Graphene oxide additive-driven widening of microporous biochar for promoting water pollutant capturing

Renewable and low-cost biochar's intrinsic porous structure limitations make its porous structure mediation highly significant for performance enhancement. We employ graphene oxide (GO) as an additive at a low dosage for the deed-leaf-derived biochar's microporous structure mediation to promote the wastewater treatment ability toward a typical antibiotic pollutant - tetracycline hydrochloride (TCH). While KOH-based activation enables biochars with a microporous structure and improved graphitization, GO transforms part of micropores of such activated biochar into mesopores (i.e. forming hierarchical porosity), driven by the thermal pyrolysis of oxygen groups enriched on GO. The GO additive-induced process overcomes the loss of surface resulted from conventional pore widening, promoting mass transport as well as improving the accessibility to pore interiors and compatibility for large-size contaminant molecules. The increase of the sp²-to-sp³ carbon ratio from 0.59 to 0.79 with GO incorporation facilitates π-π stacking interactions with aromatic pollutants (including TCH and organic dyes), which are more critical than the electrostatic attraction and hydrogen bonding interactions. Apart from superior adsorption capacities of 193.43 and 336.70 mg/g obtained at 303 and 313 K, respectively, the graphene-modified hierarchically porous biochar is recyclable, reusable, resistant to interfering metal cations, and universal for binding a range of organic pollutants bearing different charge states, demonstrating equilibrium adsorption capacities of 186.58, 190.38, 268.43, and 254.88 mg/g for TCH, methylene blue, Rhodamine B, and methyl orange, raised by 26.8%, 41.6%, 32.1%, and 105.3%, respectively, compared to the counterpart without GO-enabled micropore widening. The greater adaptability to different-size pollutants presents higher practical application viability.