Journal of Colloid and Interface Science 250, 1–4 (2002)doi:10.1006/jcis.2002.8246, available online at http://www.idealibrary.com on

Particle Size Effects on Uptake of Heavy Metals from Sewage SludgeCompost Using Natural Zeolite Clinoptilolite

Antonis A. Zorpas,1 Inglezakis Vassilis, Maria Loizidou, and Helen Grigoropoulou

Chemical Engineering Department, National Technical University of Athens, 9 Heeron Politechniou Street, Zografou Campus, 15780, Athens, Greece

E-mail: antoniszorpas@yahoo.com

Received March 1, 2001; accepted January 24, 2002; published online April 10, 2002

Land application of sewage sludge may be the least energy con-suming and the most cost-effective means of sludge disposal orutilization. However, the major technical problem with land appli-cation of sludge concerns the high concentrations of heavy metals.These metals may be leached and enter the ecosystem, the foodchain, and eventually the human population. This paper deals withthe removal of heavy metals from sewage sludge compost using nat-ural zeolite clinoptilolite, in respect to the particle size. The final re-sults indicate that heavy metals can be sufficiently removed by using25% w/w of zeolite with particle size of 3.3–4.0 mm. Pore cloggingand structural damage in smaller particle sizes is probably the rea-son for lower uptake of metals by the latter. C© 2002 Elsevier Science (USA)

Key Words: clinopltilolite; metal uptake; sludge composting; poreclogging.

INTRODUCTION

Land disposal of sewage sludge is practiced all over the worldbecause of the economic advantages it offers in comparison tosludge incineration and landfill disposal. However, human healthrisks, such as parasitic infections and heavy metals transfer inthe food chain, represent the main limitations of these practices.Environmental problems associated with sewage sludge disposalhave prompted legislative actions over the past years. At thesame time, the upgrading and expansion of wastewater treatmentplants have greatly increased the volume of sludge generated.

Composting is being increasingly considered by many munic-ipalities throughout the world because it has several advantagesover other disposal strategies. First, composting can reduce thewaste volume by 40–50% and thus require less landfill spacefor disposal (1). Second, pathogens can be killed due to the heatgenerated during the thermophilic phase (1, 2). Compost can beused as a soil conditioner as it contains major plant nutrients,such as N, P, and K, microplant nutrients such as Cu, Fe, andZn, and organic matter, which improves the physical properties

1 To whom all correspondence should be addressed: EnviTech Ltd., Lab-oratory of Environmental Trechnology, Reaserch Institute Griva Digeni 37,Paralimni, 5280, Cyprus.

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in order to have a better soil aeration and water holding capacity(1, 3).

The heavy metal content of wastewater treatment sludge lim-its the possibilities for using and even eliminating this waste,the volume of which is growing daily. Application to soils inorder to increase their organic matter is restricted since it maybe indirectly incorporated into the food chain or be washed intoaquifers. A natural zeolite, clinoptilolite, has the ability to uptakeheavy metals. Zeolites have been known worldwide in the pastdecade, for either their cations exchange or molecular sievingproperties. Natural zeolites nowadays are used in soil benefac-tion, and in water and wastewater treatment (4).

According to the theory, high rates of the ion exchange pro-cess can be achieved by using small particle size (5). However,this is not always true. It is known that surface dust clogs part ofthe pore openings in zeolite structures, leading to slower ion ex-change kinetics for smaller particles than for larger ones, in theion exchange of Pb2+/Na+ in an aqueous system (6). Further-more, structure damage in smaller particles, due to the grindingprocess, has been also reported, and may result in the same effectfor smaller particles (7).

The aim of this work is to examine particle size effects onmetal uptake by clinoptilolite in a solid state ion exchange. Also,the results of the application of clinoptilolite in the sludge com-posting process as a bulking agent, in order to remove heavymetals, is presented.

MATERIALS AND METHODS

Materials

The mineral used was collected from the Metaxades depositin the northern part of Greece. It was ground and sieved. Chem-ical analysis was performed using the SEM/EDS method. Thedewatered and anaerobically stabilized primary sewage sludge(DASPSS) samples were collected from the Psittalia wastewatertreatment plant in Athens (Greece). The samples were dried,homogenized, and stored before completing the analysis.

