PROCESS FOR THE TREATMENT OF CONTAMINATED MATERIAL

Abstract: A process for the decontamination of a medium comprising a particulate material contaminated with one or more metal species, the process comprising the steps of treating a body of the said medium with microbially produced sulphuric acid so as to solubilise themetal species as a metal sulphate; treating the leached metal sulphate by a bioprecipitation process which converts the said sulphate to an insoluble sulphide; separating hydrogen sulphide produced during the bioprecipitation from the insoluble metal sulphide; and oxidising the separated hydrogen sulphide to form a reusable source of a sulphur-containing ingredient.

Description: Process for the treatment of contaminated material The present invention relates to a process for the treatment of contaminated material, in particular a process for the removal of metal, especially heavy metal, contaminants from bulk particulate material such as land or soil using biochemical processes. World-wide, substantial amounts of land have become metal contaminated as a result of industrial, waste disposal and other activities. Examples of such contaminants include: mercury, cadmium, barium, chromium, manganese and lead, radionuclides such as actinides and fission products. Such contaminants can pose a significant threat to ground water and therefore drinking water supplies and in many cases either limit, or prevent land re-use. Additionally, as a result of recent legislation in the United States of America and likely similar legislation within the European Community and elsewhere, waste producers are becoming increasingly liable to prosecution and to meet the costs of recovery and clean up if they do not act responsibly towards their wastes. Consequently there is a growing need for technologies which can help solve the problems caused by contaminated land. To date, a number of techniques have been developed to remediate contaminated land. Examples include: soil stabilisation, electromigration, vitrification, volatilisation, incineration, soil washing, pump and treat systems, land farming, slurry phase bioremediation etc. Many of these known techniques possess several limitations including: a) Lack of a permanent solution to the problem, eg transferral of the material to a toxic landfill, or entrapment within matrixes possessing a limited life; b) Unsuitability to treat a wide range of contaminants, eg metal contaminated land in the case of currently used biological processes; c) The generation of high volume, or difficult to control secondary wastes, eg soil stabilisation and incineration; d) Lack of selectivity of in-situ or ex-situ options as appropriate to a particular site, eg as in the case of incineration and soil washing; e) High costs, eg incineration, vitrification and pump and treat systems; f) Limited ability to re-use contaminants, eg soil stabilisation systems when- applied to metals. The present invention seeks to address these problems by enabling biological systems to remediate metal contaminated land non-specifically. According to the present invention a process for the decontamination of a medium comprising a particulate material contaminated with one or more metal species comprises the steps of treating a body of the said medium with microbially produced sulphuric acid so as to solubilise and leach the metal species as a metal sulphate; treating the leached metal sulphate by a bioprecipitation process which converts the said sulphate to an insoluble sulphide; separating hydrogen sulphide produced during bioprecipitation from the insoluble metal sulphide; and oxidising the separated hydrogen sulphide to form a reusable source of a sulphur-containing ingredient. The medium to be decontaminated may comprise a particulate material such as soil, rock particles, dredgings, sediments, sludges, process residues, slags from pyrolytic processes, furnace dusts and the like. The contaminants may be contained on the surface of the particulate material or may be bound inside the particles thereof. Several metal species may be present in the said medium and these may be converted to various metal sulphates and subsequently bioprecipitated as various metal sulphides. The term "metal species" as used herein includes metals, alloys, metal salts, metalloids and metal containing compounds and complexes. The said metal species contaminants may include: i) actinides or their radioactive decay products or compounds thereof; ii) fission products; iii) heavy metals or compounds thereof. Actinides are elements having periodic numbers in the inclusive range 89 to 104. The term 'fission product 1 as used herein refers to those elements formed as direct products (or so-called 'fission fragments') in the fission of nuclear fuel and products formed from such direct products by beta decay or internal transitions. Fission products include elements in the range from selenium to cerium. Non-radioactive heavy metals desired to be separated by the process of the present invention include toxic metals such as nickel, zinc, cadmium, copper and cobalt and other common contaminants. These are commonly found as earth contaminants or in aquatic sediments near industrial plants in which chemicals containing those elements have been employed, and on waste disposal sites. The metallic contaminants separated in the process of the present invention may include a mixture of radioactive and non-radioactive metallic contaminants. As described below, the process of the present invention may be extended to include steps in which organic contaminants in the medium to be treated are also removed or detoxified. The particulate material desirably is treated by leaching with the biologically