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Migration of heavy metal in electronic waste plastics during simulated recycling on a laboratory scale
Chemosphere 245 (2020) 125645
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Migration of heavy metal in electronic waste plastics during simulated recycling on a laboratory scale Shaohua Mao a, Weihua Gu a, b, 1, Jianfeng Bai a, b, *, Bin Dong c, Qing Huang a, b, Jing Zhao a, b, Xuning Zhuang a, b, Chenglong Zhang a, b, Wenyi Yuan a, b, Jingwei Wang a, b a b c
WEEE Research Centre of Shanghai Polytechnic University, Shanghai, 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai, 201209, China School of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Recycled waste plastic products may pose environmental and human health risks. We recycled waste plastics and determined the metal contents in secondary products. High metal concentrations were present in leachate from recycled products. Metal concentrations exceeded human safety and groundwater standards.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2019 Received in revised form 5 December 2019 Accepted 11 December 2019 Available online xxx
Recycling is the primary method to handle electronic waste plastics, however, little attention has been paid to the risk posed by heavy metal migration in waste plastic products. The effect of multistage recycling processes on heavy metal migration and the environmental risk posed by heavy metals during recycling processes were investigated by: (1) Recycling waste plastics and determining the heavy metal contents in secondary products; (2) Using toxic leaching experiments to assess environmental risks of heavy metal migration in secondary products; and (3) Evaluating the effect of recycling processes on the mechanical properties and microstructure of plastics. Results showed that the contents of some harmful heavy metals in processed products exceeded the Safety of Toys Standard. Toxic leaching tests showed that Ni, Cu, Zn, Pb, and Sb migrated outward during secondary products use. With increased recycling times, concentrations of migrated Ni, Cu, Zn, Pb, and Sb increased, and the leached concentrations exceeded the limits stipulated in the Groundwater Quality Standard. Increased recycling times also accelerated waste plastics aging and caused the deterioration of mechanical properties. Furthermore, adhesion between layers decreased, stratification and cracking in polymers appeared, and adhesion of waste plastics to additives decreased. Therefore, the environmental risks of waste plastic recycling should be carefully considered. © 2019 Published by Elsevier Ltd.
Handling Editor: Shane Snyder Keywords: Waste plastics Recycling use Heavy metals Migration Environmental risks
* Corresponding author. WEEE Research Centre of Shanghai Polytechnic University, No.2360, Jinhai Road, Pudong New Area, Shanghai, 201209, China. E-mail address: [email protected] (J. Bai). 1 Co-first author. https://doi.org/10.1016/j.chemosphere.2019.125645 0045-6535/© 2019 Published by Elsevier Ltd.
1. Introduction Plastic has the characteristics of chemical resistance, high
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strength, high toughness, and malleability. That combined with its low price and accessibility, make it widely used in electronic equipment products (Panda et al., 2010). With the improvement of plastic properties and the replacement of electronic equipment products, the amount of waste plastics is increasing (Wang et al., 2012). In electronic equipment such as televisions, refrigerators, washing machines and air conditioners, the plastic mass accounts for more than 17.7% of the total, of which acrylonitrile-butadienestyrene (ABS), flame-retardant high-impact polystyrene (FRHIPS), and high-impact polystyrene (HIPS) account for the largest proportions (Oguchi et al., 2011; Schlummer et al., 2007). Statistically, global plastic production reached 8.3 billion tons in 2017, and e-waste production is expected to reach 52.2 million tons in 2021 et al., 2017). With the rapid growth in the (Geyer et al., 2017; Balde production of waste plastics, a reasonable way to deal with them is needed to reduce the pressure on the ecological environment. Waste electronic equipment products have recently received attention as a secondary source of metals and plastics, which may still have important application value (Oguchi et al., 2011). Recycling has been employed as the main treatment method of waste plastics in domestic markets, meaning that they continue to be used after modification into secondary products (Fig. S1). The processing of waste plastics includes screening, crushing, modification, granulation, injection, and moulding (Sun et al., 2015). However, waste plastics must be combined with additives (such as antioxidants, epoxy plasticizers, and colourants) during production (Liu et al., 2019a; Andrady and Rajapakse, 2016). These additives contain heavy metals (such as Pb, Cr, and Cd) as well as organic pollutants (such as dioctyl adipate (DEHA), phthalate acid easters (PAEs), polybrominated diphenyl ether (PBDEs), and tetrabromobisphenol A (TBA)) (Miloichikova et al., 2016; Cooper et al., 2011). In addition, additives may migrate to the surface of plastics because of aging during processing and subsequent usage of waste plastic secondary products, thus increasing potential hazards for humans or animals (Jaeger and Rubin, 1970; Lu et al., 2018). In the recycling process, additives are used for plastic modification, and the content of harmful substances will increases after processing. Yu et al. (2017) found that the PBDE content in waste cathode ray tube television shell plastic was 11.31 mg/kg, and that the PBDE content in new products made from recycled material was 15.20 mg/kg, suggesting that PBDEs in waste plastics are transferred into the new products (Yu et al., 2017). On comparing new and recycled plastic food containers, the concentrations of metals such as Cd and Pb in used plastic containers were found to be higher, potentially due to the lack of quality control protocols in the re-manufacturing process of used plastic containers (Waheed et al., 2012). Hence, it is necessary to control the amounts of additives and develop new additives, which do not pose an environmental risk. The additives in plastics will flow with the high temperature processing, and heavy metals in waste plastic secondary products may migrate to the environment during long-term use. Bang et al. (2012) found that plasticizers and antioxidants in plastic food containers can migrate outward and cause endocrine disruption, and that the degree of pollution depended on physical and chemical conditions, such as temperature, ultraviolet, pH value, and other factors. When polyvinyl chloride (PVC) was used as packaging for orange juice, the acidity of the orange juice promoted the dissolution of diethylhexyl phthalate (DEHP) dissolution. Furthermore, the dioctyl adipate (DEHA) in PVC film migrated to high-fat food at 40 C over 10 days, and the migration rate reached 75e90%. This was due to the fact that the plasticizer was mobile in the polymer matrix, and because DEHA is lipophilic and thus spreads easily (Coltro et al., 2014). After many cycles of waste plastic processing, high temperature will produce greater internal
