Degradation of Reactive Red 120 dye using hydrodynamic cavitation

Abstract

In the present work, degradation of Reactive Red 120 dye (RR120) has been carried out using hydrodynamic cavitation (HC) one of the upcoming advance oxidation techniques (AOPs). The effect of various operating parameters such as inlet fluid pressure, cavitation number, solution pH and addition of H₂O₂ as a supplementary oxidizing agent on the degradation rates were studied. The cavitational device used in this study has been optimized in terms of inlet fluid pressure and cavitation number. The photographic study was carried out to analyze the cavity behavior inside a transparent venturi. The degradation of RR120 was found to be dependent on the solution pH and higher degradation was achieved in acidic medium and addition of H₂O₂ further enhances the degradation rate.

Introduction

Waste water from the textile industry containing dyes causes serious environmental problem due to their intense color and potential toxicity. About 10–20% of the total dyestuff used in the dyeing process is released into the environment [1], [2]. The waste water containing colored solution is the source of aesthetic pollution, eutrophication, and perturbations in aquatic life essentially due to their organic nature. Among all types of dye used in the textile and paper industry around 50–70% dyes are of Azo class [2], [3], [4], [5]. These dyes are resistant to degradation by biological treatment methods and in fact introduce toxicity to the microbes and can be converted to hazardous by-products through oxidation, hydrolysis, or other chemical reactions taking place in the wastewater itself [1]. So these effluents need to be treated before their discharge into the environment. In the past few years many researchers have tried different methods for the degradation of textile dyes. These include carbon bed adsorption, biological methods, oxidation using chlorination and ozonation, electrochemical methods, membrane processes and other advanced oxidation techniques [6], [7]. In last decade a new technology called as hydrodynamic cavitation (HC) has been extensively studied by many researchers in the area of waste water treatment because this technique is energy efficient and also easy to scale up to industrial scale [8], [9], [10], [11]. In hydrodynamic cavitation, cavities are formed by passing the liquid through the constriction/geometry provided in line such as venturi, orifice plate. When the pressure at the throat or vena-contracta of the constriction falls below the vapor pressure of the liquid, the liquid flashes, generating number of cavities that subsequently collapse when the pressure recovers downstream of the mechanical constriction. The effects of cavity collapse are in terms of creation of hot spots, releasing highly reactive free radicals, surface cleaning and/or erosion, and enhancement in local transport (heat, mass and momentum) rates. The collapse of bubbles, generates localized “hot spots” with transient temperature of the order of 10,000 K, and pressures of about 1000 atm [12]. Under such extreme conditions water molecules are dissociated into OH and H radicals. These OH radicals then diffuse into the bulk liquid medium where they react with organic pollutants and oxidize/mineralize them. The two main mechanisms for the degradation of pollutants using hydrodynamic cavitation are the thermal decomposition/pyrolysis of the volatile pollutant molecules entrapped inside the cavity during the collapse of the cavity and secondly, the reaction of OH radicals with the pollutant occurring at the cavity–water interface. In the case of non volatile pollutant the main mechanism for the degradation of pollutants will be the attack of hydroxyl radicals on the pollutant molecules at the cavity–water interface and in the bulk fluid medium. The mechanical effects are also significant. In some cases the intensity of shockwaves generated by the collapsing cavity can break molecular bonds, especially the complex large molecular weight compounds. The broken down intermediates are more amenable to OH attack as well as biological oxidation, which can further enhance the rate of oxidation/mineralization of the pollutants.

In the present work the degradation of Reactive Red 120 dye (RR120) has been carried out using self designed hydrodynamic cavitation set-up and also the effect of solution pH and addition of H₂O₂ on the degradation rate has been studied. In this study, two important operating parameters (inlet pressure and cavitation number) for a cavitational device have been optimized in order to get maximum cavitational effects. The photographic study was also carried out to observe the cavity behavior inside the cavitating device (venturi), which offers a plausible explanation to the observation of the optimum operating conditions.