solidified waste forms

Waste Management, Vol. 17, No. 1, pp. 15-23, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0956-053X/97 $17.00 + 0.00

Pergamon PII: S0956-053X(97)00030-5

ORIGINAL CONTRIBUTION

THE LIMITATION OF THE TOXICITY CHARACTERISTIC LEACHING PROCEDURE FOR EVALUATING CEMENT-BASED STABILISED/ SOLIDIFIED WASTE FORMS

C. S. Poon* and K. W. Lio Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

ABSTRACT. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure (TCLP) is commonly used as a regulatory tool to determine whether or not a waste can be classified as a hazardous waste. The validity of the test procedure for assessing cement-based stabilized/solidified heavy metal wastes is examined in this paper. Synthetic cement-based heavy metal waste forms with different ANC were prepared and subjected to TCLP to study the effect of waste acid neutralizing capacity (ANC) on metal leaching. A "real" waste was also obtained from a local commercial treatment facility and tested to verify the findings. The results showed that as long as the stabilized/solidified waste forms have sufficient ANC to neutralize the acidity of the leachant, the leaching of metals will be small and the performance of the different waste forms cannot be differentiated. The test therefore has limited use in comparing the performance of different cement-based waste forms. A modified test procedure is proposed. © 1997 Elsevier Science Ltd INTRODUCTION

flyash, blast furnace slag and sodium silicate are often used to replace part of the cement either to lower the cost or to improve product performance. 4 Leach tests are used to study the leaching behavior of solidified wastes. 5 The U.S. EPA Toxicity Characteristics Leaching Procedure (TCLP) is one of the most commonly used leach tests. 6 It is used by regulatory agencies for classifying hazardous waste as well as by waste disposal contractors to compare the leaching performance of different waste forms. The test involves agitating a crushed waste sample in an acetic acid solution for 18 h and analyzing the leachate. The acetic acid is used to simulate the organic acid present in actual codisposal landfills (codisposing hazardous waste with municipal solid waste). This landfilling practice is being used in a number of countries including Hong Kong. When T C L P is used with cement-based waste forms, however, there may be a problem that the acid neutralizing capacity (ANC) of such waste can neutralize the acidity of the leachant rendering true assessment of leaching potential impractical. 7 This is particularly important for the assessment of the longer term leaching potential after the ANC of the waste is exhausted. Also, the elevated pH conditions can cause the leaching of some amphoteric heavy metals to occur at a highly alkaline environment instead. 8,9

The stabilization/solidification (S/S) technology is commonly used as the final treatment step for treating hazardous wastes before they are land disposed. The technology is also being used in Hong Kong. 1 It involves adding one or more solidifying agents to a waste to convert it into a monolithic solid with structural integrity. Through S/S, the waste is chemically stabilized and physically modified into a low permeability solid matrix. The leachability of the waste is therefore reduced. The resulting product is also easier to handle and transport. 2 Among the various types of binders used for S/S, cement-based systems are the most commonly used systems because of their low cost and versatility. 3 Cement-based techniques use hydraulic cement as the major solidifying reagent. Solidification relies on the reaction of the cement with the aqueous phase of the waste or with the added water. Additives such as

RECEIVED 16 DECEMBER 1996; ACCEPTED 24 APRIL 1997. *To w h o m correspondence m a y be addressed. Fax: (852)23346389; E-mail: [email protected]. Acknowledgements--The authors wish to acknowledge the financial support of the Research G r a n t Council and the H o n g K o n g Polytechnic University. 15

16

C . S . P O O N A N D K. W. L I O

In this study, synthetic cement-based waste forms with different ANC were prepared and subjected to a modified TCLP to study the effect of waste ANC on metal leaching. Cement-based waste forms with different ANC were made by solidifying a synthetic heavy metal sludge with various proportions of cement and pulverized fuel ash (PFA). Since the ANC of cement-based waste forms originates from calcium containing compounds in cement, the replacement of cement by PFA can lower the waste's ANC. A "real" stabilized/solidified waste was also obtained from a local commercial chemical waste treating facility for testing to verify the findings of the synthetic wastes. EXPERIMENTAL

