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Immobilization of some metals incontaminated sludge by zeoliteprepared from local materialsAzza M. Ashmawy a , Hanan S. Ibrahim a , Shimaa M. AbdelMoniem a & Tamer S. Saleh aa National Research Center, El Buhouth Street, P.O. Box 12311,Dokki, Cairo, EgyptAccepted author version posted online: 11 Sep 2012.Version ofrecord first published: 02 Oct 2012.

To cite this article: Azza M. Ashmawy , Hanan S. Ibrahim , Shimaa M. Abdel Moniem & Tamer S.Saleh (2012): Immobilization of some metals in contaminated sludge by zeolite prepared from localmaterials, Toxicological & Environmental Chemistry, 94:9, 1657-1669

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Toxicological & Environmental ChemistryVol. 94, No. 9, October 2012, 1657–1669

Immobilization of some metals in contaminated sludge by zeolite prepared

from local materials

Azza M. Ashmawy, Hanan S. Ibrahim*, Shimaa M. Abdel Moniem and Tamer S. Saleh

National Research Center, El Buhouth Street, P.O. Box 12311, Dokki, Cairo, Egypt

(Received 16 February 2012; final version received 2 September 2012)

In this study, zeolite was prepared from cheap local Egyptian clay (kaolin) andcharacterized by X-ray diffraction, X-ray fluorescence spectroscopy, SEM. Theprepared zeolite was used as a binder for immobilization of the metals Cd2þ,Cu2þ, Ni2þ, Pb2þ, and Zn2þ in contaminated sewage sludge. Different leachingtests were conducted to determine the efficiency of the prepared zeolite for metalstabilization. The leaching of the metals from stabilized sludge decreased as thezeolite amount increased. It was found that 10% of zeolite is sufficient for thestabilization of all metal ions under investigation. It was suggested that the metaluptake mechanism by zeolite was by an ion-exchange mechanism. Examination ofthe solidified sample for its compressive strength after curing for 28 days yielded avalue of 0.83MPa, which indicates that the treated sludge was well solidified andsafe to be used in a wide variety of applications, for instance as a raw material forpavement blocks.

Keywords: heavy metals; immobilization; zeolite; leachability; compressivestrength

Introduction

Sewage sludge produced by the conventional treatment in wastewater treatment plants(WWTP) is of great economic importance. It may contain xenobiotics, e.g. metals andpersistent organic pollutants (POPs) such as phenols and polyaromatic hydrocarbons(PAHs). In Egypt, approximately 3–5% of the municipal solid waste (MSW) originatingfrom industrial activities may be hazardous. Therefore, it represents an actual andlong-term threat to environment and groundwater resources (Guangren et al. 2006;EEAA 2010).

Various technologies have been developed to transform hazardous wastes to nontoxicforms or to reduce the potential release of toxic species into the environment (Conner1990; Pereira, Pinero, and Vale 2001; Dermatas and Meng 2003; Samaras et al. 2008;Lasheen and Ammar 2009). Recently, more attention is paid to the sludge stabilizationprocess aiming to minimize the mobility of metals using various additives where the toxicconstituents present in hazardous solid sludge are physically as well as chemically fixed toensure the compliance with existing regulatory standards (Conner 1990; Dermatas et al.2004; Fatta et al. 2004; Malviya and Chaudhary 2006; Montinaro et al. 2007; Giergicznyand Krol 2008; Buj et al. 2010). One of the promising binders is diatomaceous earth,occurring as artificial zeolites and natural sorbents (clinoptylolite), the addition of which

