Leaching behavior of trace elements from fly ashes of five Chinese coal power plants

Journal Pre-proof Leaching behavior of trace elements from fly ashes of five Chinese coal power plants

Lei Zhao, Shifeng Dai, Robert B. Finkelman, David French, Ian T. Graham, Yongchang Yang, Jixiang Li, Pan Yang PII:

S0166-5162(19)31116-4

DOI:

https://doi.org/10.1016/j.coal.2019.103381

Reference:

COGEL 103381

To appear in:

International Journal of Coal Geology

Received date:

22 November 2019

Revised date:

25 December 2019

Accepted date:

26 December 2019

Please cite this article as: L. Zhao, S. Dai, R.B. Finkelman, et al., Leaching behavior of trace elements from fly ashes of five Chinese coal power plants, International Journal of Coal Geology(2019), https://doi.org/10.1016/j.coal.2019.103381

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier.

Journal Pre-proof

Leaching behavior of trace elements from fly ashes of five Chinese coal power plants Lei Zhao1,2, Shifeng Dai1,2,* [email protected]; [email protected], Robert B. Finkelman1,3, David French4, Ian T. Graham4, Yongchang Yang1,2, Jixiang Li 1,2, Pan Yang 1,2 1

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining

and Technology, China 2

College of Geoscience and Survey Engineering, China University of Mining and

f

Technology (Beijing), Beijing 100083, China Geosciences Department, University of Texas at Dallas, Richardson, TX, United States

4

PANGEA Research Centre, School of Biological, Earth and Environmental Sciences,

oo

3

pr

University of New South Wales, Sydney, NSW 2052, Australia *

e-

Corresponding author.

Pr

Abstract

The leaching behavior of trace elements in five fly ashes from five Chinese coal-fired

al

power plants were obtained by conducting the new US EPA leaching test methods

rn

(LEAF) and the conventional EPA Toxicity Characteristic Leaching Procedure (TCLP) test method. These were done under a range of pH values and liquid to solid ratios.

Jo u

Despite different elemental and mineralogical compositions of the ashes and different ranks of the respective feed coals, the majority of elements show similar liquid/solid partitioning (LSP) curve trends among the different ash samples in the pH-dependent leaching tests. This indicates that most elements are sensitive to changes in pH. Different LSP curves exist for Cr from pH-dependent tests, probably due to different valence states of this element in the unleached fly ashes. Although most elements show similar LSP curves for different fly ash samples from the L/S dependent tests, variation exists for LSP curves of some individual elements among different ash samples. Caution should be taken when generating general L/S dependent leaching characteristics for individual elements. The leachate concentrations of the highly soluble elements such as B and Se, as well as oxyanions under alkaline conditions, are dependent on their initial concentrations in the fly ashes. The conventional EPA

Journal Pre-proof method TCLP overestimates the leachability of most hazardous elements, such as Be, B, Co, Ni, Cu, Zn, Cd, Sb, and U, at least in the studied alkaline fly ashes. LEAF tests have the advantage of being a comprehensive evaluation of the leaching characteristics, with a range of pH and L/S allowing for the examination of leachability under various conditions at different disposal or beneficial use scenarios. Key words: Coal combustion fly ash; Elemental leachability; TCLP; LEAF; Environmental impact

1. Introduction

oo

f

Coal used for electricity generation in China amounted to 4.73 Gt in 2018, accounting for 46.9% of the total amount of coal used for electricity generation in the world that

pr

year (BP, 2019). Coal combustion products (CCPs) are produced in large amounts in China every year. For example, 0.57Gt of coal combustion fly ash was produced in

e-

2015 (Bi and Liu, 2018). Although the utilization of CCPs is increasing in China, mainly

Pr

for applications such as concrete production, cement, grout, brick, road/dam construction and soil amendment, a large amount is disposed of in ponds or landfills

al

(Li et al., 2018; Yao et al., 2015).

rn

Trace elements can be much more concentrated in CCPs depending on the ash yield of the feed coals, product types of the CCPs and the volatility of the elements. Toxic

Jo u

constituents in CCPs pose potential risks to the environment and human health when being reused or disposed of. Several studies in various locations of the southeastern United States have shown the influence of CCPs on groundwater, such as the leaking of coal ash ponds to adjacent surface water and shallow groundwater aquifers (Harkness et al., 2016), and the discharge of coal ash and flue gas desulfurization (FGD) wastewaters on water quality of receiving waters (Ruhl et al., 2012). Bottom lake sediments from Sutton Lake, North Carolina, were also found to be contaminated with toxic metals derived from adjacent CCPs storage sites (Vengosh et al., 2019).

Understanding the leaching behavior of the toxic elements from the CCPs is important to diminish the adverse impacts on human health and the ecosystem either during beneficial use or land disposal. Extensive studies have been conducted on the

Journal Pre-proof leachability of elements from fly ash or other CCPs using various leaching test procedures (e.g. Izquierdo and Querol, 2012; Jankowski et al., 2006; Jones et al., 2012; Karuppiah and Gupta, 1997; Lu et al., 2009; Monroy Sarmiento et al., 2019; Oliveira et al., 2012; Querol et al., 2001; Wang et al., 2008; Wang et al., 1999; Ward et al., 2009; Zhao et al., 2018).

The conventional U.S. Environmental Protection Agency (EPA) method 1311 leaching test, the Toxicity Characteristic Leaching Procedure (TCLP) (US EPA, 1992), has been

f

widely applied to understand the leachability of elements from CCPs. However, the

oo

TCLP, as well as other single-point extraction tests, supply limited information on the elemental leaching characteristics under various environmental conditions that are

pr

expected in specific management scenarios (Kosson et al., 2014; Thorneloe et al.,

e-

2010). On the other hand, TCLP especially often overestimates metal release under conditions that are unlikely to occur in actual disposal or reuse scenarios (Izquierdo

Pr

and Querol, 2012; Kosson et al., 2002; Thorneloe et al., 2010). This is because that TCLP test was formulated for assessing the leachability of municipal waste and

al

therefore not really appropriate to characterization of the leaching behavior of CCPs.

rn

Taking into consideration the limitations of previous leaching tests, the US EPA has

Jo u

developed an integrated approach for evaluating leaching behavior of materials named the Leaching Environmental Assessment Framework (LEAF). These methods are designed to provide liquid-solid partitioning (LSP) as a function of extract pH using parallel batch extraction (LEAF method 1313), as a function of the liquid to solid ratio (L/S) under percolation conditions (LEAF Method 1314), and as a function of L/S under conditions that approach liquid solid chemical equilibrium (LEAF Method 1316). LEAF methods have been included under the US EPA compendium of leaching tests, Hazardous Waste Test Methods (SW-846) (Kosson et al., 2014).

A LEAF test was applied to a fly ash and a bottom ash from Hebei Province, China, but this was limited to only the one power plant (Zhang et al., 2019). The leaching tests in the present study were conducted in order to attempt to make comparisons of leachability of trace elements in ash samples derived from combustion of different

Journal Pre-proof ranks of feed coals and from various locations in China, and to make comparisons of the new EPA leaching tests to the TCLP test data. The present study also attempts to determine the chemical composition of the leachates, and to characterize the leaching activities of the trace elements under different management conditions, as well as to evaluate the potential risks to the environment, e.g., surface water and groundwater.

