Patent Application: US-60181206-A

Abstract:
amorphous , nanoporous silica gel having an open channel structure may be surface modified at higher loading of surface modifying ligands , e . g ., 7 . 5 mmole per gram , than known nanoporous silica gels . in one embodiment , an amorphous silica gel has a bimodal pore size distribution of pores at about 10 nanometers and at about 10 microns , and a bulk density of about 0 . 2 to about 0 . 25 g / ml . surface modification with functionalized ligand groups , effective for selective adsorption or reaction catalysis , is achieved by gelling silica sol solution to form a wet silica gel , maintaining the gel at a relatively low elevated temperature in a moist state to obtain a wet nanoporous silica gel having a plurality of open channels within the gel structure and silanol groups on the surface and reacting the surface silanol groups with the ligand group to introduce the functionalized group . the surface modifying reaction may be carried out concurrently with the gelling of a silica precursor in an aqueous alcoholic medium . 3 - mercaptopropyltrialkoxysilane is an exemplary ligand introducing compound . the chemically surface modified gel may be used , for example , to remove or concentrate metallic substances in a liquid , or to separate two or more metallic impurities from a mixture thereof , or for cleanup of oil or chemical contaminants from the surface of a body of water .

Description:
in the present invention , a silica gel is prepared from a precursor solution derived from tetraethoxyorthosilicate ( teos ), or collodial silica ( for example , ludox ), or ion - exchanged sodium silicates , and the incorporation of a surface monolayer of functionalized ligand groups is integrated with the preparation of the silica gel ( i . e . reacting during the gelation or immediately following the gelation before gel aging ), thereby making the csmg according to this invention . in order to have a compatible medium for incorporating the monolayer and a low interfacial tension for reducing the shrinkage of the gel , a specific solvent system may be chosen according to the composition of the ligand functional group . the choice of the functional group and the processing conditions of csmg , including the solvent system , will dictate the adsorption efficiency of the final products . high adsorption efficiency may be achieved by ( i ) chemiadsorption of targeted ions on the surface ; ( ii ) large surface area ; ( iii ) open porous structure , and each of these factors is described in further detail . the chemical properties of a gel surface are modified so that the target ions form chemical rather than physical bonds onto the surface . the modification with a ligand functional group increases the bonding energy of the metal ion to the silica surface sites . increasing the bond energy will exponentially decrease the residual concentration of the ion in the solution at equilibrium . for example , at room temperature , reducing residual ion concentration from ppm ( parts per million ) to ppb ( parts per billion ) would require an increase of ca . 17 kj in bonding energy . by chemically modifying the gel surface with a selected functional group the difference . in energies of bonding the metal ion with the gel surface and solvating the metal ion in water could be effectively increased . this increase in bonding energy will result in a significant reduction in the residual concentration ( ca . 6 kj for a change of one order of magnitude in residual concentration ) of the metal ion at adsorption equillibrium . the data of ion - ligand solubility product . constant ( ksp ) may be used as a direct reference for selecting appropriate functional groups to control the residual ion concentration . the accessible surface area of a low - density csmg is very large . because the silica particles are of nanometer size , the surface area of a low - density gel is in the range of from about 800 to about 1000 m 2 / g . it is two orders of magnitude higher than the surface area of ordinary ion - exchange adsorbent with a particle size of 1 micron or larger . this increase in surface area will result in a proportional increase in reaction speed of any interfacial reaction . additionally , once loaded with functional groups , a large surface area leads to a greater adsorption capacity . experimental results , as described below , clearly demonstrate this outstanding advantage of the surface modified nanogel . the present invention controls the gelation process to create the ideal gel morphology , i . e . a large surface area with many reactive silanol ( si — oh ) groups ( for the incorporation of surface functional groups ). in particular , following the gellation reaction aging is limited to only a very brief duration , usually from about 30 to about 60 minutes , sufficient to allow secondary bond formation but too short for any significant degree of cross - linking or other pore collapsing