王薪杰,王一寧,趙 儉,劉晟東,夏星輝,李 陽

河流水沙運動對微塑料運移過程影響研究進展

王薪杰,王一寧,趙 儉,劉晟東,夏星輝,李 陽*

(北京師范大學(xué)環(huán)境學(xué)院,教育部水沙科學(xué)重點實驗室,水環(huán)境模擬國家重點實驗室,北京 100875)

對河流中微塑料的分布、微塑料在河流中發(fā)生的聚沉、重懸、水平運輸和潛流交換等遷移過程進行了歸納總結(jié).由于河流中廣泛存在的泥沙會影響微塑料的遷移.本研究剖析了含沙河流中微塑料遷移,包括微塑料與泥沙的聚集和共沉降,沉積物對微塑料的再懸浮和滲透過程影響,探究了微塑料與泥沙的相互作用機制和影響因素.提出未來含沙河流中微塑料的運移研究應(yīng)重視開發(fā)可靠模型預(yù)測微塑料遷移和通量,實驗室模擬應(yīng)考慮生物和水體擾動對老化微塑料傳輸?shù)挠绊?

微塑料；納米塑料；河流；遷移；泥沙；沉積物；影響因素

微塑料(MPs)是指尺寸<5mm的一種人工合成的高分子制品[1].在尺寸上至少一個維度上小于<100nm的塑料顆粒稱為納米塑料(NPs)[1-2].通過個人護理品、光電子產(chǎn)品和靶向藥物等產(chǎn)品的使用,MPs可以直接排放到天然水體中[3].環(huán)境中的塑料也會因生物降解、機械磨損、水解、紫外輻射降解和熱降解等作用破碎成MPs和NPs,很容易富集在水生生物體內(nèi)影響其生長發(fā)育[4-5].此外,MPs能夠吸附有機污染物(如多環(huán)芳烴和有機氯農(nóng)藥等)和重金屬(如鉛和鉻等),以及釋放鄰苯二甲酸鹽、有機錫、雙酚A等有毒物質(zhì),進而加劇其對生態(tài)系統(tǒng)的危害[6-10].

泥沙是河流中重要的組成部分,負載量約為5～ 20000mg/L[11-13].全球氣候變化導(dǎo)致大暴雨的發(fā)生頻率增加,加劇水土流失和沉積物再懸浮,致使河流的高泥沙含量成為一個全球性問題[14].水中的泥沙運動顯著影響MPs的聚沉、再懸浮和滲透等重要遷移行為,進而影響環(huán)境中MPs的穩(wěn)定性、流動性和生物可利用性[15-16].MPs與懸浮泥沙發(fā)生聚集會導(dǎo)致MPs聚集體密度增加,從而影響MPs在水中的垂直分布及長期遷移[16-17].沉積物-水界面水流的剪切應(yīng)力和紊流會導(dǎo)致沉積物顆粒的再懸浮,進而引起沉積物中的MPs向上覆水體釋放[18].MPs自身特性(組成、形狀和大小等)、水體理化性質(zhì)(pH值、鹽度、離子種類和強度等)、泥沙/沉積物性質(zhì)(粒徑分布、組成和有機質(zhì)含量等)、擾動時間與擾動強度等是影響河流中MPs遷移的關(guān)鍵影響因素[19].因此,準確描述MPs的歸趨需要考慮泥沙運動和環(huán)境因素耦合作用的影響.目前針對含沙河流中MPs的遷移行為研究仍處于探索階段.

河流被視為從陸地向海洋生態(tài)系統(tǒng)輸送MPs的主要途徑[20].MPs在河流中的分布和遷移行為,決定了河流系統(tǒng)中MPs的入海通量.全球從河流進入海洋的塑料碎片數(shù)量為0.41～4×106t/a[21].一旦MPs進入海洋,將很難進行收集和清理.有效的解決方案是控制MPs在內(nèi)陸和水體(如河流)中的運輸過程.因此,研究河流中MPs濃度的時空變化、遷移轉(zhuǎn)化過程以及驅(qū)動其運輸?shù)年P(guān)鍵作用力已成為國際上環(huán)境科學(xué)領(lǐng)域的熱點問題[9,22],對闡明MPs的環(huán)境歸趨和生態(tài)風險具有重要意義.含沙水體中MPs的聚集、沉降和再懸浮受控于自身理化性質(zhì)和多種環(huán)境因素耦合的影響,這使得MPs的遷移行為復(fù)雜多樣、難以預(yù)測.因此,本文總結(jié)了全球范圍內(nèi)河流的上覆水和沉積物中MPs的分布情況,探討了河流中MPs的遷移轉(zhuǎn)化過程,總結(jié)了泥沙影響上覆水體和沉積物中MPs遷移的影響機制,并展望了未來含沙河流中MPs遷移轉(zhuǎn)化的研究方向,為更全面了解MPs在水環(huán)境中的分布規(guī)律、建立評估方法和控制MPs污染提供了科學(xué)支撐.

1 河流中微塑料的分布及賦存

研究表明,MPs在淡水、海洋環(huán)境中均廣泛分布[20,23].MPs可以通過城市污水處理廠排放、雨水徑流、輪胎和道路磨損、生活農(nóng)業(yè)垃圾不合理傾倒進入河流,或經(jīng)地表徑流匯入河流、湖泊、地下水中,并最終進入海洋[22,24].影響河流中微塑料豐度的因素包括人口密度、人類活動程度、水流流速、流量、河流類型、集水區(qū)大小、廢棄物管理方案和污水處理方法等[9,22].根據(jù)模型預(yù)測,每年大約有1533t MPs通過多瑙河流入黑海[25].歐盟環(huán)境基金會預(yù)測每年有20～30t塑料垃圾通過河流匯入北海.在意大利,每年大約有120t塑料垃圾匯入地中海[26].除了匯入海洋之外,MPs也會通過聚集、沉降等運動沉積在河床中,使得MPs在沉積物中積蓄,因此沉積物成為MPs一個重要的“匯”[16-17].

根據(jù)表1,實地測量的結(jié)果表明在河流上覆水中MPs的濃度范圍在0.1～104particles/m3數(shù)量級之間,而在沉積物中MPs的濃度范圍在10～ 104particles/ kg數(shù)量級之間[27-33].值得注意的是,在中國多個地點的河流沉積物中(海河、珠江和長江),發(fā)現(xiàn)了濃度達103～104particles/kg的MPs,比國外河流沉積物(葡萄牙Antu? River和加拿大Ottawa River)中的MPs濃度高1個數(shù)量級.不同研究報道的河流中MPs濃度存在巨大差異,可相差2～5個數(shù)量級[22],這主要是由研究地點、樣品選擇、采集方法以及樣品處理方法之間的差異所導(dǎo)致[34].由于目前沒有標準的MPs采樣方法,采樣深度,采樣裝置(濾網(wǎng)的孔徑)和度量單位等均會影響到河流中MPs濃度的測量結(jié)果.因此,目前急需開發(fā)統(tǒng)一和標準化的MPs采樣與分析方法.

