Leaching Characteristics of Arsenic from Aged Alkaline Coal Fly Ash

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Leaching Characteristics of Arsenic from Aged Alkaline Coal Fly Ash Using Column and Sequential Batch Leaching Amid P. Khodadoust,*,† Pratibha Naithani,† Thomas L. Theis,† and Ishwar P. Murarka‡ † ‡

Department of Civil & Materials Engineering, University of Illinois at Chicago, Chicago, Illinois 60607 Ish Inc., Raleigh, North Carolina 27615 ABSTRACT: Leaching of arsenic from ash in coal fly ash disposal facilities is a cause for concern due to possible contamination of groundwater. Sequential batch and column leaching experiments were carried out to determine the leaching of arsenic from ash found at a retired ash impoundment basin near a coal-fired power plant. This study assessed the leaching characteristics of arsenic from aged alkaline coal fly ash as a function of ash properties and ash depth. Comparable levels of arsenic leached from the ash in column leaching (10.5%) and sequential batch leaching (11.1%) after 14 days (7 stages) of leaching, while less arsenic (17.8%) leached for column leaching than for sequential leaching (22.0%) after 42 days (21 stages) of leaching. The leaching characteristics of arsenic in column leaching and sequential batch leaching were similar, where more than half of the leaching of arsenic occurred within 14 days of leaching with the maximum leaching of arsenic occurring during the second and third stages of leaching. More arsenic leached from the deeper ash samples in sequential batch leaching of ash samples from various depths. Arsenic leached out mainly as calcium hydrogen arsenate from the calcium arsenate present in ash. The short-term (2-day) leaching of arsenic from ash was limited by the solubility of calcium arsenate, while the long-term leaching of arsenic was limited by the leaching of calcium bicarbonate which controlled the leachate pH. The source of calcium bicarbonate in the leachate was the amorphous calcium carbonate in the ash.

’ INTRODUCTION Coal fly ash contains moderately elevated concentrations of arsenic.1 Arsenic may leach from coal fly ash as arsenite [As(III)] and arsenate [As(V)] species. Analysis of coal fly ash particles has shown that As(V) species was the principal form of arsenic produced2,3 which is a less toxic form of arsenic than As(III),4 while it has been observed that more than 90% of arsenic is in the form of arsenate.5-7 Fly ash can be categorized as either alkaline or acidic fly ash on the basis of its pH. Within the pH range 4-12, for most ash leachates the principal inorganic arsenic species are oxyanions and oxyacids.8 Arsenic in ash may be adsorbed on surfaces of oxides and oxyhydroxides of iron9 as arsenic, arsenite, and arsenate. The characteristics of ash that influence leaching include10 ash particle size and surface area exposed to leaching, permeability of the matrix, and flow rate of the leaching fluid. Chemical factors that influence leaching include dissolution and desorption reactions, pH of the ash, pH of the leaching fluid, and complexation with inorganic or organic compounds.10 The adsorption and coprecipitation on secondary minerals may be important in controlling the partitioning of trace elements between fly ash and leachate.11 Leaching behavior of ash in the environment is studied by means of field or laboratory experiments, where the dissolution of mineral components and the behavior of dissolved components are controlled by the system variables pH, redox potential (Eh), and concentrations of the dissolved species.12,13 The oxyanionic species of arsenic have been shown to have significant pH-dependent leaching from fly ash.14,5 The effect of leaching solution pH on column leaching has been shown,15 where the higher leachate pH has been shown to cause greater r 2011 American Chemical Society

