Solid-Water Interface Mineralogical transformation biomineralization dissolution precipitation Mn+ Oxidation Reduction Mn+x release Bacteria deposition Organic Matter Mineral adsorption Organic ligand desorption Soil Profile complexation Aqueous Metal Ion degradation Metal-Organic Complex Surface complex
Surface Charge Permanent Charge (so) Variable Charge (sH) >Al-OH-1/2 + H+ >Al-OH2+1/2 >Al=O-1 + H+ >Al=OHo + H+ >Al=OH2+
Ion Retention sorption desorption Aqueous Metal Ion
Types of Ion Retention Adsorption Absorption Precipitation (homogeneous) Surface (heterogeneous) Precipitation Terms Sorption Adsorption Absorption Surface precipitation Precipitation Sorptive Sorbate Sorbent
Types of Ion Retention c b a Surface precipitation Absorption Adsorption a
Adsorption: Chemical versus Electrostatic
Chemical Adsorption: Requirements 1. Chemically Reactive Ion 2. Chemically Reactive Surface
Chemical Reactivity of Ions Ion Reactive (with oxide surfaces) Cl- no SO42- partially NO3- no F- yes PO43- yes SeO32- yes SeO42- partially AsO43- yes AsO33- yes (but conditionally) CrO42- partially
Lewis Acids and Bases Hard Lewis Acids: H+, Na+, K+, Cs+, Ca2+, Mg2+, Cr3+, Fe3+, Al3+, Si4+ Intermediate Lewis Acids: Fe2+, Co2+, Ni2+, Cu2+, Zn2+ Soft Lewis Acids: Ag+, Au+, Cd2+, Hg2+ Hard Lewis Bases: H2O, OH-, O2-, CO32-, NO3-, PO43-, SO42-, F- Intermediate Lewis Bases: Br-, Cl- Soft Lewis Bases: R-SH, S2-, I-
Electrostatic Interactions + Electrostatic Interactions + + Diffuse Swarm + + + + + Outer-sphere complex
Ion Selectivity 1 Dehydrated radius (inverse to hydrated radius) Radius (nm)1 Desorption (%)2 Li+ 0.068 68 Na+ 0.097 67 K+ 0.133 49 NH4+ 0.143 50 Cs+ 0.167 37 Mg2+ 0.066 31 Ca2+ 0.099 29 Al3+ 0.051 15 Th4+ 0.102 2 1 Dehydrated radius (inverse to hydrated radius) 2 Reacted with 1 M KCl or NH4Cl
Electrostatic Interactions: Exchange Reactions
Eisenman Model rs Ca2+ ra,b
Salt Affected Soils
Acid Soils