微塑料的老化過程、產物及其環境效應研究進展

苗令占,鄧肖雅,楊 錚,李婉逸,侯 俊

微塑料的老化過程、產物及其環境效應研究進展

苗令占1*,鄧肖雅1,楊 錚2,李婉逸1,侯 俊1

(1.河海大學環境學院,淺水湖泊綜合治理與資源開發教育部重點實驗室,江蘇 南京 210098；2.廣州市城建規劃設計院有限公司云南分公司,云南 昆明 650011)

本文綜述了光氧化降解、熱降解、物理磨損和生物降解對微塑料老化過程的影響,列舉了相關實驗室研究方法及關鍵結論.在此基礎上總結了微塑料老化后顆粒態、溶解態產物的檢測方法,分析了微納米/塑料自身毒性及污染物攜帶效應、浸出液對水生生態系統及生物體的影響.最后展望了未來研究應考慮多重因素縮小與自然環境的差距,篩選降解功能的菌群治理微塑料污染,建立產物監測分析的技術方法標準,揭示產物的毒性機制.

微塑料；老化；檢測方法；浸出液；環境效應

塑料制品應用廣泛,2021年全球產量達3.9億t,其中再生塑料僅占總產量的8.32%[1],因此有大量塑料垃圾進入環境中,并在外力作用下老化破碎產生更小的顆粒,其中粒徑小于5mm的被定義為次生微塑料[2-3].此外,以小顆粒形式直接排放到環境中的小于5mm的塑料顆粒稱為原生微塑料,如清潔用品、化妝品中的微珠[4-5].調查研究表明,微塑料廣泛分布于河湖、海洋、土壤、大氣及沉積物等環境中[6-9],在生物體內[10]、人類食品[11],甚至偏遠的南極[12]和北冰洋[13]也均有檢出.隨塑料制品需求的不斷增加,排入江河湖海中的塑料量亦日漸增多[14],預計到2100年海洋中漂浮的微塑料將達到0.25～1.3億t,與2010年相比將增加50倍[15].微塑料被認為是新型的全球環境污染物[16].

微塑料中的添加劑會緩慢釋放到環境介質中,其中鄰苯二甲酸酯(PAE)、溴化阻燃劑(BFR)等多種添加劑具有內分泌干擾作用、致癌等毒性效應,向生物體遷移會對人體健康和生態安全產生威脅[17].同時,微塑料比表面積大、疏水性強,容易成為許多疏水性有機污染物、重金屬、病原體的載體,產生復合污染效應,危害生物及生態系統[18].此外,微塑料粒徑較小而容易被生物攝食,顆粒及其吸附、釋放的污染物可通過食物網富集,造成潛在的生物毒性和生態風險[19].已有研究表明,攝入微塑料可導致生物體生理代謝和生長發育異常[20-21]、產生生殖毒性[22]、免疫毒性[23]等毒性效應,使機體造成損傷.微塑料在環境中易受到海水沖刷、紫外線照射、生物膜定殖等環境因素的影響而老化[24-26],物化性質發生改變,影響其環境行為和生態效應,因此了解其老化過程對于探索微塑料對生物的毒性機制及環境效應至關重要.目前關于微塑料的綜述多側重于老化過程或毒性效應,并未對兩者進行系統歸納.鑒于此,本文總結了自然環境中微塑料經歷的主要老化過程,列舉了相關實驗室模擬研究,闡述了老化后的產物及其檢測方法、環境效應,并就微塑料的老化研究、產物檢測方法和環境效應研究進行了展望.

1 微塑料的老化過程

環境中的微塑料會在機械力、光照、溫度、生物等各種環境因素的影響下而發生老化降解,進而影響著其遷移轉化過程.已有研究中,通過對微塑料表面特性及產物監測分析進行老化表征[27-29](圖1).明確微塑料的老化過程對于理解其環境行為和風險至關重要.

