The Utilization of Carbonated Steel Slag as a Supplementary Cementitious Material in Cement.

Carbon emission reduction and steel slag (SS) treatment are challenges in the steel industry. The accelerated carbonation of SS and carbonated steel slag (CSS) as a supplementary cementitious material (SCM) in cement can achieve both large-scale utilization of SS and CO2 emission reduction, which is conducive to low-carbon sustainable development. This paper presents the utilization status of CSS. The accelerated carbonation route and its effects on the properties of CSS are described. The carbonation reaction of SS leads to a decrease in the average density, an increase in the specific surface area, a refinement of the pore structure, and the precipitation of different forms of calcium carbonate on the CSS surface. Carbonation can increase the specific surface area of CSS by about 24-80%. The literature review revealed that the CO2 uptake of CSS is 2-27 g/100 g SS. The effects of using CSS as an SCM in cement on the mechanical properties, workability, volume stability, durability, environmental performance, hydration kinetics, and microstructure of the materials are also analyzed and evaluated. Under certain conditions, CSS has a positive effect on cement hydration, which can improve the mechanical properties, workability, bulk stability, and sulfate resistance of SS cement mortar. Meanwhile, SS carbonation inhibits the leaching of heavy metal ions from the solid matrix. The application of CSS mainly focuses on material strength, with less attention being given to durability and environmental performance. The challenges and prospects for the large-scale utilization of CSS in the cement and concrete industry are described.

Effect of phytohormones on the carbon sequestration performance of CO2 absorption-microalgae conversion system under low light restriction.

Microalgae have the potential to fix CO2 into valuable compounds. Low photosynthetic efficiency caused by low light was one of the challenges faced by microalgae carbon sequestration. In this study, Melatonin (MT) and indole-propionic acid (IPA) were used to alleviate the growth inhibition of Spirulina in CAMC system under low light restriction. The results showed that MT and IPA increased biomass and carbon fixation capacity. 10 mg/L IPA group achieved the maximum biomass and carbon fixation capacity, which were 17.11% and 21.46% higher than control. MT and IPA promoted the synthesis of chlorophyll, which in turn captured more light energy for microalgae growth. The increase of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) activities enhanced the resistance of microalgae to low light stress. MT and IPA promoted the secretion of extracellular polymeric substances (EPS) which was benefit to protect cells. The maximum phycocyanin content and yield was found in 10 mg-IPA group, which was 20.67% and 46.67% higher than control. MT and IPA improved the synthesis of carbohydrates and proteins and increased carbohydrates and proteins yield. This indicated that adding phytohormones was an effective method to alleviate the growth of microalgae restricted by low light stress, which provided a theoretical guidance for the application of CAMC system in CO2 capture and resource utilization.

Pressure fermentation to boost CO2-based poly(3-hydroxybutyrate) production using Cupriavidus necator.

CO2-based poly(3-hydroxybutyrate) (PHB) can be produced by the versatile bacterium Cupriavidus necator through chemolithoautotrophic fermentation, using a gas mixture consisting of CO2, H2, and O2. Despite offering a propitious route for carbon-neutral bioplastic manufacturing, its adoption is currently hampered by the wide explosive range of the required gas mixture, as well as the limited gas-to-liquid mass transfer rates. To address these challenges, pressure fermentation was applied as a robust and effective strategy, while ensuring safe operation by adhering to the limiting O2 concentration, utilizing state-of-the-art bioreactors. Consequently, exponential growth could be prolonged, boosting CO2-based PHB production from 10.8 g/L at 1.5 bar up to 29.6 g/L at 3 bar. The production gain closely aligns with the theoretical calculations, except for when the pressure was increased up to 4 bar. Overall, the demonstrated increase in PHB production underscores the potential of pressure fermentation to enhance aerobic gas fermentation.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review.

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

On the Maximum Obtainable Purity and Resultant Maximum Useful Membrane Selectivity of a Membrane Separator.

Design considerations concerning the maximum purity of a membrane separator, and the resultant maximum effective selectivity of the membranes were explored by modeling a binary gas membrane separator (pressure-driven permeance) using a dimensionless form. Although the maximum purity has an analytical solution at the limit of zero recovery or stage cut, this solution over-predicts the obtained purity as the recovery is increased. Furthermore, at combinations of high recovery, low feed mole fraction, and low pressure ratio, the maximum purity becomes independent of selectivity above some critical selectivity. As a consequence of this purity limitation, a maximum selectivity is defined at which further increases in selectivity will result in less than a 1% change in the final purity. An equation is obtained that specifies the region in which a limiting purity is less than unity (indicating the existence of a limiting selectivity); operating at less than the limiting pressure ratio results in a purity limitation less than unity. This regime becomes larger and more significant as the inlet mole fraction decreases (e.g., inlet feed mole fraction of 10% and pressure ratio of 100 results in a maximum useful membrane selectivity of only 130 at 95% recovery). These results suggest that membrane research should focus on increasing permeance rather than selectivity for low-concentration separations. The results found herein can be used to set benchmarks for membrane development in various gas separation applications.

