L5 - Surface chemistry and adsorption

Question 1

What is the significance of a surface interface in mineral surface chemistry?

Answer 1

A surface is the outer shell of an object in contact with its environment, while an interface is the boundary separating two phases of matter (solid, liquid, or gas)

Question 2

Why does size matter in surface chemistry?

Answer 2

Smaller particles have higher surface area-to-volume (SA:V) ratios, meaning more atoms are at the surface, impacting physical and chemical properties

Question 3

What is the mineral-water interface?

Answer 3

It is the exchange location between mineral surfaces and the aqueous phase, where water interacts with mineral atoms and dissolved components.

Question 4

What are the two main features of a crystal surface?

Answer 4

Surface Topography: Physical features and textures.

Surface Structure: Atomic and molecular composition.

Question 5

How does surface topography influence chemical reactivity?

Answer 5

Imperfections like defects and impurities create irregular surfaces, altering local reactivity.

Question 6

What is surface relaxation?

Answer 6

A subtle rearrangement of atoms on the surface, differing energetically from bulk material

Question 7

How do surface ions interact with water?

Answer 7

Surface ions with low coordination bond to water molecules, which can disassociate and form hydroxylated surfaces

Question 8

What is surface charge, and how is it balanced?

Answer 8

The electrical charge of a mineral surface is balanced by adsorbed ions and ions in the diffuse layer surrounding the surface (electric double layer).

Question 9

How is surface charge generated?

Answer 9

Isomorphic Substitution: Structural changes within the crystal lattice.

Protonation/Deprotonation: Reactive surface hydroxyl groups gain or lose protons.

Ion Adsorption: From surrounding solutions

Question 10

What is permanent surface charge?

Answer 10

A pH-independent, usually negative charge found in 2:1 layer silicates like micas and smectites

Question 11

What is variable surface charge?

Answer 11

A pH-dependent charge resulting from the protonation or deprotonation of hydroxyl groups on oxides and hydroxides.

Question 12

What is the point of zero charge (PZC)?

Answer 12

The pH at which a surface has an equal amount of positive and negative charge, resulting in a net charge of zero.

Question 13

What is adsorption?

Answer 13

The enrichment of ions or molecules on the surface of solids

Question 14

What is desorption?

Answer 14

The release of ions or molecules from solid surfaces back into the surrounding medium

Question 15

What are sorbent and sorbate?

Answer 15

Sorbent: The solid phase (e.g., mineral) where adsorption occurs.

Sorbate: The adsorbed species (e.g., water, ions).

Question 16

How do electrostatic forces govern adsorption?

Answer 16

Charged ions are attracted to oppositely charged surfaces via Coulomb’s law, influencing the strength and distance of interaction

Question 17

What is ion exchange?

Answer 17

The reversible replacement of ions adsorbed to a surface by other ions in the solution, involving either cations or anions.

Question 18

What is cation exchange capacity (CEC)?

Answer 18

A measure of soil’s ability to hold and exchange positively charged ions, crucial for fertility and nutrient availability

Question 19

What are outer-sphere and inner-sphere complexes?

Answer 19

Outer-sphere: Ions are attached via electrostatic forces with hydration shells intact.

Inner-sphere: Ions form direct chemical bonds with surface, often involving ligand exchange.

Question 20

What is the electrical double layer (EDL)?

Answer 20

A two-layer structure at solid-liquid interfaces:
- Stern Layer: Fixed adsorbed ions.
- Diffuse Layer: Mobile ions in motion due to thermal energy.

Question 21

What is zeta potential?

Answer 21

The potential difference between the diffuse layer and the bulk solution, indicating surface charge properties

Question 22

What factors influence the electric potential of the EDL?

Answer 22

• Surface charge
• Ion concentration and valence
• Distance from the surface

Question 23

What are the two main types of adsorption isotherms?

Answer 23

Linear Isotherm: Adsorption is proportional to solute concentration.

Non-linear Isotherms: Freundlich (heterogeneous surfaces) and Langmuir (monolayer adsorption).

Question 24

What is the Freundlich adsorption isotherm?

Answer 24

Empirical model describing adsorption on heterogeneous surfaces with no fixed saturation point.

Question 25

What is the Langmuir adsorption isotherm?

Answer 25

Model assuming monolayer adsorption with identical, finite sites and no interaction between adsorbed molecules

Question 26

What are the limitations of Langmuir isotherms?

Answer 26

Assumes surface homogeneity.
Does not account for multilayer adsorption

Question 27

Why are adsorption isotherms important?

Answer 27

They help model adsorption capacity, predict pollutant behavior, and assess nutrient bioavailability.

Question 28

What role do mineral surfaces play in the environment?

Answer 28

They are key sites for biogeochemical cycles, nutrient mobility, pollutant transport, and soil fertility.