aT, PRl in

Keywords:Silica fumeMunicipal solid wasteFly ashStabilizationPozzolanic reaction

wassolast

for Cu, Pb and Zn of the hydrated paste cured for 7 days decreased from 0.32 mg/L to 0.05 mg/L,

on (MSstem,

mechanisms of heavy metals could be (1) sorption, (2) chemicalincorporation (surface complexation, precipitation, co-precipita-tion, diodochy), and (3) micro- or macro-encapsulation (Trusselland Spence, 1994; Glasser, 1997). During the pozzolanic reaction,metals are expected to precipitate as insoluble hydroxides or tocombine with products of y ash hydration to form complex sili-

e microstand lower

ity, compact CSH (Sakai et al., 2005; Giergiczny and Kro9lLin et al., 2008). Additionally, silica fume also acts as llersits near-perfect spheres with diameters ranging from 20 to 5(Mitchell et al., 1998; Collins and Sanjayan, 1999). It is expectedthat silica fume exhibits good xation efciency of heavy metals(Kassim and Chern, 2004; Huang and Huang, 2008; Rodella et al.,2014). To our knowledge, little work has been reported on themechanism of heavy metal incorporation and phase transforma-tion in the hydrated MSWI y ash paste using silica fume as anadditive.

Corresponding author at: School of Environment Science and Engineering,Donghua University, Shanghai 201620, PR China.

Waste Management 34 (2014) 24942504

Contents lists availab

Waste Man

elsMSWI y ash is a promising alternative (Yousuf et al., 1995;Camacho and Munson-McGee, 2006). The possible immobilization

phase. The addition of silica fume can modify thof hydrated pastes due to the formation of denserhttp://dx.doi.org/10.1016/j.wasman.2014.08.0270956-053X/ 2014 Elsevier Ltd. All rights reserved.H (gel)ructure-poros-, 2008;due to00 nmthat concentrations of heavy metals such as Cd, Cr, Cu, Pb and Znexceed the regulatory limits (Yvon et al., 2006; Shi and Kan,2009; Yang et al., 2012). It is, therefore, classied as hazardouswaste in China and requires additional detoxication processingprior to nal disposal or store. At present, over 50 thousand tonsMSWI y ash is produced annually in Shanghai. It may pollutethe groundwater due to long term leaching in the landll site.

The pozzolanic material based-stabilization of heavy metals in

large specic surface area and high amorphous silica content(Bensted and Barnes, 2008; Khan and Siddique, 2011). It has beenused in combination with y ash of coal combustion as a partialsubstitute of Portland cement, showing several advantages interms of mechanical performance and durability (Thomas andBamforth, 1999; Barbhuiya et al., 2009; Nochaiya et al., 2010;Lilkov et al., 2014). Silica fume of sub-micro size is an active com-ponent which will react with Ca(OH)2 to produce the CS1. Introduction

Municipal solid wastes incineratigely to the CaOCaCl2CaSO4SiO2 sy40.99 mg/L to 4.40 mg/L, and 6.96 mg/L to 0.21 mg/L compared with the MSWI y ash. After curing for135 days, Cd and Pb in the leachates were not detected, while Cu and Zn concentrations decreased to0.02 mg/L and 0.03 mg/L. The speciation of Pb and Cd by the modied version of the European Commu-nity Bureau of Reference (BCR) extractions showed that these metals converted into more stable state inhydrated pastes of MSWI y ash in the presence of silica fume. Although exchangeable and weak-acidsoluble fractions of Cu and Zn increased with hydration time, silica fume addition of 10% can satisfythe requirement of detoxication for heavy metals investigated in terms of the identication standardof hazardous waste of China.

2014 Elsevier Ltd. All rights reserved.

WI) y ash belongs lar-but leaching tests show

cate forms (Dusing et al., 1992). Heavy metal compounds couldbe occluded (physically encapsulated) by CSH (Chen et al.,2009).

Silica fume is a highly effective pozzolanic material due to itsTG), and MAS NMR ( Al and Si) techniques. It was found that silica fume additions could effectivelyreduce the leaching of toxic heavy metals. At the addition of 20% silica fume, leaching concentrationsStabilization of heavy metals in MSWI y

Xinying Li a,b, Quanyuan Chen a,b,, Yasu Zhou a, Marka School of Environment Science and Engineering, Donghua University, Shanghai 201620b State Environmental Protection Engineering Center for Pollution Treatment and ControcMineral Industry Research Organisation, Solihull B37 7HB, UK

a r t i c l e i n f o

Article history:Received 31 March 2014Accepted 26 August 2014Available online 30 September 2014

a b s t r a c t

The objective of this workheavy metals in municipalmeasurements, hydrated p

27

journal homepage: www.sh using silica fume

yrer c, Yang Yu a

ChinaTextile Industry, Donghua University, Shanghai 201620, PR China

to investigate the feasibility and effectiveness of silica fume on stabilizingid waste incineration (MSWI) y ash. In addition to compressive strengthes were characterized by X-ray diffraction (XRD), thermal-analyses (DTA/29

le at ScienceDirect

agement

evier .com/ locate/wasman

In this work, eight hydrated pastes with different additions ofsilica fume were examined. The objective of this work was toinvestigate the feasibility and effectiveness of silica fume on stabi-lizing heavy metals in MSWI y ash. The reduction of the heavymetal leachability by adding silica fume was evaluated by threestandard leaching procedures. The sequential extraction of heavymetals was conducted to examine the changes of metal species dis-tribution. Additionally, the solid phases were characterized bymeans of X-ray diffraction (XRD), thermo-analysis (DTA/TG), 27AlNMR and 29Si MAS NMR techniques to examine the phasetransformation.

