Novel magnetic nitrogen-doped biochar (NBC) as an activator for peroxymonosulfate to degrade naphthalene: Emphasizing the synergistic effect between NBC and MnFe2O4

[Display omitted] •MnFe2O4@NBC catalysts are synthesized with nitrogen-doped biochar as carrier via the solvothermal method.•MnFe2O4@NBC exhibits excellent performance in activating PMS to remove low molecular weight PAHs.•NBC can promote the enhancement of electron transfer and formation of BIEF to enhance the PMS activation.•The degradation of NAP in MnFe2O4@NBC/PMS system is an oxidation process dominated by the free radical pathway.•Fe2+, Mn2+, CO, pyridine nitrogen, and defective structures are important reaction sites for PMS activation. The conversion of agricultural biomass waste to value-added biochar (BC)-based catalysts receives tremendous interest because it falls under the resource recycle concept. In this work, a magnetic iron–manganese bimetallic catalyst (MnFe2O4@NBC) with nitrogen-doped BC as the carrier was prepared through the solvothermal method using agricultural waste walnut shells as the precursor. Moreover, the structure, morphology, and magnetic separation properties of the catalyst were comprehensively analyzed by various characterization methods. The prepared MnFe2O4@NBC catalyst was coupled with peroxymonosulfate (PMS) for oxidative degradation of naphthalene (NAP). Results showed that MnFe2O4@NBC could effectively activate PMS, and the removal rate of NAP could reach 80.17 % in 120 min. The degradation of NAP through the synergistic action of the free radical pathway (SO4− and OH) and the nonradical pathway (1O2), but the free radical pathway was dominant. XPS, electrochemical profiles, and DFT calculations confirmed that the catalyst surfaces of Fe2+, Mn2+, CO, and pyridine nitrogen and the defective structures are important reaction sites for PMS activation; the synergistic effect of Fe and Mn bimetals promotes the rapid cycling of metal redox pairs, whereas NBC promotes the dispersion of MnFe2O4, the direct activation of PMS, the enhancement of electron transfer, and the formation of a built-in electric field to facilitate the activation process. The catalysts showed a decrease in the removal of NAP after three times of reuse, but their activity could be restored by simple heat treatment, thus showing great potential for application. In brief, this work provides an efficient MnFe2O4@NBC heterogeneous catalyst and a new insight into PMS activation, which extends the potential application of BC-based catalysts for environmental remediation.