Assessment of the environmental toxicity of four photovoltaic module technologies in accordance with waste leaching standards

Avaliação da toxicidade ambiental de quatro tecnologias de módulos fotovoltaicos de acordo com normas de lixiviação de resíduos

ABSTRACT

In 2022, global solar energy capacity reached 1 TW and is expected to double by 2026. Predictions suggest that photovoltaic waste, which contains environmentally harmful elements such as lead and cadmium, could exceed 70 million tons by 2050. Therefore, this study aimed to assess the hazardous nature of four photovoltaic module technologies: polycrystalline silicon (p-Si), cadmium telluride (CdTe), amorphous silicon (a-Si), and copper indium gallium selenide (CIGS). This work involved chemical characterization and leaching tests for solid waste classification. The leaching procedures strictly adhered to American (TCLP 1311), European (EN 12457-2), and Chinese (HJ/T299) standards. Results indicated that the p-Si module was deemed hazardous as it exceeded the Pb tolerance limits of both the TCLP 1311 (8.69 mg/L) and HJ/T299 (7.35 mg/L) standards. The CdTe module was considered hazardous as it surpassed Cd tolerance limits in all the studied standards: TCLP 1311 (1.01 mg/L), EN 12457-2 (1.86 mg/L), and HJ/T299 (4.74 mg/L). The CIGS module was identified as hazardous according to the EN 12457-2 standard, exceeding a Se tolerance limit of 0.56 mg/L. The a-Si module did not exceed the maximum concentration levels set by the standards for the analyzed elements, although traces of toxic metals were observed in the leachate.

Keywords
photovoltaic module waste characterization; waste leaching standards; photovoltaic module waste toxicity; photovoltaic panel waste management

Resumo

Em 2022, a capacidade global de energia solar atingiu 1 TW e espera-se que duplique até 2026. Previsões indicam que os resíduos fotovoltaicos, que contêm elementos ambientalmente prejudiciais, como chumbo e cádmio, podem ultrapassar 70 milhões de toneladas até 2050. Portanto, este estudo teve como objetivo avaliar a natureza perigosa de quatro tecnologias de módulos fotovoltaicos: silício policristalino (p-Si), telureto de cádmio (CdTe), silício amorfo (a-Si) e seleneto de cobre índio gálio (CIGS). Este trabalho envolveu caracterização química e testes de lixiviação para a classificação dos resíduos sólidos. Os procedimentos de lixiviação seguiram rigorosamente os padrões americanos (TCLP 1311), europeus (EN 12457-2) e chineses (HJ/T299). Os resultados indicaram que o módulo p-Si foi considerado perigoso, pois ultrapassou os limites de tolerância ao Pb nos padrões TCLP 1311 (8,69 mg/L) e HJ/T299 (7,35 mg/L). O módulo CdTe foi considerado perigoso, pois ultrapassou os limites de tolerância ao Cd em todos os padrões estudados: TCLP 1311 (1,01 mg/L), EN 12457-2 (1,86 mg/L) e HJ/T299 (4,74 mg/L). O módulo CIGS foi identificado como perigoso de acordo com o padrão EN 12457-2, ultrapassando o limite de tolerância ao Se de 0,56 mg/L. O módulo a-Si não ultrapassou os níveis máximos de concentração definidos pelos padrões para os elementos analisados, embora metais tóxicos tenham sido observados no lixiviado.

Palavras-chave:
caracterização de resíduos de módulos fotovoltaicos; padrões de lixiviação de resíduos; toxicidade dos resíduos de módulos fotovoltaicos; gestão de resíduos de painéis fotovoltaicos

INTRODUCTION

The rapid expansion of the photovoltaic (PV) market can be attributed to the rising global demand for electricity, alongside the declining costs of photovoltaic modules. As outlined in the “Global Market Outlook for Solar Power 2022–2026” report, the global installed solar energy capacity reached 1 TW in 2022 and is anticipated to hit 2 TW by 2025 (Europe, 2022). According to data from the International Energy Agency (IEA, 2022), China emerged as the global leader in installed solar capacity in 2021, achieving 306.973 GW, which accounts for 33% of the world’s total installed capacity. Significant contributions also came from the United States (95.209 GW), Japan (74.191 GW), Germany (58.461 GW), India (49.684 GW), Italy (22.698 MW), and Australia (19.076 GW).

Hence, PV solar technology is being increasingly utilized for solar energy generation, posing no risk to human health or the environment during the usage phase (Sinha; Wade, 2015). However, when they reach the end of their operational life, they are discarded as solid waste. PV modules may contain base metals (e.g., Cu, Ni, Al, Pb, Zn, Sn, Mg, Cr, Cd, Te, Ga, In, Fe, and Sb), precious/rare metals (e.g., Ag, Au, In, Te, and Ga), metalloids (e.g., Si, As, and B), alkali metals (e.g., Na, K, and Ca), and polymeric materials used in encapsulants, backsheets, and adhesives. Some of these materials are considered environmentally hazardous, including arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver (Li et al., 2024). In the event of improper handling, these substances could pose significant environmental hazards. For example, if inadequately managed, the percolation of rainwater through discarded and broken modules in an open area or landfill can result in the leaching of metals from the modules (Ren et al., 2022). Consequently, these metals would be released into the leachate, contaminating the nearby soil and water, as emphasized by Nain and Kumar (2020a) (Zhang et al., 2023).

