EVROPSKÁ UNIE
Evropské strukturální a investiční fondy Operační program Výzkum, vývoj a vzdělávání
MINISTEHSTVO ŠKOLSTVÍ, MLÁDEŽE A TĚLOVÝCHOVY
Geochemistry on the Earth's surface for analytical geochemists
3a.
Carbonate system in the environment
&
Geochemistry of karst processes
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie č. CZ.02.2.69/0.0/0.0/16_018/0002593
Outline topic
• Importance
• Carbonate system
• Alkalinity and acidity
• Geochemistry of karst processes
— Equilibrium water-air-mineral
— Karst formations
Carbonate system
Carbonate system
• Based on C02 and derived compounds
• Essential role in weathering
• Greenhouse gas - impact on the climate
• Organic matter - biosphere
• Biogeochemistry
CO* emission and consumption are kept * in rough balance by a negative feedback resulting from the temperature-dependence of silicate weathering. The feedback operates on a million-year time scale.
(weathering)
C02 + CaSi03 ^±
CaC03 + SiO
^ C02 sources (emissions) ^ C02 sinks
(metamorphism)
Walker etal. (1981) Jour. Geophys. Res., 86, 9776.
- 20% organic matter
- 80% carbonate        *oin scrubs C02
from atmosphere
Snowball Earth.org
Carbonates
• C02(g) dissolves into water proportionally to the partial pressure in the air (PC02)
C02Cg) <-> C02(aq)
• C02reacts with water to form acid
C02(aq) + H20(/) = H2C03(aq)
• C02(aq) is about 600x more abundant, yet for simplicity we express all C02 as H2C03*
H2CO^ (aq) = C02(aq) + H2C03(aq)
Carbonic acid
We can simplify the dissolution to:
CO2(fl0 + H2O(Z) = H2CO5(ag) With the equilibrium constant
_ aH2C03(aq)
2 ~
C02
Each PC02thus corresponds to a specific aH2C03*
C02 content in water is often expressed as PC02 even if the gas phase isn't present
Dissociation
Carbonic acid dissociates to the 1st degree:
H2C03 = HC03 +H+
Kl = aHCQ*aH+ = io-6-35
aH2CO^
And to the 2nd degree:
HCO3 = COi" + H+
K2 = _E23-1_ = 10-10.33
aHC03
Speciation
0fH2CO3*/°HCO3-anC' Q,HC03-/Q,c032_ pH
dependent and can be simplified to:
= of = 10+6,35 aH+at 25 oC
"HCOg Kl
• H2C03* and HC03~are in equilibrium at 6.35 (neglecting C032~)
Speciation
Analogously for c/HC03_/aC032_
3i£°£ = 5!L = 10+10,33 aH+at 25 oC
aco|- K
C032"and HC03"are in equilibrium at pH = 10.33 (neglecting H2C03*)
Carbonate speciation
In natural waters dominates HC03"
	100 -
u	
tal Dl	80 -
of toi	60 -
'cent	40 -
01 Cl.	20 -
0
pH
FIGURE 3.15   Relative distribution of dissolved inorganic carbon species in pure water as a function of pH, at 25°C.
Adapted from Clark (2015)
Example 1
• What will be the pH of water in equilibrium with atmospheric C02at 25 °C, assuming ideal behavior and no other dissolved substances?
• The partial pressure of C02 is 4 x 10~4
• k    = in-i,47
• K^IO"6'35
• Based on the charge balance
[H+] = [OH"] + [HC03-] +2[C0321
Calcium carbonates
CaC03(s) = CO§- + Ca2+
• The solubility product is
Kc = aCa2+ aC02- = 10-8'48 calcite (25 °C) Ka = aCa2+ aC02- = 10~8'34 aragonite (25°C)
• We can solve the systems by adding the appropriate equations to the carbonate syste
- KC02, Klr K2, KW/ Kc/ PEN and one other condition
• Fixed Pc02for an open system or sum of carbonates for closed system
Example 2
What will be the pH of water in equilibrium with atmospheric C02and calcite at 25 °C, assuming ideal behavior and no other solutes?
