Abstract:
As the world population grows, factories such as textile, paper, food colouring, and leather industries are using large amounts of dyes. Small amounts of dyes can pollute large sources of water used for domestic consumption and agricultural sectors. If the dyes are not treated before being discharged, they can cause harm to the human body and the environment. Various approaches such as chemical, physical, and biological techniques, have been studied in connection with the removal of dyes from wastewater. The adsorption method has been favored in the removal of dyes since it is eco-friendly, low-cost, has high efficiency, and is easy to operate. In this research, geopolymers synthesized from fly ash (FA) from the Morupule thermal power station were used in the adsorption of cationic and anionic dyes which are pollutants found in wastewater. Fly ash is a fine waste product produced by burning coal and its high in silica and alumina. The main aim of this study was to synthesize fly ash-based geopolymers which were employed in the removal of cationic and anionic dyes which have serious environmental consequences such as pollution of water and thus are a great health hazard. Four geopolymers were synthesized using Na2SiO3 and H2O2 which were then studied for the adsorption of methylene blue (cationic dye) and Congo red dyes (anionic dye). Factors that affect the adsorption capacity and removal efficiencies such as time, concentration, dosage, and pH were investigated to find the most active geopolymers. The morphology, crystallography, and identification of major chemical components of fly ash were performed by scanning electron microscope (SEM), X-ray diffractometer (XRD), and X-ray fluorescence spectrometer (XRF), respectively. The Uv-visible spectroscopy was used to monitor the adsorption process while thermogravimetric analysis (TGA) techniques were used to study the thermal degradation of fly ash. The Brunauer-Emmett Teller (BET) analysis was used to calculate pore size distributions, pore volume, and surface area. Fourier transform infrared (FTIR) spectra were obtained and showed the presence of OH, Si-O-Si and S-O-Al bands. Four prepared geopolymers (12-M-Na2SiO3, 10-M-Na2SiO3, 12-M-H2O2 and 10-M-H2O2) and raw fly ash at the mass of 3 g adsorbents and 0.3 L of dyes were studied to determine the adsorption isotherms and adsorption kinetic models. The Langmuir isotherm, and pseudo-second-order model were favoured as they show high correlation coefficient values, R2 , and the calculated Qm values are close to experimental values. The maximum adsorption capacity of methylene XI blue (MB) using Na2SiO3-geopolymer was 17.7 mg/g which was nearly identical to the experimental value (16.3 mg/g), while Congo red (CR) achieved 3.8 mg/g using Na2SiO3- geopolymer compared to 4 mg/g experimental value. The highest removal efficiency of MB using Na2SiO3-geopolymer was 90 % and 84 % using H2O2-geopolymer. The Qe of CR based on both geopolymers displayed a low removal efficiency, but high removal efficiency using raw FA (94 %).
