II) Wood pellets for home heating can be considered ...

Microchemical Journal 127 (2016) 178–183

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II) Wood pellets for home heating can be considered environmentally friendly fuels? Heavy metals determination by inductively coupled plasma-optical emission spectrometry (ICP-OES) in their ashes and the health risk assessment for the operators Santino Orecchio a,⁎, Diana Amorello a, Salvatore Barreca b,c a b c

Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF), Università degli Studi di Palermo, Viale delle Scienze, Parco D'Orleans II, 90128 Palermo, Italy ARPA Lombardia, 20124 Milano, Italy Istituto Euro Mediterraneo di Scienza e Tecnologia (IEMEST), Via Michele Miraglia, 20, 90139 Palermo, Italy

a r t i c l e

i n f o

Article history: Received 8 March 2016 Received in revised form 18 March 2016 Accepted 18 March 2016 Available online 19 March 2016 Keywords: Metals Wood Pellet Combustion Stove

a b s t r a c t The aim of the present study was to determine the concentrations of twelve potentially hazardous elements in wood pellet ashes obtained by the combustion of 13 pellet brands for sale in Italy, the impact of adding the ashes to soils and health risk of operator due to dust exposure. Samples were analysed by Inductively Coupled Plasma Optical Emission Spectrometry. The concentrations of heavy metals in ashes from stoves ranged from 0.41 to 7.2 mg kg−1 for As, from 1.3 to 12 mg kg−1 for Sb, from 1.8 to 12 mg kg−1 for Zn, from 0.23 to 0.8 mg kg−1 for Pb, from 0.18 to 2.8 mg kg−1 for Ni, from 0.09 to 1.0 mg kg−1 for Cd, from 0.46 to 3.4 mg kg−1 for Cr, from 0.94 to 2.7 mg kg−1 for V, from 2.2 to 11 mg kg−1 for Cu, from 60 to 409 mg kg−1 for Mn, from 83 to 432 mg kg−1 for Fe and from 3484 to 15,484 mg kg−1 for Al. The total concentrations for the 12 investigated elements, expressed as the sum of the concentrations (∑me), ranged from 3703 mg kg−1 to 15,946 mg kg−1 of dry weight with a mean of 8455 mg kg−1. Considering all the metals, the results indicate that there are very low risks for operators regarding non-carcinogenic and carcinogenic elements contained in the wood pellet ashes produced during cleaning of pellet stoves in confined environments. © 2016 Elsevier B.V. All rights reserved.

1. Introduction At present, there is an increasing interest in the use of biomass for energy production [1], in particular, wood and/or their residues, have been always one source of biomass fuels, cheap in price and abundant in quantity. However, in stoves that use wood or wood pellet as combustible, a problem is due to the high ash and salt production. In biomass, the elements that form the ashes are present as salts that are chemically bonded to the carbon structure (inherent ash), or they can come with biomass as mineral soil particles that have been caught during growth or are swiped during harvest and transport (foreign ash) [1]. The main ash forming elements are: Al, Ca, Fe, K, Mg, Mn, Na, P, S, Si and Cl [2]. For residential applications, about stoves and boilers, there is an increase in the use of wood pellet worldwide as fuel that is due to the fact that, in contrast to standard fossil fuels which produce greenhouse gas emissions, wood combustion is considered as sustainable CO2 neutral energy resource. Pellets are usually made from compressed sawdust

⁎ Corresponding author. E-mail address: [email protected] (S. Orecchio).

http://dx.doi.org/10.1016/j.microc.2016.03.008 0026-265X/© 2016 Elsevier B.V. All rights reserved.

