Medium- and Short-Chain Chlorinated Paraffins in Mature Maize Plants and Corresponding Agricultural Soils

Abstract

For the most complex artificial chlorinated environmental contaminants, much less is known for medium-chain CPs than short-chain CPs. In this research, the spatial distributions of MCCPs and SCCPs in farmland soil and maize leaves near a CP production facility were found marginally influenced by seasonal winds. The levels of ∑MCCPs and ∑SCCPs were in the ranges of <1.51–188 and 5.41–381 ng/g dw for soils; and 77.6–52930 and 119–61999 ng/g dw for maize leaf, respectively. Bioaccumulation and tissue distributions of the CPs within maize plants were specifically analyzed. Most of the CPs were contained in the tissues directly exposed to airborne CPs. Though the estimated risk of CPs to humans through ingestion of kernels appears to be minimal, the edible safety of MCCPs in maize plants for cattle was nearly in the designated range of adverse effects. To our knowledge, this is the first report on bioaccumulation of CPs in mature maize plants, especially in the parts eaten by humans and domestic animals. It provides a baseline reference to the edible risks of CPs in agricultural food plants and alerts us to the problematic environmental behavior of MCCPs, a probable future replacement for SCCPs commercially.

Graphical Abstract

1. INTRODUCTION

Chlorinated paraffins (CPs) include a range of chlorinated n-alkanes divided into short-, medium-, and long chain chlorinated paraffins (SCCPs (C₁₀–C₁₃), MCCPs (C₁₄–C₁₇), and LCCPs (C_≥18)) based on the carbon chain length. CP products, such as CP-42 and CP-52, are widely used as secondary plasticizers in rubber/paints/plastics/sealants, flame retardants, and metal cutting fluids, resulting in a great amount of CPs released into the environment and entering food webs.^1–3 Current research on CPs mainly focuses on SCCPs which show characteristics of persistence, long-range transport, bioaccumulation, and toxicity.^2,4–7 Studies have shown that SCCPs could be detected in various environmental and human samples, including air,^8,9 dust,^10,11 soil,^12,13 sluge,^14,15 biota,^16–18 human blood,¹⁹ breast milk, and maternal and cord serum.^20,21 Since SCCPs have been listed in Annex A of the Stockholm Convention in 2017,²² the production and usage of MCCPs as replacement chemicals is increasing. The total registered volume of MCCPs production in Europe was about 10000–100000 t/year, much higher than that of SCCPs (1000–10000 t/year).¹ Thus, research on the environmental fates and human-ecological risks of MCCPs is urgently needed.^14,23,24

China produces the greatest amount of CPs of any country and is also the biggest market. Annual production of CPs is more than 1 million tons in China which accounts for 15% of the total global production.¹ However, information on environmental occurrences, distribution, and exposure risks of MCCPs is still lacking. Plants are an important sink of organic pollutants and play an indispensable role on the adsorption/absorption of CPs. Our previous works showed that plants absorbed SCCPs from both soil solution and air by roots and leaves, respectively, and had bidirectional translocation pathways, upward from roots to shoots and downward from shoots to roots, for SCCPs within soybeans and pumpkins. Besides the accumulation process, a fraction of CP congeners in the plants carried out dichlorination, carbon chain decomposition, and hydroxylation metabolisms, etc. devoted by plants.^25–27 The reports that focused on plants’ accumulation of CPs showed that SCCPs in pine needles ranged from 3.03 μg/g lipid weight (lw) to 40.8 μg/g lw in the urban areas of Beijing.²⁸ MCCPs and SCCPs in the ambient coniferous leaves around a production plant were 337.8–3711.1 ng/g dry weight (dw) and 218.7–1742.2 ng/g dw.²⁹ Besides, MCCPs and SCCPs were highly accumulated at 4.36 μg/g wet weight (ww) and 3.54 μg/g ww in local lettuce from an e-waste recycling industrial park.²³

Maize is planted over the largest area of any crop in the world.³⁰ In addition, maize is one of the three major food crops in China. The total planting area and the total maize output of China ranked first in the world at 42 million hectares and 259 million tons, respectively, accounting for 35.9% and 39.2% of the total crop area and total grain output in 2018.³⁰ Maize kernels are used for food, edible oil, industrial processing for ethanol and sugar, and concentrated feed for animals. Straw of maize plants is widely used as bedding and feeding materials for cattle, sheep, and pigs in animal husbandry; further, straw incorporation would make CPs enter into soil and cause adverse effects on soil organisms and groundwater. Maize straws are often burned, and the ashes are returned to the soil. Directly and indirectly, they are returned to soil after usage.

