Microplastics in water aren’t inert and make damaging chemicals on their own

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One reason plastics are used so often is that they’re chemically inert, meaning bags, food containers, and cups don’t react with what we put in them. Yet they can still break down into microplastics, which turn up in drinking water, human blood, arteries, and brains. The consequences of that are uncertain.

塑料之所以被广泛使用，原因之一是它们具有化学惰性，这意味着袋子、食品容器和杯子不会与盛放的物品发生反应。然而，它们仍会分解成微塑料，并出现在饮用水、人体血液、动脉和大脑中。其后果尚不确定。

“Whether microplastics can directly cause human disease is still being researched,” said Wei Min, a chemist at Columbia University. In a new Nature study, Min’s team took a step toward answering this question by demonstrating that microplastics are likely not as chemically inert as we might expect.

哥伦比亚大学化学家魏敏（Wei Min，音译）表示：“微塑料是否能直接导致人类疾病仍在研究中。”在《自然》杂志的一项新研究中，魏敏的团队通过证明微塑料可能并不像我们预期的那样具有化学惰性，向回答这一问题迈出了一步。

Min’s lab has spent years working on microdroplet chemistry, studying how water broken into tiny droplets behaves at its boundary with air. These droplets have been reported to generate reactive oxygen species, or ROS, a family of harmful, oxygen-containing molecules. “ROS will essentially bump into proteins, lipids, DNA, and cause damage,” Min said. “You could break a DNA strand into segments. You could mutate DNA; you could misfold proteins.”

魏敏的实验室多年来一直致力于微液滴化学研究，探讨水在分解成微小液滴后，其与空气边界处的表现。据报道，这些液滴会产生活性氧（ROS），这是一类有害的含氧分子。魏敏说：“活性氧本质上会撞击蛋白质、脂质和DNA，并造成损害。你可以将DNA链断裂成片段，可以使DNA突变，也可以使蛋白质错误折叠。”

Many animal studies have tied microplastic toxicity to oxidative stress, which is caused by ROS. So the team wondered if the boundary between water and plastic might do something similar. “We got inspired by that field,” Min said.

许多动物研究已将微塑料毒性与由活性氧引起的氧化应激联系起来。因此，研究小组想知道水与塑料之间的边界是否也会产生类似的作用。魏敏说：“我们受到了该领域的启发。”

In the first test, the team stirred polystyrene powder into water containing methyl orange, a yellow dye that ROS break apart. Over 10 days, the solution faded almost to colorless, losing nearly 90 percent of the dye. In ethanol or other solvents, the dye stayed intact. Min and his colleagues concluded that whatever chemistry polystyrene was undergoing needed water to happen. So they ran more tests.

在第一次测试中，研究小组将聚苯乙烯粉末搅拌进含有甲基橙的水中，甲基橙是一种会被活性氧分解的黄色染料。10天后，溶液几乎褪色至无色，损失了近90%的染料。而在乙醇或其他溶剂中，染料保持完好。魏敏及其同事得出结论：聚苯乙烯所经历的任何化学反应都需要水的参与。于是，他们进行了更多的测试。

First was electron spin resonance, a technique that directly detects radicals—atoms or molecules with an unpaired electron that makes them particularly reactive. The polystyrene powder and water mixture contained hydroxyl radicals, a highly reactive ROS. Next, the team used a fluorescent sensor that lights up only when it reacts with hydroxyl radicals. “You don’t see it, but once it reacts with local ROS, it turns into a bright color,” Min said.

首先是电子自旋共振技术，这是一种直接检测自由基（具有未成对电子、使其特别活泼的原子或分子）的技术。聚苯乙烯粉末和水的混合物中含有羟基自由基，这是一种高活性的活性氧。接下来，研究小组使用了一种只有在与羟基自由基反应时才会发光的荧光传感器。魏敏说：“你看不见它，但一旦它与局部的活性氧发生反应，它就会变成明亮的颜色。”

Under a microscope, the glow concentrated on the surfaces of the plastic particles and kept building over time. The effect was not unique to polystyrene either. Scientists tested six common plastics—polyethylene, polypropylene, polystyrene, PVC, PET, and acrylic—and all of them generated radicals. The most water-repellent plastics, polyethylene and polypropylene, were the most active, while PET and acrylic, which contain chemical groups that attract water, were the least.

在显微镜下，这种光芒集中在塑料颗粒的表面，并随着时间的推移不断增强。这种效应并非聚苯乙烯所独有。科学家测试了六种常见的塑料——聚乙烯、聚丙烯、聚苯乙烯、聚氯乙烯（PVC）、聚对苯二甲酸乙二醇酯（PET）和丙烯酸——它们全都产生了自由基。疏水性最强的塑料（聚乙烯和聚丙烯）活性最高，而含有亲水化学基团的PET和丙烯酸活性最低。

“The simplest answer is the most water-repellent ones change the water structure the most,” Min said. In ordinary, or bulk, water, each molecule is held in a network of hydrogen bonds with its neighbors. But a few layers of water molecules pressed against a water-repellent surface lose some of those partners. “Their structure is no longer the regular water structure. They are perturbed, and their energy is also significantly increased,” Min said.

