Treated Coal Cleans Toxic Chemicals Out of Water

Scientists have found a new way to use raw coal to remove harmful chemicals from water. They treated coal with a strong acid called p-xylene-2-sulfonic acid, which changed the coal's surface so it could trap and hold toxic pollutants. The two chemicals they focused on were bisphenol A, also called BSP-A, and 4-chlorophenol, also called 4-CL. Both of these chemicals are found in industrial wastewater and can be dangerous to people and wildlife.

Bisphenol A is often found in plastics and industrial waste, while 4-chlorophenol is a byproduct of making certain medicines and pesticides. Both chemicals are considered water pollutants because even small amounts can harm living things. Finding a cheap and effective way to remove them from water is an important goal for scientists around the world. Using coal as a starting material is attractive because coal is widely available and inexpensive.

To make SF.CA, researchers soaked raw coal in the special acid. This process is called sulfonation, and it added new chemical groups onto the surface of the coal. These groups act like tiny hooks that grab onto pollutant molecules floating in water. The process also roughened and opened up the coal's surface, creating more places for pollutants to attach.

Scientists studied the changes in the coal using several tools. One tool, called X-ray diffraction, showed that the treated coal became much less organized inside. Before treatment, the coal had a somewhat layered structure, but after treatment, that structure broke apart and became disordered. This rougher, more open structure made the coal better at trapping pollutants.

Another tool, called FT-IR, detected new chemical groups on the coal's surface after treatment. These included sulfonic acid, hydroxyl, and carboxyl groups. These groups help hold pollutant molecules in place through a type of attraction called hydrogen bonding. Hydrogen bonding is a gentle pull between molecules that keeps them together without forming a permanent bond.

Scientists also looked at the coal under a powerful microscope. The raw coal looked smooth and flat, like stacked plates. After treatment, the surface broke into rough, jagged pieces with tiny holes and cracks all over. This rough texture gave SF.CA a much larger usable surface, meaning more spots were available to catch and hold pollutants.

Tests showed that water's pH level had a big effect on how well SF.CA worked. The best pH for removing BSP-A was around 8, which is slightly basic, like baking soda in water. For 4-CL, the best pH was around 7, which is neutral. At very high pH levels, the pollutants and the coal surface both became negatively charged and pushed each other away instead of sticking together.

The time that water spent with SF.CA also mattered a great deal. The coal soaked up pollutants quickly at first, then slowed down as its surface filled up. After about 300 minutes, or five hours, the coal had absorbed nearly as much as it could hold. At that point, SF.CA had captured 120.8 milligrams of BSP-A and 92.4 milligrams of 4-CL for every single gram of coal used.

When scientists increased the amount of pollutants in the water, SF.CA captured more in total. The highest amounts captured were 243.4 milligrams of BSP-A per gram of coal and 241.4 milligrams of 4-CL per gram of coal at about 30 degrees Celsius. As water temperature increased, the coal captured slightly less each time. This confirmed that the process works better in cooler water and releases a small amount of heat, making it what scientists call exothermic.

BSP-A was captured in slightly larger amounts than 4-CL in most tests. Scientists believe this is because BSP-A has two hydroxyl groups, which are chemical features that bond well with the surface of SF.CA. The molecule of 4-CL only has one hydroxyl group, so it forms fewer bonds with the coal. This difference in structure explains why BSP-A sticks more strongly to the treated coal.

Scientists used mathematical models to understand how the pollutants stuck to the surface. Two common models, called Langmuir and Freundlich, helped describe whether the surface was smooth and uniform or rough and uneven. The data fit both models well, which means the coal has a mix of uniform spots and uneven spots for grabbing pollutants. A more advanced model using statistical physics confirmed that weak physical forces, not strong chemical bonds, hold the pollutants in place.

Because the bonds are physical and not chemical, the used coal can be washed and reused. This makes SF.CA a good candidate for real-world water treatment systems because it does not need to be replaced as often. Reusability helps keep water treatment costs low and reduces waste over time. Researchers believe this approach could one day be used in water treatment plants to help protect clean water supplies around the world.