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Chemical changes in different types of coal ash during prolonged, large scale, contact with seawater
Waste Management 23 (2003) 125–134 www.elsevier.com/locate/wasman
Chemical changes in different types of coal ash during prolonged, large scale, contact with seawater Efrat Shoham-Fridera,*, Gedaliah Shelefb, Nurit Kressa a Israel Oceanographic & Limnological Research, National Institute of Oceanography, Tel-Shikmona, PO Box 8030, Haifa 31080, Israel Division of Environmental Engineering and Water Resources, Faculty of Civil Engineering, Technion—Israel Institute of Technology, Technion City, Haifa 32000, Israel
b
Accepted 7 May 2002
Abstract In this study, we followed the chemical changes occurring in coal ash exposed to prolonged (300 days), large scale, contact with running seawater. Four major components (Al, Ca, Mg, Fe) and seven minor and trace elements (Cd, Cr, Cu, Mn, Zn, Pb, Hg) were measured in four coal ash types: fly and bottom ash freshly obtained from coal-fired power plant, and old ash (crushed and blocks) recovered from the sea after 3–5 years contact with seawater. Changes occurred in the chemical composition of the coal ash along the experiment: Fe increased in fresh ash, Al increased in old ash and Ca increased in all ash types except old ash blocks. Cu and Hg decreased in fresh fly ash while Cr increased, Cd decreased in all ash types except bottom ash, and Mn decreased in bottom ash. Most of the changes occurred in the fresh fly ash, and not in the old ash, indicating equilibrium after prior exposure to seawater. In addition, more changes occurred in fresh fly ash than in bottom ash, emphasizing the differences between the two ash types. While the changes in the concentrations of the major elements may be an indication of the integrity of the ash matrix, the only elements of environmental significance released to the environment were Hg and Cd. However, calculated seawater concentrations were much lower than seawater quality criteria and therefore the coal ash was considered suitable for marine applications concerning seawater quality. # 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Coal ash may reach and affect the marine environment as a dumped waste, or as a construction material for different marine applications such as artificial islands, artificial reefs and land reclamation of coastal areas (Woodhead, 1985; Hockley and Van Der Sloot, 1991; Homziak et al., 1993; Collins and Jensen, 1995; Pickering, 1996; among others]. Coal ash can serve as filling material, with no contact with seawater or can be in direct contact with it during dumping operations. In the latter case, besides the chemical aspects of seawater– coal ash interaction, there are additional ecological issues which concern the physical aspects of dumping (Bamber, 1980; 1983; Kress et al., 1993) its influence on * Corresponding author. Tel.: +972-4-856-5256; fax: 972-4-8511911. E-mail addresses: [email protected] (E. Shoham-Frider), [email protected] (G. Shelef), [email protected] (N. Kress).
the sediment (Kress and Herut, 1998) and on benthic organisms (Bamber, 1984; Kress and Herut, 1998). Coal ash can be mixed with cement and other pozzolanic materials to form a stabilized construction material, and their interaction with seawater is an intensive researched subject (Parker et al., 1983; Carleton and Muratore, 1985; Roethel and Oakley, 1985; Hockley and Van Der Sloot, 1991; Collins et al., 1994; Sampaolo and Relini, 1994; Kress et al., in press). Chemical and physical processes taking place within the blocks in the marine environment affect it through changes in the quality and properties of the blocks. The feasibility of using coal ash as filler in the construction of artificial islands in Israel is being studied, together with its compatibility with the marine environment (Shelef and Zimmels, 1993). In Israel about six million tons of coal fly ash were generated between the years 1982 and 1995. Sixty-five percent of this amount was utilized for cement production, 18% was used for building the embankment surrounding the ‘‘Orot Rabin’’ power plant in Hadera and
0956-053X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(02)00063-6
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17% was disposed in the deep sea, according to permits issued by the governmental authorities (Mezger, 1996). Approximately 1 million ton of coal ash was dumped at a deep-sea dumpsite in the Mediterranean Sea from 1989 to 1995. Small quantities continued to be dumped until 1998 when dumping ceased (Kress et al., 1993; Kress and Herut, 1998). From 1998 to 2000 ca. 2.8 million ton of coal fly ash was utilized for cement and concrete production and in road construction. Coal ash contains, among other components, trace elements that are potentially toxic (such as Hg, Cd). The main environmental concern in the utilization of coal ash is that these elements may leach out from the ash upon contact with an aqueous medium and reach the environment and the food chain. Several leaching studies of coal ash have been conducted with acid (Reuss, 1983; Grisafe et al., 1988; Warren and Dudas, 1988; Gutierrez et al., 1993), distilled or freshwater (Reuss, 1983; Goetz, 1983; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Garavaglia and Caramuscio, 1994), at specific pHs (Reuss, 1983; Goetz, 1983) and with seawater (Reuss, 1983; Goetz, 1983; Gurierrez et al., 1993; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Crecelius, 1985; Rose et al., 1985; Grisafe et al., 1988; Warren and Dudas, 1988; Kress, 1993; Garavaglia and Caramuscio, 1994). Most of the studies in the literature that describe leaching from coal ash during contact with seawater were conducted under laboratory, small scale, controlled conditions (Reuss, 1983; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Crecelius, 1985; Kress, 1993). Large scale leaching experiments, simulating natural environmental conditions were not performed to date in non stabilized coal ash. Laboratory experiments have shown that leaching from coal fly ash during contact with seawater is dominated by dissolution and desorption processes (Van Der Sloot and Niewendijk, 1985). At high liquid-ash ratios, leaching of major and minor components is determined by the pH of the coal fly ash, the extent of leaching being inversely proportional to pH (Van Der Sloot et al., 1985; Nathan et al., 1999). The coal ash generated in the electric power plants in Israel is of basic character and therefore less susceptible to leaching (Boker, 1989; Kress, 1993; Nathan et al., 1999). The main goal of this research was to follow changes in chemical composition and the chemical processes taking place in different types of coal ash during prolonged contact with warm and saline seawater of the Eastern Mediterranean. The experiment tried to simulate large scale, ‘‘field’’ conditions, in order to assess the environmental feasibility of using coal ash in marine projects. The types of ash studied comprised that which was freshly obtained from coal-fired power plants and that which was retrieved from a deep-sea dumpsite, namely, ash that was already in contact with seawater
for 3–5 years. This study clearly represents a worst-case scenario for the coal ash was not stabilized and used as produced or found.
2. Methods 2.1. Experimental design Four types of coal ash were used in the experiment: (1) fresh fly ash collected by the electrostatic precipitators at the Orot-Rabin (Hadera) power plant, (2) bottom ash obtained also from Orot Rabin power plant, (3) old crushed fly ash; and (4) old fly ash blocks. ‘‘Old’’ fly ash (types 3 and 4 above) was collected from the deep-sea dumpsite (Kress et al., 1993) during October 1994. Since 80% of the total fly ash was dumped at sea during 1989–1991, we estimate that the old fly ash was in contact with seawater for 3–5 years, prior to this study. The old fly ash retrieved from the sea appeared in different aggregate sizes (Kress and Herut, 1998). Some of the aggregates were crushed and sieved through 1–2 mm sieve (old crushed fly ash) and six blocks (old fly ash blocks) weighing 5–10 kg each with approximate dimensions of 402020 cm, were left intact. It should be emphasized that the initial or fresh composition of the old ash retrieved from the sea is unknown and probably different from the fresh ash used in this study. Average unburned coal content in the fly ash is 4–5%. The percentage of ignition loss at 550 C in the initial ash were: 1.85 0.6, 10.3 1.8, 8.85 1.0, 1.46 0.2 in fresh fly ash, ash blocks from the sea, crushed ash from the sea and bottom ash, respectively. Six plastic containers (111 m each) located at the yard of the institute at the seashore, were used in the experiment. A 10 cm layer of fresh fly ash was arranged at the bottom of two containers (identified as A, B) and a 10 cm layer of bottom ash was arranged at the bottom
Fig. 1. Schematic description of the experimental setup.
