Ž .International Journal of Coal Geology 40 1999 103–108

Comparative leaching experiments for traceelements in raw coal, laboratory ash, fly ash and

bottom ash

Yunquan Wang a,), Deyi Ren b, Fenghua Zhao b

a Department of Geography, Guangzhou Normal College, Guangzhou 510400, Chinab Beijing Graduate School, China UniÕersity of Mining and Technology, Beijing 100083, China

Received 13 September 1997; accepted 2 November 1998

Abstract

Samples of raw coal, fly ash and bottom ash have been collected from a Chinese power planttogether with laboratory ash obtained by ashing of the coal under 8508C. Comparative-leachingexperiments were carried out on each fraction under various pH conditions. A mathematical model

Ž .for leaching of trace elements has been developed and leaching intensity I has been calculatedl

for each element. The results show that pH of the solution, leaching time, and particularly,properties and occurrence of the elements, have a strong influence on leaching behavior. q 1999Elsevier Science B.V. All rights reserved.

Keywords: trace elements; raw coal; laboratory ash; fly ash; bottom ash; leaching experiments

1. Introduction

With the development of industry and the increasing use of coal, large quantities ofcoal combustion wastes are being generated in China. Most of the solid residues fromcoal combustion have been used to fill gullies, construct roads, or are piled up neardrinking water sources. This creates the potential for heavy metal contamination.

Ž .Therefore, scientists have paid much attention to this problem Huang and Yang, 1989 .To investigate potential water pollution, As, Zn, Pb, Ni and other elements were

leached from raw coal, laboratory ash, fly ash and bottom ash using solutions havingpHs ranging from 2.0 to 6.5.

) Corresponding author. Tel.: q86-20-86233291; Fax: q86-20-86234160

0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0166-5162 98 00062-7

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Table 1Leaching intensity of element in sample under different pH

pH Element Rawcoal Laboratory ash Fly ash Bottom ash

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .T mg L mg R % I T mg L mg R % I T mg L mg R % I T mg L mg R % Il l l l

2.0 Sr 769.2 454.0 59.02 19.67 6319.4 4707 74.48 24.83 3072.4 1272.5 41.42 13.81 1281.4 166.5 12.99 4.33Zn 470.0 86.2 18.34 6.11 1777.6 15.2 0.86 0.29 2475.0 -DL -DL -DL 725.0 8.2 1.13 0.38Pb 300.0 90.0 30.00 10.00 1500.0 250.0 16.67 5.56 2010.0 155 7.71 2.57 290.2 95.0 32.74 10.91Ni 359.2 114.2 31.79 10.60 1438.6 215.0 14.95 4.98 1585.8 138.2 8.71 2.90 2125.2 88.2 4.15 1.38As 83.6 13.0 15.55 5.18 88.8 33.8 38.06 12.69 353.0 35.8 10.14 3.38 17.0 13.2 77.65 25.88

4.0 Sr 769.2 19.6 2.55 .85 6319.4 2423 38.34 12.78 3072.4 874.6 28.47 9.49 1281.4 -DL -DL -DLZn 470.0 53.2 11.32 3.77 1777.6 39 2.19 0.73 2475.0 -DL -DL -DL 725.0 -DL -DL -DLPb 300.0 85.0 28.33 9.44 1500.0 220 14.67 4.89 2010.0 70.0 3.48 1.16 290.2 115.0 39.63 13.21Ni 359.2 23.4 6.51 2.17 1438.6 174.2 12.11 4.04 1585.8 46.4 2.93 0.98 2125.2 46.2 2.17 0.72As 83.6 9.6 11.48 3.83 88.8 20.0 22.52 7.51 353.0 28.6 8.10 2.70 17.0 2.0 11.76 3.92

6.5 Sr 769.2 8.0 1.04 0.35 6319.4 1784.6 28.24 9.41 3072.4 1057.2 34.41 11.47 1281.4 -DL -DL -DLZn 470.0 25.4 5.40 1.80 1777.6 18.4 1.04 0.35 2475.0 -DL -DL -DL 725.0 -DL -DL -DLPb 300.0 45.0 15.00 5.00 1500.0 125.0 8.33 2.78 2010.0 55.0 2.74 0.91 290.2 45.0 15.51 5.17Ni 359.2 17.4 4.84 1.61 1438.6 85.6 5.95 1.98 1585.8 68.4 4.31 1.44 2125.2 23.4 1.10 0.37As 83.6 6.2 7.42 2.47 88.8 22.0 24.78 8.26 353.0 16.6 4.70 1.57 17.0 1.6 9.41 3.14

T : total value, the amount of an element in 20 g solid samples before leaching.L: leaching value, the amount of an element in the final solution after 30 h leaching.R: leaching rate, as calculated by LrT.I : leaching intensity.l

DL: detection limit.

