Formation and Toxicity of Chlorine Species During Zeolite Regeneration by NaCl-NaClO After Stormwater Adsorption

Zeolite adsorption followed by NaCl-NaClO regeneration is an effective method for the on-site treatment of ammonia in initial stormwater. However, the formation and toxicity of chlorine species during the zeolite regeneration process need to be investigated. In this study, under intermittent and continuous operations, zeolites adsorbed NH4Cl + HA (humic acid) and actual stormwater, then regenerated with NaCl-NaClO (0.5 g/L NaCl, ClO−:N molar ratio of 1.8, pH = 10). This technology was assessed from the following three aspects: adsorption and regeneration, chlorine species formation, and toxicity. The results showed that zeolites exhibited a greater adsorption capacity for HA in stormwater compared to that in an NH4Cl + HA solution, and the presence of ammonia had a minimal impact on this process. During zeolite regeneration, ammonia had a competitive advantage over HA for ClO−. ClO3− was inevitably formed in regeneration. The formation of chlorinated organic compounds (COPs) increased over time. The order of chlorine species toxicity in zeolite regeneration solution was free chlorine > COPs > ClO3−. Controlled regeneration time was required to minimize the formation and toxicity of chlorine species. During the 10 cycles of regeneration, chlorine species continued to form and caused high toxicity hazards.