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Coprecipitation of Sodium, Magnesium, and Silicon with Calcium Carbonate

Shepherd Miller, Inc., 1600 Specht Point Dr., Suite F, Fort Collins, CO. 98525

C. Amrhein^* and M. A. Anderson

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA, 92521

A. M. Daoud

Soil Salinity Lab., Bacos, Alexandria, Egypt

*Corresponding author (christopher.amrhein{at}ucr.edu).

ABSTRACT

Calcite oversaturation in soil solutions is often attributed to overestimations of Ca²⁺ (aq) and CO^2–₃ (aq) activities, organic matter mineralization, and the presence of metastable forms of CaCO₃. Earlier research, however, has indicated that these explanations are often insufficient to explain the levels of calcite oversaturation observed in soil suspensions. This study was conducted to evaluate the solubilities of precipitates formed during evapoconcentration of synthetic irrigation waters. Solids precipitated from waters of various Mg and Si concentrations were redissolved in deionized water under controlled CO₂ conditions. Ion activities were calculated from the solution composition using the geochemical speciation model MINTEQA2. X-ray diffraction (XRD) and infrared absorption spectroscopy provided evidence for precipitation of poorly crystalline Mg silicates and hydrous Na-Mg silicates (sepiolite [Mg₂Si₃O_7.5(OH)·3H₂O], loughlinite [Na₂Mg₃Si₆ (OH)₁₆·8H₂O], stevensite [Na_0.33Mg_2.87Si₄O₁₀(OH)₂], or magadiite [Na Si₇O₁₃(OH)₃·3H₂O]). Calcite, aragonite, gypsum, and amorphous SiO₂ were also identified. The solubility of the solid phase was proportional to the amount of Si and Mg in the solid, and dissolution of the solids in deionized water under atmospheric CO₂ yielded an average CaCO₃ ion activity product value of 10^–8.0. The maximum mole fraction of Mg (X_Mg) in the calcite determined by XRD was <0.03, while X_Mg values calculated from mass balance were up to 0.53. The high solubility of the solid phases and the high Ca²⁺ (aq) and CO^2–₃ (aq) activities are attributed to poorly crystalline Mg silicates and Na silicates similar in composition to sepiolite, loughlinite, stevensite, and magadiite.

Received for publication March 11, 1995.