reactions to occur . in a gelation reaction , the backbone of the structure ( long chain bonds ) is formed rapidly at first , increasing the viscosity and slowing down additional bond formation . aging after a gelation . reaction allows cross - linking ( forming local ring - closing bonds ). forming a small ring structure enhances the mechanical strength of the gel , but also closes off some open channels . both high mechanical strength and channel openness are generally required in field applications . in the present invention , therefore , the processing conditions are controlled in order to achieve an optimized morphology : a strong but open gel structure . in order to incorporate functional groups on the surface of a low - density silica gel while it is in a wet state , it is necessary to control the gel morphology and to minimize the surface tension of the solvent system in order to preserve the high surface area and maintain a large number of open channels . replacing water with a solvent of low surface tension reduces the shrinkage . literature reports 21 also indicate that reacting surface silanol groups with organic molecules before drying could preserve the open pore structure . in the present wet gel process , the pores are filled with incompressible liquid , which provides support against capillary stress . incorporation of hs — ch2 — ch2 — ch2 — si ( ome ) 3 ( mptms ) onto the surface of a wet silica gel is achieved by using a mixture of solvents ( water and ethanol ) to lower the surface tension . additional open pores of micron sizes are created with the addition of an insoluble liquid plus an appropriate surfactant to control the pore size . these artificially created channels are intended for connecting the domains of nanopore silica in order to further facilitate the adsorption speed and efficiency . the csmg adsorbent material of this invention is essentially a tightly packed fractal - like arrangement of primary particles of approximately 10 nanometers particle size . the bulk density of the composite made by this invention is in the range of about 0 . 2 to 0 . 25 g / ml ( determined with a quantachrome mercury porosimeter ). the specific surface area of the silica before the incorporation of the ligand groups is in the range of about 600 to 1100 m 2 / g . the skeletal density of the silica was measured with a helium pycnometer ( micromeritics , pycnometer accupyc 1330 ). the specific surface area was characterized by gas adsorption ( micromeritics , gemini surface area analyzer ). other properties were calculated according to the following set of equations : pore ⁢ ⁢ volume = ( 1 bulk ⁢ ⁢ density ) - ( 1 skeletal ⁢ ⁢ density ) pore ⁢ ⁢ size = 2 × surface ⁢ ⁢ area pore ⁢ ⁢ volume porosity = ( 1 - bulk ⁢ ⁢ density skeletal ⁢ ⁢ density ) × 100 ⁢ % differences exist due to different degrees of gel shrinkage during drying before characterization . at the completion of the surface reaction , the csmg is washed several times with water , replacing the solvent mixture . two types of characterizations may be performed . first it is confirmed that the mptms has successfully formed a monolayer on the silica surface . this is done through nmr , ir , and eds spectra . compositional analysis indicates that the relative concentration of sulfur on the csmg surface is correlated to both the ratio of mptms to silica and the reaction time . as expected , higher ratios of mptms to silica and longer reaction times result in more thiol groups on the surface . this in turn , yields improved heavy metal adsorption . verification of bonding can be seen in the accompanying fig1 and 2 . eds and ir spectrometric analysis were also performed on a representative silver laden sample . the eds spectrum ( fig1 ) clearly indicates the presence of both sulfur and silver . the ir spectra of three forms of silica gel are shown in fig2 . the top curve ( a ), is for untreated silica gel . strong adsorption bands at 1089 cm − 1 and 3430 cm − 1 are attributed respectively to the stretching vibrations of si — o — si , and o — h on the surface . this should be compared to curve b , the spectrum for functionalized adsorbent . here , bands at 2924 cm − 1 , 2565 cm − 1 , 1454 cm − 1 , and 688 cm − correspond to ch 2 , sh , ch 2 s , and —( ch 2 ) 3 —, respectively , and show that mptms bonded to the surface of the silica . finally , after silver ion adsorption ( curve c ), the band at 2565 cm − 1 disappears and one at 1384 cm − 1 results from the newly formed ag — s bond . this clearly demonstrates that silver ions have bonded to thiol groups on the surface of the adsorbent . because 100 % surface coverage of ligand can be obtained with the methods disclosed , this invention allows a complete range ( from 0 to 100 %) of surface coverage with the ligand groups through the control of reaction stoichiometry and kinetics . partial coverage may be obtained with either a reduced degree of reaction ( low reaction yield ) or a