Waste Management xxx (2015) xxx–xxx
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Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk Jizhi Zhou a, Simiao Wu a, Yun Pan a, Lingen Zhang a, Zhenbang Cao a, Xiaoqiao Zhang a, Shinichi Yonemochi b, Shigeo Hosono b, Yao Wang a, Kokyo Oh b,⇑, Guangren Qian a,⇑ a b
School of Environmental and Chemical Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200444, PR China Center for Environmental Science in Saitama, 914 Kamitanadar, Kazocity, Saitama Prefecture 347-0115, Japan
a r t i c l e
i n f o
Article history: Received 10 March 2015 Revised 4 June 2015 Accepted 18 June 2015 Available online xxxx Keywords: MSWI fly ash Different particle size Heavy metal Health risk
a b s t r a c t During the pretreatment and recycling processes, the re-suspended dust from municipal solid waste incinerator (MSWI) fly ash might pose a significant health risk to onsite workers due to its toxic heavy metal content. In this work, the morphological and mineralogical characteristics of fly ash in different particle sizes are presented. The concentrations of seven trace elements (Zn, Pb, Cu, Cd, Cr, Fe and Mn) in these samples were determined. The results show that volatile metals, such as Zn, Pb, Cu and Cd, were easily concentrated in the fine particles, especially in Dp2.5-1 and Dp1, with soluble and exchangeable substances as the main chemical species. The health risk assessment illustrated that the cumulative hazard indexes for non-carcinogenic metals in Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 were 1.69, 1.41, 1.78 and 2.64, respectively, which were higher than the acceptable threshold values (1.0). The cumulative carcinogenic risk was also higher than the threshold value (106). For the onsite workers, the relatively apparent non-carcinogenic and carcinogenic effects were from Pb and Cr, respectively. The above findings suggest that fine-grained fly ash contained a considerable amount of heavy metals and exhibited a great health risk. Ó 2015 Published by Elsevier Ltd.
1. Introduction Fugitive dust in urban areas, consisting of numerous heavy metals and fine particles, is an important contributor to detrimental atmospheric particulate matter (PM) and hosts a great number of environmental pollutants. Hazardous pollutants contained in urban dust can enter human bodies and endanger human health through re-suspension–inhalation, hand-mouth ingestion and dermal contact. In particular, PM with diameter below 10 micrometer (lm) (PM10) and 2.5 lm (PM2.5) showed inhalable risk and pulmonary infection risk, respectively (Buonanno and Morawska, 2014; He et al., 2008; Pancras et al., 2013; Zereini et al., 2005; Zheng et al., 2010a). Zheng et al. (2010a,b) gave the results dealing with the concentrations, spatial distributions, source identification, and health risks of heavy metals in fine particles of dust in different districts, such as urban streets and industrial and smelting areas. It was well recognized that the concentrations of heavy metals, such as Hg, Pb, Cd, Zn and Cu, in the fine particles of dust among ⇑ Corresponding authors. E-mail (G. Qian).
addresses:
[email protected]
(K.
Oh),
[email protected]
industrial districts were significantly higher than those in street dust compared with atmospheric emissions (Alvim-Ferraz and Afonso, 2005; Chen et al., 2013; Zheng et al., 2010b; Zhu et al., 2013), which indicated that dust from industrial sources produced a greater risk for human health. Municipal solid waste incinerator (MSWI) fly ash is an aggregation of fine particles. MSWI fly ash has been classified as a hazardous waste due to the presence of toxic heavy metals and dioxin-like organic contaminants (Tian et al., 2012). How to recycle or safely dispose MSWI fly ash has become a global concern. In recent years, with the development of pretreatment and recycling technology, MSWI fly ash has been used as a potential substitute for raw civil materials by the hydration reaction at normal temperatures and the ceramic formation process at a high temperature. In these processes, fine fly ash particles can be diffused into atmosphere by wind or artificial turbulence. This portion of fine particle is defined as re-suspended dust (Ram et al., 2014), which size is usually below 10 lm and has health risk to the person who is exposed to the heavy metal bearing dust. Therefore, the secondary pollution triggered by heavy metals must be highly focused, including leaching in special landfill sites or as recycled products, potential volatilization under high temperature in ceramic
http://dx.doi.org/10.1016/j.wasman.2015.06.026 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
aggregate applications and re-suspended dust in recycling processes. (Haiying et al., 2010; Quina et al., 2008a,b; Sukandar et al., 2006). A large number of studies focused on the total amount, chemical species and potential TCLP leaching of heavy metals from MSWI fly ash in stabilization/solidification (S/S) treatment and recycling ˇ ová et al., 2013; Pan et al., processes (Cetin et al., 2012; Kubon 2013; Quina et al., 2008a; Shi and Kan, 2009; Sukandar et al., 2006). However, studies regarding the risk of MSWI fly ash in the re-suspended dust during S/S pretreatment or the recycling process are quite scarce. Moreover, studies regarding a comprehensive understanding of the distribution of heavy metals in different particle sizes of re-suspended fly ash dust and health risk assessments for fine particles are still lacking. The objectives of this work were (1) to conduct a systematic investigation of the morphology, chemical components and distributions of dominant trace elements (Pb, Zn, Cu, Cd, Cr, Fe and Mn) in different sizes of particles in MSWI fly ash; (2) to evaluate the potential human health risks including non-carcinogenic and carcinogenic risks by using heavy metal concentrations and distribution information resulting from re-suspended MSWI fly ash dust.
2. Materials and methods 2.1. Sampling of particulate fly ash and particle size distribution Samples of the bulk fly ash from a municipal solid waste incineration plant in Shanghai were screened by size. A low volume air sampler (AN-200, flow rate: 28.3 L/min) was applied to collect samples among a high concentration of fly ash. At the same ambient temperature and relative humidity, the sample was collected on a round quartz-fiber filter with 80 mm in diameter. The particle size distribution of the fly ash was measured by the LS-POP 6 Laser Particle Analyzer. For the separation of particles with different sizes, the quartz-fiber filters with mech size from 1 lm to 10 lm was used. The following PM fractions were collected: Dp > 10 (larger than 10 lm), Dp10-5, Dp5-2.5, Dp2.5-1 and Dp < 1 (smaller than 1 lm). The Dp1 sample was collected at the final stage. All of the samples were weighed at a constant temperature.