stress in the plastics and induce cracking and layering, thereby facilitating additives migration (Liu et al., 2019b). Compared with new plastic products, waste plastic has the disadvantages of having a turbid colour, strong smell, poor toughness, increased aging, and brittleness. Heavy metals in waste plastic products are more likely to migrate outward. There are many studies on the migration of organic matter in food plastic additives; however, the study of heavy metals in waste plastic products needs greater attention. The main purpose of this experiment is to explore the migration of heavy metals in electronic waste plastics during their simulated recycling at the laboratory scale. Generally, observing and evaluating the migration risk of heavy metals during the recycling processes of waste plastics requires long time frames. To simplify the experimental process and rapidly assess the environmental risk from heavy metals, waste plastics were processed three times to accelerate the recycling process and shorten the time needed to complete the study. The toxic leaching method of evaluating solid waste (China SEPA, 2007) was used to determine heavy metal migration and quantitatively determine its toxicity. Changes in the mechanical properties and microstructure of waste plastics were compared with the leaching results. The results provide an overall evaluation of the waste plastic recycling process, and assess the environmental risk posed by waste plastic secondary products to allow effective supervision of the process. 2. Materials and methods 2.1. Sample collection and reagents Waste ABS, FR-HIPS, and HIPS samples used in the experiment were obtained from Xinjinqiao Environmental Protection Co., Ltd., Shanghai, China. Samples (3000 g) were randomly collected in the disassembly workshop and loaded into a self-sealing bag (Table S1). Waste plastic was placed in 40% ethanol solution, cleaned for 15 min with deionized water in an ultrasonic cleaner, after which any paper or labels were removed from the plastic. After cleaning, samples were dried in a 70 C blast dryer in preparation for use. Nitric acid (HNO3) (65%e68%, GR), sulfuric acid (H2SO4) (98%, GR), fluoroboric acid (HBF4) (40%, AR), hydrogen peroxide (H2O2) (30%, AR), ethanol (C2H6O) (99.7%, AR), and a multi-element standard solution (100 mg/L) (Sinopharm Chemical ReagentCo., Ltd, China) were main experimental reagents used in this study. To avoid introducing foreign heavy metals in the processing and interfering with the migration study, only antioxidants (Pentaerythrite tetra [b-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]) (AO-60(1010), China) without heavy metal elements were used in the experiment. Ultrapure water was used as needed during the experiments. 2.2. Experimental methods 2.2.1. Waste plastic recycling production Recycling experiments of waste ABS, FR-HIPS and HIPS samples were performed separately following the same operation steps. The following methods were used to process the samples. The prepared sample was crushed with a multifunctional grinder, and passed through 6-mm standard screen; 2 g antioxidants were added to 2000 g sample (mass ratio of 0.1%), and mixed evenly; a co-rotating twin-screw extruder was used for granulation. The temperatures of each part of the extruder were in the range of 210e220 C (Fig. S2a), and the screw speed was 75 rpm/min. The obtained particle material was cooled and added to the injection-moulding machine for production. The temperatures of each part of the injectionmoulding machine were 220 C (Fig. S2b) (Xi et al., 2018). The cycle was repeated three times according to the methods. During each
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cycle, 500 g of injection-moulding sample was taken for analyses. The sample collection number was classified as shown in Table 1. 2.2.2. Digestion experiment One hundred gram of the injection-moulding sample (Table 1) was crushed and passed through a 0.425-mm standard screen, and 0.1000 g of the sample (Table 1) was weighed. ABS sample combined with 8 ml HNO3, 2 ml HBF4, 3 ml H2O2 and FR-HIPS, and HIPS were digested with 8 ml HNO3, 3 ml HBF4 and 1.5 ml H2O2 in a polytetrafluoroethylene (PTFE) digestion tank with microwaveassisted digestion (Anton-Paar, Multiwave PRO, Austria), as described in Table S2 (Mao et al., 2019). After the sample was cooled, it was filtered and diluted with 5% HNO3 solution to a total volume of 50 ml. Heavy metal contents were determined using an inductively coupled plasma-optical emission spectrometer (ICPOES) (Thermo Fisher Scientific, ICAP7000, USA). Each batch of samples included three replicates. 2.2.3. Toxic leaching experiment During daily use, waste plastic products may be in contact with liquid for extended periods and be affected by the acidity and basicity of the liquid, temperature, molecular characteristics, and other factors (Bang et al., 2012). Consequently, heavy metals in the products may easily migrate and pollute the liquid. Therefore, the potential environmental impacts of waste plastic products were studied by a toxic leaching experiment. The methods were as follows: the injection-moulding samples (Table 1) were used to follow the solid waste leaching method (China SEPA, 2007). The leaching experiment was continued for 30 days, and triplicate samples were taken every second day until the 27th day, and then again on the 30th day. The leaching samples were filtered and stored at 4 C (Midea, BCD-186, China) before being analysed using ICP-OES. 2.3. Analysis methods The mechanical properties of the plastics were tested using a universal testing machine (MTS, CMT6104, China) and a pendulum impact-testing machine (MTS, ZBC7000, China). The impact strength, bending strength, and tensile strength of FR-HIPS were tested according to the China National Standard (Huang et al., 2010). The surface structures of the waste plastics after recycling processes were observed using SEM (Phenom, ProX, China). The data were analysed by Microsoft Excel and Original 9.0. 3. Results 3.1. Digestion results of waste plastics recycling products Fig. 1 shows the contents of harmful heavy metals in waste plastic processing products (the detection limits of heavy metals are shown in the Table 2). After the initial processing of waste plastic products, the Cr, Pb, Sb, and As contents in ABS were 1.1, 1.5, 43.0, and 10.9 times higher than the limits set in the standards, respectively. The Cr, Pb, Sb, and As contents in FR-HIPS products were 1.4, 1.5, 426.3, and 4.7 times than the limits, and were 0.6, 1.2, 18.8, and 13.9 times higher than the limits in HIPS products,
Fig. 1. Metal concentrations in waste plastic recycling products (1: first recycle process; 2: second recycle process; 3: third recycle process).
Table 1 Sample information for each stage of cycle processing cycle. Sample
First recycle products
Second recycle products
Third recycle products
ABS FR-HIPS HIPS
ABS1 FR-HIPS1 HIPS1
ABS2 FR-HIPS2 HIPS2
ABS3 FR-HIPS3 HIPS3
Note: 1: first recycle process; 2: second recycle process; 3: third recycle process.
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Table 2 Detection limits of different heavy metals (mg/L).
Detection limits
Cr
Cd
Sb
Pb
As
Ni
Cu
Zn
0.006
0.002
0.005
0.004
0.008
0.006
0.004
0.003
Note: The detection limits are low, so the content of heavy metals can be determined accurately.
respectively. As can be seen from Fig. 1, the Cr, Pb, Sb, As, Cd, Ni, Cu and Zn contents of the products after three waste cycles, showed decreasing trends for ABS, FR-HIPS, and HIPS particularly in Sb and As. Compared with the first and third processing products of waste ABS, the Sb and As contents were reduced by 467 mg/kg and 85.2 mg/kg, respectively, corresponding to 17.7% and 28.6% decreases. Reductions in Sb and As in the first and third FR-HIPS waste processing products were 1396 mg/kg and 22.5 mg/kg (decreases of 5.5% and 19.3%), respectively. The Sb and As reductions in the first and third processing products of waste HIPS were 107 mg/kg and 55.8 mg/kg (decreases of 9.0% and 15.0%), respectively.
