Residual and ecological risk assessment of heavy metals in fly ash from co-combustion of excess sludge and coal

Co-combustion of municipal excess sludge (ES) and coal provides an alternative method for disposing ES. The present study aims to investigate the residual and ecological risk of heavy metals in fly ash from co-combustion of ES and coal. The total concentration and speciation distribution of heavy metals, characterization of SEM, EDX, XRD and leaching test were carried out to assess the fly ash in this study. The results showed that the total concentrations of Cu, Zn and Mn were higher than others in fly ash, and most heavy metals were concentrated in fine particles. For Cd, Cr and Pb, the percentages of speciation of F4 and F5 were all over 90%, suggesting the relatively lower leaching toxicity. The leaching percent of all heavy metals was lower than 5% by two diluted HNO3 solutions for fly ash. The potential ecological risks increased with the decrease of particle size of fly ash, and Cd accounted for the main fraction for ecological risk despite of lower concentration in comparison to other measured heavy metals.

www.nature.com/scientificreports/ and its potential ecological risk for subsequent final disposal. Moreover, it is necessary to exploit more reasonable technologies suitable for Chinese national conditions. In this study, the residual of heavy metals in fly ash with different particle size were examined through the determination of species of heavy metals, the characterization of SEM, EDX, XRD and anions content of fly ash were also investigated, and the potential ecological risk of residual heavy metals in fine particles was also assessed. The results would be helpful for the application and ecological risk assessment of excess sludge incineration in China.

Materials and methods
Sample preparation. In this study, excess sludge was collected each week from secondary sedimentation tank of a local municipal wastewater treatment plant (Guangzhou, China), the plant receives about 70% of domestic sewage and 30% of industrial wastewater at an average flow of approximately 250,000 m 3 /day. The produced excess sludge from the plant was disposed through co-combustion with coal in a waste incineration power plant. The predried sludge sample was prepared at 105 ℃ in an air dry oven for 24 h in triplicate until the mass no longer changed. The sludge-coal mixtures were prepared in a sludge-coal ratio of 1:3. The fly ash used in this study was sampled from the bottom of precipitator ash, respectively.
Analytical methods. The proximate analysis and ultimate analysis of ES and PC were determined by TGA2000 according to GB/T212-2008 and Vario EL cube according to ASTM D5373-08 and GB/T214-2007, respectively. The calorific values were measured by HKRL-4000B according to GB/T213-2008. The concentrations of Cl − and SO 4 2− were measured using Ion chromatography (Dionex Aquion, USA), the concentrations of HCO 3 − and CO 3 2− were measured with double-indicator neutralization titration method. The morphology characteristics of the fly ash samples were measured using a scanning electron microscopy (SEM, ZEISS, Sigma 300) at the typical accelerating voltage of 5 kV. The content of each element in fly ash was determined with a Schottky Field Emission SEM-Energy Dispersive Spectrometer (EDX, Hitachi, SU-70). X-ray diffraction (XRD) was carried out with a PANalytical XPERT-3 Powder diffractometer with copper Kα radiation operating at 40 kV and 40 mA in the 3°-80° scan range of 2θ°.
Speciation of heavy metals. In order to study the forms of Cd, Cr, Cu, Mn, Ni, Pb and Zn in bituminous coal and ES samples, the modified (five steps) Tessier extraction procedure was used 17 . Details of the procedure are given in Table 1. The extraction was carried out in glass centrifuge tubes of 50 mL capacity with an initial mass of 1 g oven dried (105 ℃) fine fraction (< 1 mm) of the samples. Supernatants were measured using atomic absorption spectroscopy. Analyses were performed in triplicate.
Ecological risk assessment method. The ecological risk index (RI) has been used to evaluate the degree of heavy metal contamination based on Eqs. (1) and (2) 18,19 .
where E i represents the monomial potential ecological risk factor for each heavy metal; T i represents the toxic factor of each heavy metal; C i and C 0 represent the measured content and background reference value for each heavy metal, respectively. As shown in Table 2, RI stands for the potential ecological risk index for each of fly ash sample with different particle size. The T i values used for calculation of RI for individual metal are Cu (5), Pb (5), Ni (5), Mn (1), Cr (2), Zn (1) and Cd (30) 18,20 . In order to accurately assess the potential ecological risk of each heavy metal, the bioavailable fractions (F1 + F2 + F3) of each heavy metal were used to calculate E i in this study.

