Abstract:
A method for reducing barium concentration in waste streams. Hydrous manganese oxide (HMO) is formed in solution/slurry and mixed with water containing barium such that the hydrous manganese oxide exhibits a negative charge at a desired pH. Barium from the water is adsorbed onto the negatively charged HMO surface. The HMO with adsorbed barium is separated from the water, producing treated effluent having a reduced barium concentration.

Description:
FIELD OF INVENTION 
     The present invention relates to methods for reducing barium concentration in water. 
     BACKGROUND 
     Barium is often released into wastewater in industrial manufacturing operations. The barium concentration of industrial wastewater is generally toxic and should be removed from the wastewater for proper disposal. If barium is not removed from wastewater before disposal, barium can seep into the groundwater and soil. Groundwater in the midwestern region of the United States contains soluble barium. Exposure to barium may cause gastrointestinal disturbances, muscular weakness, and increased blood pressure, among other things. 
     During water treatment, membrane scaling due to barium is well known. In order to protect membrane from scaling, a pretreatment for barium is required prior to pumping the water to membrane unit. Several methods have been developed to reduce barium concentration from groundwater and wastewater. 
     One method to reduce barium concentration is chemical precipitation of barium carbonate through lime softening. However, barium precipitation and removal by lime softening is a highly pH dependent process. The water must have a pH between 10.0 and 10.5 for efficient barium precipitation. Another method to reduce barium concentration is chemical precipitation of barium sulfate using coagulants such as alum or ferric sulfate. However, barium removal by a conventional coagulation process requires a two-stage precipitation system due to the slow precipitation kinetics of barium sulfate. 
     Another method to reduce barium concentration in water is the use of ion exchange systems. However, ion exchange systems require frequent resin regeneration using additional chemicals. The treatment, handling and disposal of the regenerant chemicals are a major drawback to this technique. Reverse osmosis (RO) systems have also been employed to reduce barium concentration in water. However, in RO systems, scaling often occurs on the RO membrane if the barium reacts with other contaminants in the water to form barium sulfate or barium carbonate. This reduces the efficiency of the RO unit and may damage the membrane. A final method employed to remove barium from water involves adsorption of barium onto magnesium hydroxide. However, this process is also a highly pH dependent process. The water must have a pH of approximately 11 for efficient barium adsorption and removal. 
     All of the above processes comprise several operational steps, are complicated, or are costly. Therefore, there is a need for a simple and cost effective method to remove barium from water 
     SUMMARY 
     A process is disclosed for removing barium from water. The process includes forming hydrous manganese oxide and mixing the hydrous manganese oxide with water containing barium such that the surface of hydrous manganese oxide is negatively charged at a pH greater than 5.0. The negatively charged hydrous manganese oxide contacts the water having the barium and the barium is adsorbed onto the hydrous manganese oxide. Thereafter, the hydrous manganese oxide with the adsorbed barium is separated from the water and a treated effluent is produced 
     In one embodiment, the hydrous manganese oxide with adsorbed barium is separated from the water through a conventional flocculation and separation process. In another embodiment, the hydrous manganese oxide with adsorbed barium is separated from the water through a ballasted flocculation and separation process. 
     In another embodiment, the method or process includes forming hydrous manganese oxide solution and directing the solution to a fixed-bed reactor having inert media contained therein. The hydrous manganese oxide solution is directed into the fixed-bed reactor to coat the inert media. Thereafter, the water containing the barium is directed over the coated inert media. As the water passes over the coated inert media, barium in the water is adsorbed onto the hydrous manganese oxide coated on the media. 
     In addition, while removing soluble barium by adsorption onto the hydrous manganese oxide, this process will also remove soluble iron and manganese from water 
     Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a line graph illustrating the adsorption capacity of HMO with respect to barium cation in water. 
         FIG. 2  is a line graph illustrating the pH conditions for the adsorption capacity of HMO with respect to barium cations in water. 
         FIG. 3  is a line graph illustrating the barium removal rate of HMO from water. 
         FIG. 4  is a line graph illustrating the adsorption capacity of various concentrations of HMO solution with respect to barium cations in the presence of competing cations. 
         FIG. 5  is a line graph illustrating the adsorption capacity of HMO with respect to barium cations in water without the presence of competing cations. 
         FIG. 6  is a line graph illustrating the adsorption capacity of HMO with respect to a high concentration in of barium cations in the presence of competing cations. 
