Homogenous photocatalysis using polyoxometalates (POMs) is one of the effective and efficient advanced oxidation processes (AOPs) for the degradation of a wide range of refractory organic pollutants of industrial wastewater at comparable rates to TiO2 heterogeneous photocatalysis. However, the main disadvantage of POM homogeneous photocatalysts is the separation and recycle of them due to their complete solubility in reactant solution (indeed the molecular size of the photocatalyst can be comparable to that of the pollutants). Such issue makes POM homogeneous photocatalysis unsuitable for any kind of environmental applications, thus limiting the real applications of POMs homogeneous photocatalysts in the field of industrial wastewater treatment. The current project aims to address this issue as ‘a big challenge in the literature’. Therefore, a novel approach to separate and recycle homogeneous photocatalyst with reactant solution using a cross-flow photocatalytic membrane reactor (PMR) for the treatment of industrial wastewater was proposed. The performance of homogenous photocatalyst was compared with benchmark heterogeneous photocatalyst under batch and continuous modes of operation and assessed based on the evaluating parameters (percentage primary degradation (%PD), reaction kinetics (based on a pseudo-first order reaction constant-Kapp) and mineralization (percentage total organic carbon (%TOC) removal). H3PW12O40 (denoted as POM) was the homogeneous photocatalyst and TiO2 was the benchmark heterogeneous photocatalyst. PEG1500 (denoted as PEG) was a selected polymer model of synthetic industrial wastewater. The experimental results of this work showed that:Firstly, a dead-end membrane filtration process was used to examine the ability of membrane (NF270) to reject either POM or TiO2 with PEG separately and together as feed (no UV). The used membrane could completely separate each of them from PEG reactant solution. These are very interesting results led to be used as a successful choice in the proposed cross-flow PMR approach.Secondly, the effect of operating parameters including loading (mM), pH and conventional oxidant (mgO2L-1) on POM homogeneous photocatalysis (no membrane) for the treatment of PEG in a continuously recirculating annular photoreactor was examined using a central-composite experimental design (CCED) and neural network (NN) after several control experiments to confirm the suitable range for these investigated parameters. The optimal conditions were theoretically predicted, POM loading (0.35 mM), pH (3.3) and oxidant (14 mgO2L-1), using CCED and NN and then experimentally confirmed. NN as a model fitting allowed to determine the interaction effect of these parameters in terms of saliency analysis to be in the following order: pH > loading > oxidant. In general, pH and oxidant operating parameters showed a negative impact on the %PD of PEG. So, they were not controlled under the investigation of POM homogeneous PMR. Thirdly, in terms of the concept ‘membrane enhanced photocatalysis’ is feasible using batch PMR operation under each POM, TiO2 and combined (POM-TiO2) photocatalyst. The used membrane could successfully separate and recycle these photocatalysts with complete rejection. For POM and combined photocatalysts, this concept is feasible due to the ability of membrane to concentrate the concentrations of them in the retentate (and then to the photoreactor) to that of an optimal loading (control photocatalysis-no membrane), and then increasing the %PD of PEG and Kapp. While for TiO2, this concept is not feasible because the tendency of TiO2 particles to be adsorbed onto the membrane surface increased with increasing the operating time, thus reducing the concentration in the photoreactor and then decreasing the overall %PD and Kapp. For the membrane enhanced photocatalytic mineralization of PEG (%TOC removal) under the above investigated conditions, this concept is not feasible. This is due to the formation of a broad range of products including identified and unidentified intermediates, polymeric fractions and oxidized PEG oligomers. These products expressed by TOC concentration could not considerably pass through the membrane because of the effect of a ‘secondary dynamic membrane’, thus increasing the TOC concentration in the retentate. In addition, the identified intermediates (malonic, glycolic, formaldehyde, formic, acetic and propionic acids) are so resistant to total mineralization as reported in literature. Fourthly, in terms of continuous PMR operation, it could successfully convert batch photocatalysis (control process-no membrane) to continuous photocatalysis under the optimal loading of POM (0.75 gL-1), TiO2 (0.25 gL-1) and combined (POM-TiO2) photocatalysts of Set 2 (POM-0.125 gL-1 and TiO2-0.25 gL-1) for the end course of operation, 9 h (POM or TiO2) and 12 h (combined photocatalysts) with complete rejection, high photocatalytic efficiency in the degradation of PEG and continuous ability to promote good membrane flux. However, the membrane could not enhance photocatalytic mineralization of PEG as similar to batch PMR operation.Overall, the proposed PMR approach under batch and continuous operations for the treatment of PEG confirms its ability for the complete separation and recycle of POM homogeneous photocatalyst under different investigated conditions. This successful achievement will make homogeneous photocatalysis using various types of POMs to be a suitable method for environmental applications and the starting point for the real applications of POMs in the industrial wastewater treatment field. Moreover, this approach has considerable advantages such as an environmentally friendly, economically feasible process that works under mild conditions (ambient temperature and pressure, and atmospheric oxygen is used as oxidant) and an effective process for the sustainable photocatalytic degradation of PEG that can be used as a pretreatment step before using conventional biological treatment systems. These results will be useful to facilitate easy benchmarking to real industrial wastewater process scale-up containing soluble polymers with similar properties.
|Date of Award
|31 May 2019
|Ministry of Higher Education and Scientific Research - Iraq
|Emma Emanuelsson Patterson (Supervisor) & Tom Arnot (Supervisor)