Waste Management xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH Kanokwan Komonweeraket a, Bora Cetin b,⇑, Craig H. Benson a, Ahmet H. Aydilek c, Tuncer B. Edil a a
Department of Civil and Environmental Engineering, University of Wisconsin, Madison, WI 53706, United States College of Engineering, University of Georgia, Athens, GA 30602, United States c Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, United States b
a r t i c l e
i n f o
Article history: Received 4 July 2014 Accepted 25 November 2014 Available online xxxx Keywords: pH-dependent leaching test C-type fly ash High carbon fly ash Metals Minerals Sulfate
a b s t r a c t Leaching behaviors of Arsenic (As), Barium (Ba), Calcium (Ca), Cadmium (Cd), Magnesium (Mg), Selenium (Se), and Strontium (Sr) from soil alone, coal fly ash alone, and soil-coal fly ash mixtures, were studied at a pH range of 2–14 via pH-dependent leaching tests. Seven different types of soils and coal fly ashes were tested. Results of this study indicated that Ca, Cd, Mg, and Sr showed cationic leaching pattern while As and Se generally follows an oxyanionic leaching pattern. On the other hand, leaching of Ba presented amphoteric-like leaching pattern but less pH-dependent. In spite of different types and composition of soil and coal fly ash investigated, the study reveals the similarity in leaching behavior as a function of pH for a given element from soil, coal fly ash, and soil-coal fly ash mixtures. The similarity is most likely due to similar controlling mechanisms (e.g., solubility, sorption, and solid-solution formation) and similar controlling factors (e.g., leachate pH and redox conditions). This offers the opportunity to transfer knowledge of coal fly ash that has been extensively characterized and studied to soil stabilized with coal fly ash. It is speculated that unburned carbon in off-specification coal fly ashes may provide sorption sites for Cd resulting in a reduction in concentration of these elements in leachate from soil-coal fly ash mixture. Class C fly ash provides sufficient CaO to initiate the pozzolanic reaction yielding hydrated cement products that oxyanions, including As and Se, can be incorporated into. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Soft fine-grained soils and poorly graded coarse grained soils are generally unsuitable for supporting civil works, such as roads or buildings. In areas where these types of soils exist, they are removed and replaced with stronger materials or stabilized in situ using physical or chemical methods to form a strong platform to support construction (Edil et al., 2002). Research has shown that coal fly ashes (which will be referred as fly ash here after) can be effective in stabilizing soils, providing significantly improved strength, durability, and stiffness (Cetin et al., 2010; Tasßtan et al. (2012)). Accordingly, in situ soil stabilization with fly ash has become a practical and economical solution for soil stabilization, especially in highway construction where evacuation and replacement of soft subgrades incurs substantial construction cost and time. However, because fly ash contains toxic trace elements, the use of fly ash in roadway construction presents a potential risk to the environment (Garavaglia and Caramuscio, 1994; Cetin ⇑ Corresponding author. Tel.: +1 (336) 686 1361. E-mail addresses: [email protected] (B. Cetin), [email protected] (C.H. Benson), [email protected] (A.H. Aydilek), [email protected] (T.B. Edil).
et al., 2012a,b; Cetin et al., 2014). The leaching behavior of trace elements from soil-fly ash mixtures has received more attention in recent years (Goswami and Mahanta, 2007; Cetin and Aydilek, 2013). However, none of the past studies has investigated how the leaching characteristics and behavior of different soil-fly ash mixtures varies with pH, even though pH is one of the most crucial parameters governing the leaching of metals and minerals. Understanding the pH-dependent leaching behavior and mechanisms controlling leaching of major and trace elements from soil-fly ash mixture is important for assessing the environment impacts associated with using fly ash in soil stabilization. pH-dependent leaching tests have been used to investigate leaching of major and trace elements from various solids, including fly ash (Kosson et al., 1996), stabilized waste, construction materials, steel slag (Apul et al., 2005), municipal solid waste incineration (MSWI) ash, bottom ash (Dijkstra et al. 2002, 2006) and soils (van der Sloot et al., 1996). The pH-dependent leaching test provides information on leaching characteristics of elements that is needed for geochemical modeling to identify leaching controlling mechanisms (Dijkstra et al., 2002; van der Sloot et al., 2005) which is recommended for future studies.
