Article pubs.acs.org/IECR
Remediation of Antiseptic Components in Wastewater by Photocatalysis Using TiO2 Nanoparticles Ranjana Das,† Santanu Sarkar,† Sudip Chakraborty,† Heechul Choi,‡ and Chiranjib Bhattacharjee*,† †
Chemical Engineering Department, Jadavpur University, Kolkata, 700032, India School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea
‡
ABSTRACT: Environmental awareness in both the public and regulatory sectors has necessitated proper treatment of medicinal components-rich pharmaceutical effluents. Even the presence of trace antiseptic may cause adverse health effects including development of “product resistant microbes” in the aquatic environment. The present study involves photomineralization of chlorhexidine, which belongs to the class of antiseptic drug components. This study details investigations on photocatalytic degradation of chlorhexidine in a slurry batch reactor using titanium dioxide photocatalyst. Emphases were given to study the effects of operating parameters on the degradation behavior of the targeted compound and characterization of degraded products. About 68.14% removal of chlorhexidine digluconate (CHD) was found after 1 h at 25 °C with a substrate-to-catalyst ratio of 2.5:1 under UV intensity of 50 μW·cm−2 at pH 10.5. Though the product profile illustrates several degraded products, toxicological analysis on Bacillus subtilis exhibited no inhibition zone, suggesting the eco-friendly nature of the degraded products.
1. INTRODUCTION Pharmaceuticals (PhAs) constitute a large group of medicinal compounds which have indispensable use worldwide for human and veterinary applications. Though the quantity of these pharmaceuticals in the aquatic environment is trace, for aquatic and terrestrial organisms, their continuous input may impose a potential risk and they are considered to be an emerging environmental concern. The primary route of pharmaceuticals into the environment is through their introduction into soil, surface waters, and groundwater after being excreted from humans or animals via urine or feces through the sewage system.1,2 In addition to metabolic excretion, disposal of PhAs from medical treatment organizations and common households also contributes to their entry into fresh water resources. Usually pharmaceuticals are designed in such a way that they exhibit a physiological effect on humans and animals at low concentration levels, and they become persistent against biological degradation, which is primarily responsible for their tagging as a “pollutant”.3,4 PhAs released in the environment may impose toxicity, the extent of which depends on the specific compounds and virtually on the biological structure. In addition to toxic effects, some classes of the pharmaceuticals (antibiotics) may cause long-term and irreversible changes to the microorganism, making them resistant to biodegradation even at low concentration. Several studies have exemplified the direct entering of PhAs in the sewage treatment process5−7 and their adverse effect on aquatic life.8−10 With a broad spectrum of activity, chlorhexidine (CHD) compositions have been in use throughout the world for more than 50 years as an antiseptic in various medicinal applications, including as the active component in toothpaste (0.1−0.2%); mouthwash (0.6−1%); veterinary application (1.5−2%); cosmetic preservatives (1−1.5%); and hand wash and scrubs (15−20%).11 It has been used as an antibacterial agent in commercial ophthalmic products as a replacement for © 2014 American Chemical Society
thimerosal (which is a mercury-containing bacteriostat). CHD surpasses that of the similar preparations containing providoneiodine, triclosan, hexachlorophene.12 Studies on CHD show its toxic effect on nerve tissue, and therefore its contact with brain and meninges is restricted. Intravenous administration exhibited greater toxicity because of the stromalytic effect on red blood cells resulting from its surfactant activity. Systemic oral administration of CHD formulations on rats have shown that receiving 50−200 mg/kg CHD in drinking water for 90 days produces evidence of histiocytosis of the mesenteric lymph nodes. In the bone marrow test of the mouse with dermal application of 0.2 mL of 0.5% CHD in distilled water twice daily for 28days (50 mg/kg of body mass), an increase in chromosomal aberrations was noted.13 It is also reported as environmentally hazardous, toxic to aquatic organisms and sewage microorganisms, and responsible for long-term adverse effects in the aquatic environment with an acute oral toxicity (LD50) of 6.3g/kg (mouse, oral); 2g/kg (human, oral); 3 g/kg (rat, oral).11 Increasing human and livestock populations have raised the concern of safe drinking water and nonpotable water supply. In the present context of potable water supply, the development of advanced, low-cost, and high efficiency water treatment technologies are potentially desirable. To attain the stringent international environmental standards, technological developments for removal of pharmaceuticals has become indispensable to the pharmaceutical industries. Conventional treatment technologies like adsorption and coagulation only concentrates the pollutants using phase change principle, but does not completely eliminate or destroy the components.14 To overReceived: Revised: Accepted: Published: 3012
