Chemosphere 117 (2014) 745–752

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Stabilization of simulated lead sludge with iron sludge via formation of PbFe12O19 by thermal treatment Linqiang Mao a, Hao Cui a, Hao An a, Bing Wang a, Jianping Zhai a, Yongbin Zhao b, Qin Li a,⇑ a b

State Key Laboratory of Pollution Control and Resource Reuse, and School of the Environment, Nanjing University, Nanjing 210046, PR China National Institute of Clean-and-Low-Carbon Energy (NICE), Beijing 102209, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Simulated lead sludge was stabilized

by incorporation into PbFe12O19 phase.  Iron sludge was used as a remedial additive to stabilize lead sludge.  The volumes of compressed sludge samples reduced obviously after thermal treatment.  The sintered sludge sample presented a very high stability under acidic environments.

Simulated Lead sludge Milling Fe:Pb=12

Iron-rich sludge

Thermal treatment 1050 C

PbFe12O19 Sintered product

12

25

(a)

11

PbO Sludge Sample

(b) 20

Concentration of Pb (g/L)

10 9

pH

8 7 6 5 4 3

PbO Sludge Sample

15

10

5

0

2 -2

0

2

4

6

8

10

12

14

16

18

20

22

Time(days)

24

26

28

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Time(days)

Leaching performance of PbO and sintered product

a r t i c l e

i n f o

Article history: Received 22 April 2014 Received in revised form 5 August 2014 Accepted 7 August 2014

Handling Editor: O. Hao Keywords: Lead sludge Iron sludge PbFe12O19 Thermal treatment Stabilization

a b s t r a c t This study investigated the feasibility of stabilizing lead sludge by reaction with iron sludge via the formation of PbFe12O19 through a thermal treatment process. Lead hydroxide was used to simulate leadladen sludge and the sintering procedure was performed by firing a mixture of this simulated sludge together with iron sludge at a Fe/Pb molar ratio of 12 over the temperature range from 650 to 1400 °C. The accompanying phase transformations as well as the surface characteristic of sintered samples were observed by XRD and SEM, while the leaching behavior of the stabilized sludge in an acidic environment was evaluated by a modified Toxicity Characteristic Leaching Procedure (TCLP) test. The results confirmed that PbFe12O19 acts as a stabilization phase for lead, and showed that the formation of a PbFe12O19 phase began at 750 °C with the lead completely incorporated into the PbFe12O19 phase at 1050 °C. Above 1100 °C, the PbFe12O19 phase began to decompose, accompanied by the reappearance of Fe2O3. The volumes of compressed sludge samples were reduced significantly after thermal treatment, with accompanying volume reductions of 40% at 1050 °C. This study compared the leaching of lead from PbO and sintered sludge samples using a prolonged TCLP test, and the data showed that the PbFe12O19 phase was superior to the PbO and that the sintered sludge sample exhibited very high stability under acidic environments. These results suggest a promising and reliable method of reducing lead sludge mobility and toxicity has been identified. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author. Tel./fax: +86 25 8359 2903. E-mail addresses: [email protected] (L. Mao), [email protected] (H. Cui), [email protected] (H. An), [email protected] (B. Wang), [email protected] (J. Zhai), [email protected] (Q. Li). http://dx.doi.org/10.1016/j.chemosphere.2014.08.027 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

Lead is a hazardous metal that is not biodegradable and hence tends to bioaccumulate in living systems. It is commonly used in