lowered starting concentration for the ligand ( longer processing time ). the practical lower bound of the surface coverage by this invention for each kind of ligand group may be determined by the cost - effectiveness of producing the product under the constraints of the low reaction yield or the long processing time . a second type of characterization allows for the determination of the efficiency as well as the capacity for metal ion adsorption by the csmg . atomic adsorption spectroscopy may be used to evaluate the concentration of metal ions before and after treatment with csmg . the efficiency of purification is characterized by the partition coefficient of metal ions distributed between the csmg and the solution at equilibrium ( i . e . weight % of ion in the csmg divided by the residual weight % of ion in solution ). the partition coefficient remains a constant at low adsorption concentration , equivalent to an equilibrium constant . at moderate to high adsorption , the coefficient is a function of adsorption concentration and ought to be characterized for a range of concentrations . the following method may be used to evaluate the adsorption efficiency of the csmg according to this invention . to test the ability of adsorbent for purification of metal - contaminated water , a batch adsorption experiment at room temperature was performed . 10 mg of the adsorbent produced as in the following example 2 was stirred with 50 ml of metal ion solution for 30 minute at initial concentration ranging from 5 to 10 ppm . metal ion concentrations before ( c initial ) and after ( c eq ) treatment were determined by atomic adsorption spectroscopy . results are shown in the following table . partition coefficient adsorption mg per gram solid mg / g at mg per gram c initial ( ppm ) c eq ( ppm ) equilibrium solution ag + 7 . 2 0 . 002 35 . 99 17 , 995 , 000 pb 2 + 6 . 5 0 . 028 32 . 36 1 , 155 , 714 hg 2 + 6 . 6 0 . 004 32 . 98 8 , 245 , 000 cu 2 + 6 . 6 0 . 012 32 . 94 2 , 745 , 000 the capacity of adsorbing metal ion by an adsorbent varies significantly with the ph value of the solution . for mercaptan loaded csmg , the adsorption capacity is expected to rise with the increase of the solution ph . for csmg of this invention the following tests are performed to determine the adsorption capacity of respective metal ions at ph value of three . to test the maximum adsorption of adsorbent , 140 mg of the same adsorbent was mixed with 200 ml solution , adjusted to ph = 3 , of respective metal ion for 1 hour at the initial concentrations indicated in the following table . ion concentrations before ( c initial ) and after ( c final ) treatment were determined by atomic adsorption spectroscopy . adsorption capacity c initial ( ppm ) c final ( ppm ) ( mg ) ( mg / g adsorbent ) ag + 970 475 99 707 pb 2 + 1130 953 35 . 4 253 hg 2 + 904 388 103 737 cu 2 + 930 760 34 243 it is believed that this is the highest metal ion adsorption reported for silica based adsorbents . approximately 10 , 000 tons of mercury are discharged into water or ground systems as industrial waste each year . most removal operations require the separation of mercury ions from aqueous solution . due to the high toxicity of the mercury ion , allowable concentration of the ion in the water after treatment is very low . using the novel csmg described herein would substantially lower the costs to reach the required low concentration , relative to other adsorption methods . according to the present invention , on the other hand , a gallon of the csmg can treat up to 30 , 000 gallons of wastewater , reducing mercury concentration from ppm to ppb . a test of adsorption capacity indicates that one gram ( dry weight ) of csmg substrate according to this invention can adsorb 0 . 7 g of mercury under acidic condition . from results of adsorption experiments and values of solubility product constants the inventive csmg is also effective in treating wastewater containing silver ( ag + ), lead ( pb 2 + ), cadmium ( cd 2 + ), and copper ( cu 2 + ). all of these ions are major pollutants in various industries including those which manufacture batteries , computers , and photographic films . csmg may be used for recovery use or waste clean up . precious metals and trace elements normally exist in very low concentrations . one major cost of a traditional process is extraction of these precious metals and trace elements from a low - concentration stock solution . in many cases , the preparation of high concentration and subsequent purification of the extract are the reasons for the high cost of these materials . csmg may be used to extract low concentration ( ppm level ) metal ions selectively on to its surface . due to its large surface area , csmg can adsorb an amount almost equivalent to its own weight ( see results of adsorption test ). thus , csmg may be used to reduce the concentration and purification cost of these materials significantly . in asia and elsewhere , the