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)

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

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

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

-

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

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

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

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

http://www.scielo.br/img/fbpe/jbchs/v11n5/a14fig01.gif

http://www.regional.org.au/au/asssi/supersoil2004/s3/poster/1578_srivastavap-2.gif

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.

Like       and SO    the MoO    anion is strongly adsorbed by Fe and Al oxides, which markedly increase at low pH.

http://www.ilri.org/InfoServ/Webpub/Fulldocs/Bulletin26/Molybde.htm

Mo

sorp

tion

capa

city

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

Point of Zero Charge

PZC pH at which the surface has net charge of zero: p = 01. When pH < PZC the particle surface is positively charged2. When pH > PZC the particle surface is negatively charged3. At PZC, settling of flocs occurs – important in aggregation and retention of ions during irrigation, leaching, etc.

pH below the pHZPC

http://www.gly.uga.edu/schroeder/geol6550/zpcphlow.gif

pH at the pHZPC

pH above the pHZPC

Soil components vary in PZC1. Fe and Al oxides (Oxisols, tropical soils) have high

PZC (pH 5-9)2. Soil organic matter has low PZC (pH<5)3. 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?

pH for zero point of charge for minerals Mineral  pHZPC

Gibbsite 10  Hematite  4.2 - 6.9Goethite  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