*Corresponding author. Email: drhanan@yahoo.com

ISSN 0277–2248 print/ISSN 1029–0486 online

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can improve the physicochemical properties of the sewage sludge. Also, metal contents insewage sludge decreases after the addition of sorbents, an effect typical for theimmobilization of metals on the zeolite surface and its pores (Erdem, Karapinar, andDonat 2004; Sprynskyy et al. 2007). Zeolites are naturally occurring hydrated alumino-silicate minerals. They belong to a class of minerals known as ‘‘tectosilicates.’’ The mostcommon natural zeolites are formed by the alteration of glass-rich volcanic rocks (tuff)with fresh water or by seawater (Badillo-Almaraz, Trocellier, and Davila-Rangel 2003).The structures of zeolites consist of three-dimensional frameworks of [SiO4]

4� and[AlO4]

5� tetrahedra. The aluminum ion is small enough to occupy the position in thecenter of the tetrahedron of four oxygen atoms, and the isomorphous replacement ofSi(IV) by Al(III) produces a negative charge in the lattice. The net negative charge isbalanced by the exchangeable cation (sodium, potassium, or calcium), which may give riseto a high cation exchange capacity (CEC) (Miecznikowski and Hanuza 1985; Barer 1987).Other studies argued that the immobilization of sludge was mainly affected by the CECrather than the pH value (Castaldi et al. 2008) and investigated the changes of the zeolitestructure caused by the exchange with cations of different heavy metals (Castaldi, Santona,and Melis 2005). Synthetic zeolite has been recently tested in order to evaluate their abilityfor immobilization of toxic metals. These amendments can lead to the immobilization ofmetals. First, zeolites dissolve supplying alkalinity to the acid polluted soils, causing theprecipitation of the insoluble phases. These formed phases contain metals as majorconstituents (Chen et al. 2000) or co-precipitated in hydroxides as minor components(Chlopecka and Adriano 1996; Chlopecka and Adriano 1997; Boisson et al. 1999). Second,the increase in alkalinity promotes the metal adsorption via surface complexationprocesses (Shanableh and Kharabsheh 1996; Peng, Song, and Yuan 2006; Trgo, Peric, andMedvidovic 2006; Basaldella et al. 2007; Li et al. 2009; Liu, Yang, and Li 2009; Silva andRoldan 2009; Sunarso and Ismadji 2009). Third, metal retention may also take placeregardless of the pH value due to the cation exchange in zeolite (Nissen, Lepp, andEdwards 2000; Castaldi et al. 2008).

Many research projects tend to prepare zeolites from cheap materials instead toaccommodate its economic cost (Sprynskyy et al. 2007; Ibrahim, Jamil, and Hegazy 2010).Therefore, the aim of our study is to choose Egyptian kaolin that is very cheap and theavailable source for the preparation of zeolite and investigate the use of such preparedzeolite as a binder for the immobilization of heavy metal in sludge.

Materials and methods

Materials used and reagents

All chemicals were supplied by Merck (Darmstadt, Germany) without further purificationand deionized water was used. Synthetic stock solutions of cadmium, copper, nickel,lead, and zinc were prepared using CdCl2 � 5/2H2O, CuSO4 � 5H2O, NiCl2 � 6H2O,Pb(CH3COO)2 � 3H2O, and ZnSO4 � 7H2O, respectively, in deionized water. Glacialacetic acid (99.5% CH3COOH), sulfuric acid (98% H2SO4), and nitric acid (65%HNO3) were used for leachability tests.

Sludge sampling

Integrated sludge samples were collected from secondary wastewater treatment plant ofEl-Gabal El-Asfer. Its capacity was 1,700,000m3d�1, serving municipal and industrial

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areas in the east of Cairo. Collection and preservation of all the sludge samples were doneaccording to standard methods (APHA 2005).

Preparation of zeolite

Zeolite A was prepared according to Egyptian Patent No. 23353, 2005 from EgyptianKaolin using the hydrothermal treatment method. Kaolin was analyzed via X-rayfluorescence (XRF). The chemical composition and physical properties of naturalEgyptian Kaolin are shown in Table 1. Commercial sodium silicate solution (Na2Si2O5)was used as a source of silicone and commercial sodium hydroxide was used as an alkalinesolution with different concentration percent according to the prepared zeolite.Preparation of zeolite was adopted from Selim and Abd El-Maksoud (2005); zeolite Athat was synthesized as mentioned before, was crushed, grounded, and passed through30 mm sieves and were dried in an oven at 100� 5�C for 24 h.

