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Engineering Geology 77

Removal of selected heavy metals from MSW fly ash

by the electrodialytic process

Celia Ferreiraa,b,*, Pernille Jensenb, Lisbeth Ottosenb, Alexandra Ribeiroc

aCERNAS, Escola Superior Agraria, 3040-316 Coimbra, PortugalbDepartment of Civil Engineering, Technical University of Denmark, DK-2800, Lyngby, Denmark

cDepartment of Environmental Science, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

Received 1 June 2003; accepted 1 July 2004

Available online 15 September 2004

Abstract

This paper aims to assess the applicability of the electrodialytic remediation technique for the removal of zinc, lead, copper

and cadmium from municipal solid waste (MSW) incinerator fly ash. A broad range of experimental conditions were studied

including current densities, remediation times, use of assisting agents and cell design.

Several operational problems were identified during the electrodialytic experiments, among which are formation of

precipitates, dryness of sample and partial dissolution of sample creating preferential pathways for the electric current. These

problems may explain the low remediation efficiencies obtained.

Comparison between experiments showed that generally the use of Na-gluconate as assisting agent leads to better results

than distilled water. Increasing the concentration of the assisting agent also results in higher removals.

D 2004 Elsevier B.V. All rights reserved.

Keywords: MSW fly ash; Electrodialytic treatment; Heavy metals

1. Introduction

In a municipal solid waste (MSW) incineration

unit, the flue gas resulting from combustion must be

treated before emission in order to remove pollutants,

such as particles, acid gas, nitrous and sulphuric

0013-7952/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.enggeo.2004.07.024

* Corresponding author. CERNAS, Escola Superior Agraria,

3040-316 Coimbra, Portugal. Tel.: +351 93 930 3952; fax: +351 23

980 2979.

E-mail address: celia@mail.esac.pt (C. Ferreira).

oxides, heavy metals and hazardous organics. This

treatment is performed in a series of unit operations,

resulting in a fly ash by-product.

Fly ash consists of a mixture of particles, con-

densed gases, chemicals introduced for treatment of

the flue gas and reaction products. Due to the presence

of leachable heavy metals, high concentrations of

soluble salts and residual amounts of hazardous

organics (e.g. dioxins), this waste is classified as

hazardous and must be treated prior to disposal. There

are two different approaches to solving the problem:

(2005) 339–347

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347340

(a) encapsulation of the hazardous substances or (b)

their removal from the waste.

The most commonly used treatment world-wide

for this toxic waste is stabilization/solidification,

which consists in the addition of cementitious

materials and consequent immobilization of the

contaminants in a solid matrix. The main disadvant-

age of this process is the need for additives, which

increase the cost and, in some cases, also the volumes

to be disposed. In addition, immobilization does not

always succeed and the resulting product can still fail

regulatory test. But above all, the stabilization/

immobilization process does not fit into an environ-

mental sustainable waste management perspective,

since it is not a final solution for the waste (after

deposition it must continue to be monitored) and does

not promote the reuse nor recycling of the waste.

The current work follows the second approach. Its

aim is to evaluate the use of the electrodialytic

remediation technique for the removal of heavy

metals from MSW fly ash. In case the electrodialytic

treatment succeeds, the costs of controlled deposition

could be avoided and moreover the residue obtained

could be reused in new applications which are now

out of reach due to the high loads of heavy metals,

such as: production of cement, concrete, ceramic and

glass; use in road pavement and embankments;

application in agriculture as soil amendment; use as

sorbent in industry; and for sludge conditioning

(Ferreira et al., 2003a).

2. Background

The electrodialytic remediation method was orig-

inally developed at the Technical University of

Denmark (Ottosen and Hansen, 1992) for remediation

of contaminated soil. The waste to be treated is

saturated with water (or a solution of specific

substances called bassisting agentsQ) and placed inside

the electrodialytic cell. An electric field is applied

across the cell, causing the metal ions to migrate

towards the electrodes, according to their charges.