Cement-based Waste Forms A synthetic heavy metal sludge was used as the major waste source. It was prepared by adding 6 N NaOH to a solution containing 0.1 M each of reagent grade lead nitrate, copper nitrate and zinc nitrate until the pH reached 9.00. The composition of the sludge is shown in Table 1. Ordinary Portland cement (OPC) complying with of British Standard (BS) 12 and PFA meeting the requirements of BS 3892: Part 1 were used as solidifying reagents. These were added to the sludge at a waste to reagent ratio of 0.5. Five OPC/ PFA proportions were used, 0, 20, 40, 60 and 80% PFA, respectively. Each sample was hand mixed with a spatula in a plastic cup at room temperature. After mixing, the slurries were poured into plastic cylindrical moulds (inside dia. = 4.1 cm) to a height of 7.0cm. Air bubbles in the pastes were removed by tapping the mould. The moulds were capped to retain moisture. The samples were cured in an incubator at 20°C for 56 days. After curing, the samples were removed from the moulds by knocking lightly at the bottom of the moulds with a hammer and allowing the sample to slide out gradually from the moulds. Triplicate waste samples and one blank sample were prepared for each mix. Distilled water was used instead of the heavy metal sludge in preparing the blank samples.

A "real" stabilized/solidified waste--a waste generated from the printed circuit board copper etching process that and had been stabilized by OPC, was also used. Chemical analysis of the waste revealed that the waste contained about 0.5% (5000ppm) copper by weight.

~st Me~o~ ANC test. The ANC test l° was used to determine the waste's ANC. The solidified waste sample was ground to pass a 150 #m sieve. The ground sample was oven dried at 60°C overnight. It was then cooled in a desiccator for an hour. Each Sample was subdivided into 11 sub-samples each of 3 g in weight. Each sub-sample was placed in a 50 ml centrifuge tube. An increasing amount of nitric acid was added in each successive tube. The acid addition schedule is shown in Table 2. The tubes were then tumbled at 29 rpm with a rotary tumbler at room temperature for 48 h. After the extraction, the tubes were centrifuged at 8000 rpm for 10min. The pH of the supernatant was measured. Modified Toxicity Characteristic Leaching Procedure (MTCLP) Compared with the TCLP test, in the MTCLP used in the study, the sample size was scaled down and multiple extractions were carried out. The solidified waste samples were put into different Ziploc bags and crushed with a hammer. The crushed samples were then sieved and the portion that passed through a 10mm sieve but was retained by a 5mm sieve was used. Twenty grams of crushed sample was put into a polypropylene bottle and 400ml of leachant was added. The leachant was prepared by diluting 5.7 ml glacial acetic acid to 1 liter with distilled water (leachant no. 2 in the original TCLP which is used for wastes with high ANC). The pH of the leachant was 2.88. The bottle was closed tightly and tumbled at 29rpm in a rotary extractor. The extraction was TABLE 2

ANC--Aeid Addition Schedule Tube no.

Amount of distilled water (ml)

TABLE 1

Composition of Synthetic Sludge Parameter

Value

Density Total solids pH

1.060 g cm -3 7.83% 9.00

Heavy metals Pb Zn Cu

20,055 ppm 5347 ppm 6268 ppm

0 1 2 3 4 5 6 7 8 9 10

30.0 27.0 24.0 21.0 18.0 15.0 12.0 9.0 6.0 3.0 0.0

Amount of 2N HNO3 (ml)

(meqg-I dry waste)