2.3. Chemical analysis of heavy metals The total concentrations of heavy metals in different particle-size ranges of fly ash were determined by inductively coupled plasma mass spectrometry (ICP-MS) after samples were digested using HNO3, HF and HClO4 in a heat-dispelling furnace. A four-step sequential-extraction procedure (SEP) developed by Fernández Espinosa et al. (2002) was followed to fractionate the Dp10-5, Dp5-2.5, Dp2.5-1 and bulk ash trace metals. The procedure was used in exposure studies for measuring the bio-available fractions of toxic metals associated with particulate matter (Feng et al., 2009; Gómez et al., 2007). In this sequential extraction procedure, the metals were categorized as four fractions: acid extractable (F1), reducible (F2), oxidizable (F3) and residual (F4). The detail procedure was listed in Table S1 and the dosage of sample for SEP was 2.0 g. All of the sample analyses were performed in triplicate which recovery rates were between 85% and 110%, and the average values of the results were reported. 2.4. Health risk quantification The workers worked in the municipal solid waste incinerator plants may contact with heavy metal of fly ash directly. The onsite workers contacted heavy metals through three main routes of exposure: ingestion, inhalation, and dermal contact (Wang et al., 2015). To estimate the occurrence of negative health effects from the Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 particles in re-suspended fly ash dust during the disposal and recycling processes, risk assessment models and relevant parameters from the US EPA (EPA, 1989, 2001; Schoof, 2003; Wang et al., 2013) were used in this study. The assessment was performed by using the total concentrations of heavy metals in four different particle fractions of fly ash. For the non-carcinogenic risk, the average daily dose contacted through inhalation (CDIinh) and absorbed through skin (CDIdermal) and ingestion (CDIing) were calculated as follows (EPA, 1989):
CDIinh ¼
C EF ET ED PEF AT 24
CDIdermal ¼
C EF ED SA AFd ABSd 106 BW AT
ð1Þ
ð2Þ
2.2. Physical characterization The morphology of sample with different sizes was characterized by scanning electron microscopy (S-3000N, HITACHI Co.) at 15 kV of accelerating voltage, 13–15 mm of working distance (WD), 150 mm of probe size, and 107 A of beam current. Simultaneously, an energy-dispersive X-ray spectrometry (EDX) was used to analyze the amount of element Ca, Si, Cl, Cu, Zn, Pb, Cr and Cd on the sample with the calibration of the corresponding pure metal oxides (for Cl, the pure NaCl was used). For each sample, the statistics of EDX result from 20 detected area (1 lm 1 lm) was carried out to survey the element distribution of sample with different sizes. Before the SEM/EDX analysis, a gold film with 10 nm thickness in average was coated on the sample in a vacuum coating machine after 10 min coating. X-ray diffraction (XRD, Dnmax-2550, Rigaku) was used to identify crystalline phases in the particulate matter fly ash samples. Four samples (Dp10-5, Dp5-2.5, Dp2.5-1 and bulk fly ash) were pulverized by manual grinding in agate mortar before XRD measurement. Analyses were performed on a low background plate of single crystal silicon with Cu Ka radiation at 40 kV and 40 mA. For fine particle such as Dp5-2.5 and Dp2.5-1, the sieving was carried out by several times until enough amount of the sample was gotten for the XRD measurement.
CDIing ¼
C IRfa EF ED 106 BW AT
ð3Þ
where CDIinh, CDIdermal, and CDIing are the doses of the pollutant (mg kg1 day1); C is the concentration of metals in different particle sizes and bulk fly ash (mg/kg); IRfa is the fly ash ingestion rate for the receptor (mg/day); EF is the exposure frequency (day/year); ED is the exposure duration (year), ET is exposure time (h/day); and AT is the averaging time. For the non-carcinogenic risk, AT is 25 years, and for carcinogenic risk, AT is 70 years. BW is the body weight (kg), SA is the contact area of skin surface (cm2/day), PEF is the particle emission factor (m3/kg), AFd is the dermal adherence rate (mg/cm2-skin), and ABSd is the dermal absorption factor (unitless). The above exposure parameters for the inhalation, ingestion, and dermal contact pathways were listed in Tables S2 and S3, respectively (EPA, 1989, 1996, 2001, 2004, 2011; Wang et al., 2013). After the CDIinh, CDIdermal and CDIing were calculated, the potential human health risk effect through a single exposure pathway could be evaluated by the hazard quotient (HQ). The HQ was the ratio of the average daily intake of a contaminant per unit body weight to an acceptable reference dose. A hazard index (HI) was used to estimate the risk of cumulative contamination in different particle sizes:
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
HI ¼ HQ ing þ HQ inh þ HQ dermal CDIing CDIinh CDIdermal ¼ þ þ RfDing RfCinh RfDing ABSGI
particle Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1 was discussed in details.
ð4Þ
where HQ is the hazard quotient, RfDing is the chronic oral reference dose (mg kg1 day1), RfCinh is the chronic inhalation reference concentration (mg/m3), and ABSGI is the gastrointestinal absorption factor (a.u.) used to multiply RfDing to yield the corresponding dermal values. The above toxicological characteristics of the investigated heavy metals are listed in Table S3 (EPA, 2011). If the total hazard index (HI) is greater than 1, then adverse health effects are possible. For the carcinogenic risk, the equation is described below (EPA, 1989):
RISKi ¼ CDIi SF RISK ¼
X
RISKi
3
ð5Þ ð6Þ
where RISK is a unitless probability of an individual developing cancer and i represents the three different ways of doing so. SF is the slope factor for the toxicant of concern (mg kg1 day1)1. In this paper, risk refers to the total carcinogenic risk of the above three pathways. A risk value of 106 was the upper limit of acceptable cancer risk. 3. Results and discussion 3.1. Size distribution and constituents of fly ash 3.1.1. Particle size distribution of fly ash Fig. 1 illustrates the particle size distribution of bulk fly ash. The fly ash had a board size distribution with diameter from 0.3 to 70 lm, where the amount percentage of particle increased with the increase in particle size until near 20 lm. Moreover, the cumulative distribution of fly ash size exhibited that about 36% of particle had a diameter less than 10 lm. This size distribution of fly ash was not consistent with the result reported by Boom where over 85% of fly ash had particle size larger than 38 lm in MSWI boiler with coal addition (De Boom and Degrez, 2015). As the stove applicable to MSWI without any fuel addition in our study, the difference in fly ash size distribution indicates that the MSWI stove provides numerous fine particulate matter in fly ash (Lind et al., 2007). This observation suggests that the high health risk of fly ash from the MSW combustion stove as more fine particle with diameter below 10 lm. For further contamination investigation of fine particle in fly ash, the sieve of fly ash was conducted. Fine particle was divided into five groups according to particle diameter: Dp > 10, Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1. The distributions of Dp > 10, Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1 were 63%, 17%, 9%, 7% and 4% in the total bulk fly ash, respectively. The contamination and health risk of fine
3.1.2. Mineral phases and morphological characterization Fig. 2 shows the X-ray diffraction pattern of fly ash with various particle sizes. In the bulk fly ash, anhydrite, calcite, halite, and sylvite were identified with the appearance of calcium silicate, mayenite, and calcium aluminum silicate, which was consistent with the result reported elsewhere (Li et al., 2010; Wu et al., 2012). In addition, a small portion of iron oxide was also observed in bulk fly ash. In comparison, the strength of reflection peaks of most phases were decreased in Dp10-5 and Dp5-2.5, but that of mayenite and calcium aluminum silicate as the peak of these two phases at 17.5° (2h) was sharp. This suggested that mayenite and calcium aluminum silicate became dominant in the fly ash with particle size from 2.5 to 10 lm. In particular, a board peak between 10° and 40° (2h) was observed in Dp2.5-1 without any sharp peak, indicating that the crystal of phase in this fine particulate was small and poor. This observation demonstrated that the species of heavy metal was probably different on fly ash sample with various particle sizes as the dominant phases of particle was changed. The phases evolution with various particle size in fly ash sample was further characterized by SEM/EDX technology. As shown in Fig. 3A, the SEM image of bulk fly ash exhibited the morphology of large particle. In comparison, the sample Dp10-5 and Dp5-2.5 illustrated the aggregation of small particles (Fig. 3B and C) while Dp2.5-1 showed the smaller particle in SEM image (Fig. 3D) (Li et al., 2010; Wu et al., 2012). The simultaneous EDX data of Ca, Cl, Si as well as heavy metals was collected. As shown in Fig. 4, the energy reflection of Ca in bulk fly ash was larger than those in fine particle and decreased with the decreasing of particle size. This demonstrated that Ca was predominantly in anhydrite and calcite in bulk fly ash as the weak XRD diffraction of anhydrite and calcite in fine particles (Fig. 2). In contrast, the energy reflection of Si in Dp2.5-1 was higher than those in other samples. This indicated that Si was predominantly in Ca–Si phase in the case of fine particles, which was consistent with the result of XRD (Fig. 2). It is noting that the energy reflection of Cl was increased in samples with the particle size decreasing. This observation suggested that chloride was easily bond to fine particle. It is proposed that the evolution of crystal phase in fly ash probably leads to the variety of heavy metal species (Wu et al., 2015). For the investigation of heavy metal distribution on fine particle, energy reflection of heavy metal was also performed by EDX. As shown in Fig. 4, the energy reflection of most heavy metals was higher in fine particles than that in bulk fly ash. Zinc and copper in Dp2.5-1 exhibited higher energy reflection strength than in the particle with size from 2.5 to 10 lm. In comparison, the energy reflection of Cr was similar in all samples. This observation suggested that the enrichment of Zn and Cu was achieved on the
Fig. 1. Distribution of particle sizes in bulk fly ash.
