西藏色季拉山公路沿線PAHs分布、來源及風(fēng)險(xiǎn)

黨天劍,陸光華,薛晨旺,孫文青

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西藏色季拉山公路沿線PAHs分布、來源及風(fēng)險(xiǎn)

黨天劍,陸光華*,薛晨旺,孫文青

(西藏農(nóng)牧學(xué)院水利土木工程學(xué)院,西藏 林芝 860000)

分別于2017年3月和12月沿色季拉山318國道采集表層土和冷杉(Mill.)樣品,測(cè)定了多環(huán)芳烴(PAHs)的含量.表層土和冷杉葉中∑16PAHs的含量范圍分別為30.21~366.94ng/g dw和39.53~236.42ng/g dw,組成以低環(huán)(2、3環(huán))為主.通過特征單體比值法和主成分分析法分析表明,色季拉山PAHs主要來源于化石燃料和生物質(zhì)的燃燒,同時(shí)也受到車輛石油泄漏和大氣遠(yuǎn)距離傳輸?shù)挠绊?通過反向氣團(tuán)軌跡判斷,色季拉山PAHs大氣傳輸污染主要來自于印度次大陸.色季拉山公路沿線土壤中PAHs的終生致癌風(fēng)險(xiǎn)值均低于1×10-6,說明對(duì)當(dāng)?shù)鼐用竦闹掳╋L(fēng)險(xiǎn)較小.

表層土；冷杉葉；PAHs；源解析；風(fēng)險(xiǎn)評(píng)價(jià)

多環(huán)芳烴(PAHs)是重要的環(huán)境污染物之一,因其致癌、致畸和致突變性而倍受關(guān)注[1-2].PAHs具有持久性和半揮發(fā)性,可長期存在于各環(huán)境介質(zhì)中,并通過水、大氣或其他介質(zhì)在全球范圍內(nèi)遷移.如今,除了人類活動(dòng)密集的大城市以外[3-4],在人煙稀少的兩極地區(qū)[5-6]、高山地區(qū)[7]和青藏高原[8-9]均檢測(cè)到PAHs的分布,高寒地區(qū)因存在高山冷凝捕作用,從而可能成為PAHs的儲(chǔ)蓄區(qū).

PAHs通過大氣在全球范圍內(nèi)傳輸,并通過干濕沉降至地面.地表土直接接受干濕沉降的PAHs,環(huán)境中90%以上的PAHs存儲(chǔ)在土壤中[10],同時(shí)土壤中PAHs還可以通過土壤揚(yáng)塵、皮膚接觸等對(duì)人體健康造成威脅[11],其潛在危害也不容忽視.冷杉葉同松針一樣,比表面積大且表層蠟脂含量高,對(duì)PAHs等污染物有較好的富集作用,已被很多學(xué)者用于環(huán)境中污染物的生物指示和監(jiān)測(cè)評(píng)價(jià)[12-15].

青藏高原平均海拔高于4000m,人煙稀少,幾乎沒有工業(yè)污染,是用來研究PAHs大氣傳輸?shù)牧己帽尘皡^(qū)域.近幾年,隨著旅游業(yè)的發(fā)展,越來越多的游客和車輛進(jìn)入高原,人類活動(dòng)的影響日益明顯.藏東南色季拉山擁有原始冷杉林,本研究通過分析冷杉葉和土壤中PAHs的含量水平,闡明其分布特征,解析其來源,在此基礎(chǔ)上評(píng)估其風(fēng)險(xiǎn)水平.

1 材料與方法

1.1 試劑

萘(Nap)、苊烯(Acy)、(Ace)、芴(Flu)、菲(Phe)、蒽(Ant)、熒蒽(Fla)、芘(Pyr)、苯并[a]蒽(BaA)、屈(Chr)、苯并[b]熒蒽(BbF)、苯并[k]熒蒽(BkF)、苯并[a]芘(BaP)、茚并[1.2.3-cd]芘(InP)、二苯并[a.h]蒽(DBA)、苯并[g.h.i]苝(BgP)等16種PAHs標(biāo)準(zhǔn)溶液和Ace-d10、Phe-d10和Chr-d12等3種回收率指示物均購買于百靈威科技有限公司(北京);固相萃取小柱(CNWDOND Si 1g/6mL)購于上海安譜實(shí)驗(yàn)科技股份有限公司;正己烷、二氯甲烷、甲醇和丙酮均為色譜純,購買于西隴科學(xué)股份有限公司(廣東汕頭).

