Titanium dioxide photocatalysis: A candidate for advanced wastewater treatment technology

Rao, N. N.; Mohanty, Sagarika; Khare, Pradnya; Dhodapkar, Rita S.; Chaturvedi, Vibha; Kaul, S. N.; Devotta, S.
May 2006
Chemical Business;May2006, Vol. 20 Issue 5, p31
Trade Publication
The photocatalytic (TiO2) method of treatment for the removal of COD & induction of biodegradation in case of nonbiodegradable compounds & color removal in the case of dyes & dyestuff intermediates has been examined and results are reported in this paper. The kinetics of photocatalytic (TiO2/UV) degradation of some chlorophenols, characterization of intermediates and induction of biodegradability in treated chlorophenol solutions is reported. Approximately, >95% COD exerted by cholorophenols is removed in approximately 1h at pH 5 and 0.2g TiO2/L when their concentration is ≤100 mg/L. The pseudo first order rate constant (k) is estimated to be in the range 0.0183-0.033 min-1. The biochemical oxygen demand (BOD) of treated chlorophenol solutions improved substantially. We also attempted to couple photocatalytic pretreatment and conventional activated sludge process with the objective of achieving enhanced biodegradation of H-acid. Photodegradation experiments were carried out at an optimized TiO2 dose i.e. 1g TiO2/L. The photodegradation of H-acid follows typical first-order kinetics in its concentration, in the range 50-150 mg/L (k varies from 0.0784 to 0.2055 h-1). The HPLC data showed that an intermediate builds up from the beginning and persists even in 5th hour sample; however, the parent molecule H-acid disappeared. Subsequently, it was attempted to biodegrade untreated and pretreated H-acid using activated sludge from textile industry acclimatized to H-acid. It was found that a photocatalytic treatment of H-acid for 30 min followed by biodegradation resulted in degradation of H-acid by 89-90%. Further, the photocatalytic colour removal from water containing two commercial dyes was examined under sunlight. Three types of reactors, continuously stirred batch reactor (CSBR), solar slurry reactor (SSR) and an open-trough reactor (OTR)were used. About 90% colour removal and 65-84% reduction in COD was observed during 3-4h of exposure to sunlight at an initial dye concentration of 50 mg/L of RO4 & RB5 and 0.2g TiO2/L in suspensions. In SSR, about 98% reduction in colour of dye-bath wastewater (DBWW) was observed while COD removal was 8%. In experiments using immobilized TiO2 (TiO2/PE) and open trough reactor, 99% and 97% colour removal was observed for RO 4 and RB 5 respectively in 5h. However, the COD removal was low, 19 and 7.2% respectively with RO4 and RB5. Dye bath wastewater was also treated in OTR; 97%, reduction in colour and 8% reduction in COD were achieved in 5h. Increase in biodegradability of wastewater was also observed after solar photocatalytic treatment. Finally, the energy efficiency of photooxidation & photodecolorization systems is estimated and discussed.


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