Distribution Characteristics of Heavy Metals in Different Size Fly Ash

Article pubs.acs.org/EF

Distribution Characteristics of Heavy Metals in Different Size Fly Ash from a Sewage Sludge Circulating Fluidized Bed Incinerator Yanlong Li, Ruoqi Cui, Tianhua Yang, Zhenyu Zhai, and Rundong Li* Key Laboratory of Clean Energy, College of Energy and Environment, Shenyang Aerospace University, Shenyang, Liaoning 110136, P. R. China ABSTRACT: Fly ash samples were collected from a sewage sludge incineration power plant in Zhejiang Province, China, where household sludge was mixed with industrial sludge in a ratio of 1:1. A dry-type centrifugal air classifier was used to separate and obtain particles with different sizes. According to the different air flow rates, five size fractions were obtained. The total size distribution of fly ash is described in this paper, and the chemical composition, mineralogical phase, and microstructure of the five fractions of fly ash were detected using X-ray fluorescence spectrometry, X-ray diffractometry, and scanning electron microscopy with energy-dispersive X-ray spectrometry, respectively. The heavy metal content and speciation with various particle sizes were analyzed using a sequential chemical extraction procedure. An integrated evaluation method that considers the type, content, toxicity, and stability of heavy metals was used to evaluate pollutants of the different particle size segments. An attempt was made to correlate the size, toxicity, structure, and properties.

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INTRODUCTION In recent years, sewage sludge incineration has become an irreplaceable method of disposing of sewage sludge because of its high efficiency, noticeable volume and mass reduction,1 and high energy utilization. In comparison to direct landfilling, incineration has the two main advantages of reducing the volume of sewage sludge by approximately 90% and reducing the reactivity through the nearly complete destruction of organic compounds. However, during the process of sewage sludge incineration, a large amount of fly ash can be produced, which is especially problematic because of the high content of heavy metals that are easily released. The content of fly ash accounts for 3%−5% of the total sludge incineration.2 Emissions of heavy metals from sewage sludge incineration plants are a great environmental concern because of their toxicity for both human health and the environment. Metals are not destroyed by high-temperature thermal treatment, and some vaporized metals may be emitted. Therefore, it is essential to understand the mechanism of release of heavy metals during high-temperature waste treatment to improve the understanding of their behavior and to control their emissions. Sewage sludge fly ash is listed as hazardous waste because of its high heavy metal content, which includes Cu, Pb, Zn, and Cr, as well as the presence of some toxic organic pollutants, such as polychlorodibenzo-p-dioxin (PCDD). Therefore, sewage sludge fly ash needs to be disposed of safely.3 The heavy metal toxicity of sewage sludge fly ash is not only related to the total content of heavy metals but also determined by the distribution characteristics of heavy metals. Both the biological and environmental effects differ greatly if the total content of heavy metals is the same but the distribution characteristics of the heavy metals are different.4 Currently, the study of heavy metals in fly ash is concentrated on the aspects of its content and leaching characteristics at home and abroad.5−7 Two primary questions remain to be answered. First, fly ash is assumed to be composed of uniform particles. However, because the formation conditions of fly ash © 2017 American Chemical Society

(e.g., the furnace, temperature, and turbulence properties) differ, the particle size and distribution of heavy metals are very uneven. Therefore, this work focused on the content and chemical speciation of heavy metals in fly ash with different particle sizes from sewage sludge incineration. Second, because practical applications of the evaluation of different heavy metals have shown inconsistent results,8,9 selecting a technology and optimizing processes for heavy metal pollution control is difficult. A new integrated evaluation method for heavy metal pollution control is proposed in this paper. High-precision air classification equipment was used in this work to separate particles with different sizes. The dry-type centrifugal air classifier can form an overall flow with almost no vortex or only a very small local vortex. The separated coarse and fine particles are no longer mixed but are quickly discharged. In the air flow, the energy consumption per unit output is low, the upper and lower particle sizes of the classification can be freely selected, and the mass production of the equipment uses micron or submicron ultrafine powder. In contrast to the use of a sizing screen, dry-type centrifugal air classifiers use air to break up agglomerated particles and create minimum damage to the original particles, making the results more accurate. For pretreatment separation before the use of fly ash as a resource, a dry-type centrifugal air classifier should be used.

