GB2308819A - Extraction of halocarbon contminants from hydrophobic matrices - Google Patents

Abstract

A continuous, recycling, solvent extraction process for the separation and extraction of halocarbon contaminants, particularly organochlorines such as polychlorinated biphenyls (PCBs), from hydrophobic materials, especially hydrophobic liquids and solids such as oils and fats, comprising contacting an oleophobic perfluorocarbon (PFC) fluid, in which the halocarbon contaminant is at least partially soluble, with the contaminated hydrophobic liquid or solid by means of a recycling liquid-liquid or solid-liquid continuous extraction apparatus 10 wherein the dense PFC fluid is contacted with the contaminated hydrophobic material, the contaminants are at least partially extracted into the PFC fluid, the PFC fluid is removed from the hydrophobic material into a separate PFC boiling zone 23 where the PFC fluid is converted to a vapour leaving the higher boiling point contaminants remaining in the PFC boiling zone. The contaminant-free PFC vapour is passed to a separate condenser region 30 in which the PFC vapour is condensed into a liquid and the PFC liquid is reintroduced into contact with the hydrophobic material 5, thus completing the cycle which can be continuously repeated for as long as is necessary to effect removal of the contaminant to a required extent.

Description

Process for extraction of halocarbon compounds from hydrophobic matrices.

The present invention relates to extraction of halocarbon compounds from oils and other hydrophobic, water immiscible matrices, and in particular to processes for removal of noxious organochlorine compounds such as polychlorinated biphenyls (PCBs).

Halocarbon compounds, particularly organochlorines such as PCBs, frequently occur as toxic contaminating residues in the environment, industrial wastes and foodstuffs. Safe disposal of organochlorine contaminated waste materials often requires methods of destruction of the waste in which the contaminant is also destroyed, such as high temperature incineration. Altematively, the waste can be safely disposed of by more conventional means, for example by landfill, or can be recycled following selective decomposition or detoxification of the contaminant in the waste using methods such as, in the case of industrial transformer oils, treatment with molten salt process or alkali.

It is often preferable to extract or remove the contaminant from the waste prior to its safe disposal. Such extraction methods are also applicable to the analysis of the organochlorine content of contaminated materials prior to waste disposal or, in the case of foodstuffs, to ensure that levels of contamination are within safe limits for consumption. Similarly, extraction procedures for organochlorines are routinely employed in the analysis of environmental samples for organochlorine content for the purpose of monitoring of pollution.

Because organochlorine compounds have a poor solubility in aqueous systems, procedures have been developed which utilise water immiscible organic solvent mixtures to separate, extract and concentrate halogenated organic compounds from contaminated aqueous or hydrophilic samples such as soils, sludges, wastewaters and hydrophilic animal and plant material. Solvent mixtures such as diethyl ether and hexane, in which organochlorines are known to be soluble, can thus be contacted with the contaminated matrix to extract the contaminant for the purposes of decontamination or alternatively for analytical purposes, the organochlorine-containing solvent being subsequently analysed by suitable techniques such as gas chromatography or mass spectrometry. Such methods are recommended as standard extraction procedures by various regulatory bodies, such as the United States Environmental Protection Agency. However, such organic solvents are incompatible with the extraction and concentration of organochlorine compounds from hydrophobic materials such as oils and fats because these materials are frequently themselves miscible with, or soluble in, the organic solvents.

A decontamination process involving the batch extraction and photodegradation of photodegradable contaminants by U.V. irradiation has been proposed by Stevens and Brown, Process for Treatment of Organic Contamination in Solid or Liquid Phase Wastes, US Patent 4,793,931December, 1988. They describe a process involving perfluorinated solvents, which are inert to the photodegradation step, to batch extract contaminants from hydrophilic wastes such as wastewaters, sludges and soils and decompose the contaminants in the perfiuorinated solvent (to which photooxidants could be added) by UV irradiation. Extraction efficiencies were increased by preextraction of the contaminant from a hydrophilic solid waste with hydrophilic, water soluble solvents such as acetone or methanol:water mixtures.

Thus, while several procedures exist for the solvent extraction and concentration of organochlorine compounds from hydrophilic or aqueous samples such as soils, sludges and waste waters, no solvent extraction procedures have been described for the removal of halocarbon compounds such as PCBs from hydrophobic oleaginous oils and fats.

There is, therefore, a need for a solvent extraction process by which hydrophobic halocarbon residues, particularly organochlorines such as PCBs, can be selectively extracted and separated particularly from oils and fats and which does not involve hazardous solvents miscible with the contaminated hydrophobic phase, in which the solvent can be recycled, is non-hazardous to use and can be utilised in a simple apparatus of low cost and can be applied to the extraction of residues for analytical purposes or can be applied to the large scale decontamination of industrial waste oils. Significant volumes of such PCB-contaminated mineral oils, especially transformer oils, await decontamination or safe disposal.