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batteries, electronics, plating and oil-based paints (Yang et al., 2001). A variety of technologies have been developed for the removal of lead from water, including chemical precipitation, adsorption, ion exchange, membrane processes and electrochemical flocculation (Lu and Shih, 2011; Lu et al., 2013). Some of these, however, produce large amounts of lead-contaminated sludge that may cause secondary pollution and additional operational costs (Gupta and Suhas, 2009). Current methods used to treat hazardous metal sludge generally included recovery of useful metal and the stabilization/solidification (S/S) methodology (Nagib and Inoue, 2000; Veit et al., 2006). Recovery of such metals from sludge has been accomplished using conventional methods, including solvent extraction, ion exchange, membrane separation and microbiological processes (Lu and Shih, 2011; Lu et al., 2013). The S/S method is widely used for treating hazardous waste (Wang and Vipulanandan, 2000; Yin et al., 2006) and typically involves pollutant encapsulation via cement, glass or pitch before landfill of the encapsulated waste. This process was designed to improve waste handing and to decrease the exposed surface area of the waste so as to inhibit leaching of heavy metals from the waste material. Many publications have noted that the S/S process serves to increase the final disposal volume of the sludge and hence to reduce the capacity of landfill sites. In addition, there is still the potential risk that some hazardous metals may release from the solid matrix in acidic environments (Lu et al., 2008; Li et al., 2011a,b; Shih and Tang, 2012). Thermal treatment as a disposal means for sludge has gradually attracted increasing attention, since it tends to reduce the volume of solid waste and allows the recovery of energy (Wu et al., 2010). The conversion of soluble heavy metals to stable mineral forms using a thermal process has been reported as a promising approach to the effective stabilization of hazardous metals. This technique could also significantly reduce sludge volumes and metals mobility (Shih et al., 2006a, 2006b; Ndiba et al., 2008; Shih and Tang, 2012). In previous work, some studies successfully stabilized nickel and revealed a potential mechanism for incorporating nickel within a spinel structure through sintering its oxide with alumina (Al2O3), hematite (Fe2O3) and kaolinite (Al2Si2O5(OH)4). Leaching tests demonstrated that the nickel in the spinel structure exhibited excellent resistance to leaching under acidic environments (Shih et al., 2006a,b; Shih and Leckie, 2007). Others investigated the transformation of copper into copper alumina spinel (CuAl2O4) by sintering sludge from printed circuit boards with alumina and kaolinite precursors and explored the feasibility of employing waterworks sludge as the remedial material. Copper sludge was found to react with waterworks sludge to form copper aluminum spinel, and thus the transformation of copper sludge with aluminum rich materials to produce copper aluminum spinel was also shown to represent a potentially promising strategy for stabilizing heavy metal sludge (Tang et al., 2010, 2011a; Tang and Shih, 2013; Tang et al., 2013). The stabilization of zinc-laden sludge into an alumina zinc spinel phase by high temperature processing was also successfully performed and was associated with a significant reduction in zinc mobility under acidic conditions (Tang et al., 2011b). It has thus been well documented that incorporating heavy metals into a stable mineral structure is feasible and offers a reliable strategy for the disposal of sludge containing heavy metals. Recent studies have demonstrated the successful transformation of lead-containing sludge into magnetoplumbite-like PbAl12O19. Subsequent tests found that, when Pb leaching from PbAl2O4 and PbAl12O19 was compared, the PbAl12O19 exhibited better performance in terms of its intrinsic resistance to acid attack compared with the lead alumina spinel structure (Lu and Shih, 2011; Lu et al., 2013). The similar compound PbFe12O19 is a magnetic material with excellent chemical and mechanical stability and

has a wide range of potential applications in transducers, sensors and novel memory media (Carp et al., 1997; Yang et al., 2007) and so may also represent a potential lead stabilization structure. PbFe12O19 is typically synthesized by firing a mixture of lead and iron oxides (PbO and Fe2O3) (Mountavala and Ravitz, 1962; Nevirva and Fisher, 1986). However, the lead in sludge is usually present in the form of hydrates (Chen et al., 2009; El-Sheikh and Rabbah, 2013), and there have been few studies concerning the transformation of lead-containing sludge into a PbFe12O19 phase. Another possibility is the use of iron-rich sludge from the steel industry as a substitute for iron oxide when stabilizing lead sludge, which not only greatly reduces the associated raw ingredient costs but also offers a new approach to the disposal of iron sludge (Rijay and Sihorwala, 2003; Li et al., 2007; Liu et al., 2012; Kishimoto et al., 2013). In the present study, PbFe12O19 was produced through a thermal process using lead hydroxide to simulate a lead-containing sludge in addition to industrial iron sludge as the remedial additive, with the aim of stabilizing the simulated lead sludge. The phase transformation and surface characteristics of the sintered samples during thermal processing were observed by SEM, XRD and the apparent volume shrinkage of the product was also examined. Extended leaching tests modified from U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP) were performed to assess the leaching of lead from the sintered sludge samples in a simulated acidic environment. 2. Materials and methods Lead oxide may be employed as a simulated lead sludge (Shih et al., 2006a,b; Lu and Shih, 2011; Lu et al., 2013); however, since the lead in lead-containing sludge is normally present as hydrates, lead hydroxide was used as the lead sludge in this study. Iron sludge for use as the remedial additive was obtained from Zhongtian Steel, and had been generated during rolling/pickling. This sludge was washed with water to eliminate soluble salts and dried at 105 °C for 24 h. The elemental composition of the sludge was determined with X-ray fluorescence and the main constituents are shown in Table 1. Lead incorporation experiments were conducted using Pb(OH)2 with this iron sludge. In addition, as some studies used pure oxides as simulated sludge to study the phase transformation during thermal treatment, we also used oxides mixture (PbO and Fe2O3) as reference to investigate the discrepancy between pure oxides samples and sludge samples in phase transformation and morphological structures during thermal treatment. All chemicals used in this study were reagent grade and were purchased from the Sinopharm Chemical Reagent (China). The phase composition of the PbO powder was identified by XRD to consist of a mixture of litharge (a-PbO; ICDD PDF #77-1971) and massicot (b-PbO; ICDD PDF #05-0561) phases, while the Fe2O3 powder phase was identified by XRD as iron oxide (Fe2O3; ICDD PDF #84-0307), which were confirmed by XRD analysis results (Fig. SM-1 in Supplementary Materials (SM)). The two raw ingredients (oxides samples: Fe2O3 + PbO, sludge samples: Pb(OH)2 + iron sludge) used in this study were combined at a Fe/Pb mole ratio of 12 and were mixed by ball milling in a water slurry for 18 h, after which the slurry was dried at 105 °C for 24 h. The dry solids were crushed and homogenized by mortar

Table 1 Chemical compositions of iron sludge. Components Fe2O3 SiO2 TiO2 CaO SO3 MgO MnO Al2O3 Cl P2O5 LOIa % 55.01 3.37 1.69 0.78 0.78 0.76 0.57 2.71 0.11 0.22 34 a

LOI, loss on ignition at 960 °C.