就河流中MPs的分布情況而言,大多數(shù)研究發(fā)現(xiàn)MPs在沉積物中的濃度一般高于上覆水[27-33].在河流上覆水和沉積物中,最常檢出的MPs種類為聚乙烯(PE)、聚丙烯(PP)、聚氯乙烯(PVC)、聚苯乙烯(PS)和聚對苯二甲酸乙二酯(PET)[35].其中,在上覆水中低密度PE(0.91～0.93g/cm3)占主導(dǎo),而在沉積物中,高密度PE(0.93～0.97g/cm3)占比更高[36].為了進一步探究上覆水與沉積物中MPs賦存的分布關(guān)系,同時開展同一地點的上覆水和沉積物中MPs垂直分布的研究至關(guān)重要.綜上,目前MPs在水環(huán)境中的廣泛分布,尤其是MPs在河流的上覆水和河床的沉積物中濃度較高.沉積物中的MPs是否可能受到外界作用而重新釋放到水環(huán)境中,由“匯”轉(zhuǎn)變?yōu)椤霸础?威脅生態(tài)安全和人體健康.

表1 河流的上覆水和沉積物中微塑料的分布情況

續(xù)表1

注:a為水體表面以下;b為沉積物表面以下;PES,聚酯纖維; PAN, polyacrylonitrile,聚丙烯腈;-,未提及.

2 微塑料在河流中的典型運移行為

常見污染物(有機污染物、重金屬和磷氮等營養(yǎng)物質(zhì))進入河流后,形成均質(zhì)溶液,一定時間內(nèi)會在水相和懸浮顆粒物相達到分配平衡,常用平衡分配系數(shù)p和辛醇-水分配系數(shù)ow界定其傳輸[37].而MPs進入河流后,會形成高分散和多相的懸浮液,是熱力學(xué)不穩(wěn)定的體系[18].由于MPs在河流中各相間無法處于熱力學(xué)平衡狀態(tài)[18],因此MPs傳輸需使用膠體理論來描述.MPs進入水體后會發(fā)生聚集、沉降、水平傳輸、再懸浮、埋藏和河床交換等過程(圖1).這些過程可以循環(huán)往復(fù)地發(fā)生,也可能自發(fā)終止[3,24,38].水介質(zhì)特性以及MPs自身特性會影響MPs在河流中的傳輸過程[18,39].對于水環(huán)境中MPs的聚集行為,首先要考慮MPs自身理化性質(zhì)(如疏水性、比表面積和表面電荷)、水環(huán)境條件(如光照、離子強度、組成和pH值)以及水體中共存顆粒等因素的影響[39-40].而MPs的沉降速率主要受控于MPs與水體間的密度差異[41-42].

2.1 聚集

聚集是河流中MPs的常見傳輸過程.聚集行為可分為同質(zhì)聚集(在相同類型顆粒之間發(fā)生的聚集)和異質(zhì)聚集(在不同類型顆粒之間發(fā)生的聚集)[24].由于自然水環(huán)境中其他懸浮顆粒物的濃度遠遠高于MPs的濃度,MPs異質(zhì)聚集發(fā)生的概率往往遠高于同質(zhì)聚集發(fā)生的概率[39].塑料表面常含有疏水官能團,易吸附藻類和細菌等生物體,在塑料表面形成生物膜或發(fā)生微生物定殖[17,43].MPs/NPs與生物相關(guān)的聚集已被廣泛報道[17,44].此外,有機膠體(海藻酸鹽)、黏土礦物(懸浮泥沙、Fe2O3)和納米顆粒(納米銀)也可與MPs聚集[45-46].和其他工程納米材料相似,MPs在天然水環(huán)境中的聚集行為是由布朗運動、不等速沉降和流體運動引起的[47].由于NPs及天然和人工納米顆粒具有相似的膠體性質(zhì)及高擴散系數(shù)[18,39-40],布朗運動主導(dǎo)這些小顆粒(￡300nm)的聚集[47].小顆粒布朗運動又受顆粒自身性質(zhì)和環(huán)境因素影響.具體而言,小顆粒受到液體分子碰撞后,動量傳遞與顆粒質(zhì)量成反比,顆粒越輕,碰撞后的運動速度越大.顆粒大小與運動速度成反比,即小顆粒運動更快[48-49].因此顆粒尺寸和密度越大,布朗運動越不明顯,對聚集影響程度會降低.而溫度越高,液體分子運動越劇烈、流體的黏度越低,液體內(nèi)流動阻力越小,均會加劇布朗運動,導(dǎo)致布朗運動更易影響小顆粒的聚集[48-49].

圖1 河流中微塑料的典型遷移過程和影響因素

根據(jù)Derjaguin-Landau-Verwey-Overbeek (DLVO)理論,顆粒組成、結(jié)構(gòu)、尺寸、表面電性和水環(huán)境中離子強度均可影響MPs的聚集速率[39].非DLVO力,包括空間位阻、水合作用、聚合物橋聯(lián)作用以及磁性作用等也會影響MPs的穩(wěn)定性[47].關(guān)于水環(huán)境條件、共存的固體介質(zhì)以及MPs自身性質(zhì)(如疏水性、比表面積和表面電荷)等因素對MPs在水環(huán)境中聚集行為的影響,已有綜述論文進行闡述[19,24,39],本文不再展開討論.MPs聚集將影響其遷移、降解及其在環(huán)境中的持久性和生物有效性[3,39].

