Why does activated carbon preferentially adsorb anions?

Question

Brands like Brita and Pur (in the U.S.) have made a name for themselves for the ability of their product (essentially a large-pore filter with activated carbon/charcoal) to extract the added chlorine from tap water. I visualize $\ce{C}^*$ as a vast network of channels in which ions are "trapped". I had assumed that most cations and anions were trapped effectively, but it seems to favor chlorine and iodine.

According to Wikipedia:

Activated carbon does not bind well to certain chemicals, including alcohols, glycols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.

Why would it not bind effectively to fluorine? Is it an issue of atomic radius? Does $\ce{F-}$, with a radius of 0.136 nm sneak by, while $\ce{I-}$ and $\ce{Cl-}$ at 0.181 and 0.216 nm, repectively, get caught up in the matrix? Why do the cations get passed through?

Answer 1

This answer applies to carbon filtration of water.

According to most of the sources I found, activated carbon binds to most substances through London dispersion forces (from the Wikipedia article). This should mean that it adsorbs larger molecules and non-polar molecules preferentially since they would have larger dispersion forces. As your quote indicates, it does not bind polar molecules like "alcohols, glycols, strong acids and bases" well. This source also lists neutral, non-polar compounds, along with organics and compounds with low water solubility as being effectively removed.

I think that you may have misread or misinterpreted the sentence about binding "chlorine and iodine" as saying that it traps chloride, iodide, and (by extension) other anions well. I can't find data to support this statement. It traps molecular iodine well, and as a large (in terms of electrons), non-polar molecule, that makes sense. But it removes molecular chlorine $\ce{Cl2}$ through a chemical reaction by reducing it to the chloride ion $\ce{Cl-}$ which is then soluble in water and flows through the filter.

I did find a couple of sources (1, 2) that said "activated charcoal" has a slight electropositive charge that helps it to attract negatively charged species, which would fit with your statement that it preferentially attracts anions. (However, they don't list what kind of negative species they're talking about and nitrate, sulfate, and fluoride ions were specifically excluded). The idea that it will remove ionic compounds seems to be contradicted by the statement in at least a couple of sources (one given here) that carbon filters don't remove minerals, salts, and dissolved inorganic compounds

Answer 2

Chlorine actually does not get adsorbed by activated carbon. It reacts with it and oxidizes the carbon to CO2 while the chlorine is reduced to chloride. So while activated carbon media can be regenerated when loaded with organics, it cannot be regenerated after chlorine removal applications because it actually gets depleted.