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Situational predictors of violent intentions: Results from a factorial survey
Chemosphere 138 (2015) 156–163
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Formation of lead-aluminate ceramics: Reaction mechanisms in immobilizing the simulated lead sludge Xingwen Lu, Kaimin Shih ⇑ Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
h i g h l i g h t s Formation of PbAl2O4 and PbAl12O19 for stabilizing Pb in different Pb/Al systems. Pb9Al8O21 and Pb3(CO3)2(OH)2 were detected as intermediate phases during sintering. Pb incorporation efficiency into PbAl2O4 and PbAl12O19 was quantitatively evaluated. Incorporating Pb into PbAl12O19 crystal is a preferred stabilization mechanism.
a r t i c l e
i n f o
Article history: Received 5 December 2014 Received in revised form 16 May 2015 Accepted 27 May 2015
Keywords: Sludge Lead Alumina Stabilization Leaching behavior
a b s t r a c t We investigated a strategy of blending lead-laden sludge and an aluminum-rich precursor to reduce the release of hazardous lead from the stabilized end products. To quantify lead transformation and determine its incorporation behavior, PbO was used to simulate the lead-laden sludge fired with c-Al2O3 by Pb/Al molar ratios of 1/2 and 1/12 at 600–1000 °C for 0.25–10 h. The sintered products were identified and quantified using Rietveld refinement analysis of X-ray diffraction data from the products generated under different conditions. The results indicated that the different crystallochemical incorporations of hazardous lead occurred through the formation of PbAl2O4 and PbAl12O19 in systems with Pb/Al ratios of 1/2 and 1/12, respectively. PbAl2O4 was observed as the only product phase at temperature of 950 °C for 3 h heating in Pb/Al of 1/2 system. For Pb/Al of 1/12 system, significant growth of the PbAl12O19 phase clearly occurred at 1000 °C for 3 h sintering. Different product microstructures were found in the sintered products between the systems with the Pb/Al ratios 1/2 and 1/12. The leaching performances of the PbO, Pb9Al8O21, PbAl2O4 and PbAl12O19 phases were compared using a constant pH 4.9 leaching test over 92 h. The leachability data indicated that the incorporation of lead into PbAl12O19 crystal is a preferred stabilization mechanism in aluminate-ceramics. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lead (Pb) is a type of heavy metal widely used in industry due to its versatile physical and chemical characteristics. The International Lead and Zinc Study Group estimated that global annual lead consumption approached 9 million tons in 2010. Serious environmental concerns have been raised about the release of lead into the environment from industries such as the production of lead batteries, oil-based paints, pigments, paper, pulp, and electrochemical electrodes, as well as mining, plating, hot dip galvanizing, and petroleum refining (Jalali et al., 2002; Gupta et al., 2001; Conrad and Bruun Hansen, 2007). Lead is a highly toxic and non-biodegradable metal that tends to accumulate in the cells ⇑ Corresponding author. E-mail address: [email protected] (K. Shih). http://dx.doi.org/10.1016/j.chemosphere.2015.05.090 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
of living organisms, causing severe damage to the kidneys, liver, and nervous and reproductive systems in humans (Gupta et al., 2011). The remediation of hazardous lead sludge has become a pressing challenge in recent years due to more stringent environmental regulations. Ceramic technology is a technique to create ceramic products from inorganic, non-metallic materials by the action of heat and subsequent cooling (Vincenzini, 1991). Ceramics are either formed from a molten mass that solidifies on cooling, formed and matured by the action of heat, or chemically synthesized at low temperatures using, for example, hydrothermal or sol–gel synthesis (Richerson, 1982). Ceramic technology is considered useful for rendering hazardous metal sludge inert because they can destroy organic matters and immobilize hazardous metals in a stable matrix to reduce metal leachability (Xu et al., 2008, 2009). As ceramic technology involved in the crystallization and/or vitrification
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of ceramic materials (Rawlings et al., 2006), this technology functions by binding hazardous metal ions into the framework of glass or the crystallization phases of ceramic products (Cheng, 2004; Tian et al., 2011). Previous studies have described mechanisms for stabilizing nickel and copper into aluminum-rich ceramic matrices and achieving reduced metal leachability through the intrinsic properties of spinel structures (Shih et al., 2006a,b; Tang et al., 2010; Hu et al., 2010). Several studies have reported the potential formation of lead aluminate (PbAl2O4) and a magnetoplumbite-like structure (PbAl12O19) using c-Al2O3 as an inexpensive ceramic precursor to treat PbO (Wendt et al., 1988; Park and Chang, 1993). Although the recently published PbO–Al2O3–SiO2 equilibrium phase diagram confirmed the presence of PbAl12O19 and PbAl2O4 (Chen et al., 2001), the reaction sequences involved in the incorporation of lead at different temperatures and times are still unclear. As the dwelling time of the ceramic sintering process can vary from a few minutes to several hours (Völtzke and Abicht, 2001; Aksel et al., 2005), the dominant mechanism(s) for hosting lead in crystal structures under different thermal conditions needs to be unambiguously identified, if not also quantitatively evaluated. The objective of this work was to contribute a better understanding of the phase transformations to PbAl12O19 and PbAl2O4 during ceramic sintering, which potentially play a role in lead incorporation mechanisms in aluminum-rich ceramics. The dominant reaction mechanisms at different sintering periods were determined quantitatively, together with the influence of the sintering temperature and time. Furthermore, the stabilization effects of potential lead phases in the products were evaluated via leaching tests at a constant pH value. The obtained leachability of each phase was further normalized using the sample surface area and lead content to reflect the intrinsic lead leachability and suggest a more reliable mechanism to host the hazardous lead.
2. Materials and methods PbO powder was purchased from Sigma–Aldrich. The phase composition of the PbO was identified using X-ray diffraction (XRD) for a mixture of a-PbO (litharge) and b-PbO (massicot) phases. The c-Al2O3 was prepared from PURAL SB powder with an average particle size of 45 lm fabricated by Sasol. The phase of the PURAL SB powder was identified using XRD for boehmite (AlOOH; ICDD PDF # 74-1875) and it was successfully converted to the c-Al2O3 phase after heat treatment at 650 °C for 3 h (Zhou and Snyder, 1991; Wang et al., 2005). The phase confirmation for the two materials was demonstrated by their XRD patterns, which are provided in Fig. S1 (Supporting Information). As lead usually exist as lead(II) in wastewater (Acharya et al., 2009; Deng et al., 2007; Singh et al., 2008), the treatment of lead-containing wastewater by current common techniques (i.e. chemical precipitation, ion exchange, coagulation, adsorption, and membrane processes) produce enormous amounts of lead-containing sludge that mainly composed of some insoluble lead(II) compounds (i.e. lead(II) carbonate, lead(II) oxide, lead(II) hydroxide and lead(II) sulfate) (Mao et al., 2014; Volpe et al., 2009; Yao and Naruse, 2009). However, the major portion of lead may converted into lead oxide at the high sintering temperatures. For this reason, the lead incorporation experiments were conducted using PbO to simulate the lead in sludge under sintering condition (Yao and Naruse, 2005). The c-Al2O3 precursor and PbO were mixed by ball milling in water slurry at Pb/Al molar ratios of 1/2 and 1/12 for 18 h. The slurry samples were then dried and homogenized by mortar grinding. The derived powder was pressed into 20 mm pellets at 650 MPa to ensure consistent compaction of the powder sample for the sintering process. The pellets were
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sintered at specified temperatures from 600 to 1000 °C for 0.25–10 h, and then quenched in air to room temperature. The total mass loss after sintering was less than 1 wt.%. After thermal treatment, the pellets were weighed and ground into powder with a particle size of less than 10 lm for XRD analysis. Some of the pellets were reserved for scanning electron microscopic (SEM) characterization and were polished using a submicrometer diamond lapping film and gold coated to mitigate the electron charging effect. All of the SEM investigations were performed using a Hitachi S-4800 SEM system equipped with a secondary electron detector to obtain morphological information and a backscattered electron detector for energy dispersive spectroscopy (EDS). The X-ray powder diffraction data of the samples were collected using a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu Ka radiation detector and a LynxEye detector. The diffractometer was operated at 40 kV and 40 mA, and the 2h scan range was from 10° to 80°, with a step size of 0.02° and a scan speed of 0.3 s/step. Qualitative phase identification was performed using Eva XRD Pattern Processing software (Bruker Co. Ltd.) by matching the powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF-2 Release 2008). The Rietveld refinements for quantitative analysis of the phase compositions were processed using the TOPAS (version 4.0) crystallographic program. Figs. S2–S6 (Supporting Information) present the Rietveld refinement plots for the products sintered from the PbO/c-Al2O3 system. The derived reliability values for the refinement quality of this analytic scheme are provided in Tables S1–S3 (Supporting Information). For systems potentially containing amorphous or poorly crystalline phases, a refinement method using CaF2 as the internal standard (De La Torre et al., 2001; Magallanes-Perdomo et al., 2009; Rendtorff et al., 2010) was used to quantify the amorphous content in the samples. Quantitative analysis data were collected from 10° to 120° 2h-angle, with a step width of 2h = 0.02° and a sampling time of 0.5 s per step. Before performing the leaching test, the surface areas of the single-phase powders were measured by a Beckman Coulter SA3100 Surface Area and Pore Size Analyzer using the BET method after degassing by heating at 300 °C for 3 h with He-gas purging. To distinguish the role and characteristic of each lead stabilization mechanism, the potential lead-bearing product phases were compared using the leaching test. The powder (<45 lm) samples of the target phases were subjected to batch leaching in amorphous silica vessels at a liquid/solid ratio of 50 ml/g (4 g powder sample with 200 ml of pH 4.9 acetic acid solution) for a total testing time of 92 h. Three duplicate experiments were conducted to ensure the reproducibility of the leaching test. Acid (1 N HNO3) was added into each suspension to maintain a constant pH of 4.9 using a pH-stat system. Continuous stirring was conducted to keep the mixture homogeneous and the leaching vessel was held at room temperature (23 ± 0.5 °C). At different times, the suspensions were allowed to settle for 5 min and aliquots of 5 ml of the upper liquid were taken and centrifuged. Another 5 ml of pH 4.9 acetic acid solution, together with the filtered solids, was added to the vessels. At the end of each agitation period, the leachates were filtrated using 0.2 lm syringe filters and the concentrations of lead in the leachates were analytically determined using ICP-AES (Perkin–Elmer Optima 8300 DV).
