Permeable reactive barrier
Jarno Mäkinen; VTT Technical Research Centre of Finland Ltd; P.O. Box 1000, FI-02044 VTT, Finland; jarno.makinen(at)vtt.fi
Permeable reactive barrier (PRB) is a versatile technology to treat polluted water streams, e.g. acid mine drainage (AMD), passively and in situ.
Description of the technology
The fundamental idea of PRB treatment is that polluted water is flowing gravitationally through water permeable zone which contains reactive material. Reactive material intercepts the contaminant plume immobilizing or transforming pollutants to harmless form. There are several reactive materials available for treating waters containing variety of contaminants and therefore technology is rather versatile (U.S. EPA 1998). Currently there are at least three full-scale PRB treatment systems treating acid mine drainage and the state-of-art method is biologic sulphate reduction with organic material as reactive material. However, limestone or metallic iron addition is also used in some extent in these sites (Benner et al. 2002, Ludwig et al. 2002, Jarvis et al. 2006, Gibert et al. 2011).
Biologic sulphate reduction relays on sulphate reducing bacteria (SRB), which require organic substrate and anoxic conditions to reduce sulphate to hydrogen sulphide. Hydrogen sulphide precipitates, together with raised alkalinity and pH, various heavy metals from water as poorly soluble sulphides (Liamleam & Annachhatre 2007):
3SO42− + 2CH3CHOHCOOH → 3H2S + 6HCO3−
H2S + M2+ → MS(s) + 2H+
The role of limestone is to increase alkalinity and neutralization of pH, while the metallic iron improves reducing conditions for SRBs, as well as formation of Fe oxyhydroxides for sorbents (Gibert et al. 2011).
PRBs can be utilised when contaminated water stream travels mainly underground and when the site hydrogeology is completely understood to know how the contaminant plume is spread. In addition, contaminant(s) must be identified and suitable reactive material found for treating contaminant(s). It has been reported that PRBs with different reactive materials can remove soluble sulphate, chromium, selenium, uranium, copper, zinc, lead, cadmium, strontium and nitrate. Different reactive materials have their own requirements for successful utilisation, e.g. biologic sulphate reduction requires anoxic conditions (U.S. EPA 1998, Gavaskar 1999, U.S. EPA 2000).
Advantages of the PRBs are related to passive and in situ –type of treatment. As water flow is completely managed by gravitation, there is no need for pumping or agitation. In addition, the reactive zone is filled with reactive material at construction phase so there is no need for chemical addition or removal of produced effluents. Therefore, operational costs consist only of effluent monitoring. Another advantage of the PRB systems is availability of several rather economic reactive materials, like organic matter, limestone, metallic iron and also some other emerging materials offering treatment possibilities for variety of contaminants and their mixtures. When a PRB is properly designed, these materials stay reactive for long periods, likely for years (U.S. EPA 1998).
Disadvantage of PRBs is that they cannot be used in every site: water stream should be mainly underground, with naturally occurring or constructed ground layers controlling strictly water plume. When contaminated water flow is wide or deep, or it is located very deep in soil, construction and filling costs of PRB increase quickly. To be sure the site is suitable and the PRB beyond all questions intercepts the whole plume, having also enough capacity for proper contaminant removal, heavy site characterization and laboratory test runs must be executed. In some situations investigation costs may be rather high, as well as construction of the PRB system (U.S. EPA 1998, Gavaskar 1999).
Performance of the reactive barrier is affected naturally by contaminant(s) and selection of reactive materials. However, the major factor is securing the proper contact time between contaminated water and reactive material. Therefore, the factors affecting to reduction rate are contaminated water velocity, initial concentration of contaminant(s), and finally depth, or in the other words, thickness of the reactive zone. The appropriate thickness is usually verified with laboratory tests when other parameters are known (U.S. EPA 1998, Gavaskar 1999).
There are three currently working, full-scale PRBs treating acid mine water: in Northumberland (UK), in Aznacollar (Spain) and in Nickel Rim (Canada), with initial sulphate concentration of 8.7 g/l, 1.1 g/l and 2.9 g/l, respectively. Average sulphate removal has been approximately 40%. In Northumberland also 50% of iron is removed, while in Aznacollar removal rates of aluminium, zinc and copper are 47%, 80% and 76%, respectively. In all of these three cases, biologic sulphate reduction is utilised with organic matter as reactive material. However, in Northumberland and Aznacollar there is also limestone introduced, and in Aznacollar also small amount of metallic iron (Benner et al. 2002, Ludwig et al. 2002, Jarvis et al. 2006, Gibert et al. 2011).
