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)  SOH 2 + SOH A - (aq)  SOH 2 A (s) –Where S is the surface or sorbent –Important for NO 3 -, Cl -, ClO 4 -, ~SO 4 -2, SeO 4 -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. H 4 SiO 4  H + + H 3 SiO 4 - 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) SOH A -  SA (s) + H 2 O (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

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

/a14fig01.gif Effect of pH on Cd adsorption onto kaolinite in single- (Cd concentration µM) and multi- element (Cd, Cu, Pb and Zn concentration µM each i.e. total metal concentration µM) systems.

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

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.

pH below the pH ZPC

pH at the pH ZPC

pH above the pH ZPC

Soil components vary in PZC 1.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 pH ZPC Gibbsite10 Hematite Goethite Na-feldspar 6.8 Kaolinite Montmorillonite <2 - 3 Quartz Note that Al and Fe hydroxides have a high pH ZPC Kaolinite and montmorillonite have low pH ZPC