3. Results and discussion 3.1. Lead incorporation into PbAl2O4 structure in Pb/Al 1/2 system By adding 25 wt.% CaF2 as an internal standard, the quantities of crystalline and amorphous phases in the PbO/c-Al2O3 mixtures
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with 1/2 Pb/Al molar ratio sintered at 600–1000 °C for 3 h were revealed by Rietveld refinements for the XRD patterns, as shown in Fig. 1. Table 1 summarized the quality of the Rietveld refinement analysis for quantifying the phase compositions of sintered PbO/c-Al2O3 (Pb/Al = 1/2) sample at 600–1000 °C for 3 h. The values indicate the pattern factors and the goodness-of-fit (Rp, Rwp, Rexp, and GOF). Weight fractions for the crystalline and amorphous components in the products were obtained and given in Fig. 2. In the 1/2 Pb/Al system, the PbAl2O4 phase was first generated at 700 °C with a mass content of around 12 wt.%. Increasing the sintering temperature to 750 °C led to significant PbAl2O4 development, with its phase fraction reaching 81 wt.% after the 3 h sintering. With the continuous increase in PbAl2O4, the signal of the PbO reactant eventually diminished when the temperature exceeded 800 °C. The proportion of the PbAl2O4 phase increased continuously in samples sintered at higher temperatures (800–950 °C) and reached its maximum value (near-complete reaction) at 950 and 1000 °C. Therefore, the thermal reactions for the formation of PbAl2O4 from PbO and c-Al2O3 can be expressed by the following equation.
Table 1 Quality of the Rietveld refinement analyses for quantifying the phase compositions of sintered PbO/c-Al2O3 (Pb/Al = 1/2) samples at 600–1000 °C for 3 h (Fig. 1). The values indicate the pattern factors and the goodness-of-fit (Rp, Rwp, Rexp, and GOF). Temperature (°C)
600 700 750 800 850 900 950 1000
Quality of refinement analysis RP (%)
Rwp (%)
Rexp (%)
GOF
6.03 7.83 4.20 4.95 3.83 4.58 4.49 5.52
8.00 10.35 5.42 6.56 5.15 5.92 5.75 7.26
5.13 6.36 3.74 4.16 3.15 3.70 3.69 4.66
1.56 1.63 1.45 1.58 1.63 1.60 1.56 1.56
PbO þ c-Al2 O3 ! PbAl2 O4
ð1Þ
However, an intermediate phase, Pb9Al8O21, also appeared in the reaction, as shown in Fig. 2. To further quantify the variation in lead incorporation efficiency, the mass distribution of this intermediate phase was also evaluated. The phase content of Pb9Al8O21
Fig. 1. Results of Rietveld refinement for PbO/c-Al2O3 (Pb/Al = 1/2) samples sintered at 600–1000 °C for 3 h (data lines = experimental XRD patterns at different temperatures, red lines = calculated XRD patterns, and gray lines = difference curves between the experimental and calculated patterns). Vertical bars indicate the Bragg positions of the corresponding phases. The goodness-of-fit (chi2) is provided in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Quantitative comparison of phase compositions in the products of sintering PbO/c-Al2O3 samples (1/2 Pb/Al molar ratio) at temperatures between 600 and 1000 °C for 3 h. Crystalline PbAl2O4 was observed to be the dominant product at high temperatures.
increased for temperatures ranging from 700 to 800 °C, reaching its maximum of 12.5 wt.%. The intermediate Pb9Al8O21 disappeared at 950 °C, indicating near-total incorporation of the lead into the PbAl2O4 structure in this system. The formation of Pb9Al8O21 during the sintering process was probably initiated by the reaction between PbO and c-Al2O3 in a non-equilibrium system, due to the insufficient sintering time in the lower temperature range. The efficiency of a homogeneous reaction generally depends on the encountering rate between the reactant molecules (Kukukova et al., 2009). The short sintering time and low temperature were not able to provide sufficient contact. Hence the formation of PbAl2O4 was incomplete and an intermediate compound appeared in the product (Lu and Shih, 2011). In contrast, higher temperatures enabled a more intensive interaction between reactants, allowing the enhanced molecular transport to overcome the diffusion barrier of the products and preventing the intermediate from appearing in the 950 °C sintered samples. To determine the details of the lead transformation process in the early stage of sintering the 1/2 Pb/Al system, PbO and c-Al2O3 were sintered at the low temperatures of 700 and 850 °C for 0.25–10 h. Rietveld refinement was applied to the XRD patterns of the sintered samples to quantitatively determine the lead transformation efficiency. The weight percentages of the crystalline and amorphous phases in the samples of the PbO/c-Al2O3 system are summarized in Fig. 3. Fig. 3(a) indicates that almost no reaction occurred between PbO and c-Al2O3 at 700 °C when the treatment time was less than 3 h. With increased thermal treatment time, PbAl2O4 and Pb9Al8O21 co-existed in the products, and Pb9Al8O21 even became the predominant lead-hosting phase, at around 70 wt.% in the product after the 10 h sintering process. In contrast, the PbAl2O4 phase content remained around 10 wt.% even when the thermal treatment time was increased from 3 to 10 h. The crossover of the PbO and Pb9Al8O21 phase content curves at 700 °C suggested an incomplete interaction between PbO and c-Al2O3. As more intensive interaction between reactants occurred at 800 °C, after 0.25 h sintering, the PbAl2O4 phase held as much as 80 wt.% in the product of the 1/2 Pb/Al system (Fig. 3(b)). The prominence of the Pb9Al8O21 phase then decreased with the increase in PbAl2O4 at longer sintering times. This finding may indicate that higher temperatures can greatly enhance lead incorporation into the PbAl2O4 structure in the 1/2 Pb/Al system, due to the provision of energy to overcome the diffusion barrier. However,
Fig. 3. Quantitative comparison of phase compositions in the products of sintering PbO/c-Al2O3 samples (1/2 Pb/Al molar ratio) at temperatures of (a) 700 and (b) 850 °C for 0.25–10 h.
despite the prolonged thermal treatment time (10 h), the system did not achieve the complete transformation of lead into PbAl2O4. Therefore, to more effectively facilitate the formation of PbAl2O4, the temperature of a short sintering scheme should be higher than 800 °C. According to the XRD results obtained here, with higher temperatures (950–1000 °C) and longer treatment times, the formation of PbAl2O4 phase was controlled by the equilibrium phase diagram. The reaction between the PbO and c-Al2O3 reactants could reach equilibrium under such treatment conditions. However, with lower temperatures and a shorter dwelling time, the formation of the intermediate Pb9Al8O21 species was determined by molecular kinetic and grain boundary reactions, indicating that the entire system was in non-equilibrium. The decrease of Pb9Al8O21 at higher temperatures (>850 °C) suggested that the higher temperature enhanced the rate of molecular diffusion of the reactants.
