Ind. Eng. Chem. Res. 2009, 48, 4237–4244
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Degradation of Surfactants by an Integrated Nanobubbles/VUV Irradiation Technique Tsutomu Tasaki and Tsubasa Wada JST InnoVation Satellite Miyazaki, Japan Science and Technology Agency (JST), Daiichi Miyagin Blg 6F, 1-7-4 Tachibana-dori Higashi, Miyazaki, 889-2192, Japan
Yoshinari Baba Department of Applied Chemistry, Faculty of Engineering, UniVersity of Miyazaki, 1-1, Gakuen Kibanadai Nishi, Miyazaki, 889-2192, Japan
Masato Kukizaki* Department of Material DeVelopment, Miyazaki Prefecture Industrial Technology Center, 16500-2 Higashi Kaminaka, Sadowara, Miyazaki, 880-0303, Japan
Recently, many efforts have been devoted to the elimination of alkylbenzene sulfonate (ABS) surfactants from aqueous systems. In this paper, a water and wastewater treatment technique that uses an 8-W lowpressure mercury lamp in the presence of nanobubbles (diameter ) 720 nm) was demonstrated for the decomposition of sodium dodecylbenzenesulfonate (SDBS), as a model compound in aqueous solution. Degradation experiments were conducted with an ozone lamp (185-254 nm), both with and without nanobubbles. The result shows that the oxidation and mineralization rate of SDBS were significantly enhanced under 185-254 nm irradiation by oxygen nanobubbles. Although a high concentration of surfactant was used in this study, SDBS removal is effective in the integrated nanobubbles/vacuum ultraviolet (VUV) system, via the observation of 99.8% SDBS oxidation and 76.8% total organic compound (TOC) removal after 24 h of irradiation. The current study investigates the effect of size of bubble on the mineralization rate of SDBS. Furthermore, the rates of surfactant degradation were compared with those of nonsurfactant such as benzene sulfonate (BS). It was found that the mineralization of SDBS surfactants with nanobubbles was observed to be more effective than that with microbubbles (diameter ) 75.8 µm). The comparative results show that the mineralization rate of surfactants was much faster than nonsurfactant in the presence of nanobubles under 185-254 nm irradiation. Based on the experimental results and kinetic degradation model, we concluded that the enhancement on the mineralization of surfactants is attributed to the high adsorption capability of nanobubbles, because of the small particle size offering a large surface area to facilitate the reaction. Introduction Widely used anionic surfactants such as alkylbenzene sulfonate (ABS) have been widely utilized for many years as industrial detergents, emulsifiers, and dispersing agents.1 As a consequence, they are ubiquitous in the environment. It is not uncommon to find high concentrations of ABS in municipal and industrial wastewaters, particularly from washing processes. Advanced oxidation processes (AOPs), including ozonation and photochemical methods, have been used for the treatment of ABS surfactants.2,3 Most of these works centered on the chemical or photochemical reaction with the alkyl chain and the aromatic ring of the surfactants. Although the fast oxidation of ABS was studied and proposed, knowledge of the ozone dose and the reaction time was required to achieve a complete removal of ABS. For example, Beltrain et al.4 reported that, regardless of pH, after 1 h of ozonation with an ozone dose of ∼300 mg/L of wastewater, the chemical oxygen demand (COD) and total organic compound (TOC) never decreased more than 33% and 16%, respectively, from synthetic wastewater that contained 15 mg/L sodium dodecylbenzenesulfonate (SDBS). In addition, conventional oxidative methods such as UV/H2O2 * To whom correspondence should be addressed. Tel.: +81-98574-4311. Fax: +81-985-74-4488. E-mail address:
[email protected]. miyazaki.jp.
