Decay kinetics model of available chlorine in slightly acidic electrolyzed water in storage and disinfection process

Abstract: Slightly acidic electrolyzed water (SAEW) with pH 5.0-6.5 is produced by electrolysis of dilute hydrochloric acid or salt solution in a chamber without membrane. SAEW can effectively kill various pathogenic bacteria as one of the most potential green disinfectants. However, SAEW is susceptible to be exposed to time, air, and illumination etc. To study SAEW during storage and disinfection, the variations of pH value, oxidation-reduction potential (ORP) and available chlorine concentration (ACC) were analyzed under different storage temperatures (25°C, 30°C, 35°C, 40°C, and 50°C) for 12 days. Furthermore, variations in ACC during Escherichia coli (ATCC 25922) disinfection were investigated.The results showed that the pH increased, but the ORP and ACC decreased during storage. When SAEW was stored in a transparent bottle at 25°C for 12 days, the ACC of SAEW was decreased from 20.53mg/L to 5.06mg/L, and the ORP of SAEW was also decreased from 821mv to 641 mv, while pH was increased from 6.06 to 7.45 in the same condition. Variations of ACC, ORP, and pH in a brown bottle under same stored temperatures had a similar tendency. When SAEW was stored in a brown bottle at 25°C for 12days, the ACC of SAEW was also decreased and the ORP of SAEW was decreased from 821mv to 652mv, while pH was increased from 6.06 to 7.38. The higher the stored temperature was, the quicker ACC, ORP, and pH of SAEW decayed during storage. SAEW was stored in a transparent bottle at 50°C for 12days, and the ACC of SAEW was decreased from 20.53mg/L to 0.10mg/L and the ORP from 821mv to 641mv, while pH was increased from 6.06 to 8.11 in the same condition. Similarly, when SAEW was stored in a brown bottle at 50°C for 12 days, the ACC of SAEW was decreased from 20.53mg/L to 0 and the ORP from 821mv to 583mv, while pH was increased from 6.06 to 8.03 in same condition. The same tendency of ACC was also found during E. coli disinfection, but the decay of ACC was quicker than it was presented during storage. The ACC of SAEW was reduced by 15.05 mg/L after SAEW with ACC of 25.13 mg/L was used to disinfect for 25 min., compared with the above storage condition for five days.The decay kinetics models of the ACC in SAEW during storage and disinfection were established and the correlation coefficients were above 0.90. The temperature and duration of storage and disinfection had significant impacts on the physicochemical properties of SAEW. As the stored time prolonged, the ACC and ORP of SAEW was decreased while pH increased. The higher the stored temperature was, the larger the variation of amplitude of ACC, ORP, and pH of SAEW. The decay of ACC was quicker during disinfection than it was presented during storage and active chlorine needed to be expended during the disinfection process. The decay of ACC followed first-order kinetics during the storage and disinfection process, and the values of the kinetic parameters during the storage process were higher than it were presented during the disinfection process, and the value of k′ of storage and disinfection was 0.108 (±0.044) and 0.043 (±0.005) respectively under the condition of ACC of 25.13 mg/L at 25℃.