Team develops technique for removing metals from water

Engineers at Brown University claim they have developed a system that can remove heavy metals from water.

It is reported that the team successfully reduced cadmium, copper and nickel concentrations in contaminated water samples and returned them to standards deemed acceptable by the US government.

The method, called the cyclic electrowinning/precipitation (CEP) system, is reported to collate trace-heavy metals in water by increasing their concentration, so that a proven metal-removal technique can take over.

The CEP system involves two main units: one to concentrate the cations and another to turn them into stable, solid-state metals and remove them.

In the first stage, the metal-laden water is fed into a tank in which an acid (sulphuric acid) or base (sodium hydroxide) is added to change the water’s pH, effectively separating the water molecules from the metal precipitate, which settles at the bottom. The ‘clear’ water is syphoned off and more contaminated water is brought in.

The pH swing is applied again, first redissolving the precipitate and then reprecipitating all the metal, increasing the metal concentration each time.

This process is repeated until the concentration of the metal cations in the solution has reached a point at which electrowinning can be efficiently employed.

When that point is reached, the solution is sent to a second device, called a spouted particulate electrode (SPE). This is where the electrowinning takes place and the metal cations are chemically changed to stable metal solids, so they can be easily removed.

The cleaner water is returned to the precipitation tank, where metal ions can be precipitated once again. Further cleaned, the supernatant water is sent to another reservoir, where additional processes may be employed to further lower the metal-ion concentration levels. These processes can be repeated in an automated, cyclic fashion as many times as necessary to achieve the desired performance, such as drinking-water standards.

Joseph Calo, professor of engineering at Brown, said: ‘It’s like trying to put the genie back in the bottle.’

The team, which had its work published in the Chemical Engineering Journal, believes its technique is scalable and has viable commercial applications, especially in the environmental remediation and metal-recovery fields.