余綿梓,袁 嘯,李適宇,2*,胡嘉鏜,2**

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典型PPCPs在河流沉積物中的吸附特性

余綿梓1,袁 嘯1,李適宇1,2*,胡嘉鏜1,2**

(1.中山大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣東 廣州 510275；2.廣東省環(huán)境污染控制與修復(fù)技術(shù)重點(diǎn)實(shí)驗(yàn)室,廣東 廣州510275)

選取了咖啡因、氯霉素、卡馬西平、磺胺甲噁唑和三氯生等5種PPCPs,在實(shí)驗(yàn)室條件下,近似模擬了自然河流的水/沉積物界面,應(yīng)用中心復(fù)合實(shí)驗(yàn)設(shè)計(jì),考察了溫度、pH值、有機(jī)質(zhì)含量和流速對(duì)PPCPs吸附比例的影響;利用多元回歸方程擬合實(shí)驗(yàn)數(shù)據(jù)得到PPCPs吸附模型,并利用獨(dú)立數(shù)據(jù)對(duì)模型進(jìn)行驗(yàn)證,以建立適用于自然河流的PPCPs吸附模型.實(shí)驗(yàn)結(jié)果表明,咖啡因和卡馬西平的吸附過程是放熱反應(yīng),而磺胺甲噁唑、氯霉素和三氯生的吸附為吸熱反應(yīng);pH值的升高對(duì)磺胺甲噁唑和三氯生的吸附能力產(chǎn)生抑制,但能促進(jìn)咖啡因吸附,而對(duì)氯霉素和卡馬西平的影響不大;有機(jī)質(zhì)含量、流速和初始濃度對(duì)5種PPCPs的吸附比例變化趨勢(shì)影響一致,隨著各因素濃度或速度升高,PPCPs的吸附比例均隨之增大,但影響程度有所不同.另外,吸附模型結(jié)果表明,5種PPCPs的吸附比例擬合值與實(shí)測(cè)值相關(guān)系數(shù)均達(dá)到0.8以上;獨(dú)立數(shù)據(jù)驗(yàn)證結(jié)果表明,擬合值與實(shí)測(cè)值相關(guān)系數(shù)均達(dá)到0.8以上;因此,多元回歸方程能較好地?cái)M合環(huán)境因素與PPCPs吸附之間的關(guān)系,同時(shí),在所考察的環(huán)境因素濃度范圍內(nèi),多元回歸方程較好地預(yù)測(cè)了PPCPs在自然河流沉積物中的吸附行為.

藥物和個(gè)人護(hù)理品(PPCPs)；吸附；沉積物；影響因素；多元回歸方程

藥物和個(gè)人護(hù)理品(PPCPs)的概念于1999年提出[1],由于使用量大、使用次數(shù)多,導(dǎo)致PPCPs持續(xù)不斷進(jìn)入河流、土壤和地下水等環(huán)境介質(zhì)中,尤其是在河流中的檢出率最高[2],近年來受到廣泛關(guān)注.相關(guān)研究表明,PPCPs在水環(huán)境中的暴露會(huì)對(duì)水生生物產(chǎn)生毒害作用[3],并通過食物鏈累積和人類食用行為進(jìn)入人體[4],對(duì)人類健康和生態(tài)系統(tǒng)構(gòu)成極大的威脅[5].

PPCPs進(jìn)入水環(huán)境中會(huì)發(fā)生擴(kuò)散、降解和吸附等一系列遷移轉(zhuǎn)化過程,其中PPCPs在底層水和沉積物間的吸附過程是影響PPCPs最終歸趨的關(guān)鍵過程[6],直接影響到PPCPs在水體中的分布及其生物可利用度[7].當(dāng)沉積物受到水流沖刷時(shí),水/沉積物界面的PPCPs則會(huì)通過擴(kuò)散、再懸浮等途徑,重新進(jìn)入上覆水體,引起水體二次污染[8].另外,PPCPs吸附到沉積物之后會(huì)發(fā)生垂向遷移過程,不僅會(huì)影響水流和溶質(zhì)的運(yùn)移,還對(duì)深層沉積物和地下水造成危害[9].因此,了解 PPCPs在沉積物中的吸附行為和吸附特性是一項(xiàng)重要的研究內(nèi)容,對(duì)研究水環(huán)境中PPCPs的遷移轉(zhuǎn)化和歸趨有著重要意義.

