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.

Description:
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 2 O→2H + +1/2O 2 (gas)+2 e   − (anode) 
 
2H 2 O+2 e→ 2OH − +H 2 (gas) (cathode) 
 
         [0005]     This produces an acid front (due to excess H +  ions) around the anode and an alkaline front (due to excess OH −  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™, Electro-Klean™, 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 (&lt;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.  
         [0022]     The low voltage used, coupled with the flexibility provided by the use of multiple, low cost electrodes, means that contaminated land can be sequentially treated with a series of electrode arrays, whereby the distance between individual electrodes does not exceed a few metres. In addition, the current is sufficiently low to avoid soil heating and large-scale gas generation at the electrodes. Adjustable electrode geometry means that the technique can be adapted to suit site-specific conditions, and large areas of land can be sequentially treated. It will be appreciated that the iron may be precipitated to form an impermeable coherent band, or a coating which cements soil/sediment particles, or a dispersed coating on mineral grains, between two or more electrodes. Following treatment, the iron band can simply be excavated as a coherent mass, or left in situ to provide a long-term inert, and, via reapplication of current, resealable barrier.  
         [0023]     The method of this invention provides an in situ, sustainable, cost-effective electrokinetic technology for groundwater protection and soil remediation, which can be operated in combination with, or as an alternative to, existing land remediation technologies. The technique is applicable to small sites, as well as larger areas of contaminated land, and can be implemented in ground where man-made structures are present, or where there is on-going site activity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The method of this invention will now be illustrated by the following examples and the accompanying drawings:— 
         [0025]      FIG. 1  shows a sub-vertical, 1 cm thick Fe-rich band generated in water-saturated sands after 30 hours application of a 1.5V potential difference between cast iron electrodes.  
         [0026]      FIG. 2   a  shows the generation of an Fe band in clay soil medium using the method of the present invention.  
         [0027]      FIG. 2   b  shows a diatom (marine microorganism) which, together with the underlying silt particle, has been coated and cemented by iron using the technology outlined in this application.  
         [0028]      FIGS. 3   a - d  relate to data from a hydrocarbon purging experiment, using spiked Southampton Water mud.  FIG. 3   a  shows mid IR spectrum for original engine oil used for spiking the sediment.  FIGS. 3   b, c  and  d  show FT-IR spectra for effluent drained from the cathode compartment on days 5, 12 and 13 respectively of the experiment. Note the hydrocarbon and seawater absorption lines are marked. Note also the number of FT-IR active diesel lines, and their overall intensity, increases with experimental time (e. g. day 13 CH 3  bend at 1376.66 cm −1 , appears). This indicates that the diesel within the cathode zone effluent became increasingly concentrated with experimental time.  
         [0029]      FIGS. 4   a  and  b  show  60 Co and As data for treated Ravenglass estuary mud. Fe band is located 5 cm from the anode. Note change in y-axis units, with  60 Co in Bq/g (or atomic disintegrations per second per gram) and As in ppm. Error bars on As data are smaller than the diamond marker symbol used. Note ca. ˜40% reduction of As in cathodic compartment, and ˜100% enrichment in narrow iron band. Reduction in  60 Co is less notable, but still exceeds 30% in the anode zone (compared to the untreated material). A ˜50% enrichment in the iron band compared to untreated material (which corresponds to a ˜110% enrichment in  60 Co over the anode zone) is also observed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     EXAMPLES  
       [0030]     Pilot studies have been applied at laboratory scales in 25×2×15 cm and 30×50×40 cm open topped perspex cells (i. e. effectively in two dimensional and three dimensional space). All experiments have been run at &lt;5 volts, using sacrificial cast iron electrodes. Electrodes were fabricated from 25 mm diameter cast iron rods (Grade 250), composition: C 3.48%, Si 2.87%, Mn 0.812%, S 0.099%, P 0.364%, Fe REM. Experiments have been run on a variety of contaminated muds, with groundwater and seawater interstitial pore waters, under unsaturated and saturated conditions. Time scales range from 3 to 400 hours.  
