Leaching behaviour of selected trace elements in chemically weathered alkaline fly ash

The Science of the Total Environment, 76 (1988) 229-246 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

229

LEACHING B E H A V I O U R OF S E L E C T E D TRACE E L E M E N T S IN CHEMICALLY WEATHERED A L K A L I N E F L Y ASH

C. JAMES WARREN and MARVIN J. DUDAS Department of Soil Science, The University of Alberta, Edmonton, Alta. T6G 2E3 (Canada) (Received May 17th, 1988; accepted May 23rd, 1988)

ABSTRACT In a laboratory study alkaline fly ash was leached in a series of lysimeters with dilute H2SO4. The weathered residues retrieved after leaching were analyzed for major constituents by atomic absorption spectrophotometry and trace elements by instrumental neutron activation analysis. The characteristics of the weathered residues ranged from highly leached acidified material, from which many constituent elements had been mobilized, to minimally leached alkaline material containing accumulations of newly formed secondary minerals. The leaching behavior of constituent trace elements was related to the chemical environment of the leachates and partitioning among two previously identified major phases within parent ash particles and with newly formed secondary minerals. Elements such as Rb, Cs, Pb, Ta, Ti, and Hf were enriched in the highly leached portion of the residue sequence, suggesting association with the resistant internal Si-rich glass matrix of ash particles. Between 50 and 80% of the total Mn, Sb, Th, Cr, Zn, Co, Sc, and rare earth elements was also retained in the highly leached ash residues. About 50% of the total Sr, V, and U, and more than 80% of the total As and B was dissolved from the ash under acidic conditions. With the exception of B, all elements that were mobilized from the acidified ash residues were also attenuated in the alkaline residues in association with one or more of the newly formed secondary minerals.

INTRODUCTION D i s p o s a l o f t h e v a s t q u a n t i t i e s o f fly a s h p r o d u c e d a n n u a l l y i n N o r t h A m e r i c a is a c h i e v e d p r i m a r i l y t h r o u g h b u l k d e p o s i t i o n i n t e r r e s t r i a l e n v i r o n m e n t s . M o b i l i z a t i o n o f e l e m e n t s f r o m fly a s h is r e c o g n i z e d a s a p o t e n t i a l s o u r c e of contamination of terrestrial and aquatic ecosystems. A variety of particle fractionation and depth profiling studies have shown that many trace elements, s u c h a s A s , Cd, Se, Sb, M o , a n d Zn, a r e p r e f e r e n t i a l l y c o n c e n t r a t e d o n p a r t i c l e s u r f a c e s ( D a v i d s o n e t al., 1974; N a t u s c h a n d W a l l a c e , 1974; N a t u s c h e t al., 1974; L i n t o n e t al., 1976, 1977; H a y n e s e t al., 1982). O t h e r e l e m e n t s s u c h a s Co, Cr, Cu, N i , U, a n d V a p p e a r m o d e s t l y e n r i c h e d o n p a r t i c l e s u r f a c e s ( K l e i n e t al., 1975; H a n s e n a n d F i s h e r , 1980), w h i l e r e f r a c t o r y e l e m e n t s o c c u r a l m o s t e x c l u s i v e l y w i t h i n t h e g l a s s y m a t r i x a n d m u l l i t e c r y s t a l s o f fly a s h p a r t i c l e s ( N a t u s c h e t al., 1975; H u t l e t t e t al., 1980). M o s t i n v e s t i g a t o r s e v a l u a t e t h e d i s s o l u t i o n c h a r a c t e r i s t i c s o f fly a s h t h r o u g h t h e a n a l y s i s o f l e a c h i n g s o l u t i o n s o r e x t r a c t s (e.g. T a l b o t e t al., 1978;

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© 1988 Elsevier Science Publishers B.V.

230 Phung et al., 1979; Hansen and Fisher, 1980; Kopsick and Angino, 1981; Harris and Silberman, 1983; Roy et al., 1984). Few have attempted to analyze the leached or extracted solid residues, which usually contain measurable concentrations of most elements. The advantage of analyzing solid residues, compared with aqueous leachates, lies in the ability to detect changes in the content of many trace elements of interest whose concentrations in leachates are far too dilute to detect by most analytical methods. Analyses of weathered residues are not only highly sensitive but also allow assessment of attenuation of elements by the weathered residues. Determination of the relationships of trace elements with major ash constituents (e.g. Si, A1, Fe, and Ca) within ash particles and their co-release during weathering may eventually enable, through well-founded correlations, estimation of concentrations of many trace elements in solution through measurement of dissolved major constituents. In an earlier study, Warren (1983) examined the chemical and mineralogical transformations that accompanied the weathering of alkaline fly ashes. Emphasis was placed on the physico-chemical dissolution characteristics of solid phases in fresh ash as well as characterization of secondary mineral products formed within the leached residues during weathering. Dissolution characteristics of major ash constituents in relation to physical phases of fly ash particles as well as identification of newly formed precipitation products have been presented previously (Warren and Dudas, 1984; 1985; Dudas and Warren, 1987). The present paper reports on the release of constituent trace elements from alkaline fly ash and subsequent association of mobilized elements with the identified secondary mineral substances formed in the weathered ash residues. MATERIALSAND METHODS The fresh sample of fly ash used for this investigation was obtained from Alberta Power's Battle River power plant located near Forestburg, Alberta, Canada. The ash, classified as Alkaline Modic (Roy and Griffin, 1982), was derived from subbituminous-C coal containing approximately 4 g kg 1 sulfur and 70 g kg 1sulfur and 70 g kg 1 ash. One large dry bulk sample of ~ 20 kg was obtained during normal plant operation directly from the final collection bins of the electrostatic precipitator system. The sample was composed of more than 50% silt-sized (0.002-0.05mm) particles with spherical morphologies, as observed with the scanning electron microscope (Warren, 1983). Such particles are typical of the spherical morphologies associated with fly ash produced by most coal-fired thermal electric power plants (Fisher et al., 1978). The bulk composition and mineralogy of the ash was similar to those obtained from so-called "western" coals (Klein et al., 1975) and representative of material commonly used in disposal schemes at the Battle River power plant (Warren, 1983). The fresh ash was packed into a series of five leaching columns. The first column received 50 g of ash. Each succeeding column in the sequence received an extra 25 g such that columns two through five contained 75, 100, 125 and

