Influences of ammonia contamination on leaching from air-pollution-control residues

Abstract

Application of selective non-catalytic reduction systems at municipal solid waste incinerators (MSWIs) often involves over-stoichiometric injection of ammonia into flue gases. Un-reacted ammonia may be deposited on fly ash particles and can ultimately influence the leaching behaviour of air-pollution-control (APC) residues. Batch tests were conducted to investigate the impacts of ammonia levels on leaching of a range of metals (sodium, potassium, calcium, aluminium, chromium, iron, lead, cadmium, copper, nickel and zinc), as well as chloride and dissolved organic carbon (DOC). Specific conductivity was also identified to reflect the soluble components. The results showed that with ammonia concentrations rising from a background level of 4 to 26,400 mg l⁻¹, the specific conductivity increased by 2–7 times as pH varied from alkaline to acidic values. DOC release was also significantly enhanced with high ammonia levels of 1400 mg l⁻¹ or higher at pH > 9; however at these high ammonia concentrations, the role of DOC in cadmium, copper, nickel and zinc leaching was negligible. Based on the experimental data, chloride, sodium and potassium were leached at high concentrations regardless of pH and ammonia concentrations. For aluminium, chromium, iron and lead, ammonia had little impact on their leaching behaviour. With respect to cadmium, copper, nickel and zinc, high ammonia concentrations significantly increased leaching in the pH range of 8–12 due to the formation of metal-ammonia complexes, which was also proved in the speciation calculations. However, the overall results suggest that typical levels of ammonia injection in MSWIs are not likely to affect metal leaching from APC residues.

Keywords

Air-pollution-control residues ammonia heavy metals leaching complexation metal speciation

Introduction

Within recent decades, increasing focus has been directed towards NO_x emissions from municipal solid waste incinerators (MSWIs). Most modern MSWIs have installed either SCR (selective catalytic reduction) or SNCR (selective non-catalytic reduction) gas cleaning systems for reducing NO_x emissions. The SNCR processes often involve loss of un-reacted ammonia into the flue gases, that is, ammonia slip (Jødal et al., 1991; Nguyen et al., 2010; Zandaryaa et al., 2001). This ammonia may subsequently be adsorbed onto fly ash particle surfaces, thereby increasing ammonia concentrations in ash (Furrer et al., 1998). From typical SCR and SNCR installations, ammonia deposition on ash residues may correspond to about 50–600 ppmw (parts per million by weight) (EPRI, 1997). In some cases, ammonia may be used as a conditioning agent for improving electrostatic precipitator (ESP) performance, which can also generate ammoniated ash residues. From ESP conditioning, the typical range of ammonia contamination of ash is around 700–1200 ppmw (EPRI, 1997); under high slip conditions, the level of contamination can reach 2500 ppmw (Bittner et al., 2001). Due to the fact that ammonia release from ash residues could occur upon heating or contact with moisture, much lower concentrations are expected in real-life operations.

In addition to odour generation, ammonia in ash residues can cause complexation reactions and may enhance leaching of heavy metals such as cadmium, copper, cobalt, nickel, zinc, mercury and silver, especially under alkaline conditions (Speight, 2005; Stumm and Morgan, 1996). According to the EPRI technical report (EPRI, 2005), ammonia could significantly enhance nickel, cadmium and copper leaching from ammoniated coal fly ash at alkaline pH and very high ammonia concentrations (5000 mg l⁻¹ or higher), due to the formation of metal-ammonia complexes. Wang et al. (2006, 2007) also confirmed that large amounts of ammonia addition (final ammonia concentrations greater than 5000 mg l⁻¹) could dramatically increase leaching of cadmium, copper and mercury from coal fly ash in the pH range of 8–12, but the effect of ammonia was not significant even at pH > 8 when the ammonia concentration was less than 1000 mg l⁻¹. However, concerning impacts of ammonia on leaching, most studies involve coal fly ash; air-pollution-control (APC) residues produced in MSWI, which have complex and different physicochemical characteristics (Chandler et al., 1997), have been less considered. Furthermore, literature available is mainly focused on the ammonia dependency of leaching of trace metals from fly ash, whereas very little information has been found regarding impacts of ammonia on major element leaching, for example, aluminium, potassium, sodium and chloride, which may subsequently influence the mineralogical composition of fly ash and the processes controlling leaching of metals (Astrup et al., 2006).

