ABSTRACT
Issues surrounding the impact and management of discarded or waste electronic and electrical equipment (WEEE) have received increasing attention in recent years. This attention stems from the growing quantity and diversity of electronic and electrical equipment (EEE) used by modern society, the increasingly rapid turnover of EEE with the accompanying burden on the waste stream, and the occurrence of toxic chemicals in many EEE components that can pose a risk to human and environmental health if improperly managed. In addition, public awareness of the WEEE or “e-waste” dilemma has grown in light of popular press features on events such as the transition to digital television and the exportation of WEEE from the United States and other developed countries to Africa, China, and India, where WEEE has often not been managed in a safe manner (e.g., processed with proper safety precautions, disposed of in a sanitary landfill, combusted with proper air quality procedures). This paper critically reviews current published information on the subject of WEEE. The definition, magnitude, and characteristics of this waste stream are summarized, including a detailed review of the chemicals of concern associated with different components and how this has changed and continues to evolve over time. Current and evolving management practices are described (e.g., reuse, recycling, incineration, landfilling). This review discusses the role of regulation and policies developed by governments, institutions, and product manufacturers and how these initiatives are shaping current and future management practices.
WEEE poses a challenge to environmental professionals. Recent investigations document that when managed improperly, WEEE poses an environmental and human health risk. Properly designed and operated WEEE recovery and recycling programs—implemented in many parts of the world as a result of regulatory action, producer initiative, and consumer demand—provide a safer and more sustainable WEEE management strategy. Recent research priorities have focused on quantifying environmental risks and developing efficient recovery technologies and strategies. Continued emphasis on such research is warranted for the foreseeable future.
INTRODUCTION
In the past 30 yr, a rapid growth in new technological innovations has changed the way society works, communicates, interacts, and lives. Much of this evolution centers on the design, production, and use of electronic and electrical equipment (EEE), ranging from computers to cell phones to digital cameras to smart appliances. Innovation has proceeded at such a rapid pace that EEE routinely becomes outdated after only a few years to a few months of use. The logistical, environmental, and societal challenges posed by the management of discarded EEE that has reached end of life (EOL; e.g., broken, outdated, unwanted) are the focus of this critical review.
Once viewed as a relatively minor part of the solid waste stream, one of any number of discarded commercial products (e.g., EOL computers, cell phones, televisions [TVs], video games, appliances, and the myriad of other devices powered by electricity) are now recognized as a distinct and important category of solid waste. Although colorful terms such as “electronic waste” (e-waste) and “electronic scrap” (e-scrap) are often used to describe this collective waste stream, in this review the term “waste electronic and electrical equipment” (WEEE), coined in the European Union (EU), is used. Today WEEE poses one of the major challenges facing the solid waste management community—a challenge that extends beyond the operators, engineers, and regulators who normally deal with solid waste issues and includes original equipment manufacturers (OEMs), retailers, and consumers.
Accompanying the distinction of WEEE as a unique waste stream has been the growing recognition of the potential human and environmental health challenges resulting from the mismanagement of WEEE. Although any new and expanding waste stream poses challenges with respect to storage, collection, recycling, disposal, and the environment, WEEE is particularly problematic because of the vast array of chemicals and components used to manufacture EEE. These include known toxins (e.g., lead [Pb], mercury [Hg], and polychlorinated biphenyls [PCBs]) and components that possess great recovery value (e.g., gold [Au], platinum [Pt], and copper [Cu]). Scientific research on the characterization and management of WEEE emerged in the latter half of the 1990s, including several studies assessing the potentially hazardous chemicals contained within EEE. In the first decade of the 21st century, scientific research on WEEE increased tremendously, motivated by new regulatory requirements in the developed world and several high-profile cases of human and environmental harm resulting from WEEE management in developing countries.
This review presents a broad overview of WEEE as a distinct solid waste stream, including its characterization (composition and magnitude), regulation, management, and environmental issues.
WEEE DEFINED
In a general sense, the types of devices included when defining WEEE are often apparent, but with respect to language promulgated by governments for regulating WEEE, specific detailed definitions are necessary. The EU's directive pertaining to WEEECitation1 defines WEEE as “[e]quipment which is dependent on electric currents or electromagnetic fields to work properly and equipment for the generation, transfer, and measurement of such currents and fields falling under the categories” described in as well as “designed for use with a voltage rating not exceeding 1000 Volt for alternating current and 1500 Volt for direct current.” On the basis of this definition, WEEE includes common household and personal items such as TVs, computers, cellular phones, and printers as well as devices such as large and small appliances, fluorescent lamps, power tools, toys, and drink dispensers.
Table 1. Products covered under the EU definition of WEEE.Citation1
The term WEEE was coined as part of the EU directive bearing its name, but the term is used in this review to provide a common descriptor. The types of electronic devices and electrical equipment included in government-specific regulations differ, with some rules adopting a broad array of devices similar to the EU (e.g., Japan, China), and others focused on a smaller subset (e.g., United States). The laws responsible for these definitions include those designed to enact recycling or take-back requirements, to prevent improper disposal, or a combination of both objectives. As will be discussed, no equivalent encompassing definition exists in the United States because of the absence of a comprehensive federal regulation. Thus, many individual state governments have developed their own WEEE rules that cover specific classes of WEEE items.
State regulations in the United States generally place a greater focus on WEEE known to contain components with potentially hazardous chemicals, such as display units or printed wiring boards (PWBs). For example, the state of New York defines “covered electronic equipment” as part of the Electronic Equipment Recycling and Reuse Act to include devices such as TVs, computers, computer peripherals, and small electronic equipment such as videocassette recorders (VCRs), DVD players, and electronic game consoles. The New York statute does not include many of the items on the EU WEEE list such as radios, video cameras, telephones, household appliances, or commercial medical equipment.Citation2 Covered electronic devices under California's Electronic Waste Recycling Act include devices with a cathode ray tube (CRT), liquid-crystal display (LCD), or plasma TVs.Citation3 As presented in , the 25 U.S. states with WEEE-related statutes or regulations generally target electronic devices (e.g., computers) more than electrical equipment (e.g., appliances)
Table 2. States that have passed laws pertaining to WEEE, varying from management methods to disposal bans to purchase fees.Citation199
WEEE COMPOSITION AND CHEMICAL CONTENT
WEEE can be characterized on a larger scale in terms of the types of components found within the devices (e.g., PWBs, CRTs, plastics) and on an elemental or chemical scale that describes the chemical content of toxic and precious metals or other inorganic chemicals (e.g., Pb, Hg, silver [Ag], and Au) and organic chemicals (e.g., brominated flame retardants [BFRs]).
Component Composition
The regulatory definitions presented previously describe the types of EEE that generally comprise WEEE. These devices can be characterized by several common components that make up the bulk of WEEE. summarizes the general component categories found in WEEE, such as product casing and support structures, PWBs, display devices, and memory devices. Plastics and metals (ferrous and nonferrous) are used as primary structural support and protective casing for system components, with the metal and plastic components often constituting most of the mass of WEEE. However, in the example of display devices (e.g., TVs and computer monitors), the display device (e.g., the CRT) contributes the greatest mass.
Table 3. Major WEEE component groups
The two components most commonly discussed in WEEE research studies are PWBs, also frequently referred to as printed circuit boards, and display units. PWBs consist of conductive circuitry (typically Cu) embedded in a nonconductive supporting board (typically composed of glass fibers set in an epoxy resin) and the accompanying circuitry components, including resistors, capacitors, relays, batteries, memory devices, and the conductive solder that connects the components. Display devices include CRTs, vacuum fluorescent displays (VFDs), and flat-panel display (FPD) devices such as LCDs, plasma display panels, and inorganic semiconductor liquid-emitting diodes (LEDs). Researchers and the regulatory community focus to a greater extent on PWBs and display devices because they contain the greatest amount of hazardous and valuable elements in EEE (discussed in the following section).
