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Can washing-pretreatment eliminate the health risk of municipal solid waste incineration fly ash reuse?
Ecotoxicology and Environmental Safety 111 (2015) 177–184
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Can washing-pretreatment eliminate the health risk of municipal solid waste incineration fly ash reuse? Yao Wang a,b, Yun Pan a, Lingen Zhang a, Yang Yue a, Jizhi Zhou a, Yunfeng Xu a, Guangren Qian a,n a b
Department of Environmental Science and Engineering, Shanghai University, Shanghai 200444, China Shanghai Institute of Geological Survey, Shanghai 200072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 March 2014 Received in revised form 3 September 2014 Accepted 10 September 2014
Although the reuse of washing-pretreated MSWI fly ash bas been a hot topic, the associated risk is still an issue of great concern. The present study investigated the influence of washing-pretreatment on the total contents and bioaccessibility of heavy metals in MSWI fly ash. Furthermore, the study incorporated bioaccessibility adjustment into probabilistic risk assessment, to quantify the health risk from multipathway exposure to the concerned chemicals as a result of reusing washed MSWI fly ash. The results revealed that both water-washing and acid-washing process have resulted in the concentrated heavy metal content, and have reduced the bioaccessibility of heavy metals. Besides, the acid-washing process increased the cancer risk in most cases, while the effect of water-washing process was uncertain. However, both water-washing and acid-washing pretreatment could decrease the hazard index based on bioaccesilbility. Despite the uncertainties accompanying these procedures, the results indicated that, in this application scenario, only water-washing or acid-washing process cannot reduce the actual risk from all samples to acceptable level, especially for cancer risk. & Published by Elsevier Inc.
Keywords: MSWI fly ash Washing pretreatment Bioaccessibility Probabilistic risk assessment
1. Introduction The reuse of industrial waste in building materials and civil engineering applications has undergone considerable development over a long period. Practices now commonly seen in America, Denmark, Sweden, and the Netherlands are to use coal combustion products, blast-furnace slag and municipal solid waste incineration bottom ash to repair roads and produce asphalt concrete, and ceramic materials (del Valle–Zermeño et al., 2013; Izquierdo et al., 2008; Little et al., 2008). Similarly, the published reports (Guo et al., 2014) have shown that municipal solid waste incineration (MSWI) fly ash has cementitious properties and its main chemical components belong to the system of CaO–SiO2–SO3–Al2O3. Therefore, pretreated MSWI fly ash is now increasingly used for cement manufacturing, roadbed material, and glass ceramics (Aubert et al., 2006, 2007; Francois and Pierson, 2009; Luna Galiano et al., 2011; Wu et al., 2012). Different from coal fly ash and blast-furnace slag, MSWI fly ash has been listed in the National Hazardous Waste Inventory as HW18 for containing different types of heavy metals, chlorinated organic compounds, dioxins, sulfur compounds, etc., While MSWI n
Corresponding author. Fax: þ 86 21 66137758. E-mail address: [email protected] (G. Qian).
http://dx.doi.org/10.1016/j.ecoenv.2014.09.030 0147-6513/& Published by Elsevier Inc.
fly ash is reused as the civil materials, the issue of potential environmental impact associated with the reuse has emerged. Though pretreatment technologies such as water-washing, acidwashing, have been verified that can remove a part of soluble heavy metals (Anastasiadou et al., 2012; Colangelo et al., 2012), it is still unknown that the potential health risk of pretreated MSWI fly ash reuse is acceptable or not. At present, studies mainly concentrate on using the toxicity characteristic leaching procedure (TCLP) to specify the toxicity and risk of pretreated MSWI fly ash reuse (Lee and Li, 2010; Liu et al., 2009; Yang et al., 2009), but paid little attention to the potential health risk caused by occupational exposure via inhalation, non-dietary ingestion and dermal contact pathways. Besides, approximately 18% of the particle size distributions of fly ashes are under the size of 10 μm, even a fraction of fly ash's particle diameter is around 1 μm (Shi and Kan, 2009), which means the MSWI fly ash particle is easier to adhere to skin, and generate more wind-blown dust emission than soil in the reuse process. Therefore, this paper investigated the health risk of occupational exposure to MSWI fly ash reuse with multi-exposure pathways, in order to get the overall results. The traditional health risk method based on the total content of heavy metal will lead to over-estimation of risk, since the actual health risk of heavy metals in ingested medium depend on the fraction that is soluble in the gastrointestinal tract available for absorption (Bade et al., 2012; Kördel et al., 2013). To achieve a
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sound evaluation of the health risk, the bioaccessibility or bioavailability of the heavy metals in MSWI fly ash should thus be considered. According to the literatures (Ruby et al., 1999), bioavailability is defined as the fraction of an administered dose that reaches the central (blood) compartment from the gastrointestinal tract, which should be measured by in-vivo studies; while bioaccessibility of a substance is the fraction that is soluble in the gastrointestinal environment and is available for absorption, which can be assessed by in-vitro methods. Considering the cost and time, most researchers preferred to do in-vitro methods, such as the physiologically based extraction test (PBET) (Juhasz et al., 2010, 2011), to evaluate the bioaccessibility of pollutions from matrix such as soils and dust, when assess the actual health risk of heavy metals by ingestion (Hu et al., 2013; Luo et al., 2012; Vasiluk et al., 2011). However, a wealth of studies demonstrated a strong linear relationship between bioaccessibility and exchangeable and soluble fraction, reducible fraction extracted by BCR method (Ahumada et al., 2011; Akkajit and Tongcumpou, 2010; Alvarenga et al., 2009; Baig et al., 2009; Dabek–Zlotorzynska et al., 2005; De La Calle et al., 2013; Karadaş and Kara, 2012; Poggio et al., 2009; USEPA, 2003, 2007). Hence, we used modified four-step BCR method to evaluate the oral bioaccessible fraction (bioaccessibility) in MSWI fly ash. Additionally, probabilistic approaches, such as Monte Carlo simulation and sensitivity analysis, should be taken into consideration during human health risk assessment process, which could provide the risk assessor with a flexible tool to estimate the uncertainties and stochastic properties of exposure and toxicity (Wu et al., 2011). Nowadays, probabilistic risk assessment has been successfully applied to assess the potential adverse health effects of contaminants from onsite MWS disposal and coal combustion wastes (CCW) practices (Lonati et al., 2007; Lonati and Zanoni, 2012; USEPA, 2010). Following the discussion above, the purpose of this study was two-fold: (i) to evaluate the effects of water/acid washing pretreatment on the total content and bioaccessibility of heavy metals in MSWI fly ash; (ii) to quantitatively assess the risk of occupational exposure to China MSWI fly ash reuse with a probabilistic approach.
