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Trace Metal Biogeochemistry (Marine Bioinorganic Chemistry) 12.755 Lecture 2. Last week: Four types of trace metal profiles Geochemical properties that cause these profiles shapes: solubility, inorganic speciation, organic speciation, and redox. Began Speciation lecture with

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trace metal biogeochemistry marine bioinorganic chemistry 12 755 lecture 2

Trace Metal Biogeochemistry(Marine Bioinorganic Chemistry) 12.755 Lecture 2

Last week:

Four types of trace metal profiles

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

The Conditional Stability constant

Setting up equations for inorganic species

Setting up equations for organic species

Literature: speciation of metals in seawater overview

Introduction to Minqel+ (if time)

Brief Discussion of readings

why are we talking about complexation chemistry
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?
background aquatic chemistry of trace elements a marine water column context

FROM LAST WEEK:

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-,SO42-, 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”

definitions

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
conditional stability constants specific to conditions
Conditional stability constants: specific to “conditions”

Thermodynamic constant

based on activities

Activity corrected,

Now based on concentrations

L- can interact with other ions:

Na+ K+ Ca2+ Mg2+

But we may not know anything

about their equilbrium constants

L- will have acid base chemistry

In seawater where there are many salts: Kcond = Kapp

If acid-base chemistry dominates: Kcond = Keff

slide9

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

experimental

Protonation constants of EDTA matter

Co2+ + 2HDMG  CoHDMG2

Co2+ + EDTA4-CoEDTA2-

modeling

which brings us to how do we measure metal speciation
Which brings us to:How do we measure metal speciation?
  • Use ligand exchange reactions:

Natural Ligands:

CoL Co2+ + L2-

Our “Probe” Ligand

Co2+ + 2HDMG  CoHDMG2

Net reaction:

CoL + 2HDMG  CoHDMG2 + L2-

Core Idea: There are reactions compounds we can measure

extremely sensitively in seawater using electrochemistry

They are Electroactive like CoHDMG2

There are many electroactive ligands (synthetic):

Fe: 1N,2N; TAC,

Cu: Bzac

Zn: APDC

ligand exchange
Ligand Exchange

M + L1  ML1

M + L2  ML2

ML1 + L2  ML2 +L1

ligand exchange12
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 Ca or Mg,

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 = Ca and M* = a trace metal the concentration gradient

is many orders of magnitude!

slide14

Trace Metal Speciation Calculations:

  • Inorganic speciation Terminology:
    • M’ or METAL-“PRIME” = summation of inorganic species
  • Organic speciation
    • L for unknown organic ligand (variants L1 and L2), metal-specific (?)
    • EDTA as a “model” ligand Ethylene diaminetetraacetic acid
slide17
The calculation of equilibrium between multiple chemical speciesStart 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)

species dependent on ph
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-, CO32- are pH dependent and can be ligands. Acidity constants: Ka1=6.3, Ka2=10.3

[CO32-] = [CO32-]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)

slide19

FROM LAST WEEK:

From Morel and Hering, 1993

slide21

FROM LAST WEEK:

Background Aquatic Chemistry of Trace Elements:A marine water column context

However, there can be Non-Ideal effects:

  • The effects of other solutes on the free energy of ion(s) of interest
  • Solubility product and stability constants need to be corrected, or better, determined to/at the appropriate ionic strength.
  • The activity of the metal is: {Mn+} = [Mn+]gMn+
  • The activity coefficient, gMn+, can be estimated by the Debye-Huckel correction or the Davies expression (modified Debye-Huckel)
  • Thermodynamic databases (Martell and Smith) will provide the ionic strength experimental conditions for each constant (e.g. 0.1M)
note of caution
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
history of metal speciation in seawater brief and incomplete
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