leaching of arsenic.16,17 Smaller ash particles with larger particle surface area contain more arsenic than larger ash particles.18 X-ray mapping of arsenic in fly ash indicated that arsenic was distributed as oxide within the Fe/O- and Fe/Al/Si/O-rich glass and crystalline phases in the fly ash.19The major sinks for arsenic were assumed to be oxides of iron and manganese, and silica.20 Arsenic in fly ash has been shown to be present in significant amounts in the silicate matrix.21 Arsenic in ash was associated with iron-rich glass phases, nonmagnetic phases, and nonsilicates, or was accumulated on the surfaces of fine particles.22-26 Arsenic trapped in aluminosilicate matrixes was not expected to be released at ambient conditions; some other trace metals could also be present in this phase.27 The calcium content of ash was shown to be the possible controlling factor for the leaching of arsenic.28 Fly ash of moderately alkaline character may initially contain a mixture of Fe-As and Ca-As complexes. The CaSO4, CaCO3, or Ca(OH)2 in ash may control the calcium concentration in fly ash leachate.14 Calcium arsenate has been shown to be a probable host for arsenic in alkaline fly ash.29 Column leaching experiments approximate the flow conditions, particle size distribution and pore structure, leachate flow, and solute transport found in the field more closely. Column methods are more expensive and more operationally complex, but they generate results that reflect the natural systems more closely. A basic assumption in column leaching is that the distribution of the leaching solution is uniform and that all particles Received: April 18, 2010 Accepted: December 16, 2010 Revised: November 29, 2010 Published: January 20, 2011 2204

dx.doi.org/10.1021/ie100909n | Ind. Eng. Chem. Res. 2011, 50, 2204–2213

Industrial & Engineering Chemistry Research

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Table 1. Tessier Sequential Extraction Procedure fraction I

designation exchangeable

extraction technique exchange with excess

procedure 1 M NaOAc (16 mL), pH 8.2, 1 h at room temperature, with continuous agitation

cations II

bound to carbonate

release by mild acid

1 M NaOAc (16 mL), pH 5 (pH adjusted with HOAc), 5 h at room temperature, continuous agitation

III

bound to iron and

reduction

40 mL of 0.04 M NH2OH 3 HCl in 25% (v/v) HOAc heated to 96 ( 3 °C with occasional agitation

IV

bound to organic

oxidation

1. 0.02 M HNO3 (6 mL) þ 5 mL 30% H2O2 (pH 2 adjusted with HNO3), heated at 85 ( 2 °C for 2 h

manganese oxides

for 6 h

matter

with occasional agitation. Then, 6 mL of 30% H2O2 (pH 2 adjusted with HNO3) was added and heated to 85 ( 2 °C for 3 h with agitation. 2. cooling of the mixture þ 10 mL of 3.2 M NH4OAc in 20% (v/v) HNO3. Finally, dilute to 40 mL and agitate continuously for 30 min.

V

residual

the above four fractions summed up and subtracted from the total concentration

are exposed equally to the leaching solution. Precipitation or sorption of arsenic compounds within the column may occur. An aged coal fly ash impoundment contains appreciable levels of arsenic in the deeper ash samples, where arsenic could leach out into the subsurface and contaminate the groundwater which may be used as a source of drinking water. The behavior of arsenic during disposal of coal combustion waste products is a cause for concern because of its toxicity, environmental persistence, and tendency to bioaccumulate. In order to assess the leaching of arsenic from the ash, column leaching of a deeper ash sample was carried out in this study where the deeper ash sample had a greater arsenic content than the shallower ash samples at the same location in the impoundment. The leaching characteristic of arsenic was evaluated as function of ash and leachate properties. Sequential batch leaching of the same ash sample was carried out to determine the similarity between column and batch sequential leaching. In addition, sequential batch leaching of ash samples from different depths was carried out to evaluate the leaching of arsenic as a function of ash depth and ash particle size.

