Mass Concentrations of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans (PCDD/Fs) and Heavy Metals in Different Size Fractions of Hospital Solid Waste Incinerator Fly Ash Particles

Hospital solid waste incinerator (HSWI) fly ash is an extremely complex mixture because it contains carbon constituents, toxic heavy metals, and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). We investigated the distribution of carbon constituents, PCDD/Fs, and heavy metals in the five different particle size fractions (+106 μm, –106 + 75 μm, –75 + 38 μm, –38 + 25 μm, and –25 μm) of HSWI fly ash. The loss on ignition (LOI) in the particle size fraction of –25 μm was higher than that in the particle size fractions of other sizes. Moreover, 82.63% of the fly ash particles were in the size range of –75 + 25 μm, and this particle size range was suitable for direct flotation treatment. Because powder-activated carbon adsorbed a higher amount of low-chlorinated PCDD/Fs in the gaseous phase, the concentration of each PCDD/F congener, particularly of low-chlorinated PCDD/Fs, generally increased with a decrease in the particle size. The total toxic equivalent (TEQ) of the fraction of –75 μm did not satisfy the landfill site standard (3 ng-TEQ g). The highest concentration of Pb, Zn, and Cd was observed in the fraction of –38 + 25 μm rather than in the fraction of –25 μm. The exchangeable speciation of Zn, Pb, and Cd was relatively high in the particle size fraction of –75 μm. The carbonate speciation of Pb and Zn was the highest in the particle size fraction of –38 + 25 μm. The leaching concentrations of Pb and Cd in all particle size fractions exceeded landfill allowable thresholds.


INTRODUCTION
Hospital solid waste is classified as a hazardous material in China because it contains infectious, radioactive, and toxic substances from hospitals, laboratories, and clinics (Javied et al., 2008).The annual generation of hospital solid waste in China is approximately 1.65 million tons (Xie et al., 2009).The proportion of plastic, chlorine, and heavy metals is higher in hospital solid waste than in municipal solid waste (Wang et al., 2003;Chen et al., 2013).Incineration is currently the most common technology used for hospital solid waste disposal in China.However, hospital solid waste incinerators (HSWIs) are considered as one of the major emission sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) (Yan et al., 2011).A powderactivated carbon (PAC) injection followed by baghouse filtration is a key technology to remove PCDD/Fs from flue gases (Ji et al., 2013).Therefore, numerous PAC injections are injected into the exhaust gas to absorb PCDD/Fs from flue gases so as to meet the strict emission regulations of hospital solid waste incineration (< 0.01 ng I-TEQ Nm -3 ) (Liu et al., 2013).After the adsorption of PCDD/Fs on PAC, PAC with fly ash is then collected in the bag filter and removed as fly ash (Wang et al., 2009).In addition, PCDD/Fs are easily adsorbed on the surface of unburned carbon present in HSWI fly ash.The carbon constituents including PAC and unburned carbon are indicated as the major sources of PCDD/Fs in fly ash because of their large adsorptive surface area and their role in denovo synthesis (Kakuta et al., 2007).
The resulting mixture of ash particles and injected PAC, which constitutes the physical and chemical properties of HSWI fly ash, is very complex.However, little attention has been paid to the effect of high PAC content on the distribution of PCDD/Fs and heavy metals in HSWI fly ash.Studies on HSWI fly ash have primarily focused on the determination of the bulk chemical composition of fly ash particles.The chemicals primarily evaluated were PCDD/Fs, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and some inorganic compounds such as heavy metals and metal complexes (Huang et al., 2007;Zhao et al., 2008;Kim et al., 2011;Wei et al., 2014;Xiong et al., 2014).
The particle size distribution is a crucial physical feature of fly ash.It is noted that the particle size distribution mentioned in this paper is the particle size distribution of hospital solid waste incinerator fly ash particles, which differ from the traditionally defined particle sizes in the air-quality community.A study reported that the chemical composition of fly ash is dependent on its particle size (Buekens et al., 2011).Kim et al. (2011) indicated that the particle size influenced the transport pathway of pollutants and the adsorption characteristics of toxic chemicals.Sukandar et al. (2006) reported that approximately 80% of HSWI fly ash had a particle size of -100 µm and that the concentration of Hg and Sn in fly ash tended to increase in small particle size fractions.Gidarakos et al. (2009) reported that the heavy metals were present in higher percentages in the thin sieves.However, Thipse et al. (2002) reported that the maximum values of Pb and Hg were obtained in the size range of -300 + 150 µm in municipal solid waste incinerator (MSWI) fly ash.Therefore, the classification of particles in these size fractions can significantly reduce the amount of these toxic metals in the remaining fly ash.Gilardoni et al. (2004) considered that small-size fly ash particles, which constitute most of the surface area, were the main contributors to the levels of chlorinated aromatic compounds in flue gases.Lu et al. (2007) considered that small-size MSWI fly ash particles have more active surface sites; thus, they can easily absorb reaction intermediates and catalysts to facilitate the increased formation of PCDDs/Fs.Furthermore, Lu et al. (2012) reported that all PCDDs, PCDFs, 2,3,7,8-PCDDs, and 2,3,7,8-PCDFs in MSWI fly ash increased with decreases in the particle size.Therefore, different fly ash fractions have different chemical and physical compositions and vary in their suitability for subsequent treatments (Dahl et al., 2010).
HSWI fly ash has become a crucial public concern in China because of its high toxicity.In our previous study, we developed the column flotation method, an efficient and inexpensive disposal method, for the treatment of HSWI fly ash containing a high carbon concentration (Liu et al., 2013).Numerous studies have investigated aspects of flotation kinetics while particularly focusing on particle size (Abkhoshk et al., 2010;Jorjani et al., 2013;Zhang et al., 2013).The particle size is a crucial parameter contributing to the efficient performance of the flotation process.Therefore, it is necessary to focus on PCDD/Fs and heavy metals in fly ash as a function of particle size.In this study, we investigated the relationship between particle size and PCDD/F and heavy metal distribution to evaluate the separation behavior of toxic chemicals during subsequent flotation treatments.

