Alkaline steel and cement wastewater could capture 30 million tons of CO₂ annually

Abstract. Credit: Environmental Science & Technology Letters (2026). DOI: 10.1021/acs.estlett.6c00081

Alkaline industrial wastewaters from steel or cement production are ideally suited to bind and sequester carbon dioxide (CO₂) chemically, safely, and for the long term. This is the result of a study conducted by the Helmholtz-Zentrum Hereon. Until now, this wastewater has been disposed of into rivers without using its CO₂-sequestration capacity. In the future, it could neutralize millions of tons of CO₂—offering an attractive and implementable option to mitigate climate change. The study was recently published in the journal Environment, Science & Technology Letters.

Despite the Paris Climate Agreement and all energy-saving measures, global CO₂ emissions continue to rise. Current climate protection efforts, such as expanding solar and wind energy, have so far not been sufficient to stop or even reverse this trend. For several years, climate experts have therefore been urging the removal of CO₂ from the atmosphere and its long-term sequestration.

A key focus is on a method that mimics a natural process that has controlled atmospheric CO₂ levels for billions of years: rock weathering, which chemically binds CO₂ into so-called carbonates—commonly known, for example, as baking soda. The carbonates enter the environment through the weathering of limestone-rich rocks. Rain washes them into rivers and the ocean, where they react with CO₂. In this way, the greenhouse gas CO₂ remains chemically bound for long periods and is removed from the atmosphere. Hereon researchers have now succeeded in developing an industrial-scale process based on this principle that could bind and sequester many millions of tons of carbon dioxide per year.

Reaction of carbonic acid

"Our process is essentially based on a reaction that many people will remember from chemistry class—the neutralization of a base by an acid," explains Prof Helmuth Thomas, head of the Hereon Institute for Carbon Cycles.

A textbook example is the reaction of sodium hydroxide with hydrochloric acid, producing table salt. CO₂ behaves in a similar way. CO₂ from the air reacts with water to form carbonic acid. When this carbonic acid reacts with a base—an alkaline liquid—bicarbonate is formed, which binds the CO₂ in water for the long term. The idea behind the project is not to use carbonates derived from rock for the reaction with carbonic acid, but rather alkaline industrial wastewater.

"These alkaline wastewaters are produced in large quantities—for example in cement or steel production," says Thomas. Until now, they have been mixed with sulfuric or hydrochloric acid to neutralize the base before being released into rivers. In other words: the wastewater's potential to bind CO₂ has remained completely untapped and appears wasted.

But what if, instead of sulfuric acid, the alkaline wastewater were neutralized with CO₂—or carbonic acid—in the future? This method would allow vast quantities of the greenhouse gas to be chemically bound as bicarbonate at an industrial scale. The open question was how much carbon dioxide could actually be bound using this process. Answering it required Thomas's chemical expertise, who calculated the precise CO₂ turnover for such systems.

The result was clear: neutralizing CO₂ in this way is worthwhile—especially because the energy consumption of the facilities is low. Environmental and regulatory constraints, in particular with respect to the pH conditions, are met via automatic adaptation of the released waters to the original conditions of the receiving river.

Global potential

Experts have been discussing chemical CO₂ binding with carbonates for some time. One idea has been to transport rock flour from mountains to the sea via trains and trucks, load it onto ships, and disperse it into the ocean. But the logistical effort would be enormous. Moreover, no one knows how efficiently or quickly the carbonates from the rock flour would react with CO₂ in the water—or whether they would simply sink before the reaction occurs. This is not an issue in an industrial facility: the entire reaction takes place onsite, and the mass balance can be calculated precisely.

"What's great is that the necessary technology is already available," says Thomas. It could begin immediately—unlike many other concepts for reducing atmospheric CO₂. The potential is enormous: if all alkaline industrial wastewater worldwide were used for this process, around 30 million tons of CO₂ could be bound per year. 

Provided by Helmholtz Association of German Research Centres 

by Torsten Fischer, Helmholtz Association of German Research Centres

edited by Gaby Clark, reviewed by Robert Egan 

Source: Alkaline steel and cement wastewater could capture 30 million tons of CO₂ annually