Separation and Purification Technology 40 (2004) 69–75

Separation of pollutants from tannery effluents by electro flotation

M. Murugananthan, G. Bhaskar Raju∗, S. Prabhakar

National Metallurgical Laboratory (Madras Centre), CSIR Madras Complex, Taramani, Chennai 600113, India

Received in revised form 19 January 2004; accepted 22 January 2004

Abstract

Treatment of tannery effluent samples by electroflotation technique was studied. Effects of current density and electrode material on theremoval of suspended solids, sulphides, chromium, COD and other pollutants have been discussed. Soluble anodes such as aluminum andiron were found to be very effective for the removal of suspended solids compared to stable electrodes such as titanium coated with Ir/Ta/Ruoxides and graphite. It was observed that the colloidal size suspended solids are coagulated in the presence of aluminum and iron anodes.Electroflotation has been found to be very effective especially for the elimination of pathogenic bacteria and colour of the effluent. The kineticsof flotation of suspended solids by electrolytically generated bubbles was also studied and found to obey first order rate expression.© 2004 Elsevier B.V. All rights reserved.

Keywords: Flotation kinetics; Suspended solids; Electrocoagulation; COD; Chromium; Pathogenic bacteria

1. Introduction

The tanning of hides and skins to convert them intoleather has been an important activity since antiquity. Rawhides are subjected to various processes involving wide va-riety of chemicals to obtain finished product. The tannerywastewater is a mixture of biogenic matter of hides and alarge variety of organic and inorganic chemicals. Wastewaterfrom tanneries usually contains high concentrations of chlo-rides, aliphatic sulfonates, sulfates, aromatic and aliphaticethoxylates, sulfonated poly-phenols, acrylic acid conden-sates, fatty acids, dyes, proteins, soluble carbohydrates andNa2S. The potential environmental impact of the chemicalsused in tannery operations was widely acknowledged.

Conventional physico-chemical treatment of tannery ef-fluents consist of pretreatment, coagulation/flocculation,sedimentation and sludge handling. In addition to chemi-cal coagulation treatment, secondary and tertiary treatmentmethods such as biological filters, activated sludge, etc., arebeing practised. If the nitrogen content is high, an additionalstep involving de-nitrification is essential. The existing tech-nologies are inadequate to separate low-density colloidalsize suspended solids. As fats and proteins are partiallyhydrophobic they can be easily separated by flotation rather

∗ Corresponding author. Tel.: +91-44-22542077;fax: +91-44-22541027.

E-mail address: nmlmc@vsnl.com (G. Bhaskar Raju).

than sedimentation. Separation by flotation is relativelysimple and faster compared to the coagulation and sedimen-tation. Hence, removal of such pollutants by flotation couldbe more appropriate. The technique of flotation originatedfrom mineral processing was later extended to other areas.Matis and co-workers [1] have studied the effectiveness ofdifferent flotation techniques such as dissolved air flotationand electrolytic flotation for the removal of metal ions,surfactants and other pollutants from wastewater. Since thetannery wastewater contain colloidal size precipitates andsolids, fine bubbles are essential to float them. Fine bubblesin the order of 15–80 �m can be generated by electrolysisof water. Elmore [2] has patented the use of electrolyticbubbles for the separation of minerals. Since the hydro-gen and oxygen gases produced by electrolysis are in theiratomic state at the time of liberation, they can producesignificant changes on the surface of the particles. Thoughthese forms remain in existence only for an extremely briefperiod, within this duration, electrochemical changes areimparted on the minerals. Glembotskii et al. [3] have sug-gested collectorless flotation of minerals by electrochemicaltreatment. In general, collectors (surfactants) are added toimpart hydrophobic character on mineral particles that areto be floated and separated from a other gangue minerals.However, some sulfide minerals like PbS, CuFeS2 and FeS2were made fully hydrophobic by converting sulfide ionsat the surface to elemental sulfur by electrolytic oxygen.Electrocoagulation was suggested [4] for potable water

1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.seppur.2004.01.005

70 M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75

production wherein Mo content was decreased from 9.95to 0.006 mg/l and Fe from 130 to 0.015 mg/l. Nearly 99%of suspended solids were removed from surface water byadopting three-stage process comprising electrocogulation,flotation and micro filtration [5]. Electrocoagulation andflotation techniques were reported for the separation of oilfrom emulsion [6], purification of restaurant wastewater[7], textile wastewater [8], removal of metal ions [9], latexparticles [10] from aqueous suspensions and also for thetreatment of industrial waste water [11]. Szpyrkowicz et al.[12] have suggested Ti/Pt/Ir as anode for the removal ofnitrogen from industrial wastewater. The state of the artof electroflotation [13] and its applications was reviewedby Bhaskar Raju and Khangaonkar [14]. Mraz and Krysa[15] have attempted to improve the efficiency of the pro-cess and the life of the electrodes by incorporating surfacemodifications.

