Hazardous wastes are a continuous problem in today's world, increasing in both quantity and toxicity. Besides inorganic materials industrial effluents also contain organic pollutants and also radioactive chemical toxic materials. Many treatment technologies are in use and have been proposed for recovery or destruction of these pollutants. These include activated carbon adsorption, solvent extraction for recovery of chemical, electrochemical oxidation for destruction, direct incineration, chemical destruction and even direct immobilization in matrix like cement, polymer etc. In addition management of hazardous organic mixed wastes can be done by employing techniques like wet oxidation,photochemicaloxidation,andelectrochemical oxidation. Of these, electrochemical oxidation offers an attractive way of treating solid or liquid organic waste as it uses electron as a reactant. The electrochemical destruction of organic wastes could be carried out by Direct Electrochemical Oxidation (DEO) or by Mediated Electrochemical Oxidation (MEO) for the treatment of hazardous and mixed wastes. The range of organic materials which can be destroyed by this technique is very wide. Review of research studies indicated that the process of MEO is extensively employed for nuclear industry application, rubber, some plastics, poly urethane, ion exchange resins of various types and hydraulic and lubricating oils, aliphatic and aromatic compounds, chlorinated aliphatic and aromatic compounds etc.

Mediated Electrochemical Oxidation (MEO) is a rising and one of the most promising technologies extensively used for the destruction of organics since it is capable of mineralizing the organics into carbon dioxide and water completely, without emission of any toxic materials like dioxins (Nelson, 2001; Steele, 1990; Steele et al., 1990; Farmer et al., 1991; Steele et al., 1992; Chiba et al., 1995; Farmer et al., 1992a; Farmer et al., 1992b). DEO processes have been carried out for a variety of organic compounds (Chiang et al., 1997). In these processes the organic compounds are oxidized to CO₂ and H₂O at the anode surface. The MEO process is an emerging technology for the destruction of various kinds of toxic and refractory organic pollutants (GEF, Report of UNEP, 2004). This process employs an electrochemical cell to generate the oxidizing species and uses the same to destroy the organics at ambient temperatures (below 373 K) and at atmospheric pressure (Steele, 1990; Steele et al., 1990; Galla et al., 2000; Turner, 2002). The oxidizing species (mediator ions) are produced at the anode in an acidic medium and are used to destroy the organic compounds into CO₂ and water. Since the mediated metal ions have a strong potential to oxidize, high temperature is no required for organic oxidation, and as a consequence less volatile and off gases are produced. Several metal oxidizing agents like Ag(II)/Ag(I), Ce(IV)/Ce(III), Co(III)/Co(II), etc., have been tested previously in the MEO process both in pilot and commercial scale systems ( Farmer et al., 1992a; Farmer et al., 1992b; Nelson, 2002; Matheswaran et al., 2007a). Chiba (1993) studied the MEO process taking 0.5 M Ag(II) as mediator in sulphuric acid medium for the destruction of a number of organic compounds. Bringmann et al. (1998) in their work used Ag(II)-MEO system for the destruction of hydrocarbons and pesticides in sulphuric acid medium. Research studies provide evidence that Phenol is one of the most common pollutants found in the effluents of many industries such as pharmaceuticals, dyes, synthetic chemical plants, petroleum refineries, pesticides and herbicides, and treated by several technologies (Patterson, 1985; Lippincot, 1990; Comninellis and Pulgarin, 1991; Esplugas at al., 2002; Feng and Li, 2003; Pifer et al., 1999).

Raju and Basha, (2005) in their study identified that the mediated metal ions have a strong potential to oxidize, and a high temperature is not required for organic oxidation and therefore, less volatile and off gases are produced. The organic destruction in the MEO process can be carried out in either a batch or in a continuous feeding mode. In the case of batch type reaction, the organic is added at one time (zero time) in the reactor, and the process is carried out with or without Ce(IV) regeneration. However, in real applications, the continuous organic addition is used mainly for minimizing the oxidant usage by simultaneous regeneration, and by this way more quantity of the organic materials can be destructed than in the batch process. Usually, in the continuous process, an organic substance is added for a long time (e.g. hours, days etc.) at a particular flow rate and the oxidant concentration is maintained nearly at the same level by in situ electrochemical regeneration. However, it is also possible to predict the course of the destruction process by simulation based on simple kinetic models.