Selective ion-exchange membranes placed between

the compartments prevent the metal ions from reach-

ing the electrodes. Instead, they are accumulated in

bconcentrationQ compartments from where they can be

removed. This technique has also been applied with

success to the treatment of wood waste (Ribeiro et al.,

2000) and biomass ash (Pedersen, 2002). Recently,

some studies have been presented on the use of this

technique for the treatment of electrostatic precipitator

(ESP) fly ash from incineration of municipal solid

waste (Pedersen, 2002).

Theoretically, the electrodialytic remediation of

MSW fly ash is possible. Fly ash particles have a

porous structure and a solubility of almost 40% (w/w)

(Ferreira et al., 2003b). A promising aspect is that the

majority of the toxic metals are presumably found on

the surface, as chloride compounds. This assumption

is based on the fact that during combustion and gas

transport, the thermodynamic conditions necessary for

the formation of such compounds have occurred

(Ferreira et al., 2003b). Since the great majority of

chlorides are extremely soluble, the electrodialytic

remediation may present good results.

Several conditions must be met for the successful

application of this method to this material. Firstly, the

solution used to prepare the initial slurry is important.

The use of assisting agents during electrodialytic

remediation is referred in the literature for different

media: soils and waste wood (Ottosen et al., 1998),

bioash and ESP fly ash (Pedersen, 2002). Different

types of assisting agents can be used. These agents

can normally be defined as solubilisation enhance-

ment compounds, which form stable complexes with

the substances to be removed. In order to work with

the electrodialytic technique, the complexes must be

electrically charged, so that they can be mobilized by

the electric field. In a previous investigation by

Ferreira et al. (2002), several assisting agents were

evaluated for heavy metal removal from MSW fly

ash: EDTA, ammonium acetate, ammonium citrate

and Na-gluconate. According to that study, Na-

gluconate presented the best removal efficiencies for

both zinc and lead, while performing well for the

other metals (cadmium and copper). The same study

showed that Na-gluconate presents additional charac-

teristics such as non-toxicity, good performance at

high pH values, formation of charged complexes with

the metals and reasonable price. Although this pointed

out the possibility of using Na-gluconate as an

assisting agent in the electrodialytic remediation of

fly ash, this substance has not been tested so far in

electrodialytic cells. This testing is conducted in the

current work.

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347 341

Another important aspect is the physical design of

the electrodialytic cell. Cells can have varying

numbers of compartments as well as use different

membrane types. This flexibility allows for the

development of process-specific designs. For exam-

ple, migrating ions may either be allowed to deposit

on the electrodes, or collected in a chamber by

changing from passive membranes to ion-selective

membranes. Further design possibilities were recently

proposed by Pedersen (2002) to solve issues such as

slow remediation rates, formation of precipitates and

self-hardening of fly ash, which happened when

working with ESP fly ash. This new design consists

of a cell in which the sample is continuously stirred.

3. Materials and methods

3.1. Experimental design

Several experiments were conducted under differ-

ent operational conditions, which can be divided into

three groups (see Table 1).

A preliminary testing of fly ash’s behaviour during

electrodialytic remediation without assisting agent

was done in group A. Group B refers to experiments

in which Na-gluconate was used as assisting agent. B1

and B2 were designed to compare the effect of using

two different current intensities (10 and 50 mA), and

B2 and B3 compare the effect of two different

concentrations for the assisting agent (3% and 22%).

Table 1

Summary of electrodialytic experiments conducted on MSW fly ash

Duration

(days)

Current Type

of cell

Membranesa

(mA) (mA/cm2)

A1 21 10 0.8 S Cat-PM-Cat-An

A2 20 10 0.8 S Cat-PM-PM-An

B1 20 10 0.8 S Cat-PM-PM-An

B2 20 50 4.0 S Cat-PM-PM-An

B3 20 50 4.0 S Cat-PM-PM-An

B4 20 50 4.0 S Cat-PM-PM-An

C1 12 38 0.8 L Cat-An-Cat-An

C2 12 38 0.8 L Cat-An-Cat-An

n.d.—not determined.a Cat—cation exchange membrane, PM—passive membrane, An—anb NaG—sodium gluconate.c S1—slice closest to the anode, S2—middle slice, S3—slice closest

B4 is similar to B3 the difference being that the

electrolyte solutions in II and IV were replaced every

5 days by a fresh solution, in order to study the

removal over time.