         [0031]     In experiments using sand, the initial permeability of the sands was 0.48×10 −5  m/s, post-treatment permeability (in the iron band) was recorded at 0.19×10 −5  m/s. For the mud experiments, initial permeability was typically ˜0.29×10 −7 , whereas treated material permeability (in the iron band) was recorded at 10 −9 , or less, i.e. practically impervious. In addition, clear dewatering was consistently observed in the sediment around the anode, and rewatering around the cathode.  
         [0032]     In every case, a strongly acidic zone was generated around the anode (approx. pH2), and an alkaline zone around the cathode (approx. pH13). At the point of abrupt pH change, approximately equidistant between cathode and anode, a 1-4 cm thick, coherent, iron stone was precipitated ( FIG. 1 ) having an approximate uniaxial compressive strength comparable to a moderately lithified sandstone (or the strongest Chalks in southern England). The iron stone generated consists of an amorphous iron band (see  FIG. 2   a ), or, in sandy sediments, a coating of zero valent iron and iron oxides which cement mineral grains. The presence of zero valent iron in the Fe-rich band is noteworthy since a large proportion of permeable reactive barriers employed at contaminated sites are based on the use of zero-valent iron to act as a powerful chemical reductant for chlorinated aliphatic compounds dissolved in groundwater (Younger, 2002). It is also possible to rapidly generate a dispersed sorptive coating of iron on a pre-defined area of soil without significant loss of porosity, simply by switching off the current before the Fe-band fully develops ( FIG. 2   b ). Such an approach may be desirable in situations where the sorptive properties of iron can be harnessed to reduce the concentration of specific contaminants, such as arsenic (As), in groundwater.  
         [0033]     Two specific studies are now presented which illustrate the applicability and potential of the method of the present invention for containing leachate and dissolved phase contaminants and remediating contaminated land.  
         [0034]     1. Hydrocarbon and heavy metal contaminated sediments, Southampton Water A) An estuarine mud sample, contaminated with copper (Cu) and petroleum hydrocarbons from the nearby Fawley oil refinery and from discharges from local shipping, was treated using a voltage of 2V, in a three-dimensional cell using a rectangular electrode array. A continuous iron band of up to 3 cm thickness was generated from the electrode point sources. Data for pre-and post-treatment Cu concentration indicate that the electrokinetic treatment resulted in an approximate reduction of 61% in Cu contamination in the anode zone in 16.3 days (note that a small proportion of Cu is natural background Cu held within the crystal lattice of stable minerals. This naturally occurring Cu is not influenced by the electrokinetic process). Notably, liquid hydrocarbon-rich effluent was expelled from the sediment (via electro-osmotic purging) and channeled and drained at approximately 10 ml per day from the surface of the cathodic compartment. The energy requirement for the experiment was 10.9 kW/m 3 . These values compare favourably against commonly cited energy requirements for other electrokinetic remediation systems, e. g. 500 kW/m 3  for 100% removal of metal contaminants (Virkutyte et al., 2002). The timescale for copper decontamination and hydrocarbon purging from the sediment is comparable in duration to existing technologies which employ comparatively expensive cation-selective membranes (Van Cauwenberghe, 1997). The use of cast iron electrodes (as opposed to gold coated, platinum or graphite electrodes), means that the experimental system is low cost in terms of energy, materials and electrode construction, which typically makeup ˜70% of the costs associated with any electrokinetic remediation system (Ho et al., 1997).  
