Leaching of Rare Earth Elements from fluorescent powder using the tea fungus Kombucha

In almost all modern technologies like flat screens, highly effective magnets and lasers, as well as luminescence phosphors, Rare Earth Elements (REE) are used. Unfortunately no environmentally friendly recycling-process is available so far (European 2014). Furthermore, only poor information is available regarding interactions of microorganisms with REE and there are almost no studies describing the bioleaching of REE. However, it can be assumed that microorganisms play an important role in the biogeochemistry of REE (Brisson et al. 2015; Chen et al. 2001; Goyne et al. 2010). This study investigates the potential of organic acid producing microbes to extract REE from technical waste.
In Germany, 175 tons of fluorescent phosphor (FP) are collected as a distinct fraction during the recycling of compact fluorescent lamps anually (Gallenkemper and Breer 2012; Riemann 2014). As the FP contains about 10% of REE-oxides bound in the so-called triband dyes it is a readily accessible secondary resource of REE (Haucke et al. 2011). Using the symbiotic mixed culture Kombucha, consisting of yeasts and acetic bacteria, significant leaching-rates were obtained. The highest leaching-rates were observed for the shaken cultivation using the entire Kombucha-consortium or its supernatant as leaching agent in comparison to experiments using the isolates Zygosaccharomyces lentus and Komagataeibacter hansenii as leaching organisms. During the cultivation the pH-value decreases due to the production of organic acids (mainly acetic and gluconic acid). Thus, the underlying mechanism of the triband dye solubilisation is probably connected with the carboxyl-functionality. According to the higher solubility of REE-Oxides in comparison to REE-phosphates and –aluminates, a preference in the solubilisation of the red dye Y2O3:Eu2+ containing relatively expensive REE was ascertained.
These results show that it is possible to dissolve the REE-compounds of FP by the help of microbial processes. Moreover, they provide the basis for the development of an eco-friendly alternative to the currently applied methods.