Researchers have found yet another use for the wonder material graphene. A composite material made from reduced graphene oxide and magnetite could effectively remove arsenic from drinking water, according to new work done in South Korea.

Graphene is a sheet of carbon just one atom thick that also exists as an oxide. Reduced graphene oxide (RGO) is a chemical state of the material that has gained electrons. The purification process works by dispersing a magnetite-RGO composite in water, where it soaks up arsenic. The composite is then quickly and efficiently removed from the water using a permanent magnet.

Arsenic is one of the most carcinogenic elements known and is toxic above 10 ppb. Drinking water contaminated with the element is a dangerous everyday reality for many people across the world and it can lead to chronic illness and death. The arsenic mainly comes from naturally occurring arsenic-rich rocks through which the water has filtered but it may occur in areas where arsenic is mined as well. Scientists also suspect that changes in agricultural practices, such as using groundwater wells for irrigation rather than surface water sources like rivers and ponds, may also be to blame.

Arsenic can be removed from drinking water by using activated carbon or precipitating it out with iron minerals, such as iron oxides – for example, magnetite (Fe3O4) nanocrystals. However, such particles cannot be used in rivers, or other environments where water flows, because of their small size and the fact that magnetite rapidly oxidizes when exposed to the atmosphere. Researchers have recently overcome the latter problem by combining iron oxides with carbon and carbon nanotubes, and graphene-based materials such as graphene oxide.

Superparamagnetic hybrid

Building on this work, Kwang Kim, In-Cheol Hwang and colleagues at Pohang University of Science and Technology have created a new type of magnetite composite based on RGO. The hybrid material, which is superparamagnetic at room temperature, can remove over 99.9% of arsenic in a sample and reduce its concentration to below 1 ppb.

The composite is ideal for removing arsenic (and perhaps other heavy metals) compared with bare magnetite because the presence of the graphene flakes among the magnetite particles increases the number of arsenic adsorption sites. "The reduced graphene oxide also increases the stability of magnetite so that it can be used in continuous-flow systems for longer periods," says Kim.

The researchers made their composite by first creating graphene oxide via Hummer's method. Next, the graphene oxide sheets are added to water to produce a suspension. A mixed suspension of FeCl3 and FeCl2 was then added slowly to the graphene oxide solution, and ammonia quickly introduced to precipitate Fe2+ and Fe3+ ions for creating the magnetite nanoparticles. The graphene oxide was reduced using hydrazine hydrate and the dark black coloured solution filtered, washed with water/ethanol and dried in vacuum.

The team is now looking into other large-scale graphene synthesis methods as well as making graphene-based hybrid materials for various environmental and biological applications.

The work is described in ACS Nano.