Insoluble compositions for removing mercury from a liquid medium
The present invention relates to a composition useful for the removal of mercury from liquid media wherein the active metal binding component comprises a residue of cysteine. 
     Due to the toxic nature of mercury it is extremely important to be able to reduce to a minimum the amount discharged to the environment in liquid medium. Sources of such potential polluting media are numerous; they include, for example, the depleted brine solution obtained from the manufacture of chlorine and caustic soda by electrolysis of brine using mercury as a cathode. 
     Accordingly, the present invention provides an insoluble composition comprising: 
     (i) a cysteine residue covalently fixed at the nitrogen atom thereof to the surface of, 
     (ii) a suitable insoluble carrier. 
     The present invention in particular provides an insoluble composition comprising 
     (i) a mercury binding group covalently fixed to the surface of, 
     (ii) a suitable insoluble carrier, 
     said mercury binding group having the formula 
     
         .tbd.Si--R&#39;--cys 
    
     wherein cys is a cysteine residue and R&#39; is a hydrocarbon chain of at least 3 chain atoms interrupted, if desired, by one or more heteroatoms selected from the group consisting of oxygen and nitrogen, said mercury binding group being covalently fixed to the surface of said carrier via a silica to oxygen to carrier bond, said cysteine residue being coavlently fixed at the nitrogen atom thereof to R&#39;, said carrier being an inorganic carrier. 
     The present invention in accordance with a further aspect provides a process for the preparation of an insoluble composition comprising: 
     (i) a cysteine residue covalently fixed at the nitrogen atom thereof to the surface of, 
     (ii) a suitable insoluble carrier, 
     characterized in that cysteine is reacted with a suitable reactive insoluble carrier. 
     The present invention also, in particular, provides a process for the preparation of an insoluble composition comprising: 
     (i) a mercury binding group covalently fixed to the surface of, 
     (ii) a suitable insoluble carrier, 
     said mercury binding group having the formula 
     
         .tbd.Si--R&#39;--Cys 
    
     wherein cys is a cysteine residue and R&#39; is a hydrocarbon chain of at least 3 chain atoms interrupted, if desired, by one or more heteroatoms selected from the group consisting of oxygen and nitrogen, said mercury binding group being covalently fixed to the surface of said carrier via a silica to oxygen to carrier bond, said cysteine residue being covalently fixed at the nitrogen atom thereof to R&#39;, said carrier being an inorganic carrier, characterized in that cysteine is reacted with a suitable reactive insoluble carrier, said reactive carrier having covalently fixed to its surface a reactive group of formula 
     