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2. Samples and experiments

2.1. Samples and solution preparation

Samples were ground to smaller than 400 mm. Some 20 g of each sample wereŽloaded into 10 mm diameter leaching tubes. Three different solutions pHs2.0, 4.0,

.6.5 were prepared using distilled water and HNO .3

2.2. Steps of experiments

1. All test utensils and tubes were soaked in a 14% HNO solution for 24 h and rinsed3

using distilled water.2. Each sample was leached using the three pH solutions. Some 2 cm of liquid was

maintained above the sample within each tube, and the flow velocity of leachingsolution was controlled to 2.0 mlrh.

3. Leached solutions were sampled every 10 h and their pH values were determined.The experiments lasted over 60 h.

4. Atomic fluorescence spectrometry and atomic absorption spectrometry were used todetermine concentrations of As, Zn, Pb, Ni and Sr in the solutions.

3. Results and analysis

Ž .Leaching intensity I , defined below, was used to compare the leaching behavior ofl

elements in various samples under different pH conditions:

a PVP103x

I sl A PMP tx

Ž y1 .where I is the leaching intensity h ; a is the concentration of element x in leachatel xŽ . Ž .mgrml ; V is the total volume of leached solution ml ; A is the concentration ofx

Ž . Ž .element x in original sample mgrg ; M is the total weight of sample g ; and t is theŽ .leaching time h .

The stronger the I of an element, the larger the leaching rate. The leaching intensitylŽ .I has been calculated for Sr, Zn, Pb, Ni and As after leaching for 30 h under differentl

Ž .pH conditions Table 1 .

3.1. pH of solution

Ž .The leaching intensity I of Sr, Zn, Pb, Ni and As was found to increase withlŽ .decreasing pH of the solution Table 1 . Most metal oxides dissolve in acidic solutions

Ž .Segerblom, 1957 . The stronger the acidity of solution, the larger the I of thesel

elements. Therefore, greater leaching of trace elements from outdoor coal and ash pileswould be expected in regions subject to acid rain. Moreover, coal combustion residues

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Table 2Leaching intensity class

Sample Raw coal Laboratory ash Fly ash Bottom ash

pH 2.0 4.0 6.5 2.0 4.0 6.5 2.0 4.0 6.5 2.0 4.0 6.5

Strong I 05 Sr, Pb, Zn, Ni, As Sr, Pb Pb Sr, Pb, As Sr, As Sr, As Sr Sr Sr Pb, As Pb Pbl

Medium 1( I -5 Zn, Ni, As Zn, Ni, As Ni Pb, Ni Pb, Ni Pb, Ni, As Pb, As Ni, As Ni, Sr As Asl

Weak 0.5( I -1 Zn Ni Pb Nil

Very weak I -0.5 Sr Zn Zn Zn Zn Zn Zn Zn Zn, Sr Sr, Znl

Table 3Ž .Element concentrations in leachate vary with time at pHs2.0 mgrl

UT , hs Raw coal Laboratory ash Fly ash Bottom ash

Sr Zn Pb Ni Sr Zn Pb Ni As Sr Pb Ni As Zn Pb Ni

10 7.05 1.4 2.0 1.49 75.70 0.18 6.0 2.39 0.23 18.08 2.75 1.49 0.13 -DL 1.75 1.2520 8.55 1.45 1.75 1.49 78.40 0.29 3.50 4.24 0.98 21.68 2.50 2.73 0.68 0.18 1.75 1.5530 7.10 1.46 0.75 2.73 81.25 0.29 3.0 4.12 0.48 23.88 2.50 2.69 0.98 0.23 1.25 1.6240 6.50 1.56 0.25 2.49 132.1 0.29 2.25 4.07 0.18 20.50 1.25 2.62 0.88 9.94 1.25 7.4250 5.08 6.86 -DL 2.50 123.1 0.74 -DL 2.73 0.13 17.50 1.25 2.48 0.10 9.0 -DL 4.6660 2.15 8.95 -DL 1.49 80.80 1.86 -DL 2.28 -DL 14.80 -DL 0.39 -DL 2.20 -DL 2.85Trend Curve l r _ l l r _ l l l _ l l l _ l

DL: detection limit.U Ž .T : leaching time in hours .

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from family heating and cooking are often mixed with daily rubbish, which contains alarge amount of organic matter. The organic matter would be decomposed by microbialaction into CO , H S and various organic acids. This would lower the pH of nearby2 2

Ž .water bodies, which would again promote leaching of trace elements Ling, 1984 .

