Aqueous Geochemistry-Phase Diagrams

Earth Science Extras

by Russ Colson

 

The Le Chatelier Principle: An Example Water Polllution Problem

In 2015-2016, the United States was riveted by the news that high lead (Pb) levels in the public water system in Flint Michigan was poisoning children.  The vehemence of the initial denials from officials who claimed that the accusations were from cranks with no understanding of water supply only made the local government look worse when it became clear that not only was the water supply truly dangerous, but it was dangerous for reasons that those same officials should have anticipated and prevented.

 

In fact, a little bit of understanding of the same kind of basic water phase chemistry that is important in understanding ore-forming processes and pollutant migration would have prevented the problem.   Let's take a look at it.

 

Although water chemistry can be complex, with chemical components dissolved in water interacting in complex ways, we can understand the basic problem in Flint Michigan by considering a simple chemical reaction.  

 

PbSO₄ (less soluble in water) + 2Cl- ↔ PbCl₂ (more soluble in water) + SO₄ ^2-

 

What does this reaction tell us?   The two forms of Pb shown in this equation are lead sulfate and lead chloride.  Lead chloride is much more soluble in water, meaning that it has the potential to release solid Pb that is present in many old water pipes into the water where residents would drink it.

 

The reaction also shows involvement of sulfate and chloride in the water. Perhaps the most important concept in understanding how composition affects a reaction is the idea that if you increase the chemical activity on one side of a reaction, it will cause the reaction to proceed toward the other side. Chemists call this principle the Le Chatelier Principle. (Note: chemical activity is a measure of the chemical 'potential energy' of a component, and increases, roughly, in step with concentration.)

 

Considering the Le Chatelier Principle, and the reaction above, we can infer that increasing the concentration of chlorine, or decreasing the concentration of sulfate, will drive the reaction toward the right, producing more soluble PbCl2. Said another way, increasing the Cl/sulfate ratio will drive the reaction to the right.

 

What were the series of errors that caused the serious lead problem in Flint Michigan?

 

1) In April of 2014, the town switched from getting their water from Detroit to taking it out of the Flint River (as a cost-saving measure during an economically difficult time). Because the Flint river gets lots of water runoff from roads (where salts like NaCl and CaCl2 are added in winter to melt ice), the Flint River had a much higher concentration of Cl than the water from Detroit. In fact, the Cl/sulfate ratio in the Flint River was between 3 and 4 times higher than the Detroit water supply. The increased Cl in the water caused lead present in the water pipes of Flint Michigan to dissolve in the water (see the reaction above and think about the Le Chatelier Principle). Once dissolved, the lead moved with the water into people's homes where it was present in water used for bathing and drinking.

2) E. Coli bacteria were detected in the new water supply from the Flint River, so authorities added Cl to the water as a disinfectant. This of course drove the reaction even farther to the right, causing even more significant lead solubility.

3) Cl added to the water as a disinfectant reacted with organic matter present in the river water to form unsafe levels of trihalomethanes, compounds that form when Cl and organic matter react. In order to remove the organic matter from the water to prevent formation of the trihalomethanes, authorities added ferric chloride (FeCl3) to the water. This compound acts as a coagulant, causing the organic matter to clump and settle out. Of course, it also increases the amount of Cl in the water even further, causing even more lead solubility.

4) Normally, to prevent this cascade of increasing Cl which causes Pb to dissolve in the water, authorities would add orthophosphate to the water, a compound that conbines with Pb to form an insoluble residue.

PbCl + orthophosphate ↔ Cl- + insoluble orthophosphate-lead compounds

Unfortunately, the authorities in Flint Michigan chose to not add the orthophosphate.

5) Thus, Pb in the Flint Michigan water supply rose to dangerously high levels.

 

 

 

 Another Application of the Le Chatelier Principle

 Consider the following reaction:

                              Ca^2+* + Mg^2+' ↔ Ca^2+' + Mg^2+*

 where ' indicates an ion dissolved in water, and * indicates an ion adsorbed onto a sediment particle, such as a clay mineral. Thus, this reaction portrays a Ca cation adsorbed on a sediment particle plus a Mg cation dissolved in water switching places to create a Ca cation dissolved in water plus a Mg cation adsorbed onto a sediment particle.   

 

 

Consider a river contaminated with Cd²⁺ flowing into the ocean.   Cadmium is a strong environmental poison, especially when it is dissolved in water and accessible to organism in the ecosystem.   In general, Cd oxide and Cd sulfide are very insoluble in water.   Thus, we might expect most of the Cd in the river to be associated with the sediment load and not dissolved in the water.  However, CdCl₂ is much more soluble in water.