Several investigations provide characterization data on the relative contribution of different components (e.g., battery, metals, plastics, PWBs, wires) encountered in specific WEEE devices (e.g., cell phones, color TVs, keyboards). Results from several of these efforts are compiled in . Chancerel and RotterCitation4 provide detailed characterization data on the component composition of 17 different types of WEEE. As one would expect, the relative contribution of different components depends on the design and function of the specific device. Dimitrakakis et al.Citation5 characterized the composition of small WEEE (plastics were the largest component by mass), whereas Matsuto et al.Citation6 characterized large WEEE such as refrigerators and washing machines (ferrous metals were the largest component). For CRT TVs and computers monitors, the CRT represents 60% or more of the mass of the device.Citation7 Although most of these data were gathered by manually separating and weighing WEEE components, some characterization data are provided for processed WEEE; for example, Cui and ForssbergCitation8 characterized and compared shredded TV scrap and PWB scrap, noting that PWB scrap was richer in nonferrous and precious metals compared with TV scrap.
Table 4. Composition of major EEE devices.219
Because a challenge in recycling plastics from WEEE is the many different types of polymers that have been used (), several studies have focused on characterizing the relative abundance of different WEEE plastic polymers. In the characterization of small WEEE by Dimitrakakis et al.,Citation5 the three most common plastics polymers were acrylonitrile butadiene styrene (ABS), polypropylene (PP), and polystyrene (PS; usually referred to as high-impact polystyrene, high-energy impact polystyrene (HIPS)). WEEE plastics are often blends of polymersCitation9 and in many cases include flame retardants (discussed below). In their study, Chancerel and RotterCitation4 included an assessment of the different plastic polymers and polymer blends encountered in the various WEEE types characterized. For example, dominant plastic polymers were found to be PP for coffee machines and toasters, ABS for vacuum cleaners and computers, and acrylontrile butadiene styrene/polycarbonate blend (ABS/PC) for mobile phones and laptops.
Table 5. Major plastics found in EEE
Chemical Composition
Nearly every element in the periodic table is encountered in WEEE. lists many of the elements reported in different WEEE components, which include over half of the naturally occurring elements and one of the synthetic elements (americium). Certain elements have gained notoriety because of their potentially hazardous properties, including Pb (located in CRT glass and PWB solder), Hg (located in backlighting units in FPDs, switches, relays), cadmium (Cd; located in batteries), and beryllium (Be; located in heat sinks). Others are more noted for their recovery value upon reclamation (e.g., Au, Ag, Pt, palladium [Pd]). Although most of the inorganic elements are associated with the PWBs and display units, plastics have also been found to contain heavy metals (e.g., Cd and Pb).
Table 6. Inorganic elements in EEE
Inorganic element concentration ranges encountered in WEEE components for several characterization efforts are summarized in . In some cases, the data presented were produced as the outcomes of general WEEE characterization studies.Citation4,Citation5,Citation8,Citation10 Other research focused on characterize ing a specific WEEE type such as CRT glass,Citation11 cell phone plastics,Citation12 or WEEE plastics in general.Citation13 Because metal recovery from PWBs or processed PWBs is the prime objective of PWB recycling, research investigating PWB recovery techniques often provides elemental characterization data.Citation14–16 Cu, Pb, and tin (Sn) are among the elements with the greatest concentration, whereas lower, notable concentrations of toxic (e.g., Cd, Hg) and precious (e.g., Au) elements justify the attention focused on WEEE with regard to possible environmental impact and resource recovery
Table 7. Concentrations of major toxic and precious metals measured in WEEE components
Potentially toxic organic pollutants also occur in some WEEE components. PCBs may be present in older devices in several different locations (e.g., capacitors, plastic cables).Citation10,Citation17 The most heavily researched class of or ganic chemicals in WEEE is flame retardants. Flame retardants are added to plastic casings, PWBs, and other plastic components to provide properties that inhibit or resist the spread of fire. The more commonly used flame retardants in WEEE plastics are listed in , which shows that BFRs are the most prevalent. Several studies have reported measured concentrations of flame retardants in WEEE plastics.Citation9,Citation17–19 summarizes mea sured concentrations of major flame retardant compounds and PCBs in WEEE components. TV housings can contain a relatively large mass of flame retardant (upward of 10%).
Table 8. Common flame retardants used in electrical and electronic equipment.Citation17–19 220
Table 9. Concentrations of organic chemicals measured in WEEE components
GENERATION
Several authors have attempted to quantify the amount of WEEE generated in different world regions. Because actual counts or weights of EEE reaching EOL cannot be directly measured in a reliable manner, rates for EEE reaching EOL rely on predictive methodologies that incorporate production or sales statistics and estimated product lifespan. Using this technique and predictions for product sales in upcoming years, future WEEE generation can be forecasted. In most estimates, a constant product lifespan (e.g., computer lifespan = 3 yrCitation20) is assumed; however, Babbitt et al.Citation21 noted that product lifespan often changes over time, which should be factored into forecasts.
The U.S. Environmental Protection Agency (EPA) has published several WEEE estimates for the United States.Citation22Using two different methodologies, total generation of selected EEE categories (i.e., computers, monitors, peripherals, TVs, cell phones, and printers) in 2005 was estimated at 1.9–2.2 million t (251–342 million units).Citation23 Thisamount represents slightly less than 1% of the total weight of municipal solid waste (MSW) generated in the United States in 2005.Citation24 Estimates of 2005 WEEE generation in the EU range from 8.3 to 9.1 million tCitation25; this estimate covers the broader range of products described in . When like categories in these two studies are compared, the per capita generation rates are similar. For example, EOL TVs were generated at 2.6 kg/person in the United States and 2.4 kg/person in the EU. presents the estimated WEEE composition for each of these studies for the WEEE categories investigated in the generation forecast.
Figure 1. Typical breakdown in thousand tons (t) of WEEE by device types: (a) 2005 EU WEEE on the basis of all WEEE directive categories (total of 9.1 million t)Citation25 and (b) 2005 U.S. selected WEEE categories (total of 2.2 million t).Citation23
In specific assessments of EOL cell phones, Jang and KimCitation26 estimated that 13.8 million cell phones were retired in Korea in 2005, and Osibanjo and NnoromCitation27 predicted that approximately 8 million cell phones would reach EOL in Nigeria in 2007. In comparison, 71–117 million cell phones were estimated to reach EOL in the United States in 2005.Citation23 Forecasts for computers reaching EOL in coming years were presented by Yu et al.Citation28 for different world regions and by Dwivedy and MittalCitation29 for India. These studies suggest that the magnitude of computers reaching EOL in the developing world in the future will greatly increase as a result of more recent (and upcoming) penetration of personal computer use in society.
Many of the forecasts also provide estimates for EEE disposition at EOL. Disposition end points assessed typically include disposal (landfilling, incineration), recycling, reuse, and placement into storage. EPA's study estimated that from 2003 to 2005, 44% of the targeted WEEE items were disposed, 45% were put into storage or reused, and 11% were recycled.Citation23 These estimates were derived from total EOL production predictions and data such as typical consumer disposition gathered from surveys, reported collection, and recycling statistics as well as assumptions regarding consumer behavior. It has long been recognized that household electronic devices are simply placed into storage because of their perceived value, concerns over personal information, or uncertainty regarding proper disposal method. Saphores et al.Citation30 sampled U.S. households and estimated that 470 million small electronic devices and 277 million large electronic devices were being stored, more than estimated by EPA.Citation31,Citation32
WEEE POLICY AND MANAGEMENT FUNDAMENTALS
The manner in which WEEE is managed upon EOL depends on applicable regulations and policies in place (e.g., disposal bans, recycling bans); existing infrastructure for handling such materials (e.g., available of take-back centers, collection opportunities); and the consumers understanding of and attitudes toward these programs, policies, and opportunities. In this section, U.S., EU, and international regulations pertaining to WEEE are reviewed, followed by an introduction of typical management practices.
