Integration of photocatalytic and biological processes for treatment of biorecalcitrant pharmaceuticals

Abstract

The present study facilitated exploring avenues for examining effective solutions to treat pharmaceutical model compounds and simulated effluent. The research primarily focussed on the degradation of pharmaceutical model compounds such as Aspirin, Ibuprofen, Ofloxacin and Atenolol by heterogeneous photocatalysis using various metal doped photocatalyst and its efficacy was compared with P25 TiO2 under optimized conditions. Fe-TiO2 (0.5wt %) photocatalyst was synthesized by solgel method and its efficiency was evaluated in the degradation of the Aspirin (25 ppm).The Brunauer–Emmett–Teller (BET) surface area of Fe–TiO2 was found to be 72 m2/g and it has been observed that Fe-TiO2 resulted in higher degradation (96%) when compared to P25 TiO2 (72%) after 6 h of solar irradiations.Higher activity of Fe doped catalyst was attributed to its higher surface areabecause TiO2 crystallite grain sizes decreased with the increase of Fe contents and the diffuse reflectance spectra of Fe-doped TiO2 nanoparticles displayed a red shift in the band gap transition and the absorbing band edge moved to visible range. Similarly, photocatalytic degradation of Ibuprofen (IBP) was carried out under solar irradiation (30-35W/m2) using two different catalyst viz. Bi-TiO2 and Ni-TiO2synthesized by solgel method with dopant concentration varying from 0.25wt% to 1.0wt% and were characterized using XRD, SEM and UV-reflectance spectroscopy. The rate of solar induced photocatalytic degradation of IBP was observed to be in the order of Bi–TiO2> P25 TiO2> Ni–TiO2 with rate constants as 0.0064, 0.0046 and 0.0043 min-1, respectively. Degradation of 89% was achieved with 2g/L of (0.25wt %) Bi–TiO2 photocatalyst at pH 6 after 6 h of solar illuminations, whereas, 78% degradation was attained under similar experimental condition with (0.50wt %) Ni doped TiO2. Bi–TiO2 nanoparticles exhibited higher photocatalytic activity, due to reduction in band gap(2.99 ev) when compared to band gap of P25 TiO2 (3.2 ev). Further Bi–Ni co-doped TiO2 was synthesized using sol gel method and its efficacy was observed to be higher under solar light when compared to P25 TiO2 for the degradation of 25 ppm Ofloxacin (OFL). The BET surface area was found to be 74, 55 and 18.66 m2/g for 0.25, 0.5 and 1.0 wt% of Bi - Ni co-doped TiO2,respectively and the corresponding band-gap energies were found to be 2.89, 3.09 and 3.11 eV, respectively. Degradation efficiency of 86% was attained at pH 3 with 1.5 g/L of Bi-Ni co-doped catalyst after 6 h of solar irradiations which may be attributed to decrease in band gap of Bi–Ni co-doped TiO2. Possibility of employing Graphene oxide based composites was explored with TiO2 as well as ZnO, so TiO2-G & ZnO-G composites were synthesized by hydrothermal method and their important characteristics were determined. XRD data showed the highly crystalline nature of TiO2-G. By adding graphene to TiO2, the band gap noticeably decreased from 3.2 to 2.2 eV and surface area increased to 76m2/g. Photocatalytic degradation of Atenolol (ATL) was examined using graphene TiO2 (TiO2-G) compositesunder solar simulator(75mW/cm2). Results indicated that 72% degradation of ATL (25ppm) with 1.5 g/L TiO2-Gat pH 6 under solar irradiations in 1h, whereas 56%degradation was attained with P25 TiO2. Kinetic regime was also studied by varying the catalyst dose from 0.01- 2.0g/L and the degradation rate was found to be almost constant for catalyst loading between 0.01 and 0.08 g/L and thereafter, rate drops gradually as the catalyst loading was increased from 0.08 to 2.0 g/L. Complete TOC removal (94.5%) of Atenolol solution was obtained after 7h. Further, a four factor three level Box-Benkhen design (BBD) was employed to define the photocatalytic degradation of ATL in an aqueous media under slurry mode using TiO2-G and ZnO-G as photocatalyst. The four process variables considered were catalyst dose (0.5-2.0g/L), pH (4-9), ATL concentration (5-25ppm) and light intensity (25-100mW/cm2).The surface area of ZnO-G was found to be 84m2/g. The data obtained from run of 29 experiments suggested that maximum reaction rate of 0.783min-1 was achieved with 25ppm atenolol concentration within 1h of solar irradiation (100mW/cm2)at pH 6.5 when catalyst (ZnO-G) concentration was 1.25g/L, however reaction rate obtained with TiO2-G was 0.514min-1 under similar experimental conditions. So ZnO-G was found to be better photocatalyst when compared to TiO2-G in the degradation of ATL under solar irradiations. Further, immobilization of photocatalyst (TiO2-G)was carried out on Pyrex glass plate by dip coating method and optimization of process parameters such as catalyst concentration, pH, UV intensity and substrate concentration was done using BBD technique. Moreover, efficiency of TiO2-G was compared with P25 TiO2at optimized conditions for degradation of ATLunder immobilized mode which resulted in reaction rate of 0.0722 and 0.0497 min -1,respectively.Comparing the efficacy of TiO2-G in slurry/immobilized mode indicate that degradation efficacy was 35% more in case of slurry mode under similar experimental conditions,but, photocatalytic treatment under immobilized mode increase the industrial viability of process. Simulated effluent was subjected to independent photocatalytic, biological and thereafter, integrated photocatalytic-biological sequential treatment scheme to access its degradation efficacy in terms of BOD/COD. Biodegradability (BOD5/COD ratio) increased from 0.23 to 0.42 after 4 h of photocatalytic treatment (1.5g/L TiO2) with COD removal of 69.7% under solar simulator. In the independent biological treatment, the amount of sludge was varied as 2, 5, 10 and 15%, in order to optimize sludge concentration at three different temperatures of 20, 27 and 37°C. The maximum BOD and COD reduction of 46.3 and 46.2%, respectively was achieved with 5% activated sludge at 37°C for a time period of 48h at natural pH of 6.9 under continues aeration. By employing 4h of photocatalytic treatment using TiO2 (1.5g/L) followed by 48h of biological treatment (37°C, 5% Sludge concentration) resulted in 90.5 and 80.8% removal of COD and BOD, respectively. Thus, it is concluded from the present study that the sequential photocatalytic-biologicaltreatment was more effective in the degradation of simulated effluent when compared to independent photocatalytic and biological processes.