L. Mao et al. / Chemosphere 117 (2014) 745–752

grinding, then pass through 74 lm sieve. The contact area between powders was increased by pelletizing into a small cylindrical disc with 13 mm diameter at 10 MPa in readiness for the sintering process. During sintering experiment, temperature increased at a rate of 5 °C min1 from room temperature to the targeted temperature, the samples were then placed in crucibles and were carried in muffle furnace, sintered at targeted temperature for 3 h. After sintering, the samples were air-quenched in the samples holder in the air under room temperature, and ground into powder by mortar and pestle for XRD analysis and leaching test. Phase transformations during sintering were monitored by XRD. The diffraction data were collected using a XRD-6000 diffractometer (Shimadzu) equipped with a Cu X-ray tube operated at 40 kV and 30 mA. The 2h scan range was from 10° to 80°, with a step size of 0.02 and a scan speed of 0.3 s per step. Qualitative phase identification was accomplished by matching powder XRD patterns with those retrieved from ICDD (PDF-2 Release 2008). The fracture surface morphology of each sample was observed by SEM. All SEM investigations were performed on a Hitachi S3400N system. XPS study was carried out using PHI-5000 VersaProbe (UIVAC-PHI). Sample shrinkage was determined from the differences in raw and fired samples volumes, and the volume was measured from the differences of water before and after the solids being immersed into water. The leaching characteristics of single-phase PbO and the obtained sludge samples were evaluated using a leaching test modified from the U.S. EPA TCLP, employing a pH 2.9 acetic acid solution as the leaching fluid (U.S. EPA, 1998). A leaching vial was filled with 10 mL of TCLP extraction solution and 0.5 g of sample powder and the vial was rotated end-over-end at 60 rpm for agitation periods ranging from 0.75 to 24 d (Tang et al., 2010; Shih and Tang, 2011). At the end of each agitation period, the leachates were filtered through a membrane, their pH was measured with a pH meter and the concentrations of lead and iron were determined via flame atomic absorption spectrophotometer (Thermo M6).

3. Results and discussion 3.1. Formation of PbFe12O19 PbFe12O19 has been prepared by sintering a mixture of lead oxide and iron oxide in many studies (Mountavala and Ravitz, 1962; Mexmain and Hivert, 1978; Nevirva and Fisher, 1986; Diop et al., 2010). In previous work (Nevirva and Fisher, 1986; Rivolier et al., 1993), the formation of PbFe12O19 was reported to be strongly dependent on the sintering temperature. Thus, to obtain the best possible crystalline PbFe12O19 phase, we performed a 3 h sintering process at temperatures ranging from 650 to 1400 °C (Lu et al., 2013). Fig. 1 shows the phase transformation processes of two different samples (Fig. 1a: sludge samples, Fig. 1b: oxides samples). By analysis of the XRD peaks, it is possible to monitor the formation of a well-crystallized PbFe12O19 (PbFe12O19; ICDD PDF #84-2046) phase in the oxides samples during thermal processing (Fig. 1b), and same diffraction peaks is also observed in sludge samples, which indicated that PbFe12O19 was also successfully prepared from sludge samples (Fig. 1a). Fig. 1a presents the phase transformation in the sludge samples during thermal processing. The PbFe12O19 phase was first observed at 750 °C, which is consistent with the results of past studies (Jonker, 1975; Diop et al., 2010). The Fe2O3 phase was, however, the major crystalline phase in samples sintered from 650 to 850 °C, indicating that iron hydroxide was initially dehydrated and converted to iron oxide (Fe2O3). This phenomenon is in accordance with the typical behavior of metals hydroxides, in which

