劉殿威,杜曉麗,2*,付霄宇,崔申申,代昕怡
城市地表徑流膠體與溶解性有機(jī)物結(jié)合特性
劉殿威1,杜曉麗1,2*,付霄宇1,崔申申1,代昕怡1
(1.北京建筑大學(xué)環(huán)境與能源工程學(xué)院,城市雨水系統(tǒng)與水環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,北京 100044;2.北京節(jié)能減排與城鄉(xiāng)可持續(xù)發(fā)展省部共建協(xié)同創(chuàng)新中心,北京 100044)
為探究城市地表徑流膠體與溶解性有機(jī)物(DOM)結(jié)合特性,考察了不同pH值條件下徑流膠體與DOM的吸附行為,并揭示其主要作用機(jī)制.結(jié)果表明,Langmuir模型可以較好地描述不同pH值時(shí)徑流膠體對(duì)DOM的等溫吸附過程,且隨pH值增加其最大吸附量逐漸減小,pH值為3.0的最大平衡吸附量是pH值為6.0時(shí)的4.0倍;與DOM結(jié)合后,徑流膠體Zeta電位絕對(duì)值升高,膠體更穩(wěn)定且更易遷移;徑流DOM中大分子組分更易與徑流膠體結(jié)合;徑流DOM中疏水酸性組分(HoA)和疏水中性組分(HoN)含量較高,分別占總DOC濃度的35.0%和24.3%,且這兩種組分最易與膠體結(jié)合;徑流膠體和DOM的結(jié)合作用主要通過羥基取代、表面絡(luò)合和靜電吸引實(shí)現(xiàn).
地表徑流;膠體;溶解性有機(jī)物;吸附;影響因素
膠體與溶解性有機(jī)物(DOM)是兩種極為活躍的組分,它們不僅可成為許多有機(jī)、無機(jī)污染物的遷移載體或配位體,影響其遷移轉(zhuǎn)化,還可互相結(jié)合干擾,影響對(duì)方在環(huán)境中的行為.膠體與DOM結(jié)合會(huì)改變膠體表面特性,影響膠體穩(wěn)定性和遷移能力.同時(shí),膠體顆粒會(huì)通過配位交換、靜電吸引等形式成為DOM的吸附載體,影響DOM與其他污染物的結(jié)合遷移[1].Yang等[2]研究發(fā)現(xiàn),由于強(qiáng)架橋作用高濃度腐殖酸存在會(huì)明顯促進(jìn)生物炭膠體聚集,破壞其穩(wěn)定性;Wang等[3]研究發(fā)現(xiàn)除配體交換和靜電作用外,DOM中的腐殖酸可以通過疏水性作用促進(jìn)Fe-磁性納米膠體的再懸浮.宋佳[4]研究發(fā)現(xiàn)DOM中的酚羥基、羧基會(huì)與Fe(Ⅲ)復(fù)合膠體發(fā)生作用生成含F(xiàn)e(Ⅱ)的結(jié)構(gòu),提供了更多的吸附點(diǎn)位使As(Ⅴ)更易遷移.因此,深入探究膠體與DOM之間的結(jié)合交互作用,對(duì)實(shí)現(xiàn)膠體和DOM共去除、同步控制其它污染物遷移等具有重要意義.
截至目前,國內(nèi)外對(duì)地表徑流膠體與DOM之間的結(jié)合交互作用還鮮有報(bào)道.前期,筆者課題組已對(duì)地表徑流膠體污染特性[5-6]、DOM污染特性[7-8]等進(jìn)行了深入研究,發(fā)現(xiàn)地表徑流膠體會(huì)與DOM共存,且兩者均可攜帶其他污染物共遷移.本研究通過考察徑流膠體與DOM結(jié)合特性,探究兩者結(jié)合的主要作用機(jī)制,以期為有效控制地表徑流膠體-DOM復(fù)合污染及同步控制徑流中其它污染物等提供參考.
