Alleviating the inhibitory effect of carbonate ions on ozonation to strengthen attenuation of sulfadiazine in O3/Ca(OH)2 process: Influencing factors, degradation pathways and mechanism

Sulfadiazine (SDZ), a type of antibiotics with refractory and high-mobile characteristics, can accumulate and be detected out in aquatic ecosystems all over the world. SDZ can be destroyed by ozonation, but the degradation effect is easily and negatively affected by carbonate ions (CO₃^2-) which be eliminated by introduction of Ca²⁺ in ozonation systems. Experimental results manifested that the maximum removal rate of 100 % and 94.15 % corresponding to SDZ and chemical oxygen demand (COD) were achieved under the optimum Ca(OH)₂ addition without pH adjustment. In addition, the presence of Cl^- and humic acid inhibited the SDZ degradation to some extent. It could be found that Ca²⁺ and OH^- jointly played great roles in increased oxidation of ozonation. For one thing, quenching effect of CO₃^2- on •OH was eliminated through combination of Ca²⁺ and CO₃^2- and low molecular intermediate forming insoluble precipitates. For another, OH^- caused alkaline condition which was instrumental in existence of •OH. Moreover, successive addition of Ca(OH)₂ was proposed and proved to be feasible to elevate the •OH generation and oxidizing ability in O₃/Ca(OH)₂ system. Ultimately, the three potential degradation pathways of SDZ were speculated basing on density function theory (DFT) calculations and identification of several intermediate products. Overall, this study provided promising insights for promotion of antibiotics removal and ozonation efficiency by alleviating the inhibitory effect of carbonate ions in aqueous environment.