Multi-objectives optimization on microwave-assisted-biological-based biogas upgrading and bio-succinic acid production.

This study represents the first investigation of bio-succinic acid (bio-SA) production with methane enrichment using carbon-dioxide-fixating bacteria in the co-culture of ragi tapai and macroalgae, Chaetomorpha. Microwave irradiation has also been introduced to enhance the biochemical processes as it could provide rapid and selective heating of substrates. In this research, microwave irradiation was applied on ragi tapai as a pre-treatment process. Factors such as microwave irradiation dose on ragi tapai, Chaetomorpha ratio in the co-culture, and pH value were studied. Optimal conditions were identified using Design-Expert software, resulting in optimal experimental biomethane and bio-SA production of 85.7 % and 0.65 g/L, respectively, at a microwave dose of 1.45 W/g, Chaetomorpha ratio of 0.9 and pH value of 7.8. The study provides valuable insights into microwave control for promoting simultaneous methane enrichment and bio-SA production, potentially reducing costs associated with CO2 capture and storage and biogas upgrading.

Comparing the performance of fluidized and fixed granular activated carbon beds as cathodes for microbial electrosynthesis of carboxylates from CO2.

Microbial electrosynthesis (MES) can use renewable electricity to power microbial conversion of carbon dioxide (CO2) into carboxylates. To ensure high productivities in MES, good mass transfer must be ensured, which could be accomplished with fluidization of granular activated carbon (GAC). In this study, fluidized and fixed GAC bed cathodes were compared. Acetate production rate and current density were 42 % and 47 % lower, respectively, in fluidized than fixed bed reactors. Although similar microbial consortium dominated by Eubacterium and Proteiniphilum was observed, lowest biomass quantity was measured with fixed GAC bed indicating higher specific acetate production rates compared to fluidized GAC bed. Furthermore, charge efficiency was the highest and charge recovery in carboxylates the lowest in fixed GAC beds indicating enhanced hydrogen evolution and need for enhancing CO2 feeding to enable higher production rates of acetate. Overall, fixed GAC beds have higher efficiency for acetate production in MES than fluidized GAC beds.

In Situ Spectroelectrochemical Study of Acetate Formation by CO2 Reduction Using Bi Catalyst in Amine-Based Capture Solution.

Carbon capture and utilization (CCU) are technologies sought to reduce the level of CO2 in the atmosphere. Industrial carbon capture is associated with energetic penalty, thus there is an opportunity to research alternatives. In this work, spectroelectrochemistry was used to analyze the electrochemical CO2 reduction (eCO2R) in CO2 saturated monoethanolamine (MEA)-based capture solutions, in a novel CCU process. The in situ Fourier transform infrared (FTIR) spectroscopy experiments show that at the Bi catalyst, the active species involved in the eCO2R is the dissolved CO2 in solution, and not carbamate. In addition, the products of eCO2R were evaluated under flow, using commercial Bi2O3 NP as catalyst. Formate and acetate were detected, with normalized FE for acetate up to 14.5 %, a remarkable result, considering the catalyst used. Acetate is formed either in the presence of cetrimonium bromide (CTAB) as surfactant or at higher current density (>-100 mA cm-2) and the results enabled the proposition of a pathway for its production. This work sheds light on the complex reaction environment of a capture medium electrolyte and is thus relevant for an improved understanding of the conversion of CO2 into value-added products and to evaluate the feasibility of a combined CCU approach.

Techno-Economic Assessment of Electromicrobial Production of n-Butanol from Air-Captured CO2.

