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塑料制品在我们的生活中无处不在。2017年,全球塑料产量上升至3.35亿吨[1]。最常见的塑料制品包括聚乙烯(PE)、聚丙烯(PP)、聚氯乙烯(PVC)、聚对苯二甲酸乙二酯(PET)、聚苯乙烯(PS)等。关于海洋中塑料的研究最早发表于20世纪70年代 [2]。在2004年,Thompson等[3]首次提出微塑料的概念,并引起了广泛关注。目前的研究表明微塑料存在于海洋环境[4-5]、淡水环境[6-9]、沉积物[10-11]、土壤[12-13]以及生物体内[14-15]。微塑料在被生物摄食后可造成挤压、占位等,从而导致生物摄食效率降低、生长缓慢、受伤或死亡等 [16-17]。微塑料本身也会释放塑化剂、阻燃剂、抗氧化剂等有毒物质 [18-22] 。此外,微塑料表面还能吸附环境中的疏水性有机污染物,在被水生生物摄食后,会对生物体产生毒性效应 [23-25] 。
准确、高效的分析方法是研究微塑料的环境行为及生态毒理学效应的关键前提。欧盟海洋战略框架指令(MSFD)[26]以及美国国家海洋和大气管理局[27](NOAA)分别发布了监测海洋环境中微塑料的实验方法。然而,目前微塑料的提取和分离方法尚未标准化。
本文综述了已有研究报道的水样、土壤和沉积物、水生生物样品中微塑料的前处理方法,并针对现有方法的优缺点进行比较(表1),对进一步的研究方向进行了讨论。
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筛分过滤法利用孔径较小的不锈钢或铜制滤网、筛网来截留微塑料,是水样中分离微塑料最常用的方法,也可用于样品密度分离上清液中微塑料的提取 [4, 28-30] 。在土壤或沉积物的预处理中,可通过较大的筛网进行预处理,减少样品体积,再进行密度分离,并通过过滤器或筛网过滤上清液,从而分离微塑料。过滤和筛分法采用的筛网孔径大小决定了分离微塑料的尺寸,文献报道的孔径范围一般在0.038 mm到4.75 mm之间[4, 28-30]。对于孔径较小的滤膜容易堵塞,一般在负压条件下进行,再通过异丙醇溶液(50%,体积分数)将滤膜上的微塑料洗脱,从而提高微塑料的分离效率 [67]。目前并没有标准化的孔径尺寸,导致不同研究结果之间难以进行比较。
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密度分离法的原理是利用样品中微塑料与矿物质等杂质的密度差异来实现提取分离。微塑料的密度范围从0.80 g·cm−3(如硅胶)到1.60 g·cm−3(如PVC、PET)不等,而沉积物的密度通常为2.65 g·cm−3 [4]。首先向样品中加入高密度的饱和盐溶液,充分振荡、搅拌混合均匀,随后静置沉淀直至轻组分微塑料与重组分杂质分层,最后收集上层溶液中的微塑料。目前,密度分离法广泛应用于水样、土壤和沉积物中微塑料的提取。不同的盐溶液密度不同,导致提取效率各不相同。
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NaCl作为密度分离中使用最多的盐类之一,具有价廉易得、无害等优点 [31] ,是MSFD[26]和NOAA[27]推荐使用的前处理方法。PP(密度0.8 g·cm−3)、聚酰胺(PA,密度1.13 g·cm−3)等密度较低的聚合物可通过NaCl达到分离的效果 [32]。然而,由于NaCl溶液密度(1.2 g·cm−3)的局限性,导致高密度的微塑料包括PET(密度1.37—1.45 g·cm−3)和PVC(密度1.16—1.58 g·cm−3)的提取效率较低。而PET和PVC的产量几乎占世界塑料产量的17% [1],通过NaCl溶液进行样品前处理,可能会导致环境中PET和PVC等高密度微塑料的浓度被低估。
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碘化钠(NaI,密度1.8 g·cm−3)是一种用于分离微塑料的高密度溶液。NaI价格昂贵[33-34],研究人员通过减少样品量、回收NaI等方式来降低前处理的成本。Nuelle等[31]对样品通过NaCl分离结合空气溢流(AIO)进行预处理,使初始样品的质量降低80%,再用NaI进行密度分离。通过这两个步骤,既可以有效提取PVC、PET等高密度的微塑料,还能够减少NaI的使用量。Claessens等[34]将样品首先通过洗脱柱中向上的水流和曝气,从而减少样品量,再通过NaI进行密度分离,对PVC的提取效率大幅增加。为了研究NaI的可回收利用性,Kedzierski等[35]在10个循环使用过程后,测定了NaI的溶液密度和损失,发现NaI溶液的密度没有变化,损失为35.9%,证明通过回收NaI的方法可以大大降低前处理成本。Quinn等[36]对比了几种溶液(NaCl、NaBr、ZnBr2、NaI)对沉积物中微塑料的密度分离提取效率,发现NaCl和NaBr的回收率较低(<90%),而NaI和ZnBr2能够有效分离高密度的聚合物,可重现性高。此外,使用NaI和ZnBr2分离只需要对沉淀物进行一次洗涤,而NaCl需要3次洗涤 [36] 。
