Thin-film nanocomposite membrane comprised of a novel phosphonic acid derivative of titanium dioxide for efficient boron removal

The separation of toxic boron from seawater is the main concern for the desalination industries. The membrane-based boron separation is challenging due to the existence of boron in the neutral boric acid form. According to World Health Organization, a Boron above 0.5 ppm is not recommended for human health and agricultural use. The design of NF membranes for the Boron separation is tricky due to its neutral charge. In this study, a novel modification of titanium dioxide (TiO2) nanoparticles is performed by reacting with the cyclic propyl phosphonic anhydride (T3P) reagent in the presence of triethylamine as a base to obtain the phosphonic acid derivative of TiO2 (PTiO2). The PTiO2 was used as inorganic filler during the coating of the polyamide layer on the ultrafiltration support. Better dispersion of the nanoparticles was achieved due to the effective interactions between the -POOH (of PTiO2) and -NH2 (of meta-phenylene diamine groups) as evidenced by the ATR-IR spectrum. The resultant nanocomposite membranes showed a high potential to reject scalant divalent ions (>98%) and sodium chloride (>60%) with improved permeation characteristics. The boron adsorption study revealed a strong adsorption of boric acid ~12 mg/g by the PTiO2 nanoparticles causing the pore narrowing effect. Due to the same reason, the boron diffusion coefficient of the membranes reduced at neutral pH. The boron rejection offered by the membrane with 0.1 wt% PTiO2 was ~2.5 times higher than the commercial NF90 membrane.