EC-flouride

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PROCEEDINGS OF INTERNATIONAL CONFERENCE ON ENERGY AND ENVIRONMENT MARCH 19-21, 2009 ISSN: 2070-3740

ENVIROENERGY2009

Abstract Electrocoagulation (EC) experiments were carried outin batch and continuous modes for removal of excess fluoride from

ground water. During batch studies using 2, 4 ,6 and 8 electrodes, an

optimal operating condition of 6V using 4 plates and 20 min

electrolyze time (ET) was found appropriate removing >88% of

fluoride from ground water. Repeated continuous EC runs at optimal

conditions, showed stable performance of the electrochemical reactor

(ECR) removing fluoride to the required drinking water quality

standards between 1.0 and 1.5mgL-1. Experiments were also carried

out by spiking ground water with excess fluoride from 10-20mgL -1

which showed > 90% fluoride removal. The pH of the water before

ECC was 7.2-7.4 and 8.3-8.6 depending on applied cell voltage.

Fluoride removal during continuous studies for flow rates 1, 2 and 5Lph were 86, 82, and 78% respectively from its initial value of 5.4

mgL-1.

Scanning electron micrographs (SEM) showed changes in

electrode structure before and after EC. Thermo gravimetric analysis

(TGA) of sludge showed the completion of the oxidation process at

5000C leaving behind an ash fraction of approximately 84%. Point of

zero charge (pH PZC) of fluoride bearing sludge showed a value of

8.2.

KeywordsDefluoridation, Electrocoagulation, SEM, TGA,PZC.

I. INTRODUCTION

N India, endemic fluoride occurs in many parts of India.

Fluoride concentration in ground water range from < 0.2

18 mgL-1 in the States of Jammu and Kashmir, < 0.2 - 6.5

mgL-1

in Himachal Pradesh, > 1.5 mgL-1

in Rajasthan, 0.2 - 0.6

in Haryana, 0.35 - 15 mgL-1

in Bihar, on an average 12 mgL-1

in West Bengal, 15 - 20 mgL-1 in Chattisgarh, 8.2 to 13.2

mgL-1

in Orissa and 0.7 to 6.0 in Maharashtra. In Karnataka,

fluoride hotspots are found in Raichur, Gulbarga, Bellary and

parts of Chitradurga with values ranging from 2.0 7.5 mgL-1.

Low fluoride concentration (< 1.0 mgL-1) could prevent dental

problem, but higher fluoride concentration (> 1.5 mgL-1

) will

cause dental and skeletal fluorosis [1]. Apart from these two

major effects of fluorosis, other ailments include neurological,

muscular and allergic manifestations. In addition to oxidative

stress [2, 3], commonly observed effects of fluoride in animals

include damaged sperms [4], and low birth rate [5].

The prevailing most popular processes for drinking water

Sanjeev. S is with Department of Environmental Engineering, NITK,

Surathkal 575 025 , India (e-mail: [email protected])

Rohini. J is with Department of Environmental Engineering, SJCE, Mysore

570 006 , India (e-mail: [email protected])

Naveen. S. M is with Department of Environmental Engineering, SJCE,

Mysore 570 006 , India (e-mail: [email protected])

Mahesh. S is working as Assistant Professor in the Department of

Environmental Engineering, SJCE, Mysore 570 006, India. (e-mail:

[email protected])

defluoridation are the adsorption using activated alumina [6],

bone char [7], activated carbon [8] and other adsorbents [9]

and the coagulation using aluminum salts [10]. Other major

processes for defluoridation include electro-dialysis [11]

reverse osmosis [12] and nano-filtration [13]. Recently,

Mameri, 2001, 1998 [14, 15] showed the effectiveness of ECT

in drinking water defluoridation.

The present research focuses on complete defluoridation of

ground water using electrochemical coagulation technology.

The main objective was to remove excess fluoride from

drinking water( ground water) to the prescribed drinking

water standards of 1 - 1.5 mgL-1.

A. EC Mechanism with Aluminum Electrodes

Defluoridation is achieved by forming Al (OH)3-XFX. Small

hydrogen gas bubbles evolved at the cathode enhances the Al

(OH)3-XFX flocs to form atop the ECR. Effective

defluoridation is finally achieved when the flocs are removed

by some mechanism.3+

Al + 3e at anodeAl (1)

( )3 2 33H O Al OH 3Al H + +

+ + (2)

( ) ( )- -x3 3-xAl OH xF Al OH F + xOH+ (3)-

2 22H O + 2e H + 2OH at cathode (4)

Coexisting anions such as SO42- could affect waterdefluoridation in the EC process. Since some raw water,

especially underground water may contain high concentrations

of coexisting ions; it becomes necessary to quantify the effects

of the coexisting ions on drinking water defluoridation in the

EC process. If SO42-

concentration is in excess of 250 mg L-1

,

defluoridation tends to be difficult and cause localized

corrosion of aluminum electrodes.

