Waste mass reduction
The ash contents after incinerating the peat, iron–peat and IP–lime at 850 °C and 1100 °C are presented in Fig. 1. Table 2 lists the average total concentrations of As, Cr, Cu, Fe and Zn in the ashes. Incinerating the spent sorbents at higher temperatures resulted in lower ash contents. Thus, higher contents of As, Cr, Cu and Zn were concentrated in a smaller waste body. The obtained ash contents ranged from approximately 9 –19 wt%. The addition of inorganic compounds (Fe oxides and lime) decreased the calorific value of the spent sorbents (i.e. fuel) and increased the ash content. Peat without any modifications had a calorific value of 19.7 ± 1.2 MJ kg−1, which decreased to 18.8 ± 0.8 MJ kg−1 when coated with Fe oxides and decreased even further to 17.31 ± 2.1 MJ kg−1 when lime was added.
The determined calorific values of the spent sorbents were within the range of calorific values typical for peat (13.6–25.4 MJ kg−1) (Lehtovaara and Salonen 2012). Although in this study the incineration of spent sorbents was considered to be a way to reduce the mass (and volume) of waste, waste-to-energy technology should also be explored because, in general, it not only provides renewable sources of energy but also has the potential for recycling solid wastes (preferably with a high organic content) (Kothari et al. 2010). In this study, the peat and ferric ferrous hydrosol (for coating the peat) were both waste-based materials. In addition, the lime used to reduce the leaching of contaminants from the ashes was also an industrial residue. Therefore, this process of utilising waste-based materials to clean out contaminated water, followed by the co-incineration of several waste materials at the same time, is advantageous from the circular economy and environmental points of view.
Metal(loid) leaching from ashes
A previous study (e.g. Lundholm et al. 2007) reported that the incineration of As-containing wastes was not desirable because of the low volatilisation temperature of As. The volatilisation of pure As2O5 can start at approximately 600 °C, whereas a much lower volatilisation temperature was reported (320 °C) when a mixture of As2O5 and sawdust started to smoulder. In addition, Zn also belongs to a group of semi-volatile elements, which are usually depleted in the bottom ash and enriched in the fly ash (Lundholm et al. 2007). In this study, during the evolved gas analyses, volatile substances containing metal(loid)s did not evolve during the combustion of the sample while performing TGA. It is likely that the metal(loid) content present in the spent sorbents was too low for substantial metal(loid)-gas formation. Therefore, it was assumed that the entire content of As, Cr, Cu and Zn was present merely in the ashes. However, it was not possible to explain what caused the Fe losses during the incineration of the peat and iron–peat at 1100 °C; the concentration was higher in the ashes obtained at 850 °C.
Figure 2 presents the As, Cr, Cu and Zn concentrations in the leachates from the ashes after the standardised batch leaching test at L/S = 10. The abovementioned test is also a compliance test, which can be used to confirm the appropriate type of landfill for the disposal of the waste. Based on the contaminant concentrations detected in the leachate, the waste can be deposited at landfills for (i) inert, (ii) non-hazardous or (iii) hazardous wastes. In previous studies (Kasiuliene et al. 2019a, b), it was concluded that the spent peat and iron–peat could not be landfilled at landfills for hazardous waste. The leaching of As, which intensified drastically under the reducing conditions, was one of the main factors hindering the landfilling.
The leaching limit values for waste that can be accepted at landfills for hazardous waste are as follows: 25 mg kg−1 of As, 70 mg kg−1 of Cr, 100 mg kg−1 of Cu and 200 mg kg−1 of Zn (Council Decision 2003/33/EC).
As shown in Fig. 2, the leaching of As from the ashes in all cases was below the limit value. Incinerating the spent sorbents at the higher temperature (1100 °C) slightly reduced the leaching of As from the ashes, whereas the addition of lime had a substantially higher effect. The leaching of As from the IP–lime ashes was 30 times lower than the leaching from the iron–peat ashes.
The concentration of Cu in the leachates was approximately 1 mg kg−1, which corresponded to 1% of the limit value. The incineration of the spent sorbents at different temperatures, as well as the addition of lime, did not have significant effects on the leaching of Cu.
