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塑料自出现以来极大地方便了人们的生活,其需求量和使用量不断增加,因此大量塑料垃圾进入并积累于环境中,在光照、热解、机械磨损以及生物作用下破碎形成粒径更小的塑料颗粒,其中直径小于5 mm的被定义为微塑料(microplastics,MPs)[1-2]. 微塑料作为一种新型污染物,广泛分布于水体、土壤、大气等环境介质中,在太平洋、大西洋、印度洋沿岸和深海地区,土壤及生物体内都能够检测到大量的微塑料[3-4],对生态环境安全及生物生存造成了严重的威胁[1]. 微塑料具有较大的比表面积,能够吸附大量的环境污染物,因此能够成为重金属、有机污染物等环境污染物质的运输载体,进一步增加其他环境污染物的风险[5]. 近期,重金属与微塑料之间的相互作用已成为环境领域关注的热点. 由于工业开采、金属冶炼、污泥回用等导致大量的重金属镉(Cd)进入土壤,造成严重的镉污染[6]. 据统计,我国Cd污染耕地土壤面积达2×105 km2,约占全国耕地总量的1/6,严重影响农业生产和粮食安全[7]. 重金属Cd具有高稳定性、不易降解、且易在生物体内累积的特点[8-9],严重威胁着人体健康和环境安全[10]. 同时,农业生产使用的大量农膜被遗留在环境中能够逐渐破碎形成微塑料,与重金属Cd形成复合污染[11]. 已有研究表明微塑料对水体环境中的重金属具有一定的富集作用[12],如海滩上微塑料表面镉、铬、铜、铁、锰、镍、铅、锌等重金属含量比周边海域中的重金属浓度高[13-14]. 然而,关于微塑料对Cd环境行为影响的研究仍存在不足.
微塑料吸附重金属还能影响其迁移特性与生物毒性,Boucher等[15]研究发现在潮汐带沉积物中,微塑料能够吸附重金属,而生物摄入微塑料后,重金属也随之进入食物链. 研究表明,微塑料吸附的重金属可能会在水体和肠道中被重新释放,从而使其生物毒性增加[16]. 镉与微塑料的复合污染对鲤鱼血浆中的酶活性、生化指标及免疫等具有明显的生物毒性效应,且二者表现出明显的协同作用[17]. 微塑料与重金属联合作用可导致海水青鳉肠道污染负荷增加,特定肠道微生物及肠道功能发生变化,免疫系统抵抗力受到明显抑制[18]. 目前,尚未有统一的方法对重金属与微塑料联合毒性进行测定. 研究发现,发光细菌毒性测定法具有高灵敏度、强相关性、快速、高自动化程度等特点,在重金属、有机物污染环境中的综合毒性分析中已得到广泛的应用[19]. 利用发光菌对取代苯酚和镉的混合物进行生物毒性测定,结果表明二者之间存在微弱的加成作用或近似加成作用的弱协同效应[19].
此外,环境中的微塑料在多种因素作用下逐渐发生老化,导致微塑料表面的氧化结合位点增多,对重金属及其他污染物的吸附能力增强[20],因此老化微塑料的吸附能力通常高于原始微塑料. 研究自然环境中老化微塑料对其他污染物环境行为的影响具有现实意义. 尽管已有国内外学者对实验室条件下微塑料与其他环境污染物的吸附进行了研究,但对于老化微塑料的研究仍不充分,且尚无一致定论.
微塑料高级氧化过程与自然老化过程中的氧化途径及老化产物具有极高的相似性[21],可克服自然环境中微塑料的老化速率极低的问题. 因此本研究采用热活化K2S2O8高级氧化法加速微塑料老化以获取不同老化程度的微塑料. 通过室内模拟实验,研究不同老化程度的微塑料对重金属的吸附动力学和吸附等温线的影响,以及在不同pH、微塑料浓度下对重金属的吸附,进一步探讨不同老化程度微塑料吸附行为的差异和作用机理. 采用发光菌毒性测试法对微塑料与重金属的复合污染的生物毒性进行测定,为复合污染的生物毒性评价提供科学依据.