Existing Regulations
Laws and regulations pertaining to WEEE depend on the requirements of the governing body in question. Some focus on minimizing the impact of problematic WEEE chemicals on human health and the environment through more rigorous management requirements (e.g., as hazardous waste), disposal bans, or restrictions on hazardous chemicals that may be present. Others require the development of institutions and infrastructure to promote recycling of WEEE, a major element of which is the creation of a scheme to finance collection and recovery. Although the first major WEEE legislation evolved and was implemented in Europe,Citation1 many nations now have promulgated their own legislation, with well-developed systems in AsiaCitation33 and Canada.Citation34–37
The EU has taken a progressive stand on WEEE through two major directives. The WEEE directive requires that member states develop schemes that provide for the collection and recycling of WEEE through extended producer responsibility (EPR).Citation1 Producers of EEE register and pay a fee to EU member states to provide for the take-back and recycling of the produced products at EOL. As a result of the WEEE directive, a robust infrastructure for WEEE collection, transportation, and recycling has developed a combination of retail and manufacturer take-back programs, municipal collection centers, and third-party processors and recyclers. Lauridsen and JorgensenCitation38 discuss the WEEE directive and the relationship between the disparate industries of EEE manufacture and sales and waste management. The EU's restriction of hazardous substance (RoHS) directive requires manufactures to limit the content of specific toxic chemicals in new electronic equipment sold in the EU.Citation39 With certain exceptions, RoHS-compliant components must contain a minimum level of six substances: Pb (<0.1%), Cd (<0.01%), Hg (<100 parts per million [ppm]), hexava-lent chromium (Cr; <0.01%), polybrominated biphenyls (PBBs; <0.1%), and polybrominated diphenyl ethers (PBDEs; <0.1%).
In the United States, no federal regulation specifically pertaining to the broad class of devices constituting WEEE exists, although as will be discussed below, the need for such legislation has been recognizedCitation40 and attempts have been made in the past and are currently underway to develop a national framework for WEEE management.Citation41
With respective to protection of the environment, because a discarded electronic device would be considered a solid waste under most circumstances, other applicable federal rules would apply, most notably those pertaining to hazardous waste. One possible method through which WEEE would be considered a hazardous waste under U.S. rules would be if it met the criteria of a toxicity characteristic (TC) hazardous waste. Designation as a TC waste is determined using the toxicity characteristic leaching procedure (TCLP); if TCLP concentrations for a waste are greater than the prescribed regulatory threshold (the TC limit), then the waste is hazardous (unless otherwise specifically excluded). Because several TC elements (e.g., Cd, Hg, Pb) are commonly found in WEEE, many devices have the potential to be TC hazardous wastes.
Musson et al.Citation7 tested CRTs and found that color models generally exceeded the TC limit for Pb. EPA subsequently passed federal legislation stating that color CRTs were expected to be hazardous for Pb and that unless they were managed following specific recycling protocols, they must be managed as hazardous waste.Citation42 Although the problematic nature of performing the TCLP on bulky, heterogeneous devices such as computers and similar equipment has historically precluded the routine testing of WEEE, a few researchers have examined several WEEE categories devices and found that in many cases, the TC thresholds for Pb (5 mg/L) are exceeded.Citation7,Citation43,Citation44 Although other TC elements are encountered in WEEE, they do not typically trigger the hazardous waste threshold. presents the range of TCLP Pb concentrations measured from several studies.Citation7,Citation45–48 reported TCLP Pb concentrations for cellular phones to range from 38 to 147 mg/L. The large range of TCLP Pb concentrations results from not only the different Pb content among devices, but also from the other device materials present; for example, the amount of ferrous metal present in a device has been determined to have a major impact on Pb leachability using TCLP.Citation49
Figure 2. TCLP results for various WEEE as compared with the U.S. TC hazardous waste threshold for Pb (5 mg/L).
Aside from the CRT ruleCitation42 and existing hazardous waste regulations,Citation50 U.S. federal regulations do not govern WEEE management. As a result, individual U.S. state governments have enacted their own legislation: currently 24 of the 50 states have done so (). Some states (i.e., California, Connecticut, Hawaii, Illinois, Indiana, Maine, Minnesota, New Jersey, New York, North Carolina, Oregon, Rhode Island, and South Carolina) ban the disposal of specified WEEE in municipal landfills. Other states require that OEMs or retailers take back WEEE for proper management. In most states, the costs associated with WEEE management are borne by the OEM or retailer; however, in California, consumers now pay an advanced disposal fee at the time of purchase that goes toward EOL WEEE management.Citation3 California has also enacted legislation limiting hazardous chemical content of EEE, similar to the EU RoHS directive.
The international transport of WEEE from developed countries to developing countries has been a major issue of concern in the past decade (as highlighted in Environmental Concerns). The Basel Convention for the Trans-boundary Movement of Hazardous Waste is an international treaty designed to limit the waste that developed countries can dispose of to developing countries.Citation51 Limiting exports of WEEE has been a focus of many followers of the Basel Convention, although the determination of when WEEE export constitutes shipment of products versus waste is a question of contention. The United States is not party to the Basel Convention.
Management Options and Decision-Making
Options available for managing WEEE depend substantially on location and governing regulatory structure. From the perspective of the developed world, homeowners dispose of their devices through the normal household waste system, they return them to the manufacturer or retailer, or they dispose of them at a third-party recycling operation. Businesses and institutions have similar options, although these may be limited by the regulatory classification as a hazardous waste. Currently, WEEE managed with other municipal waste will be landfilled or combusted. When recycled, WEEE will go through several processes and likely change hands several times before reaching a final disposition. All of these steps are the focus of the following sections.
Several authors have explored management issues, challenges, and options as well as governmental or public attitudes regarding WEEE from a global perspectiveCitation20 and for specific nations or states, including the United States,Citation52 Europe,Citation53,Citation54 China,Citation55–58 India,Citation59 Australia,Citation60 and Africa.Citation61 Several studies describe the application of specific decision-making tools to assess and rank appropriate management strategies, including Monte Carlo simulation,Citation62 mixed integer linear programming models,Citation63 life-cycle assessment (LCA)-based multiobjective optimization models,Citation64 fuzzy linear programming technique for multidimensional analysis of preference methodology,Citation65 a knowledge-based process planning approach,Citation66 production theory methodology,Citation67 and the analytical hierarchy process.Citation68
Several researchers have evaluated LCA principles as a means to guide the decision-making process regarding the regulation, disposition, and integrated management of WEEE. Bientinesi and PetarcaCitation69 used LCA to compare two types of thermal treatment systems for brominated WEEE plastics: co-combustion in a MSW combustion facility and staged gasification. The staged gasification system was concluded to have a smaller global environmental impact. Using a LCA analysis, Barba-Gutierrez et al.Citation70 described how for some LCA impact categories (e.g., impacts from fossil fuels and smog formation) recycling may not always be the best option relative to landfilling if required transportation distances (for collecting household appliances) were great. Dodbida et al.Citation71 used LCA methodology to compare two options for WEEE plastics: energy recovery and mechanical recycling. Mechanical recycling was found to be the more attractive option with respect to environmental burden. Hischier et al.Citation72 applied a combined approach of materials flow analysis and LCA to examine two WEEE management systems in Switzerland. Collection and processing of recovered materials into marketable products (e.g., smelting) were concluded to have a much greater negative impact compared with sorting and dismantling; WEEE recycling was determined clearly advantageous from an environmental perspective.
ENVIRONMENTAL CONCERNS
Global Concerns
The presence of potentially toxic inorganic elements and organic chemicals in WEEE components raises the concern of deleterious impacts on human health and the environment when this waste stream is improperly managed. Regulatory safeguards are in place in some locations and for some elements of the WEEE (e.g., hazardous waste classification for some WEEE). In locations such as the United States, Canada, Japan, the EU, and Australia where regulatory controls are in place to protect human health and the environment through the WEEE lifecycle (i.e., processing, recycling, disposal), there is no body of evidence currently pointing to harm caused by the chemicals in WEEE. However, sufficient safeguards have not been in place to protect human health and the environment for much of the WEEE shipped from the developed world to locations such as China, India, and Africa. The evidence of harmful impact is clear when WEEE is managed through techniques such as open burning or melting, unprotected manual separation, uncontrolled acid treatment, and open dumping.Citation73 The body of literature documenting this evidence is presented in the following subsection.