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they usually dehydrate and then transform into oxides upon heating (Shih et al., 2006a, 2006b; Tang et al., 2011a,b). The peak intensity of the PbFe12O19 phase increased as the temperature was increased from 850 to 1050 °C, at which point iron oxide was not seen, and PbFe12O19 was the major phase at 1050 °C, demonstrating almost all the lead had been transformed into PbFe12O19 at 1050 °C. This observation showed that using iron sludge as remedial materials to stabilize lead sludge carried at 1050 °C could acquire better stabilization efficiency. At sintering temperatures above 1100 °C, significant changes were observed in the XRD patterns, corresponding to the reappearance of Fe2O3, this was accompanied by a corresponding decrease in PbFe12O19, indicating that the PbFe12O19 was decomposing to generate Fe2O3. Thus sintering temperature higher than 1100 °C would reduce the stabilization efficiency of lead. Comparing the phase transformations seen in the oxide samples (Fig. 1b), it is evident that similar phase transformations occurred during thermal processing, although there are some differences between the oxide and sludge samples. In the oxide samples, only peaks corresponding to iron oxide were observed at 650 and 750 °C, indicating that the lead incorporation reaction did not occur at these temperatures, it could be attributed to the external energy in this temperature range which was insufficient to allow the diffusion of PbO and Fe2O3 and thus did not generate PbFe12O19 (Tang et al., 2010). The PbFe12O19 phase was first detected at 850 °C, a value that is 100 °C higher than that observed for the sludge samples. In addition, PbFe12O19 was still the dominant phase in the sintered sample at 1100 °C, whereas, in the sludge system, this compound began to decompose at that temperature (Fig. 1a). These results indicated that the PbFe12O19 phase generated from the sludge may decompose earlier, and thus controlling the temperature to within a narrower range will be necessary to achieve better stabilization efficiency of lead when using this method in the future. To further investigate the differences between oxide and sludge samples, the peak intensities of PbFe12O19 at a 2h of around 34.43° were examined to observe changes over the temperature range from 750 to 1400 °C (Fig. 2). The peak intensities of the PbFe12O19 phase in the sintered sludge samples were greater than those of the sintered oxides samples at temperatures below 1050 °C. This result indicated that the reactivities of metals hydroxides were higher than those of oxides as was, and it was reported in previous studies (Little et al., 2008; Stefanescu et al., 2011; El-Sheikh and Rabbah, 2013). It seems reasonable to assume that the metal hydroxide was initially dehydrated and transformed into a metal oxide with a smaller particle size (Fig. 3b(1)). As solid state reaction is usually affected by both thermodynamic conditions and the diffusion process between reactant molecules (Kukukova et al., 2009), thus smaller particles are distributed more homogeneously than larger ones, and an increased reactant grain boundary would be obtained. The associated increased contact between reactants molecules would result in a lower phase transformation temperature in the sludge samples (Hsieh et al., 2007; Li et al., 2011a,b). A common means of removing heavy metals from wastewater is usually chemical precipitation and heavy metals in sludge are typically present as hydrates or carbonates, and thus this result means that using iron sludge as an additive when stabilizing lead sludge would allow the process to be carried out at lower temperatures. However, at temperatures greater than 1100 °C, the peaks intensity of PbFe12O19 in the sludge samples were lower than those in the oxide samples, suggesting that the decomposition rate of PbFe12O19 from the sludge samples may be faster than that obtained from the oxides samples. One interesting observation was that there were no PbO or Pb(OH)2 phases detected in all XRD patterns, even though the evaporation loss of PbO seldom exceeded 2% or 3% during thermal processing (Mountavala and Ravitz, 1962). This phenomenon has

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(a) Sludge Samples

-Fe2O3

-PbFe12O19

(b) Oxide Samples

-Fe2O3

-PbFe12O19

1400 1400

1250

Relative Intensity

Relative Intensity

1250

1100

1050 950

1100

1050

950

850

850

10

20

30

40

50

60

70

750

750

650

650

80

2θ (°)

10

20

30

40

50

60

70

80

2θ (°)

Fig. 1. XRD patterns of sintered samples over sintering temperatures range from 650 to 1400 °C for 3 h. (a) Sludge samples (Pb(OH)2 + iron sludge) and (b) oxide samples (PbO + Fe2O3).

3.2. Morphological structures analyses

Oxide Samples Sludge Samples

400 350

Intensity (a.u.)

300 250 200 150 100 50 750

850

950

1050

1100

1250

Sintering temperature (

1400

)

Fig. 2. Comparison of XRD peak intensity at 2h around 34.43° from products sintered at temperatures from 650 to 1400 °C for 3 h.

also been noted in other publications (Carp et al., 1997; Diop et al., 2010; Tan and Wang, 2011). Therefore, XPS analysis was carried out to study the conformational changes of the lead and iron atoms. To investigate the associated mechanism and rule out any effects of impurities in the sludge samples, the oxide samples were used for this study. The data indicate that, during thermal processing, lead is presented in all sintered samples at all temperatures before and after firing. Fig. SM-2 shows that, compared with the raw sample, the various Fe2p binding energy values are significantly shifted to the right (i.e., increased) after thermal treatment between 600 and 1400 °C, suggesting that the vacancy of the iron species has been slightly increased. It seems reasonable to assume that Fe(III) substitutes for Pb(II) and the extra vacancies are created on the oxygen sublattice to compensate for the valence changes of the cations. This shows that, to some extent, Fe(III) replaces Pb(II) to form a solution of Fe2O3 in PbO (Diop et al., 2010; Sahu et al., 2012). Since the solubility of Fe2O3 in PbO causes some lattice distortion, the lattice becomes more open and low potential energy locations are generated, which facilitate ion migration and promote the diffusion process. As a result, the solubility of Fe2O3 in PbO might facilitate the formation of PbFe12O19.