2. Experimental

2.1. Materials

SiO2 16.4 94.2

X. Li et al. /Waste ManagemeAl2O3 4.7 0.6Fe2O3 5.9 0CaO 31.5 0MgO 6.9 0Cl 14.5 0SO42 6.3 0Sum 86.2 94.8Water content (105 C) 0.1Loss on ignition (550 C) 5.0Loss on ignition (1000 C) 27.5

Trace element contents (mg/kg)Pb 1232Cd 66added during the cleaning process of the gaseous emissions inmunicipal solid waste incineration. Silica fume (920U) used in thisstudy was purchased from the Shanghai Topken Silica Fume Com-pany. The grain size of silica fume was 95.3% passing #325 meshsieve with a specic area of 19 m2/g and a bulk density of300 kg/m3. After alkaline fusion at 750 C for 20 min in mufe fur-nace and subsequent acid dissolution, chemical analyses were car-ried out and the main chemical compositions of the MSWI y ashand silica fume are listed in Table 1.

The major chemical components of MSWI y ash were CaO, SiO2and Al2O3. Chloride and sulphate salts were important parts of they ash. Cl and SO42 constitute 14.5% and 6.3% by mass respec-tively. The loss on ignition (LOI) measured at 550 C for MSWI yash was 5.0 wt.%, indicating the presence of organics and residuallime (Ca(OH)2). The high LOI value (27.5 wt.%) of MSWI y ashmeasured at 1000 C indicates the presence of high amounts ofinorganic salts such as calcite, chlorides and sulphates.

All the chemical reagents used in this work were of analyticalgrade.

2.2. Paste preparation

Eight mixture pastes (P1-P8) were prepared with MSWI y ashto silica fume ratio of 1.00:0, 0.98:0.02, 0.96:0.04, 0.94:0.06,

Table 1Chemical characteristics of MSWI y ash and silica fume.

Major chemical composition (%) MSWI y ash Silica fumeMSWI y ash sample was collected from the bag-house hopperof Yuqiao Municipal Solid Wastes Incineration Power Plant locatedat Pudong New Area, Shanghai, PR China. Strictly speaking, itshould be called as air pollution control residue (APC residue) asresidual lime and activated carbon were present, which wereCu 553Cr 70Zn 50600.92:0.08, 0.90:0.10, 0.86:0.14 and 0.80:0.20 by weight. The MSWIy ash and silica fume mixed with distilled water in a planetarytype mortar mixer at a water-to-solid ratio of 0.4:1 by weight for5 min. All mixtures were poured into rectangular molds(100 100 100 mm), and allowed to hydrate for 24 h at ambientconditions, prior to de-molding.

The hydrated pastes were then crushed and manually grounduntil particle size was reduced to less than 0.28 mm, providingsamples for XRD, DTA/TG and NMR analyses.

2.3. Analytical methodology

2.3.1. Compressive strength testsThe 100 mm 100 mm 100 mm paste specimens were sub-

jected to the compressive strength tests in accordance with Chi-nese standard (GB/T 17671-1999) after 28 days of curing in air.The strength result reported was the average of three specimenswith a variation of no more than 10%.

2.3.2. Chemical analysesThe SiO2 content was determined using uorine potassium sil-

icate method. The Fe2O3, Al2O3, CaO and MgO contents were deter-mined using the EDTA titrimetric method. The sulphate andchloride contents were determined gravimetrically using BaCl2and AgNO3 as precipitating reagents. Minor elements (Cd, Cr, Cu,Pb and Zn) in MSWI y ash were extracted by an acid digest com-prising HNO3, H2O2 and HF (5:2:3, v/v), and heavy metal concen-trations were determined by Atomic Absorption Spectroscopy(Hitachi Z-2000 AAS).

The water content was measured by oven drying 10 g of MSWIy ash at 105 C to constant mass and re-weighing. Loss on ignition(LOI) was measured by oven drying 10 g of MSWI y ash at 105 Cto constant mass before calcining at 550 C and 1000 C for 1 h,cooling and re-weighing.

2.3.3. XRD examinationXRD analysis was performed with Cu Ka radiation in D/Max-

2550PC X-ray powder diffraction apparatus. The accelerating volt-age was 40 kV and the current was 150 mA. The scan parameters of555 2h in 0.02 increment and 10 s/step were selected to recordthe digital data. A PC-based search-match program, softwareinvolving the ICDD-PDF database as a source of reference data,was employed to identify possible crystalline phases in thesamples.

2.3.4. DTA/TG examinationThe DTA/TG measurements were performed using Boyuan DTU-

2A instrument. The Simultaneous Thermal Analysis (STA) enablesDifferential Thermal Analysis (DTA) to be carried out simulta-neously with Thermal Gravimetric Analysis (TG). The recorded out-put of each examination were temperature, sample mass loss (TG)and thermoelectric potential difference (DTA) as a function of time.During the examination, the samples were subjected to heatingrate of 10 C/min under owing nitrogen gas (80 mL/min) in thetemperature range of 301000 C.

2.3.5. NMR examinationThe solid-state 29Si {1H} CP/MAS were performed at 79.4 MHz

on a Bruker AVANCE AVIII 400 WB (9.4 T) spectrometer using aCP/MAS probe for 7 mm o.d. PSZ rotors. Because the spinning speed(vR) affects the 29Si {1H} CP efciency for weakly dipolar coupledspins (e.g. for SiOH groups), all 29Si {1H} CP/MAS experimentswere performed with a moderate spinning speed (vR = 2.0 kHz).

29

nt 34 (2014) 24942504 2495The Si MAS experiments employed an RF eld strength of cB1/2p = 40 kHz for the 90 1H pulse and the 1H decoupling, a relaxa-tion delay of 15 s, and typically 4096 scans.

Table 2Comparison of batch leaching tests applied.

Testconditions

DW SN AB

Leaching agent Distilledwater

Sulphuric acid andnitric acid

Acetic acid buffersolution

Liquid to solidratio

10 10 20

Leaching time 8 18 18

5 10 15 20 25 30 35 40 45 50 550

1000

2000

3000

4000

5000

6000

7000(a) Fly ash

Inte

nsity

2-theta (deg.)

