Aquatic Pollution and Coastal Zone Managememnt (2+1)

Unit 5 : Heavy metal pollution

 

5.4.Fate of Heavy Metals
Once in the aquatic environment, metals are partitioned among various aquatic environmental compartments (water, suspended solids, sediments and biota). The main processes governing distribution and partition are dilution, advection, dispersion, sedimentation and adsorption/desorption.
All metals in the aquatic environment may occur in dissolved, colloidal and particulate forms. The soluble forms are generally ions or unionized organometallic complexes. The colloidal and particulate metals may be found in hydroxides, oxides, silicates or sulphide forms or adsorbed to clay , silica or organic matter.
The behaviour of metals in natural waters is a function of sediment characteristics, suspended particulate composition and water chemistry. Sediment composed of fine sand and silt will generally have higher levels of metal than will quartz, feldspar and carbonate-rich sediments. Metals also have high affinity for humic acids, organo-clays and oxides coated with organic matter.
The water chemistry of the system controls the rate of adsorption and desorption of metals to and from sediment. Adsorption removes the metal from the water column and stores in the substrate. Desorption returns the metal to the water column, where recirculation and bioassimilation take place. Metals may be desorbed from the sediment if the water experiences increase in salinity, decrease in redox potential or decrease in pH.