(335f) Adsorption and Photodegradation Of Dimethyl Phthalate In An Aqueous Solution By TiO2/ Hydrotalcite Composites

Adsorption and Photodegradation of Dimethyl Phthalate in an Aqueous Solution by TiO₂/ Hydrotalcite Composites

Pingxiao Wu^*1, Zhujian Huang¹

¹College of Environment and Energy, South China University of Technology;

*pppxwu@scut.edu.cn

Environmental endocrine phthalates (PAEs) is a hydrophobic trace organic pollutant commonly existed in aquatic environment. Dimethyl phthalate (DMP) is one of the relatively stable compounds in natural environment, and its hydrolysis half-life is estimated to be about 20 years. Photocatalytic oxidation is a commonly used, advanced oxidation technology, which can effectively degrade organic pollutants. The commercial TiO₂ nanoparticles have many disadvantages for photocatalytic degrading hydrophobic trace organic pollutants. Hydrotalcite have excellent properties and are an appropriate disperser and carrier for TiO₂ nanoparticles. This paper presents a new organic hydrotalcite/TiO₂ composite in an improved preparation method to efficiently remove DMP in an aqueous solution. The hydrotalcite:TiO₂ weight ratio = 4:1, 4:2, 4:3, 4:4, 3:4, 2:4 was labeled as HT41, HT42, HT43, HT44, HT34, and HT24, respectively. The optimal hydrophobic hydrotalcite/nano-TiO₂ ratio was screened by the photocatalytic experiments of DMP, and the photocatalytic mechanism was discussed. This study aims at providing technology and theory for purifying drinking water contaminated by organic pollutant with low concentration.

The result of XRD indicated that sodium dodecyl sulfate (SDS) anions are successfully intercalated into the interlayer of hydrotalcite. The arrangement of SDS anions in the gallery of hydrotalcite is confirmed by 1-D electron density projection. The orientation angle between the SDS anions and c-axis is calculated to be 30°. As the ratio of TiO₂ to organic hydrotalcite increases in the composites, randomly immobilized TiO₂ onto the surface of high-ordered sheets forms a mixed and disordered composite. High resolution Ti 2p and O 1s XPS showed that TiO₂ dispersed on the SDS-hydrotalcite still has octahedrally coordinated anatase structure. Also, TiO₂ and SDS-hydrotalcite are not simply a physical mix, but combined together by chemical bonds. SEM image (Fig. 1) provides direct evidence that the reconstructed SDS-hydrotalcite are consisted of interconnecting nano-flakes with an average thickness of approximately 20 nm, which would provide abundant adsorption sites for adsorbate. The results (Fig. 2) of adsorption and photodegradation experiment showed that SDS-hydrotalcite/TiO₂ composites can effectively adsorb DMP in aqueous solution, which will promote the catalysts photocatalytic degrade the adsorbates in situ. Both adsorption and photocatalytic degradation contribute to removal of DMP with SDS-hydrotalcite/TiO₂ composites under UV light. The photocatalytic ability of SDS-hydrotalcite/TiO₂ composites is much higher than that of bare TiO₂. The optimum mass ratio of hydrotalcite: TiO₂ is 1:1. HR-XPS patterns of fresh and used SDS-hydrotalcite/TiO₂ composites indicated that the photogenerated electrons incline to transfer onto the surface of Mg in the hydrotalcite. Such an electron transfer leads to the depression of electron-hole recombination and the improvement of the photocatalytic activity of TiO₂.

Fig. 1 SEM image of TiO₂/hydrotalcite composites

Fig. 2 Removal of DMP by TiO₂/hydrotalcite composites.

In conclusion, the photocatalytic properties of commercial TiO₂ nanoparticles for degrading hydrophobic pollutant DMP could be effectively enhanced by a simple method through hybridizing them with the interconnecting SDS-hydrotalcite nano-flakes, and SDS-hydrotalcite/TiO₂ composites are confirmed to combine together by chemical bonds. The optimum amount of the hydrotalcite powders as a carrier for the enhancement of the photocatalytic reactivity of the SDS-hydrotalcite/TiO₂ system was estimated to be the mass ratio of 1:1. The SDS-hydrotalcite/TiO₂ composites showed higher photocatalytic reactivity than that of bare TiO₂ nanoparticles mainly because SDS-hydrotalcite with flaky structure worked as an effective adsorbent to condense DMP in the aqueous solution.