Tailoring biochar for volatile organic compounds adsorption via molten salt–assisted biomass pyrolysis

Molten salt–assisted pyrolytic carbonization was applied to convert biomass into functional biochar for volatile organic compound (VOC) adsorption. By varying molten salt types and salt-to-feedstock ratios, biochar with distinct pore structures and surface compositions was obtained. The adsorption behaviors of benzene and toluene were evaluated at 283, 298 and 313 K. For both VOCs, adsorption uptakes increased with partial pressure, while benzene adsorption decreased with increasing temperature, indicating VOC uptake is primarily governed by pore-controlled physical adsorption. Carbonate-assisted biochar (PSL-1-800 and PSL-3-800) exhibited higher adsorption capacities than other samples, which were closely associated with their enhanced microporosity and surface area. Density functional theory (DFT) calculations and wavefunction analyses indicate that adsorption on metal-free biochar is governed by weak π–π interactions, whereas metal-containing biochar exhibit strengthened adsorption through cation–π interactions. Together with the experimental observations, these results show that molten salt chemistry controls VOC adsorption primarily by regulating microporous structure and metal-assisted surface interactions. This study demonstrates molten salt–assisted pyrolysis of biomass waste is a promising approach to develop efficient VOC adsorbents, offering a promising proof-of-concept strategy for developing biochar-based adsorbents for VOC removal. • Molten salt biochar for VOCs adsorption was customized. • Carbonate biochar exhibited superior benzene adsorption. • Metal–π interactions dominated benzene and toluene adsorption. • Biochar exhibited a higher affinity toward toluene than benzene.