Photovoltaic module waste includes those degraded because of numerous factors (for example, the breakage of the outer glass and the degradation of the encapsulant due to environmental factors such as high UV radiation, humidity, and temperature) as well as nonfunctional modules that have reached the end of their operational life (Nain; Kumar, 2020c), which is typically between 25 and 30 years (Vargas; Chesney, 2021), making room for new and more efficient modules. It is anticipated that by 2050, the total solar module waste could exceed 70 million tons (Heath; Weckend; Wade, 2016). Therefore, managing the end-of-life (EOL) phase of photovoltaic modules after disposal will be an increasing concern for waste environmental regulators in the coming years, both in terms of quantity and the hazardous nature of these wastes. Redirecting these wastes to the recycling process is a waste management solution that helps prevent toxic substances from harming both the environment and human health (Heath; Weckend; Wade, 2016). Even though the European Union’s Waste Electrical and Electronic Equipment Directive has recently included photovoltaic solar panels (EU, 2012), its scope is limited to Europe. Thus, current practices for the disposal and management of end-of-life solar modules are unknown in many countries, and they generally lack established recycling methods.

With no proper management of PV waste, which is the reality in most countries, damaged and/or obsolete equipment is incorrectly discarded. Often, these devices are sent to landfill and mixed with other types of waste, without adequate environmental protection (Nain; Kumar, 2020a). As there are different types of PV with diverse components and compositions, a variety of materials can come into direct contact with soil, rivers, lakes, and groundwater (Nain; Kumar, 2020a).

The most common photovoltaic modules contain metals deemed hazardous, such as cadmium, copper, lead, aluminum, selenium, tin, and zinc, which can be released into the environment due to improper disposal (Kwak et al., 2020) (Nain; Kumar, 2020c). Therefore, it becomes necessary to conduct an environmental assessment of the different PV technologies available on the market to ascertain the polluting potential of each in the event of improper disposal. This assessment is usually carried out through standardized tests to classify any type of solid waste, including PV waste (Nain; Kumar, 2022). Various countries have issued different standards for this evaluation. Among them, the American standard (TCLP—toxicity characteristic leaching procedure), the European standard EN 12457-2 (EUROPEAN COMMITTE FOR STANDARDIZATION, 2002), the Chinese standard HJ/T299 (ENVIRONMENTAL PROTECTION INDUSTRY STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA, 2007a), and the Brazilian Standard NBR 10005 ABNT, (2004) are noteworthy. Although the procedures within these standards differ, all of them evaluate the leaching of toxic compounds from specific waste into the environment, particularly when disposed of improperly in soil or water. Therefore, a leaching test is frequently used to assess the potential environmental risks associated with hazardous chemicals, such as those present in PV modules. The TCLP standard is among the most cited in the literature. According to various studies on PV leaching, leached metal concentrations can occasionally exceed the limits specified in the TCLP standard, particularly lead from c-Si modules and cadmium from thin film modules (Nain; Kumar, 2022) (Panthi et al., 2021). When this occurs, the PV module must be considered hazardous because of its significant polluting potential if improperly discarded.

Numerous studies evaluating specific photovoltaic module technologies through standards exist in the literature, e.g., Panthi et al. (2021), Sharma et al. (2021), and Kilgo et al. (2022). However, a broader evaluation encompassing the major PV technologies on the market using a variety of standardized norms (from various countries) has yet to be conducted. Furthermore, comparing data from leaching procedures conducted for different PV technologies leads to a better understanding of the hazardous nature of this type of waste, aiding in the development of waste management policies in different countries.

This study aims to analyze amorphous silicon (a-Si), cadmium telluride (CdTe), polycrystalline silicon (p-Si), and copper indium gallium selenide (CIGS) photovoltaic modules and classify them as hazardous or nonhazardous waste from an environmental toxicity perspective. The first phase of the research involved characterizing the photovoltaic modules to identify their primary metallic components. Subsequently, standardized leaching procedures were conducted following the TCLP 1311, EN 12457-2, and HJ/T299-2007 standards. Therefore, in addition to contributing new scientific data regarding the hazardous nature of different PV technologies, the results of this study could also be relevant in assisting countries’ management policies and regulations concerning the safe disposal of solar modules.

MATERIAL AND METHODS

Four different photovoltaic modules were selected, which were chemically characterized and subjected to solid waste leaching tests. The stages of the work are detailed in Figure 1.

Figure 1
Flowchart of work stages.

Selected photovoltaic modules

In this study, four different photovoltaic module technologies were used, and the hazardous elements released after a standard leaching test were compared. The four technologies analyzed (Figure 2) were polycrystalline silicon, amorphous silicon, cadmium telluride thin film, and copper indium gallium selenide thin film.

