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Soil Syst. 2018, 2(2), 20; https://doi.org/10.3390/soilsystems2020020

A Comparison of the Solubility Products of Layered Me(II)–Al(III) Hydroxides Based on Sorption Studies with Ni(II), Zn(II), Co(II), Fe(II), and Mn(II)

Department of Earth & Environmental Sciences, Rutgers University, Newark, NJ 07102, USA
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Received: 27 February 2018 / Revised: 1 April 2018 / Accepted: 4 April 2018 / Published: 10 April 2018
(This article belongs to the Special Issue Soil Processes Controlling Contaminant Dynamics)
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Abstract

The precipitation of mixed metal–aluminum-layered double hydroxides (Me(II)–Al(III)-LDH)) may be an important control of the solubility of Mn(II), Fe(II), Ni(II), Zn(II), and Co(II) in soils, but assessment of this process is hindered by a lack of thermodynamic data. Here, we determined the solubility products (Ksp) of the Me(II)–Al(III)-LDHs formed by these metals based on long-term Me(II)–γAl2O3 sorption studies complemented with X-ray absorption spectroscopy (XAS) measurements. The LDH phases had the chemical formula Me(II)2/3Al1/3(OH)2Cl1/3. Solubility products were derived as Ksp = (Me2+)aq2/3(Al3+)aq1/3(OH)aq2(Cl)aq1/3, where brackets represent aqueous activity values determined from the equilibrium solution chemistry of the sorption samples. The Ksps are metal-dependent, with values increasing by two orders of magnitude in the order Ni(II) < Zn(II) < Co(II) < Fe(II) < Mn(II). Comparison to the solubility of β-Me(OH)2 suggests considerable thermodynamic preference of Me(II)–Al(III)-LDH over β-Me(OH)2 in soil environments and reveals a linear relation between the log-transformed Ksp values of Me(II)–Al(III)-LDH and β-Me(OH)2. Solubility plots suggest that Ni(II)–, Zn(II)–, and Co(II)–Al(III)-LDH may form in metal-polluted soils. Fe(II)–Al(III)-LDH may occur in riparian soils undergoing reduction, but precipitation of Mn(II)–Al(III)-LDH appears unlikely as it requires [Mn2+]aq much higher than commonly encountered in reducing soils. Additional thermodynamic and field studies are needed to further assess the importance of Me(II)–Al(III)-LDHs in soils and related geochemical systems. View Full-Text
Keywords: trace metals; layered double hydroxides; sorption; precipitation; solubility; aluminum trace metals; layered double hydroxides; sorption; precipitation; solubility; aluminum
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Bhattacharya, L.; Elzinga, E.J. A Comparison of the Solubility Products of Layered Me(II)–Al(III) Hydroxides Based on Sorption Studies with Ni(II), Zn(II), Co(II), Fe(II), and Mn(II). Soil Syst. 2018, 2, 20.

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