Owing to its high carbon content and renewable nature, biomass is regarded as the most sustainable and renewable source of carbonaceous materials [1,4]. Several applications of biomass-based carbon materials are being pursued, owing to their cost-effectiveness, large-scale production, and quality-controllable production in an environmentally friendly manner. Furthermore, the conversion of biomass to carbon nanomaterials, such as fullerene, carbon nanotubes, graphene, and graphene quantum dots, has revealed alluring potential for biomass valorization in contrast to the standard synthesis of biomass-derived hydrochars and biochars [8]. Carbon nanostructures and nanomaterials have received great interest for their properties and applications in the environmental, catalytic, biological, and energetic fields [4,5]. For instance, they have important applications in catalyst supports, carbon fixation, adsorbents, gas storage, electrodes, carbon fuel cells, and drug delivery [6]. Activated carbon and carbon fibres can also be prepared using additional activation and electrospinning processes, respectively [5,8]. Nonetheless, given the complex chemical components and structures of biomass, it is difficult to prepare homogeneous and controllable carbonaceous materials. Moreover, the quality, properties, and applications of carbonaceous materials differ depending on the type of biomass and conversion method used, as discussed previously [8].

The environmental applications of carbonaceous materials include CO₂ adsorbents, soil amendments, and water remediation. Increasing levels of atmospheric CO₂ need to be mitigated, as they influence climate change through the greenhouse effect. Biochar is an effective and economical strategy for capturing atmospheric CO₂ [4]. Moreover, they can remain stable in the soil for extended periods independent of mineralisation and temperature fluctuations [5]. Hydrochars derived from HTC can also act as capturing agents for CO₂ given their large surface areas and tunable porosities. Additional surface functional groups can be introduced via chemical activation for further enhancement [4]. The use of pesticides and herbicides causes soil contamination through the disposal of inorganic and organic pollutants. Anthropogenic activity can also lead to the presence of unwanted metals in the soil. These highly toxic substances are harmful to human health, living organisms, and agricultural products. Therefore, it is necessary to reduce their amounts to ensure a sustainable living environment and to protect human health. The porosity of carbon materials facilitates the adsorption of contaminants in the soil and water. Biochar has received considerable attention for its potential application as a soil amendment to reduce soil contaminants and heavy metals [4]. It acts as a soil conditioner by improving the water-retaining capacity of the soil, pH optimisation, and the total uptake of phosphorus and nitrogen. Consequently, the bioavailability of required nutrients and water increases, providing a microenvironment for the growth of essential soil microorganisms, thereby improving soil fertility [4,5,126]. Frišták et al. [127] evaluated the pretreatment of sewage sludge with sodium carbonate (Na₂CO₃) and subsequent pyrolysis at 400 °C and 500 °C for soil amendment. The authors studied the production of potential alternatives to inorganic phosphorus fertilisers and organic carbon suppliers. Peng et al. [128] produced biochar with heavy metals solidification from industrial sludge and rice straw through co-pyrolysis. This study resulted in high-quality biochar, in which the biomass composition provided energy and reduced the enrichment of heavy metals by solidification into stable forms. This approach has demonstrated potential for soil and water remediation applications. Zhu et al. [129] prepared porous carbonaceous materials from pineapple waste through HTC followed by thermal activation using alkali metal oxalates. Pineapple waste-derived porous carbons showed an enriched CO₂ adsorption performance of 1.59 mmol/g at 25 °C, indicating that these wastes are promising sorbents for capturing and separating CO₂ under real conditions.

Heterogeneous catalysts are essential for chemical synthesis and transformation. Carbonaceous materials have been shown to act as catalysts and catalyst supports. Hydrochars possess the desirable porosity and large surface area to speed up reactions, which have been explored in the form of carbonaceous nanofibers [4,5,6]. Furthermore, the combination of metallic nanoparticles results in high thermal, chemical, and mechanical stabilities [4]. The supporting materials used by heterogeneous catalysts also influence catalytic performance, which means that these materials must also have a high surface area and porosity to accelerate the reaction. HTC, followed by subsequent activation, can produce hydrochar with desirable surface area and modifiable properties for use as a catalyst or catalyst support [4]. The reusability and efficiency of these biomass-derived catalysts make them promising candidates for future industrial applications, given growing ecological concerns [4,5]. Specific catalytical applications include the use of carbon materials as supports for Pt and Pt–Ru catalysts for direct alcohol fuel cells [130], NO reduction [130], or the dehydrogenation of n-butane to olefins [131], among others.

Carbon-based materials produced at extremely high temperatures (>1200 °C) are expected to exhibit desirable electrochemical properties. They have been considered for energy conversion and storage applications owing to their versatile dimensionality, in addition to their structure, and physicochemical properties. Examples of these properties include thermal insulation, thermal conductivity, heat resistance, hardness, softness, and insulator–semiconductor–conductor properties [132]. Carbonaceous materials are often used as electrodes because of the presence of mesopores, which allow the transport of ions and electrolytes, and have a high surface area [4,5,133]. These characteristics have led to their use in supercapacitors and energy storage devices because of their high capacitance, long life span, and fast charge/discharge rates [4]. Carbon allotropes with different structures and properties, such as sp³, sp², and sp, can be obtained from combinations of carbon atom hybridisations [132]. Carbon nanotube-based materials (sp²), for instance, possess a high surface area, thermal conductivity, electron mobility, and mechanical strength, which allow them to be used for the development of solar and fuel cells. Solar cell technologies are safe, eco-friendly, inexpensive, and can transform solar energy into electric energy. Owing to their high electrical conductivity, electrocatalytic activity, high electron mobility, good optical transparency, low cost, and high abundance, carbon nanotubes have been explored to guarantee better charge conduction and increased electrode flexibility. Moreover, the properties of carbon nanotubes can be significantly affected by the number of walls, length, diameter, defect type, concentration, and synthesis method [134]. Solid biofuels usually include renewable biological materials that can be burned to generate energy, resulting in economic and social benefits, as well as a lower environmental impact [135]. Industrial waste and sewage sludge have been transformed into clean solid biofuels via hydrothermal treatment and torrefaction [4,136]. Wang et al. [137] used HTC combined with potassium hydroxide (KOH) activation for the co-production of fermentable sugars and porous carbon with oxygen-rich groups from sugarcane bagasse to be used as supercapacitor. The authors found that the pore volume distribution and surface oxygen-containing groups played a more significant role in the electrochemical performance of the carbon materials than the specific surface areas.

Carbonaceous nanomaterials can be used in various activation techniques and applications, owing to their long-term viability, adaptability, biocompatibility, safety, and biodegradability. They can be used as probes for in vivo imaging, diagnostics, and the profiling of molecules, as well as for drug and gene delivery [138]. Carbon quantum dots (CQDs) are small carbon nanoparticles (<10 nm) comprising amorphous or crystalline centres with a dominant sp² carbon. They have gained considerable attention because of their fluorescent properties, high chemical stability, good conductivity, non-blinking, and resistance to photobleaching [139,140,141]. Biomass can be used as a carbon source for CQD with the properties of chemical CQDs. Moreover, biomass-based CQDs are more environmentally friendly, potentially less toxic, and biocompatible, making them suitable for biomedical applications, and they can be used in medical bioimaging practices for disease detection or treatment [4,139,140]. Other applications include biosensing, biological labelling, medical diagnostics, and optoelectronic devices [138]. However, features related to safety, physicochemical properties, and pharmacokinetics must be further investigated before they can be used in medicine [138].