Recent advances in agricultural biotechnology and sustainable farming have drawn attention to biochar as a multifunctional material for environmental remediation. Among its emerging applications, biochar has demonstrated remarkable potential in wastewater treatment, particularly as an efficient and sustainable adsorbent for pollutant removal. Numerous studies over the past decades have highlighted its effectiveness in eliminating a wide range of contaminants. This efficiency is mainly due to its abundant feedstock availability, simple production processes, and favorable surface and structural properties. This review summarizes current developments in biochar use for wastewater treatment, emphasizing its adsorption capabilities and the underlying mechanisms responsible for pollutant removal. Key modification strategies, physical, chemical, and biological, are discussed in detail to illustrate how biochar performance can be optimized for specific treatment goals. Furthermore, the prospects of biochar-based technologies are explored, with a focus on their role in addressing both inorganic and organic pollutants. This review also describes the use of biochar in adsorbing metals, organic contaminants, and industrial waste. The integration of biochar into sustainable water management systems presents a promising pathway toward achieving long-term environmental and agricultural resilience.

High-valent metal species (iron, manganese, cobalt, copper, and ruthenium) based advanced oxidation processes (AOPs) have emerged as sustainable technologies for water remediation. These processes offer high selectivity, electron transfer efficiency, and compatibility with circular chemistry principles compared to conventional systems. This comprehensive review discusses recent advances in the synthesis, stabilization, and catalytic applications of high-valent metals in aqueous environments. This study highlights their dual functionality, not only as conventional oxidants but also as mechanistic mediators within redox cycles that underpin next-generation AOPs. In this review, the formation mechanisms of these species in various oxidant systems are critically evaluated, highlighting the significance of ligand design, supramolecular confinement, and single-atom engineering in enhancing their stability. The integration of high-valent metal-based AOPs into photocatalysis, sonocatalysis, and electrochemical regeneration is explored through a newly proposed classification framework, highlighting their potential in the development of energy efficient hybrid systems. In addition, this work addresses the critical yet underexplored area of environmental fate, elucidating the post-oxidation transformation pathways of high-valent species, with particular attention to their implications for metal recovery and nutrient valorization. This review highlights the potential of high-valent metal-based AOPs as a promising approach for zero wastewater treatment within circular economies. Future frontiers, including bioinspired catalyst design, machine learning-guided optimization, and closed loop reactor engineering, will bridge the gap between laboratory research and real-world applications.

The growing world crisis of waste, water shortage, and energy insecurity which needs sustainable and permanent solutions. Traditional nanocomposites, which are usually prepared by using high purity starting materials through energy intensive methods, are increasingly incompatible with environmental objectives. This review uniquely integrates dual functionality waste-based nanocomposites and their role in circular economy principles. The current approach highlighted in this review is the emerging waste-based nanocomposites as dual-purpose materials that can be used to meet the requirements of water purification and energy storage/conversion at the same time. We methodically explore the conversion of multiple waste streams (agricultural residues, plastic waste, industrial sludge, and biomass) into functional nanocomposites using thermal, biological, and hybrid synthesis pyrolysis, hydrothermal carbonization (HTC), microbial reduction, and artificial intelligence (AI)-directed design. Particular attention is given to multipurpose systems in which one material is used in the environmental and energy fields such as SiO ₂ /biochar composites made of rice husk in heavy metal adsorption and super capacitor electrode, or N-doped carbon made of textile waste in the dye degradation and hydrogen evolution catalysis. Besides these, some other nanomaterials have also been highlighted in this review. We critically assess synthesis pathways through life cycle assessment (LCA) and techno economic analysis (TEA) alongside structure property relationships, performance indexes, and the environmental impact of these materials. All critical issues, including the control of impurities, batch variability, and scaling, are addressed, along with innovative directions such as modular reactors, decentralized production, and their integration with wastewater energy systems. In the future, we proposed a circular nanotechnology economy in which urban waste can serve as a strategic asset to the next generation of sustainable technologies. This review offers a pathway toward the realization of high performance, low cost, and environmentally-friendly nanocomposites by sensitizing waste valorization, materials science, and system-level sustainability at the nexus of water and energy.

Recent biotechnological advancements in agro-technology and sustainable farming practices have led scientific attention in biochar as a multifunctional material for environmental remediation. Among its emerging applications, biochar has demonstrated remarkable potential in wastewater treatment, particularly as an efficient and sustainable adsorbent for pollutant removal. Numerous studies over the past decades have highlighted its effectiveness in eliminating a wide range of contaminants. This efficiency is mainly due to its abundant feedstock availability, simple production processes, and favorable surface and structural properties. This review summarizes current developments in biochar utilization for wastewater treatment, emphasizing its adsorption capabilities and the underlying mechanisms responsible for pollutant removal. Key modification strategies physical, chemical, and biological are discussed in detail to illustrate how biochar performance can be optimized for specific treatment goals. Furthermore, the future prospects of biochar-based technologies are explored, with a focus on their role in addressing both inorganic and organic pollutants. The integration of biochar into sustainable water management systems presents a promising pathway toward achieving long-term environmental and agricultural resilience.

High-valent metal species such as Fe(IV), Mn(V), Co(IV), and Cu(III) based advanced oxidation processes (AOPs) have emerged as sustainable technologies for water remediation. These processes offer high selectivity, electron transfer efficiency, and compatibility with circular chemistry principles compared to conventional radical-driven systems. This comprehensive review discusses recent advances in the synthesis, stabilization, and catalytic applications of high-valent metals in aqueous environments. Here the focus is on their role not only as conventional oxidants, but also as mechanistic mediators of redox cycles in next-generation AOPs. In this review, the formation mechanisms of these species in various oxidant systems (peroxymonosulfate, peracetic acid, periodate) are critically evaluated, highlighting the significance of ligand design, supramolecular confinement, and single-atom engineering in enhancing their stability. The integration of high-valent metal AOPs into photocatalysis, sono-catalysis, and electrochemical regeneration is examined through a newly proposed classification framework, emphasizing their potential for energy-efficient hybrid systems. Furthermore, we address the critical yet underexplored domain of environmental fate, detailing the post-oxidation transformation pathways of high-valent species and their potential for metal recovery and nutrient valorization. By bridging molecular-level reaction mechanisms with system-scale sustainability metrics, this review positions high-valent metal AOPs as a promising approach for zero-waste water treatment within circular economies. Future frontiers, including bioinspired catalyst design, machine learning-guided optimization, and closed-loop reactor engineering, will bridge the gap between laboratory research to real-world applications.