Recently, in the 2021 United Nations Climate Change Conference, around 140 countries pledged to reduce carbon dioxide emission over the years and to achieve net-zero emissions. To accomplish this goal of carbon dioxide emission, researchers have to develop sustainable alternatives for the current processes contributing to carbon dioxide. In construction industries, concrete is a widely used construction material and is composed of cement and filler materials. The cement industry is one of the largest CO2 producing industries. Hence, alternative sustainable processes for cementation that reduce the production of CO2 are the need of the hour.
Microbially induced calcite precipitation (MICP) is the process by which calcium carbonate precipitates are formed by microorganisms due to the interaction of their metabolic products with substances in the surrounding environment. This is of value and finds applications in self-healing concrete/biocementation, soil consolidation, and bio-grouting, bioremediation of groundwater, and sequestration of atmospheric CO2. Hence, MICP based cementation can be an alternative sustainable process for construction industries in the future provided economic viability at a scale. Further, MICP has advantages over chemically induced calcite precipitation (CICP) due to the problems such as difficulty to maintain the pH of CICP, narrow spaced calcium carbonate crystals etc.
In nature, some microorganisms can form calcium carbonate precipitates on the outer layer of their cell walls. These serve as nucleation sites for the crystals due to their negative surface charge density, which is the central phenomenon in MICP. Bacteria can use several mechanisms like ureolysis, photosynthesis, denitrification, etc. for MICP. The bacterium that is widely studied for this process is Sporosarcina pasteurii (S. pasteurii) for biocementation mechanisms because they are non-pathogenic, exhibit high specific urease activity, and can withstand extreme alkaline conditions. S. pasteurii expresses enzymes such as urease and carbonic anhydrase that are required for carbonate production and calcite precipitation, respectively. The understanding of different processes of MICP using S. pasteurii is useful for improving the MICP process and performing scale-up for an economically viable process.
The aim of the study, conducted by Ms. Subasree Sridhar, Dr. Nirav Bhatt, and Prof. G. K. Suraishkumar, from the Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, IIT Madras, is to build a structured model of the ureolysis process to better understand different steps involved in the process and to better understand MICP using S. pasteurii under the fixed initial concentrations of urea concentration, pH, and temperature.
In contrast to lumped kinetic models of bioprocesses, the kinetics of the ureolysis process was studied using a structured model describing important intracellular processes such as ureolysis. In the previous studies, most of the focus is on precipitation kinetics rather than ureolysis. Biomass growth, urea breakdown, etc. were all studied without considering the ureolysis kinetics. Ureolysis is important because it is the rate limiting step during calcite precipitation in S. pasteurii. The previous studies did not take into account the ureolysis process, which include urea uptake, intracellular urea breakdown, ammonium assimilation, ammonium and carbonate excretion, etc. In this study, these effects are studied and taken into account in the structured model.
Urea is hydrolysed to ammonium and carbonate ions which are very useful in this process. Ammonium ions are useful in maintaining the alkaline pH, while carbonate ions contribute to precipitation. Hence, studying detailed mechanisms of these processes of MICP is essential to better understand ureolysis and to study their roles in the overall biological process.
It was concluded that MICP is an environmentally friendly process, and the mechanism of reactions of urea uptake, ureolysis, ammonium assimilation, and ammonium excretion in S. pasteurii were postulated theoretically and the experimental results agreed with these postulations. It was found that urea uptake takes place through active transport, while ammonium excretion is passive. Urea uptake kinetics studied with minimal media will be useful in optimizing the substrate composition. Ammonium excretion kinetics can be used in understanding the pH control and regulation of MICP process. The proposed structured model is useful for developing a unified model of ureolysis processes with calcite precipitation and MICP scale-up studies in the future.
Dr. Resmi Suresh from IIT Guwahati appreciated the efforts of the team by giving the following comments: “This article focuses on developing a simple kinetic model for microbially induced calcite precipitation (MICP). If an efficient way for handling biomass can be developed, MICP would be the best method for inducing calcite precipitation. To be able to handle biomass, an accurate model that captures all processes (extracellular and intracellular) is essential. The simple structured model presented in this article could be a stepping stone towards a controlled MICP. Insights from experimental studies and stoichiometric analysis are used to make assumptions that simplified the model complexity considerably. Unlike other models in literature, the model proposed considers ammonium as a nitrogen source for the cell, active transport of urea uptake, diffusion of urea into the cell and ammonium excretion. Improving model accuracy using strategies suggested in the article and developing strategies for controlling the calcite precipitation using the model could be explored in future. With self-healing concrete being one of the applications of MICP, advances in controlled MICP could help us in developing more durable and sustainable buildings.”
Article by Akshay Anantharaman
Here is the original link to the paper:
https://www.sciencedirect.com/science/article/pii/S1369703X21002904?dgcid=coauthor
