Heterogeneous photocatalytic oxidation of organic pollutants in aqueous solution using visible light and graphitic carbon nitride photocatalysts

Z razvojem industrije, prihaja v uporabo vse več kemikalij, ki v določeni meri zaidejo v okolje in onesnažujejo pitno vodo in druge vodne vire. V državah v razvoju se je uporaba kemikalij močno povečala, skrb za okolje pa je na zelo nizki ravni, zato so pitna voda, reke, jezera in morje ponekod izredno onesnaženi. Primer škodljivih kemikalij so hormonski motilci, ki že v nizkih koncentracijah povečajo tveganje za neplodnost, pojav raka ter presnovne in kognitivne motnje. Za reševanje tega problema je pomembno, da razvijemo učinkovite metode za odstranjevanje onesnaževal iz okolja. Za čiščenje odpadnih voda so pogosto v uporabi napredni oksidacijski procesi, saj z njimi lahko neselektivno razgradimo organska onesnaževala do CO2, H2O in anorganskih ionov. Kot del naprednih oksidacijskih procesov se vse bolj raziskuje heterogena fotokataliza, kjer so v uporabi kovinski (TiO2, Cu2O, ZnO, ZnS) in nekovinski (grafitni ogljikovi nitridi (g-C3N4)) fotokatalizatorji. V okviru magistrske naloge smo iz različnih izhodiščnih spojin (dicianodiamid, tiosečnina in melamin) pripravili fotokatalizatorje na osnovi grafitnega ogljikovega nitrida, ki delujejo na vidni svetlobi. Kasneje smo z metodami karakterizacije določili njihove površinske, optične in strukturne lastnosti ter njihovo učinkovitost pri tvorbi reaktivnih kisikovih zvrsti. V šaržnem reaktorju smo na koncu izvedli še razgradnjo bisfenolov in s tem določili učinkovitost tega postopka z različnimi fotokatalizatorji. Koncentracijo bisfenolov v različnih časovnih točkah smo določili s tekočinsko kromatografijo visoke ločljivosti. Delež mineralizacije organskega ogljika pa smo določili z meritvami celotnega organskega ogljika v raztopini bisfenolov, pred in po fotokatalitski razgradnji. Pripravljeni fotokatalizatorji g-C3N4 so se med seboj pomembno razlikovali v optičnih in površinskih lastnostih ter po učinkovitosti tvorbe reaktivnih kisikovih zvrsti in fotokatalitske razgradnje bisfenolov. Razlike pri katalizatorjih g-C3N4, pripravljenih iz dicianodiamida in tiosečnine so bile manjše, medtem ko se tisti, izdelan iz melamina precej razlikuje od drugih. Katalizator iz melamina ima namreč največjo specifično površino in zato lahko tvori največ hidroksilnih radikalov. S tekočinsko kromatografijo visoke ločljivosti smo pri melaminskem katalizatorju g-C3N4 določili tudi največji delež razgradnje bisfenola A (55 %), zato smo ga uporabili tudi za razgradnjo preostalih treh modelnih bisfenolov. Pri tem smo z njegovo uporabo po 120 minutah dosegli 51 % razgradnjo bisfenola F, 28 % bisfenola AF in 8 % bisfenola S. Katalizator iz melamina je uporaben za fotokatalitsko razgradnjo BPA in BPF, precej manj pa za razgradnjo BPS in BPAF, ki sta odpornejša na tovrstno obdelavo.With industrial development, more and more chemicals are coming into use, which to a certain extent enter the environment and pollute drinking water and other water sources. In developing countries, the use of chemicals has increased significantly, while environmental concerns are low, making drinking water, rivers, lakes, and the sea extremely polluted in some places. An example of harmful chemicals is hormone disruptors, which increase the risk of infertility, cancer, metabolic and cognitive disorders even in low concentrations. To solve this problem, new effective methods for the removal of pollutants from the environment need to be developed. Advanced oxidation processes are often in use for water treatment since they can nonselectively degrade organic pollutants to H2O, CO2, and inorganic ions. As a part of advanced oxidation processes, heterogeneous photocatalysis is a promising method in which metal (TiO2, Cu2O, ZnO, ZnS) and nonmetal (graphitic carbon nitride) photocatalysts are used. In the research project, various graphitic carbon nitride photocatalysts operating in visible light were prepared from different starting compounds (dicyandiamide, thiourea and melamine). Optical, surface, structure properties, and reactive oxygen species production effectiveness of prepared catalysts were determined by different characterization methods. Photocatalytic degradation activity was determined by the degradation of bisphenols in a batch reactor. Bisphenol concentrations at certain time intervals were measured by high performance liquid cromatography. To determine the mineralization percentage of different bisphenols total organic carbon was measured in the final and initial solution. The prepared g-C3N4 photocatalysts showed different optical and surface properties. The production of reactive oxygen species and the percentage of photocatalytic degradation also differed significantly. Photocatalysts made from dicyandiamide and thiourea have more similar properties than the photocatalyst made from melamine. Photocatalyst, made from melamine, has the highest specific surface area and generates more hydroxyl radicals than other catalysts. Since melamine catalyst was most successful at removing bisphenol A from the reaction mixture (54%), we used it to degrade other bisphenols. After 120 minutes 51 % of bisphenol F, 28 % of bisphenol AF, and 8 % of bisphenol S were removed. Catalyst, made from melamine, proved to be useful for BPA and BPF removal, but inefficient for BPAF and BPS removal, as they are more resistant to photocatalytic degradation