1 1 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 [email protected]
Dec 03, 2015
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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
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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
4
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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8
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 5.7 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