Photodegradation of Tetracycline and Trimethoprim in Wastewater using Sensitized Graphite Carbon Nitride Zinc Ferrite Composite

Abstract

The presence of emerging contaminants such as antibiotics in wastewaters pose a great challenge in water reuse. Multicomponent photocatalysts such as ZnFe2O4-g-C3N4 (ZF GCN) are a promising alternative for complete mineralization of antibiotics at mild conditions. The effect of dye-sensitizers in improving the performance of photocatalysts is not adequately assessed and limited knowledge exists on the removal of antibiotics in real wastewater matrices. The main objective of this study was to evaluate the degradation of trimethoprim and tetracycline using Eosin Y sensitized ZF GCN in a solar parabolic trough reactor. The specific objectives were to: synthesize and characterize the physical and chemical properties of ZF-GCN photocatalyst; determine optimum parameters for degradation of synthetic water containing antibiotics; and evaluate the photodegradation of antibiotics in real wastewater (RWW) and assess the effect of competing ions. The study synthesized four composites of ZF-GCN at 0, 5, 15 and 25wt% loading of the ZnFe2O4 using a three-step procedure as follows: direct annealing of melamine to yield bulk g-C3N4 which was thermally and ultrasonic exfoliated to yield g-C3N4 nanosheets; a hydrothermal-precipitation reaction of g-C3N4, FeCl3.6H2O, ZnCl2.2H2O and ammonia to yield ZF-GCN; dye sensitization of resultant composite with Eosin Y dye using a mixing-adsorption method. The photocatalysts were characterized using: X-Ray diffraction (XRD), scanning electron microscope energy dispersive spectroscopy (SEM-EDX), UV-vis analysis and Pl-spectra. The composites exhibited the structural and crystalline phases of the pure photocatalysts on XRD and FTIR studies. Furthermore, the composites displayed a reduction in bandgap and suppressed charge recombination in the UV-vis and PL-spectra, respectively. This was evidence for successful creation of heterojunction. The following variables were investigated: pollutant dosage (10-25mg/l), catalyst dose (0.4-1.2g/l) and pH (4-10). The effect of competing ions was studied at various salt concentrations (100, 300, 500mg/l) in real wastewater. The central composite design was used in generating two quadratic polynomial models for degradation and TOC removal as the responses. The predicted R2 values for both models was >0.95, which signified their high accuracy in predicting the responses. The analysis of variance results revealed that all the factors were highly significant in affecting degradation and TOC removal for tetracycline while for trimethoprim, only pollutant dose and catalyst dose were the most significant factors. The optimum parameters for trimethoprim was (pollutant dose=10mg/l, pH = 7.19, catalyst = 0.72g/l, Degradation=89.52%, TOC=49.12%) while for tetracycline was (pollutant dose=10mg/l, pH=6.88, catalyst=0.737g/l, Degradation=96.07%, TOC=65.25%). Sulphate ions had the highest inhibition effect on tetracycline degradation among the three anions. Hydroxyl radical scavenging and competition for catalyst active sites could be the reason. In conclusion, a dye-sensitized photocatalyst is a potentially effective method for eliminating antibiotics in different wastewater matrices. However, it is recommended that a pretreatment step for anion removal be included in the PTR system to reduce their scavenging effects when treating real wastewater