Under leaching conditions, except for ammonia, some other ions, for example, chloride, bromide, hydroxide and sulphide, are also inclined to be bound to metal ions in ash (Stumm and Morgan, 1996) and may compete with ammonia for complexation. As is known, APC residues contain high levels of chlorine, and the chlorine concentrations can range from 4.8% to 22.0% (w/w) due to the different scrubbing processes (Hjelmar, 1996). Virtually all chlorine in fly ash is in the form of chlorides, that is, CaCl₂, NaCl and KCl (Zhu et al., 2008), which can form soluble chloride complexes with metals and may mobilize cadmium, lead, copper and zinc upon leaching (Cernuschi et al., 1990; Hjelmar, 1996). Although bromide, iodide, sulphide, etc., can act as ligands in metal complexes as well, their contents are very low in fly ash, and thus their impacts on metal complexation are negligible. As for hydroxide, the concentration of hydroxide ions is directly related to the pH value of the leachant. On the other hand, pH is often the most important factor in controlling the leachability of heavy metals from ash residues (EPRI, 2005). Previous studies have indicated that the solubility of heavy metals, such as cadmium, mercury and silver, increases with descending pH (Astrup, 2004; Fleming et al., 1996). For some amphoteric metals, namely lead, zinc, aluminium and chromium, their leaching behaviour is characterized by high leachability at low and high pH, but much lower at neutral or moderate pH (Gupta et al., 2005; Quina et al., 2009). In addition, it should be noted that in the presence of ammonia, only the free ammonia is capable of forming complexes with metal ions (Stumm and Morgan, 1996). The speciation of ammonia in aqueous solutions is also strongly dependent on pH. Besides inorganic ligands, dissolved organic carbon (DOC) in fly ash leachate also has a high affinity for binding cadmium, copper and zinc ions to form soluble metal-DOC complexes. Meima et al. (1999) reported that 95–100% of the dissolved copper was bound to DOC in leachate from MSWI bottom ash. Nevertheless, ammonia may affect DOC leaching and further change the metal speciation in leachate. Therefore, the interactions of pH, ammonia, chloride and DOC could make the leaching process more complicated. However, literature focusing on the competitive complexation of ammonia, chloride and DOC to metal ions under various pH conditions is very scarce.

The objectives of this paper are (1) to assess the effect of ammonia on leaching of selected elements (chloride, sodium, potassium, calcium, aluminium, chromium, iron, lead, cadmium, copper, nickel and zinc) from APC residues collected in MSWIs and evaluate the potential environmental impacts of ammonia-contaminated ash; (2) to determine the ammonia effect on DOC leaching in leachate; and (3) to develop improved metal speciation profiles in the ash-ammonia-chloride-metal-water system.

Materials and methods

Residue samples

The untreated semidry APC residues originated from Vestforbrænding located in Copenhagen and equipped with the ammonia-based SNCR system. During three to four weeks of the regular plant operation, about 10 kg of the residue samples were collected from an ESP, which had a dry matter ratio of 85.78%. All samples were kept in air-tight plastic containers throughout the study. The APC residues had a high natural pH of 12.05 in a suspension of liquid to solid ratio (L/S) 10 l kg⁻¹ and an ammonia concentration of 40.8 ppmw on residue samples. The composition of the residue samples was analysed by an accredited laboratory (ALS, Scandinavia, Denmark) and is shown in Table 1. From Table 1, it could be seen that calcium was the most major element in APC residues, while chlorine followed closely. The chlorine content in semidry residues was 6.4%, equal to 74,609 mg kg⁻¹ dry substance. The average amount of the leachable chloride was up to 83,113 mg kg⁻¹ dry matter leaching at L/S = 10, which was much greater than the European directive limits for hazardous waste landfill (EC, 2002). Because of different analytical means, there was some discrepancy in chlorine concentrations, but it could be inferred that most of chlorine in APC residues was in the form of soluble chlorides.

