The formation of these oxidants depends on the type of electrodes employed as anodes. The large number of materials such as Pt, graphite, ACF (Activated Carbon Fiber), RuO₂, SnO₂, PbO₂, and Boron Doped Diamond (BDD), are used as electrodes (anodes) [13-22] which produce strong oxidants in order to oxidize the organic compounds. Ricardo E. Palma-Goyes et al. [23] have reported the electrochemical degradation of crystal violet with BDD electrodes; del Rio et al. [24] have studied the behavior of Ti/SnO₂-Sb-Pt for the treatment of reactive dyes; and also Miao Li et al. [25] evaluated the two typical electrodes Ti/RuO₂-Pt and Ti/IrO₂-Pt for the treatment of phenol. These electrodes show extraordinarily good results. But the electrochemical method with low cost electrodes remains a major goal. Therefore, there is growing interest in using materials viz. carbon [26], Pt, iron [27] and aluminum [28] that have been tested. Carbon fiber electrode shows good efficiency with the Fenton's reagent [29].

In earlier studies [30], the effect of the polarity of the working electrode on the degradation efficiency was discussed; when the polarity changed to cathode played an important role in achieving good degradation efficiency. So the present study is aimed to select the suitable cathode material in order to achieve high degradation efficiency.

Experimental

Reagents

All chemicals were of analytical grade and were used without purification. The Luganil Blue N (LBN) (C.I. Acid blue 193) with molecular formula (C₄₀H₂₄N₄CrNaO₁₀S₂) [F.W. 858] and the Acid Red I (ARI) dye with molecular formula C₁₈H₁₃N₃O₈S₂.2Na [F.W. 509.43], purchased from Sigma Aldrich (Bangalore, India), were used in this study.

Electrolysis

100 mL of aqueous solution of both LBN and ARI dyes (100 mg L^-1) were used for experiments. The NaCl was used as supporting electrolyte. Electrolysis experiments were conducted in an undivided cell containing the working electrode and the counter electrode. The saturated calomel electrode was used as reference. Prior to each electrolysis experiment the electrodes were dipped in dil HCl for few minutes and then washed thoroughly with double distilled water. The solution was subjected to stirring by a magnetic stirrer with the speed of 500 rpm.

The distance between the working electrode and the counter electrode was 0.5 cm. A potentiostat/galvanostat (Model PS-618 Chemilink Systems, Mumbai, India) was used as DC source for the electrolysis. The influence of different cathodes such as platinum, copper, zinc, lead, graphite, nickel and titanium, on the degradation of LBN and ARI was studied.

Analysis methods

Cyclic voltammetry measurements were done at room temperature with a conventional three electrode cell using a computer controlled electrochemical work station (CH Instruments 660C USA). A micro platinum electrode as working electrode and a platinum wire as auxiliary electrode were used. The UV- Vis spectrophotometer (UV-1650 PC SHIMADZU) was used for recording the spectra of dye solutions electrolyzed under different conditions. The chemical oxygen demand (COD) was measured by adopting standard procedure (Open reflux Method) [31].

If X is the colour removal, then A1 is the absorbance value of untreated dye solution at 579.5 nm, and AX is the absorbance value of the treated dye solution. If X is the COD removal, then A1 is the initial COD value of the solution before electrolysis and AX is the final COD value of the solution after electrolysis.

To see the influence of cathode materials, the preliminary electrolysis experiments were conducted using a solution of 2 g L^-1 NaCl and 100 mg L^-1 dye at current densities of 25 mA cm^-2 for ARI, and 50 mA cm^-2 for LBN dye with electrolysis time ranging from 0 to 120 min.

To see the influence of the current density, the preliminary electrolysis experiments were conducted using a solution of 2 g L^-1 NaCl, taking Cu as working electrode and 100 mg L^-1 dye at current densities of 4, 13 and 25 mA cm^-2 for ARI, and of 10, 30 and 50 mA cm^-2 for LBN with electrolysis time ranging from 0 to 120 min.

The NaCl concentration was varied from 2 g L^-1 to 6 g L^-1 keeping the initial dye concentration at 100 mg L^-1 and the current density at 25 mA cm^-2 for ARI and 50 mA cm^-2 for LBN. For each concentration of NaCl, the electrochemical treatment was carried out at different time intervals starting from 10 to 120 minutes.

Further experiments were conducted using solutions with dye concentration from 50 mg L^-1 to 200 mg L^-1 keeping NaCl concentration of 2 g L^-1 and the current density at 25 mA cm^-2 for ARI and 50 mA cm^-2 for LBN with electrolysis time from 0 to 120 min.

Results and discussion

Effect of electrodes

Two types of peaks were noticed in the visible and UV region. The only one peak in the visible range was noticed for both dyes and is due to azo group.

The ARI dye shows three peaks with gradual increase in intensities with decrease in wavelength in the UV region. Whereas LBN dye shows only two peaks in the UV region. These peaks are due to the groups other than azo groups. The peaks at 515 nm and 578 nm in the visible region correspond to ARI and LBN dyes, respectively.