 

Consider the reaction:

CdO^* + 2NaCl ^' ↔ Na₂O^* + CdCl₂ ^'

where * indicates an insoluble species (part of the sediment load) and ' indicates a soluble species (present in solution in the water)

 

 

 

Phase Diagrams

Phase diagrams are graphs that portray the conditions of temperature, pressure, or chemical composition where different phases occur. One type of phase diagram important in understanding water geochemistry is an Eh-pH diagram, sometimes called a fence diagram or Pourbaix diagram. Eh is a measure of oxygen activity in the environment (or, the redox potential) and pH is a measure of acidity (or H+ ion concentration). Eh-pH diagrams are a graphical way of showing the Le Chaterlier principle for reactions that involve oxygen (Eh) and H+ ions (pH). Each line segment on an Eh-pH diagram represents a chemical reaction.

 

Generally, it is difficult to show more than one or two chemical components at a time on such a phase diagram, and so the complexities that arise when many components interact with each other can't be easily portrayed. Consequently, computer programs that calculate phase stabilities for many components simultaneously are often used in real environmental problems involving water chemistry. However, the diagrams are still highly useful in visualizing problems and conditions under which reactions might take place, and in preventing the computer programs from becoming 'magic black boxes' that the users don't fully understand (the authorities in the Flint Michigan problem would have done well to visualize a few simple conceptual problems).

 

To understand the connection between an Eh-pH diagram and the Le Chaterlier Principle, let's consider a simple reaction for the dissolution or precipitation of CaCO3, a reaction that is important in making caves, karst terrrane, or stalactites in MInnesota, in making the rocky part of a coral reef, the travertine deposits at Mammoth Hot Springs in Yellowstone, or the tufa deposits in playa lakes of California. It is an important reaction in the transition from acidic to alkaline soils that takes places as one goes from the woodlands of central Minnesota into the grassland of Eastern North Dakota, and can be a fingerprint mineral for idenfitying arid climates in the geological past. It is the reaction that results when calcite or limestone is tested with acid in a common classroom or field test.

 

CaCO3 + H+ ↔ HCO3 (in solution) + Ca2+ (in solution)

 

 

 

Considering the reaction of CaCO3 above, the Le Chatelier Principle, and the idea that a phase diagram is a visual representation of the Le Chatelier Principle, identify which of the graphs correcctly illustrates the dissolution reaction for CaCO3. On these Eh-pH diagrams, Eh is plotted on the Y axis (vertical axis), with more oxygen upward, and pH is plotted on the X axis (horizontal axis) with more H+ to the left (that is, lower pH to the left). Think about this: If pH decreases (more acidic, more H+) which phases should we get in that region? What about if pH increases? What happens if Eh (oxygen) increases?

 

Notice that, with Eh plotted on the Y axis and pH on the X axis, a reaction that involves oxygen will be a horizontal line, a reaction that involves H+ ions will be vertical line, and if both O2 and H+ are involved, the reaction will appear as a diagonal line on the Eh-pH diagram.

 

 

The reaction between ferrous iron (Fe2+, mostly soluble in water) and ferric iron (Fe3+, mostly insoluble) is an important factor in the formation of iron ore deposits in Minnesota, in the colorful reds and yellow colors often associated with a "polluted" water body, and in the deposition of an orange "ring around the bathtub" in areas that use well water. Below, I want you to think about a reaction and, based on your understanding of fence diagrams and the Le Chatelier Principle, infer the qualitative character of the phase diagram. The key idea is to remember that the phase diagram will predict how changes in oxygen or pH will affect the reaction, and the Le Chatelier Principle makes that same prediction. Think about changing Eh or pH one at a time--which species will that make more likely to form?

 

Fe(OH)2 (Ferrous, soluble) + H2 + 1/2O2 ↔ Fe(OH)3 (Ferric, insoluble) + H+

 

Although this is not the usual form for this reaction (H2 does not really exist in meaningful concentrations in Earth's natural environments), it does allow us to examine the effects of Eh and pH on the reaction. Consider this reaction for the following questions.

 

Application of the iron phase diagram and Le Chatelier Principle to an environmental problem

 

Here is a fuller version of the iron Eh-pH diagram. Find the diagonal line that represents the reaction between ferrous iron (Fe2+) and the ferric hydroxide, Fe(OH)3, that we have been considering.

 

Consider two rivers in Colorado (see the picture below taken by former student Chris Johnson).  One is flowing from a mining district with abundant sulfide minerals exposed by the mining operation.  The presence of sulfide, when oxidized in the presence of water, produces sulfuric acid (1/2FeS2 + 1.5O2 + H2O = H2SO4 + 1/2Fe).  Iron, Copper, Lead, and many other metals are more soluble under acidic conditions than under basic conditions, thus this river contains a lot of dissolved metals, including ferrous iron.  Because these metals are in solution, the water is clear.

The second river is flowing from an area dominated by limestone.  The limestone buffers the acidity of the water at a high pH (low acidity).  This is the larger of the two rivers.  Its water is also clear.

Downstream, the two rivers join (the point where two rivers join is called the confluence).  The water at the confluence turns dark and murky.  Sediment of various colors, including orange and green, is seen on the bottom and along the shores in the region. 

Explain what has happened.

 

  

 

 

Continuing to thiink about the iron phase diagram

 

 

 

last updated 8/3/2020.   Text and pictures are the property of Russ Colson unless otherwise noted in the text.