圖1 微塑料的老化表征

1.1 光氧化降解

1.1.1 光氧化降解機制 微塑料的光老化是自由基引發的聚合物鏈式反應的過程,包括鏈引發、鏈傳遞、鏈終止3個階段(圖2).在鏈引發階段中,聚合物中的不飽和鍵或發色團吸收紫外光能,生成烷基自由基(R·)[28,30].隨后進入鏈傳遞階段,R·與分子氧形成過氧自由基(ROO·),并從另一條聚合物鏈RH中奪取氫原子,形成中間體氫過氧化物(ROOH),ROOH中O—O鍵在紫外線下不穩定,吸收光能斷裂形成烷氧基自由基(RO·)和羥基自由基(·OH)[31-32].·OH攻擊ROOH和RH,分別形成ROO·和R·,此外,ROO·與RH結合通過光解作用形成過氧化氫自由基(HO2·)和R·,HO2·隨后可形成過氧化氫(H2O2)[33].RO·可經歷多種反應途徑,如從RH中奪氫生成醇、β-裂解形成酮或醛[34].酮在紫外線照射下會通過Norrish Ⅰ反應生成R·和酰基自由基(R-CO·),或通過Norrish Ⅱ反應形成末端羰基(R-CO-CH3)[35].鏈反應終止階段主要是雙分子或低分子自由基與主要產物烯烴、酮、醛之間的重組[33].

圖2 微塑料的光降解和熱降解過程[33]

RH為微塑料,R1和R2為不同長度的聚合物鏈

1.1.2 光氧化降解研究 對于微塑料的光氧化降解過程,國內外已進行大量的模擬研究[36-37],包括老化機理、光照強度及時間對老化的影響等.光氧化降解是暴露在太陽光下的微塑料老化的主要途徑[38].由紫外線輻射吸收產生的光化學反應會引起氧化,能夠導致塑料變脆、彈性降低,而容易破碎[39].老化微塑料表面形貌發生改變,如變得粗糙,出現孔隙、裂縫、凹痕,且數量、大小隨老化時間增加而不斷增多、增大,最終導致微塑料破碎[40].微塑料經紫外線照射后表面被氧化產生含氧官能團,氧碳比(O/C)和羰基指數(CI)增加[29].由于微塑料自身氧化產生的發色團產物或酚類抗氧化劑氧化產生的含醌類結構產物等會產生變色效應,微塑料的顏色通常會隨老化時間的延長而變深,如變黃甚至變黑[41].

相同光照處理條件下不同種類微塑料的老化程度有差異,44 μW/cm2輻照強度下照射90d,老化后的PS微塑料表面出現了大量的顆粒狀凸起、凹陷和微孔,而聚乙烯(PE)微塑料表面變得粗糙,產生了細小的孔隙和裂紋[42].Hebner等[27]、Fairbrother等[43]分別證實了光照時間和輻照強度也會影響微塑料老化過程,微塑料老化破碎產生的塑料顆粒隨紫外線照射時間的延長而增多,老化的變化速率隨輻照強度的增加而加快.施加機械應力也能夠加快光氧化降解的速率[44],如Sun等[45]的研究結果顯示,紫外線照射和施加機械應力更能促進PE、熱塑性聚氨酯彈性體(TPU)的降解,比單獨紫外線照射處理產生的微塑料數量更多.

紫外線輻射是導致微塑料中聚合物快速降解的重要環境因素[46].PE塑料地膜廣泛應用于農業生產,由于其在戶外陽光下使用[47],其光降解過程為學者所關注.PE薄膜的光老化為自由基鏈式反應(圖2),經過鏈引發生成高活性自由基,鏈傳遞產生ROO·或HO2·等中間產物,鏈終止階段通過Norrish Ⅰ反應或Norrish Ⅱ反應最終產生羰基、端乙烯基和羥基等[48-49].Briassoulis等[47]通過紫外輻射對PE殘留地膜進行老化試驗,并將地膜回填到土壤中觀察自然狀態下降解情況,結果表明,未經紫外照射的地膜在土壤中8.5a后降解現象不明顯,紫外照射下地膜完全分解為<1mm的塑料顆粒,證實了PE的光降解.尤里武等[50]的研究也表明了光照使PE地膜的老化速率加快,其采用干熱大氣暴露、實驗室氙燈和紫外燈暴露試驗研究了PE地膜的老化,結果顯示,3種條件下,PE的拉伸負荷和斷裂標稱應變與暴露時間均大致呈負相關性,干熱大氣暴露條件下PE老化速率與實驗室光源下呈現一定倍率關系,且紫外老化速率更快.在陸地環境中,光氧化被證明比其他類型的降解過程快了幾個數量級[21].如在Song等[51]的研究中,自然光照下聚苯乙烯泡沫(EPS)僅1個月質量就下降了5%,12個月后質量損失可達34.2%.PBAT/PLA地膜經紫外照射10d相當于在田間栽培環境老化120d[52].另有研究表明,在模擬太陽輻照下,當光強分別為北緯0°和50°自然光強的3倍和10倍時,PS微塑料可通過光降解完全礦化為CO2[53].光老化的室內模擬研究主要使用燈光模擬陽光,僅占可見光的小部分波段或光強,無法很好地反映暴露在環境中不斷變化的陽光下的真實老化行為,因此需要考慮更多因素更接近實際環境模擬老化過程.

