1. Introduction
Technological innovation promotes the high-quality development of the electronic product industry. Introducing new technologies such as 5th Generation Mobile Communication Technology (5G) and people’s desire for a better experience with electronic devices make consumer electronic products’ service life shorter and shorter [
1]. Thus, a large amount of obsolete electronic equipment, also known as electronic waste (e-waste), is being generated. E-waste is currently one of the fastest-growing waste categories in the world. Researchers estimate that by 2030, the amount of e-waste generated globally will exceed
tonnes [
2,
3]. For China, it is estimated that
tonnes of e-waste will be generated in 2030 [
3]. E-waste, also known as urban mining, contains many metal resources, such as the precious metals gold and silver and the rare metals gallium and indium [
3,
4]. However, less than
of e-waste is reportedly recycled, and the rest is incinerated and landfilled, causing a huge waste of resources and environmental health concerns [
2].
Resource recovery of e-waste is a double-edged sword. It not only has a huge metal recovery value but also contains a lot of detrimental substances such as polymers, brominated flame retardants, curing agents, and heavy metals [
5]. Polymers and brominated flame retardants provide reliable guarantees for the normal operation of electronic devices, such as moisture-proofing and flame retardancy, but they have become a huge obstacle in recycling electronic device waste [
5,
6]. Due to the complex organic compounds, improper handling of e-waste may lead to toxic compounds such as bisphenol A and polybrominated diphenyl ethers [
7,
8]. Therefore, the green removal or degradation of organic components is a prerequisite for the harmless recycling of e-waste.
A common electronic component in everyday life, the light emitting diode (LED), is attracting more and more attention to recover precious and strategic metals. LED is a light-emitting element that converts electrical energy into light energy. Its use has risen sharply over the past decade due to its energy-saving, long-life, and mercury-free properties [
9,
10]. LED is widely used in display screens, liquid crystal displays, mobile phone backlights, and general lighting [
11⇓-
13]. More importantly, for energy saving and other reasons, many countries and regions are implementing legislation to phase out incandescent lamps and promote the use of LED lamps, which means that the use of LED will inevitably grow rapidly [
14,
15]. The LED waste mainly comprises white plastic polyphthalamide (PPA), transparent epoxy resin, a metal bracket, metal lead, and a chip. It contains precious metals such as gold and silver and strategic metals such as gallium and indium, mainly concentrated on chips and metal leads [
11].
At the same time, as a new type of e-waste, LED is classified as hazardous waste in some regions of the European Union, Canada, and the United States [
16,
17]. The environmental burden of waste LED from potential toxicity is mainly related to arsenic, copper, nickel, and lead [
17⇓-
19]. The risks of an LED to the human body may originate from arsenic or inorganic arsenic compounds in waste LED. Strong epidemiological evidence shows that arsenic can cause serious health problems, such as skin or heart disease or even cancer [
20,
21]. Since LEDs are inherently energy-saving products, there is an urgent need for environmentally friendly and low-carbon disposal methods after they are scrapped, where the LED industry should contribute to realizing carbon emission reduction goals from production to disposal.
There have been some studies on metal recycling from LEDs so far. Some methods of waste LED recycling can be referenced, such as mechanical separation [
10], vacuum metallurgy separation [
11], pyrometallurgy [
22], hydrometallurgy [
23], as well as bioleaching [
24]. Nevertheless, the above methods, apart from pyrometallurgy, cannot provide solutions for the decomposition of waste LED packaging materials, and pyrometallurgy also runs the risk of dioxin formation and toxin release [
25]. Moreover, previous research on the recycling of waste LED mainly focuses on recycling metals while ignoring the removal of harmful substances such as organic packaging materials.
Furthermore, the filler
, which accounts for a large proportion of LED packing material, was also ignored in the previous recycling process.
is widely used as a filler material in polymer composites due to its high thermal conductivity (about 11.7
), low thermal expansion coefficient (8.6 parts per million
), high electrical resistivity, and non-toxic properties [
26]. Incorporating
can improve the mechanical properties and thermal stability of polymer materials, such as their tensile, flexural, and dielectric strengths [
26,
27].
in LED packaging material can improve the thermal conductivity of plastic PPA and the reflective effect, enhancing LED light output [
28]. Therefore,
fillers should also receive proper attention when recycling waste LEDs.
