Biogas contains significant quantities of undesirable and toxic compounds, such as hydrogen sulfide (H₂S), posing severe concerns when used in energy production-related applications. Therefore, biogas needs to be upgraded by removing H₂S to increase their bioenergy application attractiveness and lower negative environmental impacts. Commercially available biogas upgradation processes can be expensive for small and medium-scale biogas production plants, such as wastewater treatment facilities via anaerobic digestion process. In addition, an all-inclusive review detailing a comparison of biochar and hydrochar for H₂S removal is currently unavailable. Therefore, the current study aimed to critically and systematically review the application of biochar/hydrochar for H₂S removal from biogas. To achieve this, the first part of the review critically discussed the production technologies and properties of biochar vs. hydrochar. In addition, exisiting technologies for H₂S removal and adsorption mechanisms, namely physical adsorption, reactive adsorption, and chemisorption, responsible for H₂S removal with char materials were discussed. Also, the factors, including feedstock type, activation strategies, reaction temperature, moisture content, and other process parameters that could influence the adsorption behaviour are critically summarised. Finally, synergy and trade-offs between char and biogas production sectors and the techno-economic feasibility of using char for the adsorption of H₂S are presented. Biochar’s excellent structural properties coupled with alkaline pH and high metal content, facilitate physisorption and chemisorption as pathways for H₂S removal. In the case of hydrochar, H₂S removal occurs mainly via chemisorption, which can be attributed to well-preserved surface functional groups. Challenges of using biochar/hydrochar as commercial adsorbents for H₂S removal from biogas stream were highlighted and perspectives for future research were provided. Graphical abstract

Landfilling remains the dominant waste disposal method worldwide, particularly in developing countries, posing serious environmental, health, and climate challenges. Inefficient practices, weak regulations, and un-engineered sites contribute to massive greenhouse gas (GHG) emissions and resource loss. Transitioning to a circular economy (CE) offers a transformative path for sustainable waste management. By closing material loops, recovering energy, urban mining, controlling emissions and CE strategies can convert traditional landfills into eco-efficient systems. The integration of artificial intelligence (AI) further enhances this transition, enabling real-time monitoring, predictive management, and optimized resource recovery, thereby maximizing environmental and economic benefits. This review presents a three-level CE framework at micro (individual organizations), meso (industrial networks), and macro (national and international) levels designed to extract maximum value from waste streams and mitigate climate impacts. The proposed strategies demonstrate the potential to drastically reduce GHG emissions, promote clean energy via waste-to-energy routes, and contribute to SDGs 7, 11, 12, 13 and 15. By combining technology, innovation, and strategic management, this work highlights how AI-driven CE approaches can transform landfills from environmental liabilities into engines of sustainability and climate action. In implementing CE strategies at various levels, various challenges including technological, socio-economic, ethical, policy-based, and unintended consequences are encountered which impact sustainability initiatives. This review comprehensively discusses challenges associated with CE implementation and identifies technological advancement, social awareness and data-driven AI/ML-based modeling which could ensure success in circularity and ultimately curb climate change impacts in the long term.