1.2 熱降解

微塑料的熱降解過程與光氧化降解過程類似,均為聚合物鏈斷裂后的氧化反應[54],但在陸地和海洋環境的降解中,光氧化降解比熱降解更占主導地位[55].當微塑料受熱吸收的能量超過高分子化合物分子鏈間的化學鍵的解離能,弱位點就會發生隨機斷裂和支鏈的脫落,降解速率將會加快[56]. Fairbrother等[43]的研究中,HDPE老化降解的變化速率隨溫度的升高而加快.另有研究表明PS在70℃下發生熱裂,表面出現裂紋,而在40℃時表面仍光滑.溫度升高也導致微塑料老化浸出添加劑的速率加快,浸出濃度也越高[57].Zhou等[58]的研究也表明了一次性塑料飯盒在受熱后釋放雙酚F和雙酚S的水平在各個貯藏時間點整體上為80℃>50℃>25℃.微塑料在自然環境中發生熱降解的同時往往伴隨著光氧化降解,研究中采用的高溫多是堆肥溫度,與多變的環境溫度有所差異,因此微塑料在環境中隨溫度的熱降解需要進一步探究.

1.3 物理磨損

已有學者關注了物理磨損作用下微塑料的變化,也有研究將物理磨損與光照這一環境因素結合探究了微塑料的老化(表1).物理磨損能夠改變微塑料的表面形貌和結構,使其產生裂縫,比表面積增加,更有利于其他老化過程的進行[33,54,59].有研究顯示,物理磨損是造成微塑料表面裂紋、凹陷等紋理產生的主要因素[60].微塑料在水中會受到剪切力和拉伸力的作用,發生機械破碎和脆化形成更小的顆粒,甚至形成納米塑料[59].Enfrin等[61]研究表明,微塑料在剪切力的作用下會通過裂紋擴展破碎形成大量的納米塑料.Hebner等[27]用紫外燈照射聚丙烯(PP)、PE、聚對苯二甲酸乙二醇酯(PET)3種微塑料,并設置湍流和靜水條件進行了老化實驗,結果表明在湍流水機械應力作用下,產生的塑料顆粒比靜水條件多1.4～3倍,且產生的微塑料的數量隨粒徑的減小而增加.

微塑料還可能在沙子、石頭等其他機械力的作用下釋放顆粒到環境中[62].Song等[39]收集海灘的沙子并對其進行預處理去除有機物和塑料,將沙子與低密度聚乙烯(LDPE)、PP、EPS混合放置在滾筒攪拌機上旋轉2個月進行機械磨損實驗,LDPE、PP、EPS組分別產生(8.7±2.5)、(10.7±0.7)、(4220±33)個顆粒/粒,說明塑料的機械磨損受材料類型的影響;在研究中還對比了不同紫外燈照射時間處理后對微塑料機械磨損的影響,結果顯示,照射12個月后再進行磨損實驗LDPE、PP、EPS組分別產生(20±8.3)、(6084±1061)、(10501±1718個)顆粒/粒,PP、EPS組遠高于無紫外燈照射產生的顆粒數量,說明與單獨的機械磨損相比,紫外燈照射可以促進機械磨損釋放更多的微塑料顆粒.與塑料相互作用的海灘鵝卵石、沙子等顆粒越粗,塑料破碎的質量比例也越大[63].沙子、沉積物產生的機械應力比水產生的機械應力使微塑料的破碎率更高,釋放的顆粒或纖維更多[45,64],因此塑料顆粒的降解在陸地上比在水環境中更容易發生,特別是海灘環境更有利于微塑料的機械破碎和化學老化過程[60].目前對于微塑料物理磨損方面的研究資料有限,相關研究考慮的環境因素多是1～2個,模擬的水流速度、所用顆粒物的粒徑等在實驗過程中都是一成不變的,而實際環境中影響老化的因素眾多,情況更多變,因此與實際環境貼合設置實驗才能更好地揭示微塑料的老化過程.

2.1.5 藥品劑型的影響:同一藥品的劑型不同,其在體內的吸收也不會相同,即生物利用度相異,如果不能掌握劑量就會導致不良反應的出現。

1.4 生物降解

微塑料生物降解包括生物攝食消化和生物膜覆蓋(微生物降解)2個途徑.生物攝食消化被認為是潛在的生物降解機制,微塑料可在生物體內的酶作用下老化降解[33].微塑料為微生物提供了獨特的棲息地,微生物在其表面形成了生物膜[65],二者組合成塑料圈[66],微生物可以通過胞內或胞外解聚酶對微塑料進行降解.