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MATERIALS AND METHODS

Fly ash was collected in a sewage sludge incineration power plant in Zhejiang Province, China. Household sludge and industrial sludge (mixed 1:1) are incinerated in this plant, which is close to the practical scenario in most plants. The fly ash was collected in bag filters. First, we took random samples over the course of 3 days, and the ash content of each sample was 5 kg to guarantee a representative ash Received: October 17, 2016 Revised: January 12, 2017 Published: January 14, 2017 2044

DOI: 10.1021/acs.energyfuels.6b02676 Energy Fuels 2017, 31, 2044−2051

Article

Energy & Fuels Table 1. Equipment Technical Parameters during the Experiment classifier parameters

induced draft fan parameters

fly ash particle size (μm)

frequency (Hz)

power (kW)

rotation speed (rpm)

frequency (Hz)

power (kW)

rotation speed (rpm)

feeding speed (g/min)

50

50 50 44.8 39.6 34.5

1.1 1.1 1.1 1.1 1.1

5015 5015 4498 3972 3460

30 35 45 50 50

7.5 7.5 7.5 7.5 7.5

1770 2065 2655 2950 2950

50 50 65 100 160

neously, the morphologies of the different particle sizes were characterized using scanning electron microscopy (SEM). 2.2. Chemical Analysis of the Heavy Metals in Different Particle Size Segments. To determine the total amount of heavy metals, the analysis used heating furnace digestion with HNO3/HF/ HClO4 to digest the samples. The total concentration of the different size particles was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer Analyst 8300 spectrometer. The chemical speciation of the heavy metals was analyzed by the Bureau Communautaire de Référence (BCR) four-step sequential extraction procedure, which is related to the test procedure of the European Union. Four fractions were obtained after the four-step sequential extraction, which we called acid-extractable, reducible, oxidizable, and residual. The acid-extractable fraction represents the heavy metals that are sensitive to environmental changes and are easy to transport and transform in the weak acid environment. The reducible fraction represents heavy metals combined with iron and manganese oxide, which can be leached in a neutral salt environment that is not stable (the same as the acid-extractable fraction). The oxidizable fraction represents heavy metals that belong to organic matter, such as sulfide and carbon. The oxidizable components in fly ash indicate that heavy metals are absorbed by activated carbon, which is used to reduce nitrogen oxide. The residual fraction represents the heavy metals that can be leached only under strongly acidic conditions and belong to the stable state. 2.3. Adoption of a New Integrated Method Regarding Evaluation of the Overall Pollution Toxicity. A new integrated method regarding evaluation of the overall pollution toxicity was adopted in this paper.10 In comparison with existing protocols, such as the pollution load index (PLI)11 or the geoaccumulation index,12 the new integrated toxicity assessment is more comprehensive. It considers the type, content, toxicity, and stability of heavy metals. The establishment of the new method was based on these developed indexes. The key to this method is the calculation of the overall pollution toxicity index (OPTI) of heavy metals, indicating their

Table 2. Reference Concentrations of Heavy Metals in the Topsoil in China (C0k) and Toxic Response Factors of Heavy Metals (Trk) C0k (mg/kg) Trk

Ni

Cd

Cr

Pb

Cu

Zn

As

27 5

0.10 30

61 2

26 5

23 5

74 1

11 5

sample. According to the centrifugal principle, the air classifier was used to separate the samples into different size particles. We had a very high requirement for particle size. Adjusting only the size of the classifier’s centrifugal force was insufficient to attain a fine powder of approximately 1 μm. To guarantee the fineness and precision of the grading treatment, we had to adjust the rotation speed of the induced draft fan to regulate the size of the internal negative pressure traction of the equipment. Meanwhile, we also controlled the feeding speed to maintain the stability of the equipment’s internal gas powder concentration, and the final classification’s accuracy was improved significantly. Detailed experimental device technical parameters are shown in Table 1. Compared with the use of a sizing screen, the biggest difference is that an air classifier uses air to break up the agglomerated particles, which is less destructive than the sizing screen technique. On the basis of the research objective, the relation between heavy metals in fly ash and particle size, the experimental process included sample collection, particle size separation, particle size measurement, measurement of fly ash heavy metals, and measurement of the toxicities of different particle sizes. The specific operations are outlined below. 2.1. Physical Analysis of Different Particle Sizes. The particle size distribution was analyzed using a Mastersizer 2000 instrument, which uses a wet measurement. An X-ray fluorescence (XRF) spectrum analyzer (ZSX100e) was used to analyze the principal component. X-ray diffraction (XRD) (PRO/MPD) was used to analyze the mineral phases of the different particle sizes. Simulta-

