TY - JOUR
T1 - Calcium adsorption at the rutile-water interface
T2 - A potentiometric study in NaCl media to 250°C
AU - Ridley, Moira K.
AU - MacHesky, Michael L.
AU - Wesolowski, David J.
AU - Palmer, Donald A.
N1 - Funding Information:
MKR and MLM acknowledge the support of the National Science Foundation (EAR-9627784) as well as the Illinois State Water Survey and the Illinois Department of Natural Resources. The efforts of DAP and DJW were supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. Reviews by Wayne Nesbitt and two anonymous referees significantly improved the manuscript.
PY - 1999/10
Y1 - 1999/10
N2 - Calcium adsorption by rutile was studied potentiometrically from 25 to 250°C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca2+ was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca2+ adsorption directly. These experiments revealed that Ca2+ adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pH(znpc(NaCl)) - pH). That is, as temperature increased, Ca2+ adsorption commenced at progressively more positive pH(znpc(NaCl)) - pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca2+ adsorption at all temperatures as a result of either increased competition from Na+ or greater complexation of Ca2+ by Cl-. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H+ released / moles Ca2+ adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca2+ adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca2+ waters of hydration, are believed to be primarily responsible for the increase in adsorption with temperature.
AB - Calcium adsorption by rutile was studied potentiometrically from 25 to 250°C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca2+ was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca2+ adsorption directly. These experiments revealed that Ca2+ adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pH(znpc(NaCl)) - pH). That is, as temperature increased, Ca2+ adsorption commenced at progressively more positive pH(znpc(NaCl)) - pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca2+ adsorption at all temperatures as a result of either increased competition from Na+ or greater complexation of Ca2+ by Cl-. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H+ released / moles Ca2+ adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca2+ adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca2+ waters of hydration, are believed to be primarily responsible for the increase in adsorption with temperature.
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U2 - 10.1016/S0016-7037(99)00236-7
DO - 10.1016/S0016-7037(99)00236-7
M3 - Article
AN - SCOPUS:0033370733
SN - 0016-7037
VL - 63
SP - 3087
EP - 3096
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 19-20
ER -