Keywords

Fly ash; heavy metals; leaching behavior; volatilization ratios; sintering temperature; dioxins

5.1 Compositions of Heavy Metals in Fly Ash

Fly ash is a must by-product of municipal solid waste (MSW) incineration process, which has been classified as a hazardous waste due to its relatively high content of heavy metals all over the world. The presence of heavy metals in municipal solid waste incineration (MSWI) fly ash is of environmental concern due to their leaching potential in landfill environments. Hence, its management is becoming a challenge for incineration of MSW, and therefore attracting world attention.

The ash used in this study came from two municipal solid waste incinerators, Shanghai Pudong and Jiangqiao MSWI Plants, which handle 1500–1700 t/day. Prior to analysis for characterization of heavy metals, fly ash was dried at 105°C. Hg detector was used to measure content of Hg and ICP (IRIS Advantage 5000) used to analyze content of other heavy metals. HNO₃/H₂O₂ digestion method was applied to extract Hg from the ash, and HNO₃/HF/HClO₄ method applied for other heavy metals.

Leaching toxicity analysis was performed in accordance with HVEP (horizontal vibration extraction procedure) and ALT (available leaching toxicity) standards, in which de-ionized water is used as the agent and the liquid to solid ratio registers 10:1 for the former and 100:1 for the latter. Sequential chemical extraction has long been used as a method to analyze speciation of heavy metals. In this study, sequential chemical extraction was performed on 10 g fly ash, with a liquid to solid ratio of 10:1 in each step. Specific description of each step is shown in Table 5.1. Heavy metals of the last two speciations are hardly soluble. In the last step, the contents of Cd, Pb and Zn in the residue from Step 5 were analyzed by digestion with HCl/HNO₃/HF/HClO₄ and that of Hg by digestion with HCl/H₂O₂/HNO₃.

2 M HNO₃ was diluted to different concentrations of the solution, which were used as the extractant of different pH to extract heavy metals from the ash. Leaching solution was analyzed using ICP after 48 h horizontal vibration on rotary table concentrator (DKY-II), additionally, simulated using MINTEQA2 modeling. The fly ash was sintered at 400, 600, 800, 900, 1000, 1100 and 1200°C, respectively for 30 min in the muffle furnace (SX₂-4-10). After cooling down, the sintered fly ash went through volatilization analysis of its heavy metals.

Fig. 5.1 shows that the contents of heavy metals in fly ash follows a sequence of Zn>Pb>Cu>Cr>As>Ni>Cd≈Hg in Pudong MSWI Plant and Jiangqiao Plant, and total heavy metals accounted for around 0.68% by mass of fly ash for the former, and approximately 0.71% for the latter. Contents of heavy metals in fly ash from Jiangqiao MSWI Plant were generally higher than those from Pudong MSWI Plant, which may be resulted by the difference in waste components and incineration temperatures between both plants.

Given that hazard of heavy metals is manifested not only by their contents but also by their extraction toxicity, and thus leaching toxicity analysis is required to assess environmental risk imposed by them.

5.2 Formation and Control Technology of Dioxins in Fly Ash

In China, a total of 178 MSWs incineration plants were in operation in 2014, producing almost 2–4 million tons of fly ash and 30–46 million tons of bottom ash. Fly ash is a residue collected from the flue gas cleaning system in waste incineration plants, which is a kind of complex particles with heavy metals and persistent organic pollutants (POPs) like polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorobenzene, and polycyclic aromatic hydrocarbons (PAHs) etc., and can’t be used for landfill and building materials before treatment. Dioxin from waste incineration contains poly-chlorinated dibenzo-pdioxin (PCDDs) and poly-chlorinated dibenzofuran (PCDFs). In most of the incineration plants (including the medical waste incinerators), international toxic equivalent (I-TEQ) value in bottom ash is extremely low (only 0.55–1.78 pg I-TEQ/g), whereas the concentration in fly ash is in the range of 0.1–0.8 ng I-TEQ/g.