A maize planting area surrounding a manufacturing facility which produces commercial CP-52 was selected for this research. The environmental levels and spatial distributions of MCCPs and SCCPs in soil and mature maize samples from this area were determined. Bioaccumulation and tissue distribution of CPs in parts of individual whole maize plants were selected to understand the environmental fate of the target compounds within the mature maize. Environmental and human exposure risks were assessed through dietary intake of kernels by humans and from incorporation of straw and the kernel intake by domestic animals. To our knowledge, this is the first research focusing on the accumulation and translocation behaviors of MCCPs by maize in the edible portion and other portions of the plants. Simultaneously, we compared environmental processes in maize of MCCPs and SCCPs. Results show that MCCPs may pose risks greater than the SCCPs which they are replacing.

2. MATERIALS AND METHODS

2.1. Chemicals and Reagents.

Ultra resi-analyzed grade n-hexane, pesticide-grade dichloromethane, and cyclohexane were purchased from J. T. Baker (Phillipsburg, NJ). Standard stock solutions of MCCPs and SCCPs with different chlorine contents (C_14–17 42.0%, 52.0%, 57.0% and C_10–13 51.5%, 55.5%, 63.0%, 100 ng/μL in cyclohexane) and injection standard of ε-hexachlorocyclohexane (ε-HCH in cyclohexane, 10 ng/μL, 99.9%) were obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany). ¹³C-1,5,5,6,6,10-Hexachlorodecane (100 ng/μL, >98%) was purchased from Cambridge Isotope Laboratories (Andover, USA). Supelclean ENVI-Carb SPE columns (0.5 g, 6 mL) were acquired from Supelco (Bellefonte, USA). Silica gel 60 (1.063–0.100 mm) for column chromatography was obtained from Merck (Darmstadt, Germany) and activated at 550 °C for 12 h. Anhydrous sodium sulfate was heated at 660 °C for 6 h before use.

2.2. Sampling and Extraction.

A total of 31 sampling sites were designated within 5 km around a CP-52 manufactory (100 thousand tons annual production capacity) located in Liaocheng city of Shandong province. Surface farmland soil and leaves of maize (0–10 cm in length) were sampled from all sites (black dots in Figure 1) in September 2017 to study the occurrence and spatial distributions of CPs around the point-source atmospheric emission. Whole mature maize plants were sampled from five sampling sites (4, 10, 17, 21, 25 shown as red circles) which were located in different directions all around the manufacturing facility to study the effects of the point source on the distributions and accumulations of CPs in different maize tissues. Considering the great possibility that CPs on the particles deposited on the outer tissues (leaf, outer peel, etc.) may get partitioned with outer tissues and be subsequently translocated into the inner tissues, and also that CPs absorbed from the air or soils may be translocated to the inner tissues, whole single mature plants (plant heights ranged 1.8–2.3 m) were divided into several parts for analysis, as shown in Figure S1, including the tassel, fruit, leaf, stalk, and root. The leaf and stalk samples were subdivided into leaf 1–4 and stalk 1–4 from top to bottom. Each of the stalk samples was further divided into outside stalk, phloem, and xylem parts. The maize fruit were further determined into outer peel (the outermost layer of peel contacting with air directly), inner peel (only including the peel that covered by outer peel and not contacting with air directly), kernel, and corncob samples. For the spatial distribution study, leaf samples (same as leaf 2 in Figure S1) were collected from all sampling sites, and the leaf sample from one maize was aggregated as a sample for each site. All plant samples were washed with pure water, freeze-dried, and ground with a fine pulverizer. Farmland soil samples were dried in the shade, hand ground, and then passed through a stainless steel 80-mesh sieve (0.180 mm). The sample container with the pulverizer, pestle and mortar, and other tools used in sample preparation were all rinsed with methanol between samples. All dried samples were stored at −20 °C before analysis.

Figure 1.

Map of sampling sites around the CP-52 manufacturing facility, blue circles represent the centers of CP-52 manufacturer, black circles represent sampling sites of leaf and soil samples, red circles represent the sampling sites of whole mature maize plants. The number in the figure is the number of the sampling sites.

After being spiked with 20 ng of ¹³C-1,5,5,6,6,10-hexachlorodecane, 5–10 g of soil samples or 0.3–10 g of maize plant samples were ultrasonically extracted with 45 mL of dichloromethane/n-hexane (1:1, v/v) for 30 min, and the extraction was repeated three times. The combined extract was successively purified by 6 g of anhydrous sodium sulfate, 3.5 g of 40% (w/w) acid silica gel, and Supelclean ENVI-Carb SPE column (0.5 g, 6 mL) which was preconditioned with 5 mL of dichloromethane and 5 mL of n-hexane in sequence. The sulfur-containing compounds in soils were eliminated by adding 2.0 g of activated copper powder. Then, the CPs were eluted by 15 mL of n-hexane from the SPE column. The eluate was gradually evaporated to near dryness with a nitrogen stream; to this was added 20 ng of ¹³C-ε-HCH, and the mixture was dissolved in 200 μL of cyclohexane for instrumental analysis.