In general, the mechanical properties of a material and the leachability of lead from that material are at least partially determined by its microstructure (Xu et al., 2008, 2009; Zou et al., 2009). Fig. 3 shows the fracture surfaces of samples sintered at 750, 950, 1050 and 1250 °C. The surface of the sludge sample sintered at 750 °C shows a great deal of amorphous and porous texture (Fig. 3a(1)). Since an amorphous, porous texture with more contact area was beneficial to the diffusion process, this could explain why PbFe12O19 was formed in the sludge samples at 750 °C, i.e. at a temperature lower than that required for the sludge samples. As the sintering temperature was increased to 950 °C, amorphous and porous surfaces were fused and formed hexagonal crystals (hexaferrite is the typical PbFe12O19 crystal), indicating that lead was incorporated into PbFe12O19 crystals, based on the XRD pattern (Fig. 1b) (Tan and Wang, 2011). The size of these hexagonal crystals was about 10 nm and these observed phase change from SEM images were in accordance with the XRD results. At 1050 °C, the hexagonal crystals in the sludge samples melted slightly with a low level of homogeneity, and the molten portions agglomerated to form a more dense and more homogeneous structure with low roughness at 1250 °C. However, the products sintered at 1250 °C would likely not exhibit the best durability against acid attack due to the decomposition of PbFe12O19 suggested by the XRD results (Fig. 1a). The overall trends of microstructure changes in the sintered oxide samples were similar and also agreed with XRD patterns (Fig. 1b), although Fig. 3b(1) shows a large amount of tiny particles rather than the amorphous texture of the sludge samples. A well-crystallized lead hexaferrite form of PbFe12O19 was obtained from oxide samples at 950 and 1050 °C, the size and thickness of the hexagonal crystals both increased as the sintering temperature was elevated from 950 to 1050 °C. The resulting particle size was about 35 nm larger than that obtained with sludge samples (10 nm) at 1050 °C. The larger size and improved crystallinity of the PbFe12O19 obtained from oxide samples, together with the reduced specific surface area, could be the reason that the PbFe12O19 obtained from oxides samples remained stable and did not decompose at 1100 °C. The number of voids decreased as the crystal particles aggregated and the microstructure became more homogeneous, leading

L. Mao et al. / Chemosphere 117 (2014) 745–752

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Fig. 3. SEM images of sintered sludge samples at (a1) 750, (a2) 950, (a3) 1050, (a3) 1250 °C and sintered oxide samples at (b1) 750, (b2) 950, (b3) 1050, (b4) 1250 °C.

to shrinking of the apparent volume of the sintered samples (Xu et al., 2008, 2009). Fig. 4 presents the apparent shrinkage of samples sintered at temperatures ranging from 650 to 1400 °C. The percentage volume reduction increased with increases in sintering temperature, in agreement with the SEM results showing that amorphous particles gathered and aggregated to form a dense, homogeneous structure. It can be seen from Fig. 4 that the reduction percentage of the sludge samples was greater than that of the oxide samples, possibly due to aggregation of crystal particles and associated increases in the homogeneity of the microstructure, which was confirmed by the SEM images. For the sludge samples, the volume shrinkages were about 40% and 55% at 1050 and 1400 °C, respectively, bigger than 27% and 35% for the oxide sam-

ples. These data suggest that the volume of uncompressed sludge samples would be reduced by more than 40% after thermal treatment, a value that is consistent with the previous study (Cheeseman et al., 2003) that the volume shrinkage of sewage sludge incinerator ash after firing was about 60% in the absence of compression. This result indicates that the sludge volume could be reduced significantly after thermal treatment and thus this method represents a form of waste reduction technology. 3.3. Leaching performance of PbO and sintered sludge sample A useful means of evaluating the effects of lead immobilization after incorporation is to compare the leaching of the resulting sol-

750

L. Mao et al. / Chemosphere 117 (2014) 745–752 60

11

Oxide Samples Sludge Samples

(a)

PbO Sludge Sample

50

40 9 30

pH

Shrinkage (%)

10

20

8 3

10 2

0 600

700

800

900

24

1000 1100 1200 1300 1400 1500

Sintering temperature (

(b)

PbO Sludge Sample

) 22

2þ

PbOðsÞ þ 2Hþ ! PbðaqÞ þ H2 O

ð1Þ 2+

However, the concentration of lead ions in solution [Pb ] is also limited by potential precipitation/dissolution reactions, such as that involving Pb(OH)2(s) and shown in Eq. (2). 2þ

PbðOHÞ2ðsÞ ! PbðaqÞ þ 2OHðaqÞ

ð2Þ

-1

20 Concentration (mg L-1)

ids in a prolonged test (Lu and Shih, 2011; Lu et al., 2013). This study therefore performed leaching experiments with both PbO and sintered sludge samples (Pb(OH)2 and iron sludge). The asreceived PbO powder and a sintered sludge sample were used for these leaching test; the sintered sludge sample was made by firing a raw sludge sample at 1050 °C for 3 h. The XRD pattern of this material confirmed that PbFe12O19 was the major phase in the sample (Fig. 1a), and sintered sludge sample was ground into powder before the leaching test. Fig. 5a presents the pH values obtained from analysis of PbO and sintered sludge leachates throughout the leaching period. Within the first 18 h, the pH of the PbO leachate increased quickly from 2.9 to 9.3, after which it remained at approximately 9.5 throughout the rest of the test. In contrast, the pH of the sludge sample leachate was constant at the initial value of 3.1 and remained relatively stable during the entire leaching procedure. The increase in leachate pH is likely due to the dissolution of crystal cations through ion exchange with protons in the solution, accompanied by the decomposition of the Pb-containing crystal framework at the sample surface by the acidic fluid. These results suggest that PbFe12O19 exhibits higher inherent resistance to acidic attack and that PbO may be much more vulnerable to acidic environments. The lead concentrations in the sludge sample leachates (Fig. 5b) were lower than those in the PbO leachates and consistent with the trend observed in the leachate pH values. The lead concentration in the PbO leachate was 22.3 g L1 at the end of 24 d of leaching, a value that was significantly higher than that obtained from the sludge sample (0.5 mg L1, see the inset of Fig. 5b). This result further confirmed that the PbFe12O19 phase presented very high stability in an acidic environment compared with the PbO phase. To further investigate the leaching behavior of the PbO and the sludge sample, the potential reprecipitation and surface modification processes which could determine the leachability of the materials were examined. As a general assumption, the cation–proton exchange mechanism associated with the destruction of lead oxide by acidic attack in the solution can be expressed as in Eq. (1).