November 11, 2013 January 12, 2014 January 29, 2014 January 29, 2014 dx.doi.org/10.1021/ie403817z | Ind. Eng. Chem. Res. 2014, 53, 3012−3020
Industrial & Engineering Chemistry Research
Article
come the limitations of conventional wastewater treatment methodologies and effective utilization of available economic resources, various advanced treatment technologies have been adopted, optimized, and have been applied.15−17 Among mentioned technologies, membrane filtration, advanced oxidation processes (AOPs), and UV irradiation have been proven beneficial in removal of a wide range of challenging contaminants. Intensive studies on photocatalysis using nanoTiO2 have already been published relating the treatment of the pharmaceutical components in wastewater,18−30 but no studies have yet been reported on the photocatalytic degradation of CHD using nano-TiO2. The outstanding properties of nanoTiO2 such as high surface area, nontoxicity, photochemical stability, light absorption, charge transport, superior excitedstate lifetimes and favorable combination of electronic structure makes it the “photocatalyst of choice”.31 Considering the adverse effects of CHD on the environment, and the high efficiency of the photocatalytic degradation process, the present study aims at the photocatalytic treatment of the pharmaceutical waste to obtain an eco-friendly discharge alleviating the major environmental concerns. The novelty of the study is in the choice of the substrate as chlorhexidinde digluconate since no publication still available attempts the use of this particular class of antiseptic component. The process efficiency has been evaluated by varying the process controlling parameters using an antiseptic drug chlorhexidine digluconate (CHD) as a target component. This study also involves characterization of the process degraded products and assessment of related product toxicity.
Figure 1. Operational set up of the batch slurry reactor for photocatalytic degradation of CHD.
proper mixing of the reaction solution and even distribution of UV irradiation to the catalyst system. The distance of sample solution (upper surface) to the radiation source maintained during the photodegradation process was 7 cm, and the reaction solution thickness was 3 cm. The UV light intensity was controlled with an external controller, and inside intensity was measured using a solar UVA meter (TM 208, Tenmars, Taiwan). 2.4. Adsorption Study Chlorhexidine Digluconate on TiO2 Nanoparticle. Since adsorption of substrate (CHD) on the catalyst systems restrain the extent of photodegradation, studies on the adsorption behavior of CHD onto a catalyst system is quite relevant in the present context. To study the adsorption behavior of CHD on a catalyst system, a batch study was conducted under dark (25 °C) for a time period of 1 h with a variation of catalyst loading weight and CHD concentration. 2.5. Batch Study of the Photocatalytic Degradation of Chlorhexidine Digluconate. To study the process of photocatalytic degradation of chlorhexidine digluconate, a batch study (100 mL) was conducted with varying concentration of the substrate (CHD) and the catalyst. For overall understanding of the process, the controlling parameters such as UV intensity and the pH of the reaction mixture were also varied. The batch studies were conducted for 1 h (at 400 rpm) with sample collection at 5 min interval. To ascertain the complete catalyst removal, an aliquot was centrifuged (4 °C, 11000× g, 5 min), and supernatant was collected for further analyses. After collection of the supernatant the remaining portion was diluted with fresh ultrapure water (100 mL), and sedimentation of the catalyst was induced by adjusting the pH of the medium to the isoelectric point (IEP) of the catalyst. Though both water reuse and catalyst reuse is possible, in the present case only the catalyst was reused, as with water reuse the degradation products remain in the system, which probably will impart some product inhibition on the overall photodegradation process. 2.6. Analytical Procedures. Initially, chlorhexidine digluconate concentration of the reaction mixtures was determined using spectrophotometry at 275 nm.32 Quantification was done with respect to the standard curve of pure chlorhexidine digluconate dissolved in deionized water within the concen-
2. EXPERIMENTAL SECTION 2.1. Chemicals. Two catalyst systems of titanium dioxide photocatalyst nanopowder (pure anatase, 637254) of particle size
