Theory on trace arsenic in geothermal fluids. Final report

Technical Report · Thu Aug 01 04:00:00 EDT 1985

OSTI ID:5322948

Cobble, J W

A chemical equilibrium model has been used to determine the transport of arsenic from a geothermal reservoir throughout a typical geothermal plant. As/sub 2/S/sub 3/, orpiment, and As/sub 2/S/sub 2/, realgar, appear to be the minerals which control arsenic solubility in the reservoir and plant, and the principle soluble form in the fluid is arsenous acid, H/sub 3/AsO/sub 3/. The thermodynamic calculations indicate a soluble arsenic concentration in a 30 ppM H/sub 2/S geothermal brine of 91 ppM at 300/sup 0/C. Other H/sub 2/S concentrations will give different H/sub 3/AsO/sub 3/ concentrations. A number of arsenic species that might vaporize into steam were examined including AsH/sub 3/, AsCl/sub 3/, As/sub 4/O/sub 6/, and As/sub 4/. In the range of brine concentrations and pH values encountered in most geothermal fluids, none of these compounds can contribute detectable levels of gaseous arsenic. However, it is predicted that arsenous acid (like gaseous boric acid) will be soluble at the parts per billion to parts per trillion level in geothermal steam. Such predictions should be verified by experimental determination of gaseous H/sub 3/AsO/sub 3/ distribution coefficients. Arsenic concentrations and release rates have been calculated at a number of locations in a model geothermal plant including the well head, separator, turbine, and cooling tower. The transport of arsenic in geothermal steam leaving the separator is quite sensitive to the purity of the steam. 11 refs., 4 figs., 21 tabs.

• Research Organization: Cobble (James W.), La Jolla, CA (USA)
• OSTI ID: 5322948
• Report Number(s): EPRI-AP-4214; ON: TI85920853
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
152001* -- Geothermal Data & Theory-- Properties of Aqueous Solutions
ARSENIC
ARSENIC COMPOUNDS
ARSENIC SULFIDES
CHALCOGENIDES
CHEMICAL COMPOSITION
DISTRIBUTION FUNCTIONS
ELEMENTS
EQUILIBRIUM
FLUIDS
FUNCTIONS
GEOTHERMAL FLUIDS
MINERALS
SEMIMETALS
SOLUBILITY
SULFIDE MINERALS
SULFIDES
SULFUR COMPOUNDS
THERMODYNAMICS
TRACE AMOUNTS