Method of regenerating a tannin-adsorbent

Abstract: A method of adsorbing and separating a heavy metal element by using a tannin adsorbent comprising: (a) adjusting the pH of a solution containing a plurality of heavy metal elements to a predetermined pH; (b) contacting the adsorbent with the solution in which the pH thereof is adjusted; (c) adjusting the pH of the solution contacted with the adsorbent to a pH different from the predetermined pH; and (d) contacting the solution in which the pH thereof is adjusted at the step (d) with the adsorbent prepared at the step (a). According to the inventive method, all of the heavy metal elements can be efficiently separated and adsorbed by using a tannin adsorbent from a solution containing a number of heavy metal elements. A method of regenerating a tannin adsorbent are also disclosed.

Description: BACKGROUND OF THE INVENTION In a method in which a solution containing a number of heavy metal elements, such as, actinides, e.g., uranium, thorium, transuranium elements, and the like, as well as lead, cadmium, chromium, mercury, iron, and the like, are contacted with a tannin adsorbent to adsorb the heavy metal elements onto the adsorbent, and separating the heavy metal elements from the solution, the invention relates to a method for eluting the heavy metal elements from the adsorbent having the heavy metal elements adsorbed therein so as to regenerate the adsorbent. Unexamined Published Japanese Patent Application No. 3-206094 (U.S. Patent Application Serial No. 07/631,946) of the present inventors, discloses a method for adsorbing a heavy metal element onto an adsorbent by contacting a solution containing a heavy metal element with a tannin adsorbent. In this method, tannin is dissolved in an aldehyde aqueous solution; ammonia is added to the resulting solution to form a precipitate; and the precipitate is aged to obtain an insoluble tannin which can adsorb nuclear fuel elements or iron ions. The inventors have also disclosed a method for regenerating a tannin adsorbent having adsorbed actinides therein by contacting the adsorbent with a dilute mineral acid to elute the actinides therefrom when the adsorbing ability of the tannin adsorbent is lowered (Unexamined Published Japanese Patent Application No. 3-293597). However, a problem occurs in the former method when a waste liquid containing a number of heavy metal elements is treated, because an adsorbent capable of adsorbing all of the heavy metal elements has not been known. It has thus been impossible to efficiently separate all the heavy metal elements from the waste liquid with a single adsorbant. A problem with the latter method is that when the adsorbent is regenerated by using the dilute mineral acid, the extent of regeneration in the adsorbent varies since the elution rates of the adsorbed actinides are different each other. SUMMARY OF THE INVENTION An object of this invention is to provide a method for regenerating a tannin adsorbent by efficiently eluting heavy metal elements adsorbed on the adsorbent. This object is achieved by a method based on the present inventors' discovery that the adsorption rate of a heavy metal element adsorbed by a tannin adsorbent varies with the pH of the solution containing the heavy metal element and the elution rate of a heavy metal element eluted from the adsorbent varies with the pH of a solution contacted with the adsorbent. More particularly, the inventors have discovered a method for regenerating a tannin adsorbent as defined in Claim 1 to adsorb a desired element and to desorb the element of that group from the adsorbent into the solution; and separating the adsorbent from the solution. This method may be repeated to desorb another element from the adsorbent by adjusting the pH of the alkaline solution to a value suitable to desorb the other element(s) and contacting the adsorbent with the liquid, followed by separation of the adsorbent from the liquid. The adsorbent may be washed with pure water between each separation. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the variation of adsorption rate with pH for a tannin adsorbent and a variety of heavy metal elements in a solution; and Figure 2 is a graph similar to that of Figure 1 for another variety of heavy metal elements. DESCRIPTION OF THE PREFERRED EMBODIMENT Tannin adsorbents suitable for use in this invention include adsorbents having, as a main component, an insoluble tannin and a gelled tannin. Examples of the insoluble tannin include a tannin described in Unexamined Published Japanese Patent Application No. 3-206094 (U.S. Patent Application Serial No. 07/631,946), prepared by dissolving a tannin in an aldehyde aqueous solution, adding ammonia to the resulting solution to form a precipitate, and aging the precipitate. As the gelled tannin, a tannin prepared by dissolving a condensed tannin powder in aqueous ammonia, mixing the resulting solution with an aqueous aldehyde solution to form a gel composition, and aging the gel composition to stabilize it can be used. The present applicants have filed U.S. Patent Applications (Serial No. 07/631,946 and 07/906,273 the contents of which are incorporated herein by reference) concerning