Journal of Scientific & Industrial Research Vol. 63, May 2004, pp. 405-409
Electrochemical treatment of effluents from textile and dyeing industries Rajeev Jain*, Nidhi Sharma and Meenakshi Bhargava Department of Environmental Chemistry, Jiwaji University, Gwalior 474 011 Received 29 September 2003; accepted 20 December 2003 The textile and dyeing industry are responsible for releasing highly contaminated coloured effluent leading to intense water pollution. This paper reports the results of an efficient electrochemical removal of colour and reduction in toxicity of textile industry effluents. The electrochemical behaviour is analyzed and assessed in terms of removal of colour, decrease in absorbance, time taken to completely remove colour from the dye solution, decrease in chemical oxidation demand, total dissolved solids and disappearance of any reduction peak in colourless solution, thereby indicating the absence of electrochemically active break down product. Keywords: Electrolysis, Cyclic voltammetry, Chemical oxidation demand, Electrochemical treatment, Industrial effluents, Effluents IPC Code: Int. Cl.7: C 02 F 1/46
Introduction The textile and dyeing industry has attracted the attention of environmentalists worldwide because of its high resource consumption profile in terms of water, chemicals, and energy and release of highly contaminated coloured effluent at the end of entire production chain leading to intense water pollution. The existing wastewater treatment technology is often inadequate to control colour of effluent and toxicity of wastewater to aquatic organisms1, 2. Hence, very little decomposition of these organic molecules takes place by aerobic and anaerobic wastewater treatment processes and discharge level of COD cannot be achieved by these processes without some form of post treatment3,4. The electrochemical methods have found use in destruction of toxic and nonbiodegradable organics by direct or indirect oxidation/ reduction5,6. Electrochemical methods are very promising as they involve the controlled degradation of the pollutants. They are moreover very effective towards the reduction of chromophoric groups of dyes and colour removal, which is the main disturbing factor for water recycling in most of the industries7-12. Here the electrochemical behaviour of effluent samples was studied and the results obtained provide the optimum conditions for carrying out controlled
potential electrolysis. The electrochemical behavior is analyzed in terms of decrease in reduction peak current with time, decrease in absorbance and time taken to completely remove colour from the dye solution and decrease in COD. Experimental Procedure Effluents used for the study were sampled at yarn weaving, textile dyeing, printing and CETP plant. The characteristics of the tested effluents are given in Table 1 Due to the presence of organic dyes the samples I, II and III were intensely coloured , highly turbid and had large amount of total dissolved solids (TDS). Controlled potential electrolysis of effluents was carried out on a BAS CV-27 Cyclic Voltammograph in connection with a Digital Electronic 2000 Ominograph x-y/t recorder. Three-electrode system was used as electrochemical cell. The working electrode being used for cpe was a platinum foil of surface area 3 × 3 cm2 the reference electrode was saturated calomel electrode and counter electrode was a platinum wire. All the three were dipped directly in solution to be electrolysed. Bench-scale electrochemical treatment Table 1—The characteristics of the tested effluents Sample
pH
COD
TDS (mg/L)
Sample I Sample II Sampe III
7.8 8.2 7.6
920 320 240
863.5 332.6 233.0
________ *Corresponding author Telefax: 91-751-2346209 E-mail:
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system is shown in Figure1. Voltammetric studies were carried out on EG & G Potentiostat integrated with Applied Electrochemistry software. The pH metric measurements were made on Hach EC 40 Bench top digital pH meter. The Chemical Oxidation Demand (COD) of the dye effluent sample was determined by Open Reflux method, using COD digestion apparatus Model 2015 from Spectralab. The absorption spectra of the samples were recorded on Elico SL 159UV-Visible spectrophotometer. Materials and Methods All reagents used were AR grade. 1.0 M solution of KCl was used as supporting electrolyte. For carrying out COD studies in effluents, all the reagents were prepared as per standard methods13. Experimental Procedure Electrochemical technique was coupled with conventional processes such as clarification and filtration, in order to successfully remove the lint and solid matter present in the effluents. Thereafter, electrochemical treatment was performed on the bench scale experimental set up. The experiment was performed using an undivided cell with a capacity of 100 mL supplied with a magnetic stirrer. All the three electrodes were inserted in the electrochemical cell and small amount of KCl was added as supporting electrolyte. In our study the role of KCl is to suppress the migration current and the halide ion does not participate at all in the electrode reaction. A known volume of effluent was circulated between the electrodes separated
