MODIFIED BIOMASS FOR METAL ADSORPTION

Abstract: A method for improving the metal adsorption properties of a biomass, comprising contacting the biomass to be modified with a water-insoluble aldehyde.

Description: MODIFIED BIOMASS FOR METAL ADSORPTION Field of The Invention The present invention relates to improved modified cells useful for the adsorption of metals, and to a method for their preparation. BACKGROUND OF THE INVENTION The extraction of metals from hquid solutions is today recognized as an important problem, both from the environmental point of view, and in order to recover metals from waste streams. The amount of metals found in waste streams is substantial, and the problem is complicated by the fact that even very small concentrations of metals, in the order of ppm, represents an environmental hazard. The art is therefore constantly searching for more cost-effective means to extract metals from various solutions, and many processes and adsorbing agents are constantly developed. The Prior Art Among the adsorbants recognized in the art, biomasses have an important place. The biomasses normally employed include, e.g., yeast, algae, mold, common duckweed, etc. Such biomasses are readily available, either as by-products of a variety of processes, or can be grown by relatively inexpensive methods. While some biomasses have an inherent metal adsorption capacity, it is generally recognized that their ability to extract metals from solutions can be enhanced, sometimes dramatically, by various modification processes. For instance, WO 91/05859 teaches the preparation of beads containing an immobilized extractant which is blended with a polymeric material in an organic solvent and injecting the solution through a nozzle into water. The extractant may be, inter alia, yeast, algae, penicillium mold and Lemna sp. The beads are used to remove toxic metal contaminants from dilute aqueous solutions. US 4,021,368 teaches the use of modified myceha of fungi for the retention of metals. The biomass is stiffened by adding polymerizable components, and the product is subsequently mechanically granulated. The product is said to be particularly useful for the retention of uranium, radium, lead and similar metals. It is also known in the art (e.g., from Galun et al., Water Air Soil Pollut., 33, 359-371, 1987) to modify biomass by treating it with a water soluble aldehyde, such as formaldehyde, to improve its ability to sequester metal ions. However, the drawback of such modified biomasses lies in that, while they exhibit a substantially improved total adsorption capacity at high metal concentrations, they are not sufficiently effective in removing metal ions from solutions in which they are found in low amounts, e.g., in amounts below 5 ppm. Thus, even if such biomasses adsorb effectively from solutions containing metal concentrations above 20 ppm, when the concentration diminishes, below 20 ppm of metal, their adsorbtion power decreases substantially. Therefore, even if high concentration solutions are treated, and a high adsorption power is obtained at the beginning of the purification of the solution, a point is always reached where the adsorbing agents of the prior art become substantially ineffective when the metal concentration in the solution being treated drops. Given that it is normally desired to reduce the concentration of metal ions substantially below 0.5 ppm, it will be appreciated that the adsorption ability of an adsorbant at such low concentrations is very important. The art has so far failed to provide an adsorbant which can be used in low amounts to remove metals from low-concentration solutions, and as a result it is necessary to employ large amounts of adsorbants for this purpose, which is impractical and, of course, expensive. Summary of the Invention It has now been surprisingly found, and this is an object of the present invention, that it is possible to improve the adsorption capacity of a biomass towards low metal concentration solutions, by modifying the biomass through a treatment with a water-insoluble aldehyde. It is another object of the invention to provide modified biomass which exhibits improved adsorption ability, particulary in the range of low metal concentrations. It is still another object of the invention to provide a method for removing metals from low-concentration solutions. Other objects of the invention will become apparent as the description proceeds. The invention, in one embodiment, is directed to a method for improving the metal adsorption properties of a biomass, by contacting the biomass to be modified with a water-insoluble aldehyde. The improved biomass can be used for removing heavy metal ions, such as copper, nickel, chromium, cadmium, zinc, mercury, strontium, silver, gold, aluminum, uranium