The Influence of pH changes in chlorine disinfection process on the chlorination degradation dynamics of typical drugs

Pharmaceutical contaminants have been ubiquitously detected in various water bodies, posing a considerable threat to human and ecological health. Chlorine disinfection technology is an effective approach for eliminating pharmaceuticals from water. pH is a crucial parameter influencing the distribution of oxidizing species and the removal of contaminants during the chlorine disinfection process. Previous studies have predominantly employed phosphate buffer solutions (PBs) to regulate the initial pH of the system for investigating its impact on pollutant degradation. Nevertheless, the pH in actual water chlorination disinfection processes is constantly varying, highly nonlinear, and time-delayed, thereby making the influence on pollutant degradation more intricate. In this paper, four representative pharmaceuticals were selected to study the influence of the dynamic changes in pH of the solution after the addition of chlorine disinfectant on the variation law of their degradation kinetics. The research results indicate that the overall chlorination rate of fluoroquinolones (FQs) slows down when PBs are not added. The increase in pH of the solution is positively correlated with the increase in concentration of free available chlorine (FAC), showing a linear trend. Under this growth trend, the apparent degradation rate constant (k_obs) of FQs decreases, while the k_obs of β-blockers (BBs) increases. When the FAC concentration is the lowest (FAC=1 mg/L), the k_obs of FQs is instead the largest, with lomefloxacin and ofloxacin being 0.28 /s and 0.22 /s, respectively. The ionic form has a considerable impact on the degradation of BBs. The high proportion of the anion form of BBs at a high pH significantly promotes the chlorination degradation of BBs, with the fastest degradation occurring at pH₀=9.0. The k_obs of metoprolol and atenolol are 0.028 /s and 0.036 /s, respectively. Further studies reveal that the chlorination degradation trend of the four contaminants in drinking water is consistent with that in pure water.