2. Samples, leaching test procedures and analytical techniques

f

2.1. Samples

oo

Five fly ash samples were collected from the Shangdu (SD) power plant of Inner Mongolia, Laibin A (LBA), Laibin B (LBB), and Nanning (NN) power plants of Guangxi

pr

Province, and Weixin (WX) power plant of Yunnan Province in China (Fig. 1). The

e-

samples were derived from the combustion of low-sulfur subbituminous coal, high

Pr

sulfur anthracite, and a blend of medium sulfur low-volatile bituminous coal (Table 1).

All the power plants are equipped with electrostatic precipitators (ESP), except LBB

al

power plant which was a bag house filter, followed by wet FGD systems. All the power

rn

plants have post-combustion NOx control facilities, selective catalytic reduction (SCR), equipped before particulate removal. All the fly ashes were collected from the

Jo u

composite output going to the ash repository from the ash collection devices of respective power plants. The ash samples from each power plant were collected each day for five consecutive days. A composite sample was made by mixing the ash from each day to be more representative of the fly ash of each power plant. All five composite ashes were air-dried before subjected to further analyses, as well as the leaching tests.

2.2. Leaching tests The EPA TCLP, following the procedure outlined by US EPA (1992), and EPA LEAF methods 1313 (US EPA, 2017a) and 1316 (US EPA, 2017c) were conducted on the five composite fly ash samples. Only one fly ash sample (NN FA) was subjected to the LEAF Method 1314 test (US EPA, 2017b) due to limited time availability. Ultra-pure Milli-Q (18.2 MΩ) water was used throughout this study. All the vessels were soaked in 2-5%

Journal Pre-proof nitric acid for 18 hours and then washed with ultra-pure water before use.

For the TCLP test, the extraction fluid for all the fly ashes was acetic acid solution with pH = 4.93 ± 0.05, prepared by mixing 5.7 mL of glacial acetic acid and 64.3 mL of 1 mol/L sodium hydroxide, and diluted with deionized water to 1000 ml. The choice of extraction fluids was based on the initial pH of each fly ashes, according to the TCLP method. The L/S for each extraction was 20:1 (ml/g-wet). High-density polyethylene bottles were used as extraction vessels. The bottles were rotated end-over-end at a

oo

f

speed of 30 rpm for 18 hours.

For test Method 1313, extraction fluids for each ash extraction were prepared by

pr

mixing appropriate amounts of 0.01 M HCl and 0.01 M NaOH with ultra-pure water to

e-

provide specific final pH values (i.e., 2, 4, 5.5, 7, 8, 9, 10.5, 12, 13). Prior to this procedure, a schedule of acid and base additions was formulated from a pre-test

Pr

titration which was conducted to generate an acid/base titration curve for each ash sample. For each ash extraction, three method blank samples composed of reagent

al

water only, a mixture of reagent water and maximum volume of acid, and a mixture of reagent water and maximum volume of base, respectively, were also prepared. The

rn

nine bottles and three method blanks with a liquid to solid ratio (L/S) of 10:1 (ml/g-

Jo u

dry) were tumbled end-over-end at a speed of 28 rpm for a contact time of 24 hours. Note that L/S is the fraction of the total liquid volume (including the moisture contained in the “as used” solid sample) to the dry mass equivalent of the solid material.

For test Method 1314, ultra-pure water was introduced into a column of moderatelypacked granular material in an up-flow pumping mode, with the flow rate being maintained between 0.5-1.0 L/S per day. Leachates were collected with the cumulative L/S of 0.2, 0.5, 1, 1.5, 2, 4.5, 5, 9.5, and 10, respectively. Nine specific aliquots of varying volumes were collected for leachates of each fly ash, with each having a percolating time of 6.4, 16, 32, 48, 64, 144, 160, 304, 320 hours, respectively.

For test Method 1316, six bottles, including five test positions with L/S of 0.5, 1, 2, 5,

Journal Pre-proof and 10 (ml/g-dry), and one method blank, were tumbled end-over-end at a speed of 28 rpm for 24 hours, with the extraction fluid being ultra-pure water.

The leachates from each of the above tests were filtered through 0.2 μm pore size membrane filters using a vacuum filtration system. Each leachate was divided into two aliquots. One aliquot was tested for pH, oxidation-reduction potential (ORP) and electrical conductivity immediately. The other aliquot of each leachate was acidified with analytical grade nitric acid to pH < 2 and stored in a refrigerator at 4°C for

f

subsequent analysis of elemental composition.

oo

2.3. Analytical techniques

pr

Each composite fly ash sample was ground to a fine powder (~ 200 mesh) using a tungsten carbide mill and split into representative portions for subsequent analyses.

e-

Moisture content of the fly ashes were determined by drying the samples in an oven

Pr

at 110°C until reaching a constant mass, following ASTM Standard D2216-10 (2010). Carbon and sulfur contents of the fly ashes were determined by a LECO SC832 sulfur

al

& carbon analyzer.

rn

The composite fly ash samples were subjected to XRD analysis using a Rigaku D/max2500/PC diffractometer with Ni-filtered Cu-Kα radiation and a scintillation detector.

Jo u

The XRD patterns were recorded over a 2θ interval of 2–70°, with a step size of 0.02° and a count time of 0.6 s per step. The diffractograms were processed using the Rietveld-based Siroquant software developed by Taylor (1991). A sub-sample of each fly ash was ashed at 815°C; the resultant ashes were analyzed by X-ray fluorescence (XRF) spectrometry to determine the contents of major element oxides. Fluorine in the raw fly ash samples and all the leachates was determined using a pyrohydrolysis/fluoride ion-selective electrode technique. Mercury in the raw fly ash samples and leachates was analyzed using a Milestone DMA-80 Hg analyzer.

Concentrations of 48 trace elements in the fly ash and the leachate samples were determined by quadrupole-based inductively coupled plasma mass spectrometry (ICPMS; X series II) in a pulse counting mode. The fine powders of the fly ash samples

Journal Pre-proof were subjected to microwave dissolution in a mixed-acids reagent (2-ml 65% HNO3 and 5-ml 40% HF for each 50-mg ash sample) prior to ICP-MS analysis. Detailed description of the ICP-MS techniques for determination of trace elements has been more fully described by Dai et al. (2011). Arsenic and Se in the fly ashes and the leachates were analyzed using ICP-MS with a collision cell technique, following the method described by Li et al. (2014). Boron concentration in the samples was also determined by ICP-MS, with addition of H3PO4 to the HNO3 and HF being used in the sample digestion process to diminish boron volatilization during acid-drying after

oo

f

sample digestion.

Method detection limits for element analysis by ICP-MS (for elements except Hg) and

pr

for the Hg analyzer are listed in Table 2. Elemental concentrations in the leachates

e-

which are below detection limit (bdl) are substituted by half of the relative detection limit value, when plotted as a function of pH or L/S.