2. Materials and methods 2.1. Sampling and pretreatment Four kinds of MSWI fly ash samples (FA1, FA2, FA3, and FA4) were collected from four MSWI plants located which all exceeds 300 t MSW/d in four typical regions all over China. Besides, Both FA1 and FA2 were obtained from the grate-type incinerators, while FA3 and FA4 were obtained from the fluidized bed incinerators. The samples were stored in a desiccator at room temperature after the oven-drying process at 105 °C for 24 h. 2.1.1. Water washing pre-treatment The MSWI fly ash samples were first suspended in distilled water at a liquid–solid ratio (i.e., cm3 g 1) of 8 in a beaker, and stirred in an agitation apparatus at a rotation speed of 200 72 rpm for 5 h. After washing process, the solid/water mixtures were separated through a vacuum pump filter, and the filter cake was again washed. The resulting material was then oven dried at 105 °C for 24 h, and then stored in a desiccator until analysis. 2.1.2. Acid washing pre-treatment The MSWI fly ash samples were brought into contact with 0.5 mol/L HNO3 solutions at a liquid–solid ratio (i.e., cm3 g 1) of 20 in a beaker, and stirred in an agitation apparatus at a rotation
speed of 200 72 rpm for 1 h. After washing process, the solid/ water mixtures were separated through a vacuum pump filter, and the solids were dried in an electro-thermostatic blast oven at 105 °C over 24 h. The dried acid-washed MSWI fly ash was collected and stored in desiccators until analysis. 2.2. Chemical analysis 2.2.1. Total contents of heavy metals in raw/pretreated fly ASH Total content of heavy metal was determined by treating 0.2 g sample with HNO3/HClO4/HF acid mixture digestion method at about 120 °C until the digested solution was clear (Sun et al., 2001). Reagent blanks and analytical duplicates were included to ensure the accuracy and precision of analysis. 2.2.2. Chemical speciation of heavy metals in raw/pretreated fly ASH A modified four-step procedure sequential extraction method (Pan et al., 2013) was adopted to fractionate heavy metals in the exchangeable and acid soluble fraction (F1), reducible fraction (F2), oxidizable fraction (F3), and residual fraction (F4). As described above, the bioaccessible fraction of heavy metals have demonstrated a strong linear relationship with the exchangeable and acid soluble fraction (F1) and reducible fraction (F2). Besides, it has been proved that the excess of heavy metal leached in TCLP, was also contributed to the high content of exchangeable and acid soluble fraction (F1) and reducible fraction (F2) of heavy metal (USEPA, 2003). Consequently, we using the F1 and F2 as the oral bioaccessible fraction (bioaccessibility) in MSWI fly ash (Eq. (S1), Table 1). 2.3. Health risk assessment procedures 2.3.1. Evaluation scenario The major metals, including Zn, Pb, Cu, Cr, Cd, and Ni, are the primary concern of reuse of MSWI fly ash. The risk of worker exposure to pretreated MSWI fly ash open storage pile in landfill site was the focus in this study. The application area was set 15 m 25 m of the storage pile, and the multiple exposure pathways including non-dietary ingestion, dermal contact and inhalation routes. However, the risk caused by groundwater ingestion was not considered in this study, since the leachate from the storage pile was collected for off-site disposal. 2.3.2. Human exposure and health risk assessment model Risk assessment is a multi-step procedure comprised data collection and evaluation, exposure assessment, toxicity assessment, and risk characterization. The human exposure to heavy metals in MSWI fly ash can occur via three main pathways: (i) direct oral ingestion of substrate particles (CDIingestion); (ii) inhalation of re-suspended particulates emitted from storage pile (CDIinhalation); and (iii) dermal absorption of heavy metals in particles adhered to exposed skin (CDIdermal). The cancer risks were evaluated only for Cr, Cd, and Ni through inhalation exposure pathway, since the metals above are only classified as a known human carcinogen (Group A) or probable human carcinogen (Group B1) via the inhalation route. According to Guide for Incorporating Bioavailability Adjustments into Human Health and Ecological Risk Assessments at US Department of Defense Facilities (USEPA, 2003), modified-mathematical models and exposure parameters were listed in Table S1 and Table S2, respectively (USEPA, 2004; Wang et al., 2013; Wu et al., 2011). We treated BW, EF, ET, AFd, SA, and IRfa in Eqs. (S2)–(S4) probabilistically, and used Eq. (S5) to incorporate bioaccessibility adjustments into human health risk assessment. In addition, toxicological characteristics in the present study, for each exposure
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Table 1 The cancer risk at the 50th and 95th percentile of the cumulative probability distribution. Samples
Risk category
Original
Water washing
Acid washing
50th percentile
95th percentile
50th percentile
95th percentile
50th percentile
95th percentile
FA1
Cr Cd Ni Total
8.23 10 6 8.56 10 9 6.76 10 9 8.25 10 6
1.13 10 5 1.17 10 8 9.28 10 9 1.13 10 5
6.35 10 6 8.09 10 9 5.31 10 9 6.36 10 6
8.74 10 6 1.11 10 8 7.31 10 9 8.76 10 6
6.30 10 6 3.98 10 9 4.64 10 9 6.31 10 6
8.52 10 6 5.38 10 9 6.27 10 9 8.53 10 6
FA2
Cr Cd Ni Total
1.33 10 5 1.74 10 8 1.10 10 8 1.34 10 5
1.83 10 5 2.39 10 8 1.51 10 8 1.84 10 5
1.08 10 5 1.87 10 8 1.08 10 8 1.08 10 5
1.49 10 5 2.58 10 8 1.48 10 8 1.49 10 5
1.87 10 5 9.70 10 9 1.40 10 8 1.87 10 5
2.53 10 5 1.31 10 8 1.89 10 8 2.53 10 5
FA3
Cr Cd Ni Total
6.96 10 5 2.21 10 7 1.27 10 8 6.98 10 5
9.55 10 5 3.03 10 7 1.74 10 8 9.58 10 5
1.24 10 4 2.53 10 7 1.35 10 8 1.25 10 4
1.71 10 4 3.49 10 7 1.86 10 8 1.71 10 4
1.45 10 4 6.17 10 8 1.73 10 8 1.45 10 4
1.96 10 4 8.34 10 8 2.34 10 8 1.96 10 4
FA4
Cr Cd Ni Total
7.49 10 6 5.40 10 8 6.08 10 9 7.55 10 6
1.03 10 5 7.41 10 8 8.34 10 9 1.04 10 5
1.46 10 5 7.51 10 8 6.97 10 9 1.47 10 5
2.01 10 5 1.03 10 7 9.59 10 9 2.02 10 5
1.21 10 5 1.19 10 7 9.47 10 9 1.22 10 5
1.63 10 5 1.60 10 7 1.28 10 8 1.65 10 5
route, were normalized to account for extrapolation to a different body weight from standard of 70 kg (Table S1, Eqs. (S6) and (S7)). The chronic daily intake (CDI, mg/kg/day) for oral ingestion pathway was defined as Eq. (S2), where Cfa is the concentration of metal in MSWI fly ash (mg/kg), IRfa is MSWI fly ash ingestion rate for receptor (mg/day), EF is the exposure frequency (day/year), ED is the exposure duration (year), AT is the averaging time for carcinogens or non-carcinogens (day), BW is the bodyweight (kg). The CDI values for inhalation pathway was defined as Eq. (S3), where CPM10 is the predicted PM10 concentrations (kg/m3) of MSWI fly ash by SCREEN3 (USEPA, 1995) under the reasonable worst case scenario emitting 1000 mg MSWI fly ash PM10/m2/day (Macleod et al., 2006), ET is the exposure time (h/day). The CDI values for dermal adsorption pathway was defined as Eq. (S4), where SA is the skin surface area available for exposure (cm2/event), AFd is the dermal adherence rate (mg/cm2-skin), ABSd is the dermal absorption factor (unitless). Considering the different bioaccessibility of site-specific MSWI fly ash, the adjusted CDI values for oral ingestion route was defined as Eq. (S5). The cancer risk for inhalation pathway was defined as Eq. (S6), where the IUR is the inhalation unit risk (ug/m3) 1. The HI for three main pathways was defined as Eq. (S7), where RfDing is the chronic oral ingestion reference dose (mg/kg day), RfCinh is the chronic inhalation reference concentration (mg/m3), ABSGI is the gastrointestinal absorption factor (unitless) which is used to multiply RfDing to yield the corresponding dermal values. The above toxicological characteristics of the investigated heavy metals were list in Table S3 (TX11, 2001; US RAIS, 2011). 2.3.3. Sensitivity analysis To propagate the uncertainties and its impact throughout the whole risk assessment model, Monte Carlo simulation (n ¼5,000) were implemented using the software Crystal Ball 7.2, resulting in a final probability distribution function for the estimated individual cancer risk and hazard index due to MSWI fly ash storage pile in landfill sites. In addition, sensitivity analysis was used to determine which probability density functions have the greatest effect on the risk estimates during Monte Carlo simulations. The sensitivity of each variable relative to one another was assessed by calculating rank correlation coefficients between each input and output during
simulations and then estimating each input contribution to the output variance by squaring the output variance and normalizing to 100%. The lower end of the range of acceptable total incremental lifetime cancer risk (TILCR) was defined by a single constraint on the 95th percentile of risk distribution that must be equal or lower than 10 6 for carcinogen (Chiang et al., 2009) in industry site, and the lower end of the range of acceptable hazard index (HI) was defined by a single constraint on the 95th percentile of risk distribution that must be equal or lower than 1 (USEPA, 1989).
