Our Premium Carbon Block Cartridges are Made in the USA and are of the highest grade, acid washed, ultra pure carbon.  The carbon is acid washed to remove ash and residue as well as to reduce the carbons fines.  Further, the carbon powder is compression molded into a block that yields 0.5 micron pores (thats about 100x smaller than the width of a human hair), maximizing chemical adsorption and micro particulate filtration. 

Activated carbon is effective at removal of the Radioactive Isotope Iodine-131 and various chemical contaminants such as Pharmaceuticals, Volatile Organic Compounds, Chlorine, Oil Industry Byproducts (petrochemicals), and more. 

About Removing Radioactive Iodine 131 using Carbon:

[Activated carbon can be used as a substrate for the application of various chemicals to improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide (H₂S), ammonia (NH₃), formaldehyde (HCOH), radioisotopes iodine-131(¹³¹I) and mercury (Hg). This property is known as chemisorption.

A gram of activated carbon can have a surface area in excess of 500 m², with 1500 m² being readily achievable.^[3] 

Under an electron microscope, the high surface-area structures of activated carbon are revealed. Individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously. Tests of adsorption behaviour are usually done with nitrogen gas at 77 K under high vacuum, but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100 °C and a pressure of 1/10,000 of an atmosphere.] - Wikipedia (source) 

[Activated carbon, as viewed by an electron microscope]