Acid stable layer-by-layer nanofiltration membranes for phosphoric acid purification

The conventional wet process of extracting phosphorus using concentrated sulfuric acid produces crude phosphoric acid rich in multiple ions. High-end applications require purification of the crude phosphoric acid by removing the ions at an affordable cost. At low pH, phosphoric acid mainly exists as a neutral molecule. Nanofiltration (NF) membranes have been proposed to remove multivalent ions while allowing acid permeation, based on size and charge factors. However, commercial membranes have not been specially designed for acidic applications, and poor permselectivities were reported. In this work, we developed a novel application of the glutaraldehyde (GA) crosslinked layer-by-layer (LBL) NF membranes based on poly(allylamine hydrochloride) (PAH)/poly(sodium styrene sulfonate) (PSS) for this purpose. We identified that more coating layers contributed to lower surface charges but smaller pores with narrower distributions. The PSS/PAH2.5 membrane with 2.5 bilayers showed high permselectivity of Al rejection up to 97% and P permeation more than 90% at acid permeance of 2.5 L/m2 h bar. The steric sieving was identified as the primary mechanism for rejecting ions; thus, a NF membrane with molecule weight cut-off (MWCO) of 120–230 Da was preferred. The long-term acid stability tests demonstrated that the PSS/PAH system is an excellent candidate of stable chemistry for preparing NF membranes with high permselectivity for phosphoric acid purification. Present research showcases the new NF membranes with great potential for broad applications.