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
1 Ca12Al14O33 : Mayenite
1 7
Relative intensity (a.u.)
5 CaCO3 : Calcite
2 2 CaSO4 : Anhydrite,syn 6 NaCl : Halite,syn 4 3 3 Ca2SiO4 : Calcium Silicate7 CaAl2SiO6 : Calcium 6 3 Aluminum Silicate 4 KCl : Sylvite,syn 5 24 7 3 6 7 4 8 Fe2O3: Iron Oxide 2 1 1 7 5 354 6 3 6 Bulk
8
8 8
88
Dp10-5 Dp5-2.5
Dp2.5-1 10
20
30
40
50
60
70
80
90
2 (Degree) Fig. 2. XRD patterns of different particle sizes of MSWI fly ash.
particle with size Dp2.5-1 > Dp10-5 > Dp5-2.5 > Bulk. For instance, the content of Zn in Dp2.5-1 and Dp1 was 4006–4587 mg/kg, much larger than those in Dp > 2.5 lm and bulk sample. In comparison, the content of Cr was similar in most cases except that in Dp1 which provided a highest content of Cr. This distribution of heavy metal content was consistent with the EDX result above (Fig. 4). To evaluate the enrichment of heavy metal in fine particle of fly ash, the factor EIi (enrichment index of heavy metal, i = element of heavy metal) was proposed, which stood for the percentage of heavy metal concentration in fine particle to its total concentration in bulk sample. The value of EIi for each heavy metal in fly ash was listed in Table 1. The EIi for Zn, Cu, Pb and Cd was 87%, 76%, 62% and 75%, which indicated large portion of heavy metals in fine particles. In comparison, 42% of EICr demonstrated that small particle size did not facilitate the enrichment Cr. In fact, the enrichment of Zn, Pb, Cu and Cd in fine particles of fly ash was determined by their evaporation performance at high temperature. During a high temperature combustion process, heavy metals in the waste undergo different chemical and physical transformations. Chlorides of Zn, Pb, Cu and Cd are easy to evaporate at high temperature (700–1000 °C). For example, Zn and ZnCl2 converted to gaseous Zn(g) and ZnCl2(g) at only 500 °C and 700 °C, respectively. (Tang et al., 2013; Vejahati et al., 2010). When flue gas containing the vapor of volatile heavy metals and compounds from the furnace at high temperature (approximately 1000 °C) flowed into the low temperature flue (300–500 °C), volatile substances could promptly cool down and form metal fines (Dp2.5-1, Dp1) (Belevi and Langmeier, 2000; Belevi and Moench, 2000; Sørum et al., 2004). This could explain why Zn, Pb, Cu and Cd could be easily enriched in fine particle sizes. Coarse-particle ash, such as Dp10-5 and Dp5-2.5, was formed from small char fragmentation adhering to minerals (Buhre et al., 2006; Nelson et al., 2010;
Fig. 3. SEM photomicrograph analyses of different PM fly ash (A: bulk fly ash; B: Dp10-5; C: Dp5-2.5; D: Dp2.5-1).
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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Relative percentage (%)
J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
4
25
Ca
20
20
15
15
10
10
5
5
0
Relative percentage (%)
Cl
Bulk
Si 3 2 1
0 Dp2.5-1 Dp5-2.5 Dp10-5
0 Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
4
Zn
1.6
Pb
3
2.5
Cu
2.0
1.2
1.5
2
0.8 1.0
1
0.4
0
0.0 Dp2.5-1 Dp5-2.5 Dp10-5
Relative percentage (%)
25
Bulk
0.5 0.0 Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
0.5
Cr 0.4 0.3
Cd
3 2
0.2 1 0.1 0.0
0 Dp2.5-1 Dp5-2.5
Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Fig. 4. Relative percentage of trace metals (Ca, Cl, Si, Zn, Pb, Cu, Cr, Cd) from EDX result in each size fraction of the samples.
Table 1 Mass concentrations of seven elements in different particle sizes of fly ash (mg kg1).
Bulk Dp10-5 Dp5-2.5 Dp2.5-1 Dp1 EIi (%)
Zn
Cu
Pb
Cd
Cr
Fe
Mn
1267 ± 61 25973 ± 10 2239 ± 66 4006 ± 200 4587 ± 22
199 ± 1 357 ± 11 308 ± 4 528 ± 2 648 ± 28
852 ± 5 1399 ± 41 1170 ± 45 1477 ± 66 2145 ± 12
29 ± 0 60 ± 2 49 ± 1 54 ± 6 90 ± 2
72 ± 2 81 ± 3 62 ± 3 60 ± 2 170 ± 3
4382 ± 332 3126 ± 33 1938 ± 6 2373 ± 8 4956 ± 13
197 ± 15 184 ± 4 120 ± 2 124 ± 3 260 ± 4
87
76
62
Shim et al., 2003; Tang et al., 2013; Vejahati et al., 2010). On the other hand, heavy metal is easily bonded to Fe oxide in fly ash. As the high content of Fe in fine particle even in Dp1 (Table 1), it is also supposed that the enrichment of heavy metal is probably relative to Fe in fine particle of fly ash. 3.2.2. Chemical species analysis To further understand the chemical species of these heavy metals, a method using a four-step sequential extraction procedure was adopted. The acid extractable fraction (F1) stands for the heavy metal in chloride, carbonate and complexes with weak coordination bond, which dissolves in the acidic solution. The reducible fraction (F2) is the heavy metal bond to Fe oxide. Once the Fe oxide is reduced, the dissolution of heavy metal occurs. F1 and F2 of heavy metal usually are highly unstable and are potentially bio-available to the respiratory tract of the human body. In comparison, Oxidizable fraction (F3) is commonly the heavy metal bond to organic matter, sulfide and carbon. In fly ash collected from air clean system, heavy metal is adsorbed on activated carbon which is used to reduce NOx. Thus, F3 in the current work was
75
42
24
31
defined as the heavy metal bond to carbon. The residual fraction (F4) was the heavy metal in indissoluble mineral phase such as phosphate. As more stability of F3 and F4, these fractions is not easily bio-available and has low health risk (Feng et al., 2009; Gómez et al., 2007; Perin et al., 1985). As shown in Fig. 5, F1 of heavy metal was much low in all samples. F3 and F4 of heavy metals were the predominant fractions in bulk sample. With the particle size decreasing, F1 and F2 were increased. For instance, the percentage of F1 for Pb in the bulk ash accounted for 0% of the total chemical species, while that was increased to 1% in Dp5-2.5 and 3% in Dp2.5-1. The increase in F1 was contributed to the increase in heavy metal adsorbed Ca–Si phase and heavy metal chloride as the content of these phase was increased in EDX result (Fig. 4). Moreover, the increase in the percentage of F2 for Pb was sharply, where F2 was increased to 50% in Dp2.5-1. For Zn, the F2 was increased from 12% in bulk ash to 25–33% in fine particle. This situation was observed in most heavy metals, indicating that Fe oxide was responsible for heavy metal enrichment in fine particle instead of chloride. As the less stability of F2, it is proposed that Zn, Pb, Cu, Cd in fine particle of fly ash
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
Percentage (%)
Dp10-5
Bulk
100
100
80
80
60
60
40
40
20
20
0
0
Zn
Cu
Pb
Cd
Cr
Zn
Cu
Pb
Cr
Cd
Cr
Dp2.5-1
Dp5-2.5
Percentage (%)
Cd
100
100
80
80
60
60
40
40
20
20
0
0
Zn
Cu
Pb
Cd
Cr
acid extractable
Zn
reducible
Cu
Pb
oxidizable
residual
Fig. 5. Chemical speciation of heavy metals in different particle sizes of fly ash.