         .tbd.Si--R&#34;--ƒcn 
    
     wherein R&#34; is a hydrocarbon chain interrupted, if desired, by one or more heteroatoms selected from the group consisting of oxygen and nitrogen, and ƒcn is a functional group reactive with the amino group of cysteine to covalently fix cysteine to said carrier at the nitrogen atom thereof, said reactive group being covalently fixed to the surface of said carrier via a silica to oxygen to carrier bond, said carrier being an inorganic carrier, said group --R&#34;-ƒcn being reactive with cysteine to form the group --R&#39;--cys. 
     In accordance with another aspect the present invention provides a process for removing mercury from a liquid medium characterized in that the medium is contacted with a composition as defined above. 
     Compositions as defined above, loaded with mercury may possibly be regenerated by chemical means suitable for the removal of the bound metal; the so regenerated composition can thereafter be recycled for further use. 
     The insoluble compositions in accordance with the present invention have a very high affinity for mercury and can be used to remove mercury from solution even if mercury is present in trace amount. 
     The cysteine residue can be depicted as follows: ##STR1## 
     Having both an acidic (--CO 2  H) and a basic ##STR2## functional group, the cysteine residue can exist as a cation or anion depending upon its immediate environment. 
     The carrier can be organic or inorganic in nature. For example, the carrier may be a natural or modified natural polymer (e.g. lignin, agar, alginate, glucan, cellulose, dextran, cellulose acetate, humic acid, etc.) or a synthetic organic polymer (e.g. a polyamide, a polyamine, a polyacrylamide, a polyester, a polyurethane, a polyethylene, a polystyrene, a polypropylene, a polycarbonate, a silicone, nylon, latex, a polyfluoroolefin, etc.). An inorganic material is, however, preferred (e.g. ceria, titania, alumina, yttria, sepiolite or other such materials having surface hydroxyl groups). 
     A carrier suitable in accordance with the present invention, must of course be insoluble in the liquid medium of intended use; for example, the carrier can be water insoluble. Desirably, the carrier is also inert in the liquid medium of intended use. The carriers can be in particulate or solid form. 
     Any suitable means of covalently fixing organic coordinating sites to a carrier can be used to prepare the compositions provided that the necessary metal chelating or metal binding activity of the cysteine residue is maintained. The process of preparation is carried out such that the cysteine residue is covalently fixed via its nitrogen atom to the surface of the carrier. Cysteine can be bound in this manner to an aldehyde activated silica gel via Schiff base reaction. 
     Other known processes are also suitable for the binding of cysteine to carriers so as to preserve the chelating or complexing properties thereof. For example, the commonly used methods for covalently binding enzymes to insoluble carriers can be adapted for the immobilization of cysteine. See, for example &lt;&lt;Methods of Enzymology&gt;&gt;, XXXIV B:30 (Jakoby W. B. Ed.) Academic Press, New York (1974). 
     Carriers which may advantageously be used for the process of preparing compositions are those which already have active surfaces; the active surfaces having functional groups which can react with the amino group of cysteine. The functional group can, for example, be selected from the class consisting of ##STR3## --CH 2  --X, X being a halogen atom, for example, Br, or ##STR4## X being, as defined above, ##STR5## Any functional group can of course be used which can be made to react with the amino group of cysteine to bind cysteine to the carrier, the obtained composition having metal chelating or metal binding activity. 
     A useful carrier may need to have its surface treated in order to provide the surface with a suitable functional group which can bond to cysteine. An inorganic carrier having surface hydroxyl groups may, for example, be treated with a suitable amino silane to obtain an amino activated carrier which can in turn be treated with a bifunctional compound to provide the carrier with a functional group which can react with the amino group of cysteine. The aminosilane may, for example, have the following formula ##STR6## wherein R is a divalent organic radical of up to 20 carbon atoms (e.g. alkyl of up to 20 carbon atoms) and R 1 , R 2  and R 3  are independently selected from the class consisting of a hydrogen atom, a halogen atom (e.g. C1), a C 1-20  alkyl group, a C 1-20  alkyl group substituted by a C 6-14  aryl group, a C 6-14  aryl group and an OR 4  group, R 4  being a hydrogen atom or an organic radical of up to 20 carbon atoms (e.g. C 1-20  alkyl), provided that at least one of R 1 , R 2  and R 3  is a halogen atom or an --OR 4  group. The aryl group or moiety thereof can be mono, di or tri-cyclic e.g. phenyl, naphthyl, benzyl etc. 
     Thus silica (e.g. in the form of a silica gel) having surface hydroxyl groups can, for example, be pretreated with a suitable ω-amino (C 2  to C 10  alkyl) tri (C 1  to C 5  alkoxy) silane to provide an active surface comprising amino groups. The silane can, for example, be γ-aminopropyltriethoxysilane. See, for example, the following patents wherein silica is treated with a silane: Canadian Pat. Nos. 1,102,347, 1,103,035 and 1,102,346; U.S. Pat. Nos. 4,203,952, 3,886,080, 3,904,373, 3,519,538, 3,652,761, 4,230,803 and 4,290,892. 
     An obtained amino activated silica gel can be reacted with a suitable bifunctional compound to provide the gel with a functional group capable of reacting with the amino group of cysteine. Suitable bifunctional compounds include dialdehydes such as α,ω-(diformyl)alkanes, α,ω-(dihalo)alkanes, dicarboxylic acids and reactive derivatives of the latter such as acid halides, anhydrides, esters, etc. The acids can be α,ω-(dicarboxyl)alkanes. The alkyle moieties of the above referred to compounds can have up to 20 carbon atoms. 
     Glutaraldehyde may, for example, be used to provide a carrier having the functional group ##STR7## which can be made to react with the amino group of cysteine via a Schiff base reaction, the obtained product being stabilized by being subjected to a Schiff base reduction treatment. In accordance with the Schiff base reduction treatment the group ##STR8## is reduced to the group ##STR9## The reduction treatment can be effected with any suitable reducing agent, for example, an anhydride such as sodium borohydride. 
     It is also possible to put some distance between a cysteine residue and the surface of the carrier, in order to limit the effect on the residue of a surface characteristic of the carrier. For example, teflon may be used as a carrier. However, teflon has a highly hydrophobic surface which is non-wetting. Therefore, it is desirable to put some distance between the surface of the teflon and the cysteine residue to allow the residue to extend well into an aqueous liquid medium. 
     A spacer compound may be used to provide a spacer group to space apart a carrier and a cysteine residue. A suitable spacer compound can be a bifunctional compound such as referred to above; i.e. it has a functional group which can react with a functional group of the carrier (e.g. hydroxyl, amino, aldehyde etc.) to bind it thereto; and it has also a second functional group which can react with the amino group of cysteine to bind it thereto: see the above groups. The spacer group may alternatively have a second functional group which while not reactive with the amino group of cysteine, may be convertible into such a group. 
     A spacer compound can, for example, in addition to the above referred to functional groups, include a hydrocarbon chain, the length of which is chosen in accordance with the distance which it is desired to place between the carrier and the compound. The spacer compound used may for example be 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride salt or a straight chain dialdehyde of up to 20 carbon atoms such as glutaraldehyde. However any compound can be used which will space the compound from the carrier, the necessary or desired distance provided of course that it is bifunctional. 
     The spacer compound may be bound to a carrier by making use of conventional reactions involving the formation of ester groups, amide groups, amino groups, diazo groups, ether groups, sulphonamide groups, amidino groups; the reaction may be a carbon-carbon condensation. 
     Thus a suitable carrier for the present invention may be represented generally by the formula ##STR10## wherein n is an integer, &lt;&lt;back&gt;&gt; is a carrier backbone, R&#34;, ƒcn and the group --R&#34;--ƒcn are as defined above. For example &lt;&lt;ƒcn&gt;&gt; may be a carboxyl group and R may be a group such as ##STR11## 
     The carrier employed can have a porous structure preferably having a large specific surface. 
     When using a composition in accordance with the present invention, the conditions of use should of course be such as to avoid the break-down or decomposition of the composition; i.e. conditions such as &lt;&lt;pH, temperature, pressure, etc.&gt;&gt; should be chosen so as to avoid the breakdown of the composition. 
     As indicated above, a composition, in accordance with the present invention, can be used to remove mercury from a liquid medium. For example, the composition can be intermixed with a desired liquid medium for a suitable time, which will of course depend upon the amount of composition used, the initial mercury concentration, the desired final mercury concentration, etc. The mercury loaded composition can then be physically recovered from the medium i.e. to effect separation of mercury and medium. The affinity of the compounds for mercury can be so great that even small amounts of mercury can be removed from a liquid medium. 
     Liquid media to be treated to remove mercury can have, for example, a pH in the range of 4.5 to 9. During the contact with the composition, the temperature of the mixture can for example range from 1° C. to 50° C. and the contact can occur under atmospheric pressure. Examples of different media which can be treated with the compositions are as follows: 
     (i) Industrial water and waste water 
     incinerator scrub waters 
     effluent from chloralkali plant 
     (ii) Natural water 
     contaminated ground water. 
     As indicated previously, compositions in accordance with the present invention, may possibly be regenerated for further use by the removal of the metal therefrom by suitable chemical means. In this way, the composition can be economically used since it can be recycled for repeated use. 
     The regeneration, for example, of a composition loaded with mercury may be carried out by treating the mercury loaded composition with a suitable reagent such as 2-mercaptoethanol. Appropriate reagents and conditions should of course be chosen which will not decompose the composition or destroy the metal binding capacity thereof. 
     The insoluble compositions in accordance with the present invention thus provides for the advantageous removal of mercury from liquid media. The liquid media referred to herein may be aqueous, organic or mixtures thereof. 
     Reference will be made to a number of non-restrictive examples (infra) which deal with embodiments of the present invention. 
     The methods of preparation used in the examples were analogous to those as described by H. Weetal &amp; A. M. Filbert, Methods of Enzymology XXXIV B:59-72 1974. 
     In the examples insoluble compositions were used to remove Hg from liquid media in accordance with the following procedure: 
     For the tests 3.14 cc of insoluble composition to be tested was initially placed in a 1.0 cm (i.d.) glass Bio-Rad Econo-column (i.e. bed height of about 4.0 cm). The composition was then subjected to one or more of the following steps: 
     (a) Metal loading step: 
     The composition was washed with about 10 bed volumes of deionized (e.g. distilled) water at maximum flow rate (up to 8.3 ml/min); 1 bed volume equaling about 3.14 ml. Thereafter the composition was treated with 4 bed volumes of an appropriate mercury regenerant (e.g. 10% (v/v) 2-mercaptoethanol in 2N HCl) at a flow rate of about 0.1 ml/min. The so treated composition was then washed again with deionized water as indicated above. Thereafter the solution to be treated was then passed through the column at a desired fixed flow rate (e.g. about 1.0 ml/min.) until the desired breakthrough point of the metal was observed in the column effluent (e.g. until the observed metal concentration in the effluent was 10% of the initial metal concentration). After the composition was thus saturated it was washed with ˜10 bed volumes of deionized water to rinse off unbound metal. 
     (b) Regeneration step: 
     The metal loaded composition was treated with four bed volumes of appropriate mercury regenerant to remove Hg bound to the composition; the volume of regenerant used was in excess of amount necessary to liberate all removable Hg from the composition. 
     The combined metal loading step and regeneration step represents a complete cycle for the reuse of the composition which can be represented as follows: ##STR12## 
     In the following examples, unless otherwise indicated the metal content of treated effluent was determined by atomic absorption spectrophotometric analysis (cold vapour technique) and the metal content of the initial solution was determined by inductively coupled plasma spectrometry. The residual concentration is the observed concentration of mercury in the treated effluent at the 50% point, i.e. the point at which about 1/2 of the total volume of treated solution had passed through the column, the total volume of treated solution being the volume of solution that had passed through the column up to desired breakthrough. The content of mercury in the regenerant was compared to the quantity of mercury originally bound to the composition to determine efficiency of mercury removal. Efficiency of regeneration can be calculated by comparing the capacity of the composition for mercury in successive cycles. 
    
