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Chapter 5

PHASE INTERACTIONS IN AQUATIC

CHEMISTRY

Environmental Chemistry, 9th Edition

Stanley E. Manahan

Taylor and Francis/CRC Press

2010

For questions, contact: Stanley E. Manahan

manahans@missouri.edu

5.1 Chemical Interactions Involving Solids, Gases,

and Water (Figure 5.1)

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3

5.2 Importance and Formation of Sediments

Sediments are

• Layers of relatively finely divided matter

• Cover bottoms of various bodies of water

• Generally mixtures of clay, silt, sand, organic matter

• Various organisms

• Pollutants including heavy metals, organics

• Transfer to organisms directly or through pore water

Formation of sediments

• Physical transfer of material

• Chemical precipitation

• Biochemical processes such as photosynthesis producing

biomass and solid CaCO3, action of anoxic bacteria

producing solid FeS

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Figure 5.2 Alternate layers of CaCO3(s) and FeS(s) in lake

sediment

Organic and Carbonaceous Sedimentary Materials

• Particularly important for binding organic pollutants

• Organics may be held for many years

• Black carbon from combustion

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5.3 Solubilities

Solubilities of solids

• From solubility products (see calculation of solubility of

BaSO4 in text

• Intrinsic solubility, example of CaSO4

S = [Ca2+] + [CaSO4]

Solubilities of ionic solids affected by several factors,

example of PbCO3 (see Section 3.15)

• Increased by chelation of metal: Pb2+ + T3- PbT-

• Increased by reaction of anion:

PbCO3 + H+ Pb2+ + HCO3-

• Presence of common ion: HCO3

- H+ + CO32-

From solubility product From intrinsic solubility

Solubilities of Gases

Henry’s law: At constant temperature the solubility of a gas

in a liquid is proportional to the partial pressure of the gas

in contact with the liquid

• X(g) X(aq)

• [X(aq)] = KPX

Increased by acid-base reactions

• NH3(g) + H+ NH4+(aq)

• SO2(g) + HCO3-(alkalinity) HSO3

-(aq) + CO2

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Concentration of O2 in water in contact with air at

25˚C

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The Clausius-Clapeyron equation for gas solubilities C1 and

C2 at absolute temperatures of T1 and T2 where R is the gas

constant and DH is the heat of solution

logC2

C1=

2.303R

H 1 1T1 T2

-

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5.4 Colloidal Particles in Water

Size range of 0.001 – 1 micrometers

Include

• Minerals • Microorganisms • Organic matter

• Proteinaceous material

Important characteristics

• Light scattering (Tyndall effect) • High area

• High interfacial area • High surface/charge density ratio

Important, widespread in natural waters and wastewaters

Colloid-facilitated transport of pollutants

Colloids are widespread in water and wastewater

Kinds of colloids

• Hydrophobic • Hydrophilic • Association

Figure 5.3 Charged hydrophobic colloidal particles

surrounded by counter-ions

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+

+ +

+

+- -

-- - - -

--

++ +

+

++

+

++

++

+

+++

+- --

- - --

-- -

+ +

+

+- -

-- - - -

--

++ +

+

++

+

++

++

+

+++

+- --

- - --

-- -

+ +

+

+- -

-- - - -

--

++ +

+

++

+

++

++

+

+++

+- --

- - --

-- -

+ +

+

+- -

-- - - -

--

++

+

+

++

+

11 Figure 5.4 Colloidal soap micelle particles

C C C C C C C C C C C C C C C C C O- Na+CHH H H H H H H H H H H H H H H H H

H H H H H H H H H H H H H H H H H O

Na+Represented as -

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Colloid stability

Stabilized by attraction to water and by surface charge

Colloidal particles acquire charge by

• Surface chemical reaction, often involving H+ (see Figure

5.5, next slide)

• Ion absorption

• Ion replacement (such as Al(III) for Si(IV)) in clays

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Figure 5.5 Acquisition of surface charge by

colloidal MnO2

Mn O Mn

O

Mn O Mn

O

O

O

O

Mn

O

MnO

O O

Mn O Mn

O

O

O

O

Mn O Mn

O

Mn O Mn

O

O

O

O

Mn

O

MnO

O O

Mn O Mn

O

O

O

O

OH

H

OH

H

OH H

OH

H

OH

H

OHH

Hydration

+H 2O

Mn O Mn

O

Mn O Mn

O

O

O

O

Mn

O

MnO

O O

Mn O Mn

O

O

O

O

OH-

OH-

OH-

OH-

-HO

-HO

Mn O Mn

O

Mn O Mn

O

O

OH+

O

Mn

O

MnO

O O

Mn O Mn

OH+

+HO

OH+

OH

H

OH

H

OH H

OH

H

OH

H

OHH

+HO

Gain of H+

Loss

of H+

I I I

I I I IV

H+

5.5 The Colloidal Properties of Clays

Clays are widespread as colloidal particles in water and as

solids in sediments

• Secondary minerals

• Hydrated aluminum and silicon oxides

Common clays include

• Kaolinite: Al2(OH)4Si2O5

• Montmorillonite: Al2(OH)2Si4O10

• Nontronite: Fe2(OH)2Si4O10

• Hydrous mica: KAl2(OH)2(AlSi3)O10

Unit layers in clay structures

Clays acquire charge usually by substitution of Al(III) for

Si(IV)