3. Results and discussion 3.1. Water/acid washing effect on the total content of heavy metals To investigate the water/acid washing effects on metal removal from MSWI fly ash, the total concentrations of such harmful high content heavy metals as Zn, Pb, Cu, Cr, Cd, and Ni in original, water-washed, and acid-washed MSWI fly ash are listed in Fig. 1. In the case of original fly ash, the contents of Zn, Pb, Cu, and Cr were 1101.13–5577.52 mg/kg, 168.13–1589.33 mg/kg, 290.52– 509.75 mg/kg, and 150.58–1399.12 mg/kg, respectively, while there were relatively lower concentrations in Cd and Ni, which were 8.03–207.39 mg/kg, 43.94–82.25 mg/kg. The similar phenomenon can be observed in water-washing MSWI fly ash and acid-washing fly ash, which inferred that compared with the content of Zn, Pb, Cu, and Cr, there were relatively low levels of Cd and Ni. The water-washing process significantly increased the total contents of Zn, Pb, and Cu in all samples with the increasing range rates of 5.85–33.85%, 3.43–22.69%, and 0.34–28.69%, respectively (Fig. 1). For example, in the case of FA3, water-washing process increased Zn from 5577.52 to 6929.84 mg/kg, Pb from 1589.33 to 1762.01 mg/kg, Cu from 527.79 to 679.22 mg/kg. The above results indicated that the water-washing process resulted in concentrated heavy metal content in the ash afterwards, which was caused by the removal of soluble chlorides, especially soluble calcium salts (Wang et al., 2001). However, the contents of Cr in water-washed MSWI fly ash showed a visible reduction in FA1 and FA2. This is consistent with other reports that Cr is found to be easily extracted
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Fig. 1. The total contents of heavy metals in original and pretreated MSWI fly ash.
one among the heavy metals except for K, Ca, and Na in MSWI fly ash samples (Jiang et al., 2009; Wang et al., 2009), which infers that the removal rate of Cr is higher than the soluble chlorides, calcium salts. Besides, due to relatively low concentration of Cd and Ni, no substantial changes have been found in washed fly ash. Acid-washing process could remove calcium carbonate, calcium hydroxide and other parts of precipitates, which are difficult to be soluble by water-washing process. However, the change in total contents of heavy metals in fly ash after washing pretreatment not only depends on the relative removal amount of soluble salts and acid-soluble precipitates, but also controlled by the chemical species of heavy metals, and the chemical stability of heavy metal speciation in water or acid solutions is different. In this case, as shown in Fig. 1, similarly, acid-washing process increased the contents of Pb and Cr in FA2, FA3, and FA4. Different from water-washed fly ash, the contents of Zn, Cu in FA1, FA2, and FA3 indicated a decline after acid-washing extraction. In the case of FA1, the acid-washing process decreased the content of Zn from 1101.13 to 642.64 mg/kg, Cu from 290.52 to 195.84 mg/kg. The decline above appeared that higher amount of exchangeable and acid soluble fraction, reducible fraction of Zn, Cu have been removed with HNO3 solutions than other non-metal soluble salts. 3.2. Effect of water/acid washing pretreatment on bioaccessibility It is evident that the water/acid-washing pretreatment changed the total contents of heavy metals in MSWI fly ash. In order to evaluate the effects of water/acid-washing pretreatment on
bioaccessibility, as mentioned in Section 2.2, we defined that the exchangeable and acid soluble fraction (F1) and reducible fraction (F2) is the oral bioaccessbile fraction of heavy metals in fly ash. The bioaccessibility of heavy metals in original, water-washed and acid-washed MSWI fly ash are reported in Fig. 2. For the original MSWI fly ash, the bioaccessibilities of Zn, Cu, and Cd indicated a high level relative to other heavy metals such as Pb, Cr, and Ni. For instance, in the case of FA3, the bioaccessibility of Zn, Cu, and Pb was 39.71%, 48.94%, and 77.50%, respectively, while 10.99%, 11.77%, and 19.81%, respectively, for the Pb, Cr, and Ni. Additionally, the bioaccessibility of each heavy metal in original MSWI fly ash from FA3 and FA4 was higher than the corresponding one from FA1 and FA2. The differences may be caused by the different collecting and handling methods of municipal solid waste. Fig. 2 indicates that the water-washing process lowered the bioaccessibilities of heavy metals in FA2, FA3, and FA4. In the case of FA3, the bioaccessibility of each analyzed metal all reported a decline with the range of 2.58–23.83% after water-washing process. The phenomenon indicated that the washing process have washed out some parts of metals in the exchangeable fraction. Similarly, the acid-washing process also reduced the bioaccessibilities of heavy metals in FA1, FA2, and FA3. And, in most cases, the effects on reducing the bioaccessibility of heavy metals is better than water-washing process, since acid-washing process can remove more metals in the carbonate fraction, acid soluble fraction or reducible fraction with the support of HNO3 solutions.
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Fig. 2. The bioaccessibility of heavy metals in original and pretreated MSWI fly ash.
In the case of FA2, the bioaccessibilities of heavy metals (Zn, Pb, Cu, Cr, Cd, and Ni) reached 25.03%, 6.27%, 8.89%, 0%, 47.46%, and 10.02%, respectively, for water-washed fly ash; while 8.99%, 5.03%, 7.63%, 0.47%, 20.61%, and 5.80%, respectively, for the acid-washed fly ash. Since the water-washing and acid-washing process can remove a part of metals in the exchangeable and acid soluble fraction (F1) or reducible fraction (F2), the heavy metals of washed fly ash were more stable than original MSWI fly ash in the following recycling process. But it was still unknown whether the health risk of washed-MSWI fly ash reuse is acceptable or not. 3.3. Human exposure and health risk assessment model 3.3.1. Spatial distribution of cancer risk Since the Cr, Cd, and Ni are only classified as a known human carcinogen (Group A) or probable human carcinogen (Group B1) via the inhalation route, the carcinogenic risk of our study was focus on the lung cancer risks. Under most regulatory program, risk between 10 6 and 10 4 indicates potential risk, whereas risk larger than 10 4 indicates high potential health risk. Table 1 summarizes the probabilistic incremental lifetime cancer risk (ILCRs) for workers at MSWI fly ash open pile.
For the original fly ash, Cr was the main contributor to carcinogenic risk and the contribution rates up to 99.04–99.76%. Both the 50% probability total ILCRs (TILCRs) and the 95% probability TILCRs for all samples presented an unacceptable level, which ranged from 7.55 10 6 to 6.98 10 5, 1.04 10 5 to 9.58 10 5. It appeared that the reuse of original MSWI fly ash posed an unacceptable carcinogenic risk for the workers. As consistent with original fly ash, as shown in Table 1, the main contributor to carcinogenic risk in water-washed and acidwashed fly ash is Cr. Hence, the effects of the water-washing and acid-washing process on the spatial distribution of carcinogenic risk largely depend on the changes in total contents of Cr. The water-washing process reduced the 50% and 95% probability TILCRs of FA1 and FA2, while increased the 50% and 95% probability TILCRs of FA3 and FA4, which indicated that the effect of water-washing process is uncertain. However, the acid-washing process significantly increased 50% and 95% probability TILCRs of FA2, FA3, and FA4, and all exceeded the USEPA acceptable level of 10 6, only FA1 showed a slight decrease. The phenomena indicated that, in most cases, the acid-washing process would increase the carcinogenic risk of MSWI fly ash in the open pile scenario.
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Table 2 The hazard index at the 95th percentile of the cumulative probability distribution categorized by exposure pathway. Samples
exposure pathway
Original
Water washing
Acid washing
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
FA1
Dermal Inhalation Ingestion Total
2.20 10 2 7.00 10 3 3.41 10 1 3.70 10 1
2.20 10 2 7.00 10 3 2.00 10 2 4.90 10 2
1.80 10 2 6.00 10 3 3.18 10 1 3.41 10 1
1.80 10 2 6.00 10 3 2.20 10 2 4.60 10 2
1.80 10 2 5.00 10 3 2.66 10 1 2.89 10 1
1.80 10 2 5.00 10 3 1.30 10 2 3.50 10 2
FA2
Dermal Inhalation Ingestion Total
3.70 10 2 1.30 10 2 6.52 10 1 7.02 10 1
3.70 10 2 1.30 10 2 5.90 10 2 1.09 10 1
3.10 10 2 1.20 10 2 6.19 10 1 6.62 10 1
3.10 10 2 1.20 10 2 5.10 10 2 9.40 10 2
5.30 10 2 1.40 10 2 8.12 10 1 8.80 10 1
5.30 10 2 1.40 10 2 3.00 10 2 9.80 10 2
FA3
Dermal Inhalation Ingestion Total
1.88 10 1 8.30 10 2 3.19 3.46
1.88 10 1 8.30 10 2 1.29 1.56
3.07 10 1 1.13 10 1 4.42 4.85
3.07 10 1 1.13 10 1 1.08 1.50
3.91 10 1 8.90 10 2 5.27 5.75
3.91 10 1 8.90 10 2 9.60 10 1 1.44
FA4
Dermal Inhalation Ingestion Total
2.90 10 2 1.80 10 2 7.98 10 1 8.43 10 1
2.90 10 2 1.80 10 2 1.97 10 1 2.43 10 1
4.60 10 2 2.50 10 2 1.10 1.17
4.60 10 2 2.50 10 2 2.00 10 1 2.72 10 1
5.70 10 2 3.60 10 2 1.75 1.84
5.70 10 2 3.60 10 2 3.46 10 1 4.40 10 1
Table 1 also reports that the contribution rates of Cr in waterwashed and acid-washed fly ash were around 98.95–99.86%, therefore the removal of Cr seemed an effective way to control the carcinogenic risk in future MSWI fly ash reuse. However, predicted maximum PM10 concentration was based on the reasonable worst case scenario, which may overestimated the cancer risk.