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氯化锌(ZnCl2,密度1.6—1.7 g·cm−3)也可用于微塑料的提取和分离,通常与密度分离装置相结合使用 [32, 37-39] ,微塑料的回收率很高,而且使用成本不高。Coppock等[40]比较了NaCl、NaI和ZnCl2溶液进行样品前处理的成本和提取效率,发现ZnCl2是最有效、最便宜的方法。但是,该物质具有很大的危害性和腐蚀性。因此,在使用ZnCl2进行样品前处理时,需谨慎处置并回收利用。
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饱和甲酸钾(K(HCOO))溶液的密度为1.6 g·cm−3,具有稳定性高、成本相对较低、粘度低、可通过过滤重复使用等特点,也被用于密度分离中[41-42]。二水钨酸钠(Na2WO4 ·2H2O)和聚钨酸钠(3 Na2WO4 ·9 WO3 2 H2O)在溶液中的密度都能达到1.4 g·cm−3,因此也可用于微塑料的密度分离[43-44]。但是,聚钨酸钠的价格相对昂贵,相比之下,一些研究者更推荐使用二水钨酸钠。
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Crichton等[45]利用微塑料的亲脂性,建立了一种简单的油提取方法,从固体样品中提取微塑料。干燥的样品与水和菜籽油充分混匀,静置至油、水、矿物质完全分离,微塑料与油结合进入油层,经过转移过滤后提取微塑料,再用乙醇去除表面油脂。在不同环境样品(沉积物和海水)中,使用该油提取微塑料(纤维和碎片)的回收率达到92%—97%。近期,Mani等[46]的研究测试了蓖麻油对4种复杂环境基质中微塑料的分离效率,包括河流和海洋悬浮表面固体、海洋沙滩沉积物和农业土壤。加标回收试验中,该方法对几种微塑料的平均回收率为99%。Karlsson等[47]在盐饱和溶液中加入一滴橄榄油,促进收集上清液中的塑料颗粒,回收率从64%提高到82%。目前关于油提取的研究较少,在微塑料分离后还需洗涤剂清洗,似乎具有一定的局限性,但可以通过油与饱和溶液相结合,来提高微塑料回收率。油提取方法简单、安全、廉价、耗时短,是一种很有前景的方法,亟需进一步验证和优化。
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基于密度分离的浮选装置通常与密度分离液(如ZnCl2)结合使用,主要是通过气体或液体作为流动相,产生上升流带动样品上浮,在上浮的过程中使微塑料从沉积物中分离出来。Imhof等[32]研发了塑料沉积物分离器(MPSS,图1),配有过滤器支架的可移动样品室可将微塑料颗粒直接转移到过滤器上,从而将样品与ZnCl2密度浮选液分离,提取沉积物中的微塑料。然而,Zobkov和Esiukova[48]对MPSS装置进行了评估,发现原始塑料的回收率与Imhof等报道相似,但老化塑料的回收率却低得多,仅为13%—39%。 此外,ZnCl2具有危险性和腐蚀性,pH值低,可能与沉积物中的成分(尤其是碳酸盐)反应,从而导致起泡,严重阻碍分离过程,该MPSS装置还需进一步的测试及优化。Coppock等[40]设计了便携式的沉积物中微塑料分离装置(图2),由PVC管、PVC球阀以及磁力搅拌棒组成,与MPSS原理相似,以ZnCl2作为密度浮选液在浮选过程中分离微塑料,回收率高达92%—98%。然而该方法中PVC管的磨损可能会污染样品,从而影响环境中PVC微塑料的测定。
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样品中的有机质可能会对微塑料的测定产生干扰,因此需要在前处理过程中尽可能去除有机质,同时不影响微塑料聚合物的结构及形貌 [65,68] 。目前的研究中通常采用酸消解、碱消解、氧化消解以及酶消解等方法对样品进行预处理。
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酸消解可以去除样品中的有机质,常用的酸包括 HCl[49]、HNO3 [49]、及混合酸[69]。文献报道HCl不能破坏所有的有机质,因此消解效率不高[50-52]。HNO3被广泛用于酸性消解。然而,HNO3可能会留下油性残留物或组织碎片,导致聚合物的损失或变色[34,53-54]。此外,一些聚合物(如尼龙、PET)容易在高温和高浓度下被酸腐蚀,因此需要选择合适的浓度和温度,从而在合理的反应时间内有效去除样品中的有机质。Naidoo等[55]研究发现HNO3(55%)加热至80 ℃可使鱼组织的消化速度提高26倍。然而,当消解液加热至60℃以上时,可能会造成微塑料的损失,需格外小心[56]。总的来说,酸消解法可能会破坏样品中的聚合物,导致环境样品中的微塑料含量被低估,因此需要首先优化实验中酸的浓度及温度,并谨慎使用。
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利用NaOH或KOH等进行碱消解,可以水解化学键,使蛋白质变性从而消解水生生物组织[57]。使用KOH或NaOH[52]在60 ℃过夜[51]或60 ℃消解24 h[54],是有效的消解处理方法之一。KOH对有机质的去除和塑料的回收具有良好的效果[53,56]。Foekema等[58]研究了KOH溶液对北海鱼样品的消解,发现在2—3周后,有机质完全被破坏。但也有一些研究表明,碱消解会破坏或使塑料变色[54,56,59,60],留下油性残留物和骨质碎片[51,54],或在塑料表面重新沉积残留物,对样品的光谱信号产生干扰 [61]。
