main mechanisms include: (1) biologically controlled mineralization; (2) biologically mediated mineralization; (3) biologically induced mineralization, in which Microbially Induced Calcium Carbonate Precipitation (MICP) is one of the main biologically induced mineralization. It is very common in various environments of seawater and sediment, fresh water and soil, and it is a biological mineralization process that exists widely in nature. At the same time, the process has the characteristics of fast reaction speed, low requirements for environmental conditions, wide application range and significant greenhouse gas emission reduction effect, so that it has been widely concerned and popularized in many fields of geology, civil engineering, water conservancy and environment. The research in this field covers the intersection of biological science and inorganic chemistry, biophysics and materials science, etc. The research on biologically induced mineralization and its application in various fields are very remarkable, especially the biologically induced mineralization provides a new idea for environmental remediation. The microbial-induced precipitation of calcium carbonate has been proved for many times to effectively reduce the concentration of heavy metal elements in the environment, and its main action process is as follows: microorganisms produce metabolite CO₃²⁻ through metabolic activities, and interact with Ca²⁺ in the environment to form calcium carbonate precipitation under appropriate environmental conditions, and fix heavy metal ions in the environment during the formation of calcium carbonate.

Based on the analysis of relevant research results at home and abroad, this paper summarizes the pathway and mechanism of calcium carbonate mineralization induced by various microorganisms, such as denitrification process, sulfate reduction and urea decomposition. In the process of mineralization induced by urea decomposing bacteria, urea is hydrolyzed into carbamate and ammonia under the action of urea decomposing bacteria, and then carbamate spontaneously decomposes to produce ammonia and carbonic acid. The ammonia generated in the reaction hydrolyzes and generates OH⁻, which increases the pH of the water environment and causes the hydrolysis balance of carbonic acid to move towards the direction of generating carbonic acid, resulting in a large number of carbonic acid ions, which react with the calcium ions existing in the environment and form calcium carbonate precipitation in the supersaturated state. Compared with other MICP mechanisms, urea decomposition process has the advantage of not relying on additional nutrients and producing high concentration of calcium carbonate in a short period of time, so it has attracted more attention and research.

It is important to note that minerals induced by microbial mineralization are influenced by environmental factors. In this paper, represented by urea decomposers, the effects of environmental factors such as calcium source, ambient temperature, pH condition and ion concentration on minerals produced by MICP were discussed. The following conclusions are summarized :(1) calcium source affects the morphology, crystal size and morphology of carbonate precipitation and mineral deposition rate; (2) The optimum temperature for the formation and precipitation of urea decomposing bacteria mainly depends on the optimum temperature for bacterial growth and metabolism, which is usually between 20 ℃ and 40 ℃; (3) pH influences the plasma concentrations of HCO₃⁻, CO₃²⁻ and NH₄⁺ to change the precipitation rate, which directly determines the size of mineral crystals; (4) The presence of other ions in the environment, such as Mg, Ni and Sr, will affect the formation of minerals.

Finally, the effects and mechanisms of MICP on reducing the concentration of heavy metal elements in various environments are summarized in this paper. Research show that the process of calcium carbonate induced by urea decomposition can effectively reduce the concentration of heavy metals in the environment, and the fixation rate of copper, cadmium, zinc and other elements is above 90%. The mechanism of action is mainly divided into two kinds, for the heavy metal element like Cu²⁺, Pb²⁺ and Zn²⁺, they are fixed by forming precipitation directly with CO₃²⁻ or replacing calcium ions in calcium carbonate, while for As and Cr, they will form complex anions such as arsenate in water environment, and will be fixed by substituting carbonate ions from calcium carbonate into the solid. As a simple and efficient geological environment process, MICP has broad application prospects in the field of ecological environment remediation.