Ionised silica in the estuary of a river as supply to seawater: Identification and ionization efficiency of silica species by FAB-MS

Abstract

Measurement of the dissolution state of silicic acid is difficult. In river water, silica exists in particle form, but silica particles with a diameter of less than approximately 0.45 μm are considered as dissolved silica. In seawater, silica exists in two forms: ionic silica and particle silica. In this study, we focused on ionic silica. Using fast atom bombardment mass spectrometry (FAB-MS), the silica species in river water and seawater were detected as ionic forms. Ionic silica forms various chemical species in aquatic solutions, including the monomer ([Si(OH)3O−]) and dimer ([Si2(OH)5O2−]). The relative abundances of these species in aquatic solutions depend on the chemical and physical conditions. Silica species such as [Si(OH)2O2Na]− ([monomer-Na+]−), [Si2(OH)5O2]− ([dimer]−), [Si2(OH)4O3Na]−([dimer-Na+]−), [Si4(OH)7O5]− ([cyclic tetramer]−), [Si4(OH)6O6Na]− ([cyclic tetramer-Na+]−), [Si4(OH)9O4]− ([linear tetramer]−) and [Si4(OH)8O5Na]− ([linear tetramer-Na+]−) were directly observed by FAB-MS in river water and seawater. Some of these ionic silica species are expected to serve as “nutrients” for diatoms in seawater. Large silica particles are transported in river water, whereas in estuaries, a large amount of silica is precipitated and a small amount of silica is dissolved as ionic forms in sodium chloride solution. In river water, the concentration of silica was high, but the ionic silica species were hardly ionised by FAB-MS. In seawater, the concentration of silica was low, but the ionic silica species were well ionised. Thus, the ionization efficiency of silica species by FAB-MS indicates the type of silica species. The filtration process of silicic acid and the ionization of silicic acid to dissolve the silica species in seawater, which is an electrolyte (sodium chloride), occur in the estuary of a river. Thus, the estuary of a river plays an important role in the restructuring of silica from particle form to ionic form.