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过氧化氢(H2O2,30%—35%)作为氧化剂,可有效消解有机质,并且对聚合物几乎没有降解作用[31,50,59]。消解温度是H2O2消解效率的关键因素。例如Cole等[52]报道,在室温下用H2O2 (35%)消解7 d,仅降解25%的有机质;而Avio等[62]报道用H2O2(15%)在50 ℃过夜,可有效去除有机质。除了通过H2O2进行氧化消解外,NOAA推荐采用H2O2(30%)与0.05 mol·L-1的硫酸亚铁溶液( Fenton试剂)在75 ℃下加热消解样品。Hurley等[63]研究了不同消解方法对富含有机质的污泥和土壤样品中8种常见微塑料的提取效率差异,包括H2O2、Fenton试剂氧化消解法,以及NaOH、KOH碱消解法。结果表明,H2O2(80.2%—108%)和Fenton试剂(86.9%—106%)对土壤及污泥中有机质的去除效率均优于NaOH(60.9%—68.6%)和KOH(34.5%—56.8%)。结合提取效率、对微塑料性质的影响以及对光谱信号的影响等多个因素的比较,最终发现Fenton试剂(40 ℃以下,pH值接近3)既能有效去除土壤和污泥中的有机组分,又不会破坏微塑料中的聚合物,具备高效、成本低以及消解快速等优点。
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酶消解法包括使用纤维素酶、脂肪酶、甲壳素酶和蛋白酶等去除有机质和减少部分生物组织[52,64]。与化学消解不同,酶消解的危害性较小,并且不易对微塑料造成损害[51]。对于0.2 g的少量样品,Cole等[52]应用蛋白酶进行酶消解,97%的有机质被降解。然而,这种酶的成本较高,更适用于少量样品的消解[63]。酶消解的另一个缺点是处理样品耗时长,并且每种酶都需要最佳温度和pH值[70]。此外,根据样品的基质不同,有些有机质不能完全消化,需要后续处理去除未消解的碎片。如Karlsson等[47]使用了酶消解法结合H2O2进行再处理,才能够有效破坏所有有机质。
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Felsing等[65]利用塑料颗粒的静电行为达到样品中微塑料提取分离的目的。将样品加入静电金属-塑料分离器,在去除99%的原始样品量的同时,对几种常见微塑料的回收率高达近100%。近期的研究报道了一种磁性提取方法,利用微塑料与Fe纳米颗粒疏水性结合,进而达到磁性提取的目的[66]。该方法对于海水、淡水和沉积物中几种常见微塑料的回收率为78%—93%,可用于密度分离或消解处理后样品中微塑料的进一步提取或饮用水等基质简单的样品前处理。然而对于土壤或沉积物中存在的亲脂性物质可能会导致非特异性结合,从而降低该方法的有效性。此外,Fe纳米颗粒可能会干扰微塑料的后续分析,尽管通过超声处理可以从微塑料表面去除Fe纳米颗粒,但可能会同时破坏微塑料,还需进一步深入研究。
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环境样品和水生生物样品中微塑料的提取和分离方法并不统一,如何能够在去除样品杂质、不破坏微塑料性质的同时,保证微塑料回收率,是前处理的关键。几种提取分析方法并非独立,实验中应根据不同基质的样品,来选取最佳的前处理方法。针对水样、土壤和沉积物等样品,可使用Fenton试剂消解结合密度分离法,来提取分离微塑料。而处理生物样品时,则可使用KOH进行消解并结合密度分离法,去除杂质。未来的研究应从以下几个方面着手:
(1)结合每种方法的优势,选择更适合的方法组合,来达到最佳的提取和分离效果。比如首先通过静电分离或密度浮选装置等来大大降低样品量,再使用碘化钠等价格昂贵的密度分离浮选液,对微塑料进行进一步的提取和分离。
(2)对于文献最新报道的如油提取、磁提取、密度浮选装置等分离方法进行进一步的验证及优化。
(3)对多种提取和分离方法的一致性和准确性进行比较研究。通过开展不同介质中前处理方法比较研究和效果评价,从而筛选出最佳的预处理方法。进而分别建立水样、土壤和沉积物、生物样品等不同介质中微塑料的预处理标准方法,为深入研究微塑料的环境行为及生态毒理学效应奠定基础。
微塑料的提取分离方法研究进展
Research progress on the extraction and separation methods of microplastics
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摘要: 微塑料作为海洋环境和陆生生态系统中的新型污染物,引起了广泛关注。然而目前微塑料的分析方法尚未标准化,不同研究结果间可比性较低。如何准确、高效地提取分离样品中的微塑料,是探究微塑料的环境行为及生态毒理学效应的关键前提。本文系统地综述了环境样品和水生生物样品中微塑料的前处理分析方法,包括筛分过滤法、密度分离法、消解法以及文献报道的其他方法,并对不同方法的优缺点及研究趋势进行了讨论和分析。结合不同前处理方法的优势,开展多种方法组合、比较等研究有利于微塑料分析方法的标准化。