5th International Conference on Chemical Engineering and Applications (CCEA 2014)

Oral Presentation

Effects of Wetting Agents and Approaching Anode on Lead Migration in Electrokinetic Soil Remediation

26-27th August 2014, RegaLees Hotel, Taipei, Taiwan

Presented by:• Ng Yee Sern1,*, Mohd Ali Hashim1, Bhaskar Sen Gupta2

1 University of Malaya, Malaysia2 Queen’s University Belfast, United Kingdom

Available online at: http://dx.doi.org/10.6084/m9.figshare.1272952

2

Outline

• Introduction• Objectives• Methodology• Results and Discussion• Conclusion• Future works• References

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3

Introduction

• Lead• One of the top six toxic threats in the world [1]

– Powerful neurotoxin

• Cause acute and chronic illnesses [2-3]

– Central and peripheral nervous systems, – Cardiovascular systems – Reproductive systems– gastrointestinal and urinary tracts

4

Introduction

• Lead contaminated soil• One of the pathways for human exposure• Mining, lead-acid battery manufacturing, shooting range soil• Lead concentration as high as 751.98-138,000mg/kg [1,4-7]

5

Introduction

• Electrokinetic soil remediation• Utilize low magnitude direct current as driving force• Electromigration – For heavy metals transport• Electroosmosis – For organic compounds transport

• Advantages• No soil permeability limitation• Concentrate heavy metals into a smaller soil volume

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Introduction

• Electrokinetic Transport for Heavy Metals

DC supply

+

++

+

+

-

-

-

-

-

-

Theory [8-9]

•Main mechanism for heavy metals transport = Electromigration

• Cations move to cathode• Anions move to anode

• Cations and anions will be concentrated in cathode region and anode region, respectively

• Removal achieved by concentrating metal ions in cathode chamber

Saturated soil

Anode region Cathode region

++

-

-

+++++

+++++

-----

-----

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Introduction

• Electrokinetic Transport for Heavy Metals

DC supply

+

++

+

+

-

-

-

-

-

-

Drawback for electrokinetic soil remediation

• Electrolysis produces H+ and OH-

• Two sections are exerted • low pH (anode region) • high pH (cathode region)

• High pH section:• reduces metal migration due to metal

hydroxides precipitates• accumulate metals in the middle of the

soil [10-11]

Saturated soil

++

-

-

+++++

+++++

-----

-----

Cathode regionAnode region

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Introduction

Reduce metal hydroxides precipitation/enhance metal

migration

Buffer solution for reducing pH in cathode region[10]

Homogenize overall pH by circulation electrolyte between

anode and cathode chambers [12]

Approaching anode configuration

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Introduction

• Approaching anode [10,13-14]

– Install extra electrodes along the soil– Anode is switched towards cathode from time to time

• Induce progressive acidification for soil

Total experiment duration: 5t

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Introduction

Normal electrokinetic processNormal electrokinetic process- Electrolysis produces H+ and OH-

-H+ and OH- migrate to their respective electrodes

-Low and high pH regions are generated

-High pH region favours for metal hydroxides precipitation

pH

Soil section

+++

+++

---

---

H+ migration

OH- migration

High pH region

pH = 7

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Introduction

• Approaching anodeApproaching anode (AA)- anode switching provide progressive soil acidification

- Better for metal desorption- Reduce metal hydroxide precipitation

- Reduce migration distance- Reduce power consumption

pH

Soil section

+++

+++

---

---

pH = 7

High pH regionNormal pH trend

High pH region in AA

pH trend in AA

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Introduction

Advantages for Approaching Anode [10,13-14]

• Compressing high pH region in soil

• Reducing ions’ focusing effect

• Require lower power consumption

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Objectives

To investigate the effect of different wetting agents on Pb migration in electrokinetic soil remediation

To investigate the application of approaching anode in affecting Pb migration under different types of wetting agents

To investigate the power consumption for approaching anode assisted electrokinetic soil remediation under different types of wetting agents

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Methodology

• Soil sample– Source: Hulu Langat, Malaysia– High iron content (Contamination capacity: 1000mg/kg)– 92% sand content

• Artificial contaminated soil– Spiked with Pb(NO3)2 solution– Yield 750mg/kg Pb contaminated soil

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Methodology

Test no.

