Optimization of photocatalytic decolorization of lignin in water matrix using immobilized Nano-Ti02 catalyst by response surface methodology

Lignin is contained in pulp mill effluent and is responsible for the color of the wastewater. In this study, photocatalysis using immobilized nano-titania catalyst has been proven to be an effective process for the decolorization of lignin in water. The use of photocatalysis for the removal of lignin in wastewater can help the recycling paper mills in the Philippines comply with the DENR effluent standard for color. In order to increase the efficiency of the photocatalytic process and bring about the successful decolorization of lignin in water, there is a need to ascertain the optimum conditions of effective process parameters by means of employing a reliable optimization technique. Response Surface Methodology (RSM) is a well known efficient optimization technique that was applied in this study. This study employed the immobilized nano-TiO2 film on microscopy glass plates which was synthesized via sol-gel method. Immobilization is gaining importance in wastewater treatment area since in the large-scale application, the suspended photocatalyst powder needs to be separated from the treated wastewater before discharge which could be a time-consuming and expensive process. In this research, the use of sol-gel technique led to a production of reproducible nano-titania film with capability of adherence on glass substrates. The films were deposited on microscopy glass plates using 1, 2 and 3 coating cycles. The photocatalysts were subjected to surface characterization for elemental composition, morphology, crystal structure and size, surface area measurement, and purity. The activities of nano-titania films were investigated by photocatalytic decolorization of lignin in water matrix. Optimization of process parameters by Box- Behnken design of experiment was conducted afterwards. The optimum process parameters were applied to actual lignin-containing wastewater from the partner recycling paper mill. Afterwards, post characterization of the catalyst that was utilized in actual wastewater was done. SEM images of the surface of nano-titania films showed the good coating on the plate after repeating the deposition two times. EDX analysis confirmed that the immobilized catalysts were almost free from contaminants. Pure anatase structure of the immobilized TiO2 nanoparticles was revealed by XRD analysis. The crystallite sizes were approximated using the high-resolution TEM images of the samples and were measured to be 11.29, 14.33 and 18.54 nm for 1, 2 and 3 coatings of deposited nano-titania catalysts respectively. The findings obtained from TEM characterization were in good agreement with the results obtained from XRD wherein the crystallite sizes were computed based on Scherrer’s equation. The crystallite sizes were calculated to be 13nm, 15nm, and 21nm for 1, 2 and 3 coating cycles respectively. By BET method, the surface areas of the synthesized photocatalysts were found to be 127.5 m2/g, 109.5m2/g, and 93.79m2/g for 1, 2, and 3 coatings respectively. TGA analysis confirmed the purity of the immobilized catalysts. Good agreement of all the characterizations was achieved and showed that the crystallite sizes were found to have an increasing trend as the coating cycle was increased. The reduction in surface area of the nano-TiO2 samples is proportional to the increase in crystallite size due to longer heat treatment in higher coating cycles. The application of 2 coating cycles of immobilized nano-titania catalyst on microscopy glass plates appeared to be sufficient in saturating the surface of the substrate. The well coated substrates attained after coating the glass plates two times yield the highest photocatalytic activity for the decolorization of lignin in water.The fitted mathematical model obtained after the optimization of the process parameters is Photocatalytic Efficiency(Y) = 0.10088 – (0.018055 pH) + (4.74380 x10-4 concentration) + (2.78166 x 10-4 O2 flowrate). Considering the results of ANOVA, the fitted linear model is highly significant (with a p-value of \u3c0.0001) and adequate to represent the actual relationship between the parameters and the response. Non-significant lack of fit also proved that the model fit the experimental data. At 95% level of confidence, the initial pH (X1), initial lignin concentration (X2), and oxygen flowrate (X3) are found to be significant model terms to the response. The optimum values of the parameters to attain the highest photocatalytic efficiency (0.116 mg/L/mgcat-hr) for the decolorization of lignin in water are 3 for initial pH (X1), 100ppm for initial lignin concentration (X2), and 80mL/min for oxygen flowrate (X3). When the optimum values of initial pH and O2 flowrate were applied to 70.34 ppm of lignin in actual wastewater from the partner recycling paper mill, the values of the experimental (0.099 mg/L/mgcat-hr) and predicted (0.102 mg/L/mgcat-hr) response were in good agreement. Furthermore, the treated wastewater’s final color was reduced to 62 PCU which would already comply with the DENR limit of 100 PCU as stipulated in DAO 35 for A, B, and SB water body classification for New/Proposed Industry (NPI). Pseudo first-order kinetic reaction was fitted to the experimental data gathered in each experimental run. The pseudo first-order reaction rate constants are 0.002min-1, 0.001min-1 and 0.0004min-1 for pH 3, 5 and 7 respectively. Post characterization of the catalyst that was utilized in actual wastewater was performed. Since there was no apparent change in the characteristics, it was proven that the immobilized catalyst was stable after subjecting it to the photocatalytic treatment of actual wastewater