388 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



PRECIPITATION 



A precipitate forms when ion concentrations exceed their solu- 

 bility. The relatively large concentration of various ions in the sea 

 favors the formulation of precipitates when certain wastes are added. 

 Radioactive and normal elements precipitate together. Any excess 

 in a waste effluent of such elements as calcium, manganese, iron, and 

 nickel will precipitate. 



Although in coastal areas a more concentrated and relatively rapid 

 coagulation time is observed for sediments of terrestrial origin and 

 a substantial proportion of the mineral suspensoid is precipitated near 

 shore, the open ocean has abnormally low rates of coagulation and 

 unexpectedly long residence times for particles, this permitting wide 

 areal distribution of sediments. The average rate of accumulation 

 of sediments is believed to fall in the range of millimeters per thou- 

 sand years. Accumulation rates in the South Pacific were about 0.3 

 to 0.6 mm per thousand years (Goldberg and Koide, 1962). 



Since the sea-floor accumulation of minerals seems to be very slow, 

 and since the sinking time of fine particulate matter is in the order of 

 hundreds of years, those radioisotopes of very long half-life are of 

 principal concern to waste disposal in the open ocean. 



COPRECIPITATION 



When two elements that are chemically similar occur together in a 

 waste, these elements may be copreci pita ted. For example, calcium 

 and strontium apparently precipitate together during the formation 

 of calcium carbonate, thus assisting in the removal of any strontium 

 90 that might be present. 



ION EXCHANGE 



When an ion in an aqueous solution comes in contact with another 

 ion and the necessary conditions are met, the ions may be exchanged. 

 Ion exchanges are used in varied chemical processing, including the 

 processing of wastes. Various solid constituents in the sea have good 

 ion-exchange properties. Natural clays, including those in deep sea 

 deposits, have an appreciable ion exchange capacity. Wastes which 

 come in contact with clay particles in the waters and in sediments 

 will undergo some exchange. In such cases the waste material ap- 

 pears to be held within the lattice of the clay particles in an excep- 

 tionally firm manner. Uranium and thorium may be chelated by 

 minerals such as zeolite so that they are no longer available to the 

 biota on the ocean floor. 



MINERAL FORMATION 



Certain wastes can serve as raw materials for the formation of 

 minerals through biochemical actions and organic transfer in the 

 ocean. Carbonate secreting organisms form calcite and aragonite in 