Even though there is existing full-scale PRB treatment systems for acidic mine waters, technology is still majorly utilised for other purposes (U.S. EPA, 1998) and the first full-scale cases with acidic mine waters are showing indication of scaled-up performance and need of further development. Current performance may not be enough in the future when legislation may harden. However, there is no clear obstacle for improved removal rates of sulphate and metals if contact time is increased (U.S. EPA 1998, Gavaskar 1999).
Due to the nature of PRBs they are suitable for cases where source of pollutant itself cannot be treated or isolated completely from the environment (or do it economically enough). Moreover, treatment is planned to be long-lasting, potentially for decades. In these situations, rather high preliminary costs and capital investments can be justified by very low operating costs. In Northumberland (UK) the construction costs were approximately £60,000 and annual operation and maintenance costs less than £5,000 (Jarvis et al. 2006). However, authors have not reported if the costs contain both PRB and additional wetlands construction at site. Also the cost of the site characterization is unknown.
When designing a PRB, plenty of accurate data is required from the site. Usually work is started with studying historical or other existing data and observations from the site (U.S. EPA 1998, Gavaskar 1999). For example, in a mine site ore mineralogy and mineral processing technologies may have changed with time affecting to waste and water discharge quality, and wastes may also be disposed to multiple spots. Moreover, seasonal changes in water discharge may occur. Good background knowledge on the site therefore helps proper design of PRB.
Further site characterization studies include assessment of the hydrogeology, existing contaminants and their concentrations, geochemistry and possibly microbiology. The role of hydrogeology is to determine plume height and width, distribution, water flow direction(s), soil type and minerals and their properties (e.g. water permeability). In addition, possible leakages from the main flow path should also be investigated to find the most suitable place for PRB and its precise design to secure the interception of the whole plume. Selection of reactive material(s) and the thickness of reactive zone can be determined by water velocity, contaminants occurring in the water and their concentrations as well as geochemistry. Reactive material should be effective against contaminants, and the barrier should be thick enough to enable sufficient contact time for treatment of highest occurring concentrations and water velocities (U.S. EPA 1998, Gavaskar 1999). Moreover, geochemical phenomena affect to reactive material’s effective lifetime. For example, high calcium or oxygen concentration in waters may cause precipitation and blocking, leading to reduced efficiency and problems in water flow (U.S. EPA 1998, Liang et al. 2000). Also it must be noticed, that certain microbial activity is occurring in the mine sites and waters, especially with acid producing mineral wastes. In these cases the microbial activity can be divided to sulphur oxidizing bacteria causing mineral dissolution and acid mine drainage (Salomons 1995), and sulphur reducing bacteria causing precipitation of metal sulphides. Especially biologic sulphate reduction can be utilized together with PRB (Benner et al. 2002), but also negative impacts may be generated. For example, microbiological production of H2 can cause corrosion to metallic iron barriers and therefore shorten the lifetime of a PRB (Liang et al. 2000).
Benner, S.G., Blowes, D.W., Ptacek, C.J. & Mayer, K.U. 2002. Rates of sulphate reduction and metal sulphide precipitation in a permeable reactive barrier. Applied Geochemistry 17, 301-320
Gavaskar, A.R. 1999. Design and construction techniques for permeable reactive barriers. Journal of Hazardous Materials 68, 41-71
Gibert, O., Rötting, T., Cortina, J. L., Pablo, J., Ayora, C. & Carrera, J. 2011. In-situ remediation of acid mine drainage using a permeable reactive barrier in Aznalcollar (Sw Spain). Journal of Hazardous Materials 191, 287-295
Jarvis, A.P., Moustafa, M., Orme, P.H.A. & Younger, P.L. 2006. Effective remediation of grossly polluted acidic, and metal-rich spoil heap drainage using a novel, low-cost, permeable reactive barrier in Northumberland, UK. Environmental Pollution 143, 261-268
Liamleam, W. & Annachhatre, A.P. 2007. Electron donors for biological sulfate reduction. Biotechnology Advances 25, 452-463.
Liang, L., Korte, N., Gu, B., Puls, R. & Reeter, C. 2000. Geochemical and microbial reactions affecting the long-term performance of in situ “iron barriers”. Advances in Environmental Research 4, 273-286
Ludwig, R.D., McGregor, R.G., Blowes, D.W., Benner, S.G. & Mountjoy, K. 2002. A Permeable Reactive Barrier for Treatment of Heavy Metals. Groundwater, 40, 59-66
Salomons, W. 1995. Environmental impact of metals derived from mining activities: Processes, predictions, prevention. Journal of Geochemical Exploration 52, 5-23.
U.S. EPA. 1998. Permeable Reactive Barrier Technologies for Contaminant Remediation. United States Environmental Protection Agency.
U.S. EPA. 2000. Field Demonstration Of Permeable Reactive Barriers To Remove Dissolved Uranium From Groundwater, Fry Canyon, Utah. United States Environmental Protection Agency.