Table 2 Hazard quotients for each heavy metal and exposure pathway. Size
HQ
Zn
Pb
Cu
Cd
Cr
Cumulative
Bulk
HQinh HQdermal HQing HI
1.52 107 1.79 103 1.36 102 1.54 102
2.86 104 1.35 101 7.71 101 9.06 101
1.79 107 7.42 104 1.61 102 1.68 102
1.22 104 3.51 103 3.35 102 3.71 102
3.01 105 2.89 102 2.75 102 5.64 102
4.39 104 1.70 101 8.61 101 1.03
Dp10-5
HQinh HQdermal HQing HI
3.12 107 3.68 103 2.80 102 3.16 102
4.70 104 2.22 101 1.27 1.49
3.21 107 1.33 103 2.89 102 3.02 102
2.52 104 7.24 103 6.89 102 7.65 102
3.40 105 3.26 102 3.10 102 6.36 102
7.56 104 2.67 101 1.42 1.69
Dp5-2.5
HQinh HQdermal HQing HI
2.69 107 3.17 103 2.41 102 2.73 102
3.93 104 1.86 101 1.06 1.24
2.77 107 1.15 103 2.49 102 2.61 102
2.04 104 5.88 103 5.60 102 6.22 102
2.61 105 2.50 102 2.38 102 4.88 102
6.24 104 2.21 101 1.19 1.41
Dp2.5-1
HQinh HQdermal HQing HI
4.81 107 5.68 103 4.31 102 4.88 102
4.96 104 2.34 101 1.34 1.57
4.75 107 1.97 103 4.27 102 4.46 102
2.25 104 6.48 103 6.17 102 6.85 102
2.53 105 2.43 102 2.31 102 4.74 102
7.48 104 2.73 101 1.51 1.78
Dp1
HQinh HQdermal HQing HI
5.50 107 6.50 103 4.94 102 5.59 102
7.21 104 3.41 101 1.94 2.28
5.82 107 2.42 103 5.23 102 5.47 102
3.79 104 1.09 102 1.04 101 1.15 101
7.16 105 6.86 102 6.53 102 1.34 101
1.17 103 4.29 101 2.21 2.64
have higher risk to human beings. In comparison, the percentage of F1 and F2 for Cr was low regardless of particle size reduction. F4 of Cr was increased from 80% in bulk ash to 92% in fine particle. This suggested that Cr in fine fly ash has low risk. Therefore, it is proposed that the enrichment of most heavy metals in fine particle is relative to the Fe oxide, which probably lead to the increasing of risk to human health when the person was exposed to fine fly ash.
3.3. Health risk assessment of heavy metals for different particles of fly ash 3.3.1. Non-carcinogenic risk assessment The hazards of heavy metals on human health are widely known. A model of health risk assessment was applied to evaluate the risk of Zn, Pb, Cu, Cd, and Cr in different particle sizes of fly ash. Table 2 lists the HQ value of non-carcinogenic risks of Zn, Pb, Cu,
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx Table 3 Carcinogenic risk assessment of Cd, Cr. Risk Cd Cr Cumulative
Bulk
Dp10-5 8
(6.04 ± 0.01) 10 (7.11 ± 0.18) 106 (7.17 ± 0.18) 106
Dp5-2.5 7
(1.24 ± 0.05) 10 (8.01 ± 0.30) 106 (8.13 ± 0.31) 106
Cd, and Cr that were estimated by three exposure pathways of inhalation, dermal contact and ingestion. The results indicated that the HIs of Zn, Pb, Cu, Cd, and Cr in Dp1 were significantly higher than in bulk ash, with the ranges of 2.7 102–5.6 102, 1.2– 2.3, 2.6 102–5.5 102, 6.2 102–1.2 101 and 4.9 102– 1.3 101, respectively. Obviously, Pb was the main contributor to the total non-carcinogenic risks compared with the other elements. In addition, ingestion was the key pathway, and the total non-carcinogenic risks were approximately 84–85% in the four particle sizes. The acceptable cumulative non-carcinogenic risks threshold value is 1.0 (EPA, 1991). The cumulative non-carcinogenic risks of all four particle sizes and bulk ash all exceeded 1.0, which ranged from 1.0 to 2.6 in the order: Dp1 > Dp2.5-1 > Dp10-5 > Dp5-2.5 > Bulk. The contents of heavy metals in Dp10-5 were slightly higher than that in Dp5-2.5. Therefore, the non-carcinogenic risk in Dp10-5 was also higher than that of Dp5-2.5. In particular, the HQ of the three pathways increased rapidly in Dp2.5-1 and Dp1. For example, the HQ of Dp1 was two times more than that of bulk ash. The results above showed that fine particles, especially Dp2.5-1 and Dp1, had significant non-carcinogenic risks for onsite workers. 3.3.2. Carcinogenic risk assessment According to US EPA research result, among these seven heavy metals, only Cd and Cr have a potential carcinogenic risk (EPA, 1991). No potential carcinogenic risk is considered for Zn, Pb, Cu, according to the USEPA report (EPA, 2001). The cumulative carcinogenic risks threshold value is 106 (EPA, 1991). As presented in Table 3, the cumulative carcinogenic risks of Cd and Cr was higher than the threshold value (106). Cr was the main contributor to carcinogenic risk compared with Cd, with values ranging from 7.1 106 to 1.7 105. Any cancer risk more than the threshold value (106) was considered to produce a cancer risk for onsite workers. In addition, the carcinogenic risks of Cd and Cr in Dp1 were the highest, with maxima of 1.9 107 and 1.7 105, respectively. Based on the results of the health risks above, meaningful measures to guarantee the health of workers have been put forward and should be carried out in the future. 4. Conclusions The content and species distribution of heavy metal in MSWI fly ash with various particle size was investigation. Four groups of particle size of fly ash was sieved in the following order: Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1. The XRD and SEM/EDX revealed that anhydrite, calcite, halite, sylvite, calcium silicate, mayenite, and calcium aluminum silicate were dominant phases in fly ash, which were decreased with the particle size reduction of fly ash. The iron oxide was identified in most samples with various particle size. The evolution of crystal phase with various particle size led to 87% of Zn, 76% of Cu, 62% of Pb, and 75% of Cd was enriched in fine particle of fly ash except Cr. The analysis of heavy metal species indicated that F2 of heavy metal was increased with the reduction of particle size. This observation suggested that the iron oxide was probably responsible for the enrichment of heavy metal. The enrichment of heavy metal resulted in the high non-carcinogenic risks of fine particle as the cumulative hazard
Dp2.5-1 7
(1.01 ± 0.006) 10 (6.16 ± 0.3) 106 (6.26 ± 0.3) 106
Dp1 7
(1.11 ± 0.13) 10 (5.98 ± 0.15) 106 (6.09 ± 0.16) 106
(1.87 ± 0.03) 107 (1.69 ± 0.28) 105 (1.71 ± 0.28) 105
indexes for non-carcinogenic metals in Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 were 1.69, 1.41, 1.78 and 2.64, respectively. The cumulative carcinogenic risk of Cd and Cr was higher than the threshold value (106). Therefore, our results reveals the high health risk of heavy metal in fine particle of fly ash due to the enrichment of unstable heavy metal fractions in particle with size
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Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk Jizhi Zhou a, Simiao Wu a, Yun Pan a, Lingen Zhang a, Zhenbang Cao a, Xiaoqiao Zhang a, Shinichi Yonemochi b, Shigeo Hosono b, Yao Wang a, Kokyo Oh b,⇑, Guangren Qian a,⇑ a b
School of Environmental and Chemical Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200444, PR China Center for Environmental Science in Saitama, 914 Kamitanadar, Kazocity, Saitama Prefecture 347-0115, Japan
a r t i c l e
i n f o
Article history: Received 10 March 2015 Revised 4 June 2015 Accepted 18 June 2015 Available online xxxx Keywords: MSWI fly ash Different particle size Heavy metal Health risk
a b s t r a c t During the pretreatment and recycling processes, the re-suspended dust from municipal solid waste incinerator (MSWI) fly ash might pose a significant health risk to onsite workers due to its toxic heavy metal content. In this work, the morphological and mineralogical characteristics of fly ash in different particle sizes are presented. The concentrations of seven trace elements (Zn, Pb, Cu, Cd, Cr, Fe and Mn) in these samples were determined. The results show that volatile metals, such as Zn, Pb, Cu and Cd, were easily concentrated in the fine particles, especially in Dp2.5-1 and Dp1, with soluble and exchangeable substances as the main chemical species. The health risk assessment illustrated that the cumulative hazard indexes for non-carcinogenic metals in Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 were 1.69, 1.41, 1.78 and 2.64, respectively, which were higher than the acceptable threshold values (1.0). The cumulative carcinogenic risk was also higher than the threshold value (106). For the onsite workers, the relatively apparent non-carcinogenic and carcinogenic effects were from Pb and Cr, respectively. The above findings suggest that fine-grained fly ash contained a considerable amount of heavy metals and exhibited a great health risk. Ó 2015 Published by Elsevier Ltd.