such tannin adsorbents. The solution to be treated contains one or more and usually a plurality of heavy metal elements. Representative examples of the heavy metal elements include actinides (uranium, thorium, and the transuranium elements), lead cadmium, chromium, mercury, iron. The pH of the solution is adjusted according to the heavy metal elements in the solution, prior to contacting the solution with the tannin adsorbent. Thus, the pH is adjusted to a value which is suitable to maximizing the adsorption of the specific element or elements desired to be adsorbed in that step. For example, (1) when uranium, neptunium, americium, and curium are separated from a solution containing all of these elements, the pH of the solution is first adjusted to a range of 6 to 10 to adsorb and separate uranium and neptunium by using the tannin adsorbent and subsequently the pH of the solution is adjusted to a range of 3 to 6 to adsorb and separate americium and curium by using the tannin adsorbent. The variation of the adsorption rate in which such heavy metal elements are adsorbed by the tannin adsorbent is shown in Fig. 1. In addition, (2) when lead, cadmium, hexavalent chromium, mercury, and iron are separated from a solution containing all of these elements, first the pH of the solution is adjusted to 7 or more to adsorb and separate mainly lead and cadmium by using the tannin adsorbent and subsequently, the pH of the solution is adjusted to a range of 3 to 6 to adsorb and separate hexavalent chromium, mercury, and iron by using the tannin adsorbent. The variation of the adsorption rate in which such heavy metal elements are adsorbed by the tannin adsorbent is shown in Fig. 2. Further, when a solution which contains simultaneously both the elements of the item (1) and the elements of the item (2) is treated, or when all of the heavy metal elements are to be separated with high adsorption rate, it is preferred that the pH of the solution is more precisely adjusted and, subsequently, the adjusted solution is contacted with the tannin adsorbent. In Figs. 1 and 2, a stands for an adsorption rate which is calculated from the following formula. α = [(Co - Ct) / Co] x 100(%) wherein Co represents the heavy metal concentration of the starting liquid before adding an adsorbent thereto; Ct represents the heavy metal concentration of the solution after adsorbing the heavy metal by adding an insoluble tannin thereto. A specific method for contacting the adsorbent with the solution includes a first method comprising packing a tannin adsorbent in a column, and passing a solution containing a heavy metal element through the column. A second method comprising adding a tannin adsorbent to a solution containing a heavy metal element, e.g., in a vessel, whereby the heavy metal element is adsorbed by the adsorbent. Factors to be considered in determining whether the former method or the latter method should be used include the amount of the solution to be treated, the treating time to be required, the kinds of heavy metal elements contained in the solution, the property or form of the tannin adsorbent to be used. For example, a gelled tannin which is prepared by stabilizing the above-mentioned gel composition is used in a pulverized state when it is contacted with the solution. Since the gel tannin is superior to other adsorbents in flow resistivity, when the gelled tannin is used in the former method, it can efficiently adsorb the heavy metal elements. According to the invention, the method of eluting a heavy metal element from the adsorbent to regenerate the adsorbent may comprise the steps of: washing a tannin adsorbent having at least one heavy metal element adsorbed theron with pure water; contacting the washed adsorbent with an alkaline aqueous solution having a pH different from that used for adsorption so as to desorb the element into the solution, and separating the adsorbent therefrom; and further washing the adsorbent contacted with the aqueous solution with pure water. In another method according to the invention, at least one heavy metal element is desorbed from a column packed with the adsorbent and the adsorbent is regenerated, which may comprise the steps of: (v) washing the column with pure water; (w) passing an aqueous alkaline solution having a pH favoring desorbtion of the element and different from that of the solution used for adsorption through the washed column. (x) washing the column from step (w) with pure water; (y) adjusting the pH of the solution discharged from the column during the adsorption to a pH favoring adsorption of one or more elements different from the element(s) adsorbed the column packed with the adsorbent and different from the first pH used for adsorption; and (z) passing the solution from step (y) through the column from step (x). In the regenerating method, steps (v), (w), (x), (y) and (z) are repeated at least once and the pH of respective solutions in each step is different from one another, whereby the adjustment of the pH can be precisely carried out with a minimum number of steps. With the inventive regenerating method, adsorption of the heavy metal element and regeneration of the used adsorbent can be carried out