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  Method for soil remediation and engineeringUSPTO Application #: 20060163068Title: Method for soil remediation and engineering Abstract: An electrokinetic method for groundwater protection, soil remediation and engineering which comprises applying an electric field across an area of soil, sediment or slurry so as to generate a pH and Eh gradient and thereby promote the in situ precipitation of a stable iron-rich band. The method may be performed for the purpose of stabilisation and/or strategic dewatering/rewatering of soils, sediment and/or slurries, the improvement of the physical properties of soils and sediments for engineering purposes, the forced and directed migration of contaminated leachates, and/or electro-osmotic purging of non-polar contaminants. (end of abstract) Agent: Volpe And Koenig, P.C. - Philadelphia, PA, US Inventors: Andrew Brian Cundy, Laurence James Hopkinson USPTO Applicaton #: 20060163068 - Class: 204515000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Non-distilling Bottoms Treatment, Electrophoresis Or Electro-osmosis Processes And Electrolyte Compositions Therefor When Not Provided For Elsewhere, Inorganic Siliceous Or Calcareous Material Prepared, Separated, Or Treated (e.g., Clay, Earth, Concrete, Asbestos, Glass, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20060163068. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] This invention relates to an electrokinetic method for groundwater protection, soil remediation and engineering, and, more particularly, to such a method which involves the strategic electrokinetic placing of an iron-rich barrier in soils, sediments and slurries. BACKGROUND [0002] Contaminated soils and groundwater at industrial, waste disposal and spill sites are serious environmental problems. Although clays and silts tend to sequester large quantities of heavy metals, radionuclides, and selected organic polluants (Kovalick 1995), they are relatively resistant to remediation with traditional technologies (e. g. pump and treat, soil washing) because of their low hydraulic conductivities. This has stimulated a considerable amount of research into cost-effective, in situ techniques that can be used to remediate low-permeability, high clay content soils. One emerging technology that has received much attention is electrokinetic remediation. Electrokinetics is a process that separates and extracts heavy metals, radionuclides, and organic, inorganic, BTEX and radioactive contaminants from saturated or unsaturated clay-rich soils, sludges and sediments under the influence of an applied electrical field. Experiments have shown its applicability to a variety of organic, inorganic and radioactive wastes (Acaret et al., 1993; Kovalick 1995; Virkutyteet et al., 2002). [0003] The electrokinetic process involves the application of a low intensity direct current (DC) across electrode pairs that have been implanted in the ground on each side of the contaminated soil mass. When DC electric fields are applied to contaminated soil via electrodes placed into the ground, migration of charged ions occurs. Positive ions move towards the negatively charged cathode, while negative ions are attracted to the positively charged anode. It has been shown that non-ionic species are transported along with the electro-osmositically-induced water flow. Electrokinetic remediation is possible in both saturated and unsaturated soils. [0004] The dominant and most important electron transfer reaction that occurs at the electrodes during the electrokinetic process is the electrolysis of water. Groundwater is dissociated at the electrodes via the reactions: H.sub.2O.fwdarw.2H.sup.++1/2O.sub.2(gas)+2e.sup.-(anode) 2H.sub.2O+2e.fwdarw.2OH.sup.-+H.sub.2(gas) (cathode) [0005] This produces an acid front (due to excess H.sup.+ ions) around the anode and an alkaline front (due to excess OH.sup.- ions) at the cathode. [0006] The electric current causes electro-osmosis and ion migration, which moves both water and the aqueous phase contaminants in the subsurface from one electrode to the other. It also causes electrophoresis, which results in the migration of colloidal fractions. Sorption, precipitation and dissolution are accompanying reactions. Contaminants in the aqueous phase, and contaminants desorbed from soil particles, are transported towards the anode or cathode depending on their charge. In existing commercial electrokinetic systems, contaminants are commonly extracted by a secondary recovery system or deposited at the electrode. Recovery methods for contaminants that have migrated to the electrodes include electroplating, precipitation/co-precipitation, pumping near the electrode, or complexing with ion exchange resins. Surfactants, complexing agents and other reagents are frequently used to assist contaminant movement (Acar et al., 1993; Virkutyte et al., 2002). However, most contaminated sites contain mixtures of wastes rather than single contaminants and which makes remediation more complicated. [0007] At present there is no standardised universal soil/sediment remediation approach. Instead there are a numbers of technologies (e. g. Lasagna.TM., Electro-Klean.TM., electrochemical geooxidation), each of which has its own operational and design requirements, and limitations (Virkutyte et al., 2002). Many of these technologies are technically complex and energy intensive, and geared toward the removal of 90% or more of specific contaminants, under very specific field or laboratory-based conditions. However, in the real environment a low-tech, low-energy contaminant reduction/containment technique may be more appropriate and realistic. [0008] Electrodes that are inert to anodic dissolution are conventionally used in electrokinetic soil remediation. These include graphite, platinum, gold and silver electrodes, although less expensive electrodes made from titanium, stainless steel and plastic have also been employed. Metals such