 Crushed shells , available as a by @-@ product of the seafood canning industry , can be used to remove pollutants from water . It has been found that , as long as the water is maintained at an alkaline pH , crushed shells will remove cadmium , lead and other heavy metals from contaminated waters by swapping the calcium in their constituent aragonite for the heavy metal , and retaining these pollutants in a solid form . The rock oyster ( Saccostrea cucullata ) has been shown to reduce the levels of copper and cadmium in contaminated waters in the Persian Gulf . The live animals acted as biofilters , selectively removing these metals , and the dead shells also had the ability to reduce their concentration . 
