When Water Has Too Much of a Good Thing

In this story, kittens will be our heroes, as they help to explain how to get safe water for everyone. For this, meet Dr. Jona-Cat (orange cat) and Denny (black cat), brilliant and curious felines scientists who will guide us through the world of clean water!

Did you know that, by 2037, there will be nearly 9 billion people in the world? This huge population will make the demand for fresh water—one of the most important resources for life—enormous. However, even by 2040 there might not be enough water for everyone, and almost 40% of the global population might be at risk of running out of this vital resource. That means humans, and even our furry friends, might not have enough water to drink!

Do not panic because there is a solution—using underground sources of water called groundwater. In many dry regions, people already get their water from underground sources. Groundwater usually starts off clean, but unfortunately, it can pick up something called fluoride. Fluoride is a tiny, negatively charged particle that is okay (or even healthy) in small amounts but harmful in excess. Where does fluoride come from? As water flows through rocks and soil, fluoride dissolves into the water. Rain or farm irrigation can also carry fluoride from the soil into rivers, lakes, and even back underground, where it builds up over time. While groundwater is usually good for drinking, it sometimes needs to remove excess fluoride to make it safe for people. Then, our mission only focuses on removing the excess of fluoride from water.

You might be thinking, “If fluoride is in toothpaste, how can it be harmful”? The answer is all about how much there is. Fluoride is important for our bodies—it helps prevent cavities and makes our bones strong. But we only need a small amount, about 1 mg per liter, which we get from water or some foods. But too much fluoride can be a problem, making water unsafe. According to World Health Organization, if the water you drink every day has more than 1.5 mg of fluoride per liter, it can harm your teeth and bones, causing a condition called fluorosis [1]. To give you an idea of how small this amount is, it is like adding just one drop of ink to an Olympic-sized swimming pool!

Removing Excess Fluoride From Water

There are several ways to remove fluoride from water. One method is by reverse osmosis, which is like a super-smart strainer, where the clean water slips through, but the fluoride and other stuff get left behind. This process works well but is very expensive. Another method, called coagulation-flocculation, is like making a net with glue: a substance traps the fluoride like crumbs in sticky dough, and the big “clumps” sink or get filtered out easily. This method is cheaper, but it does not catch all the fluoride. Then there is adsorption, which uses solid materials to trap contaminants, like a powerful fishing net that grabs everything it touches as it moves through the water. The most popular material for adsorption is activated carbon, which is great at removing many pollutants. It is also affordable and easy to find but, unfortunately, it is not very good at removing fluoride [2].

Luckily, we can make activated carbon even better at removing fluoride, using an exciting new technology called electrosorption. Electrosorption combines adsorption with electrochemical principles to enhance fluoride removal. Think of it like a sponge: when a small electric current is applied, the activated carbon attracts and holds onto fluoride ions, keeping them out of the water. Although it is still an emerging technology, electrosorption shows great promise for water treatment and could play a key role in producing enough clean water in the future.

How Exactly Does Electrosorption Work?

Electrosorption requires an electrochemical cell, which you can think of as a team with four key players: the anode, the cathode, the electrolyte, and the electrical connector (Figure 1), as indicated by Dr. Jona-Cat in Figure 1.

The anode is a material that conducts electricity. When we apply voltage (electrical energy) to the anode, it becomes positively charged. Meanwhile, the cathode becomes negatively charged. These two materials, called electrodes, are usually made of metals. The energy needed to charge them can come from a battery, for example.

The electrolyte is like a transportation system. It is a liquid that contains tiny particles with positive or negative charges. These particles move between the anode (positive side) and the cathode (negative side), creating what we call an ionic current. The electrical connector, outside of the electrolyte, links the anode and cathode, allowing the electric current to flow between them (Figure 1).

Now, let us dive into the topic of electrosorption. When we apply voltage to the electrochemical cell, the ions in the water are drawn toward the electrodes. Why does this happen? Because opposite charges attract! Positive ions move toward the cathode, while negative ions—like fluoride—are pulled to the anode. This process cleans the water.

Here is a fun way to imagine it: Picture a box filled with tiny foam balls, representing fluoride. Now, let us introduce an orange kitten with short fur, called Dr. Jona-Cat. When you pet the kitten, its fur becomes electrostatically charged. Then, the curious kitten jumps into the box, and its charged fur starts attracting the foam balls, which stick to it. In this situation, the kitten acts as the anode, capturing the foam balls (fluoride) with its electric charge. By the end, our furry friend is covered in foam balls (Figure 2A)!