With respect to the developed world where controls are in place, routes of possible concern for human health and the environment include those occurring during processing and recycling, combustion, and disposal. Workers in recycling operations have the potential to be exposed to chemicals through fine particles, such as dust and size-reduced WEEE. Workers at an electronics recycling plant in Sweden were documented to have elevated blood serum concentrations of PBDEs,Citation74 which was traced to flame retardants in indoor air.Citation75 Follow-up studies conducted after changes were made to improve worker exposure documented reduced blood serum concentrations.Citation76 Similar exposures were documented for electronics dismantlers in Norway.Citation77 Thus, in recent years, efforts have focused on developing worker safety standards and facility certifications in the WEEE recycling industry.Citation78 When WEEE is combusted, the major concerns cited include the presence of polybrominated dibenzodioxins (PBDDs) and dibenzofurans (PBDFs, commonly grouped together as PBDD/Fs). The environmental exposure route cited for WEEE disposed in landfills is contamination of ground-water. The body of science regarding these concerns is presented in subsequent sections on processing, thermal treatment, and land disposal.
Other broader evaluations of environmental impacts with different WEEE management options have been explored as part of LCA or similar analysis.Citation54,Citation79 In these studies, considerations such as the presence of a toxic chemical and its possible ecological or human health significance (chemical concentrations in WEEE and their emissions as compared with risk-based concentration thresholds) or the impact on global warming (the carbon footprint resulting from recovering materials from recycling vs. that from virgin materials) are considered. For example, Macauley et al.Citation80 examined the costs of CRT computer monitor recycling and concluded that the savings in avoided health costs from CRT disposal outweighed the increase in management costs; this analysis assumed that Pb migrated from a landfill. Lim and SchoenungCitation81 examined toxicity potentials of discarded cell phones using reported data to provide information that could be used in decision-making. Lim and SchoenungCitation82 examined the potential impacts to human health and the environment from heavy metals in WEEE equipped with FPDs using industry-reported data and a LCA approach. The authors concluded that EOL FPDs posed less human health impact compared with Pb-rich CRTs, but they posed a greater ecological risk from higher amounts of Hg (LCDs) and Cu (plasma TVs)
Uncontrolled Management Concerns
At the end of the 20th century, as more and more governments, institutions, and businesses began to promote and use WEEE recycling programs, the processing capability of WEEE in the United States and other developed countries was outpaced by the amount of material accumulated. Thus, as is common with many types of solid wastes, large amounts of WEEE were shipped overseas, particularly to China, but also to Southeast Asia, India, and Africa. Several environmental groups documented evidence of recycling areas in which WEEE was processed, often in a very primitive manner, to recover valuable metals and other components from the waste.Citation83,Citation84 These processing techniques often relied on open burning of PWBs, acid extraction of metals, and manual disassembly of WEEE components without any personal protection equipment. The remnants of the processed WEEE were in many cases burned, deposited in open dumps, or placed in water bodies. When photos and videos of these operations emerged on the international setting, concerns of human health and environmental contamination prompted many research studies, particularly in China.
Several review articles have summarized recent research on the human and environmental health consequences of substandard WEEE recycling operations in developing countries and concluded that such activities have resulted in severe pollutant contamination, thus posing a considerable risk.Citation85–88 Studies on pollutant transfer in the vicinity of these facilities include measurements of soil, sediment, dust, air, and biomass (plant and animal) concentrations as well the impact on humans through measurement of pollutant concentrations in blood serum, breast milk, and placenta. The chemicals measured include those described previously as commonly found in WEEE components, such as heavy metals and flame retardants. Other measurements focus on byproducts of the processing operations (particularly burning) such as polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs; often grouped together as PCDDs/Fs), PBDDs/Fs, and polycyclic aromatic hydrocarbons (PAHs). summarizes the range of pollutant concentrations measured in many of the studies; additional description of these studies follows.
Table 10. Concentration level in soil, air, or biomass from uncontrolled recycling sites of WEEE
With respect to soils near substandard WEEE recycling areas, Cai and JiangCitation89 measured up to 600 parts per billion (ppb) of PBDEs in nearby soils, and Luo et al.Citation90 reported elevated PBDE concentrations in adjacent farmland and road soils. Shen et al.Citation91 measured PCDD/F, PCB, and PAH concentrations in agricultural soil, Luo et al.Citation92 detected PBDEs in river sediment and fish tissue, and Leung et al.Citation93 documented PBDEs and PCDDs/Fs in soils and recycling residuals. Ma et al.Citation94–96 measured chlorinated and parent PAH compounds, PCDDs/Fs, PBDDs/Fs, and PBDEs in soils and vegetation near a WEEE recycling facility and found PBDE soil concentrations 13–21% lower after plant harvesting. The potential of several plant species to phytoremediate PCB-contaminated soils near a China WEEE recycling operation was reported by Shen et al.Citation91 Dust samples in and adjacent to WEEE recycling operations in Southeast China were collected and found to have elevated heavy metal concentrations with respect to human health thresholds.Citation97 Muenhor et al.Citation98 measured BFR concentrations in dust and air from WEEE storage facilities in Thailand and found that concentrations were much lower than those encountered at WEEE recycling operations.
Pollutant emissions from burning WEEE components to recover metals are a concern, and several researchers have characterized ambient air quality, including total suspended particles (TSPs) and particulate matter (PM), in the vicinity of substandard WEEE recycling operations.Citation99–101 Bi et al.Citation100characterized PM from uncontrolled WEEE processing; triphenyl phosphate (TPP) and bisphenol A were elevated compared with typical levels and notable levels of Pb, zinc (Zn), and Sn were also observed. Li et al.Citation102 documented elevated PCDD/F and PBDD/F concentrations in ambient air surrounding a WEEE recycling area and found that organic constituents (primarily organophosphates) made up approximately one-half of the PM. Deng et al.Citation103 foundthat PBDE concentrations in TSPs and PM2.5 (PM ≤ 2.5μm in aerodynamic diameter) samples collected near a WEEE recycling facility were several orders of magnitude greater than those collected in urban reference areas; the elevated PBDE emissions were attributed to open burning or plastics containing BFRs. Chen et al.Citation104 reported the diurnal concentrations of atmospheric PBDEs in a WEEE dismantling plant and noted that the greatest concentrations were observed in the daytime. Han et al.Citation99 documented increased atmospheric PCB concentrations (PM2.5, TSP, gas phase) near a WEEE recycling region in China; although PCB concentrations had decreased over the decade, they were still much higher than a reference urban site. Tue et al.Citation105 found that settled dust collected from homes near WEEE recycling sites exhibited dioxin-like activity. An increase in heavy metal concentration in air was observed at WEEE recycling sites in Bangalore, India.Citation106
Elevated pollutant concentrations have been measured in animal and plant tissue in WEEE-impacted areas.Citation107 Elevated concentrations of PBDEs (and in some cases PCBs) have been observed in birds,Citation108 fish and shell- fish,Citation109 free-range domestic fowl,Citation110 and amphibians.Citation111 Jun-Hui and HangCitation112 documented metal contamination and toxicity to root cells in some soils associated with a WEEE recycling area. Fu et al.Citation113 measured the concentration of heavy metals in rice grown near a WEEE recycling area and concluded that recommended human intake levels were approached or exceeded, particularly for Pb and Cd. Plant uptake of PBDEs from soils collected from a WEEE recycling site was documented by Huang et al.Citation114
Several studies focused on direct human impact.Citation115 Bi et al.Citation116 compared blood serum concentrations of two populations in China and found elevated PBDE concentrations for a group located near a WEEE processing operation compared with a reference population. Testing of workers from WEEE recycling areas has found elevated concentrations of PBDEs,Citation67 hydroxylated octa- and nonbromodiphenyl ethers,Citation117 and the chlorinated flame retardant Dechlorane PlusCitation118 in blood serum. PCDD/F body burdens—as measured in human milk, placenta, and hair—were significantly higher in child-bearing-aged women at a WEEE processing site compared with a reference site.Citation119 Pregnant women near a WEEE recycling area were concluded to have higher body burdens of PBDE, PCDD/F, and PCB compounds (on the basis of measurement of maternal serum and cord blood) compared with the reference group results.Citation120 Guo et al.Citation121 measured the concentration of several heavy metals in placentas collected from a WEEE recycling area and a reference area and found a correlation between placenta Pb levels and residence in the recycling area.