Figure 1. Distribution of original fly ash. 2045


Article

Energy & Fuels Table 3. Main Components of Different Particle Size Fly Ash (wt %) particle size

Fe2O3

Al2O3

CaO

SiO2

SO3

P2O5

MgO

K2O

Cl

50 μm

28.7017 28.4121 24.7616 22.9866 25.8233

18.4541 18.3025 23.4326 15.8507 15.4982

18.2681 18.1409 18.1301 16.1066 16.2071

16.9882 17.7448 17.1379 28.5824 26.5487

5.9697 5.9710 6.2142 5.2778 4.9821

5.0624 4.9057 4.1179 3.8000 3.7233

1.6900 1.6750 1.5269 1.2095 1.1731

1.1667 1.1977 1.3085 1.4110 1.3571

0.0608 0.0502 0.0929 0.0942 0.0799

Figure 2. Fly ash samples after classification. where k denotes the type of heavy metal used, Ck is the heavy metal concentration, and C0k is the reference value for the heavy metal concentration. Usually, the reference value for the evaluation uses the highest background value for the heavy metal, which is found in topsoil before industrialization, as shown in Table 2.13,14 Toxicity reflects the toxicities of heavy metals and the sensitivity of organisms toward these heavy metals. Toxicity can be indicated by the toxic response factor (Trk) in Hakanson’s potential ecological risk index method, as shown in other references.15 Values of Trk for several heavy metals are shown in Table 2.16,17 Stability reflects the releasability of heavy metals in different forms. Stability can be determined by leaching characteristics. Only heavy

potential risk to the environment and considering the four parameters quantity, intensity, toxicity, and stability, which are described below. Quantity reflects the integrated influence of heavy metals to the environment. The hazards of multiple heavy metals should be higher than those of a single heavy metal hazard or a few heavy metal hazards. Intensity directly reflects the hazardous effects of heavy metals on the environment: the higher the concentration of heavy metals, the greater is the pollution intensity. The heavy metal pollution intensity (CIk) can be calculated as follows:

CkI = Ck − Ck0 2046


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Energy & Fuels Table 4. Heavy Metal Contents (in mg/kg) of Various Size Fractions of Fly Ash particle size

Zn

Cu

Cr

Pb

Ni

Ba

Mn

As

50 μm

3785 3307 2803 2723 2548

5013.5 4087 3386 3249 3542

3533 3609.7 2777.5 2910.5 3253

236.8 205.6 164.7 157.4 124

1106 940.3 826.15 829.9 885.2

619.5 577.7 577.15 629.85 640

1417 1250 1172 1168 1234

32.1 42.4 42.9 32.2 9.1

metals that are released to the environment can cause an ecological risk. Heavy metals usually dissolve in water prior to harming the ecological environment. Thus, to characterize the stability of a heavy metal, the leaching rate (Lk) may be defined as follows:

Lk =

mk1 mk0

size distribution is very wide, ranging from 0.316 to 2187.762 μm, but 98.07% of the mass is constituted by particles with sizes less than 500 μm. According to the cumulative distribution of the particle size, particles less than 104.713 μm in size account for 69.71% of the particulate matter. The second part, with sizes ranging from 104.713 to 549.541 μm, accounts for 28.46%, and the third part, with sizes greater than 549.541 μm, accounts for only 1.81%. This result occurs mainly because larger particles were deposited in the bottom ash due to the quality. Larger particles of fly ash account for only a very small portion, a result that may be related to the combustion of the furnace and the different types of air pollution control equipment used. 3.2. Chemical Composition of Different Particle Size Fly Ash. As shown in Table 3, the main components of fly ash include Fe2O3, Al2O3, CaO, SiO2, SO3, P2O5, MgO, and K2O. The content of SiO2 was found to decrease with decreasing particle size. However, Fe2O3, Al2O3, CaO, and MgO showed an increasing trend with decreasing particle size. The colors of the different particles appeared to be significantly different: the larger the fly ash particle, the darker the color (Figure 2). 3.3. Heavy Metal Contents of the Different Particle Sizes. To guarantee the accuracy of the heavy metal analysis by ICP-OES, we set up three parallel samples for each particle size segment and took the average value of the three parallel samples to ensure the stability of the samples. The heavy metal contents of the different particle sizes are shown in Table 4. Among them, the Pb and Zn contents decreased with increasing particle size, whereas the Cu, Cr, Ni, Ba, and Mn contents first decreased and then increased with increasing particle size. The heavy metal contents increased with increasing particle size in particles larger than 50 μm, a result that was due to heterogeneous condensation.18 In addition, coarse particles were mainly unburned mineral particles, and their heavy metal content was high. In addition, the coarse particles may have also adsorbed the fly ash of fine particles, so the heavy metal content of the fly ash was relatively large. The heavy metal content in the smaller-sized particles was generally far greater than the grain diameter of the heavy metal content, a result that occurred mainly because smaller-sized fly ash particles themselves may be composed of small heavy metal particles, representing condensation of a heavy metal atmosphere. Smaller-sized fly ash had a greater surface area and could more easily adsorb heavy metal particles.19,20 For particles smaller than 1 μm, which were composed of smaller particles, the reasons for the greater surface area are illustrated in Figure 3. The surface was made of particles with sizes of 14.22 and 26.27 nm, which was the result of agglomeration of smaller particles. To determine the origin of the high Cu, Zn, and Cr contents, we inspected the industries near the solid waste incineration plant. Some pharmacy and leather industries were built around the incineration plant. In the pharmacy industry, trace amounts of Cu had been added to the process, whereas trace amounts of

× 100%

where k is the type of heavy metal considered, m1k is the mass of the heavy metal in the leaching solution, and m0k is the mass of the heavy metal in the leaching sample. Using these four parameters, the overall pollution toxicity index of heavy metals is calculated as follows: N

OPTI =

∑ TkrCkILk k=1

where N is the number of heavy metals.

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RESULTS AND DISCUSSION 3.1. Particle Size Distribution of Fly Ash. Figure 1 shows the particle size distribution of the fly ash sample. The particle

Figure 3. SEM image of a 1 μm fly ash particle.

Figure 4. Types of oxides in the main components of fly ash. 2047


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Energy & Fuels

Figure 5. SEM photomicrographs of particles in different particle size segments.

Figure 6. Mineral phases of different particle sizes in fly ash.

Zn and Cr had been added to the process used in the leather industry. The addition of these heavy metals was the reason for the high Cu, Zn, and Cr contents. Alternatively, the particle size was generally classified as 10 μm, and the main components of fly ash were

divided into acid oxide, basic oxide, and amphoteric oxide. As shown in Figure 4, the acidic oxide increased with increasing particle size, and the basic oxide increased with decreasing particle size. Combined with the heavy metal contents in different particle sizes, these results show that the basic oxides 2048


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Energy & Fuels

Figure 7. Chemical speciation of heavy metals in different particle size fly ash.

and heavy metals were binding, whereas the acid oxides and heavy metals were the opposite. Next, by subdividing the particle sizes into five size fractions, we found that the acidic oxide content in the 10−50 μm particles was higher than that in the >50 μm particles. Similarly, the alkaline oxide content in the 10−50 μm particles was less than that in the >50 μm particles. On the basis of the analysis of the heavy metal (Cu, Cr, Ni, Ba, and Mn) contents of these two size fractions, we verified that the basic oxides were easily combined with heavy metals, and the acid oxides were the opposite. 3.4. Morphological Characterization and Mineral Phases of the Different Particle Sizes in Fly Ash. In Figure 5, the five particle size segments are labeled as 1, 2, 3, 4, and 5 from small to large. Particles in the first three particle size segments had smooth surfaces with signs of melting, and we could conclude that small particles tended to be formed by agglomeration of smaller particles (visually less than 1 μm). In contrast, particles in the two larger particle size segments had

rough surfaces. Many small particles were attached to the surfaces of larger particles in the original fly ash, and after centrifugal air classification, the amount of small particles attached to the surface of the larger particles was significantly reduced. This analysis also indicates the effect of the air classifier. According to the XRD results, shown in Figure 6, the main mineral phase of fly ash consisted of Fe2O3, Fe3O4, SiO2, and CaSO4. The peak intensity of SiO2 was stronger with increasing particle size. Si existed in the form of silicate in the small particle size segment. The silicate formation process occurs when SiO2 reacts with iron oxides during melting. The SEM images showed that the small particles had a smooth surface as a result of SiO2 melting, which was consistent with the XRD results. SiO2 was observed in the fly ash, and the strength increased with increasing particle size. This result was consistent with the XRF detection results. 2049