0.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0 27.0 30.0

0 2 4 6 8 10 12 14 16 18 20

LIMITATION OF THE TOXICITY CHARACTERISTIC LEACHING PROCEDURE TABLE3 ANC Sample OPC 20% 40% 60% 80%

12 10

A N C (meq g-1 dry waste) 15.0 12.2 8.6 6.2 2.8

PFA PFA PFA PFA

pH

84I

411

m

•

carried out at r o o m temperature for 18 h. At the end of the extraction, the leachate was filtered with W h a t m a n G F / C glass fiber filter paper. The p H of the filtrate was measured; 90 ml of the filtrate was withdrawn into a 100ml volumetric flask; l ml of concentrated nitric acid was added to acidify the solution and distilled water was added to make up to the mark. The solution was stored in a 125 ml polyethylene bottle for metal analysis. Calcium, lead, zinc and copper were measured. The residue, on the other hand, was returned to the extraction bottle to repeat the extraction. A total of five extractions were carried out. A separate experiment was carried out to monitor the change of p H and metal concentration during the 18 h period of the first extraction. The procedure was similar to that described above but small aliquots of leachate were withdrawn at selected intervals. The withdrawal schedule was: once every 15min for the

,4~8

500

~

Extraction no.

10

12

14

16

18

20% P F A

1 2 3 4 5

nd nd nd nd 3.26

0.05 nd nd 0.49 51.51

0.03 0.01 nd 0.03 11.74

40% P F A

1 2 3 4 5

nd nd nd 5.67 32.07

0.05 0.01 2.65 55.85 29.13

0.06 nd 0.19 15.75 31.93

60% P F A

1 2 3 4 5

nd nd 12.52 42.85 46.15

0.03 7.65 57.38 22.89 6.44

0.01 0.35 25.77 30.58 15.73

80% P F A

1 2 3 4 5

nd 22.19 62.07 54.78 38.37

5.27 64.36 21.68 5.47 1.46

0.24 32.78 31.36 15.34 6.98

--

--

F I G U R E 1. Acid neutralizing capacity.

10

-. i -- OPC

8

--II-- 20% PFA

pH 6

]~60%

4 2

PFA ]

I- ' l - - 80% PFA I /

I

2

3

4

5

Extraction No

F I G U R E 2. Toxicity characteristic leaching procedure leachate pH.

Cu 0.06 0.06 0.01 0.01 0.03

20

14T

Zn 0.03 0.01 nd nd 0.08

2

8

Pb 0.11 nd nd nd nd

0

meq HNO3/g dry waste

Heavy metal conc. (mg l-l)*

1 2 3 4 5

4

6

2500

OPC

60O/OP F A

6

2000

TABLE 4 Heavy Metal Concentration in TCLP Leaehates

.] ---u-. . 80% . . PFA ]

pl-I

1500

first hour; once every 30 min for the second and third hour; once every hour from the fourth to eighth hour; and a final withdrawal at the end of the 18 h extraction. At each withdrawal interval, the extraction bottle was removed from the rotary extractor. The pH of the leachate was measured by dipping a glass electrode into the extraction bottle. After allowing the leachate to settle for about 1 min, 5 ml of the leachate was withdrawn into a 50 ml centrifuge

-41-- 400/o P F ~

I0

1000

F I G U R E 3. Toxicity characteristic leaching procedure: relationship between leachate p H and calcium concentration.

-~-20o/oP~/

12

4

mmdP

Calcium Concentration (rag/L)

Sample

2

0

•

2-0~ ....... 0

0

17

USEPA standard

5

"Detection limits: Pb = 0.01 mgl-1; Zn = 0.0008 m g l 1; Cu = 0.001 m g 1-I .

18

C. S. POON AND K. W. LIO

tube. Meanwhile the extraction bottle was returned to the rotary extractor to continue the extraction. The centrifuge tube was then centrifuged at 8000 rpm for 5 min; 4 ml of the supernatant was then withdrawn and placed into a 10ml glass bottle for storage before metal analysis. The solution was acidified with 50/zl of concentrated nitric acid. Calcium and lead concentrations were determined. A total of 14 aliquots of leachate with a total volume of 70 ml were withdrawn. No replicate or blank was carried out for this experiment.

pH and Metal Analysis pH measurements were made with a Philip's PW 9420 pH meter and an Ingold U455 glass electrode calibrated using buffer solutions with pH = 4, 7 and 10. Metal analysis was carried out by flame atomic absorption spectrophotometry using a Perkin Elmer 3030 spectrophotometer.

