全氟羧酸化合物在氟化工園區(qū)土壤-植物中的分布及組成特征

伍兆誠, 王少銳, 江龍飛, 孫迎韜, 張 干, 羅春玲*

全氟羧酸化合物在氟化工園區(qū)土壤-植物中的分布及組成特征

伍兆誠1,2, 王少銳1, 江龍飛1, 孫迎韜1,2, 張 干1, 羅春玲1*

(1. 中國科學(xué)院 廣州地球化學(xué)研究所 有機(jī)地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州?510640; 2.中國科學(xué)院大學(xué), 北京?100049)

本次研究分析了典型氟化學(xué)工業(yè)園區(qū)周邊不同植物及其根際和非根際土壤中全氟羧酸(perfluorinated carboxylic acid, PFCA)的含量、組成及分布特征。結(jié)果顯示, PFCA在根際土壤和非根際土壤的濃度范圍分別為9.6～160 ng/g 和7.2～300 ng/g。根際土壤(rhizospheric soil, RS)和非根際土壤(non-rhizopsheric soil, NRS)中PFCA的濃度比值(ratio=RS/NRS)顯示, PFCA并未呈現(xiàn)出根際富集效應(yīng)。PFCA在植物地上部和根部的濃度范圍分別為 6.0～630 ng/g和6.1～570 ng/g。全氟辛酸(perfluorooctanoic acid, PFOA)是土壤和植物組織中最主要的全氟羧酸化合物, 其濃度在根際土、非根際土、植物根部和地上部分別占總PFCA的(53±18)%、(51±22)%、(80±6.7)%和(34±18)%。短鏈(C4～C8)的PFCA, 包括全氟丁酸(perfluorobutyric acid, PFBA)、全氟戊酸(perfluoropentanoic acid, PFPeA)、全氟己酸(perfluorohexanoic acid, PFHxA)、全氟庚酸(perfluoroheptanoic acid, PFHpA)和PFOA, 在植物地上部中的濃度及檢出率均高于長鏈(C9～C16)的PFCA, 說明短鏈PFCA比長鏈PFCA更容易在植物組織中富集。植物PFCA組成特征顯示, PFBA和PFHxA在植物地上部所占的比例高于根部, 說明PFBA和PFHxA等碳鏈較短的PFCA可能更容易被植物根系吸收并轉(zhuǎn)移至植物地上部; 然而PFOA在植物地上部的百分含量低于植物根部, 說明PFOA可能趨向于吸附在植物根系表皮而難以往植物地上部遷移。根部到地上部的轉(zhuǎn)運(yùn)系數(shù)與PFCA的碳鏈長度呈顯著負(fù)相關(guān)關(guān)系。

全氟羧酸化合物; 根際; 遷移

0 引 言

全氟羧酸(perfluorinated carboxylic acid, PFCA)是全氟烷基物質(zhì)和多氟烷基物質(zhì)(polyfluoroalkyl substances, PFAS)的一類, 由碳鏈中的氫原子被氟取代而成[1–2]。PFCA的特性(兩親性和碳氟鍵的穩(wěn)定性)使其在各種工業(yè)和商業(yè)環(huán)境中得到廣泛應(yīng)用, 例如, 表面活性劑[3]、防污劑和涂料等[4]。自1999年建立第一個(gè)氟化工園區(qū)起[5], 我國開始大規(guī)模生產(chǎn)PFCA, 導(dǎo)致PFCA 在各類環(huán)境介質(zhì)中均有殘留, 其中以全氟辛酸(perfluorooctanoic acid, PFOA)最為典型, 其在我國地表水[6]、飲用水[7]、海水[8]、土壤[9]、沉積物[10]和人體血清[11–15]等介質(zhì)中均有檢出。

土壤是環(huán)境PFCA的匯。土壤PFCA的主要來源包括大氣干濕沉降和地表徑流[16]。大氣中PFCA 主要來自于工業(yè)排放[17], 大氣干濕沉降過程將大氣顆粒物中的PFCA帶到陸地表面, 被土壤吸附[18–22]。由于大多數(shù)PFCA具有水溶性、兩親性和低揮發(fā)性, PFCA以水作為主要載體在環(huán)境中轉(zhuǎn)移擴(kuò)散[23]。污水處理廠等排放進(jìn)入地表徑流的PFCA通過地表徑流向下游傳輸, 由于其親油性在傳輸過程中被土壤吸附, 在世界各地水體周邊土壤和水體底泥等都能檢測(cè)到PFCA[11,24–29]。污水和污泥中被檢出高濃度的PFCA, 其在農(nóng)業(yè)生產(chǎn)工程中的施用也是PFCA進(jìn)入土壤的重要途徑[30–31]。土壤中的PFCA可通過植物吸收進(jìn)入生態(tài)圈, 進(jìn)而對(duì)人類健康產(chǎn)生威脅[32]。

