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Utilization of Coal Fly Ash for Remediation of Metal Contaminated Soil in Mining Sites


Utilization of coal fly ash for remediation of metal contaminated soil in mining sites
Chen Jiangjiang1, 2, Ouyang Tong1, Lai Limin1, Cao Wenzhi1
1)

Department of Environmental Science and Technology Xiamen University Xiamen 361005, P.R. China yz3t@xmu.edu.cn

2)

Environmental protection bureau of Quangang District Environmental protection bureau of Quanzhou City Quanzhou 362802, P. R. China

Abstract—By using a hydrothermal treatment process, coal fly ash was transferred to a series of synthesized zeolites and characterized XRD and SEM observations. The synthesized zeolites exhibited more enhanced adsorption capacities for metals compared to the raw material as the increased CEC of the synthesized zeolites apparently provides a better adsorptive ability for metals. For the three synthesized zeolite amendments, ZFAⅠshows the best adsorption capacity for metals, followed by ZFAⅡand ZFAⅢ. Soil amendment with coal fly ash and the synthesized zeolites from the fly ash reduced the leaching of Cu, Zn, Ni, Cd, Cr, and Pb effectively from contaminated soils in laboratory batch experiments. During the column experiment, the leaching amount of all the studied metals kept under a lower level below the TCLP limiting values, and remained constant, indicating that the synthesized zeolites can significantly reduce the leaching of metal pollutants from contaminated soil into groundwater even under the acid rain condition. Coal fly ash was proved a promising agent for in situ remediation of metal contaminated soil. Keywords—coal fly ash; synthesized zeolite; soil remediation; heavy metals; mining sites

investigating the optimum adsorption condition based on the batch test, lastly, using the column experiment to testify the feasibility of ZFA to reduce the leaching of metal contaminants. II. MATERIALS AND MATHODS

A. Materials Immobilization tests were performed on a sample of polluted soil collected from a Cu mining site in Fujian province in China, which operated since the 1990s. A sample of coal fly ash was obtained from a coal-fired power plant in Xiamen city. B. Zeolite-synthesizing processes Before the synthesis process, soluble materials were removed from the coal fly ash through a washing process, using a 1:5 solid/liquid ratio distilled deionized water (DDW) as the washing regent. After 6 h of shaking, the sample was settled for 2 h, afterwards the supernatant was decanted. This procedure was repeated a total of five times. The sample was then heated at 105 °C in an oven until completely dried (usually 24 h). The dried sample was sieved with an 80-mesh sieve, then stored in an airtight container as FA. A FA/caustic solution of 1/2.5 (w/v ratio of 60 g FA and 150 mL of 2 mol/L NaOH) was use for synthesizing zeolite. After 24 h of shaking in 80℃, 240 rpm water bath, the mixing solution was centrifuged, filtrated and washed with DDW for several times. The sample was then heated at 105℃ in an oven for 24 h, and stored in an airtight container as ZFA I. Using ZFAⅠ/CaCl2 (0.5 mol/L) and ZFAⅠ/Ca(H2PO3)2 (0.5 mol/L) for synthesizing zeolite, Repeat the above process, and the corresponding products were referred to as ZFAⅡ and ZFA Ⅲ. C. Experimental methods The US-EPA toxicity characteristic leaching procedure (TCLP) [7] was applied to the contaminated soil used in the experiments to determine the potential mobility of the metals. Soil sample was treated with the standardized extraction fluid (5.7 mL glacial CH3COOH added to 500 mL of DDW, plus 64.3 mL of 1 mg L-1 NaOH and diluted to 1 L, pH 4.9), and agitated on an orbital shaker for 18 h. The solid/ liquid ratio was 1:20. The product was filtered off and the filtrate was analyzed for heavy metals.

I.