于2021年4月21日降雨產(chǎn)流后在北京市大興區(qū)興華大街車行道旁雨水篦處使用塑料采樣瓶人工采集地表徑流樣品10L,采樣結(jié)束后立即送回實(shí)驗(yàn)室.取部分徑流樣品經(jīng)0.2μm微孔濾膜(有機(jī)系,津騰,中國)過濾后置于旋轉(zhuǎn)蒸發(fā)儀(RE-52AA,上海亞榮生化儀器廠,中國)濃縮至溶解性有機(jī)碳濃度(DOC)約200mg/L,保存于4℃冰箱待用.
參考Gimbert等[9-10]的研究,將剩余的徑流混合樣品依據(jù)斯托克斯定律采用離心分離法逐步提取獲得0.2~10μm膠體懸濁液,取出5mL備用,剩余懸濁液經(jīng)冷凍干燥至粉末狀后置于550℃馬弗爐中高溫煅燒5h,冷卻后置于干燥器中保存待用.
將濃縮后的徑流DOM樣品準(zhǔn)確稀釋至DOC濃度5,10,15,20,30,40,50,60,80和100mg/L后各取35mL置于50mL具塞錐形瓶中,采用0.01mol/L的HCl和NaOH溶液分別調(diào)節(jié)pH值為3.0,6.0,8.0和11.0后各加入2mg徑流膠體粉末,置于恒溫振蕩器(DDHZ-300型,太倉市實(shí)驗(yàn)設(shè)備廠,中國)在298K、120r/min下震蕩12h后取10mL過0.22μm濾膜,采用總有機(jī)碳分析儀(Jena multi N/C 3100型,德國耶拿分析儀器股份公司)測定樣品中DOC濃度.上述實(shí)驗(yàn)進(jìn)行時(shí)另有2組樣品作為平行組同時(shí)進(jìn)行,DOC濃度均取3組濃度的均值進(jìn)行分析.
分別采用Langmuir模型和Freundlich模型對(duì)不同pH值條件下徑流膠體對(duì)DOM的等溫吸附數(shù)據(jù)進(jìn)行擬合.
將與徑流DOM結(jié)合前后的0.2~10μm范圍膠體懸濁液置于超聲波清洗器中超聲分散20min(各3組,每組5mL),使用0.01mol/L HCl和NaOH精確調(diào)節(jié)膠體懸濁液pH值為3.0,6.0,8.0和11.0后,采用激光粒度Zeta電位分析儀(Zetasizer Nano ZS90,Malvern Panalytical,英國)測定膠體懸濁液的Zeta電位值.
1.4.1 分子量分布 采用水溶性凝膠滲透色譜(GPC)柱(SB-803HQ,OHpak,日本)和配備了示差檢測器(S2020A,Schambeck SFD Gmbh,德國)的液相色譜儀(Bright LC,萊伯泰科有限公司,中國),通過高壓恒流輸液泵輸入流動(dòng)相,對(duì)pH值為6.0時(shí)與徑流膠體結(jié)合前后的DOM樣品進(jìn)行分子量分布測定.色譜條件為:過0.22μm有機(jī)濾膜的0.05mol/L磷酸二氫鈉溶液作流動(dòng)相,色譜柱溫35℃,色譜柱啟用穩(wěn)定時(shí)間2h,色譜柱監(jiān)測平衡時(shí)間10min,輸液泵流速2.0mL/min.具體操作如下:取過0.22μm濾膜的徑流DOM樣品0.1mL注入進(jìn)樣器中,由化學(xué)工作站獲得徑流DOM分子量分布數(shù)據(jù),使用Origin 2018進(jìn)行DOM分子量分布曲線繪制.
1.4.2 化學(xué)組分分級(jí) 參考Du等[11]的研究,利用Amberlite XAD-8、MSC和IRA-958三種樹脂柱串聯(lián),將pH值為6.0時(shí)與徑流膠體結(jié)合前后的地表徑流DOM分別提取為親水酸性組分(HiA)、親水堿性組分(HiB)、親水中性組分(HiN)、疏水酸性組分(HoA)、疏水堿性組分(HoB)和疏水中性組分(HoN),之后測定各化學(xué)組分DOC濃度.采用DOC總量平衡的方法來控制實(shí)驗(yàn)誤差,將提取后所有組分的DOC之和與原DOM的DOC總量誤差控制在10%以內(nèi).