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Figure 8. TCLP leaching contents.

extractable fraction (ACE), the reducible fraction (RED), the oxidizable fraction (OX), and the residual fraction (RES). As shown in Figure 7, generally, the residual fraction of heavy metals is dominant. Among them, Pb, Ni, and Cr almost all appear in the residual fraction. Ni and Cr have only two fractions, ACE and RES, and ACE gradually decreases until it disappears as the particle size increases. Additionally, Pb has only two fractions, OX and RES, and OX also decreases with increasing particle size. For Cu, Mn, and Ba, ACE and RED significantly decrease and OX and RES increase with increasing particle size. Zn does not change significantly for any of the particle size segments. Generally, as the particle size becomes larger, the heavy metals in the fly ash develop toward a stable trend. 3.6. Integrated Control Efficiency of Heavy Metals in Different Particle Size Segments. To evaluate the toxicity of the heavy metals better in fly ash with different particle sizes, a new integrated method was adopted in this paper, based on the overall pollution toxicity index (OPTI). First, we adopted toxicity characteristic leaching properties (TCLP) as the leaching standard. TCLP is a method that simulates leaching of heavy metals at the time of landfilling, and it can be used to evaluate the toxicity of fly ash by simulating

Figure 9. Overall pollution toxicity index (OPTI) for different particle sizes.

3.5. Heavy Metal Chemical Speciation of Different Particle Sizes in Fly Ash. To better understand the types of heavy metals and their chemical speciation in the fly ash, the BCR four-step method of sequential extraction was used. The BCR method is divided into four steps, corresponding to the following four types of chemical heavy metal species: the acid2050


Energy & Fuels

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ACKNOWLEDGMENTS The authors acknowledge support through a research grant from the National Natural Science Foundation of China (51576134).

the harshest environment. The leaching results can be used to determine whether hazardous waste will meet the land disposal contamination limit. Figure 8 shows the leaching contents of heavy metals in different particle size segments. The leaching contents of Cu, Zn, Cr, and Ni decreased with increasing particle size. However, As, Pb, and Cd were not detected in any of the particle size segments, a result that was likely related to the fly ash properties. Two possible reasons existed for the results that were obtained from digestion and the four-step extraction procedure. First, the total contents of the three heavy metals in all of the different particle size segments were very low. Second, As and Pb were not found in the first step of BCR extraction, and even Cd was not found in all of the extraction steps, a result that was the same in digestion. The Ba leaching content showed first a decrease and then an increase with increasing sample particle size. This result also corresponded to digestion. Figure 9 shows the OPTI values for the different particle size segments. The results varied regularly with particle size: as the particle size decreased, the toxicity increased. The red line represents the overall toxicity of the original fly ash. If we separate fly ash into two segments that are larger than 10 μm and smaller than 10 μm, we can see that the toxicity of the original fly ash is somewhere between those of the two segments. Therefore, we conclude that fly ash that is less than 10 μm has more toxicity. According to the toxicities of different particle size segments, we should take resource utilization of the fly ash with different particle sizes into account.

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REFERENCES

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CONCLUSIONS The following conclusions can be reached on the basis of our results: 1. Particles with sizes of less than 1 μm are the result of the agglomeration of smaller particles. 2. According to the results of the triangle diagram of the main components of fly ash, heavy metals are easily adsorbed with basic oxide. 3. Overall, the heavy metal contents of small-sized fly ash were higher than those of large-sized fly ash. The residual fraction was dominant in the different particle size segments, and the proportion of the acid-soluble fraction became smaller and the residue increased with increasing particle size. 4. Using a new integrated method to evaluate the overall pollution toxicity for fly ash, we found that the toxicity of small-sized fly ash is higher than that of large-sized fly ash. The toxicity of fly ash less than 10 μm in size is higher than that of the original fly ash, and this material needs to be handled with caution.

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: (86) 024-89728889. Fax: (86) 024-89724558. E-mail: [email protected]. ORCID

Rundong Li: 0000-0002-8669-5397 Notes

The authors declare no competing financial interest. 2051


Distribution Characteristics of Heavy Metals in Different Size Fly Ash

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