Trace element extractability from fly ash as a function

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012

Trace element extractability from fly ash as a function of pH Mikko Mäkelä Aalto University, School of Chemical Technology P.O. Box 16400, FI-00076 Aalto, Finland [email protected]

Risto Pöykiö City of Kemi Valtakatu 26, FI-94100 Kemi, Finland [email protected]

Hannu Nurmesniemi Stora Enso Oyj, Veitsiluoto Mill FI-94800 Kemi, Finland [email protected]

Olli Dahl and Gary Watkins Aalto University, School of Chemical Technology P.O. Box 16300, FI-00076 Aalto, Finland [email protected], [email protected]

Abstract: Trace element mobility regarding waste disposition in landfill environments is largely dependent on the pH of the surrounding media. Hence, trace element (As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, Se, Sb, and Zn) and macro element (Al) extractability from fly ash was investigated as a function of pH. Fly ash samples obtained from a fluidized bed boiler of a small-sized municipal district heating plant (32 MW) incinerating peat (60 %) and forestry and sawmill residues (40 % of bark, wood chips, and sawdust) were subjected to a pH range of 0.7 - 12.3 at a liquid-to-solid ratio of 10 L·kg-1. Consequently, the following fractions were attained: (i) trace elements (As, Cd, Cr, Cu, Ni, Pb, Sb, and Zn) mainly extractable under extremely or strongly acidic conditions; (ii) elements (Ba and Al) expressing amphoteric behavior, i.e., extractable under both acidic (i.e., low pH) and alkaline conditions (i.e., high pH) but relatively insoluble at circumneutral pH; and (iii) trace elements (Mo and Se) indicating oxyanionic behavior, i.e., commonly more mobile under alkaline conditions.

Keywords: ash; pH; extractability; municipal district heating plant; solubility; waste. Introduction During combustion a majority of trace elements derived from the fuel are retained in the ash. Elements with a low volatility concentrate in the bottom ash, while more volatile elements 1193

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 concentrate in the fly ash. Owing to this separation effect in combustion plants, the individual ash fractions have a different chemical composition. During disposal and storage in landfills, the ash is most likely subjected to leaching by rainwater and comes into contact with other substances, such as chloride, sulfate, and organic matter able of complex formation with trace elements. Although the vegetation cover and physical barriers (i.e., top cover, liners) reduce the amount of leachate from the landfill, according to Sabbas et al. (2003) complete prevention of leachate formation over an extended time scale cannot be ensured. Hence, changes in pH may also occur in a landfill environment due to, e.g., sulfide oxidation, microbial activity, the buffering capacity of natural waters, acidic deposition, and atmospheric CO2. Surrounding pH is a major factor influencing element mobility in landfills. Due to atmospheric CO2, sulfur (SOx), and nitrogen oxide compounds (NOx), rainwater is an acidic solution, and therefore in tandem with watercourses provides a potential pathway for detrimental trace elements to re-enter the food chain and human life cycles from disposal sites. In real-world leaching scenarios, the pH of fly ash originating from the incineration of forest residues and peat is initially strongly alkaline (pH ~ 10 - 12.5) and the ashes have a very large acid neutralizing capacity. However, in the long-term the acid neutralizing capacity of ash in landfills decreases, allowing decrease in pH to values as low as 3 - 5. This phenomenon has also been reported by Ecke et al. (2002), especially if the ash is co-disposed with organic waste. Thus, ash disposal sites in landfills may cause unacceptably high contaminant releases into the environment as the pH decreases, even though it has been demonstrated that various types of ashes are able of maintaining alkaline pH-values over extended time scales.

Experimental Sampling The investigated fly ash samples were obtained from a municipal district heating plant located in Northern Finland. The plant has a 32 MW bubbling fluidized bed (BFB) boiler for incinerating peat and forest residues (i.e. wood chips, sawdust and bark), and it began operation in 2006. Presently, approximately 60 % of the energy produced by the BFB boiler originates from the incineration of peat, and ca. 40 % from the incineration of forest residues (i.e., bark, wood chip, sawdust). The plant produces approx. 2700 metric tons of ash (i.e., bottom ash and fly ash) per annum, which is generally disposed of in a landfill site. The fly ash was sampled from the outlet of the biomass-fired system during November 2009. The incineration temperature of the bed sand varies between 810 and 830 °C and in the upper zone of the boiler between 1100 and 1200 °C. The sampling period represented normal process operating conditions for the plant, e.g., O2 content and temperature. A coning and quartering method (Gerlach et al., 2002) was applied repeatedly to reduce the ash sample to a size suitable for the laboratory analysis. After sampling, the sample was stored in a polyethylene bottle in a refrigerator (+4 °C) until further analysis. Determination of relevant physical and chemical properties The pH and electrical conductivity (EC) of the fly ash were determined with a combined pH/EC analyzer equipped with a Thermo Orion Sure Flow pH electrode (Turnhout, Belgium) and a Phoenix conductivity electrode with a cell constant of 1.0 (Phoenix Electrode Company, USA). Determination of the pH and EC were carried out according to the European standard SFS-EN 13037 at a solid-to-liquid (S/L) ratio of 1:5. Determination of the dry matter content (DMC) of the ash was carried out according to the European standard SFS-EN 12880, in which a sample is dried overnight to a constant mass in an oven at 105 °C. The loss-on-ignition (LOI) value was determined according to the European standard SFS-EN 12879, in which an oven-dried (105 °C) sample is dry-digested overnight in a muffle furnace (Box Furnace, Lindberg, Blue M, Asheville, USA) at 550 °C. Determination of the total organic carbon (TOC) content in the ash sample was 1194

