Phosphorous acid separation and recovery from glyphosate mother liquor by two-stage bipolar membrane electrodialysis

Glyphosate is the most widely used herbicide in the world, and a large amount of mother liquor with high salinity is generated during the production of glyphosate by glycine-dimethylphosphite process. In this study, an efficient and environmentally friendly bipolar membrane electrodialysis (BMED) technology was employed for the resource recovery especially phosphorous acid from glyphosate mother liquor. Simulated wastewater as feed solution was initially employed to investigate the migration of phosphite ions in BMED process. The influences of initial pH, salt concentration of feed solution, current density and flow velocity were investigated. The results revealed that phosphites ions could effectively migrate through the anion exchange membrane at pH ≥ 9, with optimal current density of 30 mA/cm2 and flow velocity of 2.48 cm/s, respectively. It substantiated that the migration of phosphite ions during BMED process was mainly controlled by its degree of protonation, which was influenced by the solution pH. Consequently, a two-stage BMED operation was designed to separate phosphite ions from mixed NaCl and Na2HPO3 solution. The separation efficiency SBA was −84.4 % at end of the first stage and changed to 42.9 % in the second stage. Finally, real glyphosate mother liquor was employed as the feed solution to verify the performance of the two-stage BMED process, and results revealed that the two-stage operation could effectively separate phosphite ions from mixed salt solution. Two-stage operation possessed higher current efficiency and lower energy consumption than those of one-stage operation. In addition, only 0.9 % glyphosate leaked from the feed solution to acid and base solution, thus the products recovered from glyphosate mother liquor could be further processed to recycle in the production of glyphosate. These findings indicated that the two-stage BMED operation had great potential to recover resource from glyphosate mother liquor.