         FIG. 7  is a schematic illustration depicting a system and process for removing barium from water using a mixed-bed flocculation system. 
         FIG. 8  is a schematic illustration depicting a system and process for removing barium from water using a mixed-bed micro-sand ballasted flocculation system. 
         FIG. 9  is a schematic illustration depicting a system and process for removing barium from water using a fixed-bed system. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention relates to an adsorption process for removing dissolved barium from water. To reduce the barium concentration in water, the contaminated water is mixed with a hydrous manganese oxide (HMO) solution. HMO is amorphous in character and has a highly reactive surface. Upon mixing water containing barium with the HMO solution, dissolved barium is adsorbed onto the reactive surface of the HMO. Then, the HMO and adsorbed barium are separated from the water to produce a treated effluent having a reduced barium concentration. 
     The iso-electric-point of HMO i.e., point of zero charge (pH pzc ) is between 4.8 and 5.0. The point of zero charge describes the pH condition of a solution at which the HMO surface has a net neutral charge. Thus, when HMO is submerged in a solution having a pH of between approximately 4.8 and approximately 5.0, the HMO surface will have a net zero charge. However, if the pH of solution is lower than approximately 4.8, the acidic water donates more protons than hydroxide groups causing the HMO surface to become positively charged. Similarly, when the pH of solution is higher than approximately 5.0, the HMO surface becomes negatively charged and will attract positively charged cations. 
     The typical pH of untreated groundwater and industrial wastewater ranges between approximately 6.5 and approximately 8.5. Therefore, when the untreated water containing barium ions comes in contact with the HMO in solution, the surface of the HMO becomes negatively charged and will attract positively charged barium ions, Ba 2+ . The process described herein typically reduces the barium concentration in water or wastewater to approximately 50 ppb and in some circumstances may reduce the barium concentration to approximately 20 ppb or less. 
     In testing, an HMO solution was prepared at a pH of 4.0 and slowly mixed overnight. Varying dosages of HMO solution were then mixed with water having a barium concentration of 1.00 mg/L. No other cations were present in the water. Each HMO dosage was mixed with the water for 4 hours. The pH of each reaction mixture was between 7.5 and 8.0. The line graph shown in  FIG. 1  illustrates the adsorption capacity of HMO with respect to barium cations in water. As shown in the graph, the preferred concentration of the HMO solution is between approximately 5 and 10 mg/L for an influent barium concentration of approximately 1 mg/L in the untreated water. 
     Various pH conditions were also tested to determine the pH effect on the adsorption capacity of HMO. An HMO solution was prepared at a pH of 4.0 and slowly mixed overnight. HMO solution having a concentration of 10 mg/L was then added to water having a barium concentration of 1.00 mg/L. No other cations were present in the water. The HMO solution and the water were mixed together for 4 hours at various pH conditions. The line graph in  FIG. 2  illustrates the optimum pH conditions for the adsorption capacity of HMO with respect to barium cations in water. As shown in  FIG. 2 , a pH at or above 5.5 is preferred. 
     The optimum reaction kinetics for barium adsorption onto HMO was also tested. An HMO solution was mixed with water containing about 1 mg/L of barium. The line graph in  FIG. 3  indicates that barium uptake rate of the HMO is very fast. The adsorption capacity of HMO for barium in presence of other competing cation is shown in  FIG. 4 . 
     The above tests were conducted on water containing only barium cations. Therefore, additional testing was conducted to determine the impact of iron cations, Fe 2+ , on the adsorption capacity of HMO with respect to barium ions. Fe 2+  was aerated in a solution at a pH of 7.5 for 30 minutes. A 1.00 mg/L Ba 2+  solution and a 10 mg/L of HMO solution were added to the Fe 2+  solution. The mixture was mixed for 10 minutes and then filtered through a 0.45 micron filter. The barium concentration of the treated water was reduced to 15 μg/L 
     In addition, testing was conducted to determine the impact of Co-oxidized iron on the adsorption capacity of HMO with respect to barium ions. Fe 2+  and Ba 2+  were mixed together in solution. The Ba 2+  concentration was 1.00 mg/L. An HMO solution having a concentration of 10 mg/L was then added. The mixture was aerated for 30 minutes at a pH of 7.5. The mixture was then filtered through a 0.45 micron filter. The barium concentration of the treated water was reduced to 90 μg/L. 
     The barium adsorption process was also tested in the presence of several competing cations. In this example, various dosages of HMO were mixed with water containing several different cations for 10 minutes and at a pH of 7.5. The contaminants found in untreated water are given below in Table 1. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Contaminant 
                 Influent Concentrations 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ba 2+   
                 1.0 
                 mg/L 
               
               
                   
                 Ca 2+   
                 45 
                 mg/L 
               
               
                   
