Heavy metals content in ashes of wood pellets and the health risk assessment related to their presence in the environment

Collection of the samples

Ten (10) wood pellet samples were purchased from a different location in B&H, of known suppliers from the market (supermarkets, garden shops, and gas stations). The samples were accompanied by a declaration describing that nine of them were originated from B&H, and one of them was from Italy. Characteristics of collected wood pellet samples (type of wood, energetic value, declared moisture, declared and determined ash amount) are listed in Table 1. All of the samples were analyzed for moisture and ash content. Additionally, in ash samples of mentioned wood pellets, heavy metal concentration (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) was determined.

All pellet samples were originated from B&H, purchased from different cities, often used for house heating, instead of sample S3 which was from Italy.

Ash determination of wood biomass samples

The wood pellet samples were oven-dried at 105 °C for 24 h. The content of ash was determined by gravimetric method according to the procedure published by Pan and Eberhardt¹⁸ as follows: pellet samples, 1 g (± 0.1 mg) of each was weighed into a previously annealed ceramic pot (m₁) and burned in a muffle furnace (Nabertherm) for one hour at 300 °C, following by increasing the temperature to 400 °C for one hour more and then burning the samples for next six hours at 550 °C. The procedure is repeated until a constant mass (m₂) was reached. The ash content is determined by the Eq. (1):

Preparation of samples

The chemical determinations of the heavy metals in wood pellet ashes (Table 2) were made by wet digestion by soaking the samples in 25 mL of 65% HNO₃ in polytetrafluoroethylene (PTFE) vessels. After evaporation of the nitrogen oxides, the vessels were closed and allowed to react for 14 h at 80 °C, following by cooling to room temperature. Then, the digest was filtered, transferred to a 25 mL volumetric flask, and filled up with redistilled water to the mark. All samples and blank were prepared in three replicates^19,20,21.

Heavy metal analysis

Metal analyses in ash samples of mentioned wood pellets were performed using a flame atomic absorption spectrometry (Varian AA240FS) for Mn, Fe, Pb, and Zn and graphite furnace (Varian AA240Z) for Cd, Co, Cr, Cu, and Ni. A blank probe was prepared using the same digestion method to avoid the matrix effect. Standard metal solutions used for the calibration graphs were prepared by diluting 1000 mg L⁻¹ stock single-element atomic absorption standard solutions of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn (Certipur Grade, Merck, Germany). Linear calibration graphs with correlation coefficients > 0.99 were obtained for all analyzed metals. The accuracy of the method was evaluated using the standard reference materials: Fine Fly Ash (CTA-FFA-1, Institute of Nuclear Chemistry and Technology Poland) and Fly Ash from pulverized coal (BCR-038, Institute of reference materials and measurements-IRMM, Belgium). The obtained results were in the range of the reference materials. The detection limit (LOD) and limit of quantification (LOQ) for the nine analyzed metals were calculated based on X_b + 3 SD_b and X_b + 10 SD_b, respectively, where X_b is the mean concentration of the blank sample (n = 8) and SD_b is the standard deviation of the blank for eight readings²². The values of the LOD were: Cd (0.61 µg L⁻¹), Co (0.49 µg L⁻¹), Cr (0.67 µg L⁻¹), Cu (20.10 µg L⁻¹), Fe (83.85 µg L⁻¹), Mn (6.42 µg L⁻¹), Ni (1.12 µg L⁻¹), Pb (23.77 µg L⁻¹), Zn (58.68 µg L⁻¹), and LOQ values were: Cd (1.25 µg L⁻¹), Co (1.41 µg L⁻¹), Cr (1.42 µg L⁻¹), Cu (47.66 µg L⁻¹), Fe (111.2 µg L⁻¹), Mn (16.14 µg L⁻¹), Ni (2.70 µg L⁻¹), Pb (47.73 µg L⁻¹) and Zn (71.05 µg L⁻¹).