The present invention is based on the unexpected finding that perfluorochemical (PFC) fluids, which have the unique property of being essentially immiscible not only with water and hydrophilic substances but also with a wide range of hydrophobic materials, can be used to directly extract halogenated organic compounds, particularly organochlorines such as PCBs, from mineral oils and oils and fats of biological origin.

Thus, according to the present invention there is provided a continuous recycling method for the separation and extraction of halocarbon contaminants, particularly organochlorines such as PCBs, from hydrophobic materials, especially hydrophobic liquids and solids such as oils and fats, comprising contacting an oleophobic perfluorochemical (PFC) fluid, in which the halocarbon contaminant is at least partially soluble, with the contaminated hydrophobic liquid or solid by means of a recycling liquid-liquid continuous extraction apparatus wherein the dense PFC fluid is contacted with the contaminated hydrophobic material, the contaminants are at least partially extracted into the PFC fluid, the PFC fluid is removed from the hydrophobic material into a separate zone maintained at a temperature at or above the boiling point of the PFC fluid wherein the PFC fluid is converted to a vapour leaving the higher boiling point contaminants remaining in the PFC boiling zone, the contaminant-free PFC vapour is passed to a separate condenser region in which the PFC vapour is condensed into a liquid and the PFC liquid is reintroduced into contact with the hydrophobic material, thus completing the cycle which can be continuously repeated for as long as is necessary to effect removal of the contaminant to a desired extent.

The term PFC, as used herein, is used as an abbreviation for "perfluornchemical". It thus includes, but is not restricted to perfluorocarbons, i.e. compounds of the formulae: CnF2n+2 CF n 2n CF CF CnF2n-x wherein n is a positive integer from 1 to 30, preferably from 4 to 20 and x may be minus 2 or zero or a positive integer from 2 to 20, preferably from 2 to 10.

The term "PFC" further includes compounds containing atoms in addition to carbon and fluorine, including atoms of nitrogen, oxygen, and sulphur. Specific examples include perfluroalkanes, perfluorocycloalkanes, perfluoroalkyltetrahydrofurans and hydrofluorocarbons. Further examples of PFCs suitable for the purposes of the present invention are described in D.S.L. Slinn and S.W. Green (1982) "Fluorocarbon Fluids for use in the Electronics Industry", in "Preparation, Properties and Industrial Applications of Organofluorine compounds" (Ed R.E.Banks), Chapter 2, Ellis Horwood Publishers. These include perfluoropentanes (C5F,2), perfluorohexanes (C6F,4), Perfluoromethylcyclohexane (C7F,4), perfluoro(methyl)dimethylcyclohexane (C7F14/C8Fs6), perfluorodecalin (CroFas) and perfluoromethyldecalin (C"F,20). Also described are fully fluorinated alkanes, ethers and tertiary amines and perfluoro-(2-n-butyl)tetrahydrofuran. These PFC fluids are available under the tradenames Fluorinert from 3M and Galden from MontEdison SpA. It will be understood that certain alternative fluids in which the H atoms are predominantly substituted with F atoms and which exhibit suitable immiscibility with oils and a suitable solubility for halocarbon compounds such as organochlorines could also be used for the purposes of this invention even though the H atoms have not all been substituted with F atoms, for example, fluids in which all but one or two H atoms are substituted with F atoms. Examples of such fluids are the hydrofluorocarbon (HFC) series of fluids commercially available from 3M Ltd.

In accordance with the invention, contaminants can be removed from the hydrophobic solid or liquid material for analytical purposes by contacting the PFC with the contaminated hydrophobic material in one or more of a number of ways. In the case of extraction of contaminants from hydrophobic solids, the contaminants could be continuously extracted by the PFC solvent in a recycling apparatus of the Soxhlet type in which the dense PFC solvent, after having contacted the solid, percolates down through the contaminated solids matrix and is drawn off into a separate zone in which the solvent is boiled, the resulting contaminant-free vapour then being condensed in a separate zone prior to recontacting the sample thus completing the cycle. The efficiency of the extraction process can be enhanced by mechanical agitation, e.g. rotation or tumbling of the solids matrix while in contact with the PFC solvent.

In the particular case where contaminants are to be extracted from liquid samples such as oils for analytical purposes, a conventional recycling liquid/liquid continuous extraction apparatus could be used in conjunction with a suitable concentrator unit as exemplified by the Kuderna-Danish apparatus as used in conventional liquid/liquid extraction processes.