Table 1.

Composition of the semidry air-pollution-control residues.

Component	Content	Component	Content
C (%)	2.50	Zn (mg kg⁻¹)	13,490
TOC (%)	1.5	Mg (mg kg⁻¹)	10,400
H (%)	1.3	Fe (mg kg⁻¹)	7700
N (%)	<0.10	Pb (mg kg⁻¹)	1680
S (%)	3.5	Ba (mg kg⁻¹)	759
P (%)	0.59	Cu (mg kg⁻¹)	747
Cl (%)	6.4	Mn (mg kg⁻¹)	699
Ca (mg kg⁻¹)	153,000	Sr (mg kg⁻¹)	360
Si (mg kg⁻¹)	49,000	Cr (mg kg⁻¹)	148
K (mg kg⁻¹)	36,400	As (mg kg⁻¹)	97
Na (mg kg⁻¹)	31,000	Cd (mg kg⁻¹)	83
Al (mg kg⁻¹)	29,300	Ni (mg kg⁻¹)	44

TOC: total organic carbon.

Batch leaching experiments

The residue samples were mixed and quartered to get subsamples for investigation. All the batch leaching tests would comply with European standard PrEN 14429 (CEN, 2003). Preliminary determination of the acid neutralization capacity (ANC) was conducted before the pH-static leaching testing.

The pH-static leaching testing was carried out at a room temperature of 20±5^oC and divided into six batches corresponding to six different desired initial ammonia concentrations. Based on the ANC testing results, nitric acid was used to adjust the pH values to their predefined values covering the pH range of 4–12 in the same batch. During each testing, 9 g of a residue sample was added into each 100 ml polyethylene (PE) bottle and then mixed with 90 ml leachant after compensating the moisture content in the sample to maintain an L/S ratio of 10. The leachant was prepared from Mill-Q water, 25% ammonia solution and 65% nitric acid. All chemicals used were analytical reagents and from Merck, Germany. Then, all bottles were tightly screwed and agitated in an end-over-end tumbler for 48 h at a speed of 8 revolutions per minute (rpm) to approach equilibrium. After 48 h, the suspension was settled for its measurement of pH and specific conductivity directly in the bottle. The supernatant samples were further filtrated through syringe filters (0.45 μm nylon membrane) and 20 ml of eluates were immediately taken to be acidified with concentrated nitric acid to pH less than 2. Subsequently, all the eluate samples were stored in a refrigerator until analysis.

Chemical analysis

All heavy metals in eluates were analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). A MeterLab series Radiometer analytical (Hach, USA) was used for pH and specific conductivity measurements, and TitraLab 960 and 965 Titration workstations by computer-controlled silver nitrate titration (Hach, USA) for chloride analysis. DOC was measured using a Shimadzu TOC Analyzer. The total soluble ammonia concentration in the APC residue leachate was determined by Photometer Spectroquant Nova 60, test kits of Method 00683 and Method 14752 ( Merck, Germany).

Results and discussion

Ammonia impacts on conductivity and Cl, Na, K, Ca leaching

Figure 1 indicates specific conductivity variations and the leaching trends of chloride, sodium, potassium and calcium with pH under different ammonia concentrations.

Figure 1.

Ammonia impacts on conductivity and leaching of chloride, sodium, potassium and calcium from air-pollution-control residues as a function of pH under different ammonia concentrations (temperature: 20±5^oC; L/S = 10; equilibration time: 48 h).

The values of specific conductivity were basically related to soluble salts in solutions, such as NaCl, KCl and some calcium salts. From the conductivity curves, it could be seen that the specific conductivity of raw residues increased from 26.2 to 68.5 mS cm⁻¹ with pH varying from alkaline (pH = 12.05) to acidic value (pH = 3.66), and high ammonia concentrations resulted in high conductivities in suspension. It could be inferred that acidic conditions and high ammonia concentrations enhanced leaching of readily soluble components. Figure 1(a) also indicated that the conductivity curves corresponded to leaching of calcium (see Figure 1(e)).