’ MATERIALS AND METHODS Ash Samples. The coal fly ash used in column leaching was collected from the Retired Ash Basin at the Brunner Island site belonging to PPL Generation Co. in Pennsylvania. Four ash samples were obtained from the PZ8-3 location collected from a depth of 5 to 35 ft. The acid digestion of ash was performed according to U.S. EPA Method 3050B30 to determine the concentration of arsenic, calcium, iron, manganese, and boron in ash. The particle size distribution of the ash was determined (ASTM D 422). An elemental analyzer (EA) was used to determine the inorganic carbon content of the ash. The moisture content and organic carbon content were obtained using a drying oven (105 °C) and a muffle furnace (375 °C), respectively (ASTM D 2974-87).31 The pH of ash was determined by mixing 10 g of ash with 10 mL of deionized (DI) water using a 1:1 (g:mL) ratio (ASTM 4972).31 Leaching Experiments. The duplicate column leaching experiments were conducted in 30 cm long borosilicate glass columns (KONTES CHROMAFLEX, 420830-3020) with an inside diameter of 2.5 cm. A mass of 100 g of ash was packed in each glass column occupying half of the column volume. A continuous flow rate of DI water at 5.3 mL 3 h-1 was applied to two leaching columns using a Masterflex peristaltic pump (Cole Parmer, IL); at this flow rate, 254 mL of water would pass through the column to leach 100 g of ash every 48 h (2-day period).

The fly ash was leached with DI water downward from the top of the column, similar to the natural direction of leachate flow in ash impoundments. The leachate passed through a HDPE 100 mesh screen followed by a 20 μm HDPE membrane before exiting at the bottom of the leaching column as column effluent. The leachate was collected every 48 h over a period of 42 days. The same ash sample PZ8-3 (24-32 ft) was subjected to 21 successive extraction stages in sequential batch extraction for 42 days. Standard sequential batch leaching of ash was carried out by batch extraction of ash samples with DI water for 48 h per stage. For each extraction stage, 100 g of ash was mixed with 300 mL of fresh DI water using a solid:liquid extraction ratio of 1 g:3 mL, and shaken at 16 rpm. After 48 h of mixing (1 stage), the ash-water slurry was centrifuged at 12000g for 20 min to separate the ash from the supernatant solution (leachate); the leachate solution was then filtered using a 0.45 μm cellulose acetate membrane. For each subsequent leaching stage, fresh DI water was added to the same previously leached ash sample using a solid:liquid extraction ratio of 1 g:3 mL at 16 rpm for 48 h. Filtered leachate samples were analyzed for pH, alkalinity, arsenic, calcium, iron, manganese, boron, and sulfate. Samples for analysis of metals and arsenic were preserved by adding concentrated HNO3 (trace metal grade, Fisher Scientific) and were stored in polyethylene bottles. Arsenic, calcium, iron, manganese, and boron were determined using inductively coupled plasma mass spectrometry (ICP-MS), and sulfate analysis was performed using ion chromatography (IC). The detection limit for all analytes using ICP-MS was 1 μg/L. The detection limit for sulfate using IC was 100 μg/L. Tessier Extractions. Tessier sequential extractions were performed on the four samples from the PZ8-3 location to investigate the chemical association of arsenic and calcium in fly ash. The procedure was developed for the fractionation of metals into various fractions of a solid matrix such as soils, sediments, and ash.27 The Tessier sequential extraction procedure is presented in Table 1. Between each successive extraction, centrifugation was performed at 12000g for 30 min. The supernatant was removed and analyzed for arsenic and calcium using ICP-MS, whereas the residue was washed with 10 mL of DI water and centrifugation was performed for 30 min; this second supernatant was discarded and the residue was used for the next extraction step.

’ RESULTS AND DISCUSSION The properties of four ash samples from the PZ8-3 location are shown in Table 2. All ash samples were alkaline in nature, and 2205

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Table 2. Properties of Ash Samples from PZ8-3 Location depth (ft) 5-9

15-19

24-32

30.5-34.5

sand (%)

54.7

46.9

30.3

silt (%)

34.4

39.9

57.2

16.4 70.4

clay (%)

6.2

8.3

12.5

13.2

moisture (%)

14.4

20.2

20.2

22.0

pH

6.89

7.72

7.91

8.07

Eh (mV)

246

223

213

208

organic carbon (%)

0.50

0.23

0.76

0.46

inorganic carbon (%)

1.84

1.16

1.50

1.20

arsenic(mg 3 kg-1)