Pr

3. Results

al

3.1. Chemical and mineralogical characteristics of the fly ashes

rn

Moisture, carbon, and sulfur contents of the fly ashes are given in Table 3. Elemental concentrations of the composite fly ash samples are given in Table 4. As a tungsten

Jo u

mill was used for the pulverization of the fly ash samples prior to chemical analysis, the concentrations of W and probably Co in the unleached fly ash samples may not be accurate to a certain degree, depending on the abrasive components (e.g., quartz) in the ash samples. Nevetheless, these two elements are included in the present study, with intention to evaluate the change of their leachability with the change of leaching conditions.

According to ASTM Standard C618-19, all the fly ashes studied are class C fly ashes (Table 1). As suggested by Querol et al. (2001), the level of Ca concentration and Ca/S are indicative of the alkalinity of the leachate of the fly ash. Low Ca (low Ca/S ratios) and high Ca concentrations (high Ca/S ratios) will lead to leachates of moderate alkaline-near neutral and high pH values, respectively. The high concentrations of Ca

Journal Pre-proof and Ca/S ratios of all the fly ashes in this study (Table 3) indicate that all are alkaline fly ashes. This is also consistent with the ash “natural pH”, which is defined by LEAF methods as the final pH when the ash is extracted with deionized water at L/S of 10 mL/g-dry (Kosson et al., 2010). The natural pH of the fly ashes ranges from 10.54 to 11.44 (Table 3) which corresponds to the Ca-rich chemical composition of the unleached ashes.

The fly ashes in this study mainly consist of amorphous phases, the proportion of

f

which varies in the range of 77.5-89.6% (Supplementary Table 1). Crystalline phases in

oo

the fly ashes include mullite, quartz, magnetite, along with small proportions of hematite, gypsum, and anhydrite in various samples (Supplementary Table 1). Among

pr

all the fly ashes, SD fly ash has the highest proportion of amorphous phase (89.6%)

e-

and lowest mullite content (3.2%). This probably indicate that the majority of the primary clay minerals in the feed coal for SD power plant converted to amorphous

Pr

phase rather than crystalline phase (mainly mullite).

3.2. Leaching behavior as a function of pH (LEAF method 1313)

al

Leachate concentrations of trace elements from all the fly ashes as a function of pH

rn

are listed in Supplementary Table 2. Figs. 2-4 illustrate LSP curves of elements as a function of pH as well as the respective method detection limit and the maximum

Jo u

concentration levels (μg/L) in drinking water (MCL) of China (GB5749-2006, 2006) or the upper limit value of the Type III groundwater quality (GWIII) of China (GB/T 148482017, 2017), when the MCL data is not available. GWIII of China is mainly applicable to centralized domestic drinking water sources and industrial and agricultural water. The mobility of most elements from all fly ashes vary significantly with the leachate pH with most element concentrations varying by several orders of magnitude, and thus logarithmic coordinates are used in the plots (Figs. 2-4).

Elements including Co, Ni, Cu, Zn, Be, Tl, and Cd, that occur mainly as cations (US EPA, 2017a), broadly decrease in concentration with increasing pH over the studied pH range (Fig. 2). Cobalt and Ni decrease steadily with increasing pH over the entire pH range. The leachability of Cu, Be and Cd increases with decreasing pH over the acid pH

Journal Pre-proof range, and is independent of pH at near neutral to alkaline conditions (pH>7). LSP curves of Zn and Tl are similar to Cu, Be and Cd, but are pH-independent in the alkaline range (pH>8). LSP curves of Se, Mo, W, V, Sb, As, B, and U (Fig. 3), all of which tend to be dissolved as oxyanions in leachates (e.g. Neupane and Donahoe, 2013; US EPA, 2017a). Arsenic, V, Sb and U have maximum leachability in the neutral to slightly alkaline conditions (pH of 8-9), with the exception of increased dissolution under extreme acid conditions (pH of ~2). Not all the elements that typically occur as oxyanions exhibit such LSP curve

oo

f

trends. Selenium, Mo, and W have similar LSP curves to the above-mentioned elements in the acid pH range, but tend to be independent of the pH value in the

pr

neutral to alkaline range (Fig. 3).

e-

In contrast to the elements above, B in the leachates remains highly soluble at

Pr

relatively stable levels over pH greater than 7, but rapidly decreases in concentration as pH increases above 8. However, B is known to occur as oxyanions in solution and thus is more soluble in alkaline environments (Jankowski et al., 2006) and was found

al

to occur in high concentrations in fly ash leachates (e.g. Cox et al., 1978; Querol et al.,

rn

2001). There are two reasons for the decrease in soluble B in leachates at higher pH values. Boron can coprecipitate with CaCO3, and/or it can substitute in the ettringite

Jo u

structure that is formed under alkaline conditions (Iwashita et al., 2005; Jankowski et al., 2006). It is very likely that the formation of ettringite has led to the decrease in B concentrations in all the fly ash leachates at strongly alkaline conditions in the present study. The formation of ettringite in alkaline ashes, may also impact on the mobility of other components that exist as oxyanions in aqueous solution. Amphoteric species generally have LSP curves similar in shape to cationic LSP curves in the acidic pH range, with minimum concentrations at or near neutral to slightly acid pH range only to increase again for alkaline pH values (US EPA, 2017a). Lead and Sn have similar LSP curves, both of which have amphoteric behavior (Fig. 4). Both elements are extremely insoluble over mildly acid to strong alkaline conditions, despite small variations among individual pH values and ash samples. Their

Journal Pre-proof leachability only increases towards the highly acid end of the pH range. Lead tends to be mildly soluble in the strongly alkaline end of the pH (~13), due to the solubility of hydroxide complexes ( [Pb(OH 3)]-) (US EPA, 2017a).

Chromium shows two different LSP curve trends among the five studied fly ashes (Fig. 4). Its leachability does not obviously change with pH in leachates of the NN and WX fly ashes, but follows an amphoteric trend for the SD, LBA, and LBB fly ashes (Fig. 4). Both Cr (III) and Cr (VI) occur in coal combustion products with the former being

f

dominant in fly ashes (Huggins et al., 1999; Rivera et al., 2017). Trivalent Cr tends to

oo

be amphoteric (concentrations increasing towards the acid and alkaline ends of the studied pH range) and this amphoteric trend exists in the leaching behavior of the SD,

pr

LBA, and LBB fly ashes. The other two ashes (NN and WX) tend to have a maximum value at neutral to slightly alkaline conditions, which does not follow the amphoteric

e-

curve but probably indicates the existence of oxyanionic species, such as CrO 42−. This

Pr

may indicate that at least part of the Cr is hexavalent in the NN and WX ash leachates,

al

but Cr (III) is probably the dominant form of Cr in the SD, LBA, and LBB ash leachates.

