李昆(1991—),男,博士,副教授,从事微粒型污染物的环境行为及生态风险相关研究。E-mail:
轮胎磨损颗粒(tire wear particles,TWPs)作为微塑料(microplastics,MPs)的重要种类之一,当下其生态风险已受到生态学家的高度重视。通常,颗粒型污染物的环境行为过程是其生态风险的重要影响因素。然而,TWPs在土壤等多孔介质中的迁移过程及影响机制至今尚未见报道。选择冷冻破碎制备的C-TWPs(冷冻破碎轮胎磨损颗粒)以及道路磨损产生的R-TWPs(滚动摩擦轮胎磨损颗粒)和S-TWPs(滑动摩擦轮胎磨损颗粒)为典型研究对象,以石英砂柱来模拟研究TWPs在土壤等环境多孔介质中的迁移行为,并探究天然有机物腐殖酸(HA)及不同pH(4、7和10)环境对以上三种类型TWPs迁移行为的影响。结果显示:HA(50 mg·L–1)能够显著增强三种类型TWPs的迁移性,并且在HA(50 mg·L–1)存在下,不同pH(4、7和10)对TWPs迁移行为影响不同,中碱性环境(pH=7/10)更有利于TWPs的迁移。主要原因在于,HA存在或(和)中碱性环境有利于(同时)增大TWPs和石英砂颗粒表面的Zeta电位值(绝对值),此时,一方面TWPs的分散性得到改善,有较小的粒径分布,另一方面增加了TWPs和石英砂颗粒间的静电排斥力,有助于TWPs的迁移。值得注意的是,HA存在和不同pH环境条件下,低温破碎制备的C-TWPs的迁移性较R-TWPs和S-TWPs强,主要由于C-TWPs制备时携带有较多的负电荷、较小的等电点和较强的疏水性,上述性质也可促使其吸附更多的HA,从而加强其电负性;而R-TWPs和S-TWPs由于粘附了道路矿物、金属盐或灰尘而减弱以上性质,表面具有较小的电负性。研究结果揭示了不同类型TWPs在自然界中地球化学迁移行为的差异性,并暗示了研究源头性质(排放方式)以确定同种材质微塑料环境行为及生态风险内在差异的必要性。
Tire wear particles (TWPs), as one of the important types of microplastics (MPs), have received a lot of attention from ecologists for their ecological risk in recent times. Usually, the environmental behavioral processes of particulate pollutants are important influencing factors of their ecological risk. However, the migration process and influencing mechanisms of TWPs in porous media such as soil have not been reported so far.
In this paper, C-TWPs prepared by freezing crushing and R-TWPs (rolling friction) and S-TWPs (sliding friction) produced by road wear were selected as typical research objects, and quartz sand columns were used to simulate and study the migration behavior of TWPs in environmental porous media such as soil, and to investigate the effects of natural organic matter humic acid (HA) and different pH (4, 7 and 10) environments on the migration behavior of the above three types of TWPs.
The results showed that HA (50 mg·L–1) significantly enhanced the mobility of the three types of TWPs, and the migration behavior of TWPs was differently affected by different pH (4, 7 and 10) environmental conditions in the presence of HA (50 mg·L–1), with the medium-alkaline environment(pH = 7/10)being more favorable for the migration of TWPs. This was mainly due to an increase in the negative zeta potentials of the surfaces of TWPs and quartz sand particles in the presence of HA and/or the medium alkaline environment (simultaneously). Also, the dispersion of TWPs was improved for smaller particle size distribution while the electrostatic repulsion between TWPs and quartz sand particles was increased, which contributed to the migration of TWPs. It is worth noting that the migration of C-TWPs prepared by low-temperature crushing was stronger than that of R-TWPs and S-TWPs in the presence of HA and under different environmental pH conditions. This was mainly attributed to the fact that C-TWPs carried a larger negative charge, smaller isoelectric point and stronger hydrophobicity, and these properties also contributed to the adsorption of more HA, thus enhancing their electronegativity. Nevertheless, R-TWPs and S-TWPs had less electronegativity on the surface due to the adhesion of road minerals, metal salts or dust that reduced the magnitude of the mentioned properties.
These results reveal the variability of the geochemical transport behavior of different types of TWPs in nature and suggest the necessity of studying the source properties (discharge mode) to determine the inherent differences in environmental behaviors and ecological risks of microplastics of the same material.