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of two additional containers (identified as E, F). The fifth container (identified as C) contained the six old ash blocks and the sixth container (identified as D) contained old crushed ash (Fig. 1). Each container had an inlet and an outlet for fresh seawater and was covered with a plastic sheet stretched on a wooden frame in order to prevent input of atmospheric derived materials. Seawater was pumped directly from the sea, filtered through a 500-mm filter before entering the containers and flowed continuously by a system of plastic pipes and taps. The filter was cleaned periodically of macro algae. Average flow rate in every container was 0.45 m3 h 1 with calculated seawater retention time in the container of 2.2 h. The flow was sufficient to rapidly exchange water but not strong enough to cause resuspension and loss of the fine-grained ash. 2.2. Sampling The coal ash was sampled at the beginning of the experiment before the contact with the flowing seawater, 3 days after the beginning of the experiment and once a month for 12 months (March 1995–March 1996) until the end of the experiment. Ash samples from six samplings (the third and ninth of March, May, August, November 1995 and January 1996) out of the total 15 were analyzed for the different constituents. The other nine samplings were carried out so that additional analyses could be performed if necessary to explain trends or phenomena indicated by the six primary samplings. Coal ash from containers A,B,D,E,F was sampled at five points in the container until September 1995 (Fig. 1). After that, only two points (two and four) were sampled since results showed no significant differences in composition between sampling points in each container. Sampling the six ash aggregates from container C was done by scraping the surface of each aggregate with a plastic knife. Environmental conditions (temperature, pH and salinity) were measured during each sampling period. Temperature was measured directly in the container with a standard mercury thermometer. Salinity of the seawater was measured in samples taken to the laboratory, using a Autosal 8400 Guideline salinometer. Salinity values are given on a practical salinity scale that is unitless (UNESCO, 1985). The pH of seawater was measured directly using a ‘‘Mettler Toledo’’ portable pH-meter. 2.3. Sample preparation and laboratory analysis The ash samples were rinsed with DIW (deionized distilled water) to remove salt and then frozen. The frozen samples were dried by lyophilization, crushed, homogenized and kept in closed plastic containers until the chemical analysis. No Hg is lost during the drying procedure (Hornung, 1999). The dry coal ash samples were digested with a mixture of hydrofluoric acid and
aqua regia as described in the ASTM Method (1989). A separate digestion was performed in concentrated nitric acid for the determination of Hg (Hornung et al., 1992). The concentrations of the major components in the ash: Fe, Al, Ca and Mg, and of the minor and trace components: Cd, Pb, Zn, Mn, Cu and Cr, were measured by atomic absorption spectrophotometry using an IL-951 or a Perkin-Elmer 1100 B spectrophotometer equipped with flame and graphite furnace modules with D2 background correction. Hg was analyzed by cold vapor atomic absorption spectrometry on a Coleman Mercury Analyzer MAS-50. Quality control and quality assurance (QC/QA) of the results were performed with standard reference material from the National Institute of Standards and Technology (NIST—Coal Fly Ash-1633a; Table 1). The standards were digested and analyzed in the same manner as the samples, for each analytical run. The coefficient of variance for Ca and Pb in the standards was high and therefore the results were corrected mathematically to 100% recovery at each analytical run. Statistical analysis was performed using the SAS program. The procedures used were the general linear mode (GLM) using the method of least squares, t-test with unequal variance and the Duncan a-parametric test at the 95% confidence level.
Table 1 Quality control and quality assurance of the results utilizing a standard reference material, NIST 1633a—coal fly ash Al
Ca
Major components (% wt) Avg 13.6 0.47 SD 0.78 0.20 CV (%) 6 43 N 16 15 % Recovery 99 43
Fe 9.55 0.75 8 16 69
Mg 0.41 0.06 16 15 89
Certified values of NIST 1633a—coal fly ash Avg 14.3 1.11 9.4 0.455 SD 1.0 0.01 0.1 0.01 Cd Cr Cu Mn Minor and trace components (mg kg 1) Avg 0.79 154 110 174 SD 0.10 17 3 7 CV (%) 12 11 3 4 N 14 16 16 16 % Recovery 79 79 93 97 Certified values of NIST 1633a—coal fly ash Avg 1.00 196 118 179 SD 0.15 6 3 8
Pb
Zn
Hg
97.6 230 0.172 18.3 11 0.028 19 5 16 16 15 10 135 105 108
72.4 220 0.4 10
0.16 0.01
Avg, average; SD, standard deviation; CV, coefficient of variance; n, number of samples.
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3. Results 3.1. Environmental conditions Temperature, salinity and pH in the experimental containers are presented in Fig. 2. Gradual seasonal changes occurred between the minimal temperature in
January (15 C) and the maximal temperature in August (30 C). Salinity was between 39.1 in January–March and 39.7–40.0 in July–November. The pH values ranged between 8.07 in June (summer) and 8.50 in November (fall). It decreased gradually from March to June, and then increased to November. Aggregation of the powdery coal ash took place both in fresh fly ash
Fig. 2. Environmental conditions in the experimental containers.
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centration was significantly lower (Table 3). At the end of the experiment Fe increased in fresh ash (fly and bottom), Al increased in old ash from the sea (crushed ash and old ash blocks; Table 4). Ca increased in all ash types except old ash blocks. No changes in Mg were observed in any of the coal ash types (Table 4). Concentrations of the minor and trace components (Cd, Cr, Cu, Mn, Pb, Zn and Hg) in each of the coal ash types along the experiment are summarized in Table 5. The initial concentration of Pb was higher and Zn was lower in the old ash from the sea then in the fresh ash types (Table 3). Cu was similar in all the ash types while Hg was different in all types. The lowest concentration of Cd was found in bottom ash, while Cr and Mn were lowest in fresh fly ash (Table 3). At the end of the experiment Cd, Cu and Hg decreased significantly in fresh fly ash while Cr increased. Concentrations of the other elements did not change significantly in the fresh fly ash (Table 4). Mn decreased in bottom ash, while the other elements did not change significantly. The only
and old crushed ash. It is known that coal ash has a pozzolanic character and aggregates are formed during contact with aqueous medium (Bamber, 1983). A thin biological film developed on the ash bed and on the sidewalls of the containers during the year of the experiment. 3.2. Chemical composition of the coal ash Each of the six containers was sampled 15 times. A total of 405 ash samples were collected and 137 analyzed for Al, Ca, Fe, Mg, Cd, Cr, Cu, Mn, Pb, Zn and Hg. Concentrations of the major components (Al, Ca, Fe and Mg) in each of the coal ash types along the experiment are presented in Table 2. The initial concentration of Al and Mg were significantly higher in the old ash from the sea than in the fresh ash (fly and bottom) while Fe was higher in the fresh ash types. Ca was similar in all ash types except in the fresh fly ash where the con-
Table 2 Major components (% wt) in the different coal ash types during the experiment Days
Al
Ca
Mg
SD
n
Avg
SD
n
Avg
SD
n
Avg
SD
Containers AB—fresh coal fly ash 0 10 10.9 3 10 10.8 60 9 10.5 150 4 11.2 240 4 11.8 300 4 11.0
0.7 0.6 0.4 0.3 0.8 0.2
10 10 10 4 4 4
1.30 1.72 0.80 1.79 2.96 2.82
0.78 0.96 0.31 1.00 1.55 1.02
10 10 9 4 4 4
5.00 5.03 5.17 6.66 6.18 6.18
0.19 0.27 0.22 0.23 0.56 0.35
4 4 4 3 4 4
0.67 0.65 0.65 0.62 0.76 0.77
0.27 0.09 0.14 0.18 0.26 0.24
Container C—old coal ash blocks 0 6 13.7 60 6 12.6 150 6 13.9 240 5 14.3 300 5 14.8
0.7 0.6 0.3 0.7 0.6
6 6 6 5 5
4.30 4.57 6.23 5.01 6.59
0.88 1.28 1.12 1.56 1.88
6 6 6 5 5
2.73 2.67 3.30 3.17 3.26
0.26 0.37 0.6 0.39 0.39
6 6 6 5 5
2.28 2.12 2.13 2.27 2.52
0.48 0.41 0.56 0.50 0.70
Container D—old crushed coal ash 0 5 13.8 3 5 13.6 60 5 12.9 150 2 15.0 240 2 14.3 300 2 14.5
0.4 0.4 0.1 0.1 0.1 0.4
5 5 5 2 2 2
4.49 4.76 5.48 6.35 5.96 8.10
0.74 1.12 0.45 0.93 2.22 0.16
5 5 5 2 2 2
2.83 2.74 2.80 3.61 3.47 3.19
0.31 0.22 0.24 0.32 0.11 0.06
2 2 2 2 2 2
1.94 2.01 1.94 1.98 1.64 2.18
0.24 0.09 0.11 0.16 0.59 0.16
Containers EF—bottom coal ash 0 10 12.4 3 10 11.8 60 10 10.2 150 4 13.0 240 4 12.3 300 4 13.1
0.6 0.7 0.9 0.6 1.2 0.4
10 10 10 4 4 4
4.58 3.77 3.45 7.41 7.47 7.76
0.8 0.42 0.48 0.90 2.44 0.32
10 10 10 4 4 4
6.31 6.49 6.91 7.21 7.68 8.06
0.4 0.35 0.51 0.54 0.79 0.21
4 4 4 4 4 4
1.14 1.00 0.93 1.12 0.89 1.07
0.14 0.19 0.16 0.07 0.29 0.05
N
Avg
Fe
n, number of samples; Avg, average; SD, standard deviation.
