The urgent need to reduce carbon dioxide (CO2​) emissions to combat global warming has intensified research into efficient and sustainable carbon capture technologies. Traditional methods, often reliant on energy-intensive chemical solutions, highlight the demand for novel, environmentally friendly adsorbents. A recent study published in the Journal of Environmental Chemical Engineering by L. Gallego-Mena, R. Campana, A. Villardon, F. Dorado, and L. Sánchez-Silva introduces a groundbreaking approach: optimizing hydrothermal carbonization (HTC) of olive stones (OS) followed by zinc chloride (ZnCl2​) activation to create high-performance materials for CO2​ capture.

Olive stones, an abundant agricultural waste in Mediterranean countries, present an ideal feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More for producing activated biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More due to their high fixed carbon content and low ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More levels. HTC, a thermochemical conversion process, transforms biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More into hydrochar in an aqueous medium under high pressure and relatively low temperatures (180−250∘C). This method is particularly advantageous as it eliminates the need for prior drying of the biomass and operates with reduced energy consumption compared to traditional pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More.

The research systematically optimized HTC conditions to tailor the physicochemical properties of the hydrochars for subsequent activation. Key parameters investigated included residence timeResidence time refers to the duration that the biomass is heated during the pyrolysis process. The residence time can influence the properties of the biochar produced.   More (2, 4, and 8 hours), temperature (200, 220, and 240∘C), and water-to-biomass ratio (10:1 and 20:1). The findings revealed that extending the residence timeThis refers to the amount of time that the biomass is heated during the pyrolysis process. The residence time can influence the characteristics of the biochar, such as its porosity and surface area.   More to 8 hours and increasing the temperature to 240∘C significantly enhanced the hydrochar’s quality. This optimized process promoted the decomposition of hemicellulose and cellulose, increased lignin concentration, and boosted fixed carbon content to 48.73%. Concurrently, the higher heating value (HHV) improved to 23.0 MJ/kg, while the mass yield decreased to 62.8%, indicating a more energy-dense product.

The optimal hydrochar, designated OS-8-240-10, was then subjected to chemical activation with ZnCl2​. This activation step involved mixing the hydrochar with ZnCl2​ at a 1:6 ratio and heating the mixture at 700∘C for 1 hour under a nitrogen flow. This chemical activation led to a remarkable increase in the material’s surface area, reaching an impressive 1281.38 m2/g, and a total pore volume of 0.67 cm3/g. For comparison, the non-activated hydrochar had a very low surface area of 16.74 m2/g.

The dramatic improvement in textural properties was further confirmed by CO2​ adsorption analysis. The activated hydrochar exhibited a superior adsorption capacity of 2.78 mmol/g at 0∘C and 1.0 bar. This enhanced performance is attributed to the development of a highly porous structure, characterized by both micropores (0.024 cm3/g) and mesopores (0.65 cm3/g). Mesopores facilitate the transport of CO2​ molecules through the material, while micropores serve as primary sites for adsorption and storage, effectively promoting CO2​ retention.

Microscopic analyses using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) visually corroborated these structural transformations. The non-activated hydrochar displayed low porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More and amorphous carbonaceous microspheres. In stark contrast, the ZnCl2​-activated sample exhibited a much rougher surface with numerous well-defined holes, indicative of extensive meso- and micropore formation. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) further detailed the structural and chemical changes, showing the amorphous nature of the carbonaceous material and the presence of oxygen-containing functional groups that enhance CO2​ adsorption through electrostatic interactions.

This study marks a significant advancement by demonstrating that a strategic combination of optimized HTC conditions and chemical activation can effectively convert agricultural residues like olive stones into highly efficient carbon capture materials. The use of a less hazardous activating agent like ZnCl2​ further aligns this technology with green chemistry principles, offering a scalable and sustainable solution for mitigating climate change. This innovative approach provides a cost-effective and environmentally friendly alternative to conventional CO2​ capture methods, leveraging abundant agricultural waste for a crucial global challenge.

Source: Gallego-Mena, L., Campana, R., Villardon, A., Dorado, F., & Sánchez-Silva, L. (2025). Optimisation of Hydrothermal Carbonisation of Olive Stones for Enhanced CO2​ Capture: Impact of Zinc Chloride Activation. Journal of Environmental Chemical Engineering.