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Anodic Oxidation of Recalcitrant Organics in Tannery Wastewater on Carbon Anode

Posted on 29 January, 2010 | Tags: Research

Leather industry produces large quantities of wastewater with high pollution levels, which has to be treated before disposal. In this study, an electrochemical treatment method that involved oxidation of tannery wastewater at 3D electrodes was explored. The effects of pH and current were studied in detail. 3D carbon anodes were proved to have the electro catalytic properties for color and COD removal.

 

Tanning industry, one of the leading economic sectors in many countries generates large quantities of heavily polluted wastewater. Tannery wastewater owes its pollution load to a massive presence of various organic substances. However, the characteristics of tannery wastewater vary widely depending on the nature of the adopted tannin process, the amount of water used, the process of hide preservation, the hide processing capacity and the in plant measures followed to reduce pollution. Treatment of this wastewater by conventional biological methods is often inadequate to remove pollutants completely, especially ammonia and tannins (Ramaswamy and Rao, 1991), the latter being characterized by low biodegradability, common in polyphenolic compounds. In contrast, electrochemical techniques are becoming more reliable and are gaining popularity for the treatment of industrial wastewater. In recent years, there has been increasing interest in the use of electrochemical methods for treatment of wastewater. The electrochemical treatment of organic compounds in wastewater is potentially a powerful method of pollution control. Electrochemical methods have been successfully applied in the purification of wastewater from tannery industries. The reduction in color content and COD in tannery wastewater can be achieved by employing electrochemical oxidation method.
Electrochemical techniques when applied for wastewater are generally being operated at high cell potentials, and thus the anodic process occurs in the potential region of water discharge. In the anodic discharge of water, adsorbed hydroxyl radicals (OH) can be generated which are strong oxidizing agents of the majority of the organic pollutants (Simond etal.1997)
The electrochemical systems mainly comprise of 2 electrodes immersed in a suitable electrolyte. One of the electrodes is anode and the other is the cathode, depending upon the polarity connections given to D.C power supply. When requisite current and potential is applied to the system, redox reactions take place at the electrode.
Electrochemical reactors have been successfully used since the early 60's for the recovery of pollutants from metal recovery (Szpyrkowicz etal 1994, Szpyrkowicz etal, 1995). Among various applications of electrochemical processes, those in which oxidation of pollutants can be achieved is particularly important, as it can offer an alternative to traditional chemical oxidation.

Material and Methods
The tannery effluent used in the study was collected from a common effluent treatment plant (CETP) located in Tamil Nadu. The wastewater was collected from a number of tanneries that process rawhide to semi-finish stage as well as finish stage which was being treated at the CETP. The tannery wastewater sample was collected from secondary clarifier effluent outlet. Major fraction of biodegradable organic compounds has been treated in activated sludge process at CETP leaving behind non-biodegradable organic compounds.

Electrochemical Assembly
The electrochemical assembly used for the treatment of tannery wastewater comprised of a direct current power supply, a magnetic stirrer, a 3D carbon anode and cathode of the size 10 cm × 10 cm and 1.3 cm thick and an undivided electrochemical reactor (1500 ml). 
Experimental Studies
Experiments were performed for the treatment of tannery wastewater using an electrochemical assembly wherein 1liter of tannery wastewater was taken in an electrochemical vessel. 3D carbon electrodes (anode and cathode) were dipped in it. The electrochemical reaction assembly was connected to a direct current supply and the reaction medium was continuously stirred using a magnetic stirrer. A current of 0.5A was supplied for 1hour for the electrochemical reaction to take place. Samples in the interval of 10 min were collected and analyzed for pH, color and COD. Similarly investigations were carried out to study effects by varying the current supply to 1.5A. Similar procedures were carried out to study the effects by varying the pH to 4, 7.5 (natural pH of tannery wastewater) and 10.0.

Analysis
The pH of the test samples was measured using a pH meter (720A, Orion). The color removal of the samples was measured using a Data Logging Spectrophotometer (Hach). For the measurement to be reproducible within ± 5% Pt-Co units at 455nm, the spectrophotometer was standardized using Pt-Co standard consisting of potassium hexachloroplatinate and cobalt nitrate. The chemical oxygen demand (COD) of the sample was determined using a COD reactor (Hach) and Hach Portable spectrophotometer according to closed reflux method. An attempt was made to identify organic compounds in secondary clarifier effluent using GC-MS.

Results and Discussion
The physicochemical characteristics of tannery wastewater collected from secondary clarifier outlet are given in Table 1. The effluent has pH of 7.5-7.8. The concentration of suspended solids is high (100 ± 10 mgl-1). It has high TDS in the range 10-12 gl-1 that also gives rise to higher electrical conductivity obviating the need for addition of supporting electrolyte. It also contained chloride in sufficiently higher concentration (5000 ± 100 mgl-1) thereby providing means of indirect oxidation. The concentration of COD and BOD are within the standards set forth for treated effluent disposal in India. The filtered effluent still possessed brown color (300 ± 50 Pt-Co Units).