The effect of Cl-on fluoride removal is small. Cl

-can affect

defluoridation in two different ways. On one hand, Cl-

is

known to rupture the passive films of aluminum electrodes,

[16]. As Cl-

concentration increases, the current efficiency of

EC also increases, and therefore, more aluminum species get

generated. This benefits defluoridation. On the other hand, ionexchange competition is likely to occur between Cl- and F-. As

the Cl-

concentration increases, defluoridation perhaps is

slightly inhibited.

II. EXPERIMENTAL

The schematic arrangement of the experimental set up is

shown in Fig. 1. Experiments were carried out in a cubical

reactor (9.2cm x 9.2cm x 14.3cm) of capacity 1 L. Aluminum

plate electrodes (of thickness 1 mm) having dimensions of

67mm x 67mm were arranged in parallel in a monopolar

Electrocoagulation Using DC Current for

Removal of Fluoride from Ground Water

Sanjeev. S, Rohini. J, Naveen. S. M, Mahesh. S

I

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ENVIROENERGY2009

configuration in the ECR. The completely submerged

electrodes were connected to the positive and negative

terminals of the DC power supply unit. Experiments were

carried out by varying cell voltage. To attain uniform mixing,

stirring was achieved using a magnetic bar placed between the

bottom of the reactor. The gaps between the two neighboring

electrode plates were varied between 5 and 15mm throughout

the experimental runs.

Fig. 1 Schematic diagram of the experimental setup.

Batch EC experiments were carried out for a duration of 60

min in each run. Samples were retrieved and filtered at regular

time intervals, filtered and analyzed for F, pH, turbidity and

conductivity. Fluoride determinations and other parameters

were in accordance with standard methods [17]. All

experiments were carried out at ambient temperature with

different cell voltages (1, 2, 3, 4, 5, 6, 7, 8, 9,10V) as also by

varying the number of electrodes (2, 4, 6, 8, and 10). Cell

voltage was noted at regular intervals. After 60 min ET, the

contents of the ECR were emptied in to a jar and a certain

time was allowed for settling of the Al flocs in the sludge. The

sludge contains a matrix of H2, O2, Al and other materials

which settle down on little agitation in a Jar Test apparatus.The sludge was dried and subjected to further analysis. The

sludge was dried in an oven and subjected to tests such as

SEM, TGA and PZC. Filtered samples were analyzed using

fluoride ion analyzer. Optimized process and operating

parameters (4 electrodes, 6 volts for about 20 min analyses)

were then utilized for continuous experiments. Defluoridated

water was characterized for various other drinking water

parameters after ECT.

At low applied voltage, sedimentation dominates with a

slow release of coagulant and gentle agitation provided by low

bubble density and production [18, 19]. At higher current,

faster removal is expected, because of the greater bubble

density which result in a greater mass buoyed up to the surfacegiving higher rate constants [20] or greater mass flow-out

causing prolonged EC times.

III. RESULTS AND DISCUSSION

Fluoride bearing ground water prior to its use in the EC

experiments had the following characteristics: pH ~7.5-7.8,

turbidity 1.0 1.5 NTU, fluoride 5.4 6.0 mgL-1

, total

hardness 300.0 305.0 mgL-1

, chlorides 120.0 125.0 mgL-1

,

nitrates 0.3 0.5 mgL-1

, phosphates 1.25 1.50 mgL-1

, total

solids 8.0 10.0 mgL-1

, and sulphates 5.0 6.0 mgL-1

.

A. Batch EC Experiments

Fig. 2 shows fluoride degradation curves for cell voltages 1,

2, 4 and 6V respectively for an initial fluoride concentration of

5.4 mg L-1

for a 4 electrode monopolar parallel configuration

(surface area of electrodes to the volume of the bulk solution

in the ECR - SA/V = 35.92m2

m-3

). As seen, for an applied

voltage of 1V, fluoride concentration decreases to 3.3 mgL-1

from its initial value of 5.4 mgL-1

. Also, for an applied voltage

of 2V, fluoride concentration decreases from 5.4 to 2.3.Similarly, for an applied voltage of 4 V and 6V, fluoride

concentration decreases to 1.8 and 0.8 mgL-1

from its initial

concentration of 5.4 mgL-1

respectively. As seen, at higher cell

voltages (> 6 V), marginal removal of fluoride is observed.

