The Soil Chemical Environment

Reading:

General background: Sparks,Chapter 1, pp. 1-28

Additional: Essington, Chapter 1

pp. 1-21.

Chemical Interactions in Soils

Description of complex interactions involves

• Mineral Chemistry

• Colloidal Chemistry

• Physical Chemistry

• Analytical Chemistry

• Organic Chemistry

• Biochemistry

Reactions occur at interfaces

• Solid - liquid

• Liquid - gas

System is open

• fluxes of water and solutes

• fluxes of gases

• fluxes into biota and from decaying biota

• energy fluxes

Equilibrium vs. Kinetics

• Most soil chemistry is the study of reactions at equilibrium.

• This is OK for prediction of fast reactions (e.g. ion exchange and many adsorption reactions), but soil is mostly a non-equilibrium system.

• Kinetics

–Very important for many reactions (e.g. precipitation and dissolution of most minerals). Hard to study in a mixed system like soil.

–Kinetics can be used to describe the rates of all reactions but in many cases the rates are so slow that over the time of interest there is no reaction. Kinetics in mixed systems can be very complex.

Soil Chemistry and the Soil Solution

• The soil solution is central to most soil chemistry

• Soil chemistry is mostly about the interaction of ions and molecules in solution and their interaction with the gas phase and the solid phases.

• Most concepts similar to those in aquatic chemistry courses.

Soil Solution Interaction with other components (Sparks Fig. 4.1)

Inorganic anions and cations in the soil solutions (Sparks Table 4.1)

Chemical systems in soils

Properties of the Elements

• Web Elements has details on the properties and behavior of all elements

• Web Elements http://www.webelements.com/

Chemical Elements in Soil

Abundance of the elements in soils

• Major vs. minor and trace elements

–In geochemistry and soil chemistry these the definitions are a bit fuzzy.• See also plant nutrition literature

–Mostly “trace” and “minor” are used interchangeably

–Trace or minor is sometimes defined as < 1% (10 g kg-1 ). Essington says trace is < 100 mg kg-1

–Some authors consider minor as including higher concentrations than trace

Major elements

• Major: Al, Si, C, N, Fe, Ca, O, K, Ti (P in plants)

• Remainder are minor (trace) elements

see Table 2.4 in Sparks

Periodicity of the elements

• Metals

–Group 1 - alkali metals (+I), 1+ ions

–Group 2 - alkaline earths (+II), 2+ ions

–Transition elements

• Non-metals

–Group 6 chalcogens (- II)

–Group 7 halogens (- I), 1- ions

Heavy metal??

• It is such a vague word, that is often used incorrectly.

• Is boron a “heavy metal”?

Oxidation states and charge number

• Oxidation state is indicated by Roman numerals.

–E.g. Fe(III), Fe(II), Mn(IV), and Mn(II)

• Charge number on an ion.

–E.g. Ca2+ not Ca+2  

• SO42- or with oxidation state S(VI)O4

2-

Acids and Bases

See Sparks p. 65

ACIDS AND BASES

Arrhenius

Acid increases H+ concentration (activity) in solution.

First and least inclusive definition.

Base increases OH- activity.

Brönsted definition - typical definition used in environmental and soil chemistry

Acid is a proton donor.

Base is a proton acceptor

Includes Arrhenius acids and bases

Brönsted acids and Bases

– Examples

HCl + H2O = H3O+ + Cl-

acid base conjugate acid conjugate base

NH3 + H2O = NH4+ + OH-

base acid

CH3COOH + OH- = CH3COO- + H2O

acid base

Lewis Acids and Bases

• Lewis Acid – a broader definition. (Brönsted definition is subset of the Lewis

definition)

–Acid is an electron pair acceptor

–Base is an electron pair donor.– Examples:– (see Brönsted acids and bases)

e.g. H+ + H2O = H3O+

H

O

H H

O

H

H

+

– H+ is a proton --- can accept a pair of electrons from one of the two unshared pair of electrons in water to form a coordinate covalent bond.

– – water

hydronium ion

LeH+ is also a Lewis Acid

Metal ion complexes

–Lewis defines formation of complexes as acid base reactions.

e.g. Formation of Cu2+ amine complex ion.

Cu2+ + 2NH3 = Cu(NH3)22+

Ammonia has one unshared pairs of electrons that can be donated to empty bonding orbitals in Cu2+.

H

N

H

H

Metal ion complexes

• On the last slide NH3 is a ligand

• The complex is also called and adduct (addition product). Not a term used very much in environmental chemistry.

Chelation

• Chelation (multidentate complexes)• Fe(II) can accept 2 pairs of N electrons

• Bidentate complex

• This red complex is useful for detection of reduced Fe.

N N

Fe2+

orthophenathroline Fe(II) complex

EDTA complexes

H2C

N

H2 C

C H2

N

C H2

C

CH2

C

CH2

C

O

HO

OH

O

O

HO

C

O OH

H - EDTA

• N atoms can donate pairs of electrons

• Negatively charged O atoms can donate pairs of electrons.

• Negative charges contribute to ionic bonding

• Can form hexadentate complexes– e.g. Cu2+ and other first row transition metal

cations.

• Complexes with EDTA can be very strong

H2C

N

H2 C

C H2

N

C H2

C

CH2

C

CH2

C

O

O

O

O

O

O

C

O O

ionized EDTA

EDTA, is shown in 3-D at

EDTA

– Fe DTPA, a similar complex is at:

Fe-EDTA

Systeme International (SI) Units

Based on mks system

• Basic units; m, kg and s

• Examples:

–Concentration in a solid, mol kg-1

–Rate of reaction, mol L-1 s-1

• Writing units

– mol L-1 s-1 not mol/L/ s

Essington Table 1.5

Some non SI (derived) units

• We use many units that are not strictly part of the S.I. system

–e.g. L for liter and ha. for hectare.

–Land application - kg ha-1

Some equivalences

• Mg m-3 = g cm-3

• mg kg-1 or mg L-1   = ppm

• µg kg-1 or µg L-1   = ppb

• c molc kg-1 same as meq/100g

Conversion of units

• Example: Reporting 5 mg L-1 Ca extracted from a 10 g soil in 200 mL solution in units of cmolckg-1

= 0.5 mmolc kg-1

cmol c

kg soil

5mg Ca1000mL

x200mL

10g soilx

2mmol c

40mg Cax

1cmol c

10mmol cx

1000g soilkg soil

Essington Table 1.6

In class exercise

– What is the common language unit for g T-1 (gram per ton)?

Summary

• Soil solution is central to soil chemistry.

• Soil chemistry concentrates on a small fraction of the periodic table.

• Knowledge of periodicity is very useful.

• We will mostly use the Brönsted definition of acid and bases.

• Will use mostly SI units and a few derived from S.I