as lead, chromium, cadmium, copper, uranium, mercury and zinc, as well as polychlorinated biphenyls, phenols, chlorophenols, toluene, trichlorothane and acetic acid are suitable for electrokinetic remediation and recovery. [0009] The main parameters that influence the overall process are soil properties, depth and type of contamination, cost of accommodating electrodes and placing treatment zones, clean up time, and cost of labour (Virkutyte et al., 2002). Factors that influence the cost of the electrokinetic remediation process are soil characteristics and moisture, contaminant concentrations, concentration of non target ions and conductivity of pore water, depth of the remediated soil, site preparation requirements, and electricity costs (van Cauwenberghe 1997). The cost optimised distance between electrodes for commercial systems is 3 to 6 m for most soils (Lagerman 1993; Ho et et al., 1999). Given that the migration rate of contaminants is approximately 2 to 3 cm/day, the time frame for successful remediation between electrodes spaced at 2 to 3 m is of the order of 100 days, although cation-selective membranes and other technologies are commonly employed to reduce remediation periods to 10 to 20 days (van Cauwenberghe 1997). The breakdown of costs associated with an electrokinetic remediation programme are approximately 40% for electrode construction, 10 to 15% for electricity, 17% for labour, 17% for materials, and up to 16% for licenses and other fixed costs (Ho et al., 1997). SUMMARY OF THE INVENTION [0010] It is an object of the present invention to provide an improved electrokinetic method for groundwater protection, soil remediation and engineering which is low cost, efficient and flexible in its application. The method involves:-- [0011] the strategic and remote electrokinetic placement of an iron-rich barrier to a required geometry, which provides a physical and/or chemical barrier to contaminants, and improves the engineering properties of soils and sediments (contaminated or otherwise); [0012] the generation of a pH/Eh gradient to remobilise and/or trap contaminants within soils, sediments and slurries; and [0013] the stabilisation and strategic dewatering/rewatering of soils/sediments/slurries, the forced and directed migration of contaminated leachates, and the electro-osmotic purging of non-polar contaminants. [0014] Unlike existing electrokinetic techniques, the method of the present invention provides a robust, non-selective and low energy approach to contaminant reduction and containment, and is based on natural iron mineralization processes that occur in the near-surface environment. In addition, since the system mimics nature (e. g. the formation of iron pans), and iron is a common major element in rock and soil systems and is relatively non-toxic, the environmental impacts are minimal. Moreover, iron itself has well-documented contaminant-trapping properties. [0015] According to the present invention there is provided an electrokinetic method for groundwater protection, soil remediation and/or soil engineering which comprises applying an electric field between iron-rich sacrificial electrodes, which are implanted in an area of water-bearing soil, sediment or slurry, so as to generate a an abrupt pH and Eh gradient from acid to alkaline conditions, with the spontaneous in situ precipitation of a stable iron-rich band occuring at the boundary between the acid and alkaline zones. [0016] The method of this invention is characterised by increasing the mobility and solubility of contaminants through the application of an electric charge, and simultaneously arresting their migration either by fixation to an electrochemically-generated iron band which is precipitated within the area under treatment, or via forced precipitation within the imposed Eh/pH field. This approach is distinct from other remediation techniques because it is geared towards deliberately producing an iron band in situ between the cathode and the anode, which simultaneously provides a physical as well as a chemical barrier; employs a low voltage of typically less than 0.5 volts per cm distance between electrodes (with low energy requirements) to generate a strong Eh/pH gradient within soils and sediments; uses low cost, sacrificial cathode and anode materials; can produce, through differential dewatering, controlled differential subsidence and permeability reduction; and which can be generated in natural and industrial materials over laboratory timescales. In contrast, current commercial techniques have an order of magnitude higher energy requirements, actively avoid generation of a pH gradient and precipitation of iron or contaminants within the soil or sediment (e. g. current electrokinetic techniques); or use ex situ clean-up/disposal; or hard engineering technologies (e. g. permeable reactive barriers). [0017] The present invention is a low voltage (<0.5 V/cm, in most cases less than 0.2 V/cm) electro-chemical based technique, which uses electrokinetics to generate an intense pH gradient (typically from pH2-pH13) and Eh gradient in soils, sediments and sludges, destabilise/dissolve minerals and force the in situ precipitation of a stable iron-rich band. Internal electric fields of the scale used in the method of this invention commonly occur naturally in rock and soil bodies and can arise from a variety of conditions. A common result of this phenomenon is the electrical generation of bands of iron-stone in uncemented sediments. (e.g. Jacob et al., 1996). Such bands, which are found in many geological systems, can result when the electrolytic dissociation of water takes place, with the formation of an anode zone characterised by acidic ions (pH 2.0-2.5), and a cathode zone characterised by alkaline ions (pH 10.5-11.5). As a consequence of the potential difference, a sharp boundary zone is developed within which an abrupt pH change from 2.5 to 8 occurs. Where sufficient iron is present in the system, spontaneous