1.4.1 微生物降解過程 微塑料的微生物降解過程如圖3所示.首先微生物附著在微塑料上,隨后在其表面定殖并形成生物膜[67].微生物分泌的胞外聚合物(EPS)為定殖提供了粘性基質[33],給予生物膜穩定性支撐并有利于其粘附于微塑料表面.隨后,微塑料表面發生生物降解,導致微塑料裂解[68].微生物分泌的內酶和外酶促進微塑料的解聚過程,使其破碎形成具有較小分子(如單體和低聚體)的中間體并釋放添加劑[67].最后,這些小分子聚合物可以被微生物用作碳源和能源吸收代謝,通過同化、礦化作用生成代謝產物(CO2、H2O、CH4等)[69].值得注意的是,可降解微塑料區別于非降解微塑料,其生物降解過程為先吸收水分,部分高分子鏈水解為較低分子量聚合物,并在微生物體外酶作用下再分解為寡聚物或單體,最后被微生物吸收、代謝成水和二氧化碳等小分子產物[70].

圖3 微塑料的微生物降解過程[59]

1.4.2 微生物降解研究 目前微生物降解的研究包括生物膜的形成對微塑料老化的影響[67,71]、從生物體內或環境中篩選具有降解功能的菌種或菌群[72-73]等.如Bhagwat等[74]在海水中進行6個月的原位老化實驗,發現PE、PA、PES、PA這4種微塑料表面都形成了生物膜,含氧官能團增多,并檢測到了多糖類化合物光譜區的振動,微塑料不斷裂解,比表面積增加.生物膜的形成還會影響微塑料的其他老化過程.附著在微塑料表面的生物膜迅速生長,會導致微塑料浮力和疏水性顯著降低[67],生物膜分泌粘性聚合物,會促進微塑料與天然有機物的雜聚體的形成,都會使顆粒下沉處于低溫和弱光的環境中,從而可能減弱光老化和熱降解過程,影響老化速率[75-77].有研究指出,生物膜可以吸收高達99%的紫外線輻射[76],保護塑料碎片免受紫外線的輻射,減緩光化學分解過程,且能降低其他促進塑料老化的因素(如剪切力)的影響,從而保護顆粒表面,降低微塑料老化裂解的破碎率[71].另一方面,生物膜微生物群落中的細菌能夠通過光合作用產生氧氣,會加速微塑料的氧化分解過程[78].

據報道,許多菌株都具有生物降解塑料的能力,可以改變微塑料表觀形貌、使其造成質量損失,還可以使其官能團、疏水性等理化性質發生改變[69].如從蠟蟲和黃粉蟲腸道內分離出的YT1和sp. YP1被證實能夠降解PE,能夠使PE產生質量損失,表面出現孔洞,抗拉強度下降,水接觸角降低,O/C值和CI指數增加,這些結果也都表明了PE的老化[72].通過異位篩選獲得能夠有效降解微塑料的真菌菌株較困難,但也有一些真菌被篩選分離出也被證實能夠利用微塑料作為營養源進行生長,具有降解微塑料的能力[79-81].除了分離出的單個菌種,有研究關注了菌群聯合體降解微塑料的效果[82-83].如堆肥技術中的嗜熱菌群被報道能夠降解微塑料,Chen等[73]以超高溫堆肥(TC)的懸浮液為接種物,70℃條件下與PS微塑料置于瓶中培養56d后,PS表面出現大量凹痕和孔隙,質量損失約7.3%,約為未接種對照組的6.6倍,TC接種物還能夠有效地解聚或裂解PS的長鏈結構,并在高溫下形成低分子量片段,說明嗜熱菌群通過高生物氧化性能有效誘導微塑料的降解.另有研究通過分析塑料圈上的群落結構組成以篩選潛在降解功能的菌群,如Miao等[84]通過野外原位培養,分析對比了3個不同淡水系統中塑料圈上微生物群落的動態演替,通過高通量測序分析、構建網絡,并與相關文獻對比,發現可生物降解塑料具有吸引和聚集關鍵微生物的能力,包括芽孢桿菌目(Bacillales)的微小桿菌屬()和環絲菌屬()、黃單胞菌目(Xanthomonadales)的砂胞單胞菌屬()等,這些都是潛在的烴降解生物.但目前對于微生物降解微塑料的路徑資料還存在很大的空缺,研究中篩選菌種多是在實驗室優化環境下進行,且單一菌種的降解作用下通常會產生抑制其生長的副產物[85],因此未來探究微生物的降解機制,開展具有降解功能的菌群聯合體的篩選研究,繼而富集培養應用于解決微塑料污染中,是非常必要的.