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Table 3 Comparison among the initial average concentrations in the four coal ash types Al Major components (% wt) Old crushed ash 13.84 Old ash blocks 13.65 Bottom ash 12.40 Fresh fly ash 10.86
Ca A A B C
Cd Minor and trace components (mg kg 1) Old crushed ash 0.58 A Old ash blocks 0.53 A/B Bottom ash 0.23 C Fresh fly ash 0.44 B
4.49 4.30 4.58 1.30
Fe A A A B
2.83 2.73 6.31 5.00
Cr 145 126 138 89.6
Mg C C A B
Cu A B A/B C
89.0 82.5 79.6 84.5
1.94 2.28 1.14 0.67
A A B B
Mn A A A A
450 440 510 257
Pb B B A C
80.2 80.0 41.6 64.4
Zn A A C B
114 91.7 160 183
Hg B B A A
0.255 0.295 0.045 0.188
B A D C
Averages in same Duncan grouping (represented by capital letters) are not significantly different.
significant change that took place in the ash from sea, old crushed ash and old ash blocks, was a decrease in Cd concentration (Table 4). The initial composition of fresh bottom ash was different from the fresh fly ash, more heterogeneous with high standard deviation of the concentrations (Tables 2 and 5). The major elements, as well as Cr and Mn were higher in bottom ash than in fresh fly ash. The minor and trace elements (Cd, Pb, Hg), expected to be enriched at the surface due to volatilization/condensation process were lower in the bottom ash. Cu and Zn concentrations were similar in both types.
4. Discussion 4.1. Environmental conditions The temperature in the experimental containers during the year represented a normal seasonal variation. The temperature also dictated the trends of salinity. Salinity in open seawater of the Mediterranean range Table 4 Changes in concentration (percentage from initial value) in the four coal ash types during the experiment Component
Fresh fly ash
Bottom ash
Old ash blocks
Old crushed ash
Al Ca Fe Mg Cd Cr Cu Mn Pb Zn Hg
nsc +117 +24 nsc 27 +16 10 nsc nsc nsc 41
nsc +69 +28 nsc nsc nsc nsc 9 nsc nsc nsc
+8 nsc nsc nsc 30 nsc nsc nsc nsc nsc nsc
+5 +80 nsc nsc 33 nsc nsc nsc nsc nsc nsc
nsc, no significant change from initial value.
between 38.0 and 39.0 (Hecht et al., 1988) while shallow coastal areas are more affected by evaporation and rain inputs. Indeed, the lowest salinities in the experimental containers were observed during the winter with increased values during the summer. The range of 39.0– 40.0 measured during the experiment was typical of near shore waters. The pH was affected by the photosynthetic activity, since algae growth took place in the containers during the experiment. 4.2. Chemical composition and processes Elements which concentrations increased significantly during the year of the experiment were Al in old crushed ash and old ash blocks, Ca in all types of ash except old ash blocks, Fe in fresh fly ash and bottom ash, and Cr in fresh fly ash. Increase of element concentration may be due to chemical exchange processes, for example, Ca– Mg exchange (Hockley and Van Der Sloot, 1991; Kress et al., 1994) or to adsorption (Roethel and Oakley, 1985). It is known that coal ash has adsorption properties, which are magnified as the carbon content in the ash increases (Bamber, 1980). The increase of Ca found in this research contrasts with other studies where Ca was leached from coal ash into seawater or exchanged by Mg (Parker et al., 1983; Labotka et al., 1985; Hjelmar, 1990; Hockly and Van Der Sloot, 1991). In the old ash from the sea this exchange could have occurred prior to this study, during its stay at sea, and the concentration would not be expected to change. An indeed, in the ash blocks there were no changes in Ca concentration. However, Ca increased during the experiment in the old crushed ash as well in as in the fresh ash (fly and bottom). Ca increase started with a slight delay from the increase in pH, after 150 days (Table 2, Fig. 2) i.e. after the increase in photosynthetic activity and biomass growth. It is well known that a wide variety of marine organisms, for example, green and red algae, bryozoans, corals, mussels and bacteria form calcifica-
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E. Shoham-Frider et al. / Waste Management 23 (2003) 125–134 Table 5 Minor and trace components (mg kg 1) in the different coal ash types during the experiment Days
n
Cd Avg
Cr SD
Cu
Avg
SD
Avg
Containers AB—fresh coal fly ash 0 10 0.44 0.04 3 10 0.46 0.08 60 9 0.47 0.02 150 4 0.26 0.03 240 4 0.31 0.05 300 4 0.32 0.02
89.6 94.6 94.3 128 112 104
4.3 6.7 7.8 26 4 5
84.5 79.9 80.9 74.9 76.3 75.9
Container C—old coal ash blocks 0 6 0.53 0.08 60 6 0.38 0.04 150 6 0.27 0.05 240 5 0.3 0.05 300 5 0.37 0.07
126 122 158 151 138
20 17 5 21 20
Container D—old crushed coal ash 0 5 0.58 0.04 3 5 0.54 0.01 60 5 0.44 0.02 150 2 0.35 0.05 240 2 0.38 0.08 300 2 0.39 0.06
145 135 125 157 153 146
Containers EF—Bottom ash 0 10 0.23 0.15 3 10* 0.22 0.16 60 10 0.23 0.27 150 4 0.22 0.15 240 4 0.15 0.04 300 4 0.17 0.1
138 135 140 159 162 128
Mn SD
Pb
Zn
Hg
Avg
SD
Avg
SD
Avg
SD
N
Avg
SD
1.7 4.6 1.9 2.8 3.7 2.5
257 270 264 282 265 268
27 34 20 19 15 6
64.6 61.0 62.5 68.1 68.7 65.1
8.9 5.8 16.5 1.2 2.4 3.4
183 161 173 177 182 180
17.4 21.1 10.5 10.7 20.8 14.5
4 4 4 4 4 2
0.19 0.19 0.12 0.18 0.16 0.11
0.02 0.01 0.01 0.01 0.02 0.07
82.5 78.4 79.0 76.4 77.6
10.4 11.2 12.5 14.8 14.3
440 385 409 382 386
65 46 26 49 31
80.1 89.7 83.7 83.1 81.9
8.6 5.2 2.0 4.2 4.7
91.7 94.2 102 156 111
12.9 12.2 7.9 18.0 23.0
2 6 6 4 5
0.30 0.16 0.29 0.24 0.21
0.01 0.08 0.04 0.02 0.09
11 24 5 6 1 6
89.0 85.5 87.5 82.3 78.4 87.4
5.9 3.5 1.4 2.1 2.2 3.8
450 430 424 436 415 439
12 13 10 16 38 1
80.1 73.6 89.1 80.7 83.1 83.1
2.2 9.1 11.9 3.0 3.9 1.7
114 105 98.6 118 96.0 104
11.6 10.6 3.6 17.0 1.4 4.2
2 – 2 2 2 2
0.26 – 0.26 0.27 0.26 0.25
0.01 – 0.04 0.01 0.01 0.01
19 20 25 3 7 31
79.6 77.6 81.1 72.2 74.5 85.2
11.5 6.4 9.7 9.1 5.4 15.4
510 498 454 449 441 463
28 18 14 23 8 17
41.7 34.6 46.0 44.8 44.1 44.3
4.9 3.4 6.7 1.1 3.6 3.9
160 149 130 149 128 133
39.6 21.8 20.7 21.5 25.4 30.1
4 4 4 4 4 4
0.05 0.05 0.01 0.04 0.05 0.03
0.01 0.01 0.01 0.02 0.03 0.02
n, number of samples; Avg, average; SD, standard deviation. * n=9 for Cd, Cu and Pb.
