Sensitivity of grassland carbon pools to plant diversity,elevated CO2, and soil nitrogen addition over 19 years

Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO₂) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO₂, soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO₂ enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m⁻² ⋅ y⁻¹ and elevated CO₂ of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO₂ and N. Overall, our results call into question whether elevated CO₂ will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO₂ or high rates of N deposition in perennial grassland systems.

Only about half of anthropogenic carbon (C) emissions accumulate in the atmosphere, while the rest are stored by oceans and the terrestrial biosphere (1, 2). However, model projections of terrestrial C cycling in the face of climate change are highly variable, and whether the terrestrial biosphere will continue to act as a global net C sink is questionable (1, 3). A key uncertainty relates to whether the rise in atmospheric carbon dioxide (CO₂) concentration is being slowed by enhanced photosynthesis and subsequent C storage in plants and soils (1). Moreover, other factors, such as soil nitrogen (N) availability, species composition, species diversity, climate warming and associated precipitation change, and altered disturbance rates may impact terrestrial C sinks and constrain how terrestrial ecosystems respond to elevated CO₂ (eCO₂) (4–11).Elevated CO₂ increases C fixed via photosynthesis and stimulates plant growth in a wide variety of species (4, 5, 12). Additional C acquisition under eCO₂ can enhance plant and soil C pools, the latter by increasing plant-derived C inputs to soils, reducing litter quality and thereby slowing litter decomposition rates, and/or by stimulating physical protection of soil organic matter (SOM) from microbial decomposition (13–15). However, some studies have observed no C accumulation under eCO₂ (16–21).Conflicting results among studies have called into question whether short-term enhancement of plant C uptake under eCO₂ (i.e., over days or years) will translate into long-term (i.e., decades or more) ecosystem C storage. The net impact of eCO₂ on ecosystem C storage depends on the balance between eCO₂ effects on net primary productivity (NPP) and on losses of C through microbial respiration and other pathways such as leaching, fire, and herbivory. To mitigate climate change over the long term (decades to centuries to millennia), some additional C captured under eCO₂ must be transferred to biomass or SOM pools with slow turnover times (i.e., wood or physically or chemically protected SOM) (22, 23). However, eCO₂ has been observed to increase the partitioning of C to rapidly cycling pools of labile C (e.g., roots, root exudates, and detritus), resulting in enhanced C loss via soil respiration (17–19, 22, 24, 25). Elevated CO₂ can also increase soil moisture via enhanced plant water-use efficiency, thereby stimulating microbial decomposition and C loss (26). In ecosystems that are fire prone or that undergo managed fire regimes, like grasslands and savannas, enhanced biomass under eCO₂ may also increase fuel load and thus fire intensity and spread, thereby enhancing fire-mediated C losses (10, 20, 27, 28).Moreover, limited N availability often constrains positive responses of photosynthesis and plant growth to eCO₂ (4, 5, 8, 29) as N is a key driver of photosynthesis and productivity (30, 31). Under ambient CO₂, additional N inputs have been shown to promote C storage by increasing plant-derived C inputs to soils via litter and root production (32) and/or by suppressing organic matter decay (33). Under eCO₂, N addition often alleviates down-regulation of photosynthetic and growth responses (4, 5, 8), potentially enhancing positive effects of eCO₂ on plant productivity and soil C accumulation (13).Plant species richness can also affect ecosystem C accumulation and interact with eCO₂ and soil N supply (6, 34). Increased species richness has been shown to promote soil C storage in various grassland biodiversity experiments via enhanced productivity, belowground C allocation, and changes in microbial communities (35–42). For instance, at a companion experiment in Minnesota, increasing grassland species richness from 1 to 2, 4, 8, or 16 species following agricultural abandonment increased soil C storage by 60 to 178% due to greater aboveground and root biomass inputs (35). In a long-term grassland biodiversity experiment in Germany, higher plant diversity increased soil C storage by enhancing rhizosphere inputs of recently fixed C and increasing microbial biomass C (38, 40, 42). Plant growth responses to eCO₂ and N addition may be stronger in more diverse plots (6, 34), potentially fueling interactive effects with diversity on ecosystem C accumulation. However, to our knowledge, no field experiment other than that described herein concurrently manipulates CO₂, soil N supply, and species richness.It remains unclear whether the eCO₂-induced increases in plant C fixation commonly observed in short-term studies will translate into long-term C storage and whether this depends on soil N supply and/or species richness. We addressed this gap in understanding using the free-air CO₂ enrichment experiment BioCON (6, 8), which concurrently manipulates CO₂, N supply, and species richness in a frequently burned Minnesota grassland. Here, we used measurements of total soil C stocks to a depth of 60 cm coupled with long-term biomass, measures of aboveground and belowground NPP, and plant tissue chemistry data to assess how manipulations of CO₂ (ambient, +180 ppm), soil N supply (ambient, +4 g N m⁻² ⋅ y⁻¹), and planted species richness (1, 4, 9, or 16 species) have