Evaluation of waste tyre-derived char and crumb as adsorbents for gold recovery from acidic solutions

Maapola, Tshepisho Thabiso Praise (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Each year the number of vehicles is growing worldwide; therefore, the number of tyres is increasing at a faster rate daily. Tyres do not present a risk to the health on their own, the inappropriate disposal and mismanagement of large amounts can cause serious problems not only to the environment but also to human beings. Waste tyre management is therefore very important. Several ways of waste tyres management and valorisation have been attempted; of which pyrolysis is one of the most attractive technique. Pyrolysis of waste tyres produce potentially valuable products such as tyre derived pyrolytic oil, gas and char. The main aim of this project is to study the adsorption capacity of tyre-derived char and crumb as alternative low-cost adsorbents for adsorption of gold in a form of gold chloride complex ion, AuCl4−, from acidic solutions. Pyrolytic tyre char (PT-char), which is acarbonaceous solid material that constitutes 30–40 wt.% of the entire pyrolysis product, and tyre crumb (T-crumb) will be of interest in this project as alternative low-cost adsorbents. However, there is a major drawback in applications of the crude form of these adsorbents due to their high level of impurities, which could pose a threat to their economic and market value. Hence, demineralisation as a purification approach is employed to remove or reduce the ash content. Untreated PT-char and T-crumb were demineralised first with 1M NaOH and subsequently with 1M HNO3. The untreated and treated samples were characterised and used as adsorbents in adsorption of gold from acidic chloride solutions. The performance of the untreated samples was compared with that of treated PT-char and T-crumb based on adsorption capacity. Since there is no work reported yet on the application of tyre char and crumb as adsorbents for gold, then this initiative is the novelty of this project. Demineralisation of PT-char and T-crumb under acid-base medium proved to enhance the BET surface area which increased from 63.96 to 78.89 m2/g for PT-char and from 0.322 to 3.49 m2/g for T-crumb. The impurities which are regarded as ash content in this case, were significantly reduced with 90% of ash removed from UPT-char and 50% ash removed from UT-crumb. The effective pH for the adsorption of Au(III) ions with all the adsorbents was 2.0 with maximum adsorption percentage as high as 90%. The maximum capacity for untreated adsorbents was reached within 48h and for treated adsorbents was reached in 24h contact time. To study the adsorption isotherm, Langmuir and Freundlich isotherm models were used and for kinetic studies pseudo-first and second kinetic models where used. The adsorbents obeyed Langmuir isotherm model with calculated Qe of 416.7, 19.2, 47.4 and 21.1 mg/g for UPT-char, TPT-char, UT-crumb and TT-crumb, respectively. And they obeyed pseudo-second order kinetic model with calculated Qe of 434.8, 21.6, 61.7 and 21.7 mg/g for UPT-char, TPT-char, UT-crumb and TT-crumb respectively. The SEM micrograph images of the treated and untreated adsorbents after adsorption revealed that the adsorbed gold was reduced from Au(III) to Au(0) nanoparticles on the adsorbent surface. A major part of UPT-char surface was occupied with gold nanoparticles randomly distributed on specific binding sites. On the surface of the treated adsorbents the gold nanoparticles were very few which proved that the treated adsorbents had few active binding sites for gold. The reduction of gold to gold nanoparticles was due to the presence of sulphur content on the adsorbent surface which served as a reductant reducing gold via electron transfer. In conclusion, these results show that demineralisation of PT-char and T-crumb is not beneficial regarding adsorption of gold, but useful in leaching out contaminants and reducing ash content, consequently improving the surface area. The high adsorption capacity of the untreated adsorbents proved to have high affinity for gold and stand a good chance to be used as low-cost adsorbents. Sulphur content on the adsorbents surface behaved as the binding site for gold ions during adsorption. Using tyre-derived adsorbents before undergoing expensive processes such as activation and chemical treatments to manufacture expensive adsorbents such as activated carbon could be beneficial as part of valorisation of waste tyres. Recovery of gold from acidic solutions such as e-waste leachates with low-cost tyre-derived adsorbents would also be beneficial both on economic and environmental point of view. Keywords: Demineralisation; Au(III) adsorption; Adsorption capacity; Kinetics; Isotherm