研究表明,pH值[10]、有機(jī)質(zhì)含量[11]和水體擾動(dòng)程度[12]等因素對(duì)PPCPs的吸附結(jié)果均會(huì)產(chǎn)生影響.然而,目前對(duì)環(huán)境因素影響PPCPs吸附的研究多依據(jù)經(jīng)濟(jì)合作與發(fā)展組織化學(xué)品測(cè)試準(zhǔn)則OECD guideline 106批實(shí)驗(yàn)方法,且通過單因素實(shí)驗(yàn)進(jìn)行[13],該方法無法模擬真實(shí)河流中的水/沉積物界面,而且單因素實(shí)驗(yàn)難以反映自然環(huán)境下多因素構(gòu)成的復(fù)雜體系,從而導(dǎo)致實(shí)驗(yàn)研究與實(shí)際測(cè)定的結(jié)果往往出現(xiàn)偏差[14].另一方面,關(guān)于PPCPs吸附模型,目前大多采用吸附動(dòng)力學(xué)和吸附等溫式模型,但上述模型均未考慮環(huán)境因素的作用[15-16],因此在實(shí)際應(yīng)用中擬合結(jié)果與實(shí)際測(cè)定結(jié)果存在較大誤差[17].為了考慮不同環(huán)境因素對(duì)PPCPs吸附的影響,多元回歸方程的應(yīng)用越來越得到重視.例如,Franco等[18]采用多元回歸方程較好地?cái)M合了有機(jī)物吸附量與辛醇水分配系數(shù)及電離常數(shù)之間的關(guān)系;Al- Khazrajy等[19]同樣利用多元線性回歸擬合了5種PPCPs在不同土壤中的吸附,均取得較好的擬合效果.然而,上述研究中所采用的擬合數(shù)據(jù)僅限于所研究區(qū)域的沉積物,對(duì)于環(huán)境條件不同的其它區(qū)域,模型的應(yīng)用效果仍未可知.

總體而言,目前對(duì)PPCPs在河流沉積物中的吸附行為研究尚有待深入,特別是模擬真實(shí)河流環(huán)境中多種因素影響下的吸附過程,另外,適用于描述自然水體的PPCPs吸附過程的數(shù)學(xué)模型亦有待進(jìn)一步建立.因此,本文選取卡馬西平、三氯生、氯霉素、磺胺甲噁唑和咖啡因等5種典型PPCPs(理化性質(zhì)如表1)為研究對(duì)象,在實(shí)驗(yàn)室條件下近似模擬自然河流的水/沉積物界面,應(yīng)用中心復(fù)合實(shí)驗(yàn)設(shè)計(jì)(CCD),考察pH值、溫度、有機(jī)質(zhì)含量和流速等4種環(huán)境因素對(duì)不同性質(zhì)PPCPs吸附的影響;在此基礎(chǔ)上,利用多元回歸方程對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合,并結(jié)合獨(dú)立數(shù)據(jù)對(duì)擬合模型進(jìn)行預(yù)測(cè)和驗(yàn)證,以建立適用于所考察因素濃度水平內(nèi)的PPCPs在河流沉積物中的吸附模型.

表1 目標(biāo)物的理化性質(zhì)

1 材料與方法

1.1 沉積物樣品與實(shí)驗(yàn)用品

沉積物樣品:樣品采集自廣州市石井河的表層沉積物(0~5cm),采集的沉積物樣品經(jīng)風(fēng)干后過2mm 篩,在550℃下灼燒1h以去除有機(jī)質(zhì)及咖啡因,保存在4℃下待用.

實(shí)驗(yàn)藥品:咖啡因(CAF)純度99.5%、氯霉素(CAP)純度99.5%、卡馬西平(CBZ)純度99.5%、磺胺甲噁唑(SMX)純度99.5%和三氯生(TCS)純度99.5%均購自德國Dr. Ehrenstorfe公司.甲醇和乙腈(色譜純)購自德國Merck公司;腐植酸(HA)購自美國Aladdin公司.

PPCPs儲(chǔ)備液:取一定量PPCPs標(biāo)準(zhǔn)品,用甲醇溶解配成濃度為1g/L的儲(chǔ)備液,于-18℃下保存待用.

PPCPs標(biāo)準(zhǔn)使用液:取一定體積儲(chǔ)備液于50mL容量瓶中,用0.01mol/L CaCl2超純水定容,加入10mmol/L NaN3以抑制微生物作用.

實(shí)驗(yàn)儀器:三重四極桿液相色譜-質(zhì)譜聯(lián)用儀(LC-MS/MS,Thermo Fisher Scientific)、SPE固相萃取裝置(Automatic Science, China),pH計(jì)(Mettler Toledo, Swiss)、超純水儀(Millipore, Germany)、氮吹儀(Anpel Scientific, China).

1.2 實(shí)驗(yàn)方法

吸附實(shí)驗(yàn)在OECD Guideline 106[13]基礎(chǔ)上改進(jìn),以在實(shí)驗(yàn)室條件下近似模擬自然河流的水/沉積物界面,裝置如圖1所示.實(shí)驗(yàn)在一系列500mL燒杯中進(jìn)行,取20g純凈沉積物樣品平鋪于干凈燒杯中,加入20mL超純水在暗處浸潤24h使沉積物飽和[9].將上清液用針筒抽取去除.再用針筒緩慢加入100mL含100ng/L PPCPs溶液.溶液中含0.01mol/L CaCl2及100mg/L疊氮化鈉,CaCl2用以保持PPCPs的電離度,疊氮化鈉用以去除微生物影響;溶液中甲醇控制在0.1%以下以避免發(fā)生共溶現(xiàn)象.

圖1 吸附實(shí)驗(yàn)裝置

實(shí)驗(yàn)前,潔凈沉積物樣品在121℃下高壓滅菌15min 以去除微生物[24],并調(diào)節(jié)不同的pH值、溫度、有機(jī)質(zhì)含量、初始濃度和流速等條件,考察各因素對(duì)咖啡因在河流沉積物中吸附的影響.各因素的濃度水平根據(jù)響應(yīng)面分析軟件Design Expert 8.0.6中的中心復(fù)合實(shí)驗(yàn)設(shè)計(jì)確定,取值范圍和大小如表2所示,得到實(shí)驗(yàn)設(shè)計(jì)共86組.其中,pH值通過0.1mol/L HCl或0.1mol/L NaOH調(diào)節(jié).有機(jī)質(zhì)含量通過添加HA進(jìn)行調(diào)節(jié);由于自然河流沉積物中有機(jī)質(zhì)的成分復(fù)雜,在實(shí)驗(yàn)室條件下難以模擬,而前人研究表明,底泥中天然有機(jī)質(zhì)的主要成分為HA[25-26],因此本實(shí)驗(yàn)中有機(jī)質(zhì)含量的模擬通過在沉積物樣品中添加HA進(jìn)行,將一定量HA加入沉積物中,加入20mL超純水?dāng)嚢杈鶆蚝笤诎堤幗?4h,使HA與沉積物充分吸附同時(shí)使沉積物飽和[9].流速與溫度通過水浴恒溫振蕩器控制;流速大小的主要影響是改變水體的攪動(dòng)狀況,由于在實(shí)驗(yàn)室條件下難以模擬真實(shí)河流的水況,有學(xué)者將流速轉(zhuǎn)換為振蕩器轉(zhuǎn)速以模擬自然條件下不同的水體攪動(dòng)狀況[12,27].本文采用王華等[28]通過大規(guī)模同步監(jiān)測(cè)及泥沙懸浮實(shí)驗(yàn)建立的振蕩器轉(zhuǎn)速與水體流速之間的等量轉(zhuǎn)化關(guān)系,其公式為

式中:為流速, m/s,為振蕩器轉(zhuǎn)速, r/min.

實(shí)驗(yàn)以不含底泥樣品作為對(duì)照以考察水解、壁吸附等因素的影響,樣品外裹錫紙以遮光去除光解作用.

取樣后在5000r/min下離心10min,取上清液經(jīng)0.22μm濾膜過濾,濾液經(jīng)固相萃取和氮吹定容后進(jìn)行LC-MS/MS測(cè)定.咖啡因的吸附特性通過吸附比例K表示,可通過式(2)計(jì)算:

式中:K為PPCPs的吸附比例;0為PPCPs的初始濃度, ng/L;C為取樣點(diǎn)小時(shí)實(shí)驗(yàn)組上清液PPCPs濃度, ng/L;C為取樣時(shí)間點(diǎn)時(shí)對(duì)照組中咖啡因濃度, ng/L.