influenced total ecosystem C accumulation over 19 y. In herbaceous systems that lack substantial woody biomass and where aboveground biomass C turns over annually, C storage is expected to occur primarily in soils (43). We expected this system to be in the aggrading stage of perennial vegetation growth following establishment of the experiment and to therefore accumulate C over time (35, 37).Our global change treatments represent and span realistic scenarios for grassland ecosystems over the next seven or eight decades. We acknowledge it is challenging to project each factor with equal confidence and we make comparisons across experimentally selected treatments carefully. Our eCO₂ treatment simulates atmospheric concentrations projected to be reached by the end of the century (44). Our N addition treatment simulates high rates of N deposition experienced in industrialized regions of the Northern Hemisphere and biomass burning regions in the tropics, and projections of N deposition rates over the next few decades are regionally variable and dependent on changes in emission regulations (45–48). The N treatment can also be used to understand variation in soil fertility; for example, the difference in N input rate between the control and N treatment is comparable to the range in net N mineralization rates between less fertile sites (frequently burned savannas) and more fertile forests within 1 km of our study site (49). Our species richness treatments span much of the range of diversity in native and anthropogenic grasslands in central Minnesota. As noted in Reich et al. (50), nearby native grasslands averaged 16.3 species per 1.0 m² sampled, whereas BioCON plots planted with 16 species tend to have 9 to 11 species per 0.1 m² sampled, due in part to local extinctions in individual plots and sampling only a small fraction of each 4 m² plot. In contrast, postagricultural restored grasslands averaged 3.5 species (range 1 to 8) per 1.0 m² sampled (45). Our treatments with 16 planted species are thus roughly representative of high-diversity native vegetation, while our 1 and 4 species planted treatments have diversity similar to low-diversity grasslands of anthropogenic origin. Grassland species richness is projected to decline substantially by the year 2100 because of climate change and other human disturbances (51), and the range of losses we simulate is relevant to both climate-driven, local-scale species losses experienced by natural grasslands (52) and more severe losses associated with land-use change (51). Our range of species richness levels is also relevant to grassland restoration, where managers must weigh the economic and ecological effects of planting monocultures versus diverse mixtures (e.g., ref. 53).Conceptually, on a timescale of two decades, eCO₂, N addition, and increasing species richness could each increase ecosystem C accumulation because of enhanced NPP (i.e., enhanced plant-derived soil C inputs) (13, 15, 32, 35). However, pathways of C loss can also be stimulated, offsetting potential gains in ecosystem C (20, 22, 24, 25). Model projections of C storage by the end of the century show that grassland C storage may increase, decrease, or remain unchanged under future atmospheric composition and climate (3, 54, 55). Part of this uncertainty is due to a lack of empirical grounding for model assumptions regarding effects of global change factors on C cycling (56, 57). Here, the longevity of our experiment and factorial design allow us to empirically assess predictions of grassland C storage by the end of the century.We constructed hypotheses about total ecosystem C responses to two decades of CO₂, N, and species richness treatments in part based on a rich body of previous work performed in BioCON related to responses of biomass and C cycling to these treatments. After 4 y of treatments in a subexperiment of BioCON (including only 1 and 4 species plots), N addition, but not eCO₂, modestly increased total soil %C (measured to a depth of 20 cm) (58). The lack of an eCO₂ effect early in the experiment could reflect a balance between soil C inputs and losses in low-diversity plots; however, changes in soil C may take years to become apparent, may occur below a depth of 20 cm, and may be stronger with higher species richness. Past work in this experiment also indicated that these treatments stimulated both ecosystem C inputs and losses (based on measurements of photosynthesis, biomass, respiration, and fire-induced losses; 6, 8, 12, 20, 24, 34, 59, 60); for instance, early C gains in response to treatments were completely offset by greater fire-mediated C losses in years when biomass was burned, likely because enhanced litter production increased fuel loads (20). However, responses of soil C inputs and losses to global changes may be dynamic, driving periods of net positive, negative, and neutral C accumulation. Based on this prior work in the BioCON experiment, we hypothesized (H1) that neither eCO₂ nor N addition would substantially enhance total ecosystem C pools because increased soil C inputs would be largely offset by enhanced C losses. In contrast, we hypothesized (H2) that total ecosystem C pools would increase with species richness, more so over time, as increased species richness enhanced biomass several-fold more than did enriched CO₂ or N in this experiment (6). We also hypothesized (H3) that effects of eCO₂, N addition, and species richness would be synergistic (i.e., more than additive) because of past work demonstrating resource colimitation and interactions with diversity for biomass production (6, 61). For instance, substantial positive effects of eCO₂ on total ecosystem C might only be apparent at high levels of species richness or N.     

