Enhanced retention of neutral solute and charged solute with NF inorganic membrane by chemical grafting and physico-chemical treatment

Abstract

Abstract New inorganic nanofiltration membrane was obtained by covalent grafting of organo titanate agents on zirconia layer followed by hydrolysis. Grafting of zirconia by titanates with pyrophosphate groups (PP) was shown by the decrease of isoelectric point to about 5 of zirconia particles of same process history as the membranes. In order to further reduce the pore size of the grafted membrane, a pre-treatment step based on the ability of the grafted PP groups to give highly stable complexes with heavy metals is added. Accordingly, a solution of ferric ions (10 mM, pH 2) was selected as preconditioning step of the PP membrane. The reduction of membrane permeability by a factor of about 2 was a good sensor of the in situ formation of the strong complex between Fe+++ and likely two PP groups acting as chelating ligands then permeability remained stable at upper pH. Retention of 1 g l−1 (34 g l−1) sucrose with inorganic NF membrane was 32% (40%) and reached 39% (55%) with the zirconia chemically grafted with PP groups. Retention of sucrose with the Fe pre-treated PP membrane was shifted to 69% (1 g l−1) and 79% (34 g l−1), respectively. Vitamin B1 (thiamine, 265 g mol−1) was selected as positively charged solute of valence 2 at pH 3.5 and valence 1 at pH 6. Retention of monovalent thiamine was lower than that of divalent thiamine with the same membrane and pre-treatment. An enhanced retention of about 25% was achieved when using the PP membrane with the pre-treatment by Fe3+ solution: retention of divalent thiamine was then within 98% (1 g l−1; ionic strength, 14 mM) and 88% (1 g l−1+100 mM NaCl). In conclusion, combination of chemical modification (grafting of chelating ligands) followed by a stable physico-chemical pre-treatment (complex formation) permits to strongly reduce (−50%) pore size of inorganic NF membranes. Furthermore, very high retention of charged solutes was reached with an additional effect of electrostatic exclusion between the charged membrane and the solute.