Journal of Scientific & Industri al Research

Vol. 62, August 2003 , pp 813-819

Treatment and Decolourisation of an Azo Dye in Industrial Effluent

Rajeev Jain* , Meenakshi Bhargava and Nidhi Sharma

Department of Environmental Chemistry, Jiwaji University, Gwalior, 474 0 I I

Received: I 0 September 2002; accepted:09 January 2003

The electrochemical behaviour of Orange II is studied voltammetrically using Pt foil and steel foil as working electrodes, while Pt wire and Ag/AgCI were used as auxiliary and reference electrodes respectively. Controlled potenti al electrolysis was carried out at -1.20 V. The decrease in cathodic peak and peak current lead to decolouri sati on of solution (2 x I o-4 m) with a significant decrease in COD from 1840 mg/L to 174 mg/L. Orange II is found to yield a mixture of sulphanilic acid and 1-amino-2-naphthol upon electrochemical reduction. Based on voltammetric and spectrophotometric analysis, rate of the reaction was determined and found to be of first order.

Keywords: Decolourisation, Azo dye, Industrial effluent

Introduction

Most of the industries use dyes and pigments to colour their products. The largest consumers of these dyes are textile, tannery, paper and pulp, pharmaceuticals, cosmetics, food processing units and they are, in turn, the most serious polluters of our environment. The textile industry in India is one of the oldest and largest in the country. These mills require volumes of water of high purity and generate equally large volumes of wastewater, which is highly coloured, and complex. Colour removal from these effluents has been getting more attention in the last few years, not only because of its toxicity but also due to its visibility. Recent estimates indicate that, approximately 12 per cent of synthetic dyes used every year are lost during manufacture and processing operations and 20 per cent of these lost dyes enter the environment through effluents that result from the treatment of residual industrial waters 1• The proper treatment of aqueous wastes is fundamental for the preservation of environmental equilibria and the quality of aquatic systems . Electrochemical methods for pollution abatement have been shown to be viable alternative or complements to other treatment methods, in some cases, especially when pollutants

* Author for correspondence E-mai I: gwlsosmica@ sancharnet. in

are recalcitrant to biological processing, electrochemical methods are found to be useful precursors to biological treatment. In principle, any pollutant capable of undergoing oxidation or reduction in given experimental conditions at an electrode can be transformed and/or removed from wastewater at appropriate potential s2

-9

.

The present work reports the results of electrochemical studies on an azo dye Orange II, in particular reference to colour removal and wastewater treatment. It is an acid dye and the acidic group acts as auxochrome in these dyes . They are used to dye wool, silk, nylon, and polyacrylic fibres 10

.

Experimental

Instrumentation

Controlled potential electrolysis (CPE) was carried on BAS CV27 Cyclic Voltammograph in connection with a Digital Electronic 2000 Omnigraph X-Y It recorder. A three-electrode cell was used for the purpose. The cell comprises a glass container with a cap having holes for introducing electrodes and nitrogen gas . The working electrode is Pt foil (3 x 3 cm2

), reference is Ag/AgCI and auxiliary electrode, a Pt wire which was directly poured 111 the solution . Cyclic voltammograms were taken using EG&G Princeton Applied Research VersaStat-II Potentiostat.

814 J SCI IND RES VOL 62 AUGUST 2003

The pH metric measurements were made on Hach EC 40 Bench top digital pH meter fitted with a glass electrode and saturated calomel electrode as reference, which was previously standardized with buffers of known pH in acidic and alkaline medium. The electrolysed solution was subjected to TLC for end product analysis. TLC was done on ready­made plates of E-Merck (silica gel 60 F254) with overall dimensions of 20 x 20 em. For end product identification, TLC was also employed using Dioxane: Water (10: 2) as mobile phase. The initial compound could be seen with the naked eye, while for the electrolysed solution the plate was viewed under UV lamp. The reaction kinetics was studied with the help of UV-visible spectrophotometer (Elico SL-159). The rate of decrease of colour with time was continuously monitored .For measuring pollutional load of electrolysed solution, Chemical Oxygen Demand (COD) of both coloured and clecolourised/electrolysed solutions were recorded. COD Digestion Apparatus (Spectra lab-2015-S) was used for carrying out the experiment.

