Removal of mercury from waste streams
FIELD OF INVENTION 
     The present invention relates to the removal of mercury from waste streams, particularly organic mercury. 
     BACKGROUND TO THE INVENTION 
     Thimerosal is an organic mercury compound of the formula: ##STR1## This compound is used as a bacteriostatic agent or preservative in certain biological vaccines, as well as intermediates, buffers, diluents and cleaning and disinfecting reagents used in the manufacture of vaccines, and also as an inactivating agent for certain whole cell vaccine materials, and, as a result, is present in small quantities in effluent streams from vaccine manufacture. The primary contributors to the thimerosal concentration in the effluent are discarded whole cell supernatant and unused buffers. Although such effluent streams contain only small quantities of such organic mercury compound, in the 100&#39;s ppm range, such quantity is unacceptably high for disposal by simple sewering or other discharge to water bodies. Environmental regulation requires that the quantity of mercury discharged be less than 3 ppb. 
     Dilution of the effluent stream is not a practical alternative in view of the roughly 10,000 fold decrease in quantity of organic mercury required. Prior to the present invention, the applicant was unaware of any proposal which would decrease organic mercury levels in vaccine manufacture effluent streams to environmentally-acceptable acceptable levels, and which could accomplish the same in an efficient and inexpensive manner. 
     SUMMARY OF INVENTION 
     In accordance with the present invention, there is provided a method of removing thimerosal substantially completely from an aqueous effluent stream from vaccine manufacture to below environmentally-acceptable levels. The procedure involves a three-step operation, namely: 
     (a) effecting chlorination of the aqueous effluent stream under conditions to convert the thimerosal to ionic form, particularly by oxidation with sodium hypochlorite, 
     (b) removing any dissolved unreacted chlorine usually in the form of sodium hypochlorite, from the resulting aqueous solution of ionic mercury compound, and 
     (c) contacting the aqueous solution of ionic mercury compound with a material having pendant thiol groups, preferably a column of resin beads, to effect reaction of the ionic mercury with the pendant thiol groups to remove ionic mercury from the aqueous solution. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     FIG. 1 is a schematic flow sheet of a thimerosal removing process in accordance with one embodiment of the invention. 
    