produced sulphuric acid using an added aqueous solution. Where the medium to be decontaminated comprises soil or land, the soil or land may be treated in-situ or ex- situ. In the latter case the soil may be pre-treated eg to remove or crush large objects eg boulders, stones and the like. A suitable mixture of an aqueous solution containing biologically produced sulphuric acid and/or a source of sulphurous material bioconvertible into sulphuric acid may be injected into or mixed with the soil. Other ingredients such as nitrogen-rich or phosphorus-rich materials and air may optionally be added. Bioconversion to form sulphuric acid may be carried out in a known way by microbial agents present in the soil. The sulphurous material may comprise either elemental sulphur or another reduced form of sulphur. Where the soil or other particulate material, eg process residues or slag, is to be treated ex-situ it may be treated in one or more suitable known bioreactors. The aforementioned ingredients may be added to promote acid production. Where the bioconversion to produce sulphate ions is carried out in the soil to be treated it may be brought about by the action of naturally occurring sulphur oxidising organisms including: Th±obacillus ferooxidans, Thiobacilluε thiooxidans and Th±obacillus neapolitanus. These organisms may for instance be present as a consortium. These organisms obtain the energy necessary for their growth by the oxidation of reduced forms of sulphur thereby producing sulphates and sulphuric acid, or by the oxidation of ferrous iron to ferric iron. If the soil is deficient in appropriate micro-organisms, or if the particulate material is to be treated in a separate bioreactor then these micro-organisms may be added as a mixed consortium obtained from similar soil environments. In addition to acid leaching mentioned above metal release can occur by one or more of the following mechanisms: a) direct attack of metal sulphides; b) by electrochemical processes (Galvanic conversion) , resulting from contact between two dissimilar metal species immersed in a suitable electrolyte, eg sulphuric acid; or c) by the oxidative effect of ferric sulphate. As an alternative, the sulphuric acid required for the leaching step in the process embodying present invention may be produced chemically or biochemically in a separate bioreactor and added to the soil or other particulate material after production. During process start up, elemental sulphur, or sulphuric acid (bypassing in-situ biological acid production) , may be used as the acid source for leaching. Thereafter, elemental sulphur, or a combination of elemental sulphur and sulphuric acid will be the major sulphur source. Elemental sulphur, or sulphuric acid may be added to replace the available sulphur lost from the system, as metal sulphides. The leachate solution may be allowed to percolate through and drain from the body of particulate material and be collected. The leachate solution so collected may then be either re-circulated through the particulate material or pumped into a reactor to carry out the bioprecipitation stage of the process. The bioprecipitation stage employed in the process of the present invention may be similar to known sulphate reduction processes. Such processes have, for example, been used in the prior art to treat both sulphates and heavy metals, either alone or in combination, removed from waste waters. For example, the process described in EP 426254A which employs both ethanol as a substrate and methanogenic organisms to convert anaerobically produced acetate to methane is suitable. Additionally, such processes occur naturally within many anaerobic environments. The bioprecipitation process in the present invention may employ a naturally occurring consortium of disimulatory sulphate reducing bacteria (SRBs) , to convert aqueous metal sulphates to metal sulphides. Micro¬ organisms responsible for this transformation include: species of Desulfovibrio and Desulfomonas and may be grown in an enclosed above ground bioreactor system. These organisms oxidise simple organic compounds such as lactic acid and ethanol, to derive the energy necessary for their growth. However, more complex carbon sources can occasionally be used, eg phenolic compounds, or possibly organic materials leached from the soil during bioleaching. As a consequence of this oxidation, sulphates are reduced to sulphides and water. As the sulphides of many heavy metals possess low solubilities in aqueous solution, these precipitate together with some biomass as a sludge within the reactor. The metal sulphides will normally be separated as sludge and may be sold for metal recovery, or in the case of toxic, or radioactive metals, further immobilised in a subsequent process. Reduction of sulphuric acid entering the bioprecipitation reactor from the metal dissolving phase will result in the production of hydrogen sulphide and a consequent reduction in the sulphuric acid concentration. This will result in the maintenance of a pH close to neutrality within the bioprecipitation reactor and thus, an optimal pH for SRB activity. Additionally, the substantially neutral pH will cause hydrogen sulphide to remain in solution, thus maintaining a sufficiently low redox potential for SRB viability, ie <-300mV. The maintenance of a suitable redox potential by this method is common. Although, the procedure has previously been used to maintain a suitable reactor pH (eg as in EP 436254A) , it has not previously been used to buffer against influent flows as low as pH 1.0 as might be encountered from the acid leaching step described herein. As a result of the production of hydrogen sulphide and metal sulphides during bioprecipitation, three