Biosorption of thorium on the external shell surface of bivalve mollusks: The role of shell surface microtopography

External shell surface (ESS) of bivalve mollusks is known to adsorb various metals dissolved in ambient water in high concentration. It is hypothesized here that the surface microtopography of the thin organic coating layer, periostracum, or calcareous shell (if periostracum was destroyed) plays a major role in the adsorption of actinides on ESS. Thorium (natural alpha-emitter) was used in short-term biosorption experiment with shell fragments of five bivalve mollusks. After a 72 h exposure to Th (∼6 kBq L−1), thorium concentration was measured on ESS using laser ablation inductively coupled plasma mass spectrometry; the distribution and density of alpha tracks were subsequently visualized by α-track autoradiography. A trend in reduced Th concentrations on the ESS was observed depending upon the species tested: (group 1 ∼4000 μg g−1) Chlamys islandica (M.), Mercenaria mercenaria (L.), Dreissena polymorpha (P.) > (group 2 ∼1200 μg g−1) Crassostrea virginica (G.) ≫ (group 3 ∼150 μg g−1) Mytilus edulis L. The microtopography of ESS was characterized by scanning electron microscopy revealing the high porosity of the calcareous surface of C. islandica and M. mercenaria, lamellate surface of periostracum in D. polymorpha, uneven but a weakly porous surface of periostracum of C. virginica, and a nearly smooth surface of the periostracum of M. edulis. This work has demonstrated, for the first time, the presence of a strong correlation between concentration of adsorbed Th and ESS microtopography, and the role of the periostracum in this process is discussed.

► We studied shell fragments of bivalve mollusks after exposures to thorium. ► There is correlation between concentration of Th and shell surface microtopography. ► Results obtained may have direct application to the biomonitoring of actinides.