Reusable adsorbents for dilute solution separation. 6. Batch and continuous reactors for the adsorption and degradation of 1,2-dichlorobenzene from dilute wastewater streams using titania as a photocatalyst

Abstract

Two types of external lamp reactors were investigated for the titania catalyzed photodegradation of 1,2-dichlorobenzene (DCB) from a dilute water stream. The first one was a batch mixed slurry reactor and the second one was a semi-batch reactor with continuous feed recycle with titania immobilized on inert supports (quartz and low density polyethylene, LDPE). The batch reactor was used to study the intermediates and reaction kinetics of DCB degradation. Four intermediate products were observed in the degradation of DCB; these being 2-chlorophenol (CP), 2,3-dichlorophenol (DCP), 1,2-dihydroxybenzene (catechol) and, o-benzoquinone. A mechanism based on these observations is proposed. Increasing the pH of the solution increased the intial rate of photodegradation of DCB. Added oxidant (hydrogen peroxide) did not have any appreciable effect on the degradation of DCB. The Langmuir–Hinshelwood kinetic parameters for DCB, DCP and CP were obtained. The steady state removals and apparent rate constants were obtained for the plug-flow reactor with different supports and compared under similar conditions. The titania supported on LDPE showed a better rate of photocatalysis than titania supported on quartz, although the titania film thickness on LDPE was five times lower than on quartz. The modification of titania surface by adsorption of a non-photodegradable polyfluorinated surfactant vastly improved the rate of DCB degradation on both LDPE and quartz. The rate of photodegradation in the immobilized tubular reactor was mass transfer controlled for the flow regimes investigated, viz., Reynolds numbers, Re<550. The steady state removal was directly proportional to the radiant flux within the range 4–16 mW cm−2.