Effect of polyacrylic acid on polymerization and deposition of silicic acid in geothermal water: A case study at Takigami geothermal power plant, Oita Prefecture

To elucidate the effect of polyacrylic acid (PAA) on the formation of silica scales from geothermal water at geothermal power plants, two different experiments were carried out using geothermal water supplied from a production well, TP-2, at the Takigami geothermal power plant in Oita Prefecture, Japan. Polymerization of silicic acid in geothermal water and adsorption of silicic acid and aluminum on the surface of silica gel powder as a model compound of pre-formed silica scale were investigated in the presence and absence of PAA around 90?. The polymerization of silicic acid which causes changes in monosilicic acid concentration with time and in hydrodynamic size of polysilicic acid particles was not affected by PAA. On the other hand, the adsorption behavior of aluminum was considerably different in cases between the absence and presence of PAA. Only a small amount of aluminum was adsorbed in the absence of PAA, but most of aluminum was adsorbed in the presence of PAA. The amount of silicic acid adsorbed in the presence of PAA was significantly larger than that in the absence of PAA. Consequently, the Al-PAA complex may accelerate the formation of silica scales from geothermal water.