Permeable Reactive Barriers (PRB) are typically used to control the down gradient spread of contaminated plumes. A PRB can have a wide variety of designs depending on the groundwater flow characteristics, geochemistry and contaminants to be controlled. The most common barriers utilize biological treatment or reduction of contaminants by Zero Valent Iron (ZV). There are circumstances in which both are utilized in a single barrier.
The majority of installed Permeable Reactive Barrier designs use iron metal, ZVI (Feo, as the reactive media for converting contaminants to non-toxic or immobile species. Iron metal has the ability to reductively dehalogenate hydrocarbons, such as converting trichloroethene (TCE) to ethene. It can also reductively precipitate anions and oxyanions, such as converting soluble Cr(VI) oxides to insoluble Cr(III) hydroxides. Organic materials are being used as reactive media in some PRBs to biologically remediate certain other contaminants, such as nitrate and sulfate. Both laboratory and field results have shown that the rate of transformation of these and many other contaminants is sufficiently rapid for PRBs to be successfully used as full-scale PRB remediation systems. Numerous other reactive materials are being investigated, as are means to enhance both the iron and biological reactions.
Hepure provides chemical and biological Permeable Reactive Barrier solutions. Selecting the best solution for PRB depends on the specific conditions and goals of your project.
Ferox PRB® ZVI reactive iron powder has been specifically designed to optimize reactivity and longevity for reactive barrier applications. Hepure has supplied over 1.5 million pounds at hundreds of project sites within the U.S. and internationally. Reactive barriers require reactivity to insure contaminants are reduced in the time they pass through the barrier, and provide a longevity of over 10 years. This is controlled by two factors; a high purity iron with little or no rust, and balance of particle size to provide reactivity and longevity.
Ferox PRB ZVI is provided in two particle distributions, Standard and Coarse. The Standard particle mixture provides a balance of reactivity and longevity helping reduce the cost of barrier construction while providing over 10 years of barrier life. The Coarse mixture extends the barrier life by providing a coarser mixture of ZVI particles which may require a wider barrier to insure reactivity. A comparison of barrier designs can be found in the Permeable Reactive Barrier Design section.
Important Factors for PRB design
- Groundwater Flow: The design size of a barrier is greatly influenced by the groundwater flow rate. It is important to have the contaminant within the barrier for an adequate time to achieve the reductions desired.
- Defining the Matrix: Soil type (e.g. % of sand, silt or clay), fractured rock or sediment and the amount of heterogeneity.
- Type of Contaminants: A good understanding of the contaminant or mix of contaminants in the soil matrix. General categories are Chlorinated Volatile Organic Compounds (CVOC), Petroleum Hydrocarbons, Metals, Inorganics, Pesticides, Polycyclic Aromatic Hydrocarbons (PAH), and the newest categories of PFOS and PFAS. Some contaminates may be addressed with a single technology (CVOC and Metals) where others will require different approaches (Inorganics and Pesticides).
- Concentration of Contaminants: The concentration and mass of contaminant is very important. Many remedial methods work well with lower concentration and some with elevated concentration and/or free product.
- Matrix Chemistry: One of the most overlooked parameters is chemistry, specifically geochemistry. This refers to substances which normally would not be thought of as a contaminant but may interfere with implementation of remedial technologies. Soil matrix chemistry includes, major cations and anions, mineral content, pH, buffering capacity, ion exchange capacity, acidity, salinity, and ORP. The interaction of factors can be complex, such as precipitation of arsenic to arsenopyrite requires the correct pH and Eh conditions as well as the right iron and sulfate concentration which is dynamic depending on the surface reducing bacteria activity.
Permeable Reactive Barrier Design
There are two main factors which control the design of a reactive barrier to insure the contaminants are reduced to the desired effluent concentrations and insure the longevity of the barrier. Hepure utilized two models to aid in determining the amount of ZVI needed for a barrier given these conditions.
(1) Reactivity: The reactivity of the permeable reactive barrier substrate to the contaminant determines the time in which the flowing water must stay within the barrier to achieve the desired contaminant reductions.
Simple Model dc/dt=−km ρm Co
After Integration pm = Ln(C/C0) / (km t)
Where ρm = (Grams ZVI)/(Liters of Water)
In the case of a steady state flow. Liters of Water, can be stated as a relation of the Area (A), pore space (Ꜫ), and the flow velocity (µ).
Mass of ZVI = Ln(C/C0) / (km t A Ꜫ µ)
(2) Chemical Demand: The contaminants of concern are typically one of the lower demands on the barrier substrate. Other chemicals flowing with the water can also consume the barrier substrate. Common demands are Oxygen, Nitrogen, Sulfate, Hardness, and soluble metals.
High chemical demand requires both high concentration and high groundwater velocity and can be estimate for years of life based on the groundwater flow and chemical demand concentration.
ITRC. (2011). Technical / Regulatory Guidance Permeable Reactive Barrier : Technology Update PRB-5. -Interstate Technology & Regulatory Council, (June), www.itrcweb.org.
ITRC. (2005). Technical / Regulatory Guidelines Permeable Reactive Barriers : Lessons Learned / New Directions. Interstant Technology & Regulatory Council, Permeable Reactive Barrier Team, (February) www.itrcweb.org..
EPA (1998) Permeable Reactive Barrier Technologies for Contaminant Remediation, EPA/600R-98/125
Gilham, Robert W.; Vogan, John; Gui, Lai; Duchene, Michael and Son, J. (2010). Iron Barrier Walls for Chlorinated Solvent Remediation. (H. c. Stroo, Hans F.; Ward, Ed.), In Situ Remediation of Chlorinated Solvent Plumes. Springer. https://doi.org/10.1007/978-1-4419-1401-9
Wilkin, R. T., Acree, S. D., Ross, R. R., Puls, R. W., Lee, T. R., & Woods, L. L. (2014). Fifteen-year assessment of a permeable reactive barrier for treatment of chromate and trichloroethylene in groundwater. Science of the Total Environment, 468–469, 186–194. https://doi.org/10.1016/j.scitotenv.2013.08.056
Henderson, A. D., & Demond, A. H. (2007). Long-term performance of zero-valent iron permeable reactive barriers. A critical review. Environmental Engineering Science, 24(4), 401–423. https://doi.org/10.1089/ees.2006.0071