Conclusion have be made that dioxins are highly persistent in environment and are apt to bioconcentration in food chain because of the liposolubility intergenerational effect. Dioxin is a kind of chlorinated tricyclic aromatic compounds of which some have the intense carcinogenesis, tetratogenesis and mutagenesis. In Europe, tolerable daily intake (TDI) is 1–10 pg I-TEQ/kg_{body weight} where the threshold is 5 pg I-TEQ/kg_{body weight} in Sweden, Finland and Norway.

Precisely because of the existence of heavy metals and dioxins, fly ash treatment and management become one of the crucial environmental issues. In China, landfill for fly ash and reuse for the bottom ash are two dominant ways. For decades, researchers and government regulators concerns more on the concentration of heavy metals, while dioxins in the fly ash receives little attention. The reuse workspace or landfill site of fly ash and bottom ash make long at-work exposures. However, there still have relatively little data about the distribution and emission characteristics of dioxins, especially the concentration of PCDD/Fs in the workplace air.

5.3 Leaching Toxicity of Heavy Metals in Fly Ash

Leaching results of heavy metals in accordance with HVEP and ALT standards are shown in Table 5.6. It can be seen that the concentrations of most heavy metals in leaching solution, in accordance with HVEP standard, exceeded the limits required in “Identification Standard of Hazardous Waste—Identification of Leaching Toxicity”.

5.4 Speciation of Heavy Metals and Mineral Components in Fly Ash

Fig. 5.2 and Fig. 5.3 show that soluble Cd mainly existed in the acid soluble, exchangeable and water-soluble phases, soluble Hg in acid soluble and water-soluble phases, while soluble Pb appeared in acid soluble and water soluble, Zn and As in bound to organic matter and acid soluble, for fly ash at room temperature. Hence, it is relatively difficult for Pb and Zn to be leached out in environment without the existence of acids. However, in natural environment exposing to acid rain, there always exist acids and the above four heavy metals tend to be extracted by acidic liquid after years. The aim of stabilization and sintering of the fly ash is to minimize leaching toxicity of heavy metals by reducing their existence in water soluble, exchangeable, acid soluble and bound to organic matter phases, as done in this work. Content of Cr in residual phase of fly ash from Jiangqiao Plant was relatively lower than that from Pudong Plant, while for other heavy metals, it was higher in terms of the former plant. This should be caused by transformation of Cr³⁺ to Cr⁶⁺ at relatively higher incineration temperature of Pudong Plant and higher solubility of Cr⁶⁺ than Cr³⁺ (Fig 5.4, Fig 5.5).

5.5 XRD Analysis of Mineral Components of Fly Ash

To find out major mineral components which include heavy metals in their crystal lattice, XRD analysis was performed on residues derived from each step of sequential extraction procedure shown in Table 5.1. After the first step, XRD analysis of residue is shown in Fig. 5.6. Major mineral components were found to be SiO₂, Ca₃Si₂O₇, Ca₉Si₆O₂₁∙H₂O, AlCl₃∙4Al(OH)₃∙4H₂O, 5CaO∙3SiO₂∙2H₂O, Mg₇Fe₄O₁₃∙10H₂O, Al₉Cl₆(OH)₂₁∙18H₂O, Pb₂Fe₂O₅, Ca₂SiO₄∙0.35H₂O, (Mg,Al)(Si,Al)O(OH)∙8H₂O. No basic oxides and chlorides were found in the spectrum, which should be dissolved during the first washing step. Diffraction peaks of Zn and Pb compounds appeared, while those of other heavy metals can’t be found, possibly for their relatively smaller contents in fly ash under the detection limit of XRD.