3.3.2. Spatial distribution of hazard index The hazard index (HI) based on traditional method or bioaccessibility for each sample is shown in Tables 2 and 3. Table 2 presents the HI categorized by exposure pathway at the 95th percentiles of the cumulative probability distributions, and Table 3 shows HI contributed by different metals at the 95th percentiles of
Table 3 The hazard index at the 95th percentile of the cumulative probability distribution categorized by contaminants. Samples
Metal category
Original
Water washing
Acid washing
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
FA1
Zn Pb Cu Cr Cd Ni Total
1.00 10 2 1.33 10 1 2.00 10 2 1.74 10 1 2.40 10 2 8.00 10 3 3.70 10 1
3.00 10 3 8.00 10 3 2.00 10 3 2.40 10 2 8.00 10 3 2.00 10 3 4.90 10 2
1.20 10 2 1.48 10 1 2.00 10 2 1.33 10 1 2.30 10 2 6.00 10 3 3.41 10 1
5.00 10 3 1.00 10 2 2.00 10 3 1.80 10 2 9.00 10 3 2.00 10 3 4.60 10 2
6.00 10 3 1.20 10 1 1.40 10 2 1.33 10 1 1.10 10 2 5.00 10 3 2.89 10 1
1.00 10 3 7.00 10 3 1.00 10 3 2.20 10 2 3.00 10 3 1.00 10 3 3.50 10 2
FA2
Zn Pb Cu Cr Cd Ni Total
2.00 10 2 3.02 10 1 3.50 10 2 2.83 10 1 4.90 10 2 1.20 10 2 7.02 10 1
7.00 10 3 2.90 10 2 5.00 10 3 3.50 10 2 2.90 10 2 4.00 10 3 1.09 10 1
2.10 10 2 3.12 10 1 3.70 10 2 2.27 10 1 5.30 10 2 1.20 10 2 6.62 10 1
5.00 10 3 2.70 10 2 2.00 10 3 2.80 10 2 2.70 10 2 5.00 10 3 9.40 10 2
1.60 10 2 3.95 10 1 3.20 10 2 3.95 10 1 2.70 10 2 1.60 10 2 8.80 10 1
2.00 10 3 3.00 10 2 3.00 10 3 5.30 10 2 7.00 10 3 4.00 10 3 9.80 10 2
FA3
Zn Pb Cu Cr Cd Ni Total
5.20 10 2 1.26 3.70 10 2 1.47 6.24 10 1 1.40 10 2 3.46
2.40 10 2 3.24 10 1 1.60 10 2 6.48 10 1 5.46 10 1 4.00 10 3 1.56
6.50 10 2 1.40 4.70 10 2 2.61 7.10 10 1 1.50 10 2 4.85
1.90 10 2 3.20 10 1 1.60 10 2 6.70 10 1 4.70 10 1 4.00 10 3 1.50
4.40 10 2 2.43 2.80 10 2 3.06 1.70 10 1 1.90 10 2 5.75
7.00 10 3 4.80 10 1 5.00 10 3 8.60 10 1 7.60 10 2 5.00 10 3 1.44
FA4
Zn Pb Cu Cr Cd Ni Total
2.00 10 2 4.79 10 1 2.70 10 2 1.58 10 1 1.52 10 1 7.00 10 3 8.43 10 1
8.00 10 3 6.30 10 2 1.30 10 2 3.60 10 2 1.20 10 1 2.00 10 3 2.43 10 1
2.70 10 2 5.86 10 1 3.30 10 2 3.07 10 1 2.11 10 1 8.00 10 3 1.17
1.10 10 2 4.90 10 2 1.50 10 2 5.00 10 2 1.46 10 1 3.00 10 3 2.72 10 1
4.10 10 2 1.15 5.20 10 2 2.55 10 1 3.32 10 1 1.10 10 2 1.84
1.90 10 2 1.08 10 1 1.60 10 2 3.30 10 2 2.59 10 1 4.00 10 3 4.40 10 1
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the cumulative probability distributions. Under most regulatory program, the acceptable hazard index usually defined as 1. For the 95% probability HI based on total contents, as shown in Tables 2 and 3, original MSWI fly ash from FA3 presented an unacceptable risk level of 3.46, while HI of water/acid washed FA3 and FA4 not only higher than the original one, but also exceeded the USEPA acceptable level of 1, and it also indicated that the acidwashing pretreatment increased the HI of most samples except for FA1, while the effect of water-washing process is uncertain. For the method incorporated with bioaccesibility, the 95% probability HI from ingestion pathway was significantly decreased. In the case of original fly ash from FA1, the HI based on total content was 0.37, whereas the adjusted-HI was only 4.90 10 2. But the HI of FA3 from original fly ash, water-washed fly ash and acid-washed fly ash were still unacceptable with the adjustment of bioaccesibility, which is 1.56, 1.50, and 1.44, respectively. After water-washing or acid-washing pretreatment, the 95% probability HI based on bioaccesilbility in FA1, FA2, and FA3 showed a decline, but it is hard to identify the difference of water-washing and acidwashing pretreatment. Since the effect of risk reduction from acidwashing pretreatment is better than water-washing pretreatment in FA1 and FA3, while the FA2 shows an opposite phenomenon. In addition, Table 2 shows that the oral ingestion exposure pathway was the main contributor to HI. Its prevailing contribution rates based on total contents was around 98.95–99.86%, whereas only accounted for 31.15–82.69% for the adjusted one. Table 3 also reports that the Pb, Cr, and Cd are the main contributors to HI for both traditional method and bioaccessible method with the range of 85.10–99.00%, which resulted from the relative high contents of Pb, Cr, and the strong toxic effects of Cd. It appeared that the control of ingestion exposure and removal of Pb, Cr, and Cd are effective ways to control the hazard index in future MSWI fly ash reuse. 3.3.3. Sensitivity analysis A sensitivity analysis was performed to determine which probability density functions have the greatest effect on risk estimates. The contribution to the risk variance of each single in-put variable and model parameter involved in the risk assessment procedure are shown in Fig. 3. As shown in Fig. 3, for carcinogenic risk, exposure frequency (EF) and exposure time (ET) are the most influential variables (88.09%, 11.89%, respectively). For noncarcinogenic risk
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based on total content, the rate of MSWI fly ash ingestion (IR), exposure frequency (EF) are the most influential variables (73.01%, 26.51%, respectively). However, it's worth noting that the adjusted-hazard index for ingestion exposure pathway is calculated based on both the total contents and bioaccessibility, while the bioaccessibility of each metal varies in different samples, so this causes that, in the case of hazard index from multi-exposure pathways, the sensitivity analysis for each metal in different samples differ. What's more, there still exists a meaningful phenomenon that the rate of MSWI fly ash ingestion (IR) shows the prevailing influence on the estimated adjusted-hazard index, which is around or exceed 90%. Sensitivity analysis indicated that, to increase the accuracy of the results, efforts should focus on a better definition of probability distributions for exposure frequency, exposure time, and especially for rate of MSWI fly ash ingestion. Given the scarcity of exposure parameters in China, most of the probability distributions were based on US EPA data or previous papers, and this may be a limitation to the validity of the case presented. Whereas, the remaining variables in the risk equation (i.e., exposure duration, and RfD) which characterized by point estimates do not vary in a Monte Carlo simulation, hence they do not contribute to the variance in the output. This result does not mean that exposure duration, and RfD are unimportant variables in the risk.
4. Conclusions Four MSWI fly ash samples covering four typical regions all over China were used to investigate the effects of washing pretreatment process on the total contents, bioaccessibility, and the final reuse health risk distributions of heavy metals in MSWI fly ash. The results provided four concepts which are meaningful for regulatory agency to make appropriate policies of MSWI fly ash management and reuse. First, both water-washing and acidwashing process would increase the total contents of heavy metal, and reduce the bioaccessibility. Second, the acid-washing process would increase the cancer risk in most cases, while the effect of water-washing process was uncertain. Third, incorporating bioaccessbility adjustments into human health risk assessment can avoid overestimating the hazard index in oral ingestion
Fig. 3. Sensitivity analysis of health risk assessment for workers (a) hazard index based on total content, (b) cancer risk.
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exposure pathway; decreased hazard index based on bioaccesilbility could be found in FA1, FA2, and FA3 after water-washing or acid-washing pretreatment, but it is hard to identify the difference of water-washing and acid-washing pretreatment. Last, in the scenario of MSWI fly ash open storage pile, only water-washing or acid-washing process cannot reduce the actual risk from all samples to acceptable level, especially for cancer risk.
