Marine Bioinorganic Chemistry Lecture 2 Last week: 1.Four types of trace metal profiles 2.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 continued 1.The Conditional Stability constant 2.Setting up equations for inorganic species 3.Setting up equations for organic species 4.Literature: speciation of metals in seawater overview 5.Introduction to Mineql+ 6.Brief Discussion of readings 1

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The environmental importance of 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

Plasma – another environment – The battle for metals between pathogen and host Warfare: Siderophores - iron binding molecules Counterwarfare: Proteins that engulf siderophores Counter-Counter-Warfare

Background Aquatic Chemistry of Trace Elements: A marine water column context Solubility Products: Example for Fe(OH) 3(s) K sp = [Fe][OH] 3 = 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” FROM LAST WEEK: 5

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 = Kapp If acid-base chemistry dominates: Kcond = Keff 6 M 2+ + L -  ML + K = {ML + } / {M 2+ }{L - } C K = [ML + ] / [M 2+ ][L - ] (concentration constant) K app = [  ML + ] / [M 2+ ][  L - ] (apparent constant) K app = [ML + ] / [M 2+ ][  H x L - ] (effective constant)

We’ve already talked about the effects of salts Acid base chemistry also matters for complexation chemistry in seawater: Don’t know much about the acid-base chemistry of natural ligands Protonation constants of EDTA matter Co HDMG -  CoHDMG 2 Co 2+ + EDTA 4-  CoEDTA 2- modeling experimental 7 H 2 L  H + + HL - HL -  H + + L 2-

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Which brings us to: How do we measure metal speciation? Use ligand exchange reactions: Natural Ligands: CoL  Co 2+ + L 2- Our “Probe” Ligand Co HDMG  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

From Bruland

Ligand Exchange M + L 1  ML 1 M + L 2  ML 2 ML 1 + L 2  ML 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 2+ or Mg 2+, it will clog up the exchange reactions Disjunctive Adjunctive ML  M + LM* + ML  M*LM M* + L  M*LM*LM  M*L + M If M = Ca 2+ and M* = a trace metal the concentration gradient is many orders of magnitude! 11

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 Trace Metal Speciation Calculations 12

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14 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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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

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Species dependent on pH: [CoOH - ] / [Co 2+ ][OH - ] = [H + ][OH - ] = At pH 8.0: [OH - ] = / = [CoOH - ] = [Co 2+ ]10 -6 = [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 / ( [H + ] [H + ] 2 ) We typically do not assume redox equilibrium in chemical speciation reactions – instead we investigate/calculate only one redox state (Fe III) 18

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+ + K[M][A] + K[M][B] 2 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 ) 19

20 Whiteboard Equations Total Co = Co’ + CoEDTA + CoL

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

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 22

Early 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, 1992 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.,

Morel, Allen, Saito, Treatise on Geochemisrty

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Mineql installation – aquatic speciation software 26

Launches in MS-DOS command line 27

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pH can be set as a fixed solid, or calculated 31