A way has been found to purify water of heavy metals using used batteries.

Research led by Xu Zhenmin proposed using residual battery charging to remove copper, cadmium, zinc and chromium ions from aqueous solutions. Scientists have discovered that up to 90 percent of the energy from used batteries can be used to remove heavy metals from water.

Experiment

When researchers began to experiment with the recovery of aqueous solutions of copper, cadmium, and iron salts, they discovered that the recovery potential of discharged lithium-ion batteries was strongly and unevenly dependent on the acidity of the environment. The potential must be constant for a successful electrochemical process, so the scientists decided to find out why the potential of the cathode behaves strangely in a solution of heavy metal salts.

To do this, chemists prepared solutions of sodium hydroxide, sodium chloride, and sodium sulfate and immersed batteries in these solutions to do a voltammetric study. They found that the behavior of the battery in alkaline solution is due to the passivation of the iron coating of the battery. It was covered with a thin layer of oxide, which prevents further oxidation.

Later, chemists prepared solutions from copper and cadmium salts and tried to separate the metals separately. To do this, they added acid to a solution with a pH of two and then immersed a used battery in it. The iron surface began to dissolve and copper accumulated on it, and 85 percent of the battery’s remaining energy was used to release the copper. With cadmium regenerated at pH=4, the situation was worse: only 13 percent of the battery’s remaining energy was spent on this process.

Diagram of the dependence of the reaction parameters on acidity and the degree of passivation of the battery coating / Photo PNAS

The scientists were also able to recover the hexavalent chromium ions using 88% of the battery’s remaining energy.

The results show that used lithium-ion batteries can be an environmentally friendly and inexpensive solution for the treatment of water contaminated with heavy metals. The success of the experiment and the research results can be applied in practice for the treatment of wastewater and the environment in general.