土壤是PFCA在生態(tài)系統(tǒng)不同環(huán)境介質(zhì)中交換的重要周轉(zhuǎn)站, 如土壤-生物(植被)、大氣-土壤之間的界面交換和轉(zhuǎn)化。受植物根系活動(dòng)的影響, 根際土壤在物理、化學(xué)和生物學(xué)性質(zhì)上不同于非根際土壤, 這一動(dòng)態(tài)微區(qū)域是植物-土壤-微生物交互作用的場(chǎng)所, 令其成為土壤中污染物發(fā)生形態(tài)轉(zhuǎn)化、遷移運(yùn)輸及生物降解最為活躍的區(qū)域[33–35]。研究顯示PFOA前體物, 如8∶2 聚氟調(diào)醇(Fluorotelomeric ethanol, FTOH)等, 可在根際微生物的影響下進(jìn)一步轉(zhuǎn)化成PFOA, 加大其生態(tài)風(fēng)險(xiǎn)[36–37]。目前關(guān)于PFCA在根際圈的遷移轉(zhuǎn)運(yùn)過程鮮有報(bào)道。

關(guān)于PFCA在植物體內(nèi)吸收轉(zhuǎn)運(yùn)機(jī)理的研究顯示, 長鏈全氟羧酸(C9～C16的PFCA)容易富集于植物根部, 往地上部遷移能力弱, 表明長鏈全氟羧酸更易被吸附在富含脂質(zhì)的根表[38]。紫花苜蓿中PFCA的生物富集因子(bioaccumulation factor, BAF)與PFCA碳鏈長度呈顯著負(fù)相關(guān)關(guān)系[39]。小麥水培實(shí)驗(yàn)結(jié)果顯示, PFCA在小麥地上部、根部中濃度的比值與PFCA碳鏈長度呈顯著負(fù)相關(guān)關(guān)系, 并推導(dǎo)出小麥對(duì)PFCA的吸收主要通過由載體介導(dǎo)的主動(dòng)運(yùn)輸進(jìn)行[40]。然而, 這些研究均基于水培或盆栽等室內(nèi)模擬實(shí)驗(yàn), 在野外復(fù)雜環(huán)境下PFCA的吸收轉(zhuǎn)運(yùn)仍有待研究。

江蘇常熟新材料產(chǎn)業(yè)園是我國大型的氟工業(yè)園區(qū)之一, PFCA污染歷史較久[5]。因此本研究擬對(duì)江蘇常熟新材料產(chǎn)業(yè)園內(nèi)的土壤以及不同植物品種組織中PFCA的污染水平進(jìn)行調(diào)查, 通過分析PFCA在根際土壤、非根際土壤和植物根部、地上部的含量及組成特征, 旨在探索植物根際對(duì)PFCA土壤歸趨的影響以及植物吸收轉(zhuǎn)運(yùn)PFCA的潛在規(guī)律。本研究結(jié)果有助于了解PFCA在根際的遷移過程, 可為PFCA的污染控制和植物修復(fù)提供思路。

1 材料與方法

1.1 試劑與標(biāo)樣

甲醇(色譜純, Acros Organics); 乙酸銨(色譜純, Acros Organics); 碳酸氫鈉和碳酸鈉(≥98%, Sigma- Aldrich); 四丁基硫酸氫銨、甲基叔丁基醚(色譜純, Aladdin); 聚丙烯(polypropylene, PP)離心管(50 mL, Extragene Biotechnology); 玻璃器皿洗液(Chemical Products R. Borghgraef S.A.); Envi-Carb凈化柱(3 mL, 0.25 g, Supelco); 實(shí)驗(yàn)用水為超純水(Millipore), 其他未作特殊說明的試劑均為分析純。

回收率指示劑為13C4-全氟辛酸(Wellington Laboratories Inc.)。內(nèi)標(biāo)為13C8-全氟辛基磺酰胺(Wellington Laboratories Inc.)。目標(biāo)化合物包括12種全氟羧酸化合物(Wellington Laboratories Inc.): 全氟丁酸(perfluorobutyric acid, PFBA)、全氟戊酸(perfluoropentanoic acid, PFPeA)、全氟己酸(perfluorohexanoic acid, PFHxA)、全氟庚酸(perfluoroheptanoic acid, PFHpA)、全氟辛酸((perfluorooctanoic acid, PFOA)、全氟壬酸(Perfluorononanoic acid, PFNA)、全氟癸酸(perfluorodecanoic acid, PFDA)、全氟十一酸(perfluoroundecanoic acid, PFUnA)、全氟十二酸(perfluorododecanoic acid, PFDoA)、全氟十三酸(perfluorotridecanoic acid, PFTrDA)、全氟十四酸(perfluorotetradecanoic acid, PFTeDA)和全氟十六酸(perfluorohexadecanoic acid, PFHxDA)。