After the second step, diffraction peaks of Mg(OH)₂ were found, those of AlCl₃ ∙ 4Al(OH)₃ ∙ 4H₂O disappeared and diffraction peak of Al₉Cl₆(OH)₂₁18H₂O at d=2.11 Å decreased by 49% (Fig. 5.7). It was conferred that Mg²⁺ replaced Al³⁺ forming Mg(OH)₂, which reduced contents of AlCl₃ ∙ 4Al(OH)₃ ∙ 4H₂O and Al₉Cl₆(OH)₂₁ ∙ 18H₂O. Contents of other minerals increased correspondently, as reflected in the XRD spectrum.

Fig. 5.8 shows that diffraction peaks of Mg(OH), Al₉Cl₆(OH)₂₁∙18H₂O, (Mg, Al)(Si, Al)O(OH) ∙ 8H₂O and Pb₂Fe₂O₅ disappeared, and those of CaAl₂(SiO₃)₄ ∙ 6H₂O and Mg₇Fe₄O₁₃ ∙ 10H₂O appeared, indicating that the former four minerals reacted with acid in the third step and contents of latter two minerals raised consequently to within the detection limit of XRD. In addition, contents of glass phases increased compared with that of residue from the second step.

In the XRD analysis of residue from the fourth step, as shown in Fig. 5.9, Mg₇Fe₄O₁₃ ∙ 10H₂O disappeared from the spectrum, indicating that it was removed from the ash in the fourth step. Meanwhile, contents of glass phases as well as silicates increased again compared with that from the third step.

It was demonstrated that major mineral compositions of residue from the fifth step were calcium-silicates and aluminosilicates, including SiO₂, Ca₃Si₂O₇, Ca₉Si₆O₂₁∙H₂O, CaAl₂(SiO₃)₄∙6H₂O, 5CaO∙3SiO₂∙2H₂O, Al₂CaO₁₂ ∙ SI₄ ∙ 6H₂O and Ca₂SiO₄, in which heavy metals of residual phase were occluded. Besides, contents of glass phases as well as calcium-silicates and alumino-silicates increased again, compared with that from the fourth step, as shown in Fig. 5.10. It was conferred that contents of Ca, Si, and Al should influence extraction of heavy metals.

Based on the above XRD analysis of residue from each step of sequential extraction procedures, it was concluded that contents of glass phases increased steadily from the first step to the fifth step, indicating that extraction efficiency of heavy metals should decrease with the increase of glass phase contents. In addition, major mineral components of residue from the fifth step were mainly calcium-silicates and aluminosilicates, registering an increase in content from the first step to the fifth step, demonstrating that heavy metals of residual phase were occluded mainly in both silicates and content of Ca. Al and Si had a direct relationship with extraction efficiency of heavy metals. Consequently, it was conferred that fly ash may be stabilized in silicate materials containing Al and Ca, which were conducive to stabilization of heavy metals.

5.6 Effects of pH on Leaching Behavior of Heavy Metals in Fly Ash

Leaching results of heavy metals in fly ash from both plants are compared with those derived from simulation using MINTEQA2, which is an equilibrium speciation model that can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model is useful for calculating the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions

5.6.1 Effects of pH on Leaching Behavior of Zn

Leaching results of Zn are essentially consistent with the simulated ratios, indicating that simulated leaching ratios can represent actual leaching behavior of Zn. The leaching ratios were relatively higher for fly ash from Jiangqiao MSWI Plant than those from Pudong MSWI Plant, which may be caused by a higher content of residual speciation in terms of the latter plant (Fig. 5.11). At pH from 0 to 7.5, leaching ratios of Zn are approximately equal to 75%, and close to zero at pH between 8 and 12 due to formation of Zn(OH)₂ and ZnO ∙ SiO₂, demonstrating a high content of acid soluble phase which was consistent with result of the specification analysis. When pH exceeds 12, leaching ratio increases again due to transformation of Zn precipitations to soluble [Zn(OH)₃]⁻, and [Zn(OH)₄]²⁻ which is the major form in strong basic conditions.