2.2 沉降

沉降是水環(huán)境中MPs的重要環(huán)境行為之一,會影響MPs在水中的垂直分布[50].通過布朗擴散、浮力和重力沉降等作用,MPs在河流中能夠遷移到沉積物中[51].對于NPs,布朗運動是其沉降的主要機制[16].浮力和重力沉降則主要控制微米尺寸MPs和團聚體的沉積[47].沉降過程可分為3個階段:(1)初始階段,此時聚集過程仍在進行,沉降緩慢;(2)快速沉降階段;(3)減速沉積階段,此時大部分聚集體已經(jīng)沉降完全,上覆水中殘留懸浮顆粒的數(shù)量較少[52].由于MPs顆粒的物理性質(zhì)(密度和形狀)與泥沙顆粒存在很大差異,需要對傳統(tǒng)泥沙沉降(斯托克斯定律)和輸移模型進行改進才能應(yīng)用于MPs沉降輸移的模擬,沉降相關(guān)方程中MPs的沉降速度和臨界剪切應(yīng)力等關(guān)鍵參數(shù),需要通過合理的理論分析和沉降柱實驗觀測獲得.目前, Waldschlager等[53]基于斯托克斯公式建立了無泥沙的靜止淡水環(huán)境中球形和顆粒狀MPs沉降速度的修正公式(1)～式(5),可較好地預(yù)測淡水中MPs的垂直遷移過程.

(3)

式中:w為沉降速度, m/s;equi為顆粒等效粒徑, m;、、分別為單顆粒最長,中間和最短的長度, m;D為無量綱的阻力系數(shù); CSF為Corey形狀因子;為無量綱的顆粒雷諾數(shù);為流體的黏度, m2/s;為重力加速度, 9.8N/kg;為流體密度, kg/m3;s為顆粒密度, kg/m3.

此外,也可使用一階動力學(xué)沉降模型對MPs/ NPs的沉降速率進行擬合(式(6))[54].

式中:C為取樣時MPs的濃度, mg/L;0為溶液中MPs初始濃度, mg/L;res為某一時間后溶液中MPs的殘余濃度, mg/L;s為沉降速率, m/s;為采樣點的深度, m;diss為顆粒溶解常數(shù),對MPs/NPs通常為0.

MPs的形狀會影響其沉降速率.與相同密度和體積的球形顆粒相比,具有理想光滑表面的球形顆粒比不規(guī)則形狀的顆粒受到的阻力作用小,因此光滑的顆粒具有較大的下沉速度[55-56].纖維和薄膜形態(tài)的MPs顆粒比球狀或其他不規(guī)則顆粒狀的MPs具有更高的浮力和更低的沉降速度[55-56].不同長度但直徑相同的纖維狀MPs在水中具有近似的沉降速度.當纖維狀MPs的直徑增加時,沉降速度也會隨之增加[53].該現(xiàn)象的原因尚不明確.也有報道稱,具有較高暴露面積的纖維和薄膜形態(tài)的MPs更易形成生物膜,從而密度增加,沉降更快[57-58].藻類等微生物定殖會導(dǎo)致MPs聚集體密度增加,使MPs以更高的速率沉降.密度為0.89～0.91g/cm3的PP MPs與藻類聚集后密度增加到1.19g/cm3[59].水生生物的活動也會影響MPs在水中的沉降.魚類和蝌蚪等動物可吞食攝入MPs,使MPs與排泄物或生物碎屑一起遷移到沉積物中[19].電解質(zhì)濃度對PET MPs、PS NPs的沉降影響較大,沉降速率及沉降程度均隨電解質(zhì)濃度的增大而增強[16,60].二價陽離子比一價陽離子更能夠顯著促進水環(huán)境中PET MPs的沉降[60].

2.3 上浮

由于部分MPs的密度比水更小或者接近水的密度,有些MPs也會在水體中發(fā)生上浮[53,61].水與MPs之間的密度差,MPs的粒度和形狀均可影響MPs在水中的上浮過程.針對上浮過程的研究多集中在海洋環(huán)境,這是由于海洋環(huán)境密度更高,更易使MPs發(fā)生上浮[62-63].為研究河流中MPs的上浮行為,Waldschl?ger等[53]使用沉降柱確定了商業(yè)MPs在非擾動的蒸餾水中的上升速度.MPs的形狀包括球形、碎片和纖維狀,成分包括PE、PP、PS、發(fā)泡聚苯乙烯(EPS)、PVC、PET和聚酰胺(PA).該研究構(gòu)建的用于描述顆粒和球形商業(yè)MPs的上升速度w(m/s)的公式與公式(1)相同,式中僅D計算方法不同,如公式(7)所示.該研究的到MPs上升速度為0.65cm/s(直徑3mm碎片狀PE MPs)～31.4cm/s(碎片狀EPS MPs).

式中:為MPs顆粒的圓度(0～6,顆粒越圓數(shù)值越大),由不同研究人員主觀評測,最終取平均值獲得.

Waldschlagera等[61]隨后使用同樣的裝置實驗和公式,測定并計算了河流中自然風化MPs(EPS, PE, PP)的上升速度為0.18cm/s(PE MPs)～19.85cm/s (EPS MPs).其中碎片、顆粒、泡沫狀顆粒的相對平均偏差在50%及以下,但薄膜狀MPs平均偏差較高(425%).這是由于薄膜狀MPs易發(fā)生變形而使上升行為更為復(fù)雜.相比較商業(yè)MPs而言,河流風化后的MPs上升速度較低,這是由于風化后MPs顆粒表面和裂縫內(nèi)部可形成生物膜或摻入了懸浮泥沙或藻類,從而降低上升速度[58].此外,風化后顆粒表面粗糙度和表面積較大也導(dǎo)致上升減慢[61].淡水環(huán)境中MPs的上升速度與其在海水中相差不大(0.5～5.0mm 碎片MPs和纖維狀MPs上升速度分別為(0.9± 0.4)～(1.9±0.6)cm/s和(0.6±0.1)～(0.8±0.2)cm/s)[62].

2.4 推移質(zhì)移動和重懸

在水流流速較高和湍流作用下(如洪水事件),沉積物中的MPs可發(fā)生重懸浮返回到上覆水體,并構(gòu)成上覆水體中MPs的重要來源[64-65].MPs在沉積層的傳輸方式隨流體動力學(xué)條件變化而變化,當水底流速較小時,MPs顆粒主要以滑動或滾動為主,即發(fā)生推移質(zhì)移動;隨著水底的流速增大,MPs顆粒將發(fā)生跳躍并懸浮.風和水溫也是影響MPs再懸浮的重要因素.風驅(qū)動的水體振蕩混合可以促進沉積物中MPs發(fā)生再懸浮[66].降雨導(dǎo)致水體流量增加、水體擾動增強,也利于MPs發(fā)生重懸[67].對于沉降在小河和河口地區(qū)的MPs顆粒,底流作用對其傳輸?shù)挠绊懘笥跉庀髼l件.研究發(fā)現(xiàn),大尺寸的MPs上浮速度較大,因此有些地區(qū)沉積物中較小尺寸的MPs含量較多[53].沉積物中MPs也可能發(fā)生表面生物膜脫落,密度降低,再次浮到上覆水體[68].