Electromicrobial production (EMP), where electrochemically generated substrates (e.g., H2) are used as energy sources for microbial processes, has garnered significant interest as a method of producing fuels and other value-added chemicals from CO2. Combining these processes with direct air capture (DAC) has the potential to enable a truly circular carbon economy. Here, we analyze the economics of a hypothetical system that combines adsorbent-based DAC with EMP to produce n-butanol, a potential replacement for fossil fuels. First-principles-based modeling is used to predict the performance of the DAC and bioprocess components. A process model is then developed to map material and energy flows, and a techno-economic assessment is performed to determine the minimum fuel selling price. Beyond assessing a specific set of conditions, this analytical framework provides a tool to reveal potential pathways toward the economic viability of this process. We show that an EMP system utilizing an engineered knallgas bacterium can achieve butanol production costs of <$6/gal ($1.58/L) if a set of optimistic assumptions can be realized.

Artificial photosynthesis: Promising approach for the efficient production of high-value bioproducts by microalgae.

Recently, microalgae had received extensive attention for carbon capture and utilization. But its overall efficiency still could not reach a satisfactory degree. Artificial photosynthesis showed better efficiency in the conversion of carbon dioxide. However, artificial photosynthesis could generally only produce C1-C3 organic matters at present. Some studies showed that heterotrophic microalgae can efficiently synthesize high value organic matters by using simple organic matter such as acetate. Therefore, the combination of artificial photosynthesis with heterotrophic microalgae culture showed great potential for efficient carbon capture and high-value organic matter production. This article systematically analyzed the characteristics and challenges of carbon dioxide conversion by microalgae and artificial photosynthesis. On this basis, the coupling mode and development trend of artificial photosynthesis combined with microalgae culture were discussed. In summary, the combination of artificial photosynthesis and microalgae culture has great potential in the field of carbon capture and utilization, and deserves further study.

Scale-up of microalgal systems for decarbonization and bioproducts: Challenges and opportunities.

The threat of global climate change presents a significant challenge for humanity. Microalgae-based carbon capture and utilization (CCU) technology has emerged as a promising solution to this global issue. This review aims to comprehensively evaluate the current advancements in scale-up of microalgae cultivation and its applications, specifically focusing on decarbonization from flue gases, organic wastewater remediation, and biogas upgrading. The study identifies critical challenges that need to be addressed during the scale-up process and evaluates the economic viability of microalgal CCU within the carbon market. Additionally, it analyzes the commercial status of microalgae-derived products and highlights those with high market demand. This review serves as a crucial resource for researchers, industry professionals, and policymakers to develop and implement innovative approaches to enhance the efficiency of microalgae-based CO2 utilization while addressing the challenges associated with the scale-up of microalgae technologies.

Unlocking synergies: Harnessing the potential of biological methane sequestration through metabolic coupling between Methylomicrobium alcaliphilum 20Z and Chlorella sp. HS2.

A halotolerant consortium between microalgae and methanotrophic bacteria could effectively remediate in situ CH4 and CO2, particularly using saline wastewater sources. Herein, Methylomicrobium alcaliphilum 20Z was demonstrated to form a mutualistic association with Chlorella sp. HS2 at a salinity level above 3.0%. Co-culture significantly enhanced the growth of both microbes, independent of initial inoculum ratios. Additionally, increased methane provision in enclosed serum bottles led to saturated methane removal. Subsequent analyses suggested nearly an order of magnitude increase in the amount of carbon sequestered in biomass in methane-fed co-cultures, conditions that also maintained a suitable cultural pH suitable for methanotrophic growth. Collectively, these results suggest a robust metabolic coupling between the two microbes and the influence of the factors other than gaseous exchange on the assembled consortium. Therefore, multi-faceted investigations are needed to harness the significant methane removal potential of the identified halotolerant consortium under conditions relevant to real-world operation scenarios.

Techno-Economic Analysis of Gas Fermentation for the Production of Single Cell Protein.

Despite the large carbon footprint of livestock production, animal protein consumption has grown over the past several decades, necessitating new approaches to sustainable animal protein production. In this techno-economic analysis, single cell protein (SCP) produced via gas fermentation of carbon dioxide, oxygen, and hydrogen is studied as an animal feed source to replace fishmeal or soybean meal. Using wind-powered water electrolysis to produce hydrogen and oxygen with carbon dioxide captured from corn ethanol, the minimum selling price (MSP) of SCP is determined to be $2070 per metric ton. An emissions comparison between SCP, fishmeal, and soybean meal shows that SCP has a carbon intensity as low as 0.73 kg CO2-equiv/kg protein, while fishmeal and soybean meal have an average carbon intensity of 2.72 kg CO2-equiv/kg protein and 0.85 kg CO2-equiv/kg protein, respectively. Moreover, SCP production would occupy 0.4% of the land per ton of protein produced compared to soybean meal and would disturb less than 0.1% of the marine ecosystem currently disturbed by fishmeal harvesting practices. These results show promise for the future economic viability of SCP as a protein source in animal feed and indicate significant environmental benefits compared to other animal feed protein sources.

