Composition profiles, monthly changes and health risk of PCDD/F in fly ash discharged from a municipal solid waste incinerator (MSWI) in Northeast China

Abstract

One of the hazardous wastes present in municipal solid waste incinerators (MSWI) is fly ash; it threatens the health of onsite workers due to its inevitable dispersion on the ground and in the atmosphere during the process of collecting, cleaning and transporting it from the incinerator. In the present study, composition profiles and monthly changes of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash from a MSWI in Northeast China were studied. In addition, the health risk posed by these chemicals to onsite workers was quantitatively assessed based on a Monte Carlo simulation. The total and toxic equivalent concentrations of PCDD/Fs (TPCDD/Fs and TEQ) in fly ash in the sampling months were 36.5–5653.0 ng g⁻¹ (with a median value of 823.5 ng g⁻¹) and 3.2–800.1 ng TEQ g⁻¹ (with a median value of 111.1 ng TEQ g⁻¹), respectively. Although the composition of the municipal solid waste did not vary over the sampling period, a wide variation of mass concentration of PCDD/Fs in the fly ash was observed. The 95th percentile carcinogenic risk (CR) and non-carcinogenic risk (non-CR) for onsite workers were much lower than the threshold values of 10⁻⁶ and 1.0, respectively, suggesting that the potential health risk was not significant. Accidental ingestion was the largest contributor to health risk for onsite workers, at 67.7% and 68.6% of the CR and non-CR, respectively. Filter-type gas masks could be effective personal protection equipment to ameliorate this risk; not only could they reduce potential exposure to contaminants via ingestion and inhalation, but also reduce exposure to contaminated soil and ambient air. These results could be useful for health risk management of onsite workers in MSWI plant.

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1. Introduction

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are a group of ubiquitous and persistent organic pollutants, which are formed as by-products of combustion;⁷ they are widespread in environment, including air, water, soil, sediment, food and fly ash,^1–6 Due to their known toxic, carcinogenic and mutagenic properties, the fate of PCDD/Fs in environment is of great concern.^8–10

Municipal solid waste incinerators (MSWI) are one of the key means of disposal of municipal solid waste in China. In recent years, more attention has been paid to eliminate the emissions of PCDD/Fs in flue gas from MSWI in China. Some research indicated that 65.3% of total dioxin (about 1147.7 mg) produced by MSWI was discharged into the environment in the form of fly ash.¹¹ In addition, Abad et al. (2000) determined the dioxin mass balance for a Spanish MSWI and found that the PCDD/F emission from the stack gas and fly ash were 0.0048 and 2.6 g I-TEQ year⁻¹, respectively.¹²

Despite its known hazards, onsite workers in MSWI inevitably are exposed to fly ash frequently, as it disperses to the ground and floats in the atmosphere during the collection, clearing and transporting of fly ash from incinerator. Onsite workers suffer long-term exposure to PCDD/Fs in a higher dosage and frequency than does the general population; therefore, their health risk deserves special attention. However, to the best of our knowledge, the current research in this area is quite limited. Although Wu et al.¹³ studied the health risk of PCDD/Fs from exposure to fly ash from MSWI for onsite workers, the heterogeneity of the population and exposure parameters, such as accidental ingestion rate, inhalation rate and body weight, were not considered in this research.

In the present study, fly ash samples from a MSWI plant in Northeast China were collected and analyzed over a period of seven months. The composition profiles and monthly changes were studied first. Considering the heterogeneity in population and exposure parameters, the carcinogenic risk (CR) and non-carcinogenic risk (non-CR) of onsite workers in MSWI plant were quantitatively assessed based on a Monte Carlo simulation. In addition, the contributions to CR and non-CR of each pathway (accidental ingestion, inhalation and dermal contact) were quantitatively evaluated. The results were used to determine the key exposure pathway, which will help MSWI managers provide proper personal protection equipment for onsite workers.

2. Material and methods

2.1. Sample collection

The MSWI under study was located in Northeast China, and was equipped with three 75 t h⁻¹ circulating fluidized bed boilers, with total daily capacity of 1500 t of municipal solid waste (MSW). In addition, about two hundred employees worked in this MSWI plant. The operating conditions and parameters of the incinerator are listed in Table 1.