1. Introduction Fugitive dust in urban areas, consisting of numerous heavy metals and fine particles, is an important contributor to detrimental atmospheric particulate matter (PM) and hosts a great number of environmental pollutants. Hazardous pollutants contained in urban dust can enter human bodies and endanger human health through re-suspension–inhalation, hand-mouth ingestion and dermal contact. In particular, PM with diameter below 10 micrometer (lm) (PM10) and 2.5 lm (PM2.5) showed inhalable risk and pulmonary infection risk, respectively (Buonanno and Morawska, 2014; He et al., 2008; Pancras et al., 2013; Zereini et al., 2005; Zheng et al., 2010a). Zheng et al. (2010a,b) gave the results dealing with the concentrations, spatial distributions, source identification, and health risks of heavy metals in fine particles of dust in different districts, such as urban streets and industrial and smelting areas. It was well recognized that the concentrations of heavy metals, such as Hg, Pb, Cd, Zn and Cu, in the fine particles of dust among ⇑ Corresponding authors. E-mail (G. Qian).
addresses:
[email protected]
(K.
Oh),
[email protected]
industrial districts were significantly higher than those in street dust compared with atmospheric emissions (Alvim-Ferraz and Afonso, 2005; Chen et al., 2013; Zheng et al., 2010b; Zhu et al., 2013), which indicated that dust from industrial sources produced a greater risk for human health. Municipal solid waste incinerator (MSWI) fly ash is an aggregation of fine particles. MSWI fly ash has been classified as a hazardous waste due to the presence of toxic heavy metals and dioxin-like organic contaminants (Tian et al., 2012). How to recycle or safely dispose MSWI fly ash has become a global concern. In recent years, with the development of pretreatment and recycling technology, MSWI fly ash has been used as a potential substitute for raw civil materials by the hydration reaction at normal temperatures and the ceramic formation process at a high temperature. In these processes, fine fly ash particles can be diffused into atmosphere by wind or artificial turbulence. This portion of fine particle is defined as re-suspended dust (Ram et al., 2014), which size is usually below 10 lm and has health risk to the person who is exposed to the heavy metal bearing dust. Therefore, the secondary pollution triggered by heavy metals must be highly focused, including leaching in special landfill sites or as recycled products, potential volatilization under high temperature in ceramic
http://dx.doi.org/10.1016/j.wasman.2015.06.026 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
aggregate applications and re-suspended dust in recycling processes. (Haiying et al., 2010; Quina et al., 2008a,b; Sukandar et al., 2006). A large number of studies focused on the total amount, chemical species and potential TCLP leaching of heavy metals from MSWI fly ash in stabilization/solidification (S/S) treatment and recycling ˇ ová et al., 2013; Pan et al., processes (Cetin et al., 2012; Kubon 2013; Quina et al., 2008a; Shi and Kan, 2009; Sukandar et al., 2006). However, studies regarding the risk of MSWI fly ash in the re-suspended dust during S/S pretreatment or the recycling process are quite scarce. Moreover, studies regarding a comprehensive understanding of the distribution of heavy metals in different particle sizes of re-suspended fly ash dust and health risk assessments for fine particles are still lacking. The objectives of this work were (1) to conduct a systematic investigation of the morphology, chemical components and distributions of dominant trace elements (Pb, Zn, Cu, Cd, Cr, Fe and Mn) in different sizes of particles in MSWI fly ash; (2) to evaluate the potential human health risks including non-carcinogenic and carcinogenic risks by using heavy metal concentrations and distribution information resulting from re-suspended MSWI fly ash dust.
2. Materials and methods 2.1. Sampling of particulate fly ash and particle size distribution Samples of the bulk fly ash from a municipal solid waste incineration plant in Shanghai were screened by size. A low volume air sampler (AN-200, flow rate: 28.3 L/min) was applied to collect samples among a high concentration of fly ash. At the same ambient temperature and relative humidity, the sample was collected on a round quartz-fiber filter with 80 mm in diameter. The particle size distribution of the fly ash was measured by the LS-POP 6 Laser Particle Analyzer. For the separation of particles with different sizes, the quartz-fiber filters with mech size from 1 lm to 10 lm was used. The following PM fractions were collected: Dp > 10 (larger than 10 lm), Dp10-5, Dp5-2.5, Dp2.5-1 and Dp < 1 (smaller than 1 lm). The Dp1 sample was collected at the final stage. All of the samples were weighed at a constant temperature.
2.3. Chemical analysis of heavy metals The total concentrations of heavy metals in different particle-size ranges of fly ash were determined by inductively coupled plasma mass spectrometry (ICP-MS) after samples were digested using HNO3, HF and HClO4 in a heat-dispelling furnace. A four-step sequential-extraction procedure (SEP) developed by Fernández Espinosa et al. (2002) was followed to fractionate the Dp10-5, Dp5-2.5, Dp2.5-1 and bulk ash trace metals. The procedure was used in exposure studies for measuring the bio-available fractions of toxic metals associated with particulate matter (Feng et al., 2009; Gómez et al., 2007). In this sequential extraction procedure, the metals were categorized as four fractions: acid extractable (F1), reducible (F2), oxidizable (F3) and residual (F4). The detail procedure was listed in Table S1 and the dosage of sample for SEP was 2.0 g. All of the sample analyses were performed in triplicate which recovery rates were between 85% and 110%, and the average values of the results were reported. 2.4. Health risk quantification The workers worked in the municipal solid waste incinerator plants may contact with heavy metal of fly ash directly. The onsite workers contacted heavy metals through three main routes of exposure: ingestion, inhalation, and dermal contact (Wang et al., 2015). To estimate the occurrence of negative health effects from the Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 particles in re-suspended fly ash dust during the disposal and recycling processes, risk assessment models and relevant parameters from the US EPA (EPA, 1989, 2001; Schoof, 2003; Wang et al., 2013) were used in this study. The assessment was performed by using the total concentrations of heavy metals in four different particle fractions of fly ash. For the non-carcinogenic risk, the average daily dose contacted through inhalation (CDIinh) and absorbed through skin (CDIdermal) and ingestion (CDIing) were calculated as follows (EPA, 1989):
CDIinh ¼
C EF ET ED PEF AT 24
CDIdermal ¼
C EF ED SA AFd ABSd 106 BW AT
ð1Þ
ð2Þ
2.2. Physical characterization The morphology of sample with different sizes was characterized by scanning electron microscopy (S-3000N, HITACHI Co.) at 15 kV of accelerating voltage, 13–15 mm of working distance (WD), 150 mm of probe size, and 107 A of beam current. Simultaneously, an energy-dispersive X-ray spectrometry (EDX) was used to analyze the amount of element Ca, Si, Cl, Cu, Zn, Pb, Cr and Cd on the sample with the calibration of the corresponding pure metal oxides (for Cl, the pure NaCl was used). For each sample, the statistics of EDX result from 20 detected area (1 lm 1 lm) was carried out to survey the element distribution of sample with different sizes. Before the SEM/EDX analysis, a gold film with 10 nm thickness in average was coated on the sample in a vacuum coating machine after 10 min coating. X-ray diffraction (XRD, Dnmax-2550, Rigaku) was used to identify crystalline phases in the particulate matter fly ash samples. Four samples (Dp10-5, Dp5-2.5, Dp2.5-1 and bulk fly ash) were pulverized by manual grinding in agate mortar before XRD measurement. Analyses were performed on a low background plate of single crystal silicon with Cu Ka radiation at 40 kV and 40 mA. For fine particle such as Dp5-2.5 and Dp2.5-1, the sieving was carried out by several times until enough amount of the sample was gotten for the XRD measurement.
