Abstract
Water is a universal solvent essential for sustaining life and everyday use, and it is necessary for many agricultural activities and industrial processes. The ability to access this clean quality water freely (Water security) has unfortunately become a challenge in recent years due to water scarcity issues being experienced globally. The Pollution of freshwater sources with various organic and inorganic substances is the leading cause of this issue. These pollutants result from improper disposal of municipal, agricultural, and industrial waste and surface runoff. Due to certain characteristics exhibited by these pollutants, such as bioaccumulation, persistence, non-biodegradability, and more, Wastewater Treatment Plants, which employ traditional removal methods, have not always proved effective. Given the toxic effects of these pollutants on human and animal health and their negative environmental impact, it is essential to develop alternative treatment methods to overcome water scarcity issues. Developing advanced technological methods is one way to approach this, as these technologies employ nanoparticles that enhance sensitivity and removal. Adsorption is the most used method due to its efficiency over a wide variety of pollutants, ease of operation, cost-effectiveness, and the adsorbent material can be synthesised from a variety of materials and, as such, was chosen to be investigated in this study. This study aimed to test the adsorptive efficiencies of Fe3O4-ZnO and TiO2/AC/Fe3O4 on the removal of heavy metal(oid)s (As(III), Cd(III) and Pb(III)) and organic pollutants (phenol, aldicarb, carbofuran and carbaryl) respectively, to improve water quality.
To achieve this, two experimental chapters are presented, which show the application of Fe3O4-ZnO for the adsorptive removal of heavy metal(oid)s (As(III), Cd(III) and Pb(III)) and the adsorptive removal of organic pollutants using a TiO2/AC/Fe3O4 nanocomposite. The Fe3O4-ZnO was prepare using a coprecipitation method before being characterised using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), nitrogen adsorption-desorption and zeta potential. The characteristic FTIR fingerprint of the nanocomposite showed the characteristic peaks of Fe-O and Zn-O at the fingerprint region (400-610 cm-1) and Fe-OH and Zn-OH bond vibration modes at 1093 cm-1 and 1098 cm-1, respectively. Furthermore, the structural and X-ray patterns of the prepared material were compared to other studies, which showed comparable results to confirm successful incorporation. The material was then used to optimise an adsorptive method for the analytes using a central composite showing the optimum conditions to be 16 mg of adsorbent dosage, at pH of 8 and
v
sample volume of 100 mL determined at an initial analyte concentration of 10 mg/L. After which, the adsorption data was plotted onto adsorption and kinetic models which showed that the adsorption process could be explained by Langmuir adsorption for Cd(II) and Freundlich for As(III) and Pb(II), The adsorption process was found to be a chemisorption type with maximum adsorption capacities of 36.25 mg/g for As (III), 33,67 mg/g for Cd(II) and 38,50 mg/g for Pb(II).
For the adsorption of organic pollutants, a TiO2/AC/Fe3O4 was prepared by first preparing activated carbon (AC) from coal refuse using pyrolysis. The nanocomposite was then prepared using coprecipitation before characterising the resultant material using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), nitrogen adsorption-desorption and zeta potential. The central composite with response surface was used to optimise the influential factors for the adsorption of phenol, aldicarb, carbofuran and carbaryl from aqueous solutions onto the material. The adsorption process was influenced by the solution pH due to the analyte pKa values. The adsorption data at equilibrium was fitted to adsorption isotherm models (Langmuir and Freundlich) in which the Freundlich isotherm described phenol and carbaryl onto TiO2/AC/Fe3O4 adsorbent. In contrast, the Langmuir isotherm described the adsorption of aldicarb and carbofuran on the surface of TiO2/AC/Fe3O4 adsorbent. Furthermore, adsorption kinetic models were used to investigate the rate-limiting step, showing that the data could be explained by a pseudo-second order model with capacities of 6.13, 5.63, 5.84 and 9.33 mg/g for phenol, aldicarb, carbofuran and carbaryl, respectively
In conclusion, cost-effective and efficient nanoadsorbents were successfully synthesised, and it was confirmed through characterisation that these adsorbents were comparable to those recorded in literature through FTIR and XRD. The synthesised adsorbents were also porous with surface areas of 44 m2/g for Fe3O4-ZnO and 96 m2/g for TiO2/AC/Fe3O4, effective for chemical adsorption of organic analytes and metal(oid)s, respectively. The adsorption capacity of Fe3O4-ZnO against As(III) reached 135 mg/g for Pb(II), it was 144 mg/g for Cd(II) and 223 mg/g for Pb(II). The complete removal of aldicarb and carbofuran was achieved at optimal conditions, and 80% removal was achieved for phenol using TiO2/AC/Fe3O4 adsorbent. Future work may include application to real water samples and reusability studies.