WEEE RECOVERY, PROCESSING, AND RECYCLING
The stages of WEEE recycling include collection, storage, and transportation to a designated processing facility. Processing involves a combination of manual and mechanical steps to separate components. Processing is performed with some degree of size reduction in the pre- or postseparation stage or both. Once materials have been sufficiently separated and size-reduced or purified (depending on the needs of the final end use), they are converted, refined, or otherwise transformed into the desired commodity.
Collection Systems
The options available to households, institutions, or businesses for WEEE collection depend on the requirements and policies of the appropriate governing agency and the infrastructure provided by the municipality, retailer, or OEM. In developed areas, most residential MSW is collected at front of the home (i.e., curbside) or in the vicinity. Some municipalities have instituted curbside collection of WEEE, but this practice is limited. The more common practice is for the resident to bring their WEEE to a central collection point. This might be the point of sale, but the larger trend is to provide centralized drop-off facilities. A few research studies have concentrated on WEEE collection.
Darby and ObaraCitation122 assessed public attitudes toward small WEEE recycling in Wales and found that education regarding how and where to recycle was important (not why to recycle) and that integration of WEEE recycling into more widespread recycling services would increase participation. Sidique et al.Citation123 examined the factors that influenced participation in taking WEEE to drop-off sites and reported that the facility location and convenience of use were the primary drivers; greater participation was noted from higher-income residents. Bernstad et al.Citation124examined a Swedish pilot study in which waste recycling centers for consumer-separated WEEE were placed within a residential area for better access to residents. After implementation, an increase in proper WEEE management was observed compared with the previous system where recycling centers were located at the city outskirts. Gamberini et al.Citation125 described an innovative container system for collecting WEEE that was designed as an open-top box that could be partitioned and that permitted easy side-wall lowering. This container improved collection logistics because it allowed much easier loading and unloading of collected WEEE and was easily transportable.
Achillas et al.Citation63 developed a mixed integer linear programming model to optimize the reverse logistics (collection, storage, and transport to final destination) of WEEE and provided an example of the application of this approach to a region in Greece. Ahluwalia and NemaCitation64 presented a LCA-based multiobjective optimization model for the management of computer waste as a tool for decisions in determining optimal locations for collection, transportation routes, and computer lifespans. They proposed this technique for use in determining a maximum age to be accepted for donated computers intended for reuse. Nagurney and ToyasakiCitation126 modeled reverse supply chain management of WEEE and concluded that the process could result in more efficient collection.
Evaluation and Disassembly
The first step in the WEEE recovery process is an assessment of whether a piece of EEE still functions and whether it still has a direct reuse value. This involves testing to see whether the EEE works or not. With computers, the hard drives will typically be erased and reformatted. For devices that do not work or that have limited reuse value, one option that is commonly practiced is disassembly. Another option is to mechanically process the entire device for mechanical separation; this is discussed as part of the next section. In the disassembly process, the major components are separated from one another by primarily using hand tools. Some components in the device may be identified for testing and resale (e.g., computer central processing units [CPUs], memory chips, drives). The rest of the components will be separated into major material categories including plastics, ferrous metals, nonferrous metals, wire, PWBs, and other distinct components.
Basdere and SeligerCitation127 discussed the considerations for automated and manual WEEE disassembly facilities and provided examples of the modular processes and tools to handle multiple WEEE devices. Kopacek and KopacekCitation128 discussed the potential for exploring automated systems for disassembly of WEEE. Rios et al.Citation129,Citation130 examined engineering design of EEE and proposed a set of design symbols that could be use to aid in disassembly of WEEE. Stevels et al.Citation131 examined conditions for success in take-back and disassembly programs for WEEE, citing key conditions as legislative support for take-back programs and appropriate organization and certification of the end-user industry.
Separation Technologies
At some point in the WEEE recycling process, mechanical separation techniques are used. The degree of separation required and the types of equipment used depend on the properties of the incoming WEEE materials and the desired end-markets. As described previously, the materials may be relatively uniform as a result of disassembly with a processing objective of refining or concentrating one particular WEEE component (e.g., separating different types of plastics or metals from one another). In other cases, the materials may be mixed WEEE with the goal of completing separation of major components. Size reduction through grinding or shredding is an initial step in most process streams. Once in size-reduced form, materials are separated from one another using various methods, including screening, density separation, magnetic separation, and electrostatic separation. These separation processes are typically adopted from existing technologies used in ore refining or scrap-metal processing. A growing body of scientific literature has assessed the application and modification of these technologies for WEEE.
Cui and ForssbergCitation132 reviewed major processes for separation and recovery of materials from WEEE, including dismantling and mechanical separation processes such as screening, shape separation, magnetic separation, eddy current separation, electrostatic separation, and jigging. They pointed out that optimum separation will occur only after shredding to a fine particle size. Kang and SchoenungCitation52 reviewed the infrastructure and technologies available in the United States for WEEE recycling and found that material recovery facilities use sorting, size reduction, screening of ferrous and nonferrous materials, and density separation as a standard separation sequence for WEEE. Common in other areas of the world where WEEE recycling has been practiced longer, the number of these facilities in the United States has markedly increased within the past decade. Huang et al.Citation133 reviewed current technologies used for recycling PWBs in China and commented that although substandard technologies (e.g., open burning of PWBs and acid stripping) are still practiced (as described previously), modern methods that include size reduction and electrostatic separation are becoming more widely used.
Mechanical vibratory screening is a common approach for segregating different materials and has also been applied to separate metallic and nonmetallic fractions of WEEE. The difference in size, shape, and density of WEEE components controls the success of the separation process. Mohabuth and MilesCitation134 examined the use of vertical vibratory separation of ground metal and non-metal WEEE components (using an synthetic mixture of plastic and bronze) and observed that the two components did separate. Additionally, vertical vibratory separation was examined for recovering Cu from electrical wires and computer circuit boards, with 95 and 85% of the Cu recovered, respectively.Citation135 This method was identified as environmentally responsible because it used dry air to process the waste and did not produce liquid waste. Araujo et al.Citation136 examined various mechanical processes including grinding, size reduction, density separation, electrostatic separation, scrubbing, panning, and elutriation for separation of Cu from wire cables. It was concluded that Cu could be completely removed from the cables when they were ground to a granule size less than 3 mm followed by processing techniques in which separation was accomplished as a result of differing drag forces in a moving water bath (e.g., panning, elutriation).
Magnetic and electrostatic separation techniques are commonly used in the separation of metal-bearing wastes, including WEEE. Electrostatic separation selectively sorts charged and polarized components in an electric field and can be used for segregating mixed waste streams of metals and plastics. Yoo et al.Citation13 used a two-step magnetic separation technique and extracted 83% of the nickel (Ni) and Fe from whole PWBs. Li et al.Citation137 used a two-step mechanical crushing of waste PWBs followed by corona electrostatic separation and separated approximately 70% of the nonmetallic fraction in waste PWBs. Li and XuCitation138 studied a process train with a sequence of scarping, screening, corona electrostatic separation, and dust precipitation for recovering metals from waste PCBs and observed Cu recovery of more than 95%. Veit et al.Citation139 used magnetic and electrostatic separation together to separate metal fractions from polymers and ceramic fractions of PWBs, collecting more than 50, 24, and 8% of the Cu, Sn, and Pb, respectively.
Several studies have been conducted on optimizing and improving the efficiency of electrostatic separators. Park et al.Citation140 found that electrostatic separators were efficient for segregating mixed plastics using a two-stage separation process. Hou et al.Citation141 observed a reduction in effectiveness in the separation process because of the presence of nonconductive powder and moisture in the WEEE. Consequently, Hou et al.Citation141 suggested using cyclone separators to effectively manage the granule particle size and remove nonconductive powder before processing in the electrostatic separator. Wu et al.Citation142,Citation143 examined the effect of crushed PWB particle size and the use of different electrostatic separator configurations on segregation efficiency and found that a dual roll-type system could achieve greater separation efficiency compared with the traditional single-roll system. Researchers have developed models to describe the electrostatic separation process, allowing an evaluation of controlling system performance factors such as material characteristics, particle size, and operating parameters.Citation144,Citation145 Wu et al.Citation146 developed a model to reduce the inconsistencies of previous models by adding air drag force and different charging situations and reported better agreement with the experimental results as compared with previous models.