80% of fluoride removal takes place within 20 minute of ET.

The pH of the bulk solution in the ECR shows an increase

from 7.76 to 8.75. As time progresses, anode dissolution take

place, so also a small decrease in the applied current (I).

0

1

2

3

4

5

6

0 5 10 15 20 25 30

ET, min

Fluoridevalue,mgL-1

1V

2V

4V

6V

Fig. 2 Percentage fluoride removal as function of ET. F0 = 5.4 mgL-1,

Number of electrodes = 4; pH0 = 7.76; SA/V = 35.92 m2 m-3

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

ET, min

%F

luorideremoval

10 mgL-1

15 mgL-1

20 mgL-1

Fig. 3 Fluoride removal as a function of ET. No. of electrodes= 4;

pH0 = 7.76; SA/V = 35.92m2 m-3

UPS

+ -

DC rectifier

Peristaliticpump

Magnetic stirrer

ECcellStirrer

Feed tank

collecting tank

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ENVIROENERGY2009

Fig. 3 shows the fluoride removal as function of time. This

experiment was carried out at higher fluoride concentrations

of 10, 15, 20 mgL-1

at the optimal operating conditions (4

electrodes, SA/V = 35.92m2

m-3

)for 20 min ET. Sodium

fluoride was used to spike up fluoride concentrations in the

ground water samples. At a fluoride spike of 10 mgL-1

,

fluoride removal is 93%. Similarly, at 15 mgL-1

Fo, fluoride

removal is 90% and lastly at 20 mgL-1, the removal was 87%.

From this experiment, it was inferred that fluoride

concentrations in excess of 10 mgL-1 could be effectively

removed from water even at small cell voltages of 6V.

At a cell voltage of 1 V, pH increases from 7.36 to 7.76 and

for 3V, pH increases from 7.36 to 8.4, in case of third case i.e.

for 4 V, pH increases from 7.36 to 8.7, and for 6V it increased

from 7.36 to 8.9. It was observed that a higher voltage

provides a higher end pH of the bulk solution. Overall, the pH

of the cell shows increase from 7.36 to 8.9. Anode dissolution

was found to be a function of cell voltage. As the cell voltage

was increased, the anode dissolution also increased.

B. Continuous EC Experiments

Fig. 4 shows the percentage fluoride removal as a function

of time in min at optimal operating conditions for differentflow rates i.e. 1, 2, 5 Lh

-1. Samples were drawn at regular

intervals up to 300 minutes (i.e. 5 hours treatment time).

Samples so retrieved were filtered and fluoride concentration

was analyzed using fluoride ion analyzer. At the flow rate of

1 Lh-1

, fluoride removal was ~82%; at 2 Lph flow rate

fluoride removal was 82.4% and at 5 Lh-1

,82.7% as shown in

Fig 5. Higher voltages were avoided as anode dissolution was

high. A higher SA/V, i.e. over twice that of Mameri et al.,

2001 was adopted in the present study, showed more

consistent results on F removal.

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

ET, min

%

Fluorideremoval

1 Lph

2 Lph

5 Lph

Fig. 4 Fluoride removal as function of ET at optimal conditions. F 0=

5.4mgL-1; Number of electrodes=4; Applied voltage = 6V; pH0 =

7.76.

C. Sludge Analysis

The mineral ions eliminated during EC from the bulk

solution and aluminum from the electrodes were combined

with the fluoro-complexes and insoluble compounds. These

insoluble compounds get added with the aluminum hydroxide

precipitate which produces the gel like insoluble precipitate

which floats atop the ECR. Fig 5 shows sludge gel like

emulsion after each EC experiment. Low voltages provide less

sludge volume. The sludge obtained after settling in a beaker

was oven dried and subjected to further analysis for TGA,

SEM, and PZC. The pH PZC of fluoride bearing sludge showed

a value of 8.20.

Fig. 5 Gel like emulsion atop the ECR

D. Thermal Analysis of Sludge

TGA studies indicate the thermal stability of the sludge and

provide information on the nature of the complex organic

substances (Mahesh et al., 2006 Part I) in the sludge.

