In terms of trace elements released, there certainly could be differences based simply on how it is prepared and washed.
The chemical nature of the surface can impact the binding of both inorganic ions and organics.
GAC essentially has regions of both hydrophobic and hydrophilic nature. The hydrophobic sections are mostly just carbon, with some H. The hydrophilic sections contain carboxylic acids (like acetate in vinegar), and -OH groups (like in phenol and ethanol). It is the carboxylates and phenolates that can bind trace elements such as iron.
One can adjust the relative amount of these when designing how to manufacture GAC, but if you remove too many of the polar groups, the materials becomes too hydrophobic to wet properly (like adding graphite to water) and will not work well.
FWIW, I think that if one wants to make claims about impurities released and trace elements bound, one needs to do it as I mentioned above in real reef tank water AND one needs to base it on the amount of organic removed, not on a per gram or cup or whatever.
Again, maybe they did that. i just don't know without seeing the experiment and data. But a GAC that merely releases less and binds less trace elements is not necessarily an improvement if it also binds less organics.
