Leachability of toxic elements from solid wastes

Journal ofRadioanalytical andNuclear Chemistry, Articles, Vol. 161, No. 1 (1992) 181-187

LEACHABILITY OF TOXIC ELEMENTS FROM SOLID WASTES

S. S. KRISHNAN,*,** R. E. JERVIS," L. D. VELA*

*Department of Chemical Engineering, University of Toronto, Ontario, MSS 1.44 (Canada) **The Toronto Hospita L General Divisiol~ Medical Physics Department,

Room CCR W-G-803, 200 Elizabeth Street, Toronto, Ontario, M5G 2C4 (Canada)

(Received December 13, 1991)

We have examined the leachability of the toxic elements cadmium, arsenic, mercury, and selenium from solid wastes. The solid wastes studied are municipal incinerator ash, coal fly ash, hospital incinerator ash, raw sewage sludge, sewage incinerator bottom ash, and sewage incinerator lagoon ash (which is a combination of bottom and fly ashes). Cadmium displayed the greatest leachability in all waste types, with 76% leached from lhe municipal refuse incinerator ash. Although the sources of elements in the wastes are diverse, the leachability and hence the bioavailability in the incinerator ash appears mainly determined by the volatility of the element.

INTRODUCTION

Millions of tons of different types of solid wastes are stored or disposed

of in landfill sites. It is therefore important to determine and understand the

leachability of toxic elements from such wastes to determine the environmental

impact of such leachates. A number of leaching tests are now in use. The United

States Environmental Protection Agency (USEPA) uses a test known as the Toxicity

Characteristic Leaching Procedure (TCLP) ~, and the Ontario Ministry of the

Environment in Canada uses a test named Regulation 3092. Both these tests are

essentially acetic acid extraction at a specified pH and temperature. Materials

that fail the tests are regulatorily interpreted to present a hazard to the

environment, and must be appropriately disposed as hazardous wastes. Other

environmentalists have attempted to predict the environmental mobility, and hence

the bioavailability, of the elements from the perceived chemical associations

(also known as 'solid speciation') of the elements. TO achieve this, multi-step

sequential extraction procedures 3'4 have been developed. The sequential procedure

used in this work is based on that of TESSIER et al 4. This procedure isolates

the elements into five fractions, viz. (i) readily exchangeable fraction (i.e.,

readily soluble), (ii) bound to carbonates, (iii) bond to iron and manganese

oxides, (iv) bound to organics and sulfides, and (v) lattice-held residual

material. In this work both the single and sequential extraction procedures were

applied to study the leaching of different solid wastes.

Elsevier,Sequoia S. A., Lausanne Akad6miai Kiad6, Budapest

S. S. KRISHNAN el al.: LEACHABILITY OF TOXIC ELEMENTS

EXPERIMENTAL METHODS

The samples were air-dried at room temperature, homogenized, sieved through

a 200-mesh sieve, and stored in polyethylene bottles at room temperature. This

sample fraction consisted of particles less than 74 ~m in diameter.

Sinqle Extraction (Leachinq) Test: 20 mL of ammonium acetate - acetic acid

buffer at pH 5 was added to samples weighing ig. The samples were agitated on

a wrist-action shaker for 18 hours. The solid and the leachate was centrifuged

for i0 minutes, and the leachate filtered through a 0.45 gm Millipore filter.

The residues were dried under an infra-red lamp. The elements in the leachates,

residues, and unleached solids were determinedby instrumental neutron activation

analysis (INAA), and the mass balance was checked to ensure no extraneous losses

of the trace elements.

Multi-Step Sequential Extraction: The sequential extraction procedure

outlined in Table 1 was used. Five fractions were extracted, representing

different chemical species of the elements.

Instrumental Neutron Activation Analysis: The analysis for the elements

was done using INAA. Neutron irradiations were done at the University of Toronto

Slowpoke-2 Reactor facility at a neutron flux of i0 n to 1012 nslc~ 2. Irradiation

was done for 5 min., 30 min., or 16 hours on same sample so that different

radioisotopes of varying half-lives can be counted. The cooling time varied from

1 to 2 weeks for the 5 min. irradiation, 2 to 4 weeks for the 30 min. irradiation

and 3 to 6 weeks for the 16 hour irradiation. The counting was done when the

dead time was less than 10%. The counting times varied from 5 min. to 2 hours

so that statistically significant counts were accumulated. The indicator

isotopes used in this work were As ~, Cd nS, Hg l~, Hg ~, and Se 75. Each type of

solid waste and the leachates were analyzed at least in triplicates. Pure atomic

absorption liquid standards were used as comparator standards. The method was

checked and accuracy confirmed by analyzing NBS Standard Reference Materials

1633a (coal fly ash) and 1645 (river sediment). Gamma-ray spectra of samples and

standards were acquired by an APTEC/NRD hyperpure Ge detector (active volume 93

c.c.; 21.6% efficiency relative to sodium iodide detector) interfaced to an

APTEC/MCARD2 PC-based multi-channel analyzer system.

