Mineral & Organic Matter Surface Chemistry Controlling A dsorption

ENVIRONMENTAL GEOCHEMISTRY AT TEXAS A&M UNIVERSITY

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Mineral & Organic Matter Surface Chemistry

Controlling Adsorption

Bruce HerbertGeology & Geophysics



Adsorption Results in Retardation

■ It is commonly observed that contaminants and other reactive solutes migrate in porous media at slower rates than water

■ The retarded solute travels slower, because it partitions to the solid phase

■ Some of the time, the particle is attached to a solid, and does not move.

■ Idea is also applicable to chromatography and ion exchange columns



Terminology

System

rock

or

column

solids / grains

and

pores

Solution

completely filled(saturated)

partially filled(unsaturated)

consists of



Rock is like a sponge - a framework of grains around empty pores that can be filled with water

■ Porosity [n ] - total space between grains (fraction .1 to .5)

■ Grain density - density of mineral grains (g/cc 2.5 to 3.5)

■ Dry Bulk density - average density of rock (g/cc 1.2 to 1.8)

■ Water content [w ] - fraction of rock filled with water (fraction from 0 to porosity)

■ Water Saturation - fraction of void space filled with water (fraction 0 to 1)



BreakthroughCurves



Solid-Water Interface■ Sorption is the net accumulation of matter at an interface between a solid and the

aqueous phase. ■ Sorption is a generic term that does not imply any molecular-scale mechanisms or

models of the solid-water interface

To understand the chemistry of adsorption reactions we must consider the chemistry of both the solute and the chemistry of the surface. The chemistry of these components determines the type of interaction that

forms between a solute and a surface



Chemistry of Mineral Surfaces■ Important solid phases: those with high surface area

■ SA of Sand: assume sphere of 500 m, then SA = 1.2 x 104 m2/m3

■ SA of Clay: assume sphere of 2 m, then SA = 3 x 106 m2/m3

■ Important solid surfaces: clays, oxides, carbonates, sulfides, and organic matter■ crystalline: repeating structures of > 3 nm diameter■ amorphous: repeating crystalline structures of < 3 nm diameter

■ Structural regularity is important because we can predict the surface structure of minerals that are regular■ Lab-based studies typically use well defined minerals■ Field conditions: minerals may not be as well structured and characterized



Properties of Surfaces: Specific Surface Area

■ Adsorption is a function of surface area. Specific surface area is measured by operational techniques

■ where Sm is the specific surface area, S is the surface area, and m is the mass

(m2/kg)

where Sm is the specific surface area (m2 kg-1), Am is the surface area of the probe molecule (nm2), Na is Avogadro’s number (mole-1) and 10-18 is conversion factor between nm2 and m2



Specific Surface Area

■ If adsorbate is a gas (N2) and forms multilayers then Xm is calculated with Brunauer - Emmett - Teller Equation (BET)

■ Because of its size and weak interactions, N2 only adsorbs to external surfaces. Water, on the other hand, absorbs to both interior and exterior surfaces

■ Surface Area measurements are somewhat ambiguous■ Sm depends on the probe molecule used■ Interpretation of data depends on concept of mineral structure, therefore Sm is

model dependent■ Specific surface area is most useful if minerals are compared using similar

methodology



Specific Surface Areas



Surface Functional Groups

■ The surface reactivity of the solid-phase originates from the chemical behavior of surface functional groups■ Surface functional groups are molecular units bound to the

mineral structure which can react with water and aqueous solutes■ SFGs are not inorganic and organic species

■ SFGs are different from ordinary functional groups because■ SFGs can’t go to infinite dilution■ Reactivity of group depends on the state of its neighbors

■ SFGs reacts with aqueous-phase solutes to form surface complexes■ Inner-sphere complexes: no solvent molecules between the

adsorbed specie and the SFG■ Outer-sphere complexes: solvent exists between the SFG and the

adsorbed specie



Common SFGs: Hydroxides

■ Hydroxide■ found on metal oxyhydroxides, phyllosilicates, and amorphous silica

(allophane)■ The reactivity of the hydroxide group depends on the metal involved and

the structural orientation of the hydroxides

Silanol

Aluminol

Iron



Common SFGs: Hydroxides

GibbsiteGoethite



Common SFGs: Hydroxides & Acids

Organic Matter



Common SFGs: Siloxane Cavity

■ Distorted hexagonal cavity of O atoms on surface of tetrahedral plane. The cavity has a diameter of approximately 0.26 nm

Sposito G et al. PNAS 1999;96:3358-3364



Common SFGs: Siloxane Cavity

■ If there is no isomorphic cation substitution within the crystal, then the cavity functions as a very mild e- donor which can complex neutral, dipolar molecules such as water. These complexes are not very stable

■ If the isomorphic substitution is Fe2+ or Mg2+ for Al3+ in the octahedral layer, then the excess negative charge is spread over a relatively large area of the crystal surface. The siloxane cavity forms fairly strong complexes with cations

■ If the isomorphic substitution is Al3+ for Si4+ in the tetrahedral layer, then the excess negative charge is localized over a relatively small area of the crystal surface. The siloxane cavity forms very strong complexes with cations

The reactivity of the siloxane cavity depends on the nature of the charge distribution within the crystal structure

Visualization of K+ bound in an inner-sphere surface complex in the interlayer region of Wyoming montmorillonite, based on MC simulation.