tion, i.e. sedimentation of calcium into their tissues (Daws, 1998). Calcium concentrations of 6500 ppm were documented in phytoplankton (Stumm and Morgan, 1981). Therefore it is reasonable to assume that the Ca enrichment in the coal ash samples originated from external biological source, i.e. deposition of calcified remains after organisms die off on top of the coal ash bed. Deposition did not occur in the old ash blocks because of their shape (large blocks standing on the short side) and therefore Ca did not change significantly. Moreover, care was taken to sample the aggregate from parts without growth, what was not possible in the case of the coal ash beds. During the experiment Fe increased in the fresh ash (fly and bottom ash) by 24 and 28%, respectively from the initial values (Table 4). This was unexpected because Fe is known to be a part of the glass matrix or the magnetic fraction of the ash (Crecelius, 1980). However scavenging of Fe from seawater by coal ash blocks has been observed in laboratory experiments (Roethel and Oakley, 1985). Moreover, the fact that the concentra-
tion increased only in the fresh ash (fly and bottom) may support the scavenging/adsorption mechanism. Fe increase could be due also to deposition of suspended particulate matter (SPM) from the running seawater on top of the coal ash. Therefore, a rough calculation was performed to estimate how much Fe could accumulate during the experiment due to settling of SPM from seawater. The amount of seawater that passed through each container during 300 days was 3240 m3 (0.45 m3 h 1) and assuming that Fe concentration in SPM in this region is 15 mg/l (unpublished results), 48 g of Fe accumulated in an area of 1 m2. Using the specific density of coal ash 1 g cm 3 (Boker, 1989) and assuming that the deposition would affect only the top 1.5 cm of the coal ash bed, the calculated increase of Fe during the study would be 3300 mg kg 1 ash (0.33% wt) that explain only 28 and 19% of the observed concentration increase of 1.18 and 1.75% wt in fresh fly ash and bottom ash, respectively. Therefore, the increase in Fe concentration cannot be attributed only to the SPM contribution and some may be attributed to adsorption from seawater
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and maybe from biogenic source as Ca. It is interesting to note, however, that Fe concentration showed an increasing trend also in the old ash blocks and old crushed fly ash. Although these increases were not statistically significant, the amounts are in good agreement with the calculated 0.33% wt addition (Table 2). This indicates that old ash may have reached equilibrium in the marine environment concerning adsorption of Fe and the non-significant increase was due to SPM deposition. The increase of Al in the two old ash types from the sea, although statistically significant, was small (8 and 5% from the initial values, Tables 2 and 4). Al is considered to be a conservative element in coal ash (Windom et al., 1989) hence its concentration was not expected to change. However, Al decreased in concrete blocks containing coal fly ash in laboratory experiments (Sampaolo and Relini, 1994) and deployed at sea (Labotka et al., 1985; Kress et al., in press). Decrease in concentration can be explained by leaching of the element from the ash to the environment. The elements mostly prone to leaching are those enriched on the coal ash surface due to volatilization-condensation processes during coal combustion, such as Cd, Cu, Zn, Pb (Van Der Sloot and Nieuwendijk, 1985). Leaching can occur thorough chemical dissolution from the ash followed by precipitation or adsorption onto particles (Crecelius, 1980; Bourg and Alain, 1983; Van Der Sloot and Nieuwendijk, 1985), and to complexation with ligands that are present in seawater like chloride, bromide, sulfate and carbonate (Liem et al., 1983; Roethal and Oakley, 1985; Hirose, 1988; Byrne et al., 1988). Bacteria can extract constituents from the ash. Significant leaching of Mn and Cu from municipal waste ash was attributed to extraction by bacteria (32 and 57%, respectively; Bosshard et al., 1996). In this study, Hg and Cu decreased in the fresh fly ash, Mn decreased only in the bottom ash, and Cd decreased in all types of fly ash but not in bottom ash (Tables 4 and 5). Hg, due to its low boiling point, evaporates almost completely during coal combustion and is released to the atmosphere (Smith et al., 1979). However, Hg is still present in the coal ash, probably enriched at the surface as well as Cu. Surface enrichment in coal fly ash is higher than in bottom ash due to larger surface area (1–3 and 0.4 m2 g 1, respectively; Goetz, 1983; Boker, 1989) and that may explain the lower enrichment (Table 3, initial concentrations) and the lack of leaching from fresh bottom ash. Cu and Hg form complexes with carbonate and chloride in seawater, respectively, aiding and increasing leaching of these elements. Moreover, leaching occurred only in the fresh fly ash that had not interacted with seawater prior to the experiment, indicating stabilization concerning Hg and Cu in the old ash. Mn is not considered an element that is enriched at the surface during coal combustion therefore it is rea-
sonable to assume that the decrease in concentration results from dissolution. It is known that anoxic conditions cause the dissolution of oxides of manganese (Hemond and Fechner-Levy, 2000). It is hypothesized that the coarse grain size of bottom ash and its packing in the container may have created anoxic micro- environment facilitating dissolution from the ash. Changes in pH and oxygen in micro-environment that lead to changes in Mn and Al were found in the surface of stabilized coal fly ash blocks deployed at sea (Roethel and Oakley, 1985; Kress et al., in press). Cd, in contrast to Cu and Hg, leached significantly from fresh fly ash (27%) but also from old ash blocks and old crushed ash (30 and 33%, respectively) that were in prior contact with seawater, indicating a longterm, continuous process. Leaching of Cd from Israeli fly ash was shown also in laboratory experiments with seawater (Kress, 1993), at the dump site (Kress and Herut, 1998) and even from stabilized coal fly ash/ cement blocks after 33 months deployment at sea (Kress et al., in press). Leaching ranged between 20 and 55% of the initial value. As Cd was unequivocally the element that leached out of the ash and has a proven deleterious environmental effect, the addition of Cd to seawater was calculated and compared to known seawater quality standards. In the worst case, old crushed coal ash, the concentration decreased by 0.19 mg kg 1 in 300 days (Table 5) and 3240 m3 of seawater were flown through the container. Using the specific density of coal ash 1 gr cm 3 (Boker, 1989) and assuming that the leaching took place from all the ash in the container (100 kg) through a continuous process, the calculated concentration in seawater would be 0.006 mg l 1. Seawater quality standard from the US-EPA (1999) is 9.3 mg l 1, and the suggested Israeli standard is 0.5 mg l 1. These are much higher than the calculated leached concentration and therefore the use coal ash in seawater is acceptable concerning Cd leaching and seawater quality standards. The same calculation can be performed for Hg in fresh fly ash. The decrease in concentration in 300 days was 0.08 mg kg 1 (Table 5) and therefore the calculated concentration in seawater would be 0.003 mg l 1. Seawater quality standard from the US-EPA (1999) is 0.94 mg l 1 [51], and the suggested Israeli standard is 0.16 mg l 1. As for Cd, these criteria are much higher than the calculated leached concentration and therefore the use coal ash in seawater is acceptable concerning Hg leaching and seawater quality standards. In summary, coal ash is an abundant waste product that its utilization in an environmentally safe manner at land or at sea is being extensively researched. In this study we checked the suitability of four types of coal ash for use in marine applications: fly and bottom ash freshly obtained from coal-fired power plant, and old ash (crushed and blocks) after prior contact with sea-
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water. We chose to investigate the ‘worst case scenario’ since the coal ash was in its original form, non-stabilized, and in direct contact with seawater. Changes occurred in the chemical composition of the coal ash during the experiment. Most of the changes occurred in the fresh fly ash, and not in the ash that was recovered from the sea (old ash), indicating a possibility of equilibrium achieved after 3–5 years of seawater exposure. In addition, more changes occurred in fresh fly ash as compared to fresh bottom ash, emphasizing the differences between the two ash types. While the changes in the concentrations of the major elements may be an indication of integrity of the ash matrix, the only elements of environmental significance that were released to the environment were Hg from fresh fly ash and Cd from fresh fly ash and old ash from the sea (crushed and blocks). However, seawater concentrations were much lower than seawater quality criteria and therefore the coal ash was considered suitable for marine applications concerning seawater quality. However, it is inferred that if the non-stabilized, bulk ash, in complete contact with seawater did not undergo too many changes, stabilization or pre-treatment (i.e. by soaking in seawater prior to use) will even improve environmental suitability as indicated in one study with stabilized Israeli fly ash (Kress et al., in press). In this research we did not look at biotic aspects however, visual and qualitative examination of the experimental containers showed the development of a biological film. This biomass might be a potential ‘‘bridge’’ transporting elements from the ash to the environment and should be further examined in the context of marine utilization of coal ash.
Acknowledgements The generous financial help of Marco and Louise Mitrani memorial fellowship and Mailman foundation fellowship are gratefully acknowledged. We wish to thank also the two anonymous reviewers that with their thorough comments helped improve the manuscript.