Characterization of zeolite A

The prepared zeolite was characterized using X-ray diffraction (XRD) for phaseidentification, using a diffractometer with Cu-K�1 target and second monochromator at40 kV, 40mA (D8 Advance, Bruker, Rheinstetten, Germany). Surface morphology wasinvestigated under scanning electronmicroscope (SEM) ‘‘JXA-840 an Electron probeMicroAnalyzer-Japan.’’ The chemical composition of Egyptian Kaolin used in the preparation ofzeolite was determined with XRF by Axios, Sequential WD-XRF SpectrometerPANalytical 2005. Pore diameter was estimated using Gemini 2360 surface area analyzer.

The immobilization process

Metal-spiked sewage sludge samples were used for the application of the immobilizationtechniques by applying the following steps:

. The sludge samples were processed to remove the non-recyclable materials andpassed through a 2mm sieve.

. One kilogram of sludge (on dry weight basis) was spiked with 100mL of asolution containing 1000mgL�1 of Cd, Cu, Ni, Pb, and Zn in the form

Table 1. Chemical composition and physical properties of natural Egyptiankaolin (wt%).

Chemical composition (%) Physical composition (%)

SiO2 52.7 pH 7.5Al2O3 31.4 Appearance porosity (%) 41TiO2 2.2 Appearance density (g cm�3) 2.20Fe2O3 1.1 Weight of per unit volume (g cm�3) 1.30MnO 0.01 Water absorption (original) (%) 30.3MgO 0.21 Water absorption (grinding) (%) 102CaO 0.52 Original bleaching (g sample g�1 tonsil) 1.94Na2O 0.1 Active bleaching (g sample g�1 tonsil) 1.90K2O 0.04 Ignition loss 0.14P2O5 0.08SO3 0.14

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CdCl2 � 5/2H2O, CuSO4 � 5H2O, NiCl2 � 6H2O, Pb(CH3COO)2 � 3H2O, andZnSO4 � 7H2O, respectively.

. Spiked sludge was thoroughly stirred and incubated at room temperature for onemonth. After incubation, the treated sludge was again thoroughly stirred.

. The heavy metals content in the spiked sludge compared to the original sludge areshown in Table 2.

. Zeolite A was mixed thoroughly using a mechanical blender with the samples in aproportion series of 0, 2.5, 5, 10, 20, and 30% (dry weight basis) by addingapproximately 5mL of distilled water to facilitate the curing.

. The air-dried cured samples were grounded to pass through a 0.2mm sieve andtriplicate samples of the treated sludge were drawn for examining the efficiency ofthe immobilization technique.

Analytical methods

. pH measurements: sludge samples were analyzed for pH, according to theEuropean Committee for Standardization (ENV 12176, 2000).

. The digestion of samples and determination of the total contents of heavy metalswere carried out according to APHA (2005) using an atomic absorptionspectrometer (SpectrAA 220, Varian, Australia) with a graphite furnace accessoryand equipped with deuterium arc background corrector.

Quality control

The following program was applied:

(i) Blank was run with each set of analysis.(ii) Quantifications of metal were carried out using external standards with coefficient

for calibration curves higher than 0.995.(iii) The calibration program was verified on each working day by measuring one or

more standard.(iv) A random sample was run in triplicate.(v) Laboratory control sample was analyzed with each series of samples.

Evaluation of the performance of the immobilization techniques

In order to evaluate the performance of the immobilization technique, different testmethods were selected:

(1) The standard European leaching test (EN 12 457-2) according to (EuropeanCouncil Decision 2003).

Table 2. Total content of metals in original and spiked samples of sludge.

