Effect of the chloride ion on the degradation of PPCPs in UV/persulfate and UV/H2O2 and the role of radicals in these systems

This work comprehensively studied the kinetic degradation of ciprofloxacin (CIP), levofloxacin (LEV), and sulfanilamide (SAA) by UV/PS/Cl and UV/H2O2/Cl. UV contributed to the degradation of SAA to a great extent, resulting in the highest degradation, compared with the other PPCPs. 10 mM of Cl- slightly enhanced the degradation of LEV and SAA, while higher concentrations of Cl- (up to 1000 mM) inhibited the degradation of all the PPCPs in most processes. Increasing the concentration of Cl- to 1000 mM resulted in the decrease in the concentrations of •OH (which decreased by 88 – 96%) and SO4•- (which decreased by 67.4%) in UV/PS/Cl. With the increase in the concentration of Cl- to 1000 mM, the contributions of •OH and SO4•- to the degradation of the PPCPs were almost completely masked by the contribution of Cl• (except in the case of SAA). The relationship between the natural logarithms of the second-order rate constant of SO4•- and Cl• with PPCPs versus the Hammett constant was established and used to determine the second-order rate constant of SAA towards SO4•-. Acidic pH and neutral pH were conducive for the degradation of the PPCPs in the process. The presence of various constituents, including anions (i.e., HCO3-, SO42-, and NO3-) and metal cations (i.e., Fe2+, Ni2+, Co2+ and Mn2+) inhibited the degradation rate of the PPCPs to varying degrees. In the presence of 10 mM Cl- and other anions, •OH and SO4•- were consumed by Cl-, up to 71 – 90% and 8 – 10% in UV/PS/Cl, respectively. Toxicity assessment showed that 10 mM Cl- in UV/PS/Cl and UV/H2O2/Cl did not cause any significant inhibition on the growth of Chlorella vulgaris and Escherichia coli (E.Coli), compared with that in the absence of Cl-. In summary, Cl• is an important radical that can replace •OH and SO4•- in UV/PS or UV/H2O2 under saline conditions.