The hydrothermal method uses water as the solvent in a closed reaction vessel and is green, non-toxic, and non-polluting [
5]. The hydrothermal method creates cleaner, safer, and more environmentally friendly chemical processes, having different characteristics from liquid water under conventional conditions, such as a lower dielectric constant and fewer and weaker hydrogen bonds [
29⇓-
31]. Furthermore, it can support ionic, polar non-ionic, and free radical reactions, and more importantly, a single water molecule can participate in the reaction as a reactant or catalyst [
31]. (see Fig. S1 in Appendix A for further details). There is also extensive research on hydrothermal methods as reaction media for materials synthesis, biomass processing, and waste destruction [
32⇓⇓-
35]. When discussing the degradation of organic materials under hydrothermal conditions, most studies focused on the strong oxidizing power and free radical reactions [
36,
37]. However, the role of water itself is ignored, although sometimes the water is not an inert medium but an active participant in the reaction.
As an ideal solvent, water is green and cheap in chemical reactions. Therefore, this study focuses on exploring the role of water in plastic PPA degradation that current studies have overlooked and complementing the mechanism of water as a reactant in chemical reactions. The nano- encapsulated in plastic PPA was also efficiently recovered in this study. Besides, organic PPA hydrolysis and degradation processes were discussed in detail, and the degradation mechanism was speculated. The residual white powder after hydrothermal treatment was detected and analyzed, and finally, the environmental impact of recycling LED was assessed.
2. Materials and methods
2.1. Materials and chemicals
The experimental raw material used in this study was surface-mounted device LED, whose structure is illustrated in Fig. S2 in Appendix A. The main components of waste LED are listed in
Table 1. The detailed organic carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) composition of plastic PPA are listed in Table S1 in Appendix A.
was used as the oxidant, and the water used in the experiment was ultrapure. Unless otherwise specified, all chemical reagents used in the experiment are pure analytical reagents.
As depicted in Fig. S2, the chips and metal leads are encapsulated inside the LED by a transparent epoxy resin. The transparent epoxy is exposed on one side, while the other is encased in white plastic PPA. At the same time, plastic PPA tightly wraps a part of the metal bracket inside the LED. Therefore, to utilize the metal resources in LED, the white plastic PPA should be removed harmlessly.
2.2. Apparatus and analysis methods
The main experimental equipment used in this study is a Hastelloy high-pressure reactor with a volume of , produced by Shanghai Yanzheng Experiment Instrument Co., Ltd. (China). Additionally, the content of metals in the LED was examined by inductively coupled plasma mass spectrometry (ICP-MS-7800, Agilent, USA). The detailed organic elements (C, H, N, and S) of plastic PPA was analyzed via an elemental analyzer (Vario EL Cube, Element, Germany). X-ray diffraction (XRD; LabX XRD-6100, Japan) was used to characterize the white powder obtained, with an angle of , and the scanning velocity was . FTIR was also used to characterize the main functional groups of the white powder (Nicolet6700, Thermo fisher scientific, USA). During this characterization, the KBr tablet pressing method was used. The residual solids were investigated using a scanning electron microscope with X-ray energy dispersive spectrometer analysis (SEM-EDS; Sirion 200, USA), Raman imaging and SEMs (RISE-MAGNA; Tescan, the Czech Republic), and transmission electron microscopy (TEM; Talos F200X, USA).
In this study, the degradation rate of the LED packaging materials was calculated based on the following equation:
where and are the initial weight of the plastic PPA and the plastic PPA residue after the hydrothermal process, respectively.
2.3. Experimental methods
The hydrothermal experiments in this study were conducted according to Refs. [
38,
39]. Besides, a standard response surface methodology design was used, and the design type was Box-Behnken-a four-factor (A, B, C, and D), three-level experimental protocol. Temperature, rotating speed, oxygen volume ratio, and holding time were selected as variable parameters, and the degradation rate was chosen as the response. The ranges and levels of variables are reported in
Table 2. In addition, more detailed experimental design information can be found in Tables S2-S4 in Appendix A.
The hydrothermal process was implemented using a Hastelloy reactor. Each experiment involved about of waste LED, of ultrapure water, and a certain amount of oxidant. Parameters such as the temperature and holding time of the reaction kettle were set, and the switches for heating and rotating speed were turned on. Then, when the reactor reached the set temperature, the heat preservation program was started. The reaction kettle was kept warm for a period, the heating program stopped heating, and the reaction kettle was cooled to room temperature under natural conditions. The reactor was opened, and residual solids of different particle sizes and liquids were separated after sieving and vacuum filtration.
