SYNTHESIS AND CHARACTERIZATION OF SILVER OXIDE/TITANIUM (IV) OXIDE SUPPORTED ON KAOLIN NANO-FILTER FOR TREATMENT OF MINING WASTEWATER

Abstract

This study has developed a nanocomposite filter from kaolin base material with Ag2O, and TiO2 nanoparticles, for the treatment of mining wastewater. The kaolin clay was beneficiated using a sedimentation technique for 24 h to obtain slurry which was sundried, pulverized and sieved with a <150 μm mesh size. The beneficiated clay was activated with 98% H2SO4 for 2 h and was then washed, filtered, and oven-dried at 100oC for 2 h to obtain acid-activated clay (AAC). Green syntheses of Ag2O and TiO2 nanoparticles were investigated by Parkia biglobossa leaves extract using bulk silver trioxonitrate (V) and titanium isopropoxide solution respectively. The nanoparticles of Ag2O and TiO2 with AAC were incorporated in different proportions onto each other to obtain different adsorbents of Ag2O-TiO2, Ag2O-clay, TiO2-clay, and Ag2O-TiO2-clay nanocomposites. Other adsorbents that were also considered for the mine wastewater treatment are TiO2 nanoparticles, beneficiated clay and AAC. Each of the adsorbents was characterised for its functional groups, optical band, morphology/elemental composition, disparity, phase identification, surface area, and oxidation state. The mining wastewater was collected from an abandoned mine site in Chanchaga Local Government, Minna, Niger state. The wastewater was analysed for its metal ions composition and was found to have a high concentration of heavy metals such as Mn (II) (4.29 mg/L), Fe (III) (20.01mg/L), Pb (II) (0.98 mg/L), and Cu (II) ions (0.31 mg/L). Other physicochemical characteristics present in the wastewater are turbidity, low pH, TDS, colour, microbes, electrical conductivity, sulphate, BOD, and COD which are above the WHO/NIS permissible limit for drinkable water. The adsorbents were tested for the removal of the heavy metals by investigating the effects of time (20-180 min), adsorbent dosage (0.1-1.0 g), and temperature (30-70 ̊C) on the batch adsorption process. Various models of kinetics, isotherms, and thermodynamic parameters of the best adsorbent from the study were evaluated. The adsorbent with the highest percentage removal of all the metal ions was built into a water filter which was composed of 60% clay, 40% sawdust, and 0.042/2.46 mg.g-1 of Ag2O/TiO2 nanoparticles respectively. The results obtained revealed improved crystallite size of the clay from 16.65 nm to 14.92 nm and 8.08 nm for beneficiated and activated clay respectively. The nano-synthesis of TiO2 by Parkia biglobossa leaves extract yielded rutile TiO2 with 3.45 nm crystallite size. The immobilised Ag2O/TiO2 on the AAC developed as adsorbents revealed the presence of hydroxyl groups at 3697.62 cm-1, Si-O-Si band at 1000.68 cm-1, TiO2 band at 869.92 cm-1 and Ag2O band at 461.29 cm-1. The optical bands of the adsorbents confirmed the presence of Ag2O and TiO2 doping on the AAC with Ag and Ti in the +1 and +4 oxidation states respectively. The morphology of Ag2O-TiO2-clay adsorbents and nanocomposite revealed well-intercalated flat plates structure, with hemispherical edges and porosities that encourage maximum adsorption of heavy metals and other pollutants.The batch adsorption process justified the characterisation of the Ag2O/TiO2-clay nanocomposite as the most effective, among the adsorbents with percentage removal of 96.59, 99.66, 40.26 and 96.12 for Mn (II), Fe (III), Pb (II) and Cu (II) respectively. From among the isotherm models studied, Jovanovic was the best-fitted experimental data as such, the surface adsorption was homogeneous with mono-layer coverage. Also, the pseudo-second-order was the most suitable kinetic model for the experimental data, confirming multiple adsorption systems. The thermodynamic parameters justified that the adsorption process was endothermic (+ΔH◦), spontaneous (-ΔS◦), and feasible (-ΔG◦). The fabricated filter showed a better performance than the adsorbents with percentage removal of 99.5, 100, 100, and 100 for Mn (II), Fe (III), Pb (II), and Cu (II) respectively. Therefore, Ag2O/TiO2-clay nano-filter is suitable and efficient for the sequestration of pollutants from the mining wastewater.