Table 1 The operating parameters and conditions of the municipal solid waste incinerator (MSWI)

Number	Parameters and conditions	Unit	Value
1	The designed volume of municipal solid waste (MSW)	Ton per day	21.9
2	The overload volume of MSW	Ton per day	24.2
3	Designed low heat value (LHV) of MSW	kJ kg^−1	7500
4	Rated LHV of MSW	kJ kg^−1	5860
5	The range of LHV of MSW	kJ kg^−1	4100–7500
6	The operating time of incinerator	h per year	8000
7	The residence time of flue gas (>850 °C)	s	>2
8	The thermal reduction rate of residues	%	<3
9	The length of grate boiler	mm	10 000
10	The width of grate boiler	m	5700
11	The slope angel of grate boiler	°	21.1
12	The surface area of grate boiler	m^2	57.4
13	The thermal loading of grate boiler	kW m^−2	410
14	The max thermal loading of grate boiler (110%)	kW m^−2	450
15	The mechanical loading of grate boiler	kg m^−2	254
16	The max mechanical loading of grate boiler (110%)	kg m^−2	281
17	The primary air flow	m^3 h^−1	48 000
18	The secondary air flow	m^3 h^−1	6980
19	The temperature of incinerator in primary air flow	°h	180
20	The temperature of incinerator in secondary air flow	°C	37
21	The density of flue gas	kg m^−3	1.2403
22	The flue gas volume of exit	m^3 h^−1	462 982
23	The flue gas temperature of exit	°C	124

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Two samples of fly ash were collected per month from bag filters in the plant for seven consecutive months (October–December in 2014 and January–March in 2015), for a total of fourteen samples. The average concentration of the two samples was used as the monthly concentration of PCDD/Fs. The samples were transported to our laboratory and stored at −20 °C for further treatment and analysis.

2.2. Sample analysis

The samples were tested for seventeen toxic 2,3,7,8-substituted tetra- through octa-CDD and CDF congeners by high resolution gas chromatography (Thermo Trace GC) and high resolution mass spectrometry (HRGC-HRMS), according to the Chinese national standard method (HJ77.3-2008). Briefly, the fly ash samples were freeze-dried and sieved, then spiked with ¹³C₁₂-labeled PCDD/Fs internal standards (EPA-1613 LCS) and extracted by Soxhlet with toluene. The extracts were subjected to the following clean-up procedures: H₂SO₄ treatment, multi-layer silica gel column and alumina. The toluene fraction containing PCDD/Fs was collected and concentrated to dryness under a gentle N₂ purging. The injection standards (EPA-1613 ISS) were spiked, and the sample was re-dissolved in 1 mL of decane prior to analysis.

PCDD/Fs analysis was performed on an HRGC-HRMS using a fused silica capillary column DB-5 MS (60 m × 0.25 mm × 0.25 μm, J&W). The electron impact (EI) ion source was operated at 35 V electron energy, the ion accelerating voltage was up to 8000 V, and the mass spectrometer resolution was above 10 000 (10% peak valley definition). The GC temperature program was initiated at 140 °C (held for 1 min), increased to 200 °C at 20 °C min⁻¹ (held for 1 min), then to 220 °C at 5 °C min⁻¹ (held for 16 min), increased to 235 °C at 5 °C min⁻¹ (held for 7 min), and finally, increased to 310 °C at 5 °C min⁻¹ (held for 9 min). The temperatures of the inlet, source and interface were 270 °C, 250 °C and 270 °C, respectively. The injection volume was 1 μL. Quantification was performed using the external standard method, and later an internal standard correction was also made.

2.3. Quality assurance (QA) and quality control (QC)

Procedural blanks and matrix spiked samples were processed and analyzed along with field samples. Analysis of procedural blanks demonstrated that the analysis system and glassware were free of contamination. Among all the samples analyzed, five were randomly selected as parallel samples for checking repeatability. The relative standard deviations were less than 10% (n = 3) for the parallel samples. The detection limits of the 17 PCDD/Fs were estimated from a signal-to-noise ratio of 3 : 1 in blank samples and varied from 0.084 to 0.23 pg g⁻¹. The sampling recoveries of five ¹³C₁₂-PCDD/F surrogate standards were measured relative to the ¹³C₁₂-PCDD/F internal standards to obtain a measure of the collection efficiency. The recoveries for the surrogate labeled PCDD/F sampling standards were between 70% and 123% in the present study.