http://dx.doi.org/10.1016/j.wasman.2014.11.018 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
2
K. Komonweeraket et al. / Waste Management xxx (2014) xxx–xxx
The objective of this study is to investigate leaching behavior of soil-fly ash mixtures at different pH conditions. The study focused on Arsenic (As), Barium (Ba), Calcium (Ca), Cadmium (Cd), and Strontium (Sr). pH-dependent laboratory leaching tests were conducted on a range of soils and fly ashes used for soil stabilization in roadway construction projects in Wisconsin, Minnesota, and Maryland. 2. Materials Seven different fly ashes were used in this study. Three of these fly ashes were collected from Wisconsin (Dewey, Presque Isle, and Columbia fly ashes), and four of them were collected from Maryland (Brandon Shores, Paul Smith Precipitator, Dickerson Precipitator, and Morgantown fly ashes). All fly ashes, except Columbia fly ash, are referred to as ‘offspecification’ high carbon fly ashes (HCFAs) because their composition and properties do not meet the criteria for Class C or Class F ash in ASTM C 618 (Table 1). Loss on ignition (LOI) contents of HCFAs vary between 6.2% and 49%, whereas the LOI of Columbia is 0.98%. Dewey, Presque Isle (PI), and Morgantown (MT) fly ashes are alkali materials possessing a pH of 10.41, 11.68, and 9.5, respectively while Brandon Shores (BS), Paul Smith Precipitator (PSP), and Dickerson Precipitator (DP) fly ashes are neutral materials with pH of 6.1, 6.6, and 8.1, respectively. The chemical composition of Columbia fly ash meets the Class C criteria. The high CaO content also provides Columbia fly ash with strong cementing characteristics and high pH (pH 12.55). Five fine-grained soils and two sands were collected from Wisconsin, Minnesota and Maryland in this study: Lawson, Kamm, Red Wing, MnROAD, and Portage sand from Wisconsin and Minnesota,
and MDClay and MD Sand from Maryland. Organic matter contents in soils were determined using ASTM D 2974. Lawson soil has the highest organic matter (6.3%), which is consistent with its classification as organic soil. Kamm clay (2.0%), Red Wing silt (1.2%), and MnROAD clay (1.5%) have three to four times lower organic matter content than Lawson soil. MDSand, MDClay and sand materials have no measurable organic matter. Cation exchange capacities (CEC) of all soils are shown in Table 2. The CEC is the highest for the highly plastic Lawson soil (45 cmolc/kg), intermediate for the moderate to low plasticity Kamm clay (25 cmolc/kg) and MnROAD clay (19 cmolc/kg), and lowest for the Red Wing silt (8 cmolc/kg), and non-plastic Maryland soils (1.4–2.8 cmolc/kg). The CEC for some sorbing minerals in soils are strongly pH dependent (McBride, 1994; Langmuir, 1997). However, the majority constituent in all soils investigated is montmorillonite, which is less or independent of pH (Langmuir, 1997). The elemental composition of each fly ash and each soil is summarized in Table 3. The soils and fly ashes contain considerable amounts of major elements, including, Ca and Mg. The Red Wing and MnROAD soils, and Columbia fly ash contain the highest amounts of Ca and Mg. Trace elements (As, Cd, and Se) in the soils vary from non-detect to contents comparable to those in the fly ashes. 3. Methods 3.1. Sample preparation All fine-grained Wisconsin and Minnesota soils (Lawson, Kamm, RedWing, and MnROAD) were prepared by air-drying followed by
Table 1 Chemical compositions of fly ashes along with the composition of typical Class C and Class F fly ashes. Chemical requirements (%)
SiO2 + Al2O3 + Fe2O3 SO3 Moisture content Loss on ignition
ASTM requirements Class F
Class C
P70 65 63 66
P50 65 63 66
Dewey
Presque Isle
Columbia
BS
PSP
DP
MT
19(1) – 0.44 49(1)
60(1) – 0.18 32(1)
59(1) 3.7(3) 0.06 0.98(1)
75 – 0.12 6
81 – 0.08 7
87 – 0.2 16
88 – 0.34 8
Recommended Chemical Constituents (%)
Typical Class F
Typical Class C
Dewey
Presque Isle
Columbia
BS
PSP
DP
MT
CaO MgO
9 –
24 –
9.4 2.2
3 1.1
27 5.6
1 0.6
0.4 1.2
0.6 1.5
2.5 2
Dewey(2)
Presque Isle(2)
Columbia(4)
BS
PSP
DP
MT
13 83
39 49
14 96
45 –
74 –
60 –
88 –
Physical Requirements (%)
Fineness Strength activity @ 7 d
ASTM Requirements Class F
Class C
634 P75
634 P75
Note: BS: Brandon Shores fly ash, PSP: Paul Smith Precipitator fly ash, DP: Dickerson Precipitator fly ash, MT: Morgantown fly ash. (1) X-ray fluorescence (XRF) spectrometry. (2) Sauer (2006). (3) Bin-Shafique et al. (2002). (4) Acosta et al. (2003). Table 2 Physical properties of soils. Soil
Lawson
Kamm
Red wing
Mn ROAD
Sand
MDSand
MDClay
Plasticity Index (PI) Organic matter (%) Cation Exchange Capacity (cmolc/kg) USCS soil classification Quartz Montmorillonite Dolomite Calcite
31 6.3 45 OL-OH 52 15* 3 –
18 2 25 CL-CH 48 19* 2 –
17 1.2 8 ML 46 25 9 2
17 1.5 19 CL 44 29 6 2
Non-plastic – – SP – – – –
Non-plastic – 1.4 SP – – – –
Non-plastic 16 2.8 SC – – – –
Note: USCS: Unified soil classification system. * Randomly ordered mixed-layer illite/smectite when 90% smectite layer.
Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
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K. Komonweeraket et al. / Waste Management xxx (2014) xxx–xxx Table 3 pH and solid-phase concentration (mg/kg) from total elemental analysis of soils and fly ashes. Total metal & mineral content (mg/kg) Name
pH
As
Ba
Ca
Cd
Mg
Se
Sr
Soil Lawson Kamm RedWing MnROAD MDClay Sand MDSand
7 6.9 8 8 5.6 8.8 6
14 13 8.5 8.6 MDSand > Sand. The higher the CEC, the greater sorption capacity the soil is to hold cations against leaching, and the lower the concentration is in the leachate. However, only few mixtures of Columbia fly ash with soils follow this inverse relationship between the CEC and the concentrations in leachate, which can be seen in Ba at pH > 9 (Fig. 8). Furthermore, the concentration for a given element in most of the leachates highly varies from one leachate to another. No conclusive relationship was found between the CEC of soil and the concentration of element in leachate. The concentration of element in leachate is greatly affected by several factors, such as mechanism controlling the release, the presence of other chemical species in the leachates (e.g. competed ions for the reaction, sorption sites on the solid phase), and leaching conditions (e.g. pH and redox conditions). Therefore, only the CEC of soil is insufficient to predict the tendency of leaching concentration from soil-fly ash mixture. Leachate pH is believed to be a factor affecting the leaching of the element and the role of CEC of soil to retain ions. 5. Conclusions Leaching test as a function of pH was performed on soils, fly ashes and soil-fly ash mixtures and the leaching behavior of major and trace elements (As, Ca, Cd, Mg, Sr and Se) from fly ashes and soil stabilized with fly ash were investigated. Following conclusions are obtained from the current study: (1) It is observed that leaching of Ca, Cd, Mg, and Sr follows a cationic leaching pattern where the concentration decreases monotonically as pH increases. In addition, leaching of Ba from Maryland fly ashes exhibit cationic leaching pattern. Based on the literature, it is speculated that the leaching of these elements are most probably solubility controlled. (2) The leaching of As and Se from soil fly ash mixtures shows two leaching patterns including oxyanionic and amphoteric. Few of the soil-fly ash mixtures did not follow any of these leaching patterns to represent leaching behavior of As and Se. The variation in leaching behavior for As and Se could be due to several factors, a variation of dominant species of As and Se existing in the leachates for a given pH range, and effects from the presence of other dissolved ions in the leachates. Furthermore, sorption of As and Se may take place in the aqueous solutions due to their affinity to be adsorbed by the surface of soil and fly ash particles. This indicates that the leaching patterns of As and Se could be affected significantly by the sorption process. (3) Moreover leaching of Ba from Wisconsin fly ashes (Dewey, Preseque Isle and Columbia) soil-fly ash mixtures presents amphoteric-like pattern but less pH-dependent. This inconsistency observed for the leaching of Ba may occur due to different controlling mechanisms that may affect leaching behavior. (4) The aqueous concentration as a function of pH for each element shows a similar tendency regardless of whether leaching was from a fly ash alone or a soil-fly ash mixture, which is believed to be determined by similar factors, such as controlling mechanism, chemistry of a particular element, and environmental conditions (e.g. pH, redox conditions, ionic strength, and ionic composition of solute).
