Studies in Spinel Catalysts for Photocatalytic Degradation of RB21 Dye in Wastewater

Abstract

The present study aimed at colour removal and COD degradation of synthetic wastewater of RB21 dye via photocatalysis using various spinel catalysts prepared by different methods. The reactive turquoise blue (RB21) dye with strong azo bond and Cu2+ metal complex belonging to phthalocynine group is a very stable compound and difficult to degrade. The reactive dyes are widely used in textile, paper and leather industries. The present research study is essentially related to the degradation of selected RB21 dye from synthetic wastewater however it has been extended to actual industrial effluents. In order to control wastewater pollution due to dyes the UV-Photocatalytic degradation technology has been carried out by some researchers using spinel catalysts. Spinel catalysts are oxides with general formula AB2O4 where A and B are the rare earth, alkaline earth, alkali metals and transition metal cations which are expected to be able to overcome the limitations of semiconductors as photocatalysts. In the present work, emphasis has been given to develop nickel, cobalt, zinc based ferric spinel catalysts for the RB21 dye degradation. Also other spinel based substituted, doped and semiconductors were prepared for the comparison purpose. The preparation methods used for spinel catalysts were citrate complexation, co-precipitation, sol-gel and reactive grinding out of which various operating conditions of reactive grinding method using high energy planetary ball mill were optimized to synthesize pure crystalline spinel. Detailed characterization of the prepared fresh and used spinel catalysts was carried out. In order to improve the catalytic activity for RB21 dye degradation the detail study of photocatalytic reactor operating conditions were optimized for the different light sources. Based on this study, most efficient phtotocatalytic reactor was selected and optimization of catalyst dosage, pH, dye concentration, temperature of reactor, time required for the sample in dark, irradiation time, time for the withdrawing of samples rpm of stirrer etc was carried out. Finally efforts were made to identify the intermediate and final compounds formed upon degradation of RB21 dye. Spinels are mainly synthesized by ceramic and wet chemical process. These process produce low specific surface area of spinel catalyst, they are complicated and are relatively expensive to carry out. To overcome the limitations of above conventional methods used for the synthesis of spinel an improved high energy planetary ball milling for reactive grinding has been designed and used in the present study. The flexibility in varying rotation to revolution speed ratio, direction of jar, revolution of disk and other operating parameters can be adjusted which give large specific impact energy to the catalyst to form pure phase spinel without heat treatment. This reactive grinding with improved high energy planetary ball mill used to synthesize spinel in an efficient, simple and economical manner than conventional methods. In the present work spinel catalysts (NiFe2O4, ZnFe2O4, CoFe2O4, CuFe2O4, NiFe2O4/TiO2, NiFe2O4/ZnO, Zn0.8Ni0.2Fe2O4, Zn0.2Ni0.8Fe2O4, Zn0.4Ni0.6Fe2O4) were prepared by different methods. For the comparison study semiconductor TiO2 and ZnO were also used to degrade the dye. Screening study of different catalysts revealed RG3- NiFe2O4 spinel catalyst prepared by reactive grinding method as an efficient spinel among all others. The detailed operating parameters of planetary ball mill affecting synthesis of spinel were studied. The parameters like basic disc sun wheel direction and revolution speed, grinding of jars direction, rotation of speed ball to powder weight ratio used, varying milling time, extent of jars to be filled, heat and without heat treatment, were all optimized for NiFe2O4 screened catalyst. The optimized operating parameters are; jar speed: 350 rpm (clockwise), sun wheel speed: 200 rpm (anti-clockwise), ball to powder weight ratio: 15:1, milling time: 11h without heat treatment. Initially the experiments were performed under varying light sources like 600 Watt microwave oven, home-made 66 Watt 3 lamp, natural sunlight with irradiation time: 02:00 to 04:00 pm with 48 °C temperature in Ahmedabad city in the month of May, with intensity of 900 W/m2, UV-Light photocatalytic reactor made of low pressure (250nm) intensity 63.49 watt/inch and high pressure (450nm) intensity 114.299 W/inch. Among all different light sources high pressure UV-lamp irradiation exhibited promising results so UV-irradiation based photocatlaytic reactor was designed and fabricated for the detailed study. Investigation of suitability of spinel catalysts for photocatalytic dye degradation of RB21 was studied in order to screen suitable catalyst preparation method and type of spinel. The photocatalytic activity was carried out using light source (high pressure UVlight), stirring the sample (650 rpm), requirement of sample (500 mL), initial concentration of dye (50 g/L), pH (7), catalyst dosage (1.6 g/L), irradiation time (240min), temperature of the sample (35-40 °C), time interval for the collection of the samples (30 min dark followed by 60, 90, 120, 150, 180, 210, 240 min irradiation). Among all the catalysts the best screened spinel catalyst was RG3-NiFe2O4 prepared by reactive grinding method without heat treatment. The detailed characterization of catalyst and analysis of wastewater sample before and after treatment was performed using appropriate techniques like thermo gravimetric analysis (TGA), x-ray diffraction (XRD), surface area analysis (BET), electron microscopy (TEM), scanning electron microscope (SEM), X-ray energy dispersive spectrometry (EDX), vibrating sample analysis (VSM), Fourier transform infrared spectroscopy spectra (G-FTIR), liquid chromatogram with mass analysis (LC-MS), Band-gap analysis (UV-visible NIR), chemical oxygen demand (COD), UV-visible spectrophotometer. XRD results confirmed that NiFe2O4 11h-RG3 catalyst was in pure phase without any unconverted oxides or impurities with crystal size 21.10 nm comparable with TEM particle size 20 nm. The EDX result also conform only Ni and Fe elements without any other contamination. FTIR at 510 cm-1 metal tetrahedral site and 338 cm-1 metal octahedral site with stretching of vibrations proved formation of NiFe2O4 spinel. The RG3-NiFe2O4 spinel catalyst prepared by reactive grinding method milled for 11 h (without heat treatment) exhibited narrow band-gap 1.59 eV to degrade RB21 dye with 85% COD removal and 99% color removal in 240 min in UV-lamp photocatalytic reactor. It also exhibited promising results for actual industrial dye wastewater treatment. The magnetic property of RG3-NiFe2O4 was 22.22 emu/g due to which it was quite easy to separate it from the treated wastewater solution with the help of external magnetic source and could be reused for next cycle without any treatment. The kinetic study of NiFe2O4 11h-RG3 catalyst follows pseudo first order with R2=0.9833, k=0.0092 min-1.