CHEM340 Unit 2 - Sorption Processes: Ion Exchange, Specific Adsorption, Occlusion, Precipitation

sorption

process in which substances interact with soil solids

adsorption

process in which ions, molecules or gases from the soil solution adhere to the surface of soil colloids, primarily clay minerals, organic matter and metal oxides

total concentration of an element in soils is partitioned into

• element in solution (C)

• element in soil matrix (S)

ion exchange

movement of ions to and from the charged soil surface

rate of ion exchange depends on (2)

1. type and quantity of mineral and soil components

2. charge and radius of ion

cation exchange

rapid, reversible, physio-chemical process in which hydrated cation are swapped with cations attached to the surfaced of negatively charged soil colloids

outer sphere binding

the mechanism by which cation exchange occurs, in fully hydrated cations are held to the surface of soil particles through purely electrostatic attraction, not by direct bonding

two types of cations

1. basic cations

2. acidic cations

basic cations (4)

• Ca2+

• Mg2+

• K+

• Na+

acidic cations (2)

• H+

• Al3+

anion exchange

reversible chemical process in which anions in the soil solution are swapped with anions bound to positively charged sites on soil colloids (soils high in amorphous minerals like imogolite and allophane which can have variable/positive surface charge)

cation exchange capacity

number of cation adsorption sites per unit weight of soil/the total exchangeable cations a soil can adsorb

units of cation/anion exchange capacity

cmol_c kg^-1 (centimoles of positive charge per kilogram of oven dry soil)

4 determinants of cation exchange capacity and relationship

1. clay content: high clay content = higher CEC due to greater surface area and more positively charged sites

2. type of clay: 2:1 clays have high CEC due to high surface area and isomorphous substitution; 1:1 clays have low CEC

3. organic matter content: organic matter has very high CEC (200 cmolc/kg) due to the dissociation of functional groups to create negatively charged sites

4. soil pH: CEC increases as pH increases as deprotonation of functional groups creates more negatively charged sites

anion exchange capacity: number of anion adsorption sites per unit weight of soil/total exchangeable anions a soil can adsorb

2 types of surface charge

1. permanent charge

2. variable charge

permanent charge

charge that is constant and unaffected by a change in soil pH that arises through isomorphous substitution

isomorphous substitution

process in which one ion (cation) in a mineral’s crystal lattice structure is replaced by other ions (of similar size but lower charge) during weathering, resulting in permanent negative surface charge on the soil colloid

variable charge

charge that is pH dependent

point of zero charge

pH where the net charge of a soil is 0 and it doesn’t adsorb cations or anions

how can CEC or AEC be changed (3)

• addition of organic matter

• addition of minerals

• change in pH

base saturation

% of exchange sites occupied by basic cations (Ca2+, Mg2+, K+, Na+)

ion selectivity

different affinities of soil colloids to bind to certain ions over other, impacting nutrient availability and soil quality

ion selectivity formula

charge of ion/hydrated ionic radius

relationship between ionic radius and adsorption

small ionic radius = high charge density = strong attraction for water molecules = large hydrated radius = ion loosely held = weak adsorption

Gouy Chapman layer

diffuse region of counter ions adjacent to a charged soil surface, where counter-ion concentration gradually decreases with distance from the charged surface

flocculation

process in which particles suspended in soil solution aggregate to form large clusters which can settle of and clarify liquids

deflocculation

process in which large aggregates are broken down into smaller particles, dispersing particles to maintain a stable, uniform suspension

2 factors determining the thickness of the Gouy Chapman layer and their relationship

1. cation charge: higher charge = fewer cations are required to reach neutrality = shorter/thinner GCL = flocculation

2. concentration: higher concentration = fewer cations are required to reach neutrality = shorter/thinner GCL = flocculation

Specific adsorption/inner sphere binding

formation of direct chemical bonds between a solute (metal or ion) and functional groups on the soil surface

electrostatic adsorption/outer sphere binding

electrostatic attraction between a charged, hydrated ion and the soil surface

occlusion

process in which materials become physically trapped within growing mineral structures, precipitates, oxides or aggregates

process of occlusion (3)

1. ion adsorbed onto mineral surface

2. minerals in solution (e.g. Fe/Al oxyhydroxides) form layers over the ion

3. ion becomes sealed inside mineral coating

solid-phase incorporation/mineral association

chemical or physical bonding of organic compounds to the surfaces of mineral particles, forming organo-mineral associations

why is P strongly immobilized (3)

1. P forms inner-sphere complexes (directly bound)

2. can become occluded by Fe/Al oxides

3. poorly desorbed once bound

soft acid/metal

electrons loosely bound and highly polarizable

hard acid/metal

electrons tightly bound and weakly polarizable

precipitation

process in which dissolved ions react to form insoluble compounds, limiting their effect in soils

ionic potential definition

measure of an ion’s charge density which helps to predict speciation and solubility

ionic potential formula

ionic potential = charge/ionic radius (nm)

low ionic potential

exist as free cations that may be sorbed via cationic exchange

intermediate ionic potential

ions hydrolyze and precipitate

high ionic potential

forms soluble oxyanions

what happens to chemicals in soil over time

chemicals in soil typically become less soluble and less bioavailable

process of chemical immobilization over time (6)

1. dissolve in soil solution

2. outer sphere adsorption

3. inner sphere complex formation by loss of hydration shell

4. surface fixation

5. occlusion, precipitation or structural incorporation

6. adsorbed ion can diffuse back into solution

attenuation

natural reduction of contaminant concentration, toxicity, mass or mobility as they migrate through the soil matrix

K_D

distribution coefficient between soil matrix (S) and soil solution (C)

distribution coefficient between soil matrix (S) and soil solution (C) formula

K_D = S/C

distribution coefficient between soil matrix (S) and soil solution (C) definition

measure of how strongly a chemical partitions between the soil matrix and soil solution

high K_D

most chemical is bound to the soil matrix (S>C), low bioavailability, low leaching risk

low K_D

most chemical is bound to the soil solution (S<C), high bioavailability, high leaching risk

non-linearity of K_D

at low concentrations: linear relationship between concentration in matrix and solution

at high concentrations: small increases in concentration cause large increases in solution concentration because soil binding sites are saturated

what determines the environmental safety of pesticides (5)

1. toxicity

2. K_D

3. half-life

4. soil type

5. climate