Remediation of Contaminated Sites- Permeable Reactive Barriers
By Dr. Yashpal Singh
Permeable reactive barriers are emerging as an alternative to traditional pump and treat systems for ground water remediation. Laboratory studies indicate a great utility in treating ground water contaminated with a large number of inorganic substances including As, Cd, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Se, Te, U, V, NO3, PO4 and SO4 etc.
Permeable reactive barriers are put in the path of a migrating plume of contaminated ground water.
Solid phase organic carbon in the form of municipal compost has been used to remove dissolved constituents associated with acid mine drainage including SO4, Fe, Ni, Co, Cu, Cd and Zn.
Reactive materials within the barriers promote geo chemical reactions which result in the destruction or stabilization of the Ground water contaminants. These materials are sufficiently reactive to effect the treatment for years or even decades.
Biologically mediated reduction of sulphates to sulphides and consequent precipitation/formation of Metal Sulphides take place according to the following equation.
2CH2O(s) + SO4-2(aq)+2H+(aq) Yields H2S(aq)+2CO2(aq)+H2O
Me2+(aq)+H2S(aq) Yields MeS(s)+2H+(aq)
CH2O represents organic carbon provided as compost. These methods remove metals and also sulphates to a considerable extent. A range of organic substrates could be utilized for a variety of metal removals.
In 1995, a full scale continuous reactive barrier installed at an aquifer at Sudbury, Ontario and composed of municipal compost, leaf compost and wood chips resulted in a decrease of SO4 from 2400-3800 mg/L to 110-1900 mg/L and Fefrom 740-1000 Mg/L to <1-91 mg/L with substantial increase in alkalinity value which as a result of the barrier began to neutralize the pH of the receiving surface-water flow system. Concentrations of 10 mg/L up gradient of the barrier got reduced to 0.8 mg/L downgradient.
At another site in Vancouver, Canada a compost based reactive barrier was installed in the way of a plume of ground water containing Cd, Cu, Ni, Pb and Zn. Dissolved Cu concentrations were observed to decrease from 300 mg/L to <5mg/L. The concentration of Cd, Ni, Pb and Zn showed similar reduction with final concentrations generally below instrument detection limits.
A permeable insitu subsurface reactive barrier composed of 100% granular Zero valent iron (ZVI) was installed in June 1996 near Elizabeth city, North Carolina to treat overlapping plumes of Chromium and Chlorinated solvent compounds [Trichloroethylene (TCE), cis-i-chloroethylene (c-DCE) and Vinyl chloride (V.C.)]. Concentrations in excess of 10 mg/L Cr and 19 mg/L TCE had been detected since 1991. A PRB 46 meters long, 7-3 meters deep and 0.6 m wide with 3.2 m3 of Iron filings per linear meters (280 tons of iron) was installed. Performance monitoring was done by sampling 10, 5 inches dia. PVC compliance wells and 15 multi-level samplers (7 to 11 sampling points per sampler) for TCE, Cis DCE, Vinyl Chloride, Ethane, Ethene, Acetylene, Methane, Major anions, metals, Cr(Vi), Fe (II), Total Sulphides, dissolved H2, EC, pH, dissolved oxygen, specific conductance, alkalinity and turbidity.
Chromium is removed from the Ground water to less than detection limits (<0.01 mg/L) and considerably less than the regulatory targets (0.1 mg/L) within the first few centimeters of the wall. No chromate is detected down gradient of the wall. Under the Highly reducing conditions which prevail within the wall CrVI gets reduced to Cr+3 with the formation of an insoluble precipitate which has been detected on the surface of iron as a coating.
Fe0+CrO4-2+4H2o Yields (FexCr1-x) (OH)3+5OH–
There is also a reduction in chlorinated volatile organic carbon concentrations to less than regulatory limits.
Most scientists believe that most barriers have to be regenerated or retrieved in order to sustain the reactivity otherwise reoxidation and remobilization of precipitates may occur. When zero valent iron is used in a PRB,corrosion and mineral precipitation alter the surface composition which changes the reactivity of the iron and consequently the precipitation decreases the flow through the barriers.
Prediction of barrier clogging or barrier failure is therefore an important design criterion and may involve a cost-effective monitoring with predictive capability as to the circumstances under which a barrier may fail.
- An elevated D.O. in ground water will cause corrosion and clogging in the reactive barrier because of its property to assist the precipitation of ferric oxide/hydroxide.
- GW with limited buffering capacity will exhibit a high pH in the Fe0 barrier thus suppressing microbial activity which in turn will result in little risk of biofouling.
- W. with high bicarbonate buffering capacity will result in near neutralpH in theFeo barrier and a greater risk of fouling, clogging from precipitation of metal sulphides and carbonates.
- Natural organic matter will compete for iron and oxide/hydroxide surfaces, thus inhibiting reductive surface reaction with contaminants.
- When Feo contacts with ground water, oxidation reduction, mineral precipitation and associated equilibrium reactions occur. Mixed oxides (Green rusts) that include different types and amount of anion (CI, SO4-2, or CO3-2 may be formed. Various transformation may also occur as green rust to Feric oxyhydroxide or to iron sulphides.
- The amount of the Precipitate produced depends on the characteristic of the influent ground water as well as the resultant effluents.
Various configuration and installation methods have been implemented to install PRB’s into the subsurface at various depths.
- Continuous Permeable wall configuration distributes the reactive material across water. This is least sensitive to complexities in the flow field and does not alter the natural G.W. flow path.
