大野川の研究(2)河口付近おけるケイ酸の離脱

Abstract

Water samples were obtained at the mouths of the Ono and the Oita rivers and also along the coast of the Beppu Bay, and soluble silica was determined by Iwasaki's method (1960). Soluble silica was defined as the part of the total silica which formed silicomolybdic acid in the part of ammonium molybdate and sulphuric acid.When a river enters the sea, it would be expected that the high concentration of soluble silica in river water could be reduced by dilution with sea water of a low concentration of soluble silica. The data obtained, however, show that some removal of soluble silica from the river water other than dilution does occur in field samples. Several experiments were performed in the laboratory by mixing river water with Bay water showed similar results.On the basis of the field data and the laboratory experiments the mechanism of inorganic removal of soluble silica has been proposed as follows.(1) The removal of soluble silica from the river water is mainly caused by the process that soluble silica is reformed to colloidal silica during the mixing with Bay water. In the river water the initial percentage of soluble silica to total silica is over 95 per cent. This rate slightly drops with increasing chlorinity until 10gm Cl per litre but over this point the dropping rate becomes greater and when it is over 17mg Cl per litre, as in thesurface Bay water, the percentage drops down to 10-14. The difference between the concentrations of soluble and total silica indicates the amount of colloidal silica. Therefore, at a high value of chlorinity nearly all of the soluble silica change to colloidal silica by coagulation of them and are removed from the surface Bay water.(2) In addition to the mechanism mentioned in (1), the other removal of soluble silica is caused partially by suspensions in river water. In this case increasing removal occurs when the chlorinity is increased and 50-60 per cent of soluble silica removed in maximum by suspended materials.Other mechanism, i. e., removal of soluble silica due to biological uptake by diatoms and inorganic removal by electrolytes in Bay water are both negligible small.