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Permeable Reactive Barriers - The Chemistry

CHEMISTRY

The Permeable Reactive Barrier’s reactive material varies depending on the constituent(s) that need removal. The most common reactive materials used in PBRs are Zero-Valent Iron (ZVI) and Granular Activated Carbon (GAC). There are other types that are used less frequently than ZVI or GAC, however the effectiveness of these materials has not been studied as well as ZVI or GAC. Examples of these reactive materials can be found in Table 1.

 

Table 1 Reactive materials

Table 1: Reactive materials used in Permeable Reactive Barriers. (Bronstein, 2005)

 

There are four types of reactive processes used in a PRB to remove contaminants from the groundwater: (Bronstein, 2005)

1. Abiotic reduction

2. Biotic reduction-oxidation

3. Chemical precipitation

4. Sorption or ion exchange

In abiotic reduction, Zero-Valent Iron is the primary reactive material used (Bronstein, 2005) because it reduces the ionic state of passing contaminants, so the contaminants degrade to less harmful compounds that either precipitate out or flow through the barrier. (Thiruvenkatachari et al., 2008) Examples of reduction of uranium and chromium are shown in Figure 3a, below. Another common abiotic reduction by ZVI is chlorinated organic solvents. (Vogan, 1999) This process is shown in Figure 3b, below, where the chlorinated organic compound such as PCE or TCE is represented by R-Cl. 

 

Figure 3 Abiotic Reduction

Figure 3a: Example of abiotic reduction chemical processes (Bronstein, 2005)

 

Chemicals

Figure 3b: Abiotic reduction of a chlorinated organic compound (Vogan, 1999)

Biotic reduction/oxidation uses reactive materials that promote microbial growth to degrade contaminants. Some materials include Oxygen Releasing Compounds (ORC), which provide the dissolved oxygen and nutrients necessary to grow microorganisms which can then break down the contaminant. (Yeh et al., 2010) Sulfate reducing bacteria is another example of this technique, which degrades the contaminant so the metals bind with sulfur and precipitate out of the contaminated water. (Bronstein, 2005) The chemical process is shown in Figure 4.

  

Figure 4 Biotic

Figure 4: Example of biotic reduction/oxidation chemical process with sulfate reducing bacteria (Bronstein, 2005)

 

Chemical precipitation utilizes materials that react with constituents to form a solid state, which precipitates out of solution allowing for easier removal. The most common material for this technique is limestone. (Bronstein, 2005) Figure 5 gives an example of mineral precipitation using a hydroxide such as apatite and a carbonate such as limestone. 

Figure 5 Chemical Precipitation

Figure 5: Example of chemical precipitation chemical processes (Bronstein, 2005)

 

In sorption or ion exchange the reactive material promotes adsorption of the contaminant to the material for removal from groundwater. The absorption process occurs when the molecules or particles bind to a solid surface. The common materials used in this technique are ZVI, zeolites, amprphous ferric oxyhydroxide (AFO), and activated carbon. (Bronstein, 2005)  Adsorption treatment techniques can be used in activated carbon PRBs for a PCE polluted site as an alternative to abiotic reduction. (Di Nardo et al., 2010)

 

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