Raw Materials
The fly ash sample was obtained from a 20 t d -1 gyration kiln incinerator from the HSW incineration center located in northern China (Fig. 1).The incinerator was equipped with a PAC injection, and bag filters were used as air pollution control devices (APCDs) for controlling PCDD/Fs.A fresh and dry fly ash sample was collected from the hopper of the bag filter; therefore, it contained PAC that was injected into the flue gas duct before using bag filters.The ash sample was collected and dried at 105°C for 24 h.Table 1 shows the chemical composition and bulk concentration of heavy metals in the fly ash sample.Data on concentration of heavy metals in hospital waste ash from incinerators of various countries of the world has been compiled from literature and represented in Table 2.The comparison shows that concentration of heavy metals in hospital solid waste incinerator fly ash of China is relatively higher than that in fly ash of other countries of the world.The chlorine level in the fly ash sample was > 20 wt%, which was related to the usage of polyvinylchloride disposable infusion devices and NaCl frequently used in medical treatments.

Methods
The sieving analysis was performed through mechanical shaking by using a stainless steel screen with openings of 150, 200, 400, and 500 meshes (i.e., 106, 75, 38, and 25 + 38, -38 + 25, and -25 µm).The weight distribution and carbon content [loss on ignition (LOI)] of each fraction were determined.The concentration, speciation, heavy metal leaching concentration, and PCDD/F concentration and distribution were analyzed in each dimensional range of fly ash.LOI was determined as the weight lost when the subsample was kept at 600 ± 25°C for 3 h in accordance with the standard for pollution control (GB18485-2014).

Analysis of PCDD/Fs
The concentration of 17 toxic PCDD/F congeners (2,3,7,8-substituted) was analyzed using isotope dilution analysis through high-resolution gas chromatographyhigh-resolution mass spectrometry (HRGC-HRMS).HRGC was equipped with a DB-5 ms fused-silica capillary column (L = 60 m, id = 0.25 mm, and film thickness = 0.25 µm) and a splitless injection.The high-resolution mass spectrometer was equipped with a positive electron impact source.The analyzer mode of selected ion monitoring had a resolving power of 10,000.The electron energy and source temperature were 38 eV and 250°C, respectively.Prior to the analysis, the samples were ground for homogenization and subsequently subjected to treatment procedures.The sample pretreatment was conducted according to the modified version of the US EPA Method 23 (20).The 13 C 12 isotopically labeled internal standard solution was purchased from Cambridge Isotope Laboratories.The total toxic equivalents (TEQ) of dioxin congeners in the sample were calculated on the basis of international toxic equivalency factors.

