Kinetic study of Acid Orange 7 degradation using plasmas in NaNO 3 solution sustained by pulsed power An-hsuan Hsieh, Kevin C.-W. Wu, Cheng-che Hsu * Department of Chemical Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan 1. Introduction The discharge of dye-containing wastewater is an environmen- tal issue since a large amount of dyes released to the environment from industrial effluents [1]. Among commercial dyes, azo dyes are among the largest and most widely used dyes [2–4]. Not only causing visible colorization of wastewater under low concentra- tion [1], but also being toxic and carcinogenic, azo dyes threaten the ecosystem [3,5,6]. Development of processes that are able to decompose these dyes is therefore highly desired. Advanced oxidation processes (AOPs), such as photocatalytic degradation [3,7–10], photodestruction [11–14], electrochemical degradation [15–17], sonolytic oxidation [18], synergistic process- es [5,6,19,20], and plasmas [21–30], have been intensively investigated. These processes are developed as efficient alter- natives for dye-containing water treatment. These AOPs are largely based on the production of OH radicals, which is a strong oxidizer that attacks organic pollutants readily [31,32]. Among AOPs, plasma in solution is a promising process since it effectively generates several strong oxidants, including OH radicals [33]. To generate plasmas in solution with an applied voltage below 1000 V, solution with conductivity well above 0.1 mS/cm is typically required [34]. Several sodium- and potassium-containing salt solutions have been used [34,35]. Although plasmas in solutions have been widely studied [36], the kinetic path for the degradation of organic dyes requires further studies. In this study, acid orange 7 (AO7), a synthetic azo dye, is selected as the target organic substance to study the decomposi- tion kinetics under the treatment of plasmas in NaNO 3 solution sustained using pulsed power. The concentration of AO7 is quantified using UV–vis spectroscopy. Disodium salt of terephtha- lic acid (NaTA), forming 2-hydroxyterephthalic acid (HTA) when reacting with OH radicals, is chosen as the chemical probe to quantify the relative amount of OH radicals formed during the plasma treatment process. HTA is quantified using photolumines- cence (PL) spectroscopy. The AO7 decomposition kinetics upon plasma treatment, and the correlation between the decomposition rate constant and OH radical formation rate are investigated. 2. Materials and methods Fig. 1 shows the schematic of the experimental setup. A cylindrical Pyrex TM cell with a cooling water jacket is used. This cell is 6, 3, and 8 cm in its outer diameter, inner diameter, and the height, respectively. The driving electrode is made of a platinum wire 0.5 mm in diameter covered with a glass tube to precisely control the area in contact with the solution. The grounding electrode is a bare platinum wire with the same diameter immersed in the solution with a length of approximately 1 cm. The pulsed power source consists of a DC power source (GWinsek, Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx * Corresponding author. Tel.: +886 2 33663034; fax: +886 2 23623040. E-mail address: chsu@ntu.edu.tw (C.-c. Hsu). A R T I C L E I N F O Article history: Received 11 July 2013 Received in revised form 9 October 2013 Accepted 20 October 2013 Available online xxx Keywords: Plasmas in solution Dye decomposition Pulsed power plasmas A B S T R A C T The kinetic study of the degradation of Acid Orange 7 (AO7), an anionic monoazo dye, using plasmas sustained by pulsed power in NaNO 3 solution is performed. The correlation between the degradation reaction rate constants and the OH radical density are investigated. Conditions with the application of pulsed power with t on = 0.1–1 ms and a repetitive frequency of 100 Hz in 0.02 M NaNO 3 solution are examined. The level of AO7 degradation is quantified using a UV–vis spectrometer. An OH radical molecular probe, disodium salt of terephthalic acid (NaTA), is used to estimate the relative OH radical densities. Results demonstrate that the degradation of AO7 follows first-order kinetics with respect to AO7 concentration with a reaction constant increases from 0.0023 to 0.0191 min 1 when t on increases from 0.1 to 1 ms. A nearly linear relationship between the relative OH radical densities and the AO7 degradation rate constants is observed, which shows that an increase in t on is able to simultaneously increase the OH radical formation and the degradation strongly suggests that AO7 degradation by OH radicals is the dominant process. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. G Model JTICE-772; No. of Pages 6 Please cite this article in press as: Hsieh A-h, et al. Kinetic study of Acid Orange 7 degradation using plasmas in NaNO 3 solution sustained by pulsed power. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.10.005 Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers jou r nal h o mep age: w ww.els evier .co m/lo c ate/jtic e 1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.10.005