Sulphuric acid is the world’s most used chemical. It 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 fertilisers. It is therefore a worldwide challenge that sulphuric acid often contains one of the most toxic substances – mercury. Researchers at Chalmers University of Technology, Sweden, have now developed a method that can reduce the levels of mercury in sulphuric acid by more than 90 per cent – even from low levels. 

“Until now, there has been no viable method for purifying finished sulphuric acid at all. With such a radical reduction in the mercury content, we come well below the current limit values. Such pure high quality sulphuric acid is in high demand in industrial applications and an important step in reducing environmental impact,” says research leader Björn Wickman, Associate Professor at the Department of Physics at Chalmers. 

Sulphuric acid is produced either from sulphur 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. Also recycled streams in the smelters can contain mercury. 

Toxic emissions that affect all life on Earth

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 per cent 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 per cent 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,” says Wickman.

Captures the metal using electrochemistry

Five years ago, his research team at Chalmers presented a pioneering method for removing mercury from water using electrochemical processes. The method is based on a metal electrode taking up the toxic metal and forming an alloy. The mercury can then be safely removed, and the electrode reused. Now the researchers have taken this technology one step further, and in a new study they have shown how mercury can be removed from concentrated sulphuric acid.

The experiments with sulphuric 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 sulphuric acid can flow and be purified at the same time.

Potential to reduce costs and environmental impact

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

“Purifying the sulphuric 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,” says Vera Roth, doctoral student at Chalmers and first author of the recently published article in the journal ACS ES&T Engineering.

Hoping for lower limit values

According to the Statista database, the worldwide market volume for sulphuric 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 sulphuric acid, the more valuable it is. Sulphuric 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 sulphuric 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 sulphuric acid in their pilot study. 

“The limit values for how much mercury sulphuric acid may contain are based on the technology available today. With the new method for purifying sulphuric 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,” says Wickman.

Caption: Sulphuric acid is an important industrial chemical, but it often contains one of the world’s most toxic substances: the volatile heavy metal mercury, which can pollute both the air and water. Researchers at Chalmers University of Technology have shown that it is possible to reduce the levels of mercury in sulphuric acid by more than 90 per cent – even from low levels. The study was conducted in collaboration with the spin-off company Atium and mining and metals refining company Boliden. 
Image: Boliden/Unsplash, Nazrin Babashova 

More information about the method and the study:

• The purification method removes mercury from the sulphuric 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. In an episode of the program UR Samtiden from the Swedish Educational Broadcasting Company (UR), Björn Wickman demonstrates how the method works for removing mercury from water. 
   
• The article Mercury Removal from Concentrated Sulfuric Acid by Electrochemical Alloy Formation on Platinum is published in the scientific journal ACS ES&T Engineering and was authored by Vera Roth, Julia Järlebark, Alexander Ahrnens, Jens Nyberg, Justin Salminen, Teodora Retegan Vollmer and Björn Wickman. The authors are active at the Department of Physics and the Department of Chemistry and Chemical Engineering at Chalmers University of Technology, at Atium, and at Boliden.
   
• Sulphuric acid is the world’s most used chemical. It is an important reagent used in many industries and it is used in the manufacture of everything from chemicals, paper, pharmaceuticals and cosmetics to batteries, detergents and fertilisers. It is an ingredient also for many recycling processes contributing into circular economy and extractive metallurgy.
   
• The experiments in the study were performed in a lab environment, in a 50 millilitre beaker and subsequently in a 20 litre 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. 
   
• The research was funded by Formas and by the strategic innovation programme Swedish Mining Innovation – a joint venture involving Vinnova, Formas and the Swedish Energy Agency.
   

More about heavy metals in the environment:

• Heavy metals in water and watercourses are an enormous environmental problem that affects the health of millions of people worldwide. Heavy metals are toxic to all living organisms and accumulate in the food chain. According to the World Health Organization, mercury is one of the most harmful substances to human health. Among other things, it affects our nervous system and the brain’s development. The substance is therefore especially dangerous for children and foetuses.
   
• Today, there are strict regulations governing the handling of toxic heavy metals to prevent their dispersal in the natural environment. However, there are plenty of places that have already been contaminated or are affected by the deposition of airborne mercury that may have come from other countries. As a result, there are areas in our natural environment where the amounts of heavy metals have reached toxic concentrations. For example, high levels of mercury in freshwater fish is a well-known environmental problem. Even in Sweden heavy metal pollution is a serious problem, and the fish in the majority of the lakes contain more mercury than the limit value. In industries where heavy metals are used, as well as in recycling, wastewater treatment and decontamination, there is a great need for new and better methods for removing toxic heavy metals from water.