表1 微塑料老化過程相關實驗室模擬研究

續表1

續表1

2 微塑料老化產物及檢測技術

微塑料老化降解過程中會釋放顆粒態、溶解態產物,會對生態環境和生物體造成一定的效應(圖4),目前已有一些技術用于這些產物的檢測分析中(表2).

圖4 微塑料的老化產物及其毒性效應

2.1 微塑料裂解產物

微塑料在外界作用下會逐漸老化裂解產生更小粒徑的顆粒[27,63],甚至是小于1μm的納米塑料[2],即顆粒態產物.多個研究表明,光照、物理磨損等作用下,微塑料會破碎形成更小的塑料顆粒[27,39,64].大多數塑料含有增塑劑、阻燃劑、穩定劑、抗氧劑等添加劑以改善產品的性能[94],其中大部分添加劑都未與聚合物鏈以化學鍵相連,在塑料老化過程中可能會浸出到環境中[95],此外塑料單體、低聚物也會在老化過程中釋放出來[86],上述物質即為微塑料老化的溶解態產物,稱為塑料浸出液[96].已有許多研究在實驗條件下證實了塑料添加劑的浸出[97],如BFR[98]、增塑劑PAEs[99]、抗氧化劑壬基酚[100](NP)等,另有研究表明了塑料老化會釋放低聚物和其他聚合物單體[101].

微塑料自身的理化性質、外在環境因素都會影響顆粒態和溶解態產物的釋放過程.微塑料自身的添加劑可能會抑制其老化,但老化程度高的微塑料更容易滲出添加劑[102],如在模擬海水中,含添加劑PP微塑料的光老化速率明顯低于不含添加劑的微塑料原料[103],鐵紅顏料能明顯減緩微塑料的老化[104].同一條件下,由于微塑料自身特性不同微塑料破碎速率、添加劑的遷移過程會有所差異[39,45].以PP、PE、EPS為例,經紫外光照射和沙子機械磨損作用后,PP的破碎率要高于PE,EPS產生的顆粒最多[39].橡膠態聚合物中添加劑的擴散速率要高于玻璃態聚合物[105].粒徑大小也會影響微塑料的破碎速率和添加劑的浸出.由于粒徑較小的微塑料傾向于在水流作用下翻滾,受光照更均勻,運動的小顆粒表面定殖的生物膜較少,其碎裂速度比較大的顆粒更快[106].更小的微塑料擁有更大的比表面積,能夠與外界接觸的點位就越多,更容易釋放添加劑,但同時小顆粒更容易凝聚,接觸點位也會變少[107].水環境中的天然有機質(NOM)、氯離子(Cl-)、溴離子(Br-)、碳酸根(CO32-)和硝酸根(NO3-)等具有光化學活性的組分,光照下可參與到微塑料光老化自由基反應過程中,與直接暴露在空氣中的微塑料老化速率有差異,且不同水環境中的微塑料老化程度也會不同[108],如Mao等[40]的研究中,直接暴露在空氣中的PS老化程度最高,海水中次之,純水中最小.此外,環境溫度、酸堿度等條件也會影響微塑料產物的釋放.自然環境條件下,影響微塑料老化是多個因素協同、拮抗作用的結果,而目前的研究多集中在模擬條件下及單因素的分析,不能很好地契合實際環境,且其中的機理研究仍非常有限,因此需要更多更深入的研究來回答上述問題.

2.2 老化產物的檢測技術

表2 微塑料老化產物的檢測方法

對于浸出液的測定分析,需要根據微塑料類型及浸出物質種類選擇合適的方法.分光光度法和熒光光譜法可以測定浸出液中具有吸光作用的物質[116],總有機碳(TOC)分析儀可以測定TOC和溶解性有機碳(DOC)含量,兩者的測定結果可以反映有機物的變化[117].采用液/氣相色譜-質譜法可以定量分析獲取大/小分子、易揮發、不穩定、低聚物等物質的信息[116].微塑料浸出液成分復雜,且浸出濃度較小,因此準確獲取浸出液成分的信息較為困難,并且與數據庫對比工作量較大,通常存在物質濃度低而儀器未檢出、分析物質不全面等問題.如Gewert等[28]的研究中用固相萃取濃縮、液相色譜結合高分辨率質譜檢測,分析了紫外燈照射下PE、PP、PS、PET的浸出液成分,初步確定了22種降解產物,但是確認結構的僅有5種.因此,需要建立一種能夠有效定性定量分析微塑料浸出液的方法標準,且能夠對環境濃度進行定量分析,為微塑料污染防治提供數據支撐.