Concentration (g L )

Fig. 4. The volume shrinkage of oxides and sludge samples before and after sintering.

18 16

2

Sludge Sample

0.6 0.5

0.0 0

5

10 15 20 Time (d)

25

0 0

5

10

15

20

25

Time (d) Fig. 5. Leachate pH values (a) and the concentration of lead (b) leached from PbO and sludge sample. The inset diagram in (b) further provides the details of the concentration of lead in the sludge sample leachates.

The Ksp value associated with Eq. (2) is 1.6  1017 (Al-Degs et al., 2001). At pH  9.5, the product of [Pb2+] and [OH] in the PbO leachate based on a Pb concentration of 22.31 g L1 (0.108 M) was about 1.08  1010 and thus larger than the value of Ksp, suggesting that Pb2+ was not only lead species in leachate. One possible reason for this results is that Pb2+ reacted with CH3COO to form [Pb(CH3COO)3] (Chen, 2007) as shown below. 2þ



PbðaqÞ þ 3CH3 COOðaqÞ ! ½PbðCH3 COO Þ3 ðaqÞ

ð3Þ

It is clear that the lead concentration of the PbO leachates was not limited by Ksp, indicating that the PbO phase was not an ideal structure and is not sustainable in acidic environments. In contrast, when leaching the sludge sample, the congruent dissolution through the cation–proton exchange reaction can be expressed as follows.

PbFe12 O19 þ 38Hþ ! Pb

2þ

þ 12Fe3þ þ 19H2 O

ð4Þ

The Pb concentration determined in the PbFe12O19 leachates was 0.5 mg L1 (2.4  103 M) at pH 3.15, thus the product of  2 25 [Pb2+ , which indicates considerable (aq)] and [OH( aq)] was 4.8  10 undersaturation of Pb with respect to Pb(OH)2(s) as a direct result of the solid matrix controlling the release of Pb. In addition, the iron concentration measured in the PbFe12O19 leachates was very low (less than 0.03 M), either because of the formation of amorphous Fe(OH)3(s) at pH values above 2.82 (Muehe et al., 2013) or because the PbFe12O19 phase exhibited incongruent dissolution behavior (Cailleteau et al., 2008; Ohlin et al., 2010). As such, the majority of Fe–O bonds likely remain intact on the sample surfaces and the overall result indicates the existence of an Fe-rich layer on the leached sample surface, which is beneficial in terms of preventing the release of lead and could increase product durability. In summary, the PbFe12O19 phase was found to represent a stable means of immobi-

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lizing the hazardous metal lead, and the prolonged leaching tests demonstrated that the sintered sludge sample exhibits excellent resistance in an acidic environment. 4. Conclusion Lead sludge stabilization via the formation of a PbFe12O19 phase by thermal treatment with iron sludge was investigated in this study. Based on the results of XRD and SEM analyses of samples sintered at different temperatures, we may conclude that the sintering temperature had a major impact on phase transformation and microstructure changes, and that well-crystallized PbFe12O19 can be obtained at temperatures in the vicinity of 1050 °C. Above 1100 °C, PbFe12O19 began to decompose, with the concurrent reappearance of Fe2O3. The apparent volume of the compressed sludge samples was significantly altered by thermal treatment; at 1050 °C, the volume shrinkage was 40% relative to the original raw compressed lead sludge samples. TCLP tests showed the superiority of the PbFe12O19 phase over PbO with respect to its ability to resist acidic attack. Therefore, the results of this study suggest that incorporating lead sludge into a PbFe12O19 phase at an optimal processing temperature offers a promising strategy for reducing the volume, mobility and toxicity of lead sludge. Acknowledgments The authors gracefully acknowledge financial supports from the Foundation of State Key Laboratory of Pollution Control and Resource Reuse of China, the Natural Science Foundation of China (No. 51308282), Jiangsu science and technology support program social development project (No. BE2013703), and the Research Projects on Environmental Protection of Jiangsu Province (2013008). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2014.08.027. References Al-Degs, Y., Khraisheh, M.A.M., Tutunji, M.F., 2001. Sorption of lead ions on diatomite and manganese oxides modified diatomite. Water Res. 35, 3724– 3728. Cailleteau, C., Angeli, F., Devreux, F., Gin, S., Jestin, J., Jolliiivet, P., Spalla, O., 2008. Insight into silicate–glass corrosion mechanism. Nat. Mater. 7, 978–983. Carp, O., Segal, E., Brezeanu, M., Barjega, R., Stanica, N., 1997. Nonconventional methods for obtaining hexaferrites. J. Therm. Anal. Calorim. 50, 125–135. Cheeseman, C.R., Sollars, C.J., McEntee, S., 2003. Properties, microstructure and leaching of sintered sewage sludge ash. Resour. Conserv. Recycl. 40, 13–25. Chen, J.L., 2007. Basic Inorganic Chemistry. Chemical Industry Press, Beijing (in Chinese). Chen, C.L., Lo, S.L., Kuan, W.H., Hsieh, C.H., 2009. Stabilization of coppercontaminated sludge using the microwave sintering. J. Hazard. Mater. 168, 857–861. Diop, I., David, N., Fiorani, J.M., Podor, R., Vilasi, M., 2010. Experimental investigations and thermodynamic description of the PbO–Fe2O3 system. Thermochim. Acta 510, 202–212. El-Sheikh, S.M., Rabbah, M., 2013. Novel low temperature synthesis of spinel nanomagnesium chromites from secondary resources. Thermochim. Acta 568, 13– 19. Gupta, V.K., Suhas, 2009. Application of low-cost adsorbents for dye removal – a review. J. Environ. Manage. 90, 2313–2342. Hsieh, C.H., Lo, S.L., Chiueh, P.T., Kuan, W.H., Chen, C.L., 2007. Microwave enhanced stabilization of heavy metal sludge. J. Hazard. Mater. 139, 160–166. Jonker, H.D., 1975. Investigation of the phase diagram of the system PbO–B2O3– Fe2O3–Y2O3 for the growth of single crystals of Y3Fe5O12. J. Cryst. Growth. 28, 231–239. Kishimoto, N., Kitamura, T., Kato, M., Otsu, H., 2013. Reusability of iron sludge as an iron source for the electrochemical Fenton-type process using Fe2+/HOCl system. Water Res. 47, 1919–1927.