The Mediated Electrochemical Oxidation (MEO) has been identified as one of the most promising future technologies by the United Nations Environmental Programme (GEF, 2004) for the ambient temperature destruction of toxic organic pollutants and waste streams including persistent organic pollutants and dioxins (Steele, 1990; Farmer et al., 1992; Bringmann et al., 1995; Galla et al., 2000; Sequeira et al., 2006; Balaji et al., 2007a; Balaji et al., 2007b; Balaji et al., 2007c; Balaji et al., 2007d; Matheswaran et al., 2007a; Matheswaran et al., 2007b; Matheswaran et al., 2007c; Kokovkin et al., 2007). In particular, the Ag(II)/Ag(I) based MEO system has been widely studied to destroy hazardous industrial waste(Steele, 1990; Farmer et al., 1992; Bringmann et al., 1995; Galla et al., 2000; Sequeira et al., 2006; Matheswaran et al., 2007a), and to decontaminate plastic waste (Fourcade et al., 2003), It may be mentioned here that the advantages of electrochemical approach, compared to several other chemical tools, have been well recognized for the recovery of metals in their metallic form from metal ion pollutants (Kusakabe et al., 1986; Hwang et al., 1987; Kongsricharoern and Polprasert, 1996; Beauchesne et al., 2005; Chen and Lim, 2005), because the electrochemical methods are relatively simple and clean; moreover, as the conversion of the response of a chemical reaction or a process into a measurable electronic signal is direct and precise in electrochemical methods (as current or potential), regulation and automation are easier to achieve with them in comparison to the chemical techniques.

MEO process offers a number of advantages namely, the oxidation reaction takes place at ambient temperatures and pressures, the products of destruction are contained in the reaction vessel itself with the exception of gases and the production of secondary waste is minimized which avoids additional treatment methods. Hence a study was conducted to identify the best way to reduce TOC using electrochemical treatment method for resin effluents.

2.1 Materials

The synthetic anion exchange resins (amberlite R IRA 400 – strong base styrenic DVB based anion exchange resin) of different weights say 2, 3, 4 grams were mixed with 0.01M FeSO4 Solution of 100, 150, 200 ml in a reaction flask provided with a reflux condenser.

The temperature of the reaction mixture was maintained at around 95^oC by immersing the reaction flask in a boiling water bath. 50% (w/v) H₂O₂ was added to the hot reaction mixture drop wise using peristaltic pump. The dissolved resin solution was then made up to 200, 300, 400 ml respectively with distilled water.

2.2 Experimental Setup

Batch electrolyte cell used in the electro oxidation process is shown in Figure1. The cell consists of a glass beaker of 250,350,450 ml capacity closed with a PVC lid having provision to fit a cathode and an anode. Salt bridge with reference electrode was inserted through the holes provided in the lid. The current was supplied by an electric power source (Aplab 7771). Stirring was done with a magnetic stirrer.

Figure 1. Constant stirring Batch Reactor Setup

The electrochemical destruction was carried with volumes (200,300,400 ml) of dissolved resin solution which act as electrolyte in electrochemical cell. In the electrochemical cell, the electrolyte used are

Anode : Ti/RuO₂

Cathode : Stainless steel

Mediator : NaCl (g/l)

2.3 Experimental Procedure

The experiment was carried out in batch set up with different current densities of 1.25, 2.5, 3.75, 5 A/dm² and it was run for 7 hrs. Sample of about 1ml were collected for every one hour and the temperature, pH, cell voltage, electrode potentials were measured.