INTRODUCTION

Fly ash is a huge by-product of coal-fire power plants, only 40% of it is being used in China, and a considerable portion of it is disposal of without further use. To dispose of these wastes, a large amount of land will be engrossed and new contamination will be caused. Therefore, searching for new ways of its utilization is still a big task. Nowadays, utilization of fly ash or zeolited fly ash for environmental decontamination is being studied in many countries, and a big progress has been made [1-4]. Murayama et al [5] used different alkali to synthesize zeolite-like material and found that NaOH was the best alkali for synthesizing zeolite-like material. Bertocchi et al [2] used red mud and fly ash for remediation of mine sites contaminated with heavy metals, and the results showed that both waste material could significantly reduce heavy metals releasing into the eluate. Application of appropriate materials to contaminated soil can be considered as one of the option to reduce the release of heavy metals into underground water. The experimental investigation described here intends to further contribute to the development of utilization of fly ash for remediation of heavy metal contaminated soils. First, using hydrothermal treatment process to transfer raw fly ash to synthesized zeolite (ZFA), then

Sponsors: Natural Science Foundation of Fujian Province (No. D0720002, No. D0510005).

978-1-4244-4713-8/10/$25.00 ?2010 IEEE

The air-dried soil was mixed with the four amendments as following. Different portions of each amendments ranging from 5 to 20 g/100 g soil (5, 10 and 20 g/100g) were added to the soil samples. The mixture was stirred up and placed at 25℃ to dry. The samples were ground and DDW was added until the moisture reached the field capacity level. The mixtures were subjected to three cycles of the same air dry/rewetting procedure in order to allow sufficient mixing of adsorbent and soil to simulate actual field conditions. Afterwards, a batch and a column experiments, were carried out. A batch leaching test was used to estimate the water soluble fraction of heavy metals prior to and after the soil treatment following two weeks of equilibration time. A series of 10 g soil mixtures were filled into 250 mL PVC bottles and mixed with DDW at a liquid-to-solid ration (L/S) 10 L kg-1. The mixture was constantly shaken for 6 h using a reciprocating shaker (230 rpm) at 25℃ for 40 min. The solid and liquid were separated by centrifugation at 200 rpm and filtration (0.2 μm) immediately after final pH measurement. Filtrate samples were acidified and stored at 4℃ for later analysis of heavy metals. Column experiments were performed in two 30 cm high, 30 mm diameter plexiglass column which consists of a filter paper and a 1 cm layer of glass balls was kept at the bottom. Column 1 was filled with 100 g contaminated soil as an untreated group, column 2 with 100 g polluted soil and 10 g ZFAⅠat the bottom as a barrier. The columns were saturated with a counter flow of DDW and drained several times in order to release air bubbles in the columns. During the experiment, the column was leached using a pH 4.5 HNO3 solution as the simulated acid rain at a constant head of 5 cm, 160 mL of leaching solution was introduced into the column leaching for 72 h through the flow rate control. The resulting leachate was collected and after final pH measurement separated by centrifugation and filtration (0.2 μm), acidified immediately and stored at 4℃ for later analysis of heavy metals. D. Analysis Morphological observation of synthesized zeolites was performed using a scanning electron microscopy (SEM). The changes in the crystalline of the synthesized zeolites were studied by X-ray diffraction (XRD) techniques. The CECs of raw coal fly ash and the synthesized zeolites were determined using the ammounium acetate method. Metal concentrations were determined by using an inductivity coupled plasma-mass spectrophotometer (ICP-MS). III. RESULTS AND DISCUSSION

2000

a

Q M M
Q

1500

1000

Mt
500 2000 10 20 30

MS Mt

M S M M
40 50 60

S

b

Z3

1500 1000

500 2000 1500 1000 500 2500 2000 1500 1000 500 10 20 30 40

Z2 Z3
c
10 20 30 40 50 60

d

10

Z3

20

30

40

50

60

Z2 Z1
50 60

Figure 1 XRD patterns of coal fly ash (a) and hydrothermally synthesized zeolites (b-d)

accounted for the formation of new zeolites, but the quartz peak heights did not decrease significantly during the treatment [8, 9], indicating that the 80℃ water bath was probably too weak to dissolve certain components of fly ash. Moreover, the mullite peak heights also did not clearly be affected as the mullite had higher stability in the synthesizing process.