RESULTS AND DISCUSSION ANC

The ANC curves of the synthetic waste samples are plotted in Fig. 1. The higher the percentage of PFA in the sample, the more the curves shifted to the lower pH side of the graph. The ANC is defined as the amount of nitric acid required to bring the pH of the leachate down to 9. These were determined from the curves and are shown in Table 3. The results showed that the higher the percentage of PFA in the sample, the lower the ANC of the sample was.

MTCLP For the synthetic waste samples, the pH of the leachates for the waste samples were plotted in Fig. 2. The pH values were higher than the initial pH of the

OPC

7O

20%PFA 70-

60

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JO

10

LeEI

40

8

(roe/L)

30

60.

Le "1

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40 •

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Coocenl~llm

pH

(mS~)

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E x l r ~ t i ~ No.

No.

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60% PFA

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ExWazti• No.

80% PFA 70 6O

- 12

50

- I0

Lead 40 Cmlcert'~im

-8

(m~CJ.) 30

-6

20

-4

10

-2

pH

0

0 1

2

3 F.:ermion No.

4

5

F I G U R E 4. Toxicity characteristic leaching procedure: leachate lead concentration.

5

LIMITATION OF THE TOXICITY CHARACTERISTIC LEACHING P R O C E D U R E

pH of the leachate remained low up to a calcium concentration of about 1700 mg 1-1. It then increased sharply as the calcium concentration increased from about 1700 to 2200 mg 1-1, after which the pH levelled off again. The concentrations of the heavy metals in the leachates in the first extraction are shown in Table 4. The heavy metal concentrations of the first extraction can be compared to the U.S EPA standards to see whether or not the samples were classifiable as hazardous wastes. No stipulated standards are available for zinc and copper so no comparison for these metals was made. The stipulated standard in the U. S. regulation for lead is 5 mg 1-1. 3 Since lead concentrations of the first extraction were all below this standard, all the samples could be classified as nonhazardous. It could also be seen that the heavy metal concentrations were near zero for all samples in the first extraction and the results of different samples

leachant (2.88). This indicated that alkaline was being released from the samples to neutralize the acidity of the leachant. The rise in pH decreased as the percentage of PFA in the sample increased. This showed that replacement of OPC by PFA decreased the release of alkaline from the samples. The rise in pH also decreased as the extraction number increased. This indicated alkaline being released from the waste samples decreased as the extraction number increased. The rise in pH of the leachate was due to the dissolution of cement matrix which consists mainly of calcium hydroxide and calcium silicate hydrates. Calcium was released as a result of the dissolution of the cement matrix. The pH of the leachate should therefore be related to the concentration of calcium in the leachate. Figure 3 was a plot of the pH against the concentration of calcium of the leachates taken at different times during the leaching experiment. The OPC

20%PFA 70-

70

Zinc

19

6O

12

60

50

I0

50

40

Zinc 4 0 -. Concezzr~iea

8

Concentrmlon

12

pI-I

8 plt

(m~VL) 3 o +

6

(n4~L) 3o

6

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4

20

4

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1o.

2

0

0

o

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3

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5

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E x l r ~ c t i ~ No.

3

4

5

E,~h'lmtion No.

b

40% PFA

60% PFA

70-

70-

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12

60-

- 12

50-

10

50-

- 10

40 -

.8

Cm~cenlrsfaoa (mr.q.) 3o-

.6

20

20-

-4

to

I0-

-2

Zinc 4OC~cmTm=ion (m~VL)

Zinc

8 pit

30-

0 - - - - 1

pH

0 2

3

5

4

I

2

F~r~etion No.