Form transformation of Zn follows a sequence of dissolved ZnOHCl and ZnCO₃ → precipitated Zn(OH)₂ and ZnO ∙ SiO₂ → [Zn(OH)₃]⁻ and [Zn(OH)₄]²⁻, as described in Fig. 5.12. Major reactions taking place at pH ≥ 12 are: Zn(OH)₂ + OH⁻ → [Zn(OH)₃]⁻, Zn(OH)₂ + 2OH⁻ → [Zn(OH)₄]²⁻, ZnO + 2OH⁻ + H₂O → [Zn(OH)₃]⁻, ZnSO₄ + OH⁻ → [Zn(OH)₂ SO₄]²⁻.

5.6.2 Effects of pH on Leaching Behavior of Cr

Measured and simulated leaching results of Cr are not always consistent, as shown in Fig. 5.13, indicating that simulated leaching ratios can’t always represent actual leaching behavior. When pH is lower than 4, simulated value is generally higher than the measured value, and the maximum measured leaching ratio is around 35% for Jiangqiao Plant and approximately 37% for Pudong Plant. Both measured and simulated leaching results are close to zero as pH ≥4, due to formation of Cr(OH)₃ and Cr₂O₃, as shown in Fig. 5.14. It can be seen that form transformation of Cr follows a sequence of Cr²⁺ → Cr₂(OH)₂(SO₄)₂ → CrOHCl₂ → Cr(OH)₃ → Cr₂O₃ with the increase of pH.

5.6.3 Effects of pH on Leaching Behavior of Cd

As shown in Fig. 5.15, measured leaching results of Cd are relatively lower than the simulated ratios when pH≤10, in terms of fly ash from Jiangqiao Plant and Pudong Plant, and leaching ratios are relatively higher for the former plant with the same reason described for Zn. Hence, simulated value can’t always represent the actual leaching behavior in terms of Cd. At pH≥11, the leaching results are close to zero, demonstrating minimal leaching in conditions of strong alkalinity. There is one point at around pH of 10 over the simulated value, which should be caused by error. The maximum leaching ratios exceeds 80% at pH close to zero, demonstrating a high content of soluble phase of Cd. From Fig. 5.16 it can be seen that form transformation of Cd follows a sequence of Cd²⁺ → CdCO₃ and CdOHCl → Cd(OH)₂ with the increase of pH.

5.6.4 Effects of pH on Leaching Behavior of Ni

Leaching results of Ni are not consistent with the simulated value at pH≤2, as shown in Fig. 5.17, indicating that the latter can’t represent its actual leaching behavior at pH spanning 0 and 2. While, as pH≥2, the measured and simulated values are well consistent, and thus the latter can represent actual leaching behavior of Ni at pH ≥ 2. Maximum leaching ratio is relatively higher for Jiangqiao Plant with the same reason described for Zn. Form of Ni follows a sequence of Ni²⁺ → NiO ∙ SiO₂ → NiCO₃ → Ni(OH)₂ → Ni(OH)₃, as shown in Fig. 5.18.

5.6.5 Effects of pH on Leaching Behavior of Pb

Measured and simulated leaching results of Pb for both plants are essentially consistent, indicating that simulated leaching ratios can reflect actual Pb leaching behavior in the environment, as described in Fig. 5.19. Leaching ratio reduces with the increase of pH from 0 to 3, indicating its tendency to being leached out of fly ash At strong acidic environment. At pH = 3–12, it is close to 0 due to formation of insoluble Pb(OH)₂. When pH exceeds 12, it rises with the increase of pH, which is caused by transformation of Pb(OH)₂ to [Pb(OH)₃]⁻, [Pb(OH)₄]²⁻ and [Pb(OH)₂ ∙ SO₄]²⁻. Major reactions are: Pb(OH)₂ + OH⁻ → [Pb(OH)₃]⁻, Pb(OH)₂ + 2OH⁻ → [Pb(OH)₄]²⁻, PbO + 2OH⁻ + H₂O → [Pb(OH)₃]⁻, PbSO₄ + OH⁻ → [Pb(OH)₂ ∙ SO₄]²⁻. Consequently, Pb mainly exists in the form of [Pb(OH)₄]²⁻ in strong basic conditions with pH ≥ 12, as shown in Fig. 5.20.