3.2. Leaching of recycled waste plastic products Quantitative analysis of heavy metals in the leachate showed that obvious leaching of Ni, Cu, Zn, Sb, and Pb from waste ABS, FRHIPS, and HIPS products (Figs. 2-4), leaching of Cr, Cd and As were too low to be detected. The first processed products of waste plastics were leached for 30 days, and resulted in leached solution concentrations of Ni, Cu, Zn, Sb, and Pb from waste ABS products of 0.006 mg/L, 0.006 mg/L, 0.291 mg/L, 0.373 mg/L, and 0.021 mg/L, respectively, whereas the concentrations were 0.031 mg/L, 0.013 mg/L, 0.198 mg/L, 2.266 mg/L, and 0.023 mg/L in the leached solution of waste FR-HIPS products, respectively. In HIPS products, Ni, Cu, Zn, Sb, and Pb concentrations in leached solutions were 0.019 mg/L, 0.060 mg/L, 0.103 mg/L, 0.123 mg/L, and 0.045 mg/L respectively. The Sb concentrations in the leachate of waste ABS, FR-HIPS, and HIPS products were 37.3, 226.6, and 12.3 times higher, respectively, than the specified concentrations specified in the fifth class of the Groundwater Quality Standard (Table S3). The third processed products obtained from waste plastics were leached for 30 days, and the Ni, Cu, Zn, Sb, and Pb concentrations in the leachate of waste ABS products were 0.023 mg/L, 0.005 mg/L, 2.264 mg/L, 1.172 mg/L, and 0.020 mg/L, respectively. Compared with the first processed products, the leaching concentrations of Ni, Zn, and Sb increased by 2.8, 6.8, and 2.1 times, respectively. The Ni, Cu, Zn, Sb, and Pb concentrations were 0.119 mg/L, 1.041 mg/L, 0.390 mg/L, 2.413 mg/L, and 0.015 mg/L in the leachate of waste FRHIPS products, respectively. Compared with the first processed products, the leaching concentrations of Ni, Cu, Zn, and Sb increased by 2.8, 79, 0.97, and 0.06 times, respectively. Ni, Cu, Zn, Sb, and Pb concentrations of 0.140 mg/L, 0.148 mg/L, 0.813 mg/L, 0.311 mg/L, and 0.132 mg/L, respectively, were present in the leachate of waste HIPS products. Compared with the first processed products, the leaching concentration of Ni, Cu, Zn, Sb, and Pb increased by 6.4, 1.5, 1.9, 1.5, and 1.9 times, respectively. Compared with the metal content limit stipulated in the Groundwater Quality Standard, Ni, Zn, and Pb concentrations in the leachate of waste ABS products exceeded the fifth, third, and third classes of the Groundwater Quality Standard, respectively. The Ni, Cu, and Pb concentrations in the leachate of waste FR-HIPS products also exceeded the fifth, third, and third classes of the Groundwater Quality Standard, respectively, and the Ni, Cu, and Pb concentrations in the leachate of waste HIPS products exceeded the fifth, second, and fifth classes of the Groundwater Quality Standard,
Fig. 2. Metal concentrations during toxicity leaching experiments on waste ABS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
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respectively. Among them, Sb concentrations in the leachate of waste ABS, FR-HIPS, and HIPS products were 117.2, 241.3, and 31.1 times higher than the Sb concentration specified in the fifth class of the Groundwater Quality Standard, respectively. 3.3. Mechanical properties and SEM scanning The mechanical properties of thrice-recycled products of waste ABS, FR-HIPS, and HIPS are shown in Table 3. The impact, tensile, and bending strengths of waste plastic products decreased with the increase in recycling times. Comparing the recycling products between the third and first cycles, the impact, tensile, and bending properties of waste ABS, FR-HIPS, and HIPS products decreased by 35.4%, 7.2%, and 4.4%; 50.3%, 3.9%, and 6.9%; and 56.6%, 7.3%, and 6.5%, respectively. The cyclic processing of waste plastics had the greatest influence on impact properties. The cross-sectional surface layer of waste ABS, FR-HIPS, and HIPS injection-moulded sample strips at each processing stage are shown in SEM images in Fig. 5. It can be seen that the aging phenomena were aggravated by increased processing times. The stratification phenomenon of waste plastic became gradually obvious, and the adhesion between layers decreased. The size and number of pores in the surface layer in waste plastics increased. 4. Discussion 4.1. Heavy metals content of waste plastics recycling products The Cr, Pb, Sb, and As contents in the waste ABS, FR-HIPS, and HIPS products (Fig. 1) exceeded the heavy metal content limitations set by the Safety of Toys Standards (Isama et al., 2011). The samples used in this study were produced prior to 2007, at which time the Administration on the Control of Pollution Caused by Electronic Information Products was not formulated and implemented by Chinese government. There were no strict laws controlling plastic production (Song et al., 2017). Therefore, the mass use of additives was the main reason for the high content of heavy metals in waste plastic samples. Among Cr, Pb, Sb, and As in waste plastics, the Sb content exceeded the Standard the most (Fig. 1), which was mainly related to the usage requirements, Inorganic flame retardants (Sb2O3) were added to enhance high-temperature resistance when electronics were used for long durations, which increased the Sb content in plastics (Carty and Price, 2004). Nakashima et al. (2012) found that plastics may act as carriers of toxic metals and pollute the surrounding environment. Therefore, the heavy metal contents in waste plastic products should be effectively controlled. Waste plastics processing resulted in the emission of significant amounts of particle-phase pollutants (which might contain metal
Table 3 Results of mechanical properties of waste plastic recycling products. Sample ID
ABS1 ABS2 ABS3 FR-HIPS1 FR-HIPS2 FR-HIPS3 HIPS1 HIPS2 HIPS3
Impact strength/ (kJ/m2)
Tensile strength/ MPa
Bending strength/MPa
Result
RSD/%
Result
RSD/%
Result
RSD/%
7.21 5.28 4.66 6.22 4.84 3.09 8.61 7.06 3.74
0.75 4.13 3.05 4.17 4.87 4.47 1.79 1.23 1.03
42.57 40.35 39.51 28.48 27.36 25.71 29.37 28.83 27.22
1.26 0.57 3.18 1.41 1.46 4.65 2.14 2.09 0.90
63.32 62.54 60.53 41.86 41.02 38.97 47.23 46.35 44.15
0.93 0.15 0.55 2.16 3.17 1.04 0.99 0.51 3.14
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elements such as Sb and As) in high-temperature treatments. The emission of particle-phase pollutants was the main factor reducing metal contents during the recycling process (Tomsej et al., 2018). Gaseous and particles pollutants containing carcinogens affect workers’ health within and near the recycling zones (Luo et al., 2014). Therefore, dedusting and tail gas absorption devices should be added in the high-temperature stage of waste plastics processing, which could reduce the harm of pollutants to the human body and the environment. 4.2. Heavy metal migration during simulated recycling Concentrations of Ni, Cu, Zn, Sb, and Pb in the leachate increased when leaching was sustained (Figs. 2e4). When the cycle number of the same waste plastics increased, the leaching concentration of metals also increased. Existing studies have also found that Sb in polyester (PET) food packaging materials migrated outward during long-term contact with acidic food simulants, thus the concentration in the liquid increased gradually (Yao et al., 2013). When recycled waste plastic products come into contact with acidic liquid during use, heavy metals in additives may dissolve in liquid and contaminate the products. Fig. 5 indicates that with increasing processing cycles, the degradation of waste plastics is more apparent. The adhesion between layers was reduced and stratification and cracking appeared in polymers. The increased contact area between the leachate and the surface could promote the dissolution of the additives and result in metals more easily migrating outward (Fig. S3). Therefore, the migration rate of heavy metals in waste plastic products was accelerated as the degradation of waste plastics, and the migration content of some heavy metals exceeded the limitation in the Groundwater Quality Standard. This presents an environmental safety risk, which should garner more attention. 4.3. Effect of simulated recycling on mechanical properties and microstructure The cyclic processing of waste plastics had the greatest influence on impact properties, which agrees with the results published by rez et al. (2010) who studied the effect of reprocessing on ABS Pe properties. Their data showed that the impact strength of ABS samples decreased after recycling. The reduction in impact strength was related to some degradation of the polybutadiene phase, which rez et al., 2010). likely involved changes in the molecular weight (Pe Liu et al. (2019b) found that temperature accelerated destruction of composites. Resin cracking and fibre-matrix interface degradation occurred in the aged specimens. A high temperature yielded strong internal stress, resulted in debonding and interfacial damage. Comparing the microstructure of the three kinds of waste plastics, the aging rate of FR-HIPS was slower because it contained more flame retardants; therefore, its high-temperature resistance was greater than those of ABS and HIPS. During waste plastics processing, plastic was oxidized by high-temperature treatment during which the elastic rubber particles in the plastics would degrade and the grafting properties between the monomers and the plastic stability would deteriorate (Wahhab et al., 2016). After the elastic rubber particles were degraded, the toughness of rubber particles decreased, rigidity increased, the plastics gradually became brittle, and the mechanical properties deteriorated significantly. The plasticizer content in waste plastics decreased gradually after high-temperature treatment, which reduced its resistance to impact, bending, and tension. The number of pores in the surface layer of waste plastics increased after processing, due to the flow of inorganic particles, and the aging degradation of plastics was affected by the outflow of inorganic components and plasticizers
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Fig. 3. Metal concentrations during toxicity leaching experiments on waste FR-HIPS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
Fig. 4. Metal concentrations during toxicity leaching experiments on waste HIPS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
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Fig. 5. SEM images of cross-sectional surface layer of a waste plastic injection moulded sample strip (A: first recycle of ABS, B: second recycle of ABS, C: third recycle of ABS; D: first recycle of FR-HIPS, E: second recycle of FR-HIPS, F: third recycle of FR-HIPS; G: first recycle of HIPS, H: second recycle of HIPS, I: third recycle of HIPS).
