Surface complex formation

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

Integrating equations (2.37) and (2.39) under assumption that in case of direct reaction of surface complex formation (Me C ) the reaction of interaction of oxygen with surface metal atoms is the limiting stage rather than formation of physadsorbed oxygen (i.e. assuming that [02( )J = const and it does not change in time) we arrive to the respective expression for kinetics of direct and inverse reactions ... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

With a chapter on particle-particle interaction (coagulation) the characteristics of particles and colloids as chemical reactants are discussed. Since charge, and in turn the surface potential of the colloids is important in coagulation, it is illustrated how in simple cases the modelling of surface complex formation permits the calculation of surface charge and potential. The role of particle-particle interaction in natural water and soil systems and in water technology (coagulation, filtration, flotation) is exemplified. 

The central ion of a mineral surface (in this case we take for example the surface of a Fe(lll) oxide and S-OH corresponds to =Fe-OH) acts as Lewis acid and exchanges its stuctural OH against other ligands (ligand exchange). Table 2.1 lists the most important adsorption (= surface complex formation) equilibria. The following criteria are characteristic for all surface complexation models (Dzombak and Morel, 1990.)... 

Table 2.1 Adsorption (Surface Complex Formation Equilibria)...

Surface Complex Formation with Metal Ions... 

Surface complex formation of cations by hydrous oxides involves the coordination of the. metal ions with the oxygen donor atoms and the release of protons from the surface, e.g.,... 

Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (ion pair) or be in the diffuse swarm of the electric double layer. (From Sposito, 1989)... 

As we shall see (Chapter 4), the kinetics of surface complex formation is often related to the rate of H20 loss from the aquo cation. This is another (indirect) evidence for inner-sphere complex formation. 

Ligand Exchange Surface Complex Formation of Anions and Weak Acids... 

The main mechanism of ligand adsorption is ligand exchange the surface hydroxyl is exchanged by another ligand. This surface complex formation is also competitive OH ions and other ligands compete for the Lewis acid of the central ion of the hydrous oxide (e.g., the Al(iii) or the Fe(III) in aluminum or ferric (hydr)oxides). The extent of surface complex formation (adsorption) is, as with metal ions, strongly... 

Surface complex formation with ligands (anions) as a function of pH... 

Affinities of Cations and Anions for Surface Complex Formation with Oxides and Silicates... 

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

The Kurbatov plot is a convenient tool to display in a simple way adsorption (surface complex formation) data. But care must be exercised in the interpretation of the data, because n varies with pH and may vary with the adsorption coverage. For an exact analysis of the proton release stoichiometry, see Hohl and Stumm (1976) or Honeyman and Leckie (1986). 

In the surface complex formation model the amount of surface charge that can be developed on an oxide surface is restricted by the number of surface sites. (This limitation is inherently not a part of the Gouy-Chapman theory.)... 

Stipp and Hochella (1991), on the basis X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED), have shown that CaC03 exposed to water, contains at the surface =C03H and =CaOH functional groups and van Capellen (1991) has proposed a surface complex formation model for carbonates. Similarly, Ronngren et al. (1991) have proposed =SH and =ZnOH functional groups for the surface of hydrous ZnS s). 

Correcting Surface Complex Formation Constants for Surface Charge... 

Most generally for a surface complex formation reaction ... 

Furthermore the surface complex formation with Zn(II) has been determined from adsorption studies in 10 3 M suspensions of hydrous ferric oxide with dilute (10 7 M) Zn(II) solution. It can be described by the reaction... 

We resume the problem discussed in Example 2.2 and solve the same problem, but now we correct for electrostatic effects. Sumarizing the problem Calculate the pH dependence of the binding of a) a metal ion Me2+, and b) of a ligand A to a hydrous oxide, SOH, and compare the effect of a charged surface at an ionic strength I = 0.1. A specific surface area of 10 g m 2 10 4 mol surface sites per gram ( 6 sites nnrr2) concentration used 1 g e-1 (10 4 mol surface sites per liter solution). As before (Example 2.2) the surface complex formation constants are log Kj = -1 and log K = 5, respectively. 

Anion Binding. This discussion illustrates how valuable information on enthalpy changes of surface reactions (either from temperature dependence or from direct calorimetric measurements) are. Zeltner et al. (1986) have studied calorimetrically the surface complex formation of phosphate and salicylate on goethite. They show that these reactions are exothermic (at pH = 4) with AHadS values at low coverage ( 10 %) of ca. -24 kJ mol 1, they argue tentatively that these values indicate biden-tate surface complex formation. They also show that -AH decreases with increasing surface coverage. 

The conditions for the validity of a Langmuir type adsorption equilibrium are i) thermal equilibrium up to the formation of a monolayer, 0 = 1 ii) the energy of adsorption is independent of 0, (i.e., equal activity of all surface sites). There is no difference between a surface complex formation constant and a Langmuir adsorption... 

The Langmuir equation is derived here from application of the mass law, in a similar way as the surface complex formation equilibria were derived in Chapter 2. In principle at a constant pH there is no difference between a Langmuir constant and a surface complex formation constant. 

As every surface complex formation equilibrium constant can be converted into an equivalent Langmuir adsorption constant (Stumm et al., 1970), every FFG equation reflects the surface complex formation constant corrected with the interaction coeffi-... 

Example 4.1 Surface Complex Formation, Langmuir Equation and Frumkin Equation... 

We have argued that (inner-sphere) surface complex formation of a metal ion to the oxygen donor atoms of the functional groups of a hydrous oxide is in principle similar to complex formation in homogeneous solution, and we have used the same type of equilibrium constants. How far can we apply similar concepts in kinetics ... 

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