Although some studies suggest that Ba is not significantly pH-dependent and is poorly

rn

leached throughout the pH range (Izquierdo and Querol, 2012), the solubility of this

Jo u

element is sensitive to pH in the present study (Fig. 4). Barium has pH-dependent LSP curves with an amphoteric trend, with concentrations in leachates of different ashes generally having a minima at pH of 7-9. Precipitation of more insoluble sulphate BaSO4, or more likely co-precipitation as (Ba,Sr)SO4 than as SrSO4 (Fruchter et al., 1990) may occur in the present study especially in the NN ash leachates, which have a minima in both the Ba and Sr LSP curves at pH 9.

Both Rb (Fig. 4) and Cs have similar amphoteric LSP curve trends. Although the leachability of F shows some pH-dependency (Fig. 4), it is not consistent with previous studies where the extractable F is much lower under alkaline than acid conditions (Dreesen et al., 1977). The leachability of F appears to have a minimum value near neutral pH for most ashes in the present study.

Journal Pre-proof 3.3. Leaching behavior as a function of cumulative L/S ratio (LEAF Method 1314) Concentrations of trace elements in up-flow percolation column test leachates from NN FA are listed in Supplementary Table 3. Leaching behavior, as well as pH values as a function of cumulative L/S for NN FA is illustrated in Fig. 5. Although it is expected that concentrations will broadly decrease with increasing L/S due to dilution effects, different trend lines occur for some elements over specific L/S ranges. As indicated by the Method 1313 test, pH value plays an important role in controlling the leaching activities of elements, it also has an influence on the LSP leaching behavior in the

oo

f

Method 1314 test.

pr

The released concentrations of most cationic elements generally decrease in the leachability as L/S increases, either over the entire studied L/S range (such as Zn, Tl),

e-

or over the entire range except for the very initial L/S increment.

Pr

The initial sample results from this test provided an estimate of pore water concentrations (Hattaway et al., 2013), which is important in evaluating the leaching

al

activity in some management scenarios. With the initial increase in L/S from 0.2 to 1,

rn

concentrations of elements including Mo, W, Se, Cr, and Ba increase. These elements are highly soluble under alkaline conditions and can be over-saturated at low L/S, and

Jo u

thus small changes in initial L/S often does not change the leachate concentrations. As indicated by the method 1313 test, Mo, W, and Se occur as highly soluble oxya nions and increase with increasing pH over this pH range (alkaline conditions). As amphoteric species, Ba and Cr (in NN FA), also have increased solubility as pH increases over range 9.5-10.

Arsenic concentration increases only in the very initial L/S increment, but then decreases rapidly from L/S of 0.5 to 1. The respective pH ranges from 9.5 to 10 as L/S ranges from 0.2 to 1 in this test. As indicated by the LSP curve of this element as a function of pH (Fig. 5), arsenic leachability decreases as pH increases over the pH range 9.5-10. This led to the decrease in As leachability in the respective L/S range for leachate of each ash sample from L/S of 0.5 to 1.

Journal Pre-proof

Over the L/S range of 2 to 10, many elements, especially those highly soluble elements, such as B, F, Mo, Cr, Se, and W, have LSP curves with smaller slopes and have low leachate concentrations at higher L/S. This is due to quick wash-out at initial low L/S ratios (0.2-2) with a significant amount of these elements having been removed. These elements, as well as some other elements (e.g. Co, U, Pb), remain roughly at the same concentration levels as the L/S increases over L/S range of 2-10. This leaching behavior, i.e. leachate concentration is generally weakly dependent on

oo

solubility-limited over the entire studied L/S range.

f

L/S, is considered to be solubility-limited (Kosson et al., 2002). Cobalt also behaved as

3.4. Leaching behavior as a function of L/S ratio (LEAF method 1316)

pr

Concentrations of trace elements in the Method 1316 test leachates from all five fly

e-

ashes are listed in Supplementary Table4. Figs. 6 and 7 illustrate pH values as well as element LSP as a function of L/S for all five fly ashes. Although pH generally decreases

Pr

as L/S increases, it increases over the L/S range of 0.5 to 1 for the SD, LBB, and WX fly

al

ashes.

rn

The maximum change in pH over the entire L/S range (0.5-10) for each of the ash leachates by test Method 1316 (Fig. 6) is up to 0.3, which is not as significant as that in

Jo u

test Method 1314 (Fig. 5). This is because the batch test does not have the leachates removed from the system at each L/S increment as they are in the column test. This also explains why most element concentrations derived from batch tests are higher than the column tests at the same L/S ratios (Fig. 8).

Despite some variation in LSP curves of individual elements among different fly ash samples, the leaching behavior of many elements, such as Mo, Cd, Tl, Cr, Cu, Co, Sr, W, Ba as well as Se and U (over L/S 2-10), in this test is mainly controlled by dilution effects, with leachate concentrations overall steadily decreasing with increasing L/S (Figs. 6 and 7). When the limitation of maximum leachable amount of an element has been reached at lower L/S, the leaching solution is under-saturated at high L/S (Koralegedara et al., 2017). These elements probably have reached the upper limits of

Journal Pre-proof their respective soluble proportion at lower L/S values, given that pH value does not change significantly over the higher L/S range, which means pH does not play an important role in the change of element leachability over this L/S range. Concentrations of As and Sb broadly increase with L/S. Both elements are sensitive to pH changes over alkaline conditions with minimum leachability at pH of ~12, as observed in test Method 1313 (Fig. 3). With increasing solubility as pH decrease over L/S from 2 to 5, the solubility of both elements increases.

oo

f

With the exception of one ash sample (NN), Se tends to be independent of L/S. As a highly soluble element under alkaline conditions, Se is probably saturated in all the

pr

leachates of all ashes without reaching the respective soluble amounts except in the

e-

NN ash.

Pr

The boron LSP curve of LBB has a different trend from the other two samples. Boron solubility was suppressed in LBB at L/S<2. The leachate pH of LBB over this L/S range is 11.83-11.79. A sharp drop in its leachability over this pH range was observed in test

al

Method 1313 (Fig. 3). The solubility of B in the LBB leachate increases as L/S increases

rn

over L/S of 2-10 when the pH value decreases.

Jo u

For some elements, the general LSP trend cannot be generated due to significant variation among different ash samples. For example, F has very different trend lines among the studied ashes. Further investigations, especially on the modes of occurrence and leachable amount of the elements in question in the ash samples are needed to better understand the LSP characteristics.

3.5 TCLP results and comparison between TCLP and LEAF 1313 tests The TCLP test used buffered acetic acid and reached a final pH of weak acid conditions. Leachate concentrations of trace elements from the TCLP test are listed in Supplementary Table 5. The leachability for an element from a fly ash is expressed as a percentage of the concentration of that element in the unleached fly ash. The leachability is compared for selected elements which are of environmental concern

Journal Pre-proof among different LEAF 1313 tests and the TCLP test. Tests selected from the LEAF 1313 method include extractions with pH similar to TCLP, near neutral pH, highly alkaline pH, and under natural pH (alkaline conditions established by equilibrium with water) (Fig. 8).