Though the treatment of different effluents was studied byelectroflotation technique, less attention was paid in the caseof tannery effluents. In fact tannery effluents are more aptsince they contain colloidal size low-density hydrophobicsolids. Power consumption also is expected to be low asthe wastewater contains sodium chloride. Hence an attemptwas made to study the effectiveness of Electroflotation fortannery wastewater remediation.

2. Materials and methods

2.1. Samples

Samples used in this study were collected from the equal-ization tank of Common Effluent Treatment Plant (CETP).Effluents from different tanneries of small and medium scalewere generally collected in a sump and pumped to an equal-ization tank to homogenize the wastewater before subjectingto full-scale treatment. Samples were collected from three

Table 1Characteristics of tannery effluent samples

S. no. Parameter Tannery effluent

Sample 1 Sample 2 Sample 3

1 COD (mg/l) 4417 3179 30922 BOD (mg/l) 2250 1273 17503 Suspended solids (mg/l) 5080 1386 30364 Total solids (mg/l) 15294 8184 145645 Total dissolved solids(mg/l) 10214 6898 115286 Ca and Mg (mg/l) 620 231 4647 Sulfide (mg/l) 192 96 1678 SO4 (mg/l) 2400 1850 21359 Chromium (mg/l) 1.34 44.6 2.46

10 Na (mg/l) 5600 1464 592011 pH 7.09 7.00 6.8012 Conductivity (µ, ohm/cm) 2100 8360 1630013 Colour Dark brown Dark brown Dark brown14 Mean size of S.S. (�m) 23 18 21

such CETP units situated in different parts of Tamilnadu,India. An hourly samples collected over a period of 24 hwas thoroughly mixed in a drum to obtain a representativesample. The typical characteristics of these samples werepresented in Table 1.

2.2. Electroflotation cell

An electroflotation cell measuring 15 cm length, 15 cmwidth and 15 cm height fabricated out of perspex materialwas used in the present study. Front side of the cell was pro-vided with a lip typical to that of flotation cell so that thefloated material could be scooped out through this lip. Theeffective volume of the cell is about 3.0 l. Rods with a lengthof 11 cm and diameter of 0.6 cm were used as electrodes.Set of electrodes consists of both anode and cathode rodswere arranged parallel to each other in the same plane.The electrode assembly was seated on the non-conductingwedges fixed to the bottom plate of the cell. In order toavoid short-circuiting, spacers were provided between an-ode and cathode rods. The gap between anode and cathodewas maintained at 2 mm to minimize the ohmic loss. Theschematic diagram of the complete system along with iso-metric view of electro-flotation cell and electrode assemblyis shown in Fig. 1.

2.3. Experimentation

A small portion of representative sample was transferredin to a conditioning tank and the pH was adjusted to a re-quired value by adding lime. For each experiment, 3.0 l ofrepresentative sample was pumped from the conditioningtank into the electroflotation cell. The anode and cathodeleads were connected to the respective terminals of the DCrectifier. Electric power was supplied by a stabilized powersource through the DC rectifier fitted with digital amme-ter and voltmeter. After passing the required current for a

M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75 71

Fig. 1. Schematic diagram of electroflotation cell. (1) Electroflotationtank, (2) anode lead, (3) cathode lead, (4) inlet, (5) peristaltic pump, (6)conditioning tank, (7) stirrer, (8) outlet, (9) discharge valve, (10) dischargetank, (11) power source, (12) spacers, (13) wedge, (14) electrode assembly,(15) froth lip.

desired duration, the floated material was scooped out andcollected in a separate tray. Treated effluent sample waswithdrawn from the outlet of the cell for characterization.When the current (I) was varied from 5 to 10 A, the elec-trical potential (E) was found to vary between 3 and 8 Vdepending on the conductivity of the medium.

