Synergistic activation of peroxydisulfate with magnetite and copper ion at neutral condition

Magnetite is known to exhibit high catalytic reactivity in Fenton-like reactions merely at low pH conditions. Here we report the association of Cu²⁺ ion can significantly enhance peroxydisulfate (PDS) activation with magnetite under environmental aquatic conditions (near neutral pH). Cu²⁺ is able to synergistically activate PDS with magnetite to generate radicals, e.g., SO₄^·-, at neutral or slightly alkaline pH, and such synergistic activation of PDS is promising to degrade various contaminants in groundwater. In-depth study reveals Cu²⁺ ion adsorbed on magnetite plays a crucial role in PDS activation. The adsorbed Cu²⁺ is labile to be reduced by the structural Fe(II) on magnetite to generate Cu⁺, which is relatively stable in the presence of magnetite at neutral or alkaline pH, but extremely unstable at acidic pH. The generated Cu⁺ on magnetite surface, rather than Cu²⁺, contributes to PDS activation in the reaction system, and the recycling of Cu⁺/Cu²⁺ sustains continuous activation of PDS. This study is among the first to report the synergistic activation of PDS by magnetite and Cu²⁺ ion at neutral pH, and unambiguously discern the role of Cu⁺ in PDS activation. The new mechanistic knowledge provides a more accurate understanding of PDS activation by natural minerals in environmental remediation.