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.ecoenv.2014.09. 030.
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Can washing-pretreatment eliminate the health risk of municipal solid waste incineration fly ash reuse? Yao Wang a,b, Yun Pan a, Lingen Zhang a, Yang Yue a, Jizhi Zhou a, Yunfeng Xu a, Guangren Qian a,n a b
Department of Environmental Science and Engineering, Shanghai University, Shanghai 200444, China Shanghai Institute of Geological Survey, Shanghai 200072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 March 2014 Received in revised form 3 September 2014 Accepted 10 September 2014
Although the reuse of washing-pretreated MSWI fly ash bas been a hot topic, the associated risk is still an issue of great concern. The present study investigated the influence of washing-pretreatment on the total contents and bioaccessibility of heavy metals in MSWI fly ash. Furthermore, the study incorporated bioaccessibility adjustment into probabilistic risk assessment, to quantify the health risk from multipathway exposure to the concerned chemicals as a result of reusing washed MSWI fly ash. The results revealed that both water-washing and acid-washing process have resulted in the concentrated heavy metal content, and have reduced the bioaccessibility of heavy metals. Besides, the acid-washing process increased the cancer risk in most cases, while the effect of water-washing process was uncertain. However, both water-washing and acid-washing pretreatment could decrease the hazard index based on bioaccesilbility. Despite the uncertainties accompanying these procedures, the results indicated that, in this application scenario, only water-washing or acid-washing process cannot reduce the actual risk from all samples to acceptable level, especially for cancer risk. & Published by Elsevier Inc.
Keywords: MSWI fly ash Washing pretreatment Bioaccessibility Probabilistic risk assessment
1. Introduction The reuse of industrial waste in building materials and civil engineering applications has undergone considerable development over a long period. Practices now commonly seen in America, Denmark, Sweden, and the Netherlands are to use coal combustion products, blast-furnace slag and municipal solid waste incineration bottom ash to repair roads and produce asphalt concrete, and ceramic materials (del Valle–Zermeño et al., 2013; Izquierdo et al., 2008; Little et al., 2008). Similarly, the published reports (Guo et al., 2014) have shown that municipal solid waste incineration (MSWI) fly ash has cementitious properties and its main chemical components belong to the system of CaO–SiO2–SO3–Al2O3. Therefore, pretreated MSWI fly ash is now increasingly used for cement manufacturing, roadbed material, and glass ceramics (Aubert et al., 2006, 2007; Francois and Pierson, 2009; Luna Galiano et al., 2011; Wu et al., 2012). Different from coal fly ash and blast-furnace slag, MSWI fly ash has been listed in the National Hazardous Waste Inventory as HW18 for containing different types of heavy metals, chlorinated organic compounds, dioxins, sulfur compounds, etc., While MSWI n
Corresponding author. Fax: þ 86 21 66137758. E-mail address: [email protected] (G. Qian).
http://dx.doi.org/10.1016/j.ecoenv.2014.09.030 0147-6513/& Published by Elsevier Inc.
fly ash is reused as the civil materials, the issue of potential environmental impact associated with the reuse has emerged. Though pretreatment technologies such as water-washing, acidwashing, have been verified that can remove a part of soluble heavy metals (Anastasiadou et al., 2012; Colangelo et al., 2012), it is still unknown that the potential health risk of pretreated MSWI fly ash reuse is acceptable or not. At present, studies mainly concentrate on using the toxicity characteristic leaching procedure (TCLP) to specify the toxicity and risk of pretreated MSWI fly ash reuse (Lee and Li, 2010; Liu et al., 2009; Yang et al., 2009), but paid little attention to the potential health risk caused by occupational exposure via inhalation, non-dietary ingestion and dermal contact pathways. Besides, approximately 18% of the particle size distributions of fly ashes are under the size of 10 μm, even a fraction of fly ash's particle diameter is around 1 μm (Shi and Kan, 2009), which means the MSWI fly ash particle is easier to adhere to skin, and generate more wind-blown dust emission than soil in the reuse process. Therefore, this paper investigated the health risk of occupational exposure to MSWI fly ash reuse with multi-exposure pathways, in order to get the overall results. The traditional health risk method based on the total content of heavy metal will lead to over-estimation of risk, since the actual health risk of heavy metals in ingested medium depend on the fraction that is soluble in the gastrointestinal tract available for absorption (Bade et al., 2012; Kördel et al., 2013). To achieve a
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sound evaluation of the health risk, the bioaccessibility or bioavailability of the heavy metals in MSWI fly ash should thus be considered. According to the literatures (Ruby et al., 1999), bioavailability is defined as the fraction of an administered dose that reaches the central (blood) compartment from the gastrointestinal tract, which should be measured by in-vivo studies; while bioaccessibility of a substance is the fraction that is soluble in the gastrointestinal environment and is available for absorption, which can be assessed by in-vitro methods. Considering the cost and time, most researchers preferred to do in-vitro methods, such as the physiologically based extraction test (PBET) (Juhasz et al., 2010, 2011), to evaluate the bioaccessibility of pollutions from matrix such as soils and dust, when assess the actual health risk of heavy metals by ingestion (Hu et al., 2013; Luo et al., 2012; Vasiluk et al., 2011). However, a wealth of studies demonstrated a strong linear relationship between bioaccessibility and exchangeable and soluble fraction, reducible fraction extracted by BCR method (Ahumada et al., 2011; Akkajit and Tongcumpou, 2010; Alvarenga et al., 2009; Baig et al., 2009; Dabek–Zlotorzynska et al., 2005; De La Calle et al., 2013; Karadaş and Kara, 2012; Poggio et al., 2009; USEPA, 2003, 2007). Hence, we used modified four-step BCR method to evaluate the oral bioaccessible fraction (bioaccessibility) in MSWI fly ash. Additionally, probabilistic approaches, such as Monte Carlo simulation and sensitivity analysis, should be taken into consideration during human health risk assessment process, which could provide the risk assessor with a flexible tool to estimate the uncertainties and stochastic properties of exposure and toxicity (Wu et al., 2011). Nowadays, probabilistic risk assessment has been successfully applied to assess the potential adverse health effects of contaminants from onsite MWS disposal and coal combustion wastes (CCW) practices (Lonati et al., 2007; Lonati and Zanoni, 2012; USEPA, 2010). Following the discussion above, the purpose of this study was two-fold: (i) to evaluate the effects of water/acid washing pretreatment on the total content and bioaccessibility of heavy metals in MSWI fly ash; (ii) to quantitatively assess the risk of occupational exposure to China MSWI fly ash reuse with a probabilistic approach.