(Ito and Nagai, 2007). This was also the main reason that the heavy metal concentrations in the leachate increased with increasing processing times. 5. Conclusions Results indicated that after the completion of recycling, the Sb and As contents in waste plastic products decreased and leaching of Ni, Cu, Zn, Sb, and Pb was obvious. When the leaching experiment was conducted, the metal concentrations in the leaching solution increased gradually. With increasing cycles, the leaching concentrations of metal also increased after 30 days of leaching. The leaching of Ni, Cu, Zn, Sb, and Pb in waste plastic products exceeded the relevant classes of the Groundwater Quality Standard. With increased processing times, the aging degree of waste plastics accelerated and mechanical properties deteriorated, and impact strength decreased by the greatest proportion. Furthermore, the size and number of pores in the surface layer of waste plastics increased, stratification and cracking in polymers appeared, and
adhesion of the waste plastics to the additives decreased. The findings of this study can help assess the environmental risks posed by waste plastic recycling and provide some guidance for waste plastic reuse. More work is needed to assess the heavy metal contamination risk across diverse plastics sectors. This, along with an evaluation of more recent plastic waste sources, will form the basis of future research so we can begin to construct a robust framework to evaluate and understand this new environmental health and safety risk.
Author contribution section Shaohua Mao: Formal analysis; Investigation; original draft; Roles/Writing, Weihua Gu: Writing - review & editing, Jianfeng Bai: Funding acquisition; Roles/Writing; Conceptualization, Bin Dong: Funding acquisition, Qing Huang: Software, Jing Zhao: Data curation, Xuning Zhuang: Methodology, Chenglong Zhang: Validation, Wenyi Yuan: Visualization, Jingwei Wang: Project administration
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Declaration of competing interest The authors declare that they have no conflict of interests. Acknowledgments The authors thank the Gaoyuan Discipline of ShanghaiEnvironmental Science and Engineering (Resource Recycling Science and Engineering), the Key Discipline of Shanghai Polytechnic University (No. XXKZD1602), and the Construction Project of Shanghai Collaborative Innovation Center (No. ZF1224). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.125645. References Andrady, A.L., Rajapakse, N., 2016. Additives and Chemicals in Plastics. The Handbook of Environmental Chemistry. Springer International Publishing AG, USA. , C.P., Forti, V., Gray, V., Kuehr, R., Stegmann, P., 2017. The global E-waste Balde monitor 2017. https://www.itu.int/en/ITU-D/Climate-Change/Documents/GEM %202017/Global-E-waste%20Monitor%202017%20.pdf. Bang, D.Y., Kyung, M., Kim, M.J., Jung, B.Y., Cho, M.C., Choi, S.M., Kim, Y.W., Lim, S.K., Lim, D.S., Won, A.J., Kwack, S.J., Lee, Y., Kim, H.S., Lee, B.M., 2012. Human risk assessment of endocrine-disrupting chemicals derived from plastic food containers. Compr. Rev. Food Sci. Food Saf. 11, 453e470. Carty, P., Price, D., 2004. Antimony/Iron interactions in chlorinated polymer blends. J. Vinyl Addit. Technol. 10, 88e94. ^ Coltro, L., Pitta, J.B., Costa, P.A., Perez, M.A.F., Araújo, V.A., Rodrigues, R., 2014. Migration of conventional and new plasticizers from PVC films into food simulant: a comparative study. Food Control 44, 118e129. Cooper, J.E., Kendig, E.L., Belcher, S.M., 2011. Assessment of bisphenol A released from reusable plastic, aluminium and stainless steel water bottles. Chemosphere 85, 943e947. Geyer, R., Jambeck, J.R., Law, K.L., 2017. Production, use, and fate of all plastics ever made. Sci Adv 3, 1e5. Huang, Z., Lu, S., Yang, Z., Yu, C., Guo, D., 2010. Studies on the properties of epoxy resins modified with novel liquid crystalline polyurethane. Adv. Mater. Res. 150e151, 727e731. Isama, K., Kawakami, T., Nishimura, T., 2011. Migration of Eight Harmful Elements from Household Products Made of Synthetic Resin That Infants May Swallow by Mistake. Yakugaku Zasshi 131 (7), 1135e1140. Ito, M., Nagai, K., 2007. Analysis of degradation mechanism of plasticized PVC under artificial aging conditions. Polym. Degrad. Stab. 92, 260e270. Jaeger, R.J., Rubin, R.J., 1970. Plasticizers from plastic devices extraction, metabolism, and accumulation by biological systems. Science 170, 460e462. Liu, S., Ge, H., Zou, Y., Chen, J., 2019a. Properties of PP wood-plastic composites with biocompatibility additives. Mater. Sci. Forum 944, 509e514. Liu, S., Chen, Y., Chen, P., Xu, D., Xiong, X., Wang, J., 2019b. Properties of novel bismaleimide resins and thermal ageing effects on the ILSS performance of their carbon fibre-bismaleimide composites. Polym. Compos. 40, 1283e1293. Lu, K., Qiao, R., An, H., Zhang, Y., 2018. Influence of microplastics on the
accumulation and chronic toxic effects of cadmium in zebrafish (Danio rerio). Chemosphere 202, 514e520. Luo, P., Bao, L.J., Wu, F.C., Li, S.M., Zeng, E.Y., 2014. Health risk characterization for resident inhalation exposure to particle-bound halogenated flame retardants in a typical e-waste recycling zone. Environ. Sci. Technol. 48, 8815e8822. Mao, S.H., Gu, W.H., Bai, J.F., Zhao, J., Zhuang, X.N., Zhang, C.L., Yuan, W.Y., Wang, J.W., 2019. Effect analysis of different digestion methods in detecting metals from waste electrical plastic. Plastics Sci. and Technol. 3, 101e105. Miloichikova, I.A., Stuchebrov, S.G., Verigin, D.A., Krasnykh, A.A., Danilova, I.B., 2016. Simulation of the X-Ray beam absorption by the ABS-plastic filled with different metallic additives. J. Phys. Conf. Ser. 769, 1e6. Nakashima, E., Isobe, A., Kako, S., Itai, T., Takahashi, S., 2012. Quantification of toxic metals derived from macroplastic litter on Ookushi beach, Japan. Environ. Sci. Technol. 46, 10099e10105. Oguchi, M., Murakami, S., Sakanakura, H., Kida, A., Kameya, T., 2011. A preliminary categorization of end-of-life electrical and electronic equipment as secondary metal resources. Waste Manag. 31, 2150e2160. Panda, A.K., Singh, R.K., Mishra, D.K., 2010. Thermolysis of waste plastics to liquid fuel A suitable method for plastic waste management and manufacture of value added products-A world prospective. Renew. Sustain. Energy Rev. 14, 233e248. rez, J.M., Vilas, J.L., Laza, J.M., Arn n, L.M., 2010. Pe aiz, S., Mijangos, F., Bilbao, E., Leo Effect of reprocessing and accelerated weathering on ABS properties. J. Polym. Environ. 18, 71e78. €urer, A., Wolz, G., Eldik, R., 2007. Characterisation of Schlummer, M., Gruber, L., Ma polymer fractions from waste electrical and electronic equipment (WEEE) and implications for waste management. Chemosphere 67, 1866e1876. SEPA, China, 2007. State Environmental Protection Administration of China: Solid Waste-Extraction Procedure for Leaching Toxicity-Sulphuric Acid and Nitric Acid Method (HJ/T299-2007). Song, X., Wang, J., Yang, J., Lu, B., 2017. An updated review and conceptual model for optimizing WEEE management in China from a life cycle perspective. Front. Environ. Sci. Eng. 11, 1e12. Sun, Z., Shen, Z., Zhang, X., Ma, S., 2015. Co-recycling of acrylonitrileebutadieneestyrene waste plastic and nonmetal particles from waste printed circuit boards to manufacture reproduction composites. Environ. Technol. 36, 160e168. Tomsej, T., Horak, J., Tomsejova, S., Krpec, K., Klanova, J.M., Dej, Hopan, F., 2018. The impact of co-combustion of polyethylene plastics and wood in a small residential boiler on emissions of gaseous pollutants, particulate matter, PAHs and 1,3,5- triphenylbenzene. Chemosphere 196, 18e24. Waheed, S., Rahman, S., Husnain, S.M., Siddique, N., 2012. Hazardous and other element characterization of new and used domestic plastic food containers using INAA and AAS. J. Radioanal. Nucl. Chem. 292, 937e945. Wahhab, H.I.A., Dalhat, M.A., Habib, M.A., 2016. Storage stability and hightemperature performance of asphalt binder modified with recycled plastic. Road Mater. Pavement Des. 22, 1e18. Wang, H., Chen, X., Bai, Y., Guo, C., Zhang, L., 2012. Application of dissolved air flotation on separation of waste plastics ABS and PS. Waste Manag. 32, 1297e1305. Xi, Z.Y., Wang, J.W., Huang, Q., Bai, J.F., Zhang, C.L., 2018. Preparation of maleic anhydride/styrene grafting polypropylene and performance for modification with ABS/HIPS alloy. Environ. Eng. 36, 619-622, 678. Yao, C., Ai, L., Guo, C., Chen, R., Li, L., Zhang, J., 2013. Study on migration of antimony from polyester food packaging materials to food simulants by ICP-MS. Chinese journal of food hygieng 25, 12e15. Yu, D., Duan, H., Song, Q., Liu, Y., Li, Y., Li, J., Shen, W., Luo, J., Wang, J., 2017. Characterization of brominated flame retardants from e-waste components in China. Waste Manag. 68, 498e507.