or other waste materials like, working of lumber and manufacture of wood products. Moreover, by mechanical treatment, several raw biomasses can be transformed into a pellet form with improved fuel properties [3,4]. The improved density reduces the storage space and transportation costs while the homogeneous size helps the handling and feeding issues. The aim of indoor combustion of pellet in stoves and fireplaces is to transform combustible into heat, but, unfortunately, they have a higher content of minerals, including sodium, potassium, phosphorous and chloride, and in several cases, high hazardous element content [5–7]. Also, as reported in a previous paper [8], in not optimal combustion conditions, pellets may lead to the production of hazardous organic compounds such as PAHs that are in part emitted into the atmosphere, but, considerable amounts remain in the ashes. Several authors considered the emission from residential pellet wood combustion as a major contributor to ambient air pollution [5]. Indoor air of the environments in function wood stoves may contain health-damaging pollutants such as carbon monoxide, particulate matter, heavy metals [5] and polycyclic aromatic hydrocarbons [8,9]. The good maintenance of the stoves or fireplaces that burn pellets, requires the removal of the ashes daily, therefore we can assume that operators (generally the owners) are exposed and inevitably assume dusts and compounds released in indoor air from the ashes.

S. Orecchio et al. / Microchemical Journal 127 (2016) 178–183

Only few studies have been conducted on environmental risk assessment regarding the ashes produced by biomass combustion [8–10]. The biomass ashes may pose threat to the environment and humans due to the presence of hazardous substances [11]. The attention on metals, both from the analytical, environmental and toxicological viewpoint, lies with the fact that they are ubiquitous pollutants [12–15]. In the past few years, much work has been carried out to establish the concentrations and identify sources of these contaminants in common matrices and, in particular, in environmental and food ones [14]. Humans, and in particular operators, are exposed to heavy metals via many pathways because several metals are ubiquitous in the ashes of wood. The elements of the ashes can be assimilated in the human body via direct inhalation, ingestion and dermal contact absorption, and pose potentially adverse effects on the health of human beings [16]. In addition, for a sustainable use of wood pellet fuel, both domestically and on industrial scale, it is important to know that the ashes can be added to soils, replacing the extracted nutrients by plants, especially P and K and microelements [17]. An important condition for sustainable use of ashes in agriculture, however, is the evaluation of safety and possible environmental impacts. The ashes containing high concentrations of hazardous substances cannot be recycled or be used for agriculture purposes (as fertilizer and/or to neutralize soil acidification) but could be used in other fields, including, for example, as additives for the production of building materials [17]. The presence of heavy metal and their concentrations in wood pellet ashes are an issue in toxicological and environmental field because their concentrations are highly variable due to the difference in origin of raw materials, manufacturing processes, etc. Concentrations of some hazardous elements in wood ashes can be very high due to the enhanced enrichment of such elements in the combustion residue due to the high contents of organic matter in biomass. Unfortunately, there is little information regarding the potential risk for heavy metal exposure in these access routes, and their relationship with environmental factors. Whereas said before, this study aims at determining the metal content in the ashes produced from the combustion of several types of pellets in real conditions, representative of typical domestic users and to assess health risk of exposure to metals for operators. To obtain an estimate of the health risk associated with ash manipulation in indoor environment, investigations have been performed on twelve elements (Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb, V, Zn). In the ash samples, all the elements were quantified by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) [14,18]. This is the first study reporting heavy metal concentrations of ashes of wood pellet combustion in real conditions from stoves and estimation of health risk due to exposure to heavy metals from ashes. In summary, the main objectives of the current study were a) determining heavy metal concentrations of wood pellet ashes; b) determining metal uptake rates of operators via ingestion, inhalation and dermal Table 1 List of analysed samples. Sample

Country

Calorific kW/h

Ashes %

Moisture

Essence

1 2 3 4 5 6 7 8 9 10 11 12 13

– – – Austria USA – USA Italy Italy Italy Italy Canady Canady

5.7 5.3 5.2 4.8–5.3 4.8 5.2 N5.0 b5.0 4.6 5.4 N5.0 5.0 5.7

b0.4 b0.7 b0.6 b0.5 b0.5 b0.5 b0.5 b0.7 b0.6 0.9 b0.5 b0.4 b0.4

5 4 8 8 b7 b7 b7 7 b7 b10 b8 b6 4

– Fir Fir/beech Fir Conifers – Conifers Conifers Conifers + deciduous Beech Beech Fir Red fir

