Brita filters contain activated carbon and an ion-exchange resin. Certain impurities in the water bind with the carbon while the water simply passes though. The ion-exchange resin attracts heavy metals which are not effectively filtered by the carbon.
Just as Wonderflan stated, activated carbon and ion-exchange resin. I would just like to give you more details.
Activated carbon has a slightly positive charge, that allows it to attracts negative charged particles. Once the particle, say chlorine, binds, that part of the carbon molecule will not longer bind anything else, which is why you must replace filters. The molecules are tiny though, so the surface area that is able to absorb contaminants is huge. One pound of carbon is 125 acres and one tablespoon is a football field (I got these numbers from the links below). So, carbon will trap negatively charged particles (opposites attract) but not positive, so chlorine is trapped but sodium will pass right through.
The ion exchange resin works by replacing heavy metals (copper, lead and cadmium) and other positively charged molecules with a safer ion, sodium. This is the same concept as water softening (I find slimey water gross, but that’s just me) although I think for water filtering they focus on replacing magnesium and calcium. Ion exchange resins usually do not remove organic contaminants, but that is why there is also activated carbon.
Hope this helped. The ion exchange resin I looked up on wiki.
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