French researchers at the University of Strasbourg have developed a process capable of binding metal ions found in wastewater and then recovering them to transform them into metals.
Heavy metals (mercury, lead, cadmium, etc.) accumulate in the environment and are toxic to living organisms. In humans, they can cause serious health problems, even at very low concentrations. EPA regulations are very strict regarding their presence in wastewater and, in particular, in drinking water. At the same time, the supply of some of these metallic elements (palladium, platinum, ruthenium, etc.) has become a strategic issue for developed countries, particularly due to the growing demand associated with new technologies. Professors and researchers at the University of Strasbourg have developed a new process, called Metallocapt, which can fix metal ions and then recover them to transform them into metals.
Current processes are based on the use of chemicals combined with energy-intensive technologies, but they are not entirely effective because there are always traces of metal left in the treated water. These small quantities are dangerous because they can accumulate in the environment. They can also contain precious metals, and throwing them away is a waste of money.
Metallocapt is made from a polymer foam that is widely used to make seats, mattresses, car interiors, etc. It has the advantage of being cheap, robust, and flexible due to its viscoelastic properties. It is also chemically very stable in water and, thanks to its large pores, it is possible to pass a continuous flow through it without significant pressure drop. This is important because it is not necessary to exert a lot of pressure on the material to allow a liquid to pass through it.
Metallocapt is inspired by mussels
This polymer foam does not have metal filtering properties but serves as a support for a coating that can adsorb metal ions and is of biological origin. This coating was developed from research conducted by U.S. researchers in 2007. They studied the ability of mussels to adhere to almost any type of material (stones, boat hulls, wood, etc.) in an extremely robust manner. They then demonstrated that the chemical phenomenon at play was similar to that which occurs during the oxidation of polyphenols. The researchers then demonstrated that the dissolution of dopamine in an aqueous solution caused the formation of a black film of polyphenols on the surface of all materials immersed in the solution, and even on the container. The same phenomenon can be observed with tea, which leaves blackish traces on the cup.
“We tested dopamine with polymer foam and observed the formation of a black film on the surface and even inside,” explains Loïc Jierry, professor at the École Européenne de Chimie Polymères et Matériaux (ECPM) and the Institut Charles Sadron (ICS). “Our research has shown that it can adsorb certain metal ions very strongly. The compressible nature of the foam also makes it possible to increase the specific adsorption surface area for a given volume. The catechol groups involved are among the best biological ligands for binding metals.”
By immersing the foam coated with this film in a solution containing various heavy metals, the researchers have found that it is able to capture virtually all of the lead and copper present. The system is also reversible, which means that metal ions can be released and recovered. An acidic solution is used and the change in pH causes the ions to be released.
Proven selectivity for lead and copper
“Thanks to this polydopamine coating, we have been able to establish selectivity for lead and copper, continues Loïc Jierry. We are convinced that, by using other types of polyphenols, we will be able to capture other heavy metals. If we do not succeed, however, it will be possible to attach other compounds onto the polyphenols, which will make it possible to bind other types of ions such as cadmium and platinum. This however will increase the price of our process, whereas so far we have been working with foams and polyphenols that cost almost nothing.”
There are two types of companies that should be interested in this process. Those with potentially hazardous effluents and are obliged to respect maximum thresholds in their discharges. If these thresholds are exceeded, they have to pay penalties. Wastewater treatment plants should also be interested in this innovation. The Metallocapt process could help them meet the maximum thresholds for metallic elements in sewage sludge so that they can spread it on fields. When these thresholds are exceeded, the sludge is considered final waste, and spreading it is prohibited. It then has to be buried, which is an additional cost. Our process should also attract other types of companies, such as those looking for precious metals, such as platinum, which has become very expensive.
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