2.3. Instrumental Analysis and Quantification.

All target compounds were analyzed by Agilent 7200 GC-QTOF (Agilent Technologies, Santa Clara, USA) installed with negative chemical ionization source (NCI). The separation was performed on a DB-5MS capillary column (30 m ⊆ 0.25 mm ⊆ 0.25 μm, Agilent, USA). The injection port was set at 280 °C, and the injection volume was 2 μL. The oven was set at initial temperature of 100 °C for 1 min, then increased to 160 °C at 30 °C/min, kept for 5 min, increased to 310 °C at 30 °C/min, and held for 22 min. Methane (>99.99%) and helium (>99.99%) were used as reagent and carrier gases, respectively. The TOF MS was operated in full scan mode within the m/z range 50–600 at 5 spectra/s.

The quantification of MCCPs and SCCPs was carried out using the method developed by Gao et al.³¹ The method significantly reduced the interference of mass overlaps between SCCPs and MCCPs congeners by extracting accurate masses with the high resolution of TOF (10000–15000), fitting to the minimum resolution (3000) of SCCPs and MCCPs quantitation and qualification fragments. Because of the poor sensitivity of ECNI-MS for lower-chlorinated CPs, a total of 48 CP congener groups which contain 5–10 chlorine atoms and 10–17 carbon atoms were selected for determination. As shown in Table S1, the first abundance isotope ion of [M-Cl]⁻ and the second abundance isotope ion of [M-Cl]⁻ were selected as the quantitative and qualitative ions for determination, respectively.

2.4. Quality Assurance and Quality Control.

All cleaned glassware was heated at 450 °C for 6 h and rinsed with dichloromethane three times before use. Procedural blanks were detected with every batch of 6–9 samples to monitor the background contamination. None of the detectable MCCPs and SCCPs were found in those procedural blanks. Spiking recoveries of MCCPs and SCCPs were 71.3–99.0% and 87.3–111% for soil, 69.0–81.1% and 75.7–85.9% for the maize leaf, 72.5–96.1% and 68.0–92.8% for stalk, 69.2–97.5% and 79.2–91.2% for root, and 88.8–123% and 76.9–104% for kernel samples, respectively, which indicated that the pretreatment process and quantitative method were feasible. The recoveries of the surrogate standard, ¹³C₁₀-1,5,5,6,6,10-hexachlorodecane, were from 63.9% to 116% (mean 91.4%), which was used to monitor method extraction recovery and eliminate the matrix effects. All the data were quantified by the surrogate standard. The lipid contents of different plant tissues were obtained using an extraction gravimetric method, and these are summarized in Table S2. The contents of total organic carbon (TOC) of the surface soils were detected using the high temperature combustion method with a total organic carbon analyzer (SSM-5000A, Shimadzu, Japan) and are shown in Table S3. The concentrations of CPs in soil and leaf samples from 31 sampling sites were reported on a dry weight (dw) basis. The concentrations of CPs in different tissues of the whole maize plants (tissue distribution, from 5 sampling sites) were reported and normalized for lipid content. The method detection limits (MDLs) of soil and plant samples (including root, leaf, stalk and fruit, etc.) were 1.51 and 1.98–32.3 ng/g dw (or 51.7–619 ng/g lipid weight (lw)) for MCCPs (mixtures of 52.0% Cl and 57.0% Cl standards) and 1.08 and 1.61–11.5 ng/g dw (or 42.0–221 ng/g lw) for SCCPs (mixtures of 57.0% Cl and 63.0% Cl standards), respectively.

2.5. Statistical Analysis.

Agilent MassHunter Qualitative/Quantitative Analysis B.07 and Microsoft Excel 2016 were used for data analysis. Origin 2018 software (OriginLab Corp., USA) and SPSS Statistic 25.0 software (IBM Corp., Armonk, NY, USA) were used for plot drawing and statistical analysis. Statistical differences of the concentrations of CPs between maize leaves and soils or among different maize tissues were conducted with Friedman test (p < 0.05). Correlations between CPs concentrations and distances of the sampling sites to the CP-52 manufacturing facility were evaluated with Spearman’s tests (p < 0.05). To compare distributions of different congener profiles in different samples, a partial least-squares discriminant analysis (PLS-DA) and a principal component analysis (PCA) were carried out with SIMCA-P13.0 (Umetrics, Sweden). The PLS-DA model was validated with the permutation test to avoid overfitting.

