Now we can extract the critical raw material arsenic from toxic waste from groundwater purification

To invent and test their method, the researchers had to bring several kilograms of sludge back to the laboratory. The researchers received sludge from water treatment plants in Denmark, Belgium and the United States. The plants use a variety of treatment techniques, but they are all left with this sludge containing a certain amount of arsenic.

“The different levels of arsenic and the different treatment techniques meant that the sludge in our experiment was sufficiently different in structure and composition for us to test both whether our method works and whether it works better in some cases than others,” explains Case van Genuchten.

These are important insights if the method is to move out of the laboratory and into water treatment plants around the world. This makes it possible to make more realistic assessments of the types of sludge and water treatment techniques already in use, with which it is worthwhile to combine the method.

The method consists of an initial procedure and a multi-step chemical process.

First, the researchers dry the sludge at room temperature. They then sieve the sludge to remove larger pieces of material. The next step is to measure the composition of substances in the remaining dry sludge. And then the exciting part of the work can begin.

Kaifeng Wang and Case van Genuchten use a two-step chemical process: extraction and refining.

First, they remove arsenic from the surfaces of the particles in the sludge by adding a base, so that the arsenic appears in a dissolved form. This allows the sludge and arsenic to be separated by centrifugation and filtration.

They then refine the dissolved arsenic. They do this because they want to break the chemical bonds that arsenic is part of, so that they are left with arsenic in its basic form, where the atom has neither lost nor gained electrons: As(0), which is arsenic in oxidation state 0. The goal is to obtain pure arsenic that is not bound as an ion or in a compound with other substances.

To this end, the researchers have developed a process for forming As(0) in solid form from the aqueous As extracted from the sludge – chemically converting AsO₄^3- to As(0).

“We ‘make some agreements’ with AsO₄^3- about the electrons, and at the same time we have to ensure that we make As(0) stable so that it does not escape us in a new compound. You could say that we have invented a procedure that ensures that the chemical negotiation ends up with As(0) in the end,” says Case van Genuchten.

As(0) does not form compounds with other substances, but arsenic atoms can be bound to each other in a solid crystalline structure. You could say that neutral arsenic wants to play with something that looks like itself – preferably in an orderly fashion. And this is where it gets a little more exciting, because while the atoms in commercial arsenic, which is mostly used in semiconductors, are part of crystalline structures, the arsenic that Case van Genuchten and Kaifeng Wang have extracted from the sludge is amorphous, which is not quite the same thing.

To determine whether the recovered arsenic could be compared to commercial arsenic and thus have the potential to be used in the same way, the researchers had to send their extract on a trip.

The recovered arsenic and commercial arsenic were examined using the synchrotrons, MAX IV in Lund, Sweden, and DESY in Hamburg, Germany, respectively. A synchrotron is like a super microscope that allows you to see the structure of atoms. Using these advanced racetracks for electrons, the researchers were able to obtain snapshots of the structures in the outer shell of the atoms in the two types of arsenic and compare them.

“Our comparisons of commercial arsenic and recovered arsenic show that our recovered arsenic is in a solid state, but it is amorphous and not crystalline,” says Case van Genuchten.

‘Amorphous’ means that the substance is a slightly special type of solid state, where the atoms are arranged in an irregular order but still have a solid structure instead of the regular order seen in crystalline solids, such as commercial arsenic. Amorphous is therefore still in a state that we recognise as solid, but when you look very closely, you can see that it does not have the same long-range order as solids with a crystalline structure.

Amorphous is a solid state that we know from glass and bonbons. There can be little doubt that a bonbon is solid, but it still seems almost eager to melt, even under conditions that are not extreme – for instance in our mouths. And perhaps you have noticed that window glass in very old buildings can be thicker at the bottom than at the top. If you zoom in on the atoms in the glass, they are not arranged in a repeating lattice, and it is this irregularity that allows the window to change shape over time. You could say that it is a solid form that seems to be a little more adaptable.

There may be advantages to amorphous substances. For example, it is nice that bonbons become liquid at the otherwise low temperatures in our mouths.

Similarly, it may turn out that the amorphous arsenic that researchers have extracted from the sludge may have some advantages in technological applications because it may be easier to adapt. This is currently being investigated.

“Even if it turns out that amorphous arsenic is not better in, for example, semiconductors, it is not a problem to convert it into arsenic with a more crystalline structure, as we know from commercial arsenic,” says Case van Genuchten, who is therefore quite confident that arsenic, which is extracted using this method can still be used, even if the amorphous state does not turn out to have advantages that makes it a candidate to replace more traditional arsenic.

Amorphous materials are an active area of research where there is much uncharted territory.

To understand why van Genuchten’s research focuses on arsenic, we need to take a few steps back.

Arsenic is probably best known among the general population for being toxic. Many people may associate it with past and fictional murders of kings or other important figures. But arsenic does not discriminate between people, and according to the World Health Organisation (WHO), it is estimated that up to 200 million people are at risk of consuming drinking water with arsenic levels above the maximum contaminant level (MCL) of 10 micrograms per litre of water. This can lead to skin lesions, cancer and impaired cognitive development in foetuses and children, among other things. The problem is particularly acute in rural areas in Southeast Asian countries such as Bangladesh, Vietnam and India, where water purification is quite limited.

And it is the desire to do something to stop this mass poisoning that drives Case van Genuchten. Among other things, he has researched the development and testing of a water purification method in an area of India that has been plagued by the health consequences of high levels of arsenic in the water.

“It's terrible to see people with lesions on their hands and hear about people dying from this when you know it can be prevented if enough arsenic is removed from their drinking water,” says Case van Genuchten.

The fact that Case van Genuchten and his colleagues have now also managed to invent a method for extracting the dreaded but also coveted arsenic from the sludge may be an important step towards ensuring the spread of increased water purification.

“If you can turn this otherwise toxic sludge into something valuable that can be sold, it opens opportunities for funding in these areas. And funding can, amongst other things, help to ensure that water purification is carried out systematically on a larger scale,” says Case van Genuchten.