and lead, and their mixtures, from aqueous media. Detailed Description of The Invention The method for improving the metal adsorption properties of a biomass, according to the invention, comprises contacting the biomass to be modified with a water-insoluble aldehyde. The biomass can be of any suitable type, such as the aforementioned biomasses which are known in the art as useful adsorption agents. According to a preferred embodiment of the invention, however, the biomass is a yeast. In one embodiment of the method of the invention, the biomass is suspended in a water-immiscible organic solvent, to which the aldehyde has been or is then added. Contacting the biomass with the aldehyde causes a modification in the outer cell wall of the biomass. Without wishing to be bound by any specific theory, the inventors believe that it is this modification in the cell wall that is responsible for the improved adsorption properties. However, the reason for the improved adsorption capability of biomass treated with water-insoluble aldehydes at low metal ions contents, as compared with that of biomass treated with a water- soluble aldehyde, has not yet been fully elucidated. The aldehyde used to modify the biomass can be any suitable, water- insoluble aldehyde. Illustrative examples of suitable aldehydes include terephthaldehyde and benzaldehyde. As stated, different biomasses can be utilized. Preferred biomasses include, inter alia, yeast, algae, mold and common duckweed. Preferred yeasts include Saccharomyces species, e.g., Saccharomyces uυarum, Saccharomyces cereυisae or Saccharomyces lipolytica, and Candida Albicans. The invention also encompasses a modified biomass prepared by treating a biomass with a water-insoluble aldehyde, as described above. A primary object, which also forms a part of the present invention, is a process for removing metal ions from an aqueous solution which contains them, which comprises contacting the aqueous solution with a modified biomass as described above, for a period of time sufficient to reduce the metal ions content to the desired level. Specific process conditions, such as contact times, temperatures, etc., will vary according to the specific system requirements, and will be recognized by the skilled engineer. Such conditions are therefore not discussed herein in detail, for the sake of brevity. The metal ion to be removed from the solution is generally a heavy metal ion, such as of copper, nickel, chromium, cadmium, zinc, mercury, strontium, silver, gold, aluminum, uranium and lead, and their mixtures. While reference is made to "metal ions", for the sake of brevity, this term is meant to include all species containing metal ions, e.g., chlorides, nitrates, carbonates and sulfates, oxides, metallic anions such as Crθ4", and complexes such as Cu(NH3)4 Purified solutions from which metal ions have been removed by the process of the invention also form a part of the invention. AU the above and other characteristics and advantages of the invention will be better understood through the following illustrative and non- limitative examples. Example 1 Preparation of Modified Biomass Spent Saccharomyces uυarum yeasts from the beer industry were suspended in deionized water, at a 1:1 (water : yeast) ratio (v/v). The suspension so obtained was fed to a centrifuge at 8000 g for 10 minutes, and the yeast cells were separated and washed twice with isopropanol. 10 gr. of the yeast (corresponding to 2 gr on a dry basis) were then suspended in 80 ml of acetonitrile. To the suspension there was added benzaldehyde, at a ratio of 1 gr aldehyde for 1 gr of dry yeast. The suspension was stirred with a mechanical stirrer for about 6 hours at 40- 50°C, in a closed vessel. At the end of this period the sample was washed with acetonitrile until excess benzaldehyde was washed out as could be verified by a negative response to 2,4-Dinitrophenyl hydrazine or by TLC and than with water. Metal Removal Test A standard lead solution was prepared, containing 100 ppm lead in water, at a pH of 5.5. To 200 ml of this solution there was added an amount of modified yeast, corresponding to about 0.3 gr of dry matter. The suspension was stirred at room temperature for 15 minutes, and then the biomass was separated by centrifugation. The biomass was found to have adsorbed 61 mg of lead for each gram of dry matter, leaving a concentration of 0.17 ppm of lead in the final solution. The experiment was repeated several times, and the final lead concentration in the solution ranged from 0.5 ppm to 0.11 ppm. Example 2 (comparative) Preparation of Modified Biomass The same spent yeast used in Example 1 were washed with deionized water at a volume ratio of 1:3 (yeast : water). The resulting