表2 影響因素的大小及范圍

1.3 PPCPs測(cè)定

采用LC-MS/MS對(duì)實(shí)驗(yàn)樣品的PPCPs濃度進(jìn)行測(cè)定.流動(dòng)相為0.1%甲酸水溶液:乙腈=85:15,流速為0.3mL/min,進(jìn)樣量為2μL,咖啡因、卡馬西平、氯霉素、磺胺甲噁唑和三氯生的保留時(shí)間分別為1.55, 5.04,3.96,3.37和8.00min.

2 結(jié)果與討論

2.1 環(huán)境因素對(duì)PPCPs吸附的影響

2.1.1 溫度對(duì)PPCPs吸附的影響 由圖2可知,隨著溫度的升高,咖啡因和卡馬西平的吸附比例出現(xiàn)下降,磺胺甲噁唑、氯霉素和三氯生的吸附比例則上升.在pH值為7.5、初始濃度為500ng/L、有機(jī)質(zhì)含量為2.5%、流速為0.5m/s、吸附時(shí)長為36h的條件下,隨著溫度從15℃升高到40℃,咖啡因的吸附比例下降了約10%,卡馬西平的吸附比例則下降了5%;而其他3種物質(zhì)的吸附比例均隨之增加約5%.

從溫度對(duì)PPCPs吸附比例的影響可以看出,咖啡因和卡馬西平的吸附過程是放熱反應(yīng),吸附程度隨著溫度的升高而降低;這是由于溫度升高時(shí),吸附平衡會(huì)向逆反應(yīng)方向移動(dòng),同時(shí)物質(zhì)的溶解度增大,從而抑制吸附反應(yīng)[29];此外,溫度升高還一定程度上促進(jìn)了解吸過程的發(fā)生,因此,溫度的升高使咖啡因和卡馬西平的吸附比例降低[30].在天然河流的溫度范圍內(nèi)(~20℃),溫度變化對(duì)咖啡因吸附的影響較小,當(dāng)溫度大于30℃并向上遞增時(shí),咖啡因的吸附比例才出現(xiàn)較明顯的下降趨勢(shì).由于溫度的升高同樣會(huì)促進(jìn)分子的擴(kuò)散,使得咖啡因與沉積物之間接觸增多,增加吸附反應(yīng)發(fā)生的機(jī)會(huì),抵消一部分對(duì)吸附反應(yīng)的抑制作用,因此溫度較高時(shí),升高溫度對(duì)咖啡因吸附的抑制作用相比溫度較低時(shí)更明顯.研究表明,大部分PPCPs的吸附反應(yīng)是放熱過程,吸附程度隨著溫度的升高而降低.如Wang等[31]研究表明卡馬西平在石墨上的吸附過程為放熱反應(yīng).本研究中磺胺甲噁唑、氯霉素和三氯生的吸附比例隨著溫度的升高而略有增大,表明其吸附過程為吸熱反應(yīng),也可能是由于分子間無規(guī)則運(yùn)動(dòng)即分子擺脫束縛引起的[32].王凱等[33]的研究也表明,磺胺甲噁唑的吸附是吸熱反應(yīng),吸附平衡量隨著溫度升高而升高,但變化幅度不大.

圖2 溫度對(duì)PPCPs吸附的影響

2.1.2 pH值對(duì)PPCPs吸附的影響 如圖3所示.磺胺甲噁唑和三氯生的吸附比例隨著pH值升高而降低,咖啡因的吸附比例在pH值約為10左右達(dá)到最高值,而氯霉素和卡馬西平則基本不受pH值變化的影響.

pH值對(duì)PPCPs吸附的影響主要通過改變PPCPs的分子形態(tài),進(jìn)而影響其與沉積物顆粒的吸附機(jī)理從而影響吸附的程度.當(dāng)pH值低于物質(zhì)的p值時(shí),酸性物質(zhì)大部分保持質(zhì)子態(tài)[34],比電離態(tài)更容易吸附,堿性物質(zhì)則帶有正電而易于吸附;隨著pH值升高,酸性物質(zhì)更加趨向電離帶負(fù)電,吸附能力減弱,而堿性物質(zhì)則水解趨向帶正電,疏水性有所增大,吸附能力增強(qiáng)對(duì)于兩性離子,pH值較低時(shí),陽離子基團(tuán)占主導(dǎo),隨著pH值升高,去質(zhì)子化作用增強(qiáng),吸附能力下降.從實(shí)驗(yàn)結(jié)果可以看出,與沉積物之間的靜電反應(yīng)是三氯生、磺胺甲噁唑和咖啡因的吸附機(jī)理之一,而卡馬西平和氯霉素則基本不通過靜電反應(yīng)進(jìn)行吸附.

