Arsenic in geothermal power plant scales

In some pipe deposits in geothermal power plants, arsenic is associated with clays or other silicate minerals rather than sulfides or (oxy)(hydr)oxides. Pascua et al. (2005) found that about 80 % of the arsenic in pipe scales from a Japanese geothermal power plant was associated with Mg-rich smectite clays. The arsenic (mostly III) was probably located in the crystalline structures of the clays and/or present as submicron inclusions. 

Aqueous As(III) rapidly oxidizes to As(V) in the pipes and drains of some geothermal power plants. Yokoyama, Takahashi and Tarutani (1993, 109-110) speculated that rapid oxidation could be due to iron and manganese (oxy)(hydr)oxides suspended in the water or coating the walls of the pipes. Once oxidized, As(V) may sorb onto the iron (oxy)(hydr)oxides in pipe and drain scales (Webster and Nordstrom, 2003, 116). 

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