Figure 2
The selected photovoltaic modules: (A) polycrystalline silicon module—front view; (B) polycrystalline silicon module—back view; (C) amorphous silicon module—front view; (D) amorphous silicon module—back view; (E) cadmium telluride thin film module—front view; (F) cadmium telluride thin film module—back view; (G) copper indium gallium selenide thin film—front view; and (H) copper indium gallium selenide thin film—back view.

The various modules used were as follows: a Risen Solar Technology (China) polycrystalline silicon module with dimensions of 1956 × 992 × 40 mm and a weight of 22.0 kg, with its aluminum frame removed before characterization and leaching; a Sungen International Limited (Hong Kong) amorphous silicon module with dimensions of 1400 × 1100 × 7.1 mm and a weight of 26.0 kg; a First Solar (Arizona, USA) cadmium telluride thin film module with dimensions of 2000 × 1230 × 49 mm and a weight of 36 kg; and an Avancis CNBM (Germany) copper indium gallium selenide thin film with dimensions of 1595 × 672 × 45mm and a weight of 19.3 kg.

These widely adopted modules feature a multilayered design, with each layer serving specific functional and protective purposes. As illustrated in Figure 3, the polycrystalline silicon module primarily comprises an aluminum framework, glass, ethylene vinyl acetate (EVA), a solar cell, ethylene vinyl acetate (EVA), a backsheet, and a junction box (Chen et al., 2023; Li et al., 2024).

Figure 3
The general architecture of a silicon PV module composed of a glass cover, an encapsulant, a backsheet, a junction box, a frame, and solar cells encompassing electrodes and p- and n-type semiconductor materials, which have an antireflective coating.

CdTe solar cells can be manufactured in either a superstrate or substrate configuration. As depicted in Figure 4, the superstrate method entails creating a stack of transparent conductive oxide (TCO)/cadmium sulfide (CdS)/CdTe/back contact on a transparent substrate material, enabling optimal light penetration through the overlying thin layers. Similarly, the structure of CIGS solar cells includes a substrate, a rear electrode, an absorber layer (CIGS), a buffer layer (CdS), a front electrode, and an encapsulation superstrate. In contrast to both CdTe and CIGS solar cells, conventional single-junction solar cells within an a-Si PV module stack a p-layer at the bottom, followed by an i-layer, with an n-layer on top (Li et al., 2024).

Figure 4
(A) Module architecture of thin film PV technologies. The cell structure of (B) a-Si technology, (C) CdTe technology, and (d) CIGS technology (ARC and TCO) (LI et al., 2024).

Characterization of photovoltaic modules

Initially, the photovoltaic modules were manually cut using metal shears, obtaining approximately 15 × 15 cm solar cell samples for each of the four studied technologies. Subsequently, approximately 500 g of each sample was individually placed in the Retsch knife mill (model SM 300) at a rotation speed of 1500 rpm for a total milling time of 10 min. By passing the samples through successive sieves (initially a 4-mm sieve followed by a 1-mm sieve), the particle size was reduced to below 1 mm. Following this, the samples were digested using aqua regia (a mixture of HCl and HNO₃ in a 3:1 ratio). According to Veit et al. (2006), aqua regia is commonly used for dissolving metals in waste from electronic equipment. Then, 20 g of each ground sample was weighed using a precision balance and mixed with 100 mL of aqua regia in a ratio of 1:5. The temperature was set to 100°C, with agitation maintained at 60 rpm for 1 h. After this, the solution was filtered and sent for chemical analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent Technologies model 5110).

After replicating the same digestion procedure, 20 g of the newly ground sample was weighed and mixed with 150 mL of 65% nitric acid instead of aqua regia, in an attempt to accurately detect the silver content. Again, the temperature was set to 100°C, with agitation maintained at 60 rpm for 1 h. It is known that the digestion of silver in aqua regia is not efficient because of the possibility of silver chloride precipitation (Petter et al., 2014). The entire procedure was conducted in triplicate for both the aqua regia solution and nitric acid. In total, 16 chemical elements were analyzed, regardless of the type of photovoltaic module. The selection of these elements (As, Ba, Cd, Pb, Hg, Se, Ag, In, Si, Sn, Al, Cu, Ga, Ge, Te, and Zn) was based on a previous study that also conducted chemical characterization for one of the four types of technologies (Nain; Kumar, 2021).

Environmental toxicity assessment – waste leaching

The standards adopted in this study, which establish leaching procedures and tolerance limits for solid waste classification, are outlined in Table 1. The United States Environmental Protection Agency (US EPA, 1992) developed the toxicity characteristic leaching procedure (Method 1311 [US EPA, 1992]) to assess the leaching potential of solid waste when disposed of directly into the environment, as well as determining whether the material should be classified as hazardous because of its toxic properties. Accordingly, if the TCLP test results indicate that the waste exceeds the required tolerance limit for at least one chemical element, the waste must be disposed of in a hazardous waste landfill. Similarly, the standard test for leaching of waste EN 12457-2:2002 is adopted by the European Union, while the European Directive 2003/33/EC is used for landfill disposal purposes. China utilizes the HJ/T299-2007 procedure for solid waste leaching. This is also known as the sulfuric acid and nitric acid method, which is compared with the hazardous waste identification standard GB 5085.3 (ENVIRONMENTAL PROTECTION INDUSTRY STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA, 2007b) to determine the tolerance limits for disposal.

