Behaviour of Pollutant Metals in Aquatic Sediments

Abstract

So called “heavy metals”, transition metals, some of which are necessary in small concentrations to living systems, and which at higher concentrations are toxic to aquatic biota, are common pollutants because of their diverse industrial uses. Maximum permitted concentrations and quantities in aqueous effluents and in surface waters which are considered not to cause intolerable damage, have been established in the legislations of many countries. However, it is known that these metals, originally dissolved in water, are absorbed by sediments so that even after the cessation of industrial emission the sediments will contain stocks of absorbed metals which may be remobilized and become available to the biosystem with toxic effects, either by direct biological interaction with the sediment (benthic organisms) or by the subsequent physical disruption of the sediment (e.g. resuspension by dredging or during river flooding) (Duinker and Notting 1977). Sediments also contain heavy metals of natural origin, but it is likely that the chemical speciation of the metals with natural and anthropogenic origin will be different, the latter occuring in more labile forms. A considerable fraction of the metals of natural origin will occur as trace components of stable minerals. The precipitated and/or absorbed anthropogenic metals may be remobilized by variations in redox conditions or in pH.