Ultrasonic treatment of liquid waste containing EDTA
Introduction
Ethyelenediaminetetraacetic acid (EDTA) has numerous applications, based on its ability to control the action of different metal ions through complexation. It is used as an important decontaminating agent in the nuclear industry. The presence of EDTA in decontamination wastes can cause complexation of the radioactive cations resulting in interferences in their removal by various treatment processes such as chemical precipitation, ion exchange etc. Further, it might also impart elevated leachability and higher mobility to cationic contaminants from the conditioned wastes i.e. waste immobilized in cement or other matrices and can negatively influence the quality of the final form of waste [1]. EDTA is not easily biodegradable [2], scarcely degradable by chlorine [3] and hardly retained by activated carbon filters [4].
As a consequence, techniques suitable for destruction of EDTA are needed to protect the environment. In light of the increasing concern over the contamination of the environment by hazardous chemicals, there is great need to develop innovative technologies for the safe destruction of toxic pollutants. The processes must be easy to operate and capable of achieving a total or near-total mineralization.
Instead of chemical treatment, the application of high power ultrasound (US) for the destruction of organic pollutants has recently drawn much attention. It is an Advanced Oxidation process wherein it can affect organic oxidation in three different mechanisms: nucleation, growth and cavitation [5]. According to this theory, the effect of ultrasound arises from the longitudinal vibration in liquid molecules through a series of compressional and rarefactional cycles resulting in the tearing of solvent layers during rarefaction. Cavities are formed at the point where the pressure in the liquid drops well below its vapour pressure. These cavities turn into bubbles and are filled with vapour of the solvent molecules. Bubbles start reverberating with the propagating ultrasound wave and grow in size after every rarefaction cycle until an optimum stage is attained where the energy supplied by the wave is no longer capable of sustaining these bubbles. At this stage the bubble implodes, thereby allowing solvent molecules from the bulk to rush into the void space of relatively low pressure in the form of microjets. This process results in the rise of temperature as high as 5000 K and pressure of several thousand atmospheres (100 MPa) during the collapse of the micro bubbles generated by ultrasound, which now acts as a micro-reactor. These are the real sites of chemical reactivity. The typical ultrasound decomposition of toxic organics is 10,000 times faster than the natural aerobic oxidation. However in a recent economic analysis of treatment of wastewater containing organics, the cost of sonochemical oxidation is found to be comparable to incineration [6].
In wastewater treatment a bubble of cavitation may function as a micro-reactor inside which, the volatile compounds are destroyed. The cavity may also be considered as source of H, OH, HOO radicals, which have been extremely effective in the destruction of organic pollutants.
Although hydrogen peroxide (H2O2) can be produced by application of ultrasound alone to a diluted aqueous solution, the amount may be too small to be significant. These advanced chemical oxidation process (ACOP) generally use a combination of oxidising agents (such as H2O2 or O3), irradiation (such as ultraviolet or ultrasound), and catalysts (such as chemical or photo catalysts), as a means to generate hydroxyl radical [5]. The reason why H2O2 can be used for such diverse applications is the different ways in which its power can be directed i.e. selectivity. Hydrogen peroxide has none of the problems of gaseous release or chemical residues that are associated with other chemical oxidants. By simply adjusting the conditions of the reaction (e.g., pH, temperature, dose, reaction time, and/or catalyst addition) H2O2 can often be made to oxidize one pollutant over another, or even to favor different oxidation products from the same pollutant.
The degradation of EDTA has been attempted by ozonation [7], UV/H2O2 [8], phototcatalysis [1], UV/oxidants [9], radiolysis [10], radio-photocatalysis [11] and combined techniques [12] with variable results. Degradation of EDTA using H2O2 alone at alkaline pH has been reported [13]. However, in literature there are no reports available on the application of ultrasound for the degradation of EDTA.
The treatment of simulated liquid waste generated after chemical decontamination of heat exchangers of the boilers of the Pressurised Heavy Water Reactor (PHWR) was studied at Centralised Waste Management Facility (CWMF) at Kalpakkam using Advanced Oxidation Processes. The decontamination wastes contain EDTA (2%), EDA (0.6%), Ammonia (0.6%) and hydrazine (0.1%).
For the optimization of the waste treatment conditions, information on the influence of the experimental conditions on the degradation is needed. Since the concentration of EDTA is the maximum (20,000 mg/l), a systematic study of the degradation of 20,000 mg/l of EDTA alone using ultrasound at 40 and 130 KHz was attempted initially. In the present study, a comparison of kinetics of degradation of 20,000 mg/l of EDTA was carried out using stoichiometric quantities of H2O2 in all the experiments viz., using ultrasound+Fenton’s reagent (Fe+2 + H2O2) and ultrasound + H2O2 respectively without taking into consideration the formation of degradation intermediates.
Section snippets
Experimental setup and materials
Sonication experiments were performed in an ultrasonic cleaning bath of frequency 40 KHz from M/s Ultrasonic systems, model 500 and 130 KHz ELMA Transsonic industrial table top model T1-H-20-MF of power 2 KW and 300 W respectively. The samples were taken in a 1 l beaker and were placed inside the ultrasonic bath. Constant temperature was maintained at 40 °C with a temperature control system provided in the instrument. The internal temperature of the bath at both the frequencies was 55 °C ±2 °C.
Effect of frequency
The frequency and power of the sonic system are important variables, which can influence cavitation during sono activation of chemical reactions. There was no degradation of EDTA (2%) when only ultrasound of frequency 25, 40 and 130 KHz was used. When Fenton’s reagent in combination with ultrasound at 25 KHz was used, there was no significant increase in the rate of degradation when compared to the degradation using Fenton’s reagent alone. The above observation is in agreement with that of
Conclusion
From the comparative study on the kinetics of EDTA degradation it can be concluded that higher frequency of ultrasound and Fenton’s reagent favor faster kinetics of degradation. The time taken for the complete degradation of EDTA (2%) for the various sonochemical and chemical oxidation processes is shown in Table 1. From the observed pH changes during the chemical degradation and sono-Fenton processes, it can be concluded that there is loss of chelating ability of EDTA. Formation of amides in
Acknowledgements
The authors express their sincere thanks to Mrs Sharal Sarojini for having assisted in carrying out the experimental work.
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