(5) Results indicates that CECs of the soils and fly ashes do not have any major impacts on the leaching behavior of As, Ba, Ca, Cd, Mg, Se, and Sr. It is shown that leached concentrations of these elements are influenced greatly by pH of the effluent solution which directly affects the leaching controlling mechanisms and occurrence of the chemical reaction between different elements in the aqueous solutions. (6) Overall, it can be concluded that fly ash alone and soil-fly ash mixtures abundant in trace elements of concern. However, different fly ash types show different abilities in immobilizing trace elements to some extent. The abilities of fly ash to retain trace element should promote the beneficial use of fly ash in soil stabilization, especially off-specification fly ash that are rich in high unburned carbon content. (7) Geochemical modeling analysis is necessary to fully characterize the leaching behavior of elements from soil alone, fly ash alone and soil-fly ash mixtures as a function of pH. This analysis also will provide information about the dominant oxidation states of each element in the aqueous solutions and their leaching controlling mechanisms whether they are solubility or sorption controlled. Acknowledgements This study was financially supported by FHWA Recycled Materials Resource Center (RMRC), Maryland State Highway Administration (SHA), and Maryland Department of Natural Resources (MD DNR). Endorsement by RMRC, SHA, MWRRC, and MD DNR or the fly ash suppliers is not implied and should not be assumed. The authors would like to thank Dr. William Balem for his support during analyzing the results of pH-dependent leaching tests. References Acosta, H.A., Edil, T.B., Benson, C.H., 2003. Soil stabilization and drying using fly ash. Geo Engineering Report No. 03–03, University of Wisconsin-Madison, Madison. Apul, D.S., Gardner, K.H., Eighmy, T.T., Fallman, A.-M., Comans, R.N.J., 2005. Simultaneous application of dissolution/precipitation and surface complexation/surface precipitation modeling to contaminant leaching. Environ. Sci. Technol. 39–15, 5736–5741. Astrup, T., Dijkstra, J.J., Comans, R.N.J., Van der Sloot, H.A., Christensen, T.H., 2006. Geochemical modeling of leaching from MSWI air-pollution-control residues. Environ. Sci. Technol. 40 (11), 3551–3557. Bin-Shafique, M.S., Benson, C.H., Edil, T.B., 2002. Leaching of heavy metals from fly ash stabilized soils used in highway pavements. Geo Engineering Report No. 02–14, Geo Engineering Program, University of Wisconsin-Madison, Madison, WI. Cetin, B., Aydilek, A.H., 2013. PH and fly ash type effect on trace metal leaching from embankment soils. Resour. Conserv. Recycl. 80, 107–117. Cetin, B., Aydilek, A.H., Guney, Y., 2010. Stabilization of recycled base materials with high carbon fly ash. Resour. Conserv. Recycl. 54–11, 878–892. Cetin, B., Aydilek, A.H., Guney, Y., 2012a. Leaching of trace metals from high carbon fly ash stabilized highway base layers. Resour. Conserv. Recycl. 58, 8–17. Cetin, B., Aydilek, A.H., Li, L., 2012b. Experimental and numerical analysis of metal leaching from fly ash-amended highway bases. Waste Manage. 32, 965–978. Cetin, B., Aydilek, A.H., Li, L., 2014. Trace metal leaching from embankment soils amended with high-carbon fly ash. J. Geotech. Geoenviron. Eng.-ASCE 140 (1), 1–13. Cornelis, G., Poppe, S., Van Gerven, T., Van den Broeck, E., Ceulemans, M., Vandecasteele, C., 2008. Geochemical modelling of arsenic and selenium leaching in alkaline water treatment sludge from the production of nonferrous metals. J. Hazard. Mater. 159 (2–3), 271–279. Dijkstra, J.J., Van der Sloot, H.A., Comans, R.N.J., 2002. Process identification and model development of contaminant transport in MSWI bottom ash. Waste Manage. 22 (5), 531. Dijkstra, J.J., Van Der Sloot, H.A., Comans, R.N.J., 2006. The leaching of major and trace elements from MSWI bottom ash as a function of pH and time. Appl. Geochem. 21 (2), 335. Edil, T.B., Benson, C.H., Sazzad, B.-S., Tanyu, B.F., Kim, W.-H., Senol, A., 2002. Field evaluation of construction alternatives for roadways over soft subgrade. Transport. Res. Rec. 1786 (1). Fallman, A.M., 2000. Leaching of chromium and barium from steel slag in laboratory and field tests – a solubility controlled process? Waste Manage. 20 (2–3), 149–154. Fruchter, J.S., Ral, D., Zachara, J.M., 1990. Identification of solubility-controlling solid phases in a large fly ash field lysimeter. Environ. Sci. Technol. 24 (8), 1173.