Paradigm shift in Nutrient-Energy-Water centered sustainable wastewater treatment system through synergy of bioelectrochemical system and anaerobic digestion.

With advancements in research and the necessity of improving the performance of bioelectrochemical system (BES), coupling anaerobic digestion (AD) with BES is crucial for energy gain from wastewater and bioremediation. Hybridization of BES-AD concept opens new avenues for pollutant degradation, carbon capture and nutrient-resource recovery from wastewater. The strength of merging BES-AD lies in synergy, and this approach was employed to differentiate fads from strategies with the potential for full-scale implementation and making it an energy-positive system. The integration of BES and AD system increases the overall performance and complexity of combined system and the cost of operation. From a technical standpoint, the primary determinants of BES-AD feasibility for field applications are the scalability and economic viability. High potential market for such integrated system attract industrial partners for more industrial trials and investment before commercialization. However, BES-AD with high energy efficacy and negative economics demands performance boost.

Carbon capture and utilization by algae with high concentration CO2 or bicarbonate as carbon source.

Algae plays a key role in carbon capture and utilization (CCU) as it can capture and use the atmospheric CO2 for conversion of value-added products. Concentrated CO2 is common in flue gas and provides opportunities for algae cultivation. The drawbacks are mass transfer limitation, poor CO2 dissolution, and challenges to reach optimal levels for algal growth at given flue gas levels. Bicarbonate is flexible to be used as carbon source and owns the potential to enhance the efficiency of biological carbon fixation by algae. The requirements of algae strains are more stringent. To improve the industrial scale-up of CCU, system optimization is of great importance. More novel algal strains that can grow rapidly under harsh environment and provide valuable bio-products should be developed for large-scale production. Algae-driven CCU is promising for achieving carbon-neutrality.

Solar-Driven Continuous CO2 Reduction to CO and CH4 using Heterogeneous Photothermal Catalysts: Recent Progress and Remaining Challenges.

The urgent need to reduce the carbon dioxide level in the atmosphere and keep the effects of climate change manageable has brought the concept of carbon capture and utilization to the forefront of scientific research. Amongst the promising pathways for this conversion, sunlight-powered photothermal processes, synergistically using both thermal and non-thermal effects of light, have gained significant attention. Research in this field focuses both on the development of catalysts and continuous-flow photoreactors, which offer significant advantages over batch reactors, particularly for scale-up. Here, we focus on sunlight-driven photothermal conversion of CO2 to chemical feedstock CO and CH4 as synthetic fuel. This review provides an overview of the recent progress in the development of photothermal catalysts and continuous-flow photoreactors and outlines the remaining challenges in these areas. Furthermore, it provides insight in additional components required to complete photothermal reaction systems for continuous production (e. g., solar concentrators, sensors and artificial light sources). In addition, our review emphasizes the necessity of integrated collaboration between different research areas, like chemistry, material science, chemical engineering, and optics, to establish optimized systems and reach the full potential of this technology.

Lactic Acid from CO2: A Carbon Capture and Utilization Strategy Based on a Biocatalytic Approach.

The EU low-carbon economy aims to reduce the level of CO2 emission in the EU to 80% by 2050. High efforts are required to achieve this goal, where successful CCU (Carbon Capture and Utilization) technologies will have a high impact. Biocatalysts offer a greener alternative to chemical catalysts for the development of CCU strategies since biocatalysis conforms 10 of the 12 principles of green chemistry. In this study, a multienzymatic system, based on alcohol dehydrogenase (ADH), pyruvate decarboxylase (PDC), and lactate dehydrogenase (LDH), that converts CO2 and ethanol into lactic acid leading to a 100% atom economy was studied. The system allows cofactor regeneration, thus reducing the process cost. Through reaction media engineering and enzyme ratio study, the performance of the system was able to produce up to 250 µM of lactic acid under the best conditions using 100% CO2, corresponding to the highest concentration of lactic acid obtained up to date using this multienzymatic approach. For the first time, the feasibility of the system to be applied under a real industrial environment has been tested using synthetic gas mimicking real blast furnace off-gases composition from the iron and steel industry. Under these conditions, the system was also capable of producing lactic acid, reaching 62 µM.

Optimizing the Performance of Low-Loaded Electrodes for CO2-to-CO Conversion Directly from Capture Medium: A Comprehensive Parameter Analysis.

