 不久前，美国《环境科学与技术》（Environmental Science & Technology）发表了戴维·塞德拉克（David L. Sedlak）的一篇文章，这篇文章对目前全球超过10亿人口在使用的反渗透净水技术提出了一个颇为震撼的结论：反渗透技术革命的讽刺之处在于它制造了太过干净的饮用水，因为太过纯净，水中天然存在的很多有益矿物质都不存在了，因此导致人体存在着营养不良和增加患病风险。 戴维·塞德拉克是一个美国环境工程师与加州大学伯克利分校柏拉图·马洛扎莫夫讲座教授，美国化学会《环境科学与技术》与《环境科学与技术通讯》杂志主编，长期致力于化学污染物与水资源的研究，他于2016年被选为美国工程院院士。 下为厦门大学水科技与政策研究中心首席科学家蓝伟光的译文。 前言 问道者 半个多世纪以前，当Sidney Loeb和Srinivasa Sourirajan发明反渗透膜时，水革命的种子开始萌芽。 在过去的五十多年中，学术界和制造商降低了反渗透膜的生产成本与反渗透设备的能源消耗，反渗透净化水的过程变得更加简单与可靠。 如今，通过反渗透对海水和苦咸水进行脱盐淡化，世界上一些水资源紧张的城市多了一条水源供给的管道。随着加利福尼亚、德克萨斯州和新加坡的水再生利用设施投入使用，这项技术的影响已经扩展到脱盐以外的领域。 此外，由于公众对市政部门提供的自来水的质量心有疑虑，反渗透膜在家用净水装置和瓶装水的生产中也被广泛使用。 2018年，全世界约有1％的人口饮用海水淡化的水。虽然没有现成的精确估算，但相信会有千万上亿的人使用通过反渗透膜净化处理的废水，污染的河水和传统上认为不适合使用的水。 目前，通过反渗透膜净水的固定资产投资每年增长约15％，而且这种增长趋势并没有放缓的迹象。根据这些趋势，可以合理地假设，到二十一世纪中叶，超过十亿人口会使用反渗透膜净化的水。 反渗透革命为人类带来了福音，但与所有颠覆性的技术一样，它有可能产生意想不到的后果。根据反渗透膜净水的现状，世界饮用水供给的性质已经发生变化。因此，我们需要确认知识有什么差距、技术应如何改进与采取什么政策，以便保护为了保护公众健康和环境安全。 反渗透技术革命的讽刺之处在于它制造了太过干净的饮用水。人们早已认识到，长期饮用去离子水会导致营养缺乏。 由此，处理过的水通常在反渗透处理后必须再添加矿物质才适合饮用。考虑到反渗透净化的水质（呈酸性）会腐蚀供水管网，大型的反渗透净水厂都会使用便宜且容易获得的熟石灰再矿化水质（既把反渗透膜净化的水的pH值调回弱碱性，又增加了水中的钙—译者注）。 不幸的是，由于反渗透净化的水即使加熟石灰再矿化也几乎不含有镁，而人们饮用镁缺乏的水会增加患心脏疾病的风险。当这个问题第一次被发现时，以色列的供水商开始努力开发一种在再矿化过程中成本可控的添加镁的方法。 但是，在这些方法成为标准之前，水厂提供未和其他水源混合的反渗透纯净水的社区可能需要膳食补充剂。如今，已经有人把镁已被添加到反渗透产生的许多瓶装水中，也有人通过家用反渗透系统添加镁（问题是在家用反渗透纯水机中额外增加添加镁的装置非常复杂，经济上不可行。 且问，你家的反渗透净水机有调节pH、添加钙与镁的步骤吗？显然没有。所以，长期饮用反渗透膜过滤的纯净水必然会给您的家人带来健康问题—译者注）。 镁还不是反渗透膜净化的水中唯一缺少的重要营养离子。几十年来，在自然界中氟浓度水平较低的地方是否应向饮用水中添加氟化物的问题一直存在争议。 由于反渗透膜净化的纯净水会造成饮用水额外的氟缺乏，公共卫生专家将不得不更加关注增加饮食中氟化物来源的需求。这个问题在低收入的群体中尤其值得关注，因为他们很少使用含氟牙膏。 举例而言，尽管我们还无法确定反渗透膜把氟化物去除是导致中国儿童身高变矮和龋齿增加的原因，但那儿的许多中小学已经安装了反渗透膜净水机却是一个不争的事实。 考虑到饮用反渗透纯净水的人数众多，我们必须相当谨慎地思考这个问题，即人体所需要的其他微量元素也可能需要通过饮用水来摄取。 大约10年前，流行病学家报告说饮用水中锂浓度较低的群体自杀率较高。虽然并非所有后续研究都支持锂缺乏性假设，但海水淡化水的锂浓度几乎都处于检测限的低端，据报道饮用这类水的地方自杀率有所增加。 因此，可能的话，应该在反渗透纯净水中添加少量的锂或其他所需的微量元素，或在缺乏营养的人群中以其他方式补充微量元素，但至今没有进一步的研究来确定这些主意的有效性，可能没有人这样做。 几乎没有溶解性离子存在也意味着反渗透处理后的纯净水加强了矿物质溶解速率（此乃纯净水喝多了会导致人体内的矿物质被溶解流失的原因—译者注）。 因而在工程实践中，通常都要把反渗透纯净水进行再矿化以降低溶解那些覆盖在管道内壁的方解石和氧化铁层的趋势。在建设反渗透水处理厂的许多地方，此前富含矿物质离子的水已经在水管中流淌多年，一旦呈酸性的纯净水被引入，管壁中长年累月形成的固体层就会被溶解破坏，一些吸附其中的有害元素，如砷，铬和铅等，就可能释放溶解出来，引发饮水安全风险。 因此，反渗透纯净水在与老化管道接触之前必须添加石灰，以提高反渗透纯净水的pH值，从而最大限度地减少管壁中碳酸盐和氧化物的溶解。此外，反渗透处理过的水作为饮用水源注进地下加以贮藏时亦存在安全风险，因为它可能导致地下的蓄水层释放出地质砷。 此外，工程和自然系统中的微生物将受到水化学变化的影响，由此可能改变生物地球化学过程并影响水生病原体的命运。 从现在开始的一个世纪，历史学家往回看时将把反渗透的普及当作是饮用水供给发展中最重要的事件之一。科研群体面临的挑战是确保历史书中不会脚注说明反渗透革命的意外后果。 下为原文 Over half a century ago, the seeds of a water revolution were sewn when Sidney Loeb and Srinivasa Sourirajan invented the reverse osmosis membrane. Over the last five decades, academics and manufacturers have reduced the cost of producing membranes, improved their energy efficiency and made their operation simpler and more reliable. Today, desalination of seawater and brackish groundwater by reverse osmosis provides water to some of the world’s most water-stressed cities. The impact of the technology is now being extended beyond desalination as potable water reuse facilities are coming online in California, Texas, and Singapore. Reverse osmosis also has become popular in household-scale water treatment and in the production of bottled water consumed in places where the public believes that their tap water is unsafe. In 2018, about 1% of the world’s population drank desalinated seawater. Although precise estimates are not readily available, millions more used reverse osmosis to purify treated wastewater, polluted river water, and water that was deemed unsuitable for consumption. The growth in this practice shows no sign of slowing, with capital investments in reverse osmosis growing by approximately 15% per year. On the basis of these trends, it is reasonable to assume that over a billion people could be consuming reverse osmosis-treated water by the middle of the twenty-first century. The reverse osmosis revolution benefits humanity, but like all disruptive technologies, it has the potential to create unintended consequences. By considering current practices used for reverse osmosis treatment, we can identify the knowledge gaps, technology improvements, and policies needed to protect public health and the environment as the nature of the world’s drinking water supply changes. The great irony of the reverse osmosis revolution is that it has created drinking water that may be too clean. It has long been recognized that, over the long-term, consumption of ion-free water can lead to nutritional deficiencies. For