Experiments in groups A and B use the traditional

cell (S) in which the membranes and number of

compartments were chosen according to the objec-

tives of the work, described below. The new cell (L)

proposed by Pedersen (2002) is tested in group C,

with both distilled water (C1) and Na-gluconate (C2).

3.2. Sample collection and preparation

A total of 65 kg of sample were collected during

June 2001 from a MSW incinerator (mass-burn,

moving grate) in Portugal. The incinerator, with a

nominal capacity of 2016 tons/day, uses three equal

parallel trains of 28 tons/h each and includes a heat

recovery system for electricity production. During the

incineration, aqueous ammonia is injected into the

furnace for NOx removal by selective non-catalytic

reduction (SNCR), followed by the addition in the

scrubber of slaked lime and activated carbon for the

neutralization of acid gas and removal of heavy

metals. Particulates are removed by high-efficiency

baghouses. For each tonne of MSW incinerated, 200

kg of bottom ash and 30 kg of fly ash are generated.

Fly ashes collected at the boiler, scrubber and

baghouses are mixed, resulting in the material used

in the current work. The material appears as a fine,

white-grey powder. Following collection, the sample

Sample

saturationbWater content (%) Sample

initial pHStart Endc

S1 S2 S3

H2O 39.1 44.1 39.0 7.1 12.42

H2O 38.4 46.3 42.3 44.2 12.04

3% NaG 42.9 48.4 45.8 43.1 12.22

3% NaG 44.0 45.8 46.7 42.7 12.06

22% NaG 45.0 45.0 46.5 49.8 12.13

22% NaG 45.0 46.0 46.5 47.2 12.13

H2O 84.0 n.d. 12.23

3% NaG 84.0 n.d. 12.25

ion exchange membrane.

to the cathode.

Table 2

Physical properties of MSW fly ash (in Ferreira et al., 2003b)

Parameter Value

Dry matter 99.1%

Soluble fraction 39%

pH 12.47

Particle size distribution 1–1000 Am(more than half is b63 Am)

Loss at 550 8C 0.92%

Specific surface area 6.12 m2/g

External surface area 4.04 m2/g

Micropore volume 0.00110 cm3/g

Micropore area 2.1043 m2/g

Fig. 1. Scheme of the electrodialytic cell.

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347342

was introduced into a rotating drum and homogenized

for 1 week, after which it was stored in 5-l plastic

containers. Physical and chemical characteristics of

MSW fly ash are presented in Tables 2 and 3.

3.3. Electrodialytic cell

Two different types of experimental cells were

used in the current work. Cells of type bSQ, with a

smaller internal diameter of 4 cm and cells type bLQwhich presented an internal diameter of 8 cm and a

stirrer. Both consisted of a Plexiglas cylinder divided

transversely into five compartments (Fig. 1): one

central compartment (III) in which the sample is

introduced at the beginning of the experiment and two

compartments on either side: a concentration compart-

ment (II and IV) followed (outwards) by the electrode

compartment (I and V). Two inert electrodes were

placed at either end of the cell.

Compartments I, II, IV and V are filled with

electrolyte solution (NaNO3 0.01M) and separated by

membranes.

Table 3

Chemical characterization of MSW fly ash (in Ferreira et al., 2003b)

Value Value

CO32� 7.25 wt.% Zn (mg/kg) 6187

SO42� 8.0 wt.% Pb (mg/kg) 2399

PO43� 2.1 wt.% Cu (mg/kg) 546

Cl� 13.1% Mn (mg/kg) 338

F� bDLa Cd (mg/kg) 91

Na 3.4% Cr (mg/kg) 104

K 3.4% Fe (mg/kg) 1692

Ca 22.7% Mg (mg/kg) 7891

Al 2.4%

Between chambers I and II, a cation-exchange

membrane was used to prevent anions, moving under

the electric field towards the anode, of reaching

compartment I and depositing on the anode. Similarly,

on the other end of the cell, between chambers IV and

V there is a an anion-exchange membrane to prevent

cations from reaching V. Between the central compart-

ment and those immediately adjacent to it, passive

membranes were used. These consisted of a compo-

site paper/fabric filters. This basic experimental

arrangement suffered some controlled variations in

setups A1 and C. In A1, a cation-exchange membrane

was used between III and IV instead of the passive

membrane, in order to prevent energy from being

wasted on the transport towards the anode of NO3�

from the electrolyte and of OH� generated at the

cathode. In group C, the passive membranes were

replaced by ion-exchange membranes (anion-

exchange membrane between II and III and cation-

exchange membrane between III and IV), to prevent

solution in central compartment to move to adjacent

compartments due to stirring.