最近,环境介质中微塑料(microplastics,MPs)暴露的风险引起了人们的极大关注[
据报道,截至2019年,全球每年约有15亿条轮胎因磨损而报废,TWPs全球每年释放量高达600万t,而仅在欧洲和美国每年向环境中释放的TWPs量就分别高达130万和100万t[
以往研究表明,MPs等颗粒型污染物在多孔介质中的运动受诸多环境因素的影响,主要分为三类:颗粒的性质、多孔介质的性质和流体的性质[
因此,本文以城市沥青路面与胎面典型摩擦(滚动摩擦和滑动摩擦)和低温破碎条件下产生的TWPs为典型研究对象,研究三种类型TWPs在饱和石英砂柱中的运移规律,以模拟TWPs在土壤中的迁移行为,对比研究腐殖酸和不同pH(4、7和10)环境对三种类型TWPs迁移行为的影响,并探讨了相关作用机制。相关成果对深入理解轮胎不同运行工况条件下产生的TWPs的地球化学行为及其潜在迁移风险具有重要意义。
试验选取炭黑基轮胎(235/45 R17米其林PS3)为研究对象。本试验基于室内道路模拟测试设备(外转鼓路面可定制:轮胎高速匀速性测试设备——HSU500/
扫描电镜(scanning electron microscope,SEM,Hitachi,日本)配有能谱仪(energy dispersive spectroscopy,EDS)分别用于TWPs的表面形态和元素分布的检测。此外,X-射线衍射仪(XRD)(Model D8,Advanced X-ray diffractometer,德国)用以确定TWPs表面相变。利用动态光散射技术(Zetasizer Nano ZSP,马尔文仪器有限公司,英国)测定TWPs的Zeta电位(配置液为去离子水)。利用接触角分析仪(DSA 100,Kruss Ko.,德国)测量TWPs的接触角。并用全自动氮吸附比表面分析仪(ASAP 2020 Plus HD88,美国)测定三种类型TWPs的Brunauer–Emmett–Teller(BET)比表面积。最后,用橡胶密度计(ET-320,Etnaln仪特诺,北京)测定三种类型TWPs的密度值。
石英砂(SiO2纯度大于99. 9%,颗粒尺寸均匀,粒径约为500 μm,中国上海久升公司购得)作为柱填充介质。试验之前,需要对石英砂预处理去除其表面的金属氧化物和其他杂质。流程为:先经0.01 mol·L–1 NaOH超声30 min后用去离子水洗净,再经0. 01 mol·L–1 HCl超声30 min后用去离子水洗净,105℃下烘干后使用。然后用去离子水不断清洗,再烘干,通过反复清洗烘干直至石英砂pH达到7左右(6.96)。最后,室温(约25℃)条件下,进行其比表面积、表面电荷、等电点和接触角的测定。
首先,将TWPs用去离子水配置为200 mg·L–1的悬浊液,由于测定的三种类型TWPs的密度分别为1.09 g·cm–3(C-TWPs)、1.34 g·cm–3(R-TWPs)和1.75 g·cm–3(S-TWPs),均大于水的密度(1.00 g·cm–3),致使部分TWPs在去离子水中处于沉积状态(包括颗粒间吸附聚集后的沉降)。为了使颗粒达到分散状态,将悬浊液在200 W、25℃条件下超声30 min,然后静置于阴暗处24 h,以去除悬液中的TWPs较大团聚体。对悬浊液进行上述预处理是为了去除不稳定的TWPs,防止TWPs在进入石英砂之前发生聚集,影响颗粒在石英砂中的运动[
值得注意的是,本试验用去离子制备的TWPs悬浮液,在短期(数小时内)的迁移试验过程中并未检测到轮胎添加剂炭黑、金属及有机物等相关物质。所以在本研究中不考虑渗出物对于TWPs在石英砂中运动的影响。
HA是环境相关介质如水体和土壤中含量最多的天然有机物(NOM)之一[
试验装置如
试验装置的结构示意图
Schematic diagram of the construction of the experimental device
将制备好的三种类型TWPs分别与HA混合在50 mL离心管中(配置液为去离子水,并用0.1 mol·L–1 HCl和NaOH分别调节pH为4、7和10),混合液中TWPs的浓度为15 mg·L–1,HA的浓度为50 mg·L–1。室温条件下,将混合液在振荡器中以200 r·min–1条件下振荡24 h,然后在10 000
式中,
吸附试验结束后,利用Zetasizer Nano ZSP测定了TWPs的粒径分布状态,以探究HA对TWPs粒径分布状态的影响。