2.4. Health risk assessment

The chronic daily intake (CDI) of PCDD/Fs refers to the PCDD/Fs in fly ash accepted by the exposure terminal (i.e., respiratory organs of the human body). The CDI of PCDD/Fs in fly ash was calculated based on the intake methodology, including 3 pathways: inhalation, dermal contact and accidental ingestion. These pathways corresponded to the exposure processes of re-suspension–inhalation, contact and hand-into-mouth ingestion of fly ash. The CDIs were calculated by the following equations:^1,13,14where CDI_ingestion is the chronic daily intake associated with fly ash accidental ingestion (mg kg⁻¹ d⁻¹); C is the sum of converted concentrations for 17 carcinogenic PCDD/Fs in fly ash based on toxic equivalents (I-TEFs) of 2,3,7,8-T₄CDD;¹⁵ IR is the accidental ingestion rate of fly ash for onsite workers (mg d⁻¹);¹³ EF is the exposure frequency (days per year); ED is the exposure duration (year); BW is the body weight (kg); AT is the average time of onsite workers (70 years), and CF is the conversion factor (10⁻⁶ g ng⁻¹).¹⁶ The values of IR, BW, ED, EF and other parameters are presented in Table 2.where CDI_inhalation is the chronic daily intake associated with inhalation of fly ash (mg kg⁻¹ d⁻¹); HR is air inhalation rate (m³ d⁻¹); and PEF is the particle emission factor (1.36 × 10⁹ m³ kg⁻¹).^1,13,17where CDI_contact is the chronic daily intake associated with dermal contact of fly ash (mg kg⁻¹ d⁻¹); SA is the surface area of the skin that contacts fly ash (cm² d⁻¹); AF is the soil to skin adherence factor (mg cm⁻²), and ABS is the dermal absorption factor (0.03).¹⁸

Table 2 The lognormal/uniform probability with geometric mean and geometric standard deviation (LN/U (gm, gsd)) used in risk assessment for onsite workers

Definition	Unit	Onsite workers (adults)	References
Age	Year	18–70
Average time (AT)	Day	25 550	Chen et al.^26
Exposure frequency (EF)	Day per year	LN (252, 1.01)	Li et al.^14
Body weight (BW)	kg	LN (59.78, 1.07)	Man et al.^22
Exposure duration (ED)	Year	U (0, 53)	Yang et al.^27
Inhalation rate (HR)	m^3 d^−1	LN (32.73, 1.14)	Li et al.^14
Surface area (SA)	cm^2 d^−1	LN (18 182,1.10)	Yang et al.^27
Soil to skin adherence factor (AF)	mg cm^−2	LN (0.02, 2.67)	USEPA, 2001.^28

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2.4.1 Carcinogenic risk assessment. The CR was calculated by multiplying the estimated dosage and the cancer potency factor, which was adapted from USEPA (1989);¹⁹

CR_i = CDI_i × CSF (4)

where the subscript i denotes accidental ingestion, inhalation and dermal contact; and CSF is the carcinogenic slope factor of 2,3,7,8-T₄CDD. For accidental ingestion, inhalation, and dermal contact, the values of CSF are 1.3 × 10⁵, 1.5 × 10⁵ and 1.3 × 10⁵ (mg kg⁻¹ d⁻¹)⁻¹, respectively.^14,18

The total carcinogenic risk for onsite workers was the sum of the risks associated with each exposure route:

CR = CR_{accidental ingestion} + CR_inhalation + CR_contact (5)

Under most regulatory programs, a CR value below 10⁻⁶ indicates negligible cancer risk, whereas a value between 10⁻⁶ and 10⁻⁴ suggests potential cancer risk, and a value above 10⁻⁴ is an indication of high-potential risk.^20–22

2.4.2 Non-carcinogenic risk assessment. The non-CR assessment was based on the Hazard Quotient (HQ) and Hazard Index (HI). For the risk of multiple PCDD/Fs via accidental ingestion, inhalation and dermal contact pathways, the HQ and HI were calculated as in eqn (6) and (7).^14,23,24where RfD is the reference dose concentration, and the value of RfD was 7 × 10⁻¹⁰ mg kg⁻¹ d⁻¹.^14,18

