Carbon defects in biochar facilitated nitrogen doping: The significant role of pyridinic nitrogen in peroxymonosulfate activation and ciprofloxacin degradation

Abstract

Nitrogen (N) doping endows biochar with remarkable catalytic ability to activate peroxysulfate for degradation of organics. Herein, a new strategy was proposed by creating defective carbon in pinewood-derived biochar (BC800) under alkaline conditions to favor C-N configuration using 2-methylimidazole as an extrinsic N-dopant. After doping process, N presented as pyridinic N (48.5%), pyrrolic N (25.1%), and graphitic N (26.4%) in N-doped biochar (NKBC800) as per X-ray photoelectron spectroscopy (XPS). NKBC800 had a larger specific surface area (1398 m2 g−1), higher defective degree (ID/IG = 0.78), and more active N species, which facilitated peroxymonosulfate (PMS) activation performance. The removal of ciprofloxacin (CIP) was examined by BC800, NKBC800, and alkaline activated biochar (KBC800). As expected, the CIP (30 mg L−1) removal efficiency by NKBC800 (0.2 g L−1) was increased to 87%, five times greater than BC800. In contrast, KBC800 was less efficient in CIP removal (80%) with compromised degradation rate constant. Based on electron paramagnetic resonance (EPR), the degradation mechanisms comprised radical pathway (sulfate and hydroxyl radicals) and main non-radical pathway (singlet oxygen 1O2 and direct electron transfer). Degradation intermediates were recognized with UPLC-MS analysis and main possible CIP degradation pathways was proposed as cleavage of the piperazine ring attributing to 1O2 followed by the substitution of hydroxyl groups. The as-prepared NKBC800 showed good reusability even with consecutive runs and excellent pH stability between 3 and 9. These findings highlighted preparation of metal-free catalyst based on N-doped carbonaceous material with excellent PMS activation ability to remove different classes of antibiotics from wastewater.