Oxidation of urea to nitrate via persulfate activation under far-UVC light improves ultrapure water production

Low-molecular-weight organic pollutants present in feed water pose a significant challenge to maintaining ultrapure water (UPW) quality, as they are not readily removed by conventional treatment processes. Urea is a low-molecular-weight, nonionic, persistent organic pollutant resistant to conventional UPW production processes. Therefore, suitable oxidation technologies must be explored to convert urea to inorganic nitrogen ions, which are more easily removed in post-UPW treatment steps. In this study, the far-UVC (peak wavelength centered at 222 nm) coupled persulfate (PS) activation system was used for the effective oxidation of urea to nitrate (NO₃^-). In the degradation of urea (10 μM, PS 0.4 mM, pH 7), far-UVC/PS systems (k_urea,222/PDS= 1.69 × 10^-3 cm²∙mJ^-1, k_urea,222/PMS= 2.69 ×10^-4 cm²∙mJ^-1) were found to be superior to conventional 254 nm LPUV/PS systems (k_urea,254/PDS= 1.07 ×10^-4 cm²∙mJ^-1, k_urea,254/PMS= 7.01 ×10^-5 cm²∙mJ^-1). The far-UVC/PDS system showed the highest efficiency, achieving complete urea removal after 5241.0 mJ∙cm^-2 of irradiation. The dominant role of SO₄^•- in urea degradation was systematically investigated, with urea removal showing a strong correlation with the steady-state concentration of SO₄^•-. Moreover, the evolution of inorganic nitrogen species and the reduction of total organic carbon were examined to assess the applicability of the system as a pretreatment technology in UPW production. The results provide valuable insights into the integration of the far-UVC process for the removal of low-molecular weight pollutants during UPW production.