在研究HA对TWPs在石英砂柱中迁移的影响试验中,石英砂的颗粒大小约为325 μm,砂柱中的石英砂密度为1.43 g·cm–3,石英砂的孔隙率为0.41 mL。选择C-TWPs、R-TWPs和S-TWPs这三种典型轮胎颗粒研究NOM对TWPs在饱和石英砂中迁移的影响。C-TWPs、R-TWPs和S-TWPs悬浊液浓度均控制在15 mg·L–1,HA的浓度控制在0 mg·L–1和50 mg·L–1。具体迁移试验步骤为:室温条件下,通过蠕动泵(BT100-2J)分别注入3PV(空隙体积,PV=孔隙率×砂柱体积/1PV=44 mL)含有50 mg·L–1 HA的不同类型TWPs悬浊液(15 mg·L–1),以1.5 mL·min–1的流量导入砂柱中,研究NOM对TWPs在饱和石英砂中迁移的影响。上述试验环境中溶液pH可通过少量0.1 mol·L–1 HCl和NaOH进行调节,以研究pH为4、7和10时50 mg·L–1 HA对TWPs在饱和石英砂中迁移的影响。
TWPs悬浊液从砂柱的上方注入砂柱中,然后从下方收集滤出液,每隔0.5 PV收集砂柱流出液。在3PV的TWPs悬浊液注入之后,再向砂柱注入3PV不含有TWPs悬浊液的背景液,收集滤出液。TWPs的注入液和滤出液浓度分别记为
当柱迁移实验结束时(即砂柱滤出液收集完成后),随即将柱中石英砂按照其高度等分为10份(每份1cm高度),分别取出放入50 mL的离心管中,并分别加入20 mL去离子水振荡2 h,以释放滞留于石英砂颗粒上的TWPs,获取TWPs在砂柱上的垂向分布特征,用以绘制空间滞留曲线。
TWPs浓度的测定:以上TWPs滤液经过滤(0.22 μm)于坩埚中,试样经高温灰化后,用HF:HNO3(V:V= 2:8)溶解于消解管中,微波消解6 h,然后用蒸馏水冲洗消解管于烧杯中,经过滤(0.22 μm)转移至100 mL的容量瓶中,用水定容至刻度,摇匀,用电感耦合等离子体质谱分析仪(ICP-MS,PerkinElmer,ELAN DRC-e,美国)测定ZnO(Zn2+)浓度。如
轮胎磨损颗粒(TWPs)与其含有的ZnO分布曲线拟合图
Distribution curve fitting diagram of tire wear particles(TWPs)and ZnO contained in TWPs
所有检验均重复进行(
利用SEM和EDS对三种不同类型TWPs的表面结构及表面元素分布进行了表征,结果(
三种类型TWPs的表面微结构及表面元素和物质分布特征的表征
Characterization of surface structure and element and material distribution of three types of TWPs
此外,如
去离子水中(pH=7)条件下三种类型TWPs及石英砂的理化特性(
Physical and chemical properties of three types of TWPs and quartz sand in deionized water(pH=7)(
样品 |
比表面积(SBET) |
表面电位 |
零点电荷 |
接触角 |
注:同列不同字母表示样品间物化特征量度差异显著( |
||||
C-TWPs | 19.5±0.6b | –10.4±0.6c | 3.2±0.2a | 129.8±5.4d |
R-TWPs | 49.5±2.5d | –3.54±0.4b | 4.3±0.4b | 105.4±6.5c |
S-TWPs | 32.2±3.7c | –1.75±0.1a | 4.7±0.7b | 95.2±4.8b |
石英砂Quartz sand | 8.5±0.8a | –19.2±0.8d | 2.8±0.5a | 23.6±3.7a |
据报道[
三种类型TWPs对HA的吸附及其表面Zeta电位和粒径分布变化(
Adsorption of HA by three types of TWPs and their changes in surface zeta potential and particle size distribution(
pH | 样品 |
对HA的吸附量 |
吸附HA后TWPs的表面电位 |
平均粒径 |
4 | C-TWPs | 0.298±0.05c | –15.2±0.3b | 310±9g |
R-TWPs | 0.254±0.06b | –9.5±0.4a | 358±8h | |
S-TWPs | 0.205±0.12a | –9.2±0.6a | 402±6i | |
7 | C-TWPs | 0.524±0.09h | –25.2±1.2d | 259±1d |
R-TWPs | 0.485±0.10f | –18.5±0.6c | 278±3e | |
S-TWPs | 0.415±0.14d | –15.6±0.8b | 294±5f | |
