Sediments are major repositories for heavy metals. Within any given compartment of the hydrologic cycle for example, far less than 1% account for the dissolved heavy metal load (Salomons, 1995). While bulk of the heavy metals is stored in sediments (and soils), the dissolved phases are more mobile; henceforth, easily bioavalable.
Present studies dealing with particulate heavy metals in aquatic systems concern total metal concentrations in the carrier media (suspended or bottom inorganic/organic particulates). The use of total metal concentration as a sole criterion to assess the potential adverse effects of sediment contamination implies that all species of a given metal have an equal impact on the environment, which is not the case.
To evaluate the possible risk for metal mobility and consequent bioavailability, a five-step selective heavy metal extraction procedure, modified from Calmano (1983) and Calmano and Förstner (1983) was performed in the pelitic fraction of the Pasig River sediments. Results show that the moderately reducible phase (iron and manganese oxides) is the dominant controlling matrix for heavy metal partitioning. In decreasing order, the general occurrence of metal species trends as follows: residual > moderately reducible > exchangeable > organic fraction and sulfides > carbonates and easily reducible. The residual fraction comprises lattice-bound metals and is not a significant matrix of heavy metal pollutants.