CO2 enhancement of forest productivity constrained by limited nitrogen availability

Stimulation of terrestrial plant production by rising CO(2) concentration is projected to reduce the airborne fraction of anthropogenic CO(2) emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO(2), but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO(2) fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO(2) concentration over 11 y in a free-air CO(2) enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO(2) compared with NPP in plots in current ambient CO(2), and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO(2) declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO(2) fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time--an expected consequence of stand development, which was exacerbated by elevated CO(2). Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO(2); these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.     

Massive and rapid predominantly volcanic CO2 emission during the end-Permian mass extinction

The end-Permian mass extinction event (∼252 Mya) is associated with one of the largest global carbon cycle perturbations in the Phanerozoic and is thought to be triggered by the Siberian Traps volcanism. Sizable carbon isotope excursions (CIEs) have been found at numerous sites around the world, suggesting massive quantities of ¹³C-depleted CO₂ input into the ocean and atmosphere system. The exact magnitude and cause of the CIEs, the pace of CO₂ emission, and the total quantity of CO₂, however, remain poorly known. Here, we quantify the CO₂ emission in an Earth system model based on new compound-specific carbon isotope records from the Finnmark Platform and an astronomically tuned age model. By quantitatively comparing the modeled surface ocean pH and boron isotope pH proxy, a massive (∼36,000 Gt C) and rapid emission (∼5 Gt C yr⁻¹) of largely volcanic CO₂ source (∼−15%) is necessary to drive the observed pattern of CIE, the abrupt decline in surface ocean pH, and the extreme global temperature increase. This suggests that the massive amount of greenhouse gases may have pushed the Earth system toward a critical tipping point, beyond which extreme changes in ocean pH and temperature led to irreversible mass extinction. The comparatively amplified CIE observed in higher plant leaf waxes suggests that the surface waters of the Finnmark Platform were likely out of equilibrium with the initial massive centennial-scale release of carbon from the massive Siberian Traps volcanism, supporting the rapidity of carbon injection. Our modeling work reveals that carbon emission pulses are accompanied by organic carbon burial, facilitated by widespread ocean anoxia.

The end-Permian mass extinction (EPME) that occurred at 251.941 ± 0.037 Mya is considered the most severe biodiversity loss in Earth history (1, 2). The EPME coincides with the eruption of the Siberian Traps, a voluminous large igneous province (LIP) that occupies 6 million square kilometers (km²) in Siberia, Russia (3–5). The volcanic activity of this LIP is linked to SO₂ and CO₂ degassing generated by sill intrusion (6–10). The large amount of CO₂ injected into the atmosphere is thought to have led to severe global warming (11–14), catastrophic ocean anoxia (15, 16), and extreme ocean and terrestrial acidification (17–21) being lethal for life on land and in the sea (22). To date, no agreement has been reached regarding the source of the ¹³C-depleted carbon that triggered the global carbon cycle perturbation, the decrease in ocean pH, and the global warming across the EPME. Additionally, atmospheric CO₂ levels following the initial pulse of Siberian Traps volcanism and across the EPME remain poorly known (23, 24), limiting our understanding of the climate feedbacks that occur upon greenhouse gas release during this time.To address this critical gap in our knowledge, we constrain the source, pace and total amount of CO₂ emissions using an Earth system model of intermediate complexity (i.e., carbon centric-Grid Enabled Integrated Earth system model [cGENIE]; SI Appendix) forced by new astronomically tuned δ¹³C records from well-preserved lipid biomarkers preserved in sediments from the Finnmark Platform, Norway. The Finnmark Platform is located offshore northern Norway on the Eastern Barents Sea shelf, hosting an expanded shallow marine section (paleo-water depth roughly 50 to 100 m) where two drill cores were collected (7128/12-U-01 and 7129/10-U-01) spanning the Permian–Triassic transition (Fig. 1). A previously generated bulk organic carbon isotope record (δ¹³C_org) from the same core shows a two-step decline with a total carbon isotope excursion (CIE) magnitude of ∼4‰ (25). Although the sedimentary organic carbon was considered primarily of terrestrial origin, small contributions from marine organic carbon production could not be excluded. Here, we use compound-specific carbon isotope analysis of both long-chain and short-chain n-alkanes preserved in marine sediments in the Finnmark Platform to generate separate yet directly comparable records of δ¹³C for the terrestrial and the marine realm, respectively, across the EPME. Long-chain n-alkanes with a strong odd-over-even predominance (n-C₂₇ and n-C₂₉) are produced by higher plant leaf waxes, and their isotopic composition (δ¹³C_wax) relates to their main carbon source (i.e., atmospheric CO₂) (26). On the other hand, short-chain alkanes (n-C₁₇ and n-C₁₉) are derived from marine algae, and their δ¹³C values (δ¹³C_algae) represent carbon in the marine realm (27, 28). To date, only a few EPME compound-specific carbon isotope studies have been reported, all of which are limited by unfavorable sedimentary facies or high thermal maturity of the organic matter (29, 30). In the present study, the exceptionally low thermal maturity of the organic matter is evident from the yellow color of pollen and spores, indicating a color index 2 out of 7 on the thermal alteration scale of Batten (31), which is equivalent to a vitrinite reflectance R₀ of 0.3%. Moreover, the high sedimentation rate (discussed in Carbon Cycle Quantification Using Astrochronology and Earth System Model) of the siliciclastic sediments at the study site allows for studying both marine and terrestrial CIE across the EPME in unprecedented detail. Taken together, the Finnmark sedimentary records enable the reconstruction of individual yet directly comparable carbon isotope records for the terrestrial and the marine realm that can be astronomically tuned and used to quantitatively assess the source, pace, and total amount of ¹³C-depleted carbon released during the Siberian Traps eruption that led to the EPME. Using our new compound-specific carbon isotope records, rather than marine carbonates, has several advantages: 1) new astrochronology enables a 10⁴-year temporal resolution for our paired marine and terrestrial carbon isotope records; 2) we do not need to assume a constant sedimentation rate between tie point or using diachronous biozones to compare age like those used in global compilations (24) (see Fig. 4A); 3) the δ¹³C_algae data are not artificially smoothed as in ref. 32 to avoid underestimation of the CIE magnitude; and 4) our records are not affected by dissolution or truncation, a phenomenon common to shallow marine carbonates due to the presumed ocean acidification occurred during the EPME (18, 33). In addition, the directly comparable records of δ¹³C for the atmosphere and the ocean offer further insights into the size of the true CIE and rate and duration of carbon emissions.Open in a separate windowFig. 1.(A) Paleogeographical map of the Late Permian, with former and current coastlines. Indicated are 1) the location of Finnmark cores 7128/12-U-01 and 7129/10-U-01, 2) the East Greenland site at Kap Stosch discussed in ref. 52, 3) the GSSP site for the base of the Triassic at Meishan, China, and 4) the Kuh-e-Ali Bashi site of Iran (66, 107). The map was modified after ref. 61. (B) Paleogeography and paleobathymetry of the Late Permian used in cGENIE.Open in a separate windowFig. 4.Synthesized proxy records of carbon isotopes from marine carbonates and fossil C₃ land plants remains, sea surface temperature, and pH. (A) Comparison between δ¹³C_algae and global marine carbonate carbon isotopes from sites at Abadeh, Kuh-e-Ali Bashi, Shahreza, and Zal in Iran, Meishan, Wenbudangsang, and Yanggou in South China, at Bálvány North in Hungary, and at Nhi Tao in Vietnam (24). (B) Comparison between δ¹³C_leaf wax and the δ¹³C of sedimentary leaf cuticles and wood of C₃ land plants from South China (24). (C) Reconstructed sea surface temperature data using conodont fossils (circles) (24) and brachiopods (triangles) (14). The conodont-based temperature data are from sites in the Paleo-Tethys, including Chanakhchi, Kuh-e Ali Bashi, Meishan, Shangsi, and Zal. (D) Relative changes in sea surface pH based on boron isotope proxy from ref. 17 and ref. 20. Pink and red circles are data from scenario 1 and scenario 2 in ref. 17, and green and blue diamonds are data from scenario 1 and scenario 2 in ref. 20.     

