Electrochemical Technique for Industrial Wastes Treatment by Anodic Oxidation in a Batch Reactor

Some industrial effluents are toxic, posing a threat not only to human beings of surroundings, but also to the ground and surface water resources. There are three ways to reduce pollution: (1) use of new, less pollution technologies; (2) effective treatment of effluent so that it conforms to specified discharge requirements; and (3) recycling waste several times over before discharge. As a novel advanced oxidation process (AOP), electrochemical oxidation process is powerful for degrading most organic compounds including toxic and non-biodegradable ones, and, so, it has attracted great attention. In this context electro oxidation (E.O.) was tested as an alternative method for oxidation of wastewater containing dyes in a batch electrochemical reactor. Variables studied were: Initial dye concentration, pH, applied current, solution flow rate, type of anode material and time duration of anodic oxidation (A.O.).It was found that the percentage of dye removal increased with increasing the applied current, duration of anodic oxidation, solution flow rate and with decreasing the initial dye concentration at pH 7 and room temperature (25±2C). The best economic results seemed to be under the following condition: flow rate 25ml/sec. current 1 ampere, lead spheres used as anode and pH 67 at room temp., where 91.9% reduction in colour takes place in 120 min. and 85.3% at 90 min. The mass transfer coefficient was found to increase with increasing reactor current density and decreasing the initial dye concentration. The anode material also was found to play an important part in the anodic oxidation process. Lead spheres anode was more effective than stainless steel.


INTRODUCTION
Anodic oxidation (A.O.) is a powerful technique for the oxidation of many organic compounds found in wastewaters, i.e. phenol (1) ; azo-dyes (2,3,4,5) ; textile industry effluents (6) .The electrochemical treatment of textile wastewater under different condition and different type of electrode materials were studied. (7,8,9) Frthermore, many of the dyes and their intermediate products, such as aromatic amines, are Bull High Inst Public Health Vol.39 No. 3 [2009]   toxic to aquatic life, carcinogenic and mutagenic to humans.Consequently, dyes have to be removed from textile wastewater before discharge (10,11) It has been extensively studied for the synthesis of wide variety of organic compounds.
The anodic oxidation reactions have in common the fact that they involve transfer of O-atoms from H2O in the solvent phase to the oxidation products. (12)odic oxidation has been used for the removal of phenol from wastewater by De Sucre and Watkinson. (13), they found that the percentage of phenol oxidized increased with increasing current density and decreased as initial phenol concentration, electrolyte flow rate, pH and anode particle size were increased.
Alcohols can be oxidized electrochemically by various electrodes which are also a catalyst themselves. (14)uhn. (15)nsidered the use of anodic oxidation for the removal of cyanides from waste effluents, to be a valuable and a desirable technique.
The electrode material is a key factor for these processes and must fulfill the following requirements: (a) high efficiency in the organic oxidation, (b) stability in anodic polarization conditions, and (c) low production costs.Different anode materials have been suggested for the anodic oxidation process, such as lead, graphite, and platinized titanium. (12,16) this electrolytic technique, reduction of a metal ion to its elemental form occurs at the cathode within the most relevant case.When aqueous solution is used, hydrogen is also produced at the cathode.
At the anode, oxidation occurs either directly by the liberation of oxygen or indirectly by the generated oxidizing agents as chlorine and hypochlorite. (17)idation process can destroy unwanted organic and toxic pollutants to less toxic or harmless compounds.
Removal of color during electro-oxidation of leachate also has been investigated (18) .
The researchers reported that 86% of color was removed within 180 min. of electrochemical oxidation; color removal had a pseudo-second order kinetic constant.
Pollutants in wastewater can be completely mineralized by electrolysis using high oxygen over-voltage anode, such as PbO2 and boron-doped diamond. (19)Naohide. (20)treated dyestuff using PbO2 anode.In their study, orange П was decolorized completely by a 120 min.electrolysis procedure using PbO2 anode at current of 0.2Acm -2 .
The objective of the present context is to investigate the process of anodic oxidation for color removal in a batch electrochemical reactor.
In this study, the effect of the following variables on the percentage of color removal is studied: applied current, initial dye concentration, duration of oxidation, anode material, pH and flow rate.

2-Material and Methods
Experimental set-up is shown in Fig.
(    The material of anode is an important parameter for anodic oxidation process.

3-3 Anode Material
Lead spheres anode was more effective than stainless steel, especially at early stages of oxidation.
At 45min the % of color removal was 60% and 40.2% for lead and stainless steel anode respectively and it was 91.9%, and 82.0% at 2 hours duration time.The approach used in analyzing the results of the experimental work can be fully understood by applying material balance for the recirculation batch reactor.

3-4 Effect of Flow Rate
A material balance on the dye (reactant) gives: Rate of disappearance of dye = Rate of reaction-V dc/ dt = KAC which upon integration gives:

3 -
).It consisted of a simple electrochemical reactor in the form of a rectangular plexiglass container, which was divided into two compartments by a PVC porous diaphragm.The anode compartment contained the circulated dye solution in a 0.5M sodium sulfate solution (to accelerate the conductivity of the solution).At the bottom of the anode compartment, a stainless steel screen was laid to support the weight of the lead spheres which acted as anode through its connection with a regulated D.C. power supply.The cathode was made of a stainless steel electrode sheet.Dye solution containing 0.5sodium sulfate was continuously pumped from a storage tank (5litres capacity) through a centrifugal pump, to the anode compartment.The flow rate was regulated by means of a bypass line which discharged into the storage tank.Bull High Inst Public Health Vol.39 No.3 [2009] During the time for each experiment (2 hours), samples were taken from the storage tank and analyzed using visible spectrophotometer (spectrophotometer, spectronic 20D) over the wavelength range 200-600nm, at 520 nm., a wave length of maximum absorbance (λmax) which was predetermined for this dye ( Direct Pink 3B) using a 1g/l standard stock dye solution, a known volume was diluted to prepare different concentrations of the dye under investigation.Variables studied were: initial dye concentration: 10-40mg/l (the usual range for dyeing wastes); solution flow rate: 10-50 ml/s; current passed into the solution: 0.5-1.5 amps and time of anodic oxidation: 0-120 min.; and type of anode material.The temperature of the solution was kept constant during all runs (room temp.25±2C o ).Direct pink 3B (C.I.Direct Red 31) Results and discussion The disappearance of the dye by anodic oxidation was measured spectrophotometrically by measuring the optical density (absorbance) of the solution at different time intervals.As expected, the rate of dye removal was found to increase with increasing the current passing through the dye solution, and with increasing duration of anodic oxidation and circulating flow rate.

Fig. ( 2 )
Fig. (2) Shows the effect of different applied currents on the percentage of dye

Fig. ( 4 )
Fig. (4) shows the effect of anode material on the percentage of color removal in the electrochemical reactor.

Fig. ( 5 )
Fig. (5) shows that increasing the circulation flow rate increases the rate of dye removal.It was found that percentage colour reduction was 91.9%, 62.7% for 25, 10ml/sec respectively at 120 min.time duration, pH 6 and a current of 1 amp., at room temperature.

Figure 2 :Figure 3 :Figure 4 :Figure 5 :Figure 6 :
Figure 2: Percentage of colour reduction of 10mg/l dye at pH 6, flow rate 25ml/sec.using lead spheres anode for different currents at room temperature

Figure 7 :
Figure 7: Relation between Log Co/C Vs time at 1 amp.current, flow rate 10ml/sec at different dye concentration