Reagents and Solutions

Orange II of Standard Fluka Chemika was used. 2 x 1 o·' M stock solution of Orange H in 50 mL of distilled water was prepared. 1.0 M KCI was prepared in distilled water as a supporting electrolyte. Britton­Robinson buffers in the pH range 2.5-12.0 were prepared in distilled water by adding suitable amounts of 0.4 M NaOH solution (basic solution) to a stock solution composed of a mixture of phosphoric acid, boric acid, and glacial acetic acid (ac idic solution). Solution was stirred and left overnight to attain equilibrium. All the chemicals used were of AR grade.

For COD determination , potassium dichromate (0.25 N) and ferrous ammonium sulphate (0.1 N) , were prepared in distilled water. Distilled water was used for the necessary dilutions.

Procedure

For eye! ic voltammetry (CV) and controlled potential electrolysis (CPE) the solution was prepared by taking 9.0 mL of the stock solution and 1.0 mL of 1.0 M KCI (as supporting electrolyte). 1.0 mL of stock solution was eli luted to I 0.0 mL with BR buffer solutions . The solution was deareated by passing nitrogen gas for about 15 min and, thereafter, a

blanket of nitrogen gas was maintained throughout the experiment. The cyclic voltammograms were recorded in the beginning of the experiment and then the solution was subjected to electrolysis at a controlled potential of -1.20 V. Further, at the end of e lectrolysis , i.e. , when the solution became colourless and current was significantly decreased, the voltammograms were again recorded.

For coulometric determinat ion of number of electrons 'n ' consumed in the reduction, solution was prepared in the similar fashion as for cyclic voltammetric studies . With the progress of the electrolysis the colour of the solution gradually got faded and finally a colourless so lution was obtained. The current decreases exponentially with time and becomes practically zero after some time. Value of Q was read directly from the instrument and by the application of Q = nFN, the value of 'n' could be calculated .

During controlled potential electrolysis the absorbance and current values were recorded at regular interva ls with respect to time. An aliquot of the solution under study was taken out in the quartz cuvette and was put in the UV ~Vis spectrophotometer with di stilled water as reference. The decrease m

absorbance value with passage of time was noted.

Results and Discussion

Effluents, which contain azo reactive dyes , are very difficult to treat in environmental systems, due to the sulphonic acid groups, which makes the dyes

I bl d J? . J4 very water-so u e an polar - . Therefore, such

dyes are not readily adsorbed onto the non-polar carbon surface. The dye must first be degraded and its degradation products adsorbed. The reduction cleaves the azo linkage of the dye, producing the corresponding aromatic amines and resulting in decolouri sation. Direct photolysis has not been found to be effective in the degradation of azo dyes. Biological degradation is s low for many azo dyes and does not proceed at all for Orange II. Recent studies have shown that oxidation of organic compounds 15

by way of Fenton's reagent are useful in the degradation of this •type of compounds 16 17

. This low BOD 5: COD ratio values (less than 0.1) indicate their resistance to conventional bio logical treatment 18

.

Electrochemical reduction is a superior technology for physical treatment of dyes, as there is no simultaneous addition of anions, such as sulphate or

JAIN eta/.: TREATMENT AND DECOLOURISATION OF AN AZO DYE 815

chloride. The main reagent used is the electron, which is a clean reagent, and usually there is no need for adding extra chemicals .

Cyclic Voltammetry

The comparative voltammograms are shown in Figure I . The electrochemical characteristics are given in Table I . Orange ll gives a 4-electron, irreversible, diffusion controlled cathodic peak.

In aqueous medium, a peak current (ip, c) of -16.21 J..LA was observed at a scan rate 150 m V /s, which decreases to -9.0 ~LA in electrolysed so lution.

