Table 1.

Summary of the different persulfate (PS) and peroxymonosulfate (PMS) activation approaches: pathways, mechanism, and key parameters.

Activation approaches		Main reaction for sulfate radicals production	Mechanism	Key parameters	References
Physical activation	Heating	$S_2{O_8}^{2-}+heat\to 2S{O}_{\mathrm{\text{4}}}^{\mathrm{\text{-}}}·$ (PS) $HS{O_5}^{-}+heat\to S{O}_{\mathrm{\text{4}}}^{\mathrm{\text{-}}}·+HO·$ (PMS)	Fission of O–O bond	(i) Higher temperature can cleave O–O bond, whereas excessive temperature may cause side effects (ii) Owing to huge energy demand, not suitable for large-scale sewage treatment	Zhao et al., 2013; Wang and Wang, 2018
	UV	$S_2{O_8}^{2-}+h\nu \to 2S{O}_{\mathrm{\text{4}}}^{\mathrm{\text{-}}}·$ (PS) $HS{O_5}^{-}+h\nu \to S{O}_{\mathrm{\text{4}}}^{\mathrm{\text{-}}}·+HO·$ (PMS)	Fission of O–O bond	Usually under 254 nm wavelength, depending on dissolved oxygen concentration	Yang Q. et al., 2019
	Ultrasound	$S_2{O}_8^{2-}+2H_{2}O\underset{}{\overset{O{H}^{-}}{\to }}H{O}_2^{-}+2S{O}_4^{2-}+3H^{+}$ $S_2{O}_8^{2-}+2H{O}_2^{-}\to S{O}_4^{-}·+S{O}_4^{2-}+H^{+}+O_2^{-}·$ (PS)	Fission of O–O bond Hydrolysis of water molecules	Similar to heating activation	Nasseri et al., 2017
Chemical activation	Alkaline activation	$S_2{O}_8^{2-}+H_{2}O\to H{O}_2^{-}+2S{O}_4^{2-}+H^{+}$ $S_2{O}_8^{2-}+H{O}_2^{-}\to S{O}_4^{-}·+S{O}_4^{2-}+H^{+}+O_2^{-}·$ (PS) $HS{O}_5^{-}\to S{O}_5^{2-}+H^{+}$ $HS{O}_5^{-}+H_{2}O\to HS{O}_4^{-}+H_2{O}_2$ $S{O}_5^{2-}+H_{2}O\to S{O}_4^{2-}+H_2{O}_2$ $H_2{O}_2\to H{O}_2^{-}+H^{+}$ H2O2 → 2OH· $H_2{O}_2+OH·\to H{O}_2^{}·+H_{2}O$ $H{O}_2^{-}·\to H^{+}+O_2^{-}·$ $HO·+O_2^{-}·{\to }^1{O}_2+O{H}^{-}$ $2H^{+}+2O_2^{-}·{\to }^1{O}_2+H_2{O}_2$ $HS{O}_5^{-}+S{O}_5^{2-}\to HS{O}_4^{-}+S{O}_4^{-}·{+}^1{O}_2$ (PMS)	Hydrolysis of PS and PMS to hydrogen peroxide	Often pH > 11 For PMS activation, 1O2 is the predominant radical specie, whereas SO${}_4^{-}·$/OH·/1O2 for PS	Fernandes et al., 2018
	Transition metal ions	$S_2{O}_8^{2-}+M^{n+}\to M^{n+1}+S{O}_4^{2-}+S{O}_4^{-}·$ (PS) $HS{O}_{\mathrm{\text{5}}}^{-}+M^{n+}\to M^{n+1}+O{H}^{\mathrm{\text{-}}}+S{O}_4^{-}·$ (PMS)	Single electron transfer	Preparation of catalysts is more economical for a homogeneous system than that of a heterogeneous system	Liu L. et al., 2019
	Carbon-based materials	$S_{\mathrm{\text{2}}}{O}_{\mathrm{\text{8}}}^{2-}+ACsurface-OOH\to S{O}_4^{-}·+ACsurface-OO·+HS{O}_4^{-}$ $S_{\mathrm{\text{2}}}{O}_{\mathrm{\text{8}}}^{2-}+ACsurface-OH\to S{O}_4^{-}·+ACsurface-O·+HS{O}_4^{-}$ (PS) $HS{O}_5^{-}+C-\pi \to C-{\pi }^{+}+S{O}_4^{-}·+O{H}^{-}$ $HS{O}_5^{-}+C-{\pi }^{+}\to C-\pi +S{O}_5^{-}·+H^{+}$ (PMS)	Single electron transfer	Activated carbon is relatively economical	Zhao et al., 2017