Marine Bioinorganic Chemistry 12.755 Lecture 2
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Transcript of Marine Bioinorganic Chemistry 12.755 Lecture 2
Marine Bioinorganic Chemistry 12.755 Lecture 2
Last week:1. Four types of trace metal profiles2. Geochemical properties that cause these profiles shapes: solubility,
inorganic speciation, organic speciation, and redox.3. Began Speciation lecture with
– Definitions of ligands, chelates– Stability constants, solubility products, – Hard vs soft ions, Irving Williams series, – Non-ideal effects/Debye Huckel/Davies corrections, – Hydration energies of different transition metals
Today: Metal Speciation continued1. The Conditional Stability constant 2. Setting up equations for inorganic species3. Setting up equations for organic species4. Literature: speciation of metals in seawater overview5. Introduction to Mineql+ 6. Brief Discussion of readings
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Why are we talking about complexation chemistry?
• How do metals influence the biota (and carbon cycling) of seawater?
• To answer the question we have to understand:
- Natural organic-metal complexes:
FeL, CoL, NiL, CuL, ZnL, CdL
• What are the geochemical roles of these ligands?
1. Controls on “bioavailability”
- high affinity uptake systems
- ecological warfare between species
2. Protection from scavenging processes
3. Increases in solubility
• How do you study something at picomolar quantities which we don’t know much about?
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Background Aquatic Chemistry of Trace Elements:A marine water column context
Solubility Products: Example for Fe(OH)3(s)
Ksp= [Fe][OH]3 = 1042.7
Stability constants for metal complexes (where L is ligand, M is Metal):K = [ML]/[M][L]
Ligands can include inorganic chemical species:
In oxic systems: OH-, CO32-,SO4
2-, Cl-, PO43-,
In anoxic systems add: HS-,, S2-
Ligands can also include organic chemical species:EDTA, DTPA, NTA, Citrate, Tris, siderophores, cobalophores,
DFB, TETA, and the famous unknown ligand(s) “L”
FROM LAST WEEK:
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Definitions
• Ligand – an atom, ion, or molecule that donates/shares electrons with one or more central atoms or ions.
• Chelate – (from Greek chelos = crab, with two binding claws) two or more donor atoms from a single ligand to the central metal atom
FROM LAST WEEK:
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Conditional stability constants: specific to “conditions”
Thermodynamic constant based on activities
Activity corrected,Now based on concentrations
There may be a variety of L- species, the apparent constant
Aggregates this diversity.
L- will have acid base chemistry
In seawater where there are many salts: Kcond = KappIf acid-base chemistry dominates: Kcond = Keff
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M2+ + L- ML+
K = {ML+} / {M2+}{L-}
CK = [ML+] / [M2+][L-] (concentration constant)
Kapp = [ML+] / [M2+][L-] (apparent constant)
Kapp = [ML+] / [M2+][HxL-] (effective constant)
We’ve already talked about the effects of saltsAcid base chemistry also matters for complexation chemistry in seawater:
We just usually don’t know enough to correctly parameterize it
Protonation constants of EDTA matter
Co2+ + 2HDMG- CoHDMG2
Co2+ + EDTA4- CoEDTA2-
modeling
experimental
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H2L H+ + HL-
HL- H+ + L2-
Which brings us to:How do we measure metal speciation?
• Use ligand exchange reactions:
Natural Ligands:CoL Co2+ + L2-
Our “Probe” LigandCo2+ + 2HDMG CoHDMG2
Net reaction:CoL + 2HDMG CoHDMG2 + L2-
Core Idea: There are compounds we can measure extremely sensitively in seawater using electrochemistry
They adsorb to mercury when a potential is applied , and are called electroactive-ligands like CoHDMG2
There are many electroactive ligands (synthetic):Fe: 1N,2N; TAC,
Cu: BzacZn: APDC
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Ligand Exchange
M + L1 ML1
M + L2 ML2
ML1 + L2 ML2 +L1
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Ligand Exchange
M + L1 ML1
M + L2 ML2
ML1 + L2 ML2 +L1
There are kinetic considerations to this:
If in seawater and either L1 or L2 has a high affinity for Ca2+ or Mg2+, it will clog up the exchange reactions
Disjunctive Adjunctive ML M + L M* + ML M*LM
M* + L M*L M*LM M*L + M
If M = Ca2+ and M* = a trace metal the concentration gradient is many orders of magnitude!