三氯生的分子結(jié)構(gòu)上含有一個(gè)—OH和多個(gè)Cl 基團(tuán),p= 7.8,呈弱酸性.在pH為7左右時(shí),三氯生分子中83%~99%帶正電,因此吸附能力較強(qiáng)[21];當(dāng)pH>7.8時(shí),三氯生主要以陰離子形式存在,因此表面帶有負(fù)電荷的沉積物對(duì)其吸附效果較差[33].磺胺甲噁唑同樣是一種酸性物質(zhì),在天然pH值條件下同時(shí)存在離子態(tài)和質(zhì)子態(tài)[35],隨著pH值變化,不同形態(tài)的磺胺甲噁唑所占比例也隨之變化.離子態(tài)分子由于帶有負(fù)電,與沉積物之間存在靜電排斥作用而吸附能力較弱,因此磺胺甲噁唑的吸附主要依靠質(zhì)子態(tài)分子的憎水反應(yīng)[36].

咖啡因是一種堿性PPCPs,p值為10.4,因此在本研究pH值范圍內(nèi)(4~11),當(dāng)pH值小于p值時(shí),咖啡因主要呈現(xiàn)電離態(tài),在水體中帶正電,而沉積物顆粒帶負(fù)電,因此咖啡因與沉積物顆粒之間可以通過靜電反應(yīng)進(jìn)行吸附[37].從結(jié)果可以看出,pH值的升高咖啡因趨向于帶正電,與帶負(fù)電荷的沉積物顆粒之間相互作用力增強(qiáng).但pH值大于pKa時(shí),對(duì)咖啡因的水解產(chǎn)生抑制作用,從而降低吸附比例.

卡馬西平和氯霉素的pKa值均較高(分別為13.9和11.03),為中性物質(zhì),憎水性吸附是主要的吸附機(jī)理,在本研究pH值范圍內(nèi),質(zhì)子態(tài)是卡馬西平和氯霉素的主要形態(tài),pH值對(duì)其影響很小[38].

圖3 pH值對(duì)PPCPs吸附的影響

圖4 有機(jī)質(zhì)含量對(duì)PPCPs吸附的影響

2.1.3 有機(jī)質(zhì)含量對(duì)PPCPs吸附的影響 如圖4所示,有機(jī)質(zhì)含量增大,在不同程度上促進(jìn)PPCPs的吸附,其中影響最大的為卡馬西平,在吸附進(jìn)行13h未達(dá)到平衡時(shí),有機(jī)質(zhì)含量從0%升高到5%,卡馬西平的吸附比例提高約40%,但隨著接近吸附平衡,有機(jī)質(zhì)含量變化的影響逐漸減弱.但對(duì)于其他4種PPCPs,有機(jī)質(zhì)含量變化的影響程度較卡馬西平小.