231 150 g of fly ash, respectively. The five c o l u m n s were leached in sequence. The l e a c h i n g process was initiated by a d d i n g 500 ml of w h a t was initially 5 mmol l- 1 H2SO4 (pH 2.1) to the first c o l u m n in the s e q u e n c e ( c o n t a i n i n g 50 g of ash). After the s o l u t i o n h a d passed t h r o u g h the ash core and d r a i n a g e had ceased a 50 ml a l i q u o t of the l e a c h a t e was r e t a i n e d for analysis. The r e m a i n d e r of the l e a c h a t e s o l u t i o n was t h e n t r a n s f e r e d to the second c o l u m n and a second fresh 500ml i n c r e m e n t of 5 m m o l l 1 H2SO 4 was t h e n added to the first column. The p r o c e d u r e for sampling and t r a n s f e r of solutions was repeated for each i n c r e m e n t of l e a c h a t e after it h a d passed t h r o u g h each of the five c o l u m n s in the sequence. F u r t h e r details on the design of the columns, l e a c h i n g procedure, and s a m p l i n g are presented elsewhere (Warren, 1983; W a r r e n and Dudas, 1984, 1985). The a f o r m e n t i o n e d e x p e r i m e n t a l design was adopted so t h a t the l e a c h a t e s could be s y s t e m a t i c a l l y m o n i t o r e d after passing t h r o u g h and r e a c t i n g with i n c r e a s i n g volumes of ash. Ash of the first c o l u m n was leached with a t o t a l of 770 pore v o l u m e s of solution, while ash of the fifth c o l u m n was leached with only 192 pore v o l u m e s (Table 1). The ash of the first c o l u m n was leached with a r e l a t i v e l y large v o l u m e of n o n - n e u t r a l i z e d acid, while each s u b s e q u e n t c o l u m n in the sequence (each c o n t a i n i n g a g r e a t e r q u a n t i t y of ash t h a n the previous column) was leached with diminished v o l u m e s of solutions w h i c h were more t h r o u g h l y r e a c t e d with the ash and more alkaline. After ~ 90 days l e a c h i n g was terminated. At t h a t point l e a c h a t e s o b t a i n e d from the first two c o l u m n s in the s e q u e n c e were acidic and those from the t h i r d c o l u m n were b e g i n n i n g to decline t o w a r d s n e u t r a l i t y . The l e a c h a t e s o b t a i n e d from the final two c o l u m n s r e m a i n e d in the alkaline r a n g e t h r o u g h o u t the TABLE 1 Characteristics of the weathered ash residues and leachate pH valuesa Column

Leachate pH values b (increment number) 1

10

20

30

40

52

Total pore volumes passed

Minerals identified in the residuesc

Mullite, quartz, amorphous iron Mullite, quartz, amorphous iron Mullite, quartz, proto-imogolite Mullite, quartz, Mullite, quartz, Mullite, quartz

1

11.6

3.8

3.5

3.3

3.2

2.8

770

2

11.8

9.6

8.5

4.4

4.2

4.2

458

3

11.9

9.7

9.0

8.8

8.6

8.0

348

4 5 Fresh ash

11.7 12.0

10.0 10.3

9.0 9.0

9.0 8.8

8.9 8.7

8.7 8.7

255 192

oxyhydroxide gypsum, oxyhydroxide Ca-carbonate Ca-carbonate Ca-carbonate

aSummarized from Warren (1983) and Warren and Dudas (1984, 1985). bMean (n = 3) pH values for selected increments ofleachate solutions after passing through each of the columns in the sequence (Warren, 1983). cIdentification by XRD, IR, DTA and TEM (Warren and Dudas, 1985).

