Green synthesis of highly pyrrolic nitrogen-doped biochar for enhanced tetracycline degradation: New insights from endogenous mineral components and organic nitrogen synergy

The synthesis of efficient and environmentally friendly persulfate (PS) catalysts is currently a prominent research focus. This study employs single-step carbonization to investigate the interactions between endogenous mineral components (EMCs) and N-doping of nitrogen-rich biomass, aiming to design efficient biochar-based catalysts for enhanced tetracycline (TC) degradation. Biochars (PBC and SBC) derived from peanut hulls (PH) and soybean curd residue (SCR) with similar EMCs (ash ratio 16:19) but varying nitrogen content (1 % vs 1.53 %) were achieved, along with melamine-N-doped PBC (PBCN) for comparison under identical conditions. Physicochemical and quantum-chemistry analyses were conducted to investigate the synergistic effects between EMCs and organic-nitrogen in terms of morphological structures, active sites, and catalytic persulfate (PS)-mediated degradation of TC. The findings revealed that the optimal synthesis temperature for the efficient biochar-based catalyst was 800 °C. In contrast to exogenous N-doping with higher N-content (7.81 %), organic-N accelerated lattice disruption, leading to enhanced interactions with EMCs and resulting in the formation of hierarchical structures and pyrrole-N (organic-N 43.44 % > exogenous-N 17.67 %). The SBC/PS system exhibited superior degradation capability, with rapid removal of 95 % in 100 mg·L-1 TC. Density functional theory (DFT) confirmed that the non-radical pathways including 1O2 and electron transfer, facilitated by the presence of CO and pyrrole-N, were responsible for binding PS and forming biochar-PS*. This study furnishes both experimental data and theoretical insights, supporting the targeted control and efficient advancement of eco-friendly, high-value biochar-based catalysts.