Insights into the influence and mechanism for the protogenetic N, P and Fe containing biochar on peroxymonosulfate activation

The treatment and management of excess sludge is a huge challenge for environmental protection and resource utilization. In this study, the sludge-based biochar containing iron (Fe), phosphorus (P), and graphite-type nitrogen (N) was prepared (FeNC) by anaerobic carbonization process using the excess sludge. The biochar with low Fe and P content (L-FeNC) was prepared via sulfuric acid pickling. XPS spectra showed that the content of Fe, P, and graphite-type N in FeNC sample was 5.02 at%, 5.77 at%, and 1.55 at%, respectively. After sulfuric acid pickling, the Fe and P proportion in L-FeNC sample was reduced to 1.99 at% and 2.51 at%, respectively, and graphite-type N was increased to 2.14 at%. Compared with FeNC, the adsorption performance of L-FeNC was enhanced as the BET surface area was increased from 36.99 m2 g−1 to 292.86 m2 g−1 and the BJH adsorption average pore size was reduced from 24.22 nm to 11.15 nm. The sulfate radicals (•SO4−) and hydroxyl radicals (•OH) were produced through activation of peroxymonosulfate (PMS) by Fe species in FeNC. The graphite-type N can accelerate the electron transfer of adjacent carbon atoms and promote the generation of singlet oxygen (1O2) through nucleophilic reaction with PMS. Comparing with FeNC/PMS system, the 1O2 played a more prominent role in the L-FeNC/PMS system resulting from the reduced inhibitory effect from P species and higher content of graphite-type N in L-FeNC. This research mainly provides a green and effective upcycling process of excess sludge resources and removal of tetracycline pollutants in aqueous solutions.