         [0035]     To examine hydrocarbon decontamination by the method of the present invention, a sample of seawater saturated Solent mud was spiked with 0.4 litre of fresh 15 W/40 (Halfords) engine oil, and treated at 2V for 13 days. Small volumes of clean seawater were added around the anode electrodes to prevent desiccation of the sediment. Effluent was removed intermittently by pipette from a 1 cm deep trench dug in the cathode compartment. The effluent samples were analysed via Transform mid-infrared (FT-IR) spectroscopy. The resultant FT-IR spectra clearly show the hydrocarbon-rich nature of the effluent (i. e. the output solution) compared to the clean seawater added (i.e. the input solution). Essentially, the hydrocarbons (in this case engine oil) contained in the clay-rich sediment are extruded or purged via an electro-osmotic flow of water from the anode to the cathode, and replaced by clean seawater ( FIG. 3   a - d ).  
         [0036]     The natural moisture content of the untreated sedimented was 97%, compared to 69% and 88% for the anode and cathode zones respectively, consistent with the extraction of purged hydrocarbon-rich effluent from the cathode zone, and electro-osmotic flow of water from the anode to cathode zone. The bulk density of the cathode zone was recorded at 1.47 Mg/m 3  (wet), 0.78 Mg/m 3  (dry), specific gravity 2.59. Anode zone bulk density was recorded at 1.49/m 3  (wet), and 0.88 Mg/m 3  (dry), specific gravity 2.62. These differences in physical properties between the anodic and cathodic zone are consistent with the addition of iron to anodic zone sediment, during the experiment. The hand Vane shear strength of the anode sediments is 2.45 K Pa, compared to zero for cathode zone and untreated sediment. This indicates a significant improvement in the engineering properties of the anode zone sediments as a consequence of electro-osmotic dewatering, accompanied by precipitation.  
         [0037]     Radioactively-Contaminated Sediment, Ravenglass, Cumbria  
         [0038]     A clay-rich sediment sample, slightly contaminated with artificial radionuclides, was collected from the Ravenglass estuary, Cumbria and treated at 1.5 V for 410 hours in a two-dimensional perspex cell, using an electrode separation of 17 cm. A 17 mm thick Fe-rich band was generated 5 cm from the anode, at the point where a major step in pH (from pH2 to pH13) occurred. Geochemical and radiometric analysis of the treated sediment (see  FIG. 4 ) shows clear removal of radioactive cobalt ( 60 Co) from the anode zone of the cell, and precipitation of the remobilised  60 Co on the iron-rich band. This was achieved in a short 17 day timescale compared with commercial systems which typically operate over duration of 20-100 days.  
         [0039]     Manganese (Mn), calcium (Ca) and strontium (Sr) were also remobilised from the anode zone and precipitated on, or around, the iron band. Soluble ions such as iodine (I), bromine (Br) and sodium (Na) migrated towards the appropriately charged electrode. Notably, As, present as a trace contaminant in these sediments, was highly amenable to the treatment, with desorption occurring at high pHs in the cathode zone. A 100% enrichment of As occurred on the iron-rich band (see  FIG. 4 ), reflecting the strong affinity of As for the amorphous precipitated Fe. The highly particle-reactive radionuclides plutonium (Pu) and americium (Am), present at elevated activities in this sediment, were not significantly remobilised over the timescales used. The method of the present invention, however, can still be used to contain leachates contaminated with these radionuclides due to the action of the Fe band as a barrier to groundwater flow, the strong association of Pu and Am with freshly precipitated amorphous iron oxide phases, and the action of the applied electric field, which forces ionic and colloidal species to migrate towards the appropriately charged electrode.  
         [0040]     In summary, unlike existing electrokinetic techniques which actively avoid precipitation of minerals and salts in the soil mass between the two electrodes, the method of the present invention is specifically geared towards producing an iron-rich band in situ between cathode and anode. This iron band simultaneously provides a physical as well as a chemical barrier to leachate migration. The method also employs a low voltage (with low energy requirements) to generate a strong pH gradient within soils and sediments and can desorb a range of polar and ionic contaminants. It uses low cost, sacrificial cathode and anode materials, and can produce, through differential dewatering, water movement and electro-osmotic purging of non-polar organic contaminants.  
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