The leaching of Zn was also below the limit value. However, because Zn is usually more water-soluble than Cu (Kabata-Pendias 2011), incinerating the spent sorbents at the higher temperature (1100 C°) reduced the leaching of Zn from the ashes by approximately two times compared with the leaching from the ashes obtained at 850 °C. The co-incineration with lime reduced the leaching of Zn from the IP–lime ashes by three times compared with the iron–peat ashes.
Among the analysed metal(loid)s, Cr exhibited the most intensive leaching from the ashes of the spent sorbents. The Cr concentration in the leachate from the peat 850 ashes was almost 50 times above the limit value. Even though the leaching of Cr from the ashes was significantly reduced at the higher incineration temperature (1100 °C), it still exceeded the limit value by several times. Under the given experimental conditions, the leaching of Cr decreased below the leaching limit value when the spent sorbent was co-incinerated with lime.
In all cases, the addition of lime, together with the higher incineration temperature, had a positive synergistic effect on the metal(loid) stability in the ashes. Under the given experimental conditions, the co-incineration of the spent sorbents with the 10 wt% lime additive increased the ash content by 6–7% compared with the ash content before the lime addition (Fig. 2). Therefore, the increased utilisation of lime should be undertaken with caution because it might result in high ash loads from the use of inorganic lime. Additionally, facilities for incinerating hazardous waste operating at 1100 °C are less common in Sweden and the rest of Europe. Therefore, the additional transportation costs would increase the overall treatment costs and would have a negative effect on the environment. For this reason, a compromise between the slightly increased incineration efficiency, treatment costs and impact on the environment needs to be carefully considered.
Metal(loid) distribution in ashes
Figure 3 presents the different metal(loid) distribution fractions found in the ashes. It was anticipated that the fractionation of the As, Cr, Cu and Zn would be in line with the results from the standardised batch leaching. However, while this was true for Cr, Cu and Zn, it was not true for As. The main trend for Cr and Zn was that in the ashes obtained at the higher temperature (1100 °C), the exchangeable fraction was smaller, while the residual fraction was larger compared with that in the ashes obtained at the lower temperature (850 °C). The decrease in the exchangeable (water-soluble) fraction explains why Cr and Zn leached out less from the ashes obtained at 1100 °C during the standardised batch leaching test. The exchangeable fraction of Cu in all cases corresponded to less than 5%. Thus, the leaching of Cu during the batch leaching test was negligible. In contrast to Cr and Zn, the fractionation of As contradicted the results of the batch leaching test. The ashes obtained at the lower incineration temperature (850 °C) in all cases had a smaller exchangeable fraction, whereas the residual fraction was always larger compared with the ashes obtained at 1100 °C. However, in the standardised batch leaching test, significantly less As leached out from the ashes obtained at 1100 °C. At temperatures below 650 °C, the TECs (Supplementary Figs. 4,5 and 6) predicted that the As would be found in complexes with Ca, whereas K-As complexes would be dominant with increasing temperature. However, the sequential extraction analysis indicated that As was associated with Ca after the incineration of the spent peat and iron–peat at 850 °C. Upon increasing the temperature (to 1100 °C), K-As complexes were predicted (Supplementary Figs. 4, 5 and 6). Because Ca-As is less soluble than K-As (Rochette et al. 1998), a smaller fraction of the exchangeable As and larger fraction of the residual As was found in the ashes obtained at lower temperatures. This is in line with the fractionation of Ca determined by the sequential extraction (Fig. 3). The ashes obtained at the higher temperature (1100 °C) had a slightly larger fraction of the exchangeable Ca and a larger residual fraction. In addition, the Fe-Mn oxide fraction of Ca was also larger in the ashes obtained at 850 °C compared with that in the ashes obtained at 1100 °C. However, the presence of Ca-As was not detected by the XRD analysis, most likely because the Ca-As appeared to be in an amorphous phase rather than crystalline. According to the phase fitting calculations for the XRD patterns, a 16–31 wt% corresponded to amorphous phase for the peat and iron–peat ashes. The amorphous phase was slightly lower for the ashes obtained at the higher temperature.