3 老化產物的環境效應

3.1 微/納米塑料的毒性效應

3.1.1 顆粒毒性效應 老化后的塑料顆粒的形狀和顏色與生物的食物相似,其可被海綿動物、魚類等生物攝入[118-119].有研究表明,蜂海綿和沐浴海綿能夠快速累積海水中＜300mm的微塑料,且粒徑越小的顆粒越不容易被其排出體外[118].許多生物誤食微塑料后,可能會發生食道阻塞,無法正常進食,在消化道內累積,影響進一步進食[120];也可能引起假性飽腹感,導致食物攝入量減少,影響正常生理代謝和生長發育,并可能導致生物死亡[20-21].Lu等[121]研究表明,暴露于微塑料中會誘導斑馬魚體內超氧化物歧化酶和過氧化氫酶活性顯著升高,即微塑料誘導了氧化應激反應,同時,也會導致斑馬魚的肝臟代謝特征改變,并擾亂肝臟的脂質和能量代謝.PS微塑料顆粒可被底棲軟體動物攝入并在消化系統和非消化系統累積,即使經過凈化,大部分顆粒都被排出,但PS造成的神經毒性不可恢復[122].相比于微塑料,納米塑料具有更高的細胞親和力,更大的比表面積,使其更容易穿透生物屏障并在器官中積累[123],在生物體內的留存時間也更長,與微塑料相比對生物體帶來的危害有所不同,如Yin等對先前研究進行梳理總結,結果表明,在腸道中,納米塑料引起較高的炎癥和氧化應激反應,而微塑料傾向于引起更嚴重的腸道菌群失調;在肝臟中,納米塑料會產生更高的氧化應激和脂質代謝紊亂;納米塑料引起的生殖毒性和神經毒性高于微塑料[124].

對于微/納米塑料的毒性研究中塑料類型、粒徑大小、暴露時間的不同,研究結果也有所差異[125].如對于納米級PS毒性研究,一些學者研究結果表明其會誘導氧化應激、遺傳毒性、細胞毒性、壞死或炎癥等[126],Forte等[127]研究中,40nm的PS比100nm的PS在胃腺癌細胞中的累積速度更快,兩者都會影響細胞活性、炎癥基因表達和細胞形態.同時也有研究顯示納米級PS的不良影響很小或沒有[128],Fr?hlich等[129]的研究結果顯示,盡管不同粒徑(20～500nm)的PS短時間內暴露使內皮細胞溶酶體發生微小變化,但接觸時間越長,變化越不明顯.

3.1.2 污染物攜帶效應 由于尺寸小、比表面積大和疏水性強,微/納米塑料可以作為有機污染物、金屬、金屬氧化物納米顆粒等污染物的載體,并能夠將污染物輸送到生態系統中[130].吸附是影響微/納米塑料與污染物相互作用的關鍵因素,進而增強或減弱對生物的復合效應,其吸附機理如圖5所示[131].

圖5 微/納米塑料與共存污染物的吸附機理[131]

微/納米塑料吸附污染物對生物的復合毒性效應并不是絕對的.有研究表明,微/納米塑料與污染物的聯合效應會使毒性增強,如Huang等[132]發現,PS微塑料可以吸附有機磷農藥毒死蜱并將其轉移到斑馬魚體內,導致魚的氧化應激、游泳性能和組織學損傷、腸道微生物群落紊亂和多樣性改變;Araújo等[133]發現,PE微塑料與包含農藥、工/農業廢水石油等14種污染物的混合物聯合暴露對蝌蚪的誘導應激反應增強,紅細胞壞死和凋亡頻率更高,游泳活動減少;Davarpanah等[134]研究表明,將微塑料和金納米顆粒聯合暴露顯著降低了海洋微藻的生長速率,而單獨處理并未出現明顯的抑制作用.但是也有研究結果表明,微塑料與污染物共存會使毒性降低,如Fu等[135]的研究發現,老化的PVC與銅聯合暴露,銅吸附在PVC表面,引起沉淀,使其對小球藻的毒性降低,甚至促進了細胞生長;Wang等[136]研究中,小球藻單獨暴露于布洛芬中,96h-IC50= 54.5mg/L,PS納米塑料與布洛芬聯合作用下,96h- IC50=63.9mg/L,對小球藻的生長抑制作用減弱,氧化應激降低,PS處理也會導致小球藻內布洛芬累積量降低,生物降解加快.

