Synthesis and characterization of carbon nanomaterial(S) by hydrothermal carbonization of various wastes

This research investigates a sustainable and resource-efficient approach to biomass waste management by synthesizing high-value carbon nanomaterials through hydrothermal carbonization (HTC). Three distinct biomass wastes: pomegranate peel waste (PPW), marigold flower waste (MFW), and cotton fabric waste (CFW), were subjected to HTC under varying conditions to explore their conversion into functional carbon-rich products. Among the tested feedstocks, PPW emerged as the most promising precursor for carbon nanomaterial production, yielding well- defined hydrothermal carbonaceous nanospheres (HCNs) with diameters ranging from 50 to 300 nm. In contrast, the carbon materials derived from MFW and CFW exhibited irregular and poorly defined morphologies. Additionally, carbon dots (CDs) displaying strong blue luminescence were successfully recovered from the liquid phase of the PPW-HTC process, demonstrating the feasibility of dual-phase valorization from a single waste stream. To assess the environmental viability of the process, a prospective life cycle assessment (LCA) was conducted. The analysis identified electricity consumption for reactor heating (75–79%) and drying (20–22%) as the dominant contributors to environmental impact. Scenario modeling indicated that replacing the current electricity mix with renewable sources could reduce abiotic depletion and global warming potential by up to 120%. Sensitivity analysis of wastewater disposal routes revealed that while switching to treatment plants offers only minor improvements at laboratory scale (<1%), the benefits become more pronounced at industrial scales. The scenario involving CD recovery (HTCa) exhibited comparatively higher environmental burdens due to full energy allocation but avoided reliance on high-purity chemical precursors commonly used in conventional CD synthesis. This strategy highlights the environmental potential of integrating biomass-based inputs and waste valorization into emerging nanomaterial production pathways, aligning with circular economy principles. HCNs demonstrated significant potential as green antioxidants. Results from DPPH and phosphomolybdenum assays revealed strong radical-scavenging activity, comparable to ascorbic acid, a standard antioxidant. These findings support the use of HCNs as a sustainable alternative to fossil-derived nanomaterials and synthetic antioxidants, offering both environmental and human health benefits. Under the assay conditions employed in this study, HCNs exhibited minimal antibacterial activity. While these results indicate limited antibacterial effectiveness at the tested concentrations, they also suggest that HCNs may be less cytotoxic, potentially supporting their use in biocompatible or non-disruptive applications. Future studies should explore their performance under varying experimental conditions or investigate modifications to enhance antibacterial efficacy. In addition, the study evaluated the application of HCNs in cement composites. Incorporating HCNs resulted in improved workability and enhanced flexural strength in certain samples, along with a reduction in density without significant compromise in compressive strength. The method of HCNs incorporation was found to significantly influence the final properties of the composites. Although the sustainability impact of low-dosage additions is limited, these findings show the potential of HCNs as a useful addition in cement-based systems, suggesting the need for further research and optimization—particularly when combined with other sustainability-enhancing materials.