This review explores eco-friendly methods for extracting bioactive natural products from diverse sources. The introductory exploration emphasizes the increasing demand for sustainable extraction methods, with a focus on the environmental impact of conventional approaches. Addressing existing knowledge gaps, this review outlines the key objectives of evaluating various green extraction technologies, including supercritical fluid extraction, pressurized liquid extraction, ultrasound-assisted extraction, enzyme-assisted extraction, and others. The primary findings underscore the remarkable potential and advancements achieved with green solvents, specifically deep eutectic solvents and bio-based solvents. This review elucidates the synergistic effects achieved by combining different extraction techniques, exemplified by ultrasound-microwave-assisted extraction and sequential supercritical fluid and pressurized liquid extraction, among others. Notwithstanding the promising results, this review emphasizes the importance of acknowledging and addressing challenges such as standardization, selectivity, scalability, and economic viability.

The quest for sustainable and environmentally friendly fuel feedstocks has led to the exploration of green solvents for the extraction of bio-oil from various biomass sources. This review paper provides a comprehensive analysis of the challenges and future research outlooks for different categories of green extraction solvents, including bio-based solvents, water-based solvents, supercritical fluids, and deep eutectic solvents (DES). The background of each solvent category is discussed, highlighting their potential advantages and limitations. Challenges such as biomass feedstock sourcing, cost fluctuations, solvent properties variability, limited compatibility, solute solubility, high costs, and potential toxicity are identified and examined in detail. To overcome these challenges, future research should focus on alternative and abundant feedstock sources, the development of improved solubility and separation techniques, optimization of process parameters, cost-effective equipment design, standardization of DES compositions, and comprehensive toxicological studies. By addressing these challenges and advancing research in these areas, the potential of green extraction solvents can be further enhanced, promoting their widespread adoption and contributing to more sustainable and environmentally friendly industrial processes.

The push for sustainability in nanomaterials has catalyzed significant advancements in the green synthesis of carbon dots (CDs) from renewable resources. This review uniquely explores recent innovations, including the integration of hybrid techniques, such as micro-wave-assisted and ultrasonic-assisted hydrothermal methods, as well as photocatalytic synthesis. These combined approaches represent a breakthrough, offering rapid production, precise control over CD properties, and enhanced environmental sustainability. In addition, the review emphasizes the growing use of green solvents and bio-based reducing agents, which further reduce the environmental footprint of CD production. This work also addresses key challenges, such as consistently controlling CD properties—size, shape, and surface characteristics—across different synthesis processes. Advanced characterization techniques and process optimizations are highlighted as essential strategies to overcome these hurdles. Furthermore, this review pioneers the integration of circular economy principles into CD production, proposing novel strategies for sustainable material use and waste reduction. By exploring innovative precursor materials, refining doping and surface engineering techniques, and advocating for comprehensive life cycle assessments, this work sets a new direction for future research. The insights provided here represent a significant contribution to the field, paving the way for more sustainable, efficient, and scalable CD production with diverse applications in optoelectronics, sensing, and environmental remediation.

The start of art SAFs production routes and HTL potential for overcoming challenges.

This study explored the effectiveness of hydrothermal liquefaction (HTL) in converting sewage sludge (SS) into high-quality biocrude. It scrutinized the influence of various solvents, including conventional choices like dichloromethane (DCM) and hexane, alongside environmentally friendly alternatives, such as ethyl butyrate (EB) and ethyl acetate (EA). HTL experiments, conducted at 350 °C for 60 min in a 20 mL batch reactor, include solvent-based biocrude extraction. Notably, EB showed the highest extraction yield (50.1 wt %), the lowest nitrogen distribution (5.4% with 0.32 wt %), and a remarkable 74% energy recovery (ER), setting a noteworthy benchmark in nitrogen reduction. GCMS analysis reveals EB-derived biocrude's superiority in having the least heteroatoms and nitrogenous compounds compared to hexane, EA, and DCM. Solid residues from hexane, EB, and EA displayed the highest nitrogen distribution range (62-68%), hinting at potential applications in further processes. These findings significantly inform solvent selection for efficient and sustainable waste-to-energy conversion. While promising, the study emphasizes the need to explore solvent-solute interactions further to optimize biocrude quality, highlighting the pivotal role of solvent choice in advancing clean, cost-effective waste-to-energy technologies.