1.2 樣品采集

供試的土壤和植物樣品于2016年9月下旬, 采自江蘇省常熟市新材料園區(qū), 園區(qū)屬亞熱帶季風(fēng)性濕潤氣候。本研究在園內(nèi)農(nóng)田和綠化帶, 共采集了10種植物(采樣點(diǎn)分布見圖1, 采樣點(diǎn)經(jīng)緯度及其對(duì)應(yīng)植物種類信息見表1), 包括其對(duì)應(yīng)的根際土壤和非根際土壤。在7 m×7 m的區(qū)域內(nèi)采集了5個(gè)樣品混合成1個(gè)平行樣品(每個(gè)樣品包括植物、根際土壤和非根際土壤), 每個(gè)樣點(diǎn)收集３個(gè)平行樣品。

圖1 中國江蘇新材料產(chǎn)業(yè)園采樣點(diǎn)分布示意圖

表1 采樣點(diǎn)信息

采樣過程中, 先用鐵鏟將整株植物取出, 將附著根系上的土壤輕微按壓破碎, 去除部分遠(yuǎn)離根際土壤, 之后采用抖根法收集根際土壤; 非根際土壤采集于距離根系5～10 cm遠(yuǎn)處, 是未明顯受到根系影響的土壤。用錫箔紙包裹后裝入聚乙烯密封袋中, 并盡快帶回實(shí)驗(yàn)室。植物地上部(Shoot)和根部(Root)分別用自來水沖洗, 去除附著于葉片的顆粒及根系的土壤。然后用去離子水沖洗3次, 裝入聚乙烯袋子。所有土壤與植物樣品于?20 ℃冰箱冷凍過夜, 并于冷干機(jī)冷干72ｈ, 之后稱重; 土壤樣品在事先經(jīng)丙酮擦洗干凈的瑪瑙研缽中研磨, 過100目(147 μm)篩后用錫箔紙包裹后裝入聚乙烯密封袋, 于?20 ℃冰箱保存; 植物樣品經(jīng)丙酮擦洗干凈的研磨機(jī)中充分研磨至粉末狀, 用錫箔紙包裹后裝入聚乙烯密封袋, 避光于?20 ℃冰箱保存。

1.3 樣品前處理

土壤 土壤經(jīng)冷凍干燥48 h, 取0.2 g土壤樣品于50 mL 聚丙烯離心管中(使用前甲醇潤洗3次), 加12 mL甲醇, 將回收率指示劑(50 μL, 20 pg/μL)注射于溶液中, 攪拌混合, 靜止30 min后在超聲儀上超聲萃取。每次超聲萃取的時(shí)間為15 min, 每次超聲后于3000 r/min轉(zhuǎn)速下將土壤樣品離心5 min, 取上清液, 重復(fù)超聲萃取步驟2次。萃取液合并收集于50 mL 聚丙烯離心管內(nèi), 用溫柔N2濃縮至1～2 mL,然后將萃取液加載至用10 mL甲醇潤洗過的Envi- Carb柱過濾, 用10 mL甲醇洗脫, 洗脫液在溫柔N2下濃縮至130 μL, 然后轉(zhuǎn)移至裝好襯管的細(xì)胞瓶內(nèi)。細(xì)胞瓶密封時(shí), 加鋁箔層, 防止蓋子內(nèi)襯污染樣品, 在?20 ℃恒溫保存。

植物樣 將植物樣品凍干后研磨混勻, 稱取1 g樣品于15 mL 聚丙烯離心管中, 加入5 ng回收率指示劑, 加入4 mL 0.25 mol/L的碳酸鈉、碳酸氫鈉緩沖液, 1 mL 0.5 mol/L的四丁基硫酸氫銨(tetrabutylammonium bisulfate, TBAH)后, 渦旋混合, 再加入5 mL色譜純甲基叔丁基醚(methyl tertiary butyl ether, MTBE), 混合后250 r/min振蕩提取40 min,再4200 r/min離心10 min, 將上清液轉(zhuǎn)移至新的15 mL聚丙烯管中。向原聚丙烯管中加入5 mL MTBE溶液, 以上過程重復(fù)一次, 混合兩次提取液氮吹濃縮, 然后將濃縮液加載至用10 mL甲醇潤洗過的Envi-Carb柱過濾, 用10 mL甲醇洗脫, 洗脫液在溫柔N2下濃縮至130 μL, 然后轉(zhuǎn)移至裝好襯管的細(xì)胞瓶內(nèi)。細(xì)胞瓶密封時(shí), 加鋁箔層, 防止蓋子內(nèi)襯污染樣品, 在?20 ℃恒溫保存, 待測(cè)。