魏敏说：“最简单的解释是，疏水性最强的塑料对水结构改变最大。”在普通或大块水中，每个分子都通过氢键网络与邻近分子结合。但紧贴疏水表面的几层水分子会失去部分伙伴。魏敏说：“它们的结构不再是规则的水结构。它们受到了扰动，能量也显著增加。”

Water always contains a small number of hydroxide ions, and turning one into a hydroxyl radical requires stripping away an electron. In bulk water, that costs a lot of energy. But simulations run by the group showed that, at a polyethylene surface, hydroxide ions short on hydrogen bonds needed far less energy to form radicals. On top of that, the plastic surfaces build up a negative electric charge in water, which further lowered the energy cost.

水中总是含有少量的氢氧根离子，将其转化为羟基自由基需要剥离一个电子。在大块水中，这需要消耗大量能量。但该小组进行的模拟显示，在聚乙烯表面，缺乏氢键的氢氧根离子形成自由基所需的能量要少得多。此外，塑料表面在水中会积累负电荷，这进一步降低了能量成本。

Min’s earlier work on microdroplet chemistry has drawn a lot of criticism, with some researchers arguing that the reported reactive chemicals come from contamination or measurement artifacts. To avoid similar objections, Min and his colleagues used several independent detection methods. They also added a detergent-like surfactant that coats the plastic and disrupts the water layer at its surface. They found that the more surfactant they added, the weaker the plastic’s electric charge became and the fewer radicals it produced.

魏敏早期关于微液滴化学的研究曾招致许多批评，一些研究人员认为所报道的活性化学物质来自污染或测量伪影。为了避免类似的反对意见，魏敏及其同事使用了几种独立的检测方法。他们还添加了一种类似洗涤剂的表面活性剂，用于覆盖塑料并破坏其表面的水层。他们发现，添加的表面活性剂越多，塑料的电荷就越弱，产生的自由基也就越少。

“We only change one parameter while keeping all the rest [the same],” Min said. “We’re using exactly the same plastics, the same water, the same everything.” Impurities, Min explained, are typically stable molecules with no reason to spontaneously generate radicals. He is careful not to overstate it, though. “To be absolutely clear, we cannot 100 percent rule out the involvement of impurities,” he said. “But our controls and our theoretical computation studies highly suggest that this is intrinsic to the microplastic-water interface.”

魏敏说：“我们只改变了一个参数，同时保持所有其他条件不变。我们使用的是完全相同的塑料、相同的水，一切都相同。”魏敏解释说，杂质通常是稳定的分子，没有理由自发产生自由基。不过，他小心翼翼地不夸大其词。他说：“必须明确的是，我们不能百分之百排除杂质的影响。但我们的对照实验和理论计算研究强烈表明，这是微塑料与水界面所固有的特性。”

Finally, the team tested if this interface chemistry matters for biological molecules. And it most likely does. Proteins and DNA can gather on plastic surfaces, and lysozyme, a common test protein, also began clumping into aggregates there. Adding caffeine, which mops up radicals, largely stopped the clumping, pointing to the radicals as the cause. Mouse red blood cells that touched polystyrene particles broke into fragments, and the damage from each type of plastic tracked how many radicals it produced.

最后，研究小组测试了这种界面化学对生物分子是否重要。答案很可能是肯定的。蛋白质和DNA可以在塑料表面聚集，而一种常见的测试蛋白质——溶菌酶，也开始在那里聚集成团。添加能清除自由基的咖啡因后，这种聚集现象基本停止，这表明自由基是导致聚集的原因。接触聚苯乙烯颗粒的小鼠红细胞破碎成碎片，而每种塑料造成的损伤程度与其产生的自由基数量相对应。

The effect also persisted in blood serum. Min stressed, though, that this indicates that microplastics make people sick by generating ROS. While animal studies often point to ROS as possible roots of oxidative stress and chronic inflammation, ROS can be produced in many ways, including through basic metabolic activity. Min admitted comparing the magnitude of this effect to the impact of ROS our own cells make is rather tricky.

这种效应在血清中依然存在。不过，魏敏强调，这表明微塑料通过产生活性氧使人生病。虽然动物研究经常指出活性氧可能是氧化应激和慢性炎症的根源，但活性氧可以通过多种方式产生，包括通过基本的代谢活动。魏敏承认，将这种效应的强度与我们自身细胞产生的活性氧的影响进行比较是相当棘手的。

“If you are really at the interface, you could be significantly affected by this effect, whereas if you are just a few microns away, you don’t”

“如果你真的处于界面处，你可能会受到这种效应的显著影响；而如果你距离界面仅几微米，你就不会（受到影响）。”