232 progress of the experiment (Table 1). The weathered residues from each of the five columns were then dried and sectioned into three or more horizontal layers. Subsamples of each of the layers and the fresh ash were analysed for chemical and mineralogical composition (Warren, 1983). Characterization of the residues included total sample dissolution and subsequent analysis of digests for major and some selected minor elemental constituents by atomic absorption spectrophotometry (AAS), and analyses of solids by X-ray diffraction (XRD), infrared spectroscopy (IR), differential thermal analysis (DTA), scanning (SEM) and transmission (TEM) electron microscopy (Warren, 1983; Warren and Dudas, 1984, 1985; Dudas and Warren, 1987). These analyses demonstrated that the leached ash residues represented a weathered sequence ranging from strongly weathered, highly leached, acidic material (ash of the first two columns) to moderately weathered alkaline material (ash of the latter three columns) where dissolution of particles was less than in the first columns and newly formed solids had accumulated as secondary minerals. A brief summary of the characteristics of the leached ash residues is included in Table 1. Detailed discussions on the mineralogical characteristics of the ash and leachate chemistry appear elsewhere (Warren, 1983; Warren and Dudas, 1984, 1985). For determination of content of trace elements, single subsamples of the fresh ash and each of the sectioned layers (20 in total) of the weathered ash, ranging in mass from 1 to 2 g, were submitted to Nuclear Activation Service Ltd. (McMaster University, Hamilton, Ontario, Canada) for instrumental neutron activation analysis (INAA). All samples were analyzed for content of selected trace elements including As, Au, Ba, Br, Ce, C1, Co, Cr, Cs, Dy, Eu, Ga, Hf, I, In, La, Lu, Mn, Mo, Nd, Ni, Rb, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, Ti, U, V, W, Yb, and Zn. The levels of B and Gd were determined by prompt-y analysis using the same samples. Simple linear correlation coefficients were calculated for each major element/trace element pair using the analytical data for total elemental composition of all 20 subsamples. Computations were carried out using the CORR procedure of the SAS statistical package (SAS Institute Inc., 1985). Accurate data were obtained for most trace elements based on results obtained for samples of certified standards seeded among the ash subsamples submitted for INAA. The elements Ni, C1, Ga, In, I, and Au occurred in the ash at concentrations below the INAA detection limits (50.0, 100.0, 60.0, 0.5, 5.0, and 0.005 mg kg 1, respectively). Levels of Br, Mo, and W in the ash were very close to INAA detection limits (0.5. 2.0, and 1.0mgkg 1, respectively) and varied considerably. As little information could be inferred from the data obtained for these nine elements they were omitted from further discussion. RESULTS Acidic leaching caused the selective dissolution of some elements from the ash of the first column in the sequence. Selective removal resulted in the depletion of relatively mobile elements from the first column and concomitant

233 enrichment of those elements partitioned into the less-soluble ash phases of the residues. Almost all of the elements translocated from the first and second columns were deposited within the subsequent columns of the sequence. Deposition was usually expressed as enrichment relative to the levels in the fresh ash. Only boron showed no enrichment in any of the ash residues. The redistribution of all trace constituents among the five columns displayed four distinct trends, each of which were typified by the behavior of one or more major element. Correlation coefficients for each major element/ trace element pair are shown in Table 2. The four redistribution trends are identified here as categories I through IV. The typical behavior of one of the maj or constituents and a selected companion trace element from each category are shown in Figs 1-4. Data for the contents of the other elements in the fresh ash and the leached residues are shown in Table 3. For category I, the elements B, Mn, Se, and Sr followed a behavior typified by Ca and Mg. In general, these elements were preferentially removed from the ash of the first and second columns and either deposited in the third, fourth and fifth columns (Warren and Dudas, 1985) or passed completely through the five ash columns (Fig. 1). Category II elements, typified by the behavior of Si and K, included Cs, Rb, Ba, Pb, Hf, Ta, and Ti. These elements were residual and preferentially enriched in the highly leached ash of the first and second columns (Fig. 2). Category III elements, As, Sb, V and Th, behaved similar to Fe. These elements were generally dissolved and removed from the upper layers of the ash of the first column under acidic conditions and deposited in the lower layers of the first column and the upper layers of the second column resulting in accumulation in these layers (Fig. 3). The rare earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, Lu), along with Cr, Co, Zn, U, and Sc, were typified by the behavior of A1, constituting category IV. These elements were mobilized from the ash of the first column and the upper layers of the second column and subsequently accumulated in the lower layers of the second column and the upper layers of the third column (Fig. 4). DISCUSSION

Category I elements Calcium was depleted from the ash in quantities that exceeded all other major constituents. Nearly 70% of the Ca was removed from ash of the surface layer of the first column (Fig. 1). Magnesium levels in the ash displayed strong positive correlation with levels of Ca (Table 2) and was similarly removed from the ash of the first column (Table 3). Smaller amounts of Ca and Mg were removed from subsequent layers of the first column and the ash of the second and third columns. The levels of Ca and Mg in the lower layers of columns four and five were not substantially different from the content of the fresh ash. Calcium and Mg primarily from the first two (acidified) columns were deposited within the ash of the fourth and fifth columns (Fig. 1) in the form of carbonates

234 TABLE Linear

2 correlation

stituents

coefficients

in the weathered Ca

for measured

Mg

Ca

1.000

Mg

0.898**

levels of all elements

with

levels of the major

con-

ash residues Si

K

Na

Fe

A1

1.000

Si

- 0.951"*

- 0.862**

1.000

K

- 0.708**

- 0.708**

0.802**

1.000

Na

0.015

0.012

0.008

0.271

1.000

Fe

- 0.117

- 0.049

- 0.027

- 0.051

- 0.168

1.000

- 0.090

- 0.147

1.000

0.013

- 0.119

0.372

0.002

- 0.118

- 0.302

0.172

A1

0.608*

0.505

- 0.780**

- 0.775**

B

0.840**

0.697**

- 0.739**

- 0.584*

Mn

0.929**

0.910"*

- 0.914"*

- 0.756**

Se

0.441

0.403

- 0.338

- 0.354

0.550*

Sr

0.603*

0.647* - 0.023

- 0.737**

- 0.744**

0.084

0.095

Rb

- 0.474

- 0.589*

0.361

0.225

0.019

- 0.023

0.748** 0.034

Cs

- 0.778**

- 0.813"*

0.654*

0.392

0.022

0.189

- 0.250

Ba

- 0.863**

- 0.793**

0.754**

0.423

0.087

0.173

- 0.364

Pb

- 0.582*

- 0.662*

0.610"