    
     In the following examples nitrogen concentration of precursors was determined by a chloride titration technique; see L. C. Dorman, Tetrahedron Letters, 28, 2319 (1969). 
     EXAMPLE 1 
     Silica gel: amino residues covalently bonded to the surface thereof 
     Davison silica gel 62 hereinafter referred to as composition A (200 g) was suspended in 650 ml of deionized water and 88.5 gm (0.40 mole) of 3-aminopropyltriethoxysilane (Union Carbide A-1100) was added smoothly over about 1 minute. The pH of the resulting suspension was adjusted to 3.45 with concentrated aqueous hydrochloric acid (˜38 ml, 0.46 mole). The mixture was heated to 75° C. and maintained at that temperature for about 2.5 hours. Thereafter the mixture was cooled to a manageable temperature (&lt;50° C.) and the solid product recovered therefrom was water washed six times, each wash comprising suspending the solid product for about 10 to 15 minutes in about 500 ml of deonized water followed by recovery of the solid by vacuum filtration. The washed solid product was collected by vacuum filtration and dried in a forced air circulation oven at 110° C. to constant weight (about 18 hours), the so obtained dry product hereinafter being referred to as composition B. Chloride analysis of composition B showed a nitrogen concentration of 1.11 mole/kg silica. 
     The above amination can generally be described graphically as follows: ##STR13## 
     EXAMPLE 2 
     (a) Silica gel: aldehyde residue covalently bonded to the surface thereof 
     A portion of the composition B obtained from example 1 (110 g, 0.12 mole of N) was suspended in 350 ml. of a solution of 0.1M disodium hydrogen phosphate. The resulting suspension was adjusted to pH 7 with aqueous 10% sodium hydroxide solution and the system was degassed by application of water aspiration vacuum for several minutes. When gas evolution had ceased the so evacuated system was filled with an argon atmosphere and 200 ml of an aqueous 25% solution of glutaraldehyde (0.5 mole; 4.2 equiv.) was added. The mixture was stirred at 25° C. for 1.5 hours. Thereafter the obtained solid product which was orange-tan in colour was water washed six times each wash step comprising suspending the solid product for 5-10 minutes in about (500 ml) deionized water followed by recovery of the solid product (wet) by vacuum filtration. 
     The reaction outlined above can be represented graphically as follows: ##STR14## 
     (b) silica gel: cysteine residue covalently bonded to the surface thereof 
     The washed wet solid product recovered in accordance with example 2(a) above was resuspended in 150 ml of a solution of cysteine hydrochloride monohydrate (26 g, 0.148 mole, 1.23 equiv) in aqueous saturated borax of pH 8.5; the pH of the solution was adjusted to 8.5 prior to bringing it into contact with the wet solid product. The reaction flask was flushed with argon and heated to 65° C. for 2 hours. The mixture was then cooled (&lt;50° C.) and water washed three times each wash step comprising suspending the solid product for 5-10 minutes in about 500 ml deionized water, followed by recovery of the solid product (wet) by vacuum filtration. 
     The washed wet product was then resuspended in 200 ml of deionized water, and 10 g (0.26 mole; 2.2 equiv) sodium borohydride was admixed therewith. The reaction mixture was stirred for 1 hour. The solid product was then water washed twice as outlined above, washed once by a similar treatment comprising suspending the solid product for 15 minutes in 200 ml 1N aqueous hydrochloric acid followed by five water washings as outlined above. The so washed product was resuspended in 0.1N hydrochloric acid and the solid product was collected by vacuum filtration and stored as a wet cake wetted with the dilute hydrochloric acid. The obtained product is hereinafter referred to as composition C. 
     EXAMPLE 3 
     Tests were conducted to evaluate the contribution of the cysteine residue with respect to the capture of Hg and also the effect of a number of regenerants on removal of mercury from a mercury loaded insoluble composition provided with such residue. The mercury containing solution consisted of an aqueous solution of wastewater of pH 8.2 containing the following elements: 
     