2.4. Analysis

Standard methods prescribed by American Public HealthAssociation [16] were adopted for quantitative estimationof Chemical Oxygen Demand (COD), Biochemical Oxy-gen Demand (BOD), chromium, sulfide, sulfate, total solids,suspended solids, calcium, magnesium and chlorine. Theresidue of suspended solids was dried at 105 ◦C till the con-stant weight was obtained. The suspended solids thus driedwere ignited at 500 ◦C. The weight loss on ignition was re-ported as volatile matter. The specific gravity of solids wasdetermined using pycno meter. Standard procedure [17] wasfollowed for the identification of micro-organisms and theircount. The size distribution of the suspended solids was an-alyzed by using CILAS 1180 particle size analyzer.

3. Results and discussion

3.1. Removal of suspended solids

The mean size of suspended solids of tannery effluentswas estimated (Table 1) and found to be around 20 �m. Thespecific gravity and the quantity of volatile matter of thesuspended solids were also estimated to be 1.05 and 80%,

0.1 1 10 1000

20

40

60

80

100

0

1

2

3

4

5

6

7B

Cum

ulat

ive

valu

e (%

)

Suspended solids diameter (µ)

Pop

ulat

ion

dens

ity (

%)

0.1 1 10 1000

20

40

60

80

100

0

1

2

3

4

5

6

7

Cum

ulat

ive

valu

e (%

)

Suspended solids diameter (µ)P

opul

atio

n de

nsity

(%

)

0.1 1 10 1000

20

40

60

80

100

0

1

2

3

4

5

6

7

Mean diameter : 18.26µ

Cum

ulat

ive

valu

e (%

)

Suspended solids diameter (µ)

Pop

ulat

ion

dens

ity (

%)

(C)

(B)

(A)

Fig. 2. Effect of anode material on coagulation of suspended solids. (A)Raw effluent, (B) after electro flotation treatment with titanium, (C) afterelectro flotation treatment with iron as anode.

respectively. The particle size distribution of the raw efflu-ent shown in Fig. 2A clearly indicates that the suspendedsolids are distributed between 0.01 and 100 �m. From theabove results it could be inferred that the suspended solidsare low in density and more of volatile in nature. The ef-fect of different electrodes on the removal of suspendedsolids was studied and the results obtained were presentedin Table 2. From the results it is evident that the separationof suspended solids is near total in the presence of iron andaluminum and only partial in the presence of titanium elec-trode. Size distribution of suspended solids that remainedin the electroflotation cell (non-float) was analyzed and the

72 M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75

Table 2Effect of anode material on the removal of suspended solids

S. no. Electrode system Suspended solids aftertreatment (mg/l)

Removal(%)

1 Al as anode and Fe as cathode 64 95.32 Fe as anode and Al as cathode 66 95.23 Titanium as anode and cathode 658 52.14 Coagulation and sedimentation; alum

dosage: 250 ppm (plant sample)326 76.2

Initial concentration of suspended solids: 1372 mg/l, current density: 46 mA/cm2, flotation time: 900 s.

results were presented in Fig. 2B and C. From the resultsshown in Fig. 2B it is evident that colloidal solids rang-ing from 0.1 to 10 �m were remained in effluent when ti-tanium was used as anode. In other words these colloidalsolids have not responded to flotation. On the other hand,suspended solids ranging from 10 to 50 �m were found inthe resultant effluents where Fe was used as anode. Thisclearly suggests the coagulation of colloidal particles in thepresence of Fe anode. The same phenomenon was observedwhile aluminum was used as anode. Derjaguin and Dukhin[18] have observed that the probability of particle-bubblecollision will be high if the size of the bubbles and particleswere of the same order. The size of the bubbles generatedby electrolysis of water are reported to be very small andgenerally vary from 15 to 80 �m depending on the pH andcurrent density [3,19]. Better separation of suspended solidscould be attributed to coagulation of colloidal solids in thepresence of soluble anodes. It may be noted that the respec-tive metal ions are released while using soluble anodes suchas Fe and Al where as oxygen bubbles are generated by us-ing stable anodes such as titanium. Anodic dissolution ofmetal ions could be represented as

Al → Al3+ + 3e− (1)

Fe → Fe2+ + 2e− (2)

Fe2+ ions thus released in to aqueous solution may partly beconverted to Fe3+ ions either by anodic oxidation or underoxidizing environment.