Results and discussion
Proximate analyses and calorific values of materials. The technological properties (volatile matter, ash and fixed carbon) of both dried sludge and bituminous coal diverge widely, which may suggest some major differences between the corresponding combustion properties 21 . The proximate analyses of dried sludge and coal are shown in Table 3.
The ash yield of bituminous coal is similar to that of dried ES sample. And the ES sample yields higher amount of volatiles (38.15 wt%, ad), while the bituminous coal sample yields only 21.91 wt%, ad. Apparently, dried ES  Table 4 shows the contents of heavy metals of predried excess sludge and bituminous coal. The results indicated that the contents of heavy metals in the predried sludge often outweigh those in bituminous coal, and the contents of heavy metals in the predried sludge often outweigh those in bituminous coal, and the Mn, Zn and Cu contents in the predried sludge were relatively higher than others, which was normally because of the influent of industrial wastewater. The contents of heavy metals in bituminous coal were all lower than those in sludge.
Particle size distribution in fly ash. Figure 1 shows the distribution of particle sizes in fly ash. The results showed that the highest fraction of particle sizes within the range of 74-174 μm accounted for 42% of total particles in fly ash, the second fraction of particle sizes within the range of 54-74 μm accounted for 34% of total particles, and the fractions of other particle sizes were all lower than 10%.
Total content and speciation of heavy metals in fly ash. Figure 2 shows the distribution of seven heavy metals in fly ash with different particle sizes and their contribution to total content of heavy metals in fly ash. The higher contents of Cu, Zn and Mn were determined in fly ash, and mostly concentrated in fine particles    www.nature.com/scientificreports/ with particle size lower than 54 μm. The results showed that the Cd content in fine particle was higher than those in coarse particles and middle particles, which can be attributed to low boiling point of Cd (765 ℃), resulting in the volatility during incineration. The formation mechanisms for fine particles and coarse particles were homomorphic condensation nucleation and heterophase condensation, respectively 22 . The distribution of Cu, Mn, Zn and Pb was also found similar to that of Cd, and the concentration was negatively correlated to particle size. Mn had a high boiling point (2097 ℃) and was stable during incineration. The highest Mn content was about 1700 mg/kg in the fine particles of lower than 54 μm. As for lithophilic heavy metals of Ni and Cr, there was little correlation between the contents of Ni and Cr and particle size, which can be attributed to higher boiling point of Ni (2837 ℃) and Cr (2672 ℃). However, due to the dramatic turbulence in the incinerator, part of heavy metals was enwrapped in the suspended particles in flue gas and then was captured in flue gas cleaning system, meanwhile, fine ash had little effect on condensation adsorption of Ni and Cr due to their low volatility, resulting in the uneven distribution of Ni and Cr in fly ash. The ecotoxicity of heavy metals can be attributed to both concentrations and bioavailability 23 . Based on the modified Tessier extraction method, there are five speciations of heavy metals in fly ash. Heavy metals in exchangeable from (F1), carbonate bound form (F2) and Fe/Mn oxides bound form (F3) are usually considered to be mobile and bioavailable, however, heavy metals in organic form (F4) and residual form (F5) are generally stable and non-bioavailable 24 . As shown in Fig. 3, for Cd speciation distribution, the percentage of mobile and bioavailable forms mainly in F2 and F3 decreased with the increase of particle size in fly ash, and for the fly ash of particle size over 280 μm the percentage of relatively stable and non-bioavailable forms in F4 and F5 was over 92%, which suggested that Cd might show lower leaching toxicity.
As for Cu speciation distribution, there were only four speciation forms (F2, F3, F4 and F5) in fly ash. The percentage of mobile and bioavailable forms (F2 and F3) was about 18% for the particle size lower than 74 μm, and the percentage of mobile and bioavailable forms decreased to lower than 12% for the particle size over than 74 μm. The high stability of Cu-organic matter complexes was also found 24 . For Pb speciation distribution, the particle size lower than 54 μm showed the highest bioavailability of 38% in the forms of F2 and F3, and the percentage of F2 and F3 decreased to 22% for the particle size within 54-74 μm, however, the non-bioavailable forms in F4 and F5 increased to over 97% for the particle size over than 74 μm.
For Cr speciation distribution, there existed no obvious difference among all particle sizes of fly ash, the percentage of non-bioavailable form F5 was all over 88%. It was reported that the main Cr speciation in sludge were the organic and residual fractions 25 . For Ni speciation distribution, the percentage of stable and nonbioavailable forms of F4 and F5 accounted for over 82% for different particle sizes, which indicated that Cr and Ni had low leaching toxicity.
While for Mn speciation distribution, the percentage of mobile and bioavailable forms (F2 and F3) was in the range of 48-52% for the particle size lower than 74 μm, and the percentage of F2 and F3 decreased to lower than 12% for the particle size in the range of 174-900 μm. For Zn speciation distribution, the particle size lower than 174 μm showed relatively higher mobility, especially for the particle size in the range of 74-174 μm the percentage of speciation F2 and F3 reached up to 56%, however, those percentages of speciation F2 and F3 were all lower than 24% for particle size over than 174 μm. Figure 4 shows the SEM micrographs of fly ash samples with different particle sizes. It can be observed that fine particles had more porosities for Fig. 4a (< 54 μm) and Fig. 4b (54-74 μm) of fly ash samples, which mainly consisted of homogeneously dispersed irregular particles in structure. While with the increase of particle size, there existed bulky grain aggregates on the surface of fly ash samples, especially for the particles over than 280 μm, nearly no dispersed fine particles were found on the outer surface of fly ash samples. Table 5 shows the EDX results of fly ash samples with different particle sizes. And the XRD spectrums of fly ash samples are shown in Fig. 5. High contents of oxygen (O) and ferrous (Fe) were found in fly ash based on EDX analysis, which indicated the possible presence of ferric oxide. On the other hand, according to the XRD results, the presence of ferric oxide was confirmed for the particles of within 54-900 μm (Fig. 5). In addition, there also existed carbon (C), silicon (Si), calcium (Ca), alumium (Al) and sulfur (S) in fly ash, and three crystalline compounds, including silicon dioxide (SiO 2 ), calcium sulfate (CaSO 4 ) and calcium carbonate (CaCO 3 ), were also found in the XRD spectrums of fly ash (Fig. 5).

