CO2 conversion by the integration of biological and chemical methods: Spirulina sp. LEB 18 cultivation with diethanolamine and potassium carbonate addition

Advances in microalgae-based carbon sequestration: Current status and future perspectives

The advancement in carbon dioxide (CO2) sequestration technology has received significant attention due to the adverse effects of CO2 on climate. The mitigation of the adverse effects of CO2 can be accomplished through its conversion into useful products or renewable fuels. In this regard, microalgae is a promising candidate due to its high photosynthesis efficiency, sustainability, and eco-friendly nature. Microalgae utilizes CO2 in the process of photosynthesis and generates biomass that can be utilized to produce various valuable products such as supplements, chemicals, cosmetics, biofuels, and other value-added products. However, at present microalgae cultivation is still restricted to producing value-added products due to high cultivation costs and lower CO2 sequestration efficiency of algal strains. Therefore, it is very crucial to develop novel techniques that can be cost-effective and enhance microalgal carbon sequestration efficiency. The main aim of the present manuscript is to explain how to optimize microalgal CO2 sequestration, integrate valuable product generation, and explore novel techniques like genetic manipulations, phytohormones, quantum dots, and AI tools to enhance the efficiency of CO2 sequestration. Additionally, this review provides an overview of the mass flow of different microalgae and their biorefinery, life cycle assessment (LCA) for achieving net-zero CO2 emissions, and the advantages, challenges, and future perspectives of current technologies. All of the reviewed approaches efficiently enhance microalgal CO2 sequestration and integrate value-added compound production, creating a green and economically profitable process.

Enhanced carbon dioxide fixation of Chlorella vulgaris in microalgae reactor loaded with nanofiber membrane carried iron oxide nanoparticles

To improve the CO₂ dissolution and carbon fixation in the process of microalgae capturing CO₂ from flue gas, a nanofiber membrane containing iron oxide nanoparticles (NPsFe₂O₃) for CO₂ adsorption was prepared, and coupled with microalgae utilization to achieve carbon removal. The performance test results showed that the largest specific surface area and pore size were 8.148 m² g^-1 and 27.505 Å, respectively, when the nanofiber membrane had 4% NPsFe₂O₃. Through CO₂ adsorption experiments, it was found that the nanofiber membrane could prolong the CO₂ residence time and increase CO₂ dissolution. Then, the nanofiber membrane was used as a CO₂ adsorbent and semifixed culture carrier in the Chlorella vulgaris culture process. The results showed that compared with the group without nanofiber membrane (0 layer), the biomass productivity, CO₂ fixation efficiency and carbon fixation efficiency of Chlorella vulgaris with 2 layers of membranes increased by 1.4 times.

Sustainability and carbon neutrality trends for microalgae-based wastewater treatment: A review

As the global economy develops and the population increases, greenhouse gas emissions and wastewater discharge have become inevitable global problems. Conventional wastewater treatment processes produce direct or indirect greenhouse gas, which can intensify global warming. Microalgae-based wastewater treatment technology can not only purify wastewater and use the nutrients in wastewater to produce microalgae biomass, but it can also absorb CO₂ in the atmosphere or flue gas through photosynthesis, which demonstrates great potential as a sustainable and economical wastewater treatment technology. This review highlights the multifaceted roles of microalgae in different types of wastewater treatment processes in terms of the extent of their bioremediation function and microalgae biomass production. In addition, various newly developed microalgae cultivation systems, especially biofilm cultivation systems, were further characterized systematically. The performance of different microalgae cultivation systems was studied and summarized. Current research on the technical approaches for the modification of the CO₂ capture by microalgae and the maximization of CO₂ transfer and conversion efficiency were also reviewed. This review serves as a useful and informative reference for the application of wastewater treatment and CO₂ capture by microalgae, aiming to provide a reference for the realization of carbon neutrality in wastewater treatment systems.

Mechanistic and multiscale aspects of thermo-catalytic CO2 conversion to C1 products

