Nowadays,
fluoride contamination of drinking water is a major problem
for various countries, because high concentrations of fluoride pose
a risk of dental and skeletal fluorosis. Over past years, membrane
nanofiltration (NF) has been proposed as convenient defluoridation
technology. However, NF cannot be applied to water systems with high
fluoride concentration, and the disposal of the membrane concentrate
remains an issue. In this work, we compared a commercial polyester
NF membrane and a polypropylene hollow-fiber membrane distillation
(MD) module for their ability to remove fluoride ions from water in
the presence of hardness ions and organic fouling agents. The NF membrane
can offer more than 10 times higher water productivity than MD, under
realistic gradients of temperature and pressure, respectively. Despite
that, after reaching a concentration factor of about 3, fouling and
scaling caused the flux to drop to about 80% with respect to its initial
value. Moreover, F– retention decreased from 90%
to below 80%, thus providing a permeate of scarce quality. MD was
operated in the direct-contact mode on a polypropylene hollow-fiber
membrane, which was charged with a hot feed flow (average T = 58 °C) on one side and a cooled (20 °C) permeate
flow of distilled water on the other side. The concentration of fluoride
ions in the permeate was always below the detection limit of our electrode
(0.2 ppm), regardless of the fluoride concentration in the feed. Moreover,
the MD module showed higher resistance to fouling and scaling than
NF, and CaF2 crystals were recovered from the MD concentrate
after cooling. These results suggest that the synergic combination
of the two techniques might be beneficial for the purification of
fluoride-contaminated water systems: MD can be used to further concentrate
the NF retentate, thus producing high-purity water and recovering
CaF2 crystals.
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