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Available online at www.sciencedirect.com

Journal of Hazardous Materials 153 (2008) 1103–1109

Influence of pH, curing time and environmental stress on theimmobilization of hazardous waste using activated fly ash

Shefali Srivastava, Rubina Chaudhary ∗, Divya KhaleSchool of Energy and Environmental Studies, Devi Ahilya University, Takshila Campus, Khandwa Road, Indore 17, M.P., India

Received 18 November 2006; received in revised form 18 September 2007; accepted 18 September 2007Available online 25 September 2007

bstract

The current work is related to inorganic species in sludge generated from Common Effluent Treatment Plant contaminated with hazardous wastest relatively high concentration. The environmental sensitive metals studied in the sludge are Pb, Fe, Ni, Zn and Mn. The solidification/stabilizationS/S) of heavy metals within fly ash-cement-based matrix was conducted for low cost treatment and reuse of sludge. The study examines thetrength of the S/S product by predicting the effect of supplementary cementing material from efficiency factor (k) at 60 ◦C curing temperature. Theeaching test was performed at two different pH 7 and 4 to determine the efficiency of heavy metal immobilization. It was observed that replacing

6% OPC by 56% fly ash and 20% sludge for 28 days curing period shows increase in strength as well as rate of stabilization for zinc, iron andanganese at pH 7, lead and nickel were stabilized by 79 and 82%, respectively. Environmental stress test was performed to evaluate the tolerance

f extreme adverse environmental condition.2007 Elsevier B.V. All rights reserved.

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eywords: Activated fly ash; Compressive strength; Leaching; Environmental

. Introduction

The present scenario is the disposal of the waste generatedrom the industry like sludge and fly ash. Being bulky in nature,oth the sludge and fly ash face disposal problem. Presently,andfill is not the only desirable option as it causes huge financialurden to the industries. At the same time increasing load of toxicetals causes the potential threat to contaminate soil as well as

round water [1,2].The solidification/stabilization (S/S) of hazardous wastes by

ozzolan-based binders is a most familiar technology that haseen applied to many types of industrial wastes, mainly thoseontaining heavy metals [3,4]. S/S technology does not removeeavy metals from the polluted waste but has the purpose tohysically as well as chemically fix them in the solid matrixn order to reduce their mobility, to minimize the threat to the

nvironment, and to ensure compliance with existing regulatorytandards [5,6]. Earlier studies were conducted for the immo-ilization of heavy metals by ordinary Portland cement but for

∗ Corresponding author. Tel.: +91 731 2460309; fax: +91 731 2467378.E-mail address: rubina chaudhary@yahoo.com (R. Chaudhary).

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304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2007.09.065

he generation of 1 t of Portland cement about 1 t of green houseas CO2 released to the atmosphere as a result of de-carbonationf lime in the kiln during manufacturing of cement and causeajor problem of global warming [2,7]. To reduce this CO2

mission a major portion of Portland cement is replaced by flysh which itself has pozzolanic properties. This pozzolanic char-cter of fly ash immobilizes the heavy metal by the formationf well bonded and low porosity network of calcium silicatend aluminate hydrates that acts as a binding agent [8–10]. Theechanical and durable property of the final S/S of the pozzolan

ncreases by alkali activation and curing at elevated temperature10–16]. The objective of the present work is to utilize fly ash forhe immobilization of heavy metal bearing CETP sludges and totudy its treatability by the solidification/stabilization process atifferent pH, and effect of environmental stress conditions.

. Materials

The sludge samples were collected by grab-composite

ethod from the sludge drying bed of the Common Effluentreatment Plant of electroplating units, drying units, and pick-

ing units. The physico-chemical and heavy metals (Mn, Zn,b, Cu, Fe and Ni) analysis of the sludge are given in the

1104 S. Srivastava et al. / Journal of Hazardous Materials 153 (2008) 1103–1109

Table 1Physico-chemical analysis of raw sludge

pH EC (�S) Bulk density Moisture content (%) Alkalinity (mg/l) Chloride (mg/l) Sulphate (mg/l)