Mechanical,Structural and Electronic Properties of CO2 Adsorbed Graphitic Carbon Nitride (g-C3N4) under Biaxial Tensile Strain

We investigate mechanical, structural and electronic properties of CO₂ adsorbed graphitic carbon nitride (g-C₃N₄) system under biaxial tensile strain via first-principles calculations. The results show that the stress of CO₂ adsorbed g-C₃N₄ system increases and then decreases linearly with the increasing biaxial strain, reaching maximum at 0.12 strain. This is primarily caused by the plane N–C stretching of the g-C₃N₄. Furthermore, both the Perdew-Burke-Ernzerhof (PBE) and Heyd- Scuseria-Ernzerhof screened hybrid functional (HSE06) band gaps show direct-indirect transitions under biaxial tensile strain and have the maximum also at 0.12 strain. It is found that there is large dipole transition matrix element around Γ point, leading high optical absorption coefficients of the deformed adsorption system, which would be of great use for the applications of new elastic nanoelectronic and optoelectronic devices.     

Feasibility and safety of automated CO2 angiography in peripheral arterial interventions

Carbon dioxide (CO₂) gas is an established alternative to iodine contrast during angiography in patients with risk of postcontrast acute kidney injury and in those with history of iodine contrast allergy. Different CO₂ delivery systems during angiography are reported in literature, with automated delivery system being the latest. The aim of this study is to evaluate the safety, efficacy, and learning curve of an automated CO₂ injection system with controlled pressures in peripheral arterial interventions and also to study the patients’ tolerance to the system.From January 2018 to October 2019 peripheral arterial interventions were performed in 40 patients (median age-78 years, interquartile range: 69–84 years) using an automated CO₂ injection system with customized protocols, with conventional iodine contrast agent used only as a bailout option. The pain and tolerance during the CO₂ angiography were evaluated with a visual analog scale at the end of each procedure. The amount of CO₂, iodine contrast used, and radiation dose area product for the interventions were also systematically recorded for all procedures. These values were statistically compared in 2 groups, viz first 20 patients where a learning curve was expected vs the rest 20 patients.All procedures were successfully completed without complications. All patients tolerated the CO₂ angiography with a median total pain score of 3 (interquartile range: 3–4), with no statistical difference between the groups (P = .529). The 2 groups were statistically comparable in terms of comorbidities and the type of procedures performed (P = .807). The amount of iodine contrast agent used (24.60 ± 6.44 ml vs 32.70 ± 8.70 ml, P = .006) and the radiation dose area product associated were significantly lower in the second group (2160.74 ± 1181.52 μGym² vs 1531.62 ± 536.47 μGym², P = .043).Automated CO₂ angiography is technically feasible and safe for peripheral arterial interventions and is well tolerated by the patients. With the interventionalist becoming familiar with the technique, better diagnostic accuracy could be obtained using lower volumes of conventional iodine contrast agents and reduction of the radiation dose involved.     

Improved Methane Production by Photocatalytic CO2 Conversion over Ag/In2O3/TiO2 Heterojunctions

In this work, the role of In₂O₃ in a heterojunction with TiO₂ is studied as a way of increasing the photocatalytic activity for gas-phase CO₂ reduction using water as the electron donor and UV irradiation. Depending on the nature of the employed In₂O₃, different behaviors appear. Thus, with the high crystallite sizes of commercial In₂O₃, the activity is improved with respect to TiO₂, with modest improvements in the selectivity to methane. On the other hand, when In₂O₃ obtained in the laboratory, with low crystallite size, is employed, there is a further change in selectivity toward CH₄, even if the total conversion is lower than that obtained with TiO₂. The selectivity improvement in the heterojunctions is attributed to an enhancement in the charge transfer and separation with the presence of In₂O₃, more pronounced when smaller particles are used as in the case of laboratory-made In₂O₃, as confirmed by time-resolved fluorescence measurements. Ternary systems formed by these heterojunctions with silver nanoparticles reflect a drastic change in selectivity toward methane, confirming the role of silver as an electron collector that favors the charge transfer to the reaction medium.     

Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements

The phase changes in alkali-activated slag samples when exposed to supercritical carbonation were evaluated. Ground granulated blast furnace slag was activated with five different activators. The NaOH, Na₂SiO₃, CaO, Na₂SO₄, and MgO were used as activators. C-S-H is identified as the main reaction product in all samples along with other minor reaction products. The X-ray diffractograms showed the complete decalcification of C-S-H and the formation of CaCO₃ polymorphs such as calcite, aragonite, and vaterite. The thermal decomposition of carbonated samples indicates a broader range of CO₂ decomposition. Formation of highly cross-linked aluminosilicate gel and a reduction in unreacted slag content upon carbonation is observed through ²⁹Si and ²⁷Al NMR spectroscopy. The observations indicate complete decalcification of C-S-H with formation of highly cross-linked aluminosilicates upon sCO₂ carbonation. A 20–30% CO₂ consumption per reacted slag under supercritical conditions is observed.     

Shear stress-induced pH increase in plasma is mediated by a decrease in PCO2: The increase in pH enhances shear stress-induced P-selectin expression in platelets

To investigate shear stress-induced platelet activation, the cone-plate viscometer or the Couette rotational viscometer has been widely used. In a previous report, it was shown that shearing platelet-rich plasma using a Couette rotational viscometer could lead to an increase in pH by CO₂ release. However, any clear mechanism has not been provided. In this study, we examined whether shearing cell free plasma only using a cone-plate viscometer can also induce pH increase and studied the underlying mechanism of shear-induced pH increase by directly measuring total CO₂ (T_CO2) and CO₂ tension (P_CO2). When human plasma was sheared using a cone-plate viscometer, the pH of the human plasma increased time- and shear rate-dependently. Although T_CO2 of human plasma was not affected, P_CO2 was decreased by shearing, indicating that the decreased P_CO2 is associated with a pH increase of plasma. In addition, the pH of bicarbonate-containing suspension buffer was also shown to be increased by shearing; suggesting that the platelet studies using suspension buffers containing bicarbonate could be affected similarly. The effects of pH changes on shear stress-induced platelet activation were also investigated in the same in vitro systems. While shear stress-induced platelet aggregation was not affected by the pH changes, P-selectin expression was significantly increased in accordance with the pH increase. In conclusion, shear stress using a cone-plate viscometer induces pH increase in plasma or bicarbonate-containing suspension buffer through a P_CO2 decrease and the pH changes alone can contribute to platelet activation by enhancing shear stress-induced P-selectin expression.     

Effects of the Crystalline Properties of Hollow Ceria Nanostructures on a CuO-CeO2 Catalyst in CO Oxidation

The development of an efficient and economic catalyst with high catalytic performance is always challenging. In this study, we report the synthesis of hollow CeO₂ nanostructures and the crystallinity control of a CeO₂ layer used as a support material for a CuO-CeO₂ catalyst in CO oxidation. The hollow CeO₂ nanostructures were synthesized using a simple hydrothermal method. The crystallinity of the hollow CeO₂ shell layer was controlled through thermal treatment at various temperatures. The crystallinity of hollow CeO₂ was enhanced by increasing the calcination temperature, but both porosity and surface area decreased, showing an opposite trend to that of crystallinity. The crystallinity of hollow CeO₂ significantly influenced both the characteristics and the catalytic performance of the corresponding hollow CuO-CeO₂ (H-Cu-CeO₂) catalysts. The degree of oxygen vacancy significantly decreased with the calcination temperature. H-Cu-CeO₂ (HT), which presented the lowest CeO₂ crystallinity, not only had a high degree of oxygen vacancy but also showed well-dispersed CuO species, while H-Cu-CeO₂ (800), with well-developed crystallinity, showed low CuO dispersion. The H-Cu-CeO₂ (HT) catalyst exhibited significantly enhanced catalytic activity and stability. In this study, we systemically analyzed the characteristics and catalyst performance of hollow CeO₂ samples and the corresponding hollow CuO-CeO₂ catalysts.     

Thin Film Mixed Matrix Hollow Fiber Membrane Fabricated by Incorporation of Amine Functionalized Metal-Organic Framework for CO2/N2 Separation

Membrane separation technology can used to capture carbon dioxide from flue gas. However, plenty of research has been focused on the flat sheet mixed matrix membrane rather than the mixed matrix thin film hollow fiber membranes. In this work, mixed matrix thin film hollow fiber membranes were fabricated by incorporating amine functionalized UiO-66 nanoparticles into the Pebax^® 2533 thin selective layer on the polypropylene (PP) hollow fiber supports via dip-coating process. The attenuated total reflection-Fourier transform infrared (ATR-FTIR), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX) mapping analysis, and thermal analysis (TGA-DTA) were used to characterize the synthesized UiO-66-NH₂ nanoparticles. The morphology, surface chemistry, and the gas separation performance of the fabricated Pebax^® 2533-UiO-66-NH₂/PP mixed matrix thin film hollow fiber membranes were characterized by using SEM, ATR-FTIR, and gas permeance measurements, respectively. It was found that the surface morphology of the prepared membranes was influenced by the incorporation of UiO-66 nanoparticles. The CO₂ permeance increased along with an increase of UiO-66 nanoparticles content in the prepared membranes, while the CO₂/N₂ ideal gas selectively firstly increased then decreased due to the aggregation of UiO-66 nanoparticles. The Pebax^® 2533-UiO-66-NH₂/PP mixed matrix thin film hollow fiber membranes containing 10 wt% UiO-66 nanoparticles exhibited the CO₂ permeance of 26 GPU and CO₂/N₂ selectivity of 37.     

Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO

The addition of molten alkali metal salts drastically accelerates the kinetics of CO₂ capture by MgO through the formation of MgCO₃. However, the growth mechanism, the nature of MgCO₃ formation, and the exact role of the molten alkali metal salts on the CO₂ capture process remain elusive, holding back the development of more-effective MgO-based CO₂ sorbents. Here, we unveil the growth mechanism of MgCO₃ under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO₃. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO₂, a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO₃, MgCO₃ crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO₃. MgCO₃ grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO₃ and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO₃ crystals, point to the etching of surface MgO, providing dissolved [Mg²⁺…O^2–] ionic pairs for MgCO₃ growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.

Global concerns about the rising level of greenhouse gas emissions and the associated climate change require the development of efficient processes to remove CO₂ selectively from large point sources or directly from the atmosphere. Such processes are termed carbon dioxide capture and storage (CCS) (1) and can be implemented on the industrial scale in different configurations such as precombustion, postcombustion, or oxy-combustion CCS (2, 3). A large variety of both liquid and solid sorbent materials have been explored for CCS, with solid CO₂ sorbents being particularly interesting owing to their ability to capture large quantities of CO₂ from point sources with, compared to amines, favorable efficiency and cost penalty estimates (4). Yet, the development of inexpensive solid sorbents that capture CO₂ with fast rates, possess a high CO₂ capacity, and operate with high stability over many CO₂ capture and regeneration cycles remains a key challenge. To advance the current state of sorbent design in a rational fashion, an improvement of our current understanding of the interplay between a sorbent’s structural features and its CO₂ capture characteristics is required.Magnesium oxide (MgO) is an attractive solid CO₂ sorbent, in particular for precombustion CCS applications, that stands out owing to its high theoretical CO₂ uptake of 1.09 g_CO2⋅g_MgO^–1 (24.8 mmol_CO2⋅g_MgO⁻¹) and relatively low temperature for regeneration, as compared to other solid sorbents (e.g., CaO, Li₄SiO₄, and Li₂ZrO₃). Despite its high theoretical CO₂ capacity, bare MgO displays very sluggish carbonation kinetics, yielding an experimental CO₂ uptake of only 0.02 g_CO2⋅g_MgO^–1 after 1 h of exposure to CO₂ (5, 6). The low practically obtained CO₂ uptake compared to the high theoretical value has been attributed to the high lattice enthalpy of MgO reducing the kinetics of its reaction with CO₂ appreciably and to the formation of a monodentate carbonate layer on the surface of MgO, which acts as a CO₂-impermeable barrier hampering the further conversion of unreacted MgO (7–9). Encouragingly, the slow uptake of MgO can be accelerated appreciably through an engineering solution, that is, the addition of alkali metal salts (AMS; e.g., NaNO₃, KNO₃, LiNO₃, and their eutectic mixtures) which are molten at operating conditions (10–14). By optimizing the loading of AMS (ca. 20 wt. % AMS), the CO₂ uptake of MgO can be increased by a factor of 15 (0.31 g_CO2⋅g_sorbent^–1) compared to unpromoted MgO at identical carbonation durations (13). The kinetics and stability of AMS-promoted MgO can be improved even further when adding alkali earth carbonates such as SrCO₃ or CaCO₃, which have been hypothesized to act as nucleation seeds or to lead to the formation of double carbonate phases that form with faster kinetics. For such systems, CO₂ uptakes of up to 0.65 g_CO2⋅g_sorbent^–1 after 50 carbonation and calcination cycles have been reported (15–17).Owing to the impressive effect of molten AMS on the CO₂ uptake of bare MgO, the elucidation of the underlying promoting mechanism(s) has been the aim of a series of studies that have led to the postulation of a number of working hypotheses (12, 13, 16, 18–22). For example, combining thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (XRD), it has been proposed that the addition of AMS promotes the CO₂ uptake of MgO through the following two effects. Firstly, it has been argued that AMS prevent the formation of a CO₂-impermeable monodentate carbonate layer on the surface of MgO, and they dissolve CO₂ that reacts with oxide ions (O^2–) in the nitrate (23), leading to the formation of reactive carbonate ions (CO₃^2–) that subsequently react with Mg²⁺ to form MgCO₃ (13). A second hypothesis, based on TGA and density functional theory (DFT) calculations, has argued that the promoting role of the AMS is mainly due to the molten salt’s ability to lower the energy barrier associated with the high lattice enthalpy of MgO by dissolving the solid metal oxide (12, 18). The dissolution of MgO in the molten promoter yields solvated [Mg²⁺…O^2–] ionic pairs that have weaker bonds compared to the strong ionic bonds in bulk MgO (12). The DFT calculations showed that the rate-controlling step for the reaction between CO₂ and MgO is the activation of the MgO ionic bond; the energy barrier to form [Mg²⁺…O^2–] ionic pairs in the molten NaNO₃ is 5.33 eV, compared to 7.07 eV without NaNO₃ (12). The dissolved ionic pair reacts with CO₂ that is adsorbed on the MgO surface, that is, at the triple phase boundary (TPB) between MgO, CO₂, and the molten phases, to form [Mg²⁺…CO₃^2–] ionic pairs. Upon reaching saturation, the [Mg²⁺…CO₃^2–] ionic pairs precipitate as a crystalline MgCO₃ phase. It is further argued that the carbonate may precipitate away from the original dissolution site so as to not prohibit further reaction. Following this argument, the reaction would, in theory, continue until MgO is completely converted. However, in practice, MgO conversion stops at ∼70%, which has been argued to arise from a reducing TPB length. A recent in situ total scattering study points to a more multifaceted role of the AMS promoter in the MgO−CO₂ system, as the AMS do not affect only the nucleation of MgCO₃ but also the microstructure and growth of the MgCO₃ formed (20)_.Turning to the kinetics of CO₂ absorption, TGA-based studies of NaNO₃-promoted MgO powders have shown that the formation of MgCO₃ is characterized by a nucleation and growth process (16). The characteristic sigmoidal kinetic curve of MgO conversion suggests that the carbonate formation is “autocatalytic”; that is, once a stable nucleation seed is formed, the growth rate is accelerated. This interpretation was corroborated by experimental evidence that showed that the induction period, that is, the time required for the first stable nuclei to form, can be shortened by the inclusion of “inert” SrCO₃ seeds, which act as nucleation sites for MgCO₃ (16). Although there is a general agreement on the nucleation- and growth-based mechanism for the formation of MgCO₃, there are competing hypotheses on the nature of magnesium carbonate formation, that is, where it forms (at the interface AMS/MgO, at the TPB, or inside the AMS), its growth habit, and its morphology. According to an in situ TEM study of MgO nanoparticles that were physically mixed with a eutectic mixture of AMS, MgCO₃ nucleates favorably at the TPB (24, 25). Nonetheless, a different study suggests that the TPB is not a necessary condition for the absorption to take place, as carbonates (MgCO₃) were detected on the surface of a MgO(100) single crystal that was covered completely by NaNO₃ and treated under CO₂ (330 °C), as revealed by ex situ FTIR after the removal of NaNO₃ (16). Jo et al. (16) proposed that MgCO₃ is formed inside the molten promoter through nucleation and growth steps.Hence, despite extensive efforts, the mechanisms behind the promoting role of AMS on MgO-based CO₂ sorbents have not been unveiled yet. Atomic-level insight into the promoting effect of AMS would allow unlocking of the full potential of MgO-based CO₂ sorbents and design materials that approach (repeatedly) full conversion over a large number of CO₂ capture and regeneration cycles. To obtain such atomic-level insight, well-defined model systems and interrogation of them with detailed (in situ) characterization techniques are required. In this study, we utilize a single-crystal MgO(100) surface [the most stable and abundant MgO facet (21, 26, 27)] coated with NaNO₃ and probe, in detail, its structural dynamics under CO₂ capture conditions. Synchrotron-based, in situ X-ray reflectometry (XRR) and grazing incidence XRD (GIXRD) unravel the changes occurring at the surface of MgO under CO₂ capture condition (allowing study of the buried interface between the NaNO₃ promoter and MgO). We complemented the in situ X-ray−based characterizations by (ex situ) scanning electron microscopy (SEM) to characterize, in detail, the morphology of the MgCO₃ product formed (after carbonation and after removing the NaNO₃ promoter). Our studies evidence the formation of a noncrystalline carbonate layer on bare MgO(100) under CO₂ capture conditions. In contrast, MgO(100) coated with NaNO₃ exhibited an island-type growth of MgCO₃, as opposed to a homogeneous surface layer growth. MgCO₃ grows in a highly oriented fashion with a sectored-plate habit growth at the interface between NaNO₃/MgO(100), following a nucleation and growth mechanism. High-resolution TEM (HRTEM) provided atomic-level insight into the MgCO₃/MgO interface, evidencing an epitaxial arrangement between MgCO₃ and MgO whereby the lattice mismatch between MgCO₃ and MgO is relaxed through lattice misfit dislocations.     

