Novel strategies of bioleaching metals from printed circuit boards (PCBs) in mixed cultivation of two acidophiles
Introduction
Waste electrical and electronic equipments (WEEE) wastes are being produced in large amounts, which will lead to seriously environmental pollution if they are not treated properly. Status quo of WEEE wastes generation in different parts of world has been reported [1], [2], [3]. It was demonstrated that WEEE are fastest growing wastes, indicating the urgency for their treatments or recycling methods being developed [4]. Printed circuit boards (PCBs), as typical WEEE waste, its recycling has attracted attentions not only from perspective of treatments but also valuable metals [5]. To date, recycling PCBs was still limited due to heterogeneity and complexity in its components [6]. As it is known, PCBs was mainly composed of polymers, ceramics and metals. Major metals, plastics, bromine, glass and ceramics in PCBs account for 28%, 19%, 4% and 49% respectively [7]. Evidently, if PCBs is not treated or recycled appropriately, heavy metals like copper, lead, zinc and nickel will cause seriously environmental problems.
Pyrometallurgical [8], [9] and hydrometallurgical [10], [11] methods were often adopted to recycle metals from electrical wastes. Unfortunately, poisonous gases, such as dioxins and furans, or high volume of effluents were generated simultaneously, thus leading to heavy pollution [12]. In addition, although mechanical processing [13], [14] was used to recycle metals [15], it was not practical due to high-energy cost [16], [17].
Consequently, to meet environmental regulations, eco-friendly and energy-saving processes are urgently needed to recycle metals from WEEE wastes. It was reported that certain microorganisms can dissolve metals compounds and minerals, thus resulting in metals extraction [18]. Microorganisms involved in above processes principally include sulfur-and ferrous-oxidizing bacteria, such as Thiobacillus ferrooxidans and Thiobacillus thiooxidans [19]. These bacteria can grow by obtaining energy from oxidation of ferrous iron (Fe2+) or elemental sulfur (S0), and with formation of ferric iron (Fe3+) and sulphuric acid (H2SO4), thus resulting in metals solubilization [20]. Such metals bioleaching technique has been used in biomining on industrial scale [21], [22]. Moreover, these bacteria were also used to recycle metals from sewage sludge and even soils [23], [24]. However, data pertaining to recycling metals from electronic solid wastes was still scanty.
More recently, a few studies were conducted on bioleaching metals from electronic scraps [25], [26]. For example, acidophilic strains of chemolithotrophic and heterotrophic bacteria were cultivated together to extract metals from PCBs [27]. In addition, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans were mixed culture to bioleach metals from printed wire boards (PWBs) [28]. Moreover, factors influencing copper recovery from PCBs by A. ferrooxidans were studied [29]. Undoubtedly, using microorganisms to bioleach metals from electronic scraps will be a promising alternative in comparison to traditional methods. As it is known, ultimate goal for metals bioleaching from PCBs by microorganism is to maximize its addition amounts and metals recovery, which was closely related to bacterial growth. However, to date, there was no report on correlation between bacterial growth and metals recovery. Our preliminary study showed that WEEE wastes, such as PCBs, adversely inhibited bacterial growth and thus affected metals extraction. Therefore, how to minimize inhibition of bacterial growth and thus improve metals recovery is the key point. However, little is known on the methods or strategies being developed to optimize metals bioleaching from PCBs.
Accordingly, A. ferrooxidans and A. thiooxidans were used to bioleach metals from PCBs in this paper. Initially, effects of PCBs on metals extraction and bacterial growth were studied. And then based on the results that PCBs addition amounts and times are adversely proportional to biomass and metals recovery, novel strategies of multiple PCBs additions were developed. Finally, the reasons that stimulate efficient metals recovery in mixed culture was explored.
Section snippets
Source and pretreatment of printed circuit boards (PCBs)
Printed circuit boards (PCBs) was obtained from an electronics shop in Changzhou city, China. No physical or mechanical separation process was used before transportation to the laboratory. For experimental use, PCBs scrap was crushed and then ground to fine powder of 100–200 μm sieve fraction by using ring mill grinder.
Leaching microorganisms and adapted cultivation
Isolates used in this study were recovered from activated sludge taken from municipal sewage treatment plant in Changzhou, China. A. ferrooxidans JSTU-02 and A. thiooxidans
Chemical analysis of metals contents in printed circuit boards (PCBs)
Chemical analysis of PCBs was conducted to determine main metals contents, and results were shown in Table 1. It is clear that major metals in PCBs were found to be copper (12.6%), zinc (5.6%), lead (3.1%), nickel (2.4%), aluminum (1.4%) and ferric iron (1.2%). In addition to these metals, small amounts of noble metals, such as golden and silver, were also present in minor amount. In short, all metals mentioned above constituted about 26% of PCBs, and the rest of the materials were mainly
Conclusion
In this study, adding PCBs with lower level at initial stage and higher one at late phase resulted in improved PCBs addition amounts and metals recovery while minimized inhibition of bacterial growth at the same time. Mixed culture of A. ferrooxidans and A. thiooxidans showed the highest metals bioleaching capacity, suggesting application of multiple PCBs additions for metals recycle in mixed culture as being feasible. Moreover, increased redox potential and lowered pH value elucidated the
Acknowledgements
The study acknowledged the support in grants from Key Program of Ministry of Science and Technology in China (2008BAC46B04) and Jiangsu Teachers University of Technology (KYY09003).
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