Figure 1(b) showed that raw APC residues had a high chloride leaching regardless of pH, which was in good agreement with the previous studies (Jung and Osako, 2009). In addition, chloride leaching also exhibited independence on ammonia concentrations. Chloride leached from APC residues was respectively 8666, 8132, 8105, 8343, 8215 and 8359 mg l⁻¹ with ammonia concentrations increasing from 4 to 26,400 mg l⁻¹, exceeding the waste acceptance criteria for landfills (EC, 2002). However, high levels of leachable chlorides could cause many difficulties involved in management of these residues. The high concentration of chlorides not only poses a risk to surface and groundwater, but it may also enhance the leachability of heavy metals by promoting the formation of chloride complexes (Chandler et al., 1997). As shown in Figures 1(c) and (d), sodium and potassium leaching were also pH- and ammonia-independent.

Ammonia impacts on Al, Cr, Fe and Pb leaching

In the pH-static leaching procedure, a certain amount of ammonia solution was added in each batch test to reach desired ammonia contents of 300, 1000, 3000, 10,000 and 30,000 mg l⁻¹ in leachate. The average final ammonia concentrations were measured to be 67, 540, 1400, 3250 and 26,400 mg l⁻¹, respectively, in the eluate samples, which were 17–6600 times higher than ammonia leaching in the raw residues (4 mg l⁻¹). The aim of high ammonia concentrations was to better investigate the ammonia impacts on the emissions of the principal metals of concern (aluminium, chromium, iron, lead, cadmium, copper, nickel and zinc) from APC residues. The leaching behaviour of aluminium, chromium, iron and lead is plotted as a function of pH in Figure 2.

Figure 2.

Ammonia impacts on leaching of aluminium, chromium, iron and lead from air-pollution-control residues as a function of pH under different ammonia concentrations (temperature: 20±5^oC; L/S = 10; equilibration time: 48 h).

From Figure 2, ammonia did not have significant impacts on leaching of aluminium, chromium, iron and lead. It is also indicated that aluminium was the most available element for leaching, and when the residues were in contact with water, lead may be the major problem due to the amphoteric nature. From the plot of lead leaching (Figure 2(d)), the amphoteric nature (″V″ shape) was well observed, and even at its natural pH of 12.05, the legal limit was easily exceeded (EC, 2002); the solubility of lead reached its minimum at pH around 9.

Figure 2(b) shows that chromium behaved much differently than lead, and the concentration of chromium achieved the lowest value at pH of approximately 5. This could be explained by the adsorption-desorption behaviour of chromium on the ash surface. In the environment, chromium exists in Cr (III) and Cr (VI). Under leaching conditions and pH around 5, Cr (III) was almost completely adsorbed by the ash surface and the Cr (VI) adsorption ratio reached a maximum as well (EPRI, 2005), both of which resulted in a low leachability of chromium. In the presence of ammonia, there was a very minor effect in chromium leaching at any ammonia concentration over the entire pH range of this study. The reason may be that Cr (III) did not react with ammonia to form complexes, and Cr (VI) occurred as anions (CrO₄^2–, HCrO₄⁻ and Cr₂O₇^2–) that were also unaffected by ammonia. In solution, aqueous ammonia reacts with Fe²⁺ or Fe³⁺ to produce precipitates as Fe(OH)₂ or Fe(OH)₃, but not ammonia complexes. Therefore, ammonia also imposed very little impacts on the leaching behaviour of iron at any pH, which agreed with the experimental results in Figure 2(c).

