Why are phase interactions important?

Most important environmental chemical processes in water involve interactions between water and another phase.

Note: All the spheres have boundaries with all the other spheres and components of each sphere may be converted into another and back again in cycles.

Biosphere if accumulated long enough becomes the geosphere.

example: coal, oil, natural gas

Burn these geosphere components and they combine with the atmospheres O₂ and become part of the atmosphere CO₂.

Until the biosphere recycles them with energy from the sun and causes CO₂ fixation in the biosphere and providing O₂ to the atmosphere and biomass to the biosphere... etc.

Are there some other examples?

Concept and Reality Check

Question 1.

Why do we care about phases and aqueous solutions?

Question 2.

What is this chapter all about? (really)

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Important Concept!

"Fate and Transport"

What happens to the species of interest, how are they chemically transformed into other(s) {"Fate"} (sometimes temporary and transitional species), and how are they "transported" and then made available to the environment, to the public,...

Examples of Phase Interactions

Biological or Biosphere interactions

Aerobic respiration
{CH₂O} + O₂(g) ----> CO₂ + H₂O

Anaerobic respiration
2 {CH₂O} ----> CO₂(g) + CH₄ (g)

Photosynthesis
CO₂ + H₂O --(hv)--> {CH₂O} + O₂(g)

Biological Reduction in anoxic (anaerobic) conditions
(Fe⁺² to Fe⁺³ and SO₄2- to H ₂S as sulfate is the electron receptor instead of oxygen)

Fe(III) ----> Fe²⁺
SO₄^2- ----> H₂S (bacteria induced)
Fe²⁺ + H₂S + ----> FeS(s) + 2H⁺ (sediment forming rxn.)

Atmosphere

O₂ dissolution
(governed by Henry's law of partial pressure)

CO₂ and CH₄ released to the atmosphere

Particulates in Aqueous Phase
Adsorption onto metals hydroxides of Fe & Mn

Precipitation and Solubility
Ca + CO₃^2- ----> CaCO ₃(s)

Sources CaCl₂ from highway salt in the Winter
Carbon dioxide from the atmosphere or carbon dioxide from other carbonate salts from carbonate rocks or carobon dioxide from bacterial respiration dissolves in water etc.

Dissolution
Of nutrients from sediment of NH₄⁺ & PO ₄^3-

Leaching
Double Replacements Reactions
(similar to Ca and carbonate above but there are many others)
{Leaching is sometimes similar to double replacement reactions}

Sediments

Sediments are the accumulation of settled particulate mater produced by chemical, physical, biological and combinations of these mechanisms.

Containing soil, mineral, plant, animal, organic matter.

It is of varying sizes and varying consistency from sand-like, to thick organically or inorganically based goo.

Pore Water
It is water saturated and water between the particles, it is called pore water. Usually at very low pE (or E_H)

Pore water has the highest bio availability of nutrients and pollutants.

It is also where much of the chemistry of sedimentation takes place.

Biological organisms living in sediments are subject to high concentrations of nutrients (NH⁺, PO₄³⁺ ,) and pollutants (Fe ²⁺, Cd²⁺, Pb ²⁺, PCB, PNA, etc.)
{crayfish, crabs, clams, muscles, worms, bottom feeders, etc.}

Key Sediment Forming Reactions

Ca²⁺ + 2HCO₃^- ----> CaCO₃ (s) + CO₂(g) + H₂O
{when high levels of calcium ion flow into carbonate rich water or vice versa or when pH is raised by photosynthetic reaction}

4Fe²⁺ + 10H₂O + O₂ ----> 4Fe(OH)₃(s) + 8H⁺
{when a more soluble form of iron is brought in contact with oxygen rich water by joining of streams or by bottom pore water being brought to the surface in mixing with (Henry's law) soluble oxygen obtained from mixing with air.

(example, during dredging operations)

(Henry's Law P_gas = kM_gas, where: P_gas is the partial pressure of gas in the gas phase and Mgas is the molar concentration of gas in the liquid phase, k proportionality constant. O₂(g) <----> O₂ (aq)
generic X(g) <----> X(aq).

Alternative layers may be propagated based on alternative rations in different seasons or under different nutrient or flow conditions.

Example:

In lake Zurich alternative layers of
Summer
CaCO₃ produced by photosythesis in Summer and
Winter
FeS produced by bacteria reducing Fe(III) and SO₄ ^2- during Winter.
{reactions previously described}

Alternative layers of FeS and CaCO₃, in lake sediment.
This phenomenon has been observed in Lake Zurich .

Colloids and Colloidal Particles

Generally classified as
Hydrophilic
Hydrophobic
Associated

Hydrophilic generally made up of proteins, polymers humic acids, etc. Having great affinity for water

Hydrophobic clay, soil and other charged particles that have an active electrically charged double layer and are settled by the addition of salt to neutralize the charge. They do not settle or agglomerate naturally without a change in the water conditions.

