A waste-valorising pathway for circular hydrogen economies

Hydrogen is widely regarded as a key enabler of deep decarbonisation, yet conventional production and utilisation pathways remain constrained by high energy demand, carbon emissions, and feedstock limitations. Waste-derived hydrogen offers an opportunity to couple clean energy supply with waste valorisation, but its climate benefit depends critically on how both hydrogen and carbon are partitioned, upgraded, and ultimately used. This review introduces the concept of olive hydrogen as a waste-valorising hydrogen production pathway within a circular carbon economy. It provides a critical, system-level synthesis structured around semi-quantitative cross-pathway carbon/hydrogen fate mapping. Three major hydrogen-rich waste streams, biomass residues, plastic wastes, and municipal/industrial wastewater, are quantitatively assessed in terms of global availability, compositional characteristics, and practical constraints as dual hydrogen and carbon resources. The interrelations between feedstock properties, technological routes, reaction mechanisms, and hydrogen production performance are critically analysed across thermochemical, electro-/photochemical, and biological conversion pathways. Particular emphasis is placed on carbon co-product valorisation at the intersection of feedstock-technology matches, market perspectives, and policy frameworks, and environmental implications. Overall, this review aims to provide an integrated end-to-end framework to guide scalable, potentially climate-positive waste-to-hydrogen systems within a circular carbon economy.