In Kasiuliene et al. (2018), it was reported that in the spent iron–peat sorbent, As was adsorbed mainly onto ferrihydrite. However, in this study, the XRD analysis showed that haematite was the dominant crystalline Fe-bearing mineral in the ashes (Supplementary Figs. 8, 9, 10 and 11). Therefore, it is likely that upon the Fe oxide transformation from ferrihydrite to haematite due to the high-temperature oxidising atmosphere, a share of As was disassociated from the haematite and, together with Ca, went to the amorphous phase. It was shown in Zhu et al. (2006) that the solubility of Ca-As is pH dependant, with the least soluble compounds forming at pH values ranging between 11 and 13. In addition, it was shown that Ca-As compounds that precipitated under a low pH (3–5) had a more crystalline structure compared with Ca-As compounds obtained at a higher pH. This is in line with our study because it was not possible to detect crystalline Ca-As structures during the XRD analysis. Furthermore, because the solubility of Ca-As is low under high pH values, it explains the low leaching of As during the standardised batch leaching test, because the pH values of the leachates for all the samples ranged between 10 and 11. The association of As with either K or Ca was compatible with the increased As extraction in the exchangeable fraction, ammonium acetate solution under a much lower pH environment (6.5).
In the IP–lime ashes, Ca was mainly found in the Fe-Mn fraction. The TECs indicated a gradual increase in the Ca2Fe2O5 formation with an increase in the CaO content in the system (Supplementary Fig. 7). This was confirmed by the XRD results because Ca2Fe2O5 corresponded to 42–55% of the IP–lime ashes in the diffractogram (Supplementary Figs. 12, 13). It is very likely that the Ca incorporation into the Fe oxide structure altered the interaction between As and Fe, promoting the mobilisation of As. This was confirmed by the sequential extractions (Fig. 3), where As, which was found in the Fe-Mn oxide fraction (III) present in the IP–lime prior to incineration, disappeared from the IP–lime ashes. Instead, the exchangeable fraction of As increased. On the other hand, the amorphous phase in the IP–lime ashes increased (up to 43%) compared with that in the iron–peat ashes, which was poorly water-soluble during the standardised batch leaching test at the pH value of approximately 12.
The sequential extractions (Fig. 3) revealed that the Cr in the spent sorbents was mostly bound to the organic fraction, which became oxidised during the incineration; thus, a significant fraction of Cr became water-soluble. The TECs showed that water-soluble K-Cr oxides were dominant at temperatures below 800–900 °C (Supplementary Figs. 4, 5 and 6). As the temperature continued increasing, the melt fraction associated with Cr also increased, as well as the formation of corundum (Supplementary Fig. 7). For this reason, Cr leached less from the ashes obtained at the higher temperature (1100 °C). Because of the increased CaO content, corundum was no longer detectable in the IP–lime ashes. Instead, the TECs predicted that Cr would mostly be found in the form of the water-insoluble CaCr2O4 spinel (Supplementary Figs. 4, 5 and 6), which was in line with the substantially reduced leaching of Cr during the standardised batch leaching test.
The sequential extraction showed (Fig. 3) that in the peat and iron–peat ashes, irrespective of the incineration temperature, almost all of the Zn was found in the residual fraction. However, more substantial differences were observed in the Zn leaching from the ashes during the leaching test. Zinc leached more from the ashes obtained at the lower temperature (850 °C), while the exchangeable fraction of Zn during the sequential extractions was very small. It is likely that the pH of less than six that was prevalent during the sequential extractions hindered the extraction of the I–IV fractions of Zn. During the batch leaching test, a higher pH (around 11) resulted in a higher mobility for Zn. The co-incineration with lime decreased the leaching of Zn from the IP–lime ashes because of the predicted formation of (i) slag and (ii) spinel compounds with Fe (Supplementary Figs. 4, 5 and 6).
The behaviour of Cu was very similar to that of Zn. As predicted by the TECs, the formation of slag and spinel (Supplementary Figs. 4, 5 and 6) could potentially be responsible for the stabilisation of Cu in the ashes and its very weak leaching during the standardised batch leaching test.
In summary, As was associated with ferrihydrite before the incineration. Then, as it was transformed into haematite with the increasing temperatures, As became associated with Ca in the poorly water-soluble amorphous phase, which then explained the low leaching of As during the standardised batch leaching test. The formation of the water-insoluble spinel of CaCr2O4 resulted in the decreased leaching of Cr when the iron–peat was co-incinerated with lime. In the case of Cu and Zn, the formation of slag and spinel resulted in weak leaching from the ashes.