RESULTS AND DISCUSSION

Waste Composition. The total element content of various wastes are

presented in Table 2. Arsenic is the highest in coal fly ash (150 ~g/g) and next

in refuse incinerator ash (106 ~q/g). Cadmium (563 gg/g), mercury (7.5 gg/g),

and selenium (49 gg/g) are the highest in refuse incinerator ash.

18 hr. Leachinq: In the 18-hour leaching experiments the extraction of

cadmium varied from 14.2% in the case of sewage incinerator ash to 76.4% in the

case of refuse incinerator ash (Table 3). Arsenic was extracted to the extent

of 7.1% from sewage lagoon ash, and to 32.7% in the case of coal fly ash.

182

S. S. KRiSHNAN el al.: LEACHABIHTY OF TOXIC ELEMENTS

Table 1

Sequential Extraction Procedure.

Fraction

NO.

i.

2.

3.

4.

5.

Chemical Binding

Readily-Exchangeable

Carbonate-bound

Fe-Mn Oxides-bound

Organic/Sulfide-

bound

Extraction Medium

IM Amm. Acetate at pH 7

for i0 min.

Residue from 1 extracted

with IM Amm. Acetate at

pH 5 for 5 hours.


with O.04M NH2OH.HCl in

25% HAc for 6 hrs. at

96~


with 0.02M HNO3+H202 (pH

2; 4 hrs. at 85~

followed by 3.2M NH~c in

20% HAc (30m at 20~

Non-Extractable Residue from above

residue

Table 2

Total Elemental Concentration of Solid Wastes

Waste Type Cadmium Arsenic Mercury

Refuse inc ash 563 (51) 106 (5) 7.5 (0.3) 49 (i)

Coal fly ash 17 (1.6) 150 (6) 1.3 (0.2) 15 (0.6)

Hosp. inc ash 21 (2) 6.5 (0.4) 3.0 (0.2) 13 (0.6)

Sewage sludge 69 (6) 6.6 (0.3) 28 (0.9) 4.0 (0.3)

"1.8 (0.2)

Selenium ~

Sewage inc ash 37 (3) 8.8 (0.6) 32 (1.3)

Sewage lag ash 38 (4) 15 (0.6) 1.8 (0.2) 26 (1.2)

* concentrations given in ~g/g; mean (sd)

183

S. S. KRISHNAN et al.: LEACHABILITY OF TOXIC ELEMENTS

Mercury was extracted to the extent of 6.7% in the case of refuse incinerator

ash, and to 25% in the case of sewage lagoon ash. Selenium extraction varied

from 4.2%, in the case of sewage incinerator ash, to 22.9% in the case of coal

fly ash. The extraction behavlour appears to be related to the thermal chemistry

of the incineration process. Cadmium, arsenic, mercury, and selenium, which are

relatively volatile, appear to undergo a 'volatilization-condensation'

mechanism 5, where they volatilize in the high-temperature combustion zone and

then condense at lower temperatures onto the surface of less volatile metals such

as Silicon and Aluminum.

'able 3

18 hour :xtractions (%)

Waste Type Cadmium Arsenic Mercury Selenium"

Refuse ash 76.4 20.3 6.7 17.5

Coal fly ash 36.9 32.7 15.3 22.9

Hosp. inc ash 46.4 12.6 13.3 19.4

Sewage sludge 34.3 17.0 12.5 16.2

Sewage inc ash 14.2 11.5 23.1 4.2

Sewage lag. ash 20.3 7.1 25.0 8.1

Sequential Extraction: The extractability of the toxic elements under the

sequential extraction procedure are given in Tables 4 to 7.

