Danish whizz-kids produce high-grade arsenic from contaminated groundwater

Groundwater contaminated with naturally occurring but highly toxic arsenic (As) is a health problem affecting some 140 million people. Removing the toxic element from groundwater is possible, but the process required is expensive and produces large quantities of arsenic-laden sludge as a by-product. In Science Advances, chemists Kaifang Weng and Case van Genuchten of the Geological Survey of Denmark and Greenland describe a method to extract arsenic from the sludge and convert it into high-purity metallic arsenic that may be suitable for industrial applications.

Semiconductors

Pure arsenic is used in glass production, in the purification of zinc, and in high-end technological applications including gallium arsenide (GaAs) semiconductors. Solar panels on satellites and spacecraft are made from GaAs because they are highly efficient and highly resistant to radiation. Gallium arsenide semiconductor circuits can handle high frequencies and exhibit low noise.

This is one of the reasons arsenic has been on the EU’s list of Critical Raw Materials (CRM) since 2023 — materials that are important to European industry and therefore warrant European policy to secure their supply. This list includes helium, titanium, lithium, and rare earth metals, amongst others. By treating arsenic as a usable and sellable raw material, the researchers argue, countries can not only address their arsenic problem, they can even profit from it, thereby funding water purification.

Groundwater

Arsenic has been a favourite of poisoners since antiquity. In doses of 70 to 200 milligrams it causes abdominal cramps, diarrhoea, and vomiting, often with fatal consequences. At lower doses, arsenic poisoning leads to skin disorders, lung and heart problems, diabetes, and cancer. When ingested by pregnant women, the poison has negative effects on the cognitive abilities of the child.

Arsenic occurs naturally in soil and groundwater, making it a problem in India, China, Argentina, Mexico, and the United States, among other countries. Not only is pumped water unsuitable as drinking water, but crops irrigated with it and subsequently consumed also contain the poison. According to the World Health Organisation, approximately 140 million people consume water containing more than 10 micrograms of arsenic per litre, the maximum value under WHO guidelines.

Extraction

A standard process for removing arsenic from groundwater is adsorption onto iron(III) oxide particles in a fixed-bed absorption system using sand, for example. This captures arsenite (various As(III) anions with the formula As_xO_y) and arsenate (As(V) in the form of AsO₄^3-). At the same time, the process produces enormous quantities of toxic sludge — sometimes hundreds of tonnes per year per treatment plant. That sludge must then be stored, incurring costs and environmental risks. The Danish researchers now present a second process step in which the arsenite and arsenate are extracted from the sludge using 1 molar sodium hydroxide solution. The resulting arsenic salts are then reduced to metallic arsenic using the inexpensive bleaching agent thiourea dioxide (TDO, formula (NH)(NH₂)CSO₂H).

Between 98 and 99 per cent of the arsenic is captured in this way, the chemists report. The reaction yields more than 99% pure metallic As(0) nanoparticles of around 20 to 30 nanometres in diameter. Analysis using the X-ray techniques XANES and EXAFS revealed that the arsenic is amorphous, as opposed to the crystalline arsenic sold industrially. For many metals, the amorphous form has interesting properties that the crystalline form does not, the authors note. However, optimisation, scale-up, and a final reaction step to produce industrially marketable arsenic still require further development.

Copper Production

Benjamin Sprecher, a researcher at Delft University of Technology specialising in sustainable design and industrial environmental impacts, is sceptical about the usefulness of large quantities of arsenic, however pure. ‘It is of course always useful to extract arsenic from sludge because it is toxic. But they are really jumping on the CRM hype with that framing in the article,’ he says. ‘Arsenic is on the CRM list because production is highly concentrated in a few countries. It is a by-product of copper production, for example. So if anything, there is too much arsenic. If you look at it from the perspective of the societal problems caused by CRMs, I’m not sure whether recovering arsenic from sludge would make any difference there.’

K. Wang, C.M. van Genuchten, Commodifying a carcinogen: critical raw materials from arsenic-laden groundwater, Science Advances (2025), doi:10.1126/sciadv.adz5816