Analysis of Heavy Metals
The heavy metals such as Pb, Zn, and Cu were analyzed using an atomic absorption spectrometer (AA800) after sample digestion.A modified version of the sequential extraction procedure developed by Tessier was used for the partitioning of heavy metal speciation.(Liu et al., 2014).This method focused on the breakdown of materials into varied speciations that could be selectively dissolved using particular extraction agents.The chemical speciation in the fly ash samples was divided into exchangeable fraction, carbonate fraction, Fe-Mn oxide fraction, organic matter fraction, and residue fraction.The leaching test was conducted in accordance to the Chinese solid waste extraction procedure for leaching toxicity-acetic acid buffer solution (HJ/T300-2007), which is reported in our previous study (Abkhoshk et al., 2010).To ensure validity and reliability of data, all tests were performed at minimum in duplicate and results were calculated as the average of three measurements.

Particle Size Distribution
Fig. 2 shows the weight distribution and carbon content (i.e., LOI) of each particle size fraction of fly ash and PAC.Approximately 98% of the fly ash had a particle -106 µm, which was finer than that reported in the literature (Sukandar et al., 2006;Liu et al., 2013).This difference might be attributed to different feeding wastes, incinerator operating conditions, and air pollution control devices.Moreover, 89.3% of the fly ash particles had a diameter of -75 µm, and the distribution of carbon constituents in this size range was 90.4%.This could be attributed to the high amount of PAC injected into the APCDs of the incinerator.Moreover, 86.6% of the PAC particles were in the same size range.The overall LOI of raw fly ash was 12.62%, and LOI decreased with an increase in the particle size.Because fine PAC was enriched in the fraction of -25 µm, LOI in this fraction was 19.35%, which was 6.73% higher than the average LOI.This result was inconsistent with that of coal fly ash whose carbon constituents were enriched in the coarse size fraction (Liu et al., 2013).Froth flotation was primarily applicable for treating fine-grained material with a particle size range of -300 + 10 µm, and the optimum particle size range was -75 µm (Dermont et al., 2010).Therefore, the particle size of fly ash was highly suitable for froth flotation without the grinding pretreatment.

PCDD/F Congener Distribution of Different Particle Size Fractions
Fig. 4 presents the concentration of each PCDD/F congener  of different particle size fractions conducted for fly ash.Each PCDD/F congener appeared to increase when the fly ash particle size decreased; the maximum was obtained with the smallest particles (-25 µm).Furthermore, the fractions of low-chlorinated compounds, such as PeCDD, TeCDF, PeCDF, and HxCDF, increased when the particle size decreased.This can be attributed to the high PAC content in small-size fly ash particles.PAC could effectively remove gaseous-phase PCDD/Fs; however, it could not effectively remove particle-bound PCDD/Fs (Atkinson et al., 2015).The low-chlorinated PCDD/F congeners with high vapor pressure had a higher mass fraction in the gaseous phase; however, the highly chlorinated PCDD/F congeners were primarily associated with particulates (Du et al., 2013;Pan et al., 2013).Gaseous-phase PCDD/Fs were easily adsorbed onto PAC particles in the flue gas flow and subsequently filtrated by the bag filter.However, most PCDD/Fs in the solid phase were removed by the bag filter through inertial impaction, direct interception, and Brownian diffusion mechanisms (Chang et al., 2009).

Total PCDD/F Concentration in Different Particle Size Fractions
The total PCDD/F concentration in fly ash was 78.8 ng g -1 , which was equivalent to 5.61 ng-TEQ g -1 .The TEQ of PCDD/Fs was much higher than that of municipal solid waste incinerator fly ash (Liu et al., 2013).Fig. 5 shows total PCDD/F concentrations and TEQ values for the different particle size fractions of fly ash.When the particle size of fly ash decreased from +106 µm to -25 µm, the PCDD/F concentration of the fly ash increased from 42.50 ng g -1 to 103.29 ng g -1 , respectively, and the TEQ value of the particle size of -25 µm increased by 4.6 times that of the particle size +106 µm.Because the fine particles of the fly ash contained a relatively high content of PAC, a high amount of low-chlorinated PCDD/Fs was adsorbed in the gaseous phase.The low-chlorinated PCDD/Fs had a relatively high toxicity factor.This could explain the presence of high TEQ values for fine fly ash particles.All TEQ values in the fraction of -75 µm exceeded the regulation limit (3 ng I-TEQ g -1 ) for a landfill in China (Liu et al., 2014).Therefore, for the removal of PCDD/Fs, the fraction of -75 µm should be particularly considered in the flotation treatment of fly ash.