    
     GENERAL DESCRIPTION OF INVENTION 
     The first step in removing the thimerosal from the aqueous stream is to oxidize the organic mercury compound, preferably with sodium hypochlorite, to an ionic form, namely an Hg+ salt form. Such oxidation is effected by contacting the effluent stream with sodium hypochlorite or other source of active chlorine under suitable pH conditions conducive to such oxidation, generally less than about pH 3, preferably about pH 2.0 to about pH 3.0, for a time sufficient to convert all the organic mercury compound present in the effluent stream to ionic form, generally at least about 2 hours, preferably about 2 to about 4 hours. Such oxidation usually is effected at an ambient temperature (about 20° to 25° C.), although temperatures from about 5 to about 75° C. may be employed. The absence of residual unconverted organic mercury compounds in the effluent stream is determined by an atomic adsorption test for total mercury content. 
     The second step involves dechlorination of the oxidized stream containing ionic mercury compound to remove dissolved unreacted chlorine, which is generally present as sodium hypochlorite. This step is necessary since the resins employed in the subsequent ion-exchange operation are sensitive to absorption and oxidation by chlorine. Hence, if the residual chlorine were not removed, the active life of the resin material would be decreased, thereby significantly increasing costs. 
     The dechlorination procedure preferably involves a three step operation. In a first step, the oxidized stream is aerated by passing air through the oxidized stream to strip chlorine gas from the oxidized stream. Such air stripping operation is effected generally until no further chlorine can be removed in this way. Generally, such air stripping operation is effected for about 8 to about 20 hours, preferably about 12 to about 20 hours. The air stripping usually is effected at ambient temperatures (about 20° to 25° C.), although the temperature may generally range from about 5° to about 70° C. The air stripping operation generally is successful in decreasing the dissolved chlorine concentration to ppm levels. 
     It is not possible to remove all the chlorine from the oxidized solution by aeration in view of an affinity that the chlorine has for water, providing hypochlorite ions, and further processing is required. A second chlorine-removal step involves reaction of the air-stripped solution with sodium bisulfite. Such reaction is effected by mixing an aqueous solution of sodium bisulfite with the air stripped solution and is added on-line while the air-stripped solution is forwarded to the resin beds. The quantity of sodium bisulfite employed generally is about 50 ppm to about 150 ppm. The level of sodium bisulfite added is varied depending on the residual hypochlorite concentration. An on-line chlorine monitor preferably is employed to indicate the extent of residual chlorine and hence the sodium bisulfite requirement stoichiometrically to remove the residual chlorine. If the air stripping oxidation is sufficiently efficient, little or no sodium bisulfite addition may be necessary. The sodium bisulfite binds with the residual dissolved chlorine, present in the form of hypochlorite. 
     The solution resulting from the sodium bisulfite treatment is contacted with activated carbon as the third chlorine-removal step, generally by flowing the solution through a packed column of activated carbon, which results in adsorption of the sodium bisulfitebound hypochlorite to the activated carbon. 
     While the chemical treatment with sodium bisulfite and contact with activated carbon themselves are capable of removing all the dissolved unreacted chlorine from the oxidized solution, it is preferred to effect the initial air stripping operation described above to decrease the requirement for chemicals and to prolong the life of the activated carbon, thereby realizing a saving in operating costs. 
     The chlorine-removal operation is designed to remove all the unreacted chlorine contained in the aqueous ionic mercury solution prior to further processing. 
     As the third step of the overall process, the chlorine-free aqueous solution of ionic mercury compound next is subjected to an ion-exchange operation whereby the ionic mercury compound is removed from the aqueous solution. This step is effected by contacting the chlorine-free solution with a substance having pendant thiol (--SH) groups, usually in the form of a bed of resin beads . One suitable material comprises a cross-linked polystyrene matrix with thiol functionality. The thiol groups undergo ion-exchange with the ionic mercury compound and form a strong mercury-sulfur covalent bond, thereby effectively removing the mercury from the aqueous phase. 
     Depending on the amount of ionic mercury to be removed, the size of the column of ion-exchange material which contacts the aqueous solution and the density of pendant thiol groups on the resin, it may be necessary to pass the aqueous medium through a plurality of columns arranged in series flow in order to decrease the residual mercury concentration below the desired value, typically below 3 ppb. 
     When the ion-exchange resin becomes depleted and no longer effectively removes mercury from the aqueous phase, the ion-exchange resin may be disposed of by incineration, with mercury vapor being removed from the stack gases by conventional technology. 
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, a thimerosal effluent stream is mixed with sodium hypochlorite solution to provide an aqueous mixture having the desired pH conditions and the mixture is pumped by pump 10 through lines 12 and 14 to a pair of parallel reaction tanks 16 and 18. The mixture is recirculated within the reaction tanks 16 and 18 during the hypochlorite reaction period by withdrawal of mixture from the tanks 16 and 18 respectively by lines 20 and 22, recycle by line 24 and refeed by lines 24 and 26 respectively. 
     Once the reaction time for sodium hypochlorite reaction is complete, the aqueous reaction product is aerated by air or oxygen fed by lines 30 and 32 respectively to reaction tanks 16 and 18 and distributed in the reaction tanks 16 and 18 by spargers 34 and 36. A chlorine-containing gas stream is vented from each of the reaction tanks 16 and 18 by lines 38 and 40 respectively. 
     When aeration is complete, the oxidized liquid is removed from the tanks 16 and 18 by lines 20 and 22 and a combined stream is formed in line 42. Once the reaction tanks 16 and 18 are empty of oxidized solution, a new batch of aqueous mixture of thimerosal-containing solution and sodium hypochlorite may be fed to the tanks 16 and 18 for reaction and subsequent oxidation. 
     An aqueous solution of sodium metabisulfite is added to the stream in line 42 by line 44 in an amount stoichiometric to the amount of residual chlorine determined by an on-line chlorine monitor 46. The mixture of sodium bisulfite and oxidized stream passes through a mixing pipe 48 to ensure thorough mixing of the aqueous media and reaction of sodium bisulfite and residual sodium hypochlorite. 
     Downstream of the chlorine monitor 46, the mixed aqueous solution passes by line 50 to a pair of parallel feed columns 52, 54 of activated carbon, which serve to adsorb sodium bisulfite bound sodium hypochlorite from the aqueous medium. 
     The sodium hypochlorite-free solution of ionic mercury existing the activated carbon beds 52, 54 is forwarded by line 56 to series-connected ion exchange resin columns 58, 60, 62 and 64. Each of the ion-exchange resin columns contains a bed of resin beads containing pendant thiol groups, which removes the ionic mercury from the aqueous solution by ion exchange. The substantially mercury-free aqueous solution is discharged from the last of the series-linked ionexchange resin beds by line 66. 
     EXAMPLE 
     The arrangement illustrated in FIG. 1 was set up and a series of experiments was performed on an aqueous solution of thimerosal obtained as a by-product of vaccine production, arising as noted above, containing approximately 100 ppm of mercury. The carbon and resin beds were unchanged during the course of the series of experiments. The mercury concentration was determined at various locations in the operation. 
     The results obtained are summarized in the following Table 1: 
     
                       TABLE I                                                     
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                         Mercury Concentration                            
       Total    Quantity After bed                                        
                                After bed                                 
                                       Final                              
Run No.                                                                   
       Vol (gal)                                                          
                HOCl.sup.(1)                                              
                         58     60     Effluent                           
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1      450      500%                   .sup. .sup. .sup. nd.sup.(2)       
2      450      200%     0.2 ppb       nd                                 
3      450      100%                   &gt;2 ppb                             
4      450      125%                   &gt;2 ppb                             
5      450      150%     &gt;2 ppb        nd                                 
6      900      150%                   nd                                 
7      900      150%                   &gt;2 ppb                             
8      900      150%                   &gt;2 ppb                             
9      900      150%     14 ppm 20 ppb  3 ppb                             
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 Notes:                                                                   
 1. % of equivalent molar quantity of mercury.                            
 2. nd = not detected.                                                    
 
    
     These results show an efficient removal of thimerosal from 100 ppm Hg levels to a not-detectable or very low level in the final effluent. In run 9, the resin beds are becoming exhausted and need replacing to maintain an effluent discharge below 3ppb Hg. 
     SUMMARY OF DISCLOSURE 
     In summary of this disclosure, the present invention provides a novel method to effect removal of thimerosal from aqueous effluent streams from vaccine production which is particularly effective in decreasing the residual mercury concentration below environmentally-acceptable values and which is inexpensive to operate. Modifications are possible within the scope of this invention.