-100 • 0

- UO · O

- 12 0 · 0

~ " - 10 · 0

- 4 0 · 0

0 · 0

'0· 0

On reversing the scan, an anodic peak involving one electron was observed. There was increase in anodic peak current from 18.27 ~LA to 57.31 ~LA. Irreversible nature of the electrode process was indicated by a negative shift in peak potential values wi th increasing scan rate (50-200 mV/s), as shown in Figure 2. Reduction was found to be diffusion controlled, as evident by the straight-line plot of ip, c vs v112 (scan rate) (Figure 3) . A positive shift in anodic peak potential with increasing scan rate was also observed. There was also an increase in both ip,c and ip,a (i.e. peak currents) with increase in scan rate . Same trend

&O · OL-----~------~------~------~------~----~L------1 1 400·0 I 000· 0 lOD · O - J 00 ·0 -uo.a -I ooo.c -11.00

Figure 1 -Cyclic voltammograms of Orange II (2.0 X 10·4 Min distilled water) at Pt working electrode, scan rate 200 mY /s , (i) Before electrochemical treatment and (ii) After electrochemical treatment

Table 1- Electrochemical characteristics of Orange II in aqueous medium at various scan rates at Pt foil working electrode,

cone. = 2 X I 0 -J M

Working electrode Scan rate yin ip, c Ep, c ip, a Ep, a (mY/s) (I-lA) (Y) (I-lA) (Y)

50 7.07 A -6.43 -0.402 15.16 0.758 B -4.23 -0.402 17.36 0.794

100 10.0 A . 13 .61 -0.402 16.97 0.737 B -11.00 -0.402 53.40 0.804

150 12.24

Platinum Foil A - I 6.21 -0.399 18.27 0.719 B -9.00 -0.399 57.3 I 0.801

200 14.14 A - I 8.8 I -0.43 I 19.57 0.708 B -10.35 -0.420 57.31 0.790

A = Before electrochemical treatment, B = After electrochemical treatment

816 J SCI IND RES VOL 62 AUGUST 2003

was observed in both acidic and alkaline pH (3.8 and8.8, respectively).

Electrochemical Behaviour at Varying Ph

A negative shift is observed in the anodic peakwith increase in pH from 2.5 to 12.0, while thecathodic peak shows a positive shift with increase inpH. A significant cathodic current and peak in highlyalkaline or acidic medium was observed, while inbetween 5.0-8.8 pH, no significant cathodic peak was

observed. In this pH range the Ep,a (anodic peakpotential) shows a negative shift even in electrolysedsolution. In highly acidic pH (2.5, 3.8) and in highlyalkaline pH (10.5,12.0) there is a positive shift in theEp,a values in electrolysed solution. In the entire pHrange, there is significant decrease in both anodic andcathodic peak current values, as reported in Table 2.

Effect of concentration of azo dye on redoxprocess was examined and it was observed that withincrease in concentration of the azo dye, peak

-,s·OO~-----r------.------,------'-----~r------.-----'

a.oo

s.e 0 --------

toooo 60002S.DO~-----L------i-----~------~----~-------L----~UOO0 -100.0

E (m V)

Figure2 - Cyclic voltammograms of Orange II (2 X I0~4M in distilled water) at Pt working electrode at varying scan ratcs;(i) 50, (ii) 100. (iii) 150, (iv) 200 mV/s

Table 2 - Electrochemical characteristics for Orange II at different pH at scan rate = I00 mVIs, cone = 2 x I0 ~"M

pH ip,c (IlA) Ep,c (V) ip,a Ep,a

(i) (ii) (i) (ii)(uA) (V)

2.5A -20.26 -0.350 59.88 -0.518B 260.20 -0.350 0.900 95.00 -0.695

3.8A -105.00 -0.694 35.47 -0.537B -95.00 -0.694 55.00 -O.4Hl5.0A -148.70 -0.797 120.90 -0.484B 100.00 -0.5506.0A -86.51 -0.726 83.06 -().705B 60.00 -().700

10.5A -23.10 -0.384B -18.19 -0.271 74.52 -0.833

12.0A -24.31 -0.338 145.30 -0.847B -38.95 -0.224 72.60 -0.779

A = Before electrochemical treatment, B = After electrochemical treatment

JAIN eta/.: TREATMENT AN D DECOLOURI SAT ION OF AN AZO DY E 8 17

potentia l shifts towards more negati ve potential , indicating the diffi cult reduc tion and ox idat ion with concentration .. The time taken for decolourisati on a lso decreases s ignificantl y w ith decrease in concentrati on, as shown in Table 3. Decrease in ip,c (cathod ic peak current) w ith time was observed for

Orange II, as reported in Tabl e 4 .