Common SFGs: Silicates & Coatings



Cartoon of the three types of small cation adsorption by a 2:1 layer type clay mineral.




Siloxane Cavity Complexation: Spectroscopy

■ Infrared spectroscopy of complexes with H-bonding:■ Complex C2H5NH3+ and H2O to siloxane cavity in different phyllosilicate minerals■ Stretching frequency of NH, OH, and OD increases as the strength of the bond

increases

■ where E = energy, h= Plank’s constant, and = IR light frequency■ The higher the frequency, the stronger the H-bonds. The order of decreasing bond

energy was determined by the site of substitution in the mineral structure■ tetrahedral > octahedral > no isomorphic substitution



Common SFGs: Carbonates

■ These minerals have dynamic interfaces due to dissolution-precipitation reactions

■ Interaction with trace metals: (i.e. Cd(II) or Zn(II))■ First reaction: very fast reaction (1 day). The metal solute sorbs to the hydrated

mineral surface. There is an exchange of the Mn+ for Ca2+

■ Second, slower reaction: a mixed solid precipitate forms (M,Ca)CO3 during recrystallization. The kinetics of this reaction is much slower for metal ions of different ionic radius or slow dehydration



Common SFGs: Sulfides■ These mineral surfaces are important in anoxic environments. There is a

small amount of data concerning these minerals. The reactivity of the thiol or sulfhydryl group is somewhat similar to hydroxides:

■ The differences between the two SFGs:■ The hydroxyl group has a larger electronegativity difference between the

O and the H. This makes the group a harder functional group with higher acidity and easier to protonate

■ The hydroxyl group has a larger electronegativity difference between the O and the H. This makes the group a harder functional group with higher acidity and easier to protonate

■ The thiol group has a smaller electronegativity difference between the S and the H. This makes the group a softer functional group with lower acidity and is less likely to protonate



Surface Charge Density

■ Adsorption of most ions is dependent on surface charge. We can partition the total surface change of a mineral surface into several components, all of which sum to total charge

■ If we include the balancing charge due to adsorbed ions then the total charge balance of the mineral-water interface should equal 0

so+ sH + sis + sos + sD = O

so+ sH = interfacial charge that arises from unsatisfied valencies in the molecular structure of the mineral

so+ sH + sis + sos = interfacial charge of the colloid and complexed ions



Surface Charge Densities

■ Representative Values■ so = permanent structure charge (isomorphic substitution)■ sH = net proton charge due to adsorbed H+, OH-



ACQUISITION OF SURFACE CHARGE

In general, solutes interact with mineral surfaces because the latter have acquired electrical charge.

Two ways to acquire charge: Substitution for a cation in a mineral by one of lesser positive

charge. This type of charge is considered to be permanent. Reactions involving functional groups on the mineral surface

and ions in solution (surface complexation). This type of charge is variable and pH dependent.



ACQUISITION OF SURFACE CHARGE Only 2:1 clay minerals (e.g., smectites, vermiculite) acquire

significant fixed charge through ionic substitutions. Substitution of divalent cations for trivalent cations in octahedral

sites, and of trivalent cations for tetravalent cations in tetrahedral sites, results in a deficiency of positive charge, or a net negative fixed charge on the surface.

This negative charge can be balanced by the sorption of cations from solution.



ACQUISITION OF SURFACE CHARGE

Inner- and outer-sphere complexes help balance the excess negative charge.

An additional type of adsorption also helps balance the charge (diffuse double layer). Involves presence of diffuse layer of cations (counter ions) near

mineral surface. Counter ions are not bonded to surface. Counter ions are more abundant than diffuse anions (co-ions) . Net positive charge balances remaining negative charge.

(a) Counter ions (cations) in the diffuse layer tend to be concentrated close to the negatively charged ions on the surface.

(b) sP represents the net surface charge and sD represents the diffuse ion charge.

(c) Distribution of electrical potential . (d) Concentrations of positive and negative

charges with distance from the surface. (e) Distribution of charge-density with distance.

The Diffuse Double Layer



Surface Charge of Minerals



Zero Points of Charge




■ All of the surface charges components, except so, are a function of pH. These observations give rise to the concepts of points of zero charge.

■ PZC are the pH values where specific conditions are met, typically where one of the s's goes to zero.

■ PZC : point of zero charge sD = O■Determine pH at which particles don't move in an electric field. This

means the particles have zero electrophorectic mobility and will flocculate

■ PZNPC: point of zero net proton charge: sH = O■sH increases as pH increases for all minerals, all ionic strengths




■ PZNC: point of zero net charge s = sis + sos + sD

■ pH at which net absorbed ion charge, not counting H+ and OH-, vanishes. This means the roles of absorbed cations equal the moles of absorbed anions.

■ PZNC is measured through the adsorption of anions and cations at constant pH and I. PZNC is typically measured with specific index ions including Na, Cl, Li, CIO4- and NO3

-. The PZNC will vary according to the choice of index ion.■ PZSE: Point of zero salt effect

■ Measure sH by titration at several different I. The resulting sH - pH curves will usually intersect. This is the PZSE.

■ PZSE is not a pH at which surface charge vanishes therefore, it has limited use

Mineral & Organic Matter Surface Chemistry Controlling A dsorption

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