Fe2+ → Fe3+ + e− (3)

It is well known that the aluminum and iron ions formrespective oxy-hydroxy species in aqueous solutions. Thehydroxy species thus formed have a pronounced tendency

Table 3Effect of current density and flotation time on the removal of suspended solids

S. no. Flotation time (s) Removal of suspended solids (%)

Current density 31 mA/cm2 Current density 46 mA/cm2 Current density 62 mA/cm2

1 60 34 35 402 120 60 61 653 180 75 77 754 240 83 86 895 300 93 94 96

Initial concentration of suspended solids: 5080 mg/l.

to undergo polymerization due to interaction between hy-droxyl groups of adjacent molecules. In the case of iron,species such as FeOH2+, Fe(OH)2

+, Fe(OH)2, Fe(OH)3,Fe(OH)4

− and FeO(OH) are formed. Similarly aluminumions also form different species. Aqueous solutions of alu-minum ions at neutral pH have been reported to containhighly charged poly-nuclear hydroxy aluminum (III) com-plexes such as Al2(OH)2

4+, Al7(OH)174+, Al13(OH)34

5+,Al3(OH)4

5+, Al(OH)63−, Al(OH)7

4− and AlO2− [20].

Since the poly-nuclear metal hydroxyl complexes with pos-itive, negative and neutral charge are good coagulants, thesuspended colloidal solids are adsorbed on highly activepolymer molecules and form coagulum. Such coagulationwas not observed in the case of titanium electrode coatedwith Ir/Ta/Ru, since it is insoluble. Though carbon was re-leased from graphite electrode, it could not aid coagulationof suspended solids.

Generally, surface of the solid should be hydrophobic tofacilitate bubble attachment. However, bubbles generatedby electrolysis were found to be very effective to float evenhydrophilic precipitates. This was attributed to the high cap-illary pressure inside the bubbles. At the moment of bubbleformation, the pressure inside the bubble was estimatedaround 1.45 × 106 Pa [21]. Bubbles of such high pressurecould spread even on hydrophilic surface causing the forma-tion of three-phase perimeter of micro contacts [22]. Resultsshown in Table 2 also indicate that the suspended solids werebetter removed by electroflotation compared to conventionalcoagulation and sedimentation. In conventional process,alum and other organic polymers are used for effective coag-ulation whereas metal ions of either Al or Fe released fromanode are sufficient to aid coagulation in electroflotation. Inconventional process, effluents are invariably contaminatedwith sulfate ions from alum and organic molecules from

M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75 73

0 50 100 150 200 250 3000

20

40

60

80

100

% R

emov

al

Electroflotation time (s)

Experimental R=R*(1-exp(-kt)) R=R*(kt/1+kt) R=R*2(kt/1+R*kt) R=R*[1-{ln(1+kt)/kt}]

Fig. 3. Flotation kinetics-model fitting.

flocculent, which in turn increases the TDS and COD con-tent. Thus electroflotation is a cleaner technology comparedto conventional coagulation and sedimentation.

3.2. Flotation kinetics

Removal of suspended solids by electroflotation was stud-ied at different current densities using Al as anode. At eachcurrent density, samples were collected at different time in-tervals and the results are presented in Table 3. The kineticdata obtained at the current density of 31 mA/cm2 has beenfitted into the following kinetic models established by theearlier investigators [23,24].

R = R∗(1 − e−kt) (4)

R = R∗(

kt

1 + kt

)(5)

R = R∗2(

kt

1 + R∗kt

)(6)

R = R∗[

1 −{

ln(1 + kt)

kt

}](7)

where R is recovery at time t, R∗ the ultimate recovery and kthe rate constant. From the results shown in Fig. 3, it is ap-parent that the best fit to the experimental data was observed

Table 4Effect of anodic material on the treatment of tannery effluent

S. no Parameter Initial concentration (mg/l) Removal with different anode material (%)

Graphite Ti Fe Al

1 Suspended solids 3036 14.0 52.0 91.5 92.02 BOD 1750 6.0 37.0 53.0 51.03 COD 3092 12.0 31.0 52.0 52.04 Ca and Mg 464 0.0 27.0 81.0 75.05 Sulphate 2135 0.0 0.0 22.7 22.06 Sulphide 167 – – 94.5 94.07 Chromium 2.5 – – 99.0 N.A.8 Chloride ion 1850 N.A. 1.60 1.54 1.35