Gas-fed reactors for CO2 reduction processes are a solid technology to mitigate CO2 accumulation in the atmosphere. However, since it is necessary to feed them with a pure CO2 stream, a highly energy-demanding process is required to separate CO2 from the flue gasses. Recently introduced bicarbonate zero-gap flow reactors are a valid solution to integrate carbon capture and valorization, with them being able to convert the CO2 capture medium (i.e., the bicarbonate solution) into added-value chemicals, such as CO, thus avoiding this expensive separation process. We report here a study on the influence of the electrode structure on the performance of a bicarbonate reactor in terms of Faradaic efficiency, activity, and CO2 utilization. In particular, the effect of catalyst mass loading and electrode permeability on bicarbonate electrolysis was investigated by exploiting three commercial carbon supports, and the results obtained were deepened via electrochemical impedance spectroscopy, which is introduced for the first time in the field of bicarbonate electrolyzers. As an outcome of the study, a novel low-loaded silver-based electrode fabricated via the sputtering deposition technique is proposed. The silver mass loading was optimized by increasing it from 116 µg/cm2 to 565 µg/cm2, thereby obtaining an important enhancement in selectivity (from 55% to 77%) and activity, while a further rise to 1.13 mg/cm2 did not provide significant improvements. The tremendous effect of the electrode permeability on activity and proficiency in releasing CO2 from the bicarbonate solution was shown. Hence, an increase in electrode permeability doubled the activity and boosted the production of in situ CO2 by 40%. The optimized Ag-electrode provided Faradaic efficiencies for CO close to 80% at a cell voltage of 3 V and under ambient conditions, with silver loading of 565 µg/cm2, the lowest value ever reported in the literature so far.

Multiscale model to resolve the chemical environment in a pressurized CO2-captured solution electrolyzer.

The community of electrochemical CO2 reduction is almost exclusively focused on gaseous CO2-fed electrolyzers. Here, we proposed a pressurized CO2-Captured solution electrolyzer to produce solar Fuel of CO (abbreviated "CCF") without the need to regenerate gaseous CO2. Specifically, we developed an experimentally validated multiscale model to quantitatively investigate the effect of pressure-induced chemical environment and to resolve the complex relationship between this effect and the activity and selectivity of CO production. Our results show that the pressure-induced variation of the cathode pH has a negative effect on the hydrogen evolution reaction, whereas the species coverage variation positively affects CO2 reduction. These effects are more pronounced at pressures below 15 bar (1 bar = 101 kPa). Consequently, a mild increase in the pressure of the CO2-captured solution from 1 to 10 bar leads to a dramatic enhancement in selectivity. Using a commercial Ag nanoparticle catalyst, our pressurized CCF prototype achieved CO selectivity higher than 95% at a low cathode potential of -0.6 V versus reversible hydrogen electrode (RHE), comparable to that under the gaseous CO2-fed condition. This enables the demonstration of a solar-to-CO efficiency of 16.8%, superior to any known devices with an aqueous feed.

Life Cycle Assessment of Innovative Carbon Dioxide Selective Membranes from Low Carbon Emission Sources: A Comparative Study.

Carbon capture has been an important topic of the twenty-first century because of the elevating carbon dioxide (CO2) levels in the atmosphere. CO2 in the atmosphere is above 420 parts per million (ppm) as of 2022, 70 ppm higher than 50 years ago. Carbon capture research and development has mostly been centered around higher concentration flue gas streams. For example, flue gas streams from steel and cement industries have been largely ignored due to lower associated CO2 concentrations and higher capture and processing costs. Capture technologies such as solvent-based, adsorption-based, cryogenic distillation, and pressure-swing adsorption are under research, but many suffer from higher costs and life cycle impacts. Membrane-based capture processes are considered cost-effective and environmentally friendly alternatives. Over the past three decades, our research group at Idaho National Laboratory has led the development of several polyphosphazene polymer chemistries and has demonstrated their selectivity for CO2 over nitrogen (N2). Poly[bis((2-methoxyethoxy)ethoxy)phosphazene] (MEEP) has shown the highest selectivity. A comprehensive life cycle assessment (LCA) was performed to determine the life cycle feasibility of the MEEP polymer material compared to other CO2-selective membranes and separation processes. The MEEP-based membrane processes emit at least 42% less equivalent CO2 than Pebax-based membrane processes. Similarly, MEEP-based membrane processes produce 34-72% less CO2 than conventional separation processes. In all studied categories, MEEP-based membranes report lower emissions than Pebax-based membranes and conventional separation processes.