Lecture No. 3. Chemical Fundamentals II.

Continuing with measurement fundamentals:

C. Oxygenation State

This is a description of the amount of oxygen and other oxidizing agents (electron acceptors) present.

When you poke around in wet ground or in stream sediments, you generally find one of two situations: either you find black or grey, stinky mud, or you find non-stinky mud that is brown, reddish, or orange. In the former case there may be a lot of hydrogen sulfide; in the latter, there is probably some dissolved oxygen in the pore spaces.

The 3 types of geochemical environments you may find are called

1. Oxic, which means there is abundant oxygen in solution. This produces red, orange, or brown, clean-smelling mud. The color is from the presence of Fe(III) oxides or oxyhydroxides.
2. Suboxic, meaning there is a little free dissolved oxygen, usually less than 5% of saturation.
3. Anoxic, meaning there is no measurable oxygen. Anoxic environments are often characterized by black or grey, stinking mud.

In an anoxic situation the redox state is usually reduced (low E_h). This is not always true, though; in the absence of organic matter or some other reducing agent the redox state may be "perched" at high E_h even though there is no free oxygen.

D. Conductivity

This is the ability of a medium to conduct electricity. It correlates well with the total dissolved solids concentration. It sometimes correlates well with the dissolved concentration of a given element in a system. (Example: Kesterson Reservoir in California is the site of serious selenium contamination. When there was water in the lake, conductivity correlated well with dissolved Se.) In any case, it is a quick and easy, rough measurement to estimate TDS.

E. Ways to Express Concentration

We may want to express the concentration of a solute in a liquid or solid (or gas or supercritical fluid, …)

1. We can express it as a mass/volume measurement (for liquids):

For example, we may measure mg/L H₂O, m g/mL, etc.

2. We may use chemist’s measurements, which are based on gram-molecular weights, or mols

Molarity is mols of solute per liter of solution.

Molality is mols of solute per kilogram of water.

In dilute fresh-water systems, the density of water is very nearly one, and so molarity and molality are nearly the same.

3. We may use mass per unit mass. This is almost mandatory for solid solutions, and it is useful for liquids sometimes.

Parts per million (ppm) = mg/kg = m g/g

Parts per billion (ppb) = m g/kg

Note that ppm is approximately the same as mg/L in dilute aqueous solutions.
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Assignment due Tuesday, September19: Fill in the blanks in the following table.

Global Composition

Here are the crustal abundances of some selected elements. "Major" elements are those present at 0.1% or more; "trace" elements are those present at lower concentrations.

Note how the elements most important to life, except for oxygen, hydrogen, and phosphorus, are trace elements.

We often get the quantities we need of various elements from enriched deposits. "Enriched" is a relative term. An economic iron deposit is about 50% Fe; an economic copper deposit is about 0.4%; and the grade of an economic gold deposit varies somewhat with the price of gold, but it is below 1 ppm.

Major Mineral Groups

• Silicates

Quartz (SiO₂), feldspars (KAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈, and intermediate compositions), micas, amphiboles, pyroxenes, olivine, clays, feldspathoids, etc. etc.

• Oxides and Hydroxides

Corundum (Al₂O₃), hematite (Fe₂O₃), magnetite (Fe₃O₄), pyrolusite (MnO₂), ilmenite (FeTiO₃), sphene (CaTiOSiO₄), goethite and lepidochrosite (FeOOH), ferrihydrite ((FeOOH)₅(H₂O)₂), birnessite (approx. formula Na₄Mn₁₄O₂₇á8 H₂O), etc.

• Carbonates

Calcite (CaCO₃), dolomite (CaMg(CO₃)₂), siderite (FeCO₃), etc.

• Sulfides

Pyrite (FeS₂), galena (PbS), sphalerite (ZnS), etc. Here the sulfur is important environmentally, as well as the metal.

• Sulfates

Gypsum (CaSO₄) and the authigenic sulfates listed in the first lecture.

• Halides, native elements, and other classes of mineral are mostly less important in environmental geochemistry.

These minerals can be (1) sinks, (2) sources, or (3) substrates for contaminants. Silicates are especially important as substrates.

You have to rearrange your geological thinking for this course. You consider not just mineral phases, but also contaminant-mineral phases.

Definition: A contaminant-mineral phase is a mineral phase that can hold contaminants* in some form.

*For us, contaminants will usually be metals or metalloids, but in general organic contaminants also count.

Metal(loid) contaminants reside in solid phases in 3 ways.

1. within the mineral lattice, in which case availability depends on the solubility of the matrix and on environmental factors (pH, redox environment, temperature, biota, …)

Consider Zr in zircon, or Mg, Fe, Al in biotite. Elements tied up in silicates are usually hard to get out.

