Effect of Ion Water Washing of Neutralized Sludge Contaminated by High Concentrations of Chloride

Effect of Ion Water Washing of Neutralized Sludge

Contaminated by High Concentrations of Chloride

Chiharu Tokoro

1,+

, Yuji Oda

1

, Shuji Owada

1

and Hiroshi Hayashi

2

1_{Department of Resources and Environmental Engineering, School of Creative Science and Engineering,}

Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan 2_{Central Research Institute, Mitsubishi Materials Corporation, Iwaki 971-8101, Japan}

The effects of NO3¹and SO42¹ions on chloride ion removal from neutralized sludge were investigated to determine if removal of chloride

ions enabled the use of sludge for cement production. An artificial sludge prepared from iron, calcium and chloride that primarily consisted of two-line ferrihydrite was prepared. Chloride ions in the sludge were easily washed out by distilled water, NO3¹water or SO42¹water to levels

below those specified in the standard for Eco-cement production (1,000 mg/kg), but the washed sludges did not meet the standard for ordinary Portland cement (350 mg/kg). Conversely, artificial sludge prepared from aluminum, calcium and chloride mainly consisted of low crystalline boehmite with other minor components. Chloride ions in this sludge were only reduced to below those specified in the standard for the ordinary cement production if SO42¹water was used to wash the sludge. Thefiltration rate during washing using SO42¹water was faster than when

distilled water or NO3¹water was used because SO42¹ions were adsorbed onto the sludge particles and the absolute value of the zeta potential of

the sludge particles decreased. Overall, sludge washing using SO42¹water was the best process for chloride ion removal and efficientfiltration. [doi:10.2320/matertrans.M2013001]

(Received January 7, 2013; Accepted April 2, 2013; Published June 25, 2013)

Keywords: sludge recycling, chloride removal, two-line ferrihydrite, low crystalline boehmite,filtration rate

1. Introduction

Minimizing resource consumption and waste generation is a frequent topic of discussion worldwide that is especially

important in Japan because of a shortage of landfill sites

for disposal as well as a stable supply of resources. In 2010, a total of 389.746 million tons of industrial waste was discharged, while the total discharge of sludge was 173.629

million tons, which accounted for 44.5% of the industrial

waste.1)_{Accordingly, more efficient recycling and a reduction}

of the quantity of sludge generated are necessary in Japan. Several waste and by-products are utilized by the cement industry as row materials or thermal sources for cement

production.2­5) However, sludge contaminated with high

quantities of chloride cannot be utilized for cement production because the chloride ions decrease the quality of cement and make the operation unstable. As a result, the

concentration of chloride in cement is limited to 350 mg/kg

in ordinary Portland cement6) _{and 1,000 mg/kg in}

Eco-cement (Eco-cement in which municipal waste combustion ash

and sewage sludge are utilized as the main raw materials).7)

Neutralized sludge from industrial wastewater sometimes

contains high quantities of chloride and requires efficient

washing of chloride for utilization in cement production.8)

Many researchers have reported that when metal hydroxides are the main component of the sludge chloride ions cannot be removed from the sludge by washing with water alone due

to the chemical affinity between chloride ions and sludge

components.9­11) However, the mechanism for uptake of

chloride ions by sludge components and their removal during sludge washing have not been described in detail to date.

This study was conducted to investigate chloride ion removal from neutralized sludge using several types of ionic

water. Specifically, the mechanism of chloride ion removal

during the washing process was evaluated to determine if the use of ionic water for the removal of chloride ions from sludge to be used in cement production was feasible. To

study the uptake/removal mechanism of chloride ions in

sludge in detail, we used two types of artificial sludge that

were neutralized using calcium hydroxide, one prepared from

ferric and chloride ions solution (Fe­Ca­Cl sludge) and

another prepared from aluminum and chloride ions solution

(Al­Ca­Cl sludge). These ferric and aluminum ions are the

main components in actual neutralized sludge and calcium hydroxide is also the most commonly used neutralizer owing

to its good performance and effective solid/liquid separation.