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Migration of heavy metal in electronic waste plastics during simulated recycling on a laboratory scale Shaohua Mao a, Weihua Gu a, b, 1, Jianfeng Bai a, b, *, Bin Dong c, Qing Huang a, b, Jing Zhao a, b, Xuning Zhuang a, b, Chenglong Zhang a, b, Wenyi Yuan a, b, Jingwei Wang a, b a b c
WEEE Research Centre of Shanghai Polytechnic University, Shanghai, 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai, 201209, China School of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Recycled waste plastic products may pose environmental and human health risks. We recycled waste plastics and determined the metal contents in secondary products. High metal concentrations were present in leachate from recycled products. Metal concentrations exceeded human safety and groundwater standards.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2019 Received in revised form 5 December 2019 Accepted 11 December 2019 Available online xxx
Recycling is the primary method to handle electronic waste plastics, however, little attention has been paid to the risk posed by heavy metal migration in waste plastic products. The effect of multistage recycling processes on heavy metal migration and the environmental risk posed by heavy metals during recycling processes were investigated by: (1) Recycling waste plastics and determining the heavy metal contents in secondary products; (2) Using toxic leaching experiments to assess environmental risks of heavy metal migration in secondary products; and (3) Evaluating the effect of recycling processes on the mechanical properties and microstructure of plastics. Results showed that the contents of some harmful heavy metals in processed products exceeded the Safety of Toys Standard. Toxic leaching tests showed that Ni, Cu, Zn, Pb, and Sb migrated outward during secondary products use. With increased recycling times, concentrations of migrated Ni, Cu, Zn, Pb, and Sb increased, and the leached concentrations exceeded the limits stipulated in the Groundwater Quality Standard. Increased recycling times also accelerated waste plastics aging and caused the deterioration of mechanical properties. Furthermore, adhesion between layers decreased, stratification and cracking in polymers appeared, and adhesion of waste plastics to additives decreased. Therefore, the environmental risks of waste plastic recycling should be carefully considered. © 2019 Published by Elsevier Ltd.
Handling Editor: Shane Snyder Keywords: Waste plastics Recycling use Heavy metals Migration Environmental risks
* Corresponding author. WEEE Research Centre of Shanghai Polytechnic University, No.2360, Jinhai Road, Pudong New Area, Shanghai, 201209, China. E-mail address: [email protected] (J. Bai). 1 Co-first author. https://doi.org/10.1016/j.chemosphere.2019.125645 0045-6535/© 2019 Published by Elsevier Ltd.
1. Introduction Plastic has the characteristics of chemical resistance, high
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strength, high toughness, and malleability. That combined with its low price and accessibility, make it widely used in electronic equipment products (Panda et al., 2010). With the improvement of plastic properties and the replacement of electronic equipment products, the amount of waste plastics is increasing (Wang et al., 2012). In electronic equipment such as televisions, refrigerators, washing machines and air conditioners, the plastic mass accounts for more than 17.7% of the total, of which acrylonitrile-butadienestyrene (ABS), flame-retardant high-impact polystyrene (FRHIPS), and high-impact polystyrene (HIPS) account for the largest proportions (Oguchi et al., 2011; Schlummer et al., 2007). Statistically, global plastic production reached 8.3 billion tons in 2017, and e-waste production is expected to reach 52.2 million tons in 2021 et al., 2017). With the rapid growth in the (Geyer et al., 2017; Balde production of waste plastics, a reasonable way to deal with them is needed to reduce the pressure on the ecological environment. Waste electronic equipment products have recently received attention as a secondary source of metals and plastics, which may still have important application value (Oguchi et al., 2011). Recycling has been employed as the main treatment method of waste plastics in domestic markets, meaning that they continue to be used after modification into secondary products (Fig. S1). The processing of waste plastics includes screening, crushing, modification, granulation, injection, and moulding (Sun et al., 2015). However, waste plastics must be combined with additives (such as antioxidants, epoxy plasticizers, and colourants) during production (Liu et al., 2019a; Andrady and Rajapakse, 2016). These additives contain heavy metals (such as Pb, Cr, and Cd) as well as organic pollutants (such as dioctyl adipate (DEHA), phthalate acid easters (PAEs), polybrominated diphenyl ether (PBDEs), and tetrabromobisphenol A (TBA)) (Miloichikova et al., 2016; Cooper et al., 2011). In addition, additives may migrate to the surface of plastics because of aging during processing and subsequent usage of waste plastic secondary products, thus increasing potential hazards for humans or animals (Jaeger and Rubin, 1970; Lu et al., 2018). In the recycling process, additives are used for plastic modification, and the content of harmful substances will increases after processing. Yu et al. (2017) found that the PBDE content in waste cathode ray tube television shell plastic was 11.31 mg/kg, and that the PBDE content in new products made from recycled material was 15.20 mg/kg, suggesting that PBDEs in waste plastics are transferred into the new products (Yu et al., 2017). On comparing new and recycled plastic food containers, the concentrations of metals such as Cd and Pb in used plastic containers were found to be higher, potentially due to the lack of quality control protocols in the re-manufacturing process of used plastic containers (Waheed et al., 2012). Hence, it is necessary to control the amounts of additives and develop new additives, which do not pose an environmental risk. The additives in plastics will flow with the high temperature processing, and heavy metals in waste plastic secondary products may migrate to the environment during long-term use. Bang et al. (2012) found that plasticizers and antioxidants in plastic food containers can migrate outward and cause endocrine disruption, and that the degree of pollution depended on physical and chemical conditions, such as temperature, ultraviolet, pH value, and other factors. When polyvinyl chloride (PVC) was used as packaging for orange juice, the acidity of the orange juice promoted the dissolution of diethylhexyl phthalate (DEHP) dissolution. Furthermore, the dioctyl adipate (DEHA) in PVC film migrated to high-fat food at 40 C over 10 days, and the migration rate reached 75e90%. This was due to the fact that the plasticizer was mobile in the polymer matrix, and because DEHA is lipophilic and thus spreads easily (Coltro et al., 2014). After many cycles of waste plastic processing, high temperature will produce greater internal
