Purification of pesticide-rich wastewater 2,6-dichlorophenol in solar photo-oxidation reactor with visible light

Abstract

2,6-Dichlorophenol (2,6-DCP), a compound used for synthesizing medical and pharmaceutical agents, is highly toxic and potentially carcinogenic. Advanced oxidation has been used for the cleavage of strong bonds in 2,6-DCP, giving a reliable and practical option compared with the conventional wastewater treatment systems. Hence, this study focused on the application of a solar photo-oxidation process to treat herbicides wastewater containing 2,6-DCP. Bismuth chloride was combined with biochar to maintain the dual benefits of 2,6-DCP adsorption and degradation concomitantly. The O–Bi–O and Bi–O bonds were successfully detected in the synthesized Bi12O17Cl2, whereas the synergetic effect between Bi12O17Cl2 and biochar was validated by increasing the carbon-based functional groups, reducing particle size (14–32 nm), and creating more active sites of the composite. The initial 2,6-DCP concentration of 122 mg/L was reduced to 17 mg/L (86.0% removal efficiency) within 6 h of photodegradation at a pH of 2.3 and a catalyst dosage of 100 mg/L. Concurrently, the phenol concentration in the aqueous solution increased from 4.7 to 32.4 mg/L, respectively. Reusing the photocatalytic reactions three times by the biochar/Bi12O17Cl2 composite reduced the 2,6-DCP removal efficiency by only 7.7%. Biochar exhibited an adsorption energy of -14.8 kJ/mol to capture 2,6-DCP, and it accumulated photo-excited electrons that react with adsorbed oxygen and/or dissolved oxygen to generate super oxygen radicals (•O2−). Super hydroxyl radicals (•OH) could also be generated from the positive holes (h+) in the valence band of Bi12O17Cl2. These radicals could cleave C–Cl bonds of 2,6-DCP, generating Cl− ions, dechlorinated product 2-CP, and phenols. The treatment of wastewater containing 2,6-DCP by the proposed biochar-based photocatalyst could cost 10.3 USD/m3.