The effluent volume of 200,300,400 ml was taken for the experiments. Electrolysis was carried out at different parameters such as current densities viz 1.25, 2.5, 3.75 and 5.00 A/dm² with NaCl concentration as 8 g/l. During the electrolysis sample were collected at different time intervals.

2.4 Analysis

The collected samples were diluted and analyzed for Total Organic Carbon (TOC) present by using combustion TOC analyzer (Formacs^HT). TOC analyzer was used to verify that no organics remained in the spent electrolytes. To determine the quality of organically bound carbon, the organic molecules must be broken down to single carbon units and converted to a single molecular form that can be measured quantitatively. Samples were withdrawn from the electrolysis solution at different intervals. They were filtered and acidified by HCl and brought to a pH two prior to analysis. They injection volumes were 50/100µL.The temperature in the oven was 680^oC in combination with a Pt catalyst Calibration of the analyzer was achieved with potassium hydrogen phthalate standards (Merck)

Total Organic Carbon = Total carbon – Inorganic carbon.

3. Results and Discussion

Experiments were carried out at various volumes of electrolyte (200, 300, 400 ml). It was observed that good reduction in TOC (52.2 %) occurred at a current density of 3.75 A/dm² for 200 ml of electrolyte, whereas at a current density of 5 A/dm² the percentage reduction of TOC was found to decrease (Figure 2). For 300 ml of electrolyte the best reduction of TOC (73.8%) occurred at a current density of 3.75 A/dm² (Figure 3), while for 400 ml of electrolyte the best reduction of TOC (63.7%) was found to occur at 3.75 A/dm² (Figure 4) The current density was changed from 1.25 (A/dm²) to 5(A/dm²) and the best effect of TOC reduction was found to occur at 3.75(A/dm²) for the volumes of electrolyte. The concentration in the path decreased more rapidly leading to increasingly higher recovery with increase in the applied current density. Influence of current density: The electrolysis results taken with different current densities in the range 1.25 – 5 A/dm² and the corresponding charge (Amps/Lt) for various volumes of electrolyte have been calculated and graphs thus plotted (Figure 5). The study revealed that the TOC reduction had only very little effect on change in volumes of electrolyte at the end of electrolysis, since the initial concentration of effluent was kept almost same.

Figure 2. Percentage Reduction of TOC Vs Charge for 200 ml of Electrolyte

Figure 3. Percentage Reduction of TOC Vs Charge for 300 ml of Electrolyte

Figure 4. Percentage Reduction of TOC Vs Charge for 400 ml of Electrolyte

Figure 5. TOC Concentrations Vs Time for 300 ml Volume of Electrolyte.

Effect of electrode (anodic) potential: Figures 6 and 7 shows the effect of anodic potential for various current densities and volumes of electrolyte during the electrolysis time. Figure 6 depicts that the decrease in current density leads to decrease in the anode potential. Since, the anodic potential slightly increases during electrolysis due to the oxidation of organics on the anodic surface. Given a fixed current density and time duration, Figure 7 reveals that the anode potential decreases with increase in the volume of the electrolyte because of oxidation.

Figure 6. Anodic Potential Vs Time for 300 ml Volume of Electrolyte.

Figure 7. TOC Anodic Potential Vs Time for 3.75 A/dm²

Conclusion

This study focused in using electro chemical treatment based on MEO as a potential candidate to achieve mineralization of organic resins. The process is conducted for synthetic ion exchange resin materials by constant stirring batch reactor process. The experimental study was carried out in batch setup at different current densities 1.25, 2.5, 3.75, 5.00 A/dm² for various flow rates 20, 40, 60, 80, 100 LPH using RuO₂/Ti as anode and stainless steel as cathode in the electrolyser. The mediators used for this process are in situ generated OCl- ion and the Fenton's reagent (Fe²⁺/Fe³⁺ + H₂O₂),OH^o. Thus NaCl acts not only as supporting electrolyte as well as enhances the anodic oxidation of organics. The study highlighted that in batch reactor set up the best effect of TOC reduction was found to occur at 3.75 A/dm² with flow rate of 20 LPH.

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