Other processing techniques include those that separate based on shape and on density. Zhang and ForssbergCitation147 used a shape separator for separating mixed waste generated from personal computers and PWBs. Eswaraiah et al.Citation148 compared the efficiency of separating metals from nonmetals for ground PWBs using a float-sink method (specific gravity = 1.85 zinc chloride solution) and vertical column air classification. Duan et al.Citation149 examined metal recovery from PWBs using a combined process of wet impact crushing followed by centrifugal density separation. Cited advantages of wet crushing were reduced air emissions, lower temperatures, minimization of pyrolysis in the crusher, and an avoidance of overcrushing.
Methods for Extracting Metals from PWBs
The elements of greatest value in WEEE are the precious metals found in the PWBs (Au, Pt, Pd, Ag) and the Cu found in the PWBs and associated cables. In practice, these metals are recovered from the segregated WEEE components at a smelting facility already designed to extract precious metals from mined ore. A large amount of research has been conducted to explore other techniques for extracting metals from ground PWBs or from the nonmetallic fraction recovered from PWB processing. Cui and ZhangCitation150 provide a review of various metallurgic recovery techniques for metals from WEEE, including pyrometallurgical (e.g., smelting), hydrometallurical (e.g., cyanide leaching), and biometallurgical (e.g., bioleaching) methods.
In the pyrometallurgical process, the size-reduced WEEE is burned, melted, or otherwise processed under high temperatures to recover a targeted metal fraction. Most full-scale pyrometallurgical processing of WEEE scrap takes place using smelters designed for refining metals from ore or metal scrap. Research specific to pyrometallurgical processing of WEEE is limited, although Cui and ZhangCitation109 provide a good review of industrial practice and patented technology developments. Several researchers have explored modified pyrometallurgical processes for WEEE that incorporate the use of a vacuum to enhance metal separation at high temperatures. Vacuum metallurgical processes utilize differences in vapor pressures of various metals as a means to more selectively recover desired metals. Zhan et al.Citation151 demonstrated the application of vacuum metallurgy to enrich Cu from processed PWB by increasing the purity of Cu in Cu-Zn mixtures from 91% to over 99%. Long et al.Citation152 used vacuum pyrolysis (a technique also discussed under Thermal WEEE Treatment) in combination with various mechanical processes to separate PWB organic and metal fractions. A high-purity Cu product (99.5%) was obtained; the authors observed that vacuum pyrolysis could also be used to decompose organic matter into gases or liquids that can be potentially used as fuel.
In the hydrometallurgical process, processed WEEE is exposed to a chemical bath (a leaching solution) and the subsequent leachate is processed to recover the metals of interest. The most common leaching bath solutions are cyanide, halide, thiourea, and thiosulfate.Citation150 Ping et al.Citation153 used sulfuric acid (H2SO4) solution to leach Cu from the PWBs under chloride ion catalyzing conditions and observed an enhanced leaching of Cu because of the electro-oxidation technique. Liu et al.Citation154 demonstrated a process in which ground PWBs were chemically processed to recover Cu, Pb, and Sn. The ground material was first contacted with an ammonia/ammonium hydrocarbonate (NH3/NH5CO3) solution to dissolve Cu, which was refined with heating and distillation, and the remaining residue was leached with hydrochloric acid to recover Pb and Sn.
Xiu and ZhangCitation155 used supercritical water oxidation coupled with an electrokinetic process to recover Cu and Pb from PWBs. This technique decomposed the organic fraction of the PWBs and oxidized Cu and Pb, resulting in 84 and 89% recovery under optimum conditions, respectively. Veglio et al.Citation156 used H2SO4 as a leaching solution for recovering Cu and Ni from processed WEEE and observed that 94–99% of Cu and Ni were recovered at the end of process. Oh et al.Citation16 used H2SO4 and hydrogen peroxide to liberate nonmagnetic metals from segregated WEEE and extracted more than 95% of Cu, Fe, Zn, Ni, and aluminum (Al). Oh et al.Citation16 also extracted more than 95% of the precious metals Au and Ag using ammonium thiosulfate ([NH4]2S2O3), copper(II) sulfate (CuSO4), and ammonium hydroxide (NH4OH) leaching solutions.
In the biometallurgy process, micorganisms aid in the metal extraction process. Acid bioleaching uses Fe and sulfur-oxidizing bacteria (acidophiles) to extract target metals from metal-rich waste; the metals are leached as a result of the H2SO4 that forms because of the sulfur-oxidizing bacteria.Citation157 Different types of bacteria have been examined for metal extraction from WEEE. Ilyas et al.Citation158 examined the use of bioleaching of crushed PWBs using several moderately thermophilic acidophilic bacteria strains; the process successfully extracted 80% or more of the Ni, Cu, Al, and Zn originally present. A column bioleaching test using thermophilic strains of acidophilic chemolothotropic and acidophilic heterotrophic bacteria successful extracted Zn (80%), Cu (86%), Al (64%), and Ni (74%).Citation159 Yang et al.Citation160 studied the factors that influence the bioleaching of Cu from WEEE using Acidithiobacillus ferrooxidans and observed that Fe concentration, pH, and quantity of stock culture are the main controlling factors for Cu leaching. Brandl et al.Citation161 examined the use of a fungus (Aspergillus niger) and a bacteria (Thiobacillus thiooxidanes) for bioleaching metals from the fine dust produced during WEEE processing and were able to demonstrate greater than 90% mobilization for most metals because of formation of organic and inorganic acids.
Markets
Some components recovered from WEEE, such as basic (e.g., Fe, Cu, Al) and precious metals (e.g., Au, Pd), have existing scrap markets. For other recovered materials, markets are less available; thus, research has focused on developing uses for recovered fractions. WEEE fractions investigated for market development include CRT glass, glass from LCD displays, plastics, and the nonmetallic fraction from PWB processing.
The use of CRT glass as a feedstock for manufacturing of glass-ceramics was examined by Andreola et al.Citation162 Good crystallization was achieved when CRT glass was melted and mixed with lime and dolomite. The recovery of Pb from CRT glass and the production of a reduced-Pb foam glass using a pyrovacuum process were reported by Chen et al.Citation163 The separated Pb could be converted to a metal with a purity of 99.3% and the foam glass would pass a TCLP for Pb under optimal treatment conditions. Dondi et al.Citation164 examined the recycling of crushed CRT glass as an additive to clay bricks and roof tiles; addition of glass at up to 2% (by weight) did not bring about any changes in product performance. Mostaghel and SamuelssonCitation165 examined the potential for using CRT glass as a silica fluxing agent in the Cu smelting process. Gregory et al.Citation166 used material flow analysis to evaluate the economic viability of CRT-glass recycling and concluded the amount of new CRT glass required, although decreasing, is sufficient to warrant recycling and provide for the CRT-glass-to-glass recycling market demand (in which recovered CRT glass is used as an input for new CRT production) for the foreseeable future. Lin et al.Citation167–168 examinedthe recycling of thin film transistor (TFT)-LCD glass into glass ceramic products and as a pozzolanic material, recommending a blend of 10% TFT-LCD glass with 90% cement paste. Her-YungCitation169 evaluated crushed LCD glass as a sand replacement in low-strength concrete (up to 30% LCD glass) and found that engineering property requirements were met, with the 10% replacement test very close to the control (0% LCD glass).
Arnold et al.Citation170 examined the formation of voids in recycled WEEE ABS plastic, a feature with potential negative impacts on recycled product quality. Balart et al.Citation171 studied the properties of ABS and PC blends from recovered WEEE plastic, and although observing a decrease in mechanical properties (e.g., tensile strength) compared with virgin materials, the properties were still determined sufficient for use as engineering plastics. Imai et al.Citation172compared the recycling of WEEE plastics containing different flame retardants (brominated and halogen-free), and by using two different recycling scenarios, they examined the potential for PBDD/F formation during recycling and the mechanical properties and fire rating of the final property. No regulatory PBDD/F formation was noted. Recycled plastics originally treated with BFRs were found to be superior to those treated with the halogen-free flame retardants for mechanical properties and fire rating.