51

48

108

261

28 000

25 900

38 600

30 400

1220

2440

5410

5100

36 ND

36 ND

85 ND

79 24

iron (mg 3 kg-1)

calcium (mg 3 kg-1)

manganese (mg 3 kg-1) boron (mg 3 kg-1)

the pH range was 6.9-8.1. The moisture content ranged from 14 to 22%. The organic carbon and inorganic carbon content ranged from 0.23 to 0.76% and from 1.16 to 1.84%, respectively. The iron content was highest in ash samples, ranging from 25 900 to 38 600 mg 3 kg-1. The calcium content ranged from 1220 to 5410 mg 3 kg-1, while the arsenic content ranged from 48 to 261 mg 3 kg-1, and the manganese content ranged from 36 to 85 mg 3 kg-1. The boron content was 24 mg 3 kg-1 for PZ8-3 (30.5-34.5 ft), while it was below the detection limit for the other ash samples. The leaching of arsenic for column and batch sequential leaching of ash sample PZ8-3 (24-32 ft) is shown in Figure 1. The leaching of arsenic from ash in column leaching increased from the first stage (2 days) to the third stage (6 days), but decreased thereafter in the subsequent stages. Most of the arsenic leached from ash during the third stage for column leaching, whereas most of the arsenic leached during the fourth stage for sequential batch leaching. The leaching of arsenic for column and sequential batch leaching systems increased from the first stage to

Figure 1. Leaching of arsenic (C/Co) in column and sequential batch leaching. 2206



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the second stage, but decreased thereafter for the remaining leaching period. The cumulative leaching of arsenic was similar for the column and the sequential batch leaching systems up to 14 days, thereafter increasing more rapidly for sequential batch leaching. After 14 days, 10.5 and 11.1% of the arsenic content of ash (seven stages) had leached for column leaching and sequential batch leaching, respectively. After 42 days (21 stages) of leaching, 17.8 and 22.0% of the arsenic content of ash had leached for column and sequential batch leaching systems, respectively. The results presented in Figure 1 show that the pattern for leaching of arsenic was similar for column and sequential batch leaching. Table 3. Parameters for Leaching of Arsenic and Calcium after 14 days in Column and Sequential Batch Leaching of Ash Sample PZ8-3 (24-32 ft) parameter

column

sequential batch

arsenic leached (mg 3 kg ) arsenic leached (C/Co)

11.3

12.0

0.105

0.109

calcium leached (mg 3 kg-1)

235

317

calcium leached (C/Co)

0.043

0.059

arsenic leached/calcium leached

0.048

0.038

-1

The various parameters for column and sequential batch leaching for the PZ8-3 (24-32 ft) ash sample after 14 days are shown in Table 3, where the parameters for leaching of arsenic were similar for column and sequential batch leaching. The standard leaching protocol using sequential batch extraction employs seven stages of sequential extractions (2 days per extraction stage); for this leaching time period, the results from Figure 1 and Table 3 demonstrate that column leaching and sequential batch leaching of ash were comparable methods for leaching of arsenic from ash. For column leaching, the volume of water applied to 100 g of ash was 254 mL per each 48-h period. Approximately 18% more water was applied to leaching of ash for sequential batch leaching than for column leaching. After 14 days, an appreciable amount of arsenic leached per volume of leachate for column leaching (0.0083 mg 3 kg-1 3 mL-1) which was greater than that for sequential batch leaching (0.0057 mg 3 kg-1 3 mL-1). The amount of arsenic leached per volume of applied leaching solution after 42 days was 0.0053 mg 3 kg-1 3 mL-1 for column leaching and 0.0038 mg 3 kg-1 3 mL-1 for sequential batch leaching system. The results of leaching of calcium and sulfate from ash are shown in Figure 2. For both column and batch leaching systems, the maximum leaching of both calcium and sulfate occurred after 2 days (stage 1), with more calcium and sulfate leaching in

Figure 2. Leaching of calcium and sulfate in column and sequential batch leaching. 2207



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Figure 3. Leachate pH and concentrations of bicarbonate and carbonate, and ratio of bicarbonate to carbonate as a function of time.

