Chapter 15
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In Situ Fluidization for Permeable Reactive Barrier Installation and Maintenance Robert K. Niven School of Civil Engineering, The University of New South Wales at the Australian Defence Force Academy, Northcott Drive, Canberra, ACT 2600, Australia
In this study, the application of in situfluidization(ISF)for the installation of permeable reactive barriers (PRBs) - specifically zero-valent iron (ZVI) barriers - in sandy soils is investigated by laboratory experiments in columns and tanks. In this method, the soil is fluidized using a water jet, and ZVI is added to the in situ fluidized zone to create a PRB. Initially, efforts were focused on "hydraulic matching" of the ZVI particles and recipient sand, to disperse the ZVI vertically throughout the fluidized zone. However, experiments indicated serious difficulties with this approach. Instead, an "extraction method" was developed, in which hydraulically heavier ZVI is added as the jet is withdrawn. The result is a sequence of laminations of ZVI and original sand, extending throughout the former fluidized zone. Substantial cost savings are expected compared to existing PRB installation methods. The implications for PRB installation and maintenance are examined in detail.
© 2003 American Chemical Society Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction Since the 1980s, there has been considerable interest worldwide in the use of permeable reactive barriers (PRBs) - permeable zones containing reactive or catalytic solids or other fixed reagents - for the in situ treatment of contaminated groundwater. Such barriers are normally installed some distance downgradient from the contaminant source zone, enabling degradation or fixation of dissolved contaminants and protection of the downgradient groundwater quality. Initially, PRBs were envisaged as fixed installations, consisting of vertical trenches filled with solid active material. Over time, the concept has evolved to include discrete or discontinuous zones of any geometry, containing some kind of solid-bound or long-life reagent installed by any means (1). The main group of contaminants currently being addressed by PRBs are chlorinated solvents, many of which have been found to be catalytically degraded by particles of zero-valent iron (ZVI) (1,2). A wide variety of other applications of PRBs have been developed, including lime or organic-filled barriers for the treatment of heavy metals and acid mine drainage, phosphate barriers for various metals, and oxidizing or activated carbon barriers for volatile organic compounds (7). At present, most PRBs are constructed by standard engineering techniques, generally involving isolation of the treatment zone (by sheet piling or other means), excavation of natural soils, backfilling with the reactive material or soilreagent mixture, and reconnection to the aquifer (3). This requires heavy engineering equipment, and thus can only be conducted at high cost. The ZVI used for chlorinated solvent barriers is also quite expensive, and in thick barriers, is applied inefficiently. In many applications, to maintain stability of the excavation, the ZVI must be installed with a biodegradable polymer, which is later broken down using an enzyme (4). In the 1990s, efforts were directed to reduce installation costs by the development of "funnel-and-gate" systems, in which relatively impermeable zones (the "gate") were installed to direct groundwater flow through narrower PRB zones (the "funnel") (5). Cost savings are achieved by the standard engineering methods used for construction of the impermeable gates, as well as by savings in reagent costs. More recently, a variety of new installation procedures and injection methods have been developed, focussing on the PRB as a loosely-defined and often discontinuous treatment zone (6,7). An important driving force behind these methods is the desire to avoid excavation - and its associated high costs and handling difficulties - altogether. Recently, a new method, in situfluidization(ISF), has been developed for the remediation of subsurface non-aqueous phase liquid (NAPL) and/or metal contamination (8-13). The method is applicable principally to sandy soils. In this method, illustrated schematically in Figure 1, a water or water / air jet is lowered from the surface into a sandy soil, producing a sharply defined in situ
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Figure 1. Schematic diagram of ISF for contaminant remediation and/or PR installation (Reproduced with permission from reference 10. Copyright 1998 National Research Council, Canada.) fluidized zone (liquified soil) surrounding the jet. Fluidization of the soil - as distinct from in situ flushing - has the advantage that it destroys the soil skeleton, releasing both NAPL contaminants and those present on fine particles in the soil (such as heavy metals), which would otherwise remain trapped within the soil. The released contaminants are carried to the ground surface, where they can be recovered for treatment. Laboratory experiments on diesel-contaminated soils indicate that significant reductions in diesel levels (96-99.9%) may be achieved, for a wide range of initial diesel concentrations (10,000 to 150,000 mg/kg), and for fines contents of at least 10% (8,10-12). Significant reductions in lead (up to 88%) from soils containing 1500-2000 mg/kg of precipitated lead have also been achieved (8,11). The hydraulics of ISF have been examined in detail by tank experiments and pilot trials in the field (9,11,13). ISF can be applied to soils within the sand size range, and occurs quite rapidly, with development of the "asymptotic" in situ fluidized zone geometry within minutes. Systematic experiments on sandy soils and derived correlations indicate that a 50 mm (2") ID jet operated at 4.4 L/s (70 gpm) can achieve a fluidized depth of 15 m (45 feet), greater than normal remediation depths (9,10). At higher flow rates, greater depths are possible. The method also penetrates laminations of peat or clay, washing such material to the surface (13). Finally, ISF has the advantage that small-scale pump and jetting equipment can be used, reducing site mobilization requirements (10,13).
Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
220 The aim of the present study is to examine the possible application of the ISF method for the installation of PRBs, with specific attention to ZVI barriers. In this approach, the solid reagent (such as ZVI) is added to the in situ fluidized zone, producing a body of reagent-enriched sand. The desired mass loading of solid is installed by appropriate control of the fluidization and reagent addition processes. Whilst some excess soil will be generated by the process, wholesale excavation - and associated handling difficulties - is avoided. For this reason, PRB installation by ISF is expected to produce substantial cost savings over existing installation methods. The present study follows two preliminary investigations (14-15), which are further developed and expanded in detail.
Background Knowledge
Fluidization Fluidization is the suspension of a granular solid in an upwardly flowing fluid (16). Since the drag force applied to each solid particle must overcome its buoyant weight, fluidization does not occur until the upward velocity exceeds the so-called minimumfluidizationvelocity, given by Wen & Yu (17): 2{-450μ(1^ )+ /[ϊ^^ /
λ
7 Ρ/Ψ A where μ = fluid dynamic viscosity, E = porosity at minimumfluidization,p/ = fluid density; p = solid density, φ = particle shape factor (sphericity); d = particle diameter and g = acceleration due to gravity. This velocity is a superficial velocity (i.e. averaged over the cross-section of the bed). The above equation is normally simplified using the experimentally determined relations (l-e )/