0.799**

0.357

0.136

- 0.580*

Ti

- 0.623*

- 0.576*

0.555*

0.525

0.589*

- 0.514

Hf

- 0.910"*

- 0.833**

0.828**

0.565*

0.136

0.144

- 0.469

Ta

- 0.218

- 0.278

0.310

0.554*

0.100

- 0.102

- 0.381

- 0.140

As

0.210

0.305

- 0.307

- 0.309

- 0.227

0.880**

- 0.001

Sb

- 0.055

- 0.056

- 0.017

- 0.114

- 0.084

0.732**

- 0.159 0.169

0.188

0.176

- 0.347

- 0.298

- 0.210

0.882**

Th

V

- 0.304

- 0.292

0.068

- 0.137

- 0.107

0.663**

0.158

Cr

- 0.000

0.018

- 0.219

- 0.502

- 0.050

0.358

0.471

Zn

- 0.116

- 0.063

- 0.093

- 0.481

- 0.146

0.101

0.499

Co

0.403

0.505

- 0.573*

- 0.803**

- 0.064

0.069

0.728**

U

0.713"*

0.794**

- 0.786**

- 0.816"*

- 0.075

- 0.175

0.773**

Sc

0.334

0.291

- 0.531

- 0.723**

- 0.135

0.018

0.811"*

La

0.354

0.453

- 0.535

- 0.749**

0.014

0.014

0.742**

Ce

0.315

0.421

- 0.494

- 0.727**

- 0.020

0.022

0.709**

Nd

0.283

0.408

- 0.460

- 0.727**

- 0.010

0.009

0.686**

Sm

0.287

0.400

- 0.476

- 0.728**

- 0.038

0.016

0.723**

Eu

0.040

0.156

- 0.213

- 0.374

- 0.212

0.001

0.514

Gd

0.451

0.464

- 0.576*

- 0.686**

- 0.101

0.061

0.643*

Tb

0.095

0.241

- 0.233

- 0.405

- 0.066

0.288

0.305

Dy

0.696**

0.792**

- 0.792**

- 0.801"*

- 0.026

- 0.094

0.777**

Yb

0.456

0.572*

- 0.603*

- 0.821"*

0.009

- 0.046

0.744**

Lu

0.509

0.628*

- 0.657*

- 0.851"*

- 0.065

- 0.007

0.769**

* Significance

(Table

level

1; Warren

carbonates

in

quantities the

and the

ash to

Ca

and

resulting

(Warren and

Dudas,

fourth

leached,

fresh

similar

= 0.01; * * S i g n i f i c a n c e

Mg

level

1985). fifth in

The

amounts

columns

little

net

and

Dudas,

1985).

(i.e.

B,

Se,

Mn,

= 0.001.

and

were change

Trace Sr)

of

in

the

elements were

Ca

and

Mg

approximately total with

generally

deposited equal

levels leaching depleted

to

as the

relative

to

behavior from

the

235 3000

O')

2000

E o 0

1000

Control

Column 1 Column 2 Column 3 Column 4 Column 5

120 1 O0 80'

)

E 0 -,Q .>(

o

•

60

fresha s h ~ layer 1 • layer 2 layer 3 layer 4 layer 5

40

m 0

20 r~

0 Control

Column

1

Column

2

Column

3

~!~

Column

ml~_.~..,'.l 4

Column

5

Fig.1. Levels of CaO and B in the fresh ash and the weathered residues. Layer one represents the surface layer of ash residue in each column with additional layers representing increasing depths within the columns. first and second columns with little or no net a c c u m u l a t i o n in the lower columns. B o r o n was by far the most mobile ash c o n s t i t u e n t . Depletion of up to 90% (2290 mg kg -1) r e l a t i v e to the fresh ash was observed in the first c o l u m n (Fig. 1) with most of the s u b s e q u e n t ash layers depleted by at least 50% (1250 mg k g - 1). This i n d i c a t e d t h a t most of the B in the fresh ash was associated with r e a c t i v e fractions; most likely as soluble B203 or p o l y b o r a t e s (Cox et al., 1978). Loss of B from the ash in the l a t t e r columns of the s e q u e n c e i n d i c a t e d t h a t the e l e m e n t did not p r e c i p i t a t e like Ca or Mg n o r was it adsorbed in s u b s t a n t i a l quantities. This lack of a t t e n u a t i o n of b o r o n suggested t h a t its a c t i v i t y in s o l u t i o n did not exceed the solubilities of a n y B - c o n t a i n i n g solids and t h a t a d s o r p t i o n sites specific to B were not p r e s e n t in the w e a t h e r e d ash.

236 100 80

60 E

40

"0 o= ,.J

20 0 Control

Column 1

Column 2

Column 3

Column 4

Column 5

800

600

fresh ash layer 1 layer 2 layer 3 layer 4 layer 5

400 i

200

Control

Column 1

Column 2

Column 3

Column 4

Column 5

Fig. 2. Levelsof SiO2and Pb in the fresh ash and the weatheredresidues. Layer one represents the surface layer of ash residue in each columnwith additional layers representing increasing depths within the columns. Much of the B may have been leached from the ash before significant quantities of secondary minerals capable of it's attenuation could form. Any B that remained in the ash of the first column after leaching was likely contained in the less soluble interior glassy matrix of the particles (Warren and Dudas, 1985; Dudas and Warren, 1987), possibly in the form of borosilicates of limited solubility (Cox et al., 1978). Manganese displayed a leaching behavior similar to B, although the total quantities of Mn dissolved from the ash over the entire column sequence was not nearly as substantial (Table 3). Manganese remaining in the acidified ash residues was likely associated with the less soluble glass fraction and/or ferromagnetic particles (Hulett et al., 1980; Warren, 1983). Accumulation of Mn in the third, fourth, and fifth columns coincided with accumulation of carbonate (Table 3), suggesting coprecipitation with CaCO3.