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 carried out with a LECO CHN-600 analyzer (Leco Inc., USA) according to the European standard SFS-EN 13137, in which a sample is combusted and the evolved carbon dioxide is measured by infrared spectrometry. A comprehensive review of the European SFS-EN standards, analytical methods and instrumentation is given in our previous study (Nurmesniemi et al., 2005). Mineralogical characterization For investigating the mineralogy of the fly ash, an X-ray diffractogram of a powdered sample was obtained with a Siemens D 5000 diffractometer (Karlsruhe, Germany) using CuK radiation. The scan was run from 2 to 80° (2-theta-scale), with increments of 0.02° and a counting time of 1.5 seconds per step. The diffractometer was operated at an acceleration voltage of 40 kV and a current of 40 mA. Subsequent peak identification was performed with the DIFFRACplus BASIC Evaluation Package PDFMaint 12 (Bruker axs, Germany) and ICDD PDF-2 Release 2006 package software package. Table 1 - Proportions of fly ash, HNO3 (1.25 M) and H2O (ultrapure), with the pH of the extracts used for extraction of the ash as a function of pH using a liquid-to-solid ratio (L/S) of 10 L·kg-1. Extract (L/S 10 L·kg-1)

Ash (g; d.w.)

HNO3 (1.25 M; mL)

H2O (mL)

pH

1

45

0

450

12.3

2

45

22

428

10.5

3

45

45

405

9.0

4

45

55

395

7.9

5

45

90

360

6.4

6

45

110

340

5.2

7

45

135

315

4.0

8

45

270

180

3.2

9

45

315

135

2.7

10

45

405

45

1.0

12

45

450

0

0.7

Element extractability as a function of pH The extraction procedure given in Table 1 was used for the determination of extractable concentrations of elements Al, As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, Se, Sb, and Zn from the fly ash as a function of pH. During the procedure, the ash was extracted at a liquid-to-solid ratio (L/S) of 10 L kg-1 while the extractants contained different proportions of HNO3 (1.25 M) and ultrapure H2O generating respective pH values in the range of 0.7 and 12.3. This procedure simulates extremely and/or strongly acidic, circumneutral and strongly and/or extremely alkaline conditions. Nitric acid was used to acidify the extractant as it simulates acidic rainwater. Furthermore, nitric acid was chosen for minimizing the likelihood of precipitation (e.g., as occurs with sulphuric acid), complexation (e.g., with organic acids or hydrochloric acid) or analytical interferences. The extraction was carried out in polypropylene bottles by shaking 45 g of ash (d.w.) with 1195

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 450 mL of the extractant for 24 hours with an end-over-end shaker. Thus, the liquid-to-solid ratio (L/S 10 L·kg-1) and the extraction time in the extraction procedure (see Table 1) were equivalent to European standard SFS-EN 12457-4, which is a one stage batch test designed to provide information on the leachability of granular wastes (Álvarez-Ayuso et al., 2006). Subsequent to extraction, the extract was separated from the solid residue (i.e., the undissolved ash) by filtration through a 0.45 µm Shleicher and Schuell (Whatman, Dasse, Germany) membrane filter. The pH of the extract was then measured. After addition of 200 µL of 65 % HNO3 in the supernatant phase, the analytes were stored in a refrigerator (+ 4°C) until element determinations. The element concentrations in the extracts were determined either with an inductively coupled plasma mass spectrometer (ICP-MS) or with an inductively coupled plasma optical emission spectrometer (ICP-OES). In this context it is worth noting that, according to European standard SFS-EN 12457-4 (Álvarez-Ayuso et al., 2006), the extraction must be performed using undried samples. However, the extractable concentration of elements must be calculated on a dry weight (d.w.) basis. It is preferable to avoid sample drying prior to extraction as physical modification of the samples may alter their properties thus affecting the leaching behavior.