Ammonia impacts on Cd, Cu, Ni and Zn leaching

Figure 3 shows the leaching results of cadmium, copper, nickel and zinc as a function of pH under different ammonia concentrations varying from 4 to 26,400 mg l⁻¹. For raw residues without ammonia addition, the leachability of all metals of interest was negligible at weakly acidic or neutral pH, but became very effective at acidic pH lower than 6. However, in the presence of a high ammonia concentration, the leaching behaviour of APC residues changed a lot, especially in alkaline pH conditions. As the pH increased from 8 to 12, the amounts of the four metals released increased first and then decreased with raising pH, and reached their peaks at pH around 9 for 3250 and 26,400 mg l⁻¹ ammonia leachants. High levels of ammonia (greater than 1400 mg l⁻¹) could cause an increased metal leaching by several orders of magnitude in the pH range of 8–12. However, when the ammonia concentration was less than 1400 mg l⁻¹, the effect of ammonia was not significant even at pH above 8. The leaching curves of cadmium, copper, nickel and zinc exhibited similar results, which showed that the ammonia effect was only significant at alkaline pH (8–12) and very high ammonia concentrations (3250 mg l⁻¹ or higher), but little at acidic pH. In this study, ammonia leaching from industrial MSWIs did not significantly affect leaching of metals from APC residues and was not an environmental concern.

Figure 3.

Ammonia impacts on leaching of cadmium, copper, nickel and zinc from air-pollution-control residues as a function of pH under different ammonia concentrations (temperature: 20±5^oC; L/S = 10; equilibration time: 48 h).

Ammonia impacts on DOC solubility

Since DOC was important in determination of heavy metals mobilization, DOC solubility may be another process controlling leaching of metals. However, ammonia may influence DOC leaching in leachate. The relationships between DOC solubility, pH and ammonia concentrations are presented in Figure 4.

Figure 4.

Ammonia impacts on dissolved organic carbon solubility as a function of pH under different ammonia concentrations (temperature: 20±5^oC; L/S = 10; equilibration time: 48 h).

From Figure 4, it can be seen that DOC remained stable under low pH conditions and low ammonia concentrations, while high ammonia levels of 1400 mg l⁻¹ or higher at pH > 9 promoted the solubilization of DOC significantly. Moreover, the application of ammonia at concentrations of 4, 67, 540, 1400, 3250 and 26,400 mg l⁻¹ in eluates resulted in average DOC concentrations of 0.84, 1.45, 1.22, 3.08, 5.44 and 9.65 mg l⁻¹, respectively. It could be confirmed that ammonia generally increased DOC concentrations, which agreed well with Tomasiewicz and Henry’s (1985) findings. The reason may be that in the presence of ammonia, ion-exchange and complexation reactions could take place between ammonia and the functional groups (carboxyl, hydroxyl and phenolic groups) present in DOC as humic substances and thus form water-soluble ammonium humates leading to high DOC solubility.

As described previously (Figure 3), high levels of ammonia could significantly increase leaching of cadmium, copper, nickel and zinc in the pH range of 8–12. However, DOC was also extremely important in the speciation of these metals. On the other hand, ammonia may change the significance of metal-DOC complexation in metal leaching. The solubility of cadmium, copper, nickel and zinc versus DOC profiles under different ammonia concentrations is depicted in Figure 5.

Figure 5.

Relationship between dissolved organic carbon, ammonia and leaching levels of cadmium, copper, nickel and zinc (temperature: 20±5^oC; L/S = 10; equilibration time: 48 h).

From Figures 5(c) and (d) it can be seen that nickel and zinc leached out represented good linear relationships with the amounts of DOC under ammonia concentrations lower than 26,400 mg l⁻¹, and indicated that nickel and zinc leaching had a close correlation with DOC. However, as for the four metals, in particular cadmium and copper, when the ammonia concentration was higher than 1400 mg l⁻¹, their leaching properties changed a lot owing to complexation with ammonia, and complexation with DOC became negligible. In addition, most of the cadmium, copper, nickel and zinc was mobilized at DOC concentrations around 1.5 mg l⁻¹, corresponding to weakly acidic conditions. Therefore, it could be suggested that the leaching of metals was correlated more strongly with pH than with DOC concentrations.

