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Marine Bioinorganic Chemistry 12.755 Lecture 2

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

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Marine Bioinorganic Chemistry 12.755 Lecture 2

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  1. 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 Mineql+ Brief Discussion of readings

  2. 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?

  3. 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”

  4. 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

  5. Conditional stability constants: specific to “conditions” M2+ + L- ML+ K = {ML+} / {M2+}{L-} CK = [ML+] / [M2+][L-] (concentration constant) Kapp= [SML+] / [M2+][SL-] (apparent constant) Kapp= [ML+] / [M2+][SHxL-] (effective constant) 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 = Kapp If acid-base chemistry dominates: Kcond = Keff

  6. 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 H2L  H+ + HL- HL- H+ + L2- Protonation constants of EDTA matter Co2+ + 2HDMG- CoHDMG2 Co2+ + EDTA4-CoEDTA2- modeling

  7. 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 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: Bzac Zn: APDC

  8. Ligand Exchange M + L1  ML1 M + L2  ML2 ML1 + L2  ML2 +L1

  9. 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!

  10. Trace Metal Speciation Calculations • 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

  11. 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.

  12. 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

  13. 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)

  14. 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)

  15. Total M = M2++ MA + MBTotal M = M2+(1 + K[A] + K[B]2)

  16. From Bruland 1988

  17. 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

  18. 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

  19. Morel, Allen, Saito, Treatise on Geochemisrty 2003

  20. Mineql installation – aquatic speciation software

  21. Launches in MS-DOS command line

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