237 60 50 O)

E 0 ¢: Q

P ,,¢

40 30 20 10

|

0 Control

Column

1

Column

2

Column 3

i

Column

4

Column

5

8O

60 E • "0 •~ o ,,

40

• [] []

20

fresh ash layer 1 layer 2 layer 3 layer 4 layer 5

0 • Control

Column

1

Column

2

Column 3

Column

4

Column

5

Fig. 3. Levels of Fe203 and As in the fresh ash and the weathered residues. Layer one represents the surface layer of ash residue in each column with additional layers representing increasing depths within the columns. Depletion of Sr in the ash of the first column (Table 3) indicated t h a t it is also associated preferentially with accessable soluble surface phases of fly ash particles (Dudas and Warren, 1987). The presence of much of the total Sr in the fresh ash as SrO and/or SrCO3 in association with the external glassy matrix (Bauer and Natusch, 1983; Warren and Dudas, 1984; Dudas and Warren, 1987) may explain the observed behavior. Elevated levels of Sr in the second column relative to the levels in prior and subsequent columns suggested association with sulfate precipitates r a t h e r than carbonate precipitates. Accumulation of Sr as sulfate was likely an artifact of the high sulfate concent rat i on in the leachates as similar accumulation in the second column was not observed when leaching was performed with solutions not containing H2SO4 (Warren, 1983).

238 60

50 J~

E E E

,90 e.

0

40 30 20 10 0 Control

Column 1 Column 2 Column 3 Column 4 Column 5

250 200 E 0 "0 X

150 • 1O0

o 50 ¸

fresha s h ~ layer 1 I layer 2 I layer 3 I layer 4 I layer~¢li~i I

Control

Column

1

Column 2

Column 3 Column 4 Column 5

Fig. 4. Levels of A1203and Cr in the fresh ash and the weathered residues. Layer one represents the surface layer of ash residue in each column with additional layers representing increasing depths within the columns. T h e m e a s u r e d c o n t e n t of Se in the residues was variable. Selenium displayed no significant c o r r e l a t i o n s with a n y of the m a j o r ash c o n s t i t u e n t s (Table 2). The c o n t e n t of Se in the surface l a y e r of the first c o l u m n was r e d u c e d by o v e r 80% ( 4 . 5 m g k g -1) c o m p a r e d with the fresh ash (Table 3). P r e v i o u s studies suggest t h a t Se is c o n d e n s e d o n t o particle surfaces from the flue gasses d u r i n g cooling (Davidson et al., 1974; A n d r e n et al., 1975). In this study, with the e x c e p t i o n of the surface l a y e r of the first column, only ~ 0.85 mg kg-1 (15%) of the t o t a l Se was r e m o v e d from all ash cores d u r i n g the l e a c h i n g experiment, which suggested t h a t most of the Se in a l k a l i n e ash o c c u r s in forms t h a t are sparingly soluble e x c e p t u n d e r e x t r e m e l y acidic conditions.

0 0 0 0

Col. 2 5400 4600 46OO 45OO

"As CaCO 3.

89 97 89 83 89

Col. 5 3600 3800 3700 3400 3800

6.4 7.2 6.7 6.4 6.8

7.5 7.3 7.0 6.9

89 100 89 89

Col. 4 40(}0 40O0 3800 38O0

35 50 80 56 38

37 36 86 68

7.9 5 8.4 5 8.6 8 8.9 43

Col. 3 4300 108 48O0 121 45OO 121 4300 121

7.4 7.8 7.8 8.1

0 0 0

Col. 1 5600 75 5.5 55OO 80 6.2 5200 84 6.4

102 100 106 105

0 6.5 0.75 7.1 0.76 7.5 0.84

Hf

10.1 10.2 10.3 10.3

11.6 11.5 11.6 12.5 0.91 0.79 0.81 1.03

0.84 1.31 1.23 1.01 11.4 11.9 10.5 10.3

11.4 11.0 10.8 15.7

14.0 15.8 14.4 14.4

8.8 9.1 8.7 7.8 8.8

9.4 9.0 8.9 8.8

10 11 10 10

11 10 10 10

7.5 13 8.8 13 8.9 12

2.0 9.8 0.84 12.6 2.1 10.5 1.13 10.8 1.9 10.0 0.75 11.4 2.0 10.1 0.63 11.7 2.0 9.8 0.78 10.5

2.5 2.2 1.9 1.9

2.3 2.6 2.3 2.2

Gd

9.1 0.78 14.0 10

Eu

2.5 9.2 1.07 2.6 9.5 0.81 2.4 10.2 1.23 2.2 10.1 1.22

2.8 2.7 2.8

2.3

Ce Co CO~ Cs Dy (rag g 1)

Fresh 4400 103 7.9

Ba

Element

192 192 180 176 176

181 187 186 186

179 173 172 170

182 177 179 173

213 206 197

176

Lu

Mg (rag g - l )

47.8 53.3 50.2 45.2 49.1

53.2 53.7 48.6 48.2

58.8 64.5 62.9 67.1

53.3 55.7 58.0 57.0

0.96 1.02 0.98 0.93 0.99

1.06 1.05 1.01 1.00

1.17 1.33 1.31 1.48

1.05 1.11 1.17 1.20

224 234 238 236 237

226 235 245 258

219 219 228 265

206 208 223 220

41.4 0.66 154 44.6 0.76 175 46.2 0.80 177

57.1 1.16 243

K La (mg g - l )

