長江武漢段沉積物再懸浮過程中PAHs釋放的預(yù)測模型

馮精蘭,祁巧艷 ,孫劍輝* (.河南師范大學(xué)化學(xué)與環(huán)境科學(xué)學(xué)院,黃淮水環(huán)境與污染防治省部共建教育部重點(diǎn)實(shí)驗(yàn)室,河南省環(huán)境污染控制重點(diǎn)實(shí)驗(yàn)室,河南 新鄉(xiāng) 453007；2.北京師范大學(xué)環(huán)境學(xué)院,水環(huán)境模擬國家重點(diǎn)實(shí)驗(yàn)室,北京 00875)

長江武漢段沉積物再懸浮過程中PAHs釋放的預(yù)測模型

馮精蘭1,2,祁巧艷1,孫劍輝1*(1.河南師范大學(xué)化學(xué)與環(huán)境科學(xué)學(xué)院,黃淮水環(huán)境與污染防治省部共建教育部重點(diǎn)實(shí)驗(yàn)室,河南省環(huán)境污染控制重點(diǎn)實(shí)驗(yàn)室,河南 新鄉(xiāng) 453007；2.北京師范大學(xué)環(huán)境學(xué)院,水環(huán)境模擬國家重點(diǎn)實(shí)驗(yàn)室,北京 100875)

為研究長江武漢段沉積物再懸浮過程中多環(huán)芳烴的釋放,采用回歸方法建立了剪應(yīng)力與再懸浮顆粒物濃度、有機(jī)碳含量及PAHs濃度之間的定量關(guān)系模型,并結(jié)合PAHs在懸浮顆粒物-水體分配系數(shù),建立了不同剪應(yīng)力條件下再懸浮過程中上覆水體PAHs濃度預(yù)測模型,通過模型預(yù)測結(jié)果與實(shí)驗(yàn)結(jié)果的比較,表明模型具有良好的預(yù)測效果,可用來進(jìn)行不同剪應(yīng)力條件下沉積物再懸浮過程PAHs釋放的預(yù)測.

多環(huán)芳烴(PAHs)；長江；沉積物再懸??；釋放；預(yù)測模型

多環(huán)芳烴(PAHs)是環(huán)境中廣泛存在的一類具有“三致”效應(yīng)的持久性有機(jī)污染物,具有溶解度低、疏水性強(qiáng)的特點(diǎn),容易被顆粒物吸附,并在沉積物中積累[1].蓄積在沉積物中的污染物在沉積物再懸浮過程中會重新釋放進(jìn)入水體,造成二次污染[2-4].河流中沉積物再懸浮由水流、波浪等自然因素,拖網(wǎng)、疏浚等人為因素以及生物擾動等引起[5-8].研究表明,沉積物再懸浮過程中,會導(dǎo)致上覆水體污染物濃度急劇增加,且污染物濃度與剪應(yīng)力大小以及沉積物性質(zhì)存在相關(guān)性

[9-11].長江水系武漢段位于長江中游,貫穿整個武漢市,是該地區(qū)工農(nóng)業(yè)生產(chǎn)和人民生活用水的主要來源.該江段水流速快,波浪大,沉積物易于發(fā)生再懸浮,且 PAHs是沉積物中典型有機(jī)污染物[12].因此,開展長江水系武漢段沉積物再懸浮過程中 PAHs的釋放研究,考察剪應(yīng)力以及再懸浮持續(xù)時間對再懸浮過程中PAHs釋放的影響對于了解該區(qū)域水污染變化規(guī)律具有重要意義

[3,13-14].在此基礎(chǔ)上,依據(jù)本課題組前期實(shí)驗(yàn)結(jié)果[3,13-14],結(jié)合PAHs懸浮顆粒物-水分配系數(shù),建立了預(yù)測模型,對不同剪應(yīng)力條件下沉積物再懸浮過程中的總懸浮顆粒物濃度及其有機(jī)碳含量,水相、懸浮相PAHs濃度進(jìn)行了預(yù)測,并將預(yù)測結(jié)果與實(shí)驗(yàn)結(jié)果進(jìn)行了對比分析,對模型預(yù)測結(jié)果的準(zhǔn)確性進(jìn)行了討論.