• Exchangeable cations, such as H+, K+, NH4+

• Cation exchange capacity

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Aggregation of Particles

Important in water

• Example: Settling of waste biomass in wastewater

treatment

• Example: Formation of sediments from river water

entering oceans

Mechanisms of aggregation

• Coagulation from reduction of surface charge repulsion

• Flocculation with bridging compounds that produce floc

networks

• Flocculation is facilitated by synthetic and natural

polyelectrolytes

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Figure 5.6 Aggregation and restabilization of

charged colloidal particles

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Figure 5.7 Surface sorption by solids

Many of the effects of colloidal and sedimentary solids in

contact with water have to do with their sorption of solutes

Metals are sorbed by solids, particularly metal oxides

• Nonspecific ion exchange adsorption

• Complexation with surface –OH groups

• Coprecipitation with the metal oxide

• Discrete oxide or hydroxide sorbed to metal

Hydrated manganese(IV) and iron(III) oxides are good

sorbents, especially when freshly precipitated

• Freshly precipitated MnO2 may have a surface area of

several hundred square meters per gram

Anions are also sorbed by solids

• Usually with less specific bonding than metals

5.8 Solute Exchange with Bottom Sediments

Bottom sediments are important sources and sinks of

inorganic and organic matter in streams, fresh-water

impoundments, estuaries and oceans

• Generally anoxic (reducing conditions)

• Generally high levels of organic matter

Cation exchange capacity (CEC) expresses the capacity of a

sediment to sorb cations

• Expressed as milliequivalents per 100 g solid

Exchangeable cation status (ECS) refers to specific ions

held by sediments

• Common cations held by sediments are H+, K+, NH4+, Ca2+,

Mg2+, Fe2+, Mn2+, Zn2+, Cu2+, Ni2+

Sediments act as buffers by exchanging H+

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Trace-Level Metals in Suspended Matter and

Sediments

Trace metals held in sediments and colloidal suspensions

include cadmium, chromium, cobalt, copper, manganese,

molybdenum, and nickel

Metals held in suspended particles less available than those

in solution but more so than those in sediments

pE is an important factor

• High pE (oxic, oxidizing): Oxides, hydroxides, and

carbonates such as HgO, C(OH)2•CuCO3

• Low pE (anoxic, reducing): Sulfides predominate such as

CdS, PbS

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Phosphorus Exchange with Bottom Sediments

Important in algal growth eutrophication

Forms of phosphorus in sediments

• Phosphate minerals, Ca5OH(PO4)3

• Nonoccluded phosphorus such as PO43- held on mineral

surfaces

• Occluded phosphorus with orthophosphate ions contained

within mineral matrix, such as in aluminosilicates

• Organic phosphorus in biomass (usually algal or bacterial)

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Organic Compounds on Sediments and Suspended

Matter

Sediments as sinks and repositories of organic matter

Colloids may transport organic matter

Sorption affects degradation of organic matter

Common sorbents are clays, humic substances and

complexes between clays and humic substances

Sorption generally proportional to water solubility

Bound residues of organics

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Bound residues of persistent organic pollutants form during

humification of organic matter

• Immobilization

• Detoxification

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Bioavailability of Sediment Contaminants

The facility with which a substance may be taken up by

organisms

May be direct or through water (Figure 5.7 below)

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5.9 Interstitial Water

Water held in voids and pores in sediments

• Reflects chemical and biochemical conditions in sediment

• Products of decomposition and mineralization of

planktonic biomass

• Largely through activity of anoxic bacteria in sediments

Gases in interstitial water

• Usually virtually no O2

• N2 usually stripped by action of anoxic bacteria producing

CO2 and CH4

25 5.10 Phase Interactions in Chemical Fate and Transport

Hydrosphere is particularly important in fate and transport

• Rivers move dissolved and suspended substances long

distances

• Water bodies are repositories, but movement still occurs

Figure 5.8 Relatively more mixing involved with sediment in

chemical fate and transport occur in a shallow water body

(left) compared to a deeper, stratified body (right)

Wind drift

Return current

Disturbed sediment

Wind flow

Wind drift

Return currentEpilimnion layer

Quiescent hypolimnion

Undisturbed sediment

Wind flow

Exchange with the Atmosphere

Gases from air to water

• Oxygen required by fish

• Carbon dioxide required by algae

• Air pollutants including acid gases and particles

Gases from water to air

• O2 from algal photosynthesis

• CO2 from microbial degradation of organic matter

• H2S from anoxic microbial reduction of SO42-

• Volatile organic water pollutants

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27 Exchange with Sediments

Pollutants are incorporated with particles as they form and

settle in water and are placed in sediments

Figure 5.9 Sediment record of environmental lead

Replacement of leadedgasoline with unleaded

1860

1880

1900

1920

1940

1960

1980

2000

70

60

50

40

30

20

10

0

0 100 200 300 400 500 600 700 800Lead concentration in sediment, mg/kg

Yea

r of

dep

osi

tion

Dep

th into

sed

imen

t, c

m

Gradually increased industrialuse, lead paint

Rise in use of leadedgasoline

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X

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X

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X

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X

X

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