This study was conducted to compare the removal of

chloride ions from Fe­Ca­Cl sludge or Al­Ca­Cl sludge

washed with distilled water, nitric acid (NO3¹) water and

sulfuric acid (SO42¹) water, and to discuss the mechanism

by which the uptake/removal of chloride ions occurred. To

accomplish this, we repeated batch washing experimentsfive

times and then quantitatively analyzed the concentration of chloride ions in washing water and sludge. We also evaluated

the sludge before/after washing by X-ray diffraction (XRD)

and Fourier transformed infrared spectrometry (FT-IR). In general, several researches have proposed that

adsorp-tion affinity to hydroxides is SO42¹º NO3¹>Cl¹in high

concentration of acid solution.12)_{Additionally, high}

concen-tration of SO42¹ sometimes promotes dissolution of sludge

containing Cl¹ because it products new precipitation with

metallic ions in the sludge. Therefore, SO42¹or NO3¹water

has potential to remove Cl¹ from the sludge consisting of

hydroxides. After washing using these ion water, effluent is

consisted of several acid and dissolved components from the sludge and additional neutralization is needed. However, it is expected that the amount of new precipitated sludge from

effluent treatment is small.

During washing of sludge, good solid/liquid separation is

required in addition to efficient chloride removal. Therefore,

+_{Corresponding author, E-mail: tokoro@waseda.jp}

we also compared the filtration rate during the washing process and evaluated differences among sludges based on

the zeta potential of the sludge before/after washing.

2. Experimental Procedure

2.1 Standard and reagents

All chemicals and solutions used in this study were of analytical grade and were purchased from Wako Pure Chemicals, Inc., Japan. The ferric, aluminum and chloride

solutions were prepared from FeCl3·6H2O, AlCl3·6H2O and

12N HCl, respectively. Ca(OH)2 was used as neutralizer.

The NO3¹ washing water and SO42¹ washing water were

prepared from NaNO3and Na2SO4, respectively. All

experi-ments were conducted at 25°C. In addition, all experiexperi-ments were performed at least in triplicate and the error was

confirmed to be within 1%.

2.2 Preparation of artificial sludge

Artificial Fe­Ca­Cl sludge and Al­Ca­Cl sludge were

prepared as follows. Briefly, 0.6 dm3of 0.1 M Fe(III) or 0.1 M

Al(III) and 1 M Cl(-I) were initially prepared in 1 dm3beakers

and neutralized by the addition of about 0.1 dm3of 20 mass%

Ca(OH)2slurry to give a pH of 7. After agitation at 5000 rpm

for 30 min using a magnetic stirrer, the suspension was

filtered through a 0.45 µm membrane filter (Advantec,

Japan).

2.3 Washing experiments

Freshly prepared artificial sludge was prepared for all

washing experiments conducted in this study. Three types of washing water, deionized (DI) water, water containing

2000 mg/dm3 _NO

3¹ and water containing 2000 mg/dm3

SO42¹were used.

A total of 80 cm3 of artificial sludge suspension was

filtered through a 0.45 µm membrane filter and the residue

was then re-dispersed in 40 cm3 of washing water in a

100 cm3 conical beaker and stirred at 1000 rpm using a

magnetic stirrer. The suspension was then passed through a

0.45 µm membranefilter again. The re-dispersion in washing

water andfiltration were repeatedfive times, after which the

concentration of Cl¹, NO3¹or SO42¹infiltrate were analyzed

by ion chromatography (IC-2001, Tosoh, Japan) and the concentration of chloride ions in the residue were analyzed

by the Volhard method.13) _{The concentration of iron,}

aluminum, calcium and sodium in the residue was analyzed

by X-ray fluorescence (XRF) (PW1480/1404, PANalytical,

Japan).

2.4 XRD analysis

The chemical forms of the sludge were evaluated by XRD

analysis (Geigerflex RAD-IR, Rigaku, Japan). XRD patterns

of the sludge before/after washing were obtained using a

copper target (Cu K¡), a crystal graphite monochromator,

and a scintillation detector. The equipment was operated at

40 kV and 30 mA by step-scanning from 2° to 80° 2ª at

increments of 0.02° 2ªand a scan speed of 2°/min. A crystal

sample holder was used and the diffractograms were not

corrected by background diffraction. The filter residue was

freeze-dried at¹45°C for 24 h prior to analysis.

