Background: The current article is about the water treatment in which colored water contaminated
by methyl orange has been used for adsorption assisted photocatalysis. Coupling of photocatalysis with
the traditional water treatment processes has been in practice since last couple of years for the improvement
of degradation efficiencies, for example, photocatalysis coupled with ultrafilteration, adsorption, flocculation,
biological methods, photolysis, membrane distillation, etc. Among all these coupling approaches, adsorption
assisted photocatalysis being a very simple and highly efficient approach is suffering from few drawbacks on
the account of high cost, low stability and surface area of the adsorbent support. The present study is a contribution
towards improvement in this coupling approach. A low cost, highly stable spinel magnesium aluminate
(MgAl2O4) material synthesized at nanoscale is used for composite formation with antimony sulphide
(Sb2S3) material having high absorption coefficient in the visible light of solar spectrum. A review of recent
patents shows that the field of photoctalysis is dominated by the traditional TiO2 catalyst. The modification of
TiO2 by either composite formation or by doping is the main focus.
Methods: Coprecipitation method is used for the synthesis of spinel in which the desired precursors in the
respective molar ratios were mixed and annealing of the resulting precipitates was carried out at 800oC for 8 h.
Sb2S3 was synthesized by the hydrothermal method in which the required molar solution of precursors was
mixed with urea solution and the whole mixture was maintained at 105oC for 6 hrs in a Teflon lined autoclave.
The resulting suspension was then annealed at 37oC for 3 hours. The composite of Sb2S3 and MgAl2O4
has been synthesized by mixing both the materials in 1:1 and heat treated in an oven at a temperature of 200oC.
Results: Peaks in X-ray diffraction pattern correspond to both the Sb2S3 and spinel phase. All the peaks corresponding
to the Sb2S3 and spinel phase were found to be shifted to higher d-spacing values. This indicates
the expansion of unit cells of the Sb2S3 and MgAl2O4 phases. Thermal studies show that only 3% weight loss
is observed at a temperature of 200-1000oC which may be due to the loss of surface water from the sample.
Surface area, pore volume and pore size obtained from N2 adsorption were 143m2/g, 0.21cc/g and 23.26Å,
respectively. The removal efficiency of 0.1g catalyst for methyl orange solution of 5mg/L concentration after
reaction in dark conditions for the time of one hour was calculated to be 24% owing to the adsorption. The
visible light degradation efficiency of the 0.1g catalyst for 1, 5, 19, 25 and 50 mg/L concentrations of MO solutions
were 97, 93, 75, 72 and 62% respectively. The dosage of the catalyst was found to have a direct relationship
with the degradation efficiency. Lower pH was found suitable for the degradation owing to better
interaction of catalyst surface and the adsorbed dye. Percent degradation increased with the increase in the
time and temperature of reaction. The degradation kinetics followed pseudo first order rate equation; the calculated
value of rate constant was 0.0102 min-1.
Conclusion: The mechanism involves the excitation of electrons in the valence band of Sb2S3 to the conduction
band by the absorption of visible and UV light. The electrons and holes participate in the surface
reactions resulting in the formation of superoxide and hydroxyl radicals which degrade the targeted polluted.
Lower concentration of MO solutions, acidic pH, higher catalyst dosage and greater reaction times
were found suitable for the degradation efficiency.