5 10 15 20 25 30 35 40 45 50 552-theta (deg.)

5 10 15 20 25 30 35 40 45 50 552-theta (deg.)

c

sh

q

h

s ha

cqc

c cc sc

a

0

1000

2000

3000

4000

5000

6000

7000

(b) P8-28 d

Inte

nsity

sh

cs

h

sccc

qc

ga

c ch c

0

1000

2000

3000

4000

5000

tt q

hs

scc

cc

(c) P8-365 d

Inte

nsity

c

c

s h

ga

c

Fig. 1. XRD patterns of (a) y ash, (b) 28-day-old hydrated P8 paste, and (c) 365-

2496 X. Li et al. /Waste Management 34 (2014) 24942504quency of 14 kHz.29Si and 27Al chemical shifts were referenced to external sam-

ples of tetrakis (trimethylsilyl) silane (TKS) and 1 M aqueous solu-tion of AlCl36H2O, respectively.

With increasing condensation, there is an increase of diamag-netic shielding to the 29Si nuclei, from the single tetrahedral struc-ture of the monosilicates (Q0) to the end groups (Q1), to the chainmiddle groups (Q2), to the layers and the branching sites (Q3), andnally to the three-dimensional networks (Q4), which leads towell-separated, analytically useful chemical shift ranges for eachtype of SiO44 unit. Therefore, calcium silicate hydrates can besemi-quantied using chemical shifts in the 29Si nuclei in SiOXgroups, whose structures are shown below:

Si

O(m)

O(m)

O(m)(m)O

Tetrahedral (Q0)

Si

O(m)

O(m)

O(m)O

Chain end (Q1)

Si

O(m)

O(m)

OO

Chain middle (Q2)

Si

O

O(m)

OO

Chain branch (Q3)

Si

O(m)

O(m)

OO

Three-dimensional networks (Q4)

Based on NMR results, the hydration degree (a) was evaluatedby the integral intensity of the signal at 70 ppm (Q0) followedthe references (Lin et al., 1996, 1997; Wang et al., 2001):

a % 100% IQ0=I0Q0 100% 1The averaged length of linear polysilicate anions in CSH gel,27Al NMR spectra were performed at 104.2 MHz on a BrukerAVANCE AVIII 400 WB spectrometer (magnetic eld of 9.4 T) usinga probe for 4 mm o.d. ZrO2 rotors. All spectra were acquired using asingle-pulse excitation with a selective short pulse (p/18) to ensurequantitativeness with a 1 s relaxation delay and a spinning fre-

(h)Initial pH 6.80 3.20 0.05 2.64 0.05expressed as W, was estimated as follows:

w 2 IQ1 IQ2=IQ1 2

Table 3Modied BCR sequential extraction procedures.

Fraction Extraction agent (s) Extraction condition

EA 20 ml 0.11 mol/L CH3COOH Agitation (120 10 rpm), 16 h, 25 1 CRED 20 ml 0.5 mol/L NH2OHHCl (pH = 2) Agitation (120 10 rpm), 16 h, 25 1 COX 10 ml 8.8 mol/L H2O2 (pH = 2),

then 25 ml 1 mol/L NH4OAc (pH = 2)Water bath, 1 h, 85 5 CAgitation (120 10 rpm), 16 h, 25 1 C

RES HNO3/H2O2/HF (5:2:3,v/v) Microwave digestion

day-old hydrated P8 paste. (h: Halite, NaCl, c: calcite, CaCO3, s: sylvite, KCl, q:quartz, SiO2, a: anhydrite, CaSO4, g: gypsum, CaSO42H2O, and t: thaumasite,Ca3Si(OH)6(CO3)(SO4)12H2O).

eme0 200 400 600 800 100070

80

90

100(a) Fly ash

Temperature (C)

TG

DTA

DTG

TG (m

ass.%

)

DTG

(mg/

min

)

DTA

(V

)

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

-80

-60

-40

-20

0

X. Li et al. /Waste Managwhere I(Q1) and I(Q2) represent the integral intensity of signals at 80 ppm (Q1) and 87 ppm (Q2), respectively.

2.3.6. Leaching testLeachability of toxic elements was determined by applying

three Chinese solid waste-extraction procedures for leaching toxic-ity: Horizontal Vibration Method (HJ 577-2010) (DW) with Dis-tilled Water (DW, pH 6.80) or, Sulphuric Acid & Nitric Acid (HJ/T299-2007) (SN) or Acetic Acid Buffer Solution (HJ/T 300-2007)(AB) as leaching agents. In this work, SN was a dilute solution ofsulfuric acid and nitric acid at a ratio of 2:1 (pH 3.20 0.05) andAB was a mix of 17.25 mL of glacier acetic acid to 1000 mL of dis-tilled water, and then diluted to a liter of volume (pH 2.64 0.05).

The leaching tests were conducted with different leachingagents, leaching solution to solid sample (L/S) ratios, contact peri-ods of time and initial pH (see Table 2). The suspensions of theleaching agent and the solid were continuously agitated, so that

0 200 400 600Temperature (C

0 200 400 600 800 1000Temperature (C)

60

70

80

90

100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-50

-40

-30

-20

-10

0

10

(b) P1-135 d

TG

DTA

DTG

TG (m

ass.%

)

DTG

(mg/

min

)

DTA

(V

)

50

60

70

80

90

100

TG (m

ass.%

)

(e

TG

DTA

DTG

Fig. 2. The DTA/TG and DTG curves for y ash and 135-day-old h0 200 400 600 800 1000Temperature (C)

60

70

80

90

100(c) P2-135 d

TG (m

ass.%

)

TG

DTA

DTG DTG

(mg/

min

)

DTA

(V

)

-0.1

0.0

0.1

0.2

0.3

0.4

-40

-30

-20

-10

0

10

nt 34 (2014) 24942504 2497the equilibrium was reached by the end of the test. Note that therotary agitation speed was 120 rpm instead of the stipulated30 rpm. This was done to ensure a more aggressive leachingcondition.

