Author Contributions
Conceptualization, E.D.; methodology, C.C., E.D., A.P. and G.M.; validation, C.C., E.D., D.B. and Q.W.; formal analysis, C.C. and D.B.; investigation, C.C., E.D., D.B., G.C., L.P., C.B., Q.W., I.C. and F.T.; resources, C.C., E.D., D.B., C.B., Q.W. I.C., F.T. and R.B.; data curation, C.C.; writing—original draft, C.C. and A.P.; writing—review and editing, C.C., E.D., D.B., C.B., L.P., A.P. G.M. and E.B.; visualization, C.C.; supervision, E.D., G.M. and A.P.; project administration, A.P.; funding acquisition, A.P. and G.M. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on bottom ash (BA) with s ≥ 4.75 mm.
Figure 1.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on bottom ash (BA) with s ≥ 4.75 mm.
Figure 2.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on BA with 4.75 > s ≥ 2 mm.
Figure 2.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on BA with 4.75 > s ≥ 2 mm.
Figure 3.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on BA with 2 > s ≥ 1 mm.
Figure 3.
Concentrations (mg/L, log scale) versus treatment time of the main pollutants after steam washing on BA with 2 > s ≥ 1 mm.
Figure 4.
BA with s ≥ 4.75 mm before (a) and after (b) steam washing; BA with 4.75> s ≥ 2 mm before (c) and after (d) steam washing. Optical magnification 7.7×.; BA with 2 > s ≥ 1 mm before (e) and after (f) steam washing. Optical magnification 10 ×.
Figure 4.
BA with s ≥ 4.75 mm before (a) and after (b) steam washing; BA with 4.75> s ≥ 2 mm before (c) and after (d) steam washing. Optical magnification 7.7×.; BA with 2 > s ≥ 1 mm before (e) and after (f) steam washing. Optical magnification 10 ×.
Figure 5.
SEM images of solid residue removed during steam washing. SEM-EDS observations show a complex composition of both crystalline and slag phases (where no mineral could be identified the composition is reported in cement notation): 1. FeOx; 2. TiO2; 3. Pyroxene; 4. Apatite; 5. CAS; 6. Mg-spinel; 7. CA.; 8. Calcite; 9. Quartz. (Experimental conditions: W filament, accelerating voltage 15 kV, working distance 10 mm, high probe current, magnification 200×).
Figure 5.
SEM images of solid residue removed during steam washing. SEM-EDS observations show a complex composition of both crystalline and slag phases (where no mineral could be identified the composition is reported in cement notation): 1. FeOx; 2. TiO2; 3. Pyroxene; 4. Apatite; 5. CAS; 6. Mg-spinel; 7. CA.; 8. Calcite; 9. Quartz. (Experimental conditions: W filament, accelerating voltage 15 kV, working distance 10 mm, high probe current, magnification 200×).
Figure 6.
BA with s < 1 mm. Grey lines: BA not treated; black lines: BA carbonated; Solid lines: TG; Dashed lines: derivatives recorded as function of temperature for untreated sample (grey) and carbonated sample (black). The case of BA with s < 1 mm, at T = 60 °C, PCO2 = 2 bar, t = 60 min and a water content of L/S = 0.3 is shown, by way of example. In the temperature range from 30 to 540 °C, both samples exhibited qualitatively similar thermal behavior, characterized by the release of adsorbed H2O and small amounts of CO2. At higher temperatures, an important weight loss occurs for both samples (~747 °C), accompanied by a release of CO2, which can be ascribed to the decomposition of carbonate. The weight loss observed in the temperature range between 560 and 840 °C for untreated (~6.5 wt%) and carbonated BA (~11.5 wt%) yields a net sequestered CO2 amount as large as approximately 4–5 wt%.
Figure 6.
BA with s < 1 mm. Grey lines: BA not treated; black lines: BA carbonated; Solid lines: TG; Dashed lines: derivatives recorded as function of temperature for untreated sample (grey) and carbonated sample (black). The case of BA with s < 1 mm, at T = 60 °C, PCO2 = 2 bar, t = 60 min and a water content of L/S = 0.3 is shown, by way of example. In the temperature range from 30 to 540 °C, both samples exhibited qualitatively similar thermal behavior, characterized by the release of adsorbed H2O and small amounts of CO2. At higher temperatures, an important weight loss occurs for both samples (~747 °C), accompanied by a release of CO2, which can be ascribed to the decomposition of carbonate. The weight loss observed in the temperature range between 560 and 840 °C for untreated (~6.5 wt%) and carbonated BA (~11.5 wt%) yields a net sequestered CO2 amount as large as approximately 4–5 wt%.
