Photocatalytic properties of fluorinated tetraarylantimony carboxylates

A large range of organic contaminants introduced into natural water supplies or waste water treatment systems are known to be residual dyes from various sources. Organic dyes are widely used in the textile, cosmetic, pharmaceutical, tannery, paper and mining industries [1, 2] . At present, the scientific community is actively discussing the problem of extracting dyes from wastewater in order to reduce their impact on the environment [3–5]. The main problem when dyes get into water is their stability because of their complex aromatic composition and synthetic origin [6 –9] .

Another problem is their accumulation of dyes in water body resulting in the loss of dissolved oxygen, which creates anoxic environments that are toxic to marine organisms [10 –12] . According to the literature, some organic dyes are very toxic, they may cause carcinogenic, allergic and dermatic effects [13] .

Nowadays, different methods for water purification and the organic dyes decomposition are used. For example, use of biological [14 –16] , physical [17] or chemical treatment. The last includes the dyes degradation caused by polyurethane [18] , nanoparticles [19 –23] . Dye degradation generally means dye decolorization and mineralization in textile waste water. Decolorization is the destruction of the dye molecule chromophore group; organic compounds are also degraded into CO₂ and H₂O [24] .

Earlier, the photocatalytic activity of triarylantimony carboxylates has been studied [25].

In this work, we studied the possibility of photocatalytic destruction of organic dyes methylene blue and methyl violet in their aqueous solutions by adding tetraarylantimony monocarboxylates and further irradiation of the solutions with UV radiation.

ExperimentalPhotocatalytic Activity Test

The photocatalytic activity of tetraarylantimony carboxylates was estimated by photodegradation of aqueous solutions of methylene blue (MB) and methyl violet (MV) dyes at room temperature (298 K). Experiments were performed according to the general procedure [25] : 20 mg of tetraarylantimony carboxylates of the general formula Ar 4 SbOC(O)R, Ar = Ph, R = C 6 F 5 ( 1 ), CF 2 CF 2 CF 3 ( 2 ), CF 2 Br ( 3 ); Ar = p-Tol, R = CF 2 CF 3 ( 4 ), CF 2 CF 2 CF 3 ( 5 ) (synthesized by the reaction of pentaarylantimony with a proper carboxylic acid 1:1) were added to 50 ml of an aqueous solution of MB or MV (concentration 4 mg/L). To establish adsorption-desorption equilibrium, the resulting suspension was stirred in the dark for 30 minutes. Then the suspension was irradiated under a UV lamp with constant stirring for one hour. The concentration of MS and MV was measured by UV spectroscopy every 15 minutes. The collected samples were centrifuged to separate tetraarylantimony carboxylates.

The visible UV diffuse reflectance spectrum (UV-Vis.) was obtained in the region of 200 to 800 nm using a Shimadzu UV2700 spectrometer. Barium sulfate with a reflectance of 100 % was used as a standard.

Results and Discussion

A change in the peak intensity at 554 nm and 665 nm for MV and MB, respectively, in their UV spectra, indicated a change in the concentration of dyes in the solution (Fig. 1 a, b). In a control experiment carried out under similar conditions in aqueous solutions without the addition of tetraarylantimony monocarboxylates, no decomposition of the MB and MV dyes occurred.

Fig. 1. (a) Absorption spectrum of MV solution in the presence of 4;

(b) absorption spectrum of MB solution in the presence of 1

It has been found that the photocatalytic activity of the compounds under study is different. Tetraphenylantimony carboxylates 1 – 3 exhibited higher photocatalytic activity than tetra(p-tolyl)antimony carboxylates 4 , 5 (Fig. 2).

Thus, upon 60 min irradiation of solutions containing compound 1 – 3 , the decomposition of MB was 90.5 %, 97.3 %, 98.8 %, MV: 97.7 %, 99.5 %, 100 %, respectively. Tetra(p-tolyl)antimony monocarboxylates 4 , 5 exhibit relatively low photocatalytic activity: in the presence of compound 4 , the decomposition of MB and MV occurred by 53.2 % and 78.7 %, respectively; in the presence of 5 – by 55.1 % and 71.2 %, respectively. The photocatalytic efficiency of all complexes is obviously higher for the decomposition of MV than MB. In general, the photocatalytic activity of 1 – 3 is comparable to those of triphenylantimony bis (trifluoromethylbenzoates), studied earlier. Thus, triphenylantimony bis (trifluoro-2-methylbenzoate) leads to the destruction of 90.5 % MB, and 90.6 % MV after 150 min and 90 min of irradiation, respectively [25] .

To assess the possibility of the repeatable use of the photocatalysts, we studied the process of circulating photocatalysis of MB and MV under the treatment with 2, 4 and 5. After complete decay of the dyes, tetraarylantimony carboxylates were precipitated by centrifugation, the precipitates were decanted and used for the next cycle (Fig. 3). As a result of two cycles of photodegradation, the mass of tetraarylantimony carboxylates after washing and drying decreased on average by 50 %. However, as the graphs show, all the compounds were still effective towards dye decomposition and lead to the destruction of 49.5 % of MB, and 80.5 % – 90.0 % of MV after 1 hr irradiation.

Fig. 2. Change in the reduced concentration (C / C0) under UV irradiation (t, min) of solutions of MV (a) and MB (b) dyes in the presence of tetraarylantimony carboxylates 1–5

Fig. 3. Photocatalytic decomposition of MB with 2 (a), 5 (b), MV with 2 (c), 4 (d) (1 and 2 cycles)

Conclusion

Fluorinated tetraarylantimony carboxylates can be good candidate for the photocatalytic degradation of organic dyes: methylene blue and methyl violet. Meanwhile, tetraphenylantimony carboxylates are more effective towards both dyes, in comparison to tetra(p-tolyl)antimony compounds. However, methyl violet undergoes a fuller destruction than methylene blue.

Acknowledgments

We are grateful to A.N. Efremov for the compound synthesis and A.V. Bulanova, O.K. Sharutina and V.V. Sharutin for the experimental supervising.