6.5 3.8 0.55 12.70 119.0 209.9 2080

Table 2Heavy metal analysis of raw sludge and available content at pH 7 and 4

Serial no. Heavy metals Conc. (mg/kg) (total) Available conc. (mg/kg) (pH 7) Available conc. (mg/kg) (pH 4)

1 Mn 3019.8 2743.1 2986.22 Pb 381.4 362.3 375.33 Zn 395.3 349.4 371.545

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Tables 1 and 2). Low calcium fly ash and commercially avail-ble 53 grade Portland cement was used as a binder for fixationf the heavy metals. For alkali activation of fly ash, 5 M sodiumydroxide and 5% of total weight sodium silicate was used. Theercentage of waste and binders is given in Table 3.

.1. Procedure

.1.1. Unconfined compressive strengthIn order to simplify the sample screening process, the com-

ressive strength was selected as the benchmark parameter. Thiss not unusual because compressive strength has an intrinsicmportance in the structural design of concrete structures [17].

For UCS testing, cubes of 5 cm × 5 cm × 5 cm were preparedsing different waste-binder ratios. The mixture was inserted in aubic mold, compacted and vibrated to remove all the entrappedir. The cubes were unmolded after 24 h. The test was performedn a Universal Testing Machine (Model—TUN 400).

.1.2. Efficiency factorTo predict the effect of fly ash on the compressive strength of

ement as a binder, the concept of the efficiency factor (k-value)as described by Papadakis and Tsimas [18].The efficiency factor is defined as the part of fly ash that

an be considered as equivalent to Portland cement having same

roperties without fly ash. The k-value is equal to 1 for Portlandement. In this work, the efficiency factor was determined inrder to draw conclusions regarding the effectiveness of fly ashnd other activating solutions.

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able 3ercentage composition of the samples used during investigation

ample no. Sludge (%)

01014.220253350

882.0 926.25228.9 5513.0

For estimating the k-values, the expression for the compres-ive strength (fc) measured for the constructed systems:

c = K

{1

W/(C + kP)

}(1)

here K is a parameter depending on the cement type (here,0 MPa), C and P are the cement and fly ash contents, respec-ively, in the mix (kg/m3), and W is the water content (kg/m3).

.1.3. Total available leaching testThe leaching test involves testing the material in the worst

onditions to stimulate the effects of handling and other envi-onmental conditions. The rapid attainment of equilibrium wasacilitated by agitation, which prevented stratification. Theutch total available leaching test NEN 7431[19] was used touantify the elemental mass fraction available for leaching. Thextraction solubilized all readily soluble and marginally sol-ble minerals. Three grams of material (size < 125 �m) wereeached in distilled water in two serial steps: first at pH 7 withiquid/solid ratio (L/S) of 100 for 3 h and then at pH 4 for8 h. The pH was controlled by using 1 M HNO3. These twoeachate fractions were kept separately for chemical analysis and

eavy metals analysis. The instruments used for above analysisere Nephalometer, pH meter, conductivity meter (systronics),

nd atomic absorption spectrophotometer (Shimadzu AA-6300)20].

Cement (%) Fly ash (%)

30 7027 6325.7 6024 5622.5 52.520 46.615 35

ardous Materials 153 (2008) 1103–1109 1105

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Table 4Comparison of compressive strength and efficiency factor for 7 and 28 dayscuring period

Sample no. Sludge (%) UCS k-value

7 Days 28 Days 7 Days 28 Days

1 0 20 21.4 0.9 0.872 10 18.5 18.6 0.8 0.843 14.2 16.5 17.4 0.7 0.734 20 17 17.8 0.68 0.75 25 13.5 15.4 0.41 0.567

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S. Srivastava et al. / Journal of Haz

.1.4. Environmental stress testDurability relates to the long-term stability of the S/S product.

urability testing evaluates resistance of a solidified/stabilizedroduct to degradation, due to external environmental stresses.he tests are designed to mimic natural condition by stressing

he specimen through (a) freezing and thawing and (b) heat-ng and thawing. The solidified/stabilized specimen undergoesepeated cycling during the testing. Each of the test specimensas subjected for freezing at −8 ± 2 ◦C and heating at 60 ± 2 ◦C

or 12 cycles [21,22].In this study, environmental stress test was performed on

he screened ratios. Criteria of 30% weight loss were used asrejection ground for the durability test based on the study oftegemann and Cote [23]. The effect of varying temperaturesn the matrix was monitored. The importance of waste resis-ance to freeze thaw and heat thaw is strongly dependent on theeographical location of the disposal site.