Two-line ferrihydrite and low crystalline boehmite were synthesized as reference materials. Two-line ferrihydrite was

synthesized from 10 mg/dm3_{Fe(III) solution prepared using}

Fe(NO3)3·9H2O at pH 5 and an ionic strength of 0.05 using

1 M HNO3 and 1 M KOH to regulate the pH and ionic

strength, respectively. Low crystalline boehmite was

synthe-sized from 20 mg/dm3 of Al(III) solution prepared using

Al(NO3)3·9H2O at pH 9 and an ionic strength of 0.05 using

1 M HNO3 and 1 M KOH to regulate the pH and ionic

strength, respectively.

2.5 FT-IR analysis

The sludge was also analyzed during the washing experi-ments after freeze-drying using FT-IR spectroscopy (FT-IR 4200, JASCO, Japan). Pellets for FT-IR were prepared by compression molding with KBr. The mixing ratio of the

sample to KBr was fixed at 0.3 mass% and FT-IR spectra

were obtained using transmission FT-IR spectroscopy.

2.6 Zeta potential measurement

Measurement of the zeta potential was conducted using an electrophoresis light scattering spectrophotometer (ELS-8000, Otsuka Electronics, Japan). To accomplish this, suspensions of the sludge prepared during the washing

exper-iments prior tofiltration were dispersed in an ultrasonic bath

for 5 min and then rapidly analyzed by spectrophotometry.

2.7 Measurement offiltration rate

The quantity offiltrate and time offiltration were measured

during suctionfiltration of a suspension of the sludge under

0.005 MPa. For filtration, a filter holder with a 250 dm3

funnel (Sartorius, Germany) and 0.45 µm membrane filter

were used. Thefiltration rate was determined as follows:

V ¼Q_S ; ð1Þ

where Q(m3/h) is the flow rate and S (m2) is the filtration

area. In this study, the effective area of the filtration

equipment was 1.25©10¹3m2.

3. Results and Discussion

3.1 Effect of ionic water on removal of chloride from the sludge

The concentrations of chloride ions in the artificial Fe­Ca­

Cl sludge and Al­Ca­Cl sludge before/after washing using

distilled water, NO3¹ water and SO42¹ water are shown in

Figs. 1 and 2. The concentrations of iron and calcium in the

sludge are shown in Table 1. For the artificial Fe­Ca­Cl

sludge, there was little difference in chloride removal by

distilled water, NO3¹ water and SO42¹ water, and most of

the chloride ions were removed from the sludge during the

first washing process. Salingaret al.also proposed that 92%

of the chloride ions in a sludge in which Fe(III) was the main component could be removed by a single wash using

water.14) _{As shown in Table 1, the residual concentration}

of chloride in the sludge after five washes was less than

900 mg/kg, which did not meet the standard for ordinary

Portland cement (350 mg/kg), but did meet the standard for

For Al­Ca­Cl sludge, there was a large difference in

chloride removal between distilled water, NO3¹ water and

SO42¹ water. A large amount of chloride (33,000 mg/kg)

remained in the sludge when distilled water was used, while removal of additional chloride from the sludge was achieved when ionic water was used. Following washing

with NO3¹ water, the residual concentration of chloride

was 4,400 mg/kg, which exceeded the standard for cement

production. In contrast, when SO42¹ water was used, the

residual concentration of chloride was less than 300 mg/kg,

which met the standards for both ordinary Portland cement and Eco-cement.

NO3¹or SO42¹concentration taken in sludge during five

times washing was shown in Fig. 3. Both for Fe­Ca­Cl and

Al­Ca­Cl sludge, more SO42¹ ion was uptake than NO3¹

ion. Especially for Al­Ca­Cl sludge, in thefirst washing by

SO42¹ ion water, a lot of SO42¹ was uptake to the sludge.

This result suggests that new precipitates could be formed

between Al­Ca­Cl sludge and SO42¹ ion water during the

first washing. In any case, these residual SO42¹ or NO3¹

concentration in the sludge after washing have no problem for cement utilization.

In addition, pH was decreased until around 6.5 at thefirst

washing, while it was increased gradually during following washing up to 7.5.