河流水體擾動會造成水體發(fā)生紊流擴散以及MPs再懸浮[69-70].沉積物中MPs的豐度與水體流量或流速之間呈負相關(guān)[71].弱的水動力擾動有利于MPs沉積[71];當湍流存在時,再懸浮的MPs含量增加[70].雨季排水、潮汐和洪水事件等強水動力擾動均會加劇沉積物中MPs的重懸[70,72].據(jù)Hurley等[69]的估算,英格蘭河床上的MPs中約70%是通過洪水作用重新進入水體.在潮間帶地區(qū),表層沉積物中MPs的含量往往高于深層沉積物[73].造成這種現(xiàn)象的原因為,部分MPs在退潮時沉積在沉積物表面,但在漲潮時重新懸浮,因此MPs無法永久保留在較深的沉積物中[70].由于河口處水體密度和水深增加,此區(qū)域的臨界切應(yīng)力和河床切應(yīng)力更有利于MPs的重懸,MPs運輸距離相對較長[38].

2.5 水平運輸

水體中部分MPs會隨河流水平流動輸送到下游,并最終進入海洋[24,74].MPs初始密度可能高于或低于水體(海洋密度為1.02～1.04g/cm3,淡水密度為1.00g/cm3)的密度[30,36].若MPs密度低于水體密度,如PE和PP,則可能在浮力作用的主導(dǎo)下漂浮或懸浮在水中,隨水流運輸.若MPs密度大于水體,如PMMA、PVC和PET,則一方面會在水體擾動較小的水體中直接沉積到河床,另一方面在水流強度大和流體紊亂的水體中,也可在浮力作用下發(fā)生水平運輸[75].在靜態(tài)或非擾動狀態(tài)下,未團聚的MPs沉降非常緩慢,它們通常發(fā)生水平傳輸或在水體中停留數(shù)周至數(shù)年.河流中MPs的水平運輸受到風、河流形態(tài)、水體流速和植被等多種因素的影響[76].

風對水面漂浮MPs的水平輸送影響最大,尤其是對聚苯乙烯泡沫EPS MPs[75].漂浮的MPs在受到風的影響后,水平移動速度加快,移動距離更遠,更有可能被輸送停留在海灘和河岸上或輸送到海洋.針對河口處漂浮MPs運輸?shù)慕Ｑ芯勘砻?波浪和地表徑流是影響漂浮MPs空間分布的關(guān)鍵因素[77-78].河流形態(tài)會通過影響水體流速或通過彎道攔截來影響MPs的運輸.高流速會導(dǎo)致城市河流水體中MPs豐度降低[79].相比較彎曲河道而言,筆直河道內(nèi)MPs更易發(fā)生水平遷移[80].水體中的植被可通過降低水流流速,攔截作用,吸附MPs,固定沉積物從而降低沉積物中MPs的重懸等多個方式降低MPs的水平遷移[70].

目前,實測河道內(nèi)MPs水平運輸過程的研究較少.模型運算可一定程度上彌補此研究不足,Atwood等[77]利用2種模型(水動力模型和遙感模型)預(yù)測了1.5a內(nèi)意大利波河河口中直徑為1mm的球形MPs(密度為0.91g/cm3)沿海岸線的積累情況.2種模型預(yù)測的MPs累積結(jié)果近乎一致,并與9個河口樣品的測量濃度數(shù)據(jù)相符.水動力模型模擬結(jié)果表明,波河釋放的MPs中有80%以上進入海洋.遙感模型則能更好地捕捉MPs沿海岸線積累模式,具有更高的空間分辨率.但目前通過模型估算的河道內(nèi)MPs的水平輸運通量也存在較大差異[19].Mai等[74]使用基于人類發(fā)展指數(shù)的模型預(yù)測出2010～2050年全球河流塑料的外流通量值,僅是Jambeck等[81]使用基于塑料廢物排放模型預(yù)測數(shù)值的2%.

2.6 沉積、潛流交換和滲透

潛流帶是指河床界面以下的區(qū)域[82].MPs進入潛流帶后,將存在5種不同形態(tài)的顆粒:可自由移動的未發(fā)生團聚的MPs、可自由移動的MPs團聚體、吸附到沉積物上的MPs、吸附到沉積物上的MPs團聚體和河床沉積物[83].MPs在此處的遷移涉及(1)沉積(過濾)、(2)滲透和(3)潛流交換.

(1)沉積(過濾)是指膠體和沉積物間在靜電作用力和范德華力等化學(xué)作用力下發(fā)生相互作用,使膠體沉積在沉積物上[84].無論MPs在潛流帶中有沒有發(fā)生遷移,都受到一定的過濾作用.水化學(xué)條件(如離子強度、陽離子類型和pH值)主要通過改變MPs和介質(zhì)之間的靜電吸引和排斥作用而影響其遷移[85-86].土壤和NPs的Zeta電位通常與pH值成反比,低pH值條件下,NPs和土壤之間的排斥力減小,將更容易保留在固相中[85].當增大孔隙水中離子濃度時,會壓縮雙電層厚度和降低表面電位,這使得MPs更易于被沉積物截留[87].此外,在相同的離子強度下,二價的Ca2+在抑制PS NPs的遷移方面比一價的Na+表現(xiàn)出更大的抑制作用,歸因于Ca2+比Na+具有更有效的電荷中和作用[87].Ca2+還會通過橋連作用促進MPs聚集[88].當膠體在沉積物的孔隙中運動距離后,過濾作用對膠體濃度的影響可由下式求得[83]:

式中:為河流沉積物中膠體的濃度, mg/L;為膠體在任一運動軌跡中的運動距離, m;f(m-1)為河流沉積物對膠體的過濾系數(shù),其可通過下式計算得出:

式中:c為沉積物顆粒的平均直徑, μm;f為膠體顆粒與沉積物顆粒間的碰撞效率,無量綱;0為接觸效率,無量綱;s為孔隙度參數(shù),s=(2(1–5))/(2–3+35– 26),=1–,無量綱;R為顆粒粒徑比沉積物粒徑,無量綱;Pe為佩克萊數(shù),pe=c/D,無量綱;vdw為范德華數(shù),vdw=/b,無量綱;gr為重力數(shù),gr=πp4(p–f)/(3b),無量綱;為沉積物孔隙度,無量綱;為多孔介質(zhì)中流體速度,m/s;D為擴散系數(shù), m2/s;為哈梅克常數(shù),J;b為玻爾茲曼常數(shù), 1.3805×10-23J/K;為絕對溫度,K;p為顆粒半徑,μm;p為顆粒密度, kg/m3;f為流體密度, kg/m3.

