The scale of the problem the team aimed to address is considerable. Global carbon dioxide emissions since 1850 have driven average temperatures up by roughly 0.8 degrees Celsius, with even steeper rises in warmer regions, while the world generates an estimated 140 gigatonnes of biomass waste every year, much of which decomposes or burns improperly and contributes as much as 18 percent of emissions. At the same time, the biomass recycling market is projected to reach 142.3 billion dollars by 2034, and the global activated carbon market is expected to top 7.3 billion dollars by 2030. Turning one waste stream into a solution for another is precisely the kind of circular economy logic that underpins bioenergy with carbon capture and storage, and it aligns squarely with the United Nations Sustainable Development Goals on responsible consumption and climate action.

Rubber seed shell proved to be an ideal starting point because it contains about 64.5 percent carbon by mass. The researchers pre-treated the shells, ground them to a fine powder, and impregnated them with two different alkaline activating agents: potassium carbonate and potassium hydroxide, each mixed at an optimized two-to-one ratio with the biomass. After drying, the impregnated material was carbonized in a tube furnace at 800 degrees Celsius under a continuous nitrogen flow for three hours, then washed with dilute hydrochloric acid and distilled water until neutral. The yields were 37 percent for the potassium carbonate route and 29 percent for the potassium hydroxide route, conditions arrived at through systematic calibration of activator ratios, temperatures, and residence times.

The real innovation came next. Rather than leaving the carbon surface bare, the team functionalized it with ethanolammonium butyrate, an ammonium-based ionic liquid synthesized simply by adding ethanolamine dropwise to butyric acid at room temperature. This ionic liquid stands out among its peers because it is biodegradable, thermally stable, environmentally and economically sustainable, and has a moderate viscosity that eases processing. Loadings of 10, 20, and 30 weight percent were tested on both activation routes, and the samples were then subjected to a full battery of characterization techniques, including scanning electron microscopy, energy-dispersive X-ray analysis, Fourier-transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.

The performance results were striking. The best material, potassium carbonate activated carbon carrying 30 weight percent of the ionic liquid, adsorbed 3.71 millimoles of carbon dioxide per gram at 0 degrees Celsius and 2.49 millimoles per gram at 25 degrees Celsius at one bar. By contrast, the unmodified potassium hydroxide activated carbon managed only 0.95 millimoles per gram at 25 degrees Celsius. Adsorption rose with ionic liquid loading and fell with temperature, exactly as expected for an exothermic physisorption process in which higher molecular motion favors desorption. Interestingly, potassium carbonate activated samples outperformed their potassium hydroxide counterparts in carbon dioxide uptake even though the latter possessed superior textural properties, a discrepancy the authors attribute to pore structure collapse caused by the harsher activation chemistry of potassium hydroxide.

Equilibrium isotherm analysis told a consistent story. Among the four models tested, the Langmuir equation fit the experimental data best, with coefficient of determination values between 0.996 and 0.999, indicating that carbon dioxide adsorbs predominantly as a monolayer on energetically similar sites. The Langmuir affinity constant climbed with ionic liquid loading, confirming that the nitrogen-containing basic functionalities introduced by the ethanolammonium butyrate strengthen carbon dioxide affinity through weak Lewis acid-base interactions, while nitrogen is a poor match for these sites and passes through largely unretained.

Micropore analysis revealed a subtle but crucial trade-off. The pristine potassium hydroxide derived carbon boasted a specific surface area of 954 square meters per gram and a total pore volume of 0.43 cubic centimeters per gram, figures that progressively declined as ionic liquid loading increased, reaching 397 square meters per gram and 0.18 cubic centimeters per gram at 30 weight percent. Electron microscopy and elemental analysis confirmed the progressive surface coverage and the emergence of nitrogen and oxygen signals, while Raman spectroscopy showed rising defect density as the liquid infiltrated the pore network. Yet carbon dioxide uptake rose anyway, because the introduced functionalities more than compensated for the lost porosity. The team distilled this synergy into empirical equations linking the oxygen-to-carbon ratio, pore diameter, and Langmuir affinity, and derived a combined descriptor showing that surface chemistry enhancement constants were 29.25 for potassium carbonate samples and 6.60 for potassium hydroxide samples, underscoring that adsorption performance is governed by pore structure and surface chemistry together, not surface area alone.

Practical deployment demands more than raw capacity, and the optimized adsorbent passed those tests with distinction. The isosteric heat of adsorption ranged from 29.7 to 44.2 kilojoules per mole, well below the 50 kilojoule per mole threshold that signals chemisorption, confirming strong physisorption with modest regeneration energy requirements. Under simulated post-combustion conditions of 15 percent carbon dioxide and 85 percent nitrogen at one bar, the material achieved a carbon dioxide over nitrogen selectivity between 21.59 and 25.94, comparable to leading biomass-derived sorbents in recent literature. Most impressively, after ten consecutive adsorption-desorption cycles at 0 degrees Celsius, the material retained 99.26 percent of its original capacity, a figure that outperforms many previously reported cycling results and points to excellent structural durability for repeated industrial use.