AFRIKAANSE OPSOMMING: Elke jaar groei die hoeveelheid voertuie wêreldwyd. Daarom groei die hoeveelheid bande teen ʼn vinniger koers daagliks. Bande bied nie ʼn gesondheidsrisiko op hul eie nie, maar die onvanpaste wegmaking en wanbestuur van groot hoeveelhede kan ernstige probleme vir nie net die omgewing nie, maar ook vir mense, veroorsaak. Die bestuur van afvalbande is daarom baie belangrik. Verskeie maniere van bestuur van afvalbande en valorisasie is gepoog, waarvan pirolise een van die mees aantreklike tegnieke is. Pirolise van afvalbande lewer potensiële waardevolle produkte soos band-afkomstige pirolitiese olie, gas en verkoolsel. Die hoofdoel van hierdie projek is om die adsorpsie kapasiteit van band-afkomstige verkoolsel en krummel as alternatiewe lae-koste adsorbeermiddels vir adsorpsie van goud in ʼn vorm van goud chloried kompleks ioon, AuCl4-, vanuit suurvormende oplossings, te bestudeer. Pirolitiese bandverkoolsel (PT-char), wat ʼn koolstofryke vastestof materiaal is wat 30–40 wt.% van die hele pirolise produk uitmaak, en bandkrummel (T-crumb) sal van belang wees in hierdie projek as alternatiewe lae-koste adsorbeermiddels. Daar is egter ʼn groot nadeel in toepassings van die ru vorm van hierdie adsorbeermiddels as gevolg van hul hoë vlakke van onsuiwerhede, wat ʼn bedreiging vir hul ekonomiese en markwaarde kan wees. Demineralisasie as ʼn suiweringsbenadering is egter gebruik om die asinhoud te verwyder of verminder. Onbehandelde PT-char (UPT-char) en T-crumb (UT-crumb) was eers gedeminiraliseer met 1 M NaOH en gevolglik met 1 M HNO3. Die onbehandelde en behandelde monsters is gekarakteriseer en gebruik as adsorbeermiddels in adsorpsie van goud uit suur chloriedoplossings. Die werkverrigting van die onbehandelde monsters is vergelyk met dié van behandelde PT-char (TPT-char) en T-crumb (TT-crumb) gebaseer op adsorpsie kapasiteit. Aangesien daar nog geen werk gerapporteer is van die toepassing van bandverkoolsel en krummel as adsorbeermiddels vir goud nie, is hierdie inisiatief die nuutheid van hierdie projek. Demineralisasie van PT-char en T-crumb onder suurbasismedium is bewys om die BET-oppervlakarea te vergroot, wat van 63.96 na 78.89 m2/g toegeneem het vir PT-char en van 0.322 tot 3.49 m2/g vir T-crumb. Die onsuiwerhede, wat beskou word as asinhoud in hierdie geval, het beduidend verminder met 90% van die as verwyder uit UPT-char en 50% as verwyder uit UT-crumb. Die doeltreffende pH vir die adsorpsie van Au(III)-ione met al die adsorbeermiddels was 2.0 met maksimum adsorpsie persentasie so hoog soos 90%. Die maksimum kapasiteit vir onbehandelde adsorbeermiddels is binne 48 uur bereik, en binne 24 uur kontaktyd vir behandelde adsorbeermiddels. Om die adsorbsie isoterm te bestudeer is Langmuir en Freundlich isotermmodelle gebruik, en vir kinetiese studies pseudo-eerste en -tweede orde kinetiese modelle. Die adsorbeermiddels het die Langmuir isotermmodel gevolg met berekende Qe van 416.7, 19.2, 47.4 en 21.1 mg/g vir UPT-char, TPT-char, UT-crumb en TT-crumb, onderskeidelik. En dit het die pseudo-tweede orde kinetiese model gevolg met berekende Qe van 434.8, 21.6, 61.7 en 21.7 mg/g vir UPT-char, TPT-char, UT-crumb en TT-crumb, onderskeidelik. Die SEM-mikrograafbeelde van die behandelde en onbehandelde adsorbeermiddels het bekendgemaak dat die geadsorbeerde goud gereduseer is van Au(III) na Au(0)-nanopartikels op die adsorbeermiddeloppervlak. ʼn Groot deel van die UPT-char-oppervlak was betrek met goud nanopartikels wat lukraak versprei is op spesifieke verbindingsplekke. Op die oppervlak van die behandelde adsorbeermiddels was die goud nanopartikels baie min, wat bewys dat die behandelde adsorbeermiddels min aktiewe verbindingsplekke vir goud gehad het. Die reduksie van goud na goud nanopartikels is as gevolg van die teenwoordigheid van swaelinhoud op die adsorbeermiddeloppervlak wat as reduseermiddel gedien het deur elektronoordrag. Ten slotte, hierdie resultate wys dat demineralisasie van PT-char en T-crumb nie voordelig is ten opsigte van adsorpsie van goud nie, maar handig is in uitloging van onsuiwerhede en die vermindering van asinhoud, wat die oppervlakarea gevolglik verbeter. Die hoë adsorpsie kapasiteit van die onbehandelde adsorbeermiddels is bewys om hoë affiniteit vir goud te hê en staan ʼn goeie kans om as lae-koste adsorbeermiddel gebruik te word. Swaelinhoud op die adsorbeermiddeloppervlak het opgetree as die verbindingsplek vir goud-ione gedurende adsorpsie. Om band-afkomstige adsorbeermiddels te gebruik voordat duur prosesse, soos aktivering en chemiese behandeling, gebruik word om duur adsorbeermiddels, soos geaktiveerde koolstof, te vervaardig, kan voordelig wees as deel van valorisasie van afvalbande. Herwinning van goud uit suurvormende oplossings soos e-afval uitloogsels met lae-koste band-afkomstige adsorbeermiddels kan ook voordelig wees van beide ʼn ekonomiese en omgewingsstandpunt. Sleutelwoorde: Demineralisasie, Au(III), Adsorpsie, Adorspie kapasiteit, Kinetika, Isoterm.

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