For the decontamination of larger volumes of contaminated fluids, such as industrial waste mineral oils such as contaminated transformer oils, other methods more suited to industrial scale contacting of the oil with the PFC solvent may be applicable. Such means may, for example, employ counter-current flow equipment and industrial scale mixers to improve extraction efficiencies and reduce extraction times.

These means may also include the provision of spray heads through which the PFC fluids could be introduced into the vessel, the formation of droplets thereby increasing the surface area over which the two fluids can contact increasing extraction efficiency.

Alternatively, the PFC solvent may be introduced in the form of a vapour generated by boiling of the PFC prior to introduction of the vapour into the oil containing vessel or tank. Provided the contaminated medium is at a temperature below the boiling point of the PFC fluid, the PFC vapour will condense and on condensation of the PFC vapour to the liquid phase the fluid would extract contaminants on contacting the contaminated medium. The contaminantcontaining PFC solvent would then be allowed to drain to a collection point at the base of the vessel prior to recycling or further processing or analysis. When, through carrying out the process of the invention, removal of contaminants from the contaminated medium has been effected to a desired degree, the contaminant-free PFC fluid formed in the condenser unit can be diverted from being reintroduced into the contaminated medium into a suitable storage vessel for subsequent re-use. The residue remaining in the PFC boiling unit, after the PFC has been removed, will comprise a contaminant-enriched material consisting of recovered contaminants suitable for subsequent disposal or re-use.

The process of the invention may be used to extract a broad range of contaminants but is particularly useful for removing halogenated organic compounds from water immiscible solids and liquids such as oils and fats. Such contaminants must, of course, be at least partially soluble in the PFC fluid to be capable of being extracted. In this regard it has been found that PFCs are useful in decontamination processes wherein the contaminants comprise halogenated organic compounds, especially aliphatic and aromatic polychlorinated compounds such as polychlorinated biphenyls (PCBs) and related compounds. Other contaminants of interest that are amenable to extraction by the process of the invention include, but are not restricted to, pesticides, cleaning agents, additives used in the manufacture of plastics, paints, pharmaceuticals and petrochemical products and by-products produced in the manufacture or use of such materials. The organic contaminants that may be removed in accordance with the invention are generally of low molecular weight i.e. less than 3000 and contain halogen atoms. The degree of solubility of contaminants in the PFC fluid will be related to the degree of halogenation of the contaminant and PFC fluids will be particularly useful for the extraction of heavily halogenated compounds which are generally recognised as being more toxic and biologically recalcitrant than compounds containing fewer halogen atoms.

The range of the types of material from which contaminants can be removed by the process of the invention is also varied and broad. The use of PFC fluids, by virtue of its being immiscible with aqueous media and with a broad range of hydrophobic liquids and solids, allows the process of the invention to be applied to extraction of contaminants from water-immiscible material of biological, environmental or industrial origin. Thus, such material may comprise animal or vegetable fatty tissue or oils, mineral oils of petrochemical origin, pharmaceutical and cosmetic formulations and preparations, synthetic chemicals both liquid and solid in which contaminants are present, greases, waxes, paints, coatings and other materials in which hydrophobic substances are present which are normally soluble in conventional organic solvents.

It will, of course, be appreciated that the use of PFC fluids is particularly beneficial in that it allows contaminants to be extracted from materials that may otherwise be soluble in conventional organic solvents which would therefore preclude their use in the separation of contaminants.

It will be further appreciated that the process of the invention is applicable to the extraction of contaminants from materials, liquid and solid, that comprise a mixture of hydrophobic and hydrophilic phases because PFC fluids are essentially immiscible with both oils and water and as a consequence neither phase will interfere with the extraction process. Such mixed materials may, for example, include oil-in-water emulsions, water-in-oil emulsions, complex animal and plant tissue samples, cosmetic and pharmaceutical preparations and formulations, pesticide formulations, environmental samples containing oils, industrial wastes, petrochemical wastes and spent lubricants and greases. The use of PFCs which are immiscible with both the hydrophilic and hydrophobic phases is particularly advantageous in that it eliminates the need for separation of one from the other prior to subsequent treatment.