After the leaching, the solutions were ltered through a0.45 lmmembrane lter. The pH of these leachates was measuredand the solutions were then acidied with concentrated nitric acidto pH < 2 prior to analyses. The total organic carbon (TOC) contentswere measured using a Shimadzu TOC-VCPH instrument. Chemicaloxygen demand (COD) was analyzed by titrimetric method usingpotassium permanganate as the oxidant in acidic solution at100 C for 30 min. The metal concentrations in the leachates wereanalyzed using Atomic Absorption Spectroscopy.

In literature, the immobilization ratio of heavy metals havebeen evaluated with different formulas (e.g. Giergiczny and Kro9l,2008; Jin and Al-Tabbaa, 2014). In this work, the immobilizationratio (IR) of heavy metal was calculated according to the following

800 1000)

0 200 400 600 800 1000Temperature (C)

DTA

(V

)

DTA

(V

)

50

60

70

80

90

100

TG

TG (m

ass.%

)

(d) P6-135 d

DTG

DTA

DTG

(mg/

min

)

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

-60

-50

-40

-30

-20

-10

0

0.0

0.1

0.2

0.3

0.4) P8-135 d

DTG

(mg/

min

)

-40

-30

-20

-10

0

ydrated pastes. (a) Fly ash, (b) P1, (c) P2, (d) P6, and (e) P8.

eme2500

(a) Fly ash71

.73

2498 X. Li et al. /Waste Managformula, which eliminates the dilute effect of the combinedwater in the y ash hydration with or without silica fume duringa period of curing:

IR 100 ctc0

mtmn

100 3

-60 -70 -80 -90 -100 -110 -120Chemical shift,ppm

-60 -70 -80 -90 -100 -110 -120Chemical shift,ppm

-60 -70 -80 -90 -100 -110 -120Chemical shift,ppm

2000

1500

1000

500

0

1200

1000

800

600

400

200

0

2500

2000

1200

1000

500

0

Inte

nsity

, a.

u.

Inte

nsity

, a.

u.

Inte

nsity

, a.

u.

(b) P1-365 d

(c) P8-365 d

-

-84

.07

-87

.38

-10

0.04

-10

5.90

-10

9.11

-71

.86

-79

.64

-84

.48

-85

.87

-95

.74

-10

8.97

Fig. 3. Deconvolution of central components of 29Si MAS NMR spectra of (a) y ash,(b) 365-day-old hydrated P1 paste, and (c) 365-day-old hydrated P8 paste. The redline corresponds to the experimental prole and blue lines to individual compo-nents. (For interpretation of the references to colour in this gure legend, the readeris referred to the web version of this article.).of calcium carbonate, halite, sylvite, anhydrite and quartz, indicat-ing that the pozzolanic reactivity of quartz from MSWI y ash wasvery low. It should be mentioned that portlandite was not detectedby XRD. This suggests that Ca(OH)2 must be amorphous. Fig. 2shows the DTA/TG curves for y ash and hydrated pastes (P1, P2,P6 and P8) cured for 135 days. The endotherms observed at differ-ent temperature intervals can be attributed to dehydration of var-ious hydration products: calcium silicate hydrate gel (CSH) at100150 C, gehlinite hydrate or stratlingite (2CaOAl2O3SiO28H2O,C2ASH8) at 150220 C, hydrogarnet (3CaOAl2O8H2O, C3AH8) at220310 C, Friedels salt (Ca3Al2O6CaCl210H2O) at 310375 C,calcium hydroxide (CH) at 400530 C, and calcium carbonate at6007000 C (Saikia et al., 2007). According to Lilkov et al. (2014)the carbonation of CSH leads to formation of calcium carbonateswith a low degree of crystallization vaterite and aragonite, whichdissociate in the temperature interval 600700 C.

On the DTA curve for MSWI y ash and P1 paste, an endother-mic peak at 400530 C was assigned to dehydroxylation of cal-cium hydroxide, suggesting the contents of calcium hydroxidewere 11.1% and 10.3%, respectively. The progressive mass loss atwhere IR is the immobilization ratio of heavy metal (%), ct the leach-ing concentration of the hydrated paste after a period of curing (mg/L),c0 the leaching concentration of raw y ash (mg/L), mt the mass ofthe hydrated paste after a period of curing (g) andmn the mass of yash introduced to the paste (g).

2.3.7. Sequential extractionThe European Community Bureau of Reference (BCR) sequential

extraction procedure used in this work was a modied versionusing reagents and conditions given in Table 3 followed Ure et al.(1993) and Chang et al. (2009). The procedure consisted of fourextracted fractions as follows: ex-changeable and weak-acid solu-ble fraction (EA) in the rst step, reducible fraction (RED) in thesecond step, oxidizable fraction (OX) in the third step, and residue(RES) from the fourth step. Each extracted fraction was separatedby centrifugation at 5000 rpm for 10 min, collected in polyethylenebottles and stored at 4 C until analyses. The heavy metal concen-trations of each fraction were detected by Atomic AbsorptionSpectroscopy.

3. Results and discussion

3.1. Compressive strength

The average compressive strength of the P1 paste made frompure y ash was 5.4 0.1 MPa. The compressive strength of theP8 paste, incorporating silica fume in place of 20% MSWI y ash,was 22.3 1.9 MPa. A three-fold increase in strength (measuredas 331.5%) was observed compared with the P1 paste. This sug-gested that the more pozzolanic reaction between SiO2 in the silicafume and Ca(OH)2 in the MSWI y ash occurred, which producedmore CSH gel, reduced the pore size, blocked the capillary poresand improved strength (Stefanidou and Papayianni, 2012).

