Patent Application: US-52862905-A

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:
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 . 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 . 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 ( fig1 ) 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 fig2 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 ( fig2 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 . 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 . 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 ). 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 ( fig3 a - d ). 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 . 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 fig4 ) 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 . 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 fig4 ), 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 . 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 - 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