1 材料與方法

1.1 儀器和試劑

Waters1525雙泵高效液相色譜儀,配有Waters474掃描熒光檢測器和Waters2487雙波長紫外檢測器,Varian PAHs專用色譜柱(250mm×4.6mm,5μm);KQ502B數(shù)控超聲波清洗器;RV 05 basic旋轉(zhuǎn)濃縮蒸發(fā)儀;TDL-5臺式離心機(jī);Liqui TOC分析測定儀;SALD-3001激光粒度分析儀;Simplicity 185超純水系統(tǒng).

16種PAHs混合標(biāo)樣(PAH-Mix 9)購自Dr. Ehrenstorfer公司.甲醇、正己烷、二氯甲烷、丙酮(Tedia,美國)均為色譜純.無水硫酸鈉、層析硅膠(200～300目)為分析純(使用前在馬弗爐中550℃烘焙 4 h,放在干燥器中備用).實(shí)驗(yàn)用水采用Simplicity 185超純水系統(tǒng)處理.

1.2 樣品采集

圖1 采樣點(diǎn)位置示意Fig.1 Sediment sampling sites in the mainstream of the Yangtze River

沉積物樣品采自長江水系武漢段干流,使用 Van veen 抓斗式采樣器(Eijkelkamp, Netherlands)分別在2005年7月和11月采集7個沉積物樣品,采樣點(diǎn)分別為金口(JK)、東荊河(DJH)、沌口(ZHK)、武漢關(guān)(WHG)、朱家河(ZJH)、武湖(WH)、陽邏(YL),位置如圖1所示.采集后的沉積物樣品放置于潔凈的不銹鋼小桶中,4℃下遮光保存到進(jìn)行再懸浮模擬實(shí)驗(yàn).模擬實(shí)驗(yàn)用水采自沌口,經(jīng) 0.45μm醋酸纖維濾膜過濾,4℃下遮光保存.

1.3 再懸浮模擬裝置

采用的沉積物再懸浮模擬裝置如圖 2所示,其工作原理為:穿孔板通過連接桿與變速電機(jī)相連,并在電機(jī)的帶動下在水體上下擺動,在沉積物-水界面產(chǎn)生剪應(yīng)力,夾帶沉積物進(jìn)入水體,產(chǎn)生再懸浮.產(chǎn)生的剪應(yīng)力大小與穿孔板的擺幅、頻率及穿孔板距沉積物的高度有關(guān).Tsai等[15]對該模擬裝置進(jìn)行了校正,當(dāng)穿孔板在距沉積物界面5～7.5cm之間擺動,產(chǎn)生0.2,0.3,0.4,0.5N/m2剪應(yīng)力所需的電機(jī)轉(zhuǎn)速分別為375,500,600,750r/min.該裝置已被證實(shí)模擬過程重現(xiàn)性較好,可比較不同強(qiáng)度再懸浮模擬以及不同性質(zhì)沉積物再懸浮過程[16-18].

圖2 再懸浮模擬裝置結(jié)構(gòu)示意Fig.2 A schematic view of the particle entrainment simulator (PES)

1.4 釋放動力學(xué)實(shí)驗(yàn)

將采集的沉積物樣品放置于再懸浮模擬裝置中,分別在 0.2～0.5N/m2剪應(yīng)力條件下進(jìn)行沉積物再懸浮過程的實(shí)驗(yàn)?zāi)M,定時取樣,取樣量為100mL,之后重新注入100mL水樣以維持水樣體積保持恒定.水樣取出后,立即用孔徑為1.2μm的微孔玻璃纖維濾膜(GF/C, Whatman, England)過濾,測定其中PAHs含量,計算PAHs釋放量.同時,測定沉積物及再懸浮顆粒物中PAHs濃度、有機(jī)碳含量、炭黑含量.

1.5 PAHs分析

取70mL水樣置于250mL磨口錐形瓶中,加入等體積二氯甲烷超聲萃取 30min,靜置 20min,使用分液漏斗取出有機(jī)相,旋轉(zhuǎn)蒸發(fā)至約0.5mL,加入 10mL甲醇溶劑置換,旋轉(zhuǎn)蒸發(fā),定容至1.0mL,采用HPLC分析.