                 Mg 2+   
                 9.0 
                 mg/L 
               
               
                   
                 Sr 2+   
                 0.21 
                 mg/L 
               
               
                   
                 Fe 3+   
                 0.60 
                 mg/L 
               
               
                   
                 Mn 2+   
                 0.06 
                 mg/L 
               
               
                   
                 Alkalinity 
                 280 
                 mg/L as CaCO3 
               
               
                   
                   
               
             
          
         
       
     
     The line graph shown in  FIG. 4  illustrates the adsorption capacity of various concentrations of HMO solution with respect to barium cations in the presence of competing cations. 
     In the above example, when the HMO solution had a concentration of 40 mg/L, the concentration of cations in the treated water were even further decreased, as shown in Table 2. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Contaminant 
                 Effluent Concentrations 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ba 2+   
                 0.038 
                 mg/L 
               
               
                   
                 Ca 2+   
                 39 
                 mg/L 
               
               
                   
                 Mg 2+   
                 8.5 
                 mg/L 
               
               
                   
                 Sr 2+   
                 0.15 
                 mg/L 
               
               
                   
                 Fe 3+   
                 &lt;0.01 
                 mg/L 
               
               
                   
                 Mn 2+   
                 &lt;0.02 
                 mg/L 
               
               
                   
                   
               
             
          
         
       
     
     The barium adsorption process of HMO was also tested in water containing a high concentration of barium and no competing cations. HMO was mixed with water having a barium concentration of 15 mg/L. The mixture was mixed for 10 minutes at a pH of between 7.5 and 8.0. Various concentrations of HMO were observed. The line graph shown in  FIG. 5  illustrates the adsorption capacity of HMO with respect to barium cations without competing cations. As shown in the graph, one preferred concentration of the HMO solution is approximately 100 mg/L for a barium concentration of approximately 15 mg/L in the untreated water. 
     The barium adsorption process was also tested in water containing a high concentration of barium in the presence of competing cations. HMO was mixed with water having a barium concentration of 15 mg/L. The mixture was mixed for 10 minutes at a pH of between 7.5 and 8.0. Various concentrations of HMO were observed. The contaminants found in waste stream are given below in Table 3. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Contaminant 
                 Influent Concentrations 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ba 2+   
                 15 
                 mg/L 
               
               
                   
                 Ca 2+   
                 17 
                 mg/L 
               
               
                   
                 Mg 2+   
                 10 
                 mg/L 
               
               
                   
                 Sr 2+   
                 0.20 
                 mg/L 
               
               
                   
                 Fe 3+   
                 0.40 
                 mg/L 
               
               
                   
                 Mn 2+   
                 0.06 
                 mg/L 
               
               
                   
                 Alkalinity 
                 100 
                 mg/L as CaCO3 
               
               
                   
                   
               
             
          
         
       
     
     The line graph in  FIG. 6  illustrates the adsorption capacity of HMO with respect to a high concentration in of barium cations in the presence of competing cations. 
     The barium adsorption process was tested in water containing a high concentration of barium in the presence of competing cations using an HMO concentration of 90 mg/L. HMO was mixed with water having a barium concentration of 15 mg/L. The mixture was mixed for 10 minutes at a pH of between 7.5 and 8.0. The competing contaminants found in waste stream and the effluent concentrations are given below in Table 4. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Contaminant 
                 Effluent Concentrations 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ba 2+   
                 0.85 
                 mg/L 
               
               
                   
                 Ca 2+   
                 11.5 
                 mg/L 
               
               
                   
                 Mg 2+   
                 9.0 
                 mg/L 
               
               
                   
                 Sr 2+   
                 0.10 
                 mg/L 
               
               
                   
                 Fe 3+   
                 0.01 
                 mg/L 
               
               
                   
                 Mn 2+   
                 &lt;0.01 
                 mg/L 
               
               
                   
                   
               
             
          
         
       
     
     A barium removal process and system  1  that effectively reduces the barium concentration in water is illustrated in  FIG. 7 . An HMO solution is formed in HMO reactor  10 . Table 5 describes several methods to form HMO. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Methods to form HMO 
                 Redox Reactions 
               
               
                   
                   
               
             
             
               
                   
                 Oxidation of Manganous Ion 
                 3Mn 2+  + 2MnO 4   −  + 2H 2 O → 
               
               
                   
                 (Mn 2+ ) by Permanganate Ion 
                 5MnO 2(s)  + 4H +   
               
               
                   
                 (MnO 4   − ) 
               
               
                   
                 Oxidation of Manganous ion 
                 Cl 2  + Mn 2+  + 2H 2 O → 
               
               
                   