179

Table 2 ICP-OES operating conditions. RF power (W) Sample uptake flow rate (mL/min) Gas flow rates (L/min)

Viewing mode

1300 1.5 Auxiliary: 0.2 Nebulizer: 0.8 Argon: 15 Axial

contact to ashes; c) estimation of health risk posed by heavy metal exposure through wood pellet ashes; and d) valuing the impact if used in agricultural field. 2. Materials and methods 2.1. Pellet samples Metal analyses were performed on ash of 13 different samples of wood pellet listed in Table 1. For each pellet type, tree species, calorific value, and moisture (if known) are shown in Table 1. These specimens were purchased within a 50-mile radius of Palermo (Italy), from retail outlets including large supermarkets, shops garden articles and retailers of solid fuels. All purchased specimens had a label and none was purchased from occasional vendors. The selected wood pellet types were manufactured in different regions of the world and were representative of the various essences and between the most consumed in Italy. 2.2. Appliance, fuels and experimental procedures Analysis was carried out on the ashes produced with a top-feed pellet stove, representative of small household heating devices. The burner is a cast iron cup with holes for the introduction of the combustion air that is produced by an electric fan. The combustion is triggered by an electrical resistance. A heat exchanger, placed along the hot flue gases conduit, transfers the heat to a secondary air flow that heats the indoor environment. During tests, the stove was operated at about 4.0 kW h−1 in order to imitate a more realistic utilization, since the system is usually not operating at maxim power (11 kW h−1). All the collected ash samples were stored at 4 °C. Before analysis, the samples were dried at 105 °C in a hot air oven. The colour of the analysed ashes ranged from light grey to anthracite. 2.3. Laboratory equipment and chemicals Before use, all the vessels and flasks were cleaned by rinsing three times with HNO3 (3%) and several times with Milli-Q water. All the analyses of metals in wood pellet ashes samples were repeated three times and the relative standard deviation results ranged from 0.2 to 7%. Every five samples, blanks were carried out and the results demonstrate that the treatment used for cleaning vessels and flasks and chemicals (HNO3and Milli Q water) was suitable to obtain the quality Table 3 Wavelengths used for elemental determinations by ICP-OES. Element

Wavelength 1 (nm)

Wavelength 2 (nm)

Al As Cd Cr Cu Fe Mn Ni Pb Sb V Zn

396.153 193.696 228.802 267.716 327.393 238.204 257.610 231.604 220.353 206.836 292.464 206.200

308.215 188.979 214.440 205.560 324.752 239.562 259.372 221.648 217.000 217.582 310.230 213.857

180


Table 4 Metal concentrations (mg Kg−1 d.w.) in the wood pellet ashes determined by ICP-OES. Element

