12-1 Chemical Speciation Use of constants to model chemical form §Thermodynamic and kinetic §Determine property of radioelement based on speciation àChemical species in system Review §Equilibrium constants §Activity §Use of constants in equation

12-2 For a reaction §aA + bB cC + dD At equilibrium ratio of product to reactants is a constant §Constant can change with conditions àNot particularly constant §By convention, constants are expressed as products over reactants Conditions under which the constant is measured should be listed §Temperature, ionic strength Reaction Constants

12-3 Strictly speaking, activities, not concentrations should be used Activities normalize concentration to amount of anions and cations in solution At low concentration, activities are assumed to be 1 constant can be evaluated at a number of ionic strengths and overall activities fit to equations Debye-Hückel (Physik Z., 24, 185 (1923)) Z A = charge of species A µ = molal ionic strength R A = hydrated ionic radius in Å (from 3 to 11) First estimation of activity Complete picture: Activities

12-4 Debye-Hückel term can be written as: Specific ion interaction theory §Uses and extends Debye-Hückel àlong range Debye-Hückel àShort range ion interaction term  ij = specific ion interaction term Pitzer  Binary (3) and Ternary (2) interaction parameters Activities

12-5 Ca 2+ K+K+ Al 3+ Fe(CN) 6 4- Experimental Data shows change in stability constant with ionic strength Ion Specific Interaction Theory Cm-Humate at pH 6

12-6 Constants Constants can be listed by different names §Equilibrium constants (K) àReactions involving bond breaking *2 HL H 2 + 2L §Stability constants (ß), Formation constants (K) àMetal-ligand complexation *Pu 4+ + CO 3 2- PuCO 3 2+ *Ligand is written in deprotonated form §Conditional Constants àAn experimental condition is written into equation *Pu 4+ + H 2 CO 3 PuCO H + ËConstant can vary with concentration, pH Must look at equation!

12-7 Using Equilibrium Constants Constants and balanced equation can be used to evaluate concentrations at equilibrium §2 HL H 2 + 2L, §K=4E-15 §With one mole of HL initially, what are the concentration of the species at equilibrium? §write species in terms of one unknown àStart with species of lowest concentration à[H 2 ] =x, [Y]=2x, [HY]=1-2x §Since K is small, x must be small, 1-2x ≈ 1 §K=4E-15=4x 3, àx =1E-5, 2x=2E-5

12-8 Realistic Case: Uranium in Aquifer Species to consider include §free metal ion: UO 2 2+ §hydroxides: (UO 2 ) x (OH) y §carbonates: UO 2 CO 3 §humates: UO 2 HA(II), UO 2 OHHA(I) Need to get stability constants for all species §UO CO 3 2- UO 2 CO 3 Know or find conditions §Total uranium, total carbonate, pH, total humic concentration If total U concentration is low binary or tertiary species can be excluded Specieslogß UO 2 OH UO 2 (OH) UO 2 (OH) UO 2 (OH) (UO 2 ) 2 OH (UO 2 ) 2 (OH) UO 2 CO UO 2 (CO 3 ) UO 2 (CO 3 ) UO 2 HA(II)6.16 UO 2 (OH)HA(I)14.7±0.5

12-9 Equations Write concentrations in terms of species §[UO 2 ] tot = UO 2free +U-carb+U-hydroxide+U-humate §[CO 3 2- ] free =f(pH) §[OH - ] = f(pH) §[HA] tot = UO 2 HA + UO 2 OHHA+ HA free Write the species in terms of metal, ligands, and constants §[(UO 2 ) x A a B b ] = 10 -(xpUO2+apA+bpB-log(UO2)xAaBb) §pX = -log[X] free à[(UO 2 ) 2 (OH) 2 2+ ]=10 -(2pUO2+2pOH-22.0) Set up equations and solve for known terms Can use excel, incorporate solver CHESS example

12-10 U speciation with different CO 2 partial pressure 0% CO 2 1% CO 2 10% CO 2

12-11 Comparison of measured and calculated uranyl organic colloid

12-12 Energy terms Constants can be used to evaluate reaction thermodynamics From Nernst equation §∆G=-RTlnK ∆G=∆H-T∆S §-RTlnK = ∆H-T∆S §RlnK= - ∆H/T + ∆S àPlot RlnK vs 1/T