(2)滲透指在重力、湍流、潮汐和滲流作用下MPs在沉積物中發(fā)生的向下遷移[89-90].對于孔隙水中尺寸較大的膠體顆?；蚓奂w,受到重力作用后在河床介質(zhì)中發(fā)生沉降并吸附或沉積到沉積物顆粒表面,則也稱為沉降作用.沉降作用是膠體在沉積物中沉積的另一重要機制[91].研究表明,可使用斯托克斯定律對球形顆粒的沉降速度進行求解,并通過以下公式計算膠體移動路徑[83]:

式中:和分別為水平和垂直方向上的孔隙水速度, m/s;為床面波數(shù), 1/m;為沿河床的縱向坐標,m;為垂直方向坐標, m;為河床表面動壓力,無量綱;m為沙丘上方水頭變幅的半幅值,m;為導(dǎo)水率,cm/ min;u為底流速度(u=·), m/s,是河床坡度,無量綱;b是河床深度,m;s為斯托克斯沉降速度,m/s;為沉積物孔隙度,無量綱.

沉積物中MPs的滲透行為可能導(dǎo)致MPs遷移到含水層和地下水中,從而增加沉積物中MPs對生物的暴露時間,并對飲用水安全造成威脅[83,89-90].研究表明,MPs的平均濃度隨沉積物深度(0～50cm)的增加而增加,深層沉積物中的小尺寸(<2mm)MPs占比逐漸升高,最終可能遷移到地下水中[92].研究發(fā)現(xiàn),美國伊利諾伊州16個地下水樣品中均能檢測到微量MPs的存在[93].其形態(tài)均為纖維,中值濃度為6.4particle/L,最高濃度為15.2particle/L[93].沉積物中MPs的沉積(過濾)、沉降和滲透同時發(fā)生.這些過程與多孔介質(zhì)表面粗糙度、流體動力學(xué)和MPs尺寸等都有關(guān)系.

(3)潛流交換是指由于近河床區(qū)的湍流和河床表面的壓力變化,致使水和懸浮物顆粒(如MPs、小粒徑沉積物和微生物等)進出沉積物[94-95].潛流交換是調(diào)節(jié)天然水系統(tǒng)中膠體和污染物運輸?shù)闹匾^程[83,96].河床形態(tài)能夠影響河流中顆粒的潛流交換,主要歸因于兩種交換機理:平流泵吸作用/泵吸交換和沖淤交換[84].平流泵吸作用是指由于河床形狀不平整,引起河床表面壓力變化,帶動河床中孔隙水的運動,導(dǎo)致膠體進入河床內(nèi)并在其中發(fā)生遷移.沖淤交換多發(fā)生在河床沖刷區(qū).由于河床受到水流沖刷,上游泥沙被沖走,釋放孔隙水.被沖走的泥沙在下游發(fā)生沉降形成新河床并蓄積孔隙水,在此過程中孔隙水的釋放和截留會引起河床與河流間的膠體交換.

在實地取樣勘測中,發(fā)現(xiàn)不同取樣地點的沉積物中MPs種類和數(shù)量分布不均,說明可能受到了潛流交換的作用[82].另一項實測研究發(fā)現(xiàn),23% MPs的潛流交換率高于它們的沉降率[94].對于低密度MPs(如PS和聚氨酯塑料),這一比例高達42%.無論聚合物類型如何,潛流交換對于直徑小于100μm的粒子輸運和歸宿都很重要.潛流交換延長了約57% MPs的停留時間,并且這部分MPs受風暴活動才可被清除.因此,不包括潛流交換的數(shù)學(xué)模型可能大大低估了MPs在河床沉積物中的沉積時間.與沉積物中MPs的過濾和滲透保留機制相比,潛流交換對沉積物中MPs遷移過程的影響還未被廣泛研究[82,94].

總之,當MPs沉積到河流底部,可能通過潛流交換進入沉積物,穿過和離開河床.在運輸過程中,MPs可能與其他膠體發(fā)生碰撞并產(chǎn)生大的聚集體,發(fā)生沉降沉積到沉積物表面.同時,由于過濾作用,它們也可能被沉積物截留.當形成的聚集體足夠大時,它們可能會被固定在沉積物中;然而,如果是NPs或所形成的聚集體相對于沉積物的孔徑足夠小,則它們?nèi)钥梢源┩缚障栋l(fā)生滲透或在沉積物中發(fā)生水平遷移.

3 水沙運動對微塑料動態(tài)傳輸過程的影響

3.1 上覆水體中泥沙與微塑料相互作用對微塑料運移的影響

一般來說,河流中的MPs作為單分散顆粒被觀察到的可能性極低.相反,MPs會吸附水體中溶解性有機質(zhì)(DOM)和表面活性劑等物質(zhì),并可能與其他顆粒物聚集或纏繞,從而以團聚體的形式存在.如圖2a所示,通常存在于河流上覆水體的懸浮泥沙會改變MPs的穩(wěn)定性.非黏性沙粒(砂粒,50>64μm)的沖擊和拖曳會攜帶MPs進入沉積層,而黏性泥沙則可能會與MPs形成不穩(wěn)定的聚集體影響MPs的懸浮和沉積[97].由于MPs的尺寸在1nm～5mm之間,而黏性泥沙顆粒的尺寸一般為<62μm,中位徑(50)<4μm.因此MPs與黏性泥沙聚集可能存在2種情況:(1)小尺寸的MPs可以黏附在尺寸大于MPs顆粒的大泥沙顆粒表面形成聚集體;(2)小泥沙顆?？赡莛じ接诖蟪叽绲腗Ps表面形成聚集體[18].MPs與懸浮泥沙之間的相互作用受水環(huán)境條件(如水動力學(xué)特征、離子的濃度和種類、溶解或顆粒狀的有機/無機膠體、微生物和浮游植物等)和MPs/泥沙自身理化特性(如顆粒Zeta電勢大小、電荷分布和表面極性官能團等)的影響[45-46].