Many environmentally important contaminants present in soils and organic matter are susceptible to microbial degradation and the art of removing these contaminants by inducing the growth of microbial populations within the contaminated matrix and which degrade the contaminants is frequently referred to as bioremediation. However, it is recognised that many environmental materials containing toxic halogenated residues are also contaminated with oils in which the halogenated organic compounds are frequently particularly soluble which renders them unavailable to microbial attack. Moreover, the intermediates and by-products of microbial degradation of halogenated xenobiotics such as PCBs and pesticides may be themselves both toxic and soluble in hydrophobic phases. Furthermore, heavily halogenated contaminants or intermediary breakdown products are known to be generally more toxic and inhibitory to the microbial population and therefore less prone to degradation making them more recalcitrant and persistent in the environment. The process of the invention may advantageously be used for the continuous removal of halogenated contaminants or degradative by-products from material being subject to bioremediation but which contains a significant oil phase into which contaminants may solubilise which might otherwise hinder the efficiency of the bioremediation process. Examples of such bioremediation processes and the materials to which they are applied include: the growth of microbial cultures on emulsions of oil in water or water in oil, composted vegetable matter in which growth of contaminant degrading microorganisms is encouraged and the growth of contaminant-degrading microbial populations in contaminated soils and waters. Furthermore, the inherent property of PFC fluids to absorb gases such as oxygen and carbon dioxide as a function of the gas tension in the surrounding environment could be used to advantage in maintaining an optimum gas balance in a bioremediation process employing metabolising (micro)organisms, the PFC fluid being previously charged with a desired gas mixture prior to its introduction into the material undergoing bioremediation. It will of course be obvious that the non-toxicity of PFC fluids to living systems allows it to be used in the decontamination of biotic materials (e.g. soils) without detrimentally affecting the soil microflora.

The invention will now be described in more detail by way of example with particular reference to the accompanying schematic drawings of which Figure 1 illustrates apparatus according to the invention for the removal of contaminants from a hydrophobic liquid such as mineral oil.

Referring to Figure 1, decontamination apparatus 10 consists of a decontamination tank 11 connected to a PFC recirculating system 20. The recirculating system 20 comprises a pump 21 for removing the PFC fluid 1 from the base of the decontamination tank 11 which contains the contaminated material 5. The output from the pump is fed via pipe 22 to a evaporator unit 23. The evaporator unit is served by a heating element 24. The evaporator unit 23 is also provided with an tap 25 to allow material to be withdrawn from the base of the evaporator unit if required. The PFC vapour produced in the evaporator is passed via conduit 26 to a condenser 30. The recondensed PFC fluid from the condenser 30 is returned via conduit 27 to the top of the decontamination tank 11. The PFC fluid could be introduced into the decontamination tank via the optional sparger or spray head 31, if required. Conduit 27 is provided with a valve to enable fluid returning from the condenser unit 30 to be diverted to a PFC storage vessel 28 via pump 29 when the PFC fluid is required to be withdrawn from the recycling decontamination apparatus on completion of decontamination. For improved extraction efficiency the decontamination tank 11 may optionally be fitted with stirrers, tumblers or agitators 40 depending on the nature of the material 5 to be decontaminated.

In Figure 2 is shown an alternative self-regulating recycling apparatus for the liquid-liquid extraction of contaminants from PFC-immiscible hydrophobic liquids such as plant, animal or mineral oils. To the base of decontamination tank 11 is fitted an adjustable height overflow tube 50, the overflow of which leads, via conduit 51 to the evaporator unit 23 as before. (The rest of the apparatus is as in Figure 1). In priming the apparatus for operation, a volume of PFC fluid, 1, is added to the decontamination tank 11, a volume of which flows into the overflow tube 50 by virtue of its being openly connected to the decontamination tank base. Contaminated oil, 5, is then added to decontamination vessel 11. The head pressure of the column of oil forces the dense PFC fluid up the overflow tube 50 to a height determined by the weight of the column of oil in the decontamination tank. More PFC fluid can be added to the overflow tube if required.

The height of the overflow tube 50 is then adjusted so the top of the column of PFC fluid it contains is at the same level as overflow region of the tube. When PFC fluid is added to the oil column in the decontamination tank 11, the PFC at the bottom of the tank is forced up the tube 50 to overflow into the evaporator unit 23 where it boils, the contaminant-free vapour being reconverted to a liquid in condenser 30 prior to being reintroduced into the decontamination tank 11, thus completing the cycle.