rapid industrialization and population growth has endangered the supply and the quality of drinking water . as a result of ineffective pollution control and a significant increase in water consumption , cities now face a crisis in supplying drinking water with acceptable quality . one proposed solution is to separate the water supply into two systems ; one for drinking and another for other utility use . the csmg of this invention may similarly be used for purifying water used to prepare bottled drinking water . the cost of bottled water in some areas is . higher than the price of gasoline . csmg may be used in the purification of drinking water due to its high efficiency and loading capacity in ion adsorption . processing of microelectronics has become one of the fastest growing and most profitable businesses worldwide . due to the dramatic progress in device miniaturization , microelectronics products have the highest value per unit of material used . consequently , the microelectronics industry is capable of consuming many high technology and high - cost materials . one important requirement for solvent used in processing microelectronics is high purity . in particular , the ion concentration in the solvent must meet very strict standards . at present , the standard for allowable residual ions is being pushed from the sub - ppm level to the ppb level . thus , in the extreme case of semiconductor processing , the solvents may require an on - site purification to remove contaminants occurring during its shipment . reducing the ion level in a solvent from ppm to ppb is readily achievable due to the performance of csmg with its comparable ease of processing . csmg adsorbent may be used for increasing the adsorption population of one specific ion or a group of ions . the large concentration difference for the specie in the csmg adsorbent and in the solution presents application opportunities in analytical chemistry . many analytical tests use only minute quantities of the sample . when the concentration of the specie of interest is too low , the amount can not be detected . using csmg to preconcentrate the specie allows the accurate determination of the specie content even when only a small amount of sample is being analyzed . moreover , the high adsorption capacity of csmg makes it an ideal packing substrate for high efficiency liquid chromatography . a short csmg column may effectively separate ions with different partition coefficients . the present invention thus provides a novel chemically modified silica gel substrate ( csmg ) on the surface and pores of which there is incorporated a monolayer of ligand group ( e . g ., thiol ). the starting silica material for . forming a silica sol solution used to form a wet silica gel , may be , for example , an alkoxy silane , especially tetraethoxy silane ( teos ), a colloidal silica precursor ( e . g ., ludox ), or a sodium silicate . the surface modification of the gel with , for example , mercaptopropyltrimethoxysilane , is done while it is still in a wet state ( two - stage ) or during the gelation reaction ( one - stage ). the adsorption efficiency of csmg is more effective than the material made with mesoporous silicas . the invention csmg is not only considerably more effective than adsorbents with similar composition , but in addition , may be produced with a much more efficient process . using the process described above , the cost of producing the csmg substrate is many times less than the cost of any other comparative substrate . the lower cost presents a significant advantage for any particular application ( for example , wastewater treatment ). a silica gel made by a sol - gel process as described above normally contains tightly packed primary particles of size approximately 10 nanometers . as a result , the gel structure , packed from these primary particles , consists of open channels with a similar dimension ( intra - particle channel size of approximately predominantly 10 nm ). to facilitate and accelerate the diffusion of large species to and through these fine channels , a relatively small volume ( e . g ., about 10 % of the total pore volume ) of a second set of channels of micron size maybe artificially created during gelation to interconnect the finer (˜ 10 nm ) channels . an insoluble liquid ( e . g ., chloroform ) and a surfactant ( any anionic type , e . g ., sulfate , sulfonate , soap , etc .) may be used to create such an interconnecting structure . the surfactant is used to minimize the interfacial energy between the insoluble solvent phases , and its amount should be far less than required for forming micelles ( i . e . surfactant concentration sufficiently lower than critical micelle concentration to avoid micelle formation ), as used in prior art templating processes . as illustrated by the above application examples , the present invention provides liquid , e . g ., waste water and aqueous or non - aqueous solvents , purification