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

No. 7

No. 8

No. 9

No. 10

No. 11

No. 12

No. 13

Mean

As Sb Zn Pb Ni Cd Cr V Cu Mn Fe Al Total MPI

1.9 1.9 9.4 0.80 2.8 0.11 2.9 1.1 9.4 66 432 12,774 13,302 8.2

1.0 3.0 10 0.47 0.39 0.20 0.61 2.7 8.0 409 126 4318 4880 6.3

0.59 1.6 1.8 0.30 1.7 0.10 1.2 1.1 5.1 77 106 5173 5369 4.1

1.0 1.5 5.9 0.08 0.97 0.11 0.79 1.1 9.9 163 186 9141 9511 4.9

0.66 1.3 6.3 0.24 1.1 0.09 0.86 0.94 6.1 95 172 4701 4986 4.4

1.5 1.8 6.8 0.42 1.0 0.12 1.2 1.1 11 177 314 10,287 10,803 6.6

1.0 1.7 12 0.34 1.4 0.11 1.4 1.1 11 68 166 7513 7775 5.8

7.2 12 7.4 0.23 0.33 0.10 0.94 1.0 3.0 68 119 3484 3703 5.2

1.9 1.9 7.8 0.70 2.6 1.0 2.2 1.1 4.2 60 335 12,239 12,658 8.4

2.1 2.0 4.2 0.56 1.7 0.09 1.3 1.1 4.4 67 377 15,484 15,946 6.3

1.2 1.7 2.3 0.28 1.2 0.08 0.46 1.2 2.2 66 193 8881 9151 4.0

0.80 1.4 6.7 0.56 1.5 0.08 3.4 0.96 8.5 68 247 4199 4539 5.7

0.41 1.7 10 0.51 0.18 0.10 0.93 1.1 4.6 70 83 7118 7291 3.9

1.6 2.6 7.0 0.4 1.3 0.2 1.4 1.2 6.7 112 220 8101 8455 6.6

assurance required in this study. Moreover, every ten samples, calibration verification sample was analysed to quality assurance verification. The repeatability, calculated as the relative standard deviation (RSD %) of three independent measurements of a multi-elemental standard solution at 50 μg·L−1, ranged from 0.2 to 4.5%. The repeatability of the whole method, calculated as the relative standard deviation (RSD %), for three independent analysis of independent portions of the same sample, ranged from 0.5 to 6.0%. All reported data were blank corrected.

2 mL of HNO3 (69%) (Fluka, Milano) and 1 mL of H2O2 (30%) (Fluka, Milano) were added. The instrumental conditions used for the microwave digestion were: 1 min at 250 W, 1 min at 0 W, 5 min at 250 W, 5 min at 450 W, 3 min at 600 W and 5 min at 300 W. After digestion, the clear, colourless solution was brought to volume with Milli-Q water (R 20 M Ω cm−1) (Merck Millipore).

2.4. Water content analysis

Quantitative metal analyses were carried out on the solutions obtained from the mineralization of the ash samples using an ICP Optical Emission Spectrometer series Perkin Elmer Optima 2100 equipped with a Perkin S10 model autosampler. Data acquisition and processing were performed using the Win Lab 32 software (Perkin Elmer). Operating conditions are listed in Table 2. Twelve elements (Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Sb, V and Zn) were determined in each sample, chosen on the basis of their significance in toxicological and environmental fields. The ICP-OES analysis of trace elements was performed in axial viewing mode. For each element, the quantitative analysis was performed at two different spectral lines (Table 3).

The water content was determined by weight loss and was utilized to correlate all the results with dry weight. About 5 g of an aliquot of ashes was completely dried at T of 105 °C for one night. 2.5. Sample preparation A microwave oven (Milestone model MLS-1200 Mega, Milestone Laboratory Systems, Italy) with rotor of high pressure (up to 100 bar) was used for sample mineralization. About 2 g of previously homogenized samples were weighted, transferred inside Teflon vessels and

2.6. ICP-OES analysis

Fig. 1. Concentration (mg/kg d.w.) of minor elements in wood pellet ash samples.