3.2. Properties and occurrence of elements

The leachability of elements under different pH conditions can be divided into fourŽ .classes depending on the I value Table 2 . That is, strong for I 05, medium forl l

1( I -5, weak for 0.5( I -1, and very weak for I -0.5. The leachability of Sr isl l lŽ .strong, Pb and As strong to medium, Ni medium to weak, and Zn very weak Table 2 .

The I values of different elements vary not only because of their properties, but alsol

because of their mode of occurrence. Strontium often replaces Ca in CaCO in raw coal,3

whereas it is in the form of SrO in laboratory ash, fly ash and bottom ash, or as2Ž . ŽSr OH in solution. These compounds are dissolved in acidic solutions Segerblom,2

.1957 . But reaction between As, Pb, Zn and Ni compounds in various samples withŽ .HNO is more complicated Luo et al., 1990 .3

Generally speaking, leaching intensity gradually decreases with the change in elementoccurrence from water-soluble, to combination with ferromanganese oxide, to organiccomplex to presence in crystalline mineral matter. According to our experiments for

Ž .example, strontium in raw coal is mainly in organic complexes 69.25% and crystallineŽ .mineral matter 17.43% ; in laboratory ash and fly ash, strontium is mainly in ferroman-

ganese oxide, occupying 65.94 and 59.24%, respectively, while Sr in crystalline mineralmatter makes up only 1.91 and 10.64%; however, in the bottom ash, the fraction ofmineral matter increases to 43.41%. Therefore, when pHs2.0, I of Sr in raw coal,l

laboratory ash and fly ash is 19.01, 21.47 and 13.81, respectively, while in bottom ash,Ž .I is only 4.33 Table 1 . For Zn, the mineral proportion is only 46.75% in raw coal, butl

increases to 81.67, 75.95 and 76.17% in laboratory ash, fly ash and bottom ash,respectively. So the leaching grade of Zn is medium in raw coal, while very weak in

Ž .laboratory ash, fly ash and bottom ash Table 2 .

3.3. Leaching time

The change of concentrations of elements with time in leached solutions of pH 2.0 isŽ .listed in Table 3. Three different trends can be identified from Table 3: 1 a parabolic

Ž .trend is most common: Sr, Ni and As belong to this type; 2 a gradually descendingŽ .trend exhibited only by Pb; and 3 a gradually ascending trend is exhibited only by Zn.

Obviously, the first and the second are basic types conforming to objective law; whilethe third is temporal and would change into the first if experiment time were prolonged.This phenomenon has shown that different elements may have different or similar trendcurves, but the trend curve for each element is relatively consistent. The difference oftrend curves of different elements is related mainly to properties and occurrence ofelements.

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4. Conclusions

Ž .1 Different elements have different leaching behaviors because of differences inelemental properties and occurrence in various samples. The pH value of solution andleaching time also strongly influence the leaching behavior.

Ž .2 The stronger the I of an element, the larger the leaching rate.lŽ .3 Lead and arsenic are potential toxic elements having strong to medium I values.l

The highest Pb concentrations leached from raw coal, laboratory ash, fly ash and bottomash are 2.0, 6.0, 2.75 and 1.75 mgrl, respectively. For As, maximum concentrations

Ž .leached from fly ash and bottom ash are 0.98 mgrl Table 3 . Because the highestŽpermissible concentrations of Pb and As in drinking water are 0.05 mgrl GB5749-85,

.Shen, 1988 , it is clear that leaching of coal and its combustion solid residues couldcause drinking water contamination.

Acknowledgements

Ž .We thank the National Natural Science Foundation of China NSFC for its financialŽ .support No. 49372124 and Prof. Dexin Han, a member of Chinese Academy of

Engineering, for his advice. We are grateful to Dr. Robert B. Finkelman, Dr. C.-L. Chouand Dr. Curtis Palmer for their reviews which have improved this article.

References

Huang, C.H., Yang, L.J., 1989. Experimental study of pollution in fly ash to groundwater. Environ. Sci. 10,26–29, in Chinese.

Ling, X.X., 1984. Environmental hydrogeological problems about city refuse dump. J. Changchun Geol. Inst.4, 79–88, in Chinese.

Luo, S.Y., Zang, X.W., Fan, J.H., 1990. Inorganic Chemistry of Geology. Beijing Univ. Press, Beijing. pp.Ž .419–442 in Chinese .

Segerblom, W., 1957. Table Explanation of Properties of Inorganic Compound. Commercial Press, ShanghaiŽ .in Chinese .

Ž .Shen, Z.L., 1988. Hydrogeochemistry Introduction. Geological Press in Chinese .