Coagulation Behavior and Floc Properties of Dosing Different Alkaline Neutralizers into the Fenton Oxidation Effluent

Neutralization is the necessary operation to ensure the Fenton effluent pH. In situ coagulation can be induced during neutralization. In this study, three types of alkaline neutralizers (Ca(OH)2, NaOH, and Ca(OH)2 + NaOH) were added into the Fenton oxidized PSE to control the effluent pH of 6 to 9. The coagulation behavior, floc structure, and properties were investigated. The results indicated that the coagulation with the adding of three neutralizers can remove 9.68 to 24.02% of the TOC. Ca(OH)2 exhibited the highest TOC removal efficiency at the dosage of 0.4 g/L. Charge neutralization ability was in the following order: Ca(OH)2 > Ca(OH)2 + NaOH > NaOH. Ca(OH)2 and Ca(OH)2 + NaOH showed the increase of floc growth rate with the increase of agent dosage, especially for Ca(OH)2 + NaOH. Moreover, Df of NaOH flocs was higher than that of Ca(OH)2 and Ca(OH)2 + NaOH, indicating the floc formed by NaOH was more compact than that of Ca(OH)2. The main coagulation process of three neutralizers was different, and it was also affected by the agent dosage (or pH). When the dosage was 0.35 g/L (pH 6–7.5), the complexation, adsorption, and bridging were the predominant processes while charge neutralization gradually became the main coagulation process for Ca(OH)2 and Ca(OH)2 + NaOH with the increase of dosage (pH 7.5–9).