3. RESULTS AND DISCUSSION

3.1. MCCPs and SCCPs in Farmland Soils.

Several researchers have shown that CPs are semivolatile organic compounds and can be transferred to the surrounding environment by atmospheric diffusion and deposition.^28,32,33 Therefore, the manufacturing facility was considered to have a large influence on CP contamination around the point source. The concentrations of ΣMCCPs, ΣSCCPs, and ΣCPs (total concentration of MCCP and SCCP and the concentrations of different congener groups) in farmland soils of the tested area around the CP-52 manufacturing facility (shown in Figure 2, Table S3, and Table S5) were in the ranges of <1.51–188 ng/g dw (geometric mean 8.52 ng/g dw), 5.41–381 ng/g dw (geometric mean 28.4 ng/g dw), and 7.22–509 ng/g dw (geometric mean 44.0 ng/g dw), respectively, lower than those in the industrial surface soils (MCCPs, 19.3–1461 ng/g dw and SCCPs, 24.8–482 ng/g dw) reported by Xu et al.²⁹ In addition, the concentration of MCCPs was significantly lower than SCCPs in tested farmland soils (p < 0.05) in contrast to some other reports in which MCCPs showed higher concentrations than SCCPs in soils.^29,32 The ratios of MCCPs/SCCPs of widely used CP-52 products in China are in the range of 1.6–8.2,³⁴ while the ratio of MCCPs/SCCPs in the farmland soil measured here was 0.301, significantly different from the main product ratio of the CP-52 factory. Therefore, the CPs congeners with different carbon chain length (C_14–17 for MCCPs and C_10–13 forSCCPs) and chlorine atom numbers (Cl_5–10) were further explored. Short chain length congeners tended to be dominant in the farmland soils (Figure S2a), namely, C₁₄ congeners were the dominant MCCPs (23.8% of the total CPs) and C₁₀ congeners were the majority of SCCPs (53.4% of the total CPs), respectively. Chlorine atom numbers Cl₈ and Cl₇ were dominant congeners of MCCPs in soils, but for SCCPs, the percentage of less chlorinated Cl₆ and Cl₇ congeners were greatest.

Figure 2.

Box-whisker-plots of ΣMCCPs and ΣSCCPs concentrations in farmland soils and maize leaves from all sampling sites around the CP-52 manufactory. Large rectangles represent 25–75% concentrations of MCCPs or SCCPs, bold lines represent the medians, small squares represent the average value, and diamonds indicate outliers.

These results indicated that there were other factors affecting the levels of CPs in the soils of the test area besides the manufacturing facility. For example, metabolism by soil microbes and crop root exudates would decrease the total amounts of CPs and improve the percentage of congeners with shorter chain length and less chlorine atoms.³⁵ And other possible CPs pollution might be introduced with the continual cultivation activities, such as applying pesticides, agricultural films, fertilizer, and irrigation water.³⁶ The maximum level of CPs (509 ng/g dw, site 25) in farmland soil was found in a sample near the road, likely explained by multiple sources, including release of CPs by the manufacturing facility through atmospheric transportation and deposition but also other pollution sources such as tire rubber frictional dust emanating from the road.³⁷

3.2. MCCPs and SCCPs in Maize Leaf.

For the maize leaf samples, as described in Figure 2, Table S4, and Table S5, the concentrations of ΣMCCPs, ΣSCCPs, and ΣCPs were in the ranges of 77.6–52930 ng/g dw (geometric mean; 551 ng/g dw), 119–61999 ng/g dw (geometric mean; 381 ng/g dw), and 211–114929 ng/g dw (geometric mean; 960 ng/g dw), respectively, in which two extremely high concentrations of CPs in maize leaves (114929 ng/g dw of site 26 and 47770 ng/g dw of site 5) were included for geometric mean analysis. The concentrations of MCCPs and SCCPs in maize leaves herein were comparable to those in coniferous leaves obtained around another factory (MCCPs, 337.8–3711.1 ng/g dw and SCCPs, 218.7–1742.2 ng/g dw) and the SCCP levels in pine needles (11.8 μg g⁻¹ lw), and they were an order of magnitude lower than those in mustard (148 μg g⁻¹ lw and 291 μg g⁻¹ lw on mean) sampled from a mega E-Waste recycling industrial park.^23,29

MCCPs were significantly higher than SCCPs (Friedman test, p < 0.05) in maize leaves. Results showed that the geometric mean concentration of MCCPs in maize leaves was about 2 times higher than those of SCCPs, which agrees roughly with the ratios of MCCPs/SCCPs of CP-52 products in China (1.6–8.2).³⁴ However, the MCCPs/SCCPs ratio of maize leaves was greatly different from that of soil samples (0.301). The profiles of CP congeners in maize leaves showed that C₁₄ MCCP congeners were the dominant CP homologues in all leaf samples which accounted for over 40% of the total CPs, followed by C₁₅ congeners (13.7%), while C₁₆ and C₁₇ congeners only accounted for less than 1% (Figure S2b). For SCCPs in leaf samples, the average percentage of C₁₀-CPs congeners was 14.8% of the total CPs, a little bit higher than C₁₃ (10.8%) and C₁₁ congeners (10.1%), and the percentage of C₁₂ congeners was the lowest (5.19%). Similar to the soil, the predominant composition of chlorine atoms in both MCCPs and SCCPs were Cl_7–8 congeners, while the percentage of Cl₆ congeners was lower than that for soil samples.