3.2. Lead incorporation into PbAl12O19 structure in the 1/12 Pb/Al system PbAl12O19 was the only product phase reported in the PbO– Al2O3 equilibrium experiment when the PbO molar content was lower than 50% (Kuxmann and Fischer, 1974; Chen et al., 2001). A mixture of PbO and c-Al2O3 with a lower Pb/Al molar ratio of 1/12 was sintered to investigate the incorporation mechanism behind PbAl12O19 formation. Fig. 4(a) shows the variations in
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crystalline and amorphous phases produced by sintering the 1/12 Pb/Al molar ratio PbO/c-Al2O3 mixture at 600–1000 °C. The curves show that approximately 85% of the products were composed of amorphous phase(s) at temperatures of 700–950 °C. Such a large proportion occurred in the amorphous phase because of the poorly crystalline c-Al2O3 reactant, together with the formation of a poorly crystalline intermediate. Approximately 15% hydrocerussite (Pb3(CO3)2(OH)2), which was an intermediate phase during the sintering, was also measured in the products at temperatures of 700–950 °C. Intermediate Pb3(CO3)2(OH)2 has been
identified as a common lead corrosion product in existing studies (Kim et al., 2011; Kim and Herrera, 2010). Thus, the formation of the Pb3(CO3)2(OH)2 phase was probably due to the instability of the product phase(s) sintered at 700–950 °C, which may be vulnerable to the attack of CO2 and moisture in the air during the sample quenching stage. Nevertheless, the weight percentage of the PbAl12O19 exceeded 10% at 950 °C and increased to near-total incorporation at 1000 °C. A potential thermal reaction for the incorporation of lead by the c-Al2O3 precursor at the lower lead level is described by Eq. (2).
PbO þ 6 c-Al2 O3 ! PbAl12 O19
ð2Þ
The influence of thermal treatment time on the crystal growth of PbAl12O19 was quantitatively determined at temperatures of 950 and 1000 °C by examining the reactions of 1/12 Pb/Al molar ratio samples (Fig. 4(b) and (c)). At 950 °C, the phase content of PbAl12O19 increased from 5% in the first 1 h to around 35% after 10 h. As the unstable thermal treatment products were easily attacked by CO2 and moisture with the formation of hydrocerussite at 950 °C, 10 wt.% of Pb3(CO3)2(OH)2 was generally found in the sintered products. With the first observation of PbAl12O19 at 1000 °C for 0.5 h, about 8 wt.% of the PbAl12O19 phase was found in the product, together with 12 wt.% of the Pb3(CO3)2(OH)2 phase. However, the PbAl12O19 phase grew significantly with the increase in reaction time from 1 to 3 h. Fig. 4(c) illustrates the increase in PbAl12O19 crystal, together with the corresponding decrease in the Pb3(CO3)2(OH)2 and amorphous phases after the extended sintering time at 1000 °C. Longer sintering times and higher temperatures may therefore greatly enhance interaction between the reactants, allowing the complete transformation of lead into PbAl12O19. After 3 h sintering at 1000 °C, the conversion to the PbAl12O19 phase completely eliminated the formation of Pb3(CO3)2(OH)2, and PbAl12O19 was found to be the only lead-containing phase in the product. This result confirms the potential of forming PbAl12O19 to incorporate lead into aluminum-rich ceramics in systems with lower Pb/Al ratios. 3.3. Microstructures of PbO/c-Al2O3 products with 1/2 and 1/12 Pb/Al molar ratios
Fig. 4. Variation in the weight fractions of the crystalline and amorphous phases obtained from the sintering of PbO with c-Al2O3 in a 1/12 Pb/Al molar ratio (a) at temperatures from 700 to 1000 °C for 3 h, and (b) at temperatures of 950 and 1000 °C for 0.25–10 h.
The different Pb/Al molar ratios used in the PbO/Al2O3 system were found to generate lead aluminate products with distinct microstructures. Fig. 5 compares the microstructures of the samples in the 1/2 Pb/Al molar ratio PbO/Al2O3 system sintered for 3 h at 850 °C (a) and 950 °C (b), and the 1/12 Pb/Al molar ratio PbO/Al2O3 system sintered for 3 h at 950 °C (c) and 1000 °C (d). EDS (Figs. S7–10, Supporting Information) was used to confirm the chemical compositions of the samples produced in the 1/2 and 1/12 Pb/Al molar ratio PbO/Al2O3 systems. For example, Fig. 5(a) shows two compositionally distinct areas of the 1/2 Pb/Al molar ratio PbO/Al2O3 samples treated for 3 h at 850 °C, with lead present in two types of grain colors. EDS confirmed the composition of the gray grains as Pb9Al8O21 and the darker matrix as PbAl2O4, which corresponds to the crystalline phases observed in the XRD result (Fig. S1, Supporting Information). A homogeneous matrix was observed in the 950 °C sintered 1/2 Pb/Al ratio sample (Fig. 5(b)) and EDS confirmed the dark background to be PbAl2O4. In Fig. 5(c), phase separation was observed in the products. EDS confirmed that the bright areas were the lead-rich phase and the darker areas were the alumina-rich phase. The EDS results also reflected the chemical composition of the PbAl12O19, as shown in Fig. 5(d). This observation was consistent with the XRD patterns in Fig. S4 (Supporting Information).
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Fig. 5. Scanning secondary electron micrographs of the polished surfaces of samples with a 1/2 Pb/Al molar ratio sintered for 3 h at (a) 850 °C and (b) 950 °C; and for the 1/12 Pb/Al molar ratio sample treated at (c) 950 °C and (d) 1000 °C for 3 h. (e) The normalized leachability of lead from the Pb9Al8O21, PbAl2O4, and PbAl12O19 phases. The leaching experiments were conducted with TCLP extraction fluid #1 (acetic acid and sodium hydroxide solution) maintained at pH 4.9.
The product sintered from the 1/2 Pb/Al molar ratio system demonstrated distinctively different microstructure when compared with the 1/12 Pb/Al molar ratio product in the scanning secondary electron micrograph. On the electron scanning micrographs of the products sintered in the 1/2 Pb/Al system (Fig. 5(a) and (b)), the product (e.g. PbAl2O4 and Pb9Al8O21) grains appeared tightly associated and well-sintered to each other. In contrast, the product in the 1/12 Pb/Al system (Fig. 5(c) and (d)) had a very porous texture and rough morphology, even after intensive surface polishing. The product microstructure plays a vital role in determining lead incorporation efficiencies. The densely packed crystallite grains of the PbAl2O4 act as a diffusion barrier, leading to increased lead incorporation efficiency into the PbAl2O4 crystal structure in the 1/2 Pb/Al molar ratio system. Additional interfacial diffusion in the 1/12 Pb/Al molar ratio sample would also lead to enhanced lead incorporation efficiency into PbAl12O19 at higher temperatures.