Table 1
Leaching standards and tolerance limit standards used in this study for hazard characterization of solid waste.

Countries	Waste leaching standards	Tolerance limit standards
USA	TCLP 1311 (1992)	TCLP 1311 (1992)
European Union	EN 12457-2 (2002)	2003/33/EC
China	HJ/T299(2007)	GB 5085.3 (2007)

• Source: the authors (2024).

Table 2 provides a summary of the parameters (solid/liquid ratio, leaching solution, particle size, initial pH, stirring time, stirring speed, temperature, and filter size) adopted according to the leaching standards for solid waste, i.e., TCLP 1311, EN 12457-2, and HJ/T299.

Table 2
Summary of the parameters adopted for each leaching standard.

Parameters	TCLP 1311	EN 12457-2	HJ/T299
Solid/liquid ratio	1:20	1:10	1:20
Leaching solution	(Acetic acid, NaOH, deionized water)	Ultrapure deionized water	(Sulfuric acid, nitric acid, deionized water)
Granulometry	< 9.5 mm	< 4 mm	< 9.5 mm
Initial pH	4.93 ± 0.05	5 < pH < 7,5	3.2 ± 0.05
Agitation time	18 h	24 h	18 h
Shaking speed	30 RPM	10 RPM	30 RPM
Temperature	23°C	23°C	23°C
Filter size	0.7 μm	0.45 μm	μm

• Source: the authors (2024).

TCLP 1311 Method

The TCLP 1311 was conducted for all four types of photovoltaic modules, following the outlined procedures (Method 1311 [US EPA, 1992]). The standard requires samples with a particle size of less than 9.5 cm. Therefore, a Tigre hammer mill (model A4R) with a single 9.5-cm sieve was used, producing 100 g of ground samples that were subsequently homogenized and placed in a 2.2-L polyethylene extractor container. A number 1 extraction solution was used, which is a mixture of acetic acid, ultrapure deionized water, and sodium hydroxide, with a pH of 4.93 ± 0.05. Subsequently, 2 L of the extraction solution was added to the container. The slurry was mixed in a rotary extractor for 18 ± 2 h at room temperature (23°C). Following the completion of the extraction process, the solution was filtered through a 0.7-μm glass microfiber filter (GF/F, Whatman). The filtrate was collected in 1 L plastic bottles and preserved at a pH below 2 using nitric acid for subsequent ICP-OES analysis. The elements analyzed included As, Ba, Cd, Cr, Pb, Hg, and Se, with their concentrations compared to tolerance limits.

EN 12457-2

The EN-12457-2 leaching test is the method adopted by the European Union to assess the leached concentration of specific contaminants (As, Ba, Cd, Cr, Cu, Hg, Ni, Pb, Sn, Se, and Zn) present in solid waste. This standardized leaching test is based on the interaction of waste particles less than 4 mm in size with deionized water (neutral pH), which are combined in a liquid-to-solid ratio of 10 L/kg. Following this, the mixture is shaken for 24 h at 23 °C and 10 rpm. In this study, 0.95 L of ultrapure water (18.2 MΩ cm) was mixed with 0.095 kg, producing a homogenized sample. The supernatants were filtered using a 0.45-μm fiberglass filter. Finally, the solution was then analyzed by ICP-OES to determine the concentrations of dissolved contaminants and compare them with the tolerance limits established by the European Council Decision 2003/33/EC (European Council, 2003).

HJ/T299

In this Chinese standard, leaching toxicity was determined following the solid waste extraction procedure HJ/T299-2007, which involves the sulfuric acid and nitric acid method. In this procedure, approximately 150 g of homogenized waste, which was ground through a 9.5-mm sieve to achieve a particle size less than 9.5 mm, was mixed with 1.5 L of leaching solution (two drops of 2:1 concentrated 98% H₂SO₄ and 65% HNO₃ (v/v) in 1 L of ultrapure deionized water, with a pH of 3.2 ± 0.05). The mixture in the flask was agitated on a shaker at 30 rpm for 18 h at 23°C and then filtered through a 0.7-μm glass microfiber filter (GF/F, Whatman). Metal concentrations in the solution after filtration and acidification with HNO₃ to a pH less than 2 were determined using ICP-OES.

RESULTS AND DISCUSSION

Characterization of photovoltaic modules

Sixteen chemical elements were analyzed in the four types of photovoltaic modules, with the results shown in Table 3, along with the sample standard deviation for each chemical element. Mercury and germanium were not detected in any type of module. Additionally, the applied method (digestion with aqua regia or HNO₃) is unable to dissolve silicon because it forms a protective layer of silicon dioxide in the presence of these acids, as stated by Liu et al. (2017). The characterization data from this study align with other characterization studies reviewed by Nain and Kumar (2020b), such as Paiano (2015), Goe (2014), and Savvilotidou, Antoniou and Gidarakos (2017).

Table 3
Chemical composition of four technologies of photovoltaic modules.