         ______________________________________                                    
       METAL        CONC*                                                 
______________________________________                                    
       Hg           100    ppm                                            
       Mg           2.84   ppm                                            
       Na           69.3   g/l                                            
       Sr           0.24   ppm                                            
       B            8.85   ppm                                            
       Si           51.4   ppm                                            
       Ca           20.1   ppm                                            
       Zn           2.36   ppm                                            
       Fe           1.28   ppm                                            
 *by inductively coupled plasma spectrometric analysis                    
 
    
     A first series of tests were conducted using composition A, composition B and composition C. As can be seen from table 1 below compositions A and B which are precursors for composition C have a significantly lower capacity to take up Hg in relation to composition C which includes the cysteine residue. 
     In another series of tests three separate aliquots of composition C were used to treat samples of the above wastewater. The lots were each then treated with a different reagent to compare the relative mercury removal efficiencies thereof. The results are seen in table 2. 
     
                       TABLE 1                                                     
______________________________________                                    
(table 6)                                                                 
Mercury uptake                                                            
         Hg concentration (ppm)                                           
                        Metal uptake-                                     
                      treated   mg Hg                                     
                      solution -                                          
                                bound/kg                                  
                      residual  composition                               
           untreated  concentra-                                          
                                to 10%                                    
Composition                                                               
           solution   tion.sup.(a), (b)                                   
                                breakthrough                              
______________________________________                                    
A          100        ˜10 &lt;92                                       
B          100        ˜10 &lt;92                                       
C          100        0.005     4462                                      
______________________________________                                    
 .sup.(a) concentration determined by atomic absorption spectrophotometry 
 (cold vapour technique)                                                  
 .sup.(b) residual concentration defined as metal concentration in treated
 effluent at the 50% point between initial elution of treated effluent and
 10% breakthrough                                                         
 
    
     
