A critical review of biochar-enabled chain elongation for n-caproate production: From empirical addition to rational design

Microbial chain elongation (CE) offers a vital pathway to upcycle waste-derived short-chain carboxylates into medium-chain carboxylates, such as n-caproate (C₆). However, its translation to robust continuous operation remains constrained by two coupled bottlenecks: electron diversion to competing sinks and product inhibition governed by undissociated caproic acid (HCap). While biochar is widely reported to enhance CE performance, mechanistic attribution often relies on non-diagnostic indicators, such as higher titers, faster start-up, or community shifts, derived mainly from short-term batch tests. This review critically synthesizes the fundamental bottlenecks of n-caproate CE and links biochar feedstock legacy, pyrolysis conditions, and particle architecture to key interfacial functions. These functions include biomass retention, redox mediation, conductivity-related effects, micro-pH buffering, and community stabilization. Since these functions can co-occur and mimic one another at the reactor scale, we propose a CE-specific evidence-strength framework to map mechanistic claims to minimum evidence requirements and major confounders, thereby distinguishing genuine material functions from adsorption, retention, buffering, and reporting artifacts. We then examine scale-up barriers, including material heterogeneity, rheological and mass-transfer penalties, biochar aging/passivation, attrition, washout, and recovery difficulty. Finally, we outline a roadmap toward CE-grade biochar specifications, standardized reporting metrics, and reactor-separation co-design. This review aims to move biochar-enabled n-caproate CE from empirical additive use toward predictable and evidence-based design for continuous waste-to-chemicals biomanufacturing.