1.2 樣品采集

分別于2017年3和12月,沿318國道海拔每升高200m設(shè)置1個(gè)采樣點(diǎn),在東坡、西坡和山頂共設(shè)11個(gè)采樣點(diǎn)(圖1),點(diǎn)位信息見表1.

表層土采用5點(diǎn)采樣法采樣,直徑取20m,深度取0~20cm,在每個(gè)采樣點(diǎn)用潔凈不銹鋼鏟采集5個(gè)土壤子樣,混合均勻后用4分法縮分,置于聚乙烯密封袋,運(yùn)回實(shí)驗(yàn)室,-20℃冷凍后真空冷凍干燥.選取3年生冷杉葉,采樣高度為3~5m,采自4~5棵不同的冷杉樹,混勻后包入經(jīng)處理的鋁箔中,用聚乙烯塑料袋封裝,-20℃冷凍后進(jìn)行真空冷凍干燥.將干燥后的土樣和冷杉樣剔除雜物,研磨,過60目篩后密封裝袋備用.

表1 采樣點(diǎn)位信息 Table 1 Information of sampling sites

1.3 樣品預(yù)處理

稱取凍干表層土樣品5g,與0.5g活化的銅粉充分混合,加入到22mL萃取池中,剩余孔隙用硅藻土填充.用加速溶劑萃取儀(ASE350, Dionex,美國)萃取,萃取劑為二氯甲烷/正己烷(1:1,/)混合溶液,萃取條件為:萃取溫度100℃,系統(tǒng)壓強(qiáng)10.3′106Pa,加熱時(shí)間5min,靜態(tài)提取時(shí)間5min.提取液于旋轉(zhuǎn)蒸發(fā)儀上濃縮至近干,加入3mL正己烷轉(zhuǎn)換溶劑.萃取液經(jīng)固相萃取柱凈化,用5mL二氯甲烷/正己烷(2:3,/)洗脫,洗脫液氮吹濃縮至近干,再用1mL正己烷/丙酮(9:1,/)混合液定容,過0.22μm有機(jī)相濾膜后轉(zhuǎn)入進(jìn)樣瓶中待測(cè).

稱取凍干冷杉葉樣品2g,加入到22mL萃取池中,加入3g中性氧化鋁粉末,剩余孔隙用硅藻土填充.采用加速溶劑萃取儀萃取,條件與土樣相同.在提取液中加入10mL濃硫酸,靜置30min后取上層清液于旋轉(zhuǎn)蒸發(fā)儀上濃縮至近干,后續(xù)處理與土樣相同.

1.4 儀器分析

采用GC-MS/MS(ThermoFisher Scientific, USA)進(jìn)行PAHs定量分析.色譜柱為HP-5MS(30.0m× 250.0μm×0.25μm).進(jìn)樣口溫度260℃,柱初始溫度60℃,持續(xù)1min,以15℃/min升溫至120℃,持續(xù)1min,再以20℃/min升溫至180℃,之后以5℃/min升溫至290℃,持續(xù)5min.載氣為氦氣,流量1mL/min,不分流進(jìn)樣,進(jìn)樣量為1μL.質(zhì)譜條件:離子源溫度260℃,接口溫度290℃,掃描模式:SIM.

為了保證實(shí)驗(yàn)方法可靠性,實(shí)驗(yàn)通過方法空白、基質(zhì)加標(biāo)和樣品平行等來進(jìn)行質(zhì)量控制和保證.空白樣品中未檢出目標(biāo)污染物.土壤和冷杉樣品基質(zhì)加標(biāo)回收率分別為77.8%~122%和74.8%~111%,方法檢出限分別為0.01~0.16,0.13~0.33ng/g.

