Background and Objective: Lignin is a waste by-product from pulp and paper (P.P) industries and the main reason of the remaining colors in pulp and paper industries effluents. Hence, lignin should be eliminated from wastewater before being discharged to the environment, but according to its structure and properties, it is very difficult to degrade by common biological process. So, P.P industries strongly need to supplementary treatment as a powerful tool to achieve the stringent environmental regulations. Materials and Methods: In present study, the photodegradation of lignin was studied using TiO2 under UV irradiation as a supplementary treatment for pulp and paper industries. The 3-level, 3-factor Box-Behnken design and response surface methodology were applied to study the effects of main operating variables and optimization circumstances. Moreover, a quadratic model for lignin degradation was suggested. Results: The predicted model had R2 and R2adj correlation coefficients of 0.97 and 0.99 respectively, which showed goodness of fit between the experimental results and predicted values. The ANOVA results indicated that at 95% confidence level all three variables significantly affected the lignin degradation. Furthermore, the most effective variable was reaction time. Under the optimal conditions (T=115.66, TiO2=100g/L and lignin=50 mg/L) the lignin degradation efficiency was 88.24%. The results also demonstrated that the kinetic of photodegradation of lignin can be explained through the pseudo-first-order model. Conclusions: This study revealed that the Box-Behnken design was applicable and suitable model to optimize the experiments for lignin degradation and it can be used to guide and conduct the full scales studies about lignin degradation. The obtained results demonstrated that photocatalytic process using TiO2 can be used as an effective supplementary treatment for lignin degradation by biological processes.