Heavy Metal Concentration in Different Particle Size Fractions
The concentration of heavy metals in five different particle size fractions is shown in Fig. 6.The heavy metals Pb, Zn, and Cu predominantly had a higher concentration in fly ash, followed by Cd and Cr.The heavy metals such as Cu, Pb, and Zn were probably used for the production of alloys for medical equipment such as needles (Sukandar et al., 2006).Cu had a relatively high boiling point, but it was thought that Cu is partially converted into CuCl 2 and then entered into fly ash.Cr is a nonvolatile heavy metal, and a small amount of Cr is possibly carried into fly ash by solid particles in flue gases.
We observed that the concentration of heavy metals was different for different particle size fractions of fly ash.Pb and Zn had a similar concentration distribution as a function of the particle size.The highest values of both Pb and Zn were obtained for the fraction of -38 + 25 µm, followed by the fraction of -75 + 38 µm.The concentration of Cd initially increased and then decreased with a decrease in the particle size, and the maximum value was obtained in the fraction of -38 + 25 µm.This result was consistent with that reported in the literature that Cd, Pb, and Zn mainly comprised the fine fraction of fly ash (Kim et al., 2002).However, the highest values of Pb, Zn, and Cd were not observed in the smallest size fraction (-25 µm) in our study, which might be related to high PAC content in the fraction of -25 µm.Because vaporized metal chlorides condense and aggregate on the surface of fly ash before PAC injection, a high content of PAC mixed with a fly ash fraction of -25 µm might dilute heavy metals such as Pb, Zn, and Cd (Gilardoni et al., 2004).
No significant differences were observed in Cu and Cr concentrations among different particle size fractions.In particular, Cu had a high affinity for chlorine and thus played a catalytic role and easily promoted the formation of PCDD/Fs.This observation was not consistent with those reported in previous studies.For instance, Sukandar et al. (2006) reported that Cu concentration was higher in small-sized particles.Thipse et al. (2002) indicated that the Cr concentration in particles increased with an increase in the particle size.The relationship between the concentration of heavy metals and the particle size of fly ash is complex because it depends on several factors such as feeding waste composition, incinerator type, operating conditions, and flue gas cleaning device and PAC injection efficiency (Gilardoni et al., 2004;Sukandar et al., 2006;Dahl et al., 2010).

Speciation Distribution of Heavy Metals in Different Particle Size Fractions
The speciation distribution of heavy metals in different particle size fractions of fly ash is shown in Fig. 7. Pb was one of the major heavy metals in fly ash and had high toxicity.Pb retained a fairly uniform distribution in each dimensional range.Fine particles (particle size of -75 µm) had a relatively high exchangeable form.The exchangeable and carbonate-associated Pb was the highest in the particle size fraction of -38 + 25 µm.The leaching of Pb with an acid solution is a crucial concern because of its high carbonate speciation.
Zn had a high exchangeable fraction, which could be attributed to ZnCl 2 formation.Zinc oxides easily reacted with NaCl to form low-boiling point (732°C) ZnCl 2 , which adsorbed the fine fly ash particles.Carbonate-associated Zn was the highest (33.2%) in the particle size fraction of -38 + 25 µm.The residual speciation in each particle size fraction was relatively low.
The dominant and secondary speciation of Cd were exchangeable (29.8%-38.9%)and carbonate (18.2%-28.5%)fractions, respectively.Both the exchangeable and carbonate fractions of Cd were higher in the particle size fraction of -75 µm.
The exchangeable speciation of Zn, Pb, and Cd was higher in the particle size fraction of -75 µm than in the particle size fraction of +75 µm (Fig. 7).Moreover, HCl present in flue gases at combustion chamber outlets may have affected the speciation of Pb, Zn, and Cd by forming heavy metal chlorides that generated smaller particles through the volatilization-condensation processes (Gilardoni et al., 2004).Cd associated with Fe-Mn oxide, and organic and residual speciation among different particle size fractions was not remarkable.These results were inconsistent with those reported by Sukandar et al. (2006) that Fe-Mn oxide-associated Cd was the highest in the particle size fraction of -150 + 106 µm.
The residual speciation of Cu in all particle size fractions was the most dominant.Compared with other speciation, exchangeable and associated Cu was relatively less.Carbonate-associated Cu was the lowest (6.7%) in the particle size fraction of -25 µm.The behavior of Cr was similar to that of Cu, which mainly existed as a residual fraction (> 71.3%); however, some Cr existed as an exchangeable and carbonate-associated fraction.The exchangeable form of Cr was not observed in the particle size fraction of -106 + 75 µm and +106 µm.Therefore, additional studies investigating the relationship between particle size distribution and heavy metal speciation are warranted.

