Squirrels and methylene blue

 

What do squirrels have to do with removing pesticides from wastewater? You may well ask!

I’ve been thinking about competitive adsorption for cypermethrin removal and find that a useful analogy comes from an unlikely place: red squirrel conservation.

Red squirrels are smaller than grey squirrels. That difference in size can be exploited when designing feeders: by setting the mechanism around a carefully chosen weight, food can be made accessible to the smaller red squirrel while restricting access by the heavier grey squirrel.

It is, essentially, selective access.

And that is rather like what happens inside an activated carbon filter.

Granular activated carbon (GAC) isn't just a giant sponge with an infinite number of identical holes. It has a distribution of pore sizes.

The traditional iodine number gives us an indication of the carbon's capacity for adsorbing small molecules and is strongly associated with its microporosity. Methylene blue, being a considerably larger molecule, provides information about access to larger pores and is commonly used as an indicator of adsorption capacity associated with meso-/larger pores.

So you can think of them rather like two different squirrel feeders.

Now consider cypermethrin.

It is a relatively large, structurally complex pyrethroid pesticide. It can be adsorbed by activated carbon  but here’s the thing; adsorption performance is not simply a function of how much carbon you put into the process.

And wastewater makes the problem considerably more interesting (sorry water people).

The water arriving at a tertiary GAC filter isn't carrying one polite contaminant waiting patiently for its turn.

It contains dissolved organic matter, humic substances and other compounds at concentrations vastly greater than a trace pesticide. The former measured in milligrammes and the latter in nanogrammes, so essentially, there are many many more of them, like a football crowd.

And they are all competing for access to the carbon: dissolved organic matter can reduce micropollutant adsorption through both competition for adsorption sites and pore blockage.

So imagine a tiny amount of cypermethrin arriving at the entrance to the carbon It is surrounded by considerably more abundant organic molecules.

The question isn't simply:

“Does this GAC* adsorb cypermethrin?” 

*Other adsorbents media are available - more on this in a bit. 

It is:

“Can cypermethrin still get to an accessible adsorption site when everything else in the wastewater is competing for it?”

That distinction matters.

Specifying a carbon with an impressive iodine number Will not help us at all - The holes are too small.

And while high methylene blue number is a good indicator of the carbon absorption ability of molecules the size of cybermethrin, it is still not a magic pass .

The same principle applies when we start looking beyond conventional GAC.

Researchers have investigated alternative sorbents including cork, while agricultural residues such as sugarcane bagasse can also be converted into activated carbon. One study of α-cypermethrin found substantial adsorption onto both GAC and granular cork, illustrating the potential of alternative carbonaceous materials.

The interesting engineering question therefore becomes:

What are we designing the “feeder” for?

What is the size and chemistry of the molecule we actually want to capture?

What other molecules are going to arrive first?

What pores can they access?

Which pores will become blocked?

And what happens as the carbon ages?

The red squirrel analogy has another useful lesson.

A feeder that looks selective on paper isn't necessarily selective in the real world. Earlier weight-operated squirrel feeders were found not to exclude grey squirrels reliably - demonstrating that the mechanism has to be tested in the real world.

The same is true of adsorption.

The real test is what happens when the target contaminant and the media designed to remove it meet the rest of the water.

———

Perhaps that is one of the most important lessons in designing tertiary treatment:

Don't just ask whether the contaminant can fit through the door.

Ask who else is standing in the doorway.

Iodine asks:
How much access is there for the small stuff?

Methylene blue asks:
What happens when the molecule needs a bigger doorway?