氧化石墨烯使海水淡化转化为饮用水

诸平

海水淡化即利用海水脱盐生产淡水。现在所用的海水淡化方法有海水冻结法、电渗析法、蒸馏法、反渗透法、以及碳酸铵离子交换法，目前应用反渗透膜法及蒸馏法是市场中的主流。世界上有十多个国家的100多个科研机构在进行着海水淡化的研究，有数百种不同结构和不同容量的海水淡化设施在工作。一座现代化的大型海水淡化厂，每天可以生产几千、几万甚至近百万吨淡水。水的成本在不断地降低，有些国家已经降低到和自来水的价格差不多。某些地区的淡化水量达到了国家和城市的供水规模。但是英国曼彻斯特大学（University of Manchester）的研究人员利用氧化石墨烯可以实现海水淡化，使其转化为饮用水。

石墨烯氧化物膜作为发展前景的过滤新技术的候选者，已经引起了世人备受关注。现在的更受欢迎的是开发出能够实现使常见的盐分离的石墨烯氧化膜。英国曼彻斯特大学的研究结果显示，利用氧化石墨烯膜可以有望解决目前全世界面临的饮用水源短缺问题，为那些生活在海边确没有或缺少淡水资源，难以得到清洁的饮用水资源的数百万人解决饮用水危机，提供足够的清洁水源。曼彻斯特大学科研人员的新发现，2017年4月3日已经在《自然纳米技术》（Nature Nanotechnology）杂志上发表——Jijo Abraham, Kalangi S. Vasu, Christopher D. Williams, Kalon Gopinadhan, Yang Su, Christie T. Cherian, James Dix, Eric Prestat, Sarah J. Haigh, Irina V. Grigorieva, Paola Carbone, Andre K. Geim, Rahul R. Nair. Tunable sieving of ions using graphene oxide membranes. Nature Nanotechnology, (2017) doi:10.1038/nnano.2017.21. Published online: 03 April 2017.

联合国预计,到2025年世界将会有14%的人口将面临缺水问题。而英国曼彻斯特大学的此项技术有可能在全世界范围内彻底改变海水淡化，是海水中的盐份过滤问题得到解决，这对于那些担不起大规模的海水淡化厂的国家来说特别重要。更多信息请注意浏览原文：

Graphene sieve turns seawater into drinking water

April 3, 2017

New research demonstrates the real-world potential of providing clean drinking water for millions of people who struggle to access adequate clean water sources.

The new findings from a group of scientists at The University of Manchester were published today in the journal Nature Nanotechnology. Previously graphene-oxide membranes have shown exciting potential for gas separation and water filtration.

Graphene-oxide membranes developed at the National Graphene Institute have already demonstrated the potential of filtering out small nanoparticles, organic molecules, and even large salts. Until now, however, they couldn't be used for sieving common salts used in desalination technologies, which require even smaller sieves.

Previous research at The University of Manchester found that if immersed in water, graphene-oxide membranes become slightly swollen and smaller salts flow through the membrane along with water, but larger ions or molecules are blocked.

The Manchester-based group have now further developed these graphene membranes and found a strategy to avoid the swelling of the membrane when exposed to water. The pore size in the membrane can be precisely controlled which can sieve common salts out of salty water and make it safe to drink.

As the effects of climate change continue to reduce modern city's water supplies, wealthy modern countries are also investing in desalination technologies. Following the severe floods in California major wealthy cities are also looking increasingly to alternative water solutions.

When the common salts are dissolved in water, they always form a 'shell' of water molecules around the salts molecules. This allows the tiny capillaries of the graphene-oxide membranes to block the salt from flowing along with the water. Water molecules are able to pass through the membrane barrier and flow anomalously fast which is ideal for application of these membranes for desalination.

Professor Rahul Nair, at The University of Manchester said: "Realisation of scalable membranes with uniform pore size down to atomic scale is a significant step forward and will open new possibilities for improving the efficiency of desalination technology.

"This is the first clear-cut experiment in this regime. We also demonstrate that there are realistic possibilities to scale up the described approach and mass produce graphene-based membranes with required sieve sizes."

Mr. Jijo Abraham and Dr. Vasu Siddeswara Kalangi were the joint-lead authors on the research paper: "The developed membranes are not only useful for desalination, but the atomic scale tunability of the pore size also opens new opportunity to fabricate membranes with on-demand filtration capable of filtering out ions according to their sizes." said Mr. Abraham.

By 2025 the UN expects that 14% of the world's population will encounter water scarcity. This technology has the potential to revolutionise water filtration across the world, in particular in countries which cannot afford large scale desalination plants.

It is hoped that graphene-oxide membrane systems can be built on smaller scales making this technology accessible to countries which do not have the financial infrastructure to fund large plants without compromising the yield of fresh water produced.

Explore further:Researchers develop hybrid nuclear desalination technique with improved efficiency

More information: Tunable sieving of ions using graphene oxide membranes, Nature Nanotechnology, nature.com/articles/doi:10.1038/nnano.2017.21

Abstract

Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications^{1, 2, 3, 4, 5}. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ～9 Å (ref. 4), which is larger than the diameters of hydrated ions of common salts^{4, 6}. The cutoff is determined by the interlayer spacing (d) of ～13.5 Å, typical for graphene oxide laminates that swell in water^{2, 4}. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from ～9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ～10–100 kJ mol^–1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.