HI = HQ_{accidental ingestion} + HQ_inhalation + HQ_contact (7)

Under most programs, the exposed population is unlikely to experience prominent non-carcinogenic effects for an HI value below 1, whilst adverse health risk may occur if its value exceeds 1.^23–25

2.5. Monte Carlo simulation

Monte Carlo simulation is a useful tool for health risk assessment. Since it considers the heterogeneity in populations and exposure parameters, Monte Carlo simulation could overcome most of the uncertainties in health risk assessment and has been widely used to assess the health risk of pollutants in various environmental matrices.^{14,27,29–31} It uses random number generation, rather than analytic calculations, thereby mining more data than summary statistics.³² In this study, Monte Carlo simulation was operated based on Crystal Ball 7.2 software. According to the stability of the model output, 50 000 iterations of the model run was selected for the present study.

3. Results and discussion

3.1. Mass concentrations, monthly changes, composition profiles and levels of PCDD/Fs in fly ash from MSWI for sampling months

The concentrations of seventeen PCDD/Fs in fly ash from MSWI over 7 months are shown in Fig. 1. The total concentration of PCDD/Fs (TPCDD/Fs) in different months varied from 36.5 to 5653.0 ng g⁻¹ with a median value of 823.5 ng g⁻¹. The values of TPCDD/Fs in October, November and January were much higher than that in the other months. However, the monthly changes in TPCDD/Fs did not have evident regulation and significant difference. The TPCDD/Fs of each month in decreasing order were as follows: November (5653.0 ng g⁻¹) > January (5319.0 ng g⁻¹) > October (4029.0 ng g⁻¹) > April (823.5 ng g⁻¹) > February (115.1 ng g⁻¹) > March (98.7 ng g⁻¹) > December (36.3 ng g⁻¹). Although the composition of municipal solid waste did not change over the sampling period, a wide variation of mass concentration of PCDD/Fs occurred.

Fig. 1 The monthly changes of PCDD/Fs (ng g⁻¹ dry weight) in fly ash from municipal solid waste incinerator (MSWI) plant in Northeast China.

Among the individual PCDD/Fs tested, 2,3,4,7,8-P₅CDF was the dominant one. The concentration of 2,3,4,7,8-P₅CDF ranged from 2.2 to 910.0 ng g⁻¹, with a mean concentration of 341.2 ng g⁻¹, and it accounted for 14.8% of TPCDD/Fs. In addition, the concentrations of 1,2,3,4,6,7,8-H₇CDF, O₈CDD, 1,2,3,4,6,7,8-H₇CDD, 1,2,3,7,8-P₅CDF, 1,2,3,4,7,8-H₆CDF, 1,2,3,6,7,8-H₆CDF and 1,2,3,7,8,9-H₆CDF were also higher than the other PCDD/Fs. The sum of these species accounted for 79.6% of TPCDD/Fs.

Compared with our previous study, in which we studied the daily changes of PCDD/Fs in fly ash from a MSWI in Eastern China, the composition profiles of PCDD/Fs in the present study have significant differences.¹⁴ 1,2,3,4,7,8-H₆CDF, 1,2,3,4,6,7,8-H₇CDD and 1,2,3,4,6,7,8-H₇CDF were the dominant PCDD/Fs in fly ash from our previous study and their sum accounted for 45.4% of TPCDD/Fs. However, the contribution of these congeners to TPCDD/Fs in this case was quite limited. This could be due to significant differences between the two MSWI that resulted in different formation mechanisms operating in each case.

In addition, there was no evident difference between the contributions of the dominant congeners to TPCDD/Fs in different sampling months. Moreover, the TPCDD/Fs in fly ash in the present study were much higher than that in the previous studies (Such as Chen et al., 2008:³³ 19.2–236 ng g⁻¹, Wu et al., 2016:¹³ 25–450 ng g⁻¹ and Shin and Chang, 1999:³⁴ 8–2116 ng g⁻¹).

3.2. Toxic equivalent concentrations, monthly change, composition profiles and levels of PCDD/Fs in fly ash from MSWI plant for sampling months

The toxic equivalent concentrations (TEQ) of PCDD/Fs in fly ash from MSWI plant are shown in Fig. 2 based on I-TEQ. The results suggested that TEQ ranged from 3.2 to 800.1 ng TEQ g⁻¹ with a median concentration of 111.1 ng TEQ g⁻¹.