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• Inorganic speciation Terminology: – M’ or METAL-“PRIME” = summation of inorganic species
– Zn’ = Zn2+ + ZnCl+ + ZnSO4+ ZnOH+ + ZnCO3 + ZnS
• Organic speciation– L for unknown organic ligand (variants L1 and L2), metal-specific (?)
– EDTA as a “model” ligand Ethylene diaminetetraacetic acid
• [Total Dissolved Metal] = M’ + ML1 + ML2
Trace Metal Speciation Calculations
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Tables of stability constants – complied in Martell and Smith volumes/databases andreprinted in Morel and Hering and Stumm and Morgan at zero ionic strength.
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Calculations of organic speciation in seawater
• Start with mass balance the “total” equation:
[Total Dissolved Metal] = M’ + ML1 + ML2
• Write equations for inorganic and organic speciesZn’ = Zn2+ + ZnCl+ + ZnSO4+ ZnOH+ + ZnCO3 + ZnS
Total L = H4L + H3L- + H2L2- + HL3- + L4- + MgL2- + CaL2-
Simplify by removing negligible species:
Total L = H3L- + H2L2- + MgL2- + CaL2-
• Substitute in constants and abundant species to inorganic and organic (if known) equations. Then substitute those into the total equation
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Species dependent on pH:
[CoOH-] / [Co2+][OH-] = 104.3
[H+][OH-] = 10-14
At pH 8.0: [OH-] = 10-14 / 10-8 = 10-6
[CoOH-] = 104.3[Co2+]10-6
= 10-1.7 [Co2+]
Also carbonate species, H2CO3, HCO3-, CO3
2- are pH dependent and can be ligands. Acidity constants: Ka1=6.3, Ka2=10.3
[CO32-] = [CO3
2-]Total / ( 1 + 1010.3[H+]+1016.6[H+]2)
We typically do not assume redox equilibrium in chemical speciation reactions – instead we investigate/calculate only one redox state (Fe III)
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The calculation of equilibrium between multiple chemical species
Start with a simple system 3 species: M2+, MA, MB2
M + A MA K =[MA] / [M][A]
MA = K[M][A]
M + 2B MB2 K = [MB2] / [M][B]2 MB2 = K[M][B]2
Total M = M2+ + MA + MB
Total M = M2+(1 + K[A] + K[B]2)
M2+/Total M = 1 / (1 + K[A] + K[B]2)
MA/Total M = K[A] / (1 + K[A] + K[B]2)
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Total M = M2+ + MA + MB
Total M = M2+(1 + K[A] + K[B]2)
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• From Bruland 1988
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Note of caution:
• Tables in Morel and Hering and Stumm and Morgan are made for teaching
• They have been back corrected to zero ionic strength from constants
• If your application really matters, go to the literature or NIST databases for each constant
• You can use the textbooks as guidelines of species to look for though
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History of Metal Speciation in Seawater (Brief and Incomplete)
• Cu - Sunda 1983, Coale and Bruland 1988, Moffett et al., 1990• Zn - Bruland, 1988• Cd - Bruland, 1988• Fe – Gledhill and van den Berg 1994
– Rue and Bruland 1995, Wu and Luther 1995, van den Berg 1995
• Co – Saito and Moffett 2001, Ellwood and van den Berg 2001• Ni and Cr – Achterburg and van den Berg, 1997• Hg – Lamborg et al., 2004
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Morel, Allen, Saito, Treatise on Geochemisrty 2003
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Mineql installation – aquatic speciation software
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Launches in MS-DOS command line
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