precipitation of insoluble metal (mainly iron) hydroxides and oxides occurs at the point of this pH "jump" (Jacob et al., 1996). Small amounts of native (i. e. zero-valent) iron can also be present. In natural settings, such ferric iron-rich bands are commonly poorly crystalline or amorphous (e. g. Hopkinson et al., 1998). [0018] The method of the present invention thus emulates these natural iron''' mineralisation processes, but over experimental rather than geological time scales, by applying a direct electric potential to electrodes to grow bands of iron''' mineral phases in sediment and soil columns, and to harness their adsorptive properties, to trap or break down contaminants from the aqueous phase, or extracted from soil particles, during their migration in the applied electrokinetic field. Freshly precipitated amorphous or poorly crystalline Fe-rich solids, of the type generated by this method, are extremely effective scavengers of a range of heavy metals, radionuclides and organic polluants in a variety of environments (Bendell-Young and Harvey 1992, Cundy and Croudace 1995). Zero valent iron is itself an important catalyst for the dechlorination of toxic chlorinated aliphatic compounds (Haran et al., 1996). Moreover, because this method generates strongly acidic conditions at the anode and strongly alkaline conditions at the cathode, contaminants attached to soil or sediment particles (such as radio nuclides and heavy metals), which are soluble under either acidic or basic conditions are solubilised and forced to migrate towards the appropriate electrode, whence they precipitate or are co-precipitated with the iron-band. In essence, the present invention provides the opportunity to "flush" contaminants from parcels of contaminated sediments, and then retrap and concentrate them in, or adjacent to, the iron-band. This offers the potential of in situ clean-up of contaminated soils, sediments and sludges. Clean-up of the whole soil volume between the electrodes can be achieved, and plating of contaminants onto the cathode avoided, by simply reversing the polarity of the electrodes at regular intervals. [0019] The approach embodied in the method of this invention is distinct from existing in situ remediation technologies, such as permeable reactive barriers, in that rather than merely sequestering contaminants from solution, the system actually mobilises contaminants into solution prior to their subsequent trapping by the reactive band/imposed Eh/pH gradient, thus cleaning contaminated soils as well as ground waters. It differs from existing electrokinetic techniques in its use of low-cost electrodes (for example, electrodes made of cast iron, scrap iron, stainless steel or other iron-rich material), its low energy requirements and most significantly in its deliberate generation of a sorptive iron-band in the material being treated. Hence, the electrokinetic technique described here is innovative and clearly distinguished from other electrokinetic treatment systems. The precipitated iron band, however, represents much more than merely a chemical sink for toxic contaminants liberated from the sediment column via oxidation-reduction and pH reactions. The electrokinetic process that triggers iron band formation may also be used to improve the engineering properties, and massively reduce the permeability, of soils and sediments through differential dewatering of clays, and iron-band generation. Hence, electrokinetic ferric iron precipitation represents a means of physically confining waste spills, providing a reactive barrier to liquid waste spillages that can be re-sealed and strengthened by periodic applications of electrical current (for instance in physically trapping and sorbing leachate that has percolated through the base liner of a landfill). In addition, the method offers the potential, through strategic dewatering or rewatering of soils and sediments and iron-band generation, to rewater and stabilise soils for civil engineering applications (e. g. in building works). Existing dewatering techniques involve complete dewatering of large-volume slurries (e. g. Lamont-Black 2001), whereas the present technique is applied in situ to strategically rewater or dewater, and strengthen or generally improve the engineering properties of, parcels of soil, and so has a range of potential civil engineering applications (such as dealing with subsidence). [0020] The method of this invention may have direct applicability in relation to the integrity of land fill liners, permeable reactive barrier technologies, and funnel and gate systems, controlled differential subsidence, improving the engineering properties of soils and sediments, remediation of contaminated land (soils and sediments) and clean up of contaminated industrial sludges and slurries. Consequently, it will be of significant interest and potential benefit to a wide range of organisations, for example environment agencies, water companies, land fill operators, civil engineering and environmental consultants and nuclear fuel companies. [0021] The method of the present invention therefore has a number of surprising and significant benefits compared to other commercial techniques. In comparison with permeable reactive barrier technologies, it provides a resealable iron-rich barrier, which can be remotely placed (without engineering) at working sites and sites with infrastructure to physically and chemically inhibit subsurface pollutant migration, and can redirect subsurface pollutant flow. In comparison with commercial electrokinetic remediation techniques it has an order of magnitude lower energy requirements and electrode cost, does not involve the use of potentially toxic conditioning solutions, can remobilise contaminants from the solid phase and simultaneously trap and contain contaminants in the liquid phase, and can be applied on working sites, or sites containing infrastructure. Continue reading... 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