                       TABLE 2                                                     
______________________________________                                    
(table 7)                                                                 
The effect of various regenerants on mercury removal                      
         Hg conc. (ppm)                                                   
                      treated                                             
                      solution -                                          
                                % Hg                                      
                      residual  removal from                              
           untreated  concentra-                                          
                                Hg loaded                                 
Regenerant.sup.(b)                                                        
           solution   tion.sup.(a), (c)                                   
                                composition                               
______________________________________                                    
I          100        0.02      22                                        
II         100        1.0       ˜100                                
III        100        &lt;0.02     ˜95                                 
______________________________________                                    
 .sup.(a) concentration determined by atomic absorption spectrophotometry 
 (cold vapour technique)                                                  
 .sup.(b) regenerant used                                                 
 I  2N HCl                                                                
 II  10% (v/v) 2mercaptoethanol in deionized water                        
 III  10% (v/v) 2mercaptoethanol in 2N HCl                                
 .sup.(c) residual concentration defined as metal concentration in treated
 effluent at the 50% point between initial elution of treated effluent and
 1% breakthrough.                                                         
 
    
     EXAMPLE 4 
     100 g. Davison grade 12 silica gel of mean pore diameter 22 Å was suspended in 300 ml of water and stirred with a mechanical paddle type stirrer. 0.050 mole of 3-aminopropyltriethoxysilane (Union Carbide A-1100) was added smoothly over about 1 minute. The pH of the resulting suspension was adjusted to 3.45 with about 0.055 mole of concentrated aqueous hydrochloric acid. 
     The reaction mixture was heated to 75° C. and maintained at that temperature for about 2.5 hours. The mixture was then cooled to a manageable temperature (i.e. &lt;50° C.) and the recovered solid product was water washed six times, each water wash comprising suspending the solid product for 10-15 min in about 500 ml of deionized water and recovering the product as outlined above. The washed solid product was finally collected by vacuum filtration and dried in a forced air circulation oven at 110° C. to constant weight (about 18 hours). 
     Chloride analysis of the dried product showed implied nitrogen concentration of 0.55 mole/kg. 
     The solid product obtained by the above procedure was suspended in 325 ml of an aqueous 0.1M solution of disodium hydrogen phosphate. The resulting suspension was adjusted to pH 7 with aqueous 10% sodium hydroxide solution (a few milliliters) and the system was degassed by application of water aspirator vacuum for several minutes whereupon the flask was filled with air. 
     83.2 ml of a commercial aqueous 25% solution of glutaraldehyde (Aldrich, 0,208 moles at 2.5 Molar) was added rapidly. The mixture was stirred at 25° C. for 1.5 h. The solid product obtained which was orange-tan in colour was water washed six times using the wash procedure outlined above. 
     The wet solid product recovered was suspended in 250 ml of a solution of cysteine hydrochloride monohydrate (i.e. 0.080 mole of cysteine) in aqueous saturated borax. The pH of this latter solution was adjusted to 8.5 with a small amount of aqueous sodium hydroxide prior to contacting it with the aldehyde active solid product. The reaction flask was then flushed with argon, heated to 65° C. and, maintained at that temperature for 2 hours. The reaction mixture was then cooled to a manageable temperature (i.e. &lt;50° C.) and the solid product was water washed twice, each wash step comprising suspending the solid product for 5-10 min in 400 ml of deionized water followed by recovery of the product by vacuum filtration. 
     The so obtained wet solid was then resuspended in water (˜200 ml) and 0.114 mole of solid sodium borohydride was added. The mixture was stirred at 25° C. for 1 hour then washed according to the following formula; water (500 ml), 1N aqueous HCl (500 ml, 15 min. exposure time), water (5×500 ml), 0.1N aqueous HCl (500 ml) by sequential aspiration of fluids and re-suspension of solids in the specified fluid as outlined previously. 
     The solid product (hereinafter designated as composition D) was collected by vacuum filtration and stored as a wet cake wetted with the final dilute hydrochloric acid treatment. 
     EXAMPLE 5 
     200 g, (dry weight), of an aldehyde active silica gel (i.e. a silica gel having aldehyde residues covalently bound to its surface) in the form of a wet cake, (i.e. wetted with water) having 0.33 mole/kg theoretical assay of CHO, was mixed with 200 ml of an aqueous saturated borax solution; the aldehyde active silica gel was prepared as outlined below starting from a Davision Grade 12 silica gel. 100 ml of a solution comprising 17.23 g of L-cysteine hydrochloride monohydrate (0.098 mole; 1.49 equiv) in a saturated aqueous borax solution was adjusted to pH 8.5 with a few drops of 50% aqueous sodium hydroxide. This solution was added to the suspension above and the mixture was stirred and briefly subjected to reduced pressure (˜10 mm Hg). It was then heated to 65° C. and maintained at that temperature for 2 hours. The reaction mixture was then cooled to a manageable temperature (i.e. &lt;50° C.) and the fluids were removed by aspiration. The remaining solids were water washed twice each wash step comprising suspending the solid in 500 ml of deionized water for ˜10 min, followed by recovery of the solid by vacuum filtration. The so obtained wet solid was resuspended in 200 ml of deionized water and 5.5 g solid sodium borohydride (0.145 mole, 2.2 eq) was added cautiously. The mixture was stirred at 25° C. for 1 hour and then the reaction fluids were removed by vacuum aspiration. The obtained solid was washed once by resuspension for a few minutes in 400 ml of water. The wet solid recovered by filtration was then treated with 500 ml of aqueous 1N hydrochloric acid for 10 minutes, followed by water washing 5 times as above i.e. with 500 ml portions of deionized water. The recovered wet product was then suspended in 0.1N aqueous hydrochloric acid for 5-10 min. The solid was then collected by vacuum filtration and stored as a wet cake wetted with dilute HCl; the obtained product hereinafter being referred to as composition E. 
     The aldehyde active silica gel referred to above was prepared in accordance with the following procedure: 
     (a) silica gel amination stage ##STR15## wherein Et═CH 3  --CH 12  -- 
     Dry silica gel (2.00 kg) was added to 2.4 l of deionized water while stirring. 
     To the stirred suspension was added, in a steady stream, 232.6 ml of 3-aminopropyl-triethoxysilane (Union Carbide A-1100, 221 g, 1.00 mole) over a minute or so. After a few minutes (&lt;5 min) to allow complete mixing conc hydrochloric acid (approximately 83.3 ml of 12N solution, 1.00 mole) was added quickly in a steady stream (1 min). Sufficient acid was added to bring the pH to 3.45. 
     The acidified mixture was heated to 75° C. over 20 min. and maintained at that temperature for 1 hr. 
     The reaction mixture was then allowed to cool briefly and the solid product was recovered. The solid was washed with deionized water (3-4×2 L). The washed solid was then isolated on a Buchner filter and dried to constant weight in a ventilated oven at 110° C. in glass or enamelled metal pans. 
     (b) aldehyde activates silica gel ##STR16## 
     A solution of disodium hydrogen phosphate heptahydrate was prepared by dissolving the heptahydrate (67.0 g, 0.25 moles) in deionized water and diluting to a total volume of 2.50 L to make a 0.1M solution. 2.00 kg of the above aminated silica gel (wet) was added slowly to the stirring buffer. The pH of the slurry was then adjusted to 7.0 with 10% sodium hydroxide. 
     The system was de-gassed by evacuation using a water aspirator vacuum. Thereafter an inert gas (e.g. argon or nitrogen) was introduced to prevent oxidation of aldehyde in the reagent and product. 
     Sufficient glutaric dialdehyde as a commercially available 25% solution in water (Aldrich Chemical CO) was added rapidly. 
     The mixture was stirred for 1.5 hrs. at room temperature. After the reaction was complete the slurry was filtered to dryness and the obtained solid was washed with deionized water (4-5×2 L), allowing 10 minutes of stirring for each wash. The obtained wet product was then used to make the cysteine composition referred to above. 
     EXAMPLE 6 
     Tests were conducted to see the effect of flow rate on the removal of Hg from solution using cysteine composition E referred to in example 5. The tests were conducted as outlined previously. Separate aliquots of the same composition were used in each of the tests or runs. The solution treated in each case was an aqueous solution of wastewater of pH 8.2 containing the following elements. 
     