Results and discussion Mineralogical composition of sampled fly ash According to the XRD diffractogram in Figure 1, oxides (hematite), sulfates (anhydrite) and silicates (albite, biotite, microcline and quartz) were the only mineral classes and minerals which were identified in the fly ash. In this context it is worth noting that, although an XRD analysis can be useful to identify chemical species of crystalline particles in ash, in our case only a few minerals could be identified. An XRD spectrometer is unable to identify the amorphous (glass) phase (i.e., non-crystallized matter), and the respective detection limit is normally in the range of 1 - 2 % (w/w).

Bi

400

Fly ash

300

200

(Counts)

Lin (Count s)

Q An n

Bi

Bi He

Mi Al

100

Bi

Al

Bi

Al

0 5

10

20

30

40

50

60

70

80

2-Theta - Scale

Figure 1 XRD pattern of the fly ash sample. Mineral abbreviations and their abundances (%) are: Al = Albite [NaAlSi3O8 ; 40.7 %]; An = Anhydrite [CaSO4; 10.6 %]; Bi = Biotite [K(Fe,Mg)3 AlSi3O10(F,OH)2; 15.7 %]; He = Hematite [Fe2O3; 3.4 %]; Mi = Microcline [KAlSi3O8 ; 14.0 %]; Q = Quartz [SiO2 ; 15.6 %]. Relevant physical and chemical properties of the fly ash sample are given in Table 2. The results represent mean values attained from duplicate samples and are expressed on a dry weight (d.w.) 1196

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 basis. However, the standard deviations are not given to all parameters as the duplicate samples had exactly the same concentrations. As can be noted from Table 2, the pH of the ash was strongly alkaline (pH 10.8), indicating a liming effect. The low LOI value (1.1 %) indicates that the fly ash is virtually inorganic in nature. The relatively low total organic carbon concentration (TOC) value (g·kg-1, d.w. = 0.9 %), which is a combustion indicator (Lasagni et al., 1997), supports this. Nearly complete combustion of organic matter in the fluidized bed boiler is reasonable due to the fact that the incineration temperature in the bed sand varies between 810 and 830 °C and in the upper zone of the boiler between 1100 and 1200 °C. Although the loss-onignition (LOI) parameter is widely used in estimating the organic content of waste materials, it should be regarded as an index of the volatile fraction (Johansson and van Bavel, 2003; Ribbing, 2007). Additional reactions than the oxidation of organic matter can occur at temperatures below 550 °C such as dehydration of metal oxides and loss of volatile salts. Thus, LOI is an indirect measure of the organic matter content of ash. According to the electrical conductivity value (2.0 mS·cm-1), which is an index of the total dissolved electrolyte concentration, the leaching solution of the fly ash had a relatively low ionic strength indicating that only relatively small part of the dissolved metals occurs as dissolved basic metal salts, e.g., oxides and hydroxides. Table 2 - The physical and chemical properties of the fly ash. Parameter Loss-on-ignition (LOI; 550 °C) Total organic carbon (TOC) Dry matter content (105 °C) Electrical conductivity value (1:2.5) pH (1:5)

Unit % (d.w.) g·kg-1 (d.w.) % mS·cm-1 -

Fly Ash 1.1 ± 0.8 9.0 ± 9.9 99.9 2.0 ± 2.5 10.8 ± 1.4

Extractability of trace elements from fly ash as a function of pH The extractable concentrations of elements as a function of pH at a liquid-to-solid (L/S) ratio of 10 L kg.-1 are shown in Figure 2. As a result, the following fractions were obtained: (i) trace elements that were primarily extractable under extremely or strongly acidic conditions; (ii) elements that expressed amphoteric behavior and were extractable under both acidic (i.e., low pH) and alkaline (i.e., high pH) conditions, but relatively insoluble near circumneutral pH values; and (iii) trace elements indicating oxyanionic behavior and thus commonly more mobile under alkaline conditions. Based on the data, the elements with significant extractable concentrations under extremely or strongly acidic conditions were As, Cd, Cr, Cu, Ni, Pb, Sb, and Zn. Based on the acquired concentration data, the mobility of Zn was most intense under low pH conditions as the mobility of Ni showed the slowest decline towards circumneutral pH values. Mobility of Pb and Sb was only detected at very low pH values (pH 2.7) as Cd and Cu were also extracted in minor concentrations at moderately acidic pH values (pH 5.2). In this fraction trace element desorption increases as the pH decreases, respectively enabling the elements to become more mobile. Although the XRD spectra (Fig. 1) does not demonstrate the existence of oxides and/or hydroxides in the fly ash sample, the alkaline pH value of the sample indicates that part of the dissolved trace elements most likely occur as basic metal salts, oxides, or even as hydroxides and/or carbonates. In the case of As, mobility was dominated by recoveries under extremely acidic conditions, however, subsequent to a notable decline under strongly acidic conditions continued until conditions became strongly/extremely alkaline (pH 10.5). Excluding a separate recovery at pH 10.5 most likely due to the formation of anionic species CrO2 - or CrO42-, mobility of Cr was dominated by recoveries under extremely or strongly acidic conditions. The pattern of elemental release from the ash is not only a function of the extraction method 1197