Figure 7.
Average phase compositions of treated (post carb) versus non treated (NT) samples, inferred by X-ray Powder Diffraction on BA (s < 1 mm). Treatment conditions: PCO2 = 2 bar, L/S = 0.3, T = 20 °C and time = 180 min. The results shown are obtained as the average of five independent samplings and related XRPD data collections for either sample.
Figure 7.
Average phase compositions of treated (post carb) versus non treated (NT) samples, inferred by X-ray Powder Diffraction on BA (s < 1 mm). Treatment conditions: PCO2 = 2 bar, L/S = 0.3, T = 20 °C and time = 180 min. The results shown are obtained as the average of five independent samplings and related XRPD data collections for either sample.
Figure 8.
13C (150 MHz) MAS spectra of untreated (NT) and two carbonated, T = 60 °C, PCO2 = 2 bar, t = 60 min and L/S = 0.3, (Postcarb) BA samples (s < 1 mm), acquired at room temperature at a spinning speed of 20 kHz. Following a convention of NMR data, the Y axis, which reports an arbitrary unit, is missing. The broad peak at about 108 ppm is a spurious signal due to the rotor background.
Figure 8.
13C (150 MHz) MAS spectra of untreated (NT) and two carbonated, T = 60 °C, PCO2 = 2 bar, t = 60 min and L/S = 0.3, (Postcarb) BA samples (s < 1 mm), acquired at room temperature at a spinning speed of 20 kHz. Following a convention of NMR data, the Y axis, which reports an arbitrary unit, is missing. The broad peak at about 108 ppm is a spurious signal due to the rotor background.
Figure 9.
SEM images of carbonated BA samples (s < 1 mm). SEM-EDS observations show BA grains < 1 mm after treatment, highlight the complex and aggregate structure of such particles. (Experimental conditions: W filament, accelerating voltage 15 kV, working distance 10 mm, standard probe current, magnification 180×, top, and 220×, bottom).
Figure 9.
SEM images of carbonated BA samples (s < 1 mm). SEM-EDS observations show BA grains < 1 mm after treatment, highlight the complex and aggregate structure of such particles. (Experimental conditions: W filament, accelerating voltage 15 kV, working distance 10 mm, standard probe current, magnification 180×, top, and 220×, bottom).
Table 1.
Analyses of leaching tests of BA (s ≥ 4.75 mm) untreated and treated by steam washing for 240 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Standard deviations are reported. F−, Be, V, As, Se are not set out, as they lie below the detection limits. All concentration values are expressed in mg/L. LOQ for Cd and Co is 0.0005 µg/L.
Table 1.
Analyses of leaching tests of BA (s ≥ 4.75 mm) untreated and treated by steam washing for 240 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Standard deviations are reported. F−, Be, V, As, Se are not set out, as they lie below the detection limits. All concentration values are expressed in mg/L. LOQ for Cd and Co is 0.0005 µg/L.
| Cl− | SO42− | NO3− | Ni | Zn | Ba | Cd | Co | Cr | Cu | Pb |
---|
s ≥ 4.75 mm, untreated | | | | | | | | | | | |
Average | 257 | 54.3 | 0.52 | 0.015 | 0.029 | 0.02 | 0.017 | 0.0397 | 0.027 | 0.3 | 0.03 |
St. Dev. | 6.4 | 2.51 | 0.15 | 0.005 | 0.018 | 0.005 | 0.008 | 0.0107 | 0.013 | 0.024 | 0.016 |
s ≥ 4.75 mm, 240 s | | | | | | | | | | | |
Average | 40 | 19.7 | 0.25 | 0.002 | 0.001 | 0.003 | <LOQ | <LOQ | 0.002 | 0.024 | 0.003 |
St. Dev. | 4.6 | 2.13 | 0.2 | 0.001 | 0.0003 | 0.002 | <LOQ | <LOQ | 0.002 | 0.015 | 0.002 |
Italian legal limit values (mg/L) | 100 | 250 | 50 | 0.01 | 3 | 1 | 0.005 | 0.25 | 0.05 | 0.05 | 0.05 |
Table 2.
