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Sorption of Anions

Sorption of Anions. Important because: Several nutrients and agricultural chemicals are negatively charged. Nitrate, phosphate, sulfate, selenate,… Tropical, acidic and highly weathered soils exhibit notable anion sorption.

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Sorption of Anions

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  1. Sorption of Anions Important because: • Several nutrients and agricultural chemicals are negatively charged. • Nitrate, phosphate, sulfate, selenate,… • Tropical, acidic and highly weathered soils exhibit notable anion sorption. • (Particularly in soils rich in variable charged particle surfaces such as Fe, Al, and Mn oxides or allophane)

  2. Mechanisms • Outer-sphere complexation and diffuse ion swarm (pH dependent) S-OH(s) + H+(aq) SOH2+ SOH2+ + A-(aq) SOH2A(s) • Where S is the surface or sorbent • Important for NO3-, Cl-, ClO4-, ~SO4-2, SeO4-2 (selenate) • More prevalent on oxide and silicate edges than humus fraction

  3. Anion Exchange • As pH increases up to the pKa, adsorption increases. • Above the pKa, adsorption decreases. H4SiO4 H+ + H3SiO4- pKa ~ 9.5 HF  H+ + F- pKa ~ 4 • At typical conditions for most soils, anion sorption is inversely related with pH: • AEC increases as soil pH decreases.  • Ion exchange or outer-sphere sorption is greatest in soils dominated by the sesquioxides and allophane

  4. Inner-sphere complexation • ligand exchange (a.k.a. anion penetration or chemisorption) SOH2+ + A- SA(s) + H2O(l) • Important for phosphate, borate, arsenate, arsenite, silicate, selenite, molybdate • O or OH ions on mineral edges can be replaced by anions like phosphate and F- that can enter into sixfold coordination with Al+3 or Fe+3 in octahedra • Borate, B(OH)4-, can bond to humus -

  5. Surface complex structure • Monodentate – metal is bonded to only one oxygen • Bidentate – metal is bonded to two oxygens • Mononuclear – sorbed metal is associated with one metal on sorbent surface • Binuclear – sorbed metal is bonded to two sorbent metals

  6. http://www.geosc.psu.edu/envchem/1.4_files/image002.jpg

  7. http://www.nsls.bnl.gov/newsroom/science/2003/images/01-Peak-figure2.jpghttp://www.nsls.bnl.gov/newsroom/science/2003/images/01-Peak-figure2.jpg

  8. Adsorption envelopes • Plots of anion sorption vs. pH at constant concentration • Show variation in sorption behavior with pH • Important because availability of anions can be managed by managing pH (e.g., liming acid soils, acid rain, etc.) • Also shows competition between anion protonating (removing H+ from solution) and surface protonation

  9. Effect of pH on Cd adsorption onto kaolinite in single- (Cd concentration 133.33 µM) and multi-element (Cd, Cu, Pb and Zn concentration 33.33 µM each i.e. total metal concentration 133.33 µM) systems. http://www.scielo.br/img/fbpe/jbchs/v11n5/a14fig01.gif http://www.regional.org.au/au/asssi/supersoil2004/s3/poster/1578_srivastavap-2.gif

  10. Like       and SO    the MoO    anion is strongly adsorbed by Fe and Al oxides, which markedly increase at low pH. Mo sorption capacity http://www.ilri.org/InfoServ/Webpub/Fulldocs/Bulletin26/Molybde.htm

  11. http://www.freepatentsonline.com/20050156136-0-large.jpg

  12. Point of Zero Charge PZC  pH at which the surface has net charge of zero: p = 0 1. When pH < PZC the particle surface is positively charged 2. When pH > PZC the particle surface is negatively charged 3. At PZC, settling of flocs occurs – important in aggregation and retention of ions during irrigation, leaching, etc.

  13. pH below the pHZPC http://www.gly.uga.edu/schroeder/geol6550/zpcphlow.gif

  14. pH at the pHZPC

  15. pH above the pHZPC

  16. Soil components vary in PZC • Fe and Al oxides (Oxisols, tropical soils) have high PZC (pH 5-9) • Soil organic matter has low PZC (pH<5) • Silicate clays have low PZC (pH 2-5) Interpretation: low PZC = net negative charge over wider soil pH range  more cation adsorption and more CEC High PZC = net positive charge in acid conditions or in lower range of soil pH  more anion adsorption and less CEC 4. Consider the distribution of soil components in the profile – where would you expect to see more or less anion and cation adsorption?

  17. MineralpHZPC Gibbsite 10   Hematite  4.2 - 6.9 Goethite  5.9 - 6.7  Na-feldspar  6.8  Kaolinite  2 - 4.6  Montmorillonite  <2 - 3  Quartz  1 - 3 Note that Al and Fe hydroxides have a high pHZPC Kaolinite and montmorillonite have low pHZPC pH for zero point of charge for minerals

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