The process has kinetic barriers due to the slow dissolution in water and the low
concentration that drive the aggression on the rocks, even if the thermodynamics is
favorable.
The conversion of CO 2 (DG° = −396 kJ mol
−1 ) into the carbonate anion (DG
° = −530 kJ mol
−1 ) has a negative free energy of ca. 134 kJ mol
−1 . This is the
effect of the combination of a strongly negative enthalpy change and a negative
entropy change, due to the conversion of a gaseous species (CO 2 ) into a condensed
one. The carboxylation of metal oxides, and of calcium oxide in particular, is a
spontaneous reaction that means that the standard free energy (Eq. 6.18) is
negative.
CaO s
ð Þ þ CO 2 ðgÞ ! CaCO 3 s
ð Þ DG
¼ DH
À TDS
¼ À253 kJ mol
À1
ð6:18Þ
Therefore, the carboxylation of metal oxide would release energy that can be
recovered (and this is a positive fact), but needs to be carried on in solution (and this
is a bottleneck).
Based on such data, how is, thus, performed the enhanced mineral carbonation?
Let us start with the description of the materials that can be used, either natural
rocks or industrial residues, as mentioned.
Among natural rocks, “olivine, Mg 2 SiO 4 ” and “serpentine, Mg 3 Si 2 O 7 or
Mg 3 Si 2 O 5 (OH) 4 , its hydrated form” are potential sources of magnesium for mineral
carbonation. Olivine is an orthosilicate (Mg salt of orthosilicic acid H 4 SiO 4 ; by
comparison, “enstatite, MgSiO 3 ” is the Mg salt of metasilicic acid H 2 SiO 3 ) made of
isolated tetrahedral [SiO 4 ]
4− units and interstitial Mg
2+ cations (Fig. 6.10a).
Fig. 6.10 On the left a are represented the single SiO 4
2− tetrahedrals (Si = red, O = light green)
and counterions M
2+ (Mg
2+ can be substituted by a number of different cations such as Fe
2+ ); on
the right b is shown the lamellar phyllosilicate (phyllo from Greek Utkko = leaf) planar networks
of polymeric interconnected Si 2 O 5
2− units (highlighted in the red rectangle in B), with the metal
cations sitting in between two layers
6.7 Fixation of CO 2 into Long-Lasting Inorganic Materials
93
concentration that drive the aggression on the rocks, even if the thermodynamics is
favorable.
The conversion of CO 2 (DG° = −396 kJ mol
−1 ) into the carbonate anion (DG
° = −530 kJ mol
−1 ) has a negative free energy of ca. 134 kJ mol
−1 . This is the
effect of the combination of a strongly negative enthalpy change and a negative
entropy change, due to the conversion of a gaseous species (CO 2 ) into a condensed
one. The carboxylation of metal oxides, and of calcium oxide in particular, is a
spontaneous reaction that means that the standard free energy (Eq. 6.18) is
negative.
CaO s
ð Þ þ CO 2 ðgÞ ! CaCO 3 s
ð Þ DG
¼ DH
À TDS
¼ À253 kJ mol
À1
ð6:18Þ
Therefore, the carboxylation of metal oxide would release energy that can be
recovered (and this is a positive fact), but needs to be carried on in solution (and this
is a bottleneck).
Based on such data, how is, thus, performed the enhanced mineral carbonation?
Let us start with the description of the materials that can be used, either natural
rocks or industrial residues, as mentioned.
Among natural rocks, “olivine, Mg 2 SiO 4 ” and “serpentine, Mg 3 Si 2 O 7 or
Mg 3 Si 2 O 5 (OH) 4 , its hydrated form” are potential sources of magnesium for mineral
carbonation. Olivine is an orthosilicate (Mg salt of orthosilicic acid H 4 SiO 4 ; by
comparison, “enstatite, MgSiO 3 ” is the Mg salt of metasilicic acid H 2 SiO 3 ) made of
isolated tetrahedral [SiO 4 ]
4− units and interstitial Mg
2+ cations (Fig. 6.10a).
Fig. 6.10 On the left a are represented the single SiO 4
2− tetrahedrals (Si = red, O = light green)
and counterions M
2+ (Mg
2+ can be substituted by a number of different cations such as Fe
2+ ); on
the right b is shown the lamellar phyllosilicate (phyllo from Greek Utkko = leaf) planar networks
of polymeric interconnected Si 2 O 5
2− units (highlighted in the red rectangle in B), with the metal
cations sitting in between two layers
6.7 Fixation of CO 2 into Long-Lasting Inorganic Materials
93