2 結(jié)果與討論

2.1 PAHs含量水平

色季拉山表層土中∑16PAHs在3和12月分別為101.27~350.33,30.21~366.94ng/g dw,平均值分別為187.58,83.09ng/g dw(表2),與長白山PAHs含量接近[19],遠(yuǎn)低于中國西南山區(qū)[18]和歐洲山脈[20],但高于青藏高原其他背景區(qū)域[16-17].色季拉山是旅游觀光勝地,且距離林芝市不遠(yuǎn),受人類活動(dòng)影響較大.冷杉葉中∑16PAHs在3和12月分別為104.55~236.42, 39.53~119.96ng/g dw,平均值分別為194.05, 75.47ng/g dw,與Yang等[21]在藏東南研究結(jié)果相近,也與德國科隆2002年松針中PAHs含量相似[23],但遠(yuǎn)低于南京、大連松針中PAHs的濃度[13,22].植物葉片中污染物主要來源于大氣親脂性有機(jī)污染的富集,各地區(qū)PAHs濃度差異可能主要與當(dāng)?shù)卮髿馕廴舅接嘘P(guān).

表2 本研究與其他地區(qū)各介質(zhì)中PAHs的含量 Table 2 Concentrations of PAHs in different media from this study and other areas

2017年3月土壤和冷杉葉中PAHs的濃度水平均略高于2017年12月.在12月土樣中,Ace、Acy和An未檢出,其他PAHs組分檢出率為100%.12月色季拉山幾乎沒有人類活動(dòng)痕跡,污染主要來源于大氣傳輸,以中、低環(huán)PAHs為主;而3月開始游客逐漸增多,人類活動(dòng)增加,PAHs檢出種類增多、濃度升高.西坡PAHs濃度略高于東坡,但PAHs含量水平與海拔高度沒有顯著相關(guān).

2.2 PAHs組成特征

根據(jù)苯環(huán)數(shù)的不同,可將PAHs劃分為低環(huán)(2+3環(huán))、中環(huán)(4環(huán))和高環(huán)(5+6環(huán))[25].色季拉山檢出的PAHs結(jié)構(gòu)特征如圖2所示,土樣除3月S8點(diǎn)高環(huán)PAHs含量最高,達(dá)到53.63%,總體仍以低環(huán)PAHs為主,3和12月土樣中低環(huán)PAHs占比分別達(dá)到23.97%~77.21%和21.77%~82.29%.3月土壤中單體PAHs占比以Nap和Phe為主,分別占總量的28.02%和15.06%,而12月中Chr相對(duì)含量最高,達(dá)到21.9%,余下依次為Nap和Phe,分別占總量18.99%和12.18%.冷杉葉PAHs同樣以低環(huán)為主,在3和12月分別占到39.92%~73.10%和32%~70.56%,3月冷杉葉中單體PAHs占比以Nap和Phe為主,分別占總量51.68%和7.45%,12月中Phe占比最高,其次為Pry和Nap,分別占總量21.56%、19.39%和12.09%.總體上各介質(zhì)中PAHs組成為低環(huán)>中環(huán)>高環(huán).

2.3 PAHs來源解析

2.3.1 特征組分比值法,燃燒源種類和燃燒條件不同,產(chǎn)生的PAHs組分和相對(duì)含量也不同,因此PAHs特征組分之間的含量比可以用來識(shí)別PAHs的污染來源[22].本研究使用An/(An+Phe)、Flu/(Flu+Pyr)、BaA/(BaA+Chr)和IcdP/(IcdP+BghiP)來解析色季拉山PAHs的來源.

An/(An+Phe)小于0.10表明存在石油源,大于0.10表示燃燒源占主導(dǎo)地位;Flu/(Flu+Pyr)=0.50通常被定義為石油燃燒過渡點(diǎn),比率大于0.50,表明來源于生物質(zhì)和煤燃燒,比率在0.40~0.50之間符合液體化石燃料燃燒(車輛油和原油)的特征[26]. BaA/ (BaA+Chr)比率大于0.35,表明來源于煤、草和木的燃燒,0.20~0.35之間表示源于石油燃燒,小于0.2時(shí)為石油源[9].IcdP/(IcdP+BghiP)比率小于0.2,表明來源于天然石油源;在0.2~0.5之間,則為石油燃燒源;大于0.5表示來自于生物質(zhì)燃燒(草、木材、煤)[27].

