Aiming for the sustainable development of both economy and environment, a group of materials, known as “critical raw materials” (CRM), have gained importance in the European Union (European Commission. Directorate General for Internal Market, Industry, Entrepreneurship and SMEs. 2023). The European Commission update this list every three years since its creation in 2011 and, in accordance with the climate-neutrality target for 2050, the CRM list is expected to be one of the hottest topics in the years to come (Michaels 2021). These raw materials are gaining an increasing importance for the EU economy’s growth, being crucial to reduce the existing dependence of materials from non-European countries and to ensure millions of European jobs over the next few decades (Nikulski et al. 2021). Some of those elements, in particular a certain group of metals known as “strategic metals” are prominent for their scarcity in the earth’s crust, economic importance, risk of supply and low or even non-existing recycling rates (Girtan et al. 2021). Gallium belong to that category, being crucial in the semiconductor manufacturing (Chen et al. 2018). Although indium has been recently removed from the strategic classification, the research on the extraction and recovery processes for both gallium and indium must be regarded to reduce the use of natural sources and for their crucial applications (Khezerloo et al. 2023).
In relation to gallium, it is a high valuable metal whose main applications are focused on the semiconductor industry (Cenci et al. 2020). Concerning indium, is a soft silvery white metal, which is mainly employed in the electronic industry for the manufacture of photoconductors or thermistors, and in the manufacturing of liquid crystal screens (Alfantazi and Moskalyk 2003). Both metals, Ga and In, are widely applied as semiconductors (as GaN, GaAs or InGaN) in the manufacture of Light-Emitting Diode (LEDs) technology, which includes many types of displays and screens (Khezerloo et al. 2023) and lighting sources (Chen et al. 2020). LED lighting technology has established itself as an alternative to traditional bulbs (fluorescent and incandescent) (Zamprogno Rebello et al. 2020), standing out for its lower usage cost due to longer lifetimes (25.000 hours as compared with 12.000 hours for fluorescent lamps and 1.000 hours for incandescent bulbs (Richter et al. 2019)) and higher luminous effectiveness (Martins et al. 2020). LED lighting technology had a penetration rate of 47% in the global lighting market in 2019, but some studies forecast that LED lamps would have reached a market penetration of 95% for 2025 (Örtl 2019) and others assume that the 100% of market share will be reached in 2030 (Nikulski et al. 2021), just for being a more sustainable and environmentally friendly alternative. It is not only being used for household applications, but for street lighting and industry applications (i.e., automotive industry) (Balinski et al.). Both gallium and indium are important raw materials for the manufacture of copper-indium-gallium-selenide (CIGS) thin-film photovoltaic solar cells. Therefore, demand for gallium and indium is likely to increase by as much as 2.5 times between 2030 and 2050 compared to current levels (Directorate-General for Internal Market et al. 2020).
However, it is mandatory to recognize the problem associated with the increasing consumption of LED lamps. The generation of waste from electrical and electronic equipment (WEEE), also known as e-waste, is a notable problem linked to the increased consumption of these electrical devices (de Oliveira et al. 2021). LED lamps waste is no exception. Globally, more than 53.9 Mt of WEEEs was generated in 2019 (an increase of 21% since 2014) and it is estimated that over 74.7 Mt will be generated by 2030, with an annual growth rate of 2% (Ahirwar and Tripathi 2021). Luminaire waste, including LED bulbs, accounted for approximately 2% of that waste generation in 2019 (Rajesh et al. 2022). This e-waste generation is not accompanied by a proper recycling, which is crucial to meet the 12th Sustainable Development Goal (Responsible consumption and production) (Rahimifard and Trollman 2018). Only the 17.4% of the WEEE generated was properly collected and recycled in 2019 (Rajesh et al. 2022), a small fraction insufficient to overcome another problem: the increasing scarcity of raw materials for manufacturing this luminaire devices. Moreover, metals such as gallium and indium have a negligible recycling rate (less than 1% for gallium and 0% for indium) (European Commission. Directorate General for Internal Market, Industry, Entrepreneurship and SMEs. 2023). Therefore, studying the feasibility of recycling those LED lamps and the metals contained in the LED module opens a unique door for the concept of circular economy, with these wasted chips being a new source of secondary raw material for the manufacture of a new generation of lighting system way more sustainable, economical, and less harmful to environment. Otherwise, the increasing disposal and accumulation of WEEE can affect the environment (air, soil, and specially water) and cause hazardous effects to humans and other living beings and organisms (Rautela et al. 2021).
In recent years, among the alternatives for obtaining indium and gallium, their recovery from wastewater from leachates and luminaire waste is gaining attention mainly because it involves a double objective: purifying water on the one hand, and recovery the important raw materials on the other, within the framework of a circular economy and considering the Sustainable Development Goals. Among the different sources of wastewater, those coming from electronics industry are especially suitable for the recovery of gallium and indium, as they usually contain both metals simultaneously. Therefore, efforts should focus on developing a suitable technique to selectively separate these two ions from the same aqueous matrix.