column leaching than in batch leaching after 2 days. Comparable levels of calcium leached in both column and batch leaching systems after 14 days (stage 7), while comparable levels of sulfate leached in both column and batch leaching systems after 6 days (stage 3). The leaching of calcium and sulfate decreased markedly after 4 days (stage 2) in both column and batch leaching systems; leaching of sulfate subsided after 6 days while lesser leaching of calcium continued at a nearly constant level of about 0.2 mM after 14 days. The data for pH, bicarbonate, carbonate, and the ratio of bicarbonate to carbonate are shown as function of time in Figure 3 for column and sequential batch leaching systems. The concentrations of bicarbonate and carbonate anionic species were calculated from the experimental values of alkalinity and pH. The pH for sequential batch leaching (about pH 9) was higher than the pH for column leaching (about pH 8). The enhanced leaching of arsenic in sequential batch leaching after 22 days (11 stages) may be attributed to the higher leachate pH experienced in that system for the latter half of the leaching period, since greater leaching of arsenic from alkaline fly ash has been shown to occur at higher leachate pH values.16,17 Figure 4 shows the cumulative leaching of arsenic as a function of leachate pH, indicating that more arsenic leached from the ash in sequential batch leaching at higher leachate pH than in column leaching at lower leachate pH.

Comparable leaching of bicarbonate occurred in the column leaching and sequential batch leaching systems while greater leaching of carbonate occurred after 4 days in sequential batch leaching, where the ratio of bicarbonate to carbonate was higher for column leaching. The larger carbonate concentration in the leachate was conducive to the higher leachate pH values observed in sequential batch leaching. The higher concentration of carbonate in the leachate solution (and the concomitant higher leachate pH) in sequential batch leaching may be attributed to two factors. First, the larger volume of water available for extraction of ash may have led to the greater dissolution of calcium carbonate from the ash (since carbonate is less soluble than bicarbonate). In addition, the breakup and attrition of ash particles may have occurred due to the continuous shaking of the ash slurry after the third stage of sequential batch extraction, leading to the release and dissolution of carbonate species from the ash. For both column and sequential batch leaching systems, little or no iron and manganese was found in the leachate solutions while no boron or phosphate was detected. Leaching of Calcium and Consequent Leaching of Arsenic. The CaSO4, CaCO3, or CaO/Ca(OH)2 in ash may control calcium concentrations in fly ash for ash samples that were alkaline in nature.14 The precipitation of Ca3(AsO4)2 and Ba3(AsO4)2 at high pH may have limited the aqueous concentration of arsenic,6 while Ba3(AsO4)2 in ash was found to be one of the 2208



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Figure 4. Cumulative leaching of arsenic from ash as a function of leachate pH.

Figure 5. Equivalent balance for column and sequential batch leaching.

solubility-controlling phases for As(V).32 Since the pH of leachate was in the range 7-9, the main compound which controlled the pH of ash was CaCO3. Several arsenate salts which may have been present in the ash are AlAsO4, Ba3(AsO4)2, FeAsO4, Mg3(AsO4)2, Ca3(AsO4)2, and Mn3(AsO4)2. Among these arsenate salts, the calcium and magnesium salts are the most soluble (10-5-10-4 M) while the iron and barium salts are the least soluble salts (10-11-10-10 M).33 Compared to iron and manganese, calcium predominantly leached from the ash samples, which renders Ca3(AsO4)2 as the most likely source of leached arsenic. The possible sources of calcium in the ash are CaO [Ca(OH)2 in water], CaSO4, CaCO3, and Ca3(AsO4)2. When ash was leached, various calcium compounds that leached were CaSO4, CaO, and Ca3(AsO4)2. The solubility values of Ca(OH)2, CaSO4, Ca3(AsO4)2, and CaCO3 are 1.08 10-2, 7.02 10-3, 9.12 10-5, and 5.80 10-5 M, respectively.33,34 Among the species of calcium, Ca(OH)2 is the most soluble species followed by CaSO4 and Ca3(AsO4)2, while CaCO3 is the least soluble species.