將冷凍干燥得到的結(jié)合前后膠體粉末、DOM樣品粉末分別與光譜純KBr以質(zhì)量比1:100均勻混合,用壓片法制備薄膜,采用傅里葉變換紅外光譜儀(IS10FT-IR,Nicolet,美國)在波長4000~400cm-1對(duì)其進(jìn)行紅外光譜分析.
2.1.1 不同pH值條件時(shí)徑流膠體對(duì)DOM的等溫吸附過程 由圖1可知,pH值為3.0時(shí)徑流膠體對(duì)DOM的最大平衡吸附量明顯高于pH值為6.0,8.0和11.0時(shí),且pH值為3.0時(shí)的最大平衡吸附量(385.212mg/g)是pH值為6.0時(shí)(96.224mg/g)的4.0倍,同時(shí)隨pH值增加,最大吸附量逐漸減小;此外,pH值為6.0,8.0和11.0時(shí)其平衡吸附量雖有差異,但均小于100mg/g.由表1可以看出,不同pH值條件下,Langmuir模型可以更好地描述徑流膠體對(duì)DOM的等溫吸附過程,其2值均高于Freundlich模型擬合結(jié)果.通常,Langmuir方程是假設(shè)吸附劑表面吸附位點(diǎn)均勻的單分子層吸附,且認(rèn)為吸附質(zhì)之間沒有互相發(fā)生反應(yīng)[12].可見,徑流膠體對(duì)DOM的吸附過程為單層吸附,其擬合得到的最大吸附量隨pH值增加而逐漸減小,但當(dāng)pH值36.0時(shí),其最大吸附量逐漸趨于穩(wěn)定.這是因?yàn)樵趐H值較低時(shí),溶液中存在大量H+使徑流膠體表面發(fā)生質(zhì)子化,徑流膠體可通過靜電吸引作用吸附表面攜帶負(fù)電荷的DOM,因此DOM吸附量較大;隨pH值增加至6.0~8.0,徑流膠體表面質(zhì)子化作用減弱,使徑流膠體與同為負(fù)電性的DOM之間靜電斥力增加,導(dǎo)致徑流膠體對(duì)DOM的吸附量減小;隨pH值繼續(xù)增加,懸濁液中OH-離子增加本應(yīng)使膠體和DOM的靜電斥力進(jìn)一步增大而使得DOM吸附量減少,但因調(diào)節(jié)pH值時(shí)使用了NaOH溶液引入了Na+,Na+破壞了徑流膠體的壓縮雙電層結(jié)構(gòu),導(dǎo)致膠體與DOM之間的靜電斥力降低,最終使得DOM的吸附量變化不大甚至略有增加[13-14].
圖1 不同pH值條件下徑流膠體對(duì)徑流DOM的等溫吸附過程
e是吸附平衡時(shí)樣品中DOC濃度,mg/L;e是平衡吸附量,mg/g
2.1.2 不同pH值時(shí)徑流膠體的Zeta電位 通常,膠體Zeta電位的絕對(duì)值越大,膠體表面的凈負(fù)電荷總量越多,膠體越穩(wěn)定、越易發(fā)生遷移[15-16].由圖2可以看出,與徑流DOM結(jié)合前后徑流膠體Zeta電位均為負(fù)值,且與DOM結(jié)合后徑流膠體Zeta電位絕對(duì)值增大.這是因?yàn)閺搅鱀OM含有多種不飽和官能團(tuán),這些官能團(tuán)在水中易于解離使DOM攜帶較高的表面負(fù)電荷;徑流膠體與DOM結(jié)合后增加了膠體表面的負(fù)電荷密度,從而使膠體的Zeta電位絕對(duì)值升高.此外,隨pH值增加,徑流膠體及膠體-DOM體系的Zeta電位絕對(duì)值均逐漸增大.當(dāng)pH值較低時(shí),體系中的H+數(shù)量較多,膠體表面凈負(fù)電荷被中和,導(dǎo)致Zeta電位的絕對(duì)值減小;隨pH值增加,體系中H+數(shù)量減少,膠體表面凈負(fù)電荷增加,Zeta電位絕對(duì)值變大.可見,與徑流DOM結(jié)合后,徑流膠體更加穩(wěn)定并易于遷移;且pH值越高,越易發(fā)生同步遷移.