• 6 unknowns (PC02, [H2C03*], [HC03-], [C032-], [Ca2+] and [H+])
• 6 equations (PC02, Kco2' Kcand charge balance)
P    = in-3-4
rC02 ■LU
Charge balance: [H+]+2[Ca2+]=[HC031 +2[C0321+[OH-]
— At neutral pH simplified to
2[Ca2+] =[HC03-]
- By substituting the equations, we express [H+]
Example 2
What will be the pH of water in equilibrium with atmospheric C02and calcite at 25 °C, assuming ideal behavior and no other solutes?
• 6 unknowns (PC02, [H2C03*], [HCO,-], [CO,2"], [Ca2+] and [HI)
• 6 equations (PC02, KC02, K1; K2, l^and charge balance)
P   = i n-3-4
rC02 ±KJ
Charge balance: [H+]+2[Ca2+]=[HC031+2[C032-]+[OH-]
- At neutral pH simplified to
2[Ca2+] =[HC03"]
- By substituting the equations, we express [H+]
- [H+] = 10"8 2        pH = 8.2
Weathering of carbonates
GEOCHEMISTRY OF KARST PROCESSES
The importance of karst
• Landscape system
• Cultural-economic
- Tourism, aesthetic significance
- Source of raw materials (limestone, iron ore, wood)
- Agriculture
- Groundwater collectors
• A source of information about the environment in the geological past
Source: http://www.slovenia-explorerxom/wp-co^
Predjarná Castle (SLO)
Karst
Geological formation
A landscape formed by the dissolution of the bedrock - Mostly carbonate rock, but evaporites as-well Key role of CO,
Source: http://wordsmith.org/words/images/karst_large.
Karst (SLO)
9.
10.
11.
architektonické památky zříceniny hradů technické památky archeologické památky
Chrám P. Marie Bolestné Jeskyně Kůlna Hrad Holštejn
Větrný mlýn Ostrov u Macochy Hrad Blansek Větrný mlýn Rudice Huť Františka Jeskyně Býčí skála Chrám Jména P. Marie Jeskyně Pekárna Hradisko Chochola
Kňinský ADAMOV
S NAUČNÉ STEZKY
1. Sloupsko-šošůvské jeskyně
2. Macocha stezka Jana Šmardy
3. Jedovnické rybníky Rudické propadání
4. Cesia železa
5. Josefovské údolí
6. Údolí Říčky
7. Hády a Údolí Říčky
Křtinský p.
Březina
B VEŘEJNOSTI PŘfSTUFWIl JESKYNĚ
A Sloupsko-šošůvské jeskyně
B Jeskyně Balcárka
C Punkevní jeskyně, propast Macocha
D Kateřinská jeskyně
E Výpustek
□NÁRODNÍ PŘÍRODNÍ REZERVACE(NPR) NÁRODNÍ PŘÍRODNÍ PAMÁTKY (NPP)
1. NPR Vývěry Punkvy NPP Rudické propadání NPR Habrůvecká bučina NPR Býčí skála NPP Jeskyně Pekárna
Říčka
Hosten ice
Říčka
6. NPR Hádecká planinka
přírodní mzBmACE (ipr}
1. PR Sloupsko-šošůvské jeskyně
2. PR Bílá voda
3. PR Balcamva skála - Vintoky
4. PR Mokřad pod Tipečkem
5. PR Dřínová
6. PRUVýpustku 7 PR Březinka
8. PR Čihadlo
9. PR Údolí Říčky
10. PR Velký Hornek
11. PRUBrněnky
Chráněná krajinná oblast MORAVSKÝ KRAS
Moravian karst
Karst phenomena
Dissolved Joint
Subterranean River
Cave
Crack In Limestone
Cavern River
Stream Disappears
Stalagmite / Stalactite
Limestone Column\Underground Lake
Surface landforms
Surface landforms
Underground phenomena
Corrosion
Cave sediments
Cave deposits:
- allochthonous (sand, clay)
- autochthonous (cave sinters - speleothems)
Water in karst
An ideal path of water through karst
Infiltration
Flow through soi
Water retention in epikarst
Flow along fissures
Discharge into the cave
Drainage into phreatic zone
Karst processes
• Soil C02 dissolution
H20+C02^^ H2C03*
• C02 degassing Soil &
Epikras
• Carbonate rock dissolution
CaC03+H2C03 1—- Ca2++2HC03-
• Calcite precipitation ^ve
Open/closed system
CaC03 + H20 + C02 = Ca2+ + 2HC03"
• Open system: PC02 of the water remains constant, C02 from air replaces C02 consumed by dissolution
• Closed system: The ammount of calcite solution can dissolve is limited by the CO 2 initially present in the water
Dissolution
Closed system
Equlibrium
water
Only a small amount of limestone dissolves.
