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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