Pollution evaluation

The metal pollution index (MPI) as the geometric mean of the concentration of all metals found in ashes of wood samples was calculated by the following Eq. (2)²³:

where C₁ is the concentration value of the first metal, C₂ is the concentration value of the second metal, C_k is the concentration value of the kth metal.

Evaluation of the presence and the grade of anthropogenic activity were demonstrated through the calculation of the enrichment factor (EF), widely used in environmental issues²⁴. To understand which elements were relatively enriched in the different wood pellet ash samples, the heavy metal enrichment factor was calculated relative to soil values according to Eq. (3)²⁵.

where C_k is the concentration of the element in the sample or the soil, E_ref the concentration of the reference element used for normalization. A reference element is an element commonly stable in the soil characterized by the absence of vertical mobility and/or degradation phenomena. As in many studies as a reference element were Fe, Al, Mn, Sc, or total organic carbon used^26,27. Therefore Fe has been chosen as reference material in this study. Iron is one of the major constituents of soil, as well as the average chemical constituent of the upper continental crust²⁶.

Health risk assessment

The general exposure equations used in this study were adapted according to the US Environmental Protection Agency guidance^28,29,30. The daily exposure (D) to heavy metals via wood pellet ash was calculated for the three main routes of exposure: (i) direct ingestion of ash particles (D_ing); (ii) inhalation of suspended particles via mouth and nose (D_inh); and (iii) dermal absorption to skin adhered ash particles (D_dermal). Equations (4) to (6) were used to calculate exposure via ingestion, inhalation, and dermal route, respectively^22,31.

where c (mg kg⁻¹) is the heavy metals concentrations in ash samples; IngR (mg day⁻¹) is the conservative estimates of dust ingestion rates, 50 for adults, 200 for children^30,32; InhR (m³ h⁻¹) is the inhalation rate, 2.15 for adults, 1.68 for children³²; EF (h year⁻¹) is the exposure frequency, 1225 for adults and children²²; ED (years) is the exposure duration, 70 for adults, 6 for children²²; BW (kg) is the body weight, 80 for adults, 18.60 for children³²; AT (days) is the averaging time, 25,550 for adults, 2190 for children²²; PEF is the particle emission factor (m³ kg⁻¹), 6.80 × 10⁸ for adults and children³¹; SA (cm³) is the exposed skin area, 6840 for adults, 2550 for children³²; SL (mg cm⁻²) is the skin adherence factor, 0.22 for adults, 0.27 for children³²; ABS is the dermal absorption factor, 0.001 for adults and children³¹; CF1 is the unit conversation factor, 10^–6 for adults and children²².

The potential non-carcinogenic risk for each metal was estimated using the Hazard coefficient (HQ), as suggested by US EPA³³. The HQ under various routes of exposure such as ingestion (HQ_ing), inhalation (HQ_inh), and dermal (HQ_dermal) was calculated as a ratio of daily exposure (D) to reference dose of each metal (RfD) according to Eq. (7)³².

where k is ingestion, inhalation, or dermal route. The total hazard index (HI) of heavy metal for all routes of exposure was calculated as a sum of HQ_ing, HQ_inh, and HQ_dermal as given in Eq. (8)³⁴.

The carcinogenic risk (Risk) for potential carcinogenic metals was calculated by multiplying the doses by the corresponding slope factor (SF), as given in Eq. (9)³⁵. The carcinogenic oral, inhalation, and dermal SF, as well as dermal absorption toxicity values, were provided from the Integrated Risk Information System³⁰. The reference doses for Pb were taken from the Guidelines for Drinking Water Quality published by the World Health Organization³⁶.

where SF is the cancer slope factor for individually metal and k route of exposure (ingestion, inhalation, or dermal path). The total cancer risk (Risk_total) of potential carcinogens was calculated as the sum of the individual risk values using the following Eq. (10).

Source: Ecology - nature.com