將沉積物或?yàn)V膜置于 50mL聚四氟乙烯離心管中,加入 30mL正己烷/丙酮混合溶液(1:1,V: V)放置過夜.超聲(30℃)萃取 1h,4000r/min離心20min,萃取液旋轉(zhuǎn)蒸發(fā)至約1mL,過裝有1g無水硫酸鈉+2g硅膠的層析柱凈化,以8mL正己烷預(yù)洗,10mL正己烷/二氯甲烷混合溶液(1∶1,V:V)洗脫,收集洗脫液旋蒸(30℃)至干,用甲醇精確定容到1.0mL,進(jìn)HPLC分析[19].

色譜條件:柱溫:20℃;流動相:甲醇-水二元梯度;流速:1.0mL/min;進(jìn)樣量:20μL;檢測器:程序可變波長熒光檢測器,雙波長紫外檢測器.

水相及沉積物 PAHs分析的質(zhì)量控制包括方法空白、基質(zhì)空白和基質(zhì)加標(biāo)回收率.方法空白均無檢出目標(biāo)化合物.通過加標(biāo)回收實(shí)驗(yàn),水相16PAHs的回收率以及檢測限分別為 60%～100%、0.098～0.191μg/L;沉積物中16PAHs的回收率以及檢測限分別為 60%～110%(萘除外, 30±5%)、0.34～4.05μg/kg.

1.6 沉積物理化性質(zhì)的測定

沉積物或再懸浮顆粒物研磨勻質(zhì)后,以過量1.6%鹽酸酸化預(yù)處理除去 TIC(總無機(jī)碳),60℃干燥后用TOC儀進(jìn)行總有機(jī)碳的測定.

2 結(jié)果與討論

不同剪應(yīng)力條件下沉積物再懸浮過程中PAHs的釋放數(shù)據(jù)參見文獻(xiàn)[3].

2.1 不同剪應(yīng)力條件下總懸浮顆粒物濃度預(yù)測

由圖 3可知, 剪應(yīng)力及總懸浮顆粒物(TSS)濃度擬合結(jié)果較好(R=0.994, P=0.006),TSS濃度隨剪應(yīng)力增加呈指數(shù)形式增加.此外,0～0.2N/m2剪應(yīng)力條件下長江武漢段沉積物再懸浮實(shí)驗(yàn)結(jié)果表明,當(dāng)剪應(yīng)力小于0.06N/m2時,TSS濃度變化幅度很小,而剪應(yīng)力超過0.06N/m2時,TSS濃度呈現(xiàn)快速上升趨勢.因此,確定 0.06N/m2為長江武漢段沉積物發(fā)生再懸浮的臨界剪應(yīng)力,這與文獻(xiàn)報道的河流臨界剪應(yīng)力范圍 0.01～0.1N/m2相吻合[20].由圖4可見,剪應(yīng)力與TSS有機(jī)碳(TOC)含量擬合結(jié)果較好(R=0.991,P=0.009),TSS中TOC含量與剪應(yīng)力呈相反的變化趨勢.這主要是因?yàn)殡S著剪應(yīng)力的增加,TSS中大粒徑顆粒物的比例增加(63～25μm),小粒徑顆粒物的比例減少(＜25μm).對不同粒徑沉積物TOC含量分析表明,大粒徑沉積物(63～25μm)TOC含量低于小粒徑顆粒物(＜25μm)[3].綜上所述,剪應(yīng)力與 TSS及其TOC含量擬合結(jié)果較好,可以用此結(jié)果對0.2～0.5N/m2剪應(yīng)力范圍內(nèi)長江武漢段沉積物再懸浮過程中TSS及其TOC含量進(jìn)行預(yù)測.

