Pb, Cd and Hg contents of bivalves collected during the different seasons of the year

Levels of lead, cadmium and mercury were determined in talaba (Ostrea malabonensis, F.), tahong (Perna viridis, L.), halaan (Arca spp.) and tulya (Corbicula manilensis, P.) including the seawater where the saltwater bivalves were harvested, The bivalves were sampled during the dry (March-June), wet (July-October) and cold (November-February) seasons of the years 1983-1985. The heavy metals were determined by atomic absorption spectroscopy. The solutions for Pb and Cd determination were prepared by dry ashing at 400 degrees C with the use of 20% H2SO4, as ashing aid followed by wet oxidation of the residue with sulfuric acid and 30% H2O2. Samples for total mercury determination were prepared by solubilization at 50 degrees C with concentrated H2SO4, oxidized with 6% KMnO4 solution and reduced with stannous chloride solution. The average concentration of Pb, Cd and Hg in the bivalves (talaba, tahong, halaan and tulya) ranged from 0.64 to 2.24 ppm, 0.06 to 0.66 ppm and 0.62 to 11.68 ppb, respectively. However, the variations due to sample and season of the Pb and Cd contents in the bivalves were found insignificant. In contrast, the Hg content of the bivalves varied significantly with the sample and the season. The same trends were also noted in the Pb, Cd and Hg content in the seawater which ranged from 0.66 to 1.16 ppm, 0.07 to 0.10 ppm and 1.8 to 2.6 ppb, respectively. The saltwater bivalves exhibited low bio-accumulation factors for the three metals studied. Overall, the findings demonstrated that the four bivalves are safe for human consumption and that the ecosystems where they were harvested do not pose any hazard to man or to marine life. However, caution should be taken if these bivalves are to be considered for feeding children. For example, the FAO/WHO recommendation for Pb is 5 microgram/kg body weight/day for adults. However, children have about five times the gastrointestinal absorption rate of adults and consumption of these bivalves by children may predispose them to Pb exposure.