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Kress, N., Golik, A., Galil, B., Krom, M.D., 1993. Monitoring the disposal of coal fly ash at a deep water site in the eastern Mediterranean sea. Marine Pollution Bulletin 26 (8), 447–456. Kress, N., Herut, B., 1998. Disposal of coal fly ash at a deep water site in the eastern Mediterranean off Israel — six years of monitoring. Chemistry and Ecology 15, 185–198. Kress, N., Tom, M., Spanier, E. The use of coal fly ash in marine concrete for artificial reefs in the southeastern Mediterranean: compressive strength, sessile biota and chemical composition. ICES Journal of Marine Science (in press). Labotka, A.L., Duedall, I.W., Harder, P.J., Schlotter, N.J., 1985. Geochemical processes occurring in coal-waste blocks in the sea. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 717–722. (Chapter 31). Liem, H., Sandstrom, M., Wallin, T., Carne, A., Rydevik, U., Thurenius, B., Moberg, P.O., 1983. Studies on the leaching and weathering processes of coal ashes. Water Science and Technology 15, 163–191. Mezger, A., 1996. The Environmental Status of Coal Ash Produced In Israel—a Travel Report to USA, August–September. Israeli Electric Company (in Hebrew). Nathan, Y., Dvorachek, M., Pelly, I., Mimram, U., 1999. Characterization of coal fly ash from Israel. Fuel 78, 205–213. Parker, J.H., Woodhead, P.M.J., Duedall, I.W., Carlton, H.R., 1983. Ocean disposal and construction with stabilized coal waste blocks. Water Science and Technology 15, 83–95. Pickering, H., 1996. Artificial reefs of bulk waste materials: a scientific and legal review of the suitability of using the cement stabilized byproducts of coal-fired power stations. Marine Policy 6, 483–497. Reuss, M., 1983. Comparison different methods for estimating the leaching of heavy metals from coal combustion waste. Water Science and Technology 15, 193–205. Roethel, F.J., Oakley, S.A., 1985. Effects of seawater on the mineralogical and chemical composition of coal-waste blocks. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 691–704. (Chapter 30). Rose, C.D., Ward, T.J., De Pass, V.E., 1985. Ecological assessment for coal ash dumped at deepwater dumpsite- 106. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the
Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 389–414. (Chapter 16). Sampaolo, A., Relini, G., 1994. Coal ash for artificial habitats in Italy. Bulletin of Marine Science 55, 1277–1294. Shelef, G., Zimmels, Y., 1993. Construction of Artificial Islands in the Israeli Coast with the Use of Coal Ash. A report for the Period 1 August 1991–31 December 1992 (in Hebrew). Smith, R.D., Campbell, J.A., Nielson, K., 1979. Concentration dependence upon particle size of volatilized elements in fly ash. Environmental Science & Technology 13 (5), 553–558. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry. Wiley, New York. UNESCO, 1985. The international system of units (SI) in oceanography, UNESCO technical papers No. 45, IAPSO Pub. Sci. No. 32, Paris, France. United States Environmental Protection Agengy, 1999. (EPA 822-Z99–001). National recommended water quality criteria—correction, April. Van Der Sloot, H.A., Nieuwendijk, B.J.T., 1985. Release of trace elements from surface-enriched fly ash in seawater. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 449–464. (Chapter 18). Van Der Sloot, H.A., Wijkstra, J., van Stigt, C.H., Hoede, D., 1985. Leaching of Trace Elements from Coal Ash and Coal Ash Products. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 467–497. (Chapter 19). Warren, C.J., Dudas, M.J., 1988. Leaching behavior of selected trace elements in chemically weathered alkaline fly ash. Science of the Total Environment 76, 229–246. Windom, H.L., Schropp, S.J., Calder, F.D., Ryan, J.D., Smith, R.G., Burney, L.C., Lewis, F.G., Rawlinson, C.H., 1989. Natural trace metal concentrations in estuarine and coastal marine sediments of the southeastern United States. Environmental Science and Technology 23, 314–320. Woodhead, P.M.J., 1985. Biological compatability of a coal-waste block reef. In: Duedall, I.W., Kester, D.R., Park, P.K., Ketchum, B.H. (Eds.), Wastes in the Ocean, Vol. 4: Energy Wastes in the Ocean. Wiley, New York, pp. 557–571. (Chapter 23).
Chemical changes in different types of coal ash during prolonged, large scale, contact with seawater Efrat Shoham-Fridera,*, Gedaliah Shelefb, Nurit Kressa a Israel Oceanographic & Limnological Research, National Institute of Oceanography, Tel-Shikmona, PO Box 8030, Haifa 31080, Israel Division of Environmental Engineering and Water Resources, Faculty of Civil Engineering, Technion—Israel Institute of Technology, Technion City, Haifa 32000, Israel
b
Accepted 7 May 2002
Abstract In this study, we followed the chemical changes occurring in coal ash exposed to prolonged (300 days), large scale, contact with running seawater. Four major components (Al, Ca, Mg, Fe) and seven minor and trace elements (Cd, Cr, Cu, Mn, Zn, Pb, Hg) were measured in four coal ash types: fly and bottom ash freshly obtained from coal-fired power plant, and old ash (crushed and blocks) recovered from the sea after 3–5 years contact with seawater. Changes occurred in the chemical composition of the coal ash along the experiment: Fe increased in fresh ash, Al increased in old ash and Ca increased in all ash types except old ash blocks. Cu and Hg decreased in fresh fly ash while Cr increased, Cd decreased in all ash types except bottom ash, and Mn decreased in bottom ash. Most of the changes occurred in the fresh fly ash, and not in the old ash, indicating equilibrium after prior exposure to seawater. In addition, more changes occurred in fresh fly ash than in bottom ash, emphasizing the differences between the two ash types. While the changes in the concentrations of the major elements may be an indication of the integrity of the ash matrix, the only elements of environmental significance released to the environment were Hg and Cd. However, calculated seawater concentrations were much lower than seawater quality criteria and therefore the coal ash was considered suitable for marine applications concerning seawater quality. # 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Coal ash may reach and affect the marine environment as a dumped waste, or as a construction material for different marine applications such as artificial islands, artificial reefs and land reclamation of coastal areas (Woodhead, 1985; Hockley and Van Der Sloot, 1991; Homziak et al., 1993; Collins and Jensen, 1995; Pickering, 1996; among others]. Coal ash can serve as filling material, with no contact with seawater or can be in direct contact with it during dumping operations. In the latter case, besides the chemical aspects of seawater– coal ash interaction, there are additional ecological issues which concern the physical aspects of dumping (Bamber, 1980; 1983; Kress et al., 1993) its influence on * Corresponding author. Tel.: +972-4-856-5256; fax: 972-4-8511911. E-mail addresses: [email protected] (E. Shoham-Frider), [email protected] (G. Shelef), [email protected] (N. Kress).
the sediment (Kress and Herut, 1998) and on benthic organisms (Bamber, 1984; Kress and Herut, 1998). Coal ash can be mixed with cement and other pozzolanic materials to form a stabilized construction material, and their interaction with seawater is an intensive researched subject (Parker et al., 1983; Carleton and Muratore, 1985; Roethel and Oakley, 1985; Hockley and Van Der Sloot, 1991; Collins et al., 1994; Sampaolo and Relini, 1994; Kress et al., in press). Chemical and physical processes taking place within the blocks in the marine environment affect it through changes in the quality and properties of the blocks. The feasibility of using coal ash as filler in the construction of artificial islands in Israel is being studied, together with its compatibility with the marine environment (Shelef and Zimmels, 1993). In Israel about six million tons of coal fly ash were generated between the years 1982 and 1995. Sixty-five percent of this amount was utilized for cement production, 18% was used for building the embankment surrounding the ‘‘Orot Rabin’’ power plant in Hadera and
0956-053X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(02)00063-6
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17% was disposed in the deep sea, according to permits issued by the governmental authorities (Mezger, 1996). Approximately 1 million ton of coal ash was dumped at a deep-sea dumpsite in the Mediterranean Sea from 1989 to 1995. Small quantities continued to be dumped until 1998 when dumping ceased (Kress et al., 1993; Kress and Herut, 1998). From 1998 to 2000 ca. 2.8 million ton of coal fly ash was utilized for cement and concrete production and in road construction. Coal ash contains, among other components, trace elements that are potentially toxic (such as Hg, Cd). The main environmental concern in the utilization of coal ash is that these elements may leach out from the ash upon contact with an aqueous medium and reach the environment and the food chain. Several leaching studies of coal ash have been conducted with acid (Reuss, 1983; Grisafe et al., 1988; Warren and Dudas, 1988; Gutierrez et al., 1993), distilled or freshwater (Reuss, 1983; Goetz, 1983; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Garavaglia and Caramuscio, 1994), at specific pHs (Reuss, 1983; Goetz, 1983) and with seawater (Reuss, 1983; Goetz, 1983; Gurierrez et al., 1993; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Crecelius, 1985; Rose et al., 1985; Grisafe et al., 1988; Warren and Dudas, 1988; Kress, 1993; Garavaglia and Caramuscio, 1994). Most of the studies in the literature that describe leaching from coal ash during contact with seawater were conducted under laboratory, small scale, controlled conditions (Reuss, 1983; Van Der Sloot and Nieuwendijk, 1985; Van Der Sloot et al., 1985; Crecelius, 1985; Kress, 1993). Large scale leaching experiments, simulating natural environmental conditions were not performed to date in non stabilized coal ash. Laboratory experiments have shown that leaching from coal fly ash during contact with seawater is dominated by dissolution and desorption processes (Van Der Sloot and Niewendijk, 1985). At high liquid-ash ratios, leaching of major and minor components is determined by the pH of the coal fly ash, the extent of leaching being inversely proportional to pH (Van Der Sloot et al., 1985; Nathan et al., 1999). The coal ash generated in the electric power plants in Israel is of basic character and therefore less susceptible to leaching (Boker, 1989; Kress, 1993; Nathan et al., 1999). The main goal of this research was to follow changes in chemical composition and the chemical processes taking place in different types of coal ash during prolonged contact with warm and saline seawater of the Eastern Mediterranean. The experiment tried to simulate large scale, ‘‘field’’ conditions, in order to assess the environmental feasibility of using coal ash in marine projects. The types of ash studied comprised that which was freshly obtained from coal-fired power plants and that which was retrieved from a deep-sea dumpsite, namely, ash that was already in contact with seawater
for 3–5 years. This study clearly represents a worst-case scenario for the coal ash was not stabilized and used as produced or found.
