Mechanochemical activation of zero-valent iron on carbonized boron-doped graphene dots for enhanced sonochemical dyes removal

Boron-doped graphene dots (BGDs) were synthesized by a hydrothermal method using the disaccharide maltose as the carbon precursor and borax as the boron dopant. The precursors were heated to 500 °C for 5 h to obtain the carbonized BGDs (CBGDs). B doping was carried out to increase the surface area and improve the electron transfer reaction for quick and effective reactive oxygen species generation. BGDs and CBDGs showed different oxygen contents and functionalities, surface areas, morphologies, and zeta-potentials. Zero-valent iron (ZVI) was activated mechanochemically by surface passivation using BGDs and CBGDs under atmospheric conditions, and their dye removal performance via sonochemical degradation was evaluated. Owing to the greater oxygen functionality of BGDs, the ZVI@BGDs were fully decorated with ZVI, and less activation occurred that inhibited the sonochemical dye degradation properties of ZVI@BGDs. In contrast, a sheet-like structure and activation of ZVI on ZVI@CBGDs make it an active catalyst. The maximum surface area (413.82 m²/g) and high negative zeta potential (−38.25 mV) of CBGDs make it a good adsorbent. BGDs have a low surface area (20.78 m²/g), self-assembling structure, non-planer arrangement, and a twisted structure with interlocking spheres that disfavor π–π stacking between the dye and BGDs and make it non-adsorbent. The rapid generation of reactive oxygen species, the existence of iron with mixed-valence states, and the increase in surface area due to boron doping facilitate the electron transfer process to make the catalyst active and reusable even after 10 cycles.