751

Kukukova, A., Aubin, J., Kresta, S.M., 2009. A new definition of mixing and segregation: three dimensions of a key process variable. Chem. Eng. Res. Des. 87, 633–647. Li, C.W., Chen, Y.M., Chiou, Y.C., Liu, C.K., 2007. Dye wastewater treated by Fenton process with ferrous ions electrolytically generated from iron-containing sludge. J. Hazard. Mater. 144, 570–576. Li, N.H., Chen, Y.H., Hu, C.Y., Hsieh, C.H., Ching, H.H., Shang, L.L., 2011a. Stabilization of nickel-laden sludge by a high-temperature NiCr2O4 synthesis process. J. Hazard. Mater. 198, 356–361. Li, N.H., Lo, S.L., Hu, C.Y., Hsieh, C.H., Chen, C.L., 2011b. Stabilization and phase transformation of CuFe2O4 sintered from simulated copper-laden sludge. J. Hazard. Mater. 190, 597–603. Little, M.R., Adell, V., Boccaccini, A.R., Cheeseman, C.R., 2008. Production of novel materials from coal fly ash and metal finishing wastes. Resour. Conserv. Recycl. 52, 1329–1335. Liu, H., Yang, J., Shi, Y., He, S., Yang, C., Yao, H., 2012. Conditioning of sewage sludge by Fenton’s reagent combined with skeleton builders. Chemosphere 88, 235– 239. Lu, X., Shih, K., 2011. Phase transformation and its role in stabilizing simulated leadladen sludge in aluminum-rich ceramics. Water Res. 45, 5123–5129. Lu, H.C., Chang, J.E., Shih, P.H., Chiang, L.C., 2008. Stabilization of copper sludge by high-temperature CuFe2O4 synthesis process. J. Hazard. Mater. 150, 504–509. Lu, X., Shih, K., Cheng, H., 2013. Lead glass–ceramics produced from the beneficial use of waterworks sludge. Water Res. 47, 356–361. Mexmain, J., Hivert, S.L., 1978. Prepartion and characterization of lead ferrites. Ann. Chim. Fr. 3, 91–97. Mountavala, A.J., Ravitz, S.F., 1962. Phase relations and structures in the system PbO–Fe2O3. J. Am. Ceram. Soc. 45, 285–288. Muehe, E.M., Scheer, L., Daus, B., Kappler, A., 2013. Fate of arsenic during microbial reduction of biogenic versus abiogenic As–Fe (III)–mineral coprecipitates. Environ. Sci. Technol. 47, 8297–8307. Nagib, S., Inoue, K., 2000. Recovery of lead and zinc from fly ash generated from municipal incineration plants by means of acid and/or alkaline leaching. Hydrometallurgy 56, 269–292. Ndiba, P., Axe, L., Boonfueng, T., 2008. Heavy metal immobilization through phosphate and thermal treatment of dredged sediments. Environ. Sci. Technol. 42, 920–926. Nevirva, M., Fisher, K., 1986. Contribution to the binary phase diagram of the system PbO–Fe2O3. Mater. Res. Bull. 21, 1285–1290. Ohlin, C.A., Villa, E.M., Rustad, J.R., Casey, W.H., 2010. Dissolution of insulating oxide materials at the molecular scale. Nat. Mater. 9, 11–19. Rijay, V., Sihorwala, T.A., 2003. Identification and leaching characteristics of sludge generated from metal pickling and electroplating industries by Toxicity Characteristics Leaching Procedure (TCLP). Environ. Monit. Assess. 84, 193–202. Rivolier, J.L., Ferriol, M., Abraham, R., Cohenadad, M.T., 1993. Study of The PbO– Fe2O3 system. Eur. J. Solid State Inorg. Chem. 30, 727–739. Sahu, S.K., Ganesan, R., Gnanasekaran, T., 2012. Studies on the phase digram of Pb– Fe–O system and standard molar Gibbs energy of formation of PbFe5O8.5 and Pb2Fe2O5. J. Nucl. Mater. 426, 214–222. Shih, K., Leckie, J.O., 2007. Nickel aluminate spinel formation during sintering of simulated Ni-laden sludge and kaolinite. J. Eur. Ceram. Soc. 27, 91–99. Shih, K., Tang, Y., 2011. Prolonged toxicity characteristic leaching procedure for nickel and copper aluminates. J. Environ. Monitor. 13, 829–835. Shih, K., Tang, Y., 2012. Incorporating simulated zinc ash by kaolinite- and sludgebased ceramics: phase transformation and product leachability. Chin. J. Chem. Eng. 20, 411–416. Shih, K., White, T., Leckie, J.O., 2006a. Nickel stabilization efficiency of aluminate and ferrite spinels and their leaching behavior. Environ. Sci. Technol. 40, 5520– 5526. Shih, K., White, T., Leckie, J.O., 2006b. Spinel formation for stabilizing simulated nickel-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 40, 5077–5083. Stefanescu, M., Barbu, M., Vlase, T., Barvinschi, P., Barbu-Tudoran, L., Stoia, M., 2011. Novel low temperature synthesis method for nanocrystalline zinc and magnesium chromites. Thermochim. Acta 526, 130–136. Tan, G.L., Wang, M., 2011. Multiferroic PbFe12O19 ceramics. J. Electroceram. 26, 170– 174. Tang, Y., Shih, K., 2013. Stabilization mechanisms and reaction sequences for sintering simulated copper-laden sludge with alumina. ACS Sust. Chem. Eng. 1, 1239–1245. Tang, Y., Shih, K., Chan, K., 2010. Copper aluminate spinel in the stabilization and detoxification of simulated copper-laden sludge. Chemosphere 80, 375–380. Tang, Y.Y., Chui, S.S.Y., Shih, K., Zhang, L.R., 2011a. Copper stabilization via spinel formation during the sintering of simulated copper-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 45, 3598–3604. Tang, Y.Y., Shih, K., Wang, Y., Chong, T., 2011b. Zinc stabilization efficiency of aluminate spinel structure and its leaching behavior. Environ. Sci. Technol. 45, 10544–10550. Tang, Y., Lee, P.H., Shih, K., 2013. Copper sludge from printed circuit board production/recycling for ceramic materials: a quantitative analysis of copper transformation and immobilization. Environ. Sci. Technol. 47, 8609–8615. USEPA., 1998. U.S. EPA Method 1311-Toxicity Characteristic Leaching Procedure to Mineral Processing Wastes and its Salts. .