3. Results and discussion
3.1. Principal of plastic PPA hydrolysis
There are many forms of weak interaction (such as the van der Waals effect and the hydrogen bond), and the essence of weak interaction is divided into two categories: electrostatic-dominated weak interactions and dispersion-dominated weak interactions. Electrostatic weak interactions can play a role in mutual repulsion or attraction, and weak dispersion interaction acts as an attraction. PPA is a subset of thermoplastic synthetic resins in the polyamide family [
40]. The polymer chain is a poly-condensate of diamine and terephthalate, which has good heat resistance, high strength, rigidity, and excellent processability. The amide group (-CONH-) in plastic PPA is a polar and hydrophilic group, which provides the potential for hydrolytic degradation of plastic PPA. Therefore, in this study, the weak interaction was considered to play a positive role in the occurrence of the hydrolytic reaction.
When the LED is submerged in hydrothermal water, the polarity of the water molecule makes it attract the amide group (
Fig. 1(a)). Based on the Becke, three-parameter, Lee-Yang-Parr (B3LYP) method, the
basis set, the electrostatic potential map, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) orbitals were calculated via Gaussian 16 (Linux) [
41].
Figs. 1(b) and
(c) depict the HOMO and LUMO of plastic PPA. The HOMO level reflects the ability of a molecule to lose electrons, and the LUMO level is numerically equivalent to the electron affinity of the molecule. The calculated HOMO and LUMO energies are-1.926eV (losing electrons;
) and
(accepting electrons).
Figs. 1(b) and
(c) highlight that the HOMO and LUMO of plastic PPA have been distributed on the benzene ring and the amide group. It is well known that the benzene ring structure is quite stable, and therefore, the hydrolysis reaction site of plastic PPA is located in the amide group (-CONH-). The original intermolecular hydrogen bond between amide groups in the polymer chain is destroyed, and the polarity attraction weakens (Fig. S3 in Appendix A). Therefore, the rigidity and tensile strength of the plastic PPA will diminish with moisture absorption, and the ductility will increase. Furthermore, it has been reported that when moisture enters the polymer, the water acts as a plasticizer, lowering the polymer’s glass transition temperature [
42].
In addition, compared with ordinary conditions, the average kinetic energy of plastic PPA and water molecules increases under subcritical hydrothermal conditions with increased water temperature. The thermal motion of molecules intensifies, and the number of effective molecular collisions also increases. When the water temperature rises, the pressure in the reactor also increases. This further promotes the diffusion of water molecules into the interior of the plastic PPA, allowing the water molecules to enter the material’s interior. Eventually, the molecular chain will break outside and inside the material due to the high-temperature subcritical water. The simultaneous characteristics of high temperature and high pressure destroy the original lattice structure of the material, resulting in the hydrolysis of plastic PPA, as illustrated in
Fig. 1(d). Indeed, the spherical white powder filler
gradually released as the degradation rate of plastic PPA increased. Overall, the analyses presented above provide feasibility guidance for the hydrothermal degradation of plastic PPA and provide theoretical support for the recovery of titanium dioxide.
3.2. Hydrothermal degradation of plastic PPA and optimized experiment
As depicted in
Fig. 2(a), when the temperature rises from 220 to
, the average level (median) of plastic PPA degradation rate increases. The average degradation rate increased significantly from
to
when the temperature rose from 220 to
. For a temperature of
, the average degradation rate was close to
, while for
,
Fig. 2(a) reveals a mildly abnormal value corresponding to Run 12 of the experimental design (Table S2). We analyzed all experimental results at 280 °C and concluded that the insufficient oxidant addition caused the mild abnormal value.
Fig. 2(b) highlights that when the rotating speed increased from 250 to
, the average level (median) of the plastic PPA degradation rate remained almost unchanged and only increased by about
. When the rotation speed was
, the average degradation rate was nearly 100%, with Run 20 of the experimental design presenting a mildly abnormal value. By analyzing the experimental results for a rotating speed of
, we concluded that this abnormal value was due to the relatively low temperature compared with the other experiments. When the speed was
, two mild abnormal values appeared, with the lower abnormal value
caused by a relatively low temperature and insufficient oxidant and the higher abnormal value (35.97%) primarily caused by low temperature.
Fig. 2(c) infers that the average level (median) of plastic PPA degradation rate increased when oxidant was added. Specifically, when the amount of oxidant increased from 1 to
, the average degradation rate increased from 80.57% to 99.77%. Moreover, when the oxidant was
, the average degradation rate was nearly
. When
oxidant was added, mildly abnormal and extremely abnormal values were present (
Fig. 2(c)). The mildly abnormal value corresponded to Run 19 of the experimental design caused by the low temperature. Similarly, higher temperatures caused extreme outliers, indicating that temperature played a significant role in the degradation of plastic PPA compared to oxidants.
