Effect of phosphoric and hydrofluoric acid on the structure and permeation of a nanofiltration membrane

The changes have been analyzed by studying the permeability of industrial phosphoric acid aqueous solutions through a nanofiltration membrane caused by treatment with phosphoric and hydrofluoric acid solutions. The retention of phosphoric acid together with its cationic impurities were also analyzed and attributed to a complex process of active layer modification. This process was studied in terms of both pore size and surface modification, as confirmed by retention tests with known size neutral molecules and scanning probe microscopy. The very relevant changes in hydrophilic character of the membrane material were studied by using contact angle measurements. When the membrane was treated with HF solutions, the pore size remained relatively constant, while the hydrophilic character increased. Because of these changes, increases in industrial phosphoric acid permeability and in the rejection of impurities were observed. In the case of the treatment with H3PO4, the membrane also presented a significant increase in industrial phosphoric acid permeability. However, the rejections of the cationic impurities decreased. These facts are due to an increase in the pore size and in the hydrophobic character of the membrane.