Contaminant transport in soil

Question 1

What is a soil?

Answer 1

a natural body comprised of soilds, liquids and gases that occur on the land surface. Characterized bo on of the following:
- horizon or layers that are distinguishable from the initial material as a result of addition, losses, transfere and transformation of energy and matter

• the ability to support rooted plants in a natural environment

Question 2

What are the main constituents of soil?

Answer 2

• liquid and gas phases (pore volume) (ca 50%)
• solid phase: inorganic (mineral) matter (ca 45%)
• solid phase: organic matter (ca 5%)

Question 3

Whant is a sand?

Answer 3

• mainly quartz
• has uniform dimensions
• can be represented as spherical
• not necessarilt smooth, can have jagged surfaces

Question 4

What is a silt?

Answer 4

• particles size intermediate between and and clay
• higher surface area then of sand
• coated with clay
• some simular physioco-chemical properties to clay

Question 5

What is a clay?

Answer 5

• particle size <0,002 mm
• secondary mineral derived from weathering of primare minerals
• plate like a needle in shape

Question 6

What type of building blocks are the layers in a clay made of?

Answer 6

• tetraheda
• octaheda

Question 7

what is the charge of a clay mineral and what does it depend on?

Answer 7

• always negative charge
• permanent surface charge that is pH dependent

OBS! could have a positive charge at very low pH

Question 8

What is adsorption?

Answer 8

a process of accumulation of gas or liquid solute on the surface of a solid, forming a molecular or atomic layer

• clays, metal oxides and organic matter provides surface for adsorption because of:
  - large specifc surface area
  - large change per surface unit

Question 9

What is a diffuse double layer?

Answer 9

• no complexes between ions and charged surfaces
• neutralization of the surface charge
• ions and molekules can easily be exchanged for other ions and molecules present in the pore water

Question 10

What is ion exchange?

Answer 10

one ion is exchanged for onother ion at the surface of a solid particle

• cation exchange: Ca2+ –> K+
  CEC: the amount of cations a soil can hold
• anion axchange: H2PO4- –> NO3-
  AEC: the amount of anions a soil can hold

Question 11

What is CEC?

Answer 11

cation exchange capacity

ex: if CEC=8 meq/100g –> 8 mmol K+ eller 4 mmol Ca2+

OBS! sand dose not contribute to CEC!

Question 12

What is AEC?

Answer 12

anions exchange capacity

• AEC increases as soil pH decreases
• most contaminents are however positive charged

Question 13

What is soil organic matter?

Answer 13

• forms from plants and animal tissues through decomposistion by microorganisms, air and water
• new compounds, humus, are formed through humification

Question 14

What is humus?

Answer 14

composed of:
- fulvic acids: light yellow to yellow brown, most soluble

• humic acids: dark brown to grey black
• humin: black. NOT soluble, contains more carbon
• Carbon, hydrogen, oxygen and nitrogen

Question 15

How does the pH effect the cation adsorption?

Answer 15

the higher the pH is, the more negative charge –> more CEC

Question 16

How is gas present in soil?

Answer 16

• free phase
• dissolved in soil solution
• absorbed by solid matter

depends on:
- soil porosity (more pores –> more air)
- O2/CO2 penetration
- biochemical processes and reactions

Question 17

what is the aeration zone?

Answer 17

the zone where you have exchange between soil and atmosphere

• above the saturated zone

Question 18

What does hygroscopic water mean?

Answer 18

the water molecules are tightly bound to the surfaces of particles by electrostatic force
- looses mobility
- increased density
- altered physical properties

Question 19

What is capillary water?

Answer 19

remaines in the pores through surface tension forces
–> the smaler particles, the larger the capilary raise

Question 20

What is free or gravitational water?

Answer 20

the water below the groundwater table where the soil is fully saturated with water

• the groundwater layer

Question 21

How can disolved contaminants move in soil?

Answer 21

• advection: same conc al the time (moves lika an object?)
• dispersion and diffusion: high cons in middle, lower further out
• sorption/desorption: adsorption to solid phases (ex. Org matter, clay minerals, metal oxides)

Question 22

What are the assumptions for the adsorption isotherm? what is not consider?

Answer 22

• adsorption stops after all monolayer are filled
• alla surface sites are alike and can hold no more then one atom
• possibiity for a molecule to be sorbed onto surface

NOT consider:
- microbiological activity
- precipitation
- dissolution
- density difference
- complex and volatile material