3.4. Leachability of the product phases As Pb9Al8O21, PbAl2O4 and PbAl12O19 are the potential lead-containing phases in the sintered products, their lead leachability and leaching behavior need to be evaluated to optimize the lead stabilization strategy. Single phase PbAl2O4 sample was prepared from sintering the 1/2 Pb/Al molar ratio PbO/c-Al2O3 mixture at 950 °C for 3 h. The single-phase PbAl12O19 sample was obtained from the 1/12 Pb/Al molar ratio PbO/c-Al2O3 mixture sintered at 1000 °C for 3 h. The single-phase Pb9Al8O21 sample was obtained by sintering a 9/8 Pb/Al molar ratio PbO/c-Al2O3 mixture at 800 °C for 24 h. The success of fabricating the single-phase aluminate samples (Pb9Al8O21, PbAl2O4 and PbAl12O19) was reflected by their XRD patterns, given in Fig. S11 (Supporting Information). A constant pH 4.9 leaching experiment was used to determine their intrinsic leachability. The leachability of lead in PbO was also
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used as a reference to compare with that of the lead stabilized by the aluminate phases. To minimize the physical barriers to the leaching reaction, the samples were ground for the leaching experiments into powders with surface areas of 0.51 m2 g1 for PbO, 0.48 m2 g1 for Pb9Al8O21, 0.63 m2 g1 for PbAl2O4, and 3.76 m2 g1 for PbAl12O19. Starting with a pH 4.9 leaching fluid, the lead concentration in the leachate of PbO was found to be as high as 16 g/L after only 0.5 h of leaching. Compared to the lead aluminates, it was more than six times higher than Pb9Al8O21 (2.4 g/L) and more than two orders of magnitude higher than PbAl2O4 (143 mg/L) and PbAl12O19 (55 mg/L). Interestingly, despite its much higher surface area, PbAl12O19 was found to have the lowest lead concentration in its leachate, and this result is consistent with our previous study (Lu and Shih, 2011) by a prolonged TCLP test with pH 2.9 leaching fluid. PbO was completely dissolved in the pH 4.9 leaching fluid after 7 h, suggesting that the oxide form is incapable of stabilizing lead in acidic conditions, indicating that a strategy for transforming lead into a more robust hosting phase is needed. The process of leaching metals from a solid surface is primarily controlled by the rate of the leaching reaction and the availability of leaching sites for the targeted metals on the solid surface. As the amounts of lead used and the surface areas of the solid samples were known, the lead concentrations observed in the leachates could be compared by normalizing to the lead contents and surface areas of the corresponding solid samples. This normalized measure would more closely reflect the intrinsic lead leachability of the concerned lead hosting phase. To achieve this goal, this study defined the normalized leached lead per surface area of the sample (NLPbSA; m2) as follows:
NLPbSA ¼ 106
n C Pb AWPb ; k SW SA MWPhase
ð3Þ
where n is the number of Pb atoms in each molecule, k is the ratio of sample weight (g) to the extraction fluid volume (mL), CPb is the lead concentration in the leachate (mg L1), AWPb represents the atomic weight of lead, SW is the solid sample weight (g), SA is the solid sample surface area (m2 g1), and MWPhase represents the molecular weight of the tested phase. Fig. 5(e) summarizes the normalized leaching results for the Pb9Al8O21, PbAl2O4, and PbAl12O19 samples at pH 4.9 for 0.5–92 h. The normalized lead concentrations in the Pb9Al8O21 leachates increased continuously throughout the entire leaching period and were substantially higher than those of the PbAl2O4 and PbAl12O19 leachates. After the release of Pb in the initial leaching stage, which was often mostly controlled by the grain boundary property, the Pb concentrations in the PbAl2O4 leachates remained the same throughout the entire leaching period. The normalized Pb concentrations in the PbAl12O19 leachates were also nearly constant throughout the leaching process, but were at extremely low levels. After 92 h, the normalized leached lead from the PbAl2O4 phase was over five times less than that from the Pb9Al8O21 phase. PbAl12O19 showed nearly three orders of magnitude less leachability than the PbAl2O4 phase. 4. Conclusions The sintering of lead-laden waste with an aluminum-rich precursor for ceramic products can result in significant lead hosting in PbAl2O4 or PbAl12O19 structures, depending on the Pb/Al molar ratio during the sintering process. However, the QXRD result revealed that a phase with higher lead content, Pb9Al8O21, can occur as an intermediate between 800 and 900 °C, holding a maximum of 12.5 wt.% in products sintered from 1/2 Pb/Al molar ratio systems. The optimal sintering condition for the formation of
PbAl2O4 is at 950 °C for 3 h heating. Additionally, when the raw material has a lower Pb/Al molar ratio (1/12), another intermediate phase, Pb3(CO3)2(OH)2, was observed at temperatures of 700–950 °C. PbAl12O19 phase was completely formed at 1000 °C for 3 h sintering. Although a leaching test demonstrated the superiority of Pb9Al8O21, PbAl2O4, and PbAl12O19 over PbO in resisting acidic attack, PbAl12O19 was found to be the most ideal structure for hosting lead due to its superior metal stabilization capability. Acknowledgments We gratefully acknowledge the funding for this research provided by the General Research Fund Scheme of the Research Grants Council of Hong Kong (715612, 17206714) and HKU Strategic Research Themes on Clear Energy and Earth as a Habitable Planet. 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. 2015.05.090. References Acharya, J., Sahu, J.N., Mohanty, C.R., Meikap, B.C., 2009. Removal of lead (II) from wastewater by activated carbon developed from Tamarind wood by zinc chloride activation. Chem. Eng. J. 149, 249–262. Aksel, C., Kasap, F., Sesver, A., 2005. Investigation of parameters affecting grain growth of sintered magnesite refractories. Ceram. Int. 31, 121–127. Chen, S., Zhao, B., Hayes, P.C., Jak, E., 2001. Experimental study of phase equilibria in the PbO–Al2O3–SiO2 system. Metall. Mater. Trans. B 32, 997–1005. Cheng, T.W., 2004. Effect of additional materials on the properties of glass–ceramics produced from incinerator fly ashes. Chemosphere, 127–131. Conrad, K., Bruun Hansen, H.C., 2007. Sorption of zinc and lead on coir. Bioresour. Technol. 98, 89–97. De La Torre, A.G., Bruque, S., Aranda, M.A.G., 2001. Rietveld quantitative amorphous content analysis. J. Appl. Crystallogr. 34, 196–202. Deng, L., Su, Y., Su, H., Wang, X., Zhu, X., 2007. Sorption and desorption of lead (II) from wastewater by green algae Cladophora fascicularis. J. Hazard. Mater. 143, 220–225. Gupta, V.K., Gupta, M., Sharma, S., 2001. Process development for the removal of lead and chromium from aqueous solutions using red mud – an aluminium industry waste. Water Res. 35, 1125–1134. Gupta, V.K., Agarwal, S., Saleh, T.A., 2011. Synthesis and characterization of alumina-coated carbon nanotubes and their application for lead removal. J. Hazard. Mater. 185, 17–23. Hu, C.Y., Shih, K., Leckie, J.O., 2010. Formation of copper aluminate spinel and cuprous aluminate delafossite to thermally stabilize simulated copper-laden sludge. J. Hazard. Mater. 181, 399–404. Jalali, R., Ghafourian, H., Asef, Y., Davarpanah, S.J., Sepehr, S., 2002. Removal and recovery of lead using nonliving biomass of marine algae. J. Hazard. Mater. 92, 253–262. Kim, E.J., Herrera, J.E., 2010. Characteristics of lead corrosion scales formed during drinking water distribution and their potential influence on the release of lead and other contaminants. Environ. Sci. Technol. 44, 6054–6061. Kim, E.J., Herrera, J.E., Huggins, D., Braam, J., Koshowski, S., 2011. Effect of pH on the concentrations of lead and trace contaminants in drinking water: a combined batch, pipe loop and sentinel home study. Water Res. 45, 2763–2774. 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. Kuxmann, U., Fischer, P., 1974. Lead monoxide–aluminum oxide, lead monoxide– calcium oxide, and lead monoxide–silicon dioxide phase diagrams. Erzmetall 27, 533–537. Lu, X., Shih, K., 2011. Phase transformation and its role in stabilizing simulated leadladen sludge in aluminum-rich ceramics. Water Res. 45, 5123–5129. Magallanes-Perdomo, M., Carrodeguas, R.G., Pena, P., De Aza, P.N., De Aza, S., De Aza, A.H., 2009. Non-isothermal devitrification study of wollastonite-tricalcium phosphate bioeutecticÒ glass. Key Eng. Mater. 396–398, 127–130. Mao, L., Cui, H., An, H., Wang, B., Zhai, J., Zhao, Y., Li, Q., 2014. Stabilization of simulated lead sludge with iron sludge via formation of PbFe12O19 by thermal treatment. Chemosphere 117, 745–752. Park, S.E., Chang, J.S., 1993. Oxidative coupling of methane over PbO/PbAl2O4 catalysts. Stud. Surf. Sci. Catal. 75, 2233–2236. Rawlings, R.D., Wu, J.P., Boccaccini, A.R., 2006. Glass-ceramics: their production from wastes – a review. J. Mater. Sci. 41, 733–761.
X. Lu, K. Shih / Chemosphere 138 (2015) 156–163 Rendtorff, N.M., Conconi, M.S., Aglietti, E.F., Chain, C.Y., Pasquevich, A.F., Rivas, P.C., Martínez, J.A., Caracoche, M.C., 2010. Phase quantification of mullite–zirconia and zircon commercial powders using PAC and XRD techniques. Hyperfine Interact. 198, 211–218. Richerson, D.W., 1982. Modern Ceramic Engineering: Properties Processing, And Use in Design. Marcel Dekker, Inc., New York. Shih, K., White, T., Leckie, J.O., 2006a. Spinel formation for stabilizing simulated nickel-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 40, 5077–5083. Shih, K., White, T., Leckie, J.O., 2006b. Nickel stabilization efficiency of aluminate and ferrite spinels and their leaching behavior. Environ. Sci. Technol. 40, 5520–5526. Singh, C.K., Sahu, J.N., Mahalik, K.K., Mohanty, C.R., Mohan, B.R., Meikap, B.C., 2008. Studies on the removal of Pb (II) from wastewater by activated carbon developed from Tamarind wood activated with sulphuric acid. J. Hazard. Mater. 153 (1), 221–228. Tang, Y., Shih, K., Chan, K., 2010. Copper aluminate spinel in the stabilization and detoxification of simulated copper-laden sludge. Chemosphere 80, 375–380. Tian, Y., Zuo, W., Chen, D., 2011. Crystallization evolution, microstructure and properties of sewage sludge-based glass–ceramics prepared by microwave heating. J. Hazard. Mater. 196, 370–379. Vincenzini, P., 1991. Fundamentals of Ceramic Engineering. Elsevier Science Publishers, Ltd., New York.