CDIing ¼
C IRfa EF ED 106 BW AT
ð3Þ
where CDIinh, CDIdermal, and CDIing are the doses of the pollutant (mg kg1 day1); C is the concentration of metals in different particle sizes and bulk fly ash (mg/kg); IRfa is the fly ash ingestion rate for the receptor (mg/day); EF is the exposure frequency (day/year); ED is the exposure duration (year), ET is exposure time (h/day); and AT is the averaging time. For the non-carcinogenic risk, AT is 25 years, and for carcinogenic risk, AT is 70 years. BW is the body weight (kg), SA is the contact area of skin surface (cm2/day), PEF is the particle emission factor (m3/kg), AFd is the dermal adherence rate (mg/cm2-skin), and ABSd is the dermal absorption factor (unitless). The above exposure parameters for the inhalation, ingestion, and dermal contact pathways were listed in Tables S2 and S3, respectively (EPA, 1989, 1996, 2001, 2004, 2011; Wang et al., 2013). After the CDIinh, CDIdermal and CDIing were calculated, the potential human health risk effect through a single exposure pathway could be evaluated by the hazard quotient (HQ). The HQ was the ratio of the average daily intake of a contaminant per unit body weight to an acceptable reference dose. A hazard index (HI) was used to estimate the risk of cumulative contamination in different particle sizes:
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
HI ¼ HQ ing þ HQ inh þ HQ dermal CDIing CDIinh CDIdermal ¼ þ þ RfDing RfCinh RfDing ABSGI
particle Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1 was discussed in details.
ð4Þ
where HQ is the hazard quotient, RfDing is the chronic oral reference dose (mg kg1 day1), RfCinh is the chronic inhalation reference concentration (mg/m3), and ABSGI is the gastrointestinal absorption factor (a.u.) used to multiply RfDing to yield the corresponding dermal values. The above toxicological characteristics of the investigated heavy metals are listed in Table S3 (EPA, 2011). If the total hazard index (HI) is greater than 1, then adverse health effects are possible. For the carcinogenic risk, the equation is described below (EPA, 1989):
RISKi ¼ CDIi SF RISK ¼
X
RISKi
3
ð5Þ ð6Þ
where RISK is a unitless probability of an individual developing cancer and i represents the three different ways of doing so. SF is the slope factor for the toxicant of concern (mg kg1 day1)1. In this paper, risk refers to the total carcinogenic risk of the above three pathways. A risk value of 106 was the upper limit of acceptable cancer risk. 3. Results and discussion 3.1. Size distribution and constituents of fly ash 3.1.1. Particle size distribution of fly ash Fig. 1 illustrates the particle size distribution of bulk fly ash. The fly ash had a board size distribution with diameter from 0.3 to 70 lm, where the amount percentage of particle increased with the increase in particle size until near 20 lm. Moreover, the cumulative distribution of fly ash size exhibited that about 36% of particle had a diameter less than 10 lm. This size distribution of fly ash was not consistent with the result reported by Boom where over 85% of fly ash had particle size larger than 38 lm in MSWI boiler with coal addition (De Boom and Degrez, 2015). As the stove applicable to MSWI without any fuel addition in our study, the difference in fly ash size distribution indicates that the MSWI stove provides numerous fine particulate matter in fly ash (Lind et al., 2007). This observation suggests that the high health risk of fly ash from the MSW combustion stove as more fine particle with diameter below 10 lm. For further contamination investigation of fine particle in fly ash, the sieve of fly ash was conducted. Fine particle was divided into five groups according to particle diameter: Dp > 10, Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1. The distributions of Dp > 10, Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1 were 63%, 17%, 9%, 7% and 4% in the total bulk fly ash, respectively. The contamination and health risk of fine
3.1.2. Mineral phases and morphological characterization Fig. 2 shows the X-ray diffraction pattern of fly ash with various particle sizes. In the bulk fly ash, anhydrite, calcite, halite, and sylvite were identified with the appearance of calcium silicate, mayenite, and calcium aluminum silicate, which was consistent with the result reported elsewhere (Li et al., 2010; Wu et al., 2012). In addition, a small portion of iron oxide was also observed in bulk fly ash. In comparison, the strength of reflection peaks of most phases were decreased in Dp10-5 and Dp5-2.5, but that of mayenite and calcium aluminum silicate as the peak of these two phases at 17.5° (2h) was sharp. This suggested that mayenite and calcium aluminum silicate became dominant in the fly ash with particle size from 2.5 to 10 lm. In particular, a board peak between 10° and 40° (2h) was observed in Dp2.5-1 without any sharp peak, indicating that the crystal of phase in this fine particulate was small and poor. This observation demonstrated that the species of heavy metal was probably different on fly ash sample with various particle sizes as the dominant phases of particle was changed. The phases evolution with various particle size in fly ash sample was further characterized by SEM/EDX technology. As shown in Fig. 3A, the SEM image of bulk fly ash exhibited the morphology of large particle. In comparison, the sample Dp10-5 and Dp5-2.5 illustrated the aggregation of small particles (Fig. 3B and C) while Dp2.5-1 showed the smaller particle in SEM image (Fig. 3D) (Li et al., 2010; Wu et al., 2012). The simultaneous EDX data of Ca, Cl, Si as well as heavy metals was collected. As shown in Fig. 4, the energy reflection of Ca in bulk fly ash was larger than those in fine particle and decreased with the decreasing of particle size. This demonstrated that Ca was predominantly in anhydrite and calcite in bulk fly ash as the weak XRD diffraction of anhydrite and calcite in fine particles (Fig. 2). In contrast, the energy reflection of Si in Dp2.5-1 was higher than those in other samples. This indicated that Si was predominantly in Ca–Si phase in the case of fine particles, which was consistent with the result of XRD (Fig. 2). It is noting that the energy reflection of Cl was increased in samples with the particle size decreasing. This observation suggested that chloride was easily bond to fine particle. It is proposed that the evolution of crystal phase in fly ash probably leads to the variety of heavy metal species (Wu et al., 2015). For the investigation of heavy metal distribution on fine particle, energy reflection of heavy metal was also performed by EDX. As shown in Fig. 4, the energy reflection of most heavy metals was higher in fine particles than that in bulk fly ash. Zinc and copper in Dp2.5-1 exhibited higher energy reflection strength than in the particle with size from 2.5 to 10 lm. In comparison, the energy reflection of Cr was similar in all samples. This observation suggested that the enrichment of Zn and Cu was achieved on the
Fig. 1. Distribution of particle sizes in bulk fly ash.
