Researchers at Chalmers University of Technology, Sweden, have developed a method that can reduce the levels of mercury in sulfuric acid by more than 90%, even from low levels. An open-access paper on the method is published in ACS ES&T Engineering.

Sulfuric acid is an important reagent used in many industries and it is used in the manufacture of everything from paper, pharmaceuticals and cosmetics to batteries, detergents and fertilizers. It is therefore a worldwide challenge that sulfuric acid often contains one of the most toxic substances: mercury.

Until now, there has been no viable method for purifying finished sulfuric acid at all. With such a radical reduction in the mercury content, we come well below the current limit values. Such pure high quality sulfuric acid is in high demand in industrial applications and an important step in reducing environmental impact.

—Björn Wickman, Associate Professor at the Department of Physics at Chalmers, corresponding author

Sulfuric acid is produced either from sulfur from the petroleum industry or as a by-product in the mining industry’s smelters. In the latter case, mercury, which is naturally present in the ore, can end up in the finished products. Recycled streams in the smelters can also contain mercury.

Mercury dispersal is a worldwide problem, as the substance is volatile and can be dispersed by air over large areas. This toxic heavy metal is then washed into streams and lakes when it rains. It is stored in the soil, water and living organisms, impacting the entire food chain. It can damage the brains and central nervous systems of humans and animals.

According to a report from the United Nations Environment Programme (UNEP), emissions of mercury to the atmosphere increased by an estimated 20% from 2010 to 2015. In 2015, about 2,200 tonnes of mercury were emitted into the air as a result of human activities such as cement manufacture, small-scale gold mining, coal burning, metal production and other manufacturing industries.

In addition, an estimated 1,800 tonnes of mercury ended up in the soil and water in that same year. According to the report, mercury concentrations in the atmosphere may have increased by 450% in the last century.

Any and all ways we can reduce mercury emissions are good, because any mercury that is emitted accumulates in the environment and continues to pose a health threat for thousands of years.

—Björn Wickman

The purification method removes mercury from the sulfuric acid by first reducing the mercury ions to metallic form and then binding the metallic mercury into an alloy with platinum, which coated onto the electrode surface. It is then possible to remove the mercury and regenerate the electrode in a controlled way. This means that the electrode can be reused, and the toxic substance can be safely disposed of. The process is also highly energy-efficient.

The experiments in the study were performed in a lab environment, in a 50-milliliter beaker and subsequently in a 20-liter reactor. The next step will be to scale up the method into a pilot project that is closer to the actual volumes that are usual in industrial applications. Roth et al.

The experiments with sulfuric acid were done in collaboration with mining and metals refining company Boliden and the company Atium, a spin-off from the Chalmers School of Entrepreneurship with the aim of bringing the removal of mercury from water and chemicals to market. The researchers now hope to be able to move forward with their partners and develop a type of reactor through which sulfuric acid can flow and be purified at the same time.

Today, mercury is mostly removed at an earlier stage—from the concentrates and recycled streams at the smelter before sulfuric acid is produced. This is an established process, but leaves trace amounts of mercury into final products.

Purifying the sulfuric acid as well prevents additional mercury emissions, while allowing industry to operate more cost-effectively and produce a high-purity, non-toxic product. The next step will be to scale up the method into a pilot process that is closer to real-world volumes of thousands of tonnes.

—Vera Roth, first author

According to the Statista database, the worldwide market volume for sulfuric acid amounts to around 260 million tonnes per year. By 2029, this figure is expected to rise to 314 million tonnes. The lower the mercury content of the sulfuric acid, the more valuable it is. Sulfuric acid for commercial purposes is considered to be of acceptable quality when its mercury content is below 0.30 milligrams per kilogram. If the content is below 0.08 milligrams per kilogram, the sulfuric acid is considered to have a high purity. With the new method, the researchers have reduced the level of mercury to 0.02 milligrams per kilogram of sulfuric acid in their pilot study.

The limit values for how much mercury sulfuric acid may contain are based on the technology available today. With the new method for purifying sulfuric acid, our hope is that the legislation around the limit values will be tightened in a global perspective where mercury levels are generally much higher.

—Björn Wickman

The research was funded by Formas and by the strategic innovation program Swedish Mining Innovation—a joint venture involving Vinnova, Formas and the Swedish Energy Agency.

Resources

• Vera Roth, Julia Järlebark, Alexander Ahrnens, Jens Nyberg, Justin Salminen, Teodora Retegan Vollmer, and Björn Wickman (2023) “Mercury Removal from Concentrated Sulfuric Acid by Electrochemical Alloy Formation on Platinum” ACS ES&T Engineering 3 (6), 823-830 doi: 10.1021/acsestengg.2c00417