The increasing environmental concerns due to anthropogenic CO2 emissions have called for an alternate sustainable source to fulfill rising chemical and energy demands and reduce environmental problems. The thermo-catalytic activation and conversion of abundantly available CO2, a thermodynamically stable and kinetically inert molecule, can significantly pave the way to sustainably produce chemicals and fuels and mitigate the additional CO2 load. This can be done through comprehensive knowledge and understanding of catalyst behavior, reaction kinetics, and reactor design. This review aims to catalog and summarize the advances in the experimental and theoretical approaches for CO2 activation and conversion to C1 products via heterogeneous catalytic routes. To this aim, we analyze the current literature works describing experimental analyses (e.g., catalyst characterization and kinetics measurement) as well as computational studies (e.g., microkinetic modeling and first-principles calculations). The catalytic reactions of CO2 activation and conversion reviewed in detail are: (i) reverse water-gas shift (RWGS), (ii) CO2 methanation, (iii) CO2 hydrogenation to methanol, and (iv) dry reforming of methane (DRM). This review is divided into six sections. The first section provides an overview of the energy and environmental problems of our society, in which promising strategies and possible pathways to utilize anthropogenic CO2 are highlighted. In the second section, the discussion follows with the description of materials and mechanisms of the available thermo-catalytic processes for CO2 utilization. In the third section, the process of catalyst deactivation by coking is presented, and possible solutions to the problem are recommended based on experimental and theoretical literature works. In the fourth section, kinetic models are reviewed. In the fifth section, reaction technologies associated with the conversion of CO2 are described, and, finally, in the sixth section, concluding remarks and future directions are provided.

Pollution prevention and waste phycoremediation by algal-based wastewater treatment technologies: The applications of high-rate algal ponds (HRAPs) and algal turf scrubber (ATS)

Following the escalating human population growth and rapid urbanization, the tremendous amount of urban and industrial waste released leads to a series of critical issues such as health issues, climate change, water crisis, and pollution problems. With the advantages of a favorable carbon life cycle, high photosynthetic efficiencies, and being adaptive to harsh environments, algae have attracted attention as an excellent agent for pollution prevention and waste phycoremediation. Following the concept of circular economy and biorefinery for sustainable production and waste minimization, this review discusses the role of four different algal-based wastewater treatment technologies, including high-rate algal ponds (HRAPs), HRAP-absorption column (HRAP-AC), hybrid algal biofilm-enhanced raceway pond (HABERP) and algal turf scrubber (ATS) in waste management and resource recovery. In addition to the nutrient removal mechanisms and operation parameters, recent advances and developments have been discussed for each technology, including (1) Innovative operation strategies and treatment of emerging contaminants (ECs) employing HRAPs, (2) Biogas upgrading utilizing HRAP-AC system and approaches of O₂ minimization in biomethane, (3) Operation of different HABERP systems, (4) Life-cycle and cost analysis of HRAPs-based wastewater treatment system, and (5) Value-upgrading for harvested algal biomass and life-cycle cost analysis of ATS system.

Perspective of Spirulina culture with wastewater into a sustainable circular bioeconomy

Spirulina biomass accounts for 30% of the total algae biomass production globally. In conventional process of Spirulina biomass production, cultivation using chemical-based culture medium contributes 35% of the total production cost. Moreover, the environmental impact of cultivation stage is the highest among all the production stages which resulted from the extensive usage of chemicals and nutrients. Thus, various types of culture medium such as chemical-based, modified, and alternative culture medium with highlights on wastewater medium is reviewed on the recent advances of culture media for Spirulina cultivation. Further study is needed in modifying or exploring alternative culture media utilising waste, wastewater, or by-products from industrial processes to ensure the sustainability of environment and nutrients source for cultivation in the long term. Moreover, the current development of utilising wastewater medium only support the growth of Spirulina however it cannot eliminate the negative impacts of wastewater. In fact, the recent developments in coupling with wastewater treatment technology can eradicate the negative impacts of wastewater while supporting the growth of Spirulina. The application of Spirulina cultivation in wastewater able to resolve the global environmental pollution issues, produce value added product and even generate green electricity. This would benefit the society, business, and environment in achieving a sustainable circular bioeconomy.

Reuniting the Biogeochemistry of Algae for a Low-Carbon Circular Bioeconomy

Given their advantages of high photosynthetic efficiency and non-competition with land-based crops, algae, that are carbon-hungry and sunlight-driven microbial factories, are a promising solution to resolve energy crisis, food security, and pollution problems. The ability to recycle nutrient and CO₂ fixation from waste sources makes algae a valuable feedstock for biofuels, food and feeds, biochemicals, and biomaterials. Innovative technologies such as the bicarbonate-based integrated carbon capture and algae production system (BICCAPS), integrated algal bioenergy carbon capture and storage (BECCS), as well as ocean macroalgal afforestation (OMA), can be used to realize a low-carbon algal bioeconomy. We review how algae can be applied in the framework of integrated low-carbon circular bioeconomy models, focusing on sustainable biofuels, low-carbon feedstocks, carbon capture, and advances in algal biotechnology.

