From the context of your other, eye-burning, post, you appear to be assuming that the absorbent is a regular cylinder and the compound attaches as a monolayer. There are problems with the assumption:
1). Absorbents are usually porous, so they can absorb as much as possible. From your jargon, or trade name, or foreign word, materila I can't be sure what you're working with. The substance has a vendor, who will give you surface area specifications. Your single dimension (well, 3 for a regular cylinder) isn't as useful as you believe.
2). Humic acid may not absorb as a monolayer. You'll have to search peer-reviewed literature, though I suspect you'll be disappointed if you expect a simple answer. Just because it has a name, humic acid, doesn't mean it is a rigorously defined compound or well known mixture. You've acquired a molecular dimension from somewhere, but I don't believe it's absolutely valid. Furthermore, humic acid may absorb as clumps, or one humic acid synergisticly helping others to bind, or one humic acid may block adjacent absorbence sites,either stericly or based on charge repulsion, as examples.
If you're trying to make a demonstration of monolayers and Avogadro's number, you'll have to look at more traditional substances.
If you're trying to clean up a water sample, you best bet is probably a series of experiments to determine how much can be absorbed. Many people on this board perform absorption experiments -- HPLC, GC, self poured macro columns, but no one sits around with Avogadro's number and particle dimensions -- you've over simplified the process to your own detriment.