已經有學者在人體血液中發現微塑料顆粒[137],又有研究指出在人體血栓樣本[138]和痰液[139]中發現了一定數量和不同類型的微塑料,微塑料對人體可能造成的潛在健康風險需要論證.目前關于微/納米塑料的毒性研究主要集中在水生生物上,缺乏對陸生生物的毒性研究,且微塑料與納米塑料的毒性作用在不同生物上的差異是否一致還不得而知,不同塑料類型、不同粒徑的微/納米塑料的作用機制尚不清楚,因此需要就上述問題進行研究揭示塑料的毒性機理.

3.2 微塑料浸出液的環境效應

微塑料浸出液的環境效應研究主要集中于DOC和添加劑等.隨著塑料的老化破碎,塑料中溶解性有機物(DOM)會浸出到環境中[140].王琳等[141]的研究結果指出150～200μm的PS浸出的小分子有機物質是對小球藻產生毒性并抑制其生長的主要因素.DOM的浸出包括DOC的瀝濾[142].老化塑料對DOC的釋放比原始塑料浸出的DOC要高出兩個數量級[143].據估計,海洋塑料垃圾每年釋放的DOC高達2.36萬t,在高污染地區塑料浸出的DOC甚至占水體表面(40μm)DOC的10%[144].DOC通過調節光合作用和異養微生物的存在、生長和活動,在海洋食物網中發揮著重要的作用[145].天然存在的DOC是海洋食物網底端微生物的主要碳來源,并構成全球主要的碳儲存庫[146].微塑料浸出的DOC是海洋微生物的潛在重要碳源[147],影響著海洋中的微生物活動和碳循環.如Romera-Castillo等[144]的研究表明,光照和黑暗條件下不同類型的PP、PE均能夠不同程度地釋放DOC到水體中,并可被微生物吸收,刺激異養微生物的生長.微塑料不僅能夠釋放DOC,還能吸附DOC,直至吸附平衡,該過程中微塑料可能與微生物對DOC的吸收利用相競爭,從而擾亂其低營養級過程[144].因此,塑料的積累可能影響水環境中生物群落的碳循環、群落組成,進一步影響水生生態系統的結構與功能.

目前世界各地的河口和海洋中檢出的塑料添加劑濃度從pg/L～mg/L不等[95],在生物體內也有添加劑檢出,如海鳥、貽貝、大型藻類和各種魚類等[148-149].添加劑的浸出量隨微塑料在環境中停留時間的延長而增多,可能對水生生物產生負面效應[150].微塑料浸出的添加劑會抑制藻類的生長,如聚氨酯泡沫塑料(PFU)在不同天然水體和模擬水體中的熒光添加劑的釋放量隨溶液pH值和浸出時間的增加而增加,達到一定濃度時,會對小球藻的生長和細胞光合作用產生抑制作用[142],商用鉻酸鉛著色的微塑料浸出的鉻和鉛隨老化時間的延長而增加,高濃度的浸出液對銅綠微囊藻細胞的毒性更大[151]. Schrank等[152]的研究發現含增塑劑鄰苯二甲酸異壬酯(DiNP)的聚氯乙烯(PVC)浸出液會導致大型溞的體長增加,并對其產生生殖毒性導致后代數量減少.盡管某些PE塑料袋為食品級,但將其裝滿合成海水放置48h后釋放NP濃度可達(163.3±5.7) μg/L,將弗氏擬雀鯛(魚)放置于裝滿合成海水的PE袋子中暴露48h,檢測其體內NP含量為(368±39) μg/kg,魚的死亡率高達60%[100].在環境中老化后的塑料顆粒吸附了更多的污染物,這些物質與原始添加劑相互作用浸出后對貽貝的毒性作用更強[153],用攝食微塑料的貽貝喂食螃蟹,在螃蟹多個器官組織中均檢出了該塑料顆粒,表明了微塑料的營養轉移[154],這也可能導致微塑料上化學物質通過食物網轉移、富集,進一步影響人類的健康.目前微塑料浸出液的生物毒性、對生態系統的影響已有一定的基礎,但多是使用高于環境中浸出液的組分濃度進行模擬,且環境中浸出液的含量和組成是多變的,可能存在多個組分的協同毒性,作用機制可能更復雜,因此需要考慮實際環境濃度和組分來研究浸出液對生物體的毒性效應,探究其對整個生態系統的影響.