Electrode Configuration

Voltage gradient, V/cm

Wetting Agent

1 Fixed electrode 1 0.01M NaNO3

2 Fixed electrode 1 0.1M Citric acid

3 Approaching anode 1 0.01M NaNO3

4 Approaching anode 1 0.1M Citric acid

1. Soil was saturated with the wetting agent

2. DC was supplied at 1V/cm for 24 hours for Pb migration

3. For approaching anode, anode was switched to the middle electrode after 12 hours experiment

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• Pb migration

Results and Discussion

0 5 10 15 20 25

0

2

4

6

8

10

12

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Time, hr

Cu

rren

t, m

A

a

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

2

4

6

8

0.01M NaNO3 - FA0.01M NaNO3 - AA

Soil section

So

il p

H

b

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

400

800

1,200

1,600

0.01M NaNO3 - FA0.01M NaNO3 - AA

Soil section

Pb

co

nce

ntr

ati

on

, m

g/k

g

c

General trend - Pb concentration was lower in S1-S2 (anode region) and higher at S3-S4 (cathode region). - Different migration magnitude (especially S3-S4) :

- Citric acid > NaNO3 - higher current by citric acid

- lower soil pH/pH gradient for better Pb desorption [15]

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Results and Discussion

0.5 1 1.5 2 2.5 3 3.5 4 4.50

2

4

6

8

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Soil

pH

b

0.5 1 1.5 2 2.5 3 3.5 4 4.50

400

800

1,200

1,600

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Pb

conc

entr

atio

n, m

g/kg

c

NaNO3 – Fixed electrode (FA) test-Pb accumulation in S3 section - high soil pH at S3-S4 region - reduce in Pb mobilitypH at S3 is 4.61 = favours Pb adsorption [16-17]. pH at S4 is 6.62 = favours Pb hydrolysis into Pb(OH)2 and Pb(OH)3

- [18]

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Results and Discussion

0.5 1 1.5 2 2.5 3 3.5 4 4.50

2

4

6

8

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Soil

pH

b

0.5 1 1.5 2 2.5 3 3.5 4 4.50

400

800

1,200

1,600

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Pb

conc

entr

atio

n, m

g/kg

c

NaNO3 – Approaching anode (AA) test - Pb accumulation/focusing in S3 region is not observed - due to better soil acidification [10]

- Higher Pb concentration detected in S1-S2 regions - lack of electricity to maintain Pb electromigration after anode shifting

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Results and Discussion

0.5 1 1.5 2 2.5 3 3.5 4 4.50

2

4

6

8

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Soil

pH

b

0.5 1 1.5 2 2.5 3 3.5 4 4.50

400

800

1,200

1,600

0.01M NaNO3 - FA 0.01M NaNO3 - AA0.1M Citric acid - FA 0.1M Citric acid - AA

Soil section

Pb

conc

entr

atio

n, m

g/kg

c

Citric acid- No Pb accumulation in S3 region- Pb was distributed equally at cathode region (S3-S4)

-No significant difference between FA and AA tests - good soil acidification /acid front is already achieved by citric acid

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Results and Discussion

• Power consumption

0.01M NaNO3 - FA 0.01M NaNO3 - AA 0.1M Citric acid - FA 0.1M Citric acid - AA0

0.000400000000000001

0.000800000000000003

0.0012

0.00160000000000001

Pow

er co

nsum

ption

(kW

h)

Approaching anode reduces ≈20% of power consumption

Citric acid testApproaching anode maintained Pb migration at a lower power consumption

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Conclusion

• Pb migrated and concentrated to a smaller soil volume under electrokinetic process with the aid of wetting agent.

• Pb migration was highly dependent on the types of wetting agents used

– Citric acid showed better migration than NaNO3

– Pb accumulation was observed in NaNO3 tests due to higher soil pH.

• Approaching anode (AA) was useful to reduce metal hydroxide precipitation when non-acidic wetting agent was used

• AA reduced power consumption by 18.75% while maintaining efficient Pb migration when citric acid was used as wetting agent.

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