Metal (mg kg�1) Cd Cu Pb Ni Zn

Original 4.0� 0.05 538.0� 9.5 750.0� 4.9 81.0� 1.6 1204.0� 7.6Spiked sludge 407.5� 2.5 1942.0� 88.4 1500.0� 11.7 663.0� 5.99 3343.0� 22.4

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(2) Toxicity Characteristic Leaching Procedure (TCLP) according to (U.S.EPA 1986a;2005).

(3) Multiple Extraction Procedure according to (U.S.EPA 1986b).(4) Unconfined Compressive Strength test (UCS) after 28 days according to (ASTM

1991).

The percent of retained metals for remediate sludge was calculated as mentioned by Loand Chen 1990.

Retention percent of metals ¼ 100� VfF=DI� �

� 100

D: 5 g dry solids sludge; Vf: Final volume after dilution to 100mL; F: concentration ofmetal in filtrate (mgL�1); I: concentration of metal in the initial dry solid sample(mg kg�1).

Statistical analysis

Statistical analysis was done through SPSS statistical program version 14.0. For eachelement, an analysis of variance (ANOVA) and the post-hoc test least significant difference(LSD) were performed to compare between the three leaching tests. The correlationcoefficient was used to test the relationships between two quantities data (time andconcentrations).

Results and discussion

Properties of zeolite A

In this study, synthetic zeolite was prepared from Egyptian kaolin, namely LTA(Zeolite A) with the chemical formula Na12Al12Si12O48.37 H2O. This type of zeolite wascharacterized by XRD (Figure 1), which proved a high purity and crystallinity of zeolites.Also, SEM confirmed that zeolite A has a high degree of crystallinity as cubes (Figure 2).The method of preparation of LTA zeolite A from Egyptian kaolin is registered as patentwith No. 23 353/2005. The pore diameters of zeolite A type was [3.1–4.5 A].

Effect of zeolite dose on immobilization of heavy metals

The pH of stabilized sludge was slightly increased from 5.2 to 7.3, indicating that thestabilization of heavy metal using zeolite A is not pH dependant, which agrees with Trgo,Peric, and Medvidovic 2006. Also the metal concentrations decreased with increasingpercentages of zeolite. These results were expected as a result of the dilution effect and itwere also in agreement with the observations of Castaldi et al. (2008), who mentioned thatincreasing zeolite amendment rates significantly reduced the total metal contents in thesludge.

The standard European EN 12 457-2 (2002) was used to estimate the actual mobility ofheavy metals that can be obtained by applying simple one-stage leaching test. Themeasured concentrations can be compared with the values that have been issued by theEuropean Council (European Council Decision 2003/33/EC) as criteria for acceptance ofwastes in three main classes of landfill sites, i.e., landfills for inert wastes, non hazardouswastes, and hazardous wastes (European Council Decision 2003).

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Figure 3 shows that the metal leaching concentrations decrease by increasing thepercent of zeolite from 0 to 30%. The metal leaching concentrations decreased from 2.24,48.7, 5.4, 6.7, and 23.6mgL�1 to 0.014, 0.33, 0.013, 0.72, and 2.4mgL�1 for cadmium,copper, lead, nickel, and zinc, respectively. This was confirmed by our previous studies on

Figure 1. XRD of LTA (zeolite A).

Figure 2. SEM of LTA (zeolite A).

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zeolite A which determined the sorption energy E by the Dubinin–Kaganer–Radushkevichisotherms and found it lies within 8–16 kJmol�1, which revealed that the metals uptakemechanism is ion-exchange (Clifford 1990; Lo and Chen 1990; Ibrahim, Jamil, and Hegazy2010). During the ion-exchange process, the metal ions have to move not only through thepores of the zeolite mass, but also through channels of the lattice, and they have to replaceexchangeable cations (mainly sodium and calcium). This will lead to fixing the metals inthe zeolite matrix and reducing their leachability.

