Modifying nanocarbons to purify water

Heavy metals in drinking water represent a serious threat to human health in many parts of the world — particularly developing countries — with the possibility of some heavy metals accumulating in the body over time and leading to cancers and organ damage. In addition to the health threats of contaminated water linked to poor sanitation, metals such as mercury, lead, cadmium and arsenic can be present in untreated and unprotected sources of water.

Nanocarbons are currently under investigation for their ability to purify water and wastewater by adsorbing dyes, gases, organic compounds and toxic metal ions. Nanocarbons can adsorb heavy metal ions such as lead and mercury onto their surfaces via molecular attraction forces — but this attraction is weak, meaning that they aren’t efficient adsorbents on their own.

To improve adsorption, researchers are adding molecules such as amino groups to nanocarbons, which form stronger chemical bonds with heavy metals. They are also investigating ways to use all available surfaces on nanocarbons for metal ion adsorption, including the surfaces of their inner pores. This would enhance their capacity to adsorb more metal ions at a time.

Research from a team of scientists at Nagoya University, Japan may help in the effort to improve universal access to clean water via a one-step process that improves the ability of nanocarbons to remove toxic heavy metal ions from water.

Materials scientist Nagahiro Saito from Nagoya University’s Institute of Innovation for Future Society, together with colleagues, has developed a new method for synthesising an ‘amino-modified nanocarbon’ that more efficiently adsorbs several heavy metal ions compared with conventional methods. The findings are published in ACS Applied Nano Materials.

The team mixed phenol as a source of carbon, with a compound called APTES as a source of amino groups. This mixture was placed in a glass chamber and exposed to a high voltage, creating a plasma in liquid. This ‘solution plasma process’ was maintained for 20 minutes, following which black precipitates of amino-modified carbons formed and were collected, washed and dried.

A series of tests showed that the amino groups had evenly distributed over the nanocarbon surface, including into its slit-like pores.

“Our single-step process facilitates the bonding of amino groups on both outer and inner surfaces of the porous nanocarbon,” Saito said.

“This drastically increased their adsorption capacity compared to a nanocarbon on its own.”

The researchers put the amino-modified nanocarbons through 10 cycles of adsorbing copper, zinc and cadmium metal ions, washing them between each cycle. Although the capacity to adsorb metal ions decreased with repetitive cycles, the reduction was small, making them relatively stable for repetitive use.

Finally, the team compared their amino-modified nanocarbons with five others synthesised by conventional methods. Their nanocarbon had the highest adsorption capacity for the metal ions tested, indicating there are more amino groups on their nanocarbon than the others.

“Our process could help reduce the costs of water purification and bring us closer to achieving universal and equitable access to safe and affordable drinking water for all by 2030,” Saito said.

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