Current density: 46 mA/cm2, flotation time: 300 s, and pH of the effluent: 8.50.

for the first order model represented in Eq. (4). The first orderrate equation was extended to the data obtained at differentcurrent densities (Table 3). The rate constants for first orderequation were found to be 0.0077, 0.00815 and 0.00898 s−1

at current densities of 31, 46 and 62 mA/cm2, respectively.Assuming that concentrations of hydrogen bubbles and par-ticles would influence the recovery, kinetics should obeysecond order rate equation. Since the quantity of hydrogenbubbles liberating from cathode was large compared to thesuspended solids, the significance of the bubbles could benegligible. De Bruyn and Modi [25] have observed first or-der rate for the flotation of fine particles (<65 �m), if thesolids content in the pulp is less than 5.2%. Tomlinson andFleming [26] too have observed first order process when thesolids concentration in the pulp is small (uninhibited flota-tion) and zero when the concentration is high. Results of thepresent study are in good agreement with the above obser-vations.

By increasing the current density, the bubble flux and inturn the collision probability and flotation rate are expectedto increase. However, electroflotation tests conducted at dif-ferent current densities (Table 3) have shown a marginal in-crease in the removal of suspended solids. This may be dueto liberation of sufficient hydrogen bubbles even at a currentdensity of 31 mA/cm2.

3.3. Effect of electrode material

The effect of electrode material for the removal of vari-ous pollutants was studied on sample 3 and the results arepresented in Table 4. It was observed that the aluminum andiron electrodes are effective compared to graphite and tita-nium coated with oxides of Ir/Ta/Ru. It could be seen thatthe soluble electrodes are particularly effective in removingsuspended solids, sulfide and chromium. Above pollutantswere removed to the extent of 90% during electroflotation.The removal of calcium, magnesium and chromium wasalso found be significant during electroflotation. As men-tioned earlier, bubbles generated from electrolysis are quitecapable to lift metal hydroxides and oxides that are highlyhydrophilic. While using iron as anode, formation of blackcolour precipitate typical to iron sulfides was observed in

74 M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75

the flotation cell. This clearly indicates the reaction betweenFe ions released from anode and sulphide ions present inthe form of HS−, S2− and H2S. Thus the sulphide contentpresent in effluent can be effectively removed by convert-ing to iron sulfide compounds that are less soluble and hy-drophobic.

3.4. Removal of hexavalent chromium

In order to see the effectiveness of electroflotation onthe reduction of Cr6+, separate test was conducted by tak-ing potassium dichromate solution with a concentration of100 mg/l. Fe was used as anode and a current density of31 mA/cm2 was applied for a period of 600 s. Removalof Cr6+ to the extent of 99.5% along with shift in pHof the solution towards basic range was observed. The re-duction of Cr6+ to Cr3+ by electrochemical precipitationcould be explained by the following equation proposed byKongsricharoern and Polprasert [27].

3Fe +Cr2O72− + 8H2O → FeCr2O4(s) + Fe2O3(s)

+8OH− + 4H2(g) (8)

The chromite and iron oxide sludge thus produced was in-stantaneously floated and separated by electrolytically gen-erated hydrogen bubbles. The shift in pH towards basic re-gion could be explained by the generation of hydroxyl rad-icals.

3.5. Removal of COD

All the samples collected from different places were pro-cessed by electroflotation using aluminum as anode at dif-ferent current densities and the resultant effluents were an-alyzed for suspended solids, COD and BOD. The results ofthe same are presented in Table 5. Residence time of 300 swas maintained in all the tests. It is evident that above 90%removal of suspended solids and 50–70% reduction in BODand COD was achieved depending on the nature of the ef-fluent. Major contribution towards COD and BOD is ex-pected from dissolved organic substances. Generally strong

Table 5Effect of current density on the removal of COD and BOD

Sample identity Parameter Removal at different current densities (%)

Current density 31 mA/cm2 Current density 46 mA/cm2 Current density 62 mA/cm2

Sample 1 Suspended solids 93 94 96COD 62 67 68BOD 64 68 68

Sample 2 Suspended solids 85 93 93COD 50 52 53BOD 52 52 50

Sample 3 Suspended solids 92 92 –COD 46 54 53BOD 45 54 –

oxidizing agents such as hydrogen peroxide are used to ox-idize the dissolved organic molecules. The removal of CODand BOD by electroflotation could be attributed due to theremoval of suspended solids and also due to precipitation ofdissolved organic molecules as organo-metallic compounds.However, detailed studies on oxidation–reduction reactionson the surface of the electrodes and in bulk phase are essen-tial to establish the electrochemical removal of COD con-tent.