different product streams may arise from the bioprecipitation process: (a) sludge containing precipitated metal sulphides and biomass; (b) aqueous hydrogen sulphide and soluble metal sulphides together with some biomass components; (c) gaseous hydrogen sulphide. Gaseous hydrogen sulphide may be extracted by a venting means provided at or near to the top of the reactor. Aqueous hydrogen sulphide and other soluble sulphides may be separated from the sludge. The metal sulphide sludge may be removed separately via a suitable drain in the reactor. The sludge may be dewatered and recycled for metal recovery or may be treated by a suitable encapsulation process, eg a biologically enhanced metal fixation. The gaseous and aqueous hydrogen sulphide extracted is a valuable source of re-usable sulphur and may be utilised by the biochemical oxidation process described hereinafter. During the initial operation of the metal leaching stage of the process according to the present invention the leachate entering bioprecipitation will possess a substantially neutral pH. Therefore, a portion of this liquor can be used to dissolve the gaseous hydrogen sulphide effluent produced from bioprecipitation. The two aqueous hydrogen sulphide streams derived from bioprecipitation may be employed separately or combined and may be oxidised within an enclosed bioreactor system. The bioreactor system may contain a consortium of naturally occurring sulphide oxidising organisms. Examples of micro-organisms known to oxidise soluble sulphides include: Thiobacillus thioparus, T. neapolitanus, T. dentificans and Thiomicrospira species. Two routes are possible for sulphide oxidation: (a) direct oxidation to sulphuric acid and/or sulphates; (b) oxidation to elemental sulphur, which can if appropriate then be added to, eg spread on, the contaminated soil to produce sulphuric acid. Oxidation to elemental sulphur requires an oxygen limited environment, but possesses the advantage of providing a sulphide free, pH neutral liquor that can be used to dissolve effluent hydrogen sulphide gas from bioprecipitation. The sulphuric acid liquor produced by direct oxidation is more versatile for use in subsequently contacting the contaminated soil. The process of the present invention may be extended to include one or more steps for the removal of organic contaminants from the said contaminated medium and this may be by a biodegradation process employed in conjunction with the metal removal process as described in Applicants' copending UK Patent Application Nos. 9402975.8 and 9414425.0 (which process is the subject of a copending PCT application of even date herewith) . The present invention offers the following further advantages over prior art processes: (1) It provides a permanent solution to the contamination problem. (3) It allows the simultaneous treatment of metal and organic contaminants. (4) In-situ and ex-situ treatment systems can be available and selected as appropriate. (6) The size of the secondary waste streams and therefore the cost of dealing with them is minimised. (2) It minimises the use of harsh chemicals which could harm the environment. (5) An opportunity to re-use certain metal contaminants may be offered. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of a region of land being treated in-situ to provide remediation thereof by a process embodying the present invention together with equipment used in the process. As shown in Figure 1, a region of land to be treated comprises a layer 1 of soil overlying an underground aquifer 3 below a level 2. The layer 1 incorporates a metal contaminated region 4 which has been produced by migration of contaminants from a waste sump 5 provided in the surface of the layer 1. The region 4 extends into the aquifer 3. A monitoring well 6 projects downward through the region 4 to enable measurements on the extent of contamination in the region 4 to be determined. The depth and dimensions of the contaminated region 4 have previously been determined using appropriate known analytical techniques. Ground level is indicated by numeral 18. Nutrients from a nutrient source 22 and acid which may be carried in a suitable carrier liquid, eg aerated water, are applied to the base of the empty sump 5. This application is carried out by a sprayer 7. This liquid is also applied via appropriately positioned injection wells 8 and through an infiltration gallery 9, so as to permeate through the material in the contaminated region and promote soil acidification. Elemental sulphur may also be added to and mixed into areas of shallow contamination such as the base of the sump 5 further to promote in-situ bioleaching as described above. In order to enable aerobic conditions to be developed and maintained within the contaminated region 4 air is blown by an air blower 21 attached to a series of vent wells 10, (one of which is shown) either to draw air through the contaminated region 4 in the layer 1 or to inject air into the ground water in the aquifer 3 or both. Additionally, the rate of nutrient addition may be varied to avoid the creation of anaerobic conditions within the contaminated region 4. The plume or region in the layer 1 and aquifer 3 supplied with nutrients and acid is indicated by reference 20. This plume encompasses the contaminated region 4 in the layer 1 and aquifer 3. This treatment produces acid metal leaching in the region 4 in the manner described above. This may continue over weeks or months until the soil in the contaminated region 4 is substantially free of contaminating metals as determined from time-to-time by suitable analysis. The prod.ucts of the metal leaching treatment are collected within a portion of the aquifer 3, either naturally occurring or