have little effect on the removal of wastewater with high ABS concentrations. Among AOPs, VUV photolysis has been shown to be effective for the treatment of wastewater, because radiation using light with a wavelength of λ < 200 nm homolyzes water to produce OH• radicals with high efficiency. The photochemical production of OH• radicals can be performed using irradiation of liquid water using VUV lamps (for example, low-pressure mercury lamp emitting at 185 and 254 nm).5 A study shows the faster oxidation of organic compounds with the VUV process, compared to the conventional ozone system.6 However, Oppenla¨nder et al.7 reported that diffusion of reactants to small irradiated reaction sites limits the degradation rates (i.e., masstransfer-limited kinetics) in the VUV reactor. Furthermore, the efficiency of oxidation and mineralization of organic compounds is restricted markedly, because of the high absorbance of water from light with a wavelength of 185 nm.8 This photochemical condition creates an extreme heterogeneity between the irradiated reaction site and the nonirradiated bulk solution. This effect leads an oxygen deficit within irradiated reaction site, which produce byproducts such as oligomers and polymers by the recombination reaction of carbon-centered radicals during the photodegradation. The mentioned factors still remain a challenge for a wide range of applications.
10.1021/ie801279b CCC: $40.75 2009 American Chemical Society Published on Web 04/07/2009
4238 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
To this end, we have focused on the micro/nanobubbles technique to improve the bulk limited VUV process for treatment of contaminants in aqueous solutions. It was reported that micro/nanobubbles having diameters ranging from several hundreds of nanometers to 100 µm exhibit a very large gas-liquid interfacial area, gas dissolution ability, and hydrodynamic behavior. Systems containing a large number of micro/ nanobubbles have a big advantage on mass transfer through the bubble surface, because of their large surface area. Therefore, micro/nanobubbles have been shown superior performance on oxygen or ozone supply, adsorption, and catalysis immobilization.9-11 We have previously reported the degradation of Methyl Orange in the presence of oxygen microbubbles under 185-254 nm irradiation.12 In our preliminary study, we demonstrated that the degradation rate of Methyl Orange was accelerated by the oxygen microbubbles, because of the enhancement on the mass transfer of oxygen and substrate within the VUV reactor. To further explore the effect of oxygen micro/nanobubbles, we conducted an investigation of surfactant degradation under 185-254 nm irradiation, both with oxygen microbubbles (diameter ) 75.8 µm) and nanobubbles (diameter ) 720 nm). Because surface-active molecules accumulate at the interface, in particular, at the water-gas interface of micro/nanobubbles, surfactants should be a good target for this treatment. Herein, we report on a study that combined nanobubbles and VUV technologies for the first time in the treatment of surfactants. We start by first investigating the oxidation and mineralization of SDBS both with and without oxygen nanobubbles under 185-254 nm irradiation. We then reveal new insights into the surfactant adsorption onto nanobubbles for the enhancement of surfactant degradation in the VUV reactor by examining the effect of bubble size on the degradation rate of surfactant and comparative degradation test with nonsurfactant. In this article, the different experimental conditions such as gas flow rate, type of nanobubbles, initial surfactant concentration and initial pH, and their effects on the degradation of surfactants were also examined. Experimental Section Reagents. Two anionic surfactants, sodium dodecylbenzenesulfonate (SDBS, C12H25C6H4SO3Na, Tokyo Kasei, >95.0% purity) and sodium dodecyl sulfate (SDS, C12H25SO3Na, Tokyo Kasei, >95.0% purity) were chosen as model surfactants. Benzene sulfonate (BS, C6H4SO3Na, Tokyo Kasei, >96.0% purity) was chosen as a model nonsurfactant. All chemicals were of high purity and used as received. Deionized water supplied by a purification unit (SW AC-520, Shimadzu Co., Ltd., Japan) was used for the preparation and dilution of solutions. To generate oxygen microbubbles and nanobubbles, Shirasu Porous Glass (SPG) membranes13 (53 mm length × 10 mm outer diameter × 1.0 mm wall thickness) with different pore diameters (0.08 and 5.0 µm) were used in this study. The membrane was prepared by leaching of phase-separated glass in the Na2O-CaO-MgO-Al2O3-B2O3-SiO2 (Shirasu) system. The details for preparation of membrane can be found elsewhere.14 Experimental Setup. The reactor system used in this study is represented in Figure 1. The system consists of sn ultraviolet (UV) lamp, an SPG module, and a photoreactor. Photodegradation experiments were performed using an 8.0-W ozone lamp with a maximum emission at 254 nm and a smaller (
