Spectroscopic determination of selected rare earth elements (lanthanum, neodymium and dysprosium) in electronic waste samples

Abstract

Rare Earth Elements (REEs) are nowadays critical elements required in technological accessories. Their presence in electronic waste leads to environmental pollution. Therefore, there is a constant necessity for accurate data and reliable fast analytical methods. The dumping of waste electrical and electronic equipment (WEEE) and its recycling, commonly known as "urban mining," has had negative environmental impacts. The fate of rare earth elements in the environment because of the disposal of electronic waste (e-waste) and urban mining poses a threat to human health and the aquatic ecosystem. The information regarding the abundance of rare earth elements (REEs) in electronic waste components (EWC) helps the recycling industry. However, after the end of use, improper disposal may be detrimental to the environment by releasing toxic substances. The e-waste can be both valuable as secondary raw materials source and toxic if not treated and discarded improperly. Firstly, this study focuses on the development of a novel improved alkaline borate fusion application on e-waste determination. Due to the heterogeneous nature of e-waste, the composition of vast amounts of metals and the refractory nature of some elements, an improved and sensitive method was required. This study investigates the preparation, dissolution methods, optimisation of experimental particulars and instrumental techniques with a focus on selected rare earth elements (Lanthanum (La), Praseodymium (Pr), Neodymium (Nd), and Dysprosium (Dy) in electronic waste, soils and leachate derived from e-waste disposal. The optimum and safe fusion conditions for e-waste were achieved after slow thermal decomposition up to 550 oC, pulverisation to 90% of –53 μm, flux composition (90%LiBO2 + 10% Li2B4O7), 3:1 oxidant ratio of Na2CO3: NaNO3, LiBr as the non-wetting agent. Also, a sample to flux ratio of 1:15 and a total fusion time of 10 minutes was optimised. The newly improved alkaline fusion results compared better to those obtained from classical mineral acid dissolution with at most 5% RSD on REEs studied. The alkaline borate fusion results in smartphones e-waste were at least 15% and 25% higher than in four acid digest and microwave-assisted digest techniques, respectively. The results indicated enrichment of REEs in smartphones followed by non-smartphones and computer waste. Secondly, the study focuses on several microcosm studies using conditioned soil and crushed e-waste material to investigate the leaching of REEs in lab-scale experiments. For static column leaching, the H2SO4 lixiviant showed the highest accumulative recovery efficiency for Pr, Nd, and Dy, with 73.8%, 74.7%, and 52.2%, respectively. The total extractable REEs after 7 days were 73.91%, 45.79%, 16.85% and 10.55% for lixiviants H2SO4, H3PO4, C6H8O7 and rainwater, respectively. The effects of major variables on REE batch leaching were analysed, including lixiviant type and concentration, time, stirring speed, pH, and solid-to-liquid ratio. In all batch leaching experiments, the temperature was kept constant at 23  2 °C, and the agitation speed was maintained at 450 rpm. The leaching efficiency of La, Pr, Nd, and Dy was found to be significantly influenced by the lixiviant type, concentration, pH, and leaching time. The most efficient batch leaching was achieved using 1 M HNO3, a leaching time of 10 minutes, a solid-to-liquid ratio of 50 g/L, and a strongly acidic pH. For La, Pr, Nd, and Dy, a leaching efficiency of 31.4%, 74.2%, 75.7%, and 75.2%, respectively, was observed. Thirdly, In this study, we report the results of leachate quality and characterisation studies from simulated e-waste dump. The dump’s physical and chemical properties were quantified using standard methods. The sediments and liquid leachate from the waste dumps were elemental quantified at different time intervals over a period of 4 years. The effect of leaching variables on rare earth elements were evaluated. The data obtained will help with a novel predictive modelling approach of REE leaching based on several leaching parameters and to be used on a large scale and in different natural environments. The novel approach is based on REEs' reactions to their disintegration, solubility, complexation/precipitation, and permeation in the natural environment. The outcome from this study suggests that the REEs hardly leached out in the first year of exposure but gradually increased in the second and third years. The order of leaching content was Nd > Pr > La > Dy, but this was largely due to the abundance of the elements in the original waste components. Also, the levels of the leached metals in the solid sediment samples were significantly higher than in liquid leachate.