• The current application status of sludge biochar was comprehensively reviewed. • The mechanism and capacity of sludge biochar were discussed. • The key challenges in the sludge biochar application were summarized. • The strategies for overcoming sludge biochar application barriers were put forward.

An edible fungal pretreatment of rice straw was proposed for enhanced hydrogen production while reducing the chemical cost for traditional biological hydrogen production from lignocellulose. In this research, rice straw was pretreated by edible fungus Gymnopus contrarius J2 at room temperature under static conditions for 15 d at first. The highest hydrogen yield of 5.71 mmol g⁻¹-pretreated rice straw was obtained, 74% higher than the counterpart without pretreatment. Chemical composition analysis demonstrated that lignin removal was up to 22.4% with a little cellulose and hemicellulose loss of 13.3% and 17.1%, respectively, which is in favor of hydrogen production. Additionally, microscopic structure observation combined with FT-IR and XRD analysis illustrated the structural disruption of pretreated rice straw, and the crystalline index of rice straw can be decreased by 46.2% after pretreatment, which might account for the hydrogen production enhancement. The results also indicated that the hydrogen yield from pretreated rice straw was not affected without the addition of yeast extract and vitamins to the culture medium, which is substantial evidence that edible fungal pretreated rice straw could provide prerequisite nutrients for hydrogen-producing bacteria. Overall, edible fungal pretreatment has great potential under the mild conditions for high hydrogen yields and thus leads to a new direction to realize a highly efficient and economically competitive biological hydrogen production process from lignocellulosic biomass.

The conversion of carbon dioxide to medium-chain fatty acids (CO₂-to-MCFAs) through microbial processes represents a valuable technology for sequestering and exploiting CO₂, generating superior bio-chemicals from the primary contributor to the greenhouse effect. However, a comprehensive overview and generalization of microbial CO₂-to-MCFAs are presently deficient. Based on this, the present review systematically summarizes the research progress, explicates the process mechanisms, analyses the key challenges and possible solutions, and anticipates forthcoming research perspectives and priorities for the first time. We proposed two original strategies, namely the synchronous strategy and integrated strategy, from current research into microbial CO₂-to-MCFAs. The synchronous strategy concurrently achieves hydrogen (H₂) and CO₂ assimilation, as well as MCFAs production, by employing a reactor that co-cultivates predominant H₂/CO₂-utilizing microorganisms and chain elongation microorganisms. The integrated approaches involve CO₂-to-precursors (i.e., acetate and ethanol) and subsequent precursors-to-MCFAs, achieved through the use of two bioreactors for separately cultivating H₂/CO₂-utilizing microorganisms and chain elongation microorganisms. Mechanistic insights reveal that microbial CO₂-to-MCFAs predominantly encompasses two processes: H₂ and CO₂ assimilation into precursor and subsequent precursors chain elongation into MCFAs, through a Wood-Ljungdahl pathway and a two-round elongation, respectively. The analyses of key challenges and possible solutions for microbial CO₂-to-MCFAs underscore the imperative to enhance efficiency and economy and to shed light on metabolic mechanisms. Furthermore, in order to improve the strategy application potential of microbial CO₂-to-MCFAs, future research perspectives and priorities, e.g. exploitation of functional pure bacteria, screening of functional pure bacteria, multi-omics analysis, genetic modification and enhancement, enhancement of bioreactor stability, specific MCFA production, development of coupled purification technology for MCFAs, and economic benefits and ecological environmental risks, are proposed and prospected. This work is expected to offer a thorough understanding of the microbial CO₂-to-MCFAs, guide and inspire researchers to address critical challenges in-depth and propel the development of CO₂-to-MCFAs.