As depicted in
Fig. 2(d), the mean (median) level of plastic PPA degradation rate remained almost unchanged as the reaction time was extended from 5 to
. For a reaction time of
, the average plastic PPA degradation rate level was close to
, presenting a mild outlier in Run 13 of the experimental design caused by the low experimental temperature. Moreover, a mild outlier occurred at a reaction time of
, namely, Run 19, caused by a lower temperature and insufficient oxidant.
Fig. 2(e) illustrates a histogram of the degradation rate of plastic PPA. The degradation rate between 85.26% and 102.92% accounted for the largest proportion, confirming that the hydrothermal method is a compelling process for the degradation of plastic PPA. Besides,
Fig. 2(f) presents the correlation diagram between the degradation rate of plastic PPA and the related variables, revealing that the experimental variables positively correlate with the degradation rate of plastic PPA. The correlation values, from significant to minor, are temperature, oxidant, time, and rotating speed.
After clarifying the effect of each variable on the response value, the hydrothermal parameters were optimized. During the optimization process, the temperature was set within
, the rotating speed was controlled within
, and the addition of oxidant was set between
(
Fig. 3(a)). The time was controlled in a range of
, and the target degradation rate of plastic PPA was
. In addition, more detailed optimization information is shown in Table S5 in Appendix A. There were 85 solutions, and the desirability of 1-62 was 100%, as presented in
Fig. 3(b). (The meaning of the number 1 in
Fig. 3(b) is that the experimental factors and the response values can be
in line with the expectations of the experimental optimisa-tion, such as, the desirability of
was
. In
Fig. 3(b), the two colors are more for better differentiation of experimental conditions and response values without much deeper meaning.) Solutions 1-20 are listed in Table S6 in Appendix A. The photograph of the degradation process of plastic PPA is shown in Fig. S4 in Appendix A.
Fig. 4 illustrates the flowchart of the hydrothermal recycling waste LED, and the water absorption experiments of plastic PPA under low temperature and low pressure are presented in Fig. S5 in Appendix A.
3.3. Analysis of and hydrothermal residual solids
After hydrothermal treatment, the plastic PPA was degraded. White granular objects are found in hydrothermal solutions, and after vacuum filtration and drying, white solid particles were obtained. An SEM and EDS were utilized to explore residual solids’ microstructure and morphologies (
Fig. 5).
Fig. 5(a) presents the SEM image of the raw material, highlighting that the white solid particles are tightly wrapped inside the plastic PPA. The SEM images of white solid particles at different experimental temperatures (220 and
) are depicted in
Figs. 5 (b) and
(c).
Fig. 5(b) reveals that some white particles are still wrapped in plastic PPA, while after increasing the experimental temperature, the plastic PPA is effectively removed, and regular spherical particles are released (
Fig. 5(c)).
Fig. 5(d) presents the EDS layered electron image of white solid particles. The experimental results infer that the main elements of the white particles are Ti and O, as illustrated in
Figs. 5(e) and
(f). However, a small amount of
, and
is present (see
Figs. 5(g)-(i)). Finally,
Fig. 5(j) presents the map sum spectrum of the white particles, revealing the weight percentage of the different elements in the sample, such as Ti (34.94%) and O (59.34%).
The FTIR spectra of the white powder under different experimental conditions (The rotational speed was
and the reaction time was
) recorded in the range of
are presented in
Fig. 6(a). The purple spectral line was the standard spectrum of anatase
. The FTIR spectrum of white powder shows a broad absorption band at about
, and the absorption band appears for all samples. The broad absorption band could be attributed to stretching vibrations of
bonds from
molecules adsorbed on the white powder surface [
43]. The spectrum peaks around
are usually attributed to the stretching peaks of the Ti-O bond. In this study, the white powder also shows an absorption band at
. The peak around about
is usually attributed to the stretching vibrations
of
octahedrons in
[
44]. In addition, the lower intensity absorption peak located at
is attributed to
or
groups vibration [
45]. The spectrum peaks around about
are usually attributed to the existence of terephthalic acid [
46], which may be generated in the hydrolytic degradation of plastic PPA. Organic functional groups may exist because some LED packaging materials are not completely degraded, and a small part of the organic packaging materials are still attached to the
surface.
The X-ray patterns recorded in the 2 zenith angle
angles range within
, as shown in
Fig. 6(b). All white powder samples illustrate the existence of
with the phase of crystalline rutile. The spectrum’s peak intensity and width indicate the sample has high crystallinity. The crystallinity of titanium dioxide decreased continuously as the hydrothermal temperature increased. Specifically, the crystallinity at 220, 250, and 280 °C was 99.43%, 98.87%, and
, respectively. The possible reason is that higher hydrothermal temperatures mean higher energy, and the high-temperature hydrothermal environment destroys the original high-crystalline state of titanium dioxide. The TEM image of the white solid particles is presented in
Fig. 6(c), appearing as regular, spherical particles with a size of approximately
.