For the metal concentration in DASPSS and clinoptilolite,a known quantity (1 g) of sample was digested with 10 ml of

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E

Pb 340 ± 15

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TABLE 1Amounts of Clinoptilolite and DASPSS Used in the Preparation

of the Initial Substrates Placed in the Composting

Sample %, w/w clinoptilolite % w/w DASPSS

1 0 1002 10 903 15 854 20 805 25 75

conc. HNO3 and 2 ml H2SO4 as described by Zorpas et al. (9).After the completion of the digestion, the samples were vacuumfiltered, and the filtrate was used for the determination of heavymetal concentration by atomic absorption spectroscopy, using aPerkin–Elmer 2380 spectrophotometer.

In Table 1 the amounts of clinoptilolite and DASPSS used(% w/w on the dry basis of the raw materials) in the preparationof the compost samples are presented. The particle size used wasin the range of 0.16–2.5 mm.

Particle size effects were investigated for sample 5. Table 2presents the particle size of clinoptilolite used in the preparationof the compost samples.

Clinoptilolite Samples and Dust Measurements

In order to measure the clinoptilolite surface dust a givenamount of the material was washed with deionized water sev-eral times until all the surface dust was removed. The materialwas dried at 80◦C for 24 h and weighed. A reference amountwas dried under the same conditions in order to measure thewater loss of the original material. The difference between thestarting and washed material minus the water loss is the amountof surface dust. At least five measurements were performed forevery fraction.

Composting Process

The composting process was carried out in the laboratory us-ing an in-vessel reactor of 1 m3 active volume. There was anaeration system that consisted of an air fan for air supply, an airflow meter, and an air humidity meter. A temperature indica-tor controller was controlling the operation of the fan in order to

TABLE 2Particle Size of Clinoptilolite Used for the Preparation of the

Initial Substrates Placed in the Composting (Sample 5)

% w/w clinoptilolite % w/w DASPSS Particle size, mm

25 75 <0.16025 75 0.161–1.025 75 1.1–2.525 75 2.6–3.2

25 75 3.3–4.0

T AL.

TABLE 3Chemical Composition of Natural Clinoptilolite

Oxide % w/w

SiO2 66.5 ± 0.8Al2O3 12.8 ± 1.3Na2O 1.2 ± 0.4K2O 1.4 ± 0.3CaO 2.7 ± 0.6MgO 1.5 ± 0.2FeO 1.2 ± 0.4H2Oa 12.1 ± 0.2

a Water content was measured by complete dehydration of the originalmaterial.

maintain the temperature about 60◦C, according to the followingprinciple: minimum air flow (2.3 m3 per m3 active volume) wasprovided at low temperature (<30◦C) and maximum air flow(28 m3 per m3 active volume was provided at high temperature(>60◦C). The minimum air flow corresponds to the minimumoxygen demand for the microorganisms and the maximum tothe necessary air for cooling. The moisture was in the range of40–50%. The thermophilic phase lasted for 15 days in the reac-tor. After the thermophilic period in which the organic materialwas biodegraded, the compost was piled in an enclosed packagewhere it remained for about four months to mature.

RESULTS AND DISCUSSION

Materials Characterization

The characterization of the natural clinoptilolite sample isshown in Table 3. The total exchange capacity of this sample,according to its chemical composition, is estimated to 2.4 ±0.3 meq/g. This value represents the amount of exchangeablecations, which are considered to be Na+, K+, Ca2+, and Mg2+.Table 4 presents the total metal content in DASPSS on a drybasis.

Metal Uptake by Clinoptilolite and Particle Size Effects

In Fig. 1 the percentage of metal uptake, in respect to totalmetal content, by clinoptilolite–DASPSS mixtures is presented.

TABLE 4Metals Content in DASPSS

Metal Total (mg/kg) - in dry basis

Co 560 ± 30Cr 580 ± 35Cu 210 ± 15Fe 4190 ± 210Ni 50 ± 7

Zn 1800 ± 95

PARTICLE SIZE EFFECTS ON

FIG. 1. Percentage of metal uptake by clinoptilolite–DASPSS mixtures.

It is observed that the zeolite can take up to a significantamount of heavy metals. It is obvious that 25% w/w of zeolitetakes up to 12% of Co, 27% of Cu, 14% of Cr, 30% of Fe, 40%of Zn, 55% of Pb, and 60% of Ni. In Fig. 2, the percentageof metal uptake, in respect to total metal content, by differentparticle sizes of clinoptilolite is presented for sample 5.

