Characteristic and mechanism of pollution by laser cleaning high-value vehicle parts with a complex structure in remanufacturing industry

Abstract

Remanufacturing high-value vehicle parts is of substantial significance for the circular economy because it provides a second service life and reduces solid waste generation. Laser cleaning is a promising method in the remanufacturing industry owing to its high energy, precision, and efficiency. However, the pollution caused by laser cleaning remains unknown and has rarely been reported, limiting the application of this method. In this study, pollution removal and conversion using a high-energy laser were explored, and environmental life cycle and health risk assessments were conducted. The results showed that high-energy lasers might induce new organic pollutants, such as olefins, alkynes, and aldehydes. During laser cleaning, thermal elastic expansion and shock wave generation were the main mechanisms of rust removal, and evaporation and small-scope boiling were used for waste oil removal. The mean concentration of PM₁₀ was above 7000 µg/m³ in laser rust removal, and the total concentration of volatile organic compounds (VOCs) was the highest in laser oil removal (∼ 2568 µg/m³). Particulate matter contained high amounts of Fe, Al, and Mn. An environmental life cycle assessment indicated that laser cleaning for stain removal significantly reduced the total environmental impact compared with that of traditional solvent-ultrasonic cleaning (∼ 40.0%) and sandblasting (∼ 83.3%). This study provides an environmental protection basis for increasing the use of laser cleaning in the remanufacturing industry.

Graphical abstract

1. Introduction

The rapid development of the automobile industry brings convenience to peopleʼs lives but also produces many end-of-life vehicles (ELVs), which have become a global problem owing to their high recycling cost, low economic benefits, and environmental pollution [1], [2], [3]. As shown in Figure S1, the world's top five countries have a total vehicle population of approximately 780 million, with a scrap rate between 2.5% and 8%. The number of ELVs in China is increasing and is expected to reach 350–370 million units annually over the next two decades [4]. Remanufacturing is widely recognized as a key contributor to the circular economy and is widespread in many product categories, such as machinery, medical equipment, and vehicle parts [5,6]. Among these categories, vehicle parts remanufacturing dominates the remanufacturing industry, with a market share of two-thirds [7]. The global vehicle parts remanufacturing market is estimated to grow at a compound annual growth rate of 7.1% from 2018 to 2026, roughly equal to US$ 91 billion by the end of 2026 [8]. According to China's 14th Five Year Plan of Circular Economy Development Plan, the output value of China's resource recycling industry will reach 5 trillion CNY by 2025, including 200 billion CNY in the remanufacturing industry. Remanufacturing provides a new service life for every product usage cycle [9]. In the government document 2024 of Opinions of the General Office of the State Council on Accelerating the Construction of a Waste Recycling System, it is encouraged that using the remanufactured auto parts in the field of after-sales maintenance. Using remanufactured products instead of newly manufactured products not only reduces costs and saves energy and resources but also produces almost no solid waste and substantially reduces the emission of air pollutants [10,11].

The main remanufacturing processes include disassembling, cleaning, detecting, remanufacturing, processing, and assembling. [12]. The literature has focused on how to detect and renovate parts [13,14] but not on exploring and improving the cleaning process in the remanufacturing industry. In remanufacturing, cleaning is an crucial and essential step and repeated several times, and unfortunately, cleaning causes the most serious pollution of all the remanufacturing processes. Traditional cleaning methods include solvent cleaning, ultrasonic cleaning, and sandblasting (Fig. S2). These cleaning processes may exhibit poor performance, such as damaging part surfaces, incomplete cleaning, and releasing pollutants. Therefore, advanced and green cleaning technologies are in demand in the remanufacturing industry.

Laser technology has a wide range of industrial applications, including laser cutting, welding, and heat treatment [15], [16], [17], [18], [19], and the advantages of high-energy, non-contact, and fine processing. Laser cleaning has been proposed in various industries such as aerospace, automotive, nuclear, tools and dies, and carbon fiber reinforced polymers [20]. Different from traditional cleaning technology, laser cleaning is regarded as green cleaning owing to no solvent and consumable consumption, no noise, and resulting in less pollution than the former. However, laser cleaning is not commonly used in the remanufacturing industry. Additionally, a series of physical and chemical reactions will occur in stains under the action of high-energy laser, resulting in a certain number of pollutants. Thus, in the remanufacturing industry, whether laser cleaning is truly a green and applicable technology and not traditional cleaning remains unclear.

