Photoswitchable guanidinium compounds for removal of oxyanions from liquid solutions

A method for removing an oxyanion from a liquid source, the method comprising contacting said liquid source with an oxyanion precipitating compound having the following structure:

• to result in precipitation of a salt of said oxyanion precipitating compound and oxyanion;
• wherein:
• R1, R2, R3, R4, R5, R6, R7, R8, R9, and R10 are independently selected from (i) hydrogen atom, (ii) hydrocarbon groups (R) containing 1-30 carbon atoms and optionally substituted with one or more fluorine atoms, (iii) —OR′ groups, (iv) —NR′2 groups, (v) —C(O) R′ groups, and (vi) halogen atoms, wherein R′ groups are independently selected from R groups and hydrogen atoms;
• at least one of R1, R2, R3, and R4 in Formula (2) is selected from groups (iii)-(vi) provided above;
• 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 the oxyanion in said liquid source before said oxyanion precipitating compound contacts said liquid source, and Xm− is said oxyanion in said salt;
• and
• n is a number, provided that n×m=1.