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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. 

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By inserting Equation control Equation Ksp K2S04=1.212M3. 24 into Equation 22, the process 25 was developed where 

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. 1, 

The method of claim 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 