Dissolution
water flows, it can dissolve limestone.
• The amount of dissolved calcite depends critically on the availability of the gas phase with C02 at dissolution
HjC03 CONCENTRATION (mmol/liter) 0.0345 _ 0 345_345_ 34,5
C0? PRESSURE (aim.)
Fig. 2.—Changes in composition of carbonated water during equilibration with calcite at 25° C. in the presence and in the absence of a vapor phase. Curves I and II describe the behavior of a solution which was originally in equilibrium with a vapor phase with a C02 pressure of 0.10 atm., curves III and IV describe the behavior of a solution that was originally in equilibrium with a vapor phase with a C02 pressure of 0.01 atm.
Example 3
• Pure water in equilibrium with PC02 = 10~2
a) How much calcite will be dissolved in a closed system?
6 unknowns (PC02, [H2C03*], [HCCy], [C032"], [Ca2+] a [H+])
6 equations (KC02/ Kl7 K2, Kc, Ch. Bal. a IC02)
ZC02 = ZC02° + IC02 diss
IC02= [H2C03*]° + [Ca2+]
We assume negligible [C032~] in IC02and get
[H2C03*]° = [HCO3I + [H2C03*] - [Ca2+]
Example 3
• Pure water in equilibrium with P C02 = 10 ~2 a) How much calcite will appear in a closed system?
6 unknowns (P C02, [H 2 CO 3 * ], [HCO 3-], [CO 32"], [Ca 2+ ] and [H+])
6 equations (K C02, K 1, K 2, K c, PEN and I CO 2) IC02 = IC02° + IC02diss ZC02=[H2C03*]°+[Ca 2+] We assume negligible [CO 32~] in I CO 2and obtain [H 2CO 3*] ° = [HCO3-] + [H 2CO 3*] - [Ca 2+] Substituting the remaining equation gives
[Ca2+] = 3.34x 10"4mol/l Approximately 33.4 mg of calcite is dissolved per liter of water
Example 3
• Pure water in equilibrium with P C02 = 10 ~2
b) How much do you appear in the open system? - A system we have already dealt with
6 neznámých (PC02, [H2C03*], [HC03"]# [C032-], [Ca2+] a [H+]) 6 rovnic (PC02/ KC02, K1# K2, Kc and Ch. Bal.)
P    = in-2
rC02 ±u
We express [Ca2+] from the equations and substitute the parameters
[Ca2+] = 1,39 x 10"3 mol/L Approximately 139 mg ofcalcite dissolves per liter of water
Schematics of karst dissolution
Precipitation
Infiltration
Percolation
Dissolving C02from the soil
Dissolving limestone (Cave)
- Degassing and speleothem growth
Discharge
By chrislm - Own thesis , CC BY 3.0, https://c0mm0ns.wikimedia.0rg/w/index.php?curid=S462400
Karst processes
CaC03 + H20 + C02 = Ca2+ + 2HC03-
r
Very fast reaction
Always in equilibrium 1
T
Very slow reaction
2^w3
H,0 + CO, «-> H,CO, <-> HCO,- + H+    C032- + H+
+
Ca2+ 1
a CaC03(calcite)
Very slow reaction
Diagram of calcite dissolution
Bulk Solution
Surface concentration
as f(c - cs)... flux released from surface
as... constant
aD (cs - cB)... diffusional flux
aD... combined diffusion coef. and thickness of concentration boundary
FS=FD Steady state
Calcite surface processes
1. CaC03+H+ —b-+Ca2+ +HCQ
2. CaC03 + H2CO*3 —Ca2+ + 2HCO'
3- CaC03 + H2O^^Ca2+ + C02' +H+ ->Ca2+ +HC03+OH 4. Ca2++HCO~^^>CaC03 + H+
o
PWP Equation (Plummeretal. 1978)
► Dissolution rate at the surface
Fpwr —
ki(H+)s + k2 (H2C03*)J +|k3-k4(Ca2+)s(HC03-)
Reaction-transport control Surface reaction control
pH<5 pH>7
cn cn
_2
D-
pH dependent pH independent
Dolomites
CaMg(C03)2(s) = CO!" + Ca2+ + Mg2+
• The solubility product is
KD = aCa2+aMg2+aC02- = 10~17-2 (25 °C)
• The value of KD is uncertain and can display very variable values for dolomites of different origins
• Very slow dissolution under standard conditions and almost no growth (highly unsaturated/supersaturated solutions)
Formation of dolomite
