Metal-free activation of persulfate by cubic mesoporous carbons for catalytic oxidation via radical and nonradical processes

Abstract

Carbon materials have been demonstrated as effective and metal-free carbocatalysts for substituting the toxic and/or expensive transition and noble metals/oxides for various green chemical processes. In this study, three-dimensional cubic mesoporous carbon (CMK) materials were employed for adsorptive and oxidative removal (via heterogeneous persulfate activation) of toxic phenolic compounds. CMK-3 and CMK-8 are constructed with porous structures and larger specific areas of 1129 and 1072m2/g, respectively. They achieved 39.2% and 31.3% of phenol adsorption in aqueous solutions. CMK-3 and CMK-8 also demonstrated as superb persulfate (PS) activators, providing 100% phenol oxidation in 20 and 45min accordingly with high rate constants of 0.209 and 0.104min−1, respectively. The metal-free systems are more efficient than Fe2+ or Ag+ based homogeneous systems. Moreover, CMK-3/PS can be comparable to the classical zero-valent iron (ZVI)/PS system without inducing any metal ions to the waterbody. The mechanistic study indicated that persulfate activation on CMKs was different from the Fe2+/PS, Ag+/PS, and ZVI/PS systems. A nonradical pathway was unveiled in the metal-free persulfate activation for catalytic oxidation in which persulfate was activated on the carbon lattice and oxidized the adsorbed phenol molecules via a rapid electron transfer. The edging sites and kenotic groups of carbon materials would mediate persulfate to produce sulfate radicals. Both the radical pathway and nonradical process contributed to the complete phenol removal, and the great adsorption capability of CMKs further promoted the adsorption of the organic and oxidant for enhanced catalytic processes. The study dedicates to a metal-free oxidative system with cheap carbon materials for wastewater treatment and environmental remediation.