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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1 Ca12Al14O33 : Mayenite
1 7
Relative intensity (a.u.)
5 CaCO3 : Calcite
2 2 CaSO4 : Anhydrite,syn 6 NaCl : Halite,syn 4 3 3 Ca2SiO4 : Calcium Silicate7 CaAl2SiO6 : Calcium 6 3 Aluminum Silicate 4 KCl : Sylvite,syn 5 24 7 3 6 7 4 8 Fe2O3: Iron Oxide 2 1 1 7 5 354 6 3 6 Bulk
8
8 8
88
Dp10-5 Dp5-2.5
Dp2.5-1 10
20
30
40
50
60
70
80
90
2 (Degree) Fig. 2. XRD patterns of different particle sizes of MSWI fly ash.
particle with size Dp2.5-1 > Dp10-5 > Dp5-2.5 > Bulk. For instance, the content of Zn in Dp2.5-1 and Dp1 was 4006–4587 mg/kg, much larger than those in Dp > 2.5 lm and bulk sample. In comparison, the content of Cr was similar in most cases except that in Dp1 which provided a highest content of Cr. This distribution of heavy metal content was consistent with the EDX result above (Fig. 4). To evaluate the enrichment of heavy metal in fine particle of fly ash, the factor EIi (enrichment index of heavy metal, i = element of heavy metal) was proposed, which stood for the percentage of heavy metal concentration in fine particle to its total concentration in bulk sample. The value of EIi for each heavy metal in fly ash was listed in Table 1. The EIi for Zn, Cu, Pb and Cd was 87%, 76%, 62% and 75%, which indicated large portion of heavy metals in fine particles. In comparison, 42% of EICr demonstrated that small particle size did not facilitate the enrichment Cr. In fact, the enrichment of Zn, Pb, Cu and Cd in fine particles of fly ash was determined by their evaporation performance at high temperature. During a high temperature combustion process, heavy metals in the waste undergo different chemical and physical transformations. Chlorides of Zn, Pb, Cu and Cd are easy to evaporate at high temperature (700–1000 °C). For example, Zn and ZnCl2 converted to gaseous Zn(g) and ZnCl2(g) at only 500 °C and 700 °C, respectively. (Tang et al., 2013; Vejahati et al., 2010). When flue gas containing the vapor of volatile heavy metals and compounds from the furnace at high temperature (approximately 1000 °C) flowed into the low temperature flue (300–500 °C), volatile substances could promptly cool down and form metal fines (Dp2.5-1, Dp1) (Belevi and Langmeier, 2000; Belevi and Moench, 2000; Sørum et al., 2004). This could explain why Zn, Pb, Cu and Cd could be easily enriched in fine particle sizes. Coarse-particle ash, such as Dp10-5 and Dp5-2.5, was formed from small char fragmentation adhering to minerals (Buhre et al., 2006; Nelson et al., 2010;
Fig. 3. SEM photomicrograph analyses of different PM fly ash (A: bulk fly ash; B: Dp10-5; C: Dp5-2.5; D: Dp2.5-1).
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
5
Relative percentage (%)
J. Zhou et al. / Waste Management xxx (2015) xxx–xxx
4
25
Ca
20
20
15
15
10
10
5
5
0
Relative percentage (%)
Cl
Bulk
Si 3 2 1
0 Dp2.5-1 Dp5-2.5 Dp10-5
0 Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
4
Zn
1.6
Pb
3
2.5
Cu
2.0
1.2
1.5
2
0.8 1.0
1
0.4
0
0.0 Dp2.5-1 Dp5-2.5 Dp10-5
Relative percentage (%)
25
Bulk
0.5 0.0 Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
0.5
Cr 0.4 0.3
Cd
3 2
0.2 1 0.1 0.0
0 Dp2.5-1 Dp5-2.5
Dp10-5
Bulk
Dp2.5-1 Dp5-2.5 Dp10-5
Bulk
Fig. 4. Relative percentage of trace metals (Ca, Cl, Si, Zn, Pb, Cu, Cr, Cd) from EDX result in each size fraction of the samples.
Table 1 Mass concentrations of seven elements in different particle sizes of fly ash (mg kg1).
Bulk Dp10-5 Dp5-2.5 Dp2.5-1 Dp1 EIi (%)
Zn
Cu
Pb
Cd
Cr
Fe
Mn
1267 ± 61 25973 ± 10 2239 ± 66 4006 ± 200 4587 ± 22
199 ± 1 357 ± 11 308 ± 4 528 ± 2 648 ± 28
852 ± 5 1399 ± 41 1170 ± 45 1477 ± 66 2145 ± 12
29 ± 0 60 ± 2 49 ± 1 54 ± 6 90 ± 2
72 ± 2 81 ± 3 62 ± 3 60 ± 2 170 ± 3
4382 ± 332 3126 ± 33 1938 ± 6 2373 ± 8 4956 ± 13
197 ± 15 184 ± 4 120 ± 2 124 ± 3 260 ± 4
87
76
62
Shim et al., 2003; Tang et al., 2013; Vejahati et al., 2010). On the other hand, heavy metal is easily bonded to Fe oxide in fly ash. As the high content of Fe in fine particle even in Dp1 (Table 1), it is also supposed that the enrichment of heavy metal is probably relative to Fe in fine particle of fly ash. 3.2.2. Chemical species analysis To further understand the chemical species of these heavy metals, a method using a four-step sequential extraction procedure was adopted. The acid extractable fraction (F1) stands for the heavy metal in chloride, carbonate and complexes with weak coordination bond, which dissolves in the acidic solution. The reducible fraction (F2) is the heavy metal bond to Fe oxide. Once the Fe oxide is reduced, the dissolution of heavy metal occurs. F1 and F2 of heavy metal usually are highly unstable and are potentially bio-available to the respiratory tract of the human body. In comparison, Oxidizable fraction (F3) is commonly the heavy metal bond to organic matter, sulfide and carbon. In fly ash collected from air clean system, heavy metal is adsorbed on activated carbon which is used to reduce NOx. Thus, F3 in the current work was
75
42
24
31
defined as the heavy metal bond to carbon. The residual fraction (F4) was the heavy metal in indissoluble mineral phase such as phosphate. As more stability of F3 and F4, these fractions is not easily bio-available and has low health risk (Feng et al., 2009; Gómez et al., 2007; Perin et al., 1985). As shown in Fig. 5, F1 of heavy metal was much low in all samples. F3 and F4 of heavy metals were the predominant fractions in bulk sample. With the particle size decreasing, F1 and F2 were increased. For instance, the percentage of F1 for Pb in the bulk ash accounted for 0% of the total chemical species, while that was increased to 1% in Dp5-2.5 and 3% in Dp2.5-1. The increase in F1 was contributed to the increase in heavy metal adsorbed Ca–Si phase and heavy metal chloride as the content of these phase was increased in EDX result (Fig. 4). Moreover, the increase in the percentage of F2 for Pb was sharply, where F2 was increased to 50% in Dp2.5-1. For Zn, the F2 was increased from 12% in bulk ash to 25–33% in fine particle. This situation was observed in most heavy metals, indicating that Fe oxide was responsible for heavy metal enrichment in fine particle instead of chloride. As the less stability of F2, it is proposed that Zn, Pb, Cu, Cd in fine particle of fly ash
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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Percentage (%)
Dp10-5
Bulk
100
100
80
80
60
60
40
40
20
20
0
0
Zn
Cu
Pb
Cd
Cr
Zn
Cu
Pb
Cr
Cd
Cr
Dp2.5-1
Dp5-2.5
Percentage (%)
Cd
100
100
80
80
60
60
40
40
20
20
0
0
Zn
Cu
Pb
Cd
Cr
acid extractable
Zn
reducible
Cu
Pb
oxidizable
residual
Fig. 5. Chemical speciation of heavy metals in different particle sizes of fly ash.