As the particle size increases the amount of metals, whichis taken up from the clinoptilolite, increases. According to thetheory, high rates of the ion exchange process can be achievedby using a small particle size (5). However, experimental resultsshown that as the particle size increases, the metals solid con-centration increases. In order to explain this observation, dustmeasurements are determined, and in Fig. 3 the percentage ofdust (w/w) versus average diameter (mm) of the clinoptiloliteparticles is presented.

A percentage of surface dust is shown to increase with de-creasing particle size. This was expected, since crushing condi-tions are harder in order to produce smaller fractions, resultingin a higher production of dust. Pore clogging by fine particleshas been reported elsewhere as the possible cause for the dis-agreement between the theoretically expected and experimen-tally discovered ion exchange hierarchy of the various zeolitespecies (10). Furthermore, it is known that surface dust clogspart of the pore openings in a zeolite structure, leading to slower

FIG. 2. Metal uptake in %, by zeolite in respect to the particle size.

UPTAKE OF HEAVY METALS 3

0

10

20

30

40

0 1 2 3 4

d (mm)

du

st (

% w

/w)

FIG. 3. Percentage of surface dust in respect to particle size of theclinoptilolite.

ion exchange kinetics in the ion exchange of Pb2+/Na+ in anaqueous system (6). The results in Fig. 2 are in accordance withthis observation.

Structure damage in smaller particles, due to the grindingprocess, has also been reported (7, 10). In order to verify thisfact washed clinoptilolite samples were analyzed with respectto BET analysis and data on the total specific surface S (m2/g)are presented in Table 5. Washed samples were used in orderto remove surface dust and thus to eliminate the dust effect onsurface measurement (6).

Since clinoptilolite samples were washed prior to measure-ments, differences in specific surfaces are attributed to the struc-tural status. It is obvious that smaller particles (<0.16 mm) havea specific surface 18% smaller than that of larger particles (1–2 mm).

The difference in the metal uptake for different particle sizesdepicted in the present work (Fig. 2) could also be due to eitherpore clogging or structure damage in smaller particles based onthe fact that all tested particles are produced after grinding thesame mineral sample.

It is obvious especially that Ni2+ uptake is greatly affected byparticle size. The relatively poor removal of Ni2+ by a zeolite,in aqueous solutions, has been attributed to the high stability ofits aqueous complex (11). The removal of Ni2+ was measured tobe 16% and under the same conditions 70% for Zn2+ and 72%for Cu2+. For all metals the formation of stable large complexesmay explain the differences between small and large particles,since these complexes may be too large to move easily intothe zeolite pores. However, it is known that the solvation heats

TABLE 5Total Specific Surface S (m2/g) of Clinoptilolite

S (m2/g) Particle Size Range (mm)

14.23 <0.1613.43 0.16–0.616.28 0.6–1

17.28 1–2

4 ZORPAS

(around −500 kcal/mol) and the corresponding hydrated radius(around 8.3 A), which are measures of complex strength, of theabove metals are too close to explain such differences on theremoval of Ni2+ in comparison to the other metals (12). It islikely that Ni2+ forms several complexes of unknown structure,which are however very strong and large enough to exclude thismetal from small openings in zeolite structures, especially incompost–zeolite mixtures where several inorganic and organicanions are present.

SUMMARY

According to the experimental results, it is shown that the25% w/w addition of zeolite in compost is sufficiently remov-ing heavy metals, 12% of Co, 27% of Cu, 14% of Cr, 30% of Fe,40% of Zn, 55% of Pb, and 60% of Ni. The particle size of theclinoptilolite seems to affect the uptake of heavy metals. As theparticle size increases, the metals concentration taken up fromthe clinoptilolite increases. This observation could be explainedby the effect of surface dust, which clogs pores and causes struc-

tural damage in smaller particles, due to the grinding process.The Bulking agent’s (clinoptilolite) feasibility cost depends on

ET AL.

the purification, its provenance, and in the particle size. How-ever, in the market the bulking agent’s price is not stable. InGreece the cost is estimated at $ 130 per ton while in the world’smarket the cost is estimated from $ 100 to $ 1000 per tone.

REFERENCES

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(1997).5. Helfferich, F., “Ion Exchange.” Dover, New York, 1995.6. Ingezakis, V., Diamandis, N., Loizidou, M., and Grigoropoulou, H., J. Col-

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