Development of supervibration-photocatalytic reactor for treatment of lignin and 2,4-dichlorophenol in pulp and paper mill wastewater

Abstract

The photocatalytic degradation of lignin and 2,4-DCP in wastewater was studied using a newly developed supervibration-photocatalytic reactor based on photocatalytic process combined with a supervibration agitator. The effects of operating parameters as initial pH, UV intensity, vibration frequency and initial concentration on treatment of lignin and 2,4-DCP were investigated, focusing on removal efficiency to determine the optimum treatment conditions. This work also studied kinetic reaction for degradation of lignin and 2,4-DCP and extended to identify by-products of lignin and 2,4-DCP degradation. Wastewater samples used in the experiments consisted of lignin synthetic wastewater, 2,4-DCP synthetic wastewater, mixed synthetic wastewater containing lignin and 2,4-DCP and real wastewater from pulp and paper mill. From the results obtained, removal efficiencies of lignin and 2,4-DCP in all types of wastewater were similar pattern. High UV intensity and vibration frequency in a low initial pH and initial concentration increased removal efficiencies of lignin and 2,4-DCP. The optimum operating conditions of a supervibration-photocatalytic reactor for treatment of lignin and 2,4-DCP were found to be initial pH 5, UV intensity of 25.2 mW/cm² and vibration frequency of 50 Hz. For lignin synthetic wastewater, under the optimum treatment conditions, removal efficiency of lignin (100 mg/l) and color were 85.12% and 70.55%, respectively within 420 min. The apparent rate constant of first order reaction (Kap) and the initial degradation rate (r₀) for lignin degradation were 13.9 x 10-3 min⁻¹ and 1.39 mg/l min-1, respectively, after 120 min. The analysis of residue by GC-MS showed the presence of some lignin derivatives such as vanillin. In the case of 2,4-DCP synthetic wastewater, under the optimum operating conditions, 2,4-DCP degradation (0.5 mg/l) was completed within 60 min. The kap and r0 were 56.3 x 10⁻³ min-1 and 28.2 x 10⁻³ mg/l min⁻¹, respectively, after 30 min. The by-products detected by GC-MS were phenol, 2-chlorophenol and 4-chlorophenol. The degradation pathway of 2,4-DCP was also proposed. For mixed synthetic wastewater containing lignin of 400 mg/l and 2,4-DCP of 5 mg/l, the reactor could simultaneously remove lignin, color and 2,4-DCP under the optimum conditions with the efficiencies of 76.21%, 60.75% and 97.13%, respectively, after 420 min. Furthermore, removal efficiencies of lignin, color and 2,4-DCP in pulp and paper mill wastewater under the optimum conditions were 75.11%, 52.16% and 94.14%, respectively in the same time.