40 CEC/cmol kg-1 30 20 10 0 FA ZFAⅠ ZFAⅡ ZFAⅢ

A. Mineral Transformations of Zeolite-synthesizing processes X-ray diffraction. Figure 1 shows the XRD results of the raw fly ash and zeolited fly ash. Quartz and mullite were the major crystalline phases of the raw fly ash, with other minerals such as magnetite and sodalite. After the synthesizing process, two additional crystalline phases were found, identified according to peak d value or 2θ as Zeolite LTA and Zeolite X. This

Figure 2 The cation exchange capacity of coal fly ash and different synthesized zeolites

Changes in cation-exchange capacity Figure 2 shows the CEC of the raw fly ash was determined to be near zero, quickly increased after the synthesizing process using NaOH solution, and stabilized near 40 meq./100 g, then the CaCl2 or the Ca(H2PO4)2 synthesizing process did not effectively increase the CEC of the ZFA. This is because that the Ca2+ in the CaCl2 or the Ca(H2PO4)2 solution may take up

a

b

c

d

Figure 3

SEM photographs of coal fly ash and the three synthesized zeolite products, a) raw fly ash; b) ZFA I; c) ZFA II; d) ZFA III.

some hole of the zeolited fly ash which may result in the decrease of the CEC. Morphological Observation by SEM Although certain mineral crystal could not be identified definitely through the SEM photomicrograph, the formation of fly ash to different phase of zeolite was well demonstrated [6]. With the SEM results (Figure 3), it was found that raw coal fly ash looks orbicular, with a slick surface and incompact structure, and is mainly uncrystalloid (Figure 3a). After the synthesizing process, the new products lose the orbicular shape, the surface becomes rough and porous, and the structure becomes compact and presents clearly alveolate holes (Figure 3b~d), indicating the formation of zeolite minerals.. B. Soil properties Table I indicated that the collected soil contained high contents of Cu, Ni, Zn and Pb. All the six metals were detectable in the TCLP extracts. Compared with the TCLP

limiting values established by USEPA, the leachate concentrations for Cu and Zn exceeded the EPA regulatory levels; the TCLP extractable fraction of Cd and Pb was also in a substantial level, and that for Cr and Ni were much lower than the regulatory level.
Table I. General Chemical properties of the soil sample used in this study TOC Total Content (mg Kg ) TCLP level (mg L ) Limiting values (mg L ) ND, not determind.
-1 -1 -1

Cr 60 0.08 5

Ni 617 0.05 5

Cu 9150 356 15

Zn 7860 31 25

Cd 30.1 0.47 0.5

Pb 2220 2.9 5

17.2 ND ND

C. Batch experiments Table II indicates that the amount of DDW-extractable metals was significantly reduced by the addition of amendments as assessed the batch test. The removal of heavy

Table II. pH and extractable heavy metals (mg kg ) of the soil with different amendent addtion Treatment Control FA 5% FA 10% FA 20% ZFA I 5% ZFA I 10% ZFA I 20% ZFA II 5% ZFA II 10% ZFA II 20% ZFA III 5% ZFA III 10% ZFA III 20% pH 2.54 3.07 4.13 6.93 4.17 5.45 8.42 4.07 6.01 8.69 3.58 4.93 5.39 Cr 0.238 0.180 0.002 0.006 0.009 0.004 0.002 0.015 0.005 0.001 0.173 0.005 0.002 Ni 3.774 1.027 0.919 0.090 0.902 0.466 0.019 0.972 0.781 0.060 1.076 1.010 0.754 Cu 804.5 798.9 535.1 0.2 509.8 8.9 0.1 578.7 17.8 0.2 497.8 15.6 0.5 Zn 281.7 170.3 145.2 0.2 144.8 5.8 0.0 155.0 5.6 0.1 147.3 2.5 0.2 Cd 0.482 0.290 0.253 0.009 0.230 0.025 0.001 0.258 0.047 0.001 0.256 0.040 0.008 Pb 2.520 1.060 0.592 0.108 0.797 0.177 0.027 1.137 0.129 0.124 0.115 0.073 0.050