2. Materials and methods 2.1. Sampling and pretreatment Four kinds of MSWI fly ash samples (FA1, FA2, FA3, and FA4) were collected from four MSWI plants located which all exceeds 300 t MSW/d in four typical regions all over China. Besides, Both FA1 and FA2 were obtained from the grate-type incinerators, while FA3 and FA4 were obtained from the fluidized bed incinerators. The samples were stored in a desiccator at room temperature after the oven-drying process at 105 °C for 24 h. 2.1.1. Water washing pre-treatment The MSWI fly ash samples were first suspended in distilled water at a liquid–solid ratio (i.e., cm3 g 1) of 8 in a beaker, and stirred in an agitation apparatus at a rotation speed of 200 72 rpm for 5 h. After washing process, the solid/water mixtures were separated through a vacuum pump filter, and the filter cake was again washed. The resulting material was then oven dried at 105 °C for 24 h, and then stored in a desiccator until analysis. 2.1.2. Acid washing pre-treatment The MSWI fly ash samples were brought into contact with 0.5 mol/L HNO3 solutions at a liquid–solid ratio (i.e., cm3 g 1) of 20 in a beaker, and stirred in an agitation apparatus at a rotation
speed of 200 72 rpm for 1 h. After washing process, the solid/ water mixtures were separated through a vacuum pump filter, and the solids were dried in an electro-thermostatic blast oven at 105 °C over 24 h. The dried acid-washed MSWI fly ash was collected and stored in desiccators until analysis. 2.2. Chemical analysis 2.2.1. Total contents of heavy metals in raw/pretreated fly ASH Total content of heavy metal was determined by treating 0.2 g sample with HNO3/HClO4/HF acid mixture digestion method at about 120 °C until the digested solution was clear (Sun et al., 2001). Reagent blanks and analytical duplicates were included to ensure the accuracy and precision of analysis. 2.2.2. Chemical speciation of heavy metals in raw/pretreated fly ASH A modified four-step procedure sequential extraction method (Pan et al., 2013) was adopted to fractionate heavy metals in the exchangeable and acid soluble fraction (F1), reducible fraction (F2), oxidizable fraction (F3), and residual fraction (F4). As described above, the bioaccessible fraction of heavy metals have demonstrated a strong linear relationship with the exchangeable and acid soluble fraction (F1) and reducible fraction (F2). Besides, it has been proved that the excess of heavy metal leached in TCLP, was also contributed to the high content of exchangeable and acid soluble fraction (F1) and reducible fraction (F2) of heavy metal (USEPA, 2003). Consequently, we using the F1 and F2 as the oral bioaccessible fraction (bioaccessibility) in MSWI fly ash (Eq. (S1), Table 1). 2.3. Health risk assessment procedures 2.3.1. Evaluation scenario The major metals, including Zn, Pb, Cu, Cr, Cd, and Ni, are the primary concern of reuse of MSWI fly ash. The risk of worker exposure to pretreated MSWI fly ash open storage pile in landfill site was the focus in this study. The application area was set 15 m 25 m of the storage pile, and the multiple exposure pathways including non-dietary ingestion, dermal contact and inhalation routes. However, the risk caused by groundwater ingestion was not considered in this study, since the leachate from the storage pile was collected for off-site disposal. 2.3.2. Human exposure and health risk assessment model Risk assessment is a multi-step procedure comprised data collection and evaluation, exposure assessment, toxicity assessment, and risk characterization. The human exposure to heavy metals in MSWI fly ash can occur via three main pathways: (i) direct oral ingestion of substrate particles (CDIingestion); (ii) inhalation of re-suspended particulates emitted from storage pile (CDIinhalation); and (iii) dermal absorption of heavy metals in particles adhered to exposed skin (CDIdermal). The cancer risks were evaluated only for Cr, Cd, and Ni through inhalation exposure pathway, since the metals above are only classified as a known human carcinogen (Group A) or probable human carcinogen (Group B1) via the inhalation route. According to Guide for Incorporating Bioavailability Adjustments into Human Health and Ecological Risk Assessments at US Department of Defense Facilities (USEPA, 2003), modified-mathematical models and exposure parameters were listed in Table S1 and Table S2, respectively (USEPA, 2004; Wang et al., 2013; Wu et al., 2011). We treated BW, EF, ET, AFd, SA, and IRfa in Eqs. (S2)–(S4) probabilistically, and used Eq. (S5) to incorporate bioaccessibility adjustments into human health risk assessment. In addition, toxicological characteristics in the present study, for each exposure
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Table 1 The cancer risk at the 50th and 95th percentile of the cumulative probability distribution. Samples
Risk category
Original
Water washing
Acid washing
50th percentile
95th percentile
50th percentile
95th percentile
50th percentile
95th percentile
FA1
Cr Cd Ni Total
8.23 10 6 8.56 10 9 6.76 10 9 8.25 10 6
1.13 10 5 1.17 10 8 9.28 10 9 1.13 10 5
6.35 10 6 8.09 10 9 5.31 10 9 6.36 10 6
8.74 10 6 1.11 10 8 7.31 10 9 8.76 10 6
6.30 10 6 3.98 10 9 4.64 10 9 6.31 10 6
8.52 10 6 5.38 10 9 6.27 10 9 8.53 10 6
FA2
Cr Cd Ni Total
1.33 10 5 1.74 10 8 1.10 10 8 1.34 10 5
1.83 10 5 2.39 10 8 1.51 10 8 1.84 10 5
1.08 10 5 1.87 10 8 1.08 10 8 1.08 10 5
1.49 10 5 2.58 10 8 1.48 10 8 1.49 10 5
1.87 10 5 9.70 10 9 1.40 10 8 1.87 10 5
2.53 10 5 1.31 10 8 1.89 10 8 2.53 10 5
FA3
Cr Cd Ni Total
6.96 10 5 2.21 10 7 1.27 10 8 6.98 10 5
9.55 10 5 3.03 10 7 1.74 10 8 9.58 10 5
1.24 10 4 2.53 10 7 1.35 10 8 1.25 10 4
1.71 10 4 3.49 10 7 1.86 10 8 1.71 10 4
1.45 10 4 6.17 10 8 1.73 10 8 1.45 10 4
1.96 10 4 8.34 10 8 2.34 10 8 1.96 10 4
FA4
Cr Cd Ni Total
7.49 10 6 5.40 10 8 6.08 10 9 7.55 10 6
1.03 10 5 7.41 10 8 8.34 10 9 1.04 10 5
1.46 10 5 7.51 10 8 6.97 10 9 1.47 10 5
2.01 10 5 1.03 10 7 9.59 10 9 2.02 10 5
1.21 10 5 1.19 10 7 9.47 10 9 1.22 10 5
1.63 10 5 1.60 10 7 1.28 10 8 1.65 10 5
route, were normalized to account for extrapolation to a different body weight from standard of 70 kg (Table S1, Eqs. (S6) and (S7)). The chronic daily intake (CDI, mg/kg/day) for oral ingestion pathway was defined as Eq. (S2), where Cfa is the concentration of metal in MSWI fly ash (mg/kg), IRfa is MSWI fly ash ingestion rate for receptor (mg/day), EF is the exposure frequency (day/year), ED is the exposure duration (year), AT is the averaging time for carcinogens or non-carcinogens (day), BW is the bodyweight (kg). The CDI values for inhalation pathway was defined as Eq. (S3), where CPM10 is the predicted PM10 concentrations (kg/m3) of MSWI fly ash by SCREEN3 (USEPA, 1995) under the reasonable worst case scenario emitting 1000 mg MSWI fly ash PM10/m2/day (Macleod et al., 2006), ET is the exposure time (h/day). The CDI values for dermal adsorption pathway was defined as Eq. (S4), where SA is the skin surface area available for exposure (cm2/event), AFd is the dermal adherence rate (mg/cm2-skin), ABSd is the dermal absorption factor (unitless). Considering the different bioaccessibility of site-specific MSWI fly ash, the adjusted CDI values for oral ingestion route was defined as Eq. (S5). The cancer risk for inhalation pathway was defined as Eq. (S6), where the IUR is the inhalation unit risk (ug/m3) 1. The HI for three main pathways was defined as Eq. (S7), where RfDing is the chronic oral ingestion reference dose (mg/kg day), RfCinh is the chronic inhalation reference concentration (mg/m3), ABSGI is the gastrointestinal absorption factor (unitless) which is used to multiply RfDing to yield the corresponding dermal values. The above toxicological characteristics of the investigated heavy metals were list in Table S3 (TX11, 2001; US RAIS, 2011). 2.3.3. Sensitivity analysis To propagate the uncertainties and its impact throughout the whole risk assessment model, Monte Carlo simulation (n ¼5,000) were implemented using the software Crystal Ball 7.2, resulting in a final probability distribution function for the estimated individual cancer risk and hazard index due to MSWI fly ash storage pile in landfill sites. In addition, sensitivity analysis was used to determine which probability density functions have the greatest effect on the risk estimates during Monte Carlo simulations. The sensitivity of each variable relative to one another was assessed by calculating rank correlation coefficients between each input and output during
simulations and then estimating each input contribution to the output variance by squaring the output variance and normalizing to 100%. The lower end of the range of acceptable total incremental lifetime cancer risk (TILCR) was defined by a single constraint on the 95th percentile of risk distribution that must be equal or lower than 10 6 for carcinogen (Chiang et al., 2009) in industry site, and the lower end of the range of acceptable hazard index (HI) was defined by a single constraint on the 95th percentile of risk distribution that must be equal or lower than 1 (USEPA, 1989).