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Volpe, M., Oliveri, D., Ferrara, G., Salvaggio, M., Piazza, S., Italiano, S., Sunseri, C., 2009. Metallic lead recovery from lead–acid battery paste by urea acetate dissolution and cementation on iron. Hydrometallurgy 96 (1), 123–131. Völtzke, D., Abicht, H.P., 2001. Mechanistic investigations on the densification behaviour of barium titanate ceramics. Solid State Sci. 3, 417–422. Wang, Y., Suryanarayana, C., An, L., 2005. Phase transformation in nanometer-sized c-alumina by mechanical milling. J. Am. Ceram. Soc. 88, 780–783. Wendt, G., Meinecke, C.D., Schmitz, W., 1988. Oxidative dimerization of methane on lead oxide-alumina catalysts. Appl. Catal. 45, 209–220. Xu, G.R., Zou, J.L., Li, G.B., 2008. Effect of sintering temperature on the characteristics of sludge ceramsite. J. Hazard. Mater. 150, 394–400. Xu, G.R., Zou, J.L., Li, G.B., 2009. Stabilization/solidification of heavy metals in sludge ceramsite and leachability affected by oxide substances. Environ. Sci. Technol. 43, 5902–5907. Yao, H., Naruse, I., 2005. Control of trace metal emissions by sorbents during sewage sludge combustion. Proc. Combust. Inst. 30, 3009–3016. Yao, H., Naruse, I., 2009. Behavior of lead compounds during municipal solid waste incineration. Proc. Combust. Inst. 32, 2685–2691. Zhou, R.S., Snyder, R.L., 1991. Structures and transformation mechanisms of the g, c and h transition aluminas. Acta Crystallogr. Sect. B 47, 617–630.
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Formation of lead-aluminate ceramics: Reaction mechanisms in immobilizing the simulated lead sludge Xingwen Lu, Kaimin Shih ⇑ Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
h i g h l i g h t s Formation of PbAl2O4 and PbAl12O19 for stabilizing Pb in different Pb/Al systems. Pb9Al8O21 and Pb3(CO3)2(OH)2 were detected as intermediate phases during sintering. Pb incorporation efficiency into PbAl2O4 and PbAl12O19 was quantitatively evaluated. Incorporating Pb into PbAl12O19 crystal is a preferred stabilization mechanism.
a r t i c l e
i n f o
Article history: Received 5 December 2014 Received in revised form 16 May 2015 Accepted 27 May 2015
Keywords: Sludge Lead Alumina Stabilization Leaching behavior
a b s t r a c t We investigated a strategy of blending lead-laden sludge and an aluminum-rich precursor to reduce the release of hazardous lead from the stabilized end products. To quantify lead transformation and determine its incorporation behavior, PbO was used to simulate the lead-laden sludge fired with c-Al2O3 by Pb/Al molar ratios of 1/2 and 1/12 at 600–1000 °C for 0.25–10 h. The sintered products were identified and quantified using Rietveld refinement analysis of X-ray diffraction data from the products generated under different conditions. The results indicated that the different crystallochemical incorporations of hazardous lead occurred through the formation of PbAl2O4 and PbAl12O19 in systems with Pb/Al ratios of 1/2 and 1/12, respectively. PbAl2O4 was observed as the only product phase at temperature of 950 °C for 3 h heating in Pb/Al of 1/2 system. For Pb/Al of 1/12 system, significant growth of the PbAl12O19 phase clearly occurred at 1000 °C for 3 h sintering. Different product microstructures were found in the sintered products between the systems with the Pb/Al ratios 1/2 and 1/12. The leaching performances of the PbO, Pb9Al8O21, PbAl2O4 and PbAl12O19 phases were compared using a constant pH 4.9 leaching test over 92 h. The leachability data indicated that the incorporation of lead into PbAl12O19 crystal is a preferred stabilization mechanism in aluminate-ceramics. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lead (Pb) is a type of heavy metal widely used in industry due to its versatile physical and chemical characteristics. The International Lead and Zinc Study Group estimated that global annual lead consumption approached 9 million tons in 2010. Serious environmental concerns have been raised about the release of lead into the environment from industries such as the production of lead batteries, oil-based paints, pigments, paper, pulp, and electrochemical electrodes, as well as mining, plating, hot dip galvanizing, and petroleum refining (Jalali et al., 2002; Gupta et al., 2001; Conrad and Bruun Hansen, 2007). Lead is a highly toxic and non-biodegradable metal that tends to accumulate in the cells ⇑ Corresponding author. E-mail address: [email protected] (K. Shih). http://dx.doi.org/10.1016/j.chemosphere.2015.05.090 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
of living organisms, causing severe damage to the kidneys, liver, and nervous and reproductive systems in humans (Gupta et al., 2011). The remediation of hazardous lead sludge has become a pressing challenge in recent years due to more stringent environmental regulations. Ceramic technology is a technique to create ceramic products from inorganic, non-metallic materials by the action of heat and subsequent cooling (Vincenzini, 1991). Ceramics are either formed from a molten mass that solidifies on cooling, formed and matured by the action of heat, or chemically synthesized at low temperatures using, for example, hydrothermal or sol–gel synthesis (Richerson, 1982). Ceramic technology is considered useful for rendering hazardous metal sludge inert because they can destroy organic matters and immobilize hazardous metals in a stable matrix to reduce metal leachability (Xu et al., 2008, 2009). As ceramic technology involved in the crystallization and/or vitrification
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of ceramic materials (Rawlings et al., 2006), this technology functions by binding hazardous metal ions into the framework of glass or the crystallization phases of ceramic products (Cheng, 2004; Tian et al., 2011). Previous studies have described mechanisms for stabilizing nickel and copper into aluminum-rich ceramic matrices and achieving reduced metal leachability through the intrinsic properties of spinel structures (Shih et al., 2006a,b; Tang et al., 2010; Hu et al., 2010). Several studies have reported the potential formation of lead aluminate (PbAl2O4) and a magnetoplumbite-like structure (PbAl12O19) using c-Al2O3 as an inexpensive ceramic precursor to treat PbO (Wendt et al., 1988; Park and Chang, 1993). Although the recently published PbO–Al2O3–SiO2 equilibrium phase diagram confirmed the presence of PbAl12O19 and PbAl2O4 (Chen et al., 2001), the reaction sequences involved in the incorporation of lead at different temperatures and times are still unclear. As the dwelling time of the ceramic sintering process can vary from a few minutes to several hours (Völtzke and Abicht, 2001; Aksel et al., 2005), the dominant mechanism(s) for hosting lead in crystal structures under different thermal conditions needs to be unambiguously identified, if not also quantitatively evaluated. The objective of this work was to contribute a better understanding of the phase transformations to PbAl12O19 and PbAl2O4 during ceramic sintering, which potentially play a role in lead incorporation mechanisms in aluminum-rich ceramics. The dominant reaction mechanisms at different sintering periods were determined quantitatively, together with the influence of the sintering temperature and time. Furthermore, the stabilization effects of potential lead phases in the products were evaluated via leaching tests at a constant pH value. The obtained leachability of each phase was further normalized using the sample surface area and lead content to reflect the intrinsic lead leachability and suggest a more reliable mechanism to host the hazardous lead.
