Photodegradation of a persistent organic pollutant using MCWCNT-Titania composite photocatalyst / Tan Tong Ling

Persistent Organic Pollutants (POPs), particularly phthalate esters (PAEs) have been recognized as an endrocrine disrupting compound (EDC). It is a significant emerging priority contaminant that could easily be released into the environment. Hence, the health issues of PAEs exposure have raised public concern because of there are hepatoxic, teratogenic and carcinogenic in nature. In this study, a simple one-pot sol-gel technique was adopted to fabricate TiO2 nanoparticles (15 nm) and MWCNTs/TiO2 nanocomposites. The TiO2 nanoparticles were first incorporated into the MWCNTs surfaces via hydrolysis of titanium isopropoxide (TTIP), ethanol and acetic acid at a volume ratio of 2:3:1 and were crystallized into anatase nanoparticles after annealing at 450ºC. The experimental results revealed that the photocatalytic activity of MWCNTs/TiO2 nanocomposites was strongly influenced by the synthesis parameters (MWCNTs/TiO2 ratio, calcination temperature and pH). The optimum synthesis conditions for MWCNTs/TiO2 nanocomposites were achieved with 10 wt % MWCNTs at pH 9 under calcination temperature of 450°C. The photocatalytic degradation of the synthesized TiO2 and MWCNTs/TiO2 was evaluated using dimethyl phthalate esters (DMPEs) as a model compound of PAEs. The photocatalytic degradation rate of DMPEs for both MWCNTs/TiO2 nanocomposites and TiO2 nanoparticles under 96 W UV irradiation were carried out at the same time. The results show that the MWCNTs/TiO2 nanocomposites (97%) exhibit high photocatalytic degradation rate, and it was much faster than TiO2 (71%). It is obvious that the nanocomposites have induced synergy effects on the photocatalytic performance of TiO2 nanoparticles by decreasing the band gap energy of the nanocomposite. The formation of Ti-O-C bonds between the carboxylic acid groups of MWCNTs and the hydroxyl groups existent at the surface of TiO2 during an esterification reactions, contribute to a synergetic effect due to the creation of an electronic interphase interaction between MWCNTs and TiO2 phases. The ideal efficiency for removal of DMPEs was obtained when the dosage for MWCNTs/TiO2 nanocomposites was 0.5 g/L, the initial concentration and pH of DMPEs was 1 ppm and 6, respectively. The irradiation time to achieve highly degradation was 3 h. The photoproducts from PAEs degradation were elucidated through UHPLC-Orbitrap-MS/MS analysis. A total of eighteen products were identified and were used to propose a primary degradation mechanism of DMPEs. The MWCNTS/TiO2 nanocomposites is a promising catalyst for photocatalytic degradation. Basically, the functional groups present at the surface of MWCNTs promote the anchoring of the TiO2 nanoparticles as well as avoiding agglomeration and subsequently increasing the surface area of the resulting MWCNTs/TiO2 composite. In addition, this strong interface interaction also favours electron transfer pathways, thus the probability for electron/hole recombination is reduced