如圖3所示,各介質(zhì)中Flu/(Flu+Pyr)比值多數(shù)落在>0.5的區(qū)間,說明其來源為煤和生物質(zhì)燃燒,也有少量點(diǎn)分布在0.4~0.5之間,表明也受到了石油燃燒的影響.3月表層土和冷杉葉中An/(An+Phe)比值判斷結(jié)果均在>0.1的區(qū)域,表明燃燒源占主導(dǎo)地位.各介質(zhì)IcdP/(IcdP+BghiP)的比值大多分布在>0.5的區(qū)域,表明主要來源還是生物質(zhì)的燃燒,但也有部分點(diǎn)分布在0.2~0.5區(qū)間,說明石油燃燒也貢獻(xiàn)了一定的PAHs.3月各介質(zhì)BaA/(BaA+Chr)的比值判斷結(jié)果均大于0.2,表明其主要受燃燒源的影響;但12月表層土和冷杉葉中有73.68%的采樣點(diǎn)BaA/(BaA+Chr)的比值小于0.2.結(jié)果表明,不同季節(jié)樣品中特征比值相差較大,一方面是由于色季拉山PAHs來源復(fù)雜,另一方面也說明僅利用特征比值進(jìn)行PAHs來源解析存在局限.

2.3.2 主成分分析 為了進(jìn)一步解析色季拉山PAHs的來源,采用主成分分析法定量分析了各污染源對(duì)色季拉山PAHs的貢獻(xiàn).提取特征根大于1的因子,旋轉(zhuǎn)后的主成分矩陣見表3.土壤和冷杉中16種PAHs包含的信息可集中在2個(gè)或3個(gè)主成分中,且各介質(zhì)所提取的主成分總方差貢獻(xiàn)均超過80%.

土壤和冷杉葉中主成分1貢獻(xiàn)率范圍為54.70%~76.52%,其中Pyr、BaA、Chr、BbF、BkF、BaP、DBA、BgP、InP載荷突出,An、Fla載荷也較高.Fla、Pyr、BaA和Chr等4環(huán)及以上PAHs主要來源于煤的燃燒[28-29];BaA、BgP和InP是汽車尾氣的標(biāo)志性組分[30-31],BaK則是柴油機(jī)排放的主要PAHs[32];而An不僅是石油源的代表性化合物[33],也是木材等不完全燃燒的代表物質(zhì)[34].主成分2和3貢獻(xiàn)率分別達(dá)到14.73%~29.18%和8.20%~14.60%,其中2環(huán)的Nap載荷突出,Nap不穩(wěn)定、易揮發(fā),可通過大氣遠(yuǎn)距離傳輸,表明存在外來源的污染[35],此外還受石油泄漏污染的一定影響[33,36].Ace、Acy、Flu和Phe在因子2、3中均有較高載荷,其中Ace是石油源的代表性化合物[36],Acy是薪柴燃燒的標(biāo)志物[37],Flu和Phe則主要來源于焦?fàn)t排放[34,38].

主成分分析結(jié)果表明,色季拉山PAHs污染主要受化石燃料和生物質(zhì)燃燒的影響.除燃煤外,當(dāng)?shù)鼐用襁€習(xí)慣用木柴、牛糞等生物質(zhì)作燃料,符合當(dāng)?shù)啬茉蠢们闆r.色季拉山口是旅游景點(diǎn),也是去林芝魯朗小鎮(zhèn)的必經(jīng)之路,每年有大量游客駕車經(jīng)過,本研究又主要沿318國道采樣,所以交通污染源貢獻(xiàn)也較大.此外,從主成分2、3的貢獻(xiàn)來看,色季拉山PAHs還受石油源和大氣遠(yuǎn)距離傳輸?shù)挠绊?石油源主要受旅游車輛燃油泄漏影響,青藏高原作為世界第三極,而色季拉山最高海拔達(dá)到4550m,也符合高山冷凝捕條件.