Fig. 6(d) is the high-resolution transmission electron microscopy (HRTEM; Talos F200X, USA) image of the sample. Note that the interplanar spacing was obtained by measuring the lattice fringes, and the value was about
. This spacing corresponds to the crystal face of the rutile (110), which is consistent with the XRD crystal form and diffraction peak intensity analysis results. The LED undergoes hydrothermal stripping and physical separation, and finally, nano-
, metal bracket, and silver-rich group are obtained.
3.4. Environmental impact assessment and techno-economic feasibility analysis
This study was compared with previous studies regarding environmental impact, where existing studies investigated the resource disposal of waste LED via the oxidation (VS-1) [
23] and pyrolysis (VS-2) [
11] methods. All trials assessed the environmental impact of each method using the SimaPro9.0 software, and the CML-IA baseline V3.05/EU25 method was adopted. The corresponding results are reported in
Fig. 7(a), highlighting that this study’s impact on the environment is significantly lower than that of previously published studies, demonstrating the environmental friendliness of this work.
Fig. 7(b) illustrates the impact category data of this work, where the categories with the least and greatest environmental impact in this study were ozone depletion and abiotic depletion (fossil fuels), respectively. This reveals that the damage to ozone in this work was extremely weak.
Figs. 7(c) and
(d) present the world market penetration and penetration rate of LED [
47]. To align with the sustainable development scenario, the LED must account for over
of lighting product sales by 2030. Besides,
Fig. 7(d) reveals that LED technology has captured most of the lighting market since 2015, as LED costs have declined. Accordingly, the number of obsolete LEDs will increase significantly, with e-waste pollution and its increasing metal content will promote the development of recycling technologies. Urban mines will become an indispensable resource in the future.
An economic assessment of this work was conducted for the processes presented in
Fig. 4, which concerned equipment costs, depreciation (20 years of operation), electricity, and chemical raw materials.
Assume the treatment capacity is designed to handle
of waste LEDs daily. The equipment costs about
USD, and the service life is about twenty years. In Shanghai, China, the electric charge is 0.13 USD, and the charge for water is
. The average price of chemical raw materials comes from Alibaba
(), which is
. The water consumption is about seven tonnes, and the chemical material is about 0.29 tonnes. This method yielded approximately
of metal bracket,
of
, and
of silver. The main component of a metal bracket is nickel-plated iron. We calculate the metal bracket’s price according to the iron price. The market price of iron is
is
, and silver is
. Some products come from the Ministry of Commerce of the People’s Republic of China
(
http://www.alibaba.com/.).
where is depreciation cost of equipment, is cost of electric energy, is cost of chemical materials, is revenue, and is profit.
However, the resource recovery of e-waste products cannot be merely measured by money, and eliminating e-waste contamination will have profound significance from a social and environmental point of view.
4. Conclusions
In this study, the plastic PPA of waste LED was efficiently degraded by the hydrothermal method. Since the amide group (-CONH-) of plastic PPA was hydrophilic, water molecules and the amide group (-CONH-) formed hydrogen bonds first, which destroyed the hydrogen bonds between the original polymers. Water molecules reduce the stiffness and tensile properties of plastic PPA. As a plasticizer, water molecules lower the glass transition temperature of plastic PPA, thereby promoting the hydrolysis of plastic PPA. The high temperature and pressure environment encourages water molecules to enter the interior of the plastic PPA and break the molecular chain outside and inside the material due to the high-temperature subcritical water. Indeed, we demonstrated that the original lattice structure of plastic PPA was destroyed, resulting in the hydrolysis of PPA. As a result, and metal encapsulated in plastic PPA were released and recovered by suction filtration, sieving, and magnetic separation. The environmental impact assessments revealed that the proposed recycling route for waste LED significantly reduced the overall environmental impact compared to the published methods. Indeed, our method’s recovery efficiency of nano titanium dioxide exceeds . For the metal bracket, it exceeds , and for silver is more than 99.50%. For the non-metallic parts of e-waste, destructive degradation is still the major solution. Hence, new technologies that recycle non-metallic parts applicable to recycling all e-waste components may be a future research direction.
Acknowledgments
This work is partly supported by the National Natural Science Foundation of China (52270132). The authors are grateful to the reviewers who help us improve the paper by many pertinent comments and suggestions.
Compliance with ethics guidelines
Yongliang Zhang, Lu Zhan, and Zhenming Xu declare that they have no conflict of interest or financial conflicts to disclose.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2023.04.008.
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