Fig. 5.20 shows that form transformation of Pb follows a sequence of Pb²⁺ → PbSO₄ and Pb(OH)₂ → [Pb(OH)₃]⁻ and [Pb(OH)₂ SO₄]²⁻ → [Pb(OH)₄]²⁻ with the increase of pH. It is clear that Pb(OH)₂ is transformed into [Pb(OH)₄]²⁻ and [Pb(OH)₃]⁻ which finally forms [Pb(OH)₄]²⁻ under strong basic environment. In terms of PbSO₄, existing mainly at pH between 5.5 and 8.5, most of it is transformed into [Pb(OH)₄]²⁻ and only a small portion into [Pb(OH)₂ ∙ SO₄]²⁻, releasing Pb²⁺. Hence, its leaching toxicity is enhanced at both strong acidic and basic conditions, and thus inflicting a bad influence on environment.

5.6.6 Effects of pH on Leaching Behavior of Cu

Measured leaching results of Cu are well consistent with the simulated value for fly ash from both plants (Fig. 5.21), and thus the latter can reflect its actual leaching behavior in the environment. Maximum leaching ratio is over than 60%, demonstrating that Cu tends to be leached out in strong acidic environment. As pH is ≥4, its leaching ratios are close to zero, and thus a basic environment is conducive to stabilization of Cu. It is conferred that its form transformation follows sequence of dissolved Cu at pH of 1-minus → sulfides and oxides of Cu at pH = 1–4 → cuprous sulfides and oxides at pH ≥ 4.

5.6.7 Effects of pH on Leaching Behavior of As

Leaching results derived from measurement and simulation are well consistent at pH ≥ 2.5, while the ratios derived from the former are relatively lower than those from the latter at pH ≤ 2.5, demonstrating that the simulated value can represent actual leaching behavior of As only at pH of 2.5-plus, as shown in Fig. 5.22. The maximum measured leaching ratios are close to 60% at pH near zero, indicating that a strong acidic condition is conducive to extraction of As. Relatively low measured ratios compared with the simulated ones should be caused by loss of partial As in digestion process.

5.6.8 Effects of pH on Leaching Bbehavior of Hg

Like As, measured leaching ratios (Fig. 5.23), at pH of 9-minus, are relatively lower than the simulated ones for fly ash from both plants with similar reason described in terms of As. At pH between 0 and 4, measured leaching ratios are close to 50%, indicating that it is relatively easy for Hg to be leached out of fly ash and impose a bad influence on the environment. While at pH ≥ 9.5, leaching ratios by both methods are close to zero, demonstrating that Hg is relatively safe under strong basic conditions. Hence, a strong basic condition is conducive to reduction of its leaching toxicity.

Based on the above analysis of leaching results by measurement and simulation for fly ash from both plants, it is concluded that the maximum leaching results of Cr were relatively higher for Pudong Plant, while those of other heavy metals were lower compared with those for Jiangqiao Plant. It should be caused by different incineration temperatures and processes. Measured leaching results are well consistent with the simulated ones for Cu, Pb and Zn, indicating that the latter can reflect their actual leaching behavior in the environment. Leaching ratio of Cu decreases with the increase of pH until it is close to zero at pH ≥ 4, showing that a basic environment is conducive to reduction of its leaching toxicity. In terms of Zn and Pb, leaching results first decrease and finally increase with the increase of pH, demonstrating that it is easy for them to be leached out at both strong acidic and basic environment. For Ni, As, Cr, Hg, and Cd, leaching results, close to zero, are well consistent by both methods only when pH is over 1.5, 2.5, 5, 9.5, and 11 respectively, indicating that simulated ratios can’t always reflect actual leaching behavior of these heavy metals. Consequently, Ni, As, Cr, Hg, and Cd are relatively safe for the environment in strong basic conditions, as their leaching toxicity is around zero.