2. Methods 2.1. Experimental design Four types of coal ash were used in the experiment: (1) fresh fly ash collected by the electrostatic precipitators at the Orot-Rabin (Hadera) power plant, (2) bottom ash obtained also from Orot Rabin power plant, (3) old crushed fly ash; and (4) old fly ash blocks. ‘‘Old’’ fly ash (types 3 and 4 above) was collected from the deep-sea dumpsite (Kress et al., 1993) during October 1994. Since 80% of the total fly ash was dumped at sea during 1989–1991, we estimate that the old fly ash was in contact with seawater for 3–5 years, prior to this study. The old fly ash retrieved from the sea appeared in different aggregate sizes (Kress and Herut, 1998). Some of the aggregates were crushed and sieved through 1–2 mm sieve (old crushed fly ash) and six blocks (old fly ash blocks) weighing 5–10 kg each with approximate dimensions of 402020 cm, were left intact. It should be emphasized that the initial or fresh composition of the old ash retrieved from the sea is unknown and probably different from the fresh ash used in this study. Average unburned coal content in the fly ash is 4–5%. The percentage of ignition loss at 550 C in the initial ash were: 1.85 0.6, 10.3 1.8, 8.85 1.0, 1.46 0.2 in fresh fly ash, ash blocks from the sea, crushed ash from the sea and bottom ash, respectively. Six plastic containers (111 m each) located at the yard of the institute at the seashore, were used in the experiment. A 10 cm layer of fresh fly ash was arranged at the bottom of two containers (identified as A, B) and a 10 cm layer of bottom ash was arranged at the bottom
Fig. 1. Schematic description of the experimental setup.
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of two additional containers (identified as E, F). The fifth container (identified as C) contained the six old ash blocks and the sixth container (identified as D) contained old crushed ash (Fig. 1). Each container had an inlet and an outlet for fresh seawater and was covered with a plastic sheet stretched on a wooden frame in order to prevent input of atmospheric derived materials. Seawater was pumped directly from the sea, filtered through a 500-mm filter before entering the containers and flowed continuously by a system of plastic pipes and taps. The filter was cleaned periodically of macro algae. Average flow rate in every container was 0.45 m3 h 1 with calculated seawater retention time in the container of 2.2 h. The flow was sufficient to rapidly exchange water but not strong enough to cause resuspension and loss of the fine-grained ash. 2.2. Sampling The coal ash was sampled at the beginning of the experiment before the contact with the flowing seawater, 3 days after the beginning of the experiment and once a month for 12 months (March 1995–March 1996) until the end of the experiment. Ash samples from six samplings (the third and ninth of March, May, August, November 1995 and January 1996) out of the total 15 were analyzed for the different constituents. The other nine samplings were carried out so that additional analyses could be performed if necessary to explain trends or phenomena indicated by the six primary samplings. Coal ash from containers A,B,D,E,F was sampled at five points in the container until September 1995 (Fig. 1). After that, only two points (two and four) were sampled since results showed no significant differences in composition between sampling points in each container. Sampling the six ash aggregates from container C was done by scraping the surface of each aggregate with a plastic knife. Environmental conditions (temperature, pH and salinity) were measured during each sampling period. Temperature was measured directly in the container with a standard mercury thermometer. Salinity of the seawater was measured in samples taken to the laboratory, using a Autosal 8400 Guideline salinometer. Salinity values are given on a practical salinity scale that is unitless (UNESCO, 1985). The pH of seawater was measured directly using a ‘‘Mettler Toledo’’ portable pH-meter. 2.3. Sample preparation and laboratory analysis The ash samples were rinsed with DIW (deionized distilled water) to remove salt and then frozen. The frozen samples were dried by lyophilization, crushed, homogenized and kept in closed plastic containers until the chemical analysis. No Hg is lost during the drying procedure (Hornung, 1999). The dry coal ash samples were digested with a mixture of hydrofluoric acid and
aqua regia as described in the ASTM Method (1989). A separate digestion was performed in concentrated nitric acid for the determination of Hg (Hornung et al., 1992). The concentrations of the major components in the ash: Fe, Al, Ca and Mg, and of the minor and trace components: Cd, Pb, Zn, Mn, Cu and Cr, were measured by atomic absorption spectrophotometry using an IL-951 or a Perkin-Elmer 1100 B spectrophotometer equipped with flame and graphite furnace modules with D2 background correction. Hg was analyzed by cold vapor atomic absorption spectrometry on a Coleman Mercury Analyzer MAS-50. Quality control and quality assurance (QC/QA) of the results were performed with standard reference material from the National Institute of Standards and Technology (NIST—Coal Fly Ash-1633a; Table 1). The standards were digested and analyzed in the same manner as the samples, for each analytical run. The coefficient of variance for Ca and Pb in the standards was high and therefore the results were corrected mathematically to 100% recovery at each analytical run. Statistical analysis was performed using the SAS program. The procedures used were the general linear mode (GLM) using the method of least squares, t-test with unequal variance and the Duncan a-parametric test at the 95% confidence level.
Table 1 Quality control and quality assurance of the results utilizing a standard reference material, NIST 1633a—coal fly ash Al
Ca
Major components (% wt) Avg 13.6 0.47 SD 0.78 0.20 CV (%) 6 43 N 16 15 % Recovery 99 43
Fe 9.55 0.75 8 16 69
Mg 0.41 0.06 16 15 89
Certified values of NIST 1633a—coal fly ash Avg 14.3 1.11 9.4 0.455 SD 1.0 0.01 0.1 0.01 Cd Cr Cu Mn Minor and trace components (mg kg 1) Avg 0.79 154 110 174 SD 0.10 17 3 7 CV (%) 12 11 3 4 N 14 16 16 16 % Recovery 79 79 93 97 Certified values of NIST 1633a—coal fly ash Avg 1.00 196 118 179 SD 0.15 6 3 8
Pb
Zn
Hg
97.6 230 0.172 18.3 11 0.028 19 5 16 16 15 10 135 105 108
72.4 220 0.4 10
0.16 0.01
Avg, average; SD, standard deviation; CV, coefficient of variance; n, number of samples.
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3. Results 3.1. Environmental conditions Temperature, salinity and pH in the experimental containers are presented in Fig. 2. Gradual seasonal changes occurred between the minimal temperature in
January (15 C) and the maximal temperature in August (30 C). Salinity was between 39.1 in January–March and 39.7–40.0 in July–November. The pH values ranged between 8.07 in June (summer) and 8.50 in November (fall). It decreased gradually from March to June, and then increased to November. Aggregation of the powdery coal ash took place both in fresh fly ash
Fig. 2. Environmental conditions in the experimental containers.
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centration was significantly lower (Table 3). At the end of the experiment Fe increased in fresh ash (fly and bottom), Al increased in old ash from the sea (crushed ash and old ash blocks; Table 4). Ca increased in all ash types except old ash blocks. No changes in Mg were observed in any of the coal ash types (Table 4). Concentrations of the minor and trace components (Cd, Cr, Cu, Mn, Pb, Zn and Hg) in each of the coal ash types along the experiment are summarized in Table 5. The initial concentration of Pb was higher and Zn was lower in the old ash from the sea then in the fresh ash types (Table 3). Cu was similar in all the ash types while Hg was different in all types. The lowest concentration of Cd was found in bottom ash, while Cr and Mn were lowest in fresh fly ash (Table 3). At the end of the experiment Cd, Cu and Hg decreased significantly in fresh fly ash while Cr increased. Concentrations of the other elements did not change significantly in the fresh fly ash (Table 4). Mn decreased in bottom ash, while the other elements did not change significantly. The only
and old crushed ash. It is known that coal ash has a pozzolanic character and aggregates are formed during contact with aqueous medium (Bamber, 1983). A thin biological film developed on the ash bed and on the sidewalls of the containers during the year of the experiment. 3.2. Chemical composition of the coal ash Each of the six containers was sampled 15 times. A total of 405 ash samples were collected and 137 analyzed for Al, Ca, Fe, Mg, Cd, Cr, Cu, Mn, Pb, Zn and Hg. Concentrations of the major components (Al, Ca, Fe and Mg) in each of the coal ash types along the experiment are presented in Table 2. The initial concentration of Al and Mg were significantly higher in the old ash from the sea than in the fresh ash (fly and bottom) while Fe was higher in the fresh ash types. Ca was similar in all ash types except in the fresh fly ash where the con-
Table 2 Major components (% wt) in the different coal ash types during the experiment Days
Al
Ca
Mg
SD
n
Avg
SD
n
Avg
SD
n
Avg
SD
Containers AB—fresh coal fly ash 0 10 10.9 3 10 10.8 60 9 10.5 150 4 11.2 240 4 11.8 300 4 11.0
0.7 0.6 0.4 0.3 0.8 0.2
10 10 10 4 4 4
1.30 1.72 0.80 1.79 2.96 2.82
0.78 0.96 0.31 1.00 1.55 1.02
10 10 9 4 4 4
5.00 5.03 5.17 6.66 6.18 6.18
0.19 0.27 0.22 0.23 0.56 0.35
4 4 4 3 4 4
0.67 0.65 0.65 0.62 0.76 0.77
0.27 0.09 0.14 0.18 0.26 0.24
Container C—old coal ash blocks 0 6 13.7 60 6 12.6 150 6 13.9 240 5 14.3 300 5 14.8
0.7 0.6 0.3 0.7 0.6
6 6 6 5 5
4.30 4.57 6.23 5.01 6.59
0.88 1.28 1.12 1.56 1.88
6 6 6 5 5
2.73 2.67 3.30 3.17 3.26
0.26 0.37 0.6 0.39 0.39
6 6 6 5 5
2.28 2.12 2.13 2.27 2.52
0.48 0.41 0.56 0.50 0.70
Container D—old crushed coal ash 0 5 13.8 3 5 13.6 60 5 12.9 150 2 15.0 240 2 14.3 300 2 14.5
0.4 0.4 0.1 0.1 0.1 0.4
5 5 5 2 2 2
4.49 4.76 5.48 6.35 5.96 8.10
0.74 1.12 0.45 0.93 2.22 0.16
5 5 5 2 2 2
2.83 2.74 2.80 3.61 3.47 3.19
0.31 0.22 0.24 0.32 0.11 0.06
2 2 2 2 2 2
1.94 2.01 1.94 1.98 1.64 2.18
0.24 0.09 0.11 0.16 0.59 0.16
Containers EF—bottom coal ash 0 10 12.4 3 10 11.8 60 10 10.2 150 4 13.0 240 4 12.3 300 4 13.1
0.6 0.7 0.9 0.6 1.2 0.4
10 10 10 4 4 4
4.58 3.77 3.45 7.41 7.47 7.76
0.8 0.42 0.48 0.90 2.44 0.32
10 10 10 4 4 4
6.31 6.49 6.91 7.21 7.68 8.06
0.4 0.35 0.51 0.54 0.79 0.21
4 4 4 4 4 4
1.14 1.00 0.93 1.12 0.89 1.07
0.14 0.19 0.16 0.07 0.29 0.05
N
Avg
Fe
n, number of samples; Avg, average; SD, standard deviation.