Media variables testing showed that CP concentrations in maize leaves were significantly higher than those in soils (p < 0.05). The geometric mean concentrations of MCCPs and SCCPs in maize leaves were 60 and 10 times higher than those in corresponding farmland soils, respectively. Plants have a great ability to accumulate contaminants from air, water, and soil.^38–40 Leaves were reported to play important roles in adsorbing/absorbing semivolatile compounds from atmosphere.^29,41–43 With the similar MCCPs/SCCPs ratios in maize leaves and the industrial product and the much greater CP levels in maize leaves than in soil, it follows that the CPs detected in leaves mainly originated from the accumulation of atmospheric CPs diffused from the factory. The contribution of CPs translocated from the roots after uptake from soil could not be excluded, but this contribution is thought to be minor. Meanwhile, the two sites (sites 26 and 5) showing extremely high concentrations of CPs in maize leaves were likely affected by other CP pollution besides the manufacturing facility, such as the anthropogenic influence with the use of domestic polymeric products (plastics, rubber, and agricultural films, etc.) by surrounding residents and agricultural activities with farm equipment (diesel vehicles and their tire, etc.) since sites 5 and 26 were located close to a small residential village and an agricultural path, respectively.^29,34,37 In addition, the non-homogeneous diffusion of CPs from a manufacturing facility might be also one of the reasons.

PLS-DA is performed to evaluate the profiles of 48 CP groups (Figure 3a,b) and further describes the relationship between maize and soil samples. The PLS-DA model is validated with permutation test (n = 200), and the validated plot is shown in Figure 3c (R2 = (0.0, 0.199), Q2 = (0.0, −0.333)). The intercept of Y and Q2 was less than 0.05, which means the model was fit and not overfitting. The ellipse in PLS-DA score plot represents the region of hotelling’s T2 test (95%). Results outside of the ellipse, including sample S₂₅ (soil of site 25 with the maximum soil level 509 ng g⁻¹ dw), L₅, and L₂₆ (leaf of site 5 and 26 with extremely high levels of 14929 ng g⁻¹ dw and 47770 ng g⁻¹ dw, respectively) were outliers, indicating that the congener compositions of these three samples were quite different from other samples. Namely, those sites had different origins of CPs from others, further implying the existence of other pollution sources. The soil samples and leaf samples were separated primarily by the hotelling’s T2 test (Figure 3a), further illustrating that the congener compositions of the two types of samples are affected by different factors. Thus, the CPs which accumulated in maize leaves did not originate from the upward translocation after root uptake from soil, further indicating that the adsorption/absorption of airborne deposited CPs on maize leaves was the predominant source of their contamination.

Figure 3.

Score plot (a) and loading plot (b) of 48 CP congener groups in maize leaves (L₁–L₃₁) and farmland soils (S₁–S₃₁) with partial least-squares discriminant analysis (PLS-DA). The validated plot of PLS-DA with permutation test (n = 200) (c).

The great difference between maize leaves and corresponding soil might also be related to their accumulation over different lengths of time as well as the time variability in emissions from the factory. Maize leaves grew and sorbed airborne CPs for one season (several months), reflecting recent contamination by the factory into the surrounding environment. But the soil was a sink for CPs released from the factory over a period of years. Thus, soil samples reflected the comprehensive results of long-term impacts of pollution sources and the fate of CPs in soil, such as biotic transformations, volatilization, and other physical/chemical reactions.

Beside this, out of the 48 groups of compounds, C₈H₁₂Cl₆, C₉H₁₅Cl₅, C₉H₁₄Cl₆, and C₉H₁₃Cl₇ were also detected in several leaf samples without quantification because of an absence of standards. Details about the extracted ion chromatograms (EIC) of C₈–C₉ CPs congeners are shown in Figure S3. CP congeners with carbon numbers less than 10, such as CPs with 8 or 9 carbon atoms, were also reported recently in PVC products, sediments, and human serum.^20,37,44 Moreover, the toxicity of CPs has been reported to increase with an increase of number of chlorine atoms and with a decrease of number of carbon atoms.⁵ Therefore, those CPs with carbon numbers less than 10 might have more serious environmental and health risks and require more regulatory attention.

