Vacuum-UV photocatalytic oxidation for the deep treatment of industrial wastewater containing recalcitrant organic pollutants

Abstract

Under current’s wastewater regulations, the maximum allowable discharge concentrations of many organic pollutants are in ppm levels. Due to the low but still exceeding concentration and the presence of biorecalcitrant materials, most of the conventional physicochemical and biological methods are ineffective in treating industrial wastewater. Researchers are now endeavoring to develop effective treatment processes. This thesis originates from a joint project between HKU and Sinopec to develop photocatalytic oxidation technologies for the deep treatment of industrial wastewater containing recalcitrant organic pollutants. First, novel urchin-like microparticles that have photocatalytic activity and are magnetically separable were prepared using a layer-by-layer assembly process. Photocatalytic degradation was investigated using two types of mercury vapor lamps: an ozone generating lamp emitting at both 254 nm and 185 nm, and a germicidal lamp emitting at 254 nm only. These novel photocatalysts demonstrated superior photocatalytic activity in the mineralization of phenol under UVC illumination compared with the commercial P25, Degussa TiO2, especially with repeated usage. Importantly, they can be quickly separated for reuse by simply using a magnet. The merits of these 3D spiny nanostructured TiO2 microparticle photocatalysts include their high specific surface area, good permeability, reduced charge recombination rate and high catalytic activity, while the incorporation of magnetically separable property enables the rapid and easy retrieval of the suspended photocatalysts after use. Next, three different industrial wastewater samples – acrylic fiber manufacturing wastewater, reverse osmosis concentrate and petrochemical wastewater – were obtained from Sinopec and tested with various UV-based advanced oxidation processes (AOPs) including UV+TiO2, UV+Persulfate, UV+Hydrogen peroxide or their combinations. The total organic carbon (TOC) and chemical oxygen demand (COD) of the treated wastewaters were analyzed to evaluate the effectiveness of the treatments. These industrial wastewaters are laden with non-biodegradable organic matters and the effectiveness of existing treatment technology is far from satisfactory. This study explored the practical possibility of applying these UV based AOPs to treat these wastewaters. For the most challenging acrylic fiber manufacturing wastewater, an in-depth analysis of optimizing the dosages of the oxidants was carried out. In these tests, the ozone generating lamp with vacuum-UV output was adopted as the light source. Furthermore, a photocatalytic reactor using an ozone generating mercury vapor lamp with the capability to simultaneously utilize the ozone internally generated from the lamp was fabricated. The reactor had a microbubble generating mechanism to facilitate the dissolution of ozone and oxygen in the reaction liquid. Photocatalytic and photolytic degradation were investigated using both the ozone generating lamp as well as the germicidal lamp. The roles of ozone, 254 nm light and 185 nm light in the degradation of methyl orange were investigated. In particular, the mineralization of methyl orange through different processes – namely pure ozonation, photolysis under an ozone generating lamp with or without the assistance of ozone, photolysis under a germicidal lamp, TiO2 photocatalysis under an ozone generating lamp with or without the assistance of ozone and TiO2 photocatalysis under a germicidal lamp – was analyzed with the COD and UV-visible absorption measurements.