Seashells: Detoxifying agents for metal-contaminated waters

Effective treatments of industrial waste streams and toxic spills containing heavy metals depend on the rapid removal of high concentrations of metal ions. Here we describe such a method using minimally-processed waste mollusk and crustacean exoskeletons. The process of metal extraction and immobilization was studied using atomic absorption, SEM/EDAX, and x-ray diffraction to investigate both solution and mineral phases. Compared to calcium carbonates of geologic origin, biomineralized shell materials exhibit extremely rapid sequestration of metal ions: a 10,000 mg L-1 Pb solution for example, can be reduced to less than about 0.5 mg L-1 in five minutes using comminuted shell of clam (M. mercenaria), while at higher initial concentrations, both clam and oyster (C. virginica) shell can extract almost twice their weight of Pb. The uptake mechanism involves the exchange of Ca for Pb in the inorganic fraction of the shell structure. Results highlight the importance of both surface area and the presence of the organic matrix of biominerals in determining the rate and quantity of metal ions abstracted, and the uniquely coherent residues produced in the process.