300 320 320 330 320

310 330 320 310

310 310 320 320

270 280 300 300

359 370 368 355 358

364 377 374 354

376 365 350 367

368 352 369 349

200 368 230 374 230 352

300 372

30 33 31 29 30

34 33 33 33

41 48 44 48

37 37 40 40

31 34 29 28 31

35 31 26 32

36 37 29 30

39 38 32 34

2.2 2.5 2.8 2.8 2.9

2.4 2.4 2.6 3.0

2.8 2.9 2.4 2.6

3.1 2.8 2.6 2.6

5.3 6.8 4.6 6.8

3.2 3.4 5.8 3.4

5.5 2.0 3.1 6.9

1.1 3.9 5.0

5.7

14 4.8 15 2.8 14 4.9 13 6.1 14 17

15 15 14 14

17 19 17 16

17 18 19 19

26 37 2.1 11 28 34 3.1 12 29 34 3.4 13

40 32 3.1 16

Mn N a Nd Rb Sb Sc Se (mgg-1)

Sr

T a Tb Th Ti

7.65 9.05 8.27 7.39 7.93

8.95 8.34 8.43 7.98

10.1 11.5 11.4 11.7

9.14 9.50 9.93 9.80

1300 1400 1300 1200 1300

1500 1400 1400 1300

1400 1600 1500 1400

1600 1400 1700 1500

1.5 1.5 1.3 1.4 1.5

1.8 1.6 1.2 1.4

1.3 1.2 1.1 1.3

1.2 1.2 1.3 1.3

1.3 1.3 1.4 1.3 1.4

2.1 1.7 1.5 1.9

1.7 1.9 1.7 1.8

1.7 1.5 1.7 1.7

20 21 20 18 20

22 21 20 20

22 24 22 22

27 25 22 22

V Yb

2400 2600 2500 2800 2400

2600 2200 2500 2700

Zn

7.7 8.6 8.6 9.1

86 72 72 70

6.62 6.52 7.37 7.61

8.0 8.0 8.2 7.6 8.0

8.5 8.9 8.4 8.4

70 76 72 73 71

70 69 75 73

5.33 6.09 5.73 5.34 5.78

6.53 6.34 6.05 5.70

77 87 86 76 79

81 77 87 84

130 140 140 120

120 125 130 140

4.6 39 4.00 87 5.3 67 4.50 99 5.2 99 4.43 100

8.8 71 7.47 120

U

2600 9.7 75 7.40 2300 10.6 73 7.89 2600 10.7 72 8.18 2500 12.0 70 9.20

2500 2800 2700 2300

6.79 800 1.6 1.2 18 2700 7.21 1100 1.5 1.5 23 3500 7.68 1100 1.5 1.7 26 3600

9.56 1700 1.1 1.5 21 2500

Sm

T o t a l c o n c e n t r a t i o n s o f c o n s t i t u e n t s i n f r e s h a s h a n d t h e w e a t h e r e d r e s i d u e s f r o m e a c h c o l u m n . V a l u e s a r e i n m g k g -1 e x c e p t w h e r e m g g - 1 is o t h e r w i s e indicated. E n t r i e s for e a c h of t h e five c o l u m n s r e p r e s e n t a n a l y s e s of the i n d i v i d u a l l a y e r s w i t h i n t h a t c o l u m n

TABLE 3

240

Category I I elements The measured concentrations of Si, K and, to a lesser extent, Na appeared enriched in the ash of the first column (Fig. 2, Table 3), primarily due to the effects of removal of large quantities of Ca and Mg (Warren and Dudas, 1984). Levels of Si and K were negatively correlated with levels of Ca and Mg (Table 2). The amount of enrichment was less with each subsequent layer within the first and second columns. Levels in the third, fourth and fifth columns were not substantially different from that of the fresh ash. A major portion of all alkali metals in the ash are apparently associated with the less soluble internal glassy matrix of ash particles (Warren and Dudas, 1984; Dudas and Warren, 1987). The magnitude of enrichment of the larger alkali metals, Rb and Cs, in the ash of the highly leached first column was similar to K, which in turn was not as great as the enrichment of Na (Table 3). Enrichment of Cs and Rb in ash of the first column supports the suggestion of Warren (1983) that alkali metals are primarily associated with the less soluble internal matrix of fly ash particles and that their removal is accomplished through a process of solid state migration; similar to the process that occurs in other glassy materials (Jambon and Carron, 1976; White, 1983). The leaching behavior of Pb (Fig. 2), Hf, Ta, and Ti (Table 3) in the ash layers was similar to Si and appeared related to their association with the residual glass of the particles (Dudas and Warren, 1987). Levels of these elements in the residues generally showed strong positive correlations with Si and K and strong negative correlations with Ca and Mg and the other elements of category I (Table 2). Similarity in the leaching behavior of Ba (Table 2) with Si was likely due to association with SO42-. The high concentration of SO42maintained in solution through H2SO4 additions would promote BaSO t precipitation and consequently inhibit the leaching of Ba. In other studies, leaching with solutions other than H2SO4 caused Ba to behave similar to Ca and Mg (Warren, 1983). Precipitation of Pb in the form of PbSO4 could have occurred as with BaSO4, however Pb was also enriched in the ash of the first column when solutions other than H2SO4 were used for leaching (Warren, 1983). Therefore, unlike Ba, the majority of Pb in the fresh ash is likely associated with the internal glassy matrix of the ash particles and hence would not be leached even under extended acidic weathering. The refractory trace elements Ti and Hf commonly substitute for Si in the crystal structure of clay minerals (Kabata-Pendias and Pendias, 1984). As the detrital clays in coal form the bulk of the materal constituting fly ash, these would predictably be incorporated with Si into the matrix of the ash particles and similarly enriched in the highly leached residues.

