Thank you for your question. Here's the response we received from the manufacturer regarding it:
Phos-Zorb is an activated alumina oxide material which is whitish in color. Another material used to remove phosphate is iron-oxide (CFO), a reddish material. In some cases iron-oxide will remove more phosphate on a gram to gram basis when compared to alumina oxide compounds but will cost a considerable amount more.
To further explain it may take 8-ounces of activated alumina to remove 10 ppm of phosphate from a given volume of water. To remove 10 ppm of phosphate from the same volume of water may only take 6-ounces of iron oxide. However the iron oxide often cost 4 to 6 times more than activated alumina. So the cost to remove the 10 ppm of phosphate will cost the consumer considerably less when using activated alumina oxide (Phos Zorb). It is also important to remember that all activated alumina oxide materials are not equal. The activated alumina oxide material in Phos-Zorb is extremely effective at removing phosphate in the most cost effective way.
The bound phosphate and silica nutrients are permanently bound. It is not time that releases the bond but very low pH conditions (pH of 1.5) that would never occur in the aquarium.
The bound nutrients are also not consumed by plants.
We constantly test alternative resins to look for the latest and best materials. We have tested the top selling iron oxide based phosphate removers as well as other novel resins and have found none that have outperformed Phos-Zorb when balancing efficacy and cost. Phos-Zorb is an activated alumina resin and does a great job at removing phosphate and silicates.