老化微塑料吸附重金属镉及其毒性效应
Impact of ageing microplastics on heavy metal cadmium adsorption and its combined toxicity effect
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摘要: 微塑料广泛存在于各种环境介质中,且能够作为载体影响其他污染物环境行为. 本文主要探究了不同老化程度的微塑料PE和 PP对重金属镉(Cd)的吸附特性及生物毒性. 结果表明,微塑料颗粒随老化程度的增加其对Cd的吸附能力逐渐增强. 通过模型拟合发现,初始微塑料和老化微塑料对重金属Cd的吸附动力学符合准二级模型,热力学符合Freundlich吸附模型. 实验结果表明,微塑料吸附Cd的量与环境pH值及微塑料浓度密切相关. 随pH值升高,由于微塑料表面的去质子化作用Cd的吸附量逐渐增加. 然而,随着微塑料浓度升高,微塑料颗粒表面吸附位点的竞争作用则导致Cd吸附量的降低. 微塑料吸附Cd后其复合污染的生物毒性有所增加,二者的复合生物毒性与微塑料老化程度、Cd吸附量正相关,存在明显的协同效应. 本研究为充分认识微塑料及老化微塑料在环境中的行为作用提供理论依据,为准确评估微塑料与其他污染物复合污染的生态风险提供参考.Abstract: Microplastics widely exist in various environment, which can be served as a carrier to affect the environmental behavior of other pollutants. In the present study, the cadmium adsorption characteristics on different aged microplastics PE and PP were explored, and the combined biological toxicity were also investigated using Photobacterium phosphoreum T3. The adsorption capacity of Cd increased with the aging degree of microplastics increases. And the sorption kinetics was in accord with the pseudo second-order rate equation the thermodynamics was in accord with the Freundlich sorption model. In addition, the results showed that the adsorption of Cd by microplastics was strongly associated with the environmental pH and concentration of microplastics. The adsorption of Cd is positively associated with pH value, which should be attribute to the deprotonation on the surface of microplastics. On the contrary, the adsorption amount of Cd decreased with the concentration of microplastics increases, due to the competition of adsorption sites on the surface of microplastics. The toxicity of microplastics adsorbed by Cd was also determined using Photobacterium phosphoreum T3. The result showed that the biological toxicity of microplastics combined with Cd is positively associated with the ageing degree and the amount of Cd adsorbed, which indicated that there was a synergy between microplastics and cadmium. The present study provides a scientific reference for accurately assessing the ecological risk of microplastics combined with other pollutants.
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Key words:
- microplastics /
- cadmium /
- environmental behavior /
- biotoxicity.
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表 1 由准一级模型和准二级模型得到的吸附动力学参数
Table 1. Adsorption kinetic parameters obtained from the quasi-first-order model and the quasi-second-order model
微塑料
Microplastics准一级动力学方程
Quasi-first-order model准二级动力学方程
Quasi-second-order modelk1 qe/ (mg·g−1) R2 k2 qe/(mg·g−1) R2 PE0 14.4378 0.3496 0.9840 8.3893 0.3452 0.9999 PE5 6.5974 0.3652 0.9161 7.3099 0.3698 0.9999 PE10 6.8295 0.3547 0.3788 7.8927 0.3560 0.9996 PE20 6.1482 0.3813 0.8410 6.4735 0.3931 0.9998 PP0 8.5874 0.3207 0.6000 9.6525 0.3219 0.9994 PP5 10.1068 0.3250 0.7847 9.4340 0.3256 0.9997 PP10 7.6493 0.3337 0.9124 8.8810 0.3355 0.9996 PP20 5.9236 0.3504 0.9971 7.7520 0.3590 0.9998 表 2 由Freundlich模型和Langmuir模型得到的镉离子在微塑料上的吸附等温线参数
Table 2. Adsorption isotherm parameters of Cd on microplastics obtained by Freundlich model and Langmuir model
微塑料
Microplastics温度/℃
TemperatureFreundlich Langmuir KF n R2 KL Qmax R2 PE0 25 0.0301 0.6621 0.9783 0.1562 1.178 0.5861 PE20 25 0.0300 0.6958 0.9694 0.1626 1.5196 0.4516 PP0 25 0.0216 0.7354 0.9886 0.1402 1.0987 0.6591 PP20 25 0.0275 0.6887 0.9791 0.1600 1.0959 0.6592 -
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