Sorption processes in soil

1. Sorption processes in soil general term referring to the retention of material on solid surfaces - can include adsorption, surface precipitation, and polymerization

2. 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.

3. sorbate sorptive sorbent

4. Why study sorption?Important chemical process in soil affecting nutrients, contaminants,… http://www.btny.purdue.edu/Pubs/PPP/images/PPP-35.fatestransfer.jpg

5. Factors affecting the fate of soil-applied herbicides www.montana.edu/wwwpb/pubs/mt200405.html

6. Forces involved in Adsorption • Physical forces (distance and valence) • van der Waals (weak electrostatic forces between nonpolar molecules due to temporary dipole moment) • Electrostatic complexes (e.g., ion exchange) • Chemical forces (electron reconfiguration; breaking/making bonds) a. inner-sphere complexation (ligand exchange, covalent/ionic bonding, akachemisorption or specific sorption)

7. http://www.columbia.edu/cu/biology/courses/c2005/images/vdw.gifhttp://www.columbia.edu/cu/biology/courses/c2005/images/vdw.gif

8. Ion Exchange (electrostatic complex) http://www.mpi-muelheim.mpg.de/kofo/ institut/arbeitsbereiche/schueth/grafik/z_ion_exchange.gif

9. 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

10. II. Surface Functional Groups, (chemically reactive molecular unit on a solid surface) • Organic: carboxyl, carbonyl, phenolic

11. 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

12. Uranium Adsorption on Soils SFG’s www.dartmouth.edu/~soilchem/uranium.htm

13. SFG's can be protonated or deprotonated by reaction in water to form exchange sites: S-OH + H+ ↔ S-OH2+ protonation (gains protons) S-OH ↔ S-O- + H+ deprotonation (loses protons) S-OH + OH- ↔S-O- + H2O deprotonation (loses protons)

14. Surface Functional Groups on Soil Organic Matter www.humet.hu/141-vizsgalat-en.shtml

15. 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).

16. Outer Sphere Complex • weak (H-bonding) • electrostatic interaction, thus surface must be charged • rapid • reversible (exchangeable) • affected by ionic strength of the solution

17. 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

19. 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

20. IV. Adsorption Isotherms Conducting sorption experiments

21. 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

22. 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

23. 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.

24. Sorption experiments • Equilibrate sorbent (soil) with sorptive (chemical in solution over reasonable range) • Measure solution composition and volume before and after equilibration • Control or measure I, pH, T, and P ALWAYS RUN BLANKS throughout experiment • Separate solution from soil by centrifugation, settling, or filtering; or collect leachate • Calculate sorption (q) by mass balance and plot

25. 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

26. http://www.clemson.edu/agbioeng/bio/images/sumint/fen3.jpg

27. 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