Also, Figure 3 shows that 10% of the synthetics zeolite to waste could reduce metalleaching concentrations to lower than the limit values set by the European Council

Figure 3. Leachability of metals (a) cadmium, (b) copper, (c) lead, (d) nickel, and (e) zinc fromstabilized sludge by zeolite according to the standard EN 12457-2 test.

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decision 2003/33 for the acceptance of waste in landfills for non-hazardous waste.

However, the availability of Cu and Pb in the stabilized sludge under the conditions of the

EN 12 457-2 leaching test can be accepted in an inert landfill. This finding was due to the

relation between the crystal radii of metal ions (0.97 A for Cd, 0.72 A for Cu 1.2 A for Pb,

0.69 A for Ni and 0.74 for Zn) with the free dimensions of zeolite A channels where the

unhydrated ions can pass readily through the channels of zeolite A, both Cu (1.9) and Pb

(1.8) metal ions are more attracted to the zeolite surface due to their high electronegativity

(Forbes, Posner, and Quirk 1976; Elliott, Liberati, and Huang 1986; Schwertmann and

Taylor 1989).The TCLP test method no. 1311 is designed to simulate leaching that takes place in a

sanitary landfill. Figure 4 shows high retention percent of heavy metals with increasing

percentage of zeolite, which indicates that all metals were well fixed in the applied matrix.

The metal leaching concentrations were decreased from 9.0, 133, 6.8, 22.5, and 117mgL�1

to 0.27, 0.87, 0.093, 0.88, and 24.5mgL�1 for cadmium, copper, lead, nickel and zinc,

respectively.The retention percent of heavy metals at the optimum zeolite/sludge ratio (10%) was

more than 96% for Cd, Cu, Pb, and Ni and about 79% for Zn after TCLP. The order of

metal retained in the zeolite/sludge matrix was found to be as follows:

Lead4Copper4Nickel4Cadmium4Zinc

These findings can be explained by different preferences of zeolite for different cations.

Because of its high Si:Al ratio (as shown in Table 1), zeolite has a low structural charge

density. Therefore, divalent cations with low hydration energies are sorbed preferably

compared to cations with high hydration energies. Our results are substantiated by Marcus

(1991), who mentioned that the Gibbs free energies of hydration are �1425 kJmol�1 for

Pb and �1955 kJmol�1 for Zn. This explains why zeolite will prefer Pb over the other

metals.In order to assess the long-term stability of heavy metal-contaminated sludge stabilized

with zeolite, the Multiple Extraction Procedure (MEP) was applied using the U.S.EPA

0

20

40

60

80

100

120

84

86

88

90

92

94

96

98

100

102

2.5 5 10 20 30

% o

f met

al r

etai

ned

(Ni,Z

n)

% o

f met

al r

etai

ned

(Cd,

Cu,

Pb)

% of Zeolite

Cadmium Copper Lead Nickel Zinc

Figure 4. Percentage of metals retained in stabilized sludge by using after TCLP leaching test.

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Method, 1320. The results showed that the cumulative percentages of leached heavy metals

decreased with increasing leaching time and stabilized after about 7 days. This may be dueto the fact that zeolite has adsorption and ion exchange abilities for metals, so it can be

used as an additive to amend the sewage sludge and inhibit metal mobility (Chiang,

Huang, and Chang 2007; Garau et al. 2007).At the optimum zeolite/sludge ratio (10% of zeolite), the cumulative leaching percent

of Cd, Cu, Pb, Ni, and Zn were reduced from 0.57, 21.6, 0.99, 1.88, and 19.9wt % to 0.031,

0.036, 0.0044, 0.077, and 1.25wt %, respectively. The results which are graphically

represented in Figure 5 revealed that the concentration of metals retained after applyingU.S.EPA Extraction Method 1320 were very high when compared to spiked sludge. This

indicates that, even after subjecting the stabilized sludge to rigorous leaching conditions, it

exhibit good binding ability for all metals and can be considered chemically durable. This

results agreed with several authors (Oste, Lexmond, and Van Riemsdijk 1988; Shanablehand Kharabsheh 1996) where they found that zeolite can enhance the sorption capacity of

the sludge and reduce the heavy metal leaching concentrations from contaminated sludge.