3

4

F..~actiotl N o.

d

C

80% PFA 70 12 50

10

Zinc 4 0 Concmn'ztion

8

(me/L)

30

6

20

4

IO

2

plt

0

0 1

2

3

4

5

E x l r ~ i o a No. e

FIGURE 5. Characteristic leaching procedure: leachate zinc concentration.

5

C. S. POON AND K. W. LIO

20 could not be clearly differentiated. Since the samples were expected to have different leachabilities in an acidic leaching environment as a result of their different ANC, it appears that a single extraction TCLP is unable to differentiate the performance of cementbased waste forms with different ANC. This would probably be the case as long as the wastes have sufficient A N C to raise the pH of the leachate to a level at which metal solubilities are low. Heavy metal concentrations of the blank samples, on the other hand, were negligible showing that the heavy metals in the leachate originated from the sludge and not from the solidifying reagents. The concentrations of lead, zinc and copper in the leachates are plotted together with the pH of the leachates in Figs 4, 5 and 6 respectively. The heavy metals started to leach when the pH of the leachate dropped below about 7. The higher the percentage of

12

-

.

-

.

~li

r+OPC + 20%PFA + 40%PFA --o- 60%PFA --8-- 80%PFA

l0

pH

8

0

2

4

6

8

12

14

16

18

FIGURE 7. Toxicity characteristicleachingprocedure.Changesin leachate pH during the first extraction. PFA in the sample, the smaller the extraction number in which the metals started to leach. Provided that there were sufficient number of extractions remaining after the heavy metal started to leach, their concentrations would increase, reach a maximum and then decrease again, while the pH of the leachates

20%PFA

OPC 35-

14

30-

12

~

10

14

IP~a Cu cooc

30

12

"'4~-pH

10 $

Copp~ 20 pH

Ccmcmr~on

35

25

8

Copp~" 20CmrJL)

I0

Extraction Tire~ (hours)

Co~:euhl~On

t~-

6

10-

4

lO-

0

0

(~)

.6 pH

]~.

I

-4

-2

5.

J

0 l

I

2

0

3

I 4

I 1

5

4 2

•~'1~bctlcm No,

I

) ExlracticeNo.

0

t ~ 4

3

b 40% PFA

60%

PFA

3S.

14

30.

12 P"41--PH

30 -

12

25.

10

25 -

10

C~p~ 20 C~cmntioa (mr/L) l~

~

35 -

Copp~" 2 0 Coac~'.~n6o. (~/L) ts-

I'H

I0-

10

0

0 1

2

3

4

0

1

5

2

3

4

Exaction No.

~'wfiua No.

c

d 80% PFA 33

I4 ~

25 Copp~" 20

30 Co¢cem'ltlOn

10

O

h

8 12 pH

m

10-

4

5-

2

0

0 l

2

3 ExC'~ctloaNo.

4

5

t~

FIGURE 6. Toxicity characteristic leaching procedure:leachate copper concentration.

5

LIMITATION OF THE TOXICITY CHARACTERISTIC LEACHING PROCEDURE continued to decrease. One possible explanation for the decrease in metal concentration at high extraction numbers despite the continual decrease in pH was the gradual depletion of the heavy metal in the samples. Another possible explanation was the increase in the concentration of other chemical species dissolved from the cement matrix as the pH of the leachate decreased. These chemical species, which might contain various forms of silicates, might limit the solubilities of the heavy metals in the leachate. Since different leaching patterns were observed for samples containing different percentages of PFA, the modified TCLP might be used for comparing the performance of cement-based waste forms with different A N C . The change in pH during the first extraction is shown in Fig. 7. Except for the sample containing 80% PFA, the pH for all samples showed the following pattern of change: the pH first rose slowly at

21

12-

8-

•

pI[ 6 ~ 4 ~

m d l / ~ Ill

I~l

•

I I~lilllli

2 ~

0

500

I000

1500

2000

250C

Calcium Concentration (rag/L)

FIGURE 9. Toxicity characteristic leaching procedure. Relationship between leachate pH and calcium concentration during the first extraction.

the initial stage of extraction, and then rose sharply in the middle stage, and reached a plateau towards the end of the extraction. The pH at the plateau was close to the final pH of the leachate. The higher the percentage of PFA in the sample, the less rapid was

OPC

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1o

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[- m - - pH

L-*-c.o~

Time of E~r~Zlon (hoers) e

FIGURE 8. Toxicitycharacteristicleachingprocedure:Changesin calciumconcentrationduringthe firstextraction.