1.4 儀器分析

PFCA的定量使用高效液相色譜串聯(lián)質(zhì)譜(1200高效液相質(zhì)譜-6410三重串聯(lián)四極桿質(zhì)譜, Agilent Technologies)、電噴霧電離源和負(fù)離子模式進(jìn)行樣品定量分析。色譜柱選用Synergi Hydro RP 80Acolumn (150 mm × 2 mm × 4 μm)連接C18保護(hù)柱Synergi 2μ Hydro RP Mercury(20 mm × 2 mm × 2 μm)。流動(dòng)相為含10 mmol/L乙酸銨緩沖鹽的超純水和含10 mmol/L醋酸銨緩沖鹽的甲醇, 梯度洗脫條件程序?yàn)? 起始流動(dòng)相30%甲醇, 3 min后逐漸升高為70%甲醇, 4～33 min, 流動(dòng)相逐漸升高為100%甲醇, 34～40 min, 保持100%甲醇。流動(dòng)相流速為0.2 mL/min, 柱溫30 ℃, 進(jìn)樣量為10 μL。離子源參數(shù): 毛細(xì)管電壓3500 V; 毛細(xì)管出口電壓200 V; 霧化氣137.89 KPa干燥氣流量9.0 L/min; 離子源溫度350 ℃。

1.5 質(zhì)量控制與質(zhì)量保證

為了避免野外采樣期間的交叉污染, 土壤樣品被保存在聚乙烯密封袋中, 植物樣品被保存在3層密封紙袋中。為防止引入高背景值, 實(shí)驗(yàn)過程避免使用聚四氟乙烯材質(zhì)的色譜管路和器皿。本實(shí)驗(yàn)前處理過程中每9個(gè)樣品中添加1個(gè)程序空白。所有程序空白均低于檢出限(土樣的檢出限為0.002～0.01 ng/g,植物樣檢出限為 0.01～0.1 ng/g)。檢出限為信噪比(signal-noise ratio)大于3所對(duì)應(yīng)的濃度。平行樣品標(biāo)準(zhǔn)偏差(standard deviation, RSD)小于10%。在儀器分析過程中每9次進(jìn)樣后添加1個(gè)空白樣品(純甲醇), 以減少交叉污染并監(jiān)測(cè)儀器的背景污染。每10次進(jìn)樣后添加1個(gè)標(biāo)準(zhǔn)樣品(10 ng/mL), 監(jiān)測(cè)儀器漂移, 若偏離初始值±20%, 重新建標(biāo)準(zhǔn)曲線。所有PFCA的標(biāo)準(zhǔn)曲線回歸系數(shù)(2)均大于0.99。13C4-PFOA的回收率為72%～93%。

1.6 數(shù)據(jù)分析

在這項(xiàng)研究中, 土壤樣品和植物樣品中PFCA的濃度單位均基于干重, 文中引用參考文獻(xiàn)的PFAS濃度單位也均基于干重。本研究中平均值以“平均值±標(biāo)準(zhǔn)偏差”的形式表示, 引用文獻(xiàn)中部分沒能得到標(biāo)準(zhǔn)差。使用SigmaPlot 13.0 (Systat software)、OriginPro 2018 (OriginLab)和Excel 2016 (Microsoft)進(jìn)行統(tǒng)計(jì)分析和圖表繪制。通過相關(guān)性分析數(shù)據(jù)之間的聯(lián)系, 通過單因素方差分析PFCA在土壤和植物的分布差異, 統(tǒng)計(jì)顯著性閾值設(shè)為p<0.05。

1.7 根際土和非根際土分布趨勢(shì)指標(biāo)和根部到地上部的轉(zhuǎn)運(yùn)系數(shù)計(jì)算

植物對(duì)PFCA在根際土和非根際土分布的影響, 由PFCA在根際和非根際土壤總濃度的比值(ratio)表示。

式中,RS為PFCA在根際土壤的總濃度, 單位為ng/g,NRS為PFCA在非根際土壤的總濃度, 單位為ng/g。

PFCA從根部到地上部的轉(zhuǎn)移潛力, 用根部到地上部的轉(zhuǎn)運(yùn)系數(shù)(shoot to root concentration factor,SRC)進(jìn)行評(píng)估, 計(jì)算公式如下。