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Table 3 Comparison among the initial average concentrations in the four coal ash types Al Major components (% wt) Old crushed ash 13.84 Old ash blocks 13.65 Bottom ash 12.40 Fresh fly ash 10.86
Ca A A B C
Cd Minor and trace components (mg kg 1) Old crushed ash 0.58 A Old ash blocks 0.53 A/B Bottom ash 0.23 C Fresh fly ash 0.44 B
4.49 4.30 4.58 1.30
Fe A A A B
2.83 2.73 6.31 5.00
Cr 145 126 138 89.6
Mg C C A B
Cu A B A/B C
89.0 82.5 79.6 84.5
1.94 2.28 1.14 0.67
A A B B
Mn A A A A
450 440 510 257
Pb B B A C
80.2 80.0 41.6 64.4
Zn A A C B
114 91.7 160 183
Hg B B A A
0.255 0.295 0.045 0.188
B A D C
Averages in same Duncan grouping (represented by capital letters) are not significantly different.
significant change that took place in the ash from sea, old crushed ash and old ash blocks, was a decrease in Cd concentration (Table 4). The initial composition of fresh bottom ash was different from the fresh fly ash, more heterogeneous with high standard deviation of the concentrations (Tables 2 and 5). The major elements, as well as Cr and Mn were higher in bottom ash than in fresh fly ash. The minor and trace elements (Cd, Pb, Hg), expected to be enriched at the surface due to volatilization/condensation process were lower in the bottom ash. Cu and Zn concentrations were similar in both types.
4. Discussion 4.1. Environmental conditions The temperature in the experimental containers during the year represented a normal seasonal variation. The temperature also dictated the trends of salinity. Salinity in open seawater of the Mediterranean range Table 4 Changes in concentration (percentage from initial value) in the four coal ash types during the experiment Component
Fresh fly ash
Bottom ash
Old ash blocks
Old crushed ash
Al Ca Fe Mg Cd Cr Cu Mn Pb Zn Hg
nsc +117 +24 nsc 27 +16 10 nsc nsc nsc 41
nsc +69 +28 nsc nsc nsc nsc 9 nsc nsc nsc
+8 nsc nsc nsc 30 nsc nsc nsc nsc nsc nsc
+5 +80 nsc nsc 33 nsc nsc nsc nsc nsc nsc
nsc, no significant change from initial value.
between 38.0 and 39.0 (Hecht et al., 1988) while shallow coastal areas are more affected by evaporation and rain inputs. Indeed, the lowest salinities in the experimental containers were observed during the winter with increased values during the summer. The range of 39.0– 40.0 measured during the experiment was typical of near shore waters. The pH was affected by the photosynthetic activity, since algae growth took place in the containers during the experiment. 4.2. Chemical composition and processes Elements which concentrations increased significantly during the year of the experiment were Al in old crushed ash and old ash blocks, Ca in all types of ash except old ash blocks, Fe in fresh fly ash and bottom ash, and Cr in fresh fly ash. Increase of element concentration may be due to chemical exchange processes, for example, Ca– Mg exchange (Hockley and Van Der Sloot, 1991; Kress et al., 1994) or to adsorption (Roethel and Oakley, 1985). It is known that coal ash has adsorption properties, which are magnified as the carbon content in the ash increases (Bamber, 1980). The increase of Ca found in this research contrasts with other studies where Ca was leached from coal ash into seawater or exchanged by Mg (Parker et al., 1983; Labotka et al., 1985; Hjelmar, 1990; Hockly and Van Der Sloot, 1991). In the old ash from the sea this exchange could have occurred prior to this study, during its stay at sea, and the concentration would not be expected to change. An indeed, in the ash blocks there were no changes in Ca concentration. However, Ca increased during the experiment in the old crushed ash as well in as in the fresh ash (fly and bottom). Ca increase started with a slight delay from the increase in pH, after 150 days (Table 2, Fig. 2) i.e. after the increase in photosynthetic activity and biomass growth. It is well known that a wide variety of marine organisms, for example, green and red algae, bryozoans, corals, mussels and bacteria form calcifica-
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E. Shoham-Frider et al. / Waste Management 23 (2003) 125–134 Table 5 Minor and trace components (mg kg 1) in the different coal ash types during the experiment Days
n
Cd Avg
Cr SD
Cu
Avg
SD
Avg
Containers AB—fresh coal fly ash 0 10 0.44 0.04 3 10 0.46 0.08 60 9 0.47 0.02 150 4 0.26 0.03 240 4 0.31 0.05 300 4 0.32 0.02
89.6 94.6 94.3 128 112 104
4.3 6.7 7.8 26 4 5
84.5 79.9 80.9 74.9 76.3 75.9
Container C—old coal ash blocks 0 6 0.53 0.08 60 6 0.38 0.04 150 6 0.27 0.05 240 5 0.3 0.05 300 5 0.37 0.07
126 122 158 151 138
20 17 5 21 20
Container D—old crushed coal ash 0 5 0.58 0.04 3 5 0.54 0.01 60 5 0.44 0.02 150 2 0.35 0.05 240 2 0.38 0.08 300 2 0.39 0.06
145 135 125 157 153 146
Containers EF—Bottom ash 0 10 0.23 0.15 3 10* 0.22 0.16 60 10 0.23 0.27 150 4 0.22 0.15 240 4 0.15 0.04 300 4 0.17 0.1
138 135 140 159 162 128
Mn SD
Pb
Zn
Hg
Avg
SD
Avg
SD
Avg
SD
N
Avg
SD
1.7 4.6 1.9 2.8 3.7 2.5
257 270 264 282 265 268
27 34 20 19 15 6
64.6 61.0 62.5 68.1 68.7 65.1
8.9 5.8 16.5 1.2 2.4 3.4
183 161 173 177 182 180
17.4 21.1 10.5 10.7 20.8 14.5
4 4 4 4 4 2
0.19 0.19 0.12 0.18 0.16 0.11
0.02 0.01 0.01 0.01 0.02 0.07
82.5 78.4 79.0 76.4 77.6
10.4 11.2 12.5 14.8 14.3
440 385 409 382 386
65 46 26 49 31
80.1 89.7 83.7 83.1 81.9
8.6 5.2 2.0 4.2 4.7
91.7 94.2 102 156 111
12.9 12.2 7.9 18.0 23.0
2 6 6 4 5
0.30 0.16 0.29 0.24 0.21
0.01 0.08 0.04 0.02 0.09
11 24 5 6 1 6
89.0 85.5 87.5 82.3 78.4 87.4
5.9 3.5 1.4 2.1 2.2 3.8
450 430 424 436 415 439
12 13 10 16 38 1
80.1 73.6 89.1 80.7 83.1 83.1
2.2 9.1 11.9 3.0 3.9 1.7
114 105 98.6 118 96.0 104
11.6 10.6 3.6 17.0 1.4 4.2
2 – 2 2 2 2
0.26 – 0.26 0.27 0.26 0.25
0.01 – 0.04 0.01 0.01 0.01
19 20 25 3 7 31
79.6 77.6 81.1 72.2 74.5 85.2
11.5 6.4 9.7 9.1 5.4 15.4
510 498 454 449 441 463
28 18 14 23 8 17
41.7 34.6 46.0 44.8 44.1 44.3
4.9 3.4 6.7 1.1 3.6 3.9
160 149 130 149 128 133
39.6 21.8 20.7 21.5 25.4 30.1
4 4 4 4 4 4
0.05 0.05 0.01 0.04 0.05 0.03
0.01 0.01 0.01 0.02 0.03 0.02
n, number of samples; Avg, average; SD, standard deviation. * n=9 for Cd, Cu and Pb.