Guo et al.Citation173 reviewed the different options for recycling the nonmetallic fraction of waste PWBs, including physical methods (e.g., an additive to concrete) and chemical methods (e.g., pyrolysis). In addition, Guo et al.Citation174 investigated using the nonmetallic fraction of PWBs remaining after metal recovery as a material for creation of value-added products. The nonmetallic PWB particles were primarily in the form of fibers and when mixed with a resin could be used to produce a nonmetallic plate. In a separate examination of markets for the pulverized non-metal fraction of recycled PWBs, Guo et al.Citation175 found that utilization as an amendment to asphalt was promising at 25% by weight. Guo et al.Citation176 examined the use of glass nonmetals from recycled PWBs as an additive to a phenolic molding compound, observed no significant decrease in molding compound properties, and concluded this use as a promising market.
THERMAL WEEE TREATMENT
Research on the management of WEEE through thermal treatment focuses on two primary topics. First is the fate of WEEE in controlled combustion systems, for instance, when WEEE is mixed with MSW and combusted in a waste-to-energy (WTE) facility. Second is the use of thermal processing such as pyrolysis as a means to recover value from WEEE components such as plastics and PWBs. An overriding concern in all of these techniques is the production of PBDDs/Fs as a result of BFRs in the WEEE plastics.Citation177
The extent to which MSW is managed through controlled combustion depends on the region. In the United States, less than 15% of MSW is combusted in WTE facilities,Citation24 whereas in some EU countries and Japan, most MSW is managed through WTE. In most developing countries, modern WTE facilities are nonexistent, although as described earlier, uncontrolled burning may be practiced for disposal and as part of recycling operations. When WEEE is managed in modern WTE facilities with MSW, the relative contribution to the waste stream will be small, even where WEEE recycling efforts are minimal or nonexistent. However, the contribution to the mass loading of heavy metals and dioxin/furan precursors may be significant, thus attention is still warranted. Concerns would only be magnified in cases in which WEEE plastics were used as fuels in industrial operations because air pollution controls are often less stringent.
Very little research has focused on the contribution of metal from WEEE in combustion systems, although other studies on metal fate in MSW combustors may be applicable. Scharnhorst et al.Citation178 examined the partitioning of metals when PWBs were thermally treated in a quartz tube reactor at 550 and 880 °C under oxidizing and reducing conditions. Arsenic (As), Cd, Ni, gallium (Ga), Pb, and antimony (Sb) volatilized and the remaining solids were measured and compared with theoretical predictions. It was concluded that As, Cd, and Sb primarily volatilized, whereas Ni and Ga remained with the ash.
PBDD/F formation in combustion systems has received greater attention. Although research examining to PCDD/F and PBDD/F formation in specific response to WEEE combustion is limited, the formation of PBDDs/Fs has been documented in several combustion trials examining BFRs as part of solid waste mixtures.Citation179,Citation180 Barontini and CozzaniCitation181 examined the influence of heating rate and oxygen content on the thermal decomposition of the organic fraction of PWBs containing tetrabromophthalic anhydride (TBPA) and observed the formation of PBDDs/Fs in the presence of oxygen. Lai et al.Citation182 studied the formation of PBDDs/Fs during the thermal treatment of PWBs. PWBs were first melted in a high temperature furnace followed by a secondary combustion chamber. PBDD/F formation decreased when the furnace temperature was increased from 850 to 1200 °C and when lime was added to the PWB. Lime addition was concluded to remove hydrogen bromide to form solid-phase calcium bromide.
Several investigators have explored the potential of WEEE (plastics, PWB) pyrolysis (thermal treatment under conditions of reduced or absent of oxygen) as a resource recovery technique. Kantarelis et al.Citation183 examined the kinetics and activation energy of WEEE pyrolysis, whereas Quan et al.Citation184 conducted thermogravimetric analysis and examined pyrolysis kinetics of PWBs. Hall and Williams,Citation185 Marco et al.,Citation186 and Guan et al.Citation187 studied the pyrolysis of several WEEE components and evaluated the relative fraction of different pyrolysis products (i.e., liquid, gas, solids). The liquid (oil) product obtained typically accounted for the greatest conversion product by mass, although the solid residual might be large if the WEEE components originally included inorganic components (as would be the case with a PWB).
PBDD/F formation during pyrolysis has been examined. In most studies, halogens in pyrolysis products were more concentrated in the solid conversion product. Luda et al.Citation188 studied the fate of bromine in model WEEE plastics containing BFRs and found that most of the bromine partitioned to the solid residue fraction. However, thermal treatment in the absence of oxygen did not always preclude the formation of PBDDs/Fs. Molto et al.Citation189 presented data on the volatile emissions of WEEE plastics subjected to pyrolysis and combustion and reported that both conditions resulted in the formation of PCDDs/Fs.
ENGINEERED LAND DISPOSAL OF WEEE
Although many view disposal of solid waste in landfills as the least desired management method from a sustainability and environmental standpoint, landfills remain the predominant method of solid waste disposal in many parts of the world because of the current lower cost of this technology. As described in the studies pertaining to uncontrolled WEEE recycling and disposal in developing countries, WEEE and their components contain a sufficiently large concentration of potentially toxic chemicals that can impact human health and the environment because of chemical discharges during improper dumping on the land (e.g., open dumps).
In developed countries where landfilling of MSW is practiced, unless otherwise removed, WEEE is disposed of in an engineered sanitary landfill. Although modern land-fills are equipped with liners, leachate collection systems, and gas control devices to contain harmful emissions, environmental risks posed as a result of WEEE disposal have been voiced as a concern. Consequently, several studies evaluating the fate of WEEE chemicals in landfills have been explored. Jang and TownsendCitation47 evaluated Pb leaching from CRT glass and PWBs and found that Pb concentrations extracted using the TCLP were much greater than those extracted under similar conditions using leachate from operating landfills. These data suggest that the degree of Pb leaching from Pb-containing WEEE in a landfill should (under normal landfill conditions) be less than that predicted using the TCLP. Spalvins et al.Citation190 examined the mobility of Pb in landfills using simulated landfill columns containing 5% WEEE and found that Pb concentrations in columns with and without WEEE were similar. Li et al.Citation191 examined heavy metal leaching from simulated landfills containing MSW and WEEE components; Pb was found entrained in the soils under the columns. These studies focused largely on Pb because Pb was the element most likely to cause WEEE to be characterized as hazardous waste in the United States. The results indicate that the risk of Pb migration from a lined landfill will be minimal.
The potential mobility of BFRs in landfills has also been evaluated. Choi et al.Citation17 observed that BFRs from plastic TV moldings leach to a greater extent in the presence of dissolved humic matter, suggesting that the organic matter in landfill leachate might increase BFR mobility. Osako et al.Citation192 documented the presence of several BFR chemicals in landfill leachate and observed a positive relationship with the amount of organic matter in the landfill; in the presence of greater organic matter concentrations, greater BFR concentrations were observed.
A few studies have investigated the treatment of WEEE components before disposal in landfills as a means of reducing their future pollutant leachability. Under U.S. hazardous waste regulations, such treatment would be required before disposal in a hazardous waste landfill. Niu et al.Citation193 explored treatment methods for PWBs before landfill disposal including high-pressure compaction and cement stabilization, concluding that cement stabilization could greatly reduce Pb leachability under the TCLP. Kim et al.Citation194 demonstrated the use of biopolymer cross-linked concrete as a method for encapsulating CRT; Pb concentrations from the TCLP were greatly reduced compared with CRT glass mixed with concrete and no biopolymer.