by small gaps, while D C power was supplied between the electrodes. Since hydrogen is formed during the cell operation, a 10 min degassing time was allowed. Decolourisation and decomposition of organic matter in dye effluent was carried out by potentiostatic controlled potential electrolysis under batch condition using inert platinum foil working electrode. Reduction takes place directly on the inert electrode without involvement of other substances. The treated dye effluent under a given set of experimental conditions was collected and analyzed. The decolourisation was assessed from the spectral data by monitoring absorbance of the effluent at an appropriate λmax for each effluent before and after decolourisation. Results and Discussion Cyclic voltammetric behavior of effluent samples was studied at platinum working electrode by recording initial scan of all three effluent samples. Cyclic voltammograms of all the three effluents showed two cathodic peaks in the forward scan and an anodic peak in reverse scan. However, in Sample I, anodic peak is absent and in Sample II and III the difference in cathodic and anodic peak potential is more than 60 mV suggesting irreversible behaviour14-17. The effluents were finally submitted to controlled potential electrolysis. The potential was controlled at 1.20 V, which is slightly higher than reduction peak potential in all the cases. All the effluent samples got decolourised within 20 min to 5 h of electrolysis. Sample I took 3 h, sample II 5 h while sample III got
Fig. 1— Bench scale electrochemical treatment system
JAIN et al.: ELECTROCHEMICAL TREATMENT OF EFFLUENTS FROM TEXTILE & DYEING INDUSTRIES
decolourised within 20 min of electrolysis along with complete disappearance of redox peaks (Figures 2-4). The progress of electrolysis was also monitored by recording decrease in current at different time intervals (Table 2). It was observed that colour of the effluent sample gradually faded with the decrease in current. The absorption spectra of original and electrolysed sample were recorded by scanning the absorbance from 300800 nm Vs blank (distilled water). Effluent sample I, II, and III exhibited absorption bands at their respective λmax. The intensities of these absorption bands in electrolysed effluent sample faded and almost touched zero absorbance level (Figure 5 and 6). Thus, after electrolysis, all the effluent samples showed decrease in peak current and decrease in absorbance, and also decrease in TDS, decrease in COD and change in pH. Data pertaining to decrease in COD, TDS, and change in pH are presented in Table 3. The higher percentage removal of COD in sample I and II could be attributed to the greater average solution contact time with the working electrode surface area. All the effluent samples after electrolysis leading to complete decolourisation presented increase in pH. The pH increase was mainly due to cathodic water decomposition, which resulted in hydrogen evolution and shift of equilibrium towards alkaline me-
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dium. This fact is also supported by other workers18,19. Conclusions The data obtained in the present investigations reveal that high decolourization is achieved with considerable lowering of toxicity. The selected effluents intense colour and could successfully be electrolysed for 90-95 per cent colour removal (almost zero absorbance level), 60-74 per cent COD removal, and 6071 per cent TDS removal in a lab-scale batch system. Further the effluent did not require any additives like reducing agents or catalytic agents to reduce the Table 2—Gradual fading of colour and decrease in current with time for effluents Effluent Samples
Sample I
Sample II
Sample III
Time taken for electrolysis (min)
Current -ip,c
00 60 180 00 120 180 240 300 00 20
Out of range 11.88 5.15 17.31 7.51 5.82 4.37 4.30 18.84 16.43
Fig. 3—Cyclic voltammograms of effluent sample II at platinum working elec treatment and (B) After electrochemical treatment
Fig. 2— Cyclic voltammograms of effluent sample I at platinum working electrode, scanrate 500 mV/s: (A) Before electrochemical treatment and (B) After electrochemical treatment
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Table 3—Characteristics of electrochemically treated effluents
Sample I Sample II Sample III
pH
COD (mg/L)
COD per cent removal
TDS (mg/L)
TDS per cent removal
8.8 9.0 9.2
246 94 90
73.9 70.6 62.5
243.5 111.0 93.0
71.7 66 60
2
3 4 5
6 Fig. 5— Comparative absorption spectrum of effluent sample I: (A) Before electro- chemical treatment and (B) After electrochemical treatment
7
8
9 10
11
12 Fig. 6— Comparative absorption spectrum of effluent sample II: (A) before electrochemical treatment and (B) After electrochemical treatment
13
organics present in the dye effluent and due to the inert nature of electrode, consumption of electrode did not take place.
14
Acknowledgement The authors are thankful to Ministry of Environment and Forests, New Delhi, for financial support that made this study possible.
16
15
17
18
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