Fig. 6 shows the thermogravimetric analysis (TGA),

differential thermogravimetric analysis (DTGA) and

derivative thermal analysis (DTA) curves for the precipitated

sludge after ECT of fluoride contaminated drinking water. The

thermal characteristics were observed both in the oxidizing

(air) as well as inert (nitrogen) atmospheres at the heating rate

of 10 K min-1 and air/nitrogen flushing rate of 0.4 dm3

min-1

.

TempCel900800700600500400300200100

DTA

uV

20.0

0.0

-20.0

-40.0

-60.0

TG

%

120.00

115.00

110.00

105.00

100.00

95.00

90.00

85.00

DTGug/min

100.0

0.0

-100.0

-200.0

-300.0

-400.0

63Cel-1.4uV

60.9mJ/mg

24Cel99.99%

989Cel83.61

99Cel94.87%

200Cel90.35%

301Cel87.44%

401Cel85.15%500Cel

84.66%600Cel84.30%

699Cel84.11%

799Cel83.95%

899Cel83.78%

150Cel92.55%

250Cel88.74%

376Cel85.51%

66Cel97.11%

55Cel

86.2ug/min

169Cel51.4ug/min 353Cel

30.4ug/min

Fig. 6 DTA-DTG -TG plots of EC sludge at various conditions in

air atmosphere.

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ENVIROENERGY2009

Figure shows the TG/DTG and DTA behaviour of the solid

residue of the black liquor under oxidizing environment. The

TG trace shows a gradual decrease in the residual sample mass

upto a temperature of 401 oC shedding about 85% of the initial

sample mass. The weight-loss rate is found to be extremely

slow, and up to 500oC (over a temperature span of 99

oC), the

weight-loss is only 0.49%. This means that the sludge sample

loses moisture at an almost steady rate along with

volatalization of light volatiles upto 401oC and thereafter, the

sample becomes dry and stable.

The maximum weight-loss rate was 86.2 micrograms min-1

at Tmax of 55oC (see DTGA trace). The peak temperature for

the exothermic reaction as exemplified by the DTA curve was

at Tp = 63oC with heat release of 60.9 mJ mg-1. Beyond

500oC, the weight-loss is steady but very slow, giving off only

~0.88 % mass from 500-989oC (over a temperature increase

of 489oC). It is found that the organics of the precipitate get

oxidized leaving behind the ash fraction of 83.61%.

E. SEM micrographs

SEM micrographs of aluminium electrodes (anode), before

and after ECT were obtained to compare their surface texture.

The surface of the anode prior to its use in the EC experimentswas found to be uniform, bearing nano-sized crystals.

Fig. 7a Surface of the electrode before ECT

Fig. 7b Surface of the electrode after ECT

Fig. 7a. shows the SEM micrograph of the same anode plate

after several cycles of its use in EC experiments for a total

duration of ET ~ 8 h. The anode plate surface was found to be

rough, with a large number of small sized three dimensional

dents of ~100- 200 micro meters in width and depth. These

dents are formed around the nuclei of the active sites where

the anode dissolution occurs producing aluminium

hydroxides. The edges of the electrodes (anode) and the plate

surfaces wear off releasing iron hydroxide for floc formation

with an increase in the number of cycles of experiments. After

repeated cycles of EC runs, these dents increase in size all

over the active side of the plate leaving behind an eroded

surface as shown in Fig. 7b.

IV. CONCLUSION

Batch and continuous EC experiments carried out have

confirmed the efficacy of EC technique for the treatment of

fluoride rich ground water. A 4 plate arrangement in parallel

having an SA/V of 35.92 m2

m-3

operating at 6V cell voltage

was found optimal in terms of effective defluoridation (>88%

removal), minimum anode dissolution and manageable final

pH of the treated water. Continuous EC experiments with

spiking fluoride concentrations and at different flow rates 1

Lh-1, 2 Lh-1 and 5 Lh-1 showed the fluoride removal of 86%,

82%, 78% respectively. Other than the fluoride, other

parameters like sulfates, chlorides, nitrates, phosphates,

hardness were reduced to 30 -40%.

Aluminium was not detected in the treated water pointing to

fact that the electro- dissolved alumnium forms hydroxide

during ECC and complex with organics to form insoluble

precipitates the insoluble complex gets flocculated and floats a

top the ECR. SEM micrographs showed changes in electrode

structure before and after ECC. TGA of sludge showed the

completion of the oxidation process 5000C behind an ash

fraction of approximately 84%. pH PZC of fluoride bearing

sludge showed a value of 8.2.

ACKNOWLEDGMENT

The financial support by MOEF/F. No. 19/23/2008 RE

11.09.08. is gratefully acknowledged by the authors.

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