表1 Langmuir和Freundlich等溫吸附方程擬合參數(shù)
注:m是最大平衡吸附量;L是Langmuir方程吸附平衡常數(shù);F為Freundlich方程常數(shù);為反應(yīng)吸附強(qiáng)度的常數(shù).
圖2 不同pH值條件下與徑流DOM結(jié)合前后徑流膠體的Zeta電位
2.2.1 DOM分子量分布 一般而言,大分子DOM組分在凝膠色譜柱中無法進(jìn)入凝膠分子內(nèi)部,只能從凝膠分子之間的孔隙中流過,因此其出峰時(shí)間較早;而小分子DOM組分由于可以進(jìn)入凝膠分子內(nèi)部,所以其通過凝膠柱的時(shí)間要大于大分子組分,出峰時(shí)間較晚.由圖3可以看出,與徑流膠體結(jié)合前DOM樣品色譜特征峰出現(xiàn)時(shí)間為4.77,6.23和6.60min;與徑流膠體結(jié)合后,DOM色譜圖中4.77min處的特征峰消失,而6.23和6.60min處的特征峰峰強(qiáng)減弱并藍(lán)移至5.76min出峰.可見,凝膠色譜中出峰時(shí)間為4.77min時(shí)的DOM大分子組分與徑流膠體混合后被結(jié)合去除;出峰時(shí)間為6.23和6.60min時(shí)的DOM小分子組分峰強(qiáng)也明顯降低,說明小分子DOM組分也部分被結(jié)合去除,但由于所使用的凝膠色譜柱分子量監(jiān)測范圍較大,結(jié)合前體系內(nèi)的物質(zhì)未被完全區(qū)分,所以其結(jié)合后的出峰時(shí)間略發(fā)生了改變.
圖3 與徑流膠體結(jié)合前后DOM分子量分布變化
2.2.2 DOM化學(xué)組分分布變化 由圖4可以得出,地表徑流疏水性DOM組分的DOC濃度明顯高于親水性組分,占總DOC濃度的71.0%,說明地表徑流DOM以疏水性組分為主.其中,代表腐殖酸、富里酸及芳香族羧酸的HoA組分及代表碳?xì)浠衔锏腍oN組分占比較大,分別占總DOC濃度的35.0%和24.3%.這是由于地表徑流采集于城市主干道,交通密度較大,機(jī)動(dòng)車尾氣排放、汽油滴漏以及輪胎磨損導(dǎo)致徑流中含有大量多環(huán)芳烴、石油烴等物質(zhì)[17],致使徑流DOM中疏水性HoA和HoN組分含量較高.地表徑流中HoA和HoN兩種組分與徑流膠體結(jié)合率最高,分別為28.55%和22.22%,說明徑流DOM疏水性酸性和中性組分更容易被徑流膠體吸附而發(fā)生同步遷移.
圖4 與徑流膠體結(jié)合前后DOM各化學(xué)組分DOC濃度
由圖5可以看出,與DOM結(jié)合前后徑流膠體的紅外光譜圖沒有明顯差異;3409cm-1處吸收峰為羥基伸縮振動(dòng)峰;1636cm-1處吸收峰為羧基彎曲振動(dòng)峰[18]; 1147cm-1處的吸收峰為C-C伸縮振動(dòng)[19].與DOM結(jié)合后,各吸收峰的位置均發(fā)生了略微紅移,這是由于DOM與膠體發(fā)生了表面絡(luò)合反應(yīng)所致[17].
a 膠體,b DOM
此外,徑流DOM紅外光譜圖中3400cm-1附近的吸收峰為水分子羥基伸縮振動(dòng)峰,1650cm-1處的吸收峰是C=O鍵形成的,1400cm-1處的吸收峰是羧酸類化合物-COOH的拉伸峰[20],860cm-1處的彎曲峰是羧酸類化合物中-OH變形振動(dòng)產(chǎn)生的,一般由異構(gòu)化羧基和不飽和烷烴引起[21].與徑流膠體結(jié)合后,徑流DOM在860cm-1處的峰強(qiáng)明顯降低;而與DOM結(jié)合后徑流膠體在860cm-1處的振動(dòng)峰峰強(qiáng)明顯增加,這說明徑流膠體與DOM中的羧基和不飽和烷烴發(fā)生了羥基取代反應(yīng).