Fig. 2 The monthly changes of toxic equivalent concentrations (TEQ) PCDD/Fs (ng TEQ g⁻¹ dry weight) in fly ash from municipal solid waste incinerator (MSWI) plant in Northeast China.

The value of TEQ of PCDD/Fs in fly ash in sampling months varied according to the following order: November (800.1 ng TEQ g⁻¹), January (671.1 ng TEQ g⁻¹), October (539.9 ng TEQ g⁻¹), April (111.1 ng TEQ g⁻¹), February (18.1 ng TEQ g⁻¹), March (9.1 ng TEQ g⁻¹) and December (3.2 ng TEQ g⁻¹). Although the concentrations of 1,2,3,4,6,7,8-H₇CDF, O₈CDD, 1,2,3,4,6,7,8-H₇CDD, 1,2,3,7,8-P₅CDF, 1,2,3,4,7,8-H₆CDF, 1,2,3,6,7,8-H₆CDF and 1,2,3,7,8,9-H₆CDF were much higher than other those of the PCDD/Fs and contributed more than 64.8% to TPCDD/Fs; their total contribution to TEQ was quite low (24.0%). 2,3,4,7,8-P₅CDF was the dominant PCDD/Fs for TEQ and accounted for about 55.7% of its value.

According to the above discussion, we can see that the concentrations of PCDD/Fs (both mass concentration and TEQ) changed intensely over the sampling period. This phenomenon was also widely reported in previous studies. Du et al.³⁵ found that the total concentration of PCDD/Fs in fly ash from a medical waste incinerator varied over a range of 440–3800 ng g⁻¹. Our previous study found that the PCDD/F concentrations in fly ash from an MSWI changed intensely on a daily basis.¹⁴ Pan et al.³⁶ indicated that the toxic equivalent concentrations (TEQ) of PCDD/Fs from MSWIs ranged from 34 to 2500 ng WHO-TEQ g⁻¹. Liu et al.³⁷ also found that the PCDD/F concentrations varied widely in fly ash samples from an MSWI. This phenomenon may be explained by the instability of the incineration systems used by the waste incineration plants in China.³⁵

Formation of PCDD/Fs was a complicated physico-chemical process that was influenced by a number of factors. Previous study suggested that the quantity of PCDD/F synthesis was proportional to the gasifiable fraction of residual carbon and the content of Cu in the fly ash of MSWI.³⁸ A recent study indicated that aluminas and aluminosilicates were active catalytic surfaces in the formation of PCDD/Fs in fly ash.³⁹ A more detailed and systematic research about the time-dependent changes in PCDD/F formation could help us to develop a good understanding of the abovementioned phenomena.

3.3. The health risk assessment

In health risk assessment processes, the highest values of parameters may be overestimated due to simulations that cover all possible scenarios. Thus, the 95th percentile value was used instead of the maxima value.

3.3.1 The carcinogenic risk assessment. The total CR values of PCDD/Fs exposure through three pathways (accidental ingestion, inhalation and dermal contact) for onsite workers were shown in Fig. 3. The total CR of PCDD/Fs ranged from 1.79 × 10⁻¹² to 1.09 × 10⁻⁶ with a mean value of 9.00 × 10⁻⁹. The 95th percentile CR value was 2.73 × 10⁻⁸, which was much lower than the threshold value (10⁻⁶), suggesting that there was no potential cancer risk.

Fig. 3 Predicted probability density functions of total carcinogenic risk (CR) via three pathways for onsite workers.

3.3.2 The non-carcinogenic risk assessment. Based on the HI and RfD, the total non-CR of seventeen PCDD/Fs via three pathways was evaluated for onsite works and the results are shown in Fig. 4. The total non-CR values ranged from 1.97 × 10⁻⁸ to 1.20 × 10⁻², with a mean value of 9.88 × 10⁻⁵. The 95th percentile non-CR value was 3.00 × 10⁻⁴, which was lower than the threshold value (1), suggesting no evident non-carcinogenic effects.

Fig. 4 Predicted probability density functions of total non-carcinogenic risk (non-CR) via three pathways for onsite workers.