圖5 流速對(duì)PPCPs吸附的影響

圖6 初始濃度對(duì)PPCPs吸附的影響

有機(jī)質(zhì)可以通過配體交換、陽離子橋接與陰離子交換等機(jī)制吸附至沉積物顆粒[39],同時(shí)一部分以溶解態(tài)形式溶于沉積物間隙水中.沉積物中的有機(jī)質(zhì)與PPCPs進(jìn)行結(jié)合是PPCPs吸附的途徑之一,吸附態(tài)的有機(jī)質(zhì)可以給PPCPs提供特異性吸附點(diǎn)位;此外,由于多數(shù)PPCPs帶有一定的疏水性,可通過疏水反應(yīng)與有機(jī)質(zhì)進(jìn)行吸附,然后通過有機(jī)質(zhì)與沉積物顆粒的吸附間接吸附在沉積物中[40-41].從有機(jī)質(zhì)含量對(duì)卡馬西平吸附的影響可以看出,卡馬西平和沉積物中有機(jī)質(zhì)之間的反應(yīng)是其主要的吸附機(jī)理,隨著有機(jī)質(zhì)含量的增大,能夠提供更多吸附點(diǎn)位與卡馬西平進(jìn)行憎水反應(yīng)[41],極性官能團(tuán)數(shù)量也隨之增多,可以作為π受體與卡馬西平進(jìn)行吸附[42].從有機(jī)質(zhì)含量對(duì)其他4種PPCPs吸附的影響可以看出,其他4種物質(zhì)不僅通過有機(jī)質(zhì)進(jìn)行吸附,還可通過范德華力、氫鍵作用等途徑與沉積物顆粒進(jìn)行吸附[10],但有機(jī)質(zhì)含量的增大仍有一定的促進(jìn)作用,表明與有機(jī)質(zhì)進(jìn)行吸附也是這幾種PPCPs進(jìn)行吸附的途徑.

2.1.4 流速對(duì)PPCPs吸附的影響 如圖5所示,隨著流速的增大,5種PPCPs的吸附比例均隨之增大,并且增大幅度隨著反應(yīng)時(shí)間的增長而減小;除卡馬西平外,其余4種PPCPs吸附比例在57h后均達(dá)到最大值,而卡馬西平吸附比例在57h后仍隨著流速增大而有所增加.

流速是自然河流重要的水動(dòng)力指標(biāo),在不同時(shí)間、河流不同區(qū)域的流速均會(huì)存在差異,改變沉積物上覆水體的攪動(dòng)狀況,水體的攪動(dòng)會(huì)使沉積物顆粒再懸浮,增大顆粒與PPCPs之間的接觸面積,更有利于吸附過程的進(jìn)行[43].由于在實(shí)驗(yàn)室條件下難以模擬真實(shí)河流的水況,因此關(guān)于流速對(duì)吸附的影響還鮮有研究.Hajj-Mohamad等[12]通過設(shè)置靜態(tài)和振蕩條件,近似模擬了水體的攪動(dòng)和靜置狀態(tài),并對(duì)比了靜態(tài)和振蕩條件下4種藥物的吸附差異,發(fā)現(xiàn)振蕩條件下4種藥物的吸附能力均大于靜態(tài)條件;但該研究只是設(shè)置了2種條件,而沒有研究不同的攪動(dòng)狀態(tài)對(duì)藥物吸附的影響.由于流速的模擬在實(shí)驗(yàn)室條件下難以進(jìn)行,本研究將流速大小轉(zhuǎn)化為振蕩器轉(zhuǎn)速,有效地模擬了不同攪動(dòng)水況對(duì)PPCPs吸附的影響.水體的攪動(dòng)能促進(jìn)PPCPs的吸附,攪動(dòng)幅度越大,PPCPs的吸附比例越高;但這種促進(jìn)作用隨著反應(yīng)時(shí)間的增長而減弱,這是由于隨著反應(yīng)時(shí)間增長,吸附反應(yīng)已經(jīng)接近平衡,且攪動(dòng)情況下沉積物顆粒與水體接觸,也有利于解吸的進(jìn)行,因此可以從結(jié)果看到,達(dá)到吸附平衡時(shí),流速的變化對(duì)最終吸附比例沒有影響.

2.1.5 初始濃度對(duì)PPCPs吸附的影響 如圖6所示,在實(shí)驗(yàn)條件范圍內(nèi),5種PPCPs的吸附比例均隨著初始濃度的增加而增大,增大幅度約為5%,但均呈現(xiàn)出隨著吸附時(shí)間增大而幅度降低的趨勢(shì).

前人研究多用吸附等溫式對(duì)不同濃度下PPCPs的吸附進(jìn)行模擬,如Wang等[31]研究表明卡馬西平和磺胺甲噁唑的吸附等溫式更符合Langmiur模型,磺胺甲噁唑則更符合Freundlich模型;Xu等[34]通過實(shí)驗(yàn)分別對(duì)三氯生和磺胺甲噁唑的吸附等溫式進(jìn)行擬合,發(fā)現(xiàn)兩者的吸附等溫式均符合Freundlich公式,吸附能力隨著初始濃度的增大而增強(qiáng),但公式擬合結(jié)果中值不為1,說明2種物質(zhì)的吸附等溫式并不是線性的,不僅僅受到初始濃度的影響,還與其他環(huán)境因素有關(guān).