Hence, this study aimed to compare laser cleaning with traditional cleaning in the vehicle part remanufacturing industry, explore the mechanism of pollutant generation and release in different cleaning scenarios, and re-evaluate the green characteristics of laser cleaning. Moreover, an environmental life cycle assessment (LCA) and a health risk assessment of laser cleaning were conducted and compared with traditional cleaning technologies. This study objectively evaluates laser cleaning technology from the perspective of pollution release and environmental impact and provides relevant data support for its popularization in the remanufacturing industry.

2. Materials and methods

2.1. Objects and laser cleaning

All parts to be cleaned were from ELVs, mainly aluminum, cast iron, and stainless steel materials. Three typical types of stains were attached to the surface: rust, waste oil, and oil sludge. Laser cleaning was performed at a manufacturing factory in Shanghai, China. A handheld laser with a maximum average power of 3000 W (HGTECH BUZZ 0200, China) operated at a power of 85%, speed of 3500 mm/s, pulse frequency of 25 kHz, and a width of 25 mm was used for the experiment. Laser cleaning was performed in a closed operating room to ensure data accuracy.

2.2. Sampling, monitoring, and instrumental analysis

Particulate matter (PM₁₀ and PM_2.5) released during laser cleaning was collected using a medium volume air sampler (Laoying 2030 intelligent TSP sampler, China) with a 90 mm quartz fiber filter membrane. The background elemental contents of the quartz membrane are listed in Table S1. A quarter of each membrane sample was cut evenly for microwave digestion, and the heavy metal content was measured using ICP-OES (Avio 500, PerkinElmer). Additionally, PM₁₀ and PM_2.5 concentrations during laser cleaning were monitored in real time by using an aerosol monitor (TSI MODEL 8532). The monitoring sites were located approximately 30 cm above the laser cleaning site.

Volatile organic compounds (VOCs) were collected using stainless steel suma tanks (LESSMGOEM-01-3, LESHI TECH Inc.) with a volume of 3 L. The sampling site was located approximately 30 cm above the laser cleaning site. The samples were sent to an analysis laboratory immediately after collection, and the VOC composition and content were measured using the USEPA TO15-1999 method and GC–MS (7890B-5977A, Agilent Technologies Inc.).

2.3. Environmental life cycle assessment (LCA) and health risk assessment

The system boundary of LCA involves two processes: laser and traditional cleaning. The environmental LCA of the different cleaning technologies was processed using the software Simapro 9.0 [21], with the method of CML-IA baseline V3.06/EU25. Life cycle inventory data are presented in Tables S2 and S3. Nine environmental impacts indicators are used in the LCA: abiotic depletion (AD), global warming (GW), human toxicity (HT), freshwater aquatic ecotoxicity (FW), Marine aquatic ecotoxicity (MA), terrestrial ecotoxicity (TE), photochemical oxidation (PO), acidification (AC), and eutrophication (EU). The functional unit was 100 gearboxes, and all results were normalized. Details about the LCA are provided in Text S1.

Risk assessment models from the Environmental Protection Agency were adopted to assess the risks of heavy metals and VOCs exposed in different cleaning workshops [22]. Workers are exposed to heavy metals via ingestion, inhalation, and skin contact. The average daily dose (ADD) absorbed through ingestion (ADD_ing, mg/(kg × day)), inhalation (ADD_inh, mg/(kg × day)), and skin (ADD_derm, mg/(kg × day)) represent the pollutant mass exposed to workers. Additionally, the hazard quotient (HQ) and hazard index (HI) were used as indicators to evaluate the health risks of pollutants. The ADD, HQ, and HI were calculated as described in Text S2. The relevant calculation parameters are derived from the Integrated Risk Information System (IRIS) (https://www.epa.gov/iris) (Tables S4 and S5). The detailed calculation process is in our previous study [23]. For the VOC exposure risk assessment, we only considered inhalation exposure because VOCs are gases [24].

2.4. Quality assurance and quality control

Pollution monitoring and detection. In this study, an aerosol monitor (TSI MODEL 8532) was used for real-time monitoring of PM₁₀ and PM_2.5 concentrations during laser cleaning. The TSI MODEL 8532 is an accurate and widely recognized aerosol monitoring instrument in scientific research [25,26]. The aerosol monitor was calibrated before each monitoring. To determine the elemental distribution of the particles more accurately, this study analyzed the heavy metal contents thrice by using the inductively coupled plasma-atomic emission spectrometry (ICP-AES) method for each PM sampler. Statistical comparisons of pollution data were performed using IBM SPSS Statistics 26.