using csmg as a super adsorbent for heavy metal ions . using the process for producing csmg according to this invention and the resulting csmg product , it is possible to achieve the following objectives : to increase the strength of csmg material to control the structure - property relationship through processing parameters to optimize the reaction kinetics to improve the efficiency of processing to incorporate other selective functional groups onto the gel surface to develop techniques for recovering the adsorbed metal these objectives are closely linked to the quality , cost , and processing , of the new csmg product . some of the properties and characteristics of the novel csmg materials will now be described in further detail . the porosity of the silica gel is very high ( approximately 90 to 97 %). the degree of cross - linking is very low . channels among silica particles are numerous and open . these characteristics are responsible for the fast and extensive ion adsorption . however , the mechanical strength of csmg may , for the same reasons , be too low for some field applications . a weak and fragile substrate may be difficult to handle , especially for a large - scale industrial operation . fine particles detached from a csmg substrate could be a concern in an application , especially when they are loaded with toxic metal ions . aging of the silica gel will increase the degree of cross - linking and improve the material strength . however , the degree of cross - linking must be controlled , as described above , so as not to close off the pore channels . increasing the density with the use of a more concentrated sol , as described below ) will also effectively strengthen the gel structure . since the gelation of such a concentrated sol system is much faster , the reaction kinetics must be adjusted accordingly . the strength of the wet gel may be increased by , for example , taking into account the bulk modulus of the porous silica . the bulk modulus of a porous silica can be expressed by an equation 22 k = k ° ( ρ / ρ ° ) n , where ( ρ is density , k ° is modulus at the reference density , and n is from about 3 to about 4 ). increasing bulk density from 0 . 1 to about 0 . 25 will increase the modulus by a factor of approximately 15 . besides density , the strength of a gel before and after drying depends on many kinetic factors such as aging , catalysis , reaction rate , etc . the kinetics of gel formation will determine the extent of the reaction and the initial microstructure of the gel , two important factors affecting k ° . control of the kinetics of gelation to further increase k ° may also be accomplished . other techniques for increasing strength of the wet gel include using a concentrated sol solution and / or a layered silicate . so far , in all systems that have been used for making gels , teos , colloidal silica , and sodium silicate , the silica content of the starting solution was low . to increase the final density , the silica content is increased to a desired level ( e . g ., & gt ; approximately 15 %, e . g ., about 20 %) before gelation occurs , for example , by evaporating the solvent to increase the solids concentration . solvent evaporation can be achieved either at an elevated temperature or a reduced pressure . the choice between these two conditions will be based on their effects on the kinetics of gelation of a high - concentration sol . for example , to prevent premature gelation , a low - temperature ( e . g ., below room temperature ) and / or reduced pressure , evaporation might be necessary . increasing the . solid content may also be accomplished by the addition of fine particle clays ( layered silicates ) into the starting solution . layered silicates have been used to strengthen aerogels in the past , and may similarly be used to strengthen the csmg of this invention . clays of from about 20 to 30 micron particle size are preferred . according to earlier experiments , aerogels made with the addition of clay greatly improves mechanical strength . moreover , the plate geometry of a clay molecule provides a means for significantly altering physical properties in different directions with the control of the orientation of the plate molecules . nanocomposites made of clay and polymers demonstrated exceptional improvement in thermal stability , thermal expansion coefficient , and reduced gas permeation . adding layered silicates to csmg will prevent the loss of detached particles during an adsorption operation . surface modifications may also be used to improve gel strength . since the surface functional groups of nanopore silica represent a large portion of the substrate , modification of them leads to changes in bulk properties as well . in addition , the population of the ligand groups and the repulsion between them slows down the condensation of silanol groups . it is known that aerogels made with extensive surface methylation can withstand the capillary