本研究為模擬真實(shí)河流中PPCPs的濃度(ng/L級(jí)別),初始濃度的取值比一般實(shí)驗(yàn)室研究的濃度(mg/L)較小,因此吸附點(diǎn)位對(duì)于PPCPs而言相對(duì)充足[22],雖然初始濃度的增大能促進(jìn)PPCPs的吸附,但其影響較小.

2.2 多元回歸方程

PPCPs吸附模型的構(gòu)建利用多元回歸方程進(jìn)行.采用多元逐步回歸方法[44],考察PPCPs吸附比例與各因素之間的關(guān)系,并應(yīng)用主成分分析方法篩選出各PPCPs的主要影響因素,以建立較為簡化的PPCPs吸附模型.本研究模型的構(gòu)建在SPSS 22.0中完成,將pH值、溫度、有機(jī)質(zhì)、初始濃度、流速和時(shí)間作為輸入量,吸附比例K作為輸出量.實(shí)驗(yàn)數(shù)據(jù)共86組,隨機(jī)抽取其中9組作為獨(dú)立驗(yàn)證數(shù)據(jù),另外77組作為擬合數(shù)據(jù);模型的可靠性通過擬合值與實(shí)測(cè)值的相關(guān)系數(shù)分析.

得到簡化的PPCPs吸附模型如表3所示,式中參數(shù)乘積項(xiàng)表示2個(gè)因素對(duì)吸附比例的交互作用.從結(jié)果可以看出,擬合值與實(shí)測(cè)值的相關(guān)系數(shù)均大于0.8,相關(guān)性較強(qiáng),表明5種PPCPs吸附模型均可以較好地?cái)M合出環(huán)境因素與吸附比例之間的關(guān)系.從主成分分析結(jié)果可以看出,5種PPCPs吸附模型的主要影響因素分別為:咖啡因(溫度、時(shí)間和流速),氯霉素(溫度、初始濃度和時(shí)間),卡馬西平(時(shí)間、有機(jī)質(zhì)含量和流速),磺胺甲噁唑(時(shí)間和流速)及三氯生(時(shí)間和流速).可以看出,時(shí)間因素對(duì)5種PPCPs的吸附均有較大影響;除氯霉素外,其他4種PPCPs吸附的主要影響因素均包含流速;此外,溫度、初始濃度和有機(jī)質(zhì)含量也為主要的因素,而pH值對(duì)5種PPCPs的吸附影響程度均較小.

為檢驗(yàn)?zāi)Ｐ偷膽?yīng)用效果,利用所建立的模型對(duì)獨(dú)立驗(yàn)證數(shù)據(jù)進(jìn)行驗(yàn)證,結(jié)果如圖7所示.從結(jié)果可以看出,5種PPCPs吸附模型對(duì)驗(yàn)證數(shù)據(jù)的擬合值與實(shí)測(cè)值相關(guān)系數(shù)均達(dá)到0.8以上,相關(guān)性較強(qiáng),表明吸附模型可以很好地應(yīng)用于PPCPs吸附結(jié)果的預(yù)測(cè).

多元回歸方程近年來開始被應(yīng)用于吸附模型的建立.如Radka等[45]采用多元回歸方程擬合7種有機(jī)物的Freundlich系數(shù),擬合值與實(shí)測(cè)值的相關(guān)系數(shù)可達(dá)到0.97;He等[46]同樣采用多元回歸方程,建立五氯酚在不同土壤中的吸附模型,擬合值與實(shí)測(cè)值相關(guān)系數(shù)均達(dá)到0.8以上.但上述研究中,均未利用獨(dú)立數(shù)據(jù)對(duì)模型進(jìn)行驗(yàn)證,因而吸附模型的應(yīng)用效果未知.本研究中,多元回歸方程能較好地?cái)M合環(huán)境因素與PPCPs吸附之間的關(guān)系,且利用獨(dú)立數(shù)據(jù)進(jìn)行驗(yàn)證,結(jié)果表明了多元回歸方程應(yīng)用于實(shí)際預(yù)測(cè)中的可行性.

注::溫度(℃);:初始濃度(ng/L);:時(shí)間(h),:pH值;:有機(jī)質(zhì)含量(%);:流速(m/s).

