The A team

 

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 a useful analogy in 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. 

And that is rather like what happens inside a granular activated carbon filter (GAC) filter.

GAC has a distribution of pore sizes. The measures of its absorption quality are iodine number and methylene blue number.

The iodine number indicates the carbon's capacity for adsorbing small molecules, i.e the amount of microporosity. Methylene blue is a considerably larger molecule, so quantifies the adsorption capacity associated with larger meso-pores.

So they act rather like red and grey squirrel feeders

Now consider cypermethrin. It is a relatively large pyrethroid pesticide. A carbon’s capacity for absorption of it, then, is indicated by the methylene blue number .

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

Water arriving at a tertiary GAC filter isn't carrying one polite contaminant patiently waiting its turn. It contains dissolved organic matter, humic substances et.c. at concentrations vastly greater than cypermethrin. So essentially, there are many, many more of them, like a football crowd.

And they are all competing for access to the mesopores.

So a tiny amount of cypermethrin arrives at the adsorption site surrounded by considerably more abundant organic molecules.

The question isn't simply:

“Does this media 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

Specifying GAC with an impressive iodine number, like a coconut based one, will not help us at all - The holes are too small.

And while a high methylene blue number, as with a coal based GAC, is a good indicator of the carbon absorption ability of molecules the size of cypermethrin, it is still not a magic pass .

The same principle applies when we start looking beyond GAC.

Alternative sorbents including cork and sugarcane bagasse. 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 simultaneously?

What pores can they access?

Which pores will become blocked?

And what happens as the carbon ages? Could spent carbon from a water application be used again, prior to regeneration, for its unspent pesticide removal application on a wastewater plant?

The red squirrel analogy has another useful lesson.

A feeder that looks selective on paper isn't necessarily selective in practice. 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.