Sorption processes in soil
general term referring to the retention
of material on solid surfaces - can
include adsorption, surface
precipitation, and polymerization
I. Adsorption - terminology
• Accumulation of a substance between a
solid surface and the solution.
• Not including surface precipitation and/or
polymerization.
• Different from absorption which refers to
ingestion or uptake into a plant or solid
body.
sorbate
sorptive
sorbent
Why study sorption?
Important chemical process in soil
affecting nutrients, contaminants,…
http://www.btny.purdue.edu/Pubs/PPP/images/PPP-35.fatestransfer.jpg
Factors affecting the fate of soil-applied herbicides
www.montana.edu/wwwpb/pubs/mt200405.html
Forces involved in Adsorption
•
Physical forces (distance and valence)
a. van der Waals (weak electrostatic forces
between nonpolar molecules due to
temporary dipole moment)
b. Electrostatic complexes (e.g., ion exchange)
•
Chemical forces (electron reconfiguration;
breaking/making bonds)
a. inner-sphere complexation (ligand exchange,
covalent/ionic bonding, aka chemisorption or
specific sorption)
http://www.columbia.edu/cu/biology/courses/c2005/images/vdw.gif
Ion Exchange
(electrostatic complex)
http://www.mpi-muelheim.mpg.de/kofo/
institut/arbeitsbereiche/schueth/grafik/z_ion_exchange.gif
Silica-based material being attacked by water.
The process involves water physisorption (charge
attraction) onto the surface, followed by chemical
reactions with the surface (chemisorption) that
break structural bonds.
http://www.mri.psu.edu/faculty/pantano/group/leed/research.htm
II. Surface Functional Groups, (chemically
reactive molecular unit on a solid surface)
•
Organic: carboxyl,
carbonyl, phenolic
Inorganic SFG’s
•
•
O atoms of the silica tetrahedral layer (siloxane
surface) in the interlayer region of
phyllosilicates
OH groups associated with edges of minerals
http://surface.chem.uwm.edu/tysoe/research/Crown.gif
Uranium Adsorption on Soils
SFG’s
www.dartmouth.edu/~soilchem/uranium.htm
SFG's can be protonated or deprotonated by
reaction in water to form exchange sites:
S-OH + H+ ↔ S-OH2+
(gains protons)
S-OH ↔ S-O- + H+
(loses protons)
S-OH + OH- ↔S-O- + H2O
(loses protons)
protonation
deprotonation
deprotonation
Surface Functional Groups on Soil Organic
Matter
www.humet.hu/141-vizsgalat-en.shtml
III. Surface Complexes:
SFG + ion or molecule in solution = stable
molecular entity, called a surface complex.
A. Outer-sphere complex - water molecule is
present between the SFG and bound
(adsorbed) ion or molecule. Also includes
Diffuse Ion Swarms in solution.
B. Inner-sphere complex - no water molecule
present between the SFG and bound ion or
molecule.
C. Inner and outer-sphere complexation occurs
simultaneously (i.e. not mutually exclusive).
Outer Sphere Complex
•
•
•
•
•
weak (H-bonding)
electrostatic interaction, thus surface
must be charged
rapid
reversible (exchangeable)
affected by ionic strength of the solution
Inner-Sphere Complex
•
•
•
•
•
•
•
Strong (covalent and/or ionic bonding)
Mono- or polydentate (held by one or more
bonds)
May be slower than outer sphere complexation
Often irreversible or “fixed” depending upon
environmental conditions
Weakly affected by solution ionic strength
Surface charge can be changed by
complexation
Charged surface is not required for
complexation
Omoike A., J. Chorover, K. Kwon, and J.D. Kubicki (2004)
Adhesion of bacterial exopolymers to FeOOH:
Inner-sphere complexation of phosphodiester groups
www.geosc.psu.edu/envchem/2.3.htm
IV. Adsorption Isotherms
Conducting sorption experiments
Sorption experiments
• Equilibrate sorptive (chemical) with
sorbent (soil)
– Same Temperature and Pressure
– Control or measure Ionic strength and pH
• Batch equilibrium experiments
• Column or flow-through experiments
Batch equilibration – unrealistic but
easy, fast, common, maximum “q”
• Shake or stir soil with solution containing
chemical of interest (sorptive)
– Shaking or stirring too long may cause
minerals to dissolve, weather, and/or
precipitate new minerals
– Insufficient mixing means that all surface
functional groups may not be reached
– Overly vigorous mixing may break minerals
and expose new functional groups on edges
Batch sorption experiments
Solution
+ soil
0
1
5
10 … 100 mM + Blanks
Include a range of sorptive concentrations in a buffer.
Keep as many variables the same to measure sorption.
Sorption experiments
1. Equilibrate sorbent (soil) with sorptive (chemical
in solution over reasonable range)
2. Measure solution composition and volume
before and after equilibration
3. Control or measure I, pH, T, and P
ALWAYS RUN BLANKS throughout experiment
4. Separate solution from soil by centrifugation,
settling, or filtering; or collect leachate
5. Calculate sorption (q) by mass balance and plot
Flow or column experiments
• Pass solution through soil and collect
leachate.
– Columns can be packed or collected directly
from soil. No standard size or flow rate.
• Problems include macropores, cracks,
inadequate exposure of sorptive to all
functional groups.
– Column size and packing affects results
http://www.clemson.edu/agbioeng/bio/images/sumint/fen3.jpg
http://www.soilmeasurement.com/images/Tempe_cell.jpg
http://www.sce.ait.ac.th/facilities/irrlab/images/Imagep30c.jpg
http://www.hydrogeologie.tu-berlin.de/_data/images/sv2.jpg