Organic acids enhanced decoloration of azo dye in gas phase surface discharge plasma system
Introduction
A variety of synthetic organic dyes are widely used in textile industries, however, the removal of these dyes from effluents is a major environmental problem because of the non-negligible toxicity, low biodegradability and strong color of dye wastewater. It is very difficult to effectively treat dye-containing wastewater using simple biological systems [1], [2].
Currently, advanced oxidation processes (AOPs) including ultraviolet photolysis, the Fenton process and ultrasonification have been widely used to remove hazardous organic contaminants from aqueous solutions, and they were considered to be promising alternatives for dye-containing wastewater treatment and the biodegradability of wastewater could be enhanced after treatment [3], [4]. As one of the AOPs, non-thermal discharge plasma has raised considerable interest in wastewater treatment because of its high removal efficiency and environmental compatibility [5], [6]. At present, various discharge reactors and discharge types have been proposed to treat wastewater, such as point-to-plate electrode [5], [6], wire-to-plate electrode [7], wire-to-cylinder electrode [8], wire-to-wire electrode [9], and ring electrode [10]. In our previous study, an active species bubbling reactor using gas phase surface discharge was developed to treat dye-containing wastewater. In that system, active species could be rapidly injected into wastewater which resulted in the excellent performance of dye decoloration [11]. The injection rate of active species into wastewater is dependent on gas flow rate, as its lifetime in wastewater is shortened at a relatively high gas flow rate, resulting in lower utilization efficiency. Therefore, it is necessary to explore methods to improve the lifetime and mass transfer of active species in wastewater using this type of discharge plasma system.
As reported, various factors such as gas flow rate, solution pH, inorganic salt content and bubble size could affect the lifetime and mass transfer of gaseous active species in water [11], [12]. Methods to reduce bubble size have been widely studied to improve active species mass transfer, such as the ozone microbubble technique [13], [14], the foamed wastewater technique [15] and the small cloudy bubble technique [16]. In addition, Carver [17] reported that the addition of small amounts of organic acid such as lactic acid and acetic acid into solution could increase bubble number and reduce bubble size, thus enhancing gas-liquid mass transfer. Shen et al. [18] also found that ozone mass transfer was enhanced when organic acids and alcohols such as nonyl acid and n-heptyl alcohol were added into p-nitrophenol aqueous solution. Therefore, small amounts of organic acids may be able to enhance the mass transfer of gaseous active species during wastewater treatment in a gas phase surface discharge plasma system.
The aim of this study was to evaluate the feasibility of enhancing the mass transfer of active species and pollutant removal efficacy using organic acids during dye-containing wastewater treatment by gas phase surface discharge plasma. Acid orange II (AO7) was chosen as the target pollutant, which is widely used in dyeing industry. AO7 decoloration efficacy was investigated at different discharge voltages, and the effects of small organic acids such as lactic acid, acetic acid and nonoic acid on AO7 decoloration were evaluated. The possible enhancement mechanisms mediated by the addition of organic acids were discussed in terms of liquid surface tension, aqueous ozone concentration and bubble size, and the biodegradability of AO7 wastewater and decoloration mechanisms were also studied.
Section snippets
Materials
AO7, lactic acid (LA), acetic acid (AA) and nonoic acid (NA) were purchased from the Tianjin Chemical Reagent Factory in China. All organic and inorganic reagents were analytical grade and were used as purchased without further purification.
Gas phase surface discharge plasma reactor
The schematic diagram of the gas phase surface discharge plasma reactor system is shown in Fig. 1. An alternating current (50 Hz) power supply was used in the present study, and the discharge voltage was adjustable from 0 to 40 kV. The reactor was a Plexiglas
AO7 decoloration at different discharge voltages
Gas phase surface discharge plasma can generate a large number of active species (e.g., O3, O radical, N+, NO and NO2) and ultraviolet light [11], [22], which are directly related to the discharge voltage. Increasing discharge voltage can enhance pollutant degradation efficiency to a certain extent, and therefore it is important to evaluate the potential of the present discharge plasma system for AO7 decoloration at different discharge voltages.
The effect of peak discharge voltage on AO7
Conclusions
AO7 decoloration efficiency and the ratio of BOD5/COD were both enhanced by the addition of small amounts of acetic acid, lactic acid and nonoic acid. The decoloration efficiency of AO7 increased when the dosing quantity of the organic acid was increased. Aqueous surface tension decreased with organic acid addition, which led to a more uniform bubble distribution and the greater occurrence of smaller bubbles in the reactor. In addition, the ozone equivalent concentration also increased with
Acknowledgments
The authors thank the Projects funded by Natural Science Foundation of Shaanxi Province (2015JQ2040), China Postdoctoral Science Foundation (2014M562460), and the Natural Science Foundation of Shaanxi Province (2015JQ2051) for the financial supports to this research.
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