Immobilized enzime catalyzed removal of aromatic compounds from aqueous solutions

Abstract: A process for the removal of aromatic compounds from aqueous solutions using an oxidatic enzyme, such as a peroxidase enzyme, is described. The enzyme is immobilized on a surface so that preferably multiple passes of the aqueous solution with the aromatic compound can be made over the immobilized enzyme. Higher enzyme utilization with a lower oxidative substrate, such as hydrogen peroxide, requirement is achieved.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to an improved process for the precipitation of aromatic compounds from an aqueous solution using an enzyme. In particular, the present invention relates to a process wherein the enzyme is immobilized on a surface so as to cause the precipitation of the aromatic compound in the solution. (2) Prior Art Aromatic organic compounds are found in aqueous solution or suspension in groundwater, in settling ponds associated with public water purification or industrial facilities and in industrial effluents. The latter frequently derive from processes involving the combustion or transformation of fossil fuels, from the forest products industry or from the manufacture of explosives, to name a few of many sources. Aromatic compounds in general are toxic, albeit by a variety of mechanisms, and, therefore, are classed as pollutants. Existing methods for removal of aromatics from aqueous solution are mainly comprised of physical adsorption onto various media, metabolism by various micro-organisms and chemical combustion in the presence of various oxidants. Each of these methods has some or all of the following drawbacks: (a) time consuming; (b) expensive; (c) ineffective in dealing with pollutants at the levels encountered and/or in reducing pollutant levels to those desired; (d) exhibit a temperature sensitivity (therefore, large seasonal variation); and (e) have a narrow specificity with respect to the classes of aromatic compounds amenable to treatment. Klibanov has proposed an improvement in which an oxidative enzyme, peroxidase, is added to the aqueous solution to be treated along with the enzyme's co-substrate, hydrogen peroxide. This mixture converts susceptible aromatic compounds to radicals which in a subsequent, presumably non-enzymic, step couple with one another leading eventually to the formation of a precipitate. Susceptible aromatics are hydroxy- and amino-substituted compounds with various other substituents such as alkyl, alkoxy, halo and fused aryl. A series of publications and reports from Klibanov and co-workers describe these findings which are also embodied, in part, in U.S. Patent No. 4,623,465 to Klibanov . The publications are: (1) Science, Vol. 221, 259-261 (1983); (2) Klibanov, National Technical Information Service, PB84-138155, 1-18 (1983); (3) Detoxication Hazard Waste (Symp, 1981), Edited by J. H. Exner, Ann Arbor Sci. (Publisher) CA 98(12):95130c (1982); (4) Atlow, et al, Biotechnology and Bioengineering, Vo. XXVi, pp599-603 (1984); (5) Klibanov, et al, Enzyme and Microbiol. Technol. 3(2), 119-22(1981); (6) Klibanov, A. M., Enzyme Engineering 6 , 3(2), 319-325 (1982); and (7) Klibanov, A. M., et al., Journal of Applied Biochemistry 2, 414-421 (1980). The specific improvement claimed in the Klibanov patent is the ability of such a system to effect the clearance of compounds which are not substrates of the enzyme as long as there is a good substrate (a hydroxy- or amino-aromatic compound) also present. An extension is that the clearance of a poor substrate is augmented in the presence of a good substrate. There are, however, aspects to this approach which lower its economic feasibility: (a) it requires large amounts of enzyme because the enzyme appears to be progressively inactivated under the reaction conditions; (b) relatively high concentrations of hydrogen peroxide are employed which are themselves inhibitory to the enzyme; (c) the treatment requires 3 to 24 hours for completion; (d) a final filtration or centrifugation step is required to remove the precipitate generated; and (e) each of the pure compounds studied appears to require removal at a different pH. EP-A- 0 126 394 discloses a process for the removal of aromatic hydroxy compounds and /or aromatic amines from waste water by treating the water with peroxidase/ straight chain C₁ to C₄ alcohol or glucose oxidase/glucose. It is mentioned that the peroxidase and an oxidase enzyme can be immobilised and loaded into a flow reactor through which the fluid to be treated is then passed. OBJECTS It is therefore an object of the present invention to provide a process for the removal of the aromatic compounds using an enzyme which overcomes the aforementioned disadvantages of the prior art process. Further, it is an object of the present invention which is relatively simple and economical. These and other objects will become increasingly apparent by reference to the following description and the drawings. IN THE DRAWINGS Figure 1 is a schematic view of a laboratory apparatus of the present invention. Figure 2 is a schematic view of a large scale apparatus of the present invention. GENERAL DESCRIPTION The present invention relates to the process of removing an aromatic organic compound from an aqueous solution by providing one enzyme which is immobilized on a surface and one substrate for said enzyme in the aqueous solution and contacting the aqueous solution with the enzyme to cause the aromatic organic compound to form a precipitate and optionally by adding a hydroxy- or amine-substituted aromatic compound to the aqueous solution to enhance formation of the precipitate of the aromatic organic compound. Further the present invention relates to a process of enhancing the removal of a multiplicity of organic substances including an aromatic substance from an aqueous solution, including adding one enzyme and one substrate to said solution, thereby catalyzing the formation of an insoluble precipitate of said multiplicity of organic substances, wherein the precipitation of said aromatic substance is optionally enhanced by the addition of at least one other organic substance comprising a hydroxy- or amine-substituted aromatic compound, and removing said precipitate from said aqueous solution, the improvement which comprises providing the enzyme immobilised on a surface in the solution. The enzyme is immobilized on the solid phase using technology known in the fields of affinity chromatography and immunoassays. The linkage is generally covalent via, for example, amide, ester, ether, thioether amine, imine or diazo functional groups. To achieve such covalent attachment, it is usually necessary to activate the solid phase by introduction of appropriate reactive surface functional groups, for example, of the active ester, acid halide, aldehyde, isothiocyanate, cyanate, imidate, alpha-halocarbonyl, oxirane and related types. With respect to enzyme to be attached, the protein's surface carboxyl, amino or phenolic groups are frequently used as attachment sites. If the enzyme is a glycoprotein, it may alternatively be attached via carbohydate oxidation followed by imine formation with or without simultaneous or subsequent reduction. Other linking strategies involve the formation of electrostatic or hydrophobic complexes. The enzyme can be immobilized on a surface such as paper or plastic or can be immobilized on beads and like surfaces. Materials used for the solid phase encompass the whole range of polymers with some of the most prevalent being polymers of ethylenically-unsaturated monomers (e.g.-styrene, acrylic acid derivatives, ethylene, propylene), polysaccharides (e.g.-cellulose, dextran agarose), polypeptides (e.g.-gelatin/collagen, cross-linked proteins such as albumin), nylon and glass. Composites of some of the foregoing have also been used as have composites of some of the foregoing with inorganic materials other than silica, such as crystalline calcium phosphate. Preferably the surface is also useful for filtering the precipitated solids from the solution, such as with filter paper. Additional examples include other forms of cellulose, glass fiber filters and other porous solid phases of natural or synthetic occurrence. In the present invention the enzyme is used in immobilized form and preferably incorporated into a continuous flow reactor so that its contact time with the aromatic compound and peroxide mixture can be controlled by flow rate. In this way the solution can be repeatedly contacted by the enzyme through recycling or by passage through a series of reactor elements on which the enzyme is immobilized. Short contact time prolongs the operating lifetime of the enzyme and, since it is immobilized, therefore it may be used to treat a much greater volume of solution than if it had simply been included in a solution. A laboratory scale reactor is shown in Figure 1. A production reactor is shown in Figure 2. In Figure 1, line 10 is the inlet waste stream to reactor 11 containing enzyme substrates and non-substrates. Lines 12 are for co-substrate, e.g. hydrogen peroxide, addition to line 10 and then into reactor 11. The immobilized element 13 is provided in reactor 11. Figure 1 shows a second reactor 15 connected via line 14 and mixing coil 18 to reactor 11 and containing a second immobilized enzyme element 16. There may be more of such reactors in series (not shown). The aqueous stream is removed via line 17 with the enzyme substrates and non-substrates removed. The substrates are hydroxy and amino aromatic compounds. The non-substrates are other organic compounds such as naphthalene and other polycyclic aromatics which are co-precipitated in the presence of the enzyme substrates. In Figure 2, inlet line 20 is for the waste stream inlet to reactor 21 with multiple immobilized enzyme elements 22. Lines 23 are for co-substrate addition. Sensors 24 are connected to a computer system 25 for monitoring the need for co-substrate addition. The outlet 26 from the reactor 21 includes a second sensor 27 to detect remaining pollutants from the stream to the inlet line 20. The effluent is removed via line 26 and can be recycled or discharged. The present invention obviates the inhibitory action of elevated hydrogen peroxide concentration in batch processes by carrying out the clearance process at sub-stoichiometric concentrations of peroxide in a flowing stream. The stoichiometric requirement for peroxide is met in this situation by continuously adding it to the mixture between reactor elements or recycles. Under these conditions the immobilized enzyme clears phenol for instance, equally well from solution over the pH range 4 to 8. Lastly collection and removal of the precipitate is addressed in one embodiment of the present invention by allowing the precipitate to accumulate on the filter on which the enzyme is immobilized. Substantial amounts of the precipitate accumulate without drastically decreasing enzyme activity. The aromatic compounds which can be removed by the present invention, particularly include aromatic amines and hydroxides. Examples are: phenol aniline 2-chlorophenol 4-chlorophenol 2,2'-dihydroxybiphenyl 4,4'-dihydroxybiphenyl 8-hydroxyquinoline 2-aminophenol 3-aminophenol 4-aminobiphenyl 4'-amino-2,3'-dimethylazobenzene benzidine 4-bromoaniline 4-bromo-2-methylaniline 2-cresol 3-cresol 4-cresol 3,3'diaminobenzidine 3,3'-dimethoxybenzidine 3,3'-dimethylbenzidine 3,3'-dichlorobenzidine 1,3-diaminobenzene 2,3-dimethylphenol 2,6-dimethylphenol diphenylamine 4-fluoroaniline 2-methoxyphenol 3-methoxyphenol 4-methoxyphenol 2-methylphenol 5-methylresorcinol 1-naphthol 1-nitrosonaphthol 5-nitro-1-naphthylamine 1,4-diaminobenzene resorcinol 4-phenylphenol The enzymes which can be used are peroxidases, haloperoxidases, lactoperoxidase, ligninases (manganese-dependent or -independent), tyrosinase (also known as polyphenol oxidase) and cytochromes as well as heme proteins such as hemoglobin. Some of the enzymes use oxygen directly as a substrate (e.g. tyrosinase) while others use hydrogen peroxide as an oxidative substrate to activate the enzyme. All are regarded as "oxidative" or "oxidatic" enzymes. Equivalent results can be achieved where oxygen is dissolved in the aqueous solution for oxidative enzymes. The oxidative substrates for the oxidative enzymes can be oxygen, hydrogen peroxide, various alkyl hydroperoxides such as methyl hydroperoxide and/or percarboxylic acids such as peracetic acid. Illustrative are the reactions with peroxidase which is a most common and inexpensive enzyme. An example is horseradish peroxidase. Peroxidase in the resting, ferric, state reacts with hydrogen peroxide to undergo a two-electron oxidation to become peroxidase "compound I", which can return to the ferric state (via peroxidase "compound II") by two successive one-electron transfers from aromatic molecules like phenols and anilines. The latter are initially left in their one-electron oxidized forms, i.e., free radicals, which presumably by radical-radical recombinations (there being no evidence of radical chain processes) form dimers. The dimers are themselves phenols/anilines and therefore can be converted to radicals, thereby leading to trimers or tetramers: (a second H₂O equivalent is formed from the two formal H-equivalents plus the oxygen atom equivalent put on the ferric enzyme to form compound I) Analogously with anilines: Molecules like naphthalene which are not substrates of the enzyme in its compound I or compound II states are efficiently cleared from aqueous solutions in the presence of molecules, like phenols or anilines, which are. Other examples are benzene, azobenzene, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. One possible mechanism for this could involve radical addition reactions with the enzyme-generated phenoxy/anilinium radicals followed by H-atom abstraction, radical-radical recombination and/or oxidation (e.g. by air) as sketched below: If reactions of this type occur, presumably any type of molecule capable of reacting with a free radical could be a target for enzyme-catalyzed clearance in the presence of phenols/anilines. Alternatively, the non-substrate molecules can simply be cleared in the presence of substrates (phenols, anilines) by simple occlusion in the oligomers/polymers formed from the latter. SPECIFIC DESCRIPTION Example 1 Immobilization of Horseradish Peroxidase on Filter Paper Filter paper disks (Whatman #41) were treated with 20 mM sodium periodate in 0.1 M acetate buffer of pH 4.7 for 2 hours at room temperature in the dark. The disks were washed with water, then the same acetate buffer and then incubated with 100 mM adipic acid dihydrazide in the same buffer for 3 hours at room temperature. The supernatant was removed and replaced with 50 mM sodium borohydride in 0.2 M carbonate buffer of pH 9.2 and incubated one hour at room temperature. The disks were then washed and transferred to acetate buffer of pH 4.7 for exposure to oxidized enzyme. The latter was prepared by treating a 2 mg/mL solution of horseradish peroxidase (obtained from Boehringer-Mannheim; purity number >3.0) in 0.1 M acetate buffer of pH 4.7 with sodium perodate at 6 mM for one hour at room temperature. This solution was dialyzed against acetate buffer to remove periodate and iodate. The enzyme solution above at a dilution of approximately 0.2 mg/mL was incubated with filter paper disks derivatized as above such that approximately 5 to 10 micrograms of enzyme was presented per cm² of nominal surface area. Typically, 3 to 5 micrograms of enzyme was taken up per cm² of nominal surface area. Apparatus for Clearance of Aromatic Compounds From Aqueous Solution Paper disks of 2.5 cm diameter, prepared as above, were mounted in Swinnex®-type holders (obtained from Millipore Corp. (Bedford, Massachusetts); exposed nominal surface area of 3.63 cm² when mounted in holder). Aqueous solution containing buffer, aromatic compound and hydrogen peroxide was forced through the filter at flow rates in the 6 to 10 mL/min range using a peristaltic pump. If disks in separate holders were used in series additional hydrogen peroxide was added to the flowing stream between the disks and the combined stream was passed through a mixing coil (as shown in Figure 1) before encountering the next disk. The decrease in ultra-violet absorbence at the aromatic compound's maximum was used as the criterion for measuring the disappearance of the compound from the aqueous stream. Phenol Clearance A solution of phenol at 1.13 mM in 0.1 M sodium phosphate buffer of pH 7.5 (50 mL) containing 0.1 mM hydrogen peroxide was passed through two filters in series with intermediate addition (between filters) of hydrogen peroxide to increase its concentration by 0.1 mM. Several passes were made with measurement of absorbence at 272 nm used to estimate disappearance of phenol from solution. Unless stated otherwise additional hydrogen peroxide was not added to the batch before the first filter of the next pass. Pass Remaining Phenol, mM % Clearance 0 1.136 0 1 0.905 20.4 2 0.681 40.1 3 0.517 54.5 4 0.388 65.8 5 0.296 74.0 6 0.259 77.2 7 0.196 82.8 8 0.146 87.1 Example 2 The above experiment was repeated but with approximately 10-fold lower phenol concentration and 10-fold lower-hydrogen concentration (10 µM) in the starting solution and at the intermediate addition stages. Pass Remaining Phenol, mM % Clearance 0 0.105 0 1 0.084 20.2 2 0.067 35.7 3 0.052 50.1 4 0.043 59.1 5 0.040 61.9 6 0.025 76.6 7 0.012 88.4 8 0.008 92.4 The two enzyme disks showed 64 and 74% of their original activity after this experiment. Example 3 A variation of the experiment of Example 2 had the first pass as above but with the second pass peroxide was increased by 100 µM before both disks and no peroxide was added for the third pass. Pass Remaining Phenol, mM % Clearance 0 0.075 0 1 0.052 30.9 2 0.015 80.3 3 0 100 Both filters retained all of their original activity. In all of the foregoing runs precipitated material collected directly on the disks. Example 4 Naphthalene Clearance in the Presence of Phenol An experiment was carried out as in Example 1 except that phenol at 1.0 mM and naphthalene at 0.44 mM were present. In order to get the latter into solution at that level, ethanol at 10% v/v was used as cosolvent. Hydrogen peroxide was present at 0.1 mM in the initial solution and was increased by 0.1 mM by intermediate addition between filters at each pass. Naphthalene clearance was estimated from the decrease in absorbence at 312 nm. Pass Remaining Phenol, mM Remaining Naphth, mM % Clearance of Naphth 0 1.0 0.440 0 1 * 0.180 59 2 * 0.075 83 3 * 0.040 91 4 0.52 0 100 *not estimated due to naphthalene contribution to absorbence at 272 nm. Example 5 An experiment was carried out as in Example 4 but with 0.1 mM phenol plus 0.4 mM naphthalene with peroxide present in the initial solution at 10 µM and its concentration augmented by 10 µM through intermediate addition at each pass. Ethanol was used as cosolvent at 5% v/v. Pass Remaining Phenol, mM Remaining Naphth, mM % Clearance of Naphth 0 0.100 0.400 0 1 * 0.068 83 2 * 0.011 97 3 0.082 0 100 4 0.045 0 100 *not estimated due to naphthalene contribution to absorbence at 272 nm. A control experiment as above but with only phenol showed 48% clearance after 3 passes in absence of ethanol compared to 32% in the presence of ethanol at 5% v/v. As can be seen, the process of the present invention provides clearance of phenolic compounds from aqueous solution alone or along with another aromatic compound such as naphthalene. Equivalent results can be achieved with other aromatic compounds.

Claims: A process for removing at least one aromatic organic compound from an aqueous solution by providing one enzyme which is immobilised on a surface and one substrate for said enzyme in the aqueous solution and contacting the aqueous solution with the enzyme to cause the aromatic organic compound to form a precipitate. The process of claim 1 characterized in that said enzyme is a peroxidase and said substrate is a peroxide. The process of claim 1 or 2 characterized in that the first aromatic organic compound contains at least one substituent in an aromatic ring selected from -NH and -OH. The process of claims 1 to 3 characterized in that the aqueous solution is contacted in a flowing stream with the enzyme which is immobilised on the surface to cause precipitation of the aromatic compound from the solution. The process of claim 4 characterized in that the surface on which the enzyme is immobilised is a filter paper. The process of claim 4 characterized in that the enzyme is immobilised on the surface through a linking molecule. The process of claim 6 characterized in that a precursor of the linking molecule is adipic acid dihydrazide which reacts with the surface and with the enzyme to provide the linking molecule. The process of claim 7 characterized in that the surface is a filter and characterized in that the precipitated reaction product is filtered from the solution during or after contact with the enzyme immobilised on the surface. The process of claim 3 characterized in that the aromatic compound is a phenolic compound or phenol. The process of claim 3 characterized in that the aromatic compound is phenol in admixture with naphthalene. The process of claim 3 characterized in that the aromatic compound is an aromatic amine. The process of claim 11 characterized in that the aromatic amine is aniline. The process of one of the preceding claims characterized in that a second hydroxy- or amine-substituted aromatic compound is added to the aqueous solution to enhance formation of the precipitate of the aromatic organic compound. A process of enhancing the removal of a multiplicity of organic substances including an aromatic substance from an aqueous solution, including adding one enzyme and one substrate to said solution, thereby catalyzing the formation of an insoluble precipitate of said multiplicity of organic substances, wherein the precipitation of said aromatic substance is optionally enhanced by the addition of at least one other organic substance comprising a hydroxy- or amine-substituted aromatic compound, and removing said precipitate from said aqueous solution, the improvement which comprises providing the enzyme immobilised on a surface in the solution. The process of claim 14 characterized in that the formation of said insoluble precipitate is catalysed by the formation of free radicals on at least one of said organic substances. The process of claim 14 characterized in that said enzyme is peroxidase and said substrate is a peroxide. The process of claim 16 characterized in that the peroxide is hydrogen peroxide.

Inventors: TAYLOR KEITH E, TAYLOR, KEITH E.
Applicants: BIOTECH ENVIRONMENTAL INC, BIOTECH ENVIRONMENTAL, INC.
Application year: 1991
Main classification: C02F1
IPC codes: C02F3