The TCLP test, which has a final acid pH, generally produced higher levels of leachability for most elements, including Be, B, Co, Ni, Cu, Zn, Cd, Sb, and U, than the test under near neutral to alkaline conditions in LEAF tests for the studied ashes (Fig.

f

8). The difference of the leachability of these elements between the 1313 tests and

oo

the TCLP test overall reflects their pH-dependent leaching characteristics. However, the 1313 test with a pH of ~5 generally produced higher concentrations of Co, Ni, Zn,

e-

pr

Sr and Cd than the TCLP test, although the two tests have similar pH values (Fig. 8).

Some studies have found that the TCLP test underestimated the leaching ability of Sb,

Pr

As, Mo, Se and V, since these elements are unlikely to form complexes with acetate ions (Koralegedara et al., 2017). However, this was not observed in the present study.

al

The leachability of As and U is greater in the TCLP test relative to the LEAF 1313 test at similar pH in most ash samples. For other elements, consistent results are not

Jo u

4. Discussion

rn

observed among the different ashes.

4.1. Leachability vs. elemental concentrations of the fly ash samples Despite variation in elemental concentrations and modes of occurrence in the studied fly ash samples which were derived from feed coals with varying rank, the LSP curves of individual elements as a function of pH are similar among the different fly ash samples. However, the LSP curves of individual elements as a function of L/S are not always similar among different fly ash samples. This indicates that, among all the LEAF tests, pH is the major factor that controls the leaching activity of elements in the ashes.

Although the elemental concentration of the bulk fly ash is one of the factors that

Journal Pre-proof determines the concentration of the element in question in the leachate, leaching behavior of an element does not necessarily correlate with the concentration of that element in the ash (Thorneloe et al., 2010). However, the leachability of some elements, especially those highly soluble elements were observed to be related with their concentrations in the unleached fly ashes. For example, the fly ash leachability of B increases in the order: NN >LBA >LBB, which is in accordance of the order of B concentration in these fly ashes samples which is 158.8, 137.9, and 32.5 μg/g, respectively. As a readily leachable element, the leachability of B is reasonably

f

correlated with the concentrations in the ash samples. Barium was most leachable

oo

from the SD fly ash under all the extraction conditions in both the LEAF 1313 and 1316 tests. This is consistent with its significantly higher concentration (1541 μg/g) in the SD

pr

FA relative to LBA (560 μg/g), LBB (411 μg/g), NN (476 μg/g) and WX (321 μg/g) ash

e-

samples.

Pr

4.2. Implication for potential environment impact of toxic elements Caution should be paid to the mobility of Mo, which is above the MCL over the entire pH range studied, and to Se when leachate pH is above 5. Boron is above the MCL

al

over the entire pH range except in the LBB ash leachate at pH >11. Arsenic and Sb, as

rn

typical oxyanionic species, have maximum leachability at neutral to slightly alkaline conditions (pH of 8-9). These elements in the studied ashes should be of concern

Jo u

since the test pH conditions when their maximum leachability is reached are close to their natural pH values.

Tl and Ni concentrations in all the leachates exceed the MCL, except when the leachate has pH > 9-10. Exceptions for Tl were when the concentration in SD and NN leachates exceeded the MCL for the entire studied pH range. These two elements (Tl and Ni) should to be paid attention to due to their high mobility when the pH conditions of the environment are neutral or acid.

For Co, Cu, Zn, Be, Cd, and Pb in the leachates over the entire studied pH range in the LEAF 1313 test of all the fly ash samples the concentrations are below the MCL. This indicates that these elements may not have adverse impacts on the groundwater

Journal Pre-proof except under extreme pH conditions, e.g. pH of 2-5, which is unlikely, especially for alkaline fly ashes in most disposal scenarios. The concentration of Pb is generally below MCL for all the fly ashes leachates over pH range of 4-12. Lead is only significantly leachable under extremely acid (pH<4) conditions. It is below the detection limit at most studied pH values in the test pH range. This indicates that Pb presents little risk in terms of the leaching of this element.

Some elements may be environmentally safe for all but one fly ash sample. For

f

example, Cd in the NN ash exceeds the MCL over the entire pH range, but is mostly

oo

below the MCL for other ashes. Although Ba is mostly below the MCL for most fly ashes, it exceeds the MCL in SD ash over pH>10, which includes the ash’s natural pH

pr

(11.41) in this range. The unleached NN and SD ashes have the highest concentrations

e-

of Cd and Ba, respectively, among all the ashes. This may also indicate that caution should be paid to elements with high concentrations in any fly ash under investigation.

Pr

4. Conclusions

The leachability of trace elements in five Chinese fly ash samples were obtained by

al

conducting the US EPA new leaching test procedures (LEAF) and the conventional

rn

TCLP test method. Results from various leaching tests indicate that pH plays the most important role in controlling element leaching activities. Despite variation in element

Jo u

concentrations and modes of occurrence in the fly ash samples derived from feed coals with varying rank, the LSP curves for individual elements as a function of pH are broadly similar among the different fly ash samples. The liquid/solid ratio (L/S) and the element concentration in the unleached fly ash are also factors controlling the leaching activity, but their impact needs to be evaluated in conjunction wi th the change of pH in most cases. The element concentration in the unleached fly ashes is an especially important factor to control leaching activity for those highly soluble elements such as B.

All the fly ashes in the present study generate alkaline leachates when contacted with water. For alkaline fly ashes, caution should be paid to elements (e.g., Se, Mo and Sr) that showed their maximum leaching under alkaline conditions which is also their

Journal Pre-proof natural pH conditions. Those elements that occur as cations showing minimum leachability at alkaline conditions generally are of little concern.

The difference between the LEAF 1313 test and TCLP overall reflects the difference in pH conditions. TCLP uses weak acid and generates weakly acid leachates, which may not be similar to the pH of typical disposal conditions. TCLP overestimates the leachability of most hazardous elements, such as Be, B, Co, Ni, Cu, Zn, Cd, Sb, and U in the studied alkaline fly ashes.

oo

f

Acknowledgments This research was supported by the National Natural Science Foundation of China (No.

pr

41672152, 91962220, and U1810202 ), the 111 Project (No. B17042), and the Program for Changjiang Scholars and Innovative Research Team in University (No.

e-

IRT_17R104). The authors would like to thank Xiaoyun Yan for her assistance in ICP-MS

Pr

analysis of the samples. The authors would also like to thank useful comments from Editor Dr. Ralf Littke and two anonymous reviewers, which greatly improved the paper

al

quality.

rn

Credit Author Statement

Jo u

All authors have essentially contributed to the paper: Shifeng Dai proposed the ideas of the manuscript; Lei Zhao designed the experiments and drafted the manuscript; Robert Finkelman, David French, and Ian Graham reviewed the drafted and significantly edited the draft and proposed important corrections; Yongchang Yang, Jixiang Li, and Pan Yang together performed the experiments. This research was supported by the National Natural Science Foundation of China (No. 41672152, 91962220, and U1810202), the 111 Project (No. B17042), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_17R104). Conflict of interest: Authors declare that there are no conflicts of interest.