Speciation of Cd, Cu, Ni and Zn in APC residues

Since ammonia has a high affinity for complexation with many cationic metals, ammonia-promoted leaching was hypothesized to be caused by the formation of some soluble metal-ammonia complexes. Ammonia exists in two forms, as free ammonia (NH₃) and ammonium ion (NH₄⁺). At 25^oC, un-ionized ammonia (free ammonia) becomes predominant under pH greater than 9.25. The molar fraction of un-ionized ammonia and free ammonia concentration in ammonia solutions can be determined by Equation (1) and Equation (2) respectively:

f_{{NH}_{3}} = {[1 + 10^{(9.25 - pH)}]}^{- 1}

(1)

[{NH}_{3}] = f_{{NH}_{3}} {NH}_{3 T} = {[1 + 10^{(9.25 - pH)}]}^{- 1} {NH}_{3 T}

(2)

where f_NH3 is the molar fraction of un-ionized ammonia, 9.25 is the pKa (logarithms of dissociation constants) value of ammonium ion at 25^oC, [NH₃] is the concentration of free ammonia and NH_3T is the total ammonia concentration. From Equation (1), it can be concluded that at pH 9.25, the concentration of both species of ammonia is equal; at pH higher than 9.25, free ammonia becomes predominant, especially at pH above 10.5, with free ammonia up to 95%.

According to the previous studies (Soderberg and Meade, 1991), the impacts of ionic strength on the ammonia fraction were much smaller than those of pH and temperature; on the other hand, ionic strength did not have much significant effect on stability constants either (Majlesi et al., 2011). Thus in this research, the impacts of ionic strength on stability constants were not taken into account, and all the logarithms of cumulative stability constants (lg β_n) are cited from the literature (Speight, 2005; Stumm and Morgan, 1996) without any modification.

In sample handling prior to leaching testing, in order to prevent carbonation, contact time with the air was minimized. However, during the leaching tests, a series of 100 ml PE bottles was used and approximately 17 ml of headspace left above the leaching solution, which resulted in a headspace ratio (the headspace to liquid ratio) of around 0.20. The headspace above the leaching liquid could increase the pH, redox potential and the degree of carbonation, but it did not significantly affect metal leaching when the headspace ratio was lower than 0.5 (Vann, 2003). The NT TECHN report (Nordtest, 2000) also demonstrated that the presence of headspace did not have a major influence on the leaching behaviour of ashes in the pH-dependent tests. As for a headspace ratio of 0.2 in our research, the uptake of CO₂ (carbonation) from the headspace was quite limited, and its impact on metal speciation could be negligible. In addition, for simplicity, both organo-metal complexes formation and sorption processes were omitted. The modelling of metals speciation was only focused on analysis of the toxic metals that were notably affected by ammonia, that is, cadmium, copper, nickel and zinc. Based on the stability constants of the above-mentioned metal complexes and the dissociation constant of ammonia, the speciation fractions of metal-ammonia, metal-chloride and metal-hydroxide complexes could be calculated. The speciation of cadmium, copper, nickel and zinc as a function of pH in the presence of 3250 and 26,400 mg l⁻¹ ammonia is shown in Figure 6.

Figure 6.

Speciation of cadmium, copper, nickel and zinc as a function of pH in the presence of 3250 and 26,400 mg l⁻¹ ammonia (temperature: 25^oC).

From Figure 6, it can be seen that cadmium, copper, nickel and zinc exhibited similar speciation. At acidic pH lower than 6, the four metals were mainly released in the form of free metal ions and chloride complexes, which was the reason why heavy metals were easily leached out under acidic pH conditions. In the pH range of 8–12, cadmium, copper, nickel and zinc were mobilized due to the formation of soluble metal-ammonia complexes, which agreed with the leaching experimental results. At pH greater than 12, metal hydroxides were predominant species.

