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• Ethanol and acetic acid from syngas were converted into caproic acid by C. kluyveri. • Biochar (BC) and activated carbon (AC) foster bioproduction by C. kluyveri. • Differences between BC and AC translated into different fermentation profiles. • Biochar resulted in a higher initial production rate of caproic acid. • Activated carbon reduced pH-induced toxicity by partial adsorption of caproic acid. Syngas from biomass or steel mills can be fermented into a dilute stream of ethanol and acetic acid, which requires energy intensive distillation for product recovery. This can be circumvented by selective secondary fermentation of the syngas fermentation effluent to caproic acid as easier recoverable platform chemical with Clostridium kluyveri. Here, we explore the impact of biochar and activated carbon on this process. Changes during the fermentation with biochar or activated carbon were monitored, different doses were tested and the recyclability of biochar and activated carbon was assessed. Biochar decreased the lag phase and increased the caproic acid production rate (up to 0.50 g · L - 1 · h - 1 ). Upon recycling for subsequent fermentation, biochar retained this property largely. Activated carbon addition, especially at high dose, could potentially increase the conversion and selectivity towards caproic acid to 14.15 g · L - 1 (control: 11.01 g · L - 1 ) and 92% (control: 84%), respectively. [ABSTRACT FROM AUTHOR]