-1

treated soil with a 5% raw fly ash amendment was around 4.0, and with the addition of raw fly ash up to 20%, the pH value of the mixture jumps to 6.0 to 9.0. Therefore, raw coal fly ash or the synthesized zeolites can also be used to remediate the acidification soil. Considering raw fly ash is very cheap and easily obtained as a large amount of coal fire plant by-product, and the zeolite-synthesizing processes will increase the cost and may cause new pollution during the synthesizing processes, it is recommendable to use the raw fly ash for the remediation of heavy metal contaminated soil based on the consideration of “waste resource recycle” [9, 10].

metals increased with the increased adding amount of amendments. The synthesized zeolite amendments showed better adsorption capacity than the raw fly ash at the same adding amount. Before the application of amendments, the untreated control soil contained, on average, 0.40%, 0.61%, 8.79%, 3.58%, 1.60%, and 0.11% of total content of Cr, Ni, Cu, Zn, Cd, and Pb, respectively, in DDW-extractable forms. With a minimum 5% raw coal fly ash amendment, the DDW-extractable Ni, Zn, Cd, and Pb were reduced to 0.17%, 2.17%, 0.96%, and 0.05% of total content, respectively. With an increased addition of raw fly ash as to 10% amendment, the extractable Cr and Cu was also reduced to 0.01% and 5.85%, respectively. As indicated in Table II, the application of amendments also significantly increased the soil pH values, with ZFA I and ZFA II being the most effective. This could be attributed to that when the concentration of H3O+ was high enough, H3O+ will compete with metal ions for the exchange sites in the synthesized zeolites at low pH condition, as the synthesized zeolites had high selection for H3O+. Moreover, the surface functional groups of the zeolites may dissociate more anionic surface sites at high pH condition, which may make a significant contribution to the reduction of extractable metal fractions [10].Compared to the control soil, a 5% ZFA I addition reduced DDW-extractable Cr, Ni, Cd, Zn, Cu, and Pb by 91%, 88%, 89%, 79%, 95%, and 93%, respectively, and increased pH by 1.63 units. The most noticeable reduction in DDWextractable metals was observed in soil with 10% ZFA I amendment, in which DDW-ectractable Cr, Ni, Cu, Zn, Cd, and Pb were reduced to 0.007%, 0.076%, 0.097%, 0.074%, 0.083%, and 0.008% of total content, respectively. This could also be attributed to that the increased CEC of the synthesized zeolites apparently provides a better adsorptive ability for heavy metal. For the three synthesized zeolite amendments, ZFAⅠshows the best adsorption capacity for metals, followed by ZFAⅡand ZFAⅢ. The raw coal fly ash also has a preferable adsorption capacity for heavy metals, as with a 10% amendment, the DDW-extractable Cr and Pb were reduced by 99.0% and 76.5%, respectively. The soil solution’s pH value increased with the increased adding amount of raw fly ash. The initial pH value of the polluted soil was 2.03, and the pH value of the

D. Column experiments The column test can not only closely simulate field condition, but also provide detailed information about the mobility of the contaminant with respect to the treatment time [12]. Therefore, in order to simulate the acid rain condition more execrable, we use 0.1 N HNO3 to adjust distilled water to a pH value of 4.5, which is similar to that of the TCLP extraction media as execrable acid rain, and leaching experiments were conducted to study the feasibility of the synthesized zeolites as a filter wall for preventing the mobility
150 125 100 75 50 25 0 6 150 125 100 75 50 25 0 6 Ni Cu 12 12 24

