  Methylene blue (MB) is a representative of a class of dyestuffs resistant to
biodegradation. This paper presents a novel photocatalytic degradation of
methylene blue (MB) by La modified M-type strontium hexaferrite
(La0.2Sr0.7Fe12O19) compound, which is a traditional permanent magnet and
displays a large magnetic hysteresis (M-H) loop. The remnant magnetic moment
and coercive field are determined to be 52emu/g and 5876Oe, respectively.
UV-Visible optical spectroscopy reveals that La0.2Sr0.7Fe12O19 is
simultaneously a semiconductor, whose direct and indirect band gap energies are
determined to be 1.47 eV and 0.88 eV, respectively. The near infrared band gap
makes it a good candidate to harvest sunlight for photocatalytic reaction or
solar cell devices. This magnetic compound demonstrates excellent
photocatalytic activity on degradation of MB under visible illumination. The
colour of MB dispersion solution changes from deep blue to pale white and the
absorbance decreases rapidly from 1.8 down to zero when the illumination
duration extends to 6 hours. Five absorption bands did not make any blue shifts
along with the reaction time, suggesting a one-stepwise degradation process of
MB, which makes La0.2Sr0.7Fe12O19 a unique magnetic catalyst and differs from
TiO2 and other conventional catalysts.
