Synthesis of modified graphitic carbon nitride for the remediation of emerging organic contaminants

Kingsley I John

doi:10.60867/00000048

Graphitic carbon nitride (g-C3N4) is a polymeric visible-light-active semiconductor that has been a research hot spot for many years in different fields, including renewable energy, sensors, and environmental clean-up. The mainstay for the broad interest in g-C3N4 photocatalysts is its visible-light absorption, low cost, non-toxicity, and simplistic synthesis. However, the photocatalytic efficiency of bulk g-C3N4 usually falls short of satisfactory owing to the fast recombination of charge carriers, low surface area, and insufficient visible light absorption. To address this, this thesis explored several strategies for modifying the properties and photocatalytic degradation performance of bulk g-C3N4 to remove organic pollutants in actual and simulated wastewater. The objectives and results of the thesis were systematically achieved as follows: Effectiveness of various modification strategies: a systematic literature review was conducted to ascertain the most effective strategy over five years (2018 – 2023). The strategies reported in the literature are metal doping, non-metal doping, co-doping, metal decoration, defect tuning, structural engineering and heterostructure formation. Among these, findings revealed heterostructure formation and metal doping as the most practical approaches for boosting the photocatalytic performance of bulk g-C3N4. Consequently, the two strategies were adopted for tuning the properties and performance of bulk g-C3N4 in the experimental sections. Enhancing the performance of g-C3N4 heterostructure via metal decoration and co-doping: A novel photocatalyst through rational design of Lanthanum (La) doping of g-C3N4 with silver nanoparticles (AgNPs) decoration was synthesised. The optimum photocatalyst Ag-0.8/LaCN-1 showed enhanced adsorptive and photodegradation performance over the bulk g-C3N4. Several analytical characterization techniques were used to investigate the photocatalyst's properties, performance, and reaction mechanisms. Transmission electron microscopy (TEM) and high-iv resolution TEM (HRTEM) revealed the uniform distribution of AgNPs on the surface of La-doped g-C3N4. The electrostatic interaction mainly governed the improved adsorption, whilst the enhanced photocatalytic efficiency was ascribed to its improved visible light absorption, charge carrier separation and transfer. In the same vein, AgNPs were replaced by Cobalt (Co) in a co-doping strategy to prepare Co/La@g-C3N4 through a one-pot synthesis. HRTEM and electronic structure analysis using the Mott-Schottky technique displayed the formation of a p-n-p heterojunction, which was accorded the basis for the improved photodegradation performance of the Co/La@g-C3N4 sample for the removal of tetracycline. Strategic approach for improving the performance of metal-doped g-C3N4: A non-conventional method for doping was explored through the combination of in situ La doping and thermal exfoliation of bulk g-C3N4. This was achieved by combining a pre-formed g-C3N4 with metal precursors under sonication, stirring, and subsequent pyrolysis of the dried mixture. Material characterization of the modified photocatalyst using HRTEM, atomic force microscopy (AFM), and x-ray diffraction (XRD) techniques showed the formation of metal oxide deposits and the thinning of the g-C3N4. Such material design significantly improved the photocatalytic performance of La-doped g-C3N4 due to enhanced surface area and a narrowed diffusion path for charge carriers. The mechanism of synthesis was attributed to metal ion intercalation and exfoliation. This synthesis approach was employed to study the synthesis mechanism, properties, and application of Na, Fe, and Pr as representative dopants of alkali, transition, and rare earth metals, respectively. Improving the performance of g-C3N4 via composting with waste material: groundwater sludge from Perth-based local plants in Mirrabooka and Gwelup was combined with pre-formed g-C3N4, designated g-C3N4/GWS-M and g-C3N4/GWS-G, respectively. The g-C3N4/GWS-M showed superior performance, and characterization analysis revealed improved surface area, visible light absorption, charge separation and lower charge transfer resistance as crucial factors for its enhanced efficiency. Moreover, the preparation of g-C3N4/GWS-M using the approach in this thesis was better than the conventional method of combining g-C3N4 with sludge reported in the literature. Similarly, a biochar template derived from discarded pine wood was combined with g-C3N4/La2O3 for improved photocatalytic performance. HRTEM and electron diffractionspectroscopy (EDS) established the formation of g-C3N4/La2O3 heterostructure and surface coating with biochar. Consequently, the composite's surface area, visible light absorption, charge separation and charge transfer properties were improved, resulting in a better photocatalytic degradation performance. The layering of biochar notably boosted the charge separation and transfer due to its electrical conductivity associated with the conjugated SP2 carbon atoms. Apart from the performance upgrade benefits of groundwater sludge and biochar, their application can be considered a waste management and circular economy strategy for wastewater treatment. Overall, this thesis methodically addresses the limitations of bulk g-C3N4 modification strategies through a literature review for research gaps, variation of established strategies, alteration of synthesis procedures and composting of g-C3N4 with waste materials as an economic and waste management approach in wastewater treatment.

Synthesis of modified graphitic carbon nitride for the remediation of emerging organic contaminants

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