A permeable reactive barrier (PRB), also referred to as a permeable reactive treatment zone (PRTZ), is a developing technology that has been recognized as being a cost-effective technology for in situ (at the site) groundwater remediation. PRBs are barriers which allow some—but not all—materials to pass through. One definition for PRBs is an in situ treatment zone that passively captures a plume of contaminants and removes or breaks down the contaminants, releasing uncontaminated water. The primary removal methods include: (1) sorption and precipitation, (2) chemical reaction, and (3) reactions involving biological mechanisms.
Reactive processes There are a variety of ways that permeable reactive membranes can be used in order to remediate groundwater. The two main processes are immobilization (AKA sequestration) and transformation.
Immobilization Immobilization of the contaminant may occur through sorption to the barrier materials or precipitation from the dissolved state. Organic compounds tend to be undergo sorption due to hydrophobic expulsion from the surrounding water. Metals, however, tend to sorb through electrostatic attraction or surface complexation reactions. Sorption and precipitation are potentially reversible and may thus require removal of the reactive medium and gathered products in order to continue with remediation.
Transformation Transformation involves taking the contaminant and transforming it to a less harmful or non-toxic form. One of the chief benefits of transformation is that it does not necessarily require removal of the reactive medium (unless the reactive medium must be replaced due to decreased effectiveness or clogging occurs). Transformation most commonly takes the form of an irreversible redox reaction. The medium may directly supply electrons for reduction or stimulate microorganisms to facilitate electron transfer.
Reactive Materials In addition, there are several different materials which may be used. Here are the more prominent:
Zerovalent iron
Zerovalent Iron was the first material to be used in PRBs for groundwater remediation. It continues to be the main material used in the construction of these barriers. In addition to conventional scale iron, nanoscale-iron may also be used.
Biological barriers Sometimes material will be put into the ground to stimulate the growth of microbes that facilitate the groundwater remediation. Many environmental pollutants are highly reduced, thus, the oxidation of these pollutants to harmless compounds is thermodynamically viable. Other pollutants, such as chlorinated solvents, are highly oxidized and as such are easily reduced. Microorganisms commonly facilitate such redox reactions, exploiting contaminant degradation as a means to obtain energy and materials for cell synthesis. Oxidative biodegradation necessitates electron acceptors that microbes use to "respire" the electrons removed from target contaminants. This transfer of electrons releases energy to drive microbial life functions. Under aerobic conditions, molecular oxygen is used for this purpose. When oxygen is not present, a variety of other molecules can serve as electron acceptors. Oxygen is preferentially utilized over the anaerobic electron acceptors because using oxygen gives more energy and, as an added benefit, results in faster contaminants oxidation rates. Unfortunately, the available oxygen is often not sufficient for the contaminants in highly contaminated areas, and as a result the anaerobic electron acceptors must be utilized. Reactive barriers containing oxygen-releasing compounds have been used successfully to stimulate aerobic biodegradation of monoaromatic hydrocarbons.
Surfactant-modified zeolites Clays, zeolites, and other natural material have a high capacity for cation exchange. They do this by creating a net negative charge by substituting lower-valent cations (e.g. Al3+) with a higher-valent cation (e.g. Si4+) within the mineral structure. Adding sorbed surfactants can change the affinity for anions and nonpolar organic compounds. Surfactants that have accumulated at the surface will create a hydrophobic organic coating that promotes sorption of non-polar organic compounds. Surfactant Modified Zeolites (SMZs) are promising for treating non-polar organic contaminants. However, clay's low permeability means it cannot be used in flow-through PRBs, but have been proposed for use in slurry walls, landfill liners, and containment barriers. Zeolites; however, have cavities to maintain hydraulic conductivity, allowing their use in PRBs.
Peat moss Peat moss has a large specific surface area (>200 m2/g) and a high porosity. Metals are taken up by peat through an ion exchange reaction where the metal displaces a proton if the pH is low or an existing metal if the pH is high from the anionic function group. Anions, such as CrO2−4 and MnO2−4 are removed more effectively at pH < 3 because of the positively charged surface created by the addition of protons onto the surface functional groups, whereas cations, such as UO2+2, Ni2+, Cu2+, are more effectively removed at higher pH values. Peat moss seems to be an effective ion-exchange material for removing heavy metals and some anions. Removal efficiency of cations approaches 100% at low pH, but the strong dependency on pH and the initial metal ion concentration have to be considered.
Groundwater modeling Modeling groundwater flow is important for optimizing the design of a PRB. Most importantly, by modeling the flow, the hydraulic capture zone width (HCZW) and the residence time can be determined. The HCZW is the width of the zone of groundwater that will pass through the reactive cell or gate (for funnel-and-gate configurations). The residence time is the time that the contaminated groundwater will spend in the treatment zone for decontamination. Contamination outside the capture zone or that does not have a long enough residence time will not be properly decontaminated. Groundwater modeling can also be used for the following:
Configuration
Iron barriers
… excerpt ends here. Continue reading the full article.
