Unit 2 Soil Colloids - Cornell University

Unit 2Soil ColloidsglobalScalesto profile”macroscopic

Chemical Principles:First law of thermodynamics (conservation):Energy (matter) is neither created nor destroyed, it changes fromone form to another.The total amount of energy and matter in the Universe remainsconstant, merely changing from one form to another.Mass ActionA BC DCharge Balance2 2-

Soil Colloids

Soil ColloidsincreasesincreasesAtomic size (radii)

Elemental Composition of SoilsEnrichment ordepletionEF 0.5 – 2.0is not significant(large std. dev.)O, Si, and Al:the mostabundantTrace element: mass concentration in the solid phase is 100 mg kg-1

Conversion of primary minerals to secondary minerals with the release of plantnutrient elements in soluble formsPrimary Mineral(parent material)WeatheringSecondary Mineral(soils) soluble elements(“solutes”)rock-forming minerals formed at hightemperatures andpressures therefore, unstableunder currentatmospheric conditions form under currentatmospheric conditionsSoil may be unstable ifconditions change fromthose of their formationParent materialtypically clay sized: 2 µmSoil

Types of Colloids found in Soils:Crystalline silicate clays– Phylosilicates tetrahedral and octahedral crystal sheetsNon-crystalline silicate clays (Andisols)– Dominately amorphous clays (allophane and imogolite)Iron and aluminum oxides (Oxisols & )– Gibbsite (Al-oxide) and goethite (Fe-oxide)Organic (humus) colloids (Histosols & )– Colloidal sized soil organic matterPermanent charge mineralslayer silicate clays with structural negative chargeVariable (pH-dependent) charge mineralssurface charge that is pH dependent(allophanes (amorphous, high charge); Fe/Al oxide (morecrystalline, low charge); organic molecules

Illite(fine-grained mica)kaolinitemontmorillonitefulvic acid

Aim to understand the structures of soil minerals(typically represented like this):2:1 mineral

Building Blocks of Soil Minerals – molecular scaleSi and Al tetrahedra - (MO4)OAl, Fe, Mg octahedra - g(OH)6What determines the expected ion coordination?

Size (relative size)Minimum Radius RatioCN 40.225CN 60.414

Linking Tetrahedra and Octahedra to Form SheetsMain class of soil minerals (phyllosilicates) are formed by:linking Si (Al) tetrahedraform tetrahedral sheetslinking Al (Mg, Fe) octahedraform octahedral sheetsLinking Octahedra:Linking Tetrahedra:CornerEdgeBasal1 point sharing2 point sharing

Tetrahedral SheetFormed by a sharing of 3 O to form hexagonal ringsbasallinkinglinkingxyzPolyhedral viewzyxHexagonal ringSi tetrahedra only share cornersSi:O2:5

Octahedral Sheets – polyhedral viewTrioctahedral(divalent cation)Octahedrallinkinglinking3 of 3 sitesoccupied by M2 Dioctahedral(trivalent cation)2 of 3 sitesoccupied by M3

Octahedral Cation Occupancyposition of divalent and trivalent cationsTrioctahedralOH-3 of 3 sites occupied by M2 HOMgOHOH-Dioctahedral2 of 3 sites occupied by M3 HOAlOH

Tetrahedra-Octahedra linkageSharing apical oxygens in tetrahedral sheet with hydroxyls of octahedral sheet1:1 mineral2:1 mineral

Secondary Minerals: Layer Silicates or Phyllosilicatescomposed of Si, Al tetrahedra; Mg, Al, Fe octahedra“aluminosilicates”Classification based on:- number of tetrahedra and octahedra in a layer- octahedral site occupancy (octahedral composition: who and how many cations inoctahedral positions)- charge for each layer (layer charge)zplaty morphologyx-y dimensions: 10-6 myz dimensions: 10-9 mx

Secondary minerals: number of tetrahedra and octahedra in a layersimplest – single octahedra1:1 mineral(1 tetrahedra 1 octahedra)2:1 mineral(2 tetrahedra 1 octahedra)

Secondary minerals: octahedral site occupancyposition of divalent and trivalent cationsDioctahedral2 of 3 sites occupied by M3 HOAlOHTrioctahedral3 of 3 sites occupied by M2 HOMgOH