         ______________________________________                                    
METAL        CONC                                                         
______________________________________                                    
Hg           14.0          ppm*                                           
Mg           2.84          ppm                                            
Na           69.3          g/l                                            
Sr           0.24          ppm                                            
B            8.85          ppm                                            
Si           51.4          ppm                                            
Ca           20.1          ppm                                            
Zn           2.36          ppm                                            
Fe           1.28          ppm                                            
______________________________________                                    
 *Hg analysis by atomic absorption spectrophotometry (cold vapour         
 technique); other elements by inductively coupled plasma spectrometric   
 analysis.                                                                
 
    
     The results of the runs are shown in the following table 3, the residual concentration being determined relative to 1% breakthrough rather than 10% breakthrough. 
     
                       TABLE 3                                                     
______________________________________                                    
(table 8)                                                                 
           Hg conc (ppm)                                                  
        Flow                   treated solution -                         
        rate                   residue concentra-                         
Run no. ml/hr    untreated solution                                       
                               tion.sup.(e)                               
______________________________________                                    
1       31.4     14.0          &lt;0.03                                      
2       62.8     14.0          &lt;0.03                                      
3       94.2     14.0          &lt;0.03                                      
______________________________________                                    
 .sup.(e) residual concentration defined as metal concentration in treated
 effluent at the 50% point between initial elution of treated effluent and
 1% breakthrough (supra); see also .sup.(a) for Table 1.                  
 
    
     EXAMPLE 7 
     Tests were conducted in accordance with the method outlined previously using cysteine composition E wherein the composition was subjected to a number of complete cycles, i.e. a number of metal loading/regeneration steps one after the other. The solution treated was the same as that described in example 6 above. 
     The regenerant used was 10% (v/v) of mercapto-ethanol in 2N HCl. The flow rate used was 6.0 ml/hr. 
     The results of the tests are shown in Table 4 below: 
     
                       TABLE 4                                                     
______________________________________                                    
(table 9)                                                                 
                        Metal uptake                                      
Hg conc. (ppm)                                                            
mg Hg/kg                                                                  
                   treated solution -                                     
                                composition                               
       untreated   residual concen-                                       
                                to 1%                                     
Cycle no.                                                                 
       solution    tration.sup.(f)                                        
                                breakthrough                              
______________________________________                                    
1      14.0        &lt;.004        ≅1100                           
2      14.0        &lt;.004        ≅1100                           
3      14.0        &lt;.004        ≅1100                           
4      14.0        n.d..sup.(g) n.d..sup.(g)                              
5      14.0        &lt;.004        ≅1100                           
______________________________________                                    
 .sup.(f) see .sup.(e) above for Table 3                                  
 .sup.(g) not determined                                                  
 
