Browsing by Author "Khutlane, Joyce Tsepiso"
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- ItemApplications of superparamagnetic nanoparticles for the separation and recovery of PGM metals from acidic wastewater solutions(Stellenbosch : Stellenbosch University, 2018-12) Khutlane, Joyce Tsepiso; Malgas-Enus, Rehana; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: South Africa is one of the leading countries in the production of precious metals (Au and PGMs). The refinery final stages for recovering precious metals makes use of two hydrometallurgical processes: solvent extraction and extraction with ion exchangers. Even though both methods improved the recovery of these metals from the ore, they still suffer from setbacks ranging from large amounts of secondary waste solutions, expensive solvents and resins, and filtration in terms of the use of ion exchange resins for extraction. Therefore, a need for the development of simple and environmentally friendly refining processes is required to minimize the recurrence of the abovementioned setbacks. Naked magnetic iron oxide nanoparticles (MIONs) are promising materials for adsorption studies of metal ions/complexes from wastewater due to their versatile magnetic properties, which allow a facile remote control, separation and analyte recovery from solution. This work makes use of magnetite nanoparticles (MIONs) which are superparamagnetic and less expensive compared to adsorbents used by the South African refineries for extraction of Au(III)-Cl, Pd(II)-Cl and Pt(IV)-Cl complexes from acidic aqueous solutions. Synthesis of naked MIONs was carried out by using the chemical co-precipitation method. Naked MIONs successfully sorbed Au(III), Pd(II) and Pt(IV) complexes from acidic aqueous solutions, showing a higher adsorption affinity for Au(III)-Cl species compared to Pd(II) and Pt(IV) species. The adsorption kinetics of the three metal complexes using naked MIONs followed a pseudo-second-order kinetic mode, indicating that the chemical adsorption was the rate-limiting step. The equilibrium adsorption of Au(III)-Cl species onto naked MIONs at pH 1.0, pH 3.0 and pH 5.0, was fitted with Langmuir adsorption isotherms. It was found that the experimental data was in reasonably good agreement with the Langmuir model, suggesting that Au forms a monolayer coverage on the surface of naked MIONs. The adsorption capacities were as follows: 10.44 mg.g-1, 18.98 mg.g-1, and 27.25 mg.g-1 respectively. The proposed mechanism responsible for adsorption of Au(III), Pd(II) and Pt(IV) complexes onto naked MIONs was governed by the electrostatic attractions and metal reduction. These naked MIONs were found to be unstable at pH1, which is usually the conditions used for extraction in the mining industry. To circumvent this problem, the surface of MIONs was functionalised with aliphatic carboxylic acids, dendrimer micelles and then with both carboxylic acids and dendrimer micelles. Characterisation of naked MIONs and functionalised MIONs was carried out by IR, PXRD, FESEM, HRTEM and ICP-OES. Competitive adsorption of Au(III), Pd(II), and Pt(IV) complexes onto naked MIONs and functionalised MIONs was explored by varying solution pH and the contact time. The dendrimer micelles played a vital role in adsorption of Pd(II) and Pt(IV) complexes. Adsorption kinetics followed a pseudo-second-order kinetics model. The adsorption isotherms all obeyed the Langmuir model in the case of Au(III), Pd(II), and Pt(IV) complexes by naked MIONs and, adsorption isotherm for Au(III) and Pd(II) complexes using modified MIONs obeyed Langmuir, while adsorption of Pt(IV) species followed the Freundlich model. The desorption studies showed that the best desorption reagents were 1.0 M HNO3 and 1.0 M HNO3/0.5 M thiourea. The modified MIONs were stable at low pH, even at pH1, and the extraction potential of the modified MIONs were comparable to that of the naked MIONs.