2. Materials and methods PbO powder was purchased from Sigma–Aldrich. The phase composition of the PbO was identified using X-ray diffraction (XRD) for a mixture of a-PbO (litharge) and b-PbO (massicot) phases. The c-Al2O3 was prepared from PURAL SB powder with an average particle size of 45 lm fabricated by Sasol. The phase of the PURAL SB powder was identified using XRD for boehmite (AlOOH; ICDD PDF # 74-1875) and it was successfully converted to the c-Al2O3 phase after heat treatment at 650 °C for 3 h (Zhou and Snyder, 1991; Wang et al., 2005). The phase confirmation for the two materials was demonstrated by their XRD patterns, which are provided in Fig. S1 (Supporting Information). As lead usually exist as lead(II) in wastewater (Acharya et al., 2009; Deng et al., 2007; Singh et al., 2008), the treatment of lead-containing wastewater by current common techniques (i.e. chemical precipitation, ion exchange, coagulation, adsorption, and membrane processes) produce enormous amounts of lead-containing sludge that mainly composed of some insoluble lead(II) compounds (i.e. lead(II) carbonate, lead(II) oxide, lead(II) hydroxide and lead(II) sulfate) (Mao et al., 2014; Volpe et al., 2009; Yao and Naruse, 2009). However, the major portion of lead may converted into lead oxide at the high sintering temperatures. For this reason, the lead incorporation experiments were conducted using PbO to simulate the lead in sludge under sintering condition (Yao and Naruse, 2005). The c-Al2O3 precursor and PbO were mixed by ball milling in water slurry at Pb/Al molar ratios of 1/2 and 1/12 for 18 h. The slurry samples were then dried and homogenized by mortar grinding. The derived powder was pressed into 20 mm pellets at 650 MPa to ensure consistent compaction of the powder sample for the sintering process. The pellets were
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sintered at specified temperatures from 600 to 1000 °C for 0.25–10 h, and then quenched in air to room temperature. The total mass loss after sintering was less than 1 wt.%. After thermal treatment, the pellets were weighed and ground into powder with a particle size of less than 10 lm for XRD analysis. Some of the pellets were reserved for scanning electron microscopic (SEM) characterization and were polished using a submicrometer diamond lapping film and gold coated to mitigate the electron charging effect. All of the SEM investigations were performed using a Hitachi S-4800 SEM system equipped with a secondary electron detector to obtain morphological information and a backscattered electron detector for energy dispersive spectroscopy (EDS). The X-ray powder diffraction data of the samples were collected using a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu Ka radiation detector and a LynxEye detector. The diffractometer was operated at 40 kV and 40 mA, and the 2h scan range was from 10° to 80°, with a step size of 0.02° and a scan speed of 0.3 s/step. Qualitative phase identification was performed using Eva XRD Pattern Processing software (Bruker Co. Ltd.) by matching the powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF-2 Release 2008). The Rietveld refinements for quantitative analysis of the phase compositions were processed using the TOPAS (version 4.0) crystallographic program. Figs. S2–S6 (Supporting Information) present the Rietveld refinement plots for the products sintered from the PbO/c-Al2O3 system. The derived reliability values for the refinement quality of this analytic scheme are provided in Tables S1–S3 (Supporting Information). For systems potentially containing amorphous or poorly crystalline phases, a refinement method using CaF2 as the internal standard (De La Torre et al., 2001; Magallanes-Perdomo et al., 2009; Rendtorff et al., 2010) was used to quantify the amorphous content in the samples. Quantitative analysis data were collected from 10° to 120° 2h-angle, with a step width of 2h = 0.02° and a sampling time of 0.5 s per step. Before performing the leaching test, the surface areas of the single-phase powders were measured by a Beckman Coulter SA3100 Surface Area and Pore Size Analyzer using the BET method after degassing by heating at 300 °C for 3 h with He-gas purging. To distinguish the role and characteristic of each lead stabilization mechanism, the potential lead-bearing product phases were compared using the leaching test. The powder (<45 lm) samples of the target phases were subjected to batch leaching in amorphous silica vessels at a liquid/solid ratio of 50 ml/g (4 g powder sample with 200 ml of pH 4.9 acetic acid solution) for a total testing time of 92 h. Three duplicate experiments were conducted to ensure the reproducibility of the leaching test. Acid (1 N HNO3) was added into each suspension to maintain a constant pH of 4.9 using a pH-stat system. Continuous stirring was conducted to keep the mixture homogeneous and the leaching vessel was held at room temperature (23 ± 0.5 °C). At different times, the suspensions were allowed to settle for 5 min and aliquots of 5 ml of the upper liquid were taken and centrifuged. Another 5 ml of pH 4.9 acetic acid solution, together with the filtered solids, was added to the vessels. At the end of each agitation period, the leachates were filtrated using 0.2 lm syringe filters and the concentrations of lead in the leachates were analytically determined using ICP-AES (Perkin–Elmer Optima 8300 DV).
3. Results and discussion 3.1. Lead incorporation into PbAl2O4 structure in Pb/Al 1/2 system By adding 25 wt.% CaF2 as an internal standard, the quantities of crystalline and amorphous phases in the PbO/c-Al2O3 mixtures
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with 1/2 Pb/Al molar ratio sintered at 600–1000 °C for 3 h were revealed by Rietveld refinements for the XRD patterns, as shown in Fig. 1. Table 1 summarized the quality of the Rietveld refinement analysis for quantifying the phase compositions of sintered PbO/c-Al2O3 (Pb/Al = 1/2) sample at 600–1000 °C for 3 h. The values indicate the pattern factors and the goodness-of-fit (Rp, Rwp, Rexp, and GOF). Weight fractions for the crystalline and amorphous components in the products were obtained and given in Fig. 2. In the 1/2 Pb/Al system, the PbAl2O4 phase was first generated at 700 °C with a mass content of around 12 wt.%. Increasing the sintering temperature to 750 °C led to significant PbAl2O4 development, with its phase fraction reaching 81 wt.% after the 3 h sintering. With the continuous increase in PbAl2O4, the signal of the PbO reactant eventually diminished when the temperature exceeded 800 °C. The proportion of the PbAl2O4 phase increased continuously in samples sintered at higher temperatures (800–950 °C) and reached its maximum value (near-complete reaction) at 950 and 1000 °C. Therefore, the thermal reactions for the formation of PbAl2O4 from PbO and c-Al2O3 can be expressed by the following equation.
Table 1 Quality of the Rietveld refinement analyses for quantifying the phase compositions of sintered PbO/c-Al2O3 (Pb/Al = 1/2) samples at 600–1000 °C for 3 h (Fig. 1). The values indicate the pattern factors and the goodness-of-fit (Rp, Rwp, Rexp, and GOF). Temperature (°C)
600 700 750 800 850 900 950 1000
Quality of refinement analysis RP (%)
Rwp (%)
Rexp (%)
GOF
6.03 7.83 4.20 4.95 3.83 4.58 4.49 5.52
8.00 10.35 5.42 6.56 5.15 5.92 5.75 7.26
5.13 6.36 3.74 4.16 3.15 3.70 3.69 4.66
1.56 1.63 1.45 1.58 1.63 1.60 1.56 1.56
PbO þ c-Al2 O3 ! PbAl2 O4
ð1Þ
However, an intermediate phase, Pb9Al8O21, also appeared in the reaction, as shown in Fig. 2. To further quantify the variation in lead incorporation efficiency, the mass distribution of this intermediate phase was also evaluated. The phase content of Pb9Al8O21
Fig. 1. Results of Rietveld refinement for PbO/c-Al2O3 (Pb/Al = 1/2) samples sintered at 600–1000 °C for 3 h (data lines = experimental XRD patterns at different temperatures, red lines = calculated XRD patterns, and gray lines = difference curves between the experimental and calculated patterns). Vertical bars indicate the Bragg positions of the corresponding phases. The goodness-of-fit (chi2) is provided in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Quantitative comparison of phase compositions in the products of sintering PbO/c-Al2O3 samples (1/2 Pb/Al molar ratio) at temperatures between 600 and 1000 °C for 3 h. Crystalline PbAl2O4 was observed to be the dominant product at high temperatures.
increased for temperatures ranging from 700 to 800 °C, reaching its maximum of 12.5 wt.%. The intermediate Pb9Al8O21 disappeared at 950 °C, indicating near-total incorporation of the lead into the PbAl2O4 structure in this system. The formation of Pb9Al8O21 during the sintering process was probably initiated by the reaction between PbO and c-Al2O3 in a non-equilibrium system, due to the insufficient sintering time in the lower temperature range. The efficiency of a homogeneous reaction generally depends on the encountering rate between the reactant molecules (Kukukova et al., 2009). The short sintering time and low temperature were not able to provide sufficient contact. Hence the formation of PbAl2O4 was incomplete and an intermediate compound appeared in the product (Lu and Shih, 2011). In contrast, higher temperatures enabled a more intensive interaction between reactants, allowing the enhanced molecular transport to overcome the diffusion barrier of the products and preventing the intermediate from appearing in the 950 °C sintered samples. To determine the details of the lead transformation process in the early stage of sintering the 1/2 Pb/Al system, PbO and c-Al2O3 were sintered at the low temperatures of 700 and 850 °C for 0.25–10 h. Rietveld refinement was applied to the XRD patterns of the sintered samples to quantitatively determine the lead transformation efficiency. The weight percentages of the crystalline and amorphous phases in the samples of the PbO/c-Al2O3 system are summarized in Fig. 3. Fig. 3(a) indicates that almost no reaction occurred between PbO and c-Al2O3 at 700 °C when the treatment time was less than 3 h. With increased thermal treatment time, PbAl2O4 and Pb9Al8O21 co-existed in the products, and Pb9Al8O21 even became the predominant lead-hosting phase, at around 70 wt.% in the product after the 10 h sintering process. In contrast, the PbAl2O4 phase content remained around 10 wt.% even when the thermal treatment time was increased from 3 to 10 h. The crossover of the PbO and Pb9Al8O21 phase content curves at 700 °C suggested an incomplete interaction between PbO and c-Al2O3. As more intensive interaction between reactants occurred at 800 °C, after 0.25 h sintering, the PbAl2O4 phase held as much as 80 wt.% in the product of the 1/2 Pb/Al system (Fig. 3(b)). The prominence of the Pb9Al8O21 phase then decreased with the increase in PbAl2O4 at longer sintering times. This finding may indicate that higher temperatures can greatly enhance lead incorporation into the PbAl2O4 structure in the 1/2 Pb/Al system, due to the provision of energy to overcome the diffusion barrier. However,
Fig. 3. Quantitative comparison of phase compositions in the products of sintering PbO/c-Al2O3 samples (1/2 Pb/Al molar ratio) at temperatures of (a) 700 and (b) 850 °C for 0.25–10 h.