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Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH Kanokwan Komonweeraket a, Bora Cetin b,⇑, Craig H. Benson a, Ahmet H. Aydilek c, Tuncer B. Edil a a
Department of Civil and Environmental Engineering, University of Wisconsin, Madison, WI 53706, United States College of Engineering, University of Georgia, Athens, GA 30602, United States c Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, United States b
a r t i c l e
i n f o
Article history: Received 4 July 2014 Accepted 25 November 2014 Available online xxxx Keywords: pH-dependent leaching test C-type fly ash High carbon fly ash Metals Minerals Sulfate
a b s t r a c t Leaching behaviors of Arsenic (As), Barium (Ba), Calcium (Ca), Cadmium (Cd), Magnesium (Mg), Selenium (Se), and Strontium (Sr) from soil alone, coal fly ash alone, and soil-coal fly ash mixtures, were studied at a pH range of 2–14 via pH-dependent leaching tests. Seven different types of soils and coal fly ashes were tested. Results of this study indicated that Ca, Cd, Mg, and Sr showed cationic leaching pattern while As and Se generally follows an oxyanionic leaching pattern. On the other hand, leaching of Ba presented amphoteric-like leaching pattern but less pH-dependent. In spite of different types and composition of soil and coal fly ash investigated, the study reveals the similarity in leaching behavior as a function of pH for a given element from soil, coal fly ash, and soil-coal fly ash mixtures. The similarity is most likely due to similar controlling mechanisms (e.g., solubility, sorption, and solid-solution formation) and similar controlling factors (e.g., leachate pH and redox conditions). This offers the opportunity to transfer knowledge of coal fly ash that has been extensively characterized and studied to soil stabilized with coal fly ash. It is speculated that unburned carbon in off-specification coal fly ashes may provide sorption sites for Cd resulting in a reduction in concentration of these elements in leachate from soil-coal fly ash mixture. Class C fly ash provides sufficient CaO to initiate the pozzolanic reaction yielding hydrated cement products that oxyanions, including As and Se, can be incorporated into. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Soft fine-grained soils and poorly graded coarse grained soils are generally unsuitable for supporting civil works, such as roads or buildings. In areas where these types of soils exist, they are removed and replaced with stronger materials or stabilized in situ using physical or chemical methods to form a strong platform to support construction (Edil et al., 2002). Research has shown that coal fly ashes (which will be referred as fly ash here after) can be effective in stabilizing soils, providing significantly improved strength, durability, and stiffness (Cetin et al., 2010; Tasßtan et al. (2012)). Accordingly, in situ soil stabilization with fly ash has become a practical and economical solution for soil stabilization, especially in highway construction where evacuation and replacement of soft subgrades incurs substantial construction cost and time. However, because fly ash contains toxic trace elements, the use of fly ash in roadway construction presents a potential risk to the environment (Garavaglia and Caramuscio, 1994; Cetin ⇑ Corresponding author. Tel.: +1 (336) 686 1361. E-mail addresses: [email protected] (B. Cetin), [email protected] (C.H. Benson), [email protected] (A.H. Aydilek), [email protected] (T.B. Edil).
et al., 2012a,b; Cetin et al., 2014). The leaching behavior of trace elements from soil-fly ash mixtures has received more attention in recent years (Goswami and Mahanta, 2007; Cetin and Aydilek, 2013). However, none of the past studies has investigated how the leaching characteristics and behavior of different soil-fly ash mixtures varies with pH, even though pH is one of the most crucial parameters governing the leaching of metals and minerals. Understanding the pH-dependent leaching behavior and mechanisms controlling leaching of major and trace elements from soil-fly ash mixture is important for assessing the environment impacts associated with using fly ash in soil stabilization. pH-dependent leaching tests have been used to investigate leaching of major and trace elements from various solids, including fly ash (Kosson et al., 1996), stabilized waste, construction materials, steel slag (Apul et al., 2005), municipal solid waste incineration (MSWI) ash, bottom ash (Dijkstra et al. 2002, 2006) and soils (van der Sloot et al., 1996). The pH-dependent leaching test provides information on leaching characteristics of elements that is needed for geochemical modeling to identify leaching controlling mechanisms (Dijkstra et al., 2002; van der Sloot et al., 2005) which is recommended for future studies.