References BP, 2019. BP Statistical Review of World Energy. London: BP Statistical Review of World Energy. Available at: http://www.bp.com/statisticalreview. ASTM Standard C618-19, 2019. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM International, West Conshohocken, PA, www.astm.org.

Journal Pre-proof ASTM Standard D2216-10, 2010. Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass, ASTM International, West Conshohocken, PA, www.astm.org. Bi, J., Liu, M., 2018. Comprehensive utilization of fly ash resources. Chemical Industry Press, Beijing (in Chinese). Cox, J.A., Lundquist, G.L., P rzyjazny, A., Schmulbach, C.D., 1978. Leaching of boron from coal ash. Environ. Sci. Technol. 12, 722-723. Dai, S., Wang, X., Zhou, Y., Hower, J.C., Li, D., Chen, W., Zhu, X., Zou, J., 2011. Chemical and mineralogical compositions of silicic, mafic, and alkali tonsteins in the late Permian coals from the Songzao Coalfield, Chongqing, Southwest China. Chem. Geol. 282, 29-44.

oo

f

Dreesen, D.R., Gladney, E.S., Owens, J.W., Perkins, B.L., Wienke, C.L., Wangen, L.E., 1977. Comparison of levels of trace elements extracted from fly ash and levels found in effluent waters from a coal -fired power plant. Environ. Sci. Technol. 11, 1017-1019.

e-

pr

Fruchter, J.S., Rai, D., Zachara, J.M., 1990. Identification of solubility-controlling solid phases in a large fly ash field lysimeter. Environ. Sci. Technol. 24, 1173-1179.

Pr

GB5749-2006, 2006. Standards for drinking water quality. National Standard of P.R. China, p. 14 (in Chinese).

al

GB/T 14848-2017, 2017. Standard for groundwater quality. National Standard of P.R. China, p. 14 (in Chinese).

rn

Harkness, J.S., Sulkin, B., Vengosh, A., 2016. Evidence for Coal Ash Ponds Leaking in the Southeastern United States. Environ. Sci. Technol. 50, 6583-6592.

Jo u

Hattaway, J., Hardin, C.D., Daniels, J.L., 2013. Recommended guidelines for the use and application of the Leaching Environmental Assessment Framework (LEAF) for coal combustion residuals. In: World of Coal Ash (WOCA) Conference, Lexington, KY, April 22–25. Huggins, F.E., Najih, M., Huffman, G.P., 1999. Direct speciation of chromium in coal combustion by products by X-ray absorption fine-structure spectroscopy. Fuel 78, 233-242. Iwashita, A., Sakaguchi, Y., Nakajima, T., Takanashi, H., Ohki, A., Kambara, S., 2005. Leaching characteristics of boron and selenium for various coal fly ashes. Fuel 84, 479 -485. Izquierdo, M., Querol, X., 2012. Leaching behaviour of elements from coal combustion fly ash: An overview. Int. J. Coal Geol. 94, 54-66. Jankowski, J., Ward, C.R., French, D., Groves, S., 2006. Mobility of trace elements from selected Australian fly ashes and its potential impact on aquatic ecosystems. Fuel 85, 243 -256. Jones, K.B., Ruppert, L.F., Swanson, S.M., 2012. Leaching of elements from bottom ash, economizer fly ash, and fly ash from two coal-fired power plants. Int. J. Coal Geol. 94, 337-348. Karuppiah, M., Gupta, G., 1997. Toxicity of and metals in coal combustion ash leachate. J. Hazard.

Journal Pre-proof Mater. 56, 53-58. Koralegedara, N.H., Al -Abed, S.R., Arambewela, M.K.J., Dionysiou, D.D., 2017. Impact of leaching conditions on constituents release from Flue Gas Desulfurization Gypsum (FGDG) and FGDG -soil mixture. J. Hazard. Mater. 324, 83-93. Kosson, D., Garrabrants, A., Sanchez, F., Kariher, P., Thorneloe, S., van der Sloot, H., Helms, G., 2010. Use of a Leaching Assessment Framework for Evaluation of the Impacts of Coal Type and Facility Configuration on Beneficial Use Scenarios for Coal Combustion Residues, 2nd International Conference on Sustainable Construction Materials and Technologies. Kosson, D., van der Sloot, H., Sanchez, F., Garrabrants, A., 2002. An Integrated Framework for Evaluating Leaching in Waste Management and Utilization of Secondary Materials. Environ. Eng. Sci. 19, 159-204.

oo

f

Kosson, D.S., Garrabrants, A.C., DeLapp, R., van der Sloot, H.A., 2014. pH -dependent leaching of constituents of potential concern from concrete materials containing coal combustion fly ash . Chemosphere 103, 140-147.

e-

pr

Li, J., Zhuang, X., Querol, X., Font, O., Moreno, N., 2018. A review on the applications of coal combustion products in China. Int. Geol. Rev. 60, 671-716.

Pr

Li, X., Dai, S., Zhang, W., Li, T., Zheng, X., Chen, W., 2014. Determination of As and Se in coal and coal combustion products using closed vessel microwave digestion and collision/reac tion cell technology (CCT) of inductively coupled plasma mass spectrometry (ICP-MS). Int. J. Coal Geol. 124, 1-4.

al

Lu, S.G., Chen, Y.Y., Shan, H.D., Bai, S.Q ., 2009. Mineralogy and heavy metal leachability of magnetic fractions separated from some Chinese coal fly ashes. J. Hazard. Mater. 169, 246 -255.

rn

Monroy Sarmiento, L., Roessler, J.G., Townsend, T.G., 2019. Trace element mobility from coal combustion residuals exposed to landfill leachate. J. Hazard. Mater. 365, 962 -970.

Jo u

Neupane, G., Donahoe, R.J., 2013. Leachability of elements in alkaline and acidic coal fly ash samples during batch and column leaching tests. Fuel 104, 758-770. Oliveira, M.L.S., Ward, C.R., French, D., Hower, J.C., Querol, X., Silva, L.F.O., 2012. Mineralogy and leaching characteristics of beneficiated coal products from Santa Catarina, Brazil. Int. J. Coal Geol. 94, 314-325. Querol, X., Umaña, J.C., Alastuey, A., Ayora, C., Lopez-Soler, A., Plana, F., 2001. Extraction of soluble major and trace elements from fly ash in open and closed leaching systems. Fuel 80, 801 -813. Rivera, N., Hesterberg, D., Kaur, N., Duckworth, O.W., 2017. Chemical Speciation of Potentially Toxic Trace Metals in Coal Fly Ash Associated with the Kingston Fly Ash Spill. Energy Fuel. 31, 9652 -9659. Ruhl, L., Vengosh, A., Dwyer, G.S., Hsu -Kim, H., Schwartz, G., Romanski, A., Smith, S.D., 2012. The Impact of Coal Combustion Residue Effluent on Water Resourc es: A North Carolina Example. Environ. Sci. Technol. 46, 12226-12233. Thorneloe, S.A., Kosson, D.S., Sanchez, F., Garrabrants, A.C., Helms, G., 2010. Evaluating the Fate of Metals in Air Pollution Control Residues from Coal -Fired Power Plants. Environ. Sci. Technol. 44, 7351-

