Synergy of carbocatalytic and heat activation of persulfate for evolution of reactive radicals toward metal-free oxidation

Abstract

Persulfate (or peroxydisulfate, PDS) is one of green and low-cost sources of sulfate radicals (SO4radical dot−) in advanced oxidation processes (AOPs) for in situ remediation of contaminated soil and water. The key in AOPs is to develop an effective technique for PDS activation. In this paper, nitrogen-doped single-walled carbon nanotubes (N-SWCNTs) were employed as a metal-free catalyst to activate PDS for oxidation of a diversity of organic contaminants such as nitrobenzene (NB), phenol, benzoquinone and sulfachlorpyridazine. For the first time, the coupling effects of carbocatalysis and heat were investigated in a range of 5–75 °C on PDS activation, which indicated that organic oxidation efficiency was enhanced at elevated temperatures. The presence of the carbocatalyst impressively decreased the PDS activation energy from 53.4 (by heat) to 10.3–22.5 kJ/mol (heat/carbocatalysis). Intriguingly, the mechanisms of heat-assisted carbocatalysis were temperature-dependant and the synergy of the heat/carbon integrated system appeared to be phenomenal at a high temperature region (55–75 °C). The thermal stimulation promoted PDS to generate hydroxyl radicals via heterogeneous water oxidation and mutual transformation from sulfate radicals, evidenced by the selective radical quenching and spin trapping techniques. The carbocatalyst boosted the radical production by simultaneously activating the reactants (PDS and organics) with the N-doped carbon atoms and facilitating the electron transport as a conductive substrate. Therefore, this study advances the understanding in the effect of reaction temperature on persulfate activation and unveils the synergy of carbocatalysis with heat for enhanced PDS activation.