Charge:“Soils are negatively charged”“Minerals exert a charge”How?1. Isomorphic substitution (results in pH independent charge)Al3 for Si4 substitution in tetrahedral sheetMg2 /Fe2 substitution for Al3 /Fe3 in octahedral sheet2. Terminal broken bonds (pH dependent charge)Dissociation of pH-dependent functional groups: surfacehydroxyl (OH) groups in minerals and organic matterWhy is charge important?Reactivity towards everything

1. Isomorphous substitution: develops a charge within a mineral layerPermanent or Structural Charge: it is pH independentreplacement of one ion with another having a different charge but with no change inthe mineral structureDioctahedralAl3 Dioctahedral with substitutionAl3 Al3 HOHOAlAl/MgOHOHMg2

Secondary minerals: charge within a layerPermanent or Structural Charge: it is pH independent2:1 mineralAl3 Mg2 Charge sites located within the tetrahedral (Al3 for Si4 ) or/andoctahedral (Mg2 for Al3 ) sheet

2. Terminal broken bonds: pH dependent (variable) chargeaddition or release of protons from the surface results in different chargesMinerals- Al – OH ]0 H Cl-- Al – OH2 . Cl-Anion Exchange SitesLow pH (protonation) Na OH-- Al – O - . Na Cation Exchange SitesHigh pHSoil Organic MatterLow pHH OH2 Positive chargeHigh pH

2. Terminal broken bonds: pH dependent (variable) chargeWhere do they develop?goethite - α-FeOOH(silanol)(aluminol)Edges of layer silicateclaysall faces of (hydrous) oxides-- Surface hydroxyl (OH) groups of Al/Feoxides (crystalline minerals)-- Amorphous (noncrystalline) minerals(allophane)Organic Matter:

Secondary layer aluminosilicate minerals:Charge: permanent (structural) and pH-dependentsilanolPredominant ?aluminolisomorphouszplaty morphologyyxx-y dimensions: 10-6 mz dimensions: 10-9 m

(illite)Cations (K , Na , Ca2 , Mg2 ) and anions sorbed between sheets and onthe edges balance the charge

non-expanding(min. swelling)Layer Silicates:Mineral GroupsClassification based on:Number of tetrahedraand octahedraexpanding(some swelling)non-expanding(no swelling)Octahedral siteoccupancy (tri- anddioctahedral)expanding(max. swelling)Charge for each layer(magnitude and location)zplaty morphologyyxx-y dimensions: 10-6 mz dimensions: 10-9 mnon-expanding(min. swelling)

Chicago – suburb built on drained wetlandexample: engineering mediumorganic matter decomposition, “wrong” claysshoring up the house: 30,000

WeatheringMineralogical transformations - a slow transition from primary (rock-forming) tosecondary (soil-formed) mineralsChemical composition - a long-term result of losses in base cations, silicaPrimaryminerals- Si- Si2:1 layer1:1 layerFe and Alsilicate clayssilicate claysoxides- Base cations- Base cationstime, high rainfall, high temperature

RockEntisolIdeal Weathering SeriesSoil OrdersVertisolInceptisolSpodsols(mineralogy, cracks) (leached horizon)MollisolAlfisolIn general,Ultisol[ Si ][ Al, Fe ]trend is toward “desilication” and loss ofbase cations2:1 minerals stilldominate;1:1 minerals startto appearOxisolOxides ofAl and Fedominate

Reactions at the Solid-Solution Interface – Retention MechanismsSoil Solids/ColloidsAl AlAl AlAl Al Al Al Al AlAl Al Al AlAl AlAl Al Al AlAl Al Al AlSOMSoil SolutionReactions at the InterfaceIons and moleculesIon ExchangeAdsorptionα-FeOOHroot uptakeleaching Retention: net accumulation of matter at the interface between a solid phaseand an aqueous solution phase(ability of soils/colloids to remove ions from solution)

Ion Exchange on Soil Colloidsone of the mechanisms by which (soil) colloids hold ions against leachingloss, hold nutrients in ecosystems and keep pollutants out of ground andsurface waterElectroneutralityexchangeablecations andsurfacesMass Action & Charge Balancesoluble cationsand anionsIon exchange the processCEC the property of the colloid/soil