Uncertainty analysis. Uncertainties may exist in the weighing process, volume conversion, and instrument analysis, which may lead to deviations between experimental results and actual values. Moreover, uncertainty exists in the LCA results because part of the input data is based on the factory's experience estimates. The main uncertainties and deviations are listed in Table 1.

Table 1.

Uncertainties and deviation ranges in laser cleaning and life cycle assessment.

No.	Uncertainties	Deviation range
1	Laser power (kW)	± 2%
2	Laser processing time (s)	± 10%
3	Thickness of rust layer (µm)	± 30%
4	Thickness of waste oil film (µm)	± 20%
5	Pollutant content (ppm)	± 15%
6	Power consumption of sandblasting (kW·h)	± 10%
7	Power consumption of ultrasonic cleaning (kW·h)	± 5%

3. Results and discussion

3.1. Comparison of laser cleaning and traditional cleaning

3.1.1. Advantages of laser instead of traditional cleaning

ELV parts such as gearboxes and engines have complex and high-precision structures. They contain several gears, bearings, shafts, and shells of different sizes and shapes. Therefore, cleaning is a complex, difficult, and important task in the ELV remanufacturing industry. As shown in Fig. 1, when using the traditional method, parts must undergo at least three cleaning steps to satisfy the cleaning requirements. Much power is consumed during these steps, particularly for sandblasting and ultrasonic processing. Serious noise and dust pollution occur during sandblasting. A large amount of solvent is consumed, and VOCs are released during solvent cleaning. Wastewater and noise pollution are inevitable during ultrasonic processing. Thus, realizing non-destructive cleaning of high-value vehicle parts with complex structures is critical. Notably, sandblasting increases the roughness and hardness and decreases the corrosion resistance of part surfaces [27,28]. These changes may affect the performance and life of complex vehicle parts. Ultrasonic cleaning may cause cavitation erosion on the surface of a part [29]. Ultrasonic waves generate and evenly distribute cavitation implosions in liquid medium [30]. The cumulative effect of millions of tiny continuous implosions provides the mechanical energy necessary for stain removal. Cavitation implosion at a liquid-solid interface imposes severe stresses on the solid surface, with the potential for severe surface erosion.

Fig. 1.

Comparison of traditional cleaning and laser cleaning.

Different from traditional cleaning methods, laser cleaning is a one-step process for stain removal because high-energy lasers can remove the solid and liquid stains that stick firmly to the part surface. Laser cleaning has the advantages of low energy consumption, low pollution, and no solvent. Additionally, these lasers can clean parts of any shape without damaging the surface when the cleaning parameters are appropriate.

3.1.2. Disadvantages of laser cleaning in practical application

Laser cleaning has not been widely used in the remanufacturing industry owing to the limitations in technology, equipment, and operational skills. Currently, laser cleaning is mainly performed with handheld lasers, and automatic or semi-automatic intelligent laser cleaning equipment for ELV part remanufacturing are not on the market. Intelligent laser cleaning equipment requires the development of graphic recognition, visual positioning, intelligent detection, and other technologies. Additionally, the diversity of ELV part types and shapes increases the difficulty of developing intelligent laser cleaning equipment and the cost of doing so. If the existing handheld laser is used directly, factors such as laser intensity, irradiation angle, and cleaning time affect the cleaning effect, which requires workers to have high operation skills. Additionally, the cleaning efficiency of handheld lasers is lower than that of traditional cleaning methods, and some large engines or gearboxes are not suitable for laser cleaning. Thus, the development of automation and intelligence is of substantial significance for the promotion of laser cleaning technology in the remanufacturing industry.

