Biosorption of divalent Pb, Cd and Zn on aragonite and calcite mollusk shells

The potential of using mollusk shell powder in aragonite (razor clam shells, RCS) and calcite phase (oyster shells, OS) to remove Pb2+, Cd2+ and Zn2+ from contaminated water was investigated. Both biogenic sorbents displayed very high sorption capacities for the three metals except for Cd on OS. XRD, SEM and XPS results demonstrated that surface precipitation leading to crystal growth took place during sorption. Calcite OS displayed a remarkably higher sorption capacity to Pb than aragonite RCS, while the opposite was observed for Cd. However, both sorbents displayed similar sorption capacities to Zn. These could be due to the different extent of matching in crystal lattice between the metal bearing precipitate and the substrates. The initial pH of the solution, sorbent’s dosage and grain size affected the removal efficiency of the heavy meals significantly, while the organic matter in mollusk shells affected the removal efficiency to a lesser extent.

► Mollusk shells display high removal efficiency to heavy metals in contaminated water.
► Surface precipitation leading to crystal growth takes place during the sorption.
► Crystal structure similarity between precipitates and substrates affects the sorption.
► pH, sorbent dosage and grain size of adsorbent affects the removal efficiency.
► Organic matter in mollusk shells affects the removal efficiency to a less extent.