3.2. Phase identication

As shown in the XRD pattern in Fig. 1, the crystalline phases inMSWI y ash were principally halite (NaCl), sylvite (KCl), quartz(SiO2), calcite (CaCO3), and anhydrite (CaSO4). In hydrated pasteswith addition of 20% silica fume (P8 paste), gypsum (CaSO42H2O,20.62, 2h) and thaumasite (Ca3Si(OH)6(CO3)(SO4)12H2O, 9.24,16.07, 2h) were formed. The diffractograms show the existence

nt 34 (2014) 24942504400530 C from the TG curves, show that the amount of calciumhydroxide decreased with time, conrming that the pozzolanicreaction between calcium hydroxide and silica fume proceeded.

I(Q

0.40.10.4

emeOn the DTA curve for y ash, P1 and P2, there was a distinctendothermic peak at 600700 C was assigned to decompositionof calcium carbonate. But on the DTA curve for P6 and P8, theendothermic peak at 600700 C was reduced. According to themass loss at 600700 C in y ash and hydrated pastes, the con-tents of calcium carbonate in MSWI y ash, P1, P2, P6 and P8were 5.9%, 5.3%, 6.7%, 1.8% and 1.5%, respectively. This indicatedthat the content of calcium carbonate formed by carbonationdecreased signicantly in P6 and P8 during the paste curingprocess.

Fig. 3 shows 29Si MAS NMR spectra for y ash and 365-day-oldhydrated pastes (P1 and P8). The chemical shift of 29Si nuclei invarious materials was dependent on the X-group in the SiOXunits. In the spectrum of y ash, the resonance line at 71.7 ppm(Q0) was attributed to isolated silicates. The resonance lines at 84.1 and 87.4 ppm due to the non-bridging tetrahedra Q2 werevery weak. A broad peak of Q4 type at 100.1 108.9 ppm wasdue to crystalline quartz and poorly ordered silicate minerals(Ubbraco and Calabrese, 1998; Pomakhina et al., 2012). In thespectrum of the P1 paste, the resonance lines at 79.6 and 84.5 ppm due to chain-end tetrahedra Q1 and non-bridging tetra-hedra Q2 of the CSH were much stronger, while a broad Q0 com-ponent at 71.9 ppm decreased signicantly, compared with yash. In the spectrum of the P8 paste, non-bridging tetrahedra Q2

was the major silicate species. Note that the signal from Q0 at 71.9 ppmwas present in the P1 paste, but vanished in the P8 paste.The intensity of the peak at 108.9 ppm weakened signicantly,indicating that most of the amorphous silica had been consumed.Formation of CSH phases further suggested that the pozzolanicreaction occurred again when silica fume was added. Accordingto the relative peak intensities, the hydration degree and averagelength of linear polysilicates for hydrated pastes were calculatedand are summarized in Table 4. Compared to the P1 paste, thehydration degree and average chain length of polysilicate anionsin the P8 paste were increased by 48.1% and 110.1%. As the silicafume addition increased and the Ca:Si ratio in the CSH geldeclined, the relative intensity of the Q2 signal compared to theQ1 signal rose suggesting an increase in chain length (Richardson,2004).

27Al NMR shows the coordination state of Al in crystalline and

Table 4NMR analysis results for pastes curing for 365 days.

Sample Integral intensities of Qn, 29Si MAS NMR

I(Q0) I(Q1) I(Q2)

Fly ash 1 (35.87)b 0.2652 (9.51) 0.3104 (11.13)P1 0.6101 (39.96) 0.2222 (14.55) 0.2413 (15.80)P8 0.1108 (7.78) 0.1166 (8.19) 0.3942 (27.68)

a Values modied according to sample weights.b Values inside the brackets indicates percentage.

X. Li et al. /Waste Managpoorly-crystalline phases. As shown in Fig. 4, each 27Al NMR spec-trum exhibited two main peaks at around 57 ppm and 13 ppm. Theresonance at 57 ppm was assigned to tetrahedrally coordinated Al,while the resonance at 13 ppm was related to octahedrally coordi-nated Al. In contrast to the spectrum of y ash, the intensity ofpeaks around 57 ppm of the P1 paste decreased greatly, and thepeak at 13 ppm increased correspondingly, indicating thataluminum phases were converted from tetrahedral to octahedralco-ordination. As can be seen from Fig. 4c, for the P8 paste, theration of octaheadrally coordinated aluminum to tetraheadrallycoordinated aluminum was roughly even, suggesting the forma-tion of CSAH gel. Heavy metals may be incorporated either inCSH or in CSAH formed by pozzolanic reactions (Lin et al.,2008).3.3. Leaching behavior

The leaching concentrations of Cd, Cr, Cu, Pb and Zn of y ashare presented in Table 5. According to the Chinese Criteria for Haz-ardous Waste (GB 5085-2007 and HJ/T 299-2007), limit concentra-tions of Cd, Cr, Cu, Pb and Zn in leachate using sulphuric acid &nitric acid as leaching agent are 1, 5, 100, 5 and 100 mg/L, respec-tively. According to the Standard for Pollution Control on the Secu-rity Landll Site for Hazardous Wastes (GB 18598-2001 and HJ577-2010), limit concentrations of Cd, Cr, Cu, Pb and Zn in leachateusing distilled water as leaching agent, are 0.5, 12, 75, 5 and 75 mg/L, respectively. And the leachate pH should be limited in the rangeof 7.012.0. According to the Standard for Pollution Control on theLandll Site of Municipal Solid Waste (GB 16889-2008 and HJ/T300-2007), limit concentrations of Cd, Cr, Cu, Pb and Zn in leachateusing acetic acid as leaching agent are 0.15, 4.5, 40, 0.25 and100 mg/L, respectively. The leachate concentration of Pb by SNleaching exceeded the limit value (5 mg/L) stipulated in Identi-cation Standards for Hazardous Wastes-Identication for Extrac-tion Toxicity (Chinese standard, GB5085.3-2007). The leachatepH and leachate concentration of Pb by DW leaching exceededthe limit values stipulated in Standard for Pollution Control onthe Security Landll Site for Hazardous Wastes (GB18598-2001).The other heavy metals (Cd, Cr, Cu and Zn) were found to be belowthe limit values.