5.7 Effects of Temperature on Volatilization Ratios of Heavy Metals

Volatilization results of Hg and Pb with the increase of sintering temperature are shown in Fig. 5.24(A) and (B). It is clear that volatilization ratio of Hg increases considerably as sintering temperature rises from 400°C to 1,000°C, and is close to 100% after being sintered between 1,000°C and 1,200°C. Pb volatilizes slowly as it increases from 400°C to 800°C, flies away quickly from 800°C to 1,000°C, and is removed completely from fly ash Between 1,000°C and 1,200°C. Around 80% As and Cd fly away from fly ash sintered at 1,000°C, and almost total As and Cd volatilize from fly ash sintered at 1,200°C, which is shown in Fig. 5.24(C) and (D). Volatilization ratio of Zn, which is shown in Fig. 5.24 (E), is much smaller than that of the above four heavy metals after being sintered at the same temperature. Close to 60% Zn flies away from fly ash sintered at 1,200°C. Volatilization ratios of Cr, Ni and Cu are no more than 10% after being sintered at 1200°C, which is shown in Fig. 5.24(F), (G) and (H).

Based on the above analysis of volatilization of heavy metals, volatility of heavy metals decreases as: Hg > Pb > Cd, As Zn > Cr, Ni, Cu. It is easy for Hg, Cd, As and Cd to volatilize, and a little difficult for Zn and difficult for Ni, Cr, and Cu to fly away from fly ash, with the increase of sintering temperature. Hence, exhaust gas from the thermal treatment of fly ash contains a lot of heavy metals, especially Hg, Pb, Cd and As, and thus should go through further treatment.

Contents of heavy metals follow a sequence of Zn > Pb > Cu > Cr > As > Ni > Cd approximately Hg in both plants, and total heavy metals account for ≤1% by mass of fly ash. Leaching toxicity shows that major hazardous heavy metals are As, Cd, Hg, Pb and Zn. Soluble Cd mainly exists in the acid, exchangeable and water soluble phases, soluble Hg mainly in acid and water soluble phases, while soluble Pb mainly appears in acid soluble and soluble Zn and As mainly in bound to organic matter and acid soluble, in terms of fly ash At room temperature.

Contents of calcium-silicates, aluminosilicate, and glass phases increases steadily from the first step to the fifth step, indicating that extraction ratios of heavy metals should decrease with the increase of their content. In other words, content of glass phases, as well as that of Ca, Si, and Al can influence leaching behavior of heavy metals, and thus fly ash can be stabilized by increasing their content in the stabilized body.

Measured leaching results are well consistent with the simulated ones for Cu, Pb and Zn at pH between 0 and 13, while for Ni, As, Cr, Hg, and Cd, they are consistent only when pH is over 1.5, 2.5, 5, 9.5, and 11 respectively, indicating that simulated ratios reflect actual leaching behavior of the former three metals but can’t always reflect that of the latter five heavy metals. Cu, As, Cr, Hg, Cd, and Ni can be leached out in strong acidic conditions, but relatively safe under strong basic conditions. In terms of Zn and Pb, it is easy for them to be leached out at both strong acidic and basic environment.

Volatility ratio of heavy metals follows a sequence of Hg > Pb > Cd, As > Zn > Cr, Ni, and Cu. It is easy for Hg, Pb, As and Cd to volatilize, which fly away almost entirely at 1,200°C. It is relatively difficult for Zn to volatilize, around 60% of which flies away at 1,200°C, and it is difficult for Cr, Ni and Cu to volatilize, volatilization ratios of which are no more than 10%.