Leaching Concentration of Heavy Metals in Different Particle Size Fractions
Table 3 lists the leaching concentration of heavy metals in different particle size fractions.If the leaching concentration of heavy metals is under the permitted limits, fly ash can be deposited in the landfill site of municipal solid waste (GB16889-2008).The leaching concentrations of Zn, Pb, and Cd were higher than those of Cu and Cr.With regard to the particle size, the minimum leaching concentration of Pb and Cd was observed in the particle size fraction of -106 + 75 µm.The leaching concentration of Zn, Pb, and Cd was higher in the particle size fraction of -75 µm than in the particle size fraction of +75 µm.These results were in accordance with high exchangeable and carbonate fractions observed in the small particle size of Zn, Pb, and Cd (Fig. 7).No significant variations in the leaching concentration of Cu and Cr were observed for different particle size fractions.According to the landfill standard, the leaching concentration of Pb and Cd was 7-14 and 5-10 times, respectively, higher than the permitted limits.Therefore, fly ash should be treated prior to deposition in the landfill site of municipal solid waste in China.
From the viewpoint of resource recovery, the concentration of Pb and Zn and their carbonate speciation in fly ash was high; it is necessary to consider the recovery of Pb and Zn before depositing tailings in the landfill site.Pb, Zn, and Cd have high release and are converted into liquids at a low pH value.However, decarbonization flotation can only successfully remove carbon constituents and PCDD/Fs from HSWI fly ash under neutral or slightly acidic conditions (Huang et al., 2007;Liu et al., 2013;Liu et al., 2014).After decarbonization flotation, most heavy metals would be enriched into the residual slurry.Therefore, the treatment of the residual slurry by using the acid leaching-sulfide precipitation-flotation method should be considered to recover Pb and Zn from fly ash (Kuchar et al., 2007).

CONCLUSIONS
A high amount of injected PAC in HSWI fly ash made  the physical and chemical properties of fly ash extremely complex.We investigated the relationship between the particle size and chemical composition of fly ash.We observed that 89.3% of fly ash particles had a diameter of -75 µm because of the presence of a high amount of PAC.LOI in the fraction of -25 µm was higher than that in the fractions of other sizes.The particle size of fly ash was highly suitable for flotation treatment without the grinding pretreatment.Because PAC adsorbed a higher amount of low-chlorinated PCDD/Fs in the gaseous phase, the concentration of each PCDD/F congener, particularly of low-chlorinated PCDD/Fs, increased with a decrease in particle size.The TEQ value of the particle size fraction of +75 µm satisfied the landfill site standard (3 ng-TEQ g -1 ) but the particle size fraction of -75 µm did not.The fraction of -38 + 25 µm had the highest concentration of Pb, Zn, and Cd.The exchangeable speciation of Zn, Pb, and Cd was relatively high in the particle size fraction of +75 µm.The carbonate speciation of Pb and Zn was the highest in the particle size fraction of -38 + 25 µm.The leaching concentration of Pb and Cd in all particle size fractions surpassed the permitted limits of the landfill site of municipal solid waste in China.Additional studies should be conducted to better understand processes underlying the partitioning of heavy metals in the particle size fractions of fly ash.

Fig. 2 .
Fig. 2. Weight distribution and LOI of each size fraction in fly ash and PAC.

Fig. 7 .
Fig. 7. Speciation distributions of heavy metals in different particle size fractions.

Table 1 .
Chemical composition and concentration of heavy metals in the fly ash.

Table 2 .
Concentration of heavy metals in hospital solid waste incinerator ash of various countries of the world.

Table 3 .
Leaching concentration of heavy metals in different particle size fractions (mg L -1 ).