Mayumi Etou

Silica deposition induced by isolated aluminum ions bound on chelate resin as a model compound of the surface of microbes

To elucidate the mechanism of silica biodeposition in hot spring water, which is induced by Al(3+) ions bound to the surface of microbes, a chelate resin (Chelex 100) was used as a model compound of the surface of microbes. No silicic acid was adsorbed on the Na type Chelex 100, whereas silicic acids were significantly adsorbed to the Al type Chelex 100. In the Al type Chelex 100, the Al(3+) ions were present as 1:1 tridentate complex with iminodiacetate (IDA) group. After adsorption of silicic acid to Al type Chelex 100, a IDA-Al-O-Si-(OH)(3) site formed. The site acted as a template for the successive adsorption of silicic acids to form silica sheets around Al type Chelex 100 particles. In conclusion, Al(3+) ions bound to the surface of microbes play a key role as a trigger for the biodeposition of silica in hot spring water.

THE DISSOLUTION OF AMORPHOUS SILICA IN THE PRESENCE OF TROPOLONE UNDER ACIDIC CONDITIONS

Dissolution of amorphous silica or silicate is an important reaction to release silicic acid in natural water and this reaction is affected by several factors. The existence of a natural organic compound, tropolone, because of its abundance, is of particular importance. In the present study, the dissolution of amorphous silica in the presence of tropolone (HL) was investigated in an aqueous solution under acidic conditions. The dissolution is controlled by the reaction between silicic acid and tropolone (Si(OH)4 + 3HL + H+ ⇌ SiL3+ + 4H2O), where the conditional formation constant K = [SiL3+]/[Si(OH)4][HL]3[H+]). The solubility of amorphous silica in the presence of tropolone was significantly greater than that in pure water due to the formation of the Si-tropolonate complex in the solution. The acceleration of silica dissolution by tropolone depends on the reaction pH and tropolone concentration. The solubility of the amorphous silica in the tropolone solution at pH 1 was approximately ten times greater than in the 0.1 mol/dm3 HCl used as a reference. This increase in the solubility correlates to a K value of the Si-tropolone complex at each pH. From the solubility experiments, the conditional formation constants (log K) of the Si-tropolonate complex at each reaction pH 1, 2, and 3 were 6.39, 5.88, and 5.77, respectively. The significant acceleration of the dissolution of amorphous silica by tropolone at pH 1 can be attributed to the large formation constant of the Si-tropolonate complex.