A novel nanoemulsion-based microalgal growth medium for enhanced biomass production

BACKGROUND

Microalgae are well-established feedstocks for applications ranging from biofuels to valuable pigments and therapeutic proteins. However, the low biomass productivity using commercially available growth mediums is a roadblock for its mass production. This work describes a strategy to boost algal biomass productivity by using an effective CO₂ supplement.

RESULTS

In the present study, a novel nanoemulsion-based media has been tested for the growth of freshwater microalgae strain Chlorella pyrenoidosa. Two different nanoemulsion-based media were developed using 1% silicone oil nanoemulsion (1% SE) and 1% paraffin oil nanoemulsion (1% PE) supplemented in Blue-green 11 media (BG11). After 12 days of cultivation, biomass yield was found highest in 1% PE followed by 1% SE and control, i.e., 3.20, 2.75, and 1.03 g L^-1, respectively. The chlorophyll-a synthesis was improved by 76% in 1% SE and 53% in 1% PE compared with control. The respective microalgal cell numbers for 1% PE, 1% SE and control measured using the cell counter were 3.00 × 10⁶, 2.40 × 10⁶, and 1.34 × 10⁶ cells mL^-1. The effective CO₂ absorption tendency of the emulsion was highlighted as the key mechanism for enhanced algal growth and biomass production. On the biochemical characterization of the produced biomass, it was found that the nanoemulsion-cultivated C. pyrenoidosa had increased lipid (1% PE = 26.80%, 1% SE = 23.60%) and carbohydrates (1% PE = 17.20%, 1% SE = 18.90%) content compared to the control (lipid = 18.05%, carbohydrates = 13.60%).

CONCLUSIONS

This study describes a novel nanoemulsion which potentially acts as an effective CO₂ supplement for microalgal growth media thereby increasing the growth of microalgal cells. Further, nanoemulsion-cultivated microalgal biomass depicts an increase in lipid and carbohydrate content. The approach provides high microalgal biomass productivity without altering morphological characteristics like cell shape and size as revealed by field emission scanning electron microscope (FESEM) images.

Effect of different nitrogen ratio on the performance of CO2 absorption and microalgae conversion (CAMC) hybrid system

CO2 absorption hybrid with microalgae conversion (CAMC) could be a promising alternative for the conventional CO2 capture technologies. The hybrid process could avoid the challenges of thermal energy consumption in the conventional desorption process and nutrition consumption in the typical algae cultivation process. In this work, the influence of different nitrogen ratio (NH4HCO3:NaNO3) on the performance of the proposed hybrid CAMC process was investigated. Experimental results indicated that adding NH4HCO3 into cultivation solution could promote Spirulina platensis growth. When the ratio of NH4HCO3 and NaNO3 was set at 1:4, carbon utilization efficiency of the hybrid process could achieve 40.45%, which was higher than the conventional microalgae CO2 fixation processes (around 10%-30%). In addition, carbon sequestration capacity increased to 178.46 mg/L/d. It could be observed that CO2 absorption-microalgae conversion (CAMC) hybrid system has the potential for cost-effective CO2 capture and utilization.

Large-Scale Cultivation of Spirulina for Biological CO2 Mitigation in Open Raceway Ponds Using Purified CO2 From a Coal Chemical Flue Gas

In order to select excellent strains with high CO₂ fixation capability on a large scale, nine Spirulina species were cultivated in columnar photobioreactors with the addition of 10% CO₂. The two species selected (208 and 220) were optimized for pH value, total dissolved inorganic carbon (DIC), and phosphorus content with intermittent CO₂ addition in 4 m² indoor raceway ponds. On the basis of biomass accumulation and CO₂ fixation rate in the present study, the optimum pH, DIC, and phosphate concentration were 9.5, 0.1 mol L^-1, and 200 mg L^-1 for both strains, respectively. Lastly, the two strains selected were semi-continuously cultivated successfully for CO₂ mitigation in 605 m² raceway ponds aerated with food-grade CO₂ purified from a coal chemical flue gas on a large scale. The daily average biomass dry weight of the two stains reached up to 18.7 and 13.2 g m^-2 d^-1, respectively, suggesting the two Spirulina strains can be utilized for mass production.