3.2. Laser cleaning effect on rust, waste oil, and oil sludge

In remanufacturing, part surfaces have three types of stains, rust, oil, and oil sludge, and for each stain, the laser cleaning effect differs. When the laser-cleaning parameters are appropriate and the stains are small or thin, a perfect cleaning effect can be obtained (Fig. S3). All rust, waste oil, and oil sludge were completely removed without any damage to the surface. However, the cleaning effect in the actual laser cleaning process would be poor if the cleaning parameters were set inappropriately or if the stains were too thick on the part surface. Fig. 2A shows the microscopic characteristics of the part surface at different laser cleaning stages of rust removal. Before laser cleaning, rust (dark red stain) was disordered on the surface of the part. After laser cleaning, all rust was removed, and the complete surface of the stainless steel part was exposed. When the laser continued to act on the stainless steel part, high energy laser radiation ablated the metal surface, forming a metal oxide layer [31]. First, a thermal effect is created when the laser irradiates the stains. The relationship between the strain temperature and laser fluence can be obtained using a one-dimensional heat conduction equation [32]:

$$T\left(z,t\right)=\frac{2tE}{k\tau }\sqrt{\frac{kt}{\rho c}}·ierfc\frac{z}{2\sqrt{\frac{kt}{\rho c}}}+T_{amb}$$ (1)

where k, ρ, and c are the thermal conductivity, density, and specific heat capacity, respectively; T is the temperature of the strain after the laser irradiation; z is the depth of the substrate from the surface; T_amb is the ambient temperature; and t, E, τ are the action time, laser fluence, pulse width of the laser.

Fig. 2.

(A) Effect of laser cleaning on rust removal. (B) Poor effect of laser cleaning on oil sludge removal. (C) Microscopic image of oil sludge. (D) Box charts of PM₁₀ and PM_2.5 concentration during laser cleaning for rust removal. (E) Box charts of PM₁₀ and PM_2.5 concentration during laser cleaning for oil removal. (F) Box charts of PM₁₀ and PM_2.5 concentration during laser cleaning for oil sludge removal. (G) Heavy metal content in particulate matter during rust removal by laser cleaning. (H) Heavy metal content in particulate matter during oil sludge removal by laser cleaning. (I) VOCs concentration in different scenarios. (J) Thermogravimetry and differential scanning calorimetry curves of waste oil. (K) Thermogravimetry and differential scanning calorimetry curves of oil sludge. (L) FTIR spectrum of fiber in oil sludge.

Fig. 2B shows the poor effect of laser cleaning on oil sludge removal when the oil sludge on the surface was too thick. Some silicate and metal particles were present in the oil sludge, which may have remained on the surface under the action of the laser. Simultaneously, a small amount of non-volatilized waste oil with a large molecular weight was absorbed on these particles to form a spherical sludge stain. Therefore, simple laser cleaning might be insufficient for oil sludge removal, and other auxiliary cleaning methods, such as high-pressure air gun blowing, are required before laser cleaning.

3.3. Pollution characteristics during laser cleaning

3.3.1. Particulate matter releasing characteristic

During laser cleaning, stains were removed and dispersed into the atmosphere in the form of aerosols. Thus, the PM concentration of particulate matter can directly reflect the emission characteristics of pollutants. In the process of rust removal, PM₁₀ was dominated, and its concentration was mainly between 600 and 10,000 µg/m³, with a mean concentration of approximately 7000 µg/m³ (Fig. 2D). The concentration of PM_2.5 at the same site was mainly between 50 and 1000 µg/m³, with a mean concentration of approximately 800 µg/m³. The PM_2.5/PM₁₀ ratio was approximately 0.11, indicating that most of the matter was larger than 2.5 µm. This was because particulate matter was produced by the fragmentation of rust. When rust was broken to 10 µm under the action of the laser, it would smoothly leave the substrate and enter the atmosphere, achieving the cleaning effect. Therefore, large particles (particle size larger than 2.5 µm) dominated the particulate matter.

For waste oil removal, PM₁₀ concentration was mainly between 150 and 300 µg/m³, with a mean concentration of approximately 400 µg/m³ (Fig. 2E). PM_2.5 concentration in the same site was mainly between 40 and 300 µg/m³, with an average concentration of approximately 300 µg/m³. Compared with rust removal, particles in waste oil removal were mainly small particles (particle size less than 2.5 µm). This was due to differences in the formation mechanisms of the particulate matter. In the process of rust removal, particles were produced by breaking large rust pieces. In the process of oil removal, particles were small liquid oil droplets formed by the condensation and aggregation of steam after gasification. As a result, more small particles were produced during the oil removal process.