3. Results and discussion 3.1. Water/acid washing effect on the total content of heavy metals To investigate the water/acid washing effects on metal removal from MSWI fly ash, the total concentrations of such harmful high content heavy metals as Zn, Pb, Cu, Cr, Cd, and Ni in original, water-washed, and acid-washed MSWI fly ash are listed in Fig. 1. In the case of original fly ash, the contents of Zn, Pb, Cu, and Cr were 1101.13–5577.52 mg/kg, 168.13–1589.33 mg/kg, 290.52– 509.75 mg/kg, and 150.58–1399.12 mg/kg, respectively, while there were relatively lower concentrations in Cd and Ni, which were 8.03–207.39 mg/kg, 43.94–82.25 mg/kg. The similar phenomenon can be observed in water-washing MSWI fly ash and acid-washing fly ash, which inferred that compared with the content of Zn, Pb, Cu, and Cr, there were relatively low levels of Cd and Ni. The water-washing process significantly increased the total contents of Zn, Pb, and Cu in all samples with the increasing range rates of 5.85–33.85%, 3.43–22.69%, and 0.34–28.69%, respectively (Fig. 1). For example, in the case of FA3, water-washing process increased Zn from 5577.52 to 6929.84 mg/kg, Pb from 1589.33 to 1762.01 mg/kg, Cu from 527.79 to 679.22 mg/kg. The above results indicated that the water-washing process resulted in concentrated heavy metal content in the ash afterwards, which was caused by the removal of soluble chlorides, especially soluble calcium salts (Wang et al., 2001). However, the contents of Cr in water-washed MSWI fly ash showed a visible reduction in FA1 and FA2. This is consistent with other reports that Cr is found to be easily extracted
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Fig. 1. The total contents of heavy metals in original and pretreated MSWI fly ash.
one among the heavy metals except for K, Ca, and Na in MSWI fly ash samples (Jiang et al., 2009; Wang et al., 2009), which infers that the removal rate of Cr is higher than the soluble chlorides, calcium salts. Besides, due to relatively low concentration of Cd and Ni, no substantial changes have been found in washed fly ash. Acid-washing process could remove calcium carbonate, calcium hydroxide and other parts of precipitates, which are difficult to be soluble by water-washing process. However, the change in total contents of heavy metals in fly ash after washing pretreatment not only depends on the relative removal amount of soluble salts and acid-soluble precipitates, but also controlled by the chemical species of heavy metals, and the chemical stability of heavy metal speciation in water or acid solutions is different. In this case, as shown in Fig. 1, similarly, acid-washing process increased the contents of Pb and Cr in FA2, FA3, and FA4. Different from water-washed fly ash, the contents of Zn, Cu in FA1, FA2, and FA3 indicated a decline after acid-washing extraction. In the case of FA1, the acid-washing process decreased the content of Zn from 1101.13 to 642.64 mg/kg, Cu from 290.52 to 195.84 mg/kg. The decline above appeared that higher amount of exchangeable and acid soluble fraction, reducible fraction of Zn, Cu have been removed with HNO3 solutions than other non-metal soluble salts. 3.2. Effect of water/acid washing pretreatment on bioaccessibility It is evident that the water/acid-washing pretreatment changed the total contents of heavy metals in MSWI fly ash. In order to evaluate the effects of water/acid-washing pretreatment on
bioaccessibility, as mentioned in Section 2.2, we defined that the exchangeable and acid soluble fraction (F1) and reducible fraction (F2) is the oral bioaccessbile fraction of heavy metals in fly ash. The bioaccessibility of heavy metals in original, water-washed and acid-washed MSWI fly ash are reported in Fig. 2. For the original MSWI fly ash, the bioaccessibilities of Zn, Cu, and Cd indicated a high level relative to other heavy metals such as Pb, Cr, and Ni. For instance, in the case of FA3, the bioaccessibility of Zn, Cu, and Pb was 39.71%, 48.94%, and 77.50%, respectively, while 10.99%, 11.77%, and 19.81%, respectively, for the Pb, Cr, and Ni. Additionally, the bioaccessibility of each heavy metal in original MSWI fly ash from FA3 and FA4 was higher than the corresponding one from FA1 and FA2. The differences may be caused by the different collecting and handling methods of municipal solid waste. Fig. 2 indicates that the water-washing process lowered the bioaccessibilities of heavy metals in FA2, FA3, and FA4. In the case of FA3, the bioaccessibility of each analyzed metal all reported a decline with the range of 2.58–23.83% after water-washing process. The phenomenon indicated that the washing process have washed out some parts of metals in the exchangeable fraction. Similarly, the acid-washing process also reduced the bioaccessibilities of heavy metals in FA1, FA2, and FA3. And, in most cases, the effects on reducing the bioaccessibility of heavy metals is better than water-washing process, since acid-washing process can remove more metals in the carbonate fraction, acid soluble fraction or reducible fraction with the support of HNO3 solutions.
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Fig. 2. The bioaccessibility of heavy metals in original and pretreated MSWI fly ash.
In the case of FA2, the bioaccessibilities of heavy metals (Zn, Pb, Cu, Cr, Cd, and Ni) reached 25.03%, 6.27%, 8.89%, 0%, 47.46%, and 10.02%, respectively, for water-washed fly ash; while 8.99%, 5.03%, 7.63%, 0.47%, 20.61%, and 5.80%, respectively, for the acid-washed fly ash. Since the water-washing and acid-washing process can remove a part of metals in the exchangeable and acid soluble fraction (F1) or reducible fraction (F2), the heavy metals of washed fly ash were more stable than original MSWI fly ash in the following recycling process. But it was still unknown whether the health risk of washed-MSWI fly ash reuse is acceptable or not. 3.3. Human exposure and health risk assessment model 3.3.1. Spatial distribution of cancer risk Since the Cr, Cd, and Ni are only classified as a known human carcinogen (Group A) or probable human carcinogen (Group B1) via the inhalation route, the carcinogenic risk of our study was focus on the lung cancer risks. Under most regulatory program, risk between 10 6 and 10 4 indicates potential risk, whereas risk larger than 10 4 indicates high potential health risk. Table 1 summarizes the probabilistic incremental lifetime cancer risk (ILCRs) for workers at MSWI fly ash open pile.
For the original fly ash, Cr was the main contributor to carcinogenic risk and the contribution rates up to 99.04–99.76%. Both the 50% probability total ILCRs (TILCRs) and the 95% probability TILCRs for all samples presented an unacceptable level, which ranged from 7.55 10 6 to 6.98 10 5, 1.04 10 5 to 9.58 10 5. It appeared that the reuse of original MSWI fly ash posed an unacceptable carcinogenic risk for the workers. As consistent with original fly ash, as shown in Table 1, the main contributor to carcinogenic risk in water-washed and acidwashed fly ash is Cr. Hence, the effects of the water-washing and acid-washing process on the spatial distribution of carcinogenic risk largely depend on the changes in total contents of Cr. The water-washing process reduced the 50% and 95% probability TILCRs of FA1 and FA2, while increased the 50% and 95% probability TILCRs of FA3 and FA4, which indicated that the effect of water-washing process is uncertain. However, the acid-washing process significantly increased 50% and 95% probability TILCRs of FA2, FA3, and FA4, and all exceeded the USEPA acceptable level of 10 6, only FA1 showed a slight decrease. The phenomena indicated that, in most cases, the acid-washing process would increase the carcinogenic risk of MSWI fly ash in the open pile scenario.
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Table 2 The hazard index at the 95th percentile of the cumulative probability distribution categorized by exposure pathway. Samples
exposure pathway
Original
Water washing
Acid washing
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
FA1
Dermal Inhalation Ingestion Total
2.20 10 2 7.00 10 3 3.41 10 1 3.70 10 1
2.20 10 2 7.00 10 3 2.00 10 2 4.90 10 2
1.80 10 2 6.00 10 3 3.18 10 1 3.41 10 1
1.80 10 2 6.00 10 3 2.20 10 2 4.60 10 2
1.80 10 2 5.00 10 3 2.66 10 1 2.89 10 1
1.80 10 2 5.00 10 3 1.30 10 2 3.50 10 2
FA2
Dermal Inhalation Ingestion Total
3.70 10 2 1.30 10 2 6.52 10 1 7.02 10 1
3.70 10 2 1.30 10 2 5.90 10 2 1.09 10 1
3.10 10 2 1.20 10 2 6.19 10 1 6.62 10 1
3.10 10 2 1.20 10 2 5.10 10 2 9.40 10 2
5.30 10 2 1.40 10 2 8.12 10 1 8.80 10 1
5.30 10 2 1.40 10 2 3.00 10 2 9.80 10 2
FA3
Dermal Inhalation Ingestion Total
1.88 10 1 8.30 10 2 3.19 3.46
1.88 10 1 8.30 10 2 1.29 1.56
3.07 10 1 1.13 10 1 4.42 4.85
3.07 10 1 1.13 10 1 1.08 1.50
3.91 10 1 8.90 10 2 5.27 5.75
3.91 10 1 8.90 10 2 9.60 10 1 1.44
FA4
Dermal Inhalation Ingestion Total
2.90 10 2 1.80 10 2 7.98 10 1 8.43 10 1
2.90 10 2 1.80 10 2 1.97 10 1 2.43 10 1
4.60 10 2 2.50 10 2 1.10 1.17
4.60 10 2 2.50 10 2 2.00 10 1 2.72 10 1
5.70 10 2 3.60 10 2 1.75 1.84
5.70 10 2 3.60 10 2 3.46 10 1 4.40 10 1
Table 1 also reports that the contribution rates of Cr in waterwashed and acid-washed fly ash were around 98.95–99.86%, therefore the removal of Cr seemed an effective way to control the carcinogenic risk in future MSWI fly ash reuse. However, predicted maximum PM10 concentration was based on the reasonable worst case scenario, which may overestimated the cancer risk.