Untreated soil

C (μg L-1)

48

72

C (μg/L-1)

Treated with 10% ZFA I

24 time(h) Zn

48 Cd

72 Pb

Figure 4. Emissions of heavy metals from the columns with the untreated and treated soils.

of heavy metal ions. The leaching behaviors of Ni, Cu, Zn, Cd, and Pb from the untreated and 10% ZFA I amendment treated soils as a function of treatment time are shown in Figure 4. Leaching of metals occurred from all columns, but the metal flux from the treated soil was substantially lower than that from the untreated soil. Leachates from the untreated soil had metal concentration peaks for Cu, Ni, Cd, and Zn at the beginning of the experiments, then decreased from 128 to 12.1 μg L-1 for Zn,

from 77.6 to 19.8 μg L-1 for Cu, from 59.9 to 1.2 μg L-1 for Ni, and from 1.8 to 0.1 μg L-1 for Cd at the end of the 72-hour observation, respectively. Leachates from the 10% ZFA I treated soil was relatively constant during the observation period and varied between 15.8 ~ 39.2 μg L-1 for Cu and 22.7 ~ 41.2 μg L-1 for Zn. The leaching of Ni, Cd, and Pb was found at a negligible level (< 5 μg L-1) throughout the experiment. In general, the filter wall has best prevention of the mobility of Cu and Pb, the reduction rate in leaching concentration was near 99.9%, followed by Zn, Cd and Ni. The reason is that Cu and Pb were more easily adsorbed onto the surface adsorption sites of the synthesized zeolite, thus reducing the adsorption of other metals onto the adsorption sites [13]. During the treatment time, the leaching amount of all the studied metals kept under a lower level below the TCLP limiting values, and remained constant, indicating that the synthesized zeolite can significantly reduce the leaching of metal pollutants from contaminated soil into groundwater even under the acid rain condition. Compared to other adsorbent such as active carbon or ion exchange resin, coal fly ash has a variety of advantages, the most important one is that it is very cheap and easily obtained as it is a by-product of coal-fire power station. Though further field investigations are necessary to testify the results obtained the high removal rate of heavy metals in the batch test and the effective reduction of metal leaching in the column test indicated that coal fly ash is a promising agent for in situ remediation of metal contaminated soil. IV. CONCLUSIONS

the leaching of metal pollutants from contaminated soil into groundwater even under the acid rain condition. Raw coal fly ash can not only be used to remediate metal contaminated soil but also used to remediate acidificated soil. ACKNOWLEDGMENT The authors thank Ms. Li Qiurong, Mr. Liu Yaoxing and Mr. Gao Yesong, graduate students at Xiamen University, for experimental assistance. This work was supported by the Key Project of Natural Science Foundation (No. D0720002) and the Project of Natural Science Foundation (No. D0510005) of Fujian Province, China. REFERENCES
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[2]

[3]

[4]

[5]

The raw coal fly ash has a preferable adsorption capacity for heavy metals and can be used as soil amendment for reducing the leaching of a variety of heavy metals from contaminated soil. Using a hydrothermal treatment process, coal fly ash can be transferred to a series of synthesized zeolites as indicated by the XRD and SEM observations. The synthesized zeolites exhibited more enhanced adsorption capacities for metals compared to the raw material as the increased CEC of the synthesized zeolites apparently provides a better adsorptive ability for metals. For the three synthesized zeolite amendments, ZFAⅠshows the best adsorption capacity for metals, followed by ZFAⅡand ZFAⅢ. Contaminated soil amendment with coal fly ash and the synthesized zeolites from the fly ash reduced the leaching of Cu, Zn, Ni, Cd, Cr, and Pb effectively from contaminated soil in laboratory batch and column experiments. During the column experiment, the leaching amount of all the studied metals kept under a lower level below the TCLP limiting values, and remained constant, indicating that the synthesized zeolite can significantly reduce

[6] [7]

[8]

[9] [10] [11]

[12]

[13]


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