I. The History, How, and Who Cares of Surface Complexation Models, II. Application of SCM’s to the MMR

GES 166/266 Discussion 5 Feb 2004

Objectives of SCM

• To determine the chemical and electrostatic forces involved in ion retention • To provide a framework that allows such processes to be modeled • To improve problem solving – Sewage and mining discharges – Assessment of radioactive waste repositories

diffuse layer surface Stern layer

Development of SCM’s

1.Used mass law equations to describe reactions at individual surface sites (Kurbatov et al. 1951, Stanton and Maatman 1963, Dugger et al. 1964) 2. Origin and importance of surface charge on oxides (Parks and DeBruyn 1962) 3. Schindler’s group: Constant Capacitance Model and Stumm’s group: Diffuse Layer Model (~1970) 4. Leckie’s group: Triple Layer Model (Davis et al. 1978).

The 2-layer/Stern and 3-layer models

• Sorbates considered part of the solid surface (1 st specific layer) - • Electrostatic charge balanced by adjacent diffuse layer of ions in solution (2 nd layer) – nonspecific – Satisfies residual charge • Bound water layer added (T-LM) – diffuse layer is farther from solid • Surface reactions include H + exchange, cation binding, and anion binding – Need to know surface potentials!

σ p C 1 C 2 To get potential, must relate it to charge: In the Stern layer: σ = C ψ σ = charge C = Capacitance (prop. factor) ψ = potential In the diffuse layer: σ B = σ D = 2.31 n o 1/2 ψd σ B = charge on outer sphere plane σ D = charge on diffuse plane n o = concentration of counterions d = distance of interfacial region distance

The Fundamental Concepts and Assumptions of SCM’s

• Protons are the dominant potentially determining ions • All surfaces have a single site type • Each site can undergo two protonation rxns • Charges are always expressed as integers • Strict distinction between inner- and outer sphere complexes

Reactions responsible for surface charge

(1) SOH 2 + K a1 <==> SOH o + H + = [SOH o ]{H + } / [SOH 2 + ] (2) SOH o <==> SO + H + K a2 = [SO ] {H + } / [SOH o ] where {} = mol / Kg, and [] = mol / L

For a titration… (C A - [H + ]) - (C B - [OH]) = {(+) surface sites} - {(-) surface sites} = W where, W is the charge (mol ) C is [ ] of acid or base Q = charge (mol / Kg) = W / a (amount of sorbent) σ (C / m 2 ) = Q F / A where, F is Faraday A is surface area (m 2 /kg)

The zero point of charge (ZPC) is: {SO } = {SOH 2 + }

K a values must be corrected for electrostatic forces!

Following ∆G ads = ∆G coul + ∆G chem , K a = K intrinsic + (electrostatic) K a = K intrinsic exp (∆ZFψ / RT) Electrostatic or coulombic correction factor The electrostatic contribution – dependence on potential – that allows SCM’s to describe adsorption as a function of pH

The actual adsorption rxn becomes… SOH o + Me 2+ = SOMe + + H + K 1c = [SOMe + ] {H + } / [SOH o ] {Me 2+ } K 1c = K 1int exp (∆ZFψ / RT)

Part II. SCM’s at the MMR

Chemical Characteristics

• Groundwater around sewage plume has highest [DO], pH values of 5.3-5.8, low dissolved salts • High NO 3 with depth under beds, [DO] decreases • Biodegradation of OM and solute transport away from disposal beds • decreases [DO] through plume, decreases [NO 3 ], accumulates dissolved Fe(II) ---- impact pH!

• Leading edge of Zn contamination = 400 m from source; low Zn until 195 m

Assumptions

• Simple groundwater flow model unsaturated saturated pond

Assumptions (cont’d)

• Mechanical dispersion >>> molecular dispersion • Zn adsorption onto sediments is the 1° influence on Zn transport • Local equilibrium wrt to adsorption rxns achieved faster than timescale of transport

Mass action equations

• The one-site SCM Zn 2+ + >SOH = >SOZn + + H + s Q SOZn = S >SOZn γ H C H / C Zn S >SOH s Q SOZn = mass action constant S x = density of adsorption sites γ H = activity coef of H + C = conc • The two-site SCM Zn 2+ + >S s OH = >S s OZn + + H + Zn 2+ + >S w OH = >S w OZn + + H +

What happens when you cut off the source?

• Acidification • Transport: head and tail behavior • [Zn] in the core Source: Kent et al. 2000

4. Used empirical parameters associated with adsorption isotherms – Linear isotherms: c s C s = K d *c = concentration in solid phase K d = equilibrium distribution coefficient c = concentration in fluid phase – Langmuir or Freundlich isotherms: Θ i = Ba i /(1+Ba i ) Θ i = fraction of adsorption sites occupied by the adsorbate B = bonding constant c s a i = K f *c n K f = activity of the adsorbate = Freundlich adsorption constant n = Freundlich exponent

• Brusseau and Srivastava (1999) used different Kd values in different geochemical zones to compensate for variable chemistry affecting Li adsorption during transport • Kirkner et al. (1985) and Viswanathan et al. (1998) account for variable chemistry by calculating changes in aqueous speciation of adsorbing solutes with changing chemical conditions