Figure 6 also indicates that high ammonia concentrations led to more metal-ammonia complexes of high coordination numbers in the alkaline pH range. In the presence of 26,400 mg l⁻¹ ammonia, the fractions of Cd(NH₃)₄²⁺, Cu(NH₃)₄²⁺ and Zn(NH₃)₄²⁺ reached maximums at pH around 9, and levelled off at pH 10–12; the fraction of Ni(NH₃)₆²⁺ maintained a high level over a wider pH range of 9–13, and was much greater than that with 3250 mg l⁻¹ ammonia. During the pH-static leaching testing, the untreated eluate samples with 3250 and 26,400 mg l⁻¹ ammonia showed colour changes from light blue to blue as pH varied from 8 to around 11, and then light blue again at pH above 11. The reason for this may be that most of copper in APC residues was leached out by the formation of copper ammonia complexes in the pH of 8–11, among which dark blue Cu(NH₃)₄²⁺ was the dominant species under high ammonia concentrations. In addition to Cu(NH₃)₄²⁺, violet Ni(NH₃)₆²⁺ was a minor contributor to the leachate colour formation under an ammonia concentration of 26,400 mg l⁻¹. Therefore, it could be concluded that cadmium, copper, nickel and zinc dissolved mainly as Cd(NH₃)₄²⁺, Cu(NH₃)₄²⁺, Ni(NH3)6²⁺ and Zn(NH₃)₄²⁺ under alkaline pH conditions (9 to 12) and a very high ammonia concentration (26,400 mg L⁻¹).

Although ANC preservation is a potential method to immobilize most metals, ammonia on ammoniated ash could increase the risk of cadmium, copper, nickel and zinc leaching at alkaline pH, and probably influence both disposal and utilization of ash; this is where further research should be conducted.

Conclusion

In this study, fundamental information of ammonia impacts on leaching characteristics of APC residues was obtained. For six major elements, chloride, sodium, potassium, calcium, aluminium and iron, ammonia did not show significant effects on their leaching. However, ammonia could significantly increase the specific conductivity in leachate at pH 3.66–12.05. High ammonia concentrations of 1400 mg l⁻¹ or higher also enhanced DOC solubility at pH greater than 9; however, in this case the significance of DOC on cadmium, copper, nickel and zinc leaching was negligible compared to ammonia impacts.

With respect to trace metals including chromium, lead, cadmium, copper, nickel and zinc, ammonia impacts varied depending on binding affinities of ammonia towards metal ions. Specifically, ammonia imposed little impacts on chromium and lead leaching, whereas ammonia increased cadmium, copper, nickel and zinc leaching by several orders of magnitude over the pH range of 8–12 when ammonia concentrations were 3250 mg l⁻¹ or greater in the leachate.

In order to understand the leaching process, the improved speciation profiles of cadmium, copper, nickel and zinc in leachate with 3250 and 26,400 mg l⁻¹ ammonia were developed. The results showed that at acidic pH lower than 6, the four metals were mobilized in the form of free metal ions and chloride complexes, while at pH 8–12, metals were released due to the formation of ammonia complexes, and at pH greater than 12, metal hydroxides were predominant species. In addition, high ammonia concentrations favoured the formation of metal-ammonia complexes of high coordination numbers, that is, Cd(NH₃)₄²⁺, Cu(NH₃)₄²⁺, Ni(NH₃)₅²⁺, Ni(NH₃)₆²⁺ and Zn(NH₃)₄²⁺ at alkaline pH (8–12).

Based on our study, the effect of ammonia was significant only at alkaline pH and very high ammonia levels of 3250 and 26,400 mg l⁻¹, which were much higher than those in ash leachate from a typical SNCR system (for instance, ammonia leaching in the investigated ash residues from the industrial MSWI was only 4 mg l⁻¹). Therefore, it could be concluded that ammonia slip in de-NO_x processes is unlikely to cause great concern with regard to metal leaching from MSWI fly ash.

Footnotes

Declaration of conflicting interests

None declared.

Funding

This work was supported by the National Natural Science Foundation of China (No. 50874134) and the Technical University of Denmark (DTU). Support from China Scholarship Council ensured that the first author could study at DTU.

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