Controlled Potential Electrolysis and Coulometry

Contro lled potenti a l e lectrolysis of 2 x I 0-4 M soluti on of Orange II dye so lution was carri ed out at

25·0

1 20-0

<! ::].

- 15-0 u

.e- 0

1 10-0

5.0

8·0 10 ·0 12·0 14•0 16· 0 18·0

-- 1..1Y2

Figure3- Pl ot of ip.c (!JA) vs v1/ 2 lo r Orange II (2.0 x 10-4 M) at

Pt foi l worki ng electrode

Table 3- Electrochemi cal characteri sti cs for Orange II aL

\I{IITing concentration at Pt fo il working electrode

Concentrat ion o f so luti on taken for electro lysis (M)

20 X 10 -4 M

2 x l 0 -4 M

0.8 *x 10-4M

Time taken for co mpl ete removal of co lour (h)

10

5

3

I :30

Table 4 - Decrease in ip,c (cathod ic peak current) with time for Orange II at Pt fo il working elec trode, scan rate = 50 mY/s ,

cone = 2x I o·4 M

Time (h) ip ,c (pA)

00 5.22

02 2.98

04 2.86

06 2.54

e lec tro lys is potenti al of - 1.20 V . With Pt fo il as worki ng e lectrode the reaction took approximate ly 8 h for complete clecolourisati on and reducti on of cathodic peak current.

Coulometric measure ments we re carri ed out to calculate the number of e lectrons in vo lved in the overa ll e lectrode process and it was fou nd to be 4 ±

0.2. Normally, at a ll pH values azo group is reduced in two-e lectron e lec trode reac ti on to hydrazobenzene but because of the presence o f strong e lectron re leas ing substituent -OH the hyd razo de ri va tive is unstable and s ing le four-el ectron wave may result. It was a lso found that if reduc ti on was continued fo r a su ffic iently long time, a fin a l 'n.' va lue of four was a lways obta ined for -OH substituted compounds . E lec tro lys is time for compl ete removal of co lour was

extended up to I 0 h at a concentrati on o f 20 x I o-4 M.

Spectra l Studies

The kinetics of the decompos iti on was a lso monitored by recording c hanges in absorbance with time at Amn x 483 nm. The rate of dec rease of absorbance with time was noted by plotting graph between absorbance and time and log absorbance vs time. For kinetic studies the soluti on (be ing intense ly co loured) was furthe r diluted to 2 x 10-5 M. The

react ions obeyed first order rate ex press ion with rate constant 1.15 x 10-1/min and Robsfmin-1 2.3 x 10-6 •

Chromatographic Studies

End products of controll ed potential e lec tro lys is were separated by TLC in Dioxane: Water ( 10: 2) solvent system. TLC showed the formation of onl y one product , indicating that azo grouping is be ing reduced to colourless hydrazo compound.

Chemical Oxygen Demand (COD)

COD of initia l coloured and e lec tro lysed solutions was carri ed out and it was observed that the COD values show a significant dec rease from 1840 mg/L to 174 mg/L, indicating less tox ic ity of the e lec trolysi s products thus formed.

Redox Mechanism

As evident from the above studies, it can be infe rred that e lectroche mica l reduction o f Orange IT takes place in two well-defined 2-e lectron reducti on step, involving cleavage of chromophoric azo group . Cyclic voltammograms of complete ly reduced

818 J SCI IND RES VOL 62 AUGUST 2003

OH

OH

Sulphanilic acid 1-amino-2-napthol

Scheme 1-Shows electrode reduction mechanisms

solution show absence of any reduction peak, whichindicates absence of any electroactive species. Thesingle product formation, as evident by TLC, andfour- electron irreversible peak clearly indicates thereduction of -N = N-- to NH:~ groups anddecolourisation due to reduction of chrornophoric azoifOUp. Based on the results of cyclic voltamrnetry,coulometric ..;Iudic<" chromatographic .uid spectralstudie .-. flJ!!owmg electrode reduction .ucchanism, asgiven III Scheme I fOJ Orange II may be proposed atPt foil working electrode. The proposed mechanism is

t n )supported by other reported studies' .'--"

Acknowledgement

Authors are thankful to Ministry ofEnvironment and Forests, New Delhi, India forfinancial support that made this study possible.

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