Element	p-Si (ppm)	CdTe (ppm)	CIGS (ppm)	a-Si (ppm)
As	0.53 ± 0.49	0.62 ± 0.64	0.26 ± 0.43	0.47 ± 0.18
Ba	0.09 ± 0.04	0.03 ± 0.03	0.03 ± 0.02	0.03 ± 0.01
Cd	ND	26.08 ± 0.66	0.79 ± 0.22	0.4 ± 0.12
Pb	15.75 ± 2.23	ND	0.03 ± 0.05	0.59 ± 0.45
Hg	ND	ND	ND	ND
Se	2.92 ± 0.39	3.32 ± 0.45	11.09 ± 1.37	2.21 ± 0.95
Ag	2.92 ± 0.93	ND	ND	ND
In	ND	ND	5.73 ± 0.61	ND
Si	ND	ND	ND	ND
Sn	34.06 ± 1.97	11.82 ± 0.23	14.53 ± 1.49	12.5 ± 0.20
Al	173.86 ± 7.81	18.71 ± 0.84	23.9 ± 3.53	28.97 ± 6.05
Cu	77.27 ± 36.56	4.95 ± 6.15	21.22 ± 4.74	23.36 ± 9.76
Ga	ND	ND	0.66 ± 0.06	ND
Ge	ND	ND	ND	ND
Te	0.98 ± 0.71	19.0 ± 1.04	3.24 ± 1.19	1.16 ± 0.39
Zn	1.99 ± 0.59	0.99 ± 0.70	12.8 ± 2.73	1.05 ± 0.19

• Source: the authors (2024).
• ND: not detected; LOD: limit of detection (ppm): As = 0.02; Ba = 0.01; Cd = 0.01; Pb = 0.01; Hg = 0.01; Se = 0.08; Ag = 0.17; In = 0.8; Si = 1; Sn = 0.6; Al = 0.18; Cu = 0.8; Ga = 0.36; Ge = 0.1; Te = 0.12; Zn = 0.2.

In terms of mass, it is observed that the analyzed elements correspond to a very small fraction of the entire module, regardless of the technology type. It is known that these modules are mainly composed of glass and EVA polymer, which were not digested and are not shown in Table 3.

Regarding polycrystalline silicon modules, the highest measured values were the base metals aluminum and copper, with concentrations of 173.86 and 77.27 ppm, respectively. In the same module, lead and tin were also present in concentrations of 15.75 and 34.06 ppm, respectively. These likely originate from the alloy used when soldering the Cu strips. Silver, which is only present in this first-generation module, is responsible for converting sunlight into photovoltaic energy. Its concentration was determined to be 2.92 ppm.

In the CdTe module, the elements present in larger quantities are tellurium and cadmium with concentrations of 19.0 and 26.08 ppm, respectively. These elements can be found in the CdTe layer, which is several micrometers thick, as well as in the thin CdS window layer that forms the CdS/CdTe heterojunction (Zapf-Gottwick et al., 2015). Additionally, the results show that CdTe modules also contain aluminum, copper, and tin.

In the CIGS module, the elements present in larger quantities were aluminum, copper, tin, zinc, selenium, indium, and gallium, with selenium and zinc exhibiting the highest concentrations relative to the other elements. Notably, indium and gallium are unique to CIGS modules, as reported by Liu et al. (2022). Therefore, it was expected that these elements would not be detected in other technologies.

Regarding the a-Si module, while various elements were detected in its composition, lead, aluminum, copper, and tin emerged as the most significant metals. As reported by Nain and Kumar (2021) and Zapf-Gottwick et al. (2015), lead has been found in the composition of a-Si modules, albeit in smaller quantities compared to crystalline silicon modules, owing to the fewer strips present in a-Si modules.

Environmental toxicity assessment—waste leaching

The results of the leaching tests will be presented below in accordance with the standards used for the four types of photovoltaic modules. Each standard establishes different quantities of elements to be monitored, as well as specific tolerance limits.

TCLP 1311 Method

Table 4 presents the results obtained from leaching tests and the sample standard deviation for eight elements, as determined using the procedure outlined in the US standard TCLP 1311. Notably, the leachate concentration of Pb from the p-Si module was 8.69 mg/L, exceeding the maximum limit of 5 mg/L. Concerning the CdTe module, Cd exceeded the maximum limit set out by the standard, with a concentration of 1.01 mg/L. According to American standards, even if the other elements remain within their tolerance limits, the presence of just one element above its tolerance limit is sufficient to classify the waste from p-Si and CdTe modules as hazardous.

Table 4
Leaching test results of photovoltaic modules according to the NBR 10005 and TCLP 1311 standards, along with a comparison with the maximum limit of NBR 10004 and the maximum contaminant concentration of TCLP 1311.