                 (Mn 2+ ) by Chlorine (Cl 2 ) 
                 MnO 2(s)  + 2Cl −  + 4H +   
               
               
                   
                 Oxidation of Ferrous Ion (Fe 2+ ) 
                 3Fe 2+  + MnO 4   −  + 7H 2 O → 
               
               
                   
                 by Permanganate Ion (MnO 4   − ) 
                 MnO 2(s)  + 3Fe(OH) 3  + 5H +   
               
               
                   
                   
               
             
          
         
       
     
     In the embodiment described in  FIG. 7 , HMO is formed by mixing potassium permanganate (KMnO 4 ) solution and manganous sulfate (MnSO 4 ) solution in a downdraft tube  12 . In one example, 42.08 g of KMnO 4  was added to HMO reactor  10  through line  14  and 61.52 g of MnSO 4  was added to HMO reactor  10  through line  16 . These reagents were mixed in HMO reactor  10  to form an HMO solution. In this reaction, the optimum pH for HMO formation is approximately 4.0 to approximately 4.5. After HMO formation, NaOH was added to the HMO reactor  10  through line  18  to adjust the pH of the HMO solution to approximately 8.0. 
     After the HMO stock solution is prepared, a dosage of HMO solution is directed from reactor  10  into barium reactor  20  through line  28 . The dosages of the HMO solution applied to barium reactor  20  may be controlled through pump  24 . Water containing barium is added to barium reactor  20  through line  26  and mixed with the HMO solution. 
     In this embodiment, barium reactor  20  includes a downdraft tube  12  to mix the HMO solution and the water containing barium. As the HMO solution mixes with the water containing barium, the negatively charged HMO surface attracts the positively charged barium ions, which are adsorbed onto the HMO surface. Although reaction times may vary, the preferred reaction time in the barium reactor  20  is approximately 10 minutes. 
     To enhance settling and separation, the mixture of water and HMO with adsorbed barium is directed to a flocculation tank  30  where it is mixed with a flocculant to initiate floc formation. The flocculant is added through line  34 . In this embodiment, flocculation tank  30  also includes a downdraft tube  32  to mix the HMO with adsorbed barium with the flocculant. One example of a flocculant is a polymeric flocculant. 
     In some embodiments, flocculation may not be necessary. However, in some cases, mixing a flocculant and the HMO with adsorbed barium is advantageous because the flocculant causes the HMO with adsorbed barium to aggregate around the flocculant and form floc. This enhances settling and separation of the HMO with adsorbed barium from the water. 
     The treated water, including the floc, flows from flocculation tank  30  into a solids/liquid separator such as settling tank  36 . As the floc settle, treated effluent flows upward through a series of collection troughs or lamella  38  before the treated effluent is directed through line  44  for further treatment of other contaminants if required. For example, in one embodiment the treated effluent is directed through line  44  to an RO unit  40  for further clarification. Permeate from the RO unit  40  is collected through permeate line  46  and a reject stream is discharged through line  48 . While  FIG. 7  illustrates settling tank  36  having collection troughs or lamella  38 , it will be appreciated by those skilled in the art that some settling tanks may not require such structures. 
     As the floc settle to the bottom of settling tank  36 , sludge collects at the bottom of the tank. The sludge is pumped through pump  42  and into line  50 , where at least a portion of the sludge including the HMO may be directed to barium reactor  20  through line  54  and reused in the system. The recycled HMO promotes additional adsorption of barium in the waste stream by utilizing the unused reactive HMO adsorption sites. The remaining sludge may be discharged directly through line  52  or may be thickened and dewatered prior to disposal. 
     In some embodiments, ballasted flocculation systems may be used in lieu of a conventional clarifier. A ballasted flocculation system utilizes microsand or other ballast to form floc. For a detailed understanding of ballasted flocculation processes, reference is made to U.S. Pat. Nos. 4,927,543 and 5,730,864, the disclosures of which are expressly incorporated herein by reference. 
       FIG. 8  illustrates a system  100  and method for removing barium from water using a ballasted flocculation system. In this embodiment, HMO is formed in reactor  110  using a downdraft tube  112 . In this embodiment, KMnO 4  was added to HMO reactor  110  through line  114  and MnSO 4  was added to reactor  110  through line  116 . In addition, NaOH was added to the HMO solution in HMO reactor  110  through line  118  to adjust the pH of the HMO. 
     After the HMO stock solution is prepared, a dosage of HMO solution is directed from HMO reactor  110  into barium reactor  120  through line  128 . The dosages of the HMO solution applied to barium reactor  120  may be controlled through pump  124 . Water containing barium is added to barium reactor  120  through line  126  and mixed with the HMO solution. In this embodiment, barium reactor  120  includes a downdraft tube  122  to mix the HMO solution and the water containing barium. As the HMO in solution mixes with the water containing barium, the negatively charged HMO surface attracts the positively charged barium ions, which are adsorbed onto the HMO surface. Although reaction times may vary, the preferred reaction time in the barium reactor  120  is approximately 10 minutes. 