Fig. 5 shows nal leachate pH of hydrated pastes after curing 7,28 and 135 days. The leachate pH by DW and SN were very close. Itshould be pointed out that leachate of 7-day-old and 28-day-oldhydrated pastes by distilled water reached equilibration pH of ca.12.4 very fast, which was the pH value of saturated Ca(OH)2 solu-tion. The leachate pH of 135-day-old pastes by SN was slightlylower than 12.4, indicating that the sulphuric acid and nitric acidwas mainly consumed by reactions with dissolving Ca(OH)2 andCSH. Using DW and SN as leaching agents, the leachate pH val-ues were almost constant at 12.6 and 12.4 for 7-day-old and 28-day-old hydrated pastes irrespective of the addition amount of sil-ica fume, but a decrease of leachate pH from 12.6 to 10.1 wasnoticed for 135-day-old hydrated pastes as silica fume contentsincreased. Because the pH was controlled by the dissolution ofhydrated phases, especially calcium hydroxide and CSH in the

a (%) Wa

3) I(Q4) Total

584 (16.44) 0.7539 (27.04) 2.7879 (100)976 (12.94) 0.2557 (16.75) 1.5268 (100) 60.04 4.17048 (28.42) 0.3978 (27.93) 1.4240 (100) 88.92 8.76

nt 34 (2014) 24942504 2499pastes, the pH decrease may imply that the more pozzolanic reac-tion occurred with increasing addition of silica fume and time,which consumed Ca(OH)2. As the silica fume addition increased,the overall Ca:Si ratio of the pastes may decline. The equilibriumpH values of solutions buffered by low Ca:Si ratio CSH is belowthat of high Ca:Si ratio CSH (Carde et al., 1996; Stegemann andBuenfeld, 2002; Makhlou et al., 2012). Thaumasite and low Ca:Siratio of CSH can contribute to pH-buffering at a pH of around 10(Carde et al., 1996). Additionally, the pastes were matured in theair and carbonation may also lower the pH due to the reactionbetween dissolved CO2 and phases in the paste, which would alsoconsume Ca(OH)2 and CSH. After carbonation, the pH of MSWIy ash was reported to be in the range of 711 (Li et al., 2007;Wang et al., 2010; Santos et al., 2013).

eme2000

2500 X. Li et al. /Waste ManagThe solution pH values of samples leached with acetic acid werein the range of 4.55.6, which was much lower than that of heavymetal compounds at their minimum solubility. This suggested thatthe reaction between acetic acid and solid phases in the pastes wassignicant.

120 100 80 60 40 20 0 -20Chemical shift, ppm

120 100 80 60 40 20 0 -20Chemical shift, ppm

120 100 80 60 40 20 0 -20Chemical shift, ppm

1500

1000

500

0

10000

8000

6000

4000

2000

0

2000

1500

1000

500

0

Inte

nsity

, a.

u.

Inte

nsity

, a.

u.

Inte

nsity

, a.

u.

Fig. 4. Deconvolution of central components of 27Al NMR spectra of (a) y ash, (b)365-day-old hydrated P1 paste, and (c) 365-day-old hydrated P8 paste. The red linecorresponds to the experimental prole and blue lines to individual components.(For interpretation of the references to colour in this gure legend, the reader isreferred to the web version of this article.).For all the pastes, the leachate concentration of Cr was lower

Table 5The leachate concentration of y ash according to three leaching procedures.

Leaching procedure Leachate concentration (mg/L) Leachate pH

Cd Cr Cu Pb Zn

DW ND 0.04 0.32 38.54 6.63 12.4SN ND 0.04 0.32 40.99 6.96 12.5AB 1.81 0.4 5.68 5.54 11.22 6.2

ND: denotes not detected.The limit of detection for Cd is 0.005 mg/L, Cr is 0.01 mg/L, Cu is 0.01 mg/L, Pb is0.01 mg/L, Zn is 0.01 mg/L.

0 2 4 6 8 10 14 204.0

4.5

5.0

5.5

6.0

10

11

12

13

Silica fume addition (%)

pH

DW-7 d DW-28 d DW-135 d SN-7 d SN-28 d SN-135 d AB-7 d AB-28 d AB-135 d

Fig. 5. Leachate pH of hydrated pastes.

nt 34 (2014) 24942504than the detection limit (0.01 mg/L) in DW and SN extracts andin the range of 0.02-0.05 mg/L in AB extracts.

Fig. 6 depicts the amount of heavy metals (Cd, Cu, Pb and Zn)leached from hydrated pastes by different leaching procedures(DW, SN and AB). The leachate concentration of Cu, Pb and Zndecreased with increasing silica fume content of the pastes andwith curing time using DW or SN leachants. The leachate concen-trations of Cu for pastes hydrated in the presence of silica fumewere around 0.04 mg/L, while the leachate concentrations of Znwere in the range of 0.010.12 mg/L. The Cd concentration ofleachates for hydrated pastes leached by DW or SN was belowthe detection limit (0.005 mg/L) of AAS. The variation trend ofleaching concentrations of heavy metals for all pastes was alikefor DW and SN leaching. This was consistent with the nal leachatepH observations.