3.3.2. Spatial distribution of hazard index The hazard index (HI) based on traditional method or bioaccessibility for each sample is shown in Tables 2 and 3. Table 2 presents the HI categorized by exposure pathway at the 95th percentiles of the cumulative probability distributions, and Table 3 shows HI contributed by different metals at the 95th percentiles of
Table 3 The hazard index at the 95th percentile of the cumulative probability distribution categorized by contaminants. Samples
Metal category
Original
Water washing
Acid washing
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
95th percentile
BAF-95th percentile
FA1
Zn Pb Cu Cr Cd Ni Total
1.00 10 2 1.33 10 1 2.00 10 2 1.74 10 1 2.40 10 2 8.00 10 3 3.70 10 1
3.00 10 3 8.00 10 3 2.00 10 3 2.40 10 2 8.00 10 3 2.00 10 3 4.90 10 2
1.20 10 2 1.48 10 1 2.00 10 2 1.33 10 1 2.30 10 2 6.00 10 3 3.41 10 1
5.00 10 3 1.00 10 2 2.00 10 3 1.80 10 2 9.00 10 3 2.00 10 3 4.60 10 2
6.00 10 3 1.20 10 1 1.40 10 2 1.33 10 1 1.10 10 2 5.00 10 3 2.89 10 1
1.00 10 3 7.00 10 3 1.00 10 3 2.20 10 2 3.00 10 3 1.00 10 3 3.50 10 2
FA2
Zn Pb Cu Cr Cd Ni Total
2.00 10 2 3.02 10 1 3.50 10 2 2.83 10 1 4.90 10 2 1.20 10 2 7.02 10 1
7.00 10 3 2.90 10 2 5.00 10 3 3.50 10 2 2.90 10 2 4.00 10 3 1.09 10 1
2.10 10 2 3.12 10 1 3.70 10 2 2.27 10 1 5.30 10 2 1.20 10 2 6.62 10 1
5.00 10 3 2.70 10 2 2.00 10 3 2.80 10 2 2.70 10 2 5.00 10 3 9.40 10 2
1.60 10 2 3.95 10 1 3.20 10 2 3.95 10 1 2.70 10 2 1.60 10 2 8.80 10 1
2.00 10 3 3.00 10 2 3.00 10 3 5.30 10 2 7.00 10 3 4.00 10 3 9.80 10 2
FA3
Zn Pb Cu Cr Cd Ni Total
5.20 10 2 1.26 3.70 10 2 1.47 6.24 10 1 1.40 10 2 3.46
2.40 10 2 3.24 10 1 1.60 10 2 6.48 10 1 5.46 10 1 4.00 10 3 1.56
6.50 10 2 1.40 4.70 10 2 2.61 7.10 10 1 1.50 10 2 4.85
1.90 10 2 3.20 10 1 1.60 10 2 6.70 10 1 4.70 10 1 4.00 10 3 1.50
4.40 10 2 2.43 2.80 10 2 3.06 1.70 10 1 1.90 10 2 5.75
7.00 10 3 4.80 10 1 5.00 10 3 8.60 10 1 7.60 10 2 5.00 10 3 1.44
FA4
Zn Pb Cu Cr Cd Ni Total
2.00 10 2 4.79 10 1 2.70 10 2 1.58 10 1 1.52 10 1 7.00 10 3 8.43 10 1
8.00 10 3 6.30 10 2 1.30 10 2 3.60 10 2 1.20 10 1 2.00 10 3 2.43 10 1
2.70 10 2 5.86 10 1 3.30 10 2 3.07 10 1 2.11 10 1 8.00 10 3 1.17
1.10 10 2 4.90 10 2 1.50 10 2 5.00 10 2 1.46 10 1 3.00 10 3 2.72 10 1
4.10 10 2 1.15 5.20 10 2 2.55 10 1 3.32 10 1 1.10 10 2 1.84
1.90 10 2 1.08 10 1 1.60 10 2 3.30 10 2 2.59 10 1 4.00 10 3 4.40 10 1
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the cumulative probability distributions. Under most regulatory program, the acceptable hazard index usually defined as 1. For the 95% probability HI based on total contents, as shown in Tables 2 and 3, original MSWI fly ash from FA3 presented an unacceptable risk level of 3.46, while HI of water/acid washed FA3 and FA4 not only higher than the original one, but also exceeded the USEPA acceptable level of 1, and it also indicated that the acidwashing pretreatment increased the HI of most samples except for FA1, while the effect of water-washing process is uncertain. For the method incorporated with bioaccesibility, the 95% probability HI from ingestion pathway was significantly decreased. In the case of original fly ash from FA1, the HI based on total content was 0.37, whereas the adjusted-HI was only 4.90 10 2. But the HI of FA3 from original fly ash, water-washed fly ash and acid-washed fly ash were still unacceptable with the adjustment of bioaccesibility, which is 1.56, 1.50, and 1.44, respectively. After water-washing or acid-washing pretreatment, the 95% probability HI based on bioaccesilbility in FA1, FA2, and FA3 showed a decline, but it is hard to identify the difference of water-washing and acidwashing pretreatment. Since the effect of risk reduction from acidwashing pretreatment is better than water-washing pretreatment in FA1 and FA3, while the FA2 shows an opposite phenomenon. In addition, Table 2 shows that the oral ingestion exposure pathway was the main contributor to HI. Its prevailing contribution rates based on total contents was around 98.95–99.86%, whereas only accounted for 31.15–82.69% for the adjusted one. Table 3 also reports that the Pb, Cr, and Cd are the main contributors to HI for both traditional method and bioaccessible method with the range of 85.10–99.00%, which resulted from the relative high contents of Pb, Cr, and the strong toxic effects of Cd. It appeared that the control of ingestion exposure and removal of Pb, Cr, and Cd are effective ways to control the hazard index in future MSWI fly ash reuse. 3.3.3. Sensitivity analysis A sensitivity analysis was performed to determine which probability density functions have the greatest effect on risk estimates. The contribution to the risk variance of each single in-put variable and model parameter involved in the risk assessment procedure are shown in Fig. 3. As shown in Fig. 3, for carcinogenic risk, exposure frequency (EF) and exposure time (ET) are the most influential variables (88.09%, 11.89%, respectively). For noncarcinogenic risk
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based on total content, the rate of MSWI fly ash ingestion (IR), exposure frequency (EF) are the most influential variables (73.01%, 26.51%, respectively). However, it's worth noting that the adjusted-hazard index for ingestion exposure pathway is calculated based on both the total contents and bioaccessibility, while the bioaccessibility of each metal varies in different samples, so this causes that, in the case of hazard index from multi-exposure pathways, the sensitivity analysis for each metal in different samples differ. What's more, there still exists a meaningful phenomenon that the rate of MSWI fly ash ingestion (IR) shows the prevailing influence on the estimated adjusted-hazard index, which is around or exceed 90%. Sensitivity analysis indicated that, to increase the accuracy of the results, efforts should focus on a better definition of probability distributions for exposure frequency, exposure time, and especially for rate of MSWI fly ash ingestion. Given the scarcity of exposure parameters in China, most of the probability distributions were based on US EPA data or previous papers, and this may be a limitation to the validity of the case presented. Whereas, the remaining variables in the risk equation (i.e., exposure duration, and RfD) which characterized by point estimates do not vary in a Monte Carlo simulation, hence they do not contribute to the variance in the output. This result does not mean that exposure duration, and RfD are unimportant variables in the risk.
4. Conclusions Four MSWI fly ash samples covering four typical regions all over China were used to investigate the effects of washing pretreatment process on the total contents, bioaccessibility, and the final reuse health risk distributions of heavy metals in MSWI fly ash. The results provided four concepts which are meaningful for regulatory agency to make appropriate policies of MSWI fly ash management and reuse. First, both water-washing and acidwashing process would increase the total contents of heavy metal, and reduce the bioaccessibility. Second, the acid-washing process would increase the cancer risk in most cases, while the effect of water-washing process was uncertain. Third, incorporating bioaccessbility adjustments into human health risk assessment can avoid overestimating the hazard index in oral ingestion
Fig. 3. Sensitivity analysis of health risk assessment for workers (a) hazard index based on total content, (b) cancer risk.
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exposure pathway; decreased hazard index based on bioaccesilbility could be found in FA1, FA2, and FA3 after water-washing or acid-washing pretreatment, but it is hard to identify the difference of water-washing and acid-washing pretreatment. Last, in the scenario of MSWI fly ash open storage pile, only water-washing or acid-washing process cannot reduce the actual risk from all samples to acceptable level, especially for cancer risk.
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.ecoenv.2014.09. 030.
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