10 | C-TWPs | 0.546±0.16i | –32.4±1.5f | 162±3a |
R-TWPs | 0.510±0.06g | –29.4±1.7e | 198±4b | |
S-TWPs | 0.457±0.12e | –27.8±1.5e | 218±5c |
值得注意的是,不论pH如何,相对于低温破碎制备的C-TWPs,道路磨损产生的TWPs(包括R-TWPs和S-TWPs)虽然有较大的SBET(
此外,本研究表明,随着pH的增加,TWPs对HA吸附量也随之增加,HA的表面吸附量增加可增大TWPs的表面携带电荷,也降低TWPs的粒径分布范围。上述结果证实了HA的存在使TWPs间静电排斥力增加,团聚程度降低,从而增强了TWPs的分散稳定性,之前也有类似的报道[
在中性(pH=7)环境条件下,研究了HA对不同类型TWPs的迁移行为的影响(
腐殖酸(HA)对不同类型TWPs迁移性质的影响(a穿透曲线;b空间滞留曲线)
Effect of humic acid(HA)on the migration properties of different types of TWPs(a Penetration curve; b Spatial retention curve)
就不同类型TWPs而言,相对于磨损产生的R-TWPs和S-TWPs,C-TWPs在饱和石英砂中有较强的流出率,这可能是由于C-TWPs具有较小的颗粒粒径和较强的疏水性(
空间滞留曲线(
HA(50 mg·L–1)存在时,不同pH(pH = 4、7、10)条件下,TWPs在饱和石英砂柱的迁移实验中,穿透曲线和空间滞留曲线如
不同pH环境对TWPs迁移行为的影响(a~c为穿透曲线;d~f为空间滞留曲线)
Effect of different pH environments on the migration behavior of TWPs(a-c. Penetration curve; d-f. Spatial retention curve)
就TWPs的类型而言,相比于中性环境,碱性条件对道路磨损产生的TWPs(R-TWPs和S-TWPs)迁移能力的提升较强,砂柱中(0~12 cm处)滞留的TWPs有所减少;而酸性环境对低温破碎制备的C-TWPs迁移能力的抑制作用更加明显,石英砂柱中(0~16 cm处)滞留了较多的C-TWPs。
为了阐释HA和pH对三种类型TWPs迁移行为的影响机制,本文也对砂柱填充物石英砂颗粒在有无HA(50 mg·L–1)及不同pH条件下进行了Zeta电位性质的表征,测试结果如
HA和不同pH环境对石英砂Zeta电位的影响
Effect of HA and different pH environments on the Zeta potential of quartz sand
首先,
其次,在HA(50 mg·L–1)存在下,当砂柱中的中性溶液环境(pH=7)变为碱性环境时(pH=10),TWPs可吸附更多的HA,使TWPs表面的Zeta电位绝对值变得更大(负值),而且颗粒的粒径分散得较好,粒径相对较小(
就不同类型TWPs迁移行为的差异性而言,如
HA和pH环境影响不同类型TWPs迁移行为的作用机制
Schematic diagram of the mechanism of action of HA and pH environment affecting the migration behavior of different types of TWPs
本文证实冷冻破碎制备的轮胎颗粒与真实道路磨损产生的轮胎颗粒之间的初始性质差异显著(在表面结构和携带物质(
此外,现阶段众多学者在探究污染物在土壤和地下水系统中的迁移过程,通常是通过研究污染物在多孔介质(常常是石英砂)中的运移情况来模拟实现[
TWPs作为MPs环境分布的重要组分之一,其在饱和石英砂柱中的迁移行为受到天然有机物和pH的显著影响。表现为:天然有机质(HA)能显著增强TWPs的迁移性,中碱性pH环境也有助于其迁移。主要机制是HA和pH能够影响TWPs的分散性质,同时可影响环境介质石英砂的表面电荷,进而可影响TWPs砂柱中的迁移性。值得注意的是,无论在相同或不同的环境因素下,不同模式制备的不同类型TWPs表面性质不同,这就注定了不同类型TWPs在特定环境介质中迁移行为的差异性。综上,研究结果揭示弹性橡胶类MPs源头性质(排放方式)对其在自然界中地球化学迁移行为具有显著影响,并且迁移过程受土壤有机物及酸碱条件的控制。
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