Carbonyl-interfaced-biochar derived from unique capillary structures via one-step carbonization with selective methyl blue adsorption capability

Biochar dye adsorbents are touted as "green" and low-cost compared to metals/oxides and polymers. However, the most active of them require some form of multi-step chemical activation with toxic chemicals which tarnish their credentials. Going forward, there is a need to develop low-toxic chemical activators or discover new biomass sources that can generate high-value biochar adsorbents via physical activation. Herein, we demonstrate that unlike other biomass waste almond skin (WA-s) contains a unique network of lignocellulosic capillary-like structures (CN) buried inside a dense lignocellulosic shell. Via one-step carbonization at 900 °C under N2, the shell is removed exposing CN which transforms into defect-rich carbon macrostructures (AS-C-T). Data from spectroscopy analysis indicate that AS-C-T has the special ability to undergo carbonylation, at high temperatures, creating moderate acidic carbonyl (CO) interfaces that form selective hydrogen bonds with the acidic hydrogen of zwitterion methyl blue (MB), at pH 6.2 to 7.2. Such interactions induce high MB adsorption capacity (>3000 mgg−1) and selectivity comparable to the state-of-the-art. The hydrogen bonding is strong and irreversible; however, it creates secondary interfaces which ensure high recyclability. The adsorption reaction is endothermic and modeled by the pseudo-second-order kinetic, Langmuir, and Intra-particle diffusion. Practically, AS-C-T can be retrofitted to commercial filters to produce pathogen- and dye-free water. These properties make AS-C-T a potential "green" and low-cost adsorbent for dye effluents. Our results hint at the fact that biomass sources with intricate capillary structures might have unique interfacial properties which are useful for preparing high-value biochar adsorbents without chemical activation.