On the electrostatic interactions in the transfer mechanisms of iron during nanofiltration in high concentrated phosphoric acid

Concentrated phosphoric acid issued from the “wet process” contains at least 19 different contaminants, among which are metal elements, that must be removed to obtain purified acid valuable in many applications. Nanofiltration of concentrated inorganic acids is an emerging application for NF that could be used for purification of raw phosphoric acid. Nevertheless, an in-depth study of transfer mechanisms is required in order to properly understand the observed retentions of metal species during NF of raw phosphoric acid and to be able to propose a better management of the process for further purification. NF mechanisms concerning metal ions in diluted aqueous solutions are extensively studied, but up to now limited literature data deal with concentrated acids. This paper is a contribution to a better understanding of transfer of low amount of metal species in concentrated phosphoric acid, the concentration of which can be as high as 5.9 mol L−1. The study focuses on the transfer of iron that is the main metal contaminant of the raw acid.