Fighting coastal erosion: Electricity turns sand into a sea wall

New research from Northwestern University has demonstrated that a mild electrical current can significantly strengthen marine coastlines, providing a potential long-term solution to combat sand erosion caused by climate change and rising sea levels.

In this study, the researchers were inspired by marine organisms like clams and mussels, which use dissolved minerals in seawater to build their shells. 

The researchers applied a similar concept, using naturally occurring dissolved minerals to create a natural cement between grains of sand, transforming them into a rock-like solid. However, instead of using metabolic energy as mollusks do, they used electrical energy to trigger the chemical reaction.

Combating sand erosion with electricity 

Laboratory experiments showed that a mild electrical current instantly altered the structure of marine sand, making it a solid, immovable material. The researchers believe this method could offer a sustainable, low-cost solution for reinforcing coastlines around the world.

Study senior author Alessandro Rotta Loria is a civil and environmental engineer at Northwestern. 

“Over 40% of the world’s population lives in coastal areas. Because of climate change and sea-level rise, erosion is an enormous threat to these communities,” said Rotta Loria.

“Through the disintegration of infrastructure and loss of land, erosion causes billions of dollars in damage per year worldwide. Current approaches to mitigate erosion involve building protection structures or injecting external binders into the subsurface.”

Strengthening the coast without structures

Rotta Loria said that his goal was to develop an approach capable of changing the status quo in coastal protection – one that didn’t require the construction of protection structures and could cement marine substrates without using actual cement. 

“By applying a mild electric stimulation to marine soils, we systematically and mechanistically proved that it is possible to cement them by turning naturally dissolved minerals in seawater into solid mineral binders – a natural cement.”

The rise of coastal sand erosion

As climate change intensifies storms and raises sea levels, coastal erosion is becoming a more severe problem. According to a 2020 study by the European Commission’s Joint Research Center, nearly 26% of the Earth’s beaches could disappear by the end of this century.

Communities typically respond to this threat by building sea walls or injecting cement into the ground to strengthen the sandy marine substrates. However, these conventional methods have significant downsides: they are extremely expensive and often fail to last.

“Sea walls, too, suffer from erosion. So, over time, the sand beneath these walls erodes, and the walls can eventually collapse. Oftentimes, protection structures are made of big stones, which cost millions of dollars per mile. However, the sand beneath them can essentially liquify because of a number of environmental stressors, and these big rocks are swallowed by the ground beneath them,” said Rotta Loria.

“Injecting cement and other binders into the ground has a number of irreversible environmental drawbacks. It also typically requires high pressures and significant interconnected amounts of energy.”

Solutions inspired by nature

To avoid these problems, Rotta Loria and his team developed a new technique inspired by coral and mollusks. Seawater is naturally rich in ions and dissolved minerals, and when a mild electrical current (two to three volts) is applied, it triggers chemical reactions that convert some of these constituents into solid calcium carbonate, the same mineral that mollusks use to build their shells. 

A slightly higher voltage (four volts) can convert these constituents into magnesium hydroxide and hydromagnesite, common minerals found in various stones.

When these minerals form in the presence of sand, they act like glue, binding the sand particles together. In lab tests, the process worked with all types of sands, from common silica and calcareous sands to iron sands, which are often found near volcanoes.

“After being treated, the sand looks like a rock,” Rotta Loria said. “It is still and solid, instead of granular and incohesive. The minerals themselves are much stronger than concrete, so the resulting sand could become as strong and solid as a sea wall.”

Long-term protection for coastlines 

The minerals form instantaneously once the current is applied, but longer electrical stimulation produces more substantial results. “We have noticed remarkable outcomes from just a few days of stimulations,” he added. “Then, the treated sand should stay in place, without needing further interventions.”

The treated sand is expected to maintain its durability for decades, providing long-term protection for coastlines. The process is also eco-friendly and reversible. The mild voltages used in the process are harmless to sea life, and the process can be undone if necessary by switching the battery’s anode and cathode, which dissolves the minerals.

“The minerals form because we are locally raising the pH of the seawater around cathodic interfaces,” Rotta Loria explained. “If you switch the anode with the cathode, then localized reductions in pH are involved, which dissolve the previously precipitated minerals.”

Less cost, more protection 

This method offers a cost-effective alternative to traditional erosion control methods. Rotta Loria’s team estimates that their process costs just $3 to $6 per cubic meter of electrically cemented ground, compared to up to $70 for similar methods that use binders.

This approach also shows promise for repairing cracked structures made of reinforced concrete, commonly found in existing shoreside infrastructure that is deteriorating due to sea-level rise, sand erosion, and extreme weather. A single pulse of electricity could heal potentially destructive cracks without the need to rebuild the entire structure.

“The applications of this approach are countless. We can use it to strengthen the seabed beneath sea walls or stabilize sand dunes and retain unstable soil slopes. We could also use it to strengthen protection structures, marine foundations, and so many other things. There are many ways to apply this to protect coastal areas,” Rotta Loria concluded.

The study is published in the journal Nature Communications Earth & Environment.

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