Is addition of reductive metals (Mo, W) a panacea for accelerating transition metals-mediated peroxymonosulfate activation?

The interaction of reductive metal ions and peroxymonosulfate (PMS) is necessary for the generation of sulfate radials (SO₄^[rad]−), however, this process is greatly restrained by the sluggish reduction of high-valent metal ions. Here we report that commercially available reductive metal (Mo or W) powders are capable of unlocking this kinetic constraint. The reduction of Fe(III) to Fe(II), decomposition of PMS, and degradation/mineralization of 4-chlorophenol (4-CP) are all accelerated in the Mo/Fe²⁺/PMS process at a very low Fe²⁺/PMS ratio (Fe²⁺/PMS = 1/10). In such an accelerated system, common adverse effects of natural water constituents such as chloride and humic acid are largely mitigated. According to the fluorescence measurement and scavenging tests, sulfate and hydroxyl radicals dominate in Mo/Fe²⁺/PMS process. The addition of Mo or W is further confirmed to favor Cu²⁺/PMS process, but this is not the case for other metal ions (Mn²⁺, Ni²⁺, Ce³⁺ and Co²⁺). Reductive zero-valence and four-valence active sites (Mo⁰ and Mo⁴⁺; W⁰ and W⁴⁺) play key roles in overall redox reaction. Overall, our present work provides an alternative route for expediting redox cycling of transition metals in advanced oxidation processes, without useless consumption of PMS and increase of total organic carbon.