由于河流鹽度低于海水,PS NPs的流體力學(xué)直徑在天然河水中比在天然海水中幾乎減小了一倍[45].懸浮泥沙由于表面上存在有機質(zhì),含有羧酸和酚等官能團,所以在天然水中會攜帶負電荷[98].一方面,帶負電的懸浮泥沙會排斥攜帶負電荷的MPs,增強MPs在溶液中的穩(wěn)定性.在低離子強度下(<0.01mol/L NaCl),攜帶凈負電荷的黏土(蒙脫土)與帶負電荷PS NPs之間存在強靜電排斥力,從而增強了PS NPs在溶液中的穩(wěn)定性[45].但高離子強度條件下,顆粒表面負電荷被中和、雙電層被壓縮,兩者之間的靜電斥力降低,異質(zhì)聚集增強[45].并且二者表面吸附的陽離子(如Ca2+和Fe3+)可以通過橋連作用,增加聚集[24].另一方面,帶相反電荷的顆粒由于靜電吸引而發(fā)生聚集.帶負電的氧化鐵(III)礦物可吸附帶正電的PS NPs形成異質(zhì)聚集體[46].攜帶凈負電荷懸浮泥沙的邊緣等部位可能存在局部攜帶正電荷的區(qū)域.因此,在黏土礦物邊緣帶正電荷的部位能夠吸附帶負電荷的PS NPs[3].天然水體中懸浮泥沙組成、形狀和大小各異,這為MPs-懸浮泥沙相互作用提供了充足的可能,通常會導(dǎo)致異質(zhì)聚集.實驗測得近似球形的PS MPs在天然淡水中與懸浮泥沙(高嶺土與膨潤土)異質(zhì)聚集的附著效率在0.004～0.2之間,此值隨MPs大小和懸浮泥沙特性變化而變化[99].

與懸浮泥沙的異質(zhì)聚集作用可能影響塑料顆粒的沉降行為[99-100].Besseling等[99]考慮了對流傳輸、均聚、異聚、沉降-再懸浮、聚合物降解、生物膜的存在和埋藏對MPs的作用,使用模型預(yù)測得出40km河流上覆水中的近球形MPs的濃度在5d達到穩(wěn)定狀態(tài).異質(zhì)聚集主導(dǎo)了MPs在河流中的沉降率和沉降位置[100].對于小于5μm的MPs在下游14km處的沉降區(qū)達到最高濃度,而較大顆粒MPs(35μm)則在上游1km處發(fā)生沉降.MPs顆粒大小對其在沉積物中的保留率有顯著影響.直徑約5μm的中等尺寸顆粒,保留率最低(18%～25%).直徑100nm～ 1μm的MPs,保留率約50%;直徑大于50μm的MPs保留率可高達100％(大于200 μm).導(dǎo)致該現(xiàn)象的原因在于,100nm～1 μm MPs異質(zhì)聚集體的沉降主要由懸浮泥沙密度和數(shù)量決定.但隨MPs粒徑增加到幾微米,異質(zhì)聚集體沉降逐漸由MPs密度決定,異質(zhì)聚集沉降速率降低,因此導(dǎo)致低保留率.這意味著大部分NPs和毫米大小顆粒很可能保留在河流中,而微米大小的MPs則由河流運輸,進入沿海地區(qū)和海洋.

通常河口處的濁度高,懸浮泥沙組分以黏性泥沙為主,并且水體鹽度較高,MPs易發(fā)生絮凝過程.實驗室模擬河口處MPs聚沉實驗的研究發(fā)現(xiàn),懸浮泥沙的存在會降低PS NPs和PVC MPs的穩(wěn)定性[16,50].其中PS NPs(100nm)與大泥沙(150～500μm)的聚沉受鹽度和DOM的影響,當NaCl濃度從50mmol/L增加到200mmol/L時,高離子強度降低了PS NPs與大泥沙顆粒之間的靜電斥力,提高了兩者之間的聚集速率,形成了PS NPs和大泥沙的異質(zhì)聚集體,提高了PS NPs沉降速率[16].在200和500mmol/L NaCl溶液中,由于異質(zhì)聚集進入擴散控制區(qū),沉降速率沒有明顯的差異.腐殖酸(HA)能夠吸附到PS NPs或大泥沙顆粒的表面,提高了兩者之間的靜電斥力和空間阻力,降低了兩者之間的異質(zhì)聚集速率和沉降速率.碎片和細絲狀PVC MPs (63～125μm)與天然細顆粒泥沙(8μm)在2h內(nèi)發(fā)生絮凝,絮凝后PVC MPs的平均沉降速度為(0.74±0.30) mm/s,比分散MPs的平均沉降速度增加約8倍[50].但其絮凝和沉降行為不受水環(huán)境因素、MPs形狀和PVC與懸浮泥沙比例的影響.相反,PE MPs(1mm)即使在高濃度(500mg/L)小粒徑的泥沙(<10μm)存在下,仍然能夠長時間(90d)漂浮在水表面[16].這是由于PE MPs在水中的沉降行為主要受浮力的作用,PEMPs和懸浮泥沙的異質(zhì)聚集對PEMPs的沉降影響很小.

總體而言,懸浮泥沙既可能增強,也可能降低水柱中MPs的穩(wěn)定性[45,50].但現(xiàn)有的模擬實驗多是在靜水中進行,沒有考慮湍流的作用[16].實際環(huán)境中,懸浮泥沙與MPs之間的聚集和沉降往往是在湍流作用下進行,并且形成的絮體也可能由于湍流作用而發(fā)生物理破壞[100].流動水體中懸浮泥沙對MPs的聚集行為的影響,以及二者形成的團聚體在水平遷移的傳輸過程尚未開展研究.非球形顆粒和非黏性泥沙之間的作用對MPs傳輸過程的影響也缺少定論.對于纖維和非黏性泥沙,二者纏繞現(xiàn)象可能比附著聚集更易發(fā)生,從而提高水相中MPs的去除率.

3.2 沉積物對微塑料再懸浮過程的影響

水體沉積物中分布和埋藏著大量的MPs,其在河床上的運動主要由拖曳力、浮力和重力決定[101].沉積物再懸浮過程在天然河流中普遍發(fā)生,可由潮汐、風浪、暴風雪和洪澇事件等自然條件引起,還可以通過船只運行、清淤和拖網(wǎng)捕魚等人為條件引起[102].泥沙顆粒由靜止狀態(tài)變?yōu)檫\動狀態(tài)的臨界水流條件,常用起動流速或臨界剪切應(yīng)力衡量[64].起動流速和臨界起動剪切應(yīng)力是密度、粒徑和雷諾數(shù)相關(guān)的函數(shù).河流沉積物發(fā)生再懸浮時臨界剪切應(yīng)力范圍是0.2～75N/m2[103].沉積物中的MPs由于密度低(0.8～1.5kg/m3),粒徑小,在受到水流運動過程的剪切力作用時,其分散性更容易被影響,即發(fā)生再懸浮或隨沉積物的再懸浮過程重新進入水相[53].

