I just thought of sharing my old research when I was still studied geology. Yes, I've liked microbes even back then XD It's a bit of a long read but I hope you find it fascinating
Introduction
Electronic waste or e-waste is described as discarded electrical or electronic devices which have reached the end of their useful life (Das et al, 2016; Peralta, 2006). As of date, E-wastes generated increases at an exponential rate of nearly 10% yearly (Sakunda, 2013; cited in Kaya, 2016) and has been an alarming problem since e-waste manufacturing is a factor that causes metal depletion. It was also cited in the work of Torres, 2016, metals like Copper, silver, iron and gold as well as an additional 15, to 40 (Arshadi, et al., 2016), to 57 valuable elements (Kaya, 2016) contained in the PCB are either in its elemental form or exists as an alloy (Cui and Zhang, 2008; Tuncuk et al., 2012; Yang et al., 2011). Although some components to these e-wastes are harmful to the environment like an e-waste’s non-metallic components and the heavy metals which are embedded in it, it is still a feasible source of metals; a secondary source or “Secondary ores” as it’s called, in the process of urban mining (Das et al, 2016). Recovering metals from these e-wastes in the form of recycling provides a good solution to minimizing the e-wastes that go into landfill which will pose a greater problem to the environment.
Recycling methods for e-wastes include mechanical separation of components, chemical dissolution of the metals and metallurgical processes, all of which are either energy intensive and high cost or makes use of highly toxic chemicals with low metal recovery rate or if not low then are non-selective to the metal of interest. These methods have been the traditional ways of recovering metals from e-wastes for the past few decades; none which offers a solution in delaying environmental destruction but rather, worsens it. In light of these consequences, bioleaching was found to be a good alternative method for recycling e-wastes.
Bioleaching, or biological leaching, which is under biometallurgy, exploits the ability of a microorganism of either bacteria or fungi, to transform solid compounds into soluble and extractable metals which can be recovered (Krebs, 1997). The microorganism produces lixiviants or leachates which are at a biogenic level than that used in chemical processes to form water-soluble metal complexes or salts with the metals of interest. Advantages for this method, as cited by Kumar, 2017, is the lowering of demand for resources such as energy, ores and landfill space, but its application is only in its infancy (Ruan et al, 2014) since further research on bioleaching has only boomed when the first patent was published in 1958 and has continued to progress, especially in 2016 when earth was at the peak of its toxicity and numerous researches about bioleaching was done in said year. But despite its infancy, the last 20 years of research on bioleaching has led to 10 bio-heap leaching plants, 7 bio reactor plants for sulphidic concentrate and countless pilot scale operations have started worldwide (Kocadagistan, 2017). In the Philippines, the only people to have done research on bioleaching as of date is the Mines and Geosciences Bureau (MGB) which took place in 2013, and some applications of bioleaching in the mining industry but besides that, no further development has occurred in the field of bioleaching in the Philippines.
Besides the method being a good alternative to the above mentioned traditional methods, the execution of leaching itself is simple since steps involved are only culturing of microorganisms, addition of the shredded e-waste and the collection of the e-waste particles from the leaching medium after a given time. The entire leaching time usually takes 3-4 days to one week, depending on when the addition of the shredded e-wastes was and based on which bioleaching method was used. Methods for bioleaching include “one-step” where the shredded e-waste is added whilst culturing the microorganism, “two-step” which is done by adding the shredded e-waste into the culturing medium when the major lixiviant/leachate production is reached, and “spent medium” leaching where the microorganism is separated from the culturing medium and the shredded e-waste will only be added after removing the microorganism.
All methods are appropriate for all types of microorganisms that can produce metabolites which can leach metals but in bioleaching, three main microbial groups are often used in this process, as cited by Isildar, 2015, those being: chemolithotropes, those which get energy from non-carbon compounds via oxidation (Xiang et al. 2010), heterotrophic, microbes that cannot produce their own foods and instead take organic materials as sustenance (Chi et al, 2011), and thermophiles or heat loving microbes (Brandl et al, 2001). An addition to the list would be the cyanogenic microbes which are able to produce cyanogenic compounds as a metabolite or as a secondary metabolite; this microbial group is specific to recovering gold since it is a known fact that cyanide is a primary leachate to gold.
Primary sources:
Das, S., Natarajan, G., Ting, Y. P., (2016), Bio-Extraction of Precious Metals from Urban Solid Waste, AIP Conf. Proc. 1805, 020004-1–020004-8, Retrieved from: 10.1063/1.4974410
Isıldar, A., Van de Vossenbrerg, J., (2015 November 22), Two-step bioleaching of copper and gold from discarded printed circuit boards (PCB), Waste Management 57 (2015) Pp. 149-157, retrieved from: http://dx.doi.org/10.1016/j.wasman.2015.11.033
Kaya, M., (2016 August 16), Recovery of metals and non-metals from electronic waste by physical and chemical recycling processes, Waste Management 57 (2016)Pp. 64–90, Retrieved from: http://dx.doi.org/10.1016/j.wasman.2016.08.004
Brilliant