stress in the plastics and induce cracking and layering, thereby facilitating additives migration (Liu et al., 2019b). Compared with new plastic products, waste plastic has the disadvantages of having a turbid colour, strong smell, poor toughness, increased aging, and brittleness. Heavy metals in waste plastic products are more likely to migrate outward. There are many studies on the migration of organic matter in food plastic additives; however, the study of heavy metals in waste plastic products needs greater attention. The main purpose of this experiment is to explore the migration of heavy metals in electronic waste plastics during their simulated recycling at the laboratory scale. Generally, observing and evaluating the migration risk of heavy metals during the recycling processes of waste plastics requires long time frames. To simplify the experimental process and rapidly assess the environmental risk from heavy metals, waste plastics were processed three times to accelerate the recycling process and shorten the time needed to complete the study. The toxic leaching method of evaluating solid waste (China SEPA, 2007) was used to determine heavy metal migration and quantitatively determine its toxicity. Changes in the mechanical properties and microstructure of waste plastics were compared with the leaching results. The results provide an overall evaluation of the waste plastic recycling process, and assess the environmental risk posed by waste plastic secondary products to allow effective supervision of the process. 2. Materials and methods 2.1. Sample collection and reagents Waste ABS, FR-HIPS, and HIPS samples used in the experiment were obtained from Xinjinqiao Environmental Protection Co., Ltd., Shanghai, China. Samples (3000 g) were randomly collected in the disassembly workshop and loaded into a self-sealing bag (Table S1). Waste plastic was placed in 40% ethanol solution, cleaned for 15 min with deionized water in an ultrasonic cleaner, after which any paper or labels were removed from the plastic. After cleaning, samples were dried in a 70 C blast dryer in preparation for use. Nitric acid (HNO3) (65%e68%, GR), sulfuric acid (H2SO4) (98%, GR), fluoroboric acid (HBF4) (40%, AR), hydrogen peroxide (H2O2) (30%, AR), ethanol (C2H6O) (99.7%, AR), and a multi-element standard solution (100 mg/L) (Sinopharm Chemical ReagentCo., Ltd, China) were main experimental reagents used in this study. To avoid introducing foreign heavy metals in the processing and interfering with the migration study, only antioxidants (Pentaerythrite tetra [b-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]) (AO-60(1010), China) without heavy metal elements were used in the experiment. Ultrapure water was used as needed during the experiments. 2.2. Experimental methods 2.2.1. Waste plastic recycling production Recycling experiments of waste ABS, FR-HIPS and HIPS samples were performed separately following the same operation steps. The following methods were used to process the samples. The prepared sample was crushed with a multifunctional grinder, and passed through 6-mm standard screen; 2 g antioxidants were added to 2000 g sample (mass ratio of 0.1%), and mixed evenly; a co-rotating twin-screw extruder was used for granulation. The temperatures of each part of the extruder were in the range of 210e220 C (Fig. S2a), and the screw speed was 75 rpm/min. The obtained particle material was cooled and added to the injection-moulding machine for production. The temperatures of each part of the injectionmoulding machine were 220 C (Fig. S2b) (Xi et al., 2018). The cycle was repeated three times according to the methods. During each
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cycle, 500 g of injection-moulding sample was taken for analyses. The sample collection number was classified as shown in Table 1. 2.2.2. Digestion experiment One hundred gram of the injection-moulding sample (Table 1) was crushed and passed through a 0.425-mm standard screen, and 0.1000 g of the sample (Table 1) was weighed. ABS sample combined with 8 ml HNO3, 2 ml HBF4, 3 ml H2O2 and FR-HIPS, and HIPS were digested with 8 ml HNO3, 3 ml HBF4 and 1.5 ml H2O2 in a polytetrafluoroethylene (PTFE) digestion tank with microwaveassisted digestion (Anton-Paar, Multiwave PRO, Austria), as described in Table S2 (Mao et al., 2019). After the sample was cooled, it was filtered and diluted with 5% HNO3 solution to a total volume of 50 ml. Heavy metal contents were determined using an inductively coupled plasma-optical emission spectrometer (ICPOES) (Thermo Fisher Scientific, ICAP7000, USA). Each batch of samples included three replicates. 2.2.3. Toxic leaching experiment During daily use, waste plastic products may be in contact with liquid for extended periods and be affected by the acidity and basicity of the liquid, temperature, molecular characteristics, and other factors (Bang et al., 2012). Consequently, heavy metals in the products may easily migrate and pollute the liquid. Therefore, the potential environmental impacts of waste plastic products were studied by a toxic leaching experiment. The methods were as follows: the injection-moulding samples (Table 1) were used to follow the solid waste leaching method (China SEPA, 2007). The leaching experiment was continued for 30 days, and triplicate samples were taken every second day until the 27th day, and then again on the 30th day. The leaching samples were filtered and stored at 4 C (Midea, BCD-186, China) before being analysed using ICP-OES. 2.3. Analysis methods The mechanical properties of the plastics were tested using a universal testing machine (MTS, CMT6104, China) and a pendulum impact-testing machine (MTS, ZBC7000, China). The impact strength, bending strength, and tensile strength of FR-HIPS were tested according to the China National Standard (Huang et al., 2010). The surface structures of the waste plastics after recycling processes were observed using SEM (Phenom, ProX, China). The data were analysed by Microsoft Excel and Original 9.0. 3. Results 3.1. Digestion results of waste plastics recycling products Fig. 1 shows the contents of harmful heavy metals in waste plastic processing products (the detection limits of heavy metals are shown in the Table 2). After the initial processing of waste plastic products, the Cr, Pb, Sb, and As contents in ABS were 1.1, 1.5, 43.0, and 10.9 times higher than the limits set in the standards, respectively. The Cr, Pb, Sb, and As contents in FR-HIPS products were 1.4, 1.5, 426.3, and 4.7 times than the limits, and were 0.6, 1.2, 18.8, and 13.9 times higher than the limits in HIPS products,
Fig. 1. Metal concentrations in waste plastic recycling products (1: first recycle process; 2: second recycle process; 3: third recycle process).
Table 1 Sample information for each stage of cycle processing cycle. Sample
First recycle products
Second recycle products
Third recycle products
ABS FR-HIPS HIPS
ABS1 FR-HIPS1 HIPS1
ABS2 FR-HIPS2 HIPS2
ABS3 FR-HIPS3 HIPS3
Note: 1: first recycle process; 2: second recycle process; 3: third recycle process.
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Table 2 Detection limits of different heavy metals (mg/L).
Detection limits
Cr
Cd
Sb
Pb
As
Ni
Cu
Zn
0.006
0.002
0.005
0.004
0.008
0.006
0.004
0.003
Note: The detection limits are low, so the content of heavy metals can be determined accurately.
respectively. As can be seen from Fig. 1, the Cr, Pb, Sb, As, Cd, Ni, Cu and Zn contents of the products after three waste cycles, showed decreasing trends for ABS, FR-HIPS, and HIPS particularly in Sb and As. Compared with the first and third processing products of waste ABS, the Sb and As contents were reduced by 467 mg/kg and 85.2 mg/kg, respectively, corresponding to 17.7% and 28.6% decreases. Reductions in Sb and As in the first and third FR-HIPS waste processing products were 1396 mg/kg and 22.5 mg/kg (decreases of 5.5% and 19.3%), respectively. The Sb and As reductions in the first and third processing products of waste HIPS were 107 mg/kg and 55.8 mg/kg (decreases of 9.0% and 15.0%), respectively.
