Date Published: June 18, 2019
Publisher: Public Library of Science
Author(s): Jie Peng, Zhiming Liu, Varenyam Achal.
Microbially induced calcium carbonate precipitation (MICP) is a potential method for improvement of soil. A laboratory study was conducted to investigate the influence of temperatures for soil improvement by MICP. The ureolytic activity experiments, MICP experiments in aqueous solution and sand column using Sporosarcina pasteurii were conducted at different temperatures(10, 15, 20, 25 and 30°C). The results showed there were microbially induced CaCO3 precipitation at all the temperatures from 10 to 30°C. The results of ureolytic activity experiments showed that the bacterial had higher ureolytic activity at high temperatures within the early 20 hours, however, the ureolytic activity at higher temperatures decreased more quickly than at lower temperatures. The results of MICP experiments in aqueous solution and sand column were consistent with tests of ureolytic activity. Within 20 to 50 hours of the start of the test, more CaCO3 precipitation was precipitated at higher temperature, subsequently, the precipitation rate of all experiments decreased, and the higher the temperature, the faster the precipitation rate dropped. The final precipitation amount of CaCO3 in aqueous solution and sand column tests at 10 °C was 92% and 37% higher than that at 30 °C. The maximum unconfined compressive strength of MICP treated sand column at 10 °C was 135% higher than that at 30 °C. The final treatment effect of MICP at lower temperature was better than that at high temperature within the temperature range studied. The reason for better treatment effect at lower temperatures was due to the longer retention time of ureolytic activity of bacteria at lower temperatures.
Microbially induced CaCO3 precipitation (MICP) via urea hydrolysis has been proposed for soil reinforcement [1–4]. MICP refers to the process in which the metabolites of the urease-producing bacteria react with substance in the surrounding environment to synthesize calcium carbonate (CaCO3) [5, 6]. The urease-producing bacteria generate urease during the metabolic process and the urease can catalyze hydrolysis of the urea, which increases the pH of the surrounding solution and forms ammonium ions and carbonate ions (CO32- ions) [7, 8]. Because of the negatively charged bacterial surface, calcium ions (Ca2+) are adsorbed onto the cell wall surface . When CO32- and Ca2+ ions are oversaturated in the surrounding solution, Ca2+ and CO32- ions will form CaCO3 crystals with cells as the crystal nuclei [10, 11]. CaCO3 formed during the MICP process not only can fill the pores in the soil but also is an excellent cementing material that cements soil particles together . Accordingly, the processes improves soil strength , reduces the porosity, and decreases the permeability of the soil . The bacteria used in MICP process was nonpathogenic and native to the subsurface environment. Only nutrients and calcium sources are required in MICP process, Treatment methods based on this process are more environmentally friendly than traditional methods. MICP advantages include cheaper treatment in the long run , reduced environmental impacts , and potential implementation for different applications [9, 16, 17].
From previous research, it was known that the ureolytic activity of the bacterial cultures was observed to increase with increasing temperatures, which is consistent with the early stage of the experiments in MICP experiments in aqueous solution in this paper. However, the change of ureolytic activity of the bacterial cultures throughout the whole life cycle was not considered in previous research. In most previous researches on MICP, the bacteria suspension was injected into the soil once and used for several days to precipitate CaCO3, and so the change of bacteria’s ureolytic activity in the treatment stage were not able to be measured, but it shouldn’t be ignored. In this paper, the change of bacteria’s ureolytic activity and concentration of Ca2+ of MICP tests in aqueous solution under different temperatures could reflect possible change of bacteria’s ureolytic activity in practical application of MICP.
Microbially induced CaCO3 precipitation(MICP) is a complex biochemical process, the objective of this paper was to study the influence of temperatures relevant for practice to MICP. A series of ureolytic activity experiments, MICP experiments in aqueous solution and sand column using the S. pasteurii were conducted at different temperatures(10, 15, 20, 25 and 30°C).