3.2 Mineralogical changes in sludge following washing Figure 4 shows a comparison of the XRD patterns of

artificial Fe­Ca­Cl sludge before/after washing five times

using distilled water, NO3¹ water and SO42¹ water. All

XRD patterns showed two broad bands at 34° 2ª and 61°

2ª, which were the same positions as observed for two-line

ferrihydrite.14)_{These results suggest that artificial Fe­Ca­Cl}

sludge mainly consisted of two-line ferrihydrite, and that the mineral form was not changed by washing with ionic water. Chloride ions generally form outer-sphere complexes with hydro ferric oxide, and not many chloride ions can be

taken up by ferrihydrite.12,15)_{However, as shown in Table 1,}

residual chloride ions that exceeded the standard for ordinary Portland cement production were left behind in

Fe­Ca­Cl sludge after washing with any type of water.

These findings suggest that some amorphous component

other than two-line ferrihydrite strongly bound the chloride

ions into the sludge. As shown in Table 1, the artificial Fe­

Ca­Cl sludge contained 0.28%calcium, and that the amount

of calcium did not change after washing. In this study,

calcium ferrite (CaFe2O4) was thermodynamically saturated

in an artificial Fe­Ca­Cl formation16) _{as shown in Table 2.}

Moreover, there was the potential for formation of Ca­Fe

hydrotalcites, which could take up chloride ions in the Fe­

Ca­Cl sludge.

0 50 100 150 200

0 1 2 3 4 5 6

Cl Conc. in Sludge (g/kg)

Washing Cycle Number (-) Pure water

Nitrate ion water

Sulfate ion water

Fe

Fig. 1 Concentration of chloride in Fe­Ca­Cl sludge after washing with distilled water, NO3¹water and SO42¹water.

0 50 100 150 200

0 1 2 3 4 5 6

Cl Conc. in Sludge (g/kg)

Washing Cycle Number (-) Pure water

Nitrate ion water

Sulfate ion water

Al

Fig. 2 Concentration of chloride in Al­Ca­Cl sludge after washing with distilled water, NO3¹ion water and SO42¹ion water.

Table 1 Concentration of chloride, calcium and iron in sludge before/after washing.

Cl (mg/kg)

Ca (mg/kg)

Fe or Al (mg/kg)

Fe­Ca­Cl sludge Fe

Before washing 1.0©105 _2.8©104 _5.7©105

After washing with pure water 8.9©102 _2.1©104 _5.6©105

After washing with NO3¹water 8.6©102 1.9©104 5.6©105

After washing with SO42¹water 8.8©102 2.0©104 5.7©105

Al­Ca­Cl sludge Al

Before washing 1.8©105 _7.7©104 _3.7©105

After washing with pure water 3.3©104 _6.1©102 _3.6©105

After washing with NO3¹water 4.4©103 3.7©103 3.5©105

After washing with SO42¹water 2.8©102 1.3©104 2.9©105

0 0.05 0.1 0.15 0.2

0 1 2 3 4 5 6

NO

3

or SO

4

Conc. in Sludge (mg/kg)

Washing Cycle Number (-) Nitrate ion water (Fe) Nitrate ion water (Al) Sulfate ion water (Fe) Sulfate ion water (Al)

Figure 5 shows a comparison of the XRD patterns of

artificial Al­Ca­Cl sludge before/after washing five times

with distilled water, NO3¹water and SO42¹water. Thisfigure

also shows the XRD patterns of low crystalline boehmite,

which has broad bands at 29° 2ª, 39° 2ª and 48° 2ª.17)

Although the artificial Al­Ca­Cl sludge had broader and

weaker peaks at the same positions before washing as observed for low crystalline boehmite, these peaks became

sharper and clearer after five washes with distilled water or

NO3¹ water. These results suggest that artificial Al­Ca­Cl

sludge mainly consisted of low crystalline boehmite and other minor components that could easily be washed out by

distilled water or NO3¹water. These trends coincided with a

reduction in residual calcium concentration in the sludge after

washing using distilled water or NO3¹water (Table 1). Many

wastes consist of aluminum, calcium and chloride in the form

of Friedel’s salt (Ca2Al(OH)6(Cl, OH)·2H2O), which

immo-bilizes chloride ions onto the solid. Although the Friedel’s

salt was not thermodynamically saturated in this study, there was the potential for adsorption of chloride ions to boehmite

because of high chemical affinity between Al, Ca and Cl.

