Acidity of edge surface sites of montmorillonite and kaolinite

Xiandong Liu, Xiancai Lu, Michiel Sprik, Jun Cheng, Evert Jan Meijer, Rucheng Wang

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Abstract

Acid–base chemistry of clay minerals is central to their interfacial properties, but up to now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of surface groups on (0 1 0)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 and 1:1-type clay minerals, respectively. It shows that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups of kaolinite have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of titration curves, indicating that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups are the major acidic sites responsible to pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is investigated and it is found that Mg substitution increases the pKas of the neighboring triple bond; length of mdashSi–OH and triple bond; length of mdashSi–OH2 groups by 2–3 pKa units. Furthermore, our calculation shows that the pKa of edge triple bond; length of mdashMg–(OH2)2 is as high as 13.2, indicating the protonated state dominates under common pH. Together with previous adsorption experiments, our derived acidity constants suggest that triple bond; length of mdashSi–O– and triple bond; length of mdashAl–(OH)2 groups are the most probable edge sites for complexing heavy metal cations.
Original languageEnglish
Pages (from-to)180-190
Number of pages11
JournalGeochimica et Cosmochimica Acta
Volume117
Early online date24 Apr 2013
DOIs
Publication statusPublished - 15 Sep 2013

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Bentonite
Kaolin
montmorillonite
Acidity
kaolinite
acidity
Clay minerals
clay mineral
substitution
Substitution reactions
Heavy Metals
Titration
Minerals
Molecular dynamics
Cations
Energy gap
cation
heavy metal
adsorption
Adsorption

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Liu, X., Lu, X., Sprik, M., Cheng, J., Meijer, E. J., & Wang, R. (2013). Acidity of edge surface sites of montmorillonite and kaolinite. Geochimica et Cosmochimica Acta, 117, 180-190. https://doi.org/10.1016/j.gca.2013.04.008

Acidity of edge surface sites of montmorillonite and kaolinite. / Liu, Xiandong; Lu, Xiancai; Sprik, Michiel; Cheng, Jun; Meijer, Evert Jan; Wang, Rucheng.

In: Geochimica et Cosmochimica Acta, Vol. 117, 15.09.2013, p. 180-190.

Research output: Contribution to journalArticle

Liu, X, Lu, X, Sprik, M, Cheng, J, Meijer, EJ & Wang, R 2013, 'Acidity of edge surface sites of montmorillonite and kaolinite', Geochimica et Cosmochimica Acta, vol. 117, pp. 180-190. https://doi.org/10.1016/j.gca.2013.04.008
Liu, Xiandong ; Lu, Xiancai ; Sprik, Michiel ; Cheng, Jun ; Meijer, Evert Jan ; Wang, Rucheng. / Acidity of edge surface sites of montmorillonite and kaolinite. In: Geochimica et Cosmochimica Acta. 2013 ; Vol. 117. pp. 180-190.
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AU - Wang, Rucheng

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N2 - Acid–base chemistry of clay minerals is central to their interfacial properties, but up to now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of surface groups on (0 1 0)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 and 1:1-type clay minerals, respectively. It shows that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups of kaolinite have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of titration curves, indicating that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups are the major acidic sites responsible to pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is investigated and it is found that Mg substitution increases the pKas of the neighboring triple bond; length of mdashSi–OH and triple bond; length of mdashSi–OH2 groups by 2–3 pKa units. Furthermore, our calculation shows that the pKa of edge triple bond; length of mdashMg–(OH2)2 is as high as 13.2, indicating the protonated state dominates under common pH. Together with previous adsorption experiments, our derived acidity constants suggest that triple bond; length of mdashSi–O– and triple bond; length of mdashAl–(OH)2 groups are the most probable edge sites for complexing heavy metal cations.

AB - Acid–base chemistry of clay minerals is central to their interfacial properties, but up to now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of surface groups on (0 1 0)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 and 1:1-type clay minerals, respectively. It shows that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups of kaolinite have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of titration curves, indicating that triple bond; length of mdashSi–OH and triple bond; length of mdashAl–OH2OH groups are the major acidic sites responsible to pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is investigated and it is found that Mg substitution increases the pKas of the neighboring triple bond; length of mdashSi–OH and triple bond; length of mdashSi–OH2 groups by 2–3 pKa units. Furthermore, our calculation shows that the pKa of edge triple bond; length of mdashMg–(OH2)2 is as high as 13.2, indicating the protonated state dominates under common pH. Together with previous adsorption experiments, our derived acidity constants suggest that triple bond; length of mdashSi–O– and triple bond; length of mdashAl–(OH)2 groups are the most probable edge sites for complexing heavy metal cations.

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