tion, i.e. sedimentation of calcium into their tissues (Daws, 1998). Calcium concentrations of 6500 ppm were documented in phytoplankton (Stumm and Morgan, 1981). Therefore it is reasonable to assume that the Ca enrichment in the coal ash samples originated from external biological source, i.e. deposition of calcified remains after organisms die off on top of the coal ash bed. Deposition did not occur in the old ash blocks because of their shape (large blocks standing on the short side) and therefore Ca did not change significantly. Moreover, care was taken to sample the aggregate from parts without growth, what was not possible in the case of the coal ash beds. During the experiment Fe increased in the fresh ash (fly and bottom ash) by 24 and 28%, respectively from the initial values (Table 4). This was unexpected because Fe is known to be a part of the glass matrix or the magnetic fraction of the ash (Crecelius, 1980). However scavenging of Fe from seawater by coal ash blocks has been observed in laboratory experiments (Roethel and Oakley, 1985). Moreover, the fact that the concentra-
tion increased only in the fresh ash (fly and bottom) may support the scavenging/adsorption mechanism. Fe increase could be due also to deposition of suspended particulate matter (SPM) from the running seawater on top of the coal ash. Therefore, a rough calculation was performed to estimate how much Fe could accumulate during the experiment due to settling of SPM from seawater. The amount of seawater that passed through each container during 300 days was 3240 m3 (0.45 m3 h 1) and assuming that Fe concentration in SPM in this region is 15 mg/l (unpublished results), 48 g of Fe accumulated in an area of 1 m2. Using the specific density of coal ash 1 g cm 3 (Boker, 1989) and assuming that the deposition would affect only the top 1.5 cm of the coal ash bed, the calculated increase of Fe during the study would be 3300 mg kg 1 ash (0.33% wt) that explain only 28 and 19% of the observed concentration increase of 1.18 and 1.75% wt in fresh fly ash and bottom ash, respectively. Therefore, the increase in Fe concentration cannot be attributed only to the SPM contribution and some may be attributed to adsorption from seawater
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and maybe from biogenic source as Ca. It is interesting to note, however, that Fe concentration showed an increasing trend also in the old ash blocks and old crushed fly ash. Although these increases were not statistically significant, the amounts are in good agreement with the calculated 0.33% wt addition (Table 2). This indicates that old ash may have reached equilibrium in the marine environment concerning adsorption of Fe and the non-significant increase was due to SPM deposition. The increase of Al in the two old ash types from the sea, although statistically significant, was small (8 and 5% from the initial values, Tables 2 and 4). Al is considered to be a conservative element in coal ash (Windom et al., 1989) hence its concentration was not expected to change. However, Al decreased in concrete blocks containing coal fly ash in laboratory experiments (Sampaolo and Relini, 1994) and deployed at sea (Labotka et al., 1985; Kress et al., in press). Decrease in concentration can be explained by leaching of the element from the ash to the environment. The elements mostly prone to leaching are those enriched on the coal ash surface due to volatilization-condensation processes during coal combustion, such as Cd, Cu, Zn, Pb (Van Der Sloot and Nieuwendijk, 1985). Leaching can occur thorough chemical dissolution from the ash followed by precipitation or adsorption onto particles (Crecelius, 1980; Bourg and Alain, 1983; Van Der Sloot and Nieuwendijk, 1985), and to complexation with ligands that are present in seawater like chloride, bromide, sulfate and carbonate (Liem et al., 1983; Roethal and Oakley, 1985; Hirose, 1988; Byrne et al., 1988). Bacteria can extract constituents from the ash. Significant leaching of Mn and Cu from municipal waste ash was attributed to extraction by bacteria (32 and 57%, respectively; Bosshard et al., 1996). In this study, Hg and Cu decreased in the fresh fly ash, Mn decreased only in the bottom ash, and Cd decreased in all types of fly ash but not in bottom ash (Tables 4 and 5). Hg, due to its low boiling point, evaporates almost completely during coal combustion and is released to the atmosphere (Smith et al., 1979). However, Hg is still present in the coal ash, probably enriched at the surface as well as Cu. Surface enrichment in coal fly ash is higher than in bottom ash due to larger surface area (1–3 and 0.4 m2 g 1, respectively; Goetz, 1983; Boker, 1989) and that may explain the lower enrichment (Table 3, initial concentrations) and the lack of leaching from fresh bottom ash. Cu and Hg form complexes with carbonate and chloride in seawater, respectively, aiding and increasing leaching of these elements. Moreover, leaching occurred only in the fresh fly ash that had not interacted with seawater prior to the experiment, indicating stabilization concerning Hg and Cu in the old ash. Mn is not considered an element that is enriched at the surface during coal combustion therefore it is rea-
sonable to assume that the decrease in concentration results from dissolution. It is known that anoxic conditions cause the dissolution of oxides of manganese (Hemond and Fechner-Levy, 2000). It is hypothesized that the coarse grain size of bottom ash and its packing in the container may have created anoxic micro- environment facilitating dissolution from the ash. Changes in pH and oxygen in micro-environment that lead to changes in Mn and Al were found in the surface of stabilized coal fly ash blocks deployed at sea (Roethel and Oakley, 1985; Kress et al., in press). Cd, in contrast to Cu and Hg, leached significantly from fresh fly ash (27%) but also from old ash blocks and old crushed ash (30 and 33%, respectively) that were in prior contact with seawater, indicating a longterm, continuous process. Leaching of Cd from Israeli fly ash was shown also in laboratory experiments with seawater (Kress, 1993), at the dump site (Kress and Herut, 1998) and even from stabilized coal fly ash/ cement blocks after 33 months deployment at sea (Kress et al., in press). Leaching ranged between 20 and 55% of the initial value. As Cd was unequivocally the element that leached out of the ash and has a proven deleterious environmental effect, the addition of Cd to seawater was calculated and compared to known seawater quality standards. In the worst case, old crushed coal ash, the concentration decreased by 0.19 mg kg 1 in 300 days (Table 5) and 3240 m3 of seawater were flown through the container. Using the specific density of coal ash 1 gr cm 3 (Boker, 1989) and assuming that the leaching took place from all the ash in the container (100 kg) through a continuous process, the calculated concentration in seawater would be 0.006 mg l 1. Seawater quality standard from the US-EPA (1999) is 9.3 mg l 1, and the suggested Israeli standard is 0.5 mg l 1. These are much higher than the calculated leached concentration and therefore the use coal ash in seawater is acceptable concerning Cd leaching and seawater quality standards. The same calculation can be performed for Hg in fresh fly ash. The decrease in concentration in 300 days was 0.08 mg kg 1 (Table 5) and therefore the calculated concentration in seawater would be 0.003 mg l 1. Seawater quality standard from the US-EPA (1999) is 0.94 mg l 1 [51], and the suggested Israeli standard is 0.16 mg l 1. As for Cd, these criteria are much higher than the calculated leached concentration and therefore the use coal ash in seawater is acceptable concerning Hg leaching and seawater quality standards. In summary, coal ash is an abundant waste product that its utilization in an environmentally safe manner at land or at sea is being extensively researched. In this study we checked the suitability of four types of coal ash for use in marine applications: fly and bottom ash freshly obtained from coal-fired power plant, and old ash (crushed and blocks) after prior contact with sea-
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water. We chose to investigate the ‘worst case scenario’ since the coal ash was in its original form, non-stabilized, and in direct contact with seawater. Changes occurred in the chemical composition of the coal ash during the experiment. Most of the changes occurred in the fresh fly ash, and not in the ash that was recovered from the sea (old ash), indicating a possibility of equilibrium achieved after 3–5 years of seawater exposure. In addition, more changes occurred in fresh fly ash as compared to fresh bottom ash, emphasizing the differences between the two ash types. While the changes in the concentrations of the major elements may be an indication of integrity of the ash matrix, the only elements of environmental significance that were released to the environment were Hg from fresh fly ash and Cd from fresh fly ash and old ash from the sea (crushed and blocks). However, seawater concentrations were much lower than seawater quality criteria and therefore the coal ash was considered suitable for marine applications concerning seawater quality. However, it is inferred that if the non-stabilized, bulk ash, in complete contact with seawater did not undergo too many changes, stabilization or pre-treatment (i.e. by soaking in seawater prior to use) will even improve environmental suitability as indicated in one study with stabilized Israeli fly ash (Kress et al., in press). In this research we did not look at biotic aspects however, visual and qualitative examination of the experimental containers showed the development of a biological film. This biomass might be a potential ‘‘bridge’’ transporting elements from the ash to the environment and should be further examined in the context of marine utilization of coal ash.
Acknowledgements The generous financial help of Marco and Louise Mitrani memorial fellowship and Mailman foundation fellowship are gratefully acknowledged. We wish to thank also the two anonymous reviewers that with their thorough comments helped improve the manuscript.
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