Retention MechanismsOuter- and Inner- Sphere ComplexesIon ExchangeAdsorptionM2 nH2O M(H2O)n2 Ion Hydration:ionic potential z2/rsmaller and more charged ion:higher ionic potentialhydrate most strongly2 z charge; r radius 2

Ion Exchange: The Processreversiblediffusion controlled- molecular diffusion inaqueous environmentsforward and backwardreactions chiometric- ions that leave colloidsurface are replaced byequivalent amount ofother ions- exchange takes place ina charge-for-charge basismedium-long range(exchange reactions canoccur between cations ofequal or unequal lecationsMostly from permanent charge minerals and soil organic matterAl and Fe oxides, kaolinite - no real contribution to CEC of soils

Cation Exchange on Permanent Charge Minerals(Layer Aluminosilicate Clays)Exchange Equation:Colloid-Mg2 (solid) Ca2 (aq) Colloid-Ca2 (solid) Mg2 Ca2 Mg2 2Ca2 2Mg2 Mg2 General Rules:selectivity of cation by exchanger based on the ion’s charge/size(ionic potential z2/r)size:the smaller the hydrated radius the greater the affinity (same charge)(ions with small dehydrated radius have large hydrated)charge:the higher the charge the greater the exchanger preference(3 2 1 )For example: Al3 Ca2 Mg2 K NH4 Na

Cation selectivity is determined by extent of cationhydrationExtent of reaction alsoinfluenced by:2 Mass action2 Nature of colloidNature of complimentaryions

Soil Organic MatterSurface Functional Groups (hydroxyl, carboxylic, alcohols, phenols)Ion ExchangeGeneral Rule: selectivity of cation by exchanger based on the ion’s charge/sizeOrder of Selectivity:Cs Rb K Na Li Ba2 Sr2 Ca2 Mg2 Al3 M2 M

Anion Exchange Capacity (AEC)Terminal broken bonds: pH dependent (variable) chargeMineralsLow pH (protonation) H - Al – OH ]0- Al – OH2 . Cl Cl-Anion Exchange SitesSoil Organic MatterLow pHH OH2 Positive chargeAEC is generally much smaller than CECAttraction of anions to oxide minerals and SOM

Complimentary cations: Influence plant uptake and leachingAt a given %K saturation, K is more readily available for both plant uptakeand leaching in acid soils (lots of Al3 ) than in neutral to alkaline soils.Why?

CEC

Cation Exchange Capacity (CEC) – The PropertyAssessment of the quantity of cations reversibly retained (electrostatic) perunit weight of mineral/organic matter/soilmmol( ) charge / kgMagnitude of CEC determined by the nature and content of clay mineralsand organic matterCEC is related to the negative charge of the soil colloidsCEC of soils is usually dominated by Ca2 , Mg2 , Na , K , Al3 CEC (mmol charge) 2[Ca] 2[Mg] [Na] [K] 3[Al]

Determination of Cation Exchange Capacity in SoilsLeach soil with a concentrated solution of an exchanger cation (NH4 , Ba2 , Sr2 )1. determine elements in theleachate (usually Ca, Mg, Na, K, Al)2. measure exchanger cationadsorbed (requires an extra step:need to leach the exchangercation)Buffered (NH4 at pH 7, Ba2 at pH 8.2) and Unbuffered (soil pH) methodsExchangeable ions are essential for maintaining plant nutrient levels but arenot “held” strong enough to immobilize environmental pollutants

Relationships: Weathering intensity, mineralogy and CEC/AEC

Relationships: Soil pH, CEC, AEC, retention/sorption of cations/anionsretantion cationsanionslowpHhigh

Base Saturation (BS) of Cation Exchange Sitesa measure of proportion of cations (compared with hydronium ions)on exchange sitesnumber of exchange sites occupied by Ca Mg K NaBS total number of exchange sitesx 100high base saturation values are desirableSoils with low CEC and/or a small base saturation value are susceptible toacidification by either natural or anthropogenic inputs

Crystalline silicate clays – Phylosilicates tetrahedral and octahedral crystal sheets Non-crystalline silicate clays (Andisols) – Dominately amorphous clays ( allophane and imogolite) . Aim to understand the structures