The implications ripple outward in several directions. For carbon capture engineering, the study demonstrates that a deliberately greener ionic liquid, one that avoids the synthetic complexity and high viscosity of many amino acid and polymeric alternatives, can rival or exceed the performance of more exotic functionalization chemistries when paired with the right activation route. For waste valorisation, it shows that rubber seed shells, an abundant and largely untapped agricultural residue rich in carbon, can anchor a waste-to-resource supply chain for sorbent production. The authors caution that several questions remain before commercial deployment, including performance under humid conditions with mixed gas streams, adsorption kinetics and diffusion behavior, techno-economic feasibility, and life-cycle assessment at scale. They also suggest exploring other activating agents and wider operating windows. But the core message stands: with careful control of activation chemistry and a smart coating of sustainable ionic liquid, an agricultural throwaway can become a durable, selective, and regenerable weapon against rising carbon dioxide, and a template for the next generation of low-cost carbon capture materials.

Subject of Research: Ionic liquid functionalized activated carbon derived from rubber seed shell biomass for enhanced carbon dioxide capture

Article Title: Sustainable ionic liquid functionalized activated carbon for enhanced CO 2 capture: Integrated textural properties, adsorption performance and isotherms

Article References: Suleman, F., Borhan, A., Muhbat, S., Rashidi, N. A., Daood, S. S., & Abdulalh, N. H. (2026). Sustainable ionic liquid functionalized activated carbon for enhanced CO2 capture: Integrated textural properties, adsorption performance and isotherms. Cleaner Engineering and Technology, 34, Article 101314. https://doi.org/10.1016/j.clet.2026.101314

Image Credits: AI Generated

DOI: Not provided

Keywords: carbon capture, activated carbon, ionic liquids, rubber seed shell, biomass waste valorisation, CO2 adsorption, Langmuir isotherm, physisorption, CO2/N2 selectivity, microporous materials, adsorbent regeneration, sustainable materials

The work addresses a central challenge in carbon capture: finding solid materials that can bind CO₂ efficiently without requiring the large energy inputs associated with regenerating liquid solvents. Adsorption systems work by allowing gas molecules to attach to a solid surface, either through relatively weak physical forces or stronger chemical interactions. Graphene oxide is attractive because its carbon sheets contain hydroxyl, epoxy, carbonyl and carboxyl groups that can interact with CO₂. Yet its performance depends heavily on how intensely the graphite is oxidized, how the sheets restack, and how many accessible pores and chemically active sites remain after synthesis. Excessive oxidation can create defects and collapse useful structure, while insufficient oxidation can leave too few polar sites to attract the quadrupolar CO₂ molecule.

The researchers prepared graphene oxide using a modified Hummers method, an established chemical route in which graphite is oxidized by potassium permanganate in concentrated sulfuric and phosphoric acids. They varied the later preparation temperature to 60, 80 and 100 °C, producing samples called GO 60, GO 80 and GO 100. The use of phosphoric acid and the omission of sodium nitrate follow greener modifications intended to reduce the toxic nitrogen oxides associated with the original Hummers process. The oxidized material was washed repeatedly to remove residual acids and minerals, then dried. In a second step, the team extracted compounds from powdered mangosteen peel using ethanol, evaporated the solvent and mixed the dried extract with graphene oxide in water at 60 °C. The resulting composites were designated GO 60+MP, GO 80+MP and GO 100+MP.

Mangosteen peel is more than a source of biomass: it contains phenolic compounds, flavonoids, tannins, anthocyanins and xanthones, including mangostin derivatives. Many of these molecules carry hydroxyl groups and aromatic rings. Those chemical features could alter graphene oxide in two important ways. First, they can increase the polarity of the surface, strengthening interactions with CO₂. Second, the bulky organic molecules can prevent graphene sheets from packing too tightly, creating or exposing pathways through which gas molecules can diffuse. Microscopy images indicated that the extract adhered to the graphene oxide, producing more folds, wrinkles and visibly heterogeneous regions. The researchers interpret these features as evidence that mangosteen-derived compounds changed the texture and accessibility of the carbon sheets, although the study does not establish the precise molecular bonding arrangement of every extract component.

Several characterization techniques supported the chemical changes. Infrared spectra of the untreated graphene oxide showed signals associated with carbonyl and carboxyl groups, aromatic carbon–carbon bonds, epoxy bridges and alkoxy or alcohol groups. After mangosteen impregnation, the broad hydroxyl signal became more pronounced, consistent with the addition of phenolic compounds and an increase in surface polarity. Raman spectroscopy showed that the disorder-to-graphite ratio, known as the ID/IG ratio, rose from 1.45 to 1.55 as the graphene oxide preparation temperature increased from 60 to 100 °C. After adding the peel extract, the ratio increased further to 1.75, 1.76 and 1.86 for the three temperatures. In graphene-based materials, a higher ID/IG ratio generally indicates more structural disorder or a greater number of defect sites. Such defects are not automatically beneficial, but they can expose reactive edges and create additional locations where gas molecules can bind.