CeFeO3–CeO2–Fe2O3 Systems: Synthesis by Solution Combustion Method and Catalytic Performance in CO2 Hydrogenation

Rare-earth orthoferrites have found wide application in thermocatalytic reduction-oxidation processes. Much less attention has been paid, however, to the production of CeFeO₃, as well as to the study of its physicochemical and catalytic properties, in particular, in the promising process of CO₂ utilization by hydrogenation to CO and hydrocarbons. This study presents the results of a study on the synthesis of CeFeO₃ by solution combustion synthesis (SCS) using various fuels, fuel-to-oxidizer ratios, and additives. The SCS products were characterized by XRD, FTIR, N₂-physisorption, SEM, DTA–TGA, and H₂-TPR. It has been established that glycine provides the best yield of CeFeO₃, while the addition of NH₄NO₃ promotes an increase in the amount of CeFeO₃ by 7–12 wt%. In addition, the synthesis of CeFeO₃ with the participation of NH₄NO₃ makes it possible to surpass the activity of the CeO₂–Fe₂O₃ system at low temperatures (300–400 °C), as well as to increase selectivity to hydrocarbons. The observed effects are due to the increased gas evolution and ejection of reactive FeO_x nanoparticles on the surface of crystallites, and an increase in the surface defects. CeFeO₃ obtained in this study allows for achieving higher CO₂ conversion compared to LaFeO₃ at 600 °C.     

