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This article in SSSAJ

  1. Vol. 59 No. 5, p. 1258-1267
     
    Received: Mar 11, 1995
    Published: Sept, 1995


    * Corresponding author(s): christopher.amrhein@ucr.edu
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doi:10.2136/sssaj1995.03615995005900050008x

Coprecipitation of Sodium, Magnesium, and Silicon with Calcium Carbonate

  1. D. B. Levy,
  2. C. Amrhein ,
  3. M. A. Anderson and
  4. A. M. Daoud
  1. Shepherd Miller, Inc., 1600 Specht Point Dr., Suite F, Fort Collins, CO. 98525
    Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA, 92521
    Soil Salinity Lab., Bacos, Alexandria, Egypt

Abstract

Abstract

Calcite oversaturation in soil solutions is often attributed to overestimations of Ca2+ (aq) and CO2−3 (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 [Mg2Si3O7.5(OH)·3H2O], loughlinite [Na2Mg3Si6 (OH)16·8H2O], stevensite [Na0.33Mg2.87Si4O10(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 (XMg) in the calcite determined by XRD was <0.03, while XMg values calculated from mass balance were up to 0.53. The high solubility of the solid phases and the high Ca2+ (aq) and CO2−3 (aq) activities are attributed to poorly crystalline Mg silicates and Na silicates similar in composition to sepiolite, loughlinite, stevensite, and magadiite.

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