3.4. The contribution of individual exposure pathways to total health risk

3.4.1 The contribution of individual exposure pathways to total carcinogenic risk. The CR of PCDD/Fs in fly ash form MSWI plant for onsite workers from the individual exposure pathways (accidental ingestion, dermal contact and inhalation) was summarized to determine the key exposure pathway. The 95th percentiles CR for onsite workers via different exposure pathways are shown in Fig. 5.

Fig. 5 The carcinogenic risk (CR) of individual exposure pathways for teens and adults.

95th percentiles CR via accidental ingestion was 1.79 × 10⁻⁸, whereas those for inhalation and dermal contact were 5.73 × 10⁻⁹ and 2.82 × 10⁻⁹, respectively. The CR for accidental ingestion was 3.12 and 6.35 times higher than that of inhalation and dermal contact, respectively. Thus, accidental ingestion was the most important pathway for CR, contributing more than 67.7% to the total CR, followed by inhalation and dermal contact.

Earlier investigations into exposure to persistent organic pollutants (POPs) in fly ash and soils showed that the health risk via ingestion was much higher than that through dermal contact and inhalation.^13,27 The present study was also consistent with previous studies about the exposure of PCDD/Fs in some ways. Previous studies indicated that 1–5% of PCDD/Fs (mass concentration) present in human body occurred via inhalation and 0.5–2% through skin.^40,41 Moreover, the most dangerous was the oral route including ingestion of soil, water, food and fly ash. Due to the onsite workers' long-term exposure to PCDD/Fs in a higher dosage and frequency than the non-occupational population, their health risk requires special attention. As we know, proper personal protection equipment can effectively reduce the exposure to pollutants for onsite workers in MSWI plants. In the present study, the exposures of PCDD/Fs in fly ash via accidental ingestion and inhalation were much higher than that of dermal contact, suggesting that filter-type gas masks could be an effective personal protection equipment for onsite workers. Not only could they effectively reduce exposure to contaminants via ingestion and inhalation of fly ash, but also reduce exposure to contaminated soil and ambient air.

3.4.2 The contribution of individual exposure pathways to total non-carcinogenic risk. The 95th percentile values for non-CR of onsite workers for different exposure pathways are shown in Fig. 6.

Fig. 6 The non-carcinogenic risk (non-CR) of individual exposure pathways for onsite workers.

The 95th percentile value for non-CR via accidental ingestion for onsite workers was 1.97 × 10⁻⁴, whereas those for inhalation and dermal contact were 5.90 × 10⁻⁵ and 3.10 × 10⁻⁵, respectively. The values for non-CR via accidental ingestion were 3.34 and 6.35 times higher than those of inhalation and dermal contact, respectively. Thus, accidental ingestion was also an important pathway for non-CR, contributing more than 68.6% to the total non-CR, followed by inhalation and dermal contact. According to the above discussion, we can see that filter-type gas masks could also effectively reduce the non-CR of contaminants from fly ash via ingestion and inhalation for onsite workers.

4. Conclusions

In the present study, composition profiles, levels, monthly changes and health risks of PCDD/F in fly ash from a MSWI plant in Northeast China were studied. The TPCDD/Fs and TEQ of PCDD/Fs in fly ash for the sampling months were 36.5–5653.0 ng g⁻¹ and 3.2–800.1 ng TEQ g⁻¹, respectively. The 95th percentile CR and non-CR for onsite workers were much lower than the threshold value values of 10⁻⁶ and 1.0, respectively, suggesting no potential health risk. Accidental ingestion was the largest contributor to total CR and non-CR, at more than 67% of the total health risk. The exposures of PCDD/Fs in fly ash via accidental ingestion and inhalation were much higher than that of dermal contact, suggesting that filter-type gas masks could be an effective personal protection equipment for onsite workers. Not only could the masks effectively reduce exposure to contaminants via ingestion and inhalation of fly ash, but also reduce exposure to contaminated soil and ambient air. The results could be useful for health risk management of MSWI plant workers.

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The samples were tested by Jiangsu Levei Testing Company Limited of China. In addition, our study was supported by the Science and Technology Development Fund of Wuxi City (CES12N1605), the National Natural Science Foundation of China (81472920 and 81402626) and the Jiangsu Province Postdoctoral Science Foundation Grant (2015M571811 and 1402175C).

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