3.6. Removal of pathogenic bacteria

Raw effluent and after subjecting the same by electro-flotation were analysed for bacteria by presumptive coliformcount and membrane filter techniques. Bacteria such as en-terobacter species, klebsiella species, citrobacter freundii,zooplankton, algae, paramecium and chlamydomonas wereidentified in raw effluent sample to the extent of 7,180,000colonies/ml. After subjecting the above sample by elec-troflotation, the bacteria ware eliminated to the extent of99.97%. It may be attributed to the liberation of the chlorinegas as a result of secondary electro chemical reaction.

2Cl− → Cl2(g) + 2e− (9)

Hernlem and Tsai [28] have recently studied the effective-ness of electroflotation for disinfecting Escherichia coli.They have observed the generation of chlorine even at lowcurrent (0.8 A). It was further pointed out that the bacteriawere killed due to chlorine but not by electric shock. Elim-ination of bacteria during electroflotation coupled with de-crease in chloride content (around 1.5%) clearly proves thegeneration of nascent chlorine during electro-flotation.

3.7. Comparison between electroflotation andconventional coagulation

Effluent was treated using Fe as anode in the first stage(flotation time 120 s) followed by Al in the second stage(flotation time 300 s) and finally by titanium coated with ox-ides of Ir/Ta/Ru (flotation time300 s). During the first stage,Fe electrode was used to convert sulfide to iron sulfides and

M. Murugananthan et al. / Separation and Purification Technology 40 (2004) 69–75 75

Table 6Comparison between electroflotation and conventional coagulation

Parameter Removal (%)

Conventional Electroflotation

Suspended solids 71 96COD 49 68BOD 47 68Sulfide 15 85Sulfate Nil 25Pathogenic bacteria Nil 99.97Chromium 65 99.0Colour Dark brown Pale yellow

Cr6+ to Cr3+. In the second stage, aluminum was used asanode to facilitate coagulation of colloidal solids. Titaniumelectrodes were used in the final stage with a view to removeexcess aluminum and iron precipitates and to facilitate oxi-dation of complex organic molecules. Results of electroflota-tion thus obtained and results obtained by conventional co-agulation are shown in Table 6. The results clearly demon-strate the superiority of electroflotation over conventionalcoagulation. Though electroflotation alone is not sufficientto eliminate all types of contaminants of tannery effluent, theload on biological treatment could be reduced enormouslyby adopting electroflotation in the place of conventional co-agulation and sedimentation. Above all it requires less timeand space compared to conventional coagulation and sedi-mentation.

4. Summary and conclusions

Tannery effluent samples were treated by electroflotationtechnique and found to be more effective for the removalof various pollutants compared to conventional coagulation.Suspended solids, chromium and sulfides could be removedto the extent of 95% by using soluble electrodes. Elec-troflotation technique was observed to be more effective (atleast by 20%) for the removal of suspended solids, CODand BOD over conventional coagulation and sedimentationprocess. Soluble electrodes like Al and Fe were found to bevery effective compared to insoluble electrodes. Al and Feions released from anode form poly-hydroxy complexes andaid coagulation of colloidal particles. Pathogenic bacteriacould be eliminated to the extent of 99.97% by electroflota-tion technique. The kinetics of flotation of suspended solidswas found to follow first order rate equation.

Acknowledgements

The authors gratefully acknowledge the financial grant(No. BT/PR1326/PID/25/44/98) of Department of Bio

Technology, Government of India. The authors are thankfulto Professor S.P. Mehrotra Director, National MetallurgicalLaboratory, Jamshedpur for his guidance and encourage-ment. Authors are thankful to Dr. A. Rama Mohan Rao,scientist for his valuable suggestions.

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