    
     EXAMPLE 8 
     Preparation of cysteine compositions starting from silica gels of differing mean pore diameter 
     (a) Silica gel amination stage 
     Amine compositions F, G and H were each prepared as follows: 
     100 g of a respective starting silica gel specified in the table 5 was washed once with ethanol (denatured with 15% methanol) by suspension in the volume of solvent specified in the table and removal of the free fluids by aspiration. The same volume of fresh solvent was then added and the mixture stirred into suspension. Then 3-aminopropyltriethoxysilane (Union Carbide, A-1100) was added in the quantity specified in table 5. The mixture was quickly heated to and maintained at reflux for 2 hours. The reaction mixture was cooled to a manageable temperature (&lt;50° C.) and the fluids were removed by aspiration. The obtained solid was washed twice by consecutively suspending it in the volume of ethanol specified in table 5, followed by aspiration of the fluids. The so washed solid was then resuspended in aqueous 1N hydrochloric acid (250 ml) for 30 minutes then washed five times by repeated aspiration of the fluids and resuspension for several minutes in deionized water. The solid product was collected by vacuum filtration and dried in a forced circulation oven at 110° C. to constant weight (˜18 hours). 
     Chloride analysis of a sample of the dried product gave a value for the nitrogen content of each sample as specified in Table 5. 
     
                       TABLE 5                                                     
______________________________________                                    
(table 1)                                                                 
       Mean Pore                                                          
       Diam. Å                                                        
       Volume                                                             
Starting                                                                  
       Starting  of       Quantity     Amine                              
Silica Silica    Ethanol  A-1100 [N]   Composi-                           
Gel    Gel       (ml)     (mole) (mole/kg)                                
                                       tion                               
______________________________________                                    
Davison                                                                   
       22        100      0.05   0.43  F                                  
12                                                                        
Merck 60                                                                  
       60        200      0.05   0.60  G                                  
Davison                                                                   
       140       200      0.05   0.68  H                                  
62                                                                        
______________________________________                                    
 
    
     (b) Aldehyde activated silica gel 
     Aldehyde compositions I, J and K were each prepared as follows: 
     90 g of respective amine composition F, G or H prepared as described above was suspended in a quantity of aqueous disodium hydrogen phosphate (see table 6) and the pH was adjusted to 7.0 with aqueous 10% sodium hydroxide. 80 ml of a commercial aqueous 25% solution of glutaraldehyde (0.20 mole) was added and the reaction mixture was stirred at room temperature of 11/2 hours. The fluids were then removed by aspiration and the solid product washed three times by resuspension in water (5-10 min) followed by aspiration of the washings. The washed wet solid product was then used directly in the next stage of the process. 
     
                       TABLE 6                                                     
______________________________________                                    
(table 2)                                                                 
             Volume of phosphate                                          
Starting amine                                                            
             Solution       Aldehyde                                      
composition  (ml)           composition                                   
______________________________________                                    
F            100 ml         I                                             
G            200 ml         J                                             
H            250 ml         K                                             
______________________________________                                    
 
    
     (c) Cysteine composition 
     Cysteine compositions L, M and N were prepared as follows: 
     A respective starting aldehyde compositions I, J or K was suspended in an aqueous saturated solution of borax (quantity specified in table 7) to which 11.8 g of L-cysteine hydrochloride monohydrate (0.068 mole) has previously been added the pH of the borax solution having been adjusted to 8.5 with aqueous 10% sodium hydroxide prior to admixing with the solids. The suspension was then heated quickly to 65° C. under an argon atmosphere and maintained at that temperature for 2 hours. The fluids were aspirated and the solid product washed three times by successive resuspensions in 300 ml of water for about 5-10 minutes followed by aspiration of the washings. The wet solid was then suspended in the volume of water specified in table 7 and 4.25 g sodium borohydride (0.113 mole) was added. The mixture was stirred at room temperature for 24 hours. The solids were then recovered and were washed once by resuspension in 300 ml of deionized water for about 5-10 min. followed by recovery of the product (wet). The so washed wet solid product was subjected to treatment with (200 ml) aqueous 1N hydrochloric acid once and water washed thereafter with deionized water three times to gave the finished product which was collected by vacuum filtration and stored as a wet cake wetted with water. 
     
                       TABLE 7                                                     
______________________________________                                    
(table 3)                                                                 
                        Volume of                                         
           Volume of    Water for                                         
Starting Aldehyde                                                         
           Borax/Cysteine                                                 
                        reduction Cysteine                                
Composition                                                               
           Solution (ml)                                                  
                        (ml)      Composition                             
______________________________________                                    
I          100          100       L                                       
J          200          200       M                                       
K          250          250       N                                       
______________________________________                                    
 
    
     EXAMPLE 9 
     Cysteine composition E, D and C were each used to treat a starting solution having the characteristics of the solution in example no. 6 (supra). 
     The compositions were loaded with metal as outlined above. The results can be seen in Table 8: 
     
                       TABLE 8                                                     
______________________________________                                    
(table 4)                                                                 
            Hg conc. (ppm)                                                
                          Hg uptake -                                     
                            treated mg Hg/Kg                              
        Mean pore           solution -                                    
                                    composition                           
Cysteine                                                                  
        diam (Å) of     residual                                      
                                    to 1%                                 
Composi-                                                                  
        starting  untreated concentra-                                    
                                    break-                                
tion    silica gel                                                        
                  solution  tion.sup.(h)                                  
                                    through                               
______________________________________                                    
E       22        14        &lt;0.03   ≅630                        
D       22        14        &lt;0.03   ≅1500                       
C       140       14        &lt;0.005  ≅4000                       
______________________________________                                    
 .sup.(h) see .sup.(e) above for table 3.                                 
 