式中,Shoot為PFCA在地上部的濃度, 單位為ng/g,Root為PFCA在根部的濃度, 單位為ng/g, 。

2 結(jié)果與討論

2.1 PFCA在根際的分布及組成特征

如圖2所示, PFCA在根際和非根際的濃度范圍分別為9.6～160 ng/g和7.2～300 ng/g。其中紅葉石楠、棱果谷木、赤楠蒲桃、山指甲、九里香和小葉黃楊的根際及非根際土中PFCA的濃度高于其他植物。油菜、小麥、玉米和紫葉小檗的根際和非根際土壤中PFCA均低于30 ng/g。淄博市氟化學(xué)工業(yè)園區(qū)內(nèi)農(nóng)田非根際土中PFCA濃度范圍為1.8～640 ng/g, 最大總PFCA濃度高于本研究水平[41]。阜新市氟化工園區(qū)附近居民花園的非根際土PFCA總濃度為1.5～15 ng/g[42]。Lu[43]對(duì)國內(nèi)某典型氟化學(xué)工業(yè)園區(qū)蔬菜和水果的非根際土中PFCA含量調(diào)查結(jié)果顯示, 土壤中PFCA濃度范圍為16～64 ng/g, 該研究結(jié)果比本研究結(jié)果中經(jīng)濟(jì)作物和紫葉小檗的非根際土PFCA的污染水平(7.2～24 ng/g)稍高, 但遠(yuǎn)低于本研究的非根際土平均濃度。隨著距工業(yè)園區(qū)距離的增加, 土壤中的PFCA呈下降趨勢(shì)[44]。綜上, 我國氟化學(xué)等工業(yè)園區(qū)土壤中PFCA污染差異較大, 本研究區(qū)域土壤處于較高污染水平。

圖2 PFCA在根際和非根際土壤中的分布

Fig.2 Distribution of perfluorocarboxylic acids (PFCA) in rhizosphere soil (RS) and non-rhizosphere soil (NRS)

PFCA在根際和非根際土壤的濃度比值ratio如圖2所示?？傮w上看,ratio在不同植物品種之間的差異較大, 未呈現(xiàn)出一致的規(guī)律。小麥、玉米和山指甲的ratio大于1, 表明PFCA有從非根際土向根際土富集的趨勢(shì), 其他植物則相反。比如, 種植黑麥草的多環(huán)芳烴(polycyclic aromatic hydrocarbon, PAH)污染土壤中, 土壤PAH的濃度分布與離黑麥草根系的距離成反比, 離黑麥草根系越近, PAH的濃度越低[45]。此外, 有研究表明, 根際土中多氯聯(lián)苯(polychlorinated biphenyl, PCB)和PAH的含量要顯著低于非根際土, 這主要?dú)w因于根際微生物引起的生物降解。根際對(duì)土壤中的持久性有機(jī)污染物(persistent organic pollutants, POP)具有解吸或活化作用[34,46]。大量研究顯示, 根際環(huán)境能夠富集外源性化合物; 比如, Liste[47]發(fā)現(xiàn)菲(phenanthrene)和芘(pyrene)在植物根系刺激下, 能夠向根系表面轉(zhuǎn)運(yùn), 最后富集于根際圈中。Wang[48]和Wang[49]發(fā)現(xiàn)多溴聯(lián)苯醚和新型鹵代阻燃劑在14種植物根際土壤中的濃度要高于非根際土壤。PFCA在土壤中的吸附、解吸附以及植物根系的吸收可能受多種因素的影響, 如植物特征(根系脂質(zhì)含量、根系蛋白質(zhì)含量和根皮細(xì)胞膜結(jié)構(gòu)等)、環(huán)境因素(暴露濃度)和PFCA自身性質(zhì)(疏水性和鏈長等)等[50–55]。在野外采集的根際和非根際土壤可能受不同植物種類和不同土壤采集地點(diǎn)的差異影響, 因而沒有呈現(xiàn)一致的規(guī)律。