3.3. Spatial Distribution of MCCPs and SCCPs around the Manufactory.

Gaseous CPs emitted from the manufacturing facility can undergo atmospheric transport and be deposited by dry deposition (gas transfer) resulting in high sorption to plant tissues and soils. Particulate adsorbed CPs can be emitted and atmospherically transported to plant materials and soils by sedimentation and dry deposition. Both gaseous and particulate-adsorbed CPs can be transported by precipitation (wet deposition or rainout) to plant materials and (mainly) to soils. These processes determined the deposition of gaseous and particulate phases of CPs into plants and soils surrounding the manufacturing facility. The spatial distributions of MCCPs and SCCPs in farmland soils and maize leaves within 5 km distance around the factory are shown in Figure S4 (excluded are the CPs of S₂₅, L₅, and L₂₆). Maize plants were cultivated from April to October, 2017. The wind rose diagrams of the sampling area during the whole year of 2017 and during the period of maize growth (April to October) are shown in Figure S5a,b, which indicate that north and northeast wind, and south and southwest wind are the main wind directions. Concentration distributions of MCCPs and SCCPs in soil samples basically corresponded to the seasonal winds in that area. Relatively high concentrations of MCCPs and SCCPs occurred in the northeast and southwest directions of the factory. Relatively high concentrations of MCCPs (main congeners of CP-52 products) that occurred in maize leaves sampled northeast of the factory fitted well with the wind. But high leaf concentrations of MCCPs and SCCPs that occurred in the southeast area of the factory only corresponded to low frequency wind. The results illustrate that the wind exerted only marginal influence on the distribution of CPs.

The correlation and regression modeling conducted with Spearman’s tests and partial correlation analysis showed that no significant correlation existed between leaf concentrations of CPs (SCCPs or MCCPs) and the distance from the center of the CP-52 manufactory. However, the exponential regression curve (Figure S6) and the spearman correlation coefficient r (−0.51, p = 0.0055) showed a moderate negative relationship between MCCPs concentrations in soils and distances. And the correlation between SCCPs concentrations in soils and distance were not significant either (p = 0.087 > 0.05). Also, TOC contents of the soils were not correlated with the concentrations of MCCPs or SCCPs. In these results, CPs that were detected in maize leaves were mainly related to CPs in the gaseous phase and small partially to those CPs competitively partitioned, sorbed, and translocated into leaves from deposited particles. The leaf samples were washed to eliminate the effects of particulate-adsorbed CPs on the determination of leaf samples prior to pretreatment. However, CPs in soils were related to both gaseous and particulate-adsorbed CPs. CPs in the gas phase could be transported for a long-distance, leading to the observed spatial distribution of CPs in maize leaves which had no correlation with distance from the source within the few km where samples were collected. The CPs associated with particles, especially those CPs with long carbon chain length and higher chlorine degree that were likely adsorbed to particles, tended to be deposited into the soil; even those particles first deposited to leaf tended to be finally deposited to soil through wind and wet deposition. The soil near the manufacturing facility was mainly affected by the deposition of CPs on particles, and the spatial distribution of MCCPs in soil showed moderately negative relationships with the distance from the source.

3.4. Distribution of MCCPs and SCCPs in Whole Mature Maize Plants.

Different compartments of the whole maize plants were randomly sampled, and we explored the bioaccumulation, distribution, and translocation of MCCPs and SCCPs in maize. Total concentrations of MCCPs and SCCPs for different tissues of maize plants are shown in Figure 4, Table S6, and Figure S7. And the concentrations of different MCCPs and SCCPs congener groups are summarized in Table S7. The concentrations of MCCPs and SCCPs in the tassel, leaf, outside stalk, and outer peel, which directly contacted with air, were higher than other parts (Figure S7), suggesting that adsorption/absorption from atmosphere was the most important pathway for accumulation of CPs in maize in the tested area. This is consistent with the result obtained above. Concentrations of MCCPs in the leaf 4 and outside stalk 4 (lower part) were higher than those in the top leaves and outside stalks, indicating the direct effect of exposure times on the tissue concentrations of CPs (p < 0.05) (Figure 4). Furthermore, MCCPs and SCCPs in phloem and xylem were lower than those in leaves and outside stalks. For the fruit, the concentrations of MCCPs and SCCPs decreased significantly in the order of outer peel > inner peel > corncob > kernel. Higher concentrations in outer tissues and lower concentrations in inner tissues also supported the possibility to some extent that gaseous CPs and some CPs partitioned from the particles deposited on the outer maize tissues may sorb and translocate into the inner tissues. However, it was hard to evaluate the contributions of the translocation process from outer tissues to inner tissues to the distribution of CPs in the maize due to the existence of multiple uptake and translocation pathways in plants. ΣMCCPs and ΣSCCPs in maize kernels were very low, only 61.1 and 109 ng/g lw on average, indicating that the migration of CPs from root-stalk to kernels or from leave-stalk to kernels was quite weak. These results indicate that the straw incorporation of dry stalks and leaves may have caused a high amount of CPs to enter farmland because CPs were mainly accumulated in stalks and leaves.

Figure 4.

