Turning agricultural side streams into boron doped bio-carbon/silicon adsorbents for efficient dye removal: Experimental studies and theoretical calculations

Preparing sustainable and highly efficient biomass-based carbon/biosilica materials remains a challenge. Herein, carbon/biosilica materials were prepared from oat husk, a biogenic silicon source, to be employed as adsorbents for reactive black 5 dye (RB-5) removal. The carbon/silicon materials were prepared with boric acid activation under two different concentrations (1 M, BioC@Si-1M, and 2 M, BioC@Si-2M). The use of boric acid led the materials to be doped with boron, which is a highly effective carbon surface modification method, aimed at enhancing their adsorption capacity due to the introduction of Lewis acidic sites and electron deficient into the carbon's lattice, which fundamentally alters its surface chemistry and provides more adsorption active sites for molecules adsorption. The physicochemical characterization results suggested that the acid treatment had a significant impact on the material's structures. The specific surface area of Bio-C@Si-1M was 469 m 2 g -1 , while Bio-C@Si-2M was 589 m 2 g -1 . TEM and XPS analysis clearly verified the formation of a carbon-silicon composite and confirmed the successful incorporation of boron. The oxygen content was higher in Bio-C@Si-2M (14.18 at.%) compared to Bio-C@Si-1M (6.3 at.%). When tested as adsorbents, the novel materials showed excellent adsorption capacities of 338.8 and 367.0 mg g-1 for BioC@Si-1M and BioC@Si-2M, respectively. Crucially, boron doping modulated the surface charge (pH PZC ) and introduced BC 3 and BCO 2 species, which served as specific active sites that significantly enhanced π-π electron-donor-acceptor and Lewis acid-base interactions. The adsorption mechanism suggested that pore filling alongside these multiple synergistic interactions (including H-bonding and electrostatic attraction) highly contributed to the effective adsorption capacity of RB-5 on both samples. Theoretical calculations revealed different types of adsorption patterns. This research demonstrates that novel, highly efficient boron-doped carbon/silicon-based materials can be prepared from natural sources to remediate dye contamination in wastewater.