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In recent years, with the increasing research and development of the light-emitting diode (LED) industry, which contains gallium nitride (GaN), it is expected that there will be a large amount of related wastes in the future. Gallium has an extremely high economic value, therefore, it is necessary to establish a recycling system for the GaN waste. However, GaN is a direct-gap semiconductor and with its high energy gap, high hardness, and high melting point, these make it difficult to recycle. Therefore, this study will analyze the physical characteristics of LED wastes containing GaN and carry out various leaching methods to leach the valuable metals from the waste optimally. Different acids are used to find out the best reagent for gallium leaching. Different experimental parameters are discussed, such as the effect of the different acid agents, concentration, pressure, liquid-solid mass ratio, temperature and time, which influence the leaching efficiency of gallium. Finally, acid leaching under high pressure is preferred to leach the GaN waste, and hydrochloric acid is used as the leaching solution because of its better leaching efficiency of gallium. Optimally, the leaching efficiency of gallium can reach 98%. by other materials/metals, and triggers global competitions to ensure steady supply [7]. According to a recent report published by the united nations environment program (UNEP), less than 1% of end-of-life (EOL) gallium and indium bearing materials are being recycled [2,3]. Gallium is used in a wide variety of products that have microelectronic components containing either gallium arsenide (GaAs) or gallium nitride (GaN) [5]. Due to the low solubility of nitrogen in gallium and the high vapor pressure of nitrogen on GaN, the native substrate of GaN is not available in large quantities. The GaN is a crystal of high bond energy that is equal to 7.72 eV/molecule, which results in higher melting temperature and good thermal stability [8,9]. The value of sales for the GaN power device market was expected to reach $178 million by 2015 at an annual growth rate of nearly 29 percent [10]. In 2012, imports of gallium and GaAs wafers, which were valued at about $32 million, continued to satisfy almost all U.S. demand for gallium. GaAs and GaN electronic components represented about 99 percent of domestic gallium consumption [5]. The value of worldwide GaAs device consumption increased by about 7% to $7.5 billion in 2015 owing to a growing wireless telecommunications infrastructure in Asia; growth of feature-rich, application-intensive, third-and fourth-generation (3G, 4G) "smartphones", which employ up to 10 times the amount of GaAs as standard cellular handsets; and robust use in military radar and communications applications. Cellular applications accounted for approximately 53% of total GaAs device revenue and wireless communications accounted for 27%. Various automotive, consumer, fiber-optic, and military applications accounted for the remaining revenue [11]. By yearend 2016, the GaN radio frequency device mar...

14The ongoing development of new advanced technologies, created increasing demands for 15 rare earth elements (REEs) in the international market. The available conventional 16 technologies for concentration and recovery of REEs are expensive making biosorption an 17 efficient and low-cost technology for the recovery of REEs from aqueous solution. Thus, 18 the biosorption and desorption of multi-component solution containing Y(III), La(III), 19Sm(III), Dy(III), Pr(III), Nd(III), Gd(III) were investigated using dried or 250˚C and 350˚C 20 carbonized parachlorella. Evaluating the effect of pH with respect to contact time indicated 21 2 a dependency of the system with those parameters. The optimum pH for dried and 250˚C 22 carbonized parachlorella was 7 whereas 350˚C reaches it maximum uptake at pH 4. Rapid 23 adsorption within the first 5 min of contact followed by a slight variation the following 20 24 min characterized the sorption processes onto parachlorella by-products. The mechanism of 25 the biosorption is explained by a combination of complex reactions occurring 26 simultaneously in the biosorption process. 27 In addition, desorption process has been investigated using various concentrations of HCl, 28 HNO 3 , and H 2 SO 4 at different temperatures. It was found that the reversible process is rapid, 29 less temperature and pH dependent with high desorption percentage. Moreover, only light 30 REEs were desorbed regardless of the kind of acid and the solution temperature. 31Parachlorella is found to be good and low-cost biosorbent for the recovery of above REEs 32 from aqueous solutions. 33 34 35 36Rare earth elements (REEs) are often referred as the "seeds of technology" because 38 of their uses in high-tech strength permanent magnets, lasers, automotive catalytic 39 converters, fiber optics/superconductors, electronic devices, and green energy sectors [1, 2]. 40Due to the ongoing development of new advanced technologies, there is an over-increasing 41 demand for REEs in the international markets, with emphasis on identifying new resources 42 to ensure adequate supply for present and future use. 43 3The designation "rare earths" refers to the 15 elements of the periodic table known 44 as "lanthanides" with yttrium and scandium, further divided as a function of their atomic 45 number into two categories. Light rare earth elements (LREE), which accounted for 66.8% 46 of global demand in 2010 [2] referred to lanthanum, cerium, praseodymium, neodymium, 47 promethium and samarium. Heavy rare earths elements (HREE), less common and more 48 valuable, referred to the rest of lanthanides elements with yttrium. The 17 REEs are found 49 in all REE geological deposits because they share many similar properties but their 50 distribution and concentrations vary [2]. REE mineral deposits are usually rich in either 51 LREE or HREE, but rarely contain both in significant quantities [2]. The term "rare earth" 52 is actually a misnomer, because these elements are more abundant in the earth's crust 53 compared to silver, gold...

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