Category III elements Translocation of Fe was limited to the cores of the first and second columns (Warren and Dudas, 1984). Iron was dissolved primarily from the surface layer

241 of the first column and eluviated downward in a chromatographic fashion into the lower layers of the first column and the surface of the second (Fig. 3). Iron precipitated as an amorphous oxyhydroxide in response to increases in the leachate pH values (Warren and Dudas, 1985). Columns in the sequence other than the first and second displayed no evidence of Fe translocation, which suggested acidic conditions are required for mobilization of iron in the ash. Of the trace constituents detected in the weathered residues the levels of As, Sb, V, and Th were strongly correlated with Fe (Table 2). Soluble forms of As were not directly associated with all soluble Fe phases in fly ash as suggested by Theis and Wirth (1977), who found more than 90% of the total As in fly ash to be associated with an acid ammonium oxalate extractable Fe fraction. In the present study, over 80% (35 mg kg -1) of the total As was removed from the surface layer of the first column and deposited primarily in the layers immediately below it (Fig. 3). Leaching removed essentially all of the oxalate extractable Si and A1 from the ash of the surface of the first column but only ~ 60% of the extractable Fe (Warren, 1983; Warren and Dudas, 1984). Most of the total As was therefore associated with the exterior glass of the ash particles, which contained only ~ 60% of the oxalate extractable Fe. Accumulation of As in the lower layers of the first column and surface layers of the second suggested attenuation mechanisms involving adsorption and retention by precipitated Fe oxyhydroxides. Arsenic mobilized and subsequently retained by Fe precipitates in the ash residues was likely in the oxidized (As 5÷) form (Turner, 1981). Arsenate is strongly adsorbed by secondary Fe minerals above pH 5 (Guledge and O'Connor, 1973). Some leaching of As under alkaline conditions was indicated by loss of the element from the upper layers of the nonacidified columns (three, four, and five). Little or no change occurred in the content of As in the lower layers of the third, fourth, and fifth columns relative to the fresh ash, indicating that As was not leached in significant quantities through the sequence of columns. The enrichment/depletion pattern for Sb (Table 3) in the weathered residues indicated leaching behavior was very similar to As, although differences in levels of Sb among adjacent layers in the first column were not as pronounced as for As. The leaching behavior of the other three elements in the category with Fe could be explained based on formation and reaction of oxyanion hydrolysates with the secondary iron minerals formed in the weathered ash. About 45% of the total V in the fresh ash was removed from the surface layer of the first column. Accumulation of V was observed in the lower layers of the first column and the surface layers of the second column (Table 3) where secondary Fe minerals were found (Table 1). Established mechanisms of attenuation of V by Fe include precipitation as Fe(VO3)2 and/or adsorption of VO~ on Fe oxyhydroxide surfaces (Rai and Zachara, 1984). The naturally occurring actinide metal Th was slightly depleted in the surface of the first column, relative to the fresh ash, and enriched in the four adjacent layers located immmediately below (Table 3). Depletion of Th from the surface of the first column suggested

242

association with the external glassy matrix of the ash particles. Association of Th with Fe-precipitates in the ash residues may be explained by the characteristic of Th to form complex hydroxides at pH values greater than about 3.0 (Cotton and Wilkinson, 1972). Adsorption and/or coprecipitation of Th by the Fe oxyhydroxides would therefore be facilitated in a manner similar to that of other oxyanions.

Category I V elements The redistribution of A1 within the leaching columns was most evident in the first three cores of the sequence (Fig. 4). Aluminium was translocated primarily from the ash of the first column under acidic conditions and accumulated in the lower layers of the second column and the upper layers of the third as a coprecipitate with Si in the form of proto-imogolite (Warren and Dudas, 1985). Accumulation of A1 in the ash of the second and third leaching columns was accompanied by accumulation of Cr, Zn, Co, U and Sc along with the rare earth elements La, Ce, Nd, Sin, Eu, Gd, Tb, Dy, Yb and Lu. The short-range order crystalline aluminosilicate material formed in the ash likely provided adsorption sites for many of these trace elements. However, precipitation of trace elements in ash which coincided with the occurrence of proto-imogolite may have also occurred in response to decreased proton activity. The multivalent transition metal Cr usually exists in nature in one of two valence states, Cr(III) or Cr(VI). Chromium was depleted from most columns relative to the fresh ash but enriched in the bottom layer of the first column as well as the second and third columns relative to the other columns in the sequence (Fig. 4). Loss of ~ 20% ( < 10 mg kg 1) of Cr in the nonacidified fourth and fifth columns relative to the fresh ash indicated the element could also be mobile under alkaline conditions. Other researchers suggest that Cr(III) is the prevalent oxidation state in fly ash and that Cr(III) is strongly adsorbed by Fe oxyhydroxides (Rai and Zachara, 1986). In the present study the levels of Cr in the residues were not highly correlated with either A1 or Fe alone (Table 2); however, the correlation coefficient for Cr with levels of A1 plus Fe in each layer was 0.606, which was significant at the 0.001 level. Accumulation of Cr in both the second and third columns, relative to the other columns, suggested both secondary aluminosilicates and Fe-precipitates contribute to the attenuation of Cr. Zinc displayed a leaching pattern similar to A1 except that it accumulated predominantly in the ash of the third column (Table 3). Although Cd was not determined in this study the weathering behavior of Cd, which belongs to the same group in the periodic table as Zn, can be inferred from the behavior of the latter element. Based on average lithospheric contents, it is expected that levels of Cd in the fresh ash would be roughly two orders of magnitude less than the levels of Zn (i.e. ~ 1.2mgkg 1 Cd). Adsorption of Zn and Cd tends to increase with increasing pH, thus the combination of the presence of the aluminosilicate precipitates and high pH values would be ideal for retention in