Journal Pre-proof 7356. US EPA, 1992. SW-846 Test Method 1311: Toxicity characteristic leaching procedure, US EPA, Method 1311. http://www.epa.gov. US EPA, 2017a. SW-846 Test Method 1313: Liquid-Solid Partitioning as a Function of Extract pH Using a Parallel Batch Extraction Procedure. US EPA, 2017b. SW-846 Test Method 1314: Liquid -Solid Partitioning as a Function of Liquid-Solid Ratio for Constituents in Solid Materials Using An Up-Flow Percolation Column Procedure.

f

US EPA, 2017c. SW-846 Test Method 1316: Liquid -Solid Partitioning as a Function of Liquid-to-Solid Ratio in Solid Materials Using a Parallel Batch Procedure.

pr

oo

Vengosh, A., Cowan, E.A., Coyte, R.M., Kondash, A.J., Wang, Z., Brandt, J.E., Dwyer, G.S., 2019. Evidence for unmonitored coal ash spills in Sutton Lake, North Carolina: Implications for contamination of lake ecosystems. Sci. Tot. Environ. 686, 1090-1103.

e-

Wang, W., Qin, Y., Song, D., Wang, K., 2008. Column leaching of coal and its combustion residues, Shizuishan, China. Int. J. Coal Geol. 75, 81-87.

Pr

Wang, Y., Ren, D., Zhao, F., 1999. Comparative leaching experiments for trace elements in raw coal, laboratory ash, fly ash and bottom ash. Int. J. Coal Geol. 40, 103-108.

al

Ward, C.R., French, D., Jankowski, J., Dubikova, M., Li, Z., Riley, K.W., 2009. Element mobility from fresh and long-stored acidic fly ashes associated with an Australian power station. Int. J. Coal Geol. 80, 224236.

Jo u

rn

Yao, Z.T., Ji, X.S., Sarker, P.K., Tang, J.H., Ge, L.Q., Xia, M.S., Xi, Y.Q., 2015. A comprehensive review on th e applications of coal fly ash. Earth-Sci. Rev. 141, 105-121. Zhang, S., Dai, S., Finkelman, R.B., Graham, I.T., French, D., Hower, J.C., Li, X., 2019. Leaching characteristics of alkaline coal combustion by -products: A case study from a coal -fired power plant, Hebei Province, China. Fuel 255, 115710. Zhao, S., Duan, Y., Lu, J., Gupta, R., Pudasainee, D., Liu, S., Liu, M., Lu, J., 2018. Chemical speciation and leaching characteristics of hazardous trace elements in coal and fly ash from coal -fired power plants. Fuel 232, 463-469.

Fig. 1. Loction of the five power plants in China for the present study. Fig. 2. Concentrations of Co, Ni, Cu, Zn, Be, Tl, Cd and Sr in the leachates from LEAF 1313 test. MDL, method detection limit; B was only analyzed for leachates of LBA, BBB and NN fly ashes; MCL, maximum concentration levels in drinking water of China (GB5749-2006, 2006); MCL for Co is substituted with the upper limit value of the Type III groundwater quality of China (GB/T 14848-2017, 2017).

Journal Pre-proof Fig. 3. Concentrations of Se, Mo, W, V, Sb, As, U and B in the leachates from LEAF 1313 test. MDL, method detection limit; The legend is the same as Fig. 2. Fig. 4. Concentrations of Ba, Sn, Rb, Pb, Cr and F in the leachates. The legend is the same as in Fig. 2. MCL indicated in the Cr graph is for Cr (VI) rather than total Cr. Fig. 5. Concentration of trace elements in leachates of NN fly ash as function of L/S by 1314 test. MDL is indicated in case of values below MDL. Fig. 6. Concentration of trace elements in leachates as a function of L/S by the 1316 test.

oo

f

Logarithmic coordinates are used when the trend lines are difficult to view due to the low values.

pr

Fig. 7. Concentration of trace elements in leachates as a function of L/S using the 1316 test.

Pr

e-

Fig. 8. Leaching percentage of selected trace elements in the fly ashes under different pH conditions by LEAF 1313 and TCLP tests. B was not analyzed for the SD, FA, and WX FA leachates.

Table 1 Characteristics of fly ash samples and respective feed coals, and location of the power plants.

LBA LBB NN WX

al

Shangdu Power plant, Inner Mongolia Laibin A Power plant, Guangxi province Laibin B Power plant, Guangxi province Nanning power plant, Guangxi province Weixin Power plant, Yunnan province

Jo u

SD

Fly ash U ni t col l ecti on size devi ce 600 Sub-bituminous ESP MW Blend of low-volatile bituminous and 300 ESP anthracite from various sources MW Blend of low-volatile bituminous coals from 360 Bag house various sources MW Blend of low-volatile bituminous coals from 660 ESP various sources MW 600 Anthracite ESP MW Ra nk of f eed coa l

rn

Sam Power plant and location pl e

boi l er F l y as h type Cl a s s Pulverize d coal Pulverize d coal Pulverize d coal Pulverize d coal Pulverize d coal

C C C C C

Classification of coal is according to ASTM standard (2012); ESP, electrostatic precipitator; SD fly ash is a coarse fraction from the power plant, which was selected for leaching tests because it is the fraction for disposal in ash ponds rather than the fine fractions which are sold for commercial use.

Table 2 Method detection limit (MDL) for elements in leachates by ICP-MS analysis and Hg analyzer for Hg (μg/L) in the present study, and maximum concentration levels (μg/L) in drinking water (MCL) (GB5749-2006, 2006) Ele men t

Li

Be

B

Sc

V

Cr

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Rb

Sr

Y

MDL

0. 02 7

0. 00 3

1. 70 4

0. 00 9

0. 12 6

0. 02 7

0. 00 0

0. 02 1

0. 01 2

0.05 4

0. 00 3

0. 02 4

0. 10 5

0. 12 3

0. 03 9

0. 02 4

0. 05 7

2

50

20 10

100

MC

50 50

10 10

Journal Pre-proof

MDL

MC L Ele me nt MDL

0

#

*

00

0

Zr

Nb

M o

Cd

In

Sn

Sb

Cs

Ba

La

Ce

Pr

Nd

S m

Eu

Gd

Tb

0. 02 7

0. 01 8

0. 35 7

0. 00 3

0. 00 0

0. 01 2

0. 08 1

0. 00 6

0. 06 0

0.13 2

0. 15 9

0. 03 3

0. 09 6

0. 01 8

0. 00 9

0. 02 4

0. 00 9

70

5

70 0

5

Dy

Ho

Er

T m

Yb

Lu

Hf

Ta

W

Hg

Tl

Pb

Bi

Th

U

0. 01 5

0. 00 9

0. 00 6

0. 00 6

0. 00 6

0. 00 6

0. 02 1

0. 03 3

0. 22 5

0.00 000 5

0. 00 9

0. 01 5

0. 00 0

0. 09 6

0. 00 9

1

0. 1

10

oo

MC L

f

L Ele me nt

pr

#，MCL for Cr (VI); *, the upper limit value of the Type III groundwater quality (GWIII) of Chinese standard (GB/T 14848-2017, 2017), which is equivalent to maximum concentration levels in centralized drinking water source and industrial and agricultural water.

e-

Table 3 Ash yield, moisture, carbon and sulfur contents of the fly ashes (%), natural pH of the samples and Ca/S. Moi s ture ( a d)

Ca rbon ( d)

Sul f ur ( d)

N a tura l pH

Ca /S

SD FA

0.08

0.11

0.16

11.41

48.5

LBA FA

0.13

10.32

0.31

11.44

12.5

LBB FA

0.07

7.76

0.11

11.38

22.6

NN FA

0.17

1.57

0.44

10.54

6.6

WX FA

5.05

7.53

0.39

11.17

14.6

al

Pr

Sa m pl e

Jo u

rn

ad, air-dry basis; d, dry basis; Natural pH was obtained at the leachate of ashes under the condition liquid/solid=10:1 (ml/g-dry) at 28 r/min for 24 hours.