Associated colloids are made up of self assembling particles such as lipids and soaps with a hydrophobic portion and a hydrophilic portion. They self assemble into micelles in water.

Sodium stearate is an example of a soap that does this (H₃₅ C₁₇COONa⁺ ).

Representation of colloidal soap micelle particles.

Colloids also form supersaturating of solution

We have previously discussed:

Surface Sorption
Colloidal Particles

Aggregation of Particles
Coprecipitation
Supersaturation (growth and aggregation)

Surface reactions on colloids are extremely important

Clays are very important colloidal particles

Clays are composed of hydrated silicon and aluminum oxides. These secondary mineral are formed by weathering of primary rocks.

Examples of Clays

Kaolinite: Al₂(OH)₄Si₂O₅
Montmorillonite: Al₂(OH)₂Si₄O₁₀
Nontronite: Fe₂(OH)₂Si₄O₁₀
Hydrous Mica: KAl₂(OH)₂(AlSi₃)O₁₀

Clay Cation Exchange
    Mechanisms causing adsorption of clays

K⁺, Na⁺ and NH₄⁺ replace the Al(III) and Si(IV) ions in clay just as they do in glasses causing degradation there. Clays are also excellent "Cation-exchangers" with a "Cation-exchange capacity, (CEC)" rated per 100g of clay because Al is +3 and Si is +4 when these are replaced with +1 and +2 ions, an overall (negative charge forms)

The surface is then charged with other +1 and +2 ions to neutralize the charge and colloidal particles of clay are formed.

Thus clay's play a part in transport of ions in/and as part of colloidal particles.

The high surface area also adsorbs organics as well as inorganics and transports them.

Acquisition of surface charge by colloidal MnO₂ in water.

Representation of negatively charged hydrophobic colloidal particles surrounded in solution by positively charged counter-ions, forming an electrical double layer.

Aggregation, of colloids -
through Coagulation and Flocculation

Coagulation involves reduction of repulsion charges to permit aggregation (ex. - addition of salt, in estuaries where salt and fresh water mix)

Flocculation involves formation of bridges of chemical bonds forming flocs which are bridges between colloidal particles.

Polyelectolytes such are bridged by metal ions on the surface of the colloid and aggregation is achieved. These polyanions are polyvinyl alcohols, polyacrylamide, polyethylene imine, polyacrylate etc.

Examples are provided in text.

Aggregation of negatively charged colloidal particles by reaction with positive ions, followed by restabilization as a positively charged colloid.

Synthetic Polyelectrolytes and Neutral Polymers Used as Flocculants.

Bacterial Flocs - bacteria are negatively charged at pH 5-9 the pH range of natural waters. Bridging molecules are usually involved in flocing out bacteria from natural waters.
(charge from removal of hydrogen from carboxcylic acid groups (R-COOH), etc.)

Surface Sorption by Solids and Colloids

1. Complexation (Mechanism) of metal ion

2. Surface displacement of hydroxides
(Mechanism)

These are specifically favored by hydrated metals (where MtL^Z+ chelate); such as we have discussed.

Zn(H₂O)x⁺²

Mn(H₂O)^x+2

Pb(H₂O)^x+2

Generically
Displacement of either 1. H⁺ or 2. 0H^-

1.   M-OH + MtL^z+ <---> M-OMtL^(z-1) + H ⁺

2.   M-OH + MtL^z+ <---> M-MtL^(z-1) + OH ^-

Adsorption

The Mechanisms:

Chemical reactions of sorbing ions with surface functional groups is described by a surface complexation approach or metamodel. It is actually a group of models that simultaneously interact.

Charge - Fe₂O₃ Hematite is Amphoteric and can gain a proton to attain a + charge FeOH₂⁺ or lose a proton to become negatively charged FeO ^-.

The geometric model of the interface identifies surface charge distributed among two discrete planes H⁺ and OH^- incorporated in the solid. Other ions are directly bonded to the surface. This is referred to as the "triple-layer model".

Thus:
1. Sorption on oxides takes place at specific coordination sites.
2. Sorption reactions on oxides can be described quantitatively via mass law equations.
3. Surface charge results from the sorption reactions themselves.
4. Surface charge sorption can be modeled by EDL (electric double-layer theory).

XOH° denotes the surface hydroxyl group projecting into solution

Models -

Cation Surface Complexation
Cation Surface Precipitation
Surface Precipitation
Surface Acidity
Anion Surface Complexation

Cation Surface Complexation Model - (Know this one)

Surface complexation of cations by hydrous oxides involves the formation of bonds with surface oxygen atoms and the release of protons form the surface.

