Adsorption on Carbon-Based Materials

Polluted streams, both in their gas and liquid phases, constitute a potential menace to the environment and to living organisms. Among the different technologies available for the purification of streams, adsorption is still one of the most widely used due to its simplicity, low cost, and high efficiency in terms of the removal of a wide variety of hazardous pollutants.

Among the different types of adsorbents, carbon-based materials are probably the most extensively researched owing to their unique and tunable characteristics. This Special Issue seeks, therefore, to highlight cutting-edge research of adsorption technologies, addressing the urgent environmental challenges of our time. We are thrilled to present a collection of articles authored by esteemed researchers in the field of carbon-based materials with environmental applications. Key topics covered include:

• The valorization of biomass wastes as adsorbents;
• Micropollutant removal from wastewater;
• Gas sensors and H₂ storage;
• Theoretical studies.

Carbon materials are widely used in water treatment, gas purification and the protection of the environment due to their prominent adsorbent effect. The adsorption capacity of carbon materials depends on their properties, such as the specific surface area and micropore volume, studied and optimized in Contribution 1 during the development of graphite oxide by graphite oxidation. The design of carbon materials is also covered in Contribution 2, detailing the relevance of and factors affecting heating performance in the microwave fields during the synthesis of activated carbons.

Eco-friendly and cost-effective adsorbent materials can be synthesized from different biomass wastes, such as fruit peels (Contribution 3), waste masks (Contribution 4), algae (Contribution 5), or cherry seeds (Contribution 6). The resulting carbon-based materials can be used as adsorbents for wastewater treatment. This application has attracted much attention in recent years as a potential source of clean water, though there are some concerns about its safety for human use. Contribution 3 reviews the latest research focusing on the design of activated carbons from fruit peels to remove different pollutants, such as pharmaceuticals, dyes, heavy metals, and anions that are released into waste and harm human and animal health. The removal of pesticides is covered in Contributions 7 and 8, with the latter investigating a range of cellulose-based activated carbon fibers with diverse specific surface areas, pore size distributions, and elemental compositions. The adsorption results suggest that the most effective carbon-based material for pesticide removal should have a pore diameter of approximately 4 nm, a low oxygen content, a unimodal pore size distribution, and a high presence of sp² domains. The adsorption of organic dyes, pharmaceuticals, and other organic contaminants, through the use different biochars (Contributions 9 and 10), porous graphene (Contribution 11), and carbon nanofibers (Contribution 12), is also covered in this Special Issue.

The occurrence of metals in water is also a global public health concern. Several removal technologies have been developed for the removal of metals from water, with adsorption being one of the most widely used technologies. The removal of cadmium and arsenic were studied in Contributions 5 and 13, respectively. The results revealed that the variables adsorbent type, dosage, and concentration all contributed to metal removal from water, with time mediating these effects. Although less common as a metal pollutant, the recovery of palladium (II) with activated carbons sourced from cherry seeds was covered in Contribution 6. In this case, the increase in the adsorption temperature affects the palladium load suggesting that the adsorption process is associated with chemical reactions.

Despite water treatment being the main adsorption application for the carbon-based materials described in this Special Issue, their use in gas sensors and hydrogen storage is also covered. Contribution 14 describes copper-containing silicon–carbon films with excellent adsorption characteristics for different gasses, describing the adsorption kinetics of nitrogen dioxide, sulfur dioxide, and carbon monoxide and proposing an algorithm for gas recognition based on the change in the resistivity of the material. The study of pyridinic-type N-doped graphene as an adsorbent for hydrogen storage is described in Contribution 15. A DFT-based study, it points out that modified graphene can be a good support material for the stabilization of the PdNi clusters where H₂ adsorption occurs on the Pd atom.

Other theoretical methods are described and used in this Special Issue. In this regard, Contribution 16 provides a theoretical reference for the study of graphene surface localization modification and graphene-based atom/ion screening and detection. Conversely, Contribution 17 simulates the adsorption isotherms of Kr onto graphite in slit-shaped pores using a grand canonical Monte Carlo method.