Table 4. Loss on ignition (LOI, %), concentrations of major oxides (%, on organic-free basis) and trace elements (μg/g, on whole-rock basis) in the unleached fly ash samples. Sa mpl e

L OI

Si O2

SD FA

0. 29

LBA FA

10 .4 3

52 .2 1 48 .4 5

LB B FA

7. 71

63 .3

NN FA

1. 88

WX FA

8. 69

50 .9 7 44 .6 8

Ti O2 1. 15 2. 27 1. 72 1. 44 3. 56

2O

Al

Fe 2O

3

3

M g O

9.3 5

3. 34

19 .0 3 26 .7 3 23 .5 9 29 .5 9 23 .1 2

12. 51 5.0 4 9.7 7 18. 05

0. 85 5 0. 67 9 0. 71 7 0. 74 1

C a O 7. 7 7 3. 8 8 2. 4 9 2. 8 9 5. 6 7

Mn O

Na 2O

K2 O

P2 O5

S O2

Total oxide s

0.1 2

3. 44

1. 96

0. 29 3

0. 89 4

99.56

0. 55

1. 8

0. 27

2. 23

99.61

0. 26 8 0. 30 9 0. 40 9

1.1 1

0. 59 2

0. 87 9

99.73

1. 88

0. 25

1. 54

99.42

0. 87 2

0. 16 9

2. 15

99.52

0.0 60 4 0.0 60 2 0.0 63 0.1 02

Li

Be

B

F

Sc

V

C r

Co

Ni

C u

Zn

G a

Ge

As

S e

SD FA

55 .8

3. 86

14 45

21 3

65. 4

14 1

8 0

19. 6

40 .5

80 .1

17 1

29 .2

11.34

33 .1 8

LBA FA

21 3. 3

5. 46

13 8

15 8

54. 1

18 9

1 5 8

21. 9

53 .2

18 1

37 .7

6.02

8. 37

LB B FA

83 .2

3. 63

32

90

73. 7

21 5

6 9

23. 7

39 .8

22 7

29 .2

5.77

3. 45

NN FA

22 3. 5

5. 40

15 9

18 8

63. 1

13 78

22. 2

49 8

85 .3

26 5

45 .2

5.91

WX FA

96 .7

7. 27

61

10 5

65. 4

38 2

52. 8

80 .2

17 6. 3

10 8

37 .0

6.27

2. 3 1 7. 4 9 2. 6 9 6. 5 8 7. 6 7

Sa mpl e

Y

Zr

N b

M o

Ag

C d

Sn

Sb

Cs

Ba

La

Ce

Pr

SD FA

12 .3

30 7

25 .5

6. 81

1.4 2

0. 97

5.6 9

4. 50

15 .5 2

15 .4

94.3

4. 36

LBA FA

24 .6

53 3

40 .6

13 .6 4

2.0 2

1. 39

7.0 0

2. 33

3. 85

56 0

17 .3

74.9

5. 56

LB B FA

30 .6

40 1

14 .4

3. 30

1.5 9

0. 94

5.2 1

2. 34

4. 50

41 1

15 .6

74.4

5. 63

NN FA

20 .6

46 7

31 .9

37 .6 5

1.8 2

1. 59

7.8 1

4. 46

6. 94

47 6

13 .7

67.3

4. 08

WX FA

31 .4

49 5

55 .6

5. 39

1.7 7

0. 93

6.7 8

2. 15

2. 14

32 1

23 .5

128.6

7. 42

Sa mpl e

G d

Tb

D y

H o

Er

Lu

Hf

Ta

W

H g

Tl

Pb

SD FA

4. 30

0. 53

3. 03

0. 56

0.2 3

8. 30

2. 04

14 .5 3

1.09

33 .6

LBA FA

5. 67

0. 93

0.4 6

2. 66

3. 05

0.89

69 .1

LB B FA

6. 37

1. 01

0. 75

2. 49

0.43

46 .4

NN FA

4. 41

13 .8 3 10 .0 4 12 .9 0

2. 71

5. 04

1.30

96 .2

WX FA

8. 00

13 .11

4. 29

3. 06

0.56

38 .2

rn

T m

Jo u

1.6 6

0. 23

5. 70

1. 10

3.2 7

0. 48

6. 44

1. 27

3.9 6

0. 57

0. 69

4. 45

0. 87

2.7 0

0. 40

1. 18

7. 16

1. 38

4.0 1

0. 57

Pr Y b

1. 5 8 3. 1 8 3. 9 5 2. 7 4 3. 7 9

oo

0. 1 4 0. 2 4 0. 1 7 0. 2 7 0. 1 8

0.5 7 0.4 0 0.5 5

15 41

pr

In

e-

1 6 3 1 7 9

10 5. 9 11 7. 2

f

Sa mpl e

al

Journal Pre-proof

0. 13 4 0. 61 6 0. 15 6 0. 26 1 0. 62 6

12 .5 1 15 .1 3

N d 1 7. 1 2 2. 2 2 3. 9 1 6. 1 3 0. 8 Bi 0. 7 8 1. 0 5 1. 0 6 1. 6 3 0. 7 2

R b

Sr

95 .9

91 9

18 .3

59 5

16 .6

34 1

24 .0

41 0

24 .1

50 3

S m

Eu

3. 53

0. 83

5.1 1

1. 09

5. 98

1. 44

3. 87

0. 85

7. 13

1. 71

Th

U

5. 78

4. 92

9. 06

12 .5 1

9. 42

5. 05

10 .3 6

11. 54

8. 21

6. 48

Highlights    

the US EPA LEAF and TCLP test methods were conducted on five Chinese fly ashes. pH plays the most important role in controlling element leaching activities. The leaching of highly soluble elements is controlled by the concentrations in the fly ashes. The rank of feed coals does not obviously affect the leachability of the fly

Journal Pre-proof

f oo pr ePr al rn Jo u



ashes. For alkaline fly ashes, caution should be paid to elements (e.g., Se, Mo and Sr) that showed their maximum leaching under alkaline conditions which is also their natural pH conditions.