despite the prolonged thermal treatment time (10 h), the system did not achieve the complete transformation of lead into PbAl2O4. Therefore, to more effectively facilitate the formation of PbAl2O4, the temperature of a short sintering scheme should be higher than 800 °C. According to the XRD results obtained here, with higher temperatures (950–1000 °C) and longer treatment times, the formation of PbAl2O4 phase was controlled by the equilibrium phase diagram. The reaction between the PbO and c-Al2O3 reactants could reach equilibrium under such treatment conditions. However, with lower temperatures and a shorter dwelling time, the formation of the intermediate Pb9Al8O21 species was determined by molecular kinetic and grain boundary reactions, indicating that the entire system was in non-equilibrium. The decrease of Pb9Al8O21 at higher temperatures (>850 °C) suggested that the higher temperature enhanced the rate of molecular diffusion of the reactants.
3.2. Lead incorporation into PbAl12O19 structure in the 1/12 Pb/Al system PbAl12O19 was the only product phase reported in the PbO– Al2O3 equilibrium experiment when the PbO molar content was lower than 50% (Kuxmann and Fischer, 1974; Chen et al., 2001). A mixture of PbO and c-Al2O3 with a lower Pb/Al molar ratio of 1/12 was sintered to investigate the incorporation mechanism behind PbAl12O19 formation. Fig. 4(a) shows the variations in
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crystalline and amorphous phases produced by sintering the 1/12 Pb/Al molar ratio PbO/c-Al2O3 mixture at 600–1000 °C. The curves show that approximately 85% of the products were composed of amorphous phase(s) at temperatures of 700–950 °C. Such a large proportion occurred in the amorphous phase because of the poorly crystalline c-Al2O3 reactant, together with the formation of a poorly crystalline intermediate. Approximately 15% hydrocerussite (Pb3(CO3)2(OH)2), which was an intermediate phase during the sintering, was also measured in the products at temperatures of 700–950 °C. Intermediate Pb3(CO3)2(OH)2 has been
identified as a common lead corrosion product in existing studies (Kim et al., 2011; Kim and Herrera, 2010). Thus, the formation of the Pb3(CO3)2(OH)2 phase was probably due to the instability of the product phase(s) sintered at 700–950 °C, which may be vulnerable to the attack of CO2 and moisture in the air during the sample quenching stage. Nevertheless, the weight percentage of the PbAl12O19 exceeded 10% at 950 °C and increased to near-total incorporation at 1000 °C. A potential thermal reaction for the incorporation of lead by the c-Al2O3 precursor at the lower lead level is described by Eq. (2).
PbO þ 6 c-Al2 O3 ! PbAl12 O19
ð2Þ
The influence of thermal treatment time on the crystal growth of PbAl12O19 was quantitatively determined at temperatures of 950 and 1000 °C by examining the reactions of 1/12 Pb/Al molar ratio samples (Fig. 4(b) and (c)). At 950 °C, the phase content of PbAl12O19 increased from 5% in the first 1 h to around 35% after 10 h. As the unstable thermal treatment products were easily attacked by CO2 and moisture with the formation of hydrocerussite at 950 °C, 10 wt.% of Pb3(CO3)2(OH)2 was generally found in the sintered products. With the first observation of PbAl12O19 at 1000 °C for 0.5 h, about 8 wt.% of the PbAl12O19 phase was found in the product, together with 12 wt.% of the Pb3(CO3)2(OH)2 phase. However, the PbAl12O19 phase grew significantly with the increase in reaction time from 1 to 3 h. Fig. 4(c) illustrates the increase in PbAl12O19 crystal, together with the corresponding decrease in the Pb3(CO3)2(OH)2 and amorphous phases after the extended sintering time at 1000 °C. Longer sintering times and higher temperatures may therefore greatly enhance interaction between the reactants, allowing the complete transformation of lead into PbAl12O19. After 3 h sintering at 1000 °C, the conversion to the PbAl12O19 phase completely eliminated the formation of Pb3(CO3)2(OH)2, and PbAl12O19 was found to be the only lead-containing phase in the product. This result confirms the potential of forming PbAl12O19 to incorporate lead into aluminum-rich ceramics in systems with lower Pb/Al ratios. 3.3. Microstructures of PbO/c-Al2O3 products with 1/2 and 1/12 Pb/Al molar ratios
Fig. 4. Variation in the weight fractions of the crystalline and amorphous phases obtained from the sintering of PbO with c-Al2O3 in a 1/12 Pb/Al molar ratio (a) at temperatures from 700 to 1000 °C for 3 h, and (b) at temperatures of 950 and 1000 °C for 0.25–10 h.
The different Pb/Al molar ratios used in the PbO/Al2O3 system were found to generate lead aluminate products with distinct microstructures. Fig. 5 compares the microstructures of the samples in the 1/2 Pb/Al molar ratio PbO/Al2O3 system sintered for 3 h at 850 °C (a) and 950 °C (b), and the 1/12 Pb/Al molar ratio PbO/Al2O3 system sintered for 3 h at 950 °C (c) and 1000 °C (d). EDS (Figs. S7–10, Supporting Information) was used to confirm the chemical compositions of the samples produced in the 1/2 and 1/12 Pb/Al molar ratio PbO/Al2O3 systems. For example, Fig. 5(a) shows two compositionally distinct areas of the 1/2 Pb/Al molar ratio PbO/Al2O3 samples treated for 3 h at 850 °C, with lead present in two types of grain colors. EDS confirmed the composition of the gray grains as Pb9Al8O21 and the darker matrix as PbAl2O4, which corresponds to the crystalline phases observed in the XRD result (Fig. S1, Supporting Information). A homogeneous matrix was observed in the 950 °C sintered 1/2 Pb/Al ratio sample (Fig. 5(b)) and EDS confirmed the dark background to be PbAl2O4. In Fig. 5(c), phase separation was observed in the products. EDS confirmed that the bright areas were the lead-rich phase and the darker areas were the alumina-rich phase. The EDS results also reflected the chemical composition of the PbAl12O19, as shown in Fig. 5(d). This observation was consistent with the XRD patterns in Fig. S4 (Supporting Information).
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Fig. 5. Scanning secondary electron micrographs of the polished surfaces of samples with a 1/2 Pb/Al molar ratio sintered for 3 h at (a) 850 °C and (b) 950 °C; and for the 1/12 Pb/Al molar ratio sample treated at (c) 950 °C and (d) 1000 °C for 3 h. (e) The normalized leachability of lead from the Pb9Al8O21, PbAl2O4, and PbAl12O19 phases. The leaching experiments were conducted with TCLP extraction fluid #1 (acetic acid and sodium hydroxide solution) maintained at pH 4.9.
The product sintered from the 1/2 Pb/Al molar ratio system demonstrated distinctively different microstructure when compared with the 1/12 Pb/Al molar ratio product in the scanning secondary electron micrograph. On the electron scanning micrographs of the products sintered in the 1/2 Pb/Al system (Fig. 5(a) and (b)), the product (e.g. PbAl2O4 and Pb9Al8O21) grains appeared tightly associated and well-sintered to each other. In contrast, the product in the 1/12 Pb/Al system (Fig. 5(c) and (d)) had a very porous texture and rough morphology, even after intensive surface polishing. The product microstructure plays a vital role in determining lead incorporation efficiencies. The densely packed crystallite grains of the PbAl2O4 act as a diffusion barrier, leading to increased lead incorporation efficiency into the PbAl2O4 crystal structure in the 1/2 Pb/Al molar ratio system. Additional interfacial diffusion in the 1/12 Pb/Al molar ratio sample would also lead to enhanced lead incorporation efficiency into PbAl12O19 at higher temperatures.