Element	p-Si (mg/L)	CdTe (mg/L)	CIGS (mg/L)	a-Si (mg/L)	Maximum contaminant concentration TCLP 1311 (mg/L)
As	0.02 ± 0.01	0.02 ± 0.01	0.02 ± 0.02	0.03 ± 0.00	5
Ba	0.09 ± 0.01	0.03 ± 0.00	0.03 ± 0.01	0.05 ± 0.01	100
Cd	0.01 ± 0.01	1.01 ± 0.19	0.15 ± 0.03	ND	1
Cr	0.01 ± 0.00	0.01 ± 0.00	0.01 ± 0.00	0.01 ± 0.00	5
Pb	8.69 ± 6.85	0.26 ± 0.09	0.07 ± 0.02	0.27 ± 0.14	5
Hg	ND	ND	ND	ND	0.2
Se	ND	ND	ND	ND	1
Ag	ND	ND	ND	ND	5

• Source: the authors (2024).
• ND: not detected; LOD: limit of detection (mg/L): As = 0.02; Ba = 0.01; Cd = 0.01; Cr = 0.01; Pb = 0.01; Hg = 0.01; Se = 0.08; Ag = 0.17.

For all elements specified by the standard, the values obtained for CIGS thin-film modules and a-Si modules were below the tolerance limits in the leachate according to TCLP 1311. Consequently, these modules were not categorized as hazardous waste.

The presence of toxic substances in photovoltaic modules has been a subject of scientific and political debate for many years (Zapf-Gottwick et al., 2015). There are studies suggesting the potential for metals like Pb and Cd to exceed regulatory limits. However, various studies yield contradictory results regarding the leaching of hazardous substances from these modules. This is mainly because of the methodology and testing conditions used, such as the size of the module pieces to be leached, the aqueous solutions employed, the pH value of the aqueous solution, and the leaching time (Tamizhmani et al., 2018). Furthermore, as reported by Nain and Kumar (2022), variations in results may arise from the evolution of the PV manufacturing process over time or from differences between brands or origins. Consequently, literature data concerning the toxicity of photovoltaic waste is subject to limitations.

Another study in this area that illustrates the variability in procedures and obtained results is that of Dias (2015). They carried out leaching tests following the Brazilian standard NBR 10005, which is based on and is very similar to TCLP 1311. A Pb concentration of 5.5 mg/L was obtained in the leachate of polycrystalline silicon modules (Solbratec brand), while a concentration of 21.6 mg/L was detected in monocrystalline silicon modules (Solar Terra brand). These values surpassed the tolerance limits stipulated by both the Brazilian standard NBR 10004 and TCLP 1311, which are 1 and 5 mg/L, respectively. Consequently, the waste was classified as hazardous, necessitating disposal in specific landfills for hazardous waste.

EN 12457-2

Table 5 presents the results obtained from leaching tests conducted according to the European standard EN 12457-2, along with the sample standard deviation for 10 elements and the threshold values specified by 2003/33/EC. Notably, the leachate from the CdTe module had a Cd concentration of 1.86 mg/L, surpassing the threshold value of 0.1 mg/L established by the 2003/33/EC regulation. Therefore, it must be classified as hazardous waste, implying that this residue cannot be disposed of in landfills designated for nonhazardous waste. Additionally, the concentration of Se in the leachate was 0.1 mg/L for p-Si, 0.07 mg/L for CdTe, 0.56 mg/L for CIGS, and 0.17 mg/L for a-Si. Consequently, all module residues exceeded the threshold value for Se, which is 0.05 mg/L according to the European Directive 2003/33/EC. Therefore, none of these PV modules are suitable for disposal in landfills designated for nonhazardous waste, in accordance with 2003/33/EC.

Table 5
Results of the leaching test for photovoltaic modules according to the EN 12457-2 standard and a comparison with the respective threshold value.

Element	p-Si (mg/L)	CdTe (mg/L)	CIGS (mg/L)	a-Si (mg/L)	Limit values 2003/33/EC (mg/L)
As	0.02 ± 0.01	ND	ND	0.02 ± 0.01	0.2
Ba	0.03 ± 0.01	0.01 ± 0.01	0.01 ± 0.00	0.01 ± 0.01	10
Cd	0.02 ± 0.06	1.86 ± 1.05	0.09 ± 0.03	0.03 ± 0.00	0.1
Cr	0.01 ± 0.00	0.01 ± 0.01	ND	ND	1
Cu	ND	ND	0.23 ± 0.08	ND	5
Hg	ND	ND	ND	ND	0.02
Ni	ND	ND	ND	ND	1
Pb	0.31 ± 0.09	ND	ND	ND	1
Se	0.10 ± 0.02	0.07 ± 0.02	0.56 ± 0.13	0.17 ± 0.01	0.05
Zn	0.20 ± 0.07	ND	0.54 ± 0.08	0.2 ± 0.06	5

• Source: the authors (2024).
• ND: not detected; LOD: limit of detection (ppm): As = 0.02; Ba = 0.01; Cd = 0.01; Cr = 0.01; Cu = 0.8; Hg = 0.01; Ni = 0.4; Pb = 0.01; Se = 0.08; Zn = 0.2.