3.4. Experimental procedure

The fly ash sample was prepared by preliminary

saturation with either distilled water or an assisting

agent for 24 h. Subsequently, it was introduced in

compartment III. Water content and pH were meas-

ured in an aliquot (see Table 1).

The electrodialytic experiments were initiated by

application of a low voltage DC current to the

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347 343

electrodes. The electrolytes were recirculated between

the cell compartments and glass flasks using a multi-

channel peristaltic pump. During the experiment,

monitoring of the voltage drop across the cell as well

as conductivity and pH of electrolytes took place. The

pH of the electrolytes was adjusted whenever neces-

sary by addition of NaOH or HNO3, according to the

conditions defined for each experiment: pH 2 for

group A and pH 12 for groups B and C.

At the end of each experiment, the cell was put

down and the sample contained in chamber III was

analysed for pH and water content (see Table 1). For

experiments A and B, the sample was divided into

three slices and values were measured for each slice.

The electrolytes were collected and the volume

measured before the solutions were filtered through

a 0.45-Am membrane, acidified to pH 2 for metal

preservation (with nitric acid) and analyzed by AAS

for zinc, lead, copper and cadmium.

3.5. Calculations

The total amount of metal extracted is calculated

by multiplying the concentration by the volume of

each electrolyte (at the end of the experiment) and

adding the four values (Eq. (1)).

metal extracted ðmgÞ

¼X

i

concentrationi � volið Þ ð1Þ

In which i=I, II, IV and V; bconcentrationiQ is the

metal concentration in electrolyte from chamber i at

the end of remediation (in ppm); and bvoliQ is the

volume of the electrolyte collected from chamber i at

the end of remediation (in litres).

This is then divided by the mass of fly ash in order

to obtain the metal extracted per unit weight of fly

ash, according to Eq. (2).

metal extracted per unit mass ðmg=kgÞ

¼ metal extracted=mass of fly ash ð2Þ

In which bmetal extractedQ is calculated in Eq. (1) (in

mg) and bmass of fly ashQ refers to the dry weight (in

kg) of the fly ash introduced in chamber III at the

beginning of experiment.

The percentage of metal extracted is obtained

dividing the bmetal extractedQ (as calculated in Eq.

(1)) by total mass of metal originally present in fly

ash and multiplying the result by 100 (according to

Eq. (3)).

metal extracted kð Þ

¼ metal extracted ðmgÞ=total metal ðmgÞ � 100

ð3ÞIn which btotal metalQ refers to the total mass (in

mg) in the sample introduced in the beginning in the

central chamber and determined by a total digestion

procedure.

4. Results and discussion

The experiments in group Awere designed to make

a preliminary assessment of the behavior of fly ash

during the electrodialytic remediation. One of the

problems identified in A1 was that the sample nearest

to the cathode dried during the experiment, with water

content decreasing from an initial value of 39% to

7.1% (see Table 1). This may have caused poor

dissolution and low transport of ions, therefore

hindering or stopping the electrodialytic remediation

process. This problem was partly solved in experi-

ment A2, through the replacement of the membrane

between compartments III and IV by a passive

membrane, which allows the electrolyte solution to

cross over from compartment IV into III. The water

content at the slice nearest to the cathode at the end of

A2 is 44.2%, as opposed to only 7.1% in A1.

Another important aspect is the evolution of pH

during the remediation, since it is known that with

increasing acidity the solubility of metals increases.

The initial fly ash slurry introduced into the cell is

alkaline, with pH above 12 (Table 1). At the end of the

experiment, the pH is still above 12, even though the

electrolytes are kept below 2 throughout the entire

process. This is due to a high pH buffering capacity

caused by the presence of carbonates and hydroxides.