SUMMARY AND THOUGHTS FOR THE FUTURE
The class of solid waste described in this review as WEEE will continue to remain a distinct waste stream that poses unique challenges to many aspects of modern society in the foreseeable future. From the body of information gathered and presented, several points are offered to summarize the general state of WEEE as it exists today.
| 1. | The magnitude and growth potential of WEEE, coupled with the myriad of different chemicals potentially present, justify the recent interest focused on this waste stream and call for additional time and resources to be invested. Managed poorly, WEEE can pose a sizable risk to human health and the environment. Managed wisely, WEEE represents a considerable resource. | ||||
| 2. | In developed countries, risks posed by WEEE when managed using modern, regulated solid waste approaches have not manifested as a great concern. One possible exception is risk posed to workers involved in the WEEE disassembly and recycling; implementation of appropriate safety practices and institutionalization of certification programs are warranted. However, managing WEEE using traditional developed-world solid waste management approaches (combustion, landfilling) represents a waste of resources and is not the desired option from a sustainability perspective. | ||||
| 3. | Ample evidence exists that improper management of WEEE, such as has occurred in many parts of the developing world, can pose harm to human health and the environment. Development of new or expansion of existing legislation or policies to prevent WEEE from the developed world being inappropriately dumped on the developing world is required. | ||||
| 4. | Many technologies exist to process and recover value from WEEE, and the past decade has witnessed a great deal of work in this regard. Development of innovative technologies to more efficiently and more economically treat and recycle WEEE should remain a fertile research area into the future. | ||||
| 5. | The biggest challenge to increasing WEEE recovery does not lie with technical issues such as processing, but rather with societal, political, and economic issues. In many parts of the world, more attention is needed from government, OEMs, retailers, and consumers to assess the appropriate investment and involvement of each of these parties toward proper WEEE management. | ||||
Several topics that merit additional thought and discussion regarding future directions include: (1) the dilemma surrounding WEEE exportation for recycling and resource recovery; (2) the push for increased resource recovery from WEEE; and (3) the future role and direction of government, industry, and consumers in WEEE management.
Debate is still underway regarding the role of international transfer of old electronic equipment (for reuse or recycling) to developing countries.Citation61,Citation195 Certainly the donation of working computers to households and students who otherwise would not have such equipment is of benefit, but the EOL implications must be considered. Chanthy and NitivattananonCitation196 argue that the importation of second-hand computers in Cambodia does more harm than good, and that developing countries must develop regulations and controls to ensure that imported computers still have a useful life and are not simply waste. Kahhat and WilliamsCitation197 described the importation of used personal computers from the United States to Peru and their fate, noting that official trade was driven by reuse rather than recycling. Several environmental groups are adamant that shipment of WEEE to less developed parts of the world should be banned outright, whereas others cite the greater benefit through education, jobs, and resource recovery provided to developing countries as a result of international WEEE transfer.
Beyond the factors described earlier in this review (possible environmental impact, inherent value of some WEEE components) that serve as motivation for increased WEEE recycling, several other factors are beginning to play a larger motivational role. The recognition of the benefit of recycling with respect to reductions in energy demand and greenhouse gas emissions are now becoming more widely recognized and even used as justification for new regulatory requirements. The availability of scarce global resources will also play a motivational role. Recent political incidents have increased awareness of rare earth elements and their supply. Given that many of these elements are found in WEEE, it is likely that more aggressive, advanced recovery processes will be used to extract benefit from WEEE.
OEMs, retailers, governments, and consumers will all play a role in future WEEE management. The EU WEEE and RoHS directives have considerably impacted global management of WEEE.Citation198 These regulations require that OEMs contribute to WEEE management, and because many of these companies compete on a global scale, entire product lines may be altered to meet these requirements, indiscriminate of whether each country has a regulation or not. For example, it is common for devices sold in the United States to meet the RoHS requirement for minimum Pb content. An increasing number of companies are now beginning to embrace EPR and making products that are less toxic and more amenable to future recycling efforts. Although several countries have adopted EU-style strategies for WEEE, the United States has not developed such an approach at the federal level, despite efforts to bring all parties together to generate an integrated WEEE take-back and recycling infrastructure. Efforts are currently largely dictated by individual OEMs and retailers and in some locations by state and local government. Although U.S. consumers are presented with a greater number of options for managing WEEE, the approach is not consistent for the WEEE types accepted or by location. For instance, computers and cell phones often are accepted at multiple drop-off locations, whereas power tools, fans, and vacuum cleaners may have much fewer, if any, drop-off locations. Also, the number of locations available that accept WEEE varies throughout the United States, often depending on whether the state and local government has instituted any regulations or promoted recycling of WEEE through educational campaigns. Thus, increasing the quantity of WEEE recycled remains a challenge that merits continued dialogue.
ABBREVIATIONS
| ABS | = |
acrylonitrile butadiene styrene |
| ABS/PC | = |
acrylontrile butadiene styrene/polycarbonate blend |
| BAPP | = |
bisphenol A, diphenylphosphate |
| BFRs | = |
brominated flame retardants |
| CDP | = |
cresyl diphenyl phosphate |
| CPU | = |
central processing unit |
| CRT | = |
cathode ray tube Deca |
| BDE | = |
decabromodipheyl |
| EEE | = |
electronic and electrical equipment |
| EOL | = |
end of life |
| EPA | = |
U.S. Environmental Protection Agency |
| EPR | = |
extended producer responsibility |
| EPS | = |
extruded polystyrene |
| EU | = |
European Union |
| FPD | = |
flat-panel device |
| H2SO4 | = |
sulfuric acid |
| HBB | = |
hexabromobiphenyl |
| HBCD | = |
hexabromobisphenol A |
| HCl | = |
hydrogen chloride |
| HIPS | = |
high-impact polystyrene |
| HNO3 | = |
nitric acid |
| LCA | = |
lifecycle assessment |
| LCD | = |
liquid-crystal display |
| LED | = |
liquid-emitting diode |
| MSW | = |
municipal solid waste |
| OBB | = |
octabromobiphenyl Octa |
| BDE | = |
octabromodiphenyl OEM original equipment manufacturer |
| PA | = |
polyacetate |
| PAH | = |
polycyclic aromatic hydrocarbon |
| PBDD | = |
polybrominated dibenzodioxin |
| PBDD/Fs | = |
polybrominated dibenzodioxins/dibenzofurans |
| PBDE | = |
polybrominated diphenyl ether |
| PBDF | = |
polybrominated dibenzofuran |
| PBT | = |
polybutylene terephthlate |
| PBT/PET | = |
polybutylene terephthlate/polyethylene terephthlate |
| PC | = |
polycarbonate |
| PCB | = |
polychlorinated biphenyl |
| PCDD | = |
polychlorinated dibenzodioxin |
| PCDF | = |
polychlorinated dibenzofuran |
| PDBs | = |
paradichlorobenzene Penta |
| BDE | = |
pentabromodiphenyl |
| PMMA | = |
polymethylmethacrylate |
| POM | = |
polyoxymethylene |
| PP | = |
polypropylene |
| PPE/PPO | = |
polyphenylene ether/oxide |
| PPO | = |
polyphenylene oxide |
| PPO/PS | = |
polyphenylene oxide/polystyrene blend |
| PS | = |
polystyrene |
| PUR | = |
polyurethane |
| PVC | = |
polyvinyl chloride |
| PWB | = |
printed wire board |
| RDP | = |
resorcinol-bis-diphenylphosphate |
| RoHS | = |
restriction of hazardous substance |
| SAN | = |
styrene acrylonitrile |
| TBBPA | = |
tetrabromobisphenol A |
| TBBPA-adp | = |
tetrabromobisphenol-A-bis-(2,3-dibromopropyl ether) |
| TBBPA-ae | = |
tetrabromobisphenol-A-bis-(allylether) |
| TBBPA-CO3 | = |
tetrabromobisphenol A, carbonate oligomer, phenoxy-terminated |
| TBPA | = |
tetrabromophthalic anhydride |
| TBPE | = |
1,2-bis-(tribromophenoxy) ethane |
| TC | = |
toxicity characteristic |
| TCLP | = |
toxicity characteristic leaching procedure |
| TFT | = |
thin film transistor |
| TPP | = |
triphenyl phosphate |
| TPPi | = |
triarylphosphate, isopropylated |
| TV | = |
television |
| VCR | = |
videocassette recorder |
| VFDs | = |
vacuum fluorescent displays |
| WEEE | = |
waste electronic and electrical equipment |
| WTE | = |
waste-to-energy |
ACKNOWLEDGMENTS
The author thankfully recognizes the assistance of Kelly Hodoval, Hwidong Kim, Shrawan Singh, and Yu Wang in the development of this review. The historical research support of the Hinkley Center for Solid and Hazardous Waste, the Florida Department of Environmental Protection, and EPA are recognized.
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