Conversely, when samples were washed with SO42¹water,

the XRD patterns became broader and weaker. As shown

in Table 1, the aluminum concentration in the sludge was

reduced as well as the chloride ion concentration, while 17%

of calcium remained in the sludge after washing. These results suggest that a portion of the sludge dissolved and minor new components formed in the sludge after washing

with SO42¹water. In this study, gypsum (CaSO4·2H2O) and

aluminum sulfate were thermodynamically saturated during

SO42¹ water washing16) as shown in Table 3. Additionally,

ettringite (Ca6Al2(SO4)3(OH)12·26H2O) is also known to be

a stable component in Al­Ca­SO4 aqueous systems.

Although ettringite was thermodynamically unsaturated in

this study, there was the potential for adsorption of Ca or SO4

ions to boehmite because of high chemical affinity between

Al, Ca and SO4.

To evaluate mineralogical changes in the Al­Ca­Cl sludge

after washing, we also analyzed the FT-IR spectrum (Fig. 6).

The band at approximately 600 cm¹1 could be assigned to a

mixed contribution of Al­O stretching and bending for AlO6,

which has one particularly long Al­O bond with respect to

the other five bonds.18) _{Prior to washing, this band shifted}

toward a low wavenumber, which suggests that the structure of low crystalline boehmite was ambiguous in the sludge

before washing. The band at 950 cm¹1 _{corresponds to the}

0

20

40

60

80

Relati

v

e Intensity (-)

Cu

Κα

Κα

(

°

)

Two-line ferrihydrite

Before washing

Pure water

Nitrate ionwater

Sulfate ion water

Fig. 4 XRD patterns of tow-line ferrihydrite (as reference material) and Fe­Ca­Cl sludge before washing and after washing five times with distilled water, NO3¹ion water and SO42¹ion water.

Table 2 Saturation indices duringfirst washing for Fe­Ca­Cl sludge using SO4¹water.

Fe (mol/L)

Ca (mol/L)

Cl (mol/L)

SO4

(mol/L)

pH (¹)

Saturation Index (¹)

Goethite CaFe2O4 Gypsum

1.5©10¹1 _1.0©10¹2 _4.2©10¹2 _2.1©10¹2 _{6.8 10.5 11.6 ¹0.19}

0

20

40

60

80

Relati

v

e Intensity (-)

Cu

Κα

Κα

(

°

)

Poorly crystalline boehmite

Before washing

Pure water

Nitrate ionwater

Sulfate ion water

Fig. 5 XRD patterns of low crystalline boehmite (as reference material) and Al­Ca­Cl sludge before washing and after washingfive times with distilled water, NO3¹ion water and SO42¹ion water.

Table 3 Saturation indices duringfirst washing for Al­Ca­Cl sludge using SO4¹water.

Al (mol/L)

Ca (mol/L)

Cl (mol/L)

SO4

(mol/L)

pH (¹)

Saturation Index (¹)

Boehmite Al4(OH)10SO4 AlOHSO4 Gypsum Ettringite

stretching mode of Al­O,18­20) _{which is observed in both}

aluminum hydroxide and low crystalline boehmite. This band could be clearly distinguished in the sludge before and

after washing with SO42¹ water, while it shifted or became

smaller after washing using distilled water or NO3¹ water.

The small band at approximately 1050 cm¹1 corresponds to

the stretching mode of Al­O, which is only observed in low

crystalline boehmite.21)This band was unclear in the sludge

before washing or after SO42¹ water washing, while it was

clear in the sludge after washing using distilled water or

NO3¹ water. The band at approximately 1060 cm¹1 and

1400¹1_{corresponds to the bond angle bending mode of S­O}

and the stretching mode of N­O, respectively.22)_{These bands}

suggest that adsorption of NO3¹ or SO42¹ occurred during

washing with NO3¹water or SO42¹water.

All of the FT-IR spectra suggest that components of the

Al­Ca­Cl sludge before washing or after washing with

SO42¹consisted of combinations of aluminum hydroxide and

low crystalline boehmite, which had a structure that became

clearer after washing with distilled water or NO3¹ water.

Overall, the results of FT-IR analysis coincide with those of XRD analysis.

3.3 Effect of washing with ionic water onfiltration rate

The filtration rate of the Fe­Ca­Cl sludge and Al­Ca­Cl

sludge during five washes with distilled water, NO3¹water

and SO42¹water are shown in Figs. 7 and 8. In the cases of

Fe­Ca­Cl sludge and Al­Ca­Cl sludge, thefiltration rate was

largest during SO42¹water washing, followed by NO3¹water

washing and distilled water washing.