stress incurred during drying under ambient conditions 6 . the 3 - mercaptopropyl - trimethoxy - silane molecules incorporated on the surface have similar strengthening effects . as the loading of the surface monolayer is increased , mechanical strength of the csmg improves as well . other multi - functional oligomers ( e . g ., tri -[ 3 -( trimethyloxysilyl )- propyl ] isocyanurate ) may be used to cross - link the csmg for improving the strength . the size and stereochemistry of the oligomer molecules are screened so that incorporating them onto the csmg surface will not block the diffusion of target metal ions . as noted above , a great deal of the cost of producing csmg is in material processing . the properties , and therefore the performances , of the csmg material are closely correlated to the processing conditions . in order to produce the most effective ( performance / cost ) csmg , the structure - property and morphology - processing relationships of the csmg material is determined . this may be accomplished , for example , by characterization of the particular surface modification processing condition utilizing , for example , tem , nmr , and / or ir to determine the effectiveness of the surface modification scheme . the processing conditions for gels made from teos , colloidal silica , and sodium silicates , must be monitored very carefully . it is the solvent which creates the porosity in these systems . high porosity material is mechanically weak . during drying , capillary stress , resulting from surface tension of the meniscus in the pore , may shrink and crack the material . for pores of nanometer size the stress can be in the range of a hundred pounds per square inch . in the wet gel process according to this invention , the pores are filled with liquid , which is incompressible . the capillary stress will cause very little shrinkage because of the low compressibility of the liquid . during drying , however , the liquid turns into vapor , which is highly compressible , and the stress would tend to collapse the cells . shrinkage resulting from cell collapse would reduce the porosity and may close off some open channels . thus , the processing conditions of the gel may greatly affect the effectiveness of the subsequent surface modifications . while much is known about the chemistry and kinetics involved in the gelation of plain silica sol , less detail is known about the variation of those parameters with the change in solvent content , silica density , temperature , and the ph . however , by controlling the kinetics of gelation using a solvent mixture with a low surface tension , a concentrated sol solution , and / or lower temperature , the reaction speed may be controlled so that the morphology and processing are optimized . layered silicates may be used to strengthen the gels , as described above . however , care must be taken that these added ingredients do not alter the effectiveness of the modification reaction . using a series of controlled experiments and characterization work the effects of processing conditions on the overall adsorption efficiency of csmg may be readily ascertained . determination of the efficiency of adsorption in broader terms , including the extent of the adsorption , the speed of the adsorption , the effects of physical ( for example , temperature ) and chemical ( for example , ph , other ionic species , etc .) environments on the adsorption and other interfering factors will then be known . one major advantage of a silica sol - gel system in processing is that the gelation kinetics can be easily controlled with the adjustment of the ph value . for an industrial product , a faster reaction usually allows a shorter processing time and a lower fixed ( manufacturing ) cost . of course , the gelation must also be slowed down enough so that other processing procedures can catch up with the gelation . the control of gelation kinetics is critical because the microstructure and the mechanical properties of a silica gel are dictated by it . optimization of the reaction kinetics is further required due to the fact that the effectiveness of the surface modification is very sensitive to the properties and thus , the processing of the silica gel . the reaction for loading ligand groups onto silica gel surfaces and subsequent treatments generally takes a couple of hours . the reaction rate , the reaction temperature , the ph , the initial gel morphology , are factors which may be controlled to optimize the rate of the surface modification reaction and / or to achieve a higher percentage of loading within a reasonable reaction time . a higher loading of functional groups is effective to increase the adsorption efficiency and , in some cases , to improve the strength of the csmg as well . an industrial process , differing from a laboratory process , must deal with a very large quantity of materials . consequently , many issues , which are negligible on a laboratory