It might seem logical to think that the more voltage we apply, the better the cell will be at capturing contaminants, right? But it is not that simple. If we use too much voltage, unwanted electrochemical reactions can occur, using extra energy and making the process less efficient.

Imagine a mixture of fluoride and water. If the voltage is too high, we might start breaking down the water, or even damaging the electrodes. This would make the cell less effective at retaining fluoride at the anode. It is like petting the kitten so much that, instead of attracting the foam balls, it becomes so charged it starts shooting sparks around the room, burning the foam balls!

Hey, Wait… Where is the Activated Carbon?

To optimize fluoride removal, the material of the electrodes plays a crucial role. This is because the surface of the electrodes is where ions accumulate, separating the contaminant from water. To make this process more efficient, the electrodes must have a large specific surface area (measured in square meters per gram of material), which can be achieved if they have a porous structure. The pores allow the electrodes to store more ions, maximizing their interaction with the water and improving fluoride accumulation.

To meet these characteristics, carbon-based materials such as carbon fibers, carbon nanotubes, and activated carbon are used [3]. These materials stand out for their excellent electrical conductivity and their ability to be designed with an enormous specific surface area. Among them, granular activated carbon is one of the most popular options due to its high efficiency, low cost, and ease of production.

Using activated carbon is like swapping the short-haired orange cat, Dr. Jona-Cat, for a much fluffier one—Denny, the black cat (Figure 2B). This new kitty has more fur, which means there are more pores and surface area with a static charge to attract tiny foam balls (the fluoride). In this way, Denny could catch many more little balls from the box.

The Final Mission: Continuous Water Flow

The real challenge is to bring electrosorption to continuous-flow systems that can be used in large-scale water treatment plants.

How would it work? First, imagine a bunch of foam balls falling around our statically charged Denny sitting on his climbing tower. As the balls move, they become trapped in Denny’s fur and few, if any, fall out (Figure 3A). An electrochemical reactor works similarly, as a stream of fluoride-contaminated water flows in. Inside, the positively charged activated carbon anode captures the fluoride as the water passes through. In this way, the water leaving the reactor is free of the contaminant (Figure 3B).

Although this might sound simple, operating such a reactor is not that easy. Many factors must be carefully adjusted, including water flow rate, electrode size, applied voltage, initial fluoride concentration, and water temperature. Scientists are currently working hard to overcome these challenges and make this technology a reality.

In addition to being important for science, improving electrosorption could be very beneficial for everyone’s health. By helping to prevent diseases like fluorosis, we are working toward a safer future. And that is not all! Can you imagine using this technology to remove salt from seawater? This could be a revolutionary way to make seawater drinkable, helping to solve water scarcity problems. The best part is that this process could work with renewable energy, like solar power.

Where Are We? Where are We Going?

This project was created after reviewing many research papers on pollutant removal. Although the idea of electrosorption was not originally proposed by our research group, we are the first scientists trying to apply it to a continuous process—the same way water treatment plants work to produce safe water for humans (and their pets). Currently, this project is at the laboratory scale because we spent 2 years on the design and construction of the electrochemical cell. However, we hope it can be operated on an industrial scale within a few years. In fact, it might even be you, young reader, who makes it happen! Of course, this raises new questions: is it economically viable? Could it solve the global water crisis? While those answers are for another time, what is certain is that electrosorption is vast and exciting.

Glossary

Fluorosis: ↑ A health problem caused by drinking too much fluoride over time, which can damage teeth or bones.

Electrosorption: ↑ A way to clean water, in which a material uses a tiny electric current to pull ions out of the water and hold them, almost like a sponge grabbing tiny charged particles.

Electrodes: ↑ Materials that allow electricity to enter or leave a system during a process.

Porous: ↑ Containing tiny openings or cavities that allow gases or liquids to pass through easily.

Carbon Fibers: ↑ Thin strands of carbon that are very strong and light, often used to make materials that conduct electricity.

Carbon Nanotubes: ↑ Tiny tubes made of carbon atoms that are extremely small, strong, and good at carrying electricity.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Ruiz-Martinez thanks SECIHTI for his PhD scholarship (CVU: 1023516). The authors thank SECIHTI for funding the frontier science project “Heterogeneous functional materials (HeteroFoaMs)” (CF-2023-I-981) and COPOCYT convocatoria 2023-01 “Estudio e implementación a escala piloto de un proceso híbrido sostenible de electroadsorción para remediar la problemática de la elevada concentración de fluoruro en el agua subterránea de la ciudad de San Luis Potosí.

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