3.2. Leaching of recycled waste plastic products Quantitative analysis of heavy metals in the leachate showed that obvious leaching of Ni, Cu, Zn, Sb, and Pb from waste ABS, FRHIPS, and HIPS products (Figs. 2-4), leaching of Cr, Cd and As were too low to be detected. The first processed products of waste plastics were leached for 30 days, and resulted in leached solution concentrations of Ni, Cu, Zn, Sb, and Pb from waste ABS products of 0.006 mg/L, 0.006 mg/L, 0.291 mg/L, 0.373 mg/L, and 0.021 mg/L, respectively, whereas the concentrations were 0.031 mg/L, 0.013 mg/L, 0.198 mg/L, 2.266 mg/L, and 0.023 mg/L in the leached solution of waste FR-HIPS products, respectively. In HIPS products, Ni, Cu, Zn, Sb, and Pb concentrations in leached solutions were 0.019 mg/L, 0.060 mg/L, 0.103 mg/L, 0.123 mg/L, and 0.045 mg/L respectively. The Sb concentrations in the leachate of waste ABS, FR-HIPS, and HIPS products were 37.3, 226.6, and 12.3 times higher, respectively, than the specified concentrations specified in the fifth class of the Groundwater Quality Standard (Table S3). The third processed products obtained from waste plastics were leached for 30 days, and the Ni, Cu, Zn, Sb, and Pb concentrations in the leachate of waste ABS products were 0.023 mg/L, 0.005 mg/L, 2.264 mg/L, 1.172 mg/L, and 0.020 mg/L, respectively. Compared with the first processed products, the leaching concentrations of Ni, Zn, and Sb increased by 2.8, 6.8, and 2.1 times, respectively. The Ni, Cu, Zn, Sb, and Pb concentrations were 0.119 mg/L, 1.041 mg/L, 0.390 mg/L, 2.413 mg/L, and 0.015 mg/L in the leachate of waste FRHIPS products, respectively. Compared with the first processed products, the leaching concentrations of Ni, Cu, Zn, and Sb increased by 2.8, 79, 0.97, and 0.06 times, respectively. Ni, Cu, Zn, Sb, and Pb concentrations of 0.140 mg/L, 0.148 mg/L, 0.813 mg/L, 0.311 mg/L, and 0.132 mg/L, respectively, were present in the leachate of waste HIPS products. Compared with the first processed products, the leaching concentration of Ni, Cu, Zn, Sb, and Pb increased by 6.4, 1.5, 1.9, 1.5, and 1.9 times, respectively. Compared with the metal content limit stipulated in the Groundwater Quality Standard, Ni, Zn, and Pb concentrations in the leachate of waste ABS products exceeded the fifth, third, and third classes of the Groundwater Quality Standard, respectively. The Ni, Cu, and Pb concentrations in the leachate of waste FR-HIPS products also exceeded the fifth, third, and third classes of the Groundwater Quality Standard, respectively, and the Ni, Cu, and Pb concentrations in the leachate of waste HIPS products exceeded the fifth, second, and fifth classes of the Groundwater Quality Standard,
Fig. 2. Metal concentrations during toxicity leaching experiments on waste ABS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
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respectively. Among them, Sb concentrations in the leachate of waste ABS, FR-HIPS, and HIPS products were 117.2, 241.3, and 31.1 times higher than the Sb concentration specified in the fifth class of the Groundwater Quality Standard, respectively. 3.3. Mechanical properties and SEM scanning The mechanical properties of thrice-recycled products of waste ABS, FR-HIPS, and HIPS are shown in Table 3. The impact, tensile, and bending strengths of waste plastic products decreased with the increase in recycling times. Comparing the recycling products between the third and first cycles, the impact, tensile, and bending properties of waste ABS, FR-HIPS, and HIPS products decreased by 35.4%, 7.2%, and 4.4%; 50.3%, 3.9%, and 6.9%; and 56.6%, 7.3%, and 6.5%, respectively. The cyclic processing of waste plastics had the greatest influence on impact properties. The cross-sectional surface layer of waste ABS, FR-HIPS, and HIPS injection-moulded sample strips at each processing stage are shown in SEM images in Fig. 5. It can be seen that the aging phenomena were aggravated by increased processing times. The stratification phenomenon of waste plastic became gradually obvious, and the adhesion between layers decreased. The size and number of pores in the surface layer in waste plastics increased. 4. Discussion 4.1. Heavy metals content of waste plastics recycling products The Cr, Pb, Sb, and As contents in the waste ABS, FR-HIPS, and HIPS products (Fig. 1) exceeded the heavy metal content limitations set by the Safety of Toys Standards (Isama et al., 2011). The samples used in this study were produced prior to 2007, at which time the Administration on the Control of Pollution Caused by Electronic Information Products was not formulated and implemented by Chinese government. There were no strict laws controlling plastic production (Song et al., 2017). Therefore, the mass use of additives was the main reason for the high content of heavy metals in waste plastic samples. Among Cr, Pb, Sb, and As in waste plastics, the Sb content exceeded the Standard the most (Fig. 1), which was mainly related to the usage requirements, Inorganic flame retardants (Sb2O3) were added to enhance high-temperature resistance when electronics were used for long durations, which increased the Sb content in plastics (Carty and Price, 2004). Nakashima et al. (2012) found that plastics may act as carriers of toxic metals and pollute the surrounding environment. Therefore, the heavy metal contents in waste plastic products should be effectively controlled. Waste plastics processing resulted in the emission of significant amounts of particle-phase pollutants (which might contain metal
Table 3 Results of mechanical properties of waste plastic recycling products. Sample ID
ABS1 ABS2 ABS3 FR-HIPS1 FR-HIPS2 FR-HIPS3 HIPS1 HIPS2 HIPS3
Impact strength/ (kJ/m2)
Tensile strength/ MPa
Bending strength/MPa
Result
RSD/%
Result
RSD/%
Result
RSD/%
7.21 5.28 4.66 6.22 4.84 3.09 8.61 7.06 3.74
0.75 4.13 3.05 4.17 4.87 4.47 1.79 1.23 1.03
42.57 40.35 39.51 28.48 27.36 25.71 29.37 28.83 27.22
1.26 0.57 3.18 1.41 1.46 4.65 2.14 2.09 0.90
63.32 62.54 60.53 41.86 41.02 38.97 47.23 46.35 44.15
0.93 0.15 0.55 2.16 3.17 1.04 0.99 0.51 3.14
5
elements such as Sb and As) in high-temperature treatments. The emission of particle-phase pollutants was the main factor reducing metal contents during the recycling process (Tomsej et al., 2018). Gaseous and particles pollutants containing carcinogens affect workers’ health within and near the recycling zones (Luo et al., 2014). Therefore, dedusting and tail gas absorption devices should be added in the high-temperature stage of waste plastics processing, which could reduce the harm of pollutants to the human body and the environment. 4.2. Heavy metal migration during simulated recycling Concentrations of Ni, Cu, Zn, Sb, and Pb in the leachate increased when leaching was sustained (Figs. 2e4). When the cycle number of the same waste plastics increased, the leaching concentration of metals also increased. Existing studies have also found that Sb in polyester (PET) food packaging materials migrated outward during long-term contact with acidic food simulants, thus the concentration in the liquid increased gradually (Yao et al., 2013). When recycled waste plastic products come into contact with acidic liquid during use, heavy metals in additives may dissolve in liquid and contaminate the products. Fig. 5 indicates that with increasing processing cycles, the degradation of waste plastics is more apparent. The adhesion between layers was reduced and stratification and cracking appeared in polymers. The increased contact area between the leachate and the surface could promote the dissolution of the additives and result in metals more easily migrating outward (Fig. S3). Therefore, the migration rate of heavy metals in waste plastic products was accelerated as the degradation of waste plastics, and the migration content of some heavy metals exceeded the limitation in the Groundwater Quality Standard. This presents an environmental safety risk, which should garner more attention. 4.3. Effect of simulated recycling on mechanical properties and microstructure The cyclic processing of waste plastics had the greatest influence on impact properties, which agrees with the results published by rez et al. (2010) who studied the effect of reprocessing on ABS Pe properties. Their data showed that the impact strength of ABS samples decreased after recycling. The reduction in impact strength was related to some degradation of the polybutadiene phase, which rez et al., 2010). likely involved changes in the molecular weight (Pe Liu et al. (2019b) found that temperature accelerated destruction of composites. Resin cracking and fibre-matrix interface degradation occurred in the aged specimens. A high temperature yielded strong internal stress, resulted in debonding and interfacial damage. Comparing the microstructure of the three kinds of waste plastics, the aging rate of FR-HIPS was slower because it contained more flame retardants; therefore, its high-temperature resistance was greater than those of ABS and HIPS. During waste plastics processing, plastic was oxidized by high-temperature treatment during which the elastic rubber particles in the plastics would degrade and the grafting properties between the monomers and the plastic stability would deteriorate (Wahhab et al., 2016). After the elastic rubber particles were degraded, the toughness of rubber particles decreased, rigidity increased, the plastics gradually became brittle, and the mechanical properties deteriorated significantly. The plasticizer content in waste plastics decreased gradually after high-temperature treatment, which reduced its resistance to impact, bending, and tension. The number of pores in the surface layer of waste plastics increased after processing, due to the flow of inorganic particles, and the aging degradation of plastics was affected by the outflow of inorganic components and plasticizers
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Fig. 3. Metal concentrations during toxicity leaching experiments on waste FR-HIPS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
Fig. 4. Metal concentrations during toxicity leaching experiments on waste HIPS processing products (A: first recycled products; B: second recycled products; C: third recycled products).