For oil sludge removal, PM₁₀ concentration was mainly between 200 and 3000 µg/m³, with a mean concentration of approximately 5000 µg/m³ (Fig. 2F). The larger mean value of PM₁₀ concentration resulted from the presence of many outliers higher than 10,000 µg/m³. PM_2.5 concentration in the same location was mainly between 40 and 1000 µg/m³, with an average concentration of approximately 1000 µg/m³. Particulate matter concentration during oil sludge removal was significantly higher than that during waste oil removal (P < 0.01). This was because, in addition to waste oil, there were many solid organic impurities in the oil sludge, which also produced a large amount of soot under the action of the laser.

3.3.2. Heavy metals releasing characteristic

The morphologies of the particulate matter generated from laser cleaning of the three types of stains were significantly different. The particles produced during waste oil removal were mainly small liquid oil droplets, and those produced by rust and sludge removal contained many heavy metals, which greatly increased the adverse impacts. The XRD spectra indicated that SiC was the main fraction of the particulate matter (Figure S4). SiC is an additive that improves the hardness, compressive strength, and wear resistance of vehicle parts [33]. Fig. 2G,H, and S5 show the contents of the main heavy metals in the matter during or before laser cleaning. The heavy metal content in particulate matter during laser cleaning was much higher than that in the workshop before laser cleaning.

During rust removal, particulate matter contained a large amount of Fe due to the presence of rust. Fe content was as high as 140,033 ppm in PM₁₀, and its content was 80,882 ppm in PM_2.5. The Fe content of PM₁₀ was significantly higher than that of PM_2.5, indicating that Fe mainly exists as large particles. This further indicates that particulate matter was produced by crushing large pieces of rust during rust removal. In addition to Fe, particulate matter contains certain amounts of Cr, Mn, Al, and Co. These heavy metals are the main components of stainless steel and aluminum alloys. The Al content was much lower than that of Fe. There were two main reasons for this phenomenon: first, the iron material in the vehicle parts was more easily corroded by oxidation than the aluminum alloy material, and second, removing iron rust was easier than removing aluminum rust under laser radiation.

When cleaning oil sludge, the heavy metal contents in the particulate matter were much lower than those in the cleaning rust. The Fe content was 32,993 ppm in PM₁₀ 8462, and in PM_2.5, Fe content was 8462 ppm. However, the Al content here (14,394 ppm in PM₁₀) was much higher than that (3093 ppm in PM₁₀) in rust removal. When vehicles run, fine particles are generated by the wear and tear of some aluminum alloy parts, which are adsorbed into the oil on the part surface, forming oil sludge.

3.3.3. VOCs releasing characteristic

VOCs are another major category of pollutants generated during laser cleaning. Large amounts of VOCs are released during the remanufacturing process [12]. These VOCs may be from industrial solvents and lubricating oil [34,35]. Laser heating and vibrations accelerate the release of these substances, resulting in severe VOC pollution.

Fig. 2I shows the VOC concentrations in the different scenarios, and detailed data are shown in Table S6. Before laser cleaning, VOC concentration in the workshop was 1348 µg/m³. These VOC species were consistent with those found in the ultrasonic cleaning workshop. This VOC pollution may originate from solvent and ultrasonic cleaning through gas diffusion. During laser cleaning for oil sludge removal and waste oil removal, total concentrations of VOCs were 2462 and 2568 µg/m³, respectively. The concentrations and species of VOCs increased significantly, especially for alkenes and alkynes such as propylene and ethyne.

3.4. Pollutant release mechanism

3.4.1. Physical properties of stains

A laser is a strong, coherent light beam with high energy density [36]. When stains or substrates are irradiated, local temperatures rise rapidly, causing the stains to ablate and vaporize in a short time to achieve stain removal. Thus, the thermodynamic properties of stains have a substantial influence on the emission characteristics of pollutants during laser cleaning. Fig. 2J,K show the thermogravimetry and differential scanning calorimetry curves of waste oil and oil sludge attached to the part surface at the heating rate of 20 ℃/min.

As shown in Fig. 2J, waste oil removal mainly occurred in two temperature segments: 200–339 ℃ and 340–550 ℃. From room temperature to 200 ℃, waste oil decreased by 1.8%. In the temperature range of 200–339 ℃, waste oil quality decreased rapidly from 98.2% to 8.8%. As the heat continued to 550 ℃, 0.7% of the residue remained, with 99.3% of the waste oil vaporizing. When the temperature increased from 550 to 800 ℃, the residual mass was almost unchanged, maintaining at approximately 0.6%. This residue could not be removed by simple heating. It might be a spot of inorganic salts and heavy metals mixed in the waste oil. The direction of heat flow during oil removal by heating was studied as a function of temperature and heating rate. A DSC graph of the heat flow is shown in Fig. 2J. There were two endothermic peaks in the DSC curve: 323 ℃ and 527 ℃.