As can be seen in Fig. 2, the remediation results

were poor: between 2.4% and 5.2% of the metal

present in the ash was extracted. Metal mobilization

was therefore not effective and the main possible

cause was linked to a high pH value of the sample.

A2 performs slightly better than A1. The reason for

this is probably related to the use of the passive

Fig. 3. Heavy metals extracted in experiment B (20 days), as a

percentage of total present in ash.

Fig. 4. Percentage of metal extracted from fly ash in batch

experiments with sodium gluconate (NaG) and water (L/S=25)

(adapted from Ferreira et al., 2002).

Fig. 2. Heavy metals extracted in experiment A (20 days), as a

percentage of total present in ash.

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347344

membrane in A2, which prevented drying of the

sample, thus increasing efficiency.

Experiments A1 and A2 showed that when work-

ing at this high pH mobilisation of metals is low

because the metals are present as insoluble com-

pounds. Lowering pH would require large amounts of

acid to compensate for fly ash’s high buffering

capacity. Therefore, Na-gluconate, which can solubi-

lize metals at high pH, was chosen as assisting agent

for the experiments in group B at pH 12.

Two operational problems were observed in group

B experiments (with Na-gluconate). The first problem

was the formation of a white precipitate in compart-

ment IV, which deposited on all surfaces in contact

with this solution (tubes, pump, membranes, etc.).

This deposition, caused by the high pH in the

electrolyte, lead to clogging of the membranes,

making it necessary to replace them periodically.

The second problem was a partial dissolution of the

sample inside the cell, followed by a collapse of the

material, creating a lower resistance pathway for the

current in the upper part of the cell, allowing it to

bypass the sample. This leads to decreased remedia-

tion efficiencies.

The percentage of heavy metals extracted in

group B appears in Fig. 3. Contrary to what was

expected based on batch extractions tests with Na-

gluconate and water (see Fig. 4), the use of Na-

gluconate as assisting agent did not enhance the

remediation significantly. Metal removal improved

slightly when compared to group A experiments

(Fig. 2), but it is still low compared to the total mass

of metals present. The best removal ratio is for Cd,

with approximately 10%. It is possible that parts of

the studied metals are included in the precipitate and

consequently are not accounted for, partially explain-

ing the poor results. The advantage of using the

electrodialytic procedure against simple fly ash

washing/leaching is that the metals are removed

from the sample as soon as they are solubilised due

to the electromigration, whereas in leaching/washing

they remain in contact with sample, hindering further

solubilisation due to saturation of sample. Therefore,

in order to be effective, treatments based on

leaching/washing require much higher liquid to solid

ratios (L/S) and also longer treatment times. More-

over, the high volume of water used in leaching/

washing dilute the metals, making it difficult to

further capture and recover the heavy metals from

solution and creating a new problem related to the

treatment of the waste water. On the contrary, during

the electrodialytic treatment, metals are being con-

tinuously transferred from the central compartment

(sample) into bconcentrationQ compartments, result-

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347 345

ing in less volume of a more concentrated, and

therefore easier to treat, solution.

4.1. Effect of increasing the concentration of the

assisting agent

As can be seen from Fig. 3, the amount of heavy

metals extracted from the sample increases with the

concentration of Na-gluconate: extractions with 3%

Na-gluconate (B1 and B2) are approximately half of

those with 22% Na-gluconate (B3 and B4). Although

it is not possible to define the optimum concentration

for the Na-gluconate based on these experiments, the

importance of this parameter is clearly showed.

Optimization of Na-gluconate concentration is rec-

ommended for future work.