The temperature itself had a pronounced effect on the material’s composition and structure. Energy-dispersive X-ray analysis found that the oxygen content of pristine graphene oxide fell from 31.77 atomic percent in GO 60 to 24.53 percent in GO 80 and 14.03 percent in GO 100, according to the study’s discussion. X-ray diffraction measurements likewise indicated shrinking interlayer distances at higher preparation temperatures, consistent with the loss of oxygen-containing groups between carbon layers. GO 60 had a specific surface area of 52.64 square metres per gram, compared with 12.03 and 14.62 square metres per gram for GO 80 and GO 100. The researchers suggest that 60 °C created a balance: enough oxidation to expand and functionalize the layers, but not so much thermal damage that the structure became less accessible. Adding mangosteen extract raised the measured surface area of all three materials, with GO 60+MP reaching 71.27 square metres per gram.

Nitrogen adsorption measurements revealed why the GO 60 composite performed particularly well. GO 60+MP displayed a type IV adsorption isotherm with a pronounced H2 hysteresis loop, a pattern commonly associated with mesopores and complex pore networks. Mesopores range from 2 to 50 nanometres across and can provide a compromise between high surface area and rapid gas transport. The composite also contained larger voids and slit-like spaces created by partially restacked graphene oxide sheets. Its average pore diameter was 1.37 nanometres, smaller than the 1.97-nanometre average measured for GO 60, suggesting that the extract narrowed some openings while increasing the overall accessible surface. At higher synthesis temperatures, the composites exhibited weaker type III isotherms and lower surface areas—32.98 square metres per gram for GO 80+MP and 27.04 square metres per gram for GO 100+MP.

The most direct test used a fixed-bed reactor packed with 3 grams of adsorbent between layers of quartz wool. Before each experiment, the material was heated under nitrogen, cooled to 30 °C and exposed to a simulated gas stream containing nitrogen and carbon dioxide. The reported testing conditions include a gas flow of 80 millilitres per minute and CO₂ concentrations of either 20 percent in the experimental setup or 15 percent in the capacity comparison and cycling tests. Breakthrough curves track the ratio of outlet to inlet CO₂ concentration: when the ratio is low, the bed is retaining most of the gas; when it approaches one, the material is becoming saturated. Pure graphene oxide reached equilibrium after roughly 5 to 7 minutes, depending on preparation temperature. With mangosteen extract, the corresponding times extended to about 19 to 30 minutes, with GO 60+MP producing the broadest breakthrough curve. Its measured capacity was 1.86 millimoles per gram, compared with 0.51 millimoles per gram for GO 60.

The proposed explanation combines pore structure with surface chemistry rather than attributing the improvement to a single mechanism. CO₂ is a linear molecule with a substantial quadrupole moment, meaning its charge distribution allows it to interact with localized electric fields. Oxygen-containing groups on graphene oxide create surface dipoles that can attract CO₂, while hydroxyl and phenolic groups may support hydrogen-bond-like interactions. Aromatic regions in both graphene oxide and the mangosteen compounds can also interact with CO₂ through its quadrupole and the electron clouds of the rings. At the same time, mesoporous channels can speed diffusion toward internal sites. Kinetic modelling supports a mixed mechanism. Untreated graphene oxide was best described by the Avrami fractional-order model, with correlation coefficients of 0.97 to 0.99, consistent with heterogeneous sites and multiple adsorption pathways. After impregnation, pseudo-first- and pseudo-second-order models both fitted the data well, with coefficients near 0.99, suggesting that the surface treatment changed the apparent distribution and behaviour of adsorption sites rather than converting the process into purely physical or purely chemical capture.

For a carbon-capture material to be useful beyond the laboratory, it must release the stored gas without losing capacity. The researchers regenerated the composites by heating them to 200 °C under pure nitrogen and repeated adsorption and desorption three times. GO 60+MP showed complete regeneration by the third cycle, while the measured capacities remained relatively consistent across the tested cycles. That result is encouraging, but it is not yet evidence of industrial readiness. The experiments used a small fixed bed, a synthetic CO₂–nitrogen mixture and only three cycles; real flue gas contains water vapour, oxygen, sulfur compounds and other contaminants that can compete for active sites or degrade the biomass-derived coating. The chemical oxidation route also uses strong acids and oxidants, so the environmental advantages of recycling mangosteen peel will ultimately depend on reagent recovery, waste treatment, scale-up and the durability of the composite over hundreds or thousands of cycles. Even with those limitations, the study demonstrates a compelling design principle: agricultural waste can supply functional molecules that tune a graphene-based surface, while a moderate synthesis temperature preserves the porous architecture needed to capture CO₂ efficiently.