As shown in Fig. 2K, oil sludge removal mainly occurred at 150–755 ℃. From room temperature to 150 ℃, oil sludge decreased by 1.2%. In the temperature range of 150–755 ℃, oil sludge quality decreased rapidly from 98.8% to 60.4%. As the heat continued to 1000 ℃, 1.0% of oil sludge was removed. Residue at 59.4% of the mass ratio remained, and this part could not be removed by simple heating. The residue comprised inorganic salts and heavy metals from environmental dust, which was the main component of oil sludge (Fig. 2C). On the DSC curve in Fig. 2K, there were two endothermic peaks observed at approximately 305 ℃ and 506 ℃. The exothermic peak was observed at approximately 750 ℃, which might have resulted from the burning of some organic matter. As shown in Fig. 2C,L, plastic fibers were found in the oil sludge.

3.4.2. Mechanisms of laser cleaning and pollutant release

Different physicochemical reactions occur on different strains under laser radiation; therefore, the formation and release mechanisms of pollutants are also different. A high-energy laser can induce physical and chemical effects on the stains (Fig. 3A). In rust removal, pollutants mainly originate from the breaking and escaping of the rust on the surface (Fig. 3B). When the laser radiated on the rust, the temperature increased rapidly in a short time owing to the photothermal and light vibration actions, and the rust expanded and broke down [37]. Simultaneously, light vibrations further intensified the crushing of the rust. The fine rust powder formed after crushing was dispersed under expansion and shock waves, producing dust in the atmosphere.

Fig. 3.

Mechanism of pollutant generation in laser cleaning.

Many organic pollutants are produced during waste oil removal (Fig. 3C). When the laser is rayed on waste oil, it directly acts on the surface of the substrate parts through the waste oil because the substrate has a much better absorption effect for the laser than waste oil. Thus, the part surface temperature rises rapidly, causing the temperature of the waste oil at the solid–liquid interface in contact with the substrate to rise rapidly, forming a gas film layer to separate the waste oil from the part substrate. When the laser continues to radiate, it causes waste oil boiling or sputtering [38] such that the waste oil enters the environment in the form of steam or small oil droplets, resulting in atmospheric organic pollution.

In the oil sludge removal process, the cleaning mechanism was complicated owing to the complex composition of the oil sludge, which contained waste oil, rust, metal particulates, plastics, plant fibers, and inorganic salts (Fig. 2). During laser cleaning, a series of complex physicochemical reactions occur, such as the vaporization of waste oil and the breakage and deformation of solid components. Moreover, combustion and pyrolysis can occur when using a high-energy laser. As shown in Fig. 3D, when the laser radiated on the plastic fibers fully exposed to air, a combustion reaction occurred, releasing water, carbon dioxide, and other products; the burning of plastics could act as a trigger point for the burning of waste oil adsorbed on the plastic, and the incomplete combustion of waste oil can produce small molecules of organic pollutants. VOC analysis revealed that various high-concentration VOC components (e.g., ethylene, propylene, butene, pentene, hexene, benzene, acrolein) appeared in the air during laser cleaning but were not found or were at a low concentration during solvent-ultrasonic cleaning. Pyrolysis reactions may occur on plastic fibers when they have insufficient contact with oxygen, resulting in the formation of small molecular organic matter, such as unsaturated hydrocarbons or oxygen-containing organic matter [39]. The incomplete combustion of waste oil also produces small molecular organics such as alkanes, alkenes, and alkynes [40,41].