http://dx.doi.org/10.1016/j.wasman.2014.11.018 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
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K. Komonweeraket et al. / Waste Management xxx (2014) xxx–xxx
The objective of this study is to investigate leaching behavior of soil-fly ash mixtures at different pH conditions. The study focused on Arsenic (As), Barium (Ba), Calcium (Ca), Cadmium (Cd), and Strontium (Sr). pH-dependent laboratory leaching tests were conducted on a range of soils and fly ashes used for soil stabilization in roadway construction projects in Wisconsin, Minnesota, and Maryland. 2. Materials Seven different fly ashes were used in this study. Three of these fly ashes were collected from Wisconsin (Dewey, Presque Isle, and Columbia fly ashes), and four of them were collected from Maryland (Brandon Shores, Paul Smith Precipitator, Dickerson Precipitator, and Morgantown fly ashes). All fly ashes, except Columbia fly ash, are referred to as ‘offspecification’ high carbon fly ashes (HCFAs) because their composition and properties do not meet the criteria for Class C or Class F ash in ASTM C 618 (Table 1). Loss on ignition (LOI) contents of HCFAs vary between 6.2% and 49%, whereas the LOI of Columbia is 0.98%. Dewey, Presque Isle (PI), and Morgantown (MT) fly ashes are alkali materials possessing a pH of 10.41, 11.68, and 9.5, respectively while Brandon Shores (BS), Paul Smith Precipitator (PSP), and Dickerson Precipitator (DP) fly ashes are neutral materials with pH of 6.1, 6.6, and 8.1, respectively. The chemical composition of Columbia fly ash meets the Class C criteria. The high CaO content also provides Columbia fly ash with strong cementing characteristics and high pH (pH 12.55). Five fine-grained soils and two sands were collected from Wisconsin, Minnesota and Maryland in this study: Lawson, Kamm, Red Wing, MnROAD, and Portage sand from Wisconsin and Minnesota,
and MDClay and MD Sand from Maryland. Organic matter contents in soils were determined using ASTM D 2974. Lawson soil has the highest organic matter (6.3%), which is consistent with its classification as organic soil. Kamm clay (2.0%), Red Wing silt (1.2%), and MnROAD clay (1.5%) have three to four times lower organic matter content than Lawson soil. MDSand, MDClay and sand materials have no measurable organic matter. Cation exchange capacities (CEC) of all soils are shown in Table 2. The CEC is the highest for the highly plastic Lawson soil (45 cmolc/kg), intermediate for the moderate to low plasticity Kamm clay (25 cmolc/kg) and MnROAD clay (19 cmolc/kg), and lowest for the Red Wing silt (8 cmolc/kg), and non-plastic Maryland soils (1.4–2.8 cmolc/kg). The CEC for some sorbing minerals in soils are strongly pH dependent (McBride, 1994; Langmuir, 1997). However, the majority constituent in all soils investigated is montmorillonite, which is less or independent of pH (Langmuir, 1997). The elemental composition of each fly ash and each soil is summarized in Table 3. The soils and fly ashes contain considerable amounts of major elements, including, Ca and Mg. The Red Wing and MnROAD soils, and Columbia fly ash contain the highest amounts of Ca and Mg. Trace elements (As, Cd, and Se) in the soils vary from non-detect to contents comparable to those in the fly ashes. 3. Methods 3.1. Sample preparation All fine-grained Wisconsin and Minnesota soils (Lawson, Kamm, RedWing, and MnROAD) were prepared by air-drying followed by
Table 1 Chemical compositions of fly ashes along with the composition of typical Class C and Class F fly ashes. Chemical requirements (%)
SiO2 + Al2O3 + Fe2O3 SO3 Moisture content Loss on ignition
ASTM requirements Class F
Class C
P70 65 63 66
P50 65 63 66
Dewey
Presque Isle
Columbia
BS
PSP
DP
MT
19(1) – 0.44 49(1)
60(1) – 0.18 32(1)
59(1) 3.7(3) 0.06 0.98(1)
75 – 0.12 6
81 – 0.08 7
87 – 0.2 16
88 – 0.34 8
Recommended Chemical Constituents (%)
Typical Class F
Typical Class C
Dewey
Presque Isle
Columbia
BS
PSP
DP
MT
CaO MgO
9 –
24 –
9.4 2.2
3 1.1
27 5.6
1 0.6
0.4 1.2
0.6 1.5
2.5 2
Dewey(2)
Presque Isle(2)
Columbia(4)
BS
PSP
DP
MT
13 83
39 49
14 96
45 –
74 –
60 –
88 –
Physical Requirements (%)
Fineness Strength activity @ 7 d
ASTM Requirements Class F
Class C
634 P75
634 P75
Note: BS: Brandon Shores fly ash, PSP: Paul Smith Precipitator fly ash, DP: Dickerson Precipitator fly ash, MT: Morgantown fly ash. (1) X-ray fluorescence (XRF) spectrometry. (2) Sauer (2006). (3) Bin-Shafique et al. (2002). (4) Acosta et al. (2003). Table 2 Physical properties of soils. Soil
Lawson
Kamm
Red wing
Mn ROAD
Sand
MDSand
MDClay
Plasticity Index (PI) Organic matter (%) Cation Exchange Capacity (cmolc/kg) USCS soil classification Quartz Montmorillonite Dolomite Calcite
31 6.3 45 OL-OH 52 15* 3 –
18 2 25 CL-CH 48 19* 2 –
17 1.2 8 ML 46 25 9 2
17 1.5 19 CL 44 29 6 2
Non-plastic – – SP – – – –
Non-plastic – 1.4 SP – – – –
Non-plastic 16 2.8 SC – – – –
Note: USCS: Unified soil classification system. * Randomly ordered mixed-layer illite/smectite when 90% smectite layer.
Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
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K. Komonweeraket et al. / Waste Management xxx (2014) xxx–xxx Table 3 pH and solid-phase concentration (mg/kg) from total elemental analysis of soils and fly ashes. Total metal & mineral content (mg/kg) Name
pH
As
Ba
Ca
Cd
Mg
Se
Sr
Soil Lawson Kamm RedWing MnROAD MDClay Sand MDSand
7 6.9 8 8 5.6 8.8 6
14 13 8.5 8.6 MDSand > Sand. The higher the CEC, the greater sorption capacity the soil is to hold cations against leaching, and the lower the concentration is in the leachate. However, only few mixtures of Columbia fly ash with soils follow this inverse relationship between the CEC and the concentrations in leachate, which can be seen in Ba at pH > 9 (Fig. 8). Furthermore, the concentration for a given element in most of the leachates highly varies from one leachate to another. No conclusive relationship was found between the CEC of soil and the concentration of element in leachate. The concentration of element in leachate is greatly affected by several factors, such as mechanism controlling the release, the presence of other chemical species in the leachates (e.g. competed ions for the reaction, sorption sites on the solid phase), and leaching conditions (e.g. pH and redox conditions). Therefore, only the CEC of soil is insufficient to predict the tendency of leaching concentration from soil-fly ash mixture. Leachate pH is believed to be a factor affecting the leaching of the element and the role of CEC of soil to retain ions. 5. Conclusions Leaching test as a function of pH was performed on soils, fly ashes and soil-fly ash mixtures and the leaching behavior of major and trace elements (As, Ca, Cd, Mg, Sr and Se) from fly ashes and soil stabilized with fly ash were investigated. Following conclusions are obtained from the current study: (1) It is observed that leaching of Ca, Cd, Mg, and Sr follows a cationic leaching pattern where the concentration decreases monotonically as pH increases. In addition, leaching of Ba from Maryland fly ashes exhibit cationic leaching pattern. Based on the literature, it is speculated that the leaching of these elements are most probably solubility controlled. (2) The leaching of As and Se from soil fly ash mixtures shows two leaching patterns including oxyanionic and amphoteric. Few of the soil-fly ash mixtures did not follow any of these leaching patterns to represent leaching behavior of As and Se. The variation in leaching behavior for As and Se could be due to several factors, a variation of dominant species of As and Se existing in the leachates for a given pH range, and effects from the presence of other dissolved ions in the leachates. Furthermore, sorption of As and Se may take place in the aqueous solutions due to their affinity to be adsorbed by the surface of soil and fly ash particles. This indicates that the leaching patterns of As and Se could be affected significantly by the sorption process. (3) Moreover leaching of Ba from Wisconsin fly ashes (Dewey, Preseque Isle and Columbia) soil-fly ash mixtures presents amphoteric-like pattern but less pH-dependent. This inconsistency observed for the leaching of Ba may occur due to different controlling mechanisms that may affect leaching behavior. (4) The aqueous concentration as a function of pH for each element shows a similar tendency regardless of whether leaching was from a fly ash alone or a soil-fly ash mixture, which is believed to be determined by similar factors, such as controlling mechanism, chemistry of a particular element, and environmental conditions (e.g. pH, redox conditions, ionic strength, and ionic composition of solute).