圖3 總懸浮顆粒物濃度與剪應(yīng)力的擬合關(guān)系Fig.3 Relationship between concentrations of TSS and shear stress

2.2 不同剪應(yīng)力條件下再懸浮顆粒物 PAHs預(yù)測

由表1可知,剪應(yīng)力與TSS中PAHs質(zhì)量濃度擬合結(jié)果較好,可以用此結(jié)果對 0.2～0.5N/m2范圍內(nèi)沉積物再懸浮過程中懸浮顆粒物中PAHs質(zhì)量濃度進(jìn)行預(yù)測.TSS濃度與剪應(yīng)力增加呈指數(shù)增加,而TSS中PAHs的質(zhì)量濃度隨著總懸浮顆粒物濃度的增加而降低.這主要是由于高剪應(yīng)力條件下,更多大粒徑、低PAHs含量的顆粒物被夾帶使得PAHs質(zhì)量濃度趨于下降.對不同粒徑沉積物中 PAHs分布研究表明,＜25μm沉積物中PAHs的濃度＞63～25μm沉積物中 PAHs的濃度,證明了這一推論[3].因此TSS中PAHs濃度與剪應(yīng)力關(guān)系與TSS濃度與剪應(yīng)力變化不一致.此外,利用表1列出的PAHs質(zhì)量濃度與剪應(yīng)力的關(guān)系方程以及TSS濃度與剪應(yīng)力的關(guān)系方程計算得出了不同剪應(yīng)力條件下再懸浮過程中上覆水體TSS中PAHs的體積濃度,結(jié)果見表2.

2.3 不同剪應(yīng)力條件下水相PAHs預(yù)測

PAHs在懸浮顆粒物與水體之間的平衡關(guān)系通常采用分配系數(shù)表示,如下所示:

式中: Kd為PAHs分配系數(shù); Cp為再懸浮顆粒物中PAHs的濃度,μg/kg;Cd為水相中PAHs的濃度,μg/L.

圖4 總懸浮顆粒物TOC含量與剪應(yīng)力的擬合關(guān)系Fig.4 Relationship between TOC in TSS and shear stress

表1 再懸浮顆粒物中PAHs質(zhì)量濃度同剪應(yīng)力關(guān)系Table 1 Relationship between PAHs concentrations in TSS normalize to dry weight and shear stress

目前,研究認(rèn)為顆粒物中有機(jī)碳對于 PAHs在顆粒物-水之間分配起著主導(dǎo)作用[1],因此PAHs在懸浮顆粒物-水體之間的分配系數(shù)也可用下式計算:

式中: Kd為PAHs分配系數(shù); foc為有機(jī)碳質(zhì)量分?jǐn)?shù); Koc為PAHs有機(jī)碳標(biāo)化分配系數(shù)[21];Koc和foc均以有機(jī)碳為質(zhì)量單位.

由方程1及方程2可推導(dǎo)得出水相PAHs濃度為:

考慮到再懸浮過程中其它因素對PAHs在懸浮顆粒物及水相之間分配的影響,增加一個系數(shù)m(不同PAHs的校正系數(shù)m值見表3),再懸浮過程上覆水體PAHs濃度為:式中:Cd為水相中PAHs的濃度;Cp為再懸浮顆粒物中PAHs的濃度;m為校正系數(shù);Kd為PAHs分配系數(shù);foc為懸浮顆粒物有機(jī)碳質(zhì)量分?jǐn)?shù);Koc為PAHs有機(jī)碳標(biāo)化分配系數(shù)[21].

表2 不同剪應(yīng)力TSS中PAHs體積濃度的預(yù)測值(ng/L)Table 2 The predicted PAHs concentrations in TSS on a volume normalized basis during resuspension events (ng/L)

利用方程(4)及校正系數(shù)m對不同剪應(yīng)力條件下沉積物再懸浮過程中上覆水體 PAHs濃度進(jìn)行了計算,結(jié)果見表3.

表3 不同剪應(yīng)力下水相PAHs濃度的預(yù)測值(μg/L)Table 3 The predicted PAHs concentrations in overlying water during resuspension events (μg/L)

2.4 預(yù)測值與實(shí)驗(yàn)值比較

利用不同剪應(yīng)力條件下擬合得到的總懸浮顆粒物中 PAHs質(zhì)量濃度、總懸浮顆粒物濃度及其TOC含量,對不同剪應(yīng)力條件下沉積物再懸浮過程中再懸浮顆粒物PAHs體積濃度及水相PAHs濃度進(jìn)行了預(yù)測,結(jié)果分別列于表2、表3,并與實(shí)驗(yàn)得到的結(jié)果進(jìn)行了比較,結(jié)果見圖5和圖6.