Efficacy of Water-Soluble Pearl Powder Components Extracted by a CO2 Supercritical Extraction System in Promoting Wound Healing

Pearl powder is a biologically active substance that is widely used in traditional medicine, skin repair and maintenance. The traditional industrial extraction processes of pearl powder are mainly based on water, acid or enzyme extraction methods, all of which have their own drawbacks. In this study, we propose a new extraction process for these active ingredients, specifically, water-soluble components of pearl powder extracted by a CO₂ supercritical extraction system (SFE), followed by the extraction efficiency evaluation. A wound-healing activity was evaluated in vitro and in vivo. This demonstrated that the supercritical extraction technique showed high efficiency as measured by the total protein percentage. The extracts exhibited cell proliferation and migration-promoting activity, in addition to improving collagen formation and healing efficiency in vivo. In brief, this study proposes a novel extraction process for pearl powder, and the extracts were also explored for wound-healing bioactivity, demonstrating the potential in wound healing.     

Use of a Gemini-Surfactant Synthesized from the Mango Seed Oil as a CO2-Corrosion Inhibitor for X-120 Steel

A gemini surfactant imidazoline type, namely N-(3-(2-fatty-4,5-dihydro-1H-imidazol-1-yl) propyl) fatty amide, has been obtained from the fatty acids contained in the mango seed and used as a CO₂ corrosion inhibitor for API X-120 pipeline steel. Employed techniques involved potentiodynamic polarization curves, linear polarization resistance, and electrochemical impedance spectroscopy. These tests were supported by detailed scanning electronic microscopy (SEM) and Raman spectroscopy studies. It was found that obtained gemini surfactant greatly decreases the steel corrosion rate by retarding both anodic and cathodic electrochemical reactions, with an efficiency that increases with an increase in its concentration. Gemini surfactant inhibits the corrosion of steel by the adsorption mechanism, and it is adsorbed on to the steel surface according to a Langmuir model in a chemical type of adsorption. SEM and Raman results shown the presence of the inhibitor on the steel surface.     

Formation of Li2CO3 Nanostructures for Lithium-Ion Battery Anode Application by Nanotransfer Printing

Various high-performance anode and cathode materials, such as lithium carbonate, lithium titanate, cobalt oxides, silicon, graphite, germanium, and tin, have been widely investigated in an effort to enhance the energy density storage properties of lithium-ion batteries (LIBs). However, the structural manipulation of anode materials to improve the battery performance remains a challenging issue. In LIBs, optimization of the anode material is a key technology affecting not only the power density but also the lifetime of the device. Here, we introduce a novel method by which to obtain nanostructures for LIB anode application on various surfaces via nanotransfer printing (nTP) process. We used a spark plasma sintering (SPS) process to fabricate a sputter target made of Li₂CO₃, which is used as an anode material for LIBs. Using the nTP process, various Li₂CO₃ nanoscale patterns, such as line, wave, and dot patterns on a SiO₂/Si substrate, were successfully obtained. Furthermore, we show highly ordered Li₂CO₃ nanostructures on a variety of substrates, such as Al, Al₂O₃, flexible PET, and 2-Hydroxylethyl Methacrylate (HEMA) contact lens substrates. It is expected that the approach demonstrated here can provide new pathway to generate many other designable structures of various LIB anode materials.     

Experimental Research on the Preparation of K2CO3/Expanded Vermiculite Composite Energy Storage Material

Thermochemical adsorption energy storage is a potential energy utilization technology. Among these technologies, the composite energy storage material prepared by K₂CO₃ and expanded vermiculite (EVM) shows excellent performance. In this paper, the influence of the preparation process using the impregnation method and vacuum impregnation method on K₂CO₃/EVM composite material is studied. The preparation plan is further optimized with the solution concentration and the expanded vermiculite particle size as variables. In the experiment, mercury intrusion porosimetry (MIP) is used to measure the porosity and other parameters. Additionally, with the help of scanning electron microscopy (SEM), the morphological characteristics of the materials are obtained from a microscopic point of view. The effects of different preparation parameters are evaluated by comparing the experimental results. The results show that the K₂CO₃ specific gravity of the composite material increases with the increase of the vacuum degree, up to 70.440 wt.% (the vacuum degree is 6.7 kPa). Expanded vermiculite with a large particle size (3~6 mm) can carry more K₂CO₃, and content per cubic centimeter of K₂CO₃ can be as high as 0.466 g.     

Dense CO2 as a Solute,Co-Solute or Co-Solvent in Particle Formation Processes: A Review

The application of dense gases in particle formation processes has attracted great attention due to documented advantages over conventional technologies. In particular, the use of dense CO₂ in the process has been subject of many works and explored in a variety of different techniques. This article presents a review of the current available techniques in use in particle formation processes, focusing exclusively on those employing dense CO₂ as a solute, co-solute or co-solvent during the process, such as PGSS (Particles from gas-saturated solutions^®), CPF (Concentrated Powder Form^®), CPCSP (Continuous Powder Coating Spraying Process), CAN-BD (Carbon dioxide Assisted Nebulization with a Bubble Dryer^®), SEA (Supercritical Enhanced Atomization), SAA (Supercritical Fluid-Assisted Atomization), PGSS-Drying and DELOS (Depressurization of an Expanded Liquid Organic Solution). Special emphasis is given to modifications introduced in the different techniques, as well as the limitations that have been overcome.