4.2. Effect of using higher current intensities

A comparison between the first two columns in

Fig. 3 highlights the difference between using a 10

mA current (B1) and 50 mA current (B2). For zinc,

the results were similar with both current values; for

lead and cadmium, results were better using 50 mA;

copper extractions were better with 10 mA. Results

are therefore inconsistent, making it impossible to

reach a conclusion on the effect of increasing the

current intensity. Alshawabkeh et al. (1993) use

electric current densities in the order of a few tens

Fig. 5. Cumulative extraction (normalised) of Zn, Pb, Cu and Cd to the

electrodialytic experiment B4.

of milliamperes per square centimeter, which are

higher than the values used in current work (0.8–4

mA/cm2). However, Hamed and Bhadra (1997) refer

to the use of lower current density, between 0.123 and

0.615 mA/cm2. Pedersen (2002) uses values for

current densities of the same order of magnitude as

in the current work (0.8 mA/cm2). According to

Alshawabkeh et al. (1993), the appropriate current

density and electric field strength depends on the

electrochemical properties of the sample to be treated:

the higher the conductivity, the higher the electric

current density must be to maintain the electric field

strength required. The same author estimates the

necessary electric field strength to be in the order of

50 V/m, which is about two times the values in the

current work.

4.3. Time evolution of the extraction

Data collected in experiment B4 makes it possible

to assess the mass of metal that reaches the concen-

tration compartments (II and IV) over time (Fig. 5).

Fig. 5 shows that more than 40% of all metals

removed are extracted during the first five days.

Between days 5 and 10, extraction proceeds, but at a

slower rate, as is clearly shown by the reduced slope

in this period. From the 10th day onward, extraction

increases again, although not as fast as during the first

5 days.

anode side and to the cathode side as a function of time during

Fig. 6. Heavy metals extracted in experiment C (12 days), as a

percentage of total present in ash.

C. Ferreira et al. / Engineering Geology 77 (2005) 339–347346

These results indicate that remediation is fast in the

first 5 days, where a highly mobile fraction is

available and removed rapidly. The process then

slows down until the less mobile fraction begins to

reach the concentration compartments, which happens

from day 10 onward. On the cathode side, a reduced

extraction rate can be seen for the last 5 days, for Zn,

Pb and Cu, with the curve almost leveling out. This

slowing down of extraction is not seen on the anode

side, thus indicating that prolonging the remediation

time could result in additional removal.

Group C experiments tested a different cell design,

which had an agitator in the central compartment. The

pH inside the compartment III oscillated between

12.03 and 12.32. A precipitate appeared once again.

Fig. 6 presents the extraction results of experiment

C. Remarkably, the Pb extracted in C1 reached 30.7%.

Since this result was so unlike the previous experi-

ments, C1 was repeated and the results proved

identical. The high value obtained for Pb was related

to the amphoteric behavior of this metal, with

solubility increasing for very acidic or very alkaline

media. It is not understood why this only happens in

C1 with water and not in C2, which uses Na-

gluconate, since in batch extractions at pH around

12, Na-gluconate performed much better than water,

as shown in Fig. 4. One possible explanation might be

that Pb-gluconate molecules are too big to cross the

ion-exchange membrane or they take longer to do so

but it is not possible to confirm any of these

hypotheses with available data.

Unlike Pb, extraction of the other metals was

higher when the assisting agent was used (C2).

Nevertheless, extractions (except for lead) are lower

than in experiments B, because of shorter remediation

times (12 days).

5. Conclusions

Experimental results currently show low efficien-

cies for the electrodialytic removal of selected heavy

metals from MSW fly ash. Some explanations were

put forward to account for these low values, such as

the metals being in the precipitate that systematically

appeared, the possibility that complexes were too

big to cross the membranes, not enough moles

present of assisting agent, or too short remediation

times (12 days instead of 20). Nevertheless, the

exploration of these hypotheses requires further

study and experimentation.

In case the electrodialytic treatment succeeds, it

could be possible to replace the current stabilization-

landfilling procedure by a new approach based on the

reuse/recycling of fly ash, now out of reach due to the

high loads of heavy metals.

Electrodialytic remediation is expected to work

with MSW fly ash. The experiences presented in

this work are only preliminary and show concrete

breakthroughs, such as a high Pb removal in one of

the experiments. But they also reveal the nature of

the problems that face the application of this

process to the remediation of MSW fly ash, with

a number of issues remaining unexplained and

requiring further investigation at a more detailed

level.

Acknowledgements

This work was partially funded by the Portuguese

Project POCTI/AGG/45073/2002, approved by FCT

and POCTI with FEDER funds.

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