Or consider iron in oxides. Iron in magnetite is very strongly bound; in goethite it is relatively weakly held, and a change in redox conditions or pH might get it out readily. It is mainly a matter of crystallinity.

2. on surfaces, as a coating

Here the mechanism is largely ion exchange. The coating can be discontinuous, continuous over a particle, or a matrix binding multiple particles together as a matrix (like sedimentary cement). Sand (and cobble) coatings are enriched in metals and often act as sinks. This works especially well for Fe and Mn oxyhydroxides and for clays. Clays can bind elements in their interlayers.

Cation exchange capacity (CEC) measurement was developed for clays.

1. 100 g of a material is repeatedly treated with saturated NaCl at pH 7.
2. The material is then added to a concentrated KCl solution. Either K⁺ uptake or Na⁺ release is measured.

The usual units of measure are mEq/100 g.

3. by organics, especially humic substances

The usual mechanism of uptake by organics is chelation. Chelation is the binding of positive ions (normally metal ions) by multiple negatively charged radicals on the same molecule.

Humic substances are usually classified either by solubility (e.g., in NaOH solution) or by ability to be oxidized (e.g., by H₂O₂).

Example of a Procedural Definition for Humic Substances:

Brown humic acids are characteristic of humic acids in peats and spodosols (fairly heavily leached, usually acidic, cool-climate forest soils). Grey humic acids are characteristic of mollisols (cool-climate, usually alkaline, grassland soils).

Humic substances are complex organic compounds with multiple rings and multiple polar substituents (phenols, carboxylic acids, and aliphatic hydroxides). These substituent groups are well placed to chelate ions like Ca²⁺, Fe²⁺, Al³⁺, etc.

Chelated metals tend to be very tightly bound.

Solute "Phases" (Solute Fractions)

1. Metal Cations

The cations are in general hydrated. Thus they are not simply Mⁿ⁺, but M(H₂O)_xⁿ⁺.

Examples:

2. Anions

Mostly we are concerned with inorganic ions like SO₄^2–, Cl^–, NO₃^–, HPO₄^2–, H₂AsO₄^–, etc., but some organics (e.g., fulvates) may also be important. Anions in general are also hydrated.

Some metalloids (As, Se, Sb) and some metals (Mo, W, Cr) may move through the environment as anions.

3. Inorganic Complexes

These include complex cations (Cu(OH)⁺), complex anions (HAsO₄)^2–), and neutral species (H₃AsO₃).

4. Methylated Forms

These are important to the cycling of a number of metals and metalloids, including As, Sb, Se, Te, Pb, Hg, and Sn.

Some are polar, like dimethylarsinic acid ((CH₃)₂AsOOH) or monomethylmercury cation (Hg(CH₃)⁺). Others are nonpolar and often volatile, such as tetramethyllead (Pb(CH₃)₄) or trimethylarsine ((CH₃)₃As).

Note: There are also volatile compounds that are not important in solution (because they are insoluble or because they decompose rapidly in water), but which can transport metals or metalloids to an aqueous environment. Some of these are hydrides, such as arsine (AsH₃). Some are halides, most of which are not important in geochemically governed systems but which may enter the environment through industrial processes or automotive fuels, such as chromyl chloride (CrOCl), zirconyl chloride (ZrOCl₂), and lead tetrachloride (PbCl₄). Some are miscellaneous organometallic compounds. Gaseous forms are important parts of the geochemical cycles of nitrogen, sulfur, carbon, and even phosphorus (if fires are involved). Finally, elemental mercury is itself volatile.

5. Chelates

These include

• organic chelates, such as humate and fulvate complexes,
• inorganic chelates, such as pyrophosphate complexes, e.g., Ca(P₂O₇)^2–

Chemical Phases and Species

Definition: A chemical species is a form taken by an element in or out of solution.

There are two main phases that can hold contaminants: particulate and solute. Within these phases can be found various species.

How does one define the boundaries between "particulate" and "solute"?

Consider the range of particle sizes in water.

Cobbles ~10⁵ m m

Sand ~10³ m m

Clays, bacteria ~ 1 m m

Virus molecules ~ 10^-1 to 10^-2 m m

"Colloids" ~10^-1 to 10^-3 m m

Ions in solution ~ 10^-3 m m and less

The definition generally used is strictly operational: If it is <0.45 m m, it is a solute.

We will discuss this in more depth later.

Next: Chemical Reaction Types

To previous lecture: Lecture No. 2. Chemical Fundamentals

To next lecture: Lecture No. 4. Chemical Reaction Types

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