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.Gillham, R.; Vogan, J.; Gui, L.; Duchene M.; Son J. (2010). Iron barrier walls for chlorinated solvent remediation. In: Stroo, H. F.; Ward, C. H. (eds.), In Situ Remediation of Chlorinated Solvent Plumes. Springer Science+Business Media, New York, NY, p. 537. The primary removal methods include: (1) sorption and precipitation, (2) chemical reaction, and (3) reactions involving biological mechanisms.Tratnyek, P. G.; M. M. Scherer; T. J. Johnson; Matheson, L.J. (2003). Permeable reactive barriers of iron and other zero-valent metals. In: Tarr M. A. (ed.), Chemical Degradation Methods for Wastes and Pollutants; Environmental and Industrial Applications. Environmental Science and Pollution Control, Marcel Dekker, New York, pp 371-421.
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.Scherer, M. M.; Richter, S.; Valentine, R. L.; Alvarez P. J. J. (2000). "Chemistry and microbiology of permeable reactive barriers for in situ groundwater clean up." Critical Reviews in Environmental Science and Technology. 30(3): 363-411.
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.Bohn, H.L.; McNeal, B.L.; O'Connor, G.A. (1985). Soil Chemistry. Wiley Interscience, John Wiley & Sons, Inc. 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.Sheng, G.; Xu, S.; Boyd, S. (1996). Mechanism(s) controlling sorption of neutral organic contaminants by surfactant-derived and natural organic matter. Environmental Science & Technology. 30(5): 1553-1557. 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.McLellan, J. K.; Rock, C.A. (1988). Pretreating landfill leachate with peat to remove metals. Water, Air, & Soil Pollution. 37(1-2): 203-215. 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.Crist, R. H.; Martin, J. R.; Chonko, J. (1996). Uptake of metals on peat moss: an ion-exchange process. Environmental Science & Technology. 30(8): 2456-2461. Anions, such as and 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 , , , are more effectively removed at higher pH values.Morrison, S. J.; Spangler, R. R. (1992). Extraction of uranium and molybdenum from aqueous solution: a survey of industrial materials for use in chemical barriers for uranium mill tailing remediation. Environmental Science and Technology. 12(3): 1922-1931. 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:
Determining the location of the PRB Determining a suitable configuration Determining the width of the reactive cell (and funnel for funnel-and gate) Evaluating potential for underflow, overflow, or flow across aquifers Providing knowledge of groundwater flow fluctuations (velocity and direction) for use in the design Determining reactive media selection (based on hydraulic conductivity) to match the conductivity of the aquifer Evaluating possibilities for flow bypass due to reduced porosity Helping determine monitoring well locations and monitoring frequenciesFox, T. C.; Gupta, Neeraj. (1999). Hydrogeological modeling for permeable reactive barriers. Journal of Hazardous Materials. 68(1-2): 19-39.
Configuration
Iron barriers
The accompanying figure shows two approaches to application of iron particles for groundwater remediation: Fig. A, a conventional PRB made with mm-sized granular iron and Fig. B, a "reactive treatment zone" formed by sequential injection of nano-sized iron to form overlapping zones of particles absorbed by the grains of native aquifer material. In A, groundwater flows through the barrier and is remediated. In B, nanoparticles of iron are represented by black dots; the nanoparticles have little mobility in the porous medium. Note that reaction will only occur when contaminants, either dissolved in the groundwater or as DNAPL, come into contact with the iron surfaces.Tratnyek, P. G.; Johnson, R. "Remediation with Iron Metal." Center for Groundwater Research. Oregon Health and Science University, 04 Feb. 2005.
Funnel and gate Funnel and gate systems are used to channel the contaminant plume into a gate which contains the reactive material. The funnels are non-permeable, and the simplest design consists of a single gate with walls extending from both sides. The main advantage of the funnel and gate system is that a smaller reactive region can be used for treating the plume, resulting in a lower cost. In addition, if the reactive media needs to be replaced, it is much easier to do so because of the small gate.Sutherson, S. S. (1997). 'In situ' reactive walls. In: Sutherson, S. S. (ed.), Remediation Engineering: Design Concepts. CRC Press, Newtown, PA, pp. 187-213.
Implementation PRBs are typically installed by digging a long trench in the path of the flow of the contaminated groundwater. The trench is then filled with the reactive materials (typically iron, carbon, or limestone). Sand can be mixed with the reactive material to aid in allowing the water to flow through the materials. Sometimes, there will be a wall that directs the groundwater to the reactive parts of the barrier. After the trench has been filled with reactive material, soil will typically be used to cover the PRB, thus eliminating visibility from the surface.United States of America. Environmental Protection Agency. Office of Solid Waste and Emergency Response. A Citizen's Guide to Permeable Reactive Barriers. Environmental Protection Agency, Apr. 2001.
Sheet pile and excavation Sheet pile and excavation were used for the installation of earlier PRBs. This method involves containing the area of excavation using sheet piles before excavating using a trackhoe. This method may be slow (and therefore expensive) and is only viable for plumes less than 35 feet deep.
Continuous trencher Continuous trenching involves using a large cutting chain excavator system then using the trench box and hopper to continuously back-fill the trench with reactive media. Continuous trenching can be fast and thus, inexpensive, but can only be used for trenches less than 50 feet deep. In addition, the machinery used for this technique cannot be used effectively for soil with large cobbles.
Mendrel emplacement Mendrel technology involves vertically driving a long hollow beam deep into the ground. The beam is covered as it is driven in, and the cover is removed once the beam has been placed. Next, the hollow is filled with iron filings. The Mendrel is then vibrated as it is removed, allowing the iron to flow to the bottom, forming the PRB. The Mendrel is then moved one width over, the process is repeated, and a continuous PRB is made.
Hydraulic fracture