Characterization of SEM, EDX, XRD and anions content of fly ash.
Little chlorine content was measured by the EDX results, which was close to the results from the determination of anions content (Fig. 6). Based on the XRD spectrums, the crystalline compounds of LiTiO 3 , ZnFe 2 O 4 and CuMn 2 O 4 were also determined, however, the contents of Zinc (Zn), copper (Cu), manganese (Mn) were below the detection limit (0.1%) in the EDX results.
The contents of water-soluble salts in the collected fly ash samples are shown in Fig. 6. The anions contents, including Cl − , SO 4 2− , CO 3 2− and HCO 3 − , were determined to examine the effect of anions on the residue of heavy metals in fly ash. The heat reduction rate of the collected fly ash samples was in the range of 1.16-1.58%, which indicated that the content of combustible substances (including organic matter) in fly ash was low, meeting the requirements of related resource utilization. For the fly ash samples, the total salt contents of the two fly ash samples were 1.41 and 2.02 mg/L, respectively. And the main difference between two samples existed for the chlorine ion content. The low anion contents in fly ash would not cause soil salinization when considering subsequent disposal of the incineration residues 26 .
Leaching test of heavy metals. The leaching properties of heavy metals from fly ash by diluted HNO 3 are shown in Fig. 7. The leaching percent of all heavy metals was lower than 5% for two diluted HNO 3 solutions. According to sequential extraction test, the contents of carbonate-bound fraction (F2) and iron oxides bound   Ecological risk assessment of heavy metals in fly ash. Table 6 shows the E i and RI values of seven heavy metals of Cd, Cr, Cu, Mn, Ni, Pb and Zn. It was shown that the E i values of heavy metals decrease with the increase of particle size. The E i values for Cr, Cu, Mn, Ni, Pb and Zn were all less than 40, indicating low risk based on the classification of risk level (Table 2). While Cd in fly ash with lower than 74 μm was classified as moderate risk, the E i values of Cd in fly ash with other particle sizes were all lower than 40, indicating low risk. The overall potential ecological risks of seven heavy metals are shown in Table 6. The highest RI value of fly ash sample with particle size lower than 54 μm was 109.8, indicating low risk, which may be mainly due to Cd and Cu 15 . The other fly ash samples with different particle size were all classified as low risk. The bioavailable fractions of each heavy metal were used to calculate E i in this study, according to Table 6, Cd with the highest T i www.nature.com/scientificreports/ value (30) among tested metals, was enriched in fine particles, and accounted for the main fraction for ecological risk despite of lower concentration in comparison to other heavy metals measured. Therefore, the potential ecological risks increased with the decrease of particle size of fly ash.

Conclusion
The total concentrations of Cu, Zn and Mn were higher than other metals in fly ash, and most heavy metals were concentrated in fine particles. The potential ecological risks increased with the decrease of particle size of fly ash, and Cd accounted for the main fraction for ecological risk despite of lower concentration in comparison to other measured heavy metals. Therefore, considering land application of fly ash from co-combustion of ES and coal, Cd would be a main concern.   www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.