2.7. Calibration curves The detection limits (LOD) and the quantification limits (LOQ) were estimated as reported in previous papers [8,18] and ranged from 30 to 100 μg kg−1 and from 100 to 330 μg kg−1 respectively. Calibration standard solutions were prepared by dilution with HNO3 2% of a multielement calibration standard solution (Ultra Scientific, USA, catalogue number: IQC-026) that contained 26 elements. The range of concentration of the calibration curves was between 10 and 50 μg/L. For calibration, a solution of HNO3 2% as blank was used. The analysis of the five standard solutions was replicated in every 6–7 samples. Correlation coefficients of the calibration curves were in the range of 0.9992–0.9999. To eliminate memory effects related to the previous sample analysis, between two subsequent samples, a 25 s washing was settled. 3. Results and discussion The results reported in this paper have been calculated considering the average of the concentrations obtained at the two wavelengths that, for all analytes, differed by less than 5%. A total of 13 ash samples were analysed by about 500 determinations of single elements. The summary of metals concentrations in the wood pellet ash samples are reported in Table 4. The total concentrations (averages of the three analyses) for the 12 investigated elements, expressed as the sum of the concentrations (∑Me), in the ashes produced by the wood pellet combustion, ranged from 3703 to 15,946 mg kg−1 of dry weight (Table 4, Fig. 1) with a means of 8455 mg kg−1. The lowest concentration (3703 mg kg− 1) was measured in a conifer ash sample (no. 8) while the highest ones in a mixture of fir and beech wood pellet (no. 10). In relative term, Cd is the biggest contributor to total heavy metal content. Considering the mean value of all the samples, although the absolute heavy metal concentrations in the pellet ashes decrease in the order of Al N Fe N Mn N Zn N Cu N Sb N Cr N V N Pb N Cd. The concentrations of metals in the present work were lower than the literature values [19]. The European legislation on ash use in forestry and agriculture varies between the countries [20]. In Table 5 are shown the maximum concentrations of minor elements in different countries. In all analysed samples, the metal concentrations in the wood pellet ash samples were much lower than the limit values of the European countries. 3.1. Metal pollution index (MPI) To examine the overall heavy metal concentrations in all analysed pellet ash samples, metal pollution index (MPI) was calculated. This index was obtained by calculating the geometrical mean of concentrations of all the metals in the samples [14]. MPI ¼ C1 C2 C3 …Cn 1=n : where Cn = concentration of single metal in a sample. Among different analysed ashes, two sample no. 9 (conifers + deciduous) and no. 1 showed the highest values of MPI: 8.4 and 8.2 respectively, while the lowest ones were measured for ashes obtained from two well identified essence, a red fir and a beech respectively (samples no. 13 and no. 11). 3.2. The enrichment factor (EF) Generally, in environmental issue, the enrichment factor is used to discriminate the elements originating from human activities and those from natural sources, to assess the degree of anthropogenic influence [21,22]. In this paper, we used EF to value the impact of an possible addition of wood pellet ashes to soils. Element enrichment factor, evaluated relative to earth's crust values, were used to establish which elements were relatively enriched in the different wood pellet ash samples. In general, values of EF close to 1 pointed to a natural origin while those greater than 10 are considered to have a non-crustal source [14].

181

Table 5 European limit values (mg/kg) on ash utilization in forestry and agriculture for minor elements. Element

Denmark

Finland

Sweden

As Cd Cr Cu Ni Pb V Zn

– 15 100

25 1.5 300 600 100 100 – 1500

30 30 100 400 70 300 70 7000

60 120 – –

Five contamination categories are recognized on the basis of the enrichment factor (Table 6). The enrichment factor of an element in the studied sample with respect to its natural abundance in the earth's crust was calculated according to the following equation: EF ¼ ðCn =Erif Þashes =ðCn =Erif Þcrust

where Cn is the concentration of the element in the sample or in the earth's crust, Eref the concentration of reference element for normalization. In this study, Fe was adopted as reference because it is one of the largest components of soil [23]. The elemental concentrations in the crust used in this study were the background values in soil [24]. Generally, the matrices can be classified as deficiency to minimal enrichment if EF is b 2, moderate enrichment (2 ≤ EF b 5), significant enrichment (5 ≤ EF b 20), very high enrichment (20 ≤ EF b 40) or extremely high enrichment (EF ≥ 40). EFs much higher than 10 are considered to originate mainly from anthropogenic sources (Table 7). In our case, the enrichment factors ranged from 1.1 to 31,750 and, considering the mean values, decrease in the order of V b Cr b Ni N Pb b Zn b Cu b Al b As b Cd b Sb. In detail, the sample no. 8 showed the highest values of EF for antimony and arsenic (31,750 and 2100 respectively), followed by samples no. 13 for the Sb and Cd (6506 and 452 respectively) and no. 2 for Sb and Cd (7422, and 621 respectively). Considering the mean values, the EFs of all elements, excluding Ni, Cr and V, were all higher than 10, and only EF for Pb was close to 10. Therefore, the metal contents of the wood pellet ashes relative to these references indicate that this combustion residue is seriously contaminant for soil and other environmental matrices. 3.3. Estimation of daily intake of hazardous elements from wood pellet ashes and risk assessment The model used in this paper to estimate the exposure of operators to elements of wood pellet ashes is based on those proposed by the US Environmental Protection Agency [25–27]. The pathways of exposure to hazardous substances associated with indoor ashes are: chemical daily intake by direct ingestion of ash particles (CDIing); chemical daily intake through mouth and nose inhalation of suspended particles (CDIinh); chemical daily intake via absorption from skin adhered ashes particles (CDIdermal). We use the following formulas to calculate dose received through each of above listed exposure Table 6 Contamination categories based on EF values. EF b 2 EF 2–5 EF 5–20 EF 20–40 EF N 40

