Removal of toxic oxyanions by co-crystallization with sulfate

A method for removing a toxic oxyanion from an aqueous source containing sulfate and said toxic oxyanion, the method comprising:

• (i) dissolving an oxyanion precipitating compound into said aqueous source to result in precipitation of a salt containing sulfate, toxic oxyanion, and oxyanion precipitating compound, wherein the toxic oxyanion is selected from the group consisting of selenate, selenite, molybdate, arsenate, and chromate, and wherein the sulfate in the aqueous source is in a molar concentration equal to or greater than the total molar concentration of the toxic oxyanion, and the oxyanion precipitating compound is included in the aqueous source in a molar concentration equal to or greater than the total molar concentration of sulfate and toxic oxyanion;
• and
• (ii) removing the precipitated salt from the aqueous source to result in a supernatant containing at least 90% lower concentration of the toxic oxyanion compared to the aqueous source;
• wherein the oxyanion precipitating compound has the following structure:
• wherein:
• L is a bond or a hydrocarbon linker containing 1-12 carbon atoms and optionally containing one or more heteroatoms selected from O, N, and S;
• R1 and R2 are independently selected from H and hydrocarbon groups containing 1-12 carbon atoms;
• Xm− is an anionic species with a magnitude of charge m, where m is an integer of at least 1, provided that Xm− is an anionic species exchangeable with sulfate and the toxic oxyanion in said aqueous source before said oxyanion precipitating compound contacts the aqueous source in step (i), and Xm− represents a combination of sulfate and the toxic oxyanion after the oxyanion precipitating compound contacts the aqueous source;
• and
• n is an integer of at least 1;
• provided that n×m=2.