Photocatalytic Degradation of Pharmaceutical Compounds using Fixed-bed Reactors

Abstract

Pharmaceutical compounds lie within the category of emerging contaminants that have been extensively detected in the aquatic systems over the past few years. Advanced oxidation processes (AOPs) including TiO2 photocatalysis and photo-Fenton have achieved promising applications in context to the treatment of wastewater containing these types of emerging contaminants. However, each of these techniques possesses few drawbacks (frequent electron-hole recombination, increased treatment time, catalyst separation posttreatment, generation of iron sludge, high dosages of H2O2) which have restricted their successful implementation at field-scale. Thus, best efforts have been devoted in the present study to seek the commercial applications of AOPs by overcoming most of the concerns raised by these individual processes (photocatalysis or photo-Fenton). Attempts were made to integrate these two processes i.e. photocatalysis and photo-Fenton within the same treatment unit for the enhanced degradation of target recalcitrant pharmaceutical compounds i.e. Cephalexin (CEX) and Pentoxifylline (PEN). Most of the individual drawbacks of photocatalysis and photo- Fenton were effectively covered by this dual process (photocatalysis and photo-Fenton) owing to the production of hydroxyl radicals in abundance. Waste materials like foundry sand (FS) and fly ash (FA) were employed as alternative sources of iron which further defined the economic viability of our process. The systematic approach was followed in the present study to shift the dual process from slurry to fixed-bed mode for the degradation of CEX and PEN. After confirming the prominence of dual effect in slurry mode (Section 4.6), FS/FA was entrapped in small pockets made of fine cotton cloth while keeping TiO2 in suspensions. In the subsequent experiments, TiO2 was immobilized on clay beads which were used along with FS/FA pockets to bring the treatment unit completely in fixed-mode. Through these studies, significant improvement in the degradation efficiency of each compound was obtained in case of in-situ dual process resulting into the significant reduction in treatment time to almost half. Though, reasonably good amount of degradation was achieved even in fixed-bed studies incorporating dual effect, durability of the catalyst support always remains a concern. In either case, recyclability of fine cotton cloth pockets VI (entrapment material for iron source) was not up to the mark as they were recycled for only 8 times with significant reduction in the degradation efficiency of PEN (Section 4.7.1). Trials were executed to further improve the practicability of dual process by enhancing the durability and efficiency of support materials. Efforts were made to incorporate FS along with clay in order to prepare different supports for catalyst immobilization (Section 4.7.2). The support materials of different shapes (hollow circular discs, rectangular slabs and spherical beads) were prepared from clay and waste FS for TiO2 immobilization. Now both catalysts i.e. iron and TiO2 required for the dual effect study were incorporated within the same support material. Among these varying shaped catalyst immobilized supports, spherical beads depicted the best results in terms of degradation efficiency of CEX (80%) as well as catalyst recyclability (10 cycles). However, degradation efficiency continued to decrease with subsequent recycling tests. Actually, addition of FS along with clay reduced the binding efficiency of support. In the subsequent trials, the spherical beads were modified by adding FA along with FS in conjunction with clay (Section 4.7.6). Addition of FA provided sufficient compressive strength to the beads. These TiO2 immobilized beads containing FS and FA were optimized for the present study and were named as Fe-TiO2 composite beads. These Fe-TiO2 composite beads were used for carrying out the proper optimization of in-situ dual process for the degradation of CEX and PEN. The effect of several operating parameters including size of composite beads, catalyst dose, oxidant concentration, calcinations temperature and number of TiO2 coatings on the degradation efficiency of process was studied using Fe-TiO2 composite beads. This in-situ dual process using Fe-TiO2 composite beads revealed 93% degradation of CEX after 210 min and 85% degradation of PEN after 120 min of natural solar irradiations. The treatment time was reduced to less than half in case of in-situ dual process as compared to that of individual processes for the degradation of CEX and PEN. There was almost two to three fold increase in the value of rate constant, ‘k’ in case of in-situ dual process as compared to individual processes for the degradation of each compound. Moreover, overall synergy of in-situ dual process over individual processes was found to be more than 20% in case of both compounds. Fe-TiO2 composite beads were recycled for maximum number of times (> 45 recycles) as discussed in Section 4.7.9. The recyclability efficiency of composite beads was counter confirmed through different characterization techniques like SEM/EDS, XRD and DRS. VII The complete mineralization studies of each compound (CEX and PEN) were also conducted in terms of reduction in COD and TOC along with the generation of various anions like nitrite, nitrate and sulfate ions (Section 4.7.11). The intermediates formed during the degradation process of each compound were carefully identified through GC-MS analysis based on which degradation pathways were proposed in each case. Finally, the quality of the treated water in case of CEX and PEN was checked in terms of cyto-toxicity and acute toxicity analysis (Section 4.7.12). The photo-response efficiency of in-situ dual process using Fe-TiO2 composite beads was further enhanced by modifying the composite beads by replacing TiO2-P25 with doped TiO2. The coating of TiO2-P25 over the surface of beads was replaced with Ag-TiO2, N-TiO2 and N-Ag-TiO2 (Section 4.8 and Section 4.9). These modified catalysts were first comprehensively characterized through various techniques like SEM/EDS, XRD, DRS, FTIR, XPS, TGA and Raman spectroscopy before immobilizing them on the surface of beads. While after doped catalyst immobilization, the presence of iron along with catalysts and dopants was confirmed through SEM/EDS, XRD and DRS analysis. The in-situ dual process in conjunction with modified catalysts further enhanced the degradation efficiency of process resulting into the significant reduction in treatment time. The complete mechanism for this modified in-situ dual process using doped catalysts was also proposed for the degradation of pollutants. Scale-up studies were also conducted for the degradation of CEX and PEN through in-situ dual process using Fe-TiO2 composite beads (Section 4.10 and Section 4.11). Pilotscale solar reactors like re-circulation type fixed-bed glass reactor and once-through fixedbed cascade reactor incorporating in-situ dual effect were fabricated and comprehensively optimized for the degradation of target pollutants (CEX and PEN). The Fe-TiO2 composite beads demonstrated extended recyclability efficiency (> 70 recycles) even under flow conditions thus, highlighting the practical feasibility of in-situ dual process. The detailed cost of treatment process using these pilot-scale reactors was evaluated in each case to envisage the economic viability of technique. Finally, real effluent from a pharmaceutical industry was treated successfully through in-situ dual process using highly durable and inexpensive composite beads further highlights the field-scale applications of presented technique (Section 4.12). The process was optimized using Response surface methodology (RSM) based Box-Behnken design (BBD) model. An VIII almost 80% reduction in COD was achieved when the effluent was treated under solar irradiations at pilot-scale in a once-through cascade reactor at optimized conditions using Fe- TiO2 composite beads. The treated wastewater was completely non-toxic as confirmed through cyto-toxicity and acute toxicity analysis. To the best of our knowledge, this is the first reported study dealing with fixed-bed insitu dual process using waste materials along with successful scale-up trails for the treatment of pharmaceutical wastewater.