Carbondioxide capturing from industrial flue gas via calcium carbonate inducing microorganisms

Abstract

The increase in greenhouse gases, primarily caused by human activities in the past century, has led to the effects of global climate change becoming increasingly evident. Legal regulations in Europe and neighboring countries are becoming more stringent, and new taxation systems such as the Carbon Border Adjustment Mechanism are pushing industries with high carbon emissions to seek different solutions. In the near future, where conventional flue gas treatment methods alone will be insufficient, carbon capture technologies have been improved in recent years by scientists and even industry's research and development departments. Solutions involving microorganisms allow for many options with individual benefits, such as atmospheric and closed-loop systems. Carbon capture using microorganisms, especially algae, offers a promising solution. Within the scope of this study, experiments were conducted on two algae species with coccus and filamentous morphological structures obtained from Lake Salda together with Chlorella vulgaris, Spirulina, Chlamydomonas reinhardtii species. Many analyses were conducted on these algae including growth rates, growth periods, pigment contents such as chlorophyll and carotenoid, carbonic anhydrase enzyme activity values, biochemical content, and fatty acid types. The fact that Lake Salda is in the high pH category with a pH value greater than 9 was an effective factor in including the samples obtained from here in the study. In all analyses, the pH at which the relevant algae species showed the most growth between pH 8 and 11 at 0.5 intervals was examined. The two species obtained from Lake Salda, which were subjected to the same growth conditions as other algae species using BG11 medium, were not included in further studies as they grew relatively less. In addition, the measurement of carbonic anhydrase enzyme activity was considered as a parameter at least as important as growth during species selection. The presence of the relevant enzyme that enables the gaseous carbon dioxide to be converted into dissolved gas was evaluated as an factor that needs to be developed for carbon dioxide capture studies. The species selection was made as Spirulina with the help of the Analytical Hierarchy Process method, which takes into account all other effective factors as well as growth and enzyme activity. In the second phase of the experiment, the optimum mixture ratio of algae and bacteria coculture was examined in order to increase enzyme activity. Here, the Bacillus pasteurii species that secretes the relevant enzyme was selected. In addition to 100 ml of algae and bacteria monocultures in individual sterile bottles, 3:1, 1:1, 1:3 ratios were also added to the experimental set as two sets. After 12 days of incubation, the highest enzyme activity value was measured as 1.33 mU/mg for the algae-bacteria mixture ratio of 3:1. In growth values, the same culture stood out as 2.4 g/L. The same set of experiments was run for a third set, this time with only the Zarrouk medium in a different conical flask. Here, just before the 15-day incubation period was started, CaCl2 was added to the medium so that there would be 60 mM calcium ions in the medium. Advanced experiments such as XRD analysis, SEM imaging and qPCR analysis were performed on the precipitate obtained at the end of 15 days. The coculture 3:1 mixing ratio stood out again, especially due to the high impurity in the calcite it formed. In the third phase of the experiment, while moving on to pilot scale studies, the coculture mixture ratio was decided and it was progressed in a way. First, the study was carried out in atmospheric and agitated tanks positioned side by side in 200 L volume. Incubation lasting 35 days was carried out in October and November. 22 days of this was the time spent only for the growth of microorganisms in the aquatic environment, and CaCl2 was added to the medium in the last 13 days and the calcification process was followed. At the end of the total study, the VSS concentration in the coculture increased by 1.7 times, while the VSS concentration in the monoculture increased by only 1.4 times. At the end of the study, the VSS concentration in the monoculture was measured as 1.1 g/L, and the temperature has a great effect on the relatively low result. With bubble type photobioreactor, 4 days of calcification were performed in monoculture after 4 days of growth, and 4 days of calcification were performed in coculture after 8 days of growth. In the results, 2.12 g/L VSS was observed in coculture, while 4 g/L VSS was observed in monoculture. Despite the remarkable growth in monoculture, almost equal amounts of TSS were measured at the end of both studies as 14.64 g/L in monoculture and 14.42 g/L in coculture, respectively. As a result, it was determined that much more calcite was produced with much lower biomass in coculture. This study elucidates the multifaceted potential of microalgae, specifically Spirulina sp., in diverse biotechnological applications. The comprehensive analyses and experiments conducted have provided critical insights into optimizing conditions for enhanced biomass productivity and efficient CO2 utilization. The investigation into the properties of calcium carbonate (CaCO3) and the enzyme activity of carbonic anhydrase (CA) revealed valuable information for optimizing biomass productivity and CO2 utilization. Experiments conducted in closed photobioreactors demonstrated the advantages of regulating environmental parameters to maximize algal growth and productivity. The evaluation of suspended solids and volatile suspended solids yielded critical data on the overall efficiency and sustainability of the cultivation processes. Furthermore, the structural and compositional attributes of CaCO3 were found to be essential for its application in various industrial processes. The enzyme activity studies highlighted the pivotal role of CA in facilitating CO2 capture and conversion, underscoring its potential in mitigating greenhouse gas emissions. Additionally, the extracellular polymeric substances (EPS) analysis illuminated the complex nature of the extracellular matrix, suggesting avenues for further research into biofilm formation and stability. The study also compared carbon capture via algae with conventional carbon capture and storage (CCS) technologies, noting both advantages and challenges. Despite the benefits, scaling up algae-based capture in energy-intensive sectors remains challenging. Absorption and adsorption are considered more economical and advanced methods for substantial CO2 capture. Using the Analytic Hierarchy Process (AHP) method, the study evaluated various CCS technologies based on factors such as CO2 capture capacity, cost, operational difficulties, scalability, and space requirements. Algae-based CCS technologies face significant economic and spatial challenges, with costs ranging from $702 to $1,585 per ton of CO2 captured, compared to $15 to $340 for conventional methods. In conclusion, while microalgae demonstrate significant promise in addressing environmental and energy challenges, further research and development are necessary for industrial-scale applications. Continued collaboration and research will advance the field of microalgal biotechnology, fostering sustainable development.