Coupling behavior and enhancement mechanism of porous structure, graphite microcrystals, and oxygen-containing groups of activated biochar for the adsorption of phenol

Abstract

The phenol adsorbent-activated biochar (AB) has high potential. The coupling effects of porous structures, graphite microcrystals and oxygen-containing groups of biochar are key concerns for phenol removal from water. For the sake of structural diversity, three biomass feedstocks (cotton stalk, poplar wood, and bamboo) were treated with four distinct activators (KOH, K₂CO₃, ZnCl₂, and H₃PO₄) to produce ABs. The ABs are called KH-ABs, KC-ABs, ZN-ABs, and HP-ABs, respectively. The adsorption capacities of phenol vary significantly among various ABs, with ZN-ABs having the highest unit adsorption capacities (35.21–60.59 mg g⁻¹). Additionally, the storage and transportation of phenol molecules in ABs require the hierarchical macropore–mesopore-microporous porous structure that has been created. The phenol is preferentially adsorbed in pores smaller than 1.4 nm (micropores). Mesopores and macropores of ABs play important roles in the transport of phenol molecules. The graphite microcrystals of ABs enhance the adsorption of phenol through π–π EDA while the oxygen-containing functional groups enhance the adsorption through hydrogen bond and donor–acceptor interaction. In addition, the KH-ABs possess not only high occupancy of micropores and graphite microcrystals but also a richer oxygen-containing group. The porous structure of KC-ABs is similar to that of KH-ABs. However, the oxygen-containing groups and graphite microcrystals of KC-ABs were poor, leading to some extent to a limitation in the adsorption performance. The well-developed pore structure of ZN-ABs facilitates adsorption, thereby improving the content of their oxygen-containing functional groups, which could further enhance their adsorption performance. Even though HP-ABs are rich in chemical sites, they are restricted as adsorbents because of their lack of microporous structure and graphite microcrystals.