scale , must be addressed properly during scaling up . for example , voc ( volatile organic compound ), solvent recovery , fire hazards , waste treatments , scrap reuse are all important issues for industrial processing . since csmg is made from a low - density gel , the volume of solvent used for the reaction and washing is several times larger than the actual volume of the product . even though relatively safe ethanol may be used in place of benzene , the amount of solvent needed for processing may still present a problem in large scale production . however , those skilled in the art , will be able to design an efficient processing system so that all the processing issues mentioned above are satisfactorily addressed . the conditions used in batch procedures may be adopted for a semi - continuous process . in particular , the reaction rates of each individual component are adjusted so that the flow of materials for the process are synchronized . in a continuous process the majority of the production may be carried out in an extruder . the extruder may have many different zones , each one being designated for a different reaction . initial work incorporated the mercaptan functional group on the silica gel surface . metal ions that normally precipitate with the sulfide ion ( low solubility product , ksp ) are exhaustively adsorbed . by determining the solubility product value of different ion pairs other new surface modification schemes may be designed . ksp and bonding energies may be utilized to choose the appropriate functional groups to be incorporated in the csmg . as explained previously , solubility product constant ( ksp ) is a direct indication of what type of functional group to choose for a specific ion . bonding energies of a precipitate or a complex ion may be used to estimate the effectiveness of adsorption . the free energy of the adsorption may be calculated and the partition coefficient , kp ( surface ion concentration / residual ion concentration ), may be estimated , accordingly . since in most cases , only very diluted solutions will be treated with the csmg , the ideal solution scenario may be used to obtain the entropy . for the species adsorbed on the surface , the entropy may be calculated by using a two - dimensional lattice model . in qualitative analysis a group of ions can be selectively precipitated with one common ion ; likewise , a functional group incorporated at the silica gel surface may selectively adsorb a group of counter ions as desired . following traditional qualitative analytical chemistry ( of separating ions in solution ) choices of additional functional groups for ion removal may be selected . successful incorporation of new functional groups will extend the applications of csmg as a product , and will also simplify the procedures of using csmg for water or solvent purification . for instance , a multi - zone column packed with csmg of different functional groups may be used to achieve a complete purification with just one flow through the column . thus , as examples of representative suitable functional group providing ligands , mention may be made of , for example , mercaptans , such as , 3 - mercapto -( mono - or di )- alkyl ( di - or tri -) alkoxy silanes , e . g ., 3 - mercaptopropyltrimethoxysilane , 3 - mercaptopropyltriethoxy silane , 3 - mercaptopropylmethyldimethoxy silane ; amines , such as , 3 - aminopropyltrimethoxysilane , 3 - aminopropyltriethoxysilane , ethylenediamine mono -, di -, tri - or tetra - acetate , and the dithiocarbamate derivative thereof , n -[ 3 -( trimethoxysilyl ) propyl ] ethylenediamine and the triacetic acid trisodium salt thereof ; amides , such as chitin and chitin derivatives , e . g ., chitosan ; and the like . other known chelating agents , such as , for example , 1 - nitroso - 2 - naphthol , 5 - sulfodimethylisophthalate salts , e . g . na , 8 - quinolinol ; and ion - exchange resins as well known in the art may also be used as the functional group providing ligands for the csmg adsorbents of this invention . in this regard , mention is made of the literature references 23 - 39 , in the attached list of references , forming part of this application , and which references are incorporated herein in their entirities by reference thereto . the high loading capacity of csmg at saturation ( about 0 . 