Particular benefits which ensue from the process in accordance with the invention include the following: a) The PFC fluid is a solvent that is non-hazardous to use in that it is non4lammable, non-toxic, noncorrosive and chemically and physically stable.

b) The PFC fluid exhibits several physical properties that are beneficial in the process: the PFC is immiscible with the contaminated medium (whether hydrophobic, hydrophilic or mixed; liquid or solid) the PFC fluid has a low viscosity and high density which results in improved flow characteristics and percolation rates, a low surface tension for maximal contacting with the contaminated medium and a low boiling point and latent heat of evaporation which minimises energy demand in the recycling process.

c) The process allows contaminant-free PFC to be continuously recycled through the contaminated medium to maximise recovery efficiency while minimising the volume of PFC required. The resulting cumulative extraction thus compares most favourably with the otherwise low levels of PCB that partition out of oil (for example less than 1% of total contaminant concentrations in the contaminated material) when PFC is merely contacted with contaminated medium in a non-recycling, batch extraction method.

d) The process can be designed such that when applied to contaminated liquids, no pumps need be employed, self-regulation of the respective contaminated liquid and PFC fluid levels being achieved through adjustable-level overflows and in which recycling rates can be controlled simply by adjustment of the power supplied to the heating element under the PFC boiler vessel.

e) The recovered contaminants accumulate and concentrate in the PFC boiler unit facilitating subsequent re-use or safe disposal.

f) Furthermore, the materials from which the contaminants have been removed can, if required be re-used or recycled by virtue of their not having been materially affected or chemically or physically modified through contact with the substantially inert PFC fluid. Alternatively, they can subsequently be disposed of safely, the removal of contaminants having rendering the hazard level of the material sufficiently low to allow low cost and safe disposal.

Claims (18)

We claim:

1. A method for the extraction of halogenated organic contaminants from an oleaginous hydrophobic liquid or solid medium by recycling an oleophobic perfluorochemical (PFC) fluid through the contaminated medium, comprising the steps of: i) contacting the contaminated solid or liquid medium with a PFC fluid immiscible with the contaminated medium wherein said contaminant at least partially dissolves into the PFC, ii) separating the PFC fluid into which contaminants have partitioned from the contaminated medium, iii) evaporating the PFC fluid in an evaporator without evaporating the contaminant so as to accumulate and concentrate the contaminant in the evaporator unit, iv) separating the contaminant-free PFC vapour from the evaporator unit, v) condensing the PFC vapour into the liquid form, vi) reintroducing the contaminant-free PFC fluid into the contaminated liquid or solid medium, thereby continuously reducing the concentration of contaminants in a hydrophobic solid or liquid.

2. A method according to Claim 1 wherein the PFC is a perfluorocarbon.

3. A method according to Claim 1 wherein the PFC is selected from compounds of the formulae: CF 2n'x wherein n is a positive integer from 1 to 30, preferably from 4 to 20 and x is an integer from -2 to +15.

4. A method according to Claim 1 wherein the PFC is selected from compounds of the formulae: CnF2n+2 CF n 2n CnF2n-2 CF n 2n-4 CnF2n.x wherein n is a positive integer from 1 to 30, preferably from 4 to 20 and x may be minus 2 or zero or a positive integer from 2 to 20, preferably from 2 to 10.

5. A method according to Claim 1 wherein the PFC is selected from compounds containing one or more atoms in addition to carbon and fluorine, of nitrogen, oxygen, and sulphur.

6. A method according to Claim 1 wherein the PFC contains one or more atoms of hydrogen and is known as a hydrofluorocarbon fluid.

7. A method according to any preceding Claim wherein the contaminants comprise one or more halogen substituted aromatic or aliphatic compounds.

8. A method according to Claim 7 wherein the contaminants comprise one or more chlorine-substituted aromatic or aliphatic compounds.

9. A method according to Claim 1 wherein the contaminated solid or liquid is a water-immiscible material of organic or inorganic origin.

10. A method according to Claim 9 wherein the contaminated medium is a hydrophobic solid.

11. A method according to Claim 10 wherein the hydrophobic solid is a lipid, fat, grease or wax oF mineral, animal or plant origin.

12. A method according to Claim 9 wherein the contaminated medium is a hydrophobic liquid oil of mineral, animal or plant origin.

13. A method according to Claim 9 wherein the contaminants are present in the hydrophobic phase of a complex mixture, suspension or emulsion of hydrophobic and hydrophilic solids and liquids.

14. A method according to any preceding claim wherein contact between the contaminated medium and the PFC fluid is increased by agitation, mixing, stirring, tumbling or rotation.

15. A method according to Claim 1 wherein the PFC fluid is condensed on contacting the contaminated medium.

16. A method according to Claim 13 wherein the contaminants are present in the hydrophobic phase of an emulsion or complex mixture of hydrophobic and hydrophilic material being subject to a bioremediation process in which microbial degradation of at least some of the contaminants present in the complex material is encouraged by culturing or composting.

17. A method according to Claim 13 wherein halocarbon substituted pesticide residues are removed from plant or animal tissues for the purposes of analysis.

18. A method according to Claim 1 wherein polychlorinated biphenyls (PCBs) are removed from transformer oils for decontamination purposes.