     The mixture of water and HMO with adsorbed barium is then directed to ballasted flocculation tank  130  where it is mixed with a ballast, such as microsand and a flocculant in a downdraft tube  132 . The flocculant is added through line  134  and the ballast is added through line  158 . The HMO with adsorbed barium aggregates and builds up around the ballast to form floc. 
     The treated water, including the floc, flows from flocculation tank  130  into a solids separator such as a settling tank  136 . As the ballasted flocs settle, treated effluent flows upward through a series of collection troughs or lamella  138  before the treated effluent is directed for further treatment for other contaminants if required. For example, in one embodiment the treated effluent is directed to an RO unit  140  for further clarification. Permeate from the RO unit  140  is collected through permeate line  146  and a reject stream is discharged through line  148 . While  FIG. 8  illustrates a settling tank  136  having collection troughs or weirs  138 , it will be appreciated by those skilled in the art that some settling tanks may not require such structures. 
     As the ballasted floc settles to the bottom of settling tank  136 , sludge collects at the bottom of the tank. The sludge is pumped through pump  142  and at least a portion of the sludge may be directed to a separator  156 , such as a hydrocyclone. During separation in the hydrocyclone, lighter density sludge, containing HMO with adsorbed barium is separated from the higher density sludge containing ballast. At least a portion of the ballast may be directed to flocculation tank  130  and reused in the process. The recycled ballast promotes additional flocculation of HMO with adsorbed barium. The lighter density sludge containing HMO with adsorbed barium is collected from the top of the hydrocyclone, and a portion of the lighter density sludge may be directed to barium reactor  120  through line  154  and reused in the process. The recycled HMO promotes additional adsorption of barium in the waste stream. A portion of the higher density sludge containing ballast may be collected from hydrocyclone  156  and directed to flocculation tank  130  through line  158 . The remaining sludge may be discharged directly through line  152  or may be thickened and dewatered prior to disposal. 
     Another embodiment of the present invention is illustrated in  FIG. 9 . In this embodiment, barium is removed from a waste stream in a fixed-bed system  200 . In this embodiment, KMnO 4  was added to HMO reactor  210  through line  214  and MnSO 4  was added to reactor  210  through line  216 . In addition, NaOH was added to the HMO solution in HMO reactor  210  through line  218  to adjust the pH of the HMO. HMO solution is formed in reactor  210  using a downdraft tube  212 . The HMO solution is injected into a packed fixed-bed column  220  containing inert media such as sand or carbon. The HMO solution coats the inert media before the water containing barium is injected into the column. The HMO solution may be injected through line  224  into column  220 . Excess HMO is discharged from the column  220  through line  230 . Water containing barium may be injected through line  222  into column  220  at a specified hydraulic loading rate, in either a down-flow or in an up-flow mode. 
     As the water containing barium contacts the HMO coated on the inert media, the negatively charged HMO surface attracts the positively charged barium ions in the water, which are adsorbed onto the HMO surface. Depending on the configuration of the column, either down-flow or up-flow, treated effluent with a reduced barium concentration is collected either from the bottom or top of the column respectively. Treated effluent is discharged from column  220  through line  232  and may be diverted for further treatment for other contaminants if required. For example, in one embodiment the treated effluent is directed through line  232  into an RO unit  234  for further clarification. A permeate is collected through permeate line  236  and a reject stream is discharged through line  238 . HMO with adsorbed barium may be removed from the column by backwashing. Backwash fluid enters column  220  through line  226 . The sludge collected after backwashing may be directed through line  228  and collected in a sludge storage tank for disposal. 
     A fixed-bed system such as the one described above is advantageous because it may be applied as an add-on process in a plant without disturbing the existing treatment system. 
     As used herein, the term “water” refers to any water stream containing barium, including water, wastewater, groundwater, and industrial wastewater. As used herein, “HMO” refers to all types of hydrous manganese oxides, including hydrous manganese (III) oxide and hydrous manganese (II) oxide. However, hydrous manganese (IV) oxide has a higher adsorption capacity over other hydrous manganese oxides and thus hydrous manganese (IV) oxide is preferred to adsorb barium. 
     The present invention may of course be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced herein.