Table 4

Sequential Extraction of Cadmium (%)

Waste

Type

Refuse

ash

Coal fly

ash

Readily-

soluble

50.7

6.8

15.5

Carbonate

-bound

22.3

20.9

21.5

Fe/Mn -

bound

22.3

6.8

24.9

Organic-

bound

2.0

5.6

9.9

Residue

2.7

59.9

27.6 Hospital

inc ash

Sewage 6.3 14 31.4 17.3 31.0

sludge

Sewage 2.9 5.0 10.6 6.5 75.0

inc ash

Sewage 4.4 i.I 23.0 17.2 54.4

lag. ash

184

S. S. KRISHNAN ct al.: LEACHABILITY OF TOXIC ELEMENTS

Table 5

Sequential Extraction of Arsenic (%)

Waste Readily- Carbonate Fe/Mn - Organic- Residue

Type soluble -bound bound bound

Refuse 3.7 i0.i 3.6 1.7 81.1

ash

Coal fly 4.2 8.1 12.3 3.7 71.7

ash

Hospital 4.5 6.0 19.8 32.9 36.9

inc ash

Sewage 4.5 4.2 6.6 5.6 79.1

sludge

Sewage 5.5 3.3 5.6 7.5 78.1

inc ash

Sewage 0.5 4.0 4.9 4.3 86.3

lag. ash

Table 6

Sequential Extraction of Mercury (%)

Waste Readily-

Type soluble

Refuse

ash

Coal fly

ash

1.0

13

Carbonate

-bound

7.0

18

Fe/Mn -

bound

2.0

14

Organic-

bound

30

38

Residue

61

18

Hospital 8 8 13 33 38

inc ash

Sewage ii 16 4 63 6

sludge

Sewage 9 9 9 12 62

inc ash

15 1 6 21 57 Sewage

lag. ash

(i) Readily-exchanqeable form: The pH of the first extraction, being 7,

is representative of natural waters. In this fraction, the municipal refuse

incinerator ash contalns the highest percentage of cadmium (50.7%), and the

lowest percentage (0.5%) of arsenic is present in the sewage lagoon ash.

185


Table 7

Sequential Extraction of Selenium (%)

Waste Readily- Carbonate Fe/Mn - Organic- Residue

Type soluble -bound bound bound

Refuse 12 4 8 23 53

ash

Coal fly II 9 9 30 41

ash

Hospital ii 8 3 24 54

inc ash

sewage 5 2 4 16 73

sludge

Sewage 4 3 1 2 90

inc ash

Sewage i0 4 3 ii 73

lag. ash

(ii) Carbonate-bound: The elements bound to carbonates are released when

the carbonate is broken up by mild acid such as acid rain. Cadmium in refuse

incinerator ash is in this category to the greatest extent (22.3%).

(iii) Fe/Mn oxides-bound i Sewage sludge and hospital incinerator ash

contain the highest percentage of non-residual fraction of cadmium (31.4% and

24.9% respectively) in this fraction. Hospital incinerator ash and coal fly ash

contain the highest percentage of non-residual arsenic (19.8% and 12.3%

respectively), and mercury (13% and 14% respectively) is in this fraction.

(iv} Orqanic and sulfide-bound: Significant portions of non-residual

fraction of mercury (12 to 63%) and selenium (2 to 30%) are in this fraction in

many of the solid wastes.

(v) Residual fraction: Generally most of the ~lements are in this

category in most of the solid wastes studied.

Thus, the total concentration of the toxic element in the solid waste is

not an indicator of its bioavailability and hence its degree of environmental

hazard. The more volatile elements are more easily leached from these solid

wastes than the more refractory ones. A significant portion of these elements

is in the readily-extractable and in the carbonate-bound fractions, and hence

become bioavailable, especially in acid rain conditions. The disposal sites and

incineration process have to be carefully considered before landfill disposal,

so that the reentry of these toxic elements back into the environment due to

leaching is minimised or avoided.

186


ACKNOWLEDGEMENTS

Financial support from the Natural Sciences and Engineering and Research

Council of Canada is gratefully acknowledged.

References

1. US EPA, Federal Register, 51 (1986) No. 9, 1750. 2. Ontario Ministry of the Environment, Regulation, 309 (1988) 460. 3. A. WADGE, M. HUTI'ON, Environ. Pollution., 48 (1987) 8.5. 4. A. TESSIER, P. G. C. CAMPBELL, M. BISSON, Anal. Chem., 51 (1979) 844. 5. R. L. DAVIDSON, D. F. S. NATUSCH, J. R. WALLACE, C. A. EVANS, Environ. Sci. Technol., 8 (1974)

1107.

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Leachability of toxic elements from solid wastes