Speciation of dissolved silicates in natural waters containing alkaline and alkaline-earth ions - A case study - waters from arid lands (North West China)

The concentration of silica in water samples from the desert area of Xinjiang, N. W. China, has been measured by colorimetry with ammonium molybdate. The observed pattern of dependence of the concentration of silica on the concentration of sodium ion (Na+) in the water samples is consistent with the pattern obtained by experiments on in-vitro dissolution of silica gel in sodium chloride (NaCl) solution. This indicates that the dissolution of silica in the hydrologic system in this area depends on the concentration of Na+. Calcium ion (Ca2+), which is known to play an important role on the dissolution of silica on the basis of in-vitro experiments, was observed to take little part in the dissolution of silica in actual natural water samples. This implies that the Ca2+ is bound to the hydrogen carbonate anion or that the Ca2+ content of natural water containing salts is very low, owing to precipitation. In these samples silicate-Na+ was identified as the dissolution species of silica; it was also ascertained that Ca2+ did not form complexes with silicate species. These observations resulted from direct identification of dissolved chemical species by use of FAB-MS (fast atom bombardment mass spectrometry). The research indicates that in water samples in this critically arid region the concentration of dissolved silica is basically determined by the concentration of Na+, indicative of pure inorganic conditions in the desert area of Xinjiang, N.W. China.