Hybrid membrane VIS-LED photoreactor for simultaneous pharmaceuticals degradation and photocatalysts separation
Date of Issue2013
School of Civil and Environmental Engineering
Nanyang Environment and Water Research Institute
In recent years, TiO2–assisted photocatalysis has become a potentially cost–effective and environmentally–sustainable treatment technology for water reclamation and reuse. However, TiO2 could be photoexcited only under UV irradiation due to its large band gap which hampers its commercial–scale application. In addition, the key challenge for applying the TiO2 suspension system is that the TiO2 particles have to be separated from the product water. This study therefore focused on (1) developing visible–light responsive TiO2–based photocatalysts for photocatalytic degradation (PCD) of pharmaceuticals under visible–light irradiation, and (2) developing hybrid membrane photoreactor (MPR) systems for photocatalyst recovery and reuse in the photoreactor. The first phase of this study was the synthesis of C–sensitized and N–doped TiO2 (C/N–TiO2) for PCD of sulfanilamide (SNM) under visible–light–emitting diode (vis–LED) irradiation. It was found that the C/N–TiO2 calcined at 300 °C (T300) exhibited the highest photocatalytic activity for SNM degradation, due to the presence of optimal content of carbonaceous species (serve as photosensitizer) and the nitrogen doping (lead to the remarkable red shift of absorption edge). The PCD of SNM was more efficient in acidic conditions due to the carbon photosensitizing effect. Bicarbonate, phosphate and silica could inhibit the SNM mineralization to different degrees. T300 exhibited good photochemical stability and its absorption onset could be extended to ca. 600 nm. In order to harness visible light and solar light more efficiently, another novel C–N–S tridoped TiO2 was synthesized for PCD of tetracycline (TC) under visible–light and simulated solar irradiations in the second phase of this study. It was suggested that the photocatalyst with thiourea : Ti molar ratio of 0.05:1 and calcined at 450 °C (T0.05–450) exhibited the highest PCD efficiency of TC. This could be attributed to the synergistic effects of TC adsorption on T0.05–450, band–gap narrowing resulting from C–N–S tridoping, presence of carbonaceous species serving as photosensitizer, and the well–formed anatase phase. The slightly alkaline pH condition was more favorable for the PCD of TC. TC degradation was more efficient under solar irradiation. The photocatalytic activity of T0.05–450 was compared with that of T300 for the PCD of SNM and TC under visible–light irradiation. It was found that T0.05–450 exhibited higher visible–light photocatalytic activity. The third phase of this study was the development of a hybrid MPR system combining a separate vis–LED photoreactor and a cross–flow flat sheet PVDF microfiltration (MF) (mean pore size of 0.22 µm) membrane module. It was investigated for the simultaneous separation of T300 and T0.05–450 and degradation of penicillin G (PG) in both closed–loop and continuous flow–through modes. The effective separation of TiO2 particles by the MF membrane was realized. It is confirmed that the MPR system operated at a higher transmembrane pressure (TMP) or lower cross–flow velocity (CFV) in the closed–loop (recirculating) mode was more prone to induce TiO2 deposition on the membrane surface without backwashing, which resulted in the membrane fouling, the loss of TiO2 from the photoreactor and the decrease of PCD efficiency of PG. With backwashing of the membrane, the PCD efficiencies of PG could be significantly enhanced, which were almost comparable to those achieved in the batch photoreactor. The MPR system as mentioned above requires a large footprint which might not be suitable to install in a land–scare urban setting. Therefore, a more compact MPR system, namely submerged membrane photoreactor (sMPR) system, was developed in the last phase of this study. It was fabricated with a hollow fiber PVDF MF (mean pore size of 0.1 µm) membrane module submerged into a vis–LED photoreactor, and was operated in a continuous flow–through mode. The MF membrane realized effective separation of T0.05–450. A steady–state TMP was achieved after the deposition of a thin TiO2 cake layer on the membrane surface. An alkaline pH condition was favorable for the PCD of carbamazepine (CBZ) using T0.05–450 as photocatalyst. The inhibitory effects of inorganic anions on the PCD of CBZ followed the order of silica > phosphate > nitrate > bicarbonate > sulfate > chloride. The net effect of humic acid (HA) on the CBZ degradation depended on the three competitive phenomena: (i) HA as an electron shuttle promoting the generation of •O2– while reducing the electron–hole recombination, (ii) HA competing with CBZ for the active sites of the photocatalyst, and (iii) HA contributing to the light attenuation in the photoreactor. Overall, one of the major contributions arising from this study is the preparation of visible–light responsive TiO2 for the degradation of pharmaceuticals. The findings imply that the as–prepared photocatalysts could be also applied in solar photocatalysis for water/wastewater treatment. Another major contribution is the development of vis–LED irradiated hybrid MPR systems for the simultaneous pharmaceuticals degradation and TiO2 separation. These hybrid systems show the scientific merit and practical significance for treating real waters using visible–light photocatalysis and low–pressure membrane process.
DRNTU::Engineering::Environmental engineering::Water treatment