圖5 不同剪應(yīng)力下水相PAHs預(yù)測值與實(shí)測值的差異Fig.5 Comparison of predicted and measured PAHs concentrations in overlying water during resuspension experiments

圖6 不同剪應(yīng)力下再懸浮顆粒物中PAHs體積濃度預(yù)測值與實(shí)測值的差異Fig.6 Comparison of predicted and measured PAHs concentrations in TSS on volume normalized basis during resuspension experiments

由圖5和圖6可知,預(yù)測結(jié)果較好,其中低剪應(yīng)力條件下各 PAHs預(yù)測數(shù)據(jù)誤差比較大,高剪應(yīng)力(0.5N/m2)條件下各 PAHs預(yù)測數(shù)據(jù)較好.這主要是因?yàn)楦呒魬?yīng)力條件下,較強(qiáng)的擾動使得再懸浮顆粒物同水體接觸更充分,加速了 PAHs在水相及懸浮相中之間達(dá)到分配平衡,因此預(yù)測值與實(shí)驗(yàn)值的差異較小.通過預(yù)測值與實(shí)驗(yàn)值的對比可知:不同剪應(yīng)力條件下沉積物再懸浮過程中PAHs釋放預(yù)測模型可以較好的預(yù)測其在水環(huán)境中的釋放,模型具有可推廣價值.

3 結(jié)論

3.1 采用回歸分析建立了剪應(yīng)力與再懸浮顆粒物濃度、有機(jī)碳含量及PAHs質(zhì)量濃度之間的定量關(guān)系模型,應(yīng)用所得模型可預(yù)測不同剪應(yīng)力條件下再懸浮過程中上覆水體再懸浮顆粒物濃度、有機(jī)碳含量及PAHs濃度.

3.2 利用 PAHs顆粒物-水分配系數(shù),結(jié)合不同剪應(yīng)力條件下再懸浮顆粒物有機(jī)碳含量及PAHs質(zhì)量濃度定量關(guān)系模型,建立了不同剪應(yīng)力條件下再懸浮過程中上覆水體PAHs濃度預(yù)測模型.

3.3 通過模型預(yù)測結(jié)果與實(shí)驗(yàn)結(jié)果的比較,表明模型具有良好的預(yù)測效果,可用來進(jìn)行不同剪應(yīng)力條件下長江武漢段沉積物再懸浮過程中PAHs釋放的預(yù)測.

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Predictive model of PAHs release during the resuspension of sediments from Wuhan catchment of Yangtze River.

FENG Jing-lan1,2, QI Qiao-yan1, SUN Jian-hui1*(1.Henan Key Laboratory of Environmental Pollution Control, Key Laboratory for Yellow River and Huai Rviver Water Environment and Pollution Control, Ministry of Education, College of Chemistry and Environmental Sciences, Henan Normal University, Xinxiang 453007, China；2.State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China). China Environmental Science, 2011,31(12)：2065～2070

Release of PAHs during the resuspension of sediment samples at Wuhan section of Yangtze River was determined using a particle entrainment simulator (PES). Quantitative relationships between applied shear and TSS concentrations, TOC in TSS, PAHs concentrations in TSS were set up by regression analysis, respectively. Combined these quantitative relationships with PAHs partition coefficients, mathematic models were developed to predict PAHs concentrations in overlying water during sediment resuspension. It was found that PAHs concentrations measured during simulated resuspension events were good in line with predictions by models. This indicated that mathematic models could provide good predictions of PAHs concentrations in overlying water during sediment resuspension under 0.2～0.5 N/m2.

polycyclic aromatic hydrocarbons；Yangtze River；release；sediment resuspension；predictive model

X522

A

1000-6923(2011)12-2065-06

2011-03-14

國家自然科學(xué)基金資助項(xiàng)目(41103071,40701166)

* 責(zé)任作者, 教授,sunjh-hj@yahoo.com.cn

馮精蘭(1979-),女,河南新鄉(xiāng)人,副教授,博士,主要從事水體中持久性有機(jī)污染物環(huán)境行為研究.發(fā)表論文15篇.