Catalytic degradation of organic pollutants by oxysulfur radicals using transition metal-based catalysts

Abstract

The study in this thesis was designed to investigate the catalytic oxidation of organic pollutants for wastewater treatment by oxysulfur radicals generated from activated peroxymonosulfate (PMS) or sulfite. A series of transition metal-based materials were developed as catalysts for PMS or sulfite activation. To explore the catalytic reactivity of catalysts and assess the efficiency of the proposed oxidation systems, some organic pollutants derived from pharmaceutical and personal care products (PPCPs) were selected as target compounds.

The transition metal oxides, especially bimetallic oxides have shown their excellent reactivity for PMS activation. But the aggregation behavior will reduce their performance on activating PMS. Thus, the reduce graphene oxides (rGO) were introduced to help disperse nanoparticles. The CoFe2O4-rGO composites (CFGO) were synthesized as catalysts for catalytic degradation of ofloxacin and cefazolin. The CFGO202 that contains 20:2 weight ratio of CoFe2O4 to rGO exhibited the highest catalytic reactivity, indicating that the introduction of rGO can effectively improve the performance of metal oxides. The degradation of ofloxacin and cefazolin relied on the radical-based and non-radical-based oxidation, respectively. The two-electron transfer mechanism for degradation of cefazolin was revealed by quenching experiments and theoretical calculations, helping figure out the process of non-radical degradation.

The common metal oxides are poorly conductive, which will limit the electron transfer between catalysts and oxidants. Therefore, the transition-metal phosphides with high conductivity are investigated. The CoP2 encapsulated in N,P-codoped carbon (CoP2@NPC) was prepared as PMS activators by pyrolyzing phytic acid-cobalt salts-melamine precursors to avoid the generation of poisonous PH3 products during fabrication. Near-93% degradation of 40-μM NOR can be obtained within 10 min by CoP2@NPC-PMS system under optimal conditions. The CoP2 and NPC can serve as dual reactive sites to induce radical- and non-radical-based oxidations, respectively.

To further simplify the separation of catalysts from effluents, the catalysts were immobilized by flat-sheet ceramic membranes. One strategy is the surface decoration that can anchor catalysts on the surface of ceramic membrane. The CoFe2O4 nanoparticles were decorated on the surface of Al2O3 membrane via hydrothermal method, and the functionalized ceramic membrane (CFCM) was evaluated for catalytic filtration. The forced filtration endows the CFCM-PMS system with excellent degradation efficiency to degrade around 100% of 40-μM ofloxacin within 20 min.

Considering the inert substrates in decorated ceramic membranes, the other strategy is to fabricate an integrated ceramic membrane that can uniformly disperse active sites in the whole membrane. The CoAl2O4 integrated ceramic membrane (CACM) was thus prepared for catalytic degradation of ibuprofen. The CACM can effectively eliminate the pollutants that cannot be physically rejected. Its excellent stability with negligible leached cobalt ions indicates its reliability for wastewater treatment. As a cost-effective substitute for PMS, sulfite activation was also investigated. The CoNi2S4 with spinel structure was utilized for sulfite activation to decompose ibuprofen. The CoNi2S4/sulfite system can degrade around 90% of pollutants within 30 min. The S2− in sulfides can facilitate the redox cycle of metal ions during reactions. (481 words)