Exposure Assessment of Total Suspended Particulates and Heavy Metals in Anodizing and Electroplating Surface Treatment Process

Objective: This study is to evaluate exposure to harmful substances, such as particulate and heavy metals, by considering various factors, in anodizing and electroplating surface treatment process. Methods: Exposure evaluation studies on seven heavy metals (Cr, Zn, Ni, Pb, Cd, Al, and Ba) and total suspended particulates (TSP) were conducted. Heavy metals were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). This study also checked the ventilation volume of the hood with a thermal anemometer. Measurement was conducted for 8 hours and 8 days. The sample number, N, of the heavy metals and TSP was 107. Results: Geometric mean (GM) of TSP during Cr plating process was 6.15(GSD, 3.35) mg/m 3 . GM of Cr during Cr plating was 1.86(GSD, 6.65) mg/m 3 . GM of TSP and heavy metals differences were statistically signicant for each process and date variation. Average ventilation volume for all hoods ranged from 1.20 to 4.98 m 3 /s. In the hood 30 cm from bath, ventilation was 0.1 times lower. Increasing ventilation volume of the hood was the most inuential factor, followed by machine operation time and workload. Conclusion: The high concentration was due to low ventilation suction ow. We can improve health to reduce exposure by resolving the fundamental cause of risk occurrence.


Introduction
Among the metal material industry, the surface treatment industry improves the product quality of protective surfaces by using electrical, physical, and chemical treatment methods on two surface materials to prevent rust, beautify appearance, and increase wear resistance, electrical insulation, and electrical conductivity (1).
As anodizing, the cathode is made of Al alloy, Pb, or stainless steel. Anodizing is applied to improve corrosion by an anode + oxidizing, thereby undergoing degreasing, neutralization as a pre-treatment process, coloring, sealing, polishing, and non-polishing as a post-treatment process. The oxide lm is formed by soft anodizing and hard anodizing. As a post-treatment process, the product is completed through neutralization, water cleaning, drying, and packaging (2).
The most important exposed materials in the anodizing process are heavy metals. During the process of oxide lm formation, abundant Al depending on the type of Al alloy are used, and exposure to other heavy metals is possible. The arti cial exposure route of Al is through the respiratory tract, Al is related to potroom asthma, chronic bronchitis, pulmonary broids, and granulomatous lung diseases upon exposure through inhalation. Al exposure was signi cantly correlated with various neurological disorders, and contact dermatitis in workers exposed to Al alloy and Al dust also has been reported. Al toxicity can occur as a result of the interaction between Al and the plasma membrane with target established by the body (4). Al inhaled through the respiratory tract is replaced by magnesium and iron, resulting in intercellular exchange, cell growth, and secretion functions. The changes in neurons by Al are similar to the degenerative lesions observed in Alzheimer's patients, and complication of Al toxicity may have neurotoxic effects, such as nerve atrophy of the cerebellum, black matter, and striatum (5). Similarly, high concentrations of Al are very toxic to aquatic animals, especially gill-respiring organisms such as sh, and can destroy plasma and bloodstream ions, and cause osmotic disorders (6). Chromium plating was used by an electrolytic solution of chromic acid during the plating process. Workers can be exposed to Hexavalent chromium (Cr (VI)) during the mixing of acid powders and carrying products (7).
Hexavalent chromium (Cr (VI)) compounds are occupational carcinogens that cause lung, nasal, and sinus cancers (8; 9; 10). Cr (VI) compounds are produced from other airborne forms of Cr industries that use Cr (VI) compounds, such as steel passivation, electroplating, stainless steel welding, and paints production, Crbased pigments, fungicides, and anti-corrosion compounds as a by-product (11). Exposure to Cr during electroplating is a characteristic cause of occupational asthma. Sensitivity to Cr in electroplates may occur in situations where exposure levels are likely to be within the current exposure standards (12).
Previous similar studies on plating show that the metal surface is plated by shooting electrons from the cathode (Eq. (1); 13). For anodizing, the surface treatment principle is different; the surface of the metals is anodized by shooting oxygen from the anode (14). The reactions occur simultaneously during anodizing (Eq. (2); Eq. (3)).
-plating equation Where M n+ is ion, n is number of moles reacting, and e − is electron and M o is the metals.
-anodizing equation (oxidation and dissolution) This study evaluated the exposure of each process of business sites performing Al anodizing and Cr electroplating. Exposure evaluation studies on seven heavy metals (Cr, Zn, Ni, Pb, Cd, Al, and Ba) and TSP were conducted. This study also evaluated harmful substances by checking the ventilation volume of the hoods. The sample number, N, for TSP and heavy metals is 107.
Materials And Methods Table 1 shows the temperature, operations time, and electrolyte chemical composition of the bath in all the workplace processes. There are three standard surface treatment baths at this workplace which were used; the number 1 represents the medium-sized bath, and the number 2 represents the large-sized bath. Table S1 lists the values for special processes that require speci c pH or voltage adjustments and provides information on temperature and humidity during the working environment measurement period. Table S2 shows the amount of work and the number of workers during the work environment measurement period, which can be important factors for identifying variable factors according to concentration. The workplace in this study addressed all processes for anodizing and Cr plating, and the processes can be largely divided into pre-treatment, anodizing, Cr plating, and post-treatment processes (Fig. 1). Drying, packaging, and assembling processes are connected to the above processes ( Figure S1).
Samples were collected at a height of 1.5 m to represent the breathing zone of workers. The measurement time proceeded from 8:30 am to 5:30 pm for 8 days, and the measurement was conducted for total of 8 h, excluding breaktime. As a control, the anodizing and plating control sample and drying and packaging control sample were placed in the workplace to check the blank.
2.1. Air sampling and analysis of TSP PVC (Polyvinyl Chloride) lter (37mm, pore size 5um, SKC, USA) in the three-piece 37mm closed face plastic cassette (Whatman Grade QM-A, 37mm; Whatman, Maidstone, UK) was used to measure ne dust generated during the operation of forming an oxide lm and plating, which were the main processes during anodizing and electroplating. These were connected by two types of air sampling pump (Casella APEX-2, USA & Gillian, USA) operating at an air ow rate of 2 ℓ / min. The sample was wrapped with Para lm (Bemis Company, USA) before and after the sample was collected so as not to be exposed to outside air and then stored to room temperature.
To analyze the TSP, PVC lter was contained in a desiccator for more than 24 hours, and then weighed using an analytical balance having 0.01 µg sensitivity (Mettler XP6 Microbalance, Mettler Toledo, Hightstown NJ, USA). Before measuring weight of the lter, static electricity was removed and weight change according to daily temperature and humidity change was corrected using a blank lter. Weight was measured 3 times per sample, and the average value was used as the nal weight concentration. High ow rate pump was calibrated before and after sampling using a ow calibrator (Drycal, Defender 520-M, MesaLabs, USA).