3.4. Leachability of the product phases As Pb9Al8O21, PbAl2O4 and PbAl12O19 are the potential lead-containing phases in the sintered products, their lead leachability and leaching behavior need to be evaluated to optimize the lead stabilization strategy. Single phase PbAl2O4 sample was prepared from sintering the 1/2 Pb/Al molar ratio PbO/c-Al2O3 mixture at 950 °C for 3 h. The single-phase PbAl12O19 sample was obtained from the 1/12 Pb/Al molar ratio PbO/c-Al2O3 mixture sintered at 1000 °C for 3 h. The single-phase Pb9Al8O21 sample was obtained by sintering a 9/8 Pb/Al molar ratio PbO/c-Al2O3 mixture at 800 °C for 24 h. The success of fabricating the single-phase aluminate samples (Pb9Al8O21, PbAl2O4 and PbAl12O19) was reflected by their XRD patterns, given in Fig. S11 (Supporting Information). A constant pH 4.9 leaching experiment was used to determine their intrinsic leachability. The leachability of lead in PbO was also
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used as a reference to compare with that of the lead stabilized by the aluminate phases. To minimize the physical barriers to the leaching reaction, the samples were ground for the leaching experiments into powders with surface areas of 0.51 m2 g1 for PbO, 0.48 m2 g1 for Pb9Al8O21, 0.63 m2 g1 for PbAl2O4, and 3.76 m2 g1 for PbAl12O19. Starting with a pH 4.9 leaching fluid, the lead concentration in the leachate of PbO was found to be as high as 16 g/L after only 0.5 h of leaching. Compared to the lead aluminates, it was more than six times higher than Pb9Al8O21 (2.4 g/L) and more than two orders of magnitude higher than PbAl2O4 (143 mg/L) and PbAl12O19 (55 mg/L). Interestingly, despite its much higher surface area, PbAl12O19 was found to have the lowest lead concentration in its leachate, and this result is consistent with our previous study (Lu and Shih, 2011) by a prolonged TCLP test with pH 2.9 leaching fluid. PbO was completely dissolved in the pH 4.9 leaching fluid after 7 h, suggesting that the oxide form is incapable of stabilizing lead in acidic conditions, indicating that a strategy for transforming lead into a more robust hosting phase is needed. The process of leaching metals from a solid surface is primarily controlled by the rate of the leaching reaction and the availability of leaching sites for the targeted metals on the solid surface. As the amounts of lead used and the surface areas of the solid samples were known, the lead concentrations observed in the leachates could be compared by normalizing to the lead contents and surface areas of the corresponding solid samples. This normalized measure would more closely reflect the intrinsic lead leachability of the concerned lead hosting phase. To achieve this goal, this study defined the normalized leached lead per surface area of the sample (NLPbSA; m2) as follows:
NLPbSA ¼ 106
n C Pb AWPb ; k SW SA MWPhase
ð3Þ
where n is the number of Pb atoms in each molecule, k is the ratio of sample weight (g) to the extraction fluid volume (mL), CPb is the lead concentration in the leachate (mg L1), AWPb represents the atomic weight of lead, SW is the solid sample weight (g), SA is the solid sample surface area (m2 g1), and MWPhase represents the molecular weight of the tested phase. Fig. 5(e) summarizes the normalized leaching results for the Pb9Al8O21, PbAl2O4, and PbAl12O19 samples at pH 4.9 for 0.5–92 h. The normalized lead concentrations in the Pb9Al8O21 leachates increased continuously throughout the entire leaching period and were substantially higher than those of the PbAl2O4 and PbAl12O19 leachates. After the release of Pb in the initial leaching stage, which was often mostly controlled by the grain boundary property, the Pb concentrations in the PbAl2O4 leachates remained the same throughout the entire leaching period. The normalized Pb concentrations in the PbAl12O19 leachates were also nearly constant throughout the leaching process, but were at extremely low levels. After 92 h, the normalized leached lead from the PbAl2O4 phase was over five times less than that from the Pb9Al8O21 phase. PbAl12O19 showed nearly three orders of magnitude less leachability than the PbAl2O4 phase. 4. Conclusions The sintering of lead-laden waste with an aluminum-rich precursor for ceramic products can result in significant lead hosting in PbAl2O4 or PbAl12O19 structures, depending on the Pb/Al molar ratio during the sintering process. However, the QXRD result revealed that a phase with higher lead content, Pb9Al8O21, can occur as an intermediate between 800 and 900 °C, holding a maximum of 12.5 wt.% in products sintered from 1/2 Pb/Al molar ratio systems. The optimal sintering condition for the formation of
PbAl2O4 is at 950 °C for 3 h heating. Additionally, when the raw material has a lower Pb/Al molar ratio (1/12), another intermediate phase, Pb3(CO3)2(OH)2, was observed at temperatures of 700–950 °C. PbAl12O19 phase was completely formed at 1000 °C for 3 h sintering. Although a leaching test demonstrated the superiority of Pb9Al8O21, PbAl2O4, and PbAl12O19 over PbO in resisting acidic attack, PbAl12O19 was found to be the most ideal structure for hosting lead due to its superior metal stabilization capability. Acknowledgments We gratefully acknowledge the funding for this research provided by the General Research Fund Scheme of the Research Grants Council of Hong Kong (715612, 17206714) and HKU Strategic Research Themes on Clear Energy and Earth as a Habitable Planet. 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. 2015.05.090. References Acharya, J., Sahu, J.N., Mohanty, C.R., Meikap, B.C., 2009. Removal of lead (II) from wastewater by activated carbon developed from Tamarind wood by zinc chloride activation. Chem. Eng. J. 149, 249–262. Aksel, C., Kasap, F., Sesver, A., 2005. Investigation of parameters affecting grain growth of sintered magnesite refractories. Ceram. Int. 31, 121–127. Chen, S., Zhao, B., Hayes, P.C., Jak, E., 2001. Experimental study of phase equilibria in the PbO–Al2O3–SiO2 system. Metall. Mater. Trans. B 32, 997–1005. Cheng, T.W., 2004. Effect of additional materials on the properties of glass–ceramics produced from incinerator fly ashes. Chemosphere, 127–131. Conrad, K., Bruun Hansen, H.C., 2007. Sorption of zinc and lead on coir. Bioresour. Technol. 98, 89–97. De La Torre, A.G., Bruque, S., Aranda, M.A.G., 2001. Rietveld quantitative amorphous content analysis. J. Appl. Crystallogr. 34, 196–202. Deng, L., Su, Y., Su, H., Wang, X., Zhu, X., 2007. Sorption and desorption of lead (II) from wastewater by green algae Cladophora fascicularis. J. Hazard. Mater. 143, 220–225. Gupta, V.K., Gupta, M., Sharma, S., 2001. Process development for the removal of lead and chromium from aqueous solutions using red mud – an aluminium industry waste. Water Res. 35, 1125–1134. Gupta, V.K., Agarwal, S., Saleh, T.A., 2011. Synthesis and characterization of alumina-coated carbon nanotubes and their application for lead removal. J. Hazard. Mater. 185, 17–23. Hu, C.Y., Shih, K., Leckie, J.O., 2010. Formation of copper aluminate spinel and cuprous aluminate delafossite to thermally stabilize simulated copper-laden sludge. J. Hazard. Mater. 181, 399–404. Jalali, R., Ghafourian, H., Asef, Y., Davarpanah, S.J., Sepehr, S., 2002. Removal and recovery of lead using nonliving biomass of marine algae. J. Hazard. Mater. 92, 253–262. Kim, E.J., Herrera, J.E., 2010. Characteristics of lead corrosion scales formed during drinking water distribution and their potential influence on the release of lead and other contaminants. Environ. Sci. Technol. 44, 6054–6061. Kim, E.J., Herrera, J.E., Huggins, D., Braam, J., Koshowski, S., 2011. Effect of pH on the concentrations of lead and trace contaminants in drinking water: a combined batch, pipe loop and sentinel home study. Water Res. 45, 2763–2774. 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. Kuxmann, U., Fischer, P., 1974. Lead monoxide–aluminum oxide, lead monoxide– calcium oxide, and lead monoxide–silicon dioxide phase diagrams. Erzmetall 27, 533–537. Lu, X., Shih, K., 2011. Phase transformation and its role in stabilizing simulated leadladen sludge in aluminum-rich ceramics. Water Res. 45, 5123–5129. Magallanes-Perdomo, M., Carrodeguas, R.G., Pena, P., De Aza, P.N., De Aza, S., De Aza, A.H., 2009. Non-isothermal devitrification study of wollastonite-tricalcium phosphate bioeutecticÒ glass. Key Eng. Mater. 396–398, 127–130. Mao, L., Cui, H., An, H., Wang, B., Zhai, J., Zhao, Y., Li, Q., 2014. Stabilization of simulated lead sludge with iron sludge via formation of PbFe12O19 by thermal treatment. Chemosphere 117, 745–752. Park, S.E., Chang, J.S., 1993. Oxidative coupling of methane over PbO/PbAl2O4 catalysts. Stud. Surf. Sci. Catal. 75, 2233–2236. Rawlings, R.D., Wu, J.P., Boccaccini, A.R., 2006. Glass-ceramics: their production from wastes – a review. J. Mater. Sci. 41, 733–761.
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