Deficiency to minimal enrichment Moderate enrichment Significant enrichment Very high enrichment Extremely high enrichment

182


Table 7 Enrichment factors (EFs).

As Sb Zn Pb Ni Cd Cr V Cu Mn Al

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

No. 7

No. 8

No. 9

No. 10

No. 11

No. 12

No. 13

Mean

151 1358 18 9 4.0 98 3.4 1.1 20 9 22

285 7422 67 18 1.9 621 2.5 9.7 58 189 26

193 4623 14 14 10 361 6.0 4.7 44 43 36

192 2467 26 2 3.3 226 2.2 2.7 48 51 37

132 2378 30 7 4.0 212 2.5 2.5 33 32 20

163 1752 18 6 2.0 147 1.9 1.6 31 33 24

216 3216 58 10 5.2 264 4.4 3.0 58 24 34

2100 31,750 51 9 1.7 310 4.0 4.1 23 33 22

193 1741 19 10 4.8 1203 3.3 1.5 11 11 27

188 1665 9 7 2.8 91 1.8 1.3 11 10 31

208 2677 10 7 4.1 166 1.2 2.7 11 20 34

112 1768 22 11 3.9 131 7.0 1.8 31 16 13

170 6506 103 29 1.3 452 5.7 5.8 50 49 64

257 3655 26 9 3.7 317 3.2 2.5 28 30 27.5

pathways [25–27]. CDIing

¼ CUCL Ring F exp T exp 10−6 = ABW Tavrg

CDIinh ¼ CUCL Rinh F exp T exp = PEF ABW Tavrg

ð1Þ ð2Þ

CDIder ¼ CUCL SAF Askin DAF F exp T exp 10−6 = ABW Tavrg ð3Þ where CDI is the chemical daily intake (mg kg−1 day−1); Ring is the ingestion rate at 30 mg day− 1 [28]; Rinh is the inhalation rate at 20 m3 day−1 [28,29]; Fexp is exposure frequency, in this study, 2.5 day year−1, because of the lack of studies involving wood pellet ashes in indoor environments, there are no guidelines for exposure frequency, we assumed that every operator cleans the stove once a day, so that we used 20 min day for 180 day during a year as the average exposure frequency; Texp is the exposure duration, in this study, 24 years [30], Askin is the skin area, in this study, 5700 cm2 [30], SAF is the skin adherence factor, in this study, 0.1044 mg cm− 2 d−1 [29,31], DAF is the dermal absorption factor (unitless), in this study, 0.001 [29], PEF is the particle emission factor, in this study, 1.36 · 109 m3 kg−1 [29], ABW is the average body weight 70 kg for operators [28,29,31], Tavrg is the averaging time; for non-carcinogens Tavrg = Texp · 365 days; for carcinogens, Tavrg = 70 · 365 = 25,550 days [29]. CUCL (exposure-point upper confident limit content, mg kg−1) is an evaluation of the reasonable maximum exposure [25,26,28,31], which is the upper limit of the 95% confidence interval for the mean [25]. Since the concentration of most metals in the wood pellet ash samples has not a normal distribution, the 95% upper confidence limit (UCL) was calculated by employing an approach called adjusted Central Limit Theorem (CLT) [25,32]. When sample size is moderate or small, the means will not generally be normally distributed yet the non-normality is intensified by the skewness of the distribution. Therefore, it is suggested that it can be employed for non-normal distributed moderate or small size data [25]. The CUCL has been calculated as follows: h −1 i −1 STD √ n CUCL ¼ X þ Zα þ β 1 þ 2 Zα 2 6√ n