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 and the type of extractant used, but also of the element, and the mineralogy and chemistry of the solid fraction (Ryan et al., 2008). For elements that were soluble under extremely and strongly acidic conditions, the mineralogical phase of the element oxides in the ash affected their susceptibility to be attacked by the H+-ions present in the extractant. At lower pH values, there is an increase in the intensity of attack by the H+-ions on the wood ash mineral phases containing these elements thus increasing their extractability. Elements which clearly showed amphoteric behavior in this study were Ba and Al. These elements were soluble under both acidic and alkaline conditions, but were relatively insoluble at circumneutral pH values. In the case of Ba, insolubility near neutral pH values was not as pronounced as in the case of Al, however resulted in approximately a six- to four-fold decrease when compared to solubility under extremely acidic and alkaline conditions, respectively. Although As, Cu, and Zn have also been reported to demonstrate amphoteric behavior, respective solubility under acidic and alkaline conditions coupled with relative insolubility at near neutral pH values was not confirmed by our data. Trace element behavior indicating oxyanionic formation in our study was detected in the case of Mo and Se. Despite the very low recoveries of Se during the entire pH range, an increasing trend in terms of mobility can be seen at pH 6.4 – 9.0. For Mo, detected solubility in alkaline conditions is due to respective ability to form oxyanions such as MoO42-. The extractability of trace element As, which has also been reported to form oxyanions AsO33- and AsO43- under alkaline conditions, decreased rapidly at pH 10.5 as discussed earlier.

Figure 2 Extracted element concentrations (mg·kg-1, d.w.) as a function of pH at a liquid-to-solid (L/S) ratio of 10 L·kg-1. Note that the vertical axis is based on a logarithmic scale.

Conclusions The extractability of trace elements (As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, Se, Sb, and Zn) and macro element Al from fly ash was investigated as a function pH. Fly ash samples obtained from a fluidized bed boiler of a small-sized municipal district heating plant (32 MW) incinerating peat (60 %) and forestry and sawmill residues (40 %; i.e. bark, wood chips, sawdust) were subjected to a pH range of 0.7 – 12.3 at a liquid-to-solid (L/S) ratio of 10 L·kg-1. Consequently, the following main fractions were obtained: (i) trace elements (As, Cd, Cr, Cu, Ni, Pb, Sb, and Zn) 1198

The 27th International Conference on Solid Waste Technology and Managemenet Philadelphia, PA, U.S.A. – March 11-14, 2012 primarily extractable under extremely or strongly acidic conditions; (ii) elements (Ba and Al) expressing amphoteric behavior, i.e., extractable under both acidic (i.e., low pH) and alkaline conditions (i.e., high pH), but relatively insoluble at circumneutral pH values, and (iii) trace elements (Mo and Se) indicating oxyanionic behavior and thus commonly more mobile under alkaline conditions. For Mo, respective mobility under alkaline conditions is most likely due to the formation of MoO42-. Mobility of As, commonly known to occur as AsO33- or AsO43- under alkaline conditions, was dominated by recoveries under extremely acidic conditions. However, subsequent to a notable decline regarding recoveries under strongly acidic conditions, As became relatively mobile again at circumneutral pH. The results obtained in this study indicate that most of the elements in the fly ash originating from a fluidized bed boiler of a municipal district heating plant (32 MW) incinerating peat (60 %) and forestry and sawmill residues (40 %; i.e. bark, wood chips, sawdust) are not easily mobilized under normal landfill conditions.

Acknowledgements The authors wish to thank the technical staff of Suomen Ympäristöpalvelu Oy for performing the chemical analyses. The contribution of Olli Taikina-Aho from the Institute of Electron Optics (University of Oulu) regarding the XRD analysis and Jani Peurasaari from Kemin Energia Oy regarding many stimulating discussions are also gratefully acknowledged. This study was supported by Kemin Energia Oy which is deeply appreciated.

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