根際和非根際土壤中PFCA的組成特征基本一致(圖3), 根際和非根際土中各PFCA的濃度平均值呈顯著相關(guān)性(<0.01)。短鏈(C4～C8)為PFCA (PFBA、PFPeA、PFHxA、PFHpA和PFOA)的主要組成, 分別占根際土和非根際土中總PFCA濃度的(72±18)%和(67±24)%。其中, PFOA是最主要的PFCA單體, 其次是PFHxA、PFBA和PFPeA。PFOA在根際和非根際土壤的濃度范圍分別為4.8～118 ng/g和1.1～263 ng/g;其濃度占總PFCA濃度的34%～75%和6.9%～87%, 平均值為(54±18)%和(52±22)%, 高于常熟新材料產(chǎn)業(yè)園區(qū)及其周邊城鎮(zhèn)的農(nóng)業(yè)土壤(31%)[44], 低于山東氟化工園區(qū)周圍農(nóng)田土壤(83%)[56]。長鏈PFCA在根際和非根際土壤的濃度分別為0.93～78 ng/g和1.0～ 25 ng/g, 其中, PFDA、PFNA和PFTrDA是主要的長鏈PFCA單體。長鏈PFCA在根際和非根際土壤中分別占總PFCA的4.3%～61%和1.9%～54%, 平均百分含量分別為(28±18)%和(33±24)%。

2.2 PFCA在植物組織的分布及組成特征

如圖4顯示, PFCA在植物地上部的濃度范圍為6.0～632 ng/g。PFCA在植物中的組成特征顯示, 短鏈的化合物在植物地上部的濃度范圍介于6.0～526 ng/g之間; 而長鏈的化合物濃度范圍則介于n.d.～153 ng/g之間, n.d.為未檢出(not detected)。其中, PFOA是植物地上部中最主要的化合物, 其濃度范圍介于0.88～263 ng/g之間, 平均占地上部PFCA總量的(34±18)%, 而PFBA、PFPeA、PFHxA和PFHpA亦是含量相對(duì)較高的化合物, 分別占地上部PFCA的(27±15)%、(3.6±4.6)%、(20±13)%和(4.7±2.7)%。檢出率(detection frequency, DF)結(jié)果顯示, 短鏈PFCA在植物地上部的檢出率高于長鏈PFCA, 比如PFOA、PFBA、PFHxA和PFHpA的檢出率均為100%, 而PFUnA、PFDoA、PFTrDA和PFTeDA等的檢出率僅為50%左右。

PFCA植物根系中的濃度范圍為6.1～572 ng/g。其中, 短鏈化合物在植物根部的濃度范圍介于6.1～541 ng/g之間, 與地上部濃度相近。而長鏈化合物濃度范圍則為n.d.～31 ng/g, 小于地上部。與植物地上部相似, PFOA仍是植物根部中含量最高的化合物, 其濃度范圍為4.4～524 ng/g, 平均占根系PFCA濃度的(80±6.7)%, 遠(yuǎn)高于地上部的占比。然而, 與植物地上部不同, PFTeDA和PFHxDA等長鏈PFCA并未在植物根部檢出。短鏈PFCA在植物根部的檢出率依舊高于長鏈PFCA。比如, PFBA、PFHxA、PFHpA和PFOA的檢出率均大于90%; 而PFNA、PFUnA和PFTrDA的檢出率則低于50%。

圖3 PFCA在根際和非根際土壤的組成特征

Fig.3 Composition of perfluorocarboxylic acids (PFCA) in rhizosphere soil and non-rhizosphere soil

圖4 PFCA在植物組織中的組成特征

經(jīng)濟(jì)作物和景觀植物的檢出率對(duì)比見表2?？傮w而言, PFCA在景觀植物中的檢出率大于經(jīng)濟(jì)作物。對(duì)于短鏈PFCA (C4～C8), 兩者的檢出率差異不大。然而, 景觀植物中檢出了長鏈PFCA (PFNA、PFDA、PFUnA、PFDoA、PFTrDA和PFTeDA), 但在經(jīng)濟(jì)作物中卻未檢出。一方面原因可能是經(jīng)濟(jì)作物對(duì)短鏈PFCA 的優(yōu)先吸收及轉(zhuǎn)運(yùn)[55]; 另一方面, 長鏈的PFAS具有較高的lgow值(辛醇-水分配系數(shù)), 容易與土壤中的有機(jī)質(zhì)結(jié)合, 并表現(xiàn)出較低的根際遷移率和生物利用度[56]。景觀植物長達(dá)幾年甚至十年的生長周期可能為根系吸收長鏈PFCA, 并將其運(yùn)輸?shù)降厣喜刻峁┝丝赡苄?。此? 植物中PFCA亦可來源于大氣。氣相態(tài)或顆粒態(tài)PFCA可通過干濕沉降到植物葉片和樹皮上而被吸附[57]。較早研究表明, 研究區(qū)域樹葉和樹皮上的PFCA來源于工廠排放到大氣中的PFCA[58], 植物葉片中PFCA與大氣中的PFCA存在顯著相關(guān)性[59], 且葉片對(duì)PFCA的攝取與PFCA的log10oa(辛醇-氣分配系數(shù))有關(guān)[56]。

