Throughout the manuscript—particularly in Sections 5.2 and Figures 10 and 11—the authors assert that Bio-H₂ production from LB is inherently carbon-neutral and environmentally sustainable. While such outcomes are indeed theoretically plausible, the review does not present a full life cycle inventory or integrated life cycle assessment (LCA) to substantiate these claims. The reported CO₂-equivalent emissions for select pretreatment techniques (e.g., 385 kg CO₂-eq for acid hydrolysis versus 0.94 kg CO₂-eq for hot water treatment) are isolated data points. These values are presented without reference to system boundaries, functional units (e.g., per MJ of hydrogen produced), or upstream/downstream processes such as enzyme production, energy input for microbial cultivation, or digestate management.
Moreover, no comparative LCA is offered to evaluate the relative environmental burden of biological versus thermochemical pathways. Key metrics such as cumulative energy demand (CED), global warming potential (GWP), or energy return on investment (EROI), which are central to validating claims of sustainability and carbon neutrality, are not discussed.
In light of the absence of a complete life cycle framework, how do the authors justify the general conclusion that Bio-H₂ production from LB represents a carbon-neutral or sustainable alternative to fossil-based hydrogen?