The removal of Cd(ii) by the UV/permanganate process: role of continuously in situ formed MnO2 and reactive species

Abstract

The UV/permanganate process is a novel advanced oxidation process generating hydroxyl radicals (HO˙) and reactive manganese species (RMnS), followed by the formation of nano-sized manganese dioxide (MnO₂) with an average particle size of 50–60 nm. Considering the application potential of the in situ formed MnO₂, this study focused on the feasibility of heavy metal removal by the UV/permanganate process and Cd(II) was selected as a target. The removal percentage of Cd(II) reached 98.5% by the UV/permanganate process after a 60 min reaction at an initial pH of 6.0, which was much higher than those of SO₃²⁻/permanganate and Mn(II)/permanganate processes with the same dosage of permanganate. The in situ formed K-layered birnessite type MnO₂ was confirmed to be the key adsorbent of Cd(II), fitting the linear fit stage of the MnO₂ adsorption isotherm. According to X-ray photoelectron spectroscopy (XPS) and the adsorption isotherm, Cd(OH)₂ was formed during the Cd(II) adsorption via surface precipitation in the UV/permanganate process. The continuous generation of in situ formed MnO₂ and its high adsorption capacity are responsible for the gradual complete removal of Cd(II) by UV/permanganate, which shows better performance than the preformed MnO₂ with a maximum removal percentage of 55.6% at pH 6.0. RMnS was also critical to the enhanced removal of Cd(II). The Cd(II) removal percentage by UV/permanganate was only slightly decreased by the water matrix such as Ca(II), Mg(II) and natural organic matter. This study provides a better understanding of the multiple roles of UV/permanganate in water treatment, which is a more convenient way to control the formation rate of in situ formed MnO₂ for heavy metal removal.