Air sampling and analysis of heavy metals
The air sampling and analysis of heavy metals were based on the NIOSH method 7300 (15). After the weighing TSP, the remaining was also used to measure heavy metals, as described for TSP measurement.

Hood ventilation measurement
A thermal anemometer (TSI 9515, VelociCheck, TSI, USA) was used to measure the hood characteristics, ventilation volume (m 3 /s), and wind speed (m/s) according to distance. There are two types of thermal anemometers (one-way directional and omni-directional); this study used a one-way type. Figure S2 shows the overall shape of the hood; the workplace contained small, medium, and large hoods. Sampling and ventilation volume were measured at the medium and large hoods. The number 1 and 2 denotes medium and large hood, respectively. Five points on the hood were determined to measure ventilation and air ow.
Point 1, 2 and 3 were controlled from the middle. Point 4 was controlled directly under the bath, and point 5 was measured for ventilation and air ow at a distance of 30 cm from the midpoint of points 1-4. The polishing process only had extra point.

Data analysis
Air sample concentrations were expressed in terms of arithmetic mean (AM), standard deviation (SD), geometric mean (GM), geometric standard deviation (GSD), and median and range (min-max). The Shapiro-Wilk test con rmed that the TSP and heavy metals were log-normally distributed. Measured air concentrations of TSP and heavy metals in each process were compared using analysis of variance (ANOVA), and the Bonferroni method was used for post-hoc analysis.  Table S4 and Table S5 show the TSP concentrations of all the process samples measured in the workplace.

TSP
The highest GM concentrations of the samples were for Cr plating (6.15(GSD, 3.35) mg/m 3 ). As a result of ANOVA test, GM concentrations of every process were statistically signi cant, showing that the process samples have different exposure aspects during product creation. Differences in GM concentrations and daily variation were statistically signi cant among the anodizing and electroplating, post-treatment, pretreatment, and other processes. For the pre-treatment process, the average TSP concentration of degreasing was similar to that of etching and neutralization; however, the range was much higher than that of etching and neutralization. Anodizing and electroplating have similar soft and hard anodizing aspects, whereas Cr electroplating has a much higher concentration than all the processes. During the post-treatment process, the average TSP concentration of sealing has the highest average, whereas coloring and polishing have a wide concentration range. In the "other" processes, the TSP average concentration was generally lower.
However, the drying and packaging and Cr cleaning processes have su cient concentrations (Fig. 2).