- The funnel and gate configuration for insitu treatment of contaminated plumes consists of low hydraulic conductivity cut off walls with gaps that contain in situ reactors such as reactive porous media, that remove contaminants either through abiotic or biotic processes. Funnel and Gate Systems can be placed either at the front of the plumes to prevent further plume growth or just downstream of the contaminated zone to prevent contaminants from reaching the plume. Cut off walls (funnels) modify flow patterns so that ground water flows primarily through the high conductivity gaps (gates). It is a largely passive approach with little or no Maintenance over a larger period in contract to the Pump and Treat systems.
The main advantage of the funnel and gate system is that a smaller reactive zone is used for treating a plume resulting in a lower cost and also that if the media has to be replaced it is much easier to do so because of the small gate.The impermeable funnel walls maybe composed of steel sheet piles or bentonite slurry walls installed in the ground.
In order to assure complete capture of the plume, the length of the funnel and gate system is typically 1.2 to 2.5 times the plume width depending on the funnel to gate ratio and the number of gates. The insitu reactive vessel’ is an adaptation of the Funnel and gate system where the funnel collects the water and the hydraulic head build up on the upgradient side causes the contaminated water to flow in a buried vesselcontaining reactants. The Geo siphon is another innovation which is designed to passively induce ground water to flow through a permeable treatment system at an accelerated rate via a siphon taking advantage of natural head hydraulic difference between two sites.
The theoretical amount of reactive material needed in a PRB is independent of treatment system configuration since any configuration will have to treat the same mass flux.
An abandoned Uranium (U) ore upgrading facility in south eastern Utah, known as the Fry Canyon site, was selected as a long-term demonstration site to assess the performance of selective Permeable reactive barriers for the removal of Uranium and other trace metals from Ground Water.
Three PRB’s were installed in 1997 using Funnel and Gate System and ZVI (Zero Valent Iron), Amorphous Ferric Oxyhydroxide (AFO) and phosphate PO4as the reactive materials. Uranium concentration in aquifer ranged from 60 mg/L from a back ground well to 20700 mg/L in water beneath the tailings. ZVI PRB is the most effective removing almost 99% of contamination. AFO and phosphate also achieve about 90% removal efficient.
Permeable reactive barriers offer several advantages over the pump and treat systems. Because the plumes were more than 80 feet below the ground level, an alternative to “Trench and fill’ permeable reactive barriers was developed. This was called the In-situ Redox Manipulation (ISRM) which could be installed through conventional ground water wells and was useful where the ground water plumes were deep and difficult to treat by the trench emplaced permeable barriers. For the ISRM process, conventional ground water wells are placed at the site to be treated. A chemical solution of sodium dithionite is injected into the well for about 10 to 20 hours. Once it reaches the ground water it reacts with the iron present in the soil to form a large barrier. Once ground water flows through this barrier, the targeted contaminants are destroyed or immobilized. Barriers formed in this manner remain active for up to 30 years and require minimal maintenance.
Within the aquifer the injected dithionite reacts with the iron present in the medium and reduces Fe+3 to Fe+2. In case of Chromate bearing ground waters the aqueous chromate reacts with Fe+2 and is precipitated as an in soluble hydroxide [Cr (OH)3].
Similar precipitation reactions occur for other oxidized redox sensitive metal species. Bench scale tests have shown that dithionite treated soils should also be effective for treatment of dissolved Trichloro ethylene.
The funnel and gate system for in situ treatment of contaminated plumes consists of low hydraulic conductivity cut off walls with gaps that contain insitu reactors such as reactive porous media that remove contaminants by abiotic or biological processes. Funnel and gate systems can be placed at the front of plumes to prevent plume growth or at the front of the contaminated source zone to prevent the movement of contaminants into plumes. Cut off walls (the funnel) modify flow patterns so that the ground water flows primarily through the high conductivity zones (gates). It is largely a passive process with very little maintenance costs and differs from the energy intensive pump and treat system on this count.
Pump and treat systems prevent the migration of plumes beyond the extraction wells but have limitation in terms of long term remediation unless the source zone is controlled by some other means. Pump and treat systems must be operated for very long period of times, sometimes decades or centuries to prevent continued growth of contaminated plume and require a continuous input of energy and other resources and the extraction and disposal of ground water.
The width of the capture zone produced by a funnel and gate system is proportionate to the discharge through the gate which can be increased by increasing the width, length and hydraulic conductivity of the gate and width of the funnel. For a given length of cut off wall the most efficient configuration in an isotropic aquifer is a funnel with sides 1800 apart oriented perpendicular to the regional hydraulic gradient. A balance between maximizing the size of the capture zone for agate and maximizing the retention time of contaminated ground water in the gate must be achieved. In general capture zone size and retention time are inversely related. However, residence time may be easily increased by making the gates longer in the direction parallel to flow. Alternatively long residence times may also be achieved by completely surrounding a contaminant zone by cut off walls, except for a gap that contains an insitu reactor in the downstream wall. Different reactors can be put in series for different wastes.
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- Starr, R.C. and Chervy, J.A.1994.In situ remediation of contaminated ground water. The funnel and Gate system. Ground Water, Vol. 32, No. 3-May-June 1994. https://www.semanticscholar.org/paper/In-Situ-Remediation-of-Contaminated-Ground-Water%3A-Starr-Cherry/1ae91bd7dd58be0b31f0e7fb82dd393a0f74fca5
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