Marine Bioinorganic Chemistry 12.755 Lecture 2 Last week: 1.

Four types of trace metal profiles 2.

3.

Geochemical properties that cause these profiles shapes: solubility, inorganic speciation, organic speciation, and redox.

– – – – – 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 continued 1.

The Conditional Stability constant 2.

3.

Setting up equations for inorganic species Setting up equations for organic species 4.

5.

6.

Literature: speciation of metals in seawater overview Introduction to Mineql+ Brief Discussion of readings 1

2

3

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?

4

FROM LAST WEEK: Background Aquatic Chemistry of Trace Elements: A marine water column context Solubility Products: Example for Fe(OH) 3(s) K sp = [Fe][OH] 3 = 10 42.7

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 , CO 3 2 ,SO 4 2 , Cl , PO 4 3 , In anoxic systems add: HS , , S 2 Ligands can also include

organic

chemical species: EDTA, DTPA, NTA, Citrate, Tris, siderophores, cobalophores, DFB, TETA, and the famous unknown ligand(s) “L” 5

FROM LAST WEEK: 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 6

Conditional stability constants: specific to “conditions” M 2+ + L  ML +

K

= {ML + } / {M 2+ }{L } Thermodynamic constant based on activities

C K

= [ML + ] / [M 2+ ][L ] (concentration constant)

K K app app

= [ S ML = [ML + + ] / [M ] / [M 2+ 2+ ][ ][ S S H L x L ] (apparent constant) ] (effective constant) 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 = Kapp If acid-base chemistry dominates: Kcond = Keff 7

We’ve already talked about the effects of salts Acid base chemistry also matters for complexation chemistry in seawater: We just usually don’t know enough to correctly parameterize it experimental H 2 L  H + + HL HL  H + + L 2 modeling Protonation constants of EDTA matter Co 2+ Co 2+ + 2HDMG + EDTA 4  CoHDMG 2  CoEDTA 2 8

Which brings us to: How do we measure metal speciation?

• Use ligand exchange reactions: Natural Ligands: CoL  Co 2+ + L 2 Co 2+ Our “Probe” Ligand + 2HDMG  CoHDMG 2 Net reaction: CoL + 2HDMG  CoHDMG 2 + L 2 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 CoHDMG 2 There are many electroactive ligands (synthetic): Fe: 1N,2N; TAC, Cu: Bzac Zn: APDC 9

Ligand Exchange

M + L1  ML1 M + L2  ML2 ML1 + L2  ML2 +L1 10

Ligand Exchange

M + L 1  ML 1 ML M + L 2 1 + L 2   ML ML 2 2 +L 1 There are kinetic considerations to this: If in seawater and either L 1 or L 2 has a high affinity for Ca it will clog up the exchange reactions 2+ or Mg 2+ , Disjunctive ML  M + L M* + L  M*L Adjunctive M* + ML  M*LM M*LM  M*L + M If M = Ca 2+ and M* = a trace metal the concentration gradient is many orders of magnitude!

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Trace Metal Speciation Calculations • Inorganic speciation Terminology: – M’ or METAL-“PRIME” = summation of inorganic species – Zn’ = Zn 2+ + ZnCl + + ZnSO 4 + ZnOH + + ZnCO 3 + ZnS • Organic speciation – L for unknown organic ligand (variants L 1 and L 2 ), metal-specific (?) – EDTA as a “model” ligand Ethylene diaminetetraacetic acid • [Total Dissolved Metal] = M’ + ML 1 + ML 2 12

13

Tables of stability constants – complied in Martell and Smith volumes/databases and reprinted in Morel and Hering and Stumm and Morgan at zero ionic strength.

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15

Calculations of organic speciation in seawater • Start with mass balance the “total” equation: [Total Dissolved Metal] = M’ + ML 1 + ML 2 • Write equations for inorganic and organic species Zn’ = Zn 2+ + ZnCl + + ZnSO 4 + ZnOH + + ZnCO 3 + ZnS Total L = H 4 L + H 3 L + H 2 L 2 + HL 3 + L 4 + MgL 2 + CaL 2 Simplify by removing negligible species: Total L = H 3 L + H 2 L 2 + MgL 2 + CaL 2 • Substitute in constants and abundant species to inorganic and organic (if known) equations. Then substitute those into the total equation 16

Species dependent on pH: [CoOH

-

] / [Co

2+

][OH

-

] = 10

4.3

[H

+

][OH

-

] = 10

-14

At pH 8.0: [OH

-

] = 10

-14

/ 10

-8

= 10

-6

[CoOH

-

] = 10

4.3

[Co

2+

]10

-6

= 10

-1.7

[Co

2+

]

Also carbonate species, H 2 CO 3 , HCO 3 , CO 3 2 are pH dependent and can be ligands. Acidity constants: K a1 =6.3, K a2 =10.3

[CO 3 2 ] = [CO 3 2 ] Total / ( 1 + 10 10.3

[H + ]+10 16.6

[H + ] 2 ) We typically do not assume redox equilibrium in chemical speciation reactions – instead we investigate/calculate only one redox state (Fe III)

18

19

The calculation of equilibrium between multiple chemical species Start with a simple system 3 species: M 2+ , MA, MB 2

M + A



MA K =[MA] / [M][A] MA = K[M][A] M + 2B



MB

2

K = [MB

2

] / [M][B]

2

MB

2

= K[M][B]

2

Total M = M

2+

+ MA + MB Total M = M

2+

(1 + K[A] + K[B]

2

)

M 2+ /Total M = 1 / (1 + K[A] + K[B] 2 ) MA/Total M = K[A] / (1 + K[A] + K[B] 2 ) 20

Total M = M 2+ + MA + MB Total M = M 2+ (1 + K[A] + K[B] 2 ) 21

• From Bruland 1988 22

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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 24

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 25

Morel, Allen, Saito, Treatise on Geochemisrty 2003 26

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Mineql installation – aquatic speciation software 28

Launches in MS-DOS command line 29