US 10,494,270 B2  78  77  [+] = [-]  (23)  [K+] = 2[SOtJ  +  2[COtJ  $^{+ [HC0}$3 ]  The carbonate and bicarbonate concentrations are related by  pKa  and can be determined by alkalinity titration to pH 8.3  and then to pH 4.5, respectively using standard HCR solu› tion. By deriving the bicarbonate and carbonate concentra- tion equations, the potassium ion concentration can be  calculated as shown by Equation 24.  By inserting Equation  control  Equation  Ksp  K2S04=1.212M3.  24 into Equation 22, the process  25  was  developed  where  lK•  J~2lso,2-J+l2cv HcfH~s $^{3}$)CNHc1JJ+l  (V  HcfH  S  3 ~ $^{45}$)(N  Hcz)-(V HcfH~s 3 )(N  Hcz)  j  (24) Therefore a control method includes one or more of the  below:  1. Minimize the  K+  exposure to the process using the  process control Equation 25.  2. Purge Sl-SO4 aqueous phase at a rate to maintain  (So/-tample  at 80-90% of cso/-)max•  3. Be sure that none of the aqueous streams in the sulfate  circuit exceeds the comfort safety factor of 80-90% of  cso/-)max  4. Add K 2 CO 3 (aq) to S4-SO4 such that S3-SO4 and  S4-SO4 aqueous phases are s70% of  (So/-)max  and  sl % of  (So/-)max  respectively.  5. Because the operating window is narrow (about 10%  percent), each batch of filtrate should be assayed prior  to introduction to the operation at the S2-SO4 point.  While the forms of the invention herein disclosed consti› tute presently preferred embodiments, many others are pos› sible.  It  is not intended herein to mention all of the possible  equivalent forms or ramifications of the invention.  It  is to be  understood that the terms used herein are merely descriptive,  rather than limiting, and that various changes may be made  without departing from the spirit of the scope of the inven› tion.  The invention claimed is:  1.  A method for treating a first aqueous solution to remove  at least one metal cation and at least one anion from the first  aqueous solution, comprising:  (a) mixing the first aqueous solution with a first water- immiscible extractant phase in a first apparatus to form  a first unstable emulsion, wherein the extractant phase  comprises:  (i) an extractant comprising a cationic molecule and an  anionic base,  (ii) an optional diluent; and  (iii) an optional modifier for modifying phase disen› gagement,  wherein the extractant forms an ion pair with the at  least one anion from the first aqueous solution, and  the extractant also forms a floe with the at least one  metal cation; and  (b) separating the first unstable emulsion into a first  treated aqueous phase and a first loaded extractant  phase, the ion pair and the floe being present in the first  loaded extractant phase.  2.  The method of claim  1,  wherein the cationic molecule  has a carbon number from at least 8 to 34.  3.  The method of claim  1,  wherein the ratio of the first  water-immiscible extractant phase to the first aqueous solu› tion is from about 1:20 to about 20: 1 (v/v).  4.  The method of claim  1,  wherein the pH of the first  aqueous solution prior to mixing with the first water-immis› cible extractant phase is from about 2 to about 8.  5.  The method of claim  1,  wherein the first aqueous  solution is mixed with the first water-immiscible extractant  phase for a period of about 12 seconds to about 120 seconds.  6.  The method of claim  1,  wherein the level of total  dissolved solids in the first aqueous solution is above 2000  ppm, and the level of total dissolved solids in the first treated  aqueous phase is less than 250 ppm.  7.  The method of claim  1,  wherein the anionic base is  co/-, HCO 3 -, OH-, Po/-, HPo/-, H 2 PO 4 -, HS-, or s 2 -.  8.  The method of claim  1,  wherein the concentration of  the extractant in the extractant phase is from 2% to 30%.  9.  The method of claim  1,  wherein the concentration of  the anions remaining in the first treated aqueous phase after  the separating is less than 100 mg/L.  10.  The method of claim  1,  wherein the extractant phase  includes a buffer comprising carbonate and bicarbonate.  11.  The method of claim  1,  wherein carbon dioxide (CO 2 )  is produced during the mixing.  12.  The method of claim  1,  wherein the at least one anion  in the first aqueous solution is one or more of the group  consisting of sulfate, selenate, nitrate, nitrite, phosphate,  arsenate, arsenite, bromate, bromide, perchlorate, iodide,  chloride, chromate(VI), permanganate, bisulfide, and sulfide  10ns.  13.  The method of claim  1,  further comprising:  mixing the first loaded extractant phase with an aqueous  stripping solution containing an alkali metal cation and  an anionic base selected from the group consisting of  co/-, HCO3 -, OH-, Po/-, HPo/-, H2Po4-, Hs-,  and s 2 -, to produce a second unstable emulsion; and  separating the second unstable emulsion into (i) a regen› erated extractant phase stripped of the at least one anion  separated from the first aqueous solution and contain› ing the anionic base of the aqueous stripping solution,  and (ii) a second aqueous phase containing a salt  formed from the alkali metal cation and the at least one  anion separated from the first aqueous solution.  14.  The method of claim  13,  wherein the second aqueous  phase is further treated by one or more of an oil/water  separator, a solid/liquid separator, and an organic odor  sorbent, to purify and collect the salt formed from the alkali  metal cation and the at least one anion.  15.  The method of claim  13,  further comprising mixing  the regenerated extractant phase with the first aqueous  solution.  16.  The method of claim  1,  wherein the cationic molecule  of the extractant is selected from a quaternary ammonium  compound, a quaternary phosphonium compound, and an  alkylated monoguanidinium compound.  17.  A method for treating a first aqueous solution to  remove at least one anion from the first aqueous solution,  comprising:  (a) mixing the first aqueous solution with a first water› immiscible extractant phase in a first apparatus to form  a first unstable emulsion, wherein the extractant phase 