Enhanced biomass productivity of Arthrospira platensis using zeolitic imidazolate framework-8 as carbon dioxide adsorbents

In order to improve CO₂ diffusion in algae solution and conversion into dissolved HCO₃^-, zeolitic imidazolate framework-8 (ZIF-8) with zinc cores as unsaturated metal sites was first used as CO₂ adsorbents. Flue gas CO₂ from coal-chemical industry can be adsorbed and can be made available throughout cultivation to promote biomass productivity of Arthrospira platensis. The ZIF-8 adsorbent with particle size of 719 nm performed the largest pore area of 351.8 m²/g, which promoted CO₂ conversion into HCO₃^- by 72.9% compared to control condition without ZIF-8. The increased HCO₃^- concentration enhanced thylakoid membrane proportion in cell cross-sectional area by 1.3 times to 78.3%, which resulted in enhancement of photosynthesis in A. platensis cells. Relative electron transport rate increased by 9.4% accordingly, which was attributed to the improvement of chlorophyll a concentration by 110%. The biomass productivity using ZIF-8 adsorbent with particle size of 719 nm markedly increased by 64.0%.

Bio-regeneration of different rich CO2 absorption solvent via microalgae cultivation

As one of the most mature carbon capture technologies, thermal regeneration of rich CO₂ absorption solvent is a crucial challenge due to its high energy consumption (typically in the range of 3-6 MJ/kg CO₂). In this work, a concept of bio-regeneration was proposed using microalgae to convert bicarbonate (which is one of the dominant components in rich solution) into value-added biomass. To evaluate the performance of bio-regeneration, different rich solution (including NH₄HCO₃, KHCO₃ and NaHCO₃) were investigated. Experimental results indicated that NH₄HCO₃ could be a promising bicarbonate carrier for the proposed absorption-microalgae hybrid process, which had a higher biomass productivity (55.36 mg·L^-1·d^-1) compared to KHCO₃ and NaHCO₃ and carbon sequestration capacity could be up to 158.3 mg·L^-1·d^-1. Meanwhile, pH adjustment was an effective approach to further intensify the performance of hybrid process. As a result, bio-regeneration of solvents could be a promising alternative to the conventional thermal regeneration.

In situ solvent recovery by using hydrophobic/oleophilic filter during wet lipid extraction from microalgae

While lipid extraction from wet microalgae has attracted attention as an economical method for microalgal biofuel production, few studies have focused the actual separation of extract phase from the emulsified extraction mixture. Here, a novel approach which utilizes hydrophobic/oleophilic filter was developed for the efficient solvent recovery. The filter was surface-modified by coating a functional polymer via initiated vapor deposition for the selective solvent permeability. While acid-treated Chlorella sorokiniana HS1 and n-hexane was stirred for lipid extraction, tubular filter module was immersed into the mixture for separation. The mixture was kept stirred during the separation to inhibit the buildup of cell debris on the filter by inducing crossflow on the filter. Extract phase was separated directly from the raffinate phase with high separation efficiency (> 98.3%) while maintaining permeation flux. The place-, space- and energy-efficient strategy reported here could be a useful tool for the solvent extraction process.

Innovative nanofiber technology to improve carbon dioxide biofixation in microalgae cultivation

The aim of this study was to develop nanofibers containing nanoparticles with potential for the biological fixation of CO₂ together with the microalgae Chlorella fusca LEB 111. An electrospinning technique was used for the production of polymeric nanofibers with different concentrations of iron oxide nanoparticles: 0, 2, 4, 6, 8, and 10% (w v^-1). Nanofibers with a nanoparticle concentration of 4% (w v^-1) were selected for use in the microalgal cultivation due to their smaller diameter (434 nm), high specific surface area (13.8 m² g^-1) and higher CO₂ adsorption capacity (164.2 mg g^-1). The microalgae C. fusca LEB 111 presented a higher CO₂ biofixation rate of 216.2 mg L^-1 d^-1 when cultivated with these nanofibers. The results demonstrated the potential of electrospun nanofibers as physical adsorbents of CO₂ since they can increase the contact time between the gas and the microorganism and consequently increase the CO₂ biofixation by the microalgae.

Fed-batch cultivation with CO2 and monoethanolamine: Influence on Chlorella fusca LEB 111 cultivation, carbon biofixation and biomolecules production

The aim of this study was to evaluate the interaction between the periodic addition of monoethanolamine (MEA) and CO₂ during the cultivation of Chlorella fusca LEB 111. For this purpose, MEA has been added in abiotic assays, followed by fed-batch cultures with that green alga and the absorbent. BG-11 medium shown a higher potential of CO₂ absorption with MEA addition, and the bicarbonate was the chemical species of carbon prevailing in the chemical equilibrium. The periodic addition of MEA did not reduce the kinetics of growth, promoted a higher accumulation of DIC (81.4 mg L^-1) in the medium and protein (44.0% w w^-1) and lipid (30.8% w w^-1) concentrations in the biomass of C. fusca LEB 111. Therefore, it was demonstrated that fed-batch culture with MEA increased CO₂ fixation and the biomolecule synthesis as proteins and lipids.