Results of the leaching tests of BA (4.75 > s ≥ 2 mm), untreated and treated by steam washing for 600 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Standard deviations are reported. F−, Be, V, As, Se are not set out, as they lie below the detection limits. All concentration values are expressed in mg/L.
Table 2.
Results of the leaching tests of BA (4.75 > s ≥ 2 mm), untreated and treated by steam washing for 600 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Standard deviations are reported. F−, Be, V, As, Se are not set out, as they lie below the detection limits. All concentration values are expressed in mg/L.
| Cl− | SO42− | NO3− | Ni | Zn | Ba | Cd | Co | Cr | Cu | Pb |
---|
4.75 > s ≥ 2 mm, untreated | | | | | | | | | | | |
Average | 252 | 115 | 0.4 | 0.004 | 0.013 | 0.005 | 0.001 | 0.005 | 0.007 | 0.27 | 0.001 |
St. Dev | 21.4 | 8.15 | 0.1 | 0.001 | 0.002 | 0.002 | 0.001 | 0.001 | 0.001 | 0.048 | 0.002 |
4.75 > s ≥ 2 mm, 600 s | | | | | | | | | | | |
Average | 82 | 44 | 0.2 | 0.002 | 0.003 | 0.001 | 0.001 | 0.002 | 0.004 | 0.045 | 0.001 |
St. Dev. | 6.6 | 14.6 | 0.08 | 0.001 | 0.003 | 0.001 | 0.003 | 0.001 | 0.002 | 0.012 | 0.001 |
Italian legal limit values (mg/L) | 100 | 250 | 50 | 0.01 | 3 | 1 | 0.005 | 0.25 | 0.05 | 0.05 | 0.05 |
Table 3.
Results of leaching test of BA (2 > s ≥ 1 mm) untreated and treated by steam washing for 600 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Statistical data are reported. F−, Be, V, As, Se are not reported, as they lie below the detection limits. All concentration values are expressed in mg/L.
Table 3.
Results of leaching test of BA (2 > s ≥ 1 mm) untreated and treated by steam washing for 600 s, and comparison with Italian legal limits for reuse (Min. Dec. 186/2006). Statistical data are reported. F−, Be, V, As, Se are not reported, as they lie below the detection limits. All concentration values are expressed in mg/L.
| Cl− | SO42− | NO3− | Ni | Zn | Ba | Cd | Co | Cr | Cu | Pb |
---|
2 > s ≥ 1 mm untreated | | | | | | | | | | | |
Average | 594 | 248 | 0.4 | 0.007 | 0.032 | 0.03 | 0.004 | 0.005 | 0.07 | 1.24 | 0.003 |
St. Dev. | 18.5 | 3.5 | 0.08 | 0.001 | 0.003 | 0.01 | 0.001 | 0.003 | 0.02 | 0.13 | 0.001 |
2 > s ≥ 1 mm, 600 s | | | | | | | | | | | |
Average | 16 | 11.8 | 0.2 | 0.004 | 0.01 | 0.03 | 0.001 | 0.002 | 0.05 | 0.07 | 0.001 |
St. Dev. | 1.13 | 0.9 | 0.081 | 0.001 | 0.001 | 0.01 | 0.001 | 0.001 | 0.01 | 0.01 | 0.001 |
Italian legal limit values (mg/L) | 100 | 250 | 50 | 0.01 | 3 | 1 | 0.005 | 0.25 | 0.05 | 0.05 | 0.05 |
Table 4.
Wastewater composition of steam washing on BA with s ≥ 4.75 mm, 4.75 > s ≥ 2 mm, 2 > s ≥ 1 mm (240, 600, 600 s). Be, V, As, Se and Hg are not reported, as they are below the detection limit. All concentration values are expressed in mg/L. LOQ for Cd, Co, Ba, Pb is 0.0005 µg/L, for Ni 0.002 µg/L.
Table 4.
Wastewater composition of steam washing on BA with s ≥ 4.75 mm, 4.75 > s ≥ 2 mm, 2 > s ≥ 1 mm (240, 600, 600 s). Be, V, As, Se and Hg are not reported, as they are below the detection limit. All concentration values are expressed in mg/L. LOQ for Cd, Co, Ba, Pb is 0.0005 µg/L, for Ni 0.002 µg/L.