Nain and Kumar (2021) assessed three different types of commercially available solar modules: polycrystalline silicon, amorphous silicon, and CIGS. Initially, the external aluminum frame, cables, and junction box (if present) were manually removed. To simulate realistic leaching conditions, i.e., to develop a custom adaptation that does not strictly follow the standard, real solar modules with all layers, including the glass and encapsulation layers, were used in all experiments. To conduct the leaching study, the layered solar module was cut into pieces measuring 2–5 cm using a metal shear machine. The size of the pieces was manually reduced with scissors to meet the requirements of different leaching standard tests. A stainless-steel mixer-grinder was used to obtain homogenized samples. It is important to note that the authors used a sample size of 1 cm for EN 12457-4. Moreover, they used only 50 g of the total sample, and all extracts were filtered using 0.45 mm cellulose acetate filters (Whatman). Regarding EN 12457-4, Nain and Kumar (2021) reported a maximum value of 0.9 mg/L for Pb in a-Si leachate and 0.244 mg/L in p-Si leachate. Additionally, the maximum values of Pb and Se in the leachate of the CIGS module were 1.038 and 0.7 mg/L, respectively.

HJ/T299-2007

Table 6 provides the results obtained for 12 elements, the measurement standard deviation, and the tolerance limits stipulated by the GB 5085.3 (standard, as well as a comparison with the Chinese leaching standard HJ/T299-2007. For the p-Si module, a Pb concentration of 7.35 mg/L in the leachate was obtained, which is above the tolerance limit established by the GB 5085.3 standard, set at 5 mg/L. Furthermore, the leachate from the CdTe module contained 4.74 mg/L of Cd, which is 4.74 times above the tolerance limit. Therefore, according to the GB 5085.3 standard, the residues from both the p-Si and CdTe modules were classified as hazardous waste. However, based on these tests, the CIGS and a-Si modules were classified as nonhazardous as the concentrations of the elements present in the leachate were all below the tolerance limits.

Table 6
Leaching test results of photovoltaic modules according to the HJ/T299-2007 standard and a comparison with the respective tolerance limit.

Element	p-Si (mg/L)	CdTe (mg/L)	CIGS (mg/L)	a-Si (mg/L)	Tolerance limit GB 5085.3-2007 (mg/L)
Cu	ND	ND	ND	ND	100
Zn	1.44 ± 0.65	0.28 ± 0.12	9.23 ± 0.54	3.00 ± 0.30	100
Cd	0.09 ± 0.03	4.74 ± 1.33	0.43 ± 0.09	0.09 ± 0.02	1
Pb	7.35 ± 1.48	0.44 ± 0.22	0.03 ± 0.05	0.12 ± 0.06	5
Total Cr	0.01 ± 0.01	0.13 ± 0.02	0.01 ± 0.01	0.02 ± 0.00	5
Hg	ND	ND	ND	ND	0.1
Ba	0.05 ± 0.03	0.03 ± 0.01	0.04 ± 0.00	0.04 ± 0.01	100
Ni	ND	ND	ND	ND	5
Ag	ND	ND	ND	ND	5
As	0.02 ± 0.01	0.02 ± 0.01	0.02 ± 0.01	0.02 ± 0.02	5
Se	ND	ND	0.13 ± 0.01	ND	1

• Source: the authors (2024).
• ND: not detected. LOD: limit of detection (ppm): (ppm): Cu = 0.8; Zn = 0.2; Cd = 0.01; Pb = 0.01; Cr = 0.01; Hg = 0.01; Ba = 0.01; Ni = 0.4; Ag = 0.17; As = 0.02; Se = 0.08.

The experiment by Su et al. (2019) is the only study that aimed to determine the leaching toxicity of PV cells following the solid waste extraction procedure HJ/T299-2007, i.e., the sulfuric acid and nitric acid method. Employing a CIGS module, the authors subjected the materials to crushing with a plastic file-wrapped hammer, followed by mixing with 2 L of leaching reagent and stirring for 18 h at 23 °C. After filtration with a 0.22 μm filter, metal concentrations in the solution were determined using ICP-OES. The results showed that the Zn concentration surpassed that of all other metals, which is consistent with the results presented in Table 6. Moreover, no elemental concentrations exceeded the tolerances set by GB 5085.3. Therefore, CIGS modules could be defined as nonhazardous solid waste (Su et al., 2019). Furthermore, these authors conducted experiments that involved burying modules in different types of synthetic soil for a period of 60 d, observing a positive correlation between increased metal concentrations in the soil and the amounts of added CIGS. The metals with the highest concentrations in these soils were Ga, Zn, Pb, Cu, Ni, In, and Cr. The results of this study confirmed that when buried, CIGS modules have a direct pollutant potential in the soil (Su et al., 2019).

Assessment of leaching standards

It is evident that different standards consider different quantities of elements when classifying waste as hazardous. However, most published studies predominantly adopt the TCLP 1311 standard for waste leaching, which evaluates a smaller number of elements compared to the European and Chinese standards (Petroli et al., 2024). It is suggested that other toxic elements, such as tellurium, should be included in the regulations of various countries. Tellurium is considered to be a toxic element present in CdTe modules, but it was not evaluated by any standard applied in this study.

Metals such as Ba, Ag, As, and Cr leached in insignificant quantities for all solar modules and all standards, as reported in different studies (Nain; Kumar, 2022). It is worth noting that the high proportion of materials considered nontoxic, such as glass and EVA, can interfere with leaching results as they were also subjected to standardized waste leaching tests.