微塑料在沉積物的重懸受其特性影響.研究學(xué)者利用大型水槽裝置模擬實驗,研究了14種不同粒徑(0.5～8mm)、材質(zhì)和形狀(球形、纖維和碎片)的MPs在非黏性沉積物中的推移質(zhì)運動,發(fā)現(xiàn)直徑為4.8mm的PS MPs在光滑無泥沙沉積層的臨界剪切應(yīng)力最小(0.002N/m2),直徑為2.7mm的PET MPs在尺寸為0.3～4mm的無黏性大沙粒沉積層的臨界剪切應(yīng)力最大(0.233N/m2)[64].粒徑和形狀與MPs的臨界剪切應(yīng)力無顯著相關(guān)性.但MPs臨界剪切應(yīng)力與其密度成正相關(guān),剪切應(yīng)力符合如下順序:PVC和PET>聚酰胺(PA)>PS.利用數(shù)值模擬方法(TUFLOW Finite Volume)研究MPs在河流沉積物中的運輸過程結(jié)果也證實,密度較低的MPs具有較高的遷移率[38].因此,沉積物中低密度的PE MPs和PP MPs更易重懸進而隨河流運輸.而高密度的PA MPs和PET MPs會傾向于沉積在靠近其釋放源點的位置,并可能停留數(shù)年.

沉積物的粒徑結(jié)構(gòu)、粗糙度和有機質(zhì)組成等可影響MPs的遷移和分配[33,64,80].泥沙粗糙度與MPs的臨界剪切應(yīng)力顯著正相關(guān)(<0.001).在光滑的沉積面上,MPs更易發(fā)生滑行,而在沉積物床上,MPs運動主要受控于阻力,MPs更易發(fā)生跳躍.不同大小的MPs在沉積物上具有隱藏暴露效應(yīng).依據(jù)隱藏暴露效應(yīng)建立了沉積物中MPs的臨界剪切應(yīng)力的計算公式.公式可適用于各種形狀、密度和大小的天然沉積物(均勻和不均勻)和MP顆粒.粒徑較小沉積物處積累的MPs比粒徑較大的沉積物處更多,這可能歸因于隱藏-暴露效應(yīng)(圖2b),沉積在粗顆粒沉積物的MPs更易懸浮[64,80].沉積在松散沙質(zhì)沉積物的MPs更易于重懸返回上覆水中,而黏性較高且富含有機物的沉積物被視為塑料碎片的儲存庫,此處的MPs較少發(fā)生重懸,會在此積累[66,80].然而,也有研究發(fā)現(xiàn),渥太華河沉積物中MPs的含量與沉積物的粒徑或有機質(zhì)含量沒有顯著相關(guān)性[33].

3.3 沉積物對微塑料滲透過程的影響

沉積物主要由礫石(2～64mm)、砂(0.0625～ 2mm)、粉砂(0.0039～0.0625mm)和黏土(<0.0039mm)組成[104].MPs能夠在河流沉積物立體多孔隙結(jié)構(gòu)中遷移(自上向下、水平或自下向上)[89].在天然沉積物中MPs的滲透遷移過程首先受到自身理化性質(zhì)[105]、環(huán)境因素和其他膠體顆粒等的影響[106],當前研究主要針對MPs在沉積物中自上向下的滲透過程展開研究.

如圖2c所示,小尺寸的MPs可相對自由在多孔介質(zhì)中滲透遷移,較大粒徑的MPs或者MPs聚集體的滲透遷移則會受到顯著抑制.根據(jù)膠體過濾理論,隨著MPs粒徑與多孔介質(zhì)粒徑中值之比的增加, MPs的截留率也會增加[107].Fan等[108]發(fā)現(xiàn),在中國珠江流域,MPs多分布在表層沉積物,但隨著沉積物深度增加,小顆粒MPs(<0.45mm)占比逐漸提高,在42cm處可達到100%,表明小粒徑MPs更易向下滲透.同樣,在城市河流(秦淮河)采集的沉積物越深,小尺寸MPs占比越高[92].在淺層(0～10cm)沉積物中,大顆粒MPs(>4mm)的占比為40.5%,在深層沉積物(41～50cm)中小顆粒(<2mm)MPs為主要成分(63.5%)[92].據(jù)估計,深層沉積物中埋藏的MPs總量是表層沉積物中的5倍[109].沉積物中MPs和聚集體的保留積累,會逐漸堵塞沉積物顆粒間的孔隙,抑制甚至完全阻礙沉積物中MPs的滲透遷移[110].

通常使用DLVO理論來描述微塑料與多孔介質(zhì)間的相互作用.MPs和多孔介質(zhì)的表面電性會影響MPs的遷移過程[111-112].如表面帶正電荷的MPs將被表面帶負電荷的砂子等吸附截留[111-112].相反,表面帶負電荷的MPs與帶負電荷的砂子之間的靜電斥力作用能夠增強MPs的穩(wěn)定性和遷移量(圖2c).天然有機質(zhì)通過疏水作用、配體交換和靜電作用吸附在MPs或多孔介質(zhì)表面[113].在無金屬離子或存在一價陽離子電解液的情況下,吸附在表面的天然有機質(zhì)會在顆粒之間提供負電荷與空間位阻作用,從而提高顆粒的穩(wěn)定性[114-115].當二價或三價金屬陽離子存在時,天然有機質(zhì)通過與金屬離子絡(luò)合的橋聯(lián)作用影響顆粒聚集并進一步影響MPs的遷移[116].有機質(zhì)構(gòu)象和表面性質(zhì)、MPs的粒徑、表面官能團都會影響沉積物中MPs的穩(wěn)定性[117].研究表明,在干凈的淺層砂中,較大的顆粒態(tài)有機質(zhì)可以提高NPs的穩(wěn)定性和遷移率,而較小的溶解態(tài)有機質(zhì)則會促進團聚[111].HA顯著增加了PS MPs在氧化錳覆膜砂中的滲透,隨HA濃度從0增加到10mg/L,滲透率也相應(yīng)增加[118].