Wastewater | Cl− | SO42− | NO3− | Ni | Zn | Ba | Cd | Co | Cr | Cu | Pb |
---|
s ≥ 4.75 mm (240 s) | | | | | | | | | | | |
Average | 214 | 33 | 0.26 | 0.001 | 0.002 | <LOQ | 0.002 | <LOQ | 0.02 | 0.2 | <LOQ |
St. Dev. | 16.26 | 6.66 | 0.02 | 0.001 | 0.001 | <LOQ | 0.001 | <LOQ | 0.01 | 0.01 | <LOQ |
4.75 > s ≥ 2 mm (600 s) | | | | | | | | | | | |
Average | 150 | 51 | 0 | 0.001 | 0.01 | <LOQ | < LOQ | <LOQ | 0.003 | 0.3 | <LOQ |
St. Dev. | 43.59 | 17.93 | 0 | 0.001 | 0.02 | <LOQ | < LOQ | <LOQ | 0.004 | 0.001 | <LOQ |
2 > s ≥ 1 mm (600 s) | | | | | | | | | | | |
Average | 504 | 250 | 0.2 | <LOQ | 0.001 | < LOQ | 0.002 | 0.001 | 0.05 | 1.05 | <LOQ |
St. Dev. | 53.95 | 0.83 | 0.01 | <LOQ | 0.001 | < LOQ | 0.001 | 0.001 | 0.03 | 0.001 | <LOQ |
Table 5.
Mass balance of the steam washing treatment referred to the three grain size classes investigated. The weight loss refers the BA weight difference after and before treatment; retained water, dissipated steam and wastewater fractions are referred to the initial water balance. Weight loss is expressed on dry weight.
Table 5.
Mass balance of the steam washing treatment referred to the three grain size classes investigated. The weight loss refers the BA weight difference after and before treatment; retained water, dissipated steam and wastewater fractions are referred to the initial water balance. Weight loss is expressed on dry weight.
Grain Size BA | Time (s) | Retained Water (wt%) | Dissipated Steam (wt%) | Wastewater (wt%) | Weight Loss (wt%) |
---|
≥4.75 mm | 240 | 3 | 91 | 6 | 0.4 |
4.75 mm > s ≥ 2 mm | 600 | 2 | 85 | 13 | 5 |
2 mm > s ≥ 1 mm | 600 | 13 | 75 | 12 | 10 |
Table 6.
Carbonation of BA s < 1 mm (T = 6, 20, 60 °C; t = 60, 180 min; L/S = 0.3), PCO2 = 2 bar. Standard deviations are set out. Concentration values are expressed in mg/L.
Table 6.
Carbonation of BA s < 1 mm (T = 6, 20, 60 °C; t = 60, 180 min; L/S = 0.3), PCO2 = 2 bar. Standard deviations are set out. Concentration values are expressed in mg/L.
Time, Temperature | Cl− | SO42− | NO3− | Ni | Zn | Ba | Cd | Co | Cr | Cu | Pb |
---|
Untreated, 20 °C | 1223 | 378 | 1.15 | 0.063 | 0.065 | 0.068 | 0.118 | 0.148 | 0.15 | 3.065 | 0.02 |
St. Dev. s < 1 mm | 124.1 | 51 | 1.04 | 0.01 | 0.044 | 0.039 | 0.013 | 0.021 | 0.014 | 0.093 | 0.008 |
60 min, 20°C | 1185 | 354 | 0.63 | 0.056 | 0.107 | 0.133 | 0.142 | 0.156 | 0.135 | 0.786 | 0.097 |
St. Dev. s < 1 mm | 120.8 | 24.2 | 0.05 | 0.001 | 0.013 | 0.012 | 0.041 | 0.007 | 0.014 | 0.118 | 0.028 |
60 min, 60 °C | 792 | 152 | 3.48 | 0.002 | 0.006 | 0.067 | 0.003 | 0.001 | 0.006 | 0.483 | 0.001 |
St. Dev. | 2 | 1 | 0.03 | 0.001 | 0.012 | 0.006 | 0.006 | 0.001 | 0.006 | 0.042 | 0.008 |
180 min, 20 °C | 1116 | 253 | 0.57 | 0.007 | 0.11 | 0.046 | 0.001 | 0.002 | 0.02 | 0.045 | 0.003 |
St. Dev. | 210 | 68 | 0.05 | 0.005 | 0.009 | 0.075 | 0 | 0.001 | 0.01 | 0.005 | 0.002 |
Italian legal limit values (mg/L) | 100 | 250 | 50 | 0.01 | 3 | 1 | 0.005 | 0.25 | 0.05 | 0.05 | 0.05 |