 

The treatment of the tannery wastewater using an electrochemical system was evident through the loss of color in the test solutions. The reaction accompanied only slight change in the pH i.e. 6.8 vis-à-vis a pH of 7.5 of the untreated effluent. The observations that were common to all the experiments conducted in the study were water electrolysis leading to gas evolution at anode and cathode, smell of chlorine gas, foam (froth) collection above the liquid level in the reactor and color removal. The treated effluents became slightly acidic compared to their initial pH (ΔpH = 1.0-1.5) and conductivity also increased by 3-4 mscm-1. There was accumulation of dirt on the carbon anode surface and off-white salty deposit on cathode at the end of electrolytic treatment.
Tannery wastewater contained several organic compounds as identified by GC-MS. The various major compounds and corresponding retention times are given in Table 2. Several of these compounds were absent in the electrochemically treated wastewater, notably the low molecular weight compounds. Some higher molecular weight compounds appear to resist oxidation and some new compounds form under the reaction conditions.

 

Effect of pH
The pH of the electrolyte is known to influence the rate of electro-oxidation. Moreover the pH adjustment can also lead to competitive reactions such as enhanced water electrolysis, enhanced deposition of metals at cathode and other reactions. Leaving apart changes in the electrolyte, pH also manifests in changing the electrode potential, e.g. 3D anode / electrolyte interface. Therefore, the effect of pH on the treatment of tannery wastewater was examined for different pH i.e. 4.0, 7.5 (natural pH of secondary clarifier effluent) and 10.0.

Effect of pH on Color and COD Removal
The reduction in color was found to be maximum at pH 4.0 (>80% removal). With respect to the color removal, pH 10 was only slightly better than pH 7.5. However, the color reduction was very slow at pH = 7.5. It was clear that the acidic pH (4.0) is more effective than the natural pH of the secondary clarifier effluent. The COD reduction was not significantly influenced at pH 10.0. It was observed that the COD removal was initially fast, then reached a plateau and then started to increase as a function of treatment time. This could be attributed to anode fouling due to the deposition of polymeric material as the formation of stable intermediates such as polyphenols.
The kinetics indicated that the color removal reaction can be approximately described as pseudo first order type with respect to color concentration. This may be explained by the following equation;
-d[color] / dt = k[color]
At higher pH (10.0), the reaction does not obey expected linearity. The corresponding k values are shown in the Table 3.
The kinetic constants for the electro-oxidative removal of COD are depicted in the Table 3. The -log [(COD) t / (COD)o] vs. time plots are fairly linear (R2  = 0.98) at pH 7.5, whereas the linearity is unsatisfactory at pH 4 (R2  = 0.674) and pH 10.0 (R2  = 0.77). The deviation of the data from the expected trend may be attributed to the simultaneous occurrence of more than one reactions facilitated by the H+ or the OH-. The pseudo first order rate constants (k) are as shown in the Table 3. The rate expression can be explained as;
-d[COD] / dt = k[COD]

 

Effect of Current
Anode current is yet another parameter that strongly controls the rate of the reaction in any electrochemical reactor. In the present study, experiments at current 0.5A and 1.5A were conducted and the changes in color and COD were measured as a function of reaction time. The study was carried out at pH 4.0 as well as 7.5

Effect of Current on Color Removal
There is a slight increase in color removal percentage at higher current. However the overall cooler removal within 40 min. is similar at both the currents. The corresponding rate constants are shown in Table 4. Similarly, at pH 4 also the rate of color removal is initially the same within 30 min. However, the overall percentage of color removal was observed to be 74.8% at 1.5A as against 64.7% at 0.5A. In general, the rate constants were higher at high current and at low pH.

Effect of Current on COD Removal
The effect of current on COD removal is shown in the Fig 2.(c-d). It is observed that applying higher current is not particularly useful at least initially. Only at 50-60 min, did the COD removal exceed the value at a lower current. Thus, 71.5% COD removal was obtained at 1.5A in a period of 1h as compared to 50% at 0.5A current. The corresponding k values are shown in the Table 4.  It can be inferred that the COD removal was not significantly improved at higher current.

Conclusions
Tannery wastewater is a complex wastewater with several pollutants present in it. The electrochemical method as described in the study can be efficiently used to treat the wastewater for reducing color and COD. As a general approach, maintaining a lower pH, say pH 4 and lower current (0.5A) can ensure 70-75% color removal and 50-60% COD removal. The reactions of color reduction and COD removal follow first order kinetics. The method is applicable with moderate energy input and medium to high capital cost.

Acknowledgements
DST, New Delhi for financial support.
Dr. S. Devotta, Director, NEERI for encouragement.

References

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NN Rao is Corresponding Author, is associated with Wastewater Technology Division NEERI, Nagpur
contact: nn_rao@neeri.res.in
Dr. SN Kaul is Principal MIT College of Engineering Pune
contact: principal_mitcoe@mitpune.com