4 結語

塑料制品的使用量不斷增加,但其的回收率依舊很低,導致環境中的塑料垃圾越來越多.自然環境中塑料垃圾破碎產生微塑料,微塑料會在水環境中物理、化學和生物作用下老化破碎成更小的顆粒,并釋放化學物質,在環境中賦存,對水生生物產生毒性作用,影響著水生生態系統的平衡.因此,研究環境中微塑料的老化過程及產物對開展微塑料的污染防治及生態風險評估具有重要意義,但目前相關研究還存在諸多不足,今后的研究可從以下幾個方面開展.

4.1 微塑料老化過程實驗室研究應更貼合實際環境進行,縮小兩者之間的差距.目前光老化研究使用的燈光僅是可見光的小部分波段或光強,熱降解研究中多是采用高于自然環境的溫度,實驗中物理磨損模擬的水流速度、所用沙子的粒徑等都是不變的,但自然環境條件更多變復雜,應關注多個環境因素協同老化的自然作用過程,更好地揭示微塑料的老化過程.

4.2 探究微生物的降解機制,篩選具有降解功能的菌群聯合體,并富集培養應用于解決微塑料污染問題.當前研究篩選菌種多是在實驗室優化環境下進行,對于微生物降解微塑料的路徑資料也還存在很大的空缺,今后應對降解機制展開深入研究,致力于篩選可降解塑料的菌群,并為微塑料污染防治服務.

4.3 建立微/納米塑料、浸出液的定性定量分析的技術方法標準.目前對于顆粒太、溶解態產物的檢測仍然缺乏高效、低成本的分析方法,因此建立完整的評估方法、定性定量分析方案能夠更全面地研究產物的毒性效應、了解產物在環境中的賦存濃度.

4.4 探究微/納米塑料對陸生、水生生物的毒性效應及作用機制,微塑料與納米塑料的毒性差異.目前關于微/納米塑料的毒性主要集中在水生生物上,缺乏對陸生生物的毒性效應資料,且微塑料與納米塑料的毒性作用在不同生物上的差異還不清楚,不同塑料類型、不同粒徑的微/納米塑料的作用機制尚不明晰,因此需要就上述問題進行研究揭示塑料的毒性機理.

4.5 考慮浸出液的實際環境濃度和組分來研究老化產物對生物體的毒性效應.目前研究多是使用高于環境中浸出液的組分濃度進行模擬,且環境中浸出液的含量和組成是不定的,可能存在多個組分的協同毒性,作用機制可能更復雜,因此需要考慮實際環境污染來研究浸出液對生物體的毒性效應,探究其對整個生態系統的影響.

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Research progress on the aging process, leachates of microplastics and their environmental effects.

MIAO Ling-zhan1*, DENG Xiao-ya1, YANG Zheng2, LI Wan-yi1, HOU Jun1

(1.Key Laboratory of Integrated Regulation and Resources Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China；2.Yunnan branch of Guangzhou Urban Construction Planning and Design Institute Co., Ltd., Kunming 650011, China).2023,43(11)：6156～6171

Effects of photo-degradation, thermal-degradation, physical abrasion and biodegradation on the aging process of microplastics were reviewed. The relevant laboratory research methods and the key conclusions were discussed. Furthermore, the detection methods of the particulate and dissolved leachates of aged microplastics were summarized, and the toxicity of micro/nano-plastics, their pollutant carrying effects and the impacts of plastic’s leachates on aquatic ecosystem and organisms were analyzed. Finally, the future research focuses were proposed. It is suggested that future research should take into account the multiple factors to simulate the natural environment, screen the plastic degrading microbes, establish the technical and methodological standards for plastic’s leachates, and reveal the toxic mechanism of leachates from microplastics.

microplastics；aging；analytical methods；leachate；environmental effect

X131

A

1000-6923(2023)11-6156-16

苗令占(1988-),男,河南省鄭州人,教授,博士,主要從事河湖水質改善與生態修復工程、河流微生物生態學、新污染物特性及環境效應等方面的研究與應用工作.發表論文60余篇.lzmiao@hhu.edu.cn.

苗令占,鄧肖雅,楊 錚,等.微塑料的老化過程、產物及其環境效應研究進展 [J]. 中國環境科學, 2023,43(11):6156-6171.

Miao L Z, Deng X Y, Yang Z, et al. Research progress on the aging process, leachates of microplastics and their environmental effects [J]. China Environmental Science 2023,43(11):6156-6171.

2023-03-16

西藏科技廳重點研發計劃項目(XZ202101ZY0016G)

* 責任作者, 教授, lzmiao@hhu.edu.cn