7 gram hg / gram csmg ) provides an opportunity for recovering the metals from csmg after wastewater treatment . there are - at least two options available for removing metal ions from the surface of csmg . one is to change the partition coefficient of the adsorbed metal ion by varying the temperature and / or the ph of the solution . regeneration of used csmg material with recovery of adsorbed metal ions may be achieved using , for example , a concentrated hcl solution . this will significantly increase the concentration of the metal ion in the solution and lead to a regeneration of the csmg surface . the regenerated materials will retain high loading capacity and remain effective even after several cycles . dissolving the csmg in a hot basic solution will also result in a separation of the metal ion from the csmg surface . after being relieved from the csmg surface , the metal ions can be reduced to metal through chemical reaction or electrolysis . with a recovery option , the csmg wastewater treatment bridges a complete cycle for the use of heavy metal materials . the following experimental procedures are disclosed merely as examples of practicing this invention according to the detailed principles described above . many variations in practices of this invention within the boundary of the working principles and the scope of the claims may be recognized by those skilled in the art according to the principles and examples disclosed . silica sol is prepared from teos , h 2 o , ethanol and hcl , in the total molar ratio 1 : 2 : 4 : 0 . 0007 . the mixture of teos , h 2 o , ethanol and hcl is stirred at 60 ° c . for 2 hours . a nh 4 oh solution and variable amount of water is added to adjust the ph to 6 to 7 and to gel the mixture . gelation normally occurs within a few minutes . the obtained wet silica gel is aged at 60 ° c . briefly ( about 30 to 60 minutes ) and washed with ethanol and water separately . the mixture of 50 g of wet silica gel and a variable amount ( depending on the desired % of ligand loading ) of 3 - mercaptopropyltrimethoxysilane is added into a reaction vessel equipped with agitator , heating mantel , thermometer and nitrogen purge system . a solution of water and ethanol is used as the reaction medium . the amount of ethanol in this mixed solvent should be adjusted according to the amount of ligand desired in the mixture . the reaction mixture is heated to 50 - 60 ° c . for from 1 to 2 hours . after cooling down to room temperature , the product is filtered and washed thoroughly with ethanol and water successively . silica sol is prepared from teos , h 2 o , ethanol and hcl , in the total molar ratio 1 : 2 : 4 : 0 . 0007 . the mixture of 50 ml of silica sol and a variable amount ( depending on the desired % of ligand loading ) of 3 - mercaptopropyltrimethoxysilane is added into a reaction vessel equipped with agitator , heating mantel , thermometer and nitrogen purge system . additional amount of water or ethanol is used to adjust the water / ethanol ratio in the solvent mixture so that their proportions are suitable for the amount of ligand desired . the reaction mixture is heated to 50 - 60 ° c . from 1 to 2 hours . then , a nh 4 oh solution is added to the mixture to induce gelation . after cooling the csmg is filtered and washed thoroughly with ethanol and water successively . separately following the procedures of example 1 and example 2 , 3 - aminopropyltrimethoxysilane or chitosan are incorporated onto the surface of the silica gel with high loading , respectively , by the two - phase or one - phase embodiments . following procedures in example 2 to create a one - phase mixture of ligand and silica sol , the reaction mixture is heated to from 50 to 60 ° c . for from 1 to 2 hours . after the mixture is cooled down to room temperature , 2 ml of chloroform . and 0 . 2 to 0 . 5 gram of sodium dodecyl sulfate in water ( 2 to 5 ml ) is added to the mixture . the mixture is heated to 30 to 40 ° c . with vigorous stirring for 1 hour . then , a nh 4 oh ( 1n ) solution is slowly added to the mixture until gelation occurs . after aging at 30 to 40 ° c ., the product is filtered and washed thoroughly with ethanol and water successively . silica sol is prepared from 100 g nalcol 115 by adding 10 ml of 1m h 2 so 4 to adjust the ph to 6 . 78 . the mixture gels within 30 minutes at room temperature . a mixture of 50 g of wet silica gel and a variable amount ( depending on the desired % of ligand loading ) of 3 - mercaptopropyltrimethoxysilane is added into a reaction vessel equipped with agitator , heating mantel , thermometer and nitrogen purge system . a solution of water and ethanol is used as the reaction medium . the amount of ethanol in this mixture solvent is adjusted according to the amount of ligand desired in the mixture . the reaction mixture is heated to 50 to 60 ° c . for from 1 to 2 hours . after cooling down to room temperature , the product is filtered and washed thoroughly with ethanol and water successively . while the invention has been described above in connection with silica based csmg and silica gel precursors , the invention is equally applicable to other metal oxide adsorbents , such as , for example , alumina , zirconia , titania , and the like , including mixtures of metal oxides . as well known in the art gels of the metal oxides may be prepared similarly to the preferred silica gels , such as , for example , from the corresponding metal hydroxide precursors . 1 . l . mercier and t . pinnavaia , adv . mater ., 9 , no . 65 , pp 500 - 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