結(jié)合不同pH值條件下的徑流膠體與DOM的結(jié)合行為可知,徑流膠體可通過羥基取代、表面絡(luò)合、靜電吸引等途徑實(shí)現(xiàn)與DOM結(jié)合(圖6).
圖6 徑流膠體與DOM的吸附結(jié)合機(jī)理
3.1 地表徑流膠體與DOM可發(fā)生結(jié)合作用,Langmuir模型可以較好地描述不同pH值時(shí)徑流膠體對(duì)DOM的等溫吸附過程,且隨pH值增加其最大吸附量逐漸減小,當(dāng)pH值36時(shí),其最大吸附量逐漸趨于穩(wěn)定.
3.2 與DOM結(jié)合后,徑流膠體Zeta電位絕對(duì)值升高;且隨pH值增加,徑流膠體-DOM體系Zeta電位絕對(duì)值逐漸增大.說明與徑流DOM結(jié)合后,徑流膠體更加穩(wěn)定并易于遷移;且pH值越高,兩者越易發(fā)生同步遷移.
3.3 與徑流DOM中的小分子組分相比,DOM大分子組分更易與徑流膠體結(jié)合;與親水性組分相比,DOM疏水性酸性和中性組分更易與徑流膠體結(jié)合而發(fā)生同步遷移.徑流膠體和DOM的結(jié)合作用主要通過羥基取代、表面絡(luò)合和靜電吸引實(shí)現(xiàn).
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Characteristics of the combination between colloids and dissolved organic matter in urban surface runoff.
LIU Dian-wei1,DU Xiao-li1,2*,FU Xiao-yu1,CUI Shen-shen1,DAI Xin-yi1
(1.Key Laboratory of Urban Stormwater System and Water Environment,Ministry of Education,School of Environment and Energy Engineering,Beijing University of Civil Engineering and Architecture,Beijing 100044,China;2.Beijing Energy Conservation & Sustainable Urban and Rural Development Provincial and Ministry Co-construction Collaboration Innovation Center,Beijing 100044,China).,2022,42(8):3690~3695
To characterize the combination between colloids and dissolved organic matter (DOM) in urban runoff,the adsorption behaviors of both colloids and DOM under different pH conditions were examined to reveal its mechanisms. The results showed that the Langmuir model described the isothermal sorption of DOM by runoff colloids at different pH values better,and the maximum sorption capacity decreased gradually with an increase in pH. The maximum equilibrium adsorption capacity at pH 3 was 4.0 times more than that at pH 6. The absolute value of zeta potential of runoff colloids increased after adsorption with DOM and then the runoff colloids became more stable and easier to migrate. HoA and HoN are two main fractions in the runoff DOM and account for 35.0% and 24.3% of the total DOC concentration,respectively,and can be combined with runoff colloids most easily,implying that the DOM fractions with high molecular weight are more likely combined with runoff colloids. The combination between runoff colloids and DOM can be achieved through hydroxyl substitution,surface complexation and electrostatic attraction.
runoff;colloids;dissolved organic matter;adsorption;influencing factors
X143
A
1000-6923(2022)08-3690-06
2022-01-17
國家自然科學(xué)基金資助項(xiàng)目(51878024);北京建筑大學(xué)金字塔人才培養(yǎng)工程(JDJQ20200302);北京建筑大學(xué)科學(xué)研究基金資助項(xiàng)目(X20148)
* 責(zé)任作者,教授,duxiaoli@bucea.edu.cn
杜曉麗(1980-),女,山東高密人,教授,博士,主要從事城市雨洪控制利用理論與技術(shù)、水污染控制與水質(zhì)轉(zhuǎn)化技術(shù)等研究.發(fā)表論文40余篇.