22

C.S. POON AND K. W. LIO

the rise in pH. For the sample containing 80% PFA, the pH rose steadily (no rapid increase) throughout the 18 h of extraction. The continuous increase in pH in the leachate showed that alkaline was continually being released from the waste. The higher the ANC of the waste the more rapid was the rise in the pH. The change in the calcium concentration is shown in Fig. 8(a)-(e). For all the samples, the calcium concentration increased rapidly initially but the rate of increase decreased as the extraction time increased. It appears that calcium in the leachate slowed down further release of calcium from the sample. The calcium concentration levelled off towards the end of the extraction showing that the maximum amount of calcium had been leached. The relationship between the pH and the calcium concentrations during the extraction is shown in Fig. 9. This curve is very similar to Fig. 3, showing that the

pH was related to the calcium concentration in the same way whether it was the final leachate or the leachate in the course of an extraction. The changes in the lead concentration are shown in Fig. 10(a)-(e). For all the samples, the lead concentration increased rapidly initially and reached a maximum. However, it decreased again after the maximum and returned to very low levels again. The decrease in lead concentration was probably due to the continuous rise in pH of the leachate which caused the reprecipitation of the metal. The maximum concentration of lead increased as the percentage of the PFA in the sample increased. This was because the pH of the leachate was lower for the PFA blended samples. The relationship between the lead concentration and pH during the extraction is shown in Fig. 11. The lead concentration rose rapidly as the pH dropped below 7. This meant that the concentration of lead would be high at the initial

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(houri)

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Time of Exa-at'fioa (hour)

e

FIGURE 10. Toxicity characteristic leaching procedure: changes in lead concentration during the first extraction.

LIMITATION OF THE TOXICITY CHARACTERISTIC LEACHING PROCEDURE 20 -

iB

15+

D ,L

5

0

2

4

6

pH

8

10

12

14

FIGURE 11. Toxicity characteristic leaching procedure: relationship between lead concentration and leachate pH during the first extraction.

90 80

i

70

-

14

~Cu

12

~pH

Conc j

10

60

8 ~

40

6

~ ~

3o 2o

4

~

10

2

0

0 1

2

3

4

5

Extraction No.

FIGURE 12. Toxicitycharacteristic leaching procedure: results of real waste.

stage of extraction when the pH of the leachate was low but it would be low at the later stage of extraction when the pH of the leachate was high. For the "real" waste sample, a similar trend of M T C L P results was also obtained (Fig. 12). Appreciable leaching of copper could only be seen at the latter part of the multiple extraction process. CONCLUSION This study indicates the limitation of the use of the T C L P for evaluating cement-based waste forms. As long as the waste has sufficient A N C to neutralize the acidity of the leachant, the pH of the leachate will be high and the leaching of metals will be negligible and the performance of the waste cannot be differentiated.

23

Actually, heavy metals were leached initially during the early part of the extraction but reprecipitated in the latter part of the extraction when the pH increased. The peak of the metal concentration was different for wastes with different ANC. Care must therefore be taken in the interpretation of the results of TCLP when they are used to compare the efficiency of different cement-based solidification processes. On the other hand, it has been shown that when multiple extraction is carried out, waste forms with different A N C showed different leaching patterns as a result of the continual consumption of alkalinity. Thus a multiple extraction test may be more useful for evaluating cement-based waste forms. Finally, it must be stressed that in the above discussion, pH has been taken as the sole factor governing leaching of heavy metals from cementbased waste forms. However, other factors like redox potential may also play a role in leaching, particularly in a real landfill environment.

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