Furthermore, Querol et al. (2006) reported that the addition of zeolite drastically decreasesthe metal mobility, even at the lowest zeolite dose applied to the contaminated sludge.

Statistical analysis

Statistical analysis was done using ANOVA to compare between the different leaching

tests and heavy metals leaching concentrations in sludge stabilized with different doses ofzeolite. The obtained results showed that there were significant variations in the

concentrations of all metals where p5 0.0001. It can be concluded that there was no

significant variation between the two compared leaching standard methods for estimatingthe mobility of heavy metals under their specific condition, while the MEP was

significantly higher than both EN 12 457-2 and TCLP.

84%

86%

88%

90%

92%

94%

96%

98%

100%

2.55

12

3

The

cum

ulat

ive

wei

ght p

erce

ntag

e

% of Zeolite

Lead

Nickel

Cadmium

Copper

Zinc

Figure 5. The cumulative weight percentage of metals in sludge stabilized using zeolite after MEPleaching test.

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Furthermore, applying the MEP test, statistical results showed that the metal leachateconcentrations were significantly affected by two factors, the percentage of zeolite usedand the time of the test. Where, the leaching metal concentrations were inversely correlatedwith both percentage of zeolite and time of MEP test.

Compressive strength

The minimal compressive strength required for solid waste disposal at landfills in ResourceConservation & Recovery Act’s (RCRA) is 0.35MPa (ASTM 1991; U.S.EPA 1989).

In order to solidify the stabilized sludge with the optimum dose of zeolite A, 20% ofsolidified sludge was mixed with a cement mixture prepared by fixed weight basis ofcement and sand with percentages of 20 and 60, respectively, and adequate mass ratioof water/cementatious material (cement and zeolite) 0.55 was used to maintain the properworkability of cement mixture as recommended by American Society of Testing andMaterials (ASTM) C-230-90 (ASTM 1991). The mixture was placed in a mould of5 cm� 5 cm� 5 cm size as recommended by ASTM designation C190-90 (ASTM 1991),and allowed to cure for 28 days to simulate the worst conditions of actual field practice.Our results revealed that the UCS was 0.83MPa, greater than the minimal compressivestrength required for solid waste disposal at landfills (0.35MPa). Our results confirm thoseof Bagosi and Csetenyi (1999); they found that zeolites increase the binding properties ofcement-based materials for contaminants and produce a more durable cement system.

Conclusions

On assessment of the utilization of zeolite prepared from local materials for immobili-zation of hazardous metal contaminated sludge, we can conclude that:

. It was found that only 10% of zeolite is required to produce a waste form withhigh physical stability. The leachability of cadmium, copper, lead, nickel, and zincin the stabilized sludge under the conditions of the EN 12 457-2 leaching test wasconsiderably lower than the limit values set by the European Council decision forthe acceptance of a waste in a landfill for non-hazardous waste. Also TCLPleaching test confirmed that the investigated metals are well stabilized and fixedby zeolite. The average metal concentrations in the leachate did not exceed thelimit specified by the U.S.EPA, which is as same as the limit set by the EgyptianRegulations law 4/1994 for hazardous waste management. MEP confirms highretention percentages of metals in the stabilized sludge.

. The unconfined compressive strength, of the solidified/stabilized (S/S) sludge, byusing an optimum ratio of zeolite, was 0.83MPa, which indicates that theremediate sludge was well solidified and safe to be used in a wide variety ofapplications, such as a raw material used in concrete blocks, to be sold aspavement blocks, or used in roadbeds and/or parking lots.

. Finally, zeolite prepared from Egyptian kaolin can be used as a low-cost andeffective material for metal immobilization.

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