Graphene and biomass-based carbocatalysts as high-performance peroxymonosulfate activator for the removal of recalcitrant pollutants in water

• Main Author: Chen, Xiao
• Other Authors: Lim Teik Thye
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/136926
• Institution: Nanyang Technological University

Water contamination by refractory organics is one of the most critical and challenging problems in industrialization. Advanced oxidation processes based on peroxymonosulfate (PMS) activation is increasingly becoming popular due to its high ability to completely decompose toxic and refractory organic pollutants. Heteroatom-doped nanocarbons have attracted considerable attention over the conventional metal-based catalysts (e.g., Fe- or Co-based catalyst, etc.) to activate PMS, because carbocatalysts with the environmentally benign nature, corrosion resistance and biocompatibility can overcome the sintering and metal leaching problems caused by metal-based catalysts. The main objective of this study is to fabricate carbocatalysts to activate PMS for the degradation of recalcitrant organic contaminants, especially sulfonamide antibiotics in the aqueous environment. In the first part of this study, nitrogen-doped graphenes (NG) was fabricated to activate PMS for sulfacetamide (SAM) degradation. The contents of reactive functional groups and catalytic performance of NG were delicately controlled by adjusting thermal annealing temperature. NG600 (NG thermally annealed at 600°C) with the optimized amount of N species and C=O group exhibited a better PMS-activating activity than NGs prepared under other thermal annealing temperatures or via other optimized synthesis methods. Quenching experiment, electron paramagnetic resonance (EPR) study and Density Functional Theory (DFT) calculations revealed that non-radical pathway with surface activated PMS as the key reactive oxygen species (ROS) contributed more to SAM degradation than radical pathway in the NG/PMS/SAM system. The effect of catalyst loading, PMS dosage and common matrix species on PMS activation by NG600 for SAM degradation, the SAM degradation pathway, and the reusability of NG600 were investigated. In the second part of this study, nitrogen and boron-co-doped graphene was synthesized through two-step thermal annealing (2sNBG) and one-step thermal annealing (1sNBG). Boron-doped graphene was also synthesized via thermal annealing (BG). The carbocatalysts were employed as PMS activators to degrade SAM. The concentration of the main reactive functionalities and catalytic activity of 2sNBGs were delicately maneuvered through tuning the thermal annealing temperatures. 2sNBG800 (prepared at 800°C) with the highest N and B doping levels, the highest contents of pyridinic N and BC3 (substitutional B) that serve as the main active sites, and absence of hexagonal boron nitride (h-BN), performed best to activate PMS for SAM degradation. By contrast, the 1sNBG contained h-BN which could hamper its catalytic activity. The catalytic performances of the various doped graphenes prepared in this study followed the order of 2sNBG800 > 2sNBG900 > 2sNBG700 > 2sNBG600 > NG600 > 1sNBGs > BG800. Both radical quenching experiment and DFT calculation revealed that the introduction of B into NG can facilitate the shift of reaction pathway from a non-radical oxidation dominating in the NG/PMS system to the coexistence of non-radical and radical oxidations in the 2sNBG/PMS system. The synergistic coupling effect from bonding configuration of B-C-C-C-pyridinic N was the main reason for the enhanced catalytic activity of 2sNBG800 to activate PMS for SAM degradation. The SAM degradation was negligibly influenced by NO3- in the 2sNBG800/PMS/SAM system, while Cl- and humic acid led to 33% and 64% decrease in kapp, respectively. The transformation of the aromatic amino group and subsequent mineralization of SAM can effectively minimize the hazardous potentials of sulfonamides to the environment. Nevertheless, the adsorbed intermediates could deactivate 2sNBG to some extent. In the third part of this study, nitrogen-doped chitosan-derived carbon nanosheets (CNUs) were synthesized as a renewable, cheap and easily accessible alternative to the graphene-based carbocatalyst. The contents of reactive functionalities, graphitization degree and porous structure of CNU can be effectively tailored by pyrolysis temperature (Tp). The outstanding PMS-activating activity of CNU800 (prepared at Tp = 800°C) for SAM degradation can be attributed to its high level of C=O/C and graphitic N/C, relatively high graphitization degree, and its large specific surface area and hierarchically porous structure. The introduction of urea with the presence of NaHCO3 during chitosan pyrolysis facilitated formation of the graphene-like carbocatalyst with hierarchically porous structure and an enhanced PMS-activating activity. Quenching experiment and EPR collectively revealed that non-radical oxidation with singlet oxygen (1O2) as the main ROS was the dominant catalytic pathway in the CNU800/PMS/SAM system. The effect of catalyst loading, PMS dosage and common matrix species on PMS activation by CNU800 for SAM degradation, SAM degradation pathway, and reusability of CNU800 were probed.