Applications of superparamagnetic nanoparticles for the separation and recovery of PGM metals from acidic wastewater solutions

Khutlane, Joyce Tsepiso (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Suid-Afrika is een van die voorste lande in die vervaardiging van edelmetale (Au en PGM's). Die finale stappe van die raffinadery vir die herstel van edelmetale, maak gebruik van twee hidrometallurgiese prosesse: oplosmiddel ekstraksie en ekstraksie met ioonuitruilers. Alhoewel beide metodes die herstel van hierdie metale van die erts verbeter het, ly hulle steeds aan terugslae wat wissel van groot hoeveelhede sekondêre afvaloplossings, duur oplosmiddels en harse, en filtrasie in terme van die gebruik van ioonruilharse vir ekstraksie. Daarom is 'n behoefte aan die ontwikkeling van eenvoudige en omgewingsvriendelike verfyningsprosesse nodig om die herhaling van bogenoemde terugslae te verminder. Naakte magnetiese ysteroksiednanodeeltjies (MION's) is ‘n belowende materiaal vir adsorpsiestudies van metaalione/komplekse uit afvalwater as gevolg van hul veelsydige magnetiese eienskappe, wat 'n maklike afstandbeheer, skeiding en analietherstel van oplossing moontlik maak. Hierdie werk maak dus gebruik van magnetietnanodeeltjies (MION's) wat superparamagneties en goedkoper is in vergelyking met adsorbente wat deur die Suid-Afrikaanse raffinaderye gebruik word as ekstraktante vir Au (III)-Cl, Pd(II)-Cl en Pt(IV)]-Cl-komplekse uit suurwaterige oplossings. Sintese van naakte MION's is uitgevoer met behulp van die chemiese ko-presipitasie metode. Naakte MIONs het die Au(III), Pd(II) en Pt(IV) komplekse suksesvol ekstrak van suurwaterige oplossings, die Au(III)Cl spesies toon ‘n hoër absorpsie affiniteit as vir Pd(II) en Pt(IV) spesies. Die adsorpsiekinetika van die drie metaalkomplekse wat naakte MION's gebruik het, volg ‘n pseudo-tweede-orde kinetiese modus, wat aandui dat die chemiese adsorpsie die tempobeperkende stap was. Die ewewigsabsorpsie van die Au(III)-Cl spesies op naakte MION's by pH 1.0, pH 3.0 en pH 5.0, was aangepas met Langmuir-adsorpsie-isoterme. Daar is bevind dat die eksperimentele data in 'n redelike goeie ooreenkoms met die Langmuir-model was, wat monolaag dekking van Au op die oppervlak van naakte MIONs voorgestel het. Die adsorpsiekapasiteite was soos volg: 10.44 mg.g-1, 18.98 mg.g-1 en 27.25 mg.g-1 onderskeidelik. Die voorgestelde meganisme wat verantwoordelik is vir die adsorpsie van Au(III), Pd(II) en Pt(IV) komplekse op naakte MION's, is beheer deur die elektrostatiese aantreklikhede en metaalreduksie. Hierdie naakte MIONs was onstabiel by pH1, wat gewoonlik die kondisie is wat gebruik word in die mynbedryf. Om hierdie probleem te omseil, is die oppervlak van MION's gefunksionaliseer met alifatiese karboksielsure, dendrimeermiselle en dan met beide karboksielsure en dendrimeermiselle. Karakterisering van naakte MION's en gefunksionaliseerde MION's is uitgevoer deur IR, PXRD, FESEM, HRTEM en ICP-OES. Kompeterende adsorpsie van Au(III), Pd(II) en Pt(IV) komplekse op naakte MION's en gefunksionaliseerde MION's is ondersoek deur verskillende oplossing pH en die kontaktyd. Die dendrimeermiselle het 'n belangrike rol gespeel in die adsorpsie van Pd(II) en Pt(IV) komplekse. Adsorbsiekinetika volg die pseudo-tweede-orde kinetika model. Die adsorpsie-isoterme was gehoorsaam aan die Langmuir-model in die geval van Au(III), Pd(II) en Pt(IV) komplekse deur naakte MION's en adsorptie isoterm vir Au(III) en Pd(II) komplekse deur gefunksionaliseerde MIONs volg Langmuir, terwyl adsorpsie van Pt(IV) spesies die Freundlich-model gevolg het. Die desorptiestudies het getoon dat die beste desorpsie-reagense 1,0 M HNO3 en 1.0 M HNO3 / 0.5 M tiokoors was. Die gefunksionaliseerde MIONs was stabiel teen lae pH en selfs teen pH1, terwyl die ekstraksie potensiaal vergelykbaar was met dit van die naakte MIONs.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105139
This item appears in the following collections: