METHOD FOR THE REMOVAL OF METALS FROM STARTING MATERIALS COMPRISING PROTEINACEOUS SUBSTANCES AS WELL AS MATERIAL OBTAINABLE ACCORDING TO SUCH A METHOD

Abstract: The invention relates to a method for the removal of metals from starting materials comprising proteinaceous substances by labilizing and solubilizing the metal with a treatment with chemical agent which includes leaching out, extraction, removal by means of redox and/or acid-base reactions, wherein this chemical treatment is combined with a treatment causing cross-linking of the proteins in the proteinaceous material. The invention allows for the treatment of e.g. leather wastes for the removal of chromium therein. After the appropriate treatment collagen is obtained. This collagen is structurally undamaged, free of chromium and suitable for uses in cosmetics, drugs, high quality gelatin, food, etc.

Description: Method for the removal of metals from starting materials comprising proteinaceous substances as well as material obtainable according to such a method. The invention relates to a method for the removal of metals from starting materials comprising proteinaceous substances by treatment with chemical agents including leaching out, extraction, removal by means of redox and/or acid-base reactions. Methods of this type are generally known. In particular these methods are used for the removal of chromium from leather, e.g. leather wastes in the wet-blue stage, which have undergone tanning with chromium salts only or with chromium salts in combination with other substances such as aluminum salts and complexing agents. One of the known methods involves the use of enzymes to remove chromium from leather. This method yields hydrolyzed collagen. Known methods involve the use of specific chemical substances such as sodium glycolate, or the use of acid and/or base. However, these methods require a treatment during a long period and/or yield a hydrolyzed collagen product, e.g. gelatin. Chem. Abstr. Vol. 102, No. 104, January 1985, page 126, abstract No. 26833y (JP-A-59179600) mentions the removal of chromium from chrome containing leather shavings by emersing succesively in milk of lyme sulfuric acid, sodium hydroxide, hydrogen peroxide, and sulfuric acid. Although this method provides a complete removal of chromium, the proteinaceous material (leather shavings) is destroyed in the sense that it is highly hydrolyzed. It is an object of the present invention to provide a method for the removal of metals from starting materials comprising proteinaceous substances by treatment with chemical agents including leaching out, extraction, removal by means of redox and/or acid-base reactions, which method yields a proteinaceous material such as collagen which is not hydrolyzed, not denaturated, in general un¬ damaged. Another object is to obtain a product containing as less metal - such as chromium - as possible. These objects are achieved by the present invention, which provides a method of the type as mentioned in the preamble, wherein the treatment is a combination of processes causing cross-linking of the proteins in the proteinaceous material, and labilization-solubi- lization of the metal. Consequently, the present invention relates to a method for the removal of metals from starting materials comprising proteinaceous substances by labilizing and solubilizing the metal with a treatment with chemical agent which includes leaching out, extraction, removal by means of redox and/or acid-base reactions, wherein this chemical treatment is combined with a treatment causing cross-linking of the proteins in the proteinaceous material. The expressions "cross-linking", "solubilization" and "labil- ization" are well known in the art. With respect to the last- mentioned expression reference is made to e.g. Cotton and Wilkinson, "Advanced Inorganic Chemistry", . 4th Ed. , John Wiley & Sons, New York, 1980, page 1184; J.E. Huhee, "Inorganic Chemistry", 2nd Ed., Harper International Edition, New York, 1978, page 497 and Purcell and Kotz, "Inorganic Chemistry", W.B. Saunders Company, Philadelphia, 1977, page 660. The method of the invention has the combined effect of cross- linking, * labilization and solubilization. Preferably, the treatment of the invention comprises a treatment with free radical producing means including ionizing radiation, UV, ultrasonic treatment, heat treatment and/or treatment with chemical cross-linkers. However, it should be noted that the present invention is based on the finding that cross-linking of the proteinaceous substances brings the starting material in an apparently slightly modified condition which may be responsible for the effectiveness with respect to the removal of metal ions such as chromium, allowing use of strong labilizing and solubilizing agents, such as strong bases, without destruction of the protein. According to the invention it has been found that the effec¬ tiveness of the removal of undesirable metals in the starting material can. further be improved by a pretreatment with an acid. In other words, in the method of the invention the treatment causing