Incidence of pretreatment by potassium permanganate on hazardous laboratory wastes photodegradability

Abstract

In this paper we present the results of the application of conventional and photocatalytic methods for the treatment of hazardous laboratory wastes. Liquid wastes from the spectrophotometric determination of NH+4 as indophenol have been chosen as model samples. The waste composition is rather heterogeneous, as phenol, ethanol, nitroprusside and citrate can be found, among others. High organic carbon contents (thousands of ppm) are found, part of which is volatile organic carbon (VOC) up to 68%. Three different photocatalytic methods have been tested: TiO2-photocatalysis, photoFenton reaction and the combination of H2O2-UV light. The degradation process was monitored by Total Organic Carbon (TOC) reduction. The effect of experimental parameters, such as pH and TiO2, H2O2 and Fe2+ concentrations, has been investigated. The strongest TOC reduction was achieved employing the photoFenton reaction with 2.5 mM Fe2+ and 8 mM H2O2. The effect of solar TiO2-photocatalysis and the photoFenton reaction was also studied. TOC reductions of 76.6 and 46.8% were achieved by the photoFenton reaction and TiO2-photocatalysis, respectively. The mineralization of individual components of the samples: nitroprusside, phenol and citrate (1000 ppm of C) was also attempted by KMnO4 and the three photomethods. Nitroprusside was the most resistant component to be mineralized. PhotoFenton and the combination of H2O2–UV light gave the best results for nitroprusside degradation. Due to the high TOC content of the samples, an oxidative pretreatment with KMnO4 before the photocatalytic treatment was also investigated. KMnO4 pretreatment gave reductions of initial TOC as high as 90% and VOC reductions from 30 to 77%. The remaining solution must be treated with KMnO4 if the photocatalytic methods are to be applied later. If the sample is not previously treated with KMnO4, not one of the photomethods tested is able to reduce the remaining non-purgeable (NPOC) organic carbon of the sample. This suggests that KMnO4 oxidizes the organic pollutants to other organics compounds that can be more readily mineralized by photodegradation. The recommended method for the treatment of these wastes consists of a pretreatment with KMnO4, followed by the photocatalytic treatment by means of photoFenton reaction.