Examples

XOH° + M²⁺ + H₂O <--------> XOMOH₂⁺ + H ⁺

or equivalently, XOH° + M ²⁺ <--------> XOM ⁺ + H ⁺

Cation Surface Precipitation Models

A new surface phase results form these reactions

Adsorption of M²⁺ on X(OH)_3(s)

XOH° + M²⁺ + 2H₂O <--------> X(OH)_3(s) + <--------> MOH₂ ⁺ + H⁺ Precipitation of M²⁺

MOH₂⁺ + M²⁺ + 2H₂O <--------> M(OH)_2(s) + <--------> MOH₂⁺ + 2H⁺

Precipitation of X³⁺

XOH° + X³⁺ + 3H₂O <--------> X(OH)_3(s) <--------> XOH° + 3H⁺

Surface Precipitation Model X(OH)_3(s)

It is difficult to distinguish between precipitation, complexation and adsorption at the surface in microscale.

Precipitation of X(OH)_3(s)

XOH° + X^3- + 3H₂O <--------> XOH° + 3H⁺

Precipitation of XA_(s)

XHA^- + X³⁺ + A^3- <--------> XA_(s) + XHA^-

Surface Acidity Model

General form of the surface acidity model is described thus:

XOH₂⁺ <--------> XOH° + H⁺ K_a1

XOH° <--------> XO^- + H⁺ K_a2

Where XOH₂⁺ , XOH° , and XO^- represent positively charged, neutral, and negatively charged surface hydroxyl groups, and K_a1 and K_a2 are apparent acidity constants.

Anion Surface Complexation Model

Specific sorption of anions occur via ligand exchange reactions in which hydroxyl surface groups are replaced by the sorbing ions.

Examples

XOH° + A^3- + H⁺ <--------> XA^2- + H₂O

XOH° + A^3- + 2H⁺ <--------> XHA^- + H₂O

Ref. Surface Complexation Modeling - Hydrous Ferric Oxide, David Dzombak and Francois Morel, John Wiley & Sons, Inc., NY, NY, pgs. 104-105, 192, 1990. pg. 1-41 overview

Solution Activity

Solution Activity Coefficients
Chemical equilibrium calculations are usually performed using molar concentrations rather than activities, and these quantities can deviate significantly form each other at high ionic strengths.

The corrections of equilibrium constants should be made to correct for high ionic strength solutions. Many natural waters are high ionic strength solutions such as sea and estuary water.

A table (2.13) of correction constants is provided in the reference on page 39 and the method of correcting equilibrium constants is discussed.

Ref. Surface Complexation Modeling - Hydrous Ferric Oxide, David Dzombak and Francois Morel, John Wiley & Sons, Inc., NY, NY, pgs. 104-105, 192, 1990. pg. 1-41 overview

Trace Metals in Oxidizing and Reducing Conditions

The interaction of water with oxides such as AlO₂, MnO ₂, FeO_x, etc. OH^- and O-H⁺ charged species are formed and are capable of interacting with other metal ions.

Organic Compound Adsorption

Like dissolves like is the rule of thumb.

Non-polar organics are adsorbed by non-polar sediment components

Polar organics are absorbed by more polar groups

Usually attributed to:

• van der Waals forces
  
• induced dipole interactions
  
• hydrogen bonding
  
• charge transfer complexation, and
  
• hydrophobic interactions
  

Example:
2,4-D (2,4-dichlorophenoxyacetic acid)

The Freundlich isotherm
X = K C_n

Where:
X is the amount of sorbed organic per unit weight or solid sediment
organioclay complex, n
and K a constant and the intercept

Reaction for the coupling of pollutant 2,4-dichlorophenol to an aromatic ring on a humic substance molecule.

Covalent bonding mechanisms of organics

Occurs creating bound residues
(usually to humic acids)

Through extracellular enzymes
(frequently the source)

Example:
oxidoreductases covalently bonds 2,4-dichlorophenol to humic acid molecules

For this reason covalent bonding and degradation is a consideration of new pesticides

These non-polar organic and aromatics are also bioacumulators that are accumulated in fat and adapose tissue and in aquatic food chain constituents

Sediments in anaerobic conditions (called anerobic fermentation)
CH₄ is the dominant product not CO₂.
2{CH₂O} ----> CH₄(g) + CO₂ (g)

Gases in sediment in the Chesapeake Bay Sediment

Gas	Depth	Gas conc., mL/L
N2	Surface	13.5
N2	1 m	2.4
Ar	Surface	0.35
Ar	1 m	0.12
CH4	Surface	0.00
CH4	1 m	1.4 x 10 2

The interaction of water and different phases is a very important concept inenvironmental chemistry

X. What reason did the text give for the reactivity of colloidal material?

Components of Answer:
"Colloidal material is involved with many significant aquatic chemical phenomenon. It is very reactive because of its high surface and area to voulme ratio."

Adsorption of ⁶⁰Co(II) ions.

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Revised 4/23/99.