Zapf-Gottwick et al. (2015) reported that lower pH levels correlate with increased leaching of Cd from crushed CdTe modules and Pb from crystalline silicon modules, a trend verified in this study despite the different methodological procedures for each standard. Additionally, Nover et al. (2017) suggested that the highest risk of metal contamination from photovoltaic modules occurs in acidic and oxidizing conditions.

It is important to note that waste leaching standards were created with the aim of assessing worst-case scenarios for waste to be classified as hazardous. However, all potential risk analyses were conducted assuming that solar modules would only be disposed of in authorized landfills, overlooking potential disposal in other environments (Nain; Kumar, 2021). The testing methods evaluate small fragments (centimeter scale) to simulate the potential crushing of waste by landfill equipment (Panthi et al., 2021). Therefore, this study strictly followed the methodologies of the TCLP 1311, EN-12457-2, and HJ/T299-2007 standards, adopting a prevention-based approach to assess the worst-case scenario for environmental risks (crushed samples that are not cut, resulting in cell release) while remaining realistic within the criteria established by the standard (simulating impact fracture). Thus, the results of this study hold significance for waste managers and policymakers, providing an understanding of photovoltaic waste from various environmental perspectives.

It is crucial that photovoltaic solar modules receive appropriate end-of-life management (through recycling), not only to reduce the impact of waste on the environment but also to prevent future shortages of critical metals like In or Ga (Avarmaa et al., 2019). According to Nain and Kumar (2020b), various stakeholders highlight the main barriers hindering safe end-of-life management of photovoltaic cells, including a lack of recycling centers, low profitability, a lack of incentives for recycling, the absence of regulations, and low environmental awareness. Therefore, all countries should adopt appropriate policy measures to manage photovoltaic waste and minimize the potential for environmental pollution.

Different photovoltaic module technologies have distinct metals and compositions, which are subject to change as research advances in manufacturing processes to improve efficiency and minimize material use. The adoption of more modern and advanced solder materials, such as SnCu, CuSnZn, or SnCuAg, can significantly reduce the use and emission of Pb when crystalline silicon modules reach their end-of-life (Song et al., 2019). Consequently, older solar modules may leach higher concentrations of toxic metals compared to newer modules.

Accurate quantification of toxic elements prone to leaching from photovoltaic modules is essential for manufacturers, plant owners, operators, and stakeholders to determine appropriate end-of-life disposal strategies. To achieve this, an acceptable sample removal method needs to be developed, where consistency in sample particle sizes is ensured to quantify toxic element levels accurately and reproducibly during standardized tests. Furthermore, for a more in-depth assessment of the effects of the residues, research analyzing the specific local exposure conditions, including the destination of the modules and transportation methods, is necessary to assess the dispersion and environmental impacts of toxic metals, facilitating a deeper understanding of residue effects.

CONCLUSIONS

The characterization of photovoltaic modules indicated the presence of various toxic metals, such as As, Pb, Cd, Te, and Se, alongside base metals like Cu, Al, Sn, and Zn, across p-Si, CdTe, CIGS, and a-Si modules. Therefore, to assess the potential leaching of residues, especially toxic metals, from solar modules after end-of-life disposal, the American TCLP 1311, European EN 12457-2, and Chinese HJ/T299-2007 standards were studied. Regarding the toxicity classification of photovoltaic module residues, the toxicity assessment results revealed that the p-Si module was classified as hazardous by the TCLP 1311 and GB 5085.3 standards due to excessive levels of lead, registering concentrations of 8.68 and 7.35 mg/L, respectively. The variation in the measured lead concentration is due to the different procedures of each standard. Moreover, it was also deemed hazardous by EN 12457-2 due to excess selenium, with a concentration of 0.1 mg/L. The CdTe module was classified as hazardous in all analyzed standards due to high Cd levels, with concentrations of 1.01 mg/L (TCLP 1311), 1.86 mg/L (EN 12457-2), and 4.74 mg/L (HJ/T299). Furthermore, selenium exceeded the tolerance limit with a measure concentration of 0.07 mg/L for EN 12457-2. On the other hand, the CIGS and a-Si modules were identified as hazardous waste only according to the EN 12457-2 standard due to elevated selenium levels, registering concentrations of 0.56 and 0.17 mg/L, respectively. Therefore, improper disposal of solar modules, regardless of technology, may result in the release of toxic metals into the environment.

It can also be concluded that the same module may be classified as hazardous waste in one country and not in another, as is the case with both CIGS and a-Si modules. This demonstrates the need to revise the existing regulatory waste tests to standardize the methodologies and tolerance limits adopted by major leaching and solid waste classification standards. Furthermore, it is prudent not to dispose of solar modules with municipal solid waste regardless of the type of module and the standard used as all technologies analyzed in this study contain one or more toxic metals in their composition.

ACKNOWLEDGMENTS

This research is supported by the CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico, 156825/2021-0 and 140764/2021-6.

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• Funding:
  CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico, 156825/2021-0 and 140764/2021-6.

Publication Dates

• Publication in this collection
  14 Apr 2025
• Date of issue
  2025

History

• Received
  02 Sept 2024
• Accepted
  17 Oct 2024
• Corrected
  30 June 2025