The concentrations of Cu, Pb and Zn during leaching of y ashby DW were 0.32 mg/L, 38.54 mg/L and 6.63 mg/L; whereas Cu,Pb and Zn leaching concentrations for the 7-day-old hydrated yash paste (P1 paste) were somewhat lower at 0.18 mg/L,15.62 mg/L and 2.64 mg/L. This indicated that the immobilizedproportions of Cu, Pb and Zn were 22.55%, 44.19% and 45.17%,respectively. The immobilization of these heavy metals increasedwith time, such that in the 135-day-old hydrated y ash paste,the immobilized proportions of Cu, Pb and Zn were 79.03%,88.61% and 97.57%. In the addition of 20% silica fume (P8 paste),leaching concentrations for Cu, Pb and Zn for the 7-day-old pasteby SN leaching decreased from 0.32 mg/L to 0.05 mg/L, 40.99 mg/L to 4.40 mg/L, and 6.96 mg/L to 0.21 mg/L compared with yash. After curing 135 days, Cd and Pb in the leachate were belowthe detection limits (0.005 mg/L and 0.01 mg/L, respectively),while Cu and Zn concentrations decreased to 0.02 mg/L and0.03 mg/L. These results indicated that silica fume additionimproved the stabilization of Cu, Pb and Zn in MSWI y ashsignicantly.

eme0.04

0.08

0.12

0.16

0.20 7 d 28 d 135 d

Cu

conc

entr

atio

n (m

g/L

)

DW

X. Li et al. /Waste ManagFor 28 day old hydrated paste with a silica fume addition over10%, the leachate pH and leachate concentrations of Cd, Cr, Cu,Pb and Zn by SN were lower than limit values. It can be disposedas non-hazardous waste according to the Chinese Criteria for Haz-ardous Waste (GB 5085-2007). For 135 day old hydrated pasteswith a silica fume addition over 10%, the leachate pH and leachateconcentrations of Cd, Cr, Cu, Pb and Zn by DW were lower than thelimit values stipulated in Standard for Pollution Control on theSecurity Landll Site for Hazardous Wastes (GB 18598-2001). Itshould be noted that leaching concentrations of Cd and Zn by ABincreased with time in the addition of silica fume. Zn release forthe 135-day-old hydrated paste increased up to approximately 4times. This could be attributed to the reduction of alkalinity dueto pozzolanic reaction and the complex coordination ability of ace-tic acid for Cd and Zn. An increased Zn mobility in waste treated

0 2 4 6 8 10 12 14 16 18 200.00

Silica fume addition (%)

0 2 4 6 8 10 12 14 16 18 202

3

4

5

6

7

7 d 28 d 135 d

Cu

conc

entr

atio

n (m

g/L

)

Silica fume addition (%)

AB

0 2 4 6 8 10 12 14 16 18 20

0

4

8

12

16

20

Pb c

once

ntra

tion

(mg/

L)

Silica fume addition (%)

7 d 28 d 135 d

SN

Fig. 6. Leaching concentratio0.04

0.08

0.12

0.16

0.20

7 d 28 d 135 d

Cu

conc

entr

atio

n (m

g/L

)

SN

nt 34 (2014) 24942504 2501with cement was also observed by Hale et al. (2012). We tenta-tively suggest that leaching by acetic acid is unrealistically aggres-sive, as an experimental model of encapsulated wastes in serviceand should be considered as the extreme limit of likely chemicalenvironment.

Table 6 shows TOC and CODMn values in leachates by the DWleaching procedure. TOC and CODMn decreased with silica fumeaddition and curing time. After curing for 135 days, the CODMn ofleachates decreased 67.2% and 93.1% for P1 and P8 pastes in com-parison with y ash, whereas TOC decreased 58.74% and 74.4% cor-respondingly. The TOC in the leachate decreased 61.12% and63.21% for 365-day-old pastes (P1 and P8) in comparison with28-day-old pastes. This indicated that the xation of organicswas improved with the increment of curing time. The decrease ofCOD and TOC in leachates could be attributed to the degradation

0 2 4 6 8 10 12 14 16 18 200.00

Silica fume addition (%)

0 2 4 6 8 10 12 14 16 18 20

0

4

8

12

16

Pb c

once

ntra

tion

(mg/

L)

Silica fume addition (%)

7 d 28 d 135 d

DW

0 2 4 6 8 10 12 14 16 18 204

6

8

10

12

14

Pb c

once

ntra

tion

(mg/

L)

Silica fume addition (%)

7 d 28 d 135 d

AB

ns of Cu, Pb, Zn and Cd.

eme0 2 4 6 8 10 12 14 16 18 20-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Zn c

once

ntra

tion

(mg/

L)

Silica fume addition (%)

7 d 28 d 135 d

DW

110 AB

2502 X. Li et al. /Waste Managof organics during paste curing process, or possibly sorption oforganics on hydration products in paste. The xation mechanismof organics may need further study in future.

3.4. Chemical speciation of heavy metals

The distributions of heavy metal speciation in y ash andhydrated pastes cured for 28 and 135 days are shown in Fig. 7.According to Johannesson and Utgenannt (2001), the exchangeableand weak-acid soluble fraction is the amount of heavy metal thatwould be released into the environment if it was attacked by acidicuids such as aggressive ground waters. The reducible fraction ofheavy metal represents the contents of metal bound to iron andmanganese oxides that could be released if the substrate was sub-jected to more reducing conditions (Usero et al., 1998). The oxidis-able fraction of heavy metal shows the amount of metal bound tothe organic matter and sulfur species which would be released intothe environment if condition became oxidising. The residual frac-tion of heavy metals is bound with the strongest association tothe crystalline structures of the minerals. The oxidisable fractionand residual fraction could be leached out only in strongly oxidiz-

0 2 4 6 8 10 12 14 16 18 2020

30

40

50

60

70

80

90

100

Silica fume addition (%)

7 d 28 d 135 d

Zn c

once

ntra

tion

(mg/

L)

Fig. 6 (cont

Table 6TOC and CODMn of leachates by DW.

