Lecture 15: Adsorption; Soil Acidity

Lecture 15: Adsorption; Soil Acidity

Surface Complexation (Your textbook calls this adsorption )

Surface Complexation Both cations and anions can bind to sites on the external surfaces of soil minerals They form surface complexes

General Surface Complexation Behavior Cations show increasing adsorption with increasing ph Anions show increasing adsorption with decreasing ph This results from: Reaction stoichiometry ph-dependent surface charge Adsorption of ions on likecharged surfaces indicates a specific, chemical interaction PZC + + - PZC

Behavior of Specific Elements

Behavior Controlled by Cation Exchange Cation exchange has a major influence on alkaline and alkaline earth elements (nonacid/base cations): Na, Mg, K, Ca, Rb, Sr, Cs, Ba The main minerals in soil responsible for most cation exchange sites are: 2:1 clays: smectite and vermiculite Manganese oxides Zeolites Organic matter is also important

Cs + Exchange into Clays Radioactive 137 Cs released from nuclear accidents can be trapped in clays via cation exchange McKinley et al. (2004) Environ. Sci. Technol.

Behavior Controlled by Surface Complexation Elements that occur as anions only adsorb to soils through surface complexation: S, P, As, Se, Cr(VI), Mo, Sb The main minerals responsible for surface complexation is soils are: Iron oxides Aluminum oxides Kaolinite

Phosphate Fixation: Adsorption to Oxide Minerals in Soils Limits Availability for Plants

Ultisols and Oxisols have a High Capacity to Adsorb Sulfate Important in soils rich in kaolinite and oxides Rodríguez-Lado et al. (2007) Water Air Soil Pollut.

Behavior Controlled by Both Cation Exchange and Surface Complexation Many elements, especially heavy metals, can adsorb to soils by both cation exchange and surface complexation Co, Ni, Cu, Zn, Cd, Hg, Pb, U The dominant process depends on the composition of the soil solution and the minerals that are present Calcium and high concentrations of sodium can suppress cation exchange of these elements

Cation Exchange and Surface Complexation of Uranium on a Smectite U on Exchange Site U Surface Complex Surface complexation still occurs when cation exchange is suppressed by excess Na or Ca Turner et al. (1996) Geochim. Cosmochim. Acta

Heavy Metal Adsorption to Iron Oxide Coatings on Wetland Grass Roots Wetland grasses pump oxygen to their roots, causing iron to oxidize and form iron oxide plaques Heavy metals adsorb to these iron oxide plaques through surface complexation Hansel et al. (2001) Environ. Sci. Technol.

Heavy Metals Often Adsorbed to Oxides and Clays in Soils Heavy metals in natural and contaminated soils are distributed heterogeneously May occur as discrete phases, but more commonly associated with oxides and clays Iron Copper Zinc Iron oxide Nachtegaal et al. (2005) Geochim. Cosmochim. Acta

Key Concepts in Adsorption Cation exchange is a physical adsorption process where positively charged cations attracted to negatively charged sites on a soil solid Surface complexation is a chemical adsorption processes involving the making and breaking of bonds Surface complexed ions are more strongly bound Affects both cations and anions Ions can adsorb onto like-charged solids Surface complexation plays an important role in contaminant fate and the availability of sulfate, phosphate, and micronutrients in soils

Soil Acidity

ph Range of Soils ph = -log [H+] ph X means [H + ] = 10 -X moles per liter

ph Range of Soils Range of Most Soils ph = -log [H+] ph X means [H + ] = 10 -X moles per liter

ph Range of Soils Spodosols Ultisols Alfisols Mollisols Aridisols ph = -log [H+] ph X means [H + ] = 10 -X moles per liter

Sources of Acid (H + ) in Soils Carbon dioxide CO 2 + H 2 O H 2 CO 3 HCO 3- + H + Organic acids from organic matter decay OM Decay RCOOH RCOO - + H + RCOOH + Ca 2+ RCOOCa + + H + Nitrification (Oxidation of Ammonium) NH 4+ + 2O 2 H 2 O + NO 3- + 2H + R = large organic molecule (soil organic matter is complicated!)

Sources of Acid (H + ) in Soils Oxidation of Sulfur H 2 S + 2O 2 SO 4 2- + 2H + 2FeS 2(pyrite) + 7O 2 +2H 2 O Fe 2+ + 2SO 4 2- + 2H + Acids in precipitation (rain) H 2 SO 4 SO 4 2- + 2H + HNO 3 NO 3- + H +

Plant Uptake of Cations May Cause Acidification Plants must maintain charge balance when they take up ions from the soil solution Plants take up cations more frequently than anions Root Interior Soil Solution NH 4 + H + Ca 2+ 2H + Cation uptake balanced by H + release: acidification Ca 2+ SO 4 2- Cation uptake balanced by anion uptake: no acidification

Alkalinizing (H + -consuming) Processes Plant uptake of anions HCO 3- + H + CO 2 + H 2 O Root Interior NO 3 - Soil Solution HCO 3 - Mineral weathering CaAl 2 Si 2 O 8,feldspar + 2H 2 O + 2H + Ca 2+ + 2Al(OH) 3,gibbsite + 2SiO 2,quartz CaCO 3,calcite + H + Ca 2+ + HCO 3 -

Plant Uptake of Nutrients in Fertilized Agricultural Soils Often Causes Acidification

Relationship Between Rainfall, Acidity, and Soil Leaching

Rain is Acidic!!!

Rain used to be more Acidic!!!

High Rainfall Promotes Acid Soils

ph value ~4 ~5.5 ~7 ~8

Soil Acidification Promotes Leaching e.g., Rain! Rainfall and other acid-forming processes produce H +, which replaces base cations on exchange site Base cations are lost when water percolates through soil

Linking Rainfall, Acidity, and Soil Development Rainfall brings in acidity (H + ), which initially weathers primary minerals (feldspar, amphibole, pyroxene), forming secondary minerals (smectite, vermiculite) Entisols/Inceptisols + Acidity in Rain = Mollisols/Alfisols with smectites As a soil ages, primary minerals are completely consumed and the acidity starts leaching secondary minerals, forming new clays of different composition Mollisols/Alfisols with smectites + Acidity in Rain = Ultisols with kaolinite