this and other reasons, treated water is typically remineralized after reverse osmosis treatment. At full-scale water treatment plants, where corrosion of water distribution pipes is a major concern, lime (i.e., Ca(OH)2(s)) is used for remineralization because it is inexpensive and readily available. Unfortunately, the near absence of magnesium in water produced by this process has resulted in deficiencies in magnesium that increase the risks of heart disease. When this problem first came to light, water providers in Israel initiated an effort to develop cost-effective and reliable approaches for introducing magnesium during remineralization. But until such systems become the norm, dietary supplements may be needed in communities where treatment plants deliver reverse osmosis-treated water that has not blended with water from other sources. (Magnesium is already added to many bottled waters produced by reverse osmosis. It is also added by some household reverse osmosis systems.) Magnesium may not be the only nutritionally important ion missing from reverse-osmosis-treated water. The issue of whether or not to add fluoride to drinking water in places where the naturally occurring levels are low has been a matter of controversy for decades. As reverse osmosis creates additional fluoride-deficient drinking water supplies, public health experts will have to pay more attention to the need to augment dietary fluoride sources. This issue is particularly important in lower income communities, where fluoride-containing toothpaste is less common. For example, failure to appreciate the impact of reverse osmosis on fluoride led to decreases in height and increases in caries among children in communities in China where reverse osmosis systems had been installed at primary schools. Considering the number of people who rely upon reverse osmosis-treated water, it is prudent to look more carefully at the possibility that other trace elements are derived from drinking water. About 10 years ago, epidemiologists reported increased rates of suicide in communities where lithium concentrations in drinking water are low. Although not all of the subsequent studies supported the lithium deficiency hypothesis, the concentrations of lithium in desalinated seawater are at the low end of the range reported in places where increased suicide rates have been observed. It would be possible to add a small amount of lithium, or other needed trace elements, to reverse osmosis water or to supplement diets in other ways in deficient populations, but without additional research to establish the validity of these ideas, this is unlikely to happen. The near absence of dissolved ions also means that reverse osmosis-treated water enhances rates of mineral dissolution. The remineralization process, which decreases the tendency of reverse osmosis-treated water to dissolve the calcite and iron oxide layers that coat the inner walls of pipes, was adapted from engineering practices developed in places where the local water supply contained low concentrations of dissolved ions. In many of the locations where reverse osmosis treatment plants are being installed, water from ion-rich sources had been flowing through the pipes for decades prior to introduction of the desalinated water. Adding lime and raising the pH of reverse osmosis-treated water prior to its contact with the aged pipes may minimize dissolution of carbonates and oxides, but exposure to remineralized water could still release adsorbed trace elements, like arsenic, chromium, and lead. Reverse-osmosis-treated water could also pose risks during water storage as illustrated by the release of geogenic arsenic from an aquifer where remineralized water was used to recharge a drinking water aquifer. Furthermore, the microbes in engineered and natural systems will be affected by the change in water chemistry in a manner that could alter biogeochemical processes and affect the fate of waterborne pathogens. A century from now, historians will look back on the popularization of reverse osmosis as one of the most significant events in the development of drinking water supplies. The challenge for the research community is to make certain that the history books do not include a footnote about the unintended consequences of the reverse osmosis revolution.  |
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