3.5. Environmental LCA and health risk assessment

An environmental LCA was used to quantitatively compare the effects of different cleaning technologies on the environment during the remanufacturing process. The impact category data were calculated using Simapro 9.0, the leading software tool used for LCA [42]. The data used were from physical measurements and factory experience estimates, which may have resulted in some uncertainties (listed in Table 1). In traditional cleaning technologies, sand blasting is used only for rust removal, whereas solvent-ultrasonic cleaning is used only for oil and oil sludge removal. Therefore, we compared the environmental impacts of laser cleaning and traditional cleaning technologies on rust and oil (sludge) removal. The total environmental impact is shown in Fig. 4A,B, and the detailed data are shown in Tables S7 and S8. Traditional cleaning technologies had a higher total environmental impact than laser cleaning. For rust removal, the total normalized environmental impact of sand blasting was 1.81 × 10⁻⁸, which was approximately 6.0 times as much as that of laser cleaning (total normalized environmental impact of 3.02 × 10⁻⁹). For oil (sludge) removal, the total normalized environmental impact of solvent-ultrasonic cleaning was 7.55 × 10⁻⁹, which was approximately 1.67 times as much as that of laser cleaning (4.53 × 10⁻⁹). These two multiples were very close to the electricity consumption multiples, indicating that the total environmental impact of the cleaning process was mainly caused by electricity consumption. This inference can also be drawn from Fig. 4C,D. The environmental impact category showed that the largest impact of laser cleaning and traditional cleaning technologies was marine aquatic ecotoxicity, which was mainly caused by pollutants released from power generation.

Fig. 4.

(A) Total environmental impact for rust removal, (B) total environmental impact for oil (sludge) removal, (C) impact category data of laser cleaning for rust removal, (D) impact category data of laser cleaning for oil (sludge) removal, and (E) health risk assessment of different cleaning technologies.

Safeguarding workers’ health is critical; thus, evaluating the health risks to workers in different cleaning scenarios is necessary. Fig. 4E shows the exposure risks of workers in different cleaning workshops to several major pollutants. For heavy metals, the HI of Al was greater than 1 in the laser cleaning and the sandblasting workshops. Al might damage the human nervous system and cause Parkinson's disease, cognitive disfunction, and memory decline [43,44]. Avoiding the health risks posed by Al requires protective measures, such as wearing a mask and installing a dust collector. During the removal of waste oil and oil sludge using traditional cleaning technology, the hazard indices of all VOCs species were below 1, indicating no exposure risk. However, acrolein was detected during laser cleaning, and its HI was much larger than 1, indicating a high chronic risk. Although laser cleaning can reduce the release of pollutants, waste oil and oil sludge would undergo complex chemical reactions under high-energy lasers, which may produce more harmful substances. Thus, the corresponding protection during laser cleaning should be considered and strengthened.

4. Conclusion

In summary, this study proposed a high-energy laser to replace traditional cleaning technology in the high-value vehicle part remanufacturing industry. Pollution characteristics and pollutant generation during the laser cleaning process were discussed systematically. Laser rust removal and sludge removal would produce a high concentration of particulate matter, and the instantaneous concentration of PM₁₀ might exceed 10,000 µg/m³. Particulate matter contained high concentrations of iron and aluminum. High levels of ethylene, ethylene, butene, pentene, hexene, benzene, and acrolein were detected during oil and oil sludge removal. The main mechanisms of rust removal are thermal expansion and vibration crushing caused by the laser thermal effect. The primary mechanism of oil removal is the evaporation of waste oil under heat. The mechanism of oil sludge removal involves the evaporation of waste oil, thermal expansion, and vibration of the solid fraction, which are accompanied by complicated combustion or pyrolysis processes. The LCA results indicated that laser cleaning, instead of traditional cleaning, could effectively reduce the environmental impact by reducing power consumption. Health exposure risk assessment showed that laser cleaning could not effectively reduce the health risk to workers and might produce more harmful substances; therefore, implementing corresponding protective measures during laser cleaning is necessary.

CRediT authorship contribution statement

Rui Wang: Methodology, Conceptualization, Investigation, Visualization, Writing – original draft. Lu Zhan: Visualization, Formal analysis, Methodology, Writing – review & editing. Zhenming Xu: Methodology, Supervision.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Rui Wang is a Ph.D. student in School of Environmental Science and Engineering, Shanghai Jiao Tong University. The main research work is focused on pollution emission, migration and transformation during secondary resource recycling.

Lu Zhan is an associate professor and Ph.D. supervisor in School of Environmental Science and Engineering, Shanghai Jiao Tong University. He obtained his Ph.D. degree in School of Environmental Science and Engineering, Shanghai Jiao Tong University in 2011. His-research interests are e-waste recycling technology, solid waste recycling and pollution controlling.