Adsorption Made Simple: How Materials Clean Up Impurities

What is adsorption?

In the last article, I introduced biochar, which could be used as an adsorbent. Adsorbents are used for a wide range of separations. The most familiar application might be chromatography. They are usually in the form of powders, small pellets, beads or granules. Adsorbents have many pores. Accordingly, they can take more molecules per volume than others because many pores contribute to a larger surface area. You might notice how often porous material scientists mention surface area. You may not happen to meet them, though, as I have not heard about the surface area and have not met them since I graduated. Anyway, adsorbents are a kind of sponge.

Then, how do adsorbents separate the chemicals from others? Let's think about blue-colored water in the fountain in the backyard. I put too much blue dye into the water. I knew that the charcoal could attract the blue dye like Pond Blue. So, I dump the charcoal into the water. Now, the blue dye diffuses from the water to the exterior surface of the charcoal. Then, it diffuses inside the pore, where it is attracted to the pore's surface.

In this scenario

• Charcoal is an adsorbent
• Blue dye is adsorbate

What if the dye is not removed enough? Just add more adsorbent or find better one. Commercial adsorbent is

1. Activated carbon: Black. If you want to melt or burn a microwave oven, put this material inside and run it for10 minutes.
2. Silica gel: You might find this in the bag to dry nori.
3. Zeolite:
4. Resins

What conditions make adsorption faster?

Under that circumstance, water and charcoal don't move at all, so the driving force is only diffusion. Therefore, removing the dye will take a long time. Agitating the water and adsorbent makes the adsorption faster or pumping water to circulate through a charcoal filter to the fountain. However, we still don't know the amount of charcoal and adsorption time required to finish this process.

We can simply try trial and error: Add a certain amount and wait, then add additional amount and wait, repeat. If we are rich, we can take this approach by letting others do it. But I don't think most scientists and engineers have the luxury enough to do that task in a large fountain. They have to come up with a cost-efficient way to determine it: Doing experiments with a small beaker and estimating the requirement from that data.

To estimate how much dye will be removed by a specific amount of charcoal, we need to collect data on how much dye attaches to the charcoal at different dye concentrations. Then, we can get the relationship between the concentration of dye in water and the removed dye on the charcoal: Adsorption isotherm.

From this relation, we can get the adsorption capacity by fitting an isotherm model. The unit is usually g of impurities per kg of adsorbent. So, now that we know the capacity and concentration of dye, we can estimate the required amount of charcoal.

To estimate the time requirement, we should determine the relationship between the removed amount of dye and time: Adsorption Kinetic.

We can estimate the required time to remove impurities to the specific concentration. For example, if the fish in the fountain can tolerate 150g/L of dye material, we don't need to run the whole time to remove 100%.

A breakthrough experiment is required for more practical applications using a column filled with adsorbents, but it is not covered here. In short, the experiment is for determining how the impurities flow through columns and determining the size of columns for continuous removal, like pumping water through the column for circulation or single pass as the second option I mentioned above.

Those graphs are generated from https://drspchoi.pythonanywhere.com/. That app was developed during my Ph.D training for fun but never deployed on the web because I assumed that was an easy task that could be done by Excel or simple code. However, I learned that some tasks are not as easy for others as I thought. So, I hope this web app is helpful for educational purposes for someone or for pre-check purposes during experiments. You can simply upload csv file, choose any model and click submit. Graphs will be shown.

That's it. I hope this article helps you learn about adsorption, adsorbent, adsorbate, adsorption isotherm and kinetic.