近年來, 關(guān)于PFAS污染場(chǎng)址周邊植物組織中PFAS污染水平的研究工作逐漸展開。Yoo[60]對(duì)種植于PFAS污染土壤植物組織中的PFAS進(jìn)行監(jiān)測(cè), 其中PFOA是最主要的同系物, 濃度達(dá)到10～ 200 ng/g; PFDA的濃度次之。對(duì)中國常熟氟化學(xué)工業(yè)園區(qū)下游蔬菜水果的PFAS檢測(cè)結(jié)果顯示, 短鏈PFCA是主要成分。其中, PFBA是主要化合物(占總PFAS的45%, 最高濃度為12 ng/g)。PFCA檢出率結(jié)果顯示, PFOA在49%的樣品中檢出, 而PFBA和PFHpA分別只有26%和13%。長鏈PFCA (PFNA、PFUnDA、PFTrDA、PFTeDA和PFDOA)亦有檢出, 其檢出率小于7%, 表明植物樣品中短鏈PFCA的檢出率大于長鏈PFCA[44], 該結(jié)果與本研究結(jié)果相符。Bao[61]研究顯示, 中國某大型氟化工業(yè)園區(qū)周圍作物中PFAS為58～8085 ng/g, 短鏈PFCA, 尤其PFBA, 是多種作物中主要的PFCA單體。綜上, 我國氟化工園區(qū)周邊植物組織中以短鏈PFCA為主。

2.3 PFCA在土壤-植物體系中的遷移

如圖5所示, PFCA在根際和非根際土壤之間的組成特征基本一致, 以PFOA、PFHxA和PFBA為主。植物組織中PFBA、PFHxA和PFOA三者的百分含量之和超過了總PFCA的50%。PFHxA和PFBA在地上部所占的百分含量顯著高于植物根部。PFOA則呈現(xiàn)出不同的規(guī)律。PFOA在植物根部占的百分含量(80±6.7)%顯著高于根際土 (54±18)%, 但在植物地上部占的百分含量(34±18%)顯著低于植物根部。PFCA受蒸騰作用主導(dǎo), 可以隨著水流被植物吸收[62]。通常, PFCA可通過質(zhì)外體或共質(zhì)體途徑穿過根系皮層, 然而凱氏帶的存在對(duì)部分大分子結(jié)構(gòu)物質(zhì), 如碳鏈較長的PFCA, 起到了阻滯作用[63]。PFOA由于較強(qiáng)的疏水性能與細(xì)胞膜、細(xì)胞壁中的脂質(zhì)或者蛋白質(zhì)結(jié)合, 被凱氏帶截留在根系表皮而難以往植物地上部遷移, PFHxA和PFBA等碳鏈較短的PFCA則可能進(jìn)入蒸騰流向地上部遷移[64]。研究顯示, 玉米對(duì)碳鏈長度為4～7的PFAS的轉(zhuǎn)運(yùn)系數(shù)均大于 2, 而碳鏈長度大于7的PFAS, 如PFNA、PFDA、PFOS和PFHxS等, 其轉(zhuǎn)運(yùn)系數(shù)則小于1, 說明碳鏈較短的PFAS相對(duì)容易被轉(zhuǎn)移至植物地上部,而碳鏈較長的PFAS則主要富集于玉米根部。此外, 生菜轉(zhuǎn)運(yùn)系數(shù)隨著鏈長的增加呈指數(shù)下降, 只有PFBA和PFPeA在葉片中的濃度高于根部[65], 此發(fā)現(xiàn)與本文研究結(jié)果一致。

表2 經(jīng)濟(jì)作物(n = 3)和景觀植物(n = 7)的PFCA檢出率(%)

注:表示植物種類數(shù)量。NRS–非根際土壤; RS–根際土壤; Root–根部; Shoot–地上部。

圖5 PFCA在土壤-植物體系的組成特征

Fig.5 Composition of perfluorocarboxylic acids (PFCA) in soils and plants

RS–根際土; NRS–非根際土; Root–根部; Shoot–地上部。

PFCA在植物地上部和根部的不同組成特征說明PFCA在植物組織中的累積可能受到其碳鏈長度的影響。因此, 我們分析了根部到地上部的轉(zhuǎn)運(yùn)系數(shù)(shoot to root concentration factor,SRC)與短鏈PFCA的碳鏈長度之間的關(guān)系, 如圖6所示。結(jié)果顯示, 短鏈PFCA的SRC與碳鏈長度存在顯著負(fù)相關(guān)(= 0.0042,2= 0.21)[40]。Wen[66]發(fā)現(xiàn)小麥對(duì)短鏈PFAA的轉(zhuǎn)運(yùn)能力與PFAA碳鏈長度呈顯著負(fù)相關(guān), 這與本研究一致。同時(shí), Zhang[40]指出植物根系中蛋白質(zhì)和脂質(zhì)的存在抑制了碳鏈較長的PFCA向地上部的遷移, 并將其儲(chǔ)存于植物根部。