243 the third column. Loss of some Zn (~ 30% of fresh ash levels) from the alkaline fourth and fifth columns may be attributed to the formation of soluble Zncarbonate species such as Zn(CO3)~ or ZnCO3 (Rai and Zachara, 1984). Carbonates of Cd are not as soluble as those of Zn, hence Cd would be retained in the ash more so than Zn under alkaline leaching conditions. Much of the Co removed from the ash of the first column was deposited in the lower layers of the third column (Table 3), indicating that the majority of the total Co was associated with the relatively less soluble internal matrix of the particles. Cobalt is normally strongly adsorbed onto Fe oxyhydroxides (Kabata-Pendias and Pendias, 1984), but this was not evident from the present data. The pattern of accumulation of Co in the third column suggested attenuation by aluminosilicate material rather than by adsorption onto the precipitated Fe oxyhydroxides. Nearly half of the U in layers of the first column was dissolved and then deposited primarily within the ash of the second and third columns (Table 3). Approximately a half of total U was therefore associated with the more soluble phases of the ash particles. Under acidic leaching conditions, UO~ ÷ is the stable species in solution (Harmsen and de Haam, 1980). The solubility of carbonate complexes of U increases with increasing pH, while the solubility of UO~ ÷ decreases. Accumulation of U in the third column of the sequence was therefore attributed to precipitation reactions in response to increases in the leachate pH values or adsorption by the secondary aluminosilicates. Of the 14 naturally occurring rare earth elements (REE) the concentration ofLa, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, and Lu were determined for the fresh ash and each of the residue samples (Table 3). The levels of most of the REE were strongly correlated with the levels of A1 and negatively correlated with K (Table 2). The leaching behavior for all REE and Sc was very similar and typified by La (Table 3). No differences in leachability were evident among the heavy and light REE. The greatest quantities were removed from the ash of the first column. Losses also occurred in the fourth and fifth columns, indicating some mobility under alkaline conditions. The second and third columns contained some accumulation of REE, especially in the lower layers of the third column below the layers enriched in A1. Deposition in the ash cores at a position later in the sequence than for the deposition of A1 indicated that attenuation of REE and Sc by the ash was primarily due to precipitation reactions in response to increases in pH values rather than adsorption or coprecipitation with aluminosilicates. CONCLUSIONS This study provided empirical data on the potential dissolution and attenuation behavior of trace elements during the weathering of alkaline fly ash. Although specific trace element phases could not be identified, the correlations point out several similarities that exist among the weathering behavior of trace elements and the major ash constituents. There is a definite partitioning of all

244 trace elements among the relatively soluble and insoluble phases that make-up fly ash particles. The potential exists for mobilization and removal of major portions of the total contents of those trace elements that are preferentially enriched in soluble phases. Enrichment of mobilized trace elements in the residues where secondary minerals arc also observed indicated that adsorption, coprecipitation, and/or solid solution formation involving secondary minerals and trace elements are likely active during the weathering process. Based on the analyses of the highly leached residues from the first column it is apparent that the largest fraction of most trace elements in alkaline fly ash is associated with the more soluble Si-rich internal glass matrix of fly ash particles. With the exception of B, Se, and As, > 50% of the total quantities of all trace constituents remained in the ash residues of the highly leached first column. Hence, the greatest proportion of most trace elements in fly ash are not likely to be readily mobilized even with acidic leaching. The elements Rb, Cs, Pb, Ta, Ti, and Hf were significantly enriched in the highly leached acidified ash, suggesting almost exclusive partitioning into the internal matrix of ash particles. The less soluble phase in the ash also contained between 50 and 80% of the total Mn, Sb, Th, Cr, Zn (Cd by inference), Co, Sc and the rare earth elements. The elements Sr, V and U were partitioned approximately equally between the readily soluble and relatively insoluble phases of the ash particles. Of the three exceptionally mobile elements, only B was not attenuated in significant quantities by precipitates and secondary minerals formed during ash weathering. The pronounced leachability of boron in this study highlights the special consideration that should be given this element in disposal schemes involving alkaline fly ashes. Secondary minerals formed from the major elements in the ash contributed to trace element retention, as did trace element response to changes in leachate pH values. Although much of the As is in a soluble form in the fresh ash it was either adsorbed onto secondary Fe minerals or precipitated as sparingly soluble compounds in this leaching experiment. The elements Sb, V, Th and Cr were similarly adsorbed or coprecipitated by Fe oxyhydroxides which formed within the ash during weathering. Chromium was also retained by protoimogolite which formed in ash in a separate layer from the Fe minerals. Attenuation of mobilized U, Sc, and the REE was apparently not related to reaction with secondary minerals but' to direct precipitation in response to increases in the leachate pH values. Further investigations must focus on the specific role of secondary weathering products in attenuation processes. ACKNOWLEDGEMENTS We wish to thank the Electric Power Research Institute (EPRI) for financial support of this project through RP2485-01.

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