Contaminant transport in soil Flashcards
What is a soil?
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
What are the main constituents of soil?
- liquid and gas phases (pore volume) (ca 50%)
- solid phase: inorganic (mineral) matter (ca 45%)
- solid phase: organic matter (ca 5%)
Whant is a sand?
- mainly quartz
- has uniform dimensions
- can be represented as spherical
- not necessarilt smooth, can have jagged surfaces
What is a silt?
- particles size intermediate between and and clay
- higher surface area then of sand
- coated with clay
- some simular physioco-chemical properties to clay
What is a clay?
- particle size <0,002 mm
- secondary mineral derived from weathering of primare minerals
- plate like a needle in shape
What type of building blocks are the layers in a clay made of?
- tetraheda
- octaheda
what is the charge of a clay mineral and what does it depend on?
- always negative charge
- permanent surface charge that is pH dependent
OBS! could have a positive charge at very low pH
What is adsorption?
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
What is a diffuse double layer?
- 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
What is ion exchange?
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
What is CEC?
cation exchange capacity
ex: if CEC=8 meq/100g –> 8 mmol K+ eller 4 mmol Ca2+
OBS! sand dose not contribute to CEC!
What is AEC?
anions exchange capacity
- AEC increases as soil pH decreases
- most contaminents are however positive charged
What is soil organic matter?
- forms from plants and animal tissues through decomposistion by microorganisms, air and water
- new compounds, humus, are formed through humification
What is humus?
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
How does the pH effect the cation adsorption?
the higher the pH is, the more negative charge –> more CEC
How is gas present in soil?
- 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
what is the aeration zone?
the zone where you have exchange between soil and atmosphere
- above the saturated zone
What does hygroscopic water mean?
the water molecules are tightly bound to the surfaces of particles by electrostatic force
- looses mobility
- increased density
- altered physical properties
What is capillary water?
remaines in the pores through surface tension forces
–> the smaler particles, the larger the capilary raise
What is free or gravitational water?
the water below the groundwater table where the soil is fully saturated with water
- the groundwater layer
How can disolved contaminants move in soil?
- 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)
What are the assumptions for the adsorption isotherm? what is not consider?
- 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
