proton exchange

CVEN 6414 – Lecture 3: 

CVEN 6414 – Lecture 3 Proton exchange on oxide minerals review reading Stumm (1992) Chapter 3 Davis and Kent (1990) Models for Adsorption-Desorption Equilibrium surface complexation models proton stoichiometry parameter estimation

Adsorption Chapters: 

Adsorption Chapters Stumm and Morgan (1996) Aquatic Chemistry Chapter 9 Benjamin (2001) Water Chemistry Chapter 10 Morel and Hering (1993) Principles and Applications of Aquatic Chemistry Chapter 8 Langmuir (1997) Aqueous Environmental Geochemistry Chapter 10

Proton Exchange: 

Proton Exchange Models (surface complexation) single site model >SOH2+ = H+ + >SOH >SOH = H+ + >SO-

Proton Exchange: 

Proton Exchange Models (surface complexation) multi-site model (e.g., MUSIC) singly-coordinated surface oxygen atoms bound to one or two protons doubly-coordinated surface oxygen atoms bound to one or two protons triply-coordinated surface oxygen atoms bound to one or two protons better represents heterogeneous nature of surface sites crystal morphology crystal faces crystal defects

Proton Exchange: 

Proton Exchange MUSIC Hiemstra et al. (1989)

Proton Exchange: 

Proton Exchange Measuring surface charge titration bulk suspensions acidimetric conductimetric microelectrophoresis zeta potential surface force AFM, “EFM” flow cytometry adsorption of fluorescent molecules

Proton Exchange: 

Proton Exchange Microelectrophoresis charged particles electric field electrophoretic mobility velocity/field strength units of m s-1/V cm-1 zeta potential related to “surface potential” (but how?) + -

Proton Exchange: 

Proton Exchange Measuring particle velocity Microscope, grid, stopwatch Laser Doppler frequency shift

Proton Exchange: 

Proton Exchange  potential is potential at the “shear plane”

Proton Exchange: 

Proton Exchange “Velocity” is electrophoretic mobility U Mobility — zeta Potential  Big particle thin double layer Small particle thick double layer

Proton Exchange: 

Proton Exchange Double layer high ionic strength p 0 d -1

Proton Exchange: 

Proton Exchange Double layer low ionic strength p 0 d -1

Proton Exchange: 

Proton Exchange Microelectro-phoresis pHpzc or pHiep surface charge (?) via surface potential approximation

Proton Exchange: 

Proton Exchange What is the pH of a goethite suspension in pure water (closed system)? 1 g L-1 goethite (-FeOOH) 50 m2 g-1 surface area 0.2 C m-2 site density (maximum surface charge) TOT FeOH = 0.1 mM pKa values of 7.5 and 10.2 proton condition use FeOH as the reference species

Proton Exchange: 

Proton Exchange Proton condition TOTH equation [H+] – [OH-] + [FeOH2+] - [FeO-] = 0 [H+] + [FeOH2+] = [OH-] + [FeO-]

Proton Exchange: 

Proton Exchange [H+] + [FeOH2+] = [OH-] + [FeO-] log C-pH

Proton Exchange: 

Proton Exchange Accounting for electrostatic model adsorption has two components chemical – intrinsic electrostatic – depends on surface potential new way to express equilibrium constant

Proton Exchange: 

Proton Exchange S—O- S—OH S—O- S—OH S—OH H+ H+ electrostatic: from solution to surface chemical: binding at surface 0

Proton Exchange: 

Proton Exchange S—OH S—OH S—OH S—OH S—OH H+ H+ 0

Proton Exchange: 

Proton Exchange Surface potential from surface charge Which electrostatic model? constant capacitance double layer triple layer others…

Proton Exchange: 

Proton Exchange Incorporating electrostatics tableau method (Morel and Hering, 1993) special accounting for surface charge manual calculations (!) geochemical equilibrium code Visual MINTEQ MINEQL+ etc.

Proton Exchange: 

Proton Exchange Goethite suspension in pure water 1 g L-1 goethite (-FeOOH) 50 m2 g-1 surface area 3.84 sites nm-2 site density [FeOH]tot = 0.32 mM 1 mM sodium chloride solution pKa1int = 7.5 pKa2int = 10.2 Nuñez et al. (2000)

Proton Exchange: 

Proton Exchange Visual MINTEQ use FeOH2+, FeOH, FeO- as species use double layer, 2 pKa model calculate sweep from pH 0 to 14, 0.25 pH intervals

Proton Exchange: 

Proton Exchange Goethite suspension in pure water (MINTEQ)

Proton Exchange: 

Proton Exchange Goethite suspension in pure water (MINTEQ)

Problem Session: 

Problem Session Problem Set 1 Examine effect of ionic strength on proton exchange for goethite I = 10-5 M I = 10-3 M I = 10-1 M

Paper Presentations: 

Paper Presentations Audrey: Gaboriaud and Ehrhardt (2003) Effects of different crystal faces on the surface charge of colloidal goethite (-FeOOH) particles: An experimental and modeling study. Geochimica et Cosmochimica Acta 67, 967-983. Chase: Kosmulski (2002) The significance of the difference in the point of zero charge between rutile and anatase. Advances in Colloid and Interface Science 99, 255-264.

Next Class Meeting: 

Next Class Meeting Papers for student presentation Rönngrenn et al. (1991) Sokolov et al. (2001) Review reading Davis and Kent (1990) Stumm (1992)