Decontamination of Groundwater In Situ Remediation

Scientists have developed a promising in situ technique for treating groundwater contaminated by volatile (mainly C5 and Cz) chlorinated organics. They construct an underground permeable "wall" of material (mostly coarse sand) along the path of the water. The water is cleansed as a result of its passage through the wall and never has to be pumped out of the ground (see Figure 14-7).

The ingredient that is placed within the sand bed and that chemically cleans the water is metallic iron, Fe°, in the form of small granules, a common

FIGURE 14-7 In situ purification of groundwater using an "iron wall."

Permeable subsurface treatment wall composed of iron filings and sand

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• Stream of • chemicals ###

Direction of groundwater flow waste product of manufacturing processes. When placed in contact with certain chlorinated organics dissolved in water, the iron acts as a reducing agent, giving up electrons to form the ferrous, or Fe(II), ion Fe2+, which dissolves in the water:

Usually, these electrons are donated to chloroorganic molecules that are temporarily adsorbed onto the metal's surface; the chlorine atoms contained in these molecules are consequently reduced to chloride ions, Cl~, which are released into aqueous solution. This technique is an example of reductive degradation. For example, the reduction of trichloroethene to its completely dechlorinated form, ethene, can be written in unbalanced form as

C2HCl3->C2H4 + 3Cr

Upon application of a standard redox balancing technique for alkaline solution, we obtain the balanced half-reaction

CzHC13 + 3 HzO + 6 e"-> C2H4 + 3 CI" + 3 OH"

Combination of the half-reactions (after tripling the oxidation step to ensure the number of electrons lost and gained is the same) yields the overall reaction

3 Fe(s) + C2HC13 + 3 H20-> 3 Fez+(aq) + C2H4 + 3 CI" 4- 3 OH"

One of the by-products of the reaction is hydroxide ion, OH". Recall from Chapter 13 that in limestone-rich areas, the groundwater contains significant concentrations of dissolved calcium bicarbonate, Ca(HC03)2. The hydroxide ions produced in the groundwater remediation reaction react with bicarbonate to produce carbonate ion, C0 52 , which combines with dissolved calcium ions to produce insoluble calcium carbonate, CaCO ,, which then precipitates in the sand-iron mixture.

Field trials indicate that this new technology can work successfully for several years at least, and it may replace the pump-and-treat methods for many applications involving chlorinated methanes and ethanes dissolved in underground water.

Recently, it has been found that coating the iron filings with nickel speeds up the rate of degradation of the organic compounds by a factor of 10; with this modification, the technique may be even more useful than first imagined. In addition, it has been discovered that the elemental iron in the barriers will reduce soluble Cr6+ ions to insoluble Cr3+ oxides and can therefore remediate groundwater contaminated by Cr6+. (The environmental chemistry of chromium is discussed in more detail in Chapter 15). A technique for the in situ creation of elemental iron from its ions (Fe2+ and Fe3+) by the injection of aqueous reducing agents has also been tested.

An in situ technique of treating TCE and PCE by hydrogenation has been developed. The process uses dissolved H2 gas to rapidly dechlorinate these two organics, eventually forming ethane and HC1. The reaction, which uses a palladium catalyst, can be done within a well bore so that the water need not be brought up to the surface.

PROBLEM 14-7

The dissolution of iron in the process described above produces some molecular hydrogen gas, H2. Show by balanced equation(s) how the hydrogen could arise from the reduction of water rather than of TCE.

PROBLEM 14-8

Suppose that this "iron wall" technology reduced an appreciable fraction of TCE to vinyl chloride rather than completely to ethenc. Why would this be an unacceptable result environmentally? (Note that in practice a sufficiently thick wall of iron is used to convert any vinyl chloride by-product to ethene.)

PROBLEM 14-9

Deduce the overall reaction by which perchloroethene is converted to ethene by metallic iron.

PROBLEM 14-10

At one test site for this remediation process, the water contained 270 ppm TCE and 53 ppm perehloroethene. Calculate the mass of iron required to remediate 1 L of this groundwater.

Continue reading here: The Chemical Contamination and Treatment of Wastewater and Sewage

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