Selective Separation Behavior of Nanofiltration Membranes for Phosphorus-Fluoride-Containing Acidic Wastewater

During the production of wet-process phosphoric acid, large quantities of acidic wastewater rich in phosphorus, fluoride and other components are inevitably generated. Selective separation and efficient enrichment of phosphorus and fluoride from such wastewater using appropriate technologies are conducive to resource recovery and recycling, thereby contributing to the sustainable development of the wet-process phosphoric acid industry. In this study, four commercial nanofiltration membranes with different molecular weight cutoffs were selected to treat acidic water from a phosphorus-chemical plant in Guizhou Province, China. The results show that metal ion rejection is governed not only by the membrane pore structure but also by the speciation of the ions in the acidic water system, whereas anion rejection is determined jointly by size sieving and ionic speciation. Among the four membranes, NF7 possesses the smoothest surface, the best hydrophilicity, a positive charge under the experimental condition, and a relatively broad pore size distribution. These characteristics enable NF7 to exhibit the highest pure water flux (62 L·m−2·h−1) and maintain >90% rejection of Fe3+ and Mg2+ while allowing >70% permeation of phosphorus and fluoride, demonstrating considerable potential for the purification of phosphorus-fluoride-containing acidic wastewater generated during wet-process phosphoric acid production.