After electrolysis the peak intensities of electrolyzed solution in the visible region decrease to zero for cathodes Cu, Pb, Pt, indicating complete removal of colour. Whereas for steel, zinc, titanium, nickel and graphite cathodes, 93 to 97 % colour removal was noticed. Also peaks at 375 and 230 nm of LBN, and 314 and 238 of ARI dyes were decreased in the order copper, lead, platinum, graphite, nickel, titanium, zinc and steel.

For the Cu electrode, not only the rate of degradation of dyes (ARI and LBN) is faster but also higher rate of COD removal is achieved. At Cu electrode as cathode, 87% COD removal for LBN dye and 74% COD removal for ARI dye were achieved with electrolysis time of 70 and 50 min, respectively. But 43%, 50%, 65%, 72%, 76%, 80% and 84% (LBN) and 33%, 42%, 50%, 56%, 66% and 69% (ARI) of COD removal was obtained at lead, platinum, graphite, nickel, titanium, zinc and steel electrodes.

From the results one can infer that the maximum degradation was observed at Cu electrode for both the dyes. Even though the electrochemical oxidation of organics depends on the type of anode materials, the cathode materials also play a role in enhancing the degradation. This may be due to the good electro catalytic property of copper, i.e., it reduces the dissolved oxygen present in the dye solution to produce H₂O₂, a good oxidant which enhances the oxidation reaction to occur at higher rate, or may be higher cathodic reaction takes place at copper which forces the anodic reaction to take place at higher rate compared to other cathodes. Hence it indirectly supports the anode and makes the electrochemical oxidation reaction to proceed at faster rate.

So in the present case the rate of oxidation reaction was accelerated when Cu was used as cathode. Among all electrodes, Cu was considered as the best cathode material for the present electrochemical investigation and it was used for further studies.

Effect of current densitys

In electrochemical degradation studies the current density is key design parameter and process manipulator of electrochemical applications. In order to assess the effect of applied current density on the rate of COD removal, another set of experiments was carried out with Cu electrode as cathode and varying current densities ranging from 4-25 mA cm^-2 for ARI and 10-50 mA cm^-2 for LBN dye.

Each curve in both figures shows three types of variation with respect to time for a given current density. In the first stage COD removal proceeds at faster rate, i.e., within 10 minutes, but in the second stage up to 60 min of electrolysis a gradual rise in rate of COD removal was noticed. In the final stage it reaches a limiting value of 74% COD removal for ARI at 25 mA cm^-2 and it was 87% at 50 mA cm^-2 for LBN dye. Further at higher current densities a slight increase in solution temperature was noticed. For further studies the current density was fixed at 25 mA cm^-2 for ARI dye and 50 mA cm^-2 for LBN dye as the optimum current densities.

Effect of electrolyte concentration

A general trend was observed for both dyes, an increase of the COD removal with increasing the electrolyte concentration up to 2 g L^-1, above which there was no additional effect of NaCl.

In other words, the maximum degradation of both ARI and LBN dyes and COD removal was achieved at 2 g L^-1 NaCl.

The higher concentration of chloride ions was not favourable for greater mineralisation. At higher concentration of NaCl, more chloride based radicals were produced at the vicinity of the anode and these radicals are capable of cleaving the -N=N-, azo group which is the most active site of dye molecule rather than the aromatic as well as the aliphatic linkages [32]. Hence the decolourisation of dye takes place at faster rate. However lower COD removal at higher concentration of NaCl indicated that, only less number of hydroxyl and other related radicals are produced which are known to oxidize the aromatic and aliphatic linkages leading to lower value of COD.

Effect of initial dye concentration

It was observed that the degradation decreased from 74% to 59% as the concentration of dye increased from 100 mg L^-1 to 200 mg L^-1 for ARI and 87% to 73.6% for LBN. Correspondingly, the amount of COD removal increased sharply at low concentrations of dyes.

The decreasing trend of decolourization and degradation efficiencies at higher dye concentrations may be due to the combined effects of increased dye concentration and decreased transparency of dye solution and in addition increase in dye concentration utilizes more and more oxidants which results in low rate of degradation of dye.

Conclusions

ARI and LBN dyes can be degraded by using Cu electrode as cathode with appreciable COD removal. The electrochemical property of cathode material affects remarkably the reaction rate for both the dyes. The reaction rate is reasonably higher on the Cu electrode, while the reaction rate is relatively slow on the Pb, Pt, C cathodes and the slowest on the steel cathode. However the rate of reaction is in the order of Steel < Zinc < Titanium < Nickel < Graphite < Platinum < Lead < Copper. The optimized conditions for the present degradation process are 25 mA/cm² current density for ARI and 50 mA cm^-2 current density for LBN, 2 g L^-1 NaCl concentration, 100 mg L^-1 dye concentration. The maximum COD removal of 74.24% and 87% was achieved for ARI and LBN respectively.

References