Thefiltration rate generally depends on the crystallinity or

particle size of the components in the sludge and the surface

electric potential at the solid/liquid interface between the

sludge and the washing water. As shown in Figs. 9 and 10, the zeta potential of the sludge decreased considerably after

washing with SO42¹water, which suggests that many SO42¹

ions were adsorbed onto the surface of the sludge particles.

In the case of NO3¹water washing, the zeta potential of the

sludge also decreased because of the adsorption of NO3¹ions

onto the sludge; however, this decrease was not as great as

that observed during washing with SO42¹water. Conversely,

the zeta potential showed increases or decreases during

washing with distilled water owing to the desorption of chloride, calcium and aluminum ions from the sludge.

The relationship between the zeta potential and the

filtration rate was found to be negatively correlated

(Fig. 11), which suggests that when the absolute value of the zeta potential was small, particles of sludge were agglomerated together and the pathway of the washing water could form inside the sludge.

Before washing

Pure water

Nitrate ionwater

Sulfate ion water SO4

NO3

Al-O in boehmite

Al-O

AlO6

H2O

Fig. 6 FT-IR patterns of Al­Ca­Cl sludge before washing and after washingfive times with distilled water, NO3¹water and SO42¹water.

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016

0 1 2 3 4 5 6

Filtration Rate (m/h)

Washing Cycle Number (-) Pure water Nitrate ion water Sulfate ion water Fe

Fig. 7 Filtration rate of Fe­Ca­Cl sludge during washing with distilled water, NO3¹water and SO42¹water.

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016

0 1 2 3 4 5 6

Filtration Rate (m/h)

Washing Cycle Number (-) Pure water Nitrate ion water Sulfate ion water Al

Fig. 8 Filtration rate of Al­Ca­Cl sludge during washing with distilled water, NO3¹water and SO42¹water.

0 10 20 30 40 50 60

0 1 2 3 4 5 6

Zeta P

otential (mV)

Washing Cycle Number (-) Pure water

Nitrate ion water Sulfate ion water

Fe

4. Conclusion

The effects of NO3¹or SO42¹on removal of chloride ions

from sludge during washing were investigated. Artificial

sludge prepared from iron, calcium and chloride mainly consisted of two-line ferrihydrite and chloride ions at levels exceeding the standards for cement utilization. However, the

chloride ions in the Fe­Ca­Cl sludge were easily removed

to levels below the standard for Eco-cement production

(1,000 mg/kg) by washing with distilled water, NO3¹water

or SO42¹water, although they did not meet the standard for

ordinary Portland cement (350 mg/kg).

The artificial sludge prepared from aluminum, calcium and

chloride mainly consisted of low crystalline boehmite with other minor components and chloride ions at levels exceed-ing the standards for cement utilization. Washexceed-ing with

distilled water or NO3¹ water removed a portion of the

chloride ions as well as minor components of the sludge, but the concentration of chloride remained above the standards

for cement utilization. However, when SO42¹water was used

for washing, the chloride levels were reduced to below the standard for ordinary cement production.

The effects of NO3¹ or SO42¹ ions on the filtration rate

during washing of sludge were also investigated. During

washing using SO42¹water, thefiltration rate was faster than

that of distilled water or NO3¹ water because SO42¹ ions

were adsorbed onto the sludge particles and the absolute value of the zeta potential of the sludge particles decreased.

Overall, sludge washing using SO42¹water produced the best

results for chloride ion removal andfiltration.

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0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016

0 10 20 30 40 50 60

Filtration Rate (m/h)

Zeta Potential (mV)

Fe-pure water Fe-nitrate ion water Fe-sulfate ion water Al-pure water Al-nitrate ion water Al-sulfate ion water

Fig. 11 The relationship betweenfiltration rate and zeta potential for Fe­ Ca­Cl sludge and Al­Ca­Cl sludge before/after washing with distilled water, NO3¹water and SO42¹water.

0 10 20 30 40 50 60

0 1 2 3 4 5 6

Zeta P

otential (mV)

Washing Cycle Number (-)

Pure water Nitrate ion water Sulfate ion water

Al