Toxic metals in WEEE: Characterization and substance flow analysis in waste treatment processes
Highlights
► Appropriate management of toxic metals contained in WEEE is important during recycling and treatment of WEEE. ► CRT TVs contain large amount of toxic metals with high concentration and thus appropriate management is highly important. ► Mid-sized equipment is a future target for managing toxic metals in WEEE because the total amount is not negligible. ► Changes in the flows of toxic metals will occur when treatment processes are modified to emphasize resource recovery. ► The flows of toxic metals and valuable materials should be managed simultaneously in recycling and treatment of WEEE.
Introduction
Waste electrical and electronic equipment (WEEE), or e-waste, has recently received extensive attention as a secondary resource (UNEP, 2009), and recovery of materials such as valuable metals contained in WEEE is currently popular in both developed and developing countries. At the same time, the hazardous nature of WEEE is also widely recognized (Tsydenova and Bengtsson, 2011), so WEEE is an important target for appropriate management for the prevention of environmental pollution.
European countries have increased their recycling of WEEE under the directive on WEEE (European Council, 2003), which covers all electrical and electronic equipment for consumer and professional use (Huisman et al., 2007). Since the properties of different types of WEEE differ and the mixture of end-of-life products is highly heterogeneous, sorting WEEE by specific characteristics is necessary to optimize processes for resource recovery (Chancerel and Rotter, 2008). Similarly, sorting is also useful for appropriate hazard management in the recycling and treatment of WEEE, because the characteristics regarding contained toxic materials are also different by types of WEEE.
In the United States, WEEE is commonly managed in municipal solid waste (MSW) treatment systems, although several types of electronics are covered in WEEE legislation and producer take-back programs in some states (Electronics Takeback Coalition, 2011). In Japan, several schemes for the separate collection and recycling of WEEE have been introduced either through legislation or on a voluntary basis (Aizawa et al., 2008, Murakami et al., 2008, Yoshida et al., 2009) as shown in Table 1. Many other types of WEEE, however, are not covered by recycling laws and are generally subject to MSW treatment. According to an estimate by Oguchi et al. (2008), those types of WEEE account for one-third of the total generation of WEEE in Japan on a weight basis. Better management of toxic substances in these items is therefore also needed. Overall, a better understanding of substances contained in various types of WEEE is necessary to improve systems of collection and recycling according to each item's specific characteristics.
An understanding of the fate of substances in WEEE recycling and treatment processes is also important for appropriate hazard management as well as for improved resource recovery. Several experimental studies have been conducted on substance flows of selected metals and brominated flame retardants associated with dismantling, shredding, and separation of WEEE in Europe (Chancerel and Rotter, 2009, Chancerel et al., 2009, Morf et al., 2005, Morf et al., 2007). Oguchi et al. (2012) investigated the fate of 55 metals contained in WEEE in shredding and separation processes at an MSW plant in Japan. Residue from such processes, however, is often landfilled either directly or after being incinerated. Further study is needed on the fate of substances throughout the entire recycling and treatment system for WEEE, including the treatment of residues.
As a basis for discussion toward better management of substances in WEEE, this article presents a characterization and substance flow analysis of toxic metals in WEEE. First, by using data on product composition and the amount of end-of-life products generated, this study examined the relative importance of various types of WEEE in terms of managing both toxic and valuable metals. Second, this study investigated the distribution behavior of metals in a typical process for MSW thermal treatment in Japan. Based on that, a case study was conducted to estimate the final destinations of selected metals contained in WEEE under different treatment scenarios.
Section snippets
Characterization of WEEE in terms of contained valuable and toxic metals
The authors, along with other colleagues, previously discussed the relative importance of various types of WEEE as targets for metal recovery (Oguchi et al., 2011). From an analogy between natural resource development and metal recovery from WEEE, it was pointed out that not only metal concentration but also the total amount of metals contained in WEEE should be considered in the characterization of WEEE as secondary metal resources. In the analysis, various types of WEEE were categorized into
WEEE characteristics in terms of contained toxic metals
Table 2 shows the median weight fractions of the materials in 24 types of WEEE, and Table 3 shows the concentrations of six toxic metals in printed circuit boards in these items. Note that the concentrations for Cr were those of total chromium, not hexavalent chromium. The median concentrations for Ba in CRT panel glass and Pb in CRT funnel glass were 79,000 and 215,000 mg/kg-glass, respectively. Concentrations of selected common metals and precious metals that were reported in Oguchi et al.
Characteristics of WEEE in terms of contained valuable and toxic metals
Table 4 summarizes the characteristics of each WEEE item based on the categorization results for toxic metals. The relative characteristics of the examined items may be similar in other countries because the compositional breakdown of WEEE items was found not to be very different between Japan and other countries from the comparison of the estimates by Oguchi et al. (2008) and Huisman et al. (2007). It would, of course, be possible to conduct similar characterizations in other countries as data
Conclusions
This study discussed the relative importance of various types of WEEE in managing toxic metals from the viewpoint of both the concentration and the total amount of contained toxic metals. Appropriate recycling and disposal of CRT televisions should notably contribute to better management of toxic metals contained in WEEE. A future challenge is discovering how to manage toxic metals in mid-sized items such as audio/video and ICT equipment because, although they contain moderate concentrations of
Acknowledgments
The authors thank the city of Hitachi for the cooperation in the investigation. The authors thank the editor and two anonymous reviewers for their helpful comments. This work was partly supported by the Environment Research and Technology Development Fund from the Ministry of the Environment, Japan (K113011) and the Japan Society for the Promotion of Science KAKENHI (22360387).
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