752

L. Mao et al. / Chemosphere 117 (2014) 745–752

Veit, H.M., Bernardes, A.M., Ferreira, J.Z., Tenorio, J.A.S., Malfatti, C.F., 2006. Recovery of copper from printed circuit boards scraps by mechanical processing and electrometallurgy. J. Hazard. Mater. 137, 1704–1709. Wang, S., Vipulanandan, C., 2000. Solidification/stabilization of Cr (VI) with cement: leachability and XRD analyses. Cem. Concr. Res. 30, 385–389. Wu, C., Zhang, H., He, P., Shao, L., 2010. Thermal stabilization of chromium slag by sewage sludge: effects of sludge quantity and temperature. J. Environ. Sci. 22, 1110–1115. Xu, G.R., Zou, J., Li, G., 2008. Ceramsite made with water and wastewater sludge and its characteristics affected by SiO2 and Al2O3. Environ. Sci. Technol. 42, 7417– 7423. Xu, G.R., Zou, J., Li, G., 2009. Stabilization/solidification of heavy metals in sludge ceramsite and leachability affected by oxide substances. Environ. Sci. Technol. 43, 5902–5907.

Yang, J., Mosby, D.E., Casteel, S.W., Blanchar, R.W., 2001. Lead immobilization using phosphoric acid in a smelter-contaminated urban soil. Environ. Sci. Technol. 35, 3553–3559. Yang, N., Yang, H., Jia, J., Pan, X., 2007. Formation and magnetic properties of nanosized PbFe12O19 particles synthesized by citrate precursor technique. J. Alloy. Compd. 438, 263–267. Yin, C.Y., Mahmud, H.B., Shaaban, M.G., 2006. Stabilization/solidification of leadcontaminated soil using cement and rice husk ash. J. Hazard. Mater. 137, 1758– 1764. Zou, J.L., Xu, G.R., Li, G.B., 2009. Ceramsite obtained from water and wastewater sludge and its characteristics affected by Fe2O3, CaO and MgO. J. Hazard. Mater. 165, 995–1001.