ð4Þ

X = mean concentration in mg kg−1; STD = standard deviation; β =

skewness; α = the probability of making Type I error (false positive); Zα = (1 − α)th quantile of the standard normal distribution. For the 95% confidence level Zα = 1.645; n = number of samples. The potential non-carcinogenic and carcinogenic risks for each metal were calculated using the following equations [28,33,34]: Hazard Quotient ðHQ Þ ¼ ðCDI BAFÞ=RfDo

ð5Þ

Carcinogenic Risk ¼ CDI BAF SLF:

ð6Þ

where BAF is the ratio of metal content that is bioavailable to the total contents in the wood pellet ashes. Considering that no study on the bioavailable fractions of the metals in indoor ashes samples is available, we use similar literature values (Table 8) [28,35]. Reference doses (RfDo) and Slope factors (SLF) values are from Regional Screening Levels [36] and are listed in Table 8. Cr toxicity depends on its valence states and the SLF and RfDo of Cr(VI) are assumed as for total Cr. US EPA [36] has not established an RfDo value for Pb, so RfDo used in this study was 3.5 × 10−3 mg kg− 1 day−1 calculated from the provisional tolerable weekly Pb intake limit, 25 μg kg−1 body weight−1, recommended by the FAO/WHO for adults [28]. Hazard Index (HI) is the sum of calculated hazard quotients (HQ). Values greater than 1 indicate that there is a chance that noncarcinogenic effects may occur, whereas HI values lower than 1 show no significant risk of non-carcinogenic effects. Therefore, greater the HI value higher the probability of non-carcinogenic effects [30]. The estimated value of carcinogenic risk indicates the probability of an individual developing any kind of cancer from lifetime exposure to carcinogenic hazards and acceptable or tolerable risk for regulatory purposes is given in the range of 1 × 10−6 to 1 × 10−4 [30]. In the current study, HI and carcinogenic risk were calculated to assess operator health risk of metal exposure to indoor wood pellet ashes. HQ, HI and carcinogenic risk values are shown in Table 8. Among the carcinogenic metals, only As and Crtotal were considered. Regarding no carcinogenic risks, the HI values of the single metals considered in this study are from 5 to 9 orders of magnitude lower than 1 (Table 8). The ingestion pathway posed the highest risk, followed by dermal contact. Exposure through the three pathways resulted in health risks in the following order: As

Table 8 Evaluation parameters and health risk of trace elements in the studied wood pellet ashes. Element

CUCL (mg kg−1)