(5) Results indicates that CECs of the soils and fly ashes do not have any major impacts on the leaching behavior of As, Ba, Ca, Cd, Mg, Se, and Sr. It is shown that leached concentrations of these elements are influenced greatly by pH of the effluent solution which directly affects the leaching controlling mechanisms and occurrence of the chemical reaction between different elements in the aqueous solutions. (6) Overall, it can be concluded that fly ash alone and soil-fly ash mixtures abundant in trace elements of concern. However, different fly ash types show different abilities in immobilizing trace elements to some extent. The abilities of fly ash to retain trace element should promote the beneficial use of fly ash in soil stabilization, especially off-specification fly ash that are rich in high unburned carbon content. (7) Geochemical modeling analysis is necessary to fully characterize the leaching behavior of elements from soil alone, fly ash alone and soil-fly ash mixtures as a function of pH. This analysis also will provide information about the dominant oxidation states of each element in the aqueous solutions and their leaching controlling mechanisms whether they are solubility or sorption controlled. Acknowledgements This study was financially supported by FHWA Recycled Materials Resource Center (RMRC), Maryland State Highway Administration (SHA), and Maryland Department of Natural Resources (MD DNR). Endorsement by RMRC, SHA, MWRRC, and MD DNR or the fly ash suppliers is not implied and should not be assumed. The authors would like to thank Dr. William Balem for his support during analyzing the results of pH-dependent leaching tests. References Acosta, H.A., Edil, T.B., Benson, C.H., 2003. Soil stabilization and drying using fly ash. Geo Engineering Report No. 03–03, University of Wisconsin-Madison, Madison. Apul, D.S., Gardner, K.H., Eighmy, T.T., Fallman, A.-M., Comans, R.N.J., 2005. Simultaneous application of dissolution/precipitation and surface complexation/surface precipitation modeling to contaminant leaching. Environ. Sci. Technol. 39–15, 5736–5741. Astrup, T., Dijkstra, J.J., Comans, R.N.J., Van der Sloot, H.A., Christensen, T.H., 2006. Geochemical modeling of leaching from MSWI air-pollution-control residues. Environ. Sci. Technol. 40 (11), 3551–3557. Bin-Shafique, M.S., Benson, C.H., Edil, T.B., 2002. Leaching of heavy metals from fly ash stabilized soils used in highway pavements. Geo Engineering Report No. 02–14, Geo Engineering Program, University of Wisconsin-Madison, Madison, WI. Cetin, B., Aydilek, A.H., 2013. PH and fly ash type effect on trace metal leaching from embankment soils. Resour. Conserv. Recycl. 80, 107–117. Cetin, B., Aydilek, A.H., Guney, Y., 2010. Stabilization of recycled base materials with high carbon fly ash. Resour. Conserv. Recycl. 54–11, 878–892. Cetin, B., Aydilek, A.H., Guney, Y., 2012a. Leaching of trace metals from high carbon fly ash stabilized highway base layers. Resour. Conserv. Recycl. 58, 8–17. Cetin, B., Aydilek, A.H., Li, L., 2012b. Experimental and numerical analysis of metal leaching from fly ash-amended highway bases. Waste Manage. 32, 965–978. Cetin, B., Aydilek, A.H., Li, L., 2014. Trace metal leaching from embankment soils amended with high-carbon fly ash. J. Geotech. Geoenviron. Eng.-ASCE 140 (1), 1–13. Cornelis, G., Poppe, S., Van Gerven, T., Van den Broeck, E., Ceulemans, M., Vandecasteele, C., 2008. Geochemical modelling of arsenic and selenium leaching in alkaline water treatment sludge from the production of nonferrous metals. J. Hazard. Mater. 159 (2–3), 271–279. Dijkstra, J.J., Van der Sloot, H.A., Comans, R.N.J., 2002. Process identification and model development of contaminant transport in MSWI bottom ash. Waste Manage. 22 (5), 531. Dijkstra, J.J., Van Der Sloot, H.A., Comans, R.N.J., 2006. The leaching of major and trace elements from MSWI bottom ash as a function of pH and time. Appl. Geochem. 21 (2), 335. Edil, T.B., Benson, C.H., Sazzad, B.-S., Tanyu, B.F., Kim, W.-H., Senol, A., 2002. Field evaluation of construction alternatives for roadways over soft subgrade. Transport. Res. Rec. 1786 (1). Fallman, A.M., 2000. Leaching of chromium and barium from steel slag in laboratory and field tests – a solubility controlled process? Waste Manage. 20 (2–3), 149–154. Fruchter, J.S., Ral, D., Zachara, J.M., 1990. Identification of solubility-controlling solid phases in a large fly ash field lysimeter. Environ. Sci. Technol. 24 (8), 1173.
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Please cite this article in press as: Komonweeraket, K., et al. Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.11.018
