TiO2-photocatalyzed degradation of phenol in saline media: lumped kinetics, intermediates, and acute toxicity

Abstract

The photocatalytic degradation of phenol in aqueous suspensions of TiO2 under different salt concentrations has been investigated. In all cases, complete removal of phenol and mineralization degrees above 90% were achieved. However, the time needed to effectively remove phenol and degrade organics was considerably increased in high salinity medium. The photodegradation of the organics followed a Langmuir–Hinshelwood–Hougen–Watson lumped kinetics, showing rate and adsorption/desorption constants that decrease as salinity increases. From GC/MS analyses, various hydroxylated intermediates formed during oxidation have been identified. The main ones were catechol, hydroquinone, and 4,4′-dihydroxybiphenyl, in this order. It was shown that salinity inhibited catechol formation and promoted 4,4′-dihydroxybiphenyl production. The formation of negligible concentrations of 4-chlorophenol was observed only in high salinity medium. Acute toxicity during degradation process was determined by using Artemia sp. as the test organism, which is suited for saline environments. Intermediate products were all less toxic than phenol and a significant removal of the overall toxicity was accomplished, regardless of the salt concentration.Photocatalytic phenol degradation in the absence of salt and in low salinity media (2gL−1) was quite the same (100% for a 150min irradiation time). However, high salinity media (50gL−1) showed a significant difference (81%). Organic matter mineralization showed a similar behavior regarding the media (94 and 44%, respectively).