. Results and discussions

The samples were studied for UCS, leachability andnvironmental stress test. The results are discussed on themmobilization of heavy metals at pH 4 and 7, for differentaste-binder ratios cured for 7 and 28 days.

.1. Unconfined compressive strength

Strength test data often used to provide a baseline comparisonetween unstabilized and stabilized wastes. Prepared samples

f different waste-binder ratio were cured for 7 and 28 days at0 ◦C and were subjected to unconfined compressive strengthest. Unstabilized waste materials generally do not exhibit goodompressive strength; however, if the waste is stabilized into

[Lt[

Fig. 1. Effect of pH 7 on metal fi

33 11 13.4 0.18 0.350 10.5 12.2 0.02 0.09

ement-like form the strength characteristics can increase sig-ificantly [5].

Table 4 shows the sludge percentage and strength of theample. It was observed that, for 7 and 28 days of curingeriod, strength for reference sample increased whereas strengthecreased with increase in sludge from 0 to 50% for 7 days cur-ng. For 28 days curing, strength of the sample increased withncrease in sludge from 10 to 20% and then decreases on furtherddition of sludge. For 25–50% sludge, the strength was almostame for both 7 and 28 days curing period (Table 4). Initial curingemperature of 60 ◦C, accelerates pozzolanic reaction and henceecreases setting days. Strength for all W/B ratios was lowers compared to reference sample. Low strength may be due toaste addition that affects the C–S–H hydration. Metal ions formrecipitates on cement clinker grain are believed to be respon-ible for the retardation of cement hydration reaction (CSH)

24]. Another reason for the low strength may be increased/S ratio. With the increase in W/B ratio the water demand of

he sample increases resulting in lower compressive strength25,26].

xation at 7 days of curing.

1106 S. Srivastava et al. / Journal of Hazardous Materials 153 (2008) 1103–1109

etal fi

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Fig. 2. Effect of pH 4 on m

In the frame of this study, where a small replacement ofy ash by alkali took place, P was considered as the sumf fly ash and alkali in each blend. Applying in the Eq. (1),he measured values of the compressive strength, the k-values,or the activated systems were calculated and presented inable 4.

For the pure binder specimen, the efficiency factor increasesith increase of curing days as the hydration procedure evolved.

able 4 shows that on increasing sludge from 10 to 25% the-value was nearer to unity which indicates that compressivetrength is comparable to cement and can be further studied fornvironmental stress test.

masp

Fig. 3. Effect of pH 7 on metal fi

xation at 7 days of curing.

.2. Heavy metal leaching

Leaching test is probably the single most important measuref the effectiveness of heavy metals immobilization within flysh-based matrix. Table 2 presents maximum leachable portionf the heavy metals from raw sludge, available under aggressiveeaching condition. The S/S products showed a high retentionf heavy metal. The predominant metals in the sludge are iron,

anganese and nickel accomplished by lesser amount of lead

nd zinc (Table 2). In terms of total amount of metals thesepecies were more abundant in sludge. Figs. 1 and 2 presentercentage of heavy metals fixed in the alkali-activated matrix

xation at 28 days of curing.

S. Srivastava et al. / Journal of Hazardous Materials 153 (2008) 1103–1109 1107

tal fix

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Fig. 4. Effect of pH 4 on me

t 7 days and Figs. 3 and 4 at 28 days of curing under aggressiveeaching conditions.

Zinc possesses amphoteric properties and its stability is low-st at pH range 9–11. In neutral environmental condition (pH) zinc was completely stabilized for all the waste-binder ratiosfter 7 days curing period. Bhatty [27] also showed excellentxation of zinc by the tricalcium silicate component of OPC.n drastic environmental conditions (pH 4) the zinc was com-letely immobilized with increase of W/B ratios after 7 or 28ays curing periods (Figs. 1 and 2). Complete stabilization was

bserved for manganese after 28 days curing period at pH 7. Mnrecipitation as Mn(OH)2 is expected at pH 8–13 [3]. At pH 4he leaching of Mn increased for all the W/B ratios in optimizeduring period (Figs. 2 and 4).

wi(

Fig. 5. Weight loss duri

ation at 28 days of curing.

