Highly permeable acid-stable poly (sulfonamide-urea) nanofiltration membrane for purifying phosphoric acid

The traditional wet-process phosphoric acid has higher economic benefits than the thermal-process phosphoric acid, but introduces many metal ion impurities, which limits the application of phosphoric acid. In highly concentrated phosphoric acid solutions, phosphoric acid mainly exists in molecular form. Due to the surface charge and appropriate pore size of the nanofiltration (NF) membrane, it exhibits high selectivity for phosphoric acid and its impurities. However, because of the uncontrolled diffusion of monomers and rapid polymerization, the pore size distribution becomes highly uneven. Meanwhile the extremely low pH and high viscosity of the phosphoric acid compromise the stability and permeance of commercial membranes. We identified that the organic monomer benzene-1,3-disulfonyl chloride (BDSC) exhibits a capping effect when reacting with the aqueous phase of branched polyethyleneimine (PEI) which allows the newly formed membrane to regulate the subsequent diffusion and reaction of hexamethylene diisocyanate (HDI), thereby optimizing the membrane structure. By adjusting the concentration of BDSC, the pore size of the membrane layer was optimized. The final average pore size was determined to be 0.33 nm. The membrane exhibited an average ion rejection of over 65 % when tested in a 20 wt% P 2 O 5 solution, while maintaining a very low rejection rate of only 1.74 % for phosphorus. The permeance reached 1.21 L/h m 2 bar, surpassing that of traditional commercial membrane operating at less than 1 L/h m 2 bar under 10 wt% P 2 O 5 conditions. This study employed the capping effect to adjust the membrane structure and pore size, demonstrating that spatial sieving is the main mechanism for ions purification in phosphoric acid. Long-term acid resistance tests confirmed the stability of   the poly (sulfonamide-urea) NF membrane during phosphoric acid purification process. This work shows that this new nanofiltration membrane exhibits promising prospects for application within acidic high-viscosity systems.