Degradation of selected pesticides using Fenton's reaction based advanced oxidation processes

Abstract

One of the promising alternatives to conventional wastewater remediation processes is the Fenton process, which has been emerged as widely applicable water purification strategy. The current project was focused on the development of an economical, highly efficient, non-selective and easily applicable degradation method for the recalcitrant micro-pollutants, present in wastewater. The degradation method, based on heterogeneous Fenton reaction, has been successfully applied using different metal ferrites and their graphene oxide composites. Graphene oxide (GO) was prepared using modified Hummer’s method while Graphene oxide based metal ferrites (GO-MFe2O4, where M = Fe3+, Co2+, Mn2+ and Ni2+) were prepared by one-pot-hydrothermal co-precipitation method. The as-prepared composites were morphologically and chemically characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR). The composites were checked for their catalytic potential against degradation of persistent organic micro-pollutants such as pesticides, present in water bodies. Different influencing factors, such as pH, oxidant dose, catalyst dose, pesticide load and irradiation time, were also optimized in order to enhance pesticide’s degradation. All of the catalysts exhibited remarkable pollutant’s degradation potential in the presence of hydrogen peroxide, under UV light irradiation. Three insecticides, widely used in Pakistan, were selected as model pollutants viz. acetamiprid, carbofuran and nitenpyram. For comparative purposes, metal ferrites alone (without GO) were also catalytically investigated. Under optimal conditions, acetamiprid and nitenpyram were degraded about 90% using magnetite, GO-magnetite, cobalt ferrite, GO-cobalt ferrite, manganese ferrite, GO-manganese ferrite, nickel ferrite and GO-nickel ferrite within 60 minutes of irradiation. On the other hand, approximately 80% degradation of carbofuran was achieved using all metal ferrites and their composites. Furthermore, the study concluded that GO assists in remediation of aquatic pollutants by acting both as supportive catalyst immobilization material and electron transporter between the two catalytic active sites. Thus, GO indirectly promotes ≡M3+/≡M2+ cycle and also avoids e-/h+ pair recombination by capturing and transporting the electron. At optimized conditions of pH, doses of Fenton’s reagent (various catalysts and oxidant), irradiation time and pollutant load, ultrasound assisted photo-Fenton process degraded all the pollutants (acetamiprid, carbofuran and nitenpyram) more than 95% within a very short time span. This was confirmed with the help of UV/Vis spectra. The magnetic property of catalysts enabled their easy recovery xiii from the aqueous medium by applying external magnetic field. Suitable kinetic models, as a function of operating conditions were also proposed for wastewater remediation process. Major potential factors that affect the pesticide degradation were investigated statistically by applying Central Composite Design (CCD – a type of RSM). Statistical analysis helped in evaluating the interactions among controlling parameters such as pH, oxidant dose and pollutant load. The response surfaces revealed that highly acidic pH, moderate oxidant dose and lower pollutant load favored the enhanced pesticide’s degradation. The reusability of the catalysts was also studied up to five cycles. It has been observed that heterogeneous catalysts can be reused multiple times without substantial decrease in the degradation potential. The outcome of the study is to fabricate novel, effective, magnetic, durable and economically feasible catalysts. The project also concluded that graphene oxide and ultrasound assisted strategy played a vital role in enhancing the degradation potential of heterogeneous Fenton-like catalysts.