圖2 泥沙/沉積物對微塑料(a)聚集沉降、(b)再懸浮和(c)滲透過程的影響

沉積物的礦物組成和孔隙度在MPs遷移中起著重要作用.在石英砂(300～425μm)柱中添加鐵氧化物(針鐵礦和赤鐵礦)后,尺寸為0.2μm的MPs穿透率從73%降低至27%,尺寸為2μm的MPs穿透率從83%降低至2%[119].PS NPs在土壤中保留量與鐵/鋁氧化物含量呈正相關(guān),這是由于鐵/鋁氧化物表面在中性pH值下產(chǎn)生正電荷,從而通過靜電吸引促進帶負電的MPs吸附在飽和土壤表面[85].Waldschl?ger等[89]使用玻璃珠模擬天然沉積物進行MPs滲透實驗,發(fā)現(xiàn)MPs的滲透深度隨玻璃球直徑的增加而增加.當MPs與玻璃球直徑比大于0.32時,幾乎觀察不到任何滲透,MPs僅停留在沉積物表面(1.2cm);當MPs與玻璃球直徑比<0.11,MPs可以進入沉積物的深度為13.1cm.形狀不規(guī)則的MPs易在孔中纏結(jié),因此球形MPs顆粒的滲透要比碎片和纖維深,直徑較細的纖維滲透得較深(圖2c).

4 研究趨勢與展望

微塑料作為一種新污染物,當進入水環(huán)境后,由于物理、化學(xué)和生物等作用的耦合影響,其運移過程十分復(fù)雜,可能對水生生物、淡水和海洋生態(tài)系統(tǒng)甚至人類健康產(chǎn)生不利影響.目前,研究學(xué)者對于河流中,尤其是含沙河流中MPs的聚集、沉降和再懸浮等遷移過程和微觀機理研究仍不夠透徹,研究方法不足,模型建立尚不準確.在今后的研究中,應(yīng)該著重加強以下4個方面的研究:

4.1 加強不同MPs在含沙河流中遷移行為的研究工作.不同單體聚合而成的MPs的吸光性、密度和剛性等性質(zhì)不盡相同,因此MPs的理化性質(zhì)能夠影響其在水中和沉積物中的分散性和運輸過程.實際環(huán)境中的MPs受到溫度、紫外線輻射、生物降解和湍流等作用的影響,易發(fā)生老化和微生物定殖.微塑料表面上生物膜的生長、降解會改變其密度和表面性質(zhì),強烈影響MPs的聚集沉降等過程.可降解塑料產(chǎn)量預(yù)計每年將以超過30%的速度增長.未來的工作應(yīng)探究不同材質(zhì)的MPs顆粒,尤其是可降解MPs和老化MPs在水中的遷移過程,以更全面和真實地描述自然水域中MPs的遷移行為.

4.2 深入開展泥沙運動、水動力和水化學(xué)等多因素耦合作用下MPs遷移過程的實驗室模擬研究. MPs在水體中的遷移是泥沙/沉積物特性、河床面特征、水動力學(xué)特征、水化學(xué)特征和MPs自身特性多因素共同作用的結(jié)果.然而,目前絕大部分的研究均以單因素為主考察MPs的遷移過程,多因素系統(tǒng)性研究還相對欠缺.此外,考慮到真實環(huán)境中復(fù)雜的泥沙運動和水文條件,并且泥沙/沉積物等介質(zhì)具有非均質(zhì)性,未來研究中需加強多因素耦合作用對河流中MPs遷移過程影響的研究,確定關(guān)鍵過程,完善微觀機理.

4.3 進行含沙河流中微塑料水平運輸和河床交換過程模擬研究.目前關(guān)于水中MPs聚集和沉降的研究已經(jīng)廣泛開展,MPs的水平運輸和河床交換過程尚未進行系統(tǒng)性的研究.實驗室中通常以玻璃微珠和石英砂作為填充物的柱體模擬研究塑料顆粒在沉積物中的遷移和沉積行為,很難代表復(fù)雜的自然環(huán)境.泵吸交換、沖淤交換作用和過濾作用均未開展研究.未來的研究可考慮使用大型水槽裝置、利用熒光MPs開展定量研究,以完善水環(huán)境中MPs水平運輸和沉積物中遷移的基本過程和闡明作用機制.

4.4 開展河流中微塑料遷移的數(shù)值模擬和實地勘測研究,計算MPs在水體中的傳輸通量.由于室內(nèi)模擬實驗與天然環(huán)境仍存在較大差異,室內(nèi)模擬實驗不能準確說明真實含沙河流中MPs的輸運方式.數(shù)學(xué)模型是了解MPs遷移過程的重要研究手段.現(xiàn)有的模型往往僅針對球形的MPs顆粒在上覆水體和沉積物表面的遷移過程.未來的研究需要結(jié)合水動力模型、遙感模型、MPs自身形貌、降解和老化等過程,以及實驗室模擬所獲得的附著效率和過濾系數(shù)等參數(shù),嘗試開發(fā)模擬真實河流和深層沉積物中MPs的遷移和交換模型,對MPs輸移和傳輸通量進行精確描述和科學(xué)評估.實地勘測可準確獲得MPs在水體各層的分布特征,推測其傳輸過程,檢驗?zāi)Ｐ偷臏蚀_性.

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Recent progress of the effect of suspended sediment movement on the transport of microplastics in rivers.

WANG Xin-jie, WANG Yi-ning, ZHAO Jian, LIU Sheng-dong, XIA Xing-hui, LI Yang*

(Key Laboratory of Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China)., 2022,42(2)：863～877

This review summarized the occurrence and abundance of microplastics (MPs) firstly and then the main transformation processes of MPs in rivers, including aggregation, settling, rising, resuspension, horizontal migration, and hyporheic exchange. In this paper, we critically evaluated the transportation of MPs in sediment-laden river, such as the heteroaggregation and co-settling of MPs with suspended sediments, influence of sediments on the resuspension and infiltration of MPs. The interaction mechanisms between suspended sediments and MPs concerning influencing factors of MP transportation have been discussed. Finally, recommendations for future research were discussed: (1) reliable models for predicting the migration process and flux of MPs in rivers are needed to develop; (2) researchers are suggested to use aged-MPs to conduct experiments, and consider the effect of turbulent water and organisms on the transportation behavior of MPs during laboratory work.

microplastics；nanoplastics；river；transport；suspended sediment；sediment；influence factors

X522

A

1000-6923(2022)02-0863-15

王薪杰(1994-),女,山東煙臺人,北京師范大學(xué)博士研究生,主要從事水環(huán)境中微塑料遷移轉(zhuǎn)化方面研究.發(fā)表論文11篇.

2021-06-28

國家重點研發(fā)計劃項目課題(2021YFC3200401);國家自然科學(xué)基金資助項目(52170024,21677015)

* 責任作者, 副教授, liyang_bnu@bnu.edu.cn