Scientists pilot new means of removing toxic metals from rivers
Toxic metals, including nickel, which enters rivers and oceans as run-off from mining projects and ends up in the human food chain, could be removed from water through the use of galvanised steel charged with an electric current. Exposure to nickel has been linked to type 2 diabetes and cancer. Lab tests by Australian and French scientists have now shown that it is possible to trap metal ions in calcified deposits that grow on electrodes. Pilot projects in New Caledonia are underway with results expected next spring.
There are few efficient, rapid and cheap methods to remove toxic metals from contaminated waters. Here we hypothesised that cathodic protection, an existing method used to control the corrosion of metallic structures, may trap toxic metals. Indeed, in seawater, the application of a cathodic current on a metallic structure induces the precipitation of limestone (CaCO 3 ) and magnesium dihydroxyde (Mg(OH) 2 ), thus forming a calcareous deposit on the metal surface. We therefore presumed that such calcareous deposit may trap metals dissolved in waters. Actually calcareous deposit formation has never been studied in the presence of dissolved metallic contaminants such as nickel. Here we tested ionic nickel (Ni2+) precipitation in calcareous deposit with a galvanized steel electrode by spiking artificial seawater with a NiCl 2 salt during 7 days of applied current. We analysed deposit surface and cross section by µ-Raman spectroscopy and scanning electron microscopy (SEM) with X-ray microanalysis. Ni concentration in the deposit was quantified by inductively coupled plasma analysis, after deposit dissolution in 60% HNO 3 . Results show that in 7 days up to 24% of nickel can be trapped in the calcareous deposit. Scanning electron microscopy reveals that Ni is trapped under a pure CaCO 3 layer of aragonite. Raman spectra show that nickel is incorporated as nickel dihydroxyde (Ni(OH) 2 ), as evidence by vibration bands at 446 and 510 cm−1. Overall our findings disclose a new and efficient method, calcareous electrochemical precipitation, which has potential applications to remove toxic metals from contaminated waters.