4.7 Article

Copper isotope fractionation during adsorption onto kaolinite: Experimental approach and applications

Journal

CHEMICAL GEOLOGY
Volume 396, Issue -, Pages 74-82

Publisher

ELSEVIER
DOI: 10.1016/j.chemgeo.2014.12.020

Keywords

Cu isotopes; Isotope fractionation; Adsorption; Kaolinite; Experiment

Funding

  1. National Natural Foundation of China [41473017]
  2. [2-9-2014-068]

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The adsorption of copper and other heavy metals onto clay minerals is an important process that controls the distribution of trace metals in natural environments. Copper isotopes are a potentially useful tool to track the source of contaminated metals in soils formed in natural systems, but Cu isotope fractionation during adsorption onto clay minerals, the major component in soils, has not been thoroughly studied. In this study, we carried out for the first time a series of experiments to investigate the isotope fractionation of Cu during adsorption onto kaolinite for a wide range of conditions, including the contact time (t = 10360 min), temperature (T = 150 degrees C), initial Cu concentration of the starting solution (C-0 = 2-100 mu g/g), pH value (4.0-6.0) and ionic strength (NaNO3; I = 0-0.1 mol/L). Our results indicate that Cu isotopes are significantly fractionated with preferential adsorption of the light isotope (Cu-63) onto the mineral surface. The fractionation factors (Delta Cu-65(adsorbed-solution) = delta Cu-65(adsorbed) - delta Cu-65(solution)) weakly depend on the pH and temperature with a constant value of approximately - 0.27 parts per thousand at C0 = 20 mu g/g and in the absence of NaNO3. Addition of NaNO3 into the starting solution has a dramatic negative influence on the Delta Cu-65(adsorbed-solution) values that range from -1.46 parts per thousand to -0.29 parts per thousand. Such results are useful for interpreting Cu isotopic variations observed in sediments, soils and water from estuarine settings or industrial sewage pollution areas. The Delta Cu-65(adsorbed-solution) values significantly increase with increasing initial Cu concentration of the starting solutions at C-0 < 30 mu g/g, but approach a stable value of -0.17 +/- 0.10 parts per thousand (2SD) when the kaolinite has reached its maximum adsorption capacity at C-0 > 30 mu g/g. The results imply that the isotopic compositions of the Cu adsorbed onto natural soils may vary greatly at relatively low Cu concentrations of the soil solutions. Furthermore, the pore waters after draining kaolinite-bearing rocks would become isotopically heavier due to the preferential adsorption of Cu-63 onto kaolinite. Given that no redox change occurred in all experiments, we propose that the most likely mechanism responsible for such Cu isotope fractionation is the different adsorption capacities of isotopically different species in aqueous solutions and the formation of outer-sphere surface Cu(II) complexes. Our study represents one important step for future studies to use Cu isotopes to trace the source of metal contaminants in natural soils. (C) 2014 Elsevier B.V. All rights reserved.

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