圖6 PFCA從植物根部到地上部的轉(zhuǎn)運(yùn)系數(shù)(fSRC)與PFCA碳鏈長度之間的關(guān)系

Fig.6 Relationship between the logarithmic transfer factors of perfluorocarboxylic acids (PFCA) and their carbon chain length

3 結(jié) 論

(1) 短鏈和長鏈PFCA在土壤中均有檢出, 其中短鏈?zhǔn)侵饕奈廴疚? PFOA是最主要的PFCA單體。

(2) PFCA在植物組織中的分布趨勢(shì)與在土壤中的分布趨勢(shì)基本一致, 以PFOA的濃度為最高。

(3) PFBA和PFHxA等鏈長較短的PFCA可能更容易被植物根系所吸收并轉(zhuǎn)移至植物地上部, 而PFOA則可能趨向于吸附在植物根系表皮而難以往植物地上部遷移。

衷心感謝審稿專家和編輯老師對(duì)本論文提出的寶貴意見和建議, 使文章質(zhì)量得以明顯提升。感謝課題組成員李繼兵、唐嬌、怡欣、胡蓓蓓、趙軒、劉藝等在樣品采集、分析測(cè)試、文字潤色、圖件繪制等方面的幫助。本文受廣東省青年拔尖人才項(xiàng)目(2016TQ03Z938)、廣東省“珠江人才計(jì)劃”本土創(chuàng)新科研團(tuán)隊(duì)項(xiàng)目(2017BT01Z134)和廣東省自然科學(xué)基金(2017A030313043)資助。

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Distribution and composition of perfluorocarboxylic acid substances in the soil-plant system around a fluorochemical manufacturing park

WU Zhao-cheng1,2, WANG Shao-rui1, JIANG Long-fei1, SUN Ying-tao1,2, ZHANG Gan1and LUO Chun-ling1*

1.State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, GuangdongGuangzhou?510640, China; 2. University of Chinese Academy of Sciences, Beijing?100049, China

We investigated the distribution and composition of perfluorocarboxylic acids (PFCA) in soils and plants around a fluorochemical manufacturing park. The concentrations of total PFCA in rhizosphere soil (RS) and non-rhizosphere soil (NRS) were in the ranges of 9.6–160 ng/g and 7.2–300 ng/g, respectively. The variation inratio(RS/NRS) among plant species indicated that PFCA was not enriched in the rhizosphere. The concentration of total PFCA ranged from 6.0 ng/g to 630 ng/g in shoots and from 6.1 ng/g to 570 ng/g in roots. The predominance of perfluorooctanoic acid (PFOA) was consistently observed in NRS, RS, roots, and shoots, with percentages of (53±18)%, (51±22)%, (80±6.7)%, and (34±1.8)%, respectively. Short-chained PFCA (C4–C8) showed higher concentrations and detection frequencies in shoots than long-chained PFCA (C>8), thereby indicating that short-chained PFCA are more prone to be enriched in plant tissues compared with long-chained PFCA. The higher percentages of perfluorobutanic acid (PFBA) and perfluorohexanoic acid (PFHxA) in shoots than in roots indicated that short-chained PFCA could be absorbed by roots and acropetally transferred to shoots. In contrast, PFOA was adsorbed onto roots and difficult to be translocated to the plant shoots, given that a lower percentage of PFOA was observed in shoots than in roots. These findings are supported by the significant and negative correlation observed between the translocation factor and chain length. The results of this study will help understand the fate of PFCA within the soil-plant system.

perfluorocarboxylic acid; rhizosphere; translocation

P593; X53

A

0379-1726(2021)05-0525-11

10.19700/j.0379-1726.2021.01.014

2020-01-06;

2020-03-11;

2020-03-13

廣東省青年拔尖人才項(xiàng)目(2016TQ03Z938); 廣東省“珠江人才計(jì)劃”本土創(chuàng)新科研團(tuán)隊(duì)項(xiàng)目(2017BT01Z134); 廣東省自然科學(xué)基金(2017A030313043)

伍兆誠(1995–), 男, 碩士研究生, 環(huán)境工程專業(yè)。E-mail: wuzhaocheng17@mails.ucas.ac.cn

通訊作者(Corresponding author): LUO Chun-ling, E-mail: clluo@gig.ac.cn; Tel: +86-20-85290290