Tissue distributions of MCCPs and SCCPs in the whole maize plants.

Regarding the relationship of MCCPs to SCCPs, the average concentration of MCCPs was about 1.42–3.89 times higher than that of SCCPs in maize tissues which were in contact with the air directly, including the tassel, leaf, outside stalk, and outer peel (Figure S7). However, the concentrations of MCCPs were not significantly higher than those of SCCPs among the inner parts of phloem 1–4 and xylem 1–4 (p > 0.05), and MCCPs were lower than SCCPs in the corncob, kernels, and root. The ratio of MCCPs/SCCPs in roots was similar to that in soil and significantly different than that in maize leaves.

To further assess the translocation behaviors of CP congeners, PCA was used to describe the main characteristics of congener composition in different maize tissues (Figure 5). The average relative abundance (%) of carbon congeners and chlorine congeners of SCCPs and MCCPs in different maize tissues are shown in Figure S8. The CP congeners loading plot (Figure 5a) shows that higher chlorinated congeners with longer carbon chains cluster in the third and fourth quadrants, while CP congeners with lower degrees of chlorination and carbon chain length are in the first and second quadrants; especially, Cl_5–7 congers were dominant in the second quadrant. The congener compositions of kernels and corncobs, which did not contact airborne CPs directly, clustered in the second quadrant of Figure 5a,b with the dominant congeners of C₁₀-CPs accounting for over 60% of total CPs. Correspondingly, leaves, outside stalks, tassels, and outer peels clustered in the third and fourth quadrants in Figure 5a,b; xylems and inner peels were located mainly in the first and second quadrants. It appears that CP congeners in leaves and outside stalks may have the same origin, and the CPs in phloem, xylem, and inner peel were mainly transported from other tissues. Besides, as shown in the PCA scoring plot, roots were an outlier in hotelling’s T2 test region, showing a quite different congener composition from other plant tissues. But the congener profile of SCCPs in the root was similar to that of the corresponding soil sample (Figure S9).

Figure 5.

Loading plot (a) and score plot (b) of 48 CP congener groups in different tissues of the whole maize plants with principal component analysis (PCA), the C₁₇Cl₅ and C₁₇Cl₁₀ congeners were eliminated with an insignificant importance for catching main variance with the PCA model.

Hence, it was concluded that roots simply adsorbed/absorbed CPs from soils, and the CPs taken-up by roots from the soil did not affect the CPs of aboveground parts. CPs in phloem and xylem were mainly transported from leaves and outside stalks. However, the dominant congeners (lower chlorinated degrees and less carbon atoms) in the phloem, xylem, kernel, and corncob were different from those in the leaf and stalk, which were likely caused by translocation differences of CP congeners or by the dechlorination and carbon-chain decomposition degradation behaviors of CPs in maize plants.^25,26

3.5. Ingestion Safety Assessment.

In China, maize kernel is a popular staple food. Kernel and maize straw are also used as the main roughage forage for livestock farming and concentrated poultry feed. Huang et al. measured SCCPs (mean: 129 ± 4.1 ng g⁻¹ wet weight) and MCCPs (mean: 5.7 ± 0.59 ng g⁻¹ wet weight) in meat and meat products from 20 provinces of China.⁴⁵ They found that the concentrations of CPs were consistent with the geographical distribution of CP production plants, meaning that the CP production plants could impact local poultry meat. Due to the high concentrations of CPs in maize straw, the use of straw as field animal derived food may be an important contributing factor to CP exposure of poultry. Therefore, the ingestion safety of CPs in maize plants to surrounding residents and animals was assessed.

The resident and poultry daily exposures to SCCPs and MCCPs through dietary intake were calculated with estimated daily intake (EDI) equation, as eq,¹⁰

$${\text{EDI}}_{\text{diet }}=\frac{{\text{C}}_{\text{diet }}{\text{I}}_{\mathrm{R}}}{\text{BW}}$$ (1)

where C_diet(ng/g) was the concentration of SCCPs or MCCPs in kernels or maize plants, I_R was the mass of daily consumption (g/day), and BW was the body weight (kg).

For nearby (local) residents, maize kernel was taken as a part of their staple food. According to the recommended daily consumption of grain, staple food is 50–150 g/day (>6 years old) per Chinese dietary guidelines. For worst case conditions, 150 g/day was selected as a high level of daily consumption of maize kernel. Two groups of people, young people (6–18 years) and adults (>18 years) were separately considered to assess exposure risks. The mean and 95th percentile concentrations of CPs in kernels were used to assess the mean and the highest estimated daily intakes with the values of 4.17 ng/g dw and 5.40 dw ng/g for SCCPs and 2.34 ng/g dw and 5.27 ng/g dw for MCCPs, respectively. Average body weights of young people and adults were 37.1 and 70.0 kg. If SCCPs and MCCPs in kernels were assumed to be completely absorbed by the human body, exposure risks of residents around the factory to SCCPs and MCCPs through ingestion of kernels are shown in Table 1. The ingestion exposure levels of SCCPs and MCCPs were in the range of 16.9–21.8 and 9.46–21.3 ng/kg BW/day for young people, and 8.94–11.6 and 5.01–11.3 ng/kg BW/day for adults.