cross-linking may start with soaking in acid. Preferably, the mini- mum concentration of the acid used is 10 "3 normal (0.001 n) . This concentration may correspond with pH 3 (if strong acids are used) . A maximum value of the acid concentration may be 1 normal. In general, the concentration of the acid used may depend on the size of the particles, the origin of the material treated, the nature of the protein, the* used chemicals, etc. Of course, there is also an interdependency between the acid strength and the period of the treating time. So, it is possible to use strong acids or weak acids such as acetic acid. The selection can be carried out by an average expert in the present field, i.e. empirically with the main criterium that the protein is not destroyed during this stage (or at any later stage) . According to a specific embodiment the invention relates to a method as defined in the above, specifically directed to the removal of chromium from a starting material such as leather comprising the steps of a) subjecting the starting material to the treatment causing cross-linking and partial labilization and solubilization in an acid medium, b) separating off the acid medium from the material produced at step a) , c) subjecting the material produced at step b) to a treatment with a strong base in order to labilize and solubilize Cr(III), leaving the cross-linked protein undamaged, d) separating off the basic medium from the material produced at step c) , and optionally, e) subjecting the material produced at step d) to a treatment with an oxidizing agent such as hydrogen peroxide or a reducing agent to obtain collageneous material. In such a specific embodiment it is preferred that in step c) a strong base such as sodium hydroxide having a normality in the range of 0.5-2 n, e.g. 0.8-1 n is used and that in step e) the oxi¬ dizing agent is used at a pH-value between 6 and 8, but it can also be used at higher pH-values. The oxidizing agent may be a hydrogen peroxide solution having a concentration of 0.1-30 % . Suitable reducing agents are: chromium(II) , chromium metal, zinc metal, magnesium metal, all in acidic medium. Reducing can also be carried out electrochemically. The method of the above specific embodiment may be comple- mented by treating the collageneous material obtained with water at a temperature of 50~8θ ° C to obtain sterilized and, high Bloom number gelatin or at higher temperature up to 100°C to obtain sterilized, lower Bloom number and technical gelatin. It should be noted that without separations, purifications, and additional sterilization the material obtained according to the invention is already sterilized. In the method of the invention it is also possible to replace the oxidant by a reductant. In such a case the presence of air will be avoided. The expression "proteinaceous substance" comprises a variety of materials such as leather, fur, and other skin products originating from animals, but also composites of collagen with other polymers, e.g. biodegradable combinations thereof. Starting materials may be of biological origin or not. Examples of metals which may be efficiently removed from the starting material are chromium and aluminum, examples of proteins thus obtained are collagen, elastin, keratines. The known methods for the removal of metals from materials such as leather require a relatively long time for the chemical treatment. Furthermore, the reagents are often expensive and/or toxic or the protein structure is destroyed. This results in a decrease of the commercial value of the product or in products polluting the environment as the removal of metals such as chromium is insufficient and the contents of such a metal remains above the allowed limits for food products. The invention solves the problem of the quantitative sepa¬ ration between metal such as chromium and collagen in the solid wastes of the tanneries. In this separation the collagen remains intact and in excellent quality and accordingly it can be used in the preparation of cosmetics, medical materials, edible and industrial gelatins of high quality, in food preparations and in animal feed including fodder for fish and chickens. For these pur¬ poses a very low content of metals such as chromium is required. By virtue of the present invention purified materials con¬ taining a metal content less than 100 ppm, e.g. a chromium content of 2-20 ppm or lower, if so desired (by prolonging the treatment) may be obtained. In this respect it should be noted that chromium relates to chromium(III) . The chromium(VI) contents can " be .zero in the material which has been treated according to the invention is zero. The treatment of the present invention can be carried out under mild conditions, at room temperature and with small consumption of the chemical reagents used. Moreover, chromium may be regained as chromium(III) , which may be directly recycled in the tannery. In case the specific methods comprising the above steps a)-e) is used, in which method the basic medium contains sodium hydroxide and the acid medium contains sulfuric acid, the only chemical