    
     EXAMPLE 10 
     Cysteine composition L, M and N were each used to treat a starting solution having the following characteristics: 
     wastewater of pH 8.2 containing the following: 
     
         ______________________________________                                    
       METAL  CONC                                                        
______________________________________                                    
       Hg     100          ppm                                            
       Mg     2.84         ppm                                            
       Na     69.3         g/l                                            
       Sr     0.24         ppm                                            
       B      8.85         ppm                                            
       Si     51.4         ppm                                            
       Ca     20.1         ppm                                            
       Zn     2.36         ppm                                            
       Fe     1.28         ppm                                            
______________________________________                                    
 
    
     The metal loading was carried out as outlined above and the results are shown in the following table 9: 
     
                       TABLE 9                                                     
______________________________________                                    
(table 5)                                                                 
            Hg conc. (ppm)                                                
                          Hg uptake                                       
                            treated mg Hg/Kg                              
        Mean pore           solution -                                    
                                    composition                           
Cysteine                                                                  
        diam. (Å) of    residual                                      
                                    to 10%                                
Composi-                                                                  
        starting  untreated concentra-                                    
                                    break-                                
tion    silica gel                                                        
                  solution  tion.sup.(i)                                  
                                    through                               
______________________________________                                    
L       22        100       &lt;1.0    783                                   
M       60        100       &lt;1.0    4960                                  
N       140       100       &lt;1.0    6992                                  
______________________________________                                    
 .sup.(i) see .sup.(b) above for table 1.                                 
 
    
     EXAMPLE 11 
     A number of tests were conducted with cysteine composition C and cysteine composition N using the metal loading step outlined previously, using various types of solutions containing Hg. The results of the tests are outlined in table 10, the composition of the solutions being outlined in table 11 below. 
     
                       TABLE 10                                                    
______________________________________                                    
(table 11)                                                                
        Hg conc.                                                          
Cysteine  untreated treated solution -                                    
Composition                                                               
          solution.sup.(m)                                                
                    residual conc..sup.(j)                                
                                  Solution.sup.(l)                        
______________________________________                                    
C         0.9    ppb    N.D..sup.(k)                                      
                                    I                                     
N         100    ppm    &lt;1     ppm    II                                  
C         0.21   ppm    &lt;0.01  ppm    III                                 
C         3.35   ppm    &lt;0.1   ppm    IV                                  
C         7.54   ppm    &lt;0.1   ppm    V                                   
______________________________________                                    
 .sup.(j) see .sup.(b) above for table 1                                  
 .sup.(k) not detectable                                                  
 .sup.(l) see table 11 below                                              
 .sup.(m) analysis by inductively coupled plasma spectrometry.            
 
    
     
                       TABLE 11                                                    
______________________________________                                    
(Annexe V)                                                                
Solution                                                                  
        Nature of                                                         
no.     solution     Ph     Element                                       
                                   Conc..sup.(n)                          
______________________________________                                    
I       polished     8.3    Hg     0.9   ppb                              
        water               Fe     0.33  ppm                              
                            Si     1.4   ppm                              
                            B      0.57  ppm                              
                            Na     0.2   ppm                              
II      wastewater   8.3    Hg     100   ppm                              
                            Mg     2.84  ppm                              
                            Na     69.3  g/l                              
                            Sr     0.24  ppm                              
                            B      8.85  ppm                              
                            Si     51.4  ppm                              
                            Ca     20.1  ppm                              
                            Zn     2.36  ppm                              
                            Fe     1.28  ppm                              
III     contaminated 7      Hg     0.21  ppm                              
        ground water        Ca     37.8  ppm                              
                            Zn     0.04  ppm                              
                            Sr     0.95  ppm                              
                            Mg     18.2  ppm                              
                            Na     2168  ppm                              
                            Si     7.81  ppm                              
                            B      0.51  ppm                              
                            Mn     0.014 ppm                              
                            Fe     0.62  ppm                              
IV      chloroalkaline                                                    
                     7                                                    
g       3.35         ppm                                                  
        process waste       Al     0.72  ppm                              
        water               Ca     5.66  ppm                              
                            Zn     1.16  ppm                              
                            Sr     0.13  ppm                              
                            Na     4068  ppm                              
                            Si     16.8  ppm                              
                            B      0.26  ppm                              
                            Fe     0.19  ppm                              
V       dechlorinated                                                     
                     7      Hg     7.54  ppm                              
        water               Al     0.60  ppm                              
                            Ca     1.83  ppm                              
                            Zn     0.10  ppm                              
                            Sr     0.03  ppm                              
                            Na     3674  ppm                              
                            Si     10.9  ppm                              
                            B      0.21  ppm                              
                            Fe     0.18  ppm                              
______________________________________                                    
 .sup.(n) analysis by inductively coupled plasma spectrometry.