Table 1.

Estimated Ingestion Exposure Risks of SCCPs and MCCPs for Residents and Cattle in the Vicinity of the CPs Manufactory

Young people
	Cdiet (ng/g dw)		CdietIR (ng/day)		EDI values (ng/kg BW/day)		MOE values
	mean	max	mean	max	mean	max	mean	max
SCCPs	4.17	5.40	626	810	16.9	21.8	5.93 × 106	4.58 × 106
MCCPs	2.34	5.27	351	791	9.46	21.3	2.40 × 106	1.08 × 106
Adults
	Cdiet (ng/g dw)		CdietIR (ng/day)		EDI values (ng/kg BW/day)		MOE values
	mean	max	mean	max	mean	max	mean	max
SCCPs	4.17	5.40	626	810	8.94	11.6	1.12 × 107	8.64 × 106
MCCPs	2.34	5.27	351	791	5.01	11.3	4.59 × 106	2.04 × 106
Cattle
	Cdiet (ng/g dw)		CdietIR (μg/day)		EDI values (μg/kg BW/day)		MOE values
	meana	maxb	mean	max	mean	max	mean	max
SCCPs	187	832	935	4161	1.91	8.32	5.21 × 104	1.20 × 104
MCCPs	469	2917	2344	11720	4.69	23.4	4.90 × 103	9.81 × 102

^aThe mean concentrations of leave1–4, outside stalk1–4, phloem1–4, and xylem1–4.

^bThe average 95th percentile concentrations of leave1–4, outside stalk1–4, phloem1–4, and xylem1–4.

Margin of exposure (MOE), the standard method proposed by the European Food Safety Authority (EFSA), was used to evaluate the safety of ingestion exposure of CPs to residents. If the MOE value is less than 1000, the target compound is considered to lead to a potential health risk for humans. The equation was given as follows

$$\text{MOE}=\frac{\text{NOAEL}}{\text{EDI}}$$ (2)

where NOAEL means the level without observed adverse effects.

The NOAELs for SCCPs and MCCPs given in the European risk assessment are 100 mg/kg/d and 23 mg/kg/d. Then MOE values calculated for young people and adults through corn ingestion in our work ranged from 4.58 × 10⁶ to 5.93 × 10⁶ and from 8.64 × 10⁶ to 1.12 × 10⁷ for SCCPs, respectively, and 1.08 × 10⁶ to 2.40 × 10⁶ and from 2.04 × 10⁶ to 4.59 × 10⁶ for MCCPs, respectively. Thus, the MOE values indicated that exposure to CPs through the ingestion of kernels would not induce health risks in young people or adults.

For the animals, all the above-ground parts of the whole maize plants are generally ground up and used as food. Taking adult cattle (500 kg) as an example, the daily intake amount of food is about 2.5–3.0% of its weight, including 3–5 kg of roughage forage (straw in dry weight) and 3–4 kg of concentrated animal feed (kernels in dry weight). In comparison to the high concentrations of CPs in straws, the amount of CPs in kernel was negligible. So the concentration of CPs in leaves and stalks were used to evaluate the safety to cattle (Table 1). The MOE values of SCCPs (1.20 × 10⁴–5.21 × 10⁴) showed that the ingestion of maize was safe for cattle, but the MOE values of MCCPs (9.81 × 10²–4.90 × 10³) were on the edge of the safety limit, suggesting the potential for adverse effect to cattle. Several reports showed that SCCPs have quite low concentrations in broiler/rat meat, but they tend to be deposited in liver and fat, and can be eliminated through feces and biliary excretion. No adverse effects were calculated due to the high concentrations of SCCPs by oral exposure to broilers or rats (100 mg/kg BW, et al).^46–48 Our research indicates that CP production facilities do not cause ingestion exposure risks to surrounding residents but may have the potential for adverse effects on nearby cattle.

In summary, this is the first report focusing on the bioaccumulation and tissue distribution of MCCPs and SCCPs in mature maize plants around a CPs manufacturing facility. High concentrations of CPs, especially MCCPs, were found in maize straws and mainly originated from the accumulation of atmospheric CPs diffused from the factory. Though CPs in straw and maize kernels would not affect the health of human via direct ingestion, the possible adverse effects to cattle and other domestic animals should be considered. More work is needed to explore the problematic environmental behavior and risk assessment of CPs in the pollutant field due to their high concentrations with increasing trends in recent years, especially MCCPs, which are proposed future replacements for SCCPs commercially.