Several alternatives have been studied in the literature, when dealing with metal-contaminated wastewater. Some of them are filtration (Lahti et al. 2020), chemical precipitation (Janin et al. 2009), membrane processes (Lahti et al. 2020), solvent extraction (Liu et al. 2006; Song et al. 2020; Drzazga et al. 2021) or electrochemical methods (Grevtsov et al. 2021). However, these processes have significant drawbacks, such as sludge production, large operation costs or incapability of reaching a complete removal. Adsorption has emerged as a promising technology, especially suitable when a high selectivity for a specific metal (as is the case of this study) or the pre-concentration of trace metal amounts is required.
The adsorbents used for adsorption are very varied, an include zeolites, activated carbon, minerals, oxides, and many others. The main drawback of an adsorption process is that the overall cost-effectiveness of the process depends mainly on the cost of the adsorbent (Saravanan et al. 2021; Shrestha et al. 2021). Therefore, recent efforts have concentrated on finding relatively inexpensive and available adsorbents. Low-cost sorbents such as industrial and agricultural wastes and by-products (sawdust, straws, fly ash, mud, oil, bagasse, shells) and natural materials stand out for being relatively cheap, as they are highly and naturally accessible (Worch 2012). Among natural materials, one especially adequate is a natural zeolite, chabazite.
Chabazite is a natural zeolite member of chabazite group that can be found in the cavities of basaltic rocks all around the world. Its generic chemical formula is (Ca0,5,Na,K)4[Al4Si8O24]·12H2O, with a silicon to aluminum ratio ranging from 2 to 5, what confers thermal stability to the zeolite. The crystal structure of chabazite consist of a three-dimensional pore structure with an opening of 3.8 Å, occupied by water and exchangeable cations, such as Na+ or K+. These characteristics make chabazite a suitable adsorbent for metal ions removal from wastewater.
Previous studies have employed chabazite as an adsorbent for different compounds, which gives an idea of its potential applicability. Aysan et al. (Aysan et al. 2016), and Solisio and Aliakbarian (Solisio and Aliakbarian 2017) employed this natural zeolite to successfully remove methylene blue from aqueous solutions. It has also been employed to separate different compounds from gas streams, such as N2, O2 and Ar (Singh and Webley 2005) or N2, CO2 and CH4 (Watson et al. 2012). Other applications of this solid as adsorbent have been CO2 capture (Zhang et al. 2008; Pham et al. 2014), separation of CO2/CH4 mixtures (Shang et al. 2020) or alkane adsorption (Denayer et al. 2008; Göltl and Hafner 2011).
Despite its suitable characteristics, chabazite has not been studied in depth in literature as potential adsorbent for metal removal. Only few references can be found. Gallant et al. (Gallant et al. 2009) compared chabazite and clinoptilolite as adsorbents to remove Cs, Co, Sr, Cu, Cd and Zn, found in effluents of nuclear operations. They concluded that the better adsorption capacity of chabazite could be attributed to its larger pore volume as well as its high silicon to aluminum ratio. Egashira et al. (Egashira et al. 2012) employed chabazite, mordenite and clinoptilolite, to adsorb Cu, Zn and Mn. They also concluded that the cation exchange capacity of natural zeolite increases whith increasing aluminum content. Yakout and Borai (Yakout and Borai 2014) employed chabazite to remove Cd from aqueous solutions, obtaining a maximum cadmium adsorption capacity of 120 mg/g, and concluding that the adsorption process is strongly pH dependent in the 2.5–8.5 range. Dwairi et al. (Dwairi et al. 2015) employed two Jordanian natural zeolites of type phillipsite-chabazite, for continuous removal of Pb and Li ions from industrial wastewater effluents with adsorption capacities of 34.7 and 23.64 mg/g for Pb ions, and 18.65 and 21.43 mg/g for Li ions. Finally, Ibrahim et al. (Ibrahim et al. 2016) employed phillipsite–chabazite tuffs to adsorb Mo and Ni from aqueous solutions, reaching a Mo removal efficiency of 76% and a Ni removal efficiency above 90%.
Currently, most of the world's gallium supply comes from bauxite mining and sediment-hosted lead-zinc (Pb-Zn) resources, with an average content of Ga in bauxite about 50 mg/kg (Qi et al. 2023). The main source of indium is zinc concentrates (indium containing 0.0001–0.1%), recovered as a byproduct in the smelting of zinc ore and tin smelting process. However, the LED module, according to our measurements, contains about 7000 mg/kg of gallium and above 400 mg/kg of indium. Other sources indicate that the Ga content in LEDs is around 0.25–4 wt% (Balinski et al.). This is a much higher gallium content than the gallium present in bauxite, demonstrating the importance of LED waste as secondary sources of these metals. Therefore, the main aim of this work is to study the selective removal of two metals present in leachate of spent LED, gallium, and indium, from aqueous solutions by sorption onto a low-cost zeolite, chabazite, barely studied in the literature. A comparison between zeolite and natural and organic sorbents, such as mesoporous carbon, bagasse, coffee grounds and others will be provided, in which chabazite has emerged as the most effective option. The removal of both metals to a large extent and the possibility of removing one of them separately will be studied in depth. A complete characterization of the chabazite will be carried out, for a better understanding of the adsorption mechanism.
Due to the nature of some of the adsorbents used, the removal of cations from aqueous solution was expected to occur by both adsorption and ion exchange. Therefore, the term sorption will be used since adsorption and ion exchange are sorption processes (Haan 2015).