BAF %

RfDo

CDIing

CDIinh

CDIderm

HQ ing

HQ inh

HQ der

HI

HI canc

As Sb Zn Pb Ni Cd Cr V Cu

2.9 4.8 8.3 0.5 1.7 0.4 1.9 1.5 8.0

38.8 20 60 47 15.7 74.5 5.8 11.2 29.8

0.003 0.0005 0.3 0.0035 0.02 0.001 0.003 0.005 0.04

8.5E−09 1.4E−08 2.4E−08 1.5E−09 5.0E−09 1.1E−09 5.6E−09 4.5E−09 2.4E−08

3.3E−12 5.4E−12 9.5E−12 5.9E−13 1.9E−12 4.3E−13 2.2E−12 1.7E−12 9.2E−12

1.7E−10 2.8E−10 4.9E−10 3.0E−11 9.9E−11 2.2E−11 1.1E−10 8.9E−11 4.7E−10

1.1E−05 5.6E−06 4.9E−08 2.0E−07 3.9E−08 8.2E−07 1.1E−08 1.0E−07 1.8E−07

4.3E−09 2.2E−09 1.9E−11 8.0E−11 1.5E−11 3.2E−10 4.2E−12 3.9E−11 6.8E−11

2.2E−07 1.1E−07 9.7E−10 4.1E−09 7.7E−10 1.6E−08 2.2E−10 2.0E−09 3.5E−09

1.1E−05 5.7E−06 5.0E−08 2.1E−07 4.0E−08 8.3E−07 1.1E−08 1.0E−07 1.8E−07

1.8E−09

5.9E−12


N ≈ Mn N Sb N Cd N Pb N Cu N V N Zn N Ni N Cr N Fe. Considering all the metals, the total HI is 2.9 · 10−5 indicating that there are very low noncarcinogenic risk for the heavy metals in wood pellet ashes produced during cleaning of pellet stoves in confined environments. For the cancer risks, the HQ values of As are 1.7 · 10−9, 2.7 · 10−14, 7.8 · 10−11 for ingestion, inhalation and dermal exposure, respectively, and of Cr are 5.6 · 10−12, 9.1 · 10−17, 3.2 · 10−13 for ingestion, inhalation and dermal exposure, respectively. These data show that the cancer risk levels of As and Cr are very lower than the EPA's acceptable range (10−6–10−4), indicating that the carcinogenic risks of As and Cr (VI) for indoor wood pellet ashes are negligible. Therefore, based on the results of the non-carcinogenic and carcinogenic risks, regarding the metals investigated in the present study, the health risks from the wood pellet ashes for the operators in indoor environments are not significant. 4. Conclusions Like other hazardous compounds [8], the heavy metals in wood pellet ashes have got immense environmental issues. The results reported in this paper represent the first quantitative investigations of metals in pellet ashes from thirteen different types fired in domestic stoves. Twelve heavy metals, including Sb, Zn, Pb, Ni, Cd, V, Cu, Mn, Fe and Al, as well as As and Cr, which can have serious health risks, were investigated. Their concentrations are also highly variable due to the difference in origin and composition of wood species. Concentrations of metals found in this paper will also serve as a baseline for the future monitoring campaigns and/or environmental. In general, the addition of the ashes in soil (as recommended by ecologists and manufacturers of pellet stoves) can also help to improve its physical, chemical and biological properties to the benefit of agricultural production. It is known that the ashes can supply most of the essential macro- and micro-elements required for plant growth. Therefore, considering the results of this study, before any adding of pellet ashes to soils, it is need to know the ashes and soil composition and how different crops can respond to the soil environment created by added ash. In particular, it is to avoid growing plants destined for human and animal those can bioaccumulate several of studied elements. Considering all the metals, the results indicate that there are very low risks for operators regarding non-carcinogenic and carcinogenic elements contained in the wood pellet ashes produced during cleaning of pellet stoves in confined environments. Acknowledgments This work was performed thanks to a financial source from the University of Palermo (ex 60% 2007). The data and information reported in this publication do not depend on the activities of ARPA Lombardia, which therefore cannot in any event be held responsible for the contents of the publication, and the conclusions by the authors. References [1] L.J.R. Nunes, J.C.O. Matias, J.P.S. Catalão, Mixed biomass pellets for thermal energy production: a review of combustion models, Appl. Energy 127 (2014) 135–140. [2] J. Werkelin, B.-J. Skrifvars, M. Zevenhoven, B. Holmbom, M. Hupa, Chemical forms of ash-forming elements in woody biomass fuels, Fuel 89 (2010) 481–493. [3] G. Toscano, D. Duca, A. Amato, A. Pizzi, Emission from realistic utilization of wood pellet stove, Energy 68 (2014) 644–650. [4] G. Toscano, G. Riva, E. Foppa Pedretti, F. Corinaldesi, C. Mengarelli, D. Duca, Investigation on wood pellet quality and relationship between ash content and the most important chemical elements, Biomass Bioenergy 56 (2013) 317–322. [5] A. Williams, J.M. Jones, L. Ma, M. Pourkashanian, Pollutants from the combustion of solid biomass fuels, Prog Energy Combust 38 (2012) 113–137. [6] R.E. Masto, E. Sarkar, J. George, K. Jyoti, P. Dutta, L.C. Ram, PAHs and potentially toxic elements in the fly ash and bed ash of biomass fired power plants, Fuel Process. Technol. 132 (2015) 139–152.

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