Sample Leachate concentration (mg/L)

TOC (28 d) TOC (365 d) CODMn (365 d)

Fly ash 37.69 30.56 17.8P1 32.54 12.61 5.84P8 21.23 7.81 1.220 2 4 6 8 10 12 14 16 18 20-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

7 d 28 d 135 d

Zn C

once

ntra

tion

(mg/

L)

Silica fume addition (%)

SN

0.8

1.2

1.6

2.0

2.4

2.8 7 d 28 d 135 d

Cd

conc

entra

tion

(mg/

L)

AB

nt 34 (2014) 24942504ing or strongly acid environments. It can be seen from Fig. 7 that Cdmainly existed in the reducible phase and Cu, Pb and Zn in bothreducible and oxidizable phases of the y ash. The exchangeableand weak-acid soluble fractions of Cd, Cu, Pb and Zn in y ash were0%, 0%, 0.49% and 0.07% respectively, whilst the stable-state frac-tions of these metals in y ash were 2.75%, 52.50%, 86.07% and55.79%. After hydration, the form in which the heavy metals existin MSWI y ash altered signicantly. The substantial increase inCd, Cu, Pb and Zn concentrations in exchangeable and weak-acidsoluble fractions could, to some extent explain the decrease inpH, presumably due to the formation of CSH with a low Ca:Siratio in the hydration process.

It could be seen that the stable state fractions of Cd in thehydrated pastes (P2, P6 and P8) increased with both silica fumecontent and curing time. The stable state fractions of Cd were2.23%, 1.43% and 3.88% for 28-day-old pastes, and after 135 days,6.77%, 7.16% and 14.69% respectively, implying that the potentialadverse impact on the environment was reduced. It is believed thatcadmium was mainly immobilized in the form of Cd(OH)2, whichwas encapsulated on a microscopic scale within CSH (Haleet al., 2012; Pandey et al., 2012).

The percentage of the exchangeable and weak-acid soluble frac-tions of Cu changed slightly and was independent on the amount ofsilica fume and curing time. The result was agreement with thetrend of Cu release by acetic acid during 135 days of hydration per-iod. Copper may also exist as hydroxides or more complex com-pounds (such as basic carbonates) in the hydrated pastes.

The percentage of stable fractions of Pb in the hydrated pastes(P2, P6 and P8) increased with increasing the amount of silica fumeaddition and curing time. The stable state fractions of Pb were78.11%, 84.99% and 86.07% for 28-day-old pastes, while 78.59%,

0 2 4 6 8 10 12 14 16 18 200.4

Silica fume addtion (%)

inued)

50

on p 50 per

100

emeFA P2 P6 P8 P2 P6 P80

10

20

30

40

Cd

extra

cti

135 d135 d28 d28 d28 d 135 d

10

20

30

40

50

60

70

80

90

100

Cu

extra

ctio

n pe

rcen

tage

(%)60

70

80

90

100

EA RED OX RES

erce

ntag

e, %

X. Li et al. /Waste Manag85.62% and 87.53% after 135 days. This nding was consistent withthe decreased Pb released during acetic acid leaching. Huang et alreported that silica fume powder showed good xation efciencythrough the major mechanism of the formation of PbSiO3 precipi-tates (Huang and Huang, 2008). Calciumleadsilicate sulfatehydrates (CPbSH) may form, where lead would be incorporatedin the silicate matrix (Lee, 2006; Gineys et al., 2010).

With increasing silica fume content, the percentage ofexchangeable and weak acid leachable Zn increased, while the per-centage of residual Zn decreased, contributing to an overallenhancement of the mobility of Zn. After aging, the increasing lea-ched concentration suggested that either much of Zn was trappedin soluble form within a disconnected pore network or that it wasassociated with hydration products (Asavapisit and Macphee,2007), which presumably explained the higher Zn concentrationsin acetic acid extracts from hydrated pastes.

In y ash and hydrated pastes, Cr was almost in the form ofresidual fraction. It was deduced that the most chromium com-pounds were silicates and silico-aluminates.

4. Conclusions

The hydration of MSWI y ash with the addition of silica fume,improved mechanical strength of hydrated pastes and heavy metalimmobilization signicantly, largely due to its pozzolanic reactionwith calcium hydroxide. In addition to variations due to the leach-ing agents, curing time and the amount of silica fume added, thepH value of leachate was greatly inuenced by the dissolution ofCSH and thaumasite. In comparison with MSWI y ash, heavymetals (especially Pb) became stable in the hydrated pastes with

FA P2 P6 P8 P2 P6 P80

135 d135 d28 d28 d28 d 135 d

Fig. 7. Chemical speciation of Cd, Cu, Pb a10

20

30

40

50

60

70

80

90

Zn e

xtra

ctio

n pe

rcen

tage

(%)FA P2 P6 P8 P2 P6 P80

10

20

30

40

Pb e

xtra

ctio

n

135 d135 d28 d28 d28 d 135 d60

70

80

90

100

cent

age

(%)

nt 34 (2014) 24942504 2503increasing silica fume addition. It was found that the leaching ofheavy metals was reduced signicantly which we suggest is dueto both physical entrapment (occlusion) and chemical incorpora-tion as precipitates. An addition of 10% silica fume can satisfy therequirement of detoxication for heavy metals in MSWI y ashin terms of the identication standard of hazardous waste of China.The XRD, DTA/TG and NMR results demonstrate that the pozzola-nic reaction in the presence of silica fume generated more CSHgel compared with the hydrated y ash paste and that aluminiumbecame incorporated in this gel phase.

Acknowledgements

This work was supported by Innovation Program of ShanghaiMunicipal Education Commission under Contract Number 11ZZ61.The nancial support from theNational Science Foundation of Chinaunder Contract Number 21277023 is gratefully acknowledged. Thisworkwas also supported by the PhD thesis innovation foundation ofthe Donghua University (No. 2232013I-44).

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Stabilization of heavy metals in MSWI fly ash using silica fume1 Introduction2 Experimental2.1 Materials2.2 Paste preparation2.3 Analytical methodology2.3.1 Compressive strength tests2.3.2 Chemical analyses2.3.3 XRD examination2.3.4 DTA/TG examination2.3.5 NMR examination2.3.6 Leaching test2.3.7 Sequential extraction

3 Results and discussion3.1 Compressive strength3.2 Phase identification3.3 Leaching behavior3.4 Chemical speciation of heavy metals

4 ConclusionsAcknowledgementsReferences