substance remaining after the treatment is sodium sulfate in neutral aqueous solution, which can be treated by the usual de¬ salination methods in order to recycle water and to obtain solid sodium sulfate, which has commercial value. With respect to the removal of metals from leather it should be noted that the present method can be used in old leather products, such as old shoes, clothing, leather suitcases, etc. after removal of the materials which have been applied after tanning, such as varnishes, paints, tannines etc. The interrelated effects of the combined treatment on wet- blue wastes can be described as follows: in the pretreatment with the acid there is protonation of the amino group of glycine, which constitutes one third of the amino acids of collagen, and of the hydroxyl groups of hydroxyproline, which is ca. 11 % (g per 100 g of dry protein) . The repulsion between positive charges thus created is very strong, and becomes evident even macroscopically, since the material swells. The duration of the pretreatment and the concentra¬ tion of the acid depend on the origin of the material, the metals it contains and the particle size, and they must be adjusted so as to loosen up chromium-protein bonding, but without significant protein damage. The presence of chromium is essential in this respect. Under the conditions of the treatment of the invention collagen without chromium is damaged. However, the presence of chromium is not enough for preserving collagen at the subsequent stage of treating with strong base. Consequently, some additional strengthening is required, and this is done by a limited cross-linking. During cross- linking there is a parallel labilization too, caused by the free radical oxidation of Cr 111 or its reduction. The cross-linked material can then be treated with strong base such as alkali hydroxides, magnesium hydroxide, barium hydroxide, trimethyl ammonium hydroxide etc., in order to dissolve Cr 111 , while the pro¬ tein is essentially intact. The remaining of chromium is removed with the redox reagents, oxidizing or reducing (examples: hydrogen peroxide and air-free Cr 2*" , respectively). Mechanistically the treat¬ ments with the oxidizing agent and base are similar, namely they both involve negatively charged ions, which are repelled by the negatively charged sites of the protein. With respect to the method for the removal of chromium from leather the following important aspects should be noted with respect to the method involving the above steps a) - e) . High temperatures are not required and as a result the pro¬ tein does not break down. During the treatment with base, chromium remains in solution while the protein is in the solid state. The opposite happens in the prior art methods - which means decomposition of the protein to soluble products such as amino acids and peptides or solubilization e.g. by formation of soluble gelatin. The solution of the chemicals used may be recycled several times, since the solubility of chromium(III) is more than a thousand times larger compared to the amount of chromiu (III) extracted in each cycle, which is of course a matter of great economic importance. The combinations of the chemicals that can be selected to be used allow complete neutralization at the end of the cycles of the treatment, and chromium(III) precipitation can be combined with this neutralization. Chromium is obtained in a form that can be recycled directly in. the tanneries essentially quantitatively. The total time required for the treatment is relatively short. The method can be applied to any kind of leather waste (old shoes, leather goods in general) after the appropriate pre-treat¬ ment. In general, the method of the invention can be used for any sensitive macromolecules of biological or other origin, which have been cross-linked using salts or oxides or other inorganic compounds and not just chromium salts. A non-destructive recovery of the treated materials is achieved. Ionizing radiation may be used for the purposes of the present invention. Such radiation is already used on a large scale for sterilization of various objects such as syringes or for cross- linking in polymers, etc. For the application of the present inven¬ tion the "excess of the radiation" from such sources can be used, i.e. the radiation passing through the irradiated objects, which hit the wall of the chamber containing the radiation source and which normally get lost. The necessary doses for the present method are small. Hence, the already existing facilities can be used, which means that it is possible to have a combination of different technologies. It is also possible to use weakened sources for which there is a problem of disposal and storage. This implies minimization of the costs but also a contribution towards the solution of another environmental problem, i.e. the utilization of otherwise undesired radiation sources. The present invention is elucidated in the following examples. Example I Particles (ca. 1 mm) of the wet-blue leather waste are soaked in 1 n sulfuric acid (8 % suspension) for three hours under stirring, and