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 center dot OH (which decreased by 88 - 96%) and SO4 center dot- (which decreased by 67.4%) in UV/PS/Cl. With the increase in the concentration of Cl- to 1000 mM, the contributions of center dot OH and SO4 center dot- to the degradation of the PPCPs were almost completely masked by the contribution of Cl center dot (except in the case of SAA). The relationship between the natural logarithms of the second-order rate constant of SO4 center dot- and Cl center dot with PPCPs versus the Hammett constant was established and used to determine the second-order rate constant of SAA towards SO4 center dot- . 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 - , SO4 2-, 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, center dot OH and SO4 center dot- 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 center dot is an important radical that can replace center dot OH and SO4 center dot- in UV/PS or UV/H2O2 under saline conditions.