artificially created in a direction X, and are received by and returned to the surface above the layer 1 via a series of recovery wells 11 (one shown) using appropriate pumps (not shown) . The level 2 of the aquifer 3 may be adjusted by addition of water through an infiltration gallery 24 to assist water flow in the direction X. The collected liquor is then delivered to a selected one of: (a) a buffer tank 12 for aeration and addition of appropriate nutrients before re-application to the contaminated area. This is the principal route during initial operation of the process; (b) a bioprecipitation reactor 13; (c) a gas liquid contactor 14 to scrub hydrogen sulphide from the gaseous effluent from bioprecipitation. Liquor enters the reactor 13 at its base and flows upward through the reactor 13. As it does so, sulphate reducing organisms present in the reactor 13 convert the influent sulphates to sulphides in the manner described above. The gaseous effluent produced during bioprecipitation in the reactor 13 is passed through the gas liquid contactor 14 connected to the reactor 13. The contactor 14 permits hydrogen sulphide recovery. The gas stream leaving the contactor 14 is passed through a secondary scrubber unit 19 and discharged to atmosphere. Bioprecipitated sludge containing insoluble sulphides is collected in the base of the reactor 13 and transferred via a pipeline 15 to a separate treatment process, eg biologically enhanced metal fixation or is dewatered and collected and delivered to another site for metal recovery. The liquor obtained by dewatering the sludge may either be returned for re-use in the bioleaching process, or further treated and discharged. The effluent liquor containing dissolved sulphides arising from bioprecipitation is extracted and combined with the aqueous sulphide stream arising from the gas/liquid contactor 14. The combined aqueous sulphide stream is then pumped through a gas/liquid contactor 16 and into a sulphide oxidation reactor 17. Contactor 16 ensures that any gaseous hydrogen sulphide released by acid in the reactor 17 is re-dissolved by the alkaline influent liquor. Within the oxidation reactor 17, the sulphide containing liquor is intimately mixed with suitable micro¬ organisms and oxidised to sulphate in the manner described above. The acid liquor produced is then transferred to the buffer tank or bioreactor 12 where further elemental sulphur may be added from a sulphur source 23 if required, and oxidised to sulphuric acid, by micro-organisms carried over from reactor 17 before re-addition to the contaminated material in the soil 1 in the manner described above (via the wells 8 and gallery 9 and sprayer 7). The metal removal treatment process is therefore cyclical and metal contaminants in the portion 3 of the soil layer 1 are, during various cycles of the metal removal process, gradually leached by the leachate solution containing biochemically formed sulphuric acid and recovered as an insoluble sulphide formed in the bioprecipitation reactor 13. A proportion of the sulphur is recovered by oxidation of sulphides in the oxidation reactor 17 and is re-used in the soil acid leaching of metal contaminants.

Claims: Claims 1. A process for the decontamination of a medium comprising a particulate material contaminated with one or more metal species, the process comprising the steps of treating a body of the said medium with microbially produced sulphuric acid so as to solubilise the metal species as a metal sulphate; treating the leached metal sulphate by a bioprecipitation process which converts the said sulphate to an insoluble sulphide; separating hydrogen sulphide produced during the bioprecipitation from the insoluble metal sulphide; and oxidising the separated hydrogen sulphide to form a reusable source of a sulphur-containing ingredient. 2. A process as in Claim 1 and wherein several metal species are present in the said medium and these are converted to various metal sulphates and subsequently bioprecipitated as various metal sulphides. 3. A process as in Claim 1 or Claim 2 and wherein the said metal or metals include radioactive species. 4. A process as in any one of Claims 1 to 3 and wherein the medium to be decontaminated comprises a particulate material selected from soil, rock particles, dredgings, sediment, sludge, process residue, slag from a pyrolytic process and furnace dust, the contaminants may be contained on the surface of the particulate material or being bound inside the particles thereof. 5. A process as in any one of the preceding claims and wherein the particulate material is treated by leaching with the microbially produced sulphuric acid using an aqueous leachant solution. 6. A process as in Claim 5 and wherein the sulphuric acid is produced microbially in-situ in the material comprising the particulate material. 7. A process as in Claim 5 and wherein the sulphuric acid is produced in a separate bioreactor and is added to the material comprising the particulate material. 8. A process as in Claim 5 or Claim 6 and wherein the process is applied to treat soil or land in-situ. 9. A process as in any one of the preceding claims and wherein the insoluble sulphide or sulphides produced in the bioprecipitation process are extracted and encapsulated. 10. A process as in Claim 1 and wherein the process is operated cyclically, the said reusable source of sulphur- containing ingredient being re-applied to the medium being decontaminated to provide further sulphuric acid metal . leaching therein.

Inventors: ECCLES HARRY, ECCLES, HARRY
Applicants: BRITISH NUCLEAR FUELS PLC, ECCLES HARRY, ECCLES, HARRY
Application year: 1995
Main classification: A62D3
IPC codes: A62D3, A62D101, B09C1, C02F3, C02F11, C22B3