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

[摘要] 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

[发布机构] Stellenbosch University