slurry was introduced into a centrifuge bucket and spun at 8000 g for about 15 minutes. The yeasts were separated and washed again with an aqueous solution containing 50% by volume of acetone, followed by an additional aqueous solution containing 30% by volume of acetone. The second stage was carried out by treating the biomass with an aqueous solution containing 3% (by volume) of glutaraldehyde, at a volume ratio of 1:10 (washed yeast : aldehyde solution). After a thorough mixing, the pH was adjusted to about 7 by the addition of a 2.5N sodium hydroxide solution. The mixture was stirred overnight at 32°C. The stabilized yeasts were separated by centrifugation, and then washed several times with water and kept refrigerated in a closed vessel until used. Metal Removal Test The standard lead solution of Example 1 was used. To 200 ml of this solution there were added 2.5 gr of modified yeast, corresponding to about 0.5 gr of dry matter. The suspension was stirred at room temperature for 15 minutes, and then the biomass was separated by centrifugation. The biomass was found to have adsorbed 20 mg of lead for each gram of dry matter, leaving a concentration of 25 ppm of lead in the final solution. Several solutions were treated. The final concentration of lead in the solutions ranged from 20 ppm to 40 ppm. Example 3 Example 1 was repeated, but using Candida Albicans as the biomass, and benzaldehyde as the fixation agent. The biomass adsorbed 32.4 mg of lead for each gram of dry matter, under the standard conditions of Example 1. Example 4 Example 1 was repeated, except that terephthaldehyde was used instead of benzaldehyde. The biomass adsorbed 35.5 mg of lead for each gram of dry matter. Example 5 Example 1 was repeated, except that the solution contained 90 ppm of mercury, instead of lead. The mortified biomass was found to have adsorbed 38.5 mg Hg per 1 gr of dry matter. As will be appreciated by the skilled person, the invention provides considerable advantages, particularly when removal of metal ions, down to fractions of ppm, are desired. The skilled chemist will understand that many modifications can be carried out in the process, products and methods of the invention described above. For instance, different water- insoluble aldehydes can be used, different solvents can be employed in the modification process, and different biomasses can be utilized, which can further be treated according to procedures known in the art (e.g., immobilization or pelletization), all without exceeding the scope of the invention.

Claims: CLAIMS 1. A method for improving the metal adsorption properties of a biomass, comprising contacting the biomass to be modified with a water-insoluble aldehyde. 2. A method according to claim 1, wherein the biomass is suspended in a water-immiscible organic solvent, to which the aldehyde has been or is then added. 3. A method according to claim 1 or 2, wherein the aldehyde is selected from terephthaldehyde and benzaldehyde. 4. A method according to any one of claims 1 to 3, wherein the biomass is selected from among yeast, algae, mold and common duckweed. 5. A method according to claim 4, wherein the yeast is Saccharomyces species or Candida Albicans. 6. A method according to claim 5, wherein the Saccharomyces is Saccharomyces uυarum. 7. A modified biomass prepared by treating a biomass with a water- insoluble aldehyde. 8. A modified biomass according to claim 7, which is a yeast. 9. A modified biomass according to claim 7 or 8, wherein the aldehyde is terephthaldehyde or benzaldehyde. 10. A process for removing metal ions from an aqueous solution which contains them, comprising contacting the aqueous solution with a modified biomass according to any one of claims 7 to 9, for a period of time sufficient to reduce the metal ions content to the desired level. 11. A process according to claim 10, wherein the metal ion is selected from among the ions of copper, nickel, chromium, cadmium, zinc, mercury, strontium, silver, gold, aluminum, uranium and lead, and their mixtures. 12. A process for removing metal ions from an aqueous solution, substantially as described and with particular reference to the examples. 13. A method for improving the metal adsorption properties of a biomass, substantially as described and with particular reference to the examples. 14. A modified biomass, whenever prepared by the method of any one of claims 1 to 6. 15. A purified solution from which metal ions have been removed by the process of any one of claims 10 to 12.

Inventors: GOTTLIEB LEVI, RAHMAN RON, GOTTLIEB, LEVI, RAHMAN, RON
Applicants: GOTTLIEB LEVI, RAHMAN RON, VERDECO TECHNOLOGIES LTD, GOTTLIEB, LEVI, RAHMAN, RON, VERDECO TECHNOLOGIES LTD.
Application year: 1997
Main classification: C02F3
IPC codes: C02F1