Abstract: Microplastic as an emerging pollutant in the marine environment and terrestrial ecosystems has attracted widespread attention. However, the analysis methods of microplastics have not been standardized at present, which hampered the comparability between different research results. How to accurately and efficiently extract microplastics in samples is a crucial prerequisite for exploring the environmental behavior and ecotoxicological effects of microplastics. The pretreatment analysis methods of microplastics in environmental samples and aquatic biological samples were systematically reviewed in this paper, including sieve filtration method, density separation method, digestion method and other methods reported in the literature. Besides, the advantages and disadvantages of different methods and research trends were discussed. Combining the advantages of different pretreatment methods, carrying out studies with various method combinations and comparisons is conducive to the standardization of microplastic analysis methods.
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Key words:
- microplastics /
- extraction and separation /
- flotation /
- density separation /
- digestion
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表 1 样品前处理方法汇总
Table 1. Summary of methods for sample pretreatment.
前处理方法
Pretreatment methods样品基质
Sample matrix优点
Advantages缺点
Disadvantage参考文献
Reference筛分过
滤法过滤筛分 水、固体样品浮选上清液 可快速分离;通过不同孔径滤网,
可对微塑料按照粒径分类没有标准化的孔径尺寸,
不同研究可比性低[4, 28-30] 密度分
离法NaCl 水、土、沉积物、
生物无毒、无害、成本低 对高密度微塑料提取效率低 [26, 27, 31- 32] NaI 水、土、沉积物、
生物密度高、安全、可重复使用、
提取效率高价格昂贵 [31, 33-36] ZnCl2 水、土、沉积物、
生物密度高、提取效率高、
成本低腐蚀性、危害性 [32, 37-40] 甲酸钾 水、土、沉积物、
生物稳定性好、成本低 目前应用研究较少 [41-42] 聚钨酸钠 水、土、沉积物、
生物密度高、成本较低 吸湿性强 [43-44] 油 土、沉积物 成本低、易操作 需要对微塑料进行进一步清洗;
目前应用研究较少[45-47] 密度分离
浮选装置土、沉积物 直接分离,能够有效
减小样品量需要与密度浮选液结合,
还需进一步验证及优化[32, 40, 48] 消解法 酸消解
(HCl)水、土、沉积物、
生物− 不能破坏所有有机质 [49-52] 酸消解
(HNO3)水、土、沉积物、
生物能够去除大部分有机质 可能会造成PET等聚合物溶解 [34, 49, 53-56] 碱消解
(NaOH/KOH)水、土、沉积物、
生物能去除大部分有机质;对大
部分聚合物没有破坏性可能使塑料变色;沉积残留物
对光谱信号产生干扰[51-54, 56-61] 氧化消解
(H2O2)水、土、沉积物、
生物能去除大部分有机质; 对部分聚合物有破坏性 [31, 50, 52, 59, 62] 氧化消解
(Fenton试剂)水、土、沉积物、
生物能去除有机质、提取效率高、
对光谱信号无影响− [27, 63] 酶消解 水、土、沉积物、
生物危害小、
不会对聚合物造成损害成本高、耗时长 [51-52, 63-64] 其他
方法静电分离装置 沉积物 能够将样品量减小99% 不适用于少量样品 [65] 磁提取法 水、沉积物 对大部分聚合物提取效率高 对于复杂样品需与其他方法结合,
更适用于饮用水等基质简单
的样品;需进一步优化[66] -
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