3 結(jié)論

3.1 在實(shí)驗(yàn)室條件下近似模擬了自然河流中的水/沉積物界面,綜合考察了不同環(huán)境因素對(duì)咖啡因、氯霉素、卡馬西平、磺胺甲噁唑和三氯生等5種PPCPs在沉積物中吸附的影響,并通過擬合和驗(yàn)證多元回歸方程,得到適用于自然河流的PPCPs吸附模型.實(shí)驗(yàn)結(jié)果表明,咖啡因和卡馬西平的吸附過程是放熱反應(yīng),而磺胺甲噁唑、氯霉素和三氯生的吸附為吸熱反應(yīng);pH值升高抑制磺胺甲噁唑和三氯生的吸附能力,而促進(jìn)咖啡因的吸附過程,但對(duì)氯霉素和卡馬西平的影響不大.隨著有機(jī)質(zhì)含量、流速和初始濃度值的增大,5種PPCPs的吸附比例均隨之增大,但增大程度有所不同.

3.2 通過對(duì)77組實(shí)驗(yàn)數(shù)據(jù)進(jìn)行多元回歸方程擬合,5種PPCPs的吸附比例擬合值與實(shí)測(cè)值相關(guān)系數(shù)均達(dá)到0.8以上.利用模型對(duì)9組獨(dú)立驗(yàn)證數(shù)據(jù)進(jìn)行預(yù)測(cè),擬合值與實(shí)測(cè)值相關(guān)系數(shù)也均達(dá)到0.8以上;結(jié)果表明,多元回歸方程能較好地?cái)M合環(huán)境因素與PPCPs吸附之間的關(guān)系,驗(yàn)證結(jié)果表明,在所考察的環(huán)境因素取值范圍內(nèi),將其應(yīng)用于預(yù)測(cè)自然河流中PPCPs在沉積物中的吸附是可行的.

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致謝：中山大學(xué)環(huán)境科學(xué)與工程學(xué)院樊青娟老師對(duì)本研究中PPCPs的測(cè)定提供了幫助,劉廣州和徐闖同學(xué)在現(xiàn)場(chǎng)采樣過程中提供了幫助,在此一并表示感謝！

Experimental and modeling study of sorption characteristics of selected PPCPs onto river sediments.

YU Mian-zi1, YUAN Xiao1, LI Shi-yu1,2*, HU Jia-tang1,2**

(1.School of Environmental Science and Engineering, Sun Yat-sun University, Guangzhou 510275, China；2.Guangdong Province Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China)., 2019,39(4)：1724~1733

A water/sediment interface was roughly simulated in laboratory, and a central composite design was applied to investigate the influence of temperature, pH, organic content, and velocity on the sorption proprotion of five selected PPCPs (caffeine, chloramphenicol, carbamazepine, sulfamethoxazole and triclosan). Based on the experimental data, multiple regression equations were fitted and calibrated to establish favorable sorption models for PPCPs in the river systems. The results showed that the sorption process of caffeine and carbamazepine were exothermic reaction, and the sorption of sulfamethoxazole, chloramphenicol and triclosan were endothermic reaction. The increase of pH inhibited the sorption capacity of sulfamethoxazole and triclosan, but it could promote the sorption of caffeine, while it had little effect on the sorption of chloramphenicol and carbamazepine. The organic content, velocity and initial concentration had consistent effect on the sorption ratio of PPCPs. With the increase of the values of the factors, the sorption ratio of the five PPCPs increased with different degrees. The fitting and calibration results showed that the correlation coefficients between fitted and measured values of PPCPs sorption proportions were both above 0.8. Therefore, within the ranges of concentration of the investigated factors, the multiple regression equations were able to reasonably model and predict the sorption of PPCPs onto rivers sediment.

PPCPs；sorption；sediment；parameters；multiple regression equation

X522

A

1000-6923(2019)04-1724-10

2018-08-03

廣東省自然科學(xué)基金資助項(xiàng)目(2018A030313135);中央高校基本科研業(yè)務(wù)費(fèi)(20133800031650007);國家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFC0502803)

*責(zé)任作者, 教授, eeslsy@mail.sysu.edu.cn; 副教授, hujtang@mail.sysu. edu.cn

余綿梓(1993-),男,廣東潮州人,中山大學(xué)碩士研究生,主要從事PPCPs吸附模擬研究.發(fā)表論文1篇.