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Fig. 5. SEM images of cross-sectional surface layer of a waste plastic injection moulded sample strip (A: first recycle of ABS, B: second recycle of ABS, C: third recycle of ABS; D: first recycle of FR-HIPS, E: second recycle of FR-HIPS, F: third recycle of FR-HIPS; G: first recycle of HIPS, H: second recycle of HIPS, I: third recycle of HIPS).
(Ito and Nagai, 2007). This was also the main reason that the heavy metal concentrations in the leachate increased with increasing processing times. 5. Conclusions Results indicated that after the completion of recycling, the Sb and As contents in waste plastic products decreased and leaching of Ni, Cu, Zn, Sb, and Pb was obvious. When the leaching experiment was conducted, the metal concentrations in the leaching solution increased gradually. With increasing cycles, the leaching concentrations of metal also increased after 30 days of leaching. The leaching of Ni, Cu, Zn, Sb, and Pb in waste plastic products exceeded the relevant classes of the Groundwater Quality Standard. With increased processing times, the aging degree of waste plastics accelerated and mechanical properties deteriorated, and impact strength decreased by the greatest proportion. Furthermore, the size and number of pores in the surface layer of waste plastics increased, stratification and cracking in polymers appeared, and
adhesion of the waste plastics to the additives decreased. The findings of this study can help assess the environmental risks posed by waste plastic recycling and provide some guidance for waste plastic reuse. More work is needed to assess the heavy metal contamination risk across diverse plastics sectors. This, along with an evaluation of more recent plastic waste sources, will form the basis of future research so we can begin to construct a robust framework to evaluate and understand this new environmental health and safety risk.
Author contribution section Shaohua Mao: Formal analysis; Investigation; original draft; Roles/Writing, Weihua Gu: Writing - review & editing, Jianfeng Bai: Funding acquisition; Roles/Writing; Conceptualization, Bin Dong: Funding acquisition, Qing Huang: Software, Jing Zhao: Data curation, Xuning Zhuang: Methodology, Chenglong Zhang: Validation, Wenyi Yuan: Visualization, Jingwei Wang: Project administration
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Declaration of competing interest The authors declare that they have no conflict of interests. Acknowledgments The authors thank the Gaoyuan Discipline of ShanghaiEnvironmental Science and Engineering (Resource Recycling Science and Engineering), the Key Discipline of Shanghai Polytechnic University (No. XXKZD1602), and the Construction Project of Shanghai Collaborative Innovation Center (No. ZF1224). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.125645. References Andrady, A.L., Rajapakse, N., 2016. Additives and Chemicals in Plastics. The Handbook of Environmental Chemistry. Springer International Publishing AG, USA. , C.P., Forti, V., Gray, V., Kuehr, R., Stegmann, P., 2017. The global E-waste Balde monitor 2017. https://www.itu.int/en/ITU-D/Climate-Change/Documents/GEM %202017/Global-E-waste%20Monitor%202017%20.pdf. Bang, D.Y., Kyung, M., Kim, M.J., Jung, B.Y., Cho, M.C., Choi, S.M., Kim, Y.W., Lim, S.K., Lim, D.S., Won, A.J., Kwack, S.J., Lee, Y., Kim, H.S., Lee, B.M., 2012. Human risk assessment of endocrine-disrupting chemicals derived from plastic food containers. Compr. Rev. Food Sci. Food Saf. 11, 453e470. Carty, P., Price, D., 2004. Antimony/Iron interactions in chlorinated polymer blends. J. Vinyl Addit. Technol. 10, 88e94. ^ Coltro, L., Pitta, J.B., Costa, P.A., Perez, M.A.F., Araújo, V.A., Rodrigues, R., 2014. Migration of conventional and new plasticizers from PVC films into food simulant: a comparative study. Food Control 44, 118e129. Cooper, J.E., Kendig, E.L., Belcher, S.M., 2011. Assessment of bisphenol A released from reusable plastic, aluminium and stainless steel water bottles. Chemosphere 85, 943e947. Geyer, R., Jambeck, J.R., Law, K.L., 2017. Production, use, and fate of all plastics ever made. Sci Adv 3, 1e5. Huang, Z., Lu, S., Yang, Z., Yu, C., Guo, D., 2010. Studies on the properties of epoxy resins modified with novel liquid crystalline polyurethane. Adv. Mater. Res. 150e151, 727e731. Isama, K., Kawakami, T., Nishimura, T., 2011. Migration of Eight Harmful Elements from Household Products Made of Synthetic Resin That Infants May Swallow by Mistake. Yakugaku Zasshi 131 (7), 1135e1140. Ito, M., Nagai, K., 2007. Analysis of degradation mechanism of plasticized PVC under artificial aging conditions. Polym. Degrad. Stab. 92, 260e270. Jaeger, R.J., Rubin, R.J., 1970. Plasticizers from plastic devices extraction, metabolism, and accumulation by biological systems. Science 170, 460e462. Liu, S., Ge, H., Zou, Y., Chen, J., 2019a. Properties of PP wood-plastic composites with biocompatibility additives. Mater. Sci. Forum 944, 509e514. Liu, S., Chen, Y., Chen, P., Xu, D., Xiong, X., Wang, J., 2019b. Properties of novel bismaleimide resins and thermal ageing effects on the ILSS performance of their carbon fibre-bismaleimide composites. Polym. Compos. 40, 1283e1293. Lu, K., Qiao, R., An, H., Zhang, Y., 2018. Influence of microplastics on the
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