L5 - Surface chemistry and adsorption Flashcards
What is the significance of a surface interface in mineral surface chemistry?
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)
Why does size matter in surface chemistry?
Smaller particles have higher surface area-to-volume (SA:V) ratios, meaning more atoms are at the surface, impacting physical and chemical properties
What is the mineral-water interface?
It is the exchange location between mineral surfaces and the aqueous phase, where water interacts with mineral atoms and dissolved components.
What are the two main features of a crystal surface?
Surface Topography: Physical features and textures.
Surface Structure: Atomic and molecular composition.
How does surface topography influence chemical reactivity?
Imperfections like defects and impurities create irregular surfaces, altering local reactivity.
What is surface relaxation?
A subtle rearrangement of atoms on the surface, differing energetically from bulk material
How do surface ions interact with water?
Surface ions with low coordination bond to water molecules, which can disassociate and form hydroxylated surfaces
What is surface charge, and how is it balanced?
The electrical charge of a mineral surface is balanced by adsorbed ions and ions in the diffuse layer surrounding the surface (electric double layer).
How is surface charge generated?
Isomorphic Substitution: Structural changes within the crystal lattice.
Protonation/Deprotonation: Reactive surface hydroxyl groups gain or lose protons.
Ion Adsorption: From surrounding solutions
What is permanent surface charge?
A pH-independent, usually negative charge found in 2:1 layer silicates like micas and smectites
What is variable surface charge?
A pH-dependent charge resulting from the protonation or deprotonation of hydroxyl groups on oxides and hydroxides.
What is the point of zero charge (PZC)?
The pH at which a surface has an equal amount of positive and negative charge, resulting in a net charge of zero.
What is adsorption?
The enrichment of ions or molecules on the surface of solids
What is desorption?
The release of ions or molecules from solid surfaces back into the surrounding medium
What are sorbent and sorbate?
Sorbent: The solid phase (e.g., mineral) where adsorption occurs.
Sorbate: The adsorbed species (e.g., water, ions).
How do electrostatic forces govern adsorption?
Charged ions are attracted to oppositely charged surfaces via Coulomb’s law, influencing the strength and distance of interaction
What is ion exchange?
The reversible replacement of ions adsorbed to a surface by other ions in the solution, involving either cations or anions.
What is cation exchange capacity (CEC)?
A measure of soil’s ability to hold and exchange positively charged ions, crucial for fertility and nutrient availability
What are outer-sphere and inner-sphere complexes?
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.
What is the electrical double layer (EDL)?
A two-layer structure at solid-liquid interfaces:
- Stern Layer: Fixed adsorbed ions.
- Diffuse Layer: Mobile ions in motion due to thermal energy.
What is zeta potential?
The potential difference between the diffuse layer and the bulk solution, indicating surface charge properties
What factors influence the electric potential of the EDL?
- Surface charge
- Ion concentration and valence
- Distance from the surface
What are the two main types of adsorption isotherms?
Linear Isotherm: Adsorption is proportional to solute concentration.
Non-linear Isotherms: Freundlich (heterogeneous surfaces) and Langmuir (monolayer adsorption).
What is the Freundlich adsorption isotherm?
Empirical model describing adsorption on heterogeneous surfaces with no fixed saturation point.
What is the Langmuir adsorption isotherm?
Model assuming monolayer adsorption with identical, finite sites and no interaction between adsorbed molecules
What are the limitations of Langmuir isotherms?
Assumes surface homogeneity.
Does not account for multilayer adsorption
Why are adsorption isotherms important?
They help model adsorption capacity, predict pollutant behavior, and assess nutrient bioavailability.
What role do mineral surfaces play in the environment?
They are key sites for biogeochemical cycles, nutrient mobility, pollutant transport, and soil fertility.