Heavy metals
The heavy metal totals were analyzed with 17 types, including the existing seven (Cr, Mn, Zn, Ni, Pb, Cd, Al, and Ba) types and ten other types. Table S6, Table S7, Table S8, Table S9, Table S10, Table S11, Table S12 and Table S13 show the exposure to seven types of heavy metals in all workplace processes. GM of Cr the GMs of heavy metals were statistically signi cant. When the proportion of heavy metals was con rmed, the analysis was performed with seven basic heavy metals, excluding the 10 other heavy metals. Pretreatment processes con rmed that Al occupied the largest proportion of heavy metals, followed by Cr, and occupied most of the anodizing and electroplating processes; for Cr plating, however, Cr occupied the largest proportion. The post-treatment process revealed that the Cr concentration occupied a high rate while polishing. In other processes, Al occupied the highest rate, and Ni occupied the highest rate in the drying and packaging processes (Fig. 3).
Relationship between TSP and heavy metals Figure 4 shows the correlation distribution between TSP and heavy metals(N = 107) concentrations. The coe cient of determination r 2 was 0.994, indicating a strong relationship. The ratio of seven kinds of heavy metals causing adverse health effects relative to the total measured substances in the air was not relatively high. The Cr concentration accounted for 62.65% in degreasing, drying and packaging, and Cr cleaning and for 20-40% in soft and hard anodizing (Fig. 5).

Ventilation
The workplace was equipped with a hood for every process, and the total suction ow rate for all the processes was 600 m 3 /s. The hood had a total of four slots, and the polishing process involved a total of ve slots. Table S14 and Table S15 show   Aspects of TSP and heavy metals in workplace The exposure values of the anodizing and electroplating processes, which handle more heavy metals were higher. The concentrations of some types of heavy metals in all the processes were below the OEL of Korea; Additionally, creatinine concentrations were 30.1 ± 19.5 and 76.3 ± 54.5 µg/g in urine. Individual exposure was not very high in air, and the correlation with urine concentrations was weak (20). The factors in uencing individual Cr concentration in Cr plating factories were con rmed by regression analysis. The exposure coe cients for wearing / not wearing gloves, smoking, and exposure after recently had a skin disease were 103.5 E − 6 , 39.6 E − 6 ± 20.9 E − 6 , 23.8 E − 6 ± 22.6 E − 6 , and 161.6 E − 6 ± 65.6 E − 6 mg/m 3 , respectively. Although smaller particle size resulted in lower concentration, the adverse effect on health was greater at smaller particle sizes. Additionally, ner particles can move the alveoli of the lungs (17; 22). The diameter distribution of Cr (VI) less than 10 µm in the Cr plating plant was investigated, and the mass median diameters of Cr particles in the two electroplating tanks were 5.11 µm and 6.38 µm, respectively. The Cr (VI) diameter distribution in the general manufacturing industry was 1.67 µm. Therefore, the electroplating tank had a relatively high mass distribution (23).

Limitations
Calculating the workload of the detailed process the precise amounts of chemical substances used in the electrolytes is di cult, because most materials are trade secrets. TSP was investigated in all the processes while making metal products. The quantitative composition of heavy metals particle size PM0.3 to PM10 using a real-time measuring instrument (Aerotrak Particle counter, TSI, USA) found that smaller particle sizes correlate to higher quantitative composition concentration in all the processes; among these, a higher quantitative composition concentration was measured as the particle size decreased in degreasing, Cr plating, and Ni plating (24).
Checking the concentration of dust by particle size to provide information that can explain the extent of inhalation of smaller particles into the human body is necessary. A study for sampling nanoparticle size in electroplating revealed that the generation of nanoparticles were 64,327 particles/cm 3 with a passivating bath process and 33,249 particles/cm 3 without a passivating bath process based on 8 hour TWA (Time-Weighted Average). Additionally, 5,645 and 4,947 particles/cm 3 were generated when working with and without the rotary abraser bath process, respectively. Thus, further research on nanoparticles is important (25).

Conclusions
Most of the work environment measurement data at this workplace were undetected. Firstly, identifying the fundamental cause of the high concentration in the processes is di cult. Work environment measurement system is intensively focused on personal exposure. Obtaining a su cient number of samples N of data is hard. Therefore, to con rm the reliability of work environment measurement system, conducting simultaneous personal and area exposure measurements would improve the results. The suction ow rate was exceptionally low and di cult to collect when measured from a distance of 30 cm. Therefore, improving the suction ow rate of the hood is necessary.
Few studies have measured and evaluated exposure to TSP and heavy metals related to anodizing and electroplating. In this study, we have con rmed that the exceptionally high concentrations of TSP and heavy metals are primarily caused by low ventilation suction ow rates. Concentrations were similar among pretreatment and anodizing processes, especially high in the Cr plating process, and primarily high in the coloring and polishing processes among the post-treatment process. Concentrations in other processes remained relatively high. Although in most processes, the Al concentration was the highest, Cr concentration was high in the Cr plating and polishing processes. This study provides data on the risk of exposure for anodizing and electroplating and improves the reliability of work environment measurement data, thereby improving health through the need for fundamental exposure. Determining the cause of the difference in concentration for varying dates was di cult, and the concentrations of each variable showed particularly poor trends. Room humidity, temperature, and product characteristics were predicted the variables affecting date concentration.

Declarations Data availability
Additional information associated with this article can be found in the supplementary data.  Distribution of the total heavy metals and TSP. All the concentrations are arithmetic means(N=107).