then irradiated in a Co 60 source at a dose of 60 krad, in the presence or absence of air, and after settling for 8 hours are filtered and washed with water. The solid is then added to a sodium hydroxide solution of 1 n and after 3 minutes filtered off and washed. After that follows the treatment with 3 % H 2 0 2 at pH 7. for 5 minutes, filtration and washing again. The treatment with the peroxide is repeated two more times, and finally the solid is washed with plenty of water. The solutions of the chemicals are used several times and finally they are mixed to the appropriate pH in order to have neu¬ tralization and simultaneously precipitation of chromium(III) . The organic product is white, the protein retains its tex- ture, and as a result of all tests the chromium(III) content is 2-20 ppm. Drying is done by the known methods, e.g. freeze-drying. If drying is not thorough, the product is kept under refrigeration or by using preservatives. Variations of the above operation involve use of X-rays or electron beams, instead of gamma rays and yield the same favourable results. Example II With 0.1 mm particles the pretreatment is done with 0.1 N acid - sulfuric, hydrochloric or other - for 8 hours. The rest of the operation is in accordance with example I. Similar results are obtained. Example III 1 mm Particles tanned partly with aluminum salts are treated as in example II, but with shorter time of contact with the base. The results are the same as in the preceding examples. Example IV Use of 20 kHz ultrasound instead of gamma-rays, for 30 minutes. The results are the same as in the preceding examples. Example V As in example I, but using 1 % or 0.3 %, or 0.1 % peroxide for longer times (e.g. 30 minutes for 0.3 % peroxide) at pH 10. The results are the same as in the preceding examples. Other variations include: use of other acids or strong bases, different percentage of the suspension, different doses of radiation, different concentrations of the chemicals, different times of treatment. The choice of the specific parameters depends in each case on the specific properties of the material (i.e. kind of animal and its age, the content of the leather in chromium). Also, instead of oxidizing agent one can use labilizing reducing agents such as Cr 2 ", zinc flakes etc., in air-free suspensions. All of the tests using the various parameters - which are within the scope of the claims - yield the favourable results indi¬ cated in the Examples. The invention is further elucidated by means of the following flow chart which is based on a process as described in Example I. "Once", "X 2" and "X 3" mean that washing is done once, twice or three times. The possibilities of recycling are clearly indicated.

Claims: C L A I M S 1. A method for the removal of metals from starting materials comprising proteinaceous substances by labilizing and solubilizing the metal with a treatment with chemical agent which includes leaching out, extraction, removal by means of redox and/or acid-base reactions, wherein this chemical treatment is combined with a treat¬ ment causing cross-linking of the proteins in the proteinaceous material. 2. The method according to claim 1, wherein the treatment having " the combined effect of cross-linking, labilization, and solubili¬ zation comprises a treatment with free radical producing means including ionizing radiation, UV, ultrasonic treatment, heat treatment, and/or treatment with chemical cross-linkers. 3- The method according to claim 1 or 2, wherein the treatment causing cross-linking starts with soaking in acid. 4. The method according to any of claims 1-3 for the removal of chromium from a starting material such as leather comprising the steps of a) subjecting the starting material to the treatment causing cross-linking and partial labilization and solubilization in an acid medium, b) separating off the acid medium from the material produced at step a) , c) subjecting the material produced at step b) to a treatment with a strong base in order to labilize and solubilize Cr 111 leaving the cross-linked protein undamaged, d) separating off the basic medium from the material produced at step c) , and optionally, e) subjecting the material produced at step d) to a treatment with an oxidizing agent such as hydrogen peroxide or a — reducing agent to obtain collageneous material. . 5. The method according to claim 4, wherein in step e) the oxi¬ dizing agent is used at a pH-value between 6 and 8. 6. The method according to claim 4, complemented by treating the collageneous material obtained with water at a temperature of 50-80°C to obtain sterilized and, high Bloom number gelatin or at higher temperature up to 100°C to obtain sterilized, lower Bloom number and technical gelatin. 7- Material obtainable according to any of the preceding claims.

Inventors: KATAKIS DIMITRIS, KONSTANTATOS JOHN, POULOPOULOU VASSILIKI, VRACHNOU-DORIER ERSI, KATAKIS, DIMITRIS, KONSTANTATOS, JOHN, POULOPOULOU, VASSILIKI, VRACHNOU-DORIER, ERSI
Applicants: INVENTE S A, INVENTE S.A.
Application year: 1992
Main classification: C09H1
IPC codes: A23J1, C09H1, C14C3