The investigation showed that some of the studied compo-ents could be considerably demobilized by alkali activated flysh. Concentration of some metals like Mn and Zn were below orot quit above the determination and were considered as com-letely locked. At pH 7, for 7 and 28 days curing period theeaching of lead increases steadily with increase in sludge con-ent. Fixation of lead was 79–89% for 10–50% sludge contentor 28 days of curing.

At pH 4, specimens cured for 28 days showed comparativelyess leaching than 7 days.

Even when exposed to most drastic condition the leadas found stable from 91.2 to 79.4%, i.e., insignificant with

ncreasing sludge from 14.2 to 25% in 28 days curing periodFigs. 3 and 4) and for 7 days curing period leaching was very

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108 S. Srivastava et al. / Journal of Haz

igh. It was up to 94% for the same sludge percentage. Leadeaching is significantly elevated as pH falls below 4.8. In pHange 8–9 lead leaching is lowest, addition of fly ash results inhe wider pH range (7–11) of Pb immobilization [25]. Leachingate increases as the pH decreases or increases beyond this pHange [28].

At 7 days curing period with increase of sludge from 14.2o 25% nickel stabilization decreases by 77–57%. Result showshat after 28 days curing period Ni stabilization was improvedy 65–80% (Figs. 3 and 4). At pH 4, Ni shows almost samexation in 7 and 28 days curing period for all the samples. Onddition of sludge content the leaching of Ni increases.

AT pH 7, maximum fixation of iron was observed at curingeriod of 28 days. With the increase of sludge content the leach-ng of iron increases. With increasing sludge content from 14o 25%, iron stabilization decreases by 8581 and 98–88% for

and 28 days of curing period, respectively (Figs. 1–4). Withhe increase of sludge content from 10 to 50%, the rate of fix-tion decreases from 85.4 to 16.7%, respectively, for 28 daysuring.

.3. Environmental stress test

The ratios for environmental stress test were screened on theasis of higher strength achieved in curing days. As per EPAuidelines [29], for the S/S product 0.35 MPa is the minimumtrength required for the landfill. The strength obtained was fiveimes higher for the screened ratios in both the curing days.

Durability relate to the long-term stability of the S/S product.he solidified samples when subjected to freezing thawing cyclend heating thawing cycle process withstand all 12 cycles. Theverage cumulated, corrected mass loss for 14–50% sludge was6, 24, 27, 35 and 98% for freezing thawing cycle and 1, 2, 4, 6nd 8% for heating thawing cycle (Fig. 5).

. Conclusions

Results indicate that, with the increase of sludge from 14.2o 25% the UCS was almost same for both the curing days. Theeplacement of 76% OPC by 56% fly ash and 20% sludge for8 days curing period shows increase in strength as well as ratef stabilization within range of 95–99% for zinc, iron and man-anese at pH 7. Leaching of heavy metals in the S/S matrix can beonsidered as pH-dependent and corresponding metal hydrox-de solubility controlled process. The pH-dependent leach testllows a good characterization of the environmental propertiesf stabilized product. The order of fixation of toxic metals inhe alkali activated fly ash matrix was Zn > Mn > Fe > Ni > Pb.n environmental stress conditions, freeze thaw was found to beore deteriorating than heating thaw. It also withstands condi-

ions with higher threshold limit.Hence, alkali activation of fly ash for solidifica-

ion/stabilization can be an alternative technology for the

ltimate disposal of heavy metal sludge wastes. In conditionf sanitary landfill environment, numerous interacting factorsan simultaneously affect desorption of heavy metal in theinder.

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s Materials 153 (2008) 1103–1109

cknowledgements

Authors are thankful to School of Energy and Environ-ent Studies. Authors also extend sincere thanks to Institute ofngineering Technology, Indore for providing strength testing

acility during course of this study.

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