Dolomite is often formed by alteration of calcite CaMg(C03)2 + Ca2+ = 2CaC03 + Mg2+
_ aMg2+ KCD ---
aCa2+
In solutions where the Mg/Ca ratio is greater than K CD/ dolomite is more stable than calcite and vice versa
The process is kinetically very slow
Calcite changes to dolomite only under large excess of magnesium
Dissolution of dolomite
• Analogous to calcite
• Significantly slower
• Extreme times needed to reach "equilibrium" (thousands of years)
— Significant uncertainty in equlibrium values
• At low temperatures, based on kinetics and thermodynamics, it should dissolve incongruently
Magnesium calcite
Ca(1_x)MgxC03(s) = C0§" + (1 - x)Ca2++xMg2+
• Low magnesium Mg-calcite (<5%)
• High magnesium Mg-calcite (>10%) - recent deep-sea sediments
• All high-magnesium calcites are unstable (transformation to dolomite and low-magnesium calcite), but under surface conditions this is inhibited by dolomite growth kinetics
• Mg-calcite in seawater more stable than pure calcite
Dissolution of Mg-calcite
Equilibrium with solution definable as cation exchange or dissolution
- As a result, both processes must be in balance
The correctness of the approaches is not clarified, the relationships are very complex
The determination of Ks complicates the combination of congruent and incongruent dissolution
o 3.5E-03 E
£ 3.0E-03
CaTot [mol L"1]
EVROPSKÁ UNIE
Evropské strukturální a investiční fondy Operační program Výzkum, vývoj a vzdělávání
MINISTERSTVO ŠKOLSTVÍ, MLÁDEŽE A TELOVÝCHOVY
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia
chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Resources
• Unmarked images are copyrighted or taken with permission from doc. Zeman and doc. Faimon
Clark, I. (2015). Ground water Geochemistry and Isotopes . CRC Press . 442 p. ISBN 978-1-4665-9174-5 ( e-book - PDF)
• Drever, J. (1997): The geochemistry of natural waters: surface and underground waters environment. (I have a book from our library with me on long-term loan)
• Dreybrodt, W. & Eisenlohr, L. (2000): Limestone dissolution great rates _ environment. - In: Klimchouk, AB ( ed .): Speleogenesis development of karst aquifers . - National speleological society. Huntsville, Alabama. 527 p
- , P. (2014). Environmental and Low Temperature Geochemistry. John Wiley and Sons . 402 p. ISBN 978-1-4051-8612-4 ( pbk •)
- Appelo, C. A. J., & Postma, D. (2005). Geochemistry groundwater and pollution : (2nd ed.). Leiden:
• Holland HD, Kirsipu TV ., Huebner JS, Oxburgh UM (1964) On some aspects of the chemical evolution of cave waters. J Geol 72:36-67. doi : 10.1086/626964
• Plummer LN, Parkhurst DL, Wigley TML (1979) A critical review of the kinetics of calcite dissolution and precipitation.
• Plummer LN, Wigley TML, Parkhurst DL (1978) Kinetics of calcite dissolution in C02-water systems at 5 degrees to 60 degrees C and 0.0 to 1.0 atm C02. Am J Sci 278:179-216. doi : 10.2475/ajs.278.2.179
• Pracný P, Faimon J, Všianský D, Kabelka L (2017) Evolution of Mg/Ca Ratios during Limestone Dissolution Under Epikarstic Conditions. Water Geochemistry 1-21. doi : 10.1007/sl0498-017-9313-y
• Pracný P, Faimon J, Kabelka L, Hebelka J (2016) Variation of carbon dioxide in the air and scapular waters of Punkevni caves (Moravský karst, Czech Republic). Carbonates and Evaporites 31:375-386. doi : 10.1007/sl3146-015-0259-0
• Stumm , W. and Morgan, J. (1995): Water chemistry: chemical equilibria and rates in natural waters.