Arsenic is one of the most carcinogenic elements known and is toxic above 10 ppb (the World Health Organisation's standard). Drinking water contaminated with arsenic 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 now also believe 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. Indeed, this might explain the elevated levels of drinking-water arsenic in countries like Bangladesh, which has seen massive epidemics of arsenic poisoning in recent years.

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 oxidises 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 synthesised a new type of magnetite composite based on reduced graphene oxide (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 – as measured by inductively coupled plasma (ICP) techniques.

The magnetite-RGO composite can be dispersed in water. Once it has adsorbed arsenic, it can quickly be removed from a sample using a permanent hand-held magnet (with a strength of 20 mT) within a fraction of a minute.

The composite is ideal for removing arsenic (and perhaps other heavy metals) compared to bare magnetite because the presence of the graphene flakes among the magnetite particles increases the number of arsenic adsorption sites. And both As(III) and As(V) can be strongly adsorbed. "The reduced graphene oxide also increases the stability of magnetite so that it can be used in continuous-flow systems for longer periods," Kim told nanotechweb.org.

The researchers made their composite by first synthesising graphene oxide via Hummer's method (see figure). Next, they exfoliated the graphene oxide in water to produce a suspension of graphene oxide sheets. 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 synthesising 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 present results were reported in ACS Nano.