Table 2 Hazard quotients for each heavy metal and exposure pathway. Size
HQ
Zn
Pb
Cu
Cd
Cr
Cumulative
Bulk
HQinh HQdermal HQing HI
1.52 107 1.79 103 1.36 102 1.54 102
2.86 104 1.35 101 7.71 101 9.06 101
1.79 107 7.42 104 1.61 102 1.68 102
1.22 104 3.51 103 3.35 102 3.71 102
3.01 105 2.89 102 2.75 102 5.64 102
4.39 104 1.70 101 8.61 101 1.03
Dp10-5
HQinh HQdermal HQing HI
3.12 107 3.68 103 2.80 102 3.16 102
4.70 104 2.22 101 1.27 1.49
3.21 107 1.33 103 2.89 102 3.02 102
2.52 104 7.24 103 6.89 102 7.65 102
3.40 105 3.26 102 3.10 102 6.36 102
7.56 104 2.67 101 1.42 1.69
Dp5-2.5
HQinh HQdermal HQing HI
2.69 107 3.17 103 2.41 102 2.73 102
3.93 104 1.86 101 1.06 1.24
2.77 107 1.15 103 2.49 102 2.61 102
2.04 104 5.88 103 5.60 102 6.22 102
2.61 105 2.50 102 2.38 102 4.88 102
6.24 104 2.21 101 1.19 1.41
Dp2.5-1
HQinh HQdermal HQing HI
4.81 107 5.68 103 4.31 102 4.88 102
4.96 104 2.34 101 1.34 1.57
4.75 107 1.97 103 4.27 102 4.46 102
2.25 104 6.48 103 6.17 102 6.85 102
2.53 105 2.43 102 2.31 102 4.74 102
7.48 104 2.73 101 1.51 1.78
Dp1
HQinh HQdermal HQing HI
5.50 107 6.50 103 4.94 102 5.59 102
7.21 104 3.41 101 1.94 2.28
5.82 107 2.42 103 5.23 102 5.47 102
3.79 104 1.09 102 1.04 101 1.15 101
7.16 105 6.86 102 6.53 102 1.34 101
1.17 103 4.29 101 2.21 2.64
have higher risk to human beings. In comparison, the percentage of F1 and F2 for Cr was low regardless of particle size reduction. F4 of Cr was increased from 80% in bulk ash to 92% in fine particle. This suggested that Cr in fine fly ash has low risk. Therefore, it is proposed that the enrichment of most heavy metals in fine particle is relative to the Fe oxide, which probably lead to the increasing of risk to human health when the person was exposed to fine fly ash.
3.3. Health risk assessment of heavy metals for different particles of fly ash 3.3.1. Non-carcinogenic risk assessment The hazards of heavy metals on human health are widely known. A model of health risk assessment was applied to evaluate the risk of Zn, Pb, Cu, Cd, and Cr in different particle sizes of fly ash. Table 2 lists the HQ value of non-carcinogenic risks of Zn, Pb, Cu,
Please cite this article in press as: Zhou, J., et al. Enrichment of heavy metals in fine particles of municipal solid waste incinerator (MSWI) fly ash and associated health risk. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.026
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J. Zhou et al. / Waste Management xxx (2015) xxx–xxx Table 3 Carcinogenic risk assessment of Cd, Cr. Risk Cd Cr Cumulative
Bulk
Dp10-5 8
(6.04 ± 0.01) 10 (7.11 ± 0.18) 106 (7.17 ± 0.18) 106
Dp5-2.5 7
(1.24 ± 0.05) 10 (8.01 ± 0.30) 106 (8.13 ± 0.31) 106
Cd, and Cr that were estimated by three exposure pathways of inhalation, dermal contact and ingestion. The results indicated that the HIs of Zn, Pb, Cu, Cd, and Cr in Dp1 were significantly higher than in bulk ash, with the ranges of 2.7 102–5.6 102, 1.2– 2.3, 2.6 102–5.5 102, 6.2 102–1.2 101 and 4.9 102– 1.3 101, respectively. Obviously, Pb was the main contributor to the total non-carcinogenic risks compared with the other elements. In addition, ingestion was the key pathway, and the total non-carcinogenic risks were approximately 84–85% in the four particle sizes. The acceptable cumulative non-carcinogenic risks threshold value is 1.0 (EPA, 1991). The cumulative non-carcinogenic risks of all four particle sizes and bulk ash all exceeded 1.0, which ranged from 1.0 to 2.6 in the order: Dp1 > Dp2.5-1 > Dp10-5 > Dp5-2.5 > Bulk. The contents of heavy metals in Dp10-5 were slightly higher than that in Dp5-2.5. Therefore, the non-carcinogenic risk in Dp10-5 was also higher than that of Dp5-2.5. In particular, the HQ of the three pathways increased rapidly in Dp2.5-1 and Dp1. For example, the HQ of Dp1 was two times more than that of bulk ash. The results above showed that fine particles, especially Dp2.5-1 and Dp1, had significant non-carcinogenic risks for onsite workers. 3.3.2. Carcinogenic risk assessment According to US EPA research result, among these seven heavy metals, only Cd and Cr have a potential carcinogenic risk (EPA, 1991). No potential carcinogenic risk is considered for Zn, Pb, Cu, according to the USEPA report (EPA, 2001). The cumulative carcinogenic risks threshold value is 106 (EPA, 1991). As presented in Table 3, the cumulative carcinogenic risks of Cd and Cr was higher than the threshold value (106). Cr was the main contributor to carcinogenic risk compared with Cd, with values ranging from 7.1 106 to 1.7 105. Any cancer risk more than the threshold value (106) was considered to produce a cancer risk for onsite workers. In addition, the carcinogenic risks of Cd and Cr in Dp1 were the highest, with maxima of 1.9 107 and 1.7 105, respectively. Based on the results of the health risks above, meaningful measures to guarantee the health of workers have been put forward and should be carried out in the future. 4. Conclusions The content and species distribution of heavy metal in MSWI fly ash with various particle size was investigation. Four groups of particle size of fly ash was sieved in the following order: Dp10-5, Dp5-2.5, Dp2.5-1 and Dp1. The XRD and SEM/EDX revealed that anhydrite, calcite, halite, sylvite, calcium silicate, mayenite, and calcium aluminum silicate were dominant phases in fly ash, which were decreased with the particle size reduction of fly ash. The iron oxide was identified in most samples with various particle size. The evolution of crystal phase with various particle size led to 87% of Zn, 76% of Cu, 62% of Pb, and 75% of Cd was enriched in fine particle of fly ash except Cr. The analysis of heavy metal species indicated that F2 of heavy metal was increased with the reduction of particle size. This observation suggested that the iron oxide was probably responsible for the enrichment of heavy metal. The enrichment of heavy metal resulted in the high non-carcinogenic risks of fine particle as the cumulative hazard
Dp2.5-1 7
(1.01 ± 0.006) 10 (6.16 ± 0.3) 106 (6.26 ± 0.3) 106
Dp1 7
(1.11 ± 0.13) 10 (5.98 ± 0.15) 106 (6.09 ± 0.16) 106
(1.87 ± 0.03) 107 (1.69 ± 0.28) 105 (1.71 ± 0.28) 105
indexes for non-carcinogenic metals in Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 were 1.69, 1.41, 1.78 and 2.64, respectively. The cumulative carcinogenic risk of Cd and Cr was higher than the threshold value (106). Therefore, our results reveals the high health risk of heavy metal in fine particle of fly ash due to the enrichment of unstable heavy metal fractions in particle with size