with a minimal amount of equipment. By using a regenerating method other than a method in which a solution containing heavy metal elements is passed through a column packed with the adsorbent, it is also possible to regenerate the adsorbent which has adsorbed a heavy metal element and was subsequently filtered. This regenerating method comprises the steps of adding a tanning adsorbent to a first solution containing a heavy metal element in which the pH thereof is adjusted to thereby adsorb the heavy metal element; filtering the mixed solution to leave a residue on a filter; washing the residue with pure water, separating the residue from the washings; contacting the separated residue with an alkaline solution, each having a pH different from that of the first solution and favoring desorbtion of the element separating the residue and washing the separated residue with pure water. When the adsorbent is regenerated, for example, when the adsorbent is contacted with a solution containing a heavy metal having pH 5, the adsorbent which adsorbs the heavy metal element is contacted with an alkaline aqueous solution having a pH different from the pH 5, for example, pH 8, thereby eluting or desorbing the heavy metal element therefrom. As mentioned above, according to this invention, by taking advantage of the pH dependence of the adsorption rate and elution rate of a tannin adsorbent, all of the heavy metal elements, efficiently adsorbed on the adsorbent, can be independently eluted therefrom with high elution rates. In the inventive method, both continuous and batch methods can be easily carried out by using a single adsorbent and by only adjusting the pH, so that the apparatus to be used for the inventive method is not complicated. Further, the inventive method can rapidly handle a solution containing both uranium and thorium, which are generated from a nuclear fuel manufacturing process; a solution including simultaneously the transuranium elements generated from fuel reprocessing process, such as, curium, americium, neptunium, and plutonium; as well as a solution containing simultaneously lead, cadmium, hexavalent chromium, mercury, and iron, which are generated from processes handling a heavy metal element. The present invention is described in greater detail with reference to the following example. EXAMPLE An aqueous solution (250 ml) of chromium trioxides (hexavalent chromium concentration 18.0 ppm) in which the pH thereof is adjusted to 5.0 by using ammonia was prepared. To the aqueous solution were added 550 mg (dry weight) of a wattle tannin adsorbent as described in Unexamined Published Japanese Patent Application No. 3-206094 (U.S. Patent Application Serial No. 07/631,946 or EP-A-438 776), followed by stirring for two hours at room temperature. The stirred mixed solution was filtered through filter paper (Toyo Filter Paper No. 6), and the concentration of the hexavalent chromium in the filtrate was measured. As a result, the hexavalent concentration was found to be 0.2 ppm and the adsorption rate thereof was 98.9 %. The tannin adsorbent thus filtered was thoroughly washed with pure water, followed by filtering again. The refiltered adsorbent was added to 250 ml of aqueous sodium hydroxide solution of pH 10, followed by stirring for 30 minutes. The stirred solution was filtered, and the hexavalent chromium concentration of the filtrate was measured. The hexavalent chromium concentration was found to be 17.5 ppm and the elution rate thereof was 98.3 %. The tannin adsorbent from which the hexavalent chromium was eluted was thoroughly washed with pure water, followed by filtering again to obtain a rewashed tannin adsorbent. In addition, 250 ml of an aqueous solution of cadmium chloride (cadmium concentration 15.0 ppm) in which the pH was adjusted to 10 with an aqueous sodium hydroxide solution. To the aqueous solution was added the rewashed tannin adsorbent, followed by stirring for two hours. The stirred mixed solution was filtered and the cadmium concentration of the filtrate was measured. As a result, the cadmium concentration was found to be 0.3 ppm and the adsorption rate thereof was 98.0 %.

Claims: A method for regenerating a tannin adsorbent having at least one of heavy metal elements adsorbed thereon, the tannin adsorbent being selected from an insoluble tannin and a gelled tannin, comprising : contacting the adsorbent with an aqueous alkaline solution having a pH suitable to desorb the metal element from the adsorbent, the pH for desorption being different from the pH of adsorption ; and washing the adsorbent contacted with the aqueous alkaline solution with pure water. A method according to claim 1 wherein the heavy metal is selected from the group consisting of uranium, thorium, curium, americium, neptunium, lead, cadmium, chromium, mercury, iron and mixtures thereof.

Inventors: KAMEI YOSHINOBU, SHIRATO WATARU, KAMEI, YOSHINOBU, SHIRATO, WATARU, Kamei, Yoshinobu, c/o Tokai Seisakusho, Mitsubishi, Shirato, Wataru, c/o Tokai Seisakusho, Mitsubishi
Applicants: MITSUBISHI NUCLEAR FUEL, MITSUBISHI NUCLEAR FUEL COMPANY, LTD.
Application year: 1992
Main classification: B01D15
IPC codes: G21C19, B01J20, C02F1, G21F9, C02F101, B01D15