2.3.3 反向氣團(tuán)軌跡分析 為了追蹤色季拉山PAHs的大氣傳輸來源,利用美國國家海洋和大氣管理局(NOAA)空氣資源實(shí)驗(yàn)室和澳大利亞氣象局聯(lián)合研發(fā)的HYSPLIT-4模型分析了到達(dá)色季拉山3個(gè)采樣點(diǎn)(山頂和兩邊山腳)距離地面1000,1500和2000m高處的氣團(tuán)運(yùn)動(dòng)軌跡(圖4).與青藏高原中、北部在6~9月開始受印度季風(fēng)的影響不同[39],色季拉山地處藏東南地區(qū),在2月起就開始受印度季風(fēng)影響,南風(fēng)將印度和孟加拉國等地區(qū)的污染物通過雅魯藏布江大峽谷傳輸?shù)角嗖馗咴喜?并于6月結(jié)束;7~8月主要受來自青藏高原內(nèi)部氣團(tuán)的影響,帶來拉薩、林芝等地區(qū)污染物,此外也受到青海、甘肅等內(nèi)陸省份的影響;從9月~次年1月,則受冬季西風(fēng)帶的影響,這與Chen等[40]提出藏東南地區(qū)冬季主要受西部氣團(tuán)影響的結(jié)論相似,西風(fēng)繞道喜馬拉雅山脈,將印度北部和巴基斯坦等地區(qū)的污染物傳輸?shù)缴纠絒39].

綜上所述,雖然在7,8月色季拉山受到青藏高原內(nèi)部和內(nèi)陸省份的影響,但色季拉山氣團(tuán)主要還是受印度季風(fēng)和冬季西風(fēng)的影響,而二者皆經(jīng)過印度次大陸,因此色季拉山PAHs污染主要還是受印度次大陸污染源的遠(yuǎn)距離傳輸影響.

表3 方差最大旋轉(zhuǎn)后16種PAHs的主成分因子載荷 Table 3 Factor loading for PAHs on principle component analysis with varimax rotation

注:-為未檢出.

2.4 表層土中PAHs的健康風(fēng)險(xiǎn)評(píng)估

美國環(huán)保署將BaA、BbF、BkF、BaP、Chr、DBA和InP等7類PAHs歸類為可能的人類致癌物,因此有必要對(duì)色季拉山PAHs進(jìn)行風(fēng)險(xiǎn)評(píng)估.本文采用終生致癌風(fēng)險(xiǎn)(ILCRs)模型評(píng)估了PAHs通過直接攝入、呼吸攝入和皮膚接觸等對(duì)兒童、青少年和成人的潛在風(fēng)險(xiǎn).各暴露途徑PAHs致癌風(fēng)險(xiǎn)計(jì)算公式如(1)~(3)所示[41].

式中:CS為總的BaP當(dāng)量(BaPeq), mg/kg;CSF為致癌斜率因子mg/(kg×d);BW為人平均體重kg;EF為暴露頻率d/a;IR攝取為土壤攝取速率mg/d;ED為暴露時(shí)間a;SA為與土壤接觸的皮膚面積cm2/d;AF為土壤的皮膚黏附因子mg/cm2;ABS為皮膚吸收因子;PEF為顆粒物排放因子m3/kg.各參數(shù)取值參考文獻(xiàn)[41].

式(1)~(3)計(jì)算結(jié)果表明,色季拉山PAHs在3和12月通過直接攝入、呼吸攝入和皮膚接觸的致癌風(fēng)險(xiǎn)范圍分別為:2.82×10-8~2.76×10-7、7.44× 10-12~ 1.20×10-10、1.04×10-8~7.46×10-8和1.97×10-9~ 7.87×10-8、5.20×10-13~3.43×10-11、7.28×10-10~ 2.13×10-8.研究區(qū)域土壤中PAHs對(duì)不同人群的致癌風(fēng)險(xiǎn)如圖5所示.3月色季拉山PAHs通過不同暴露途徑對(duì)不同人群的影響均高于12月,但PAHs的終生致癌風(fēng)險(xiǎn)影響卻類似,均是直接攝入>接觸攝入>呼吸攝入.受直接攝入影響最大的是兒童群體,成人群體則受呼吸攝入影響最大,不同群體受皮膚接觸攝入影響接近.對(duì)不同人群而言,成人受不同暴露途徑的總影響最高,其次是兒童,青少年受影響最小.

當(dāng)ILCRs值在10-6~10-4之間時(shí),被認(rèn)為存在潛在危險(xiǎn),而ILCRs值￡10-6時(shí),可忽略其風(fēng)險(xiǎn)[42].色季拉山不同年齡段人群總ICLRs值均小于1×10-6,因此,色季拉山土壤對(duì)當(dāng)?shù)鼐用癫⒉淮嬖谥掳╋L(fēng)險(xiǎn).但隨著色季拉山周邊城鎮(zhèn)和旅游業(yè)的發(fā)展,色季拉山PAHs污染可能逐漸加重,仍需注意.

3 結(jié)論

3.1 藏東南色季拉山公路沿線土壤和冷杉中普遍檢出PAHs,含量分別為30.21~366.94,39.53~ 236.42ng/g dw,單體特征以低環(huán)為主,其中Nap和Phe含量最高.

3.2 色季拉山PAHs主要來自于化石燃料和生物質(zhì)的燃燒,這與當(dāng)?shù)鼐用裆盍?xí)慣和旅游業(yè)的發(fā)展情況相符;同時(shí)色季拉山還受到大氣遠(yuǎn)距離傳輸?shù)挠绊?主要為印度次大陸的污染輸入.

3.3 健康風(fēng)險(xiǎn)評(píng)估結(jié)果表明,色季拉山土壤中PAHs對(duì)當(dāng)?shù)鼐用竦闹掳╋L(fēng)險(xiǎn)較小.

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Distribution, sources and risk of polycyclic aromatic hydrocarbons along the highway in Shergyla Mountain in Tibet.

DANG Tian-jian, LU Guang-hua*, XUE Chen-wang, SUN Wen-qing

(Department of Water Resources and Civil Engineering, Tibet Agriculture and Animal Husbandry University, Linzhi 860000, China)., 2019,39(3)：1109~1116

The surface soil and fir (Mill.) samples were collected in March and December 2017 along the China National Highway 318 in Shergyla Mountain, respectively. The contents of polycyclic aromatic hydrocarbons (PAHs) in the samples were measured. The concentrations of ∑16PAHs ranged from 30.21 to 366.94ng/g (dry weight) in the surface soil and 39.53 to 236.42ng/g (dry weight) in the fir leaves, respectively, and the lower rings (2- or 3-ring) constituents were dominants. The results of diagnostic ratios and principal component analysis suggested that the PAHs mainly originated from the combustion of fossil fuel and biomass, and also affected by oil leaks and atmospheric transmission. The atmospheric transmission pollution of PAHs could mainly result from the Indian subcontinent based on the backward air mass trajectories. The incremental lifetime cancer risks of PAHs in the soils along the highway in Shergyla Mountain were lower than 1×106, indicating a lower carcinogenic risk to the local residents.

surface soil；fir leaves；PAHs；source diagnosis；risk assessment

X53

A

1000-6923(2019)03-1109-08

黨天劍(1994-),男,甘肅武威人,西藏農(nóng)牧學(xué)院碩士研究生,主要研究方向?yàn)槲廴旧鷳B(tài)化學(xué).發(fā)表論文3篇.

2018-08-24

國家自然科學(xué)基金資助項(xiàng)目(51879228);西藏自治區(qū)高等學(xué)?？蒲袆?chuàng)新團(tuán)隊(duì)項(xiàng)目;西藏農(nóng)牧學(xué)院研究生創(chuàng)新計(jì)劃項(xiàng)目(YJS2017-04)

* 責(zé)任作者, 教授, ghlu@hhu.edu.cn