A leaching kinetics study was conducted for the deepest ash sample in the PZ8-3 location. The first-order kinetics parameters for leaching during the first 48 h of leaching are presented in Table 4 as first-order rate constant k (h-1) and final leachate concentration Cf (mM) according to the equation C = Cf[1 - e-kt]. The kinetics study for the deepest ash sample from the location PZ8-3 (30.5-34.5 ft) showed that calcium and sulfate leached at similar leaching rates with first-order rate constants of 2.42 and 2.37 h-1, respectively. The leaching rates of arsenic and carbonate were similar, whereas the bicarbonate leaching rate was slightly higher. This observation led to the conclusion that calcium first leached as calcium sulfate followed by leaching of calcium bicarbonate. Leaching of arsenic occurred as calcium hydrogen arsenate along with calcium carbonate. Equivalent Balances for Calcium in Leachate Based on Electrical Neutrality. Calcium may have leached from ash as calcium sulfate, calcium carbonate, calcium bicarbonate, and calcium hydrogen arsenate. The equivalent balances shown in 2209



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Table 4. First-Order Leaching Rates for the First 48 h of Batch Leaching for Ash Sample PZ8-3 (30.5-34.5 ft) plot

R2

Cf (mM)

k (h-1)

SO42- leached vs time

0.99

0.29

2.42

Ca2þ leached vs time HCO3- leached vs time

0.95 0.92

0.55 0.20

2.37 1.04

CO32- leached vs time

0.89

0.001

0.73

AsO43- leached vs time

0.93

0.007

0.77

Table 5. Tessier Extractions for Arsenic (Percentage of Arsenic in Ash) in Ash Sample PZ8-3 depth (ft) ash fraction

5-9

15-19

24-32

30.5-34.5

I, exchangeable

1.4

4.2

2.6

1.9

II, carbonate

8.0

29.2

19.4

8.7

III, iron/manganese oxides

54.5

60.9

28.6

18.0

IV, organic

2.5

1.5

1.9

1.6

V, aluminosilicate

33.7

4.2

47.6

69.9

Table 6. Leaching of Arsenic and Calcium from Ash after 14 Days for PZ8-3 Location as a Function of Ash Depth arsenic leached sample depth (ft)

-1

mg 3 kg

C/Co

calcium leached mg 3 kg-1

C/Co

5-9

2.0

0.02

183

0.03

15-19

5.5

0.05

285

0.05

24-32

12

0.11

317

0.06

30.5-34.5

19

0.18

350

0.07

Figure 5 for column and sequential batch leaching are based on electrical neutrality in the leachate solution. The ratio of equivalent concentration of the calcium cation to the sum of equivalent concentration of sulfate, carbonate, bicarbonate, and arsenate anions was nearly 1.0 for column leaching (Figure 5a). The results indicate that the dominant cations and anions in the leachate solution were calcium, sulfate, carbonate, bicarbonate, and arsenate. This ratio was nearly constant throughout 28 days (14 stages) of leaching. The second plot presented in Figure 5b for the ratio of equivalents of calcium to equivalents of carbonate species (carbonate and bicarbonate) shows that the ratio for first 48 h of leaching was nearly 2.0 for column leaching, indicating there were other anions present besides carbonate species. Since in Figure 5b sulfate was not considered, the ratio was nearly 2.0 for the first stage of leaching. However, after the first stage of leaching, the ratio was close to 1.0 throughout the next 26 days of leaching. Calcium first leached as calcium sulfate, and subsequently there was continuous leaching of carbonate and bicarbonate. The data indicate that the long-term leaching of calcium was controlled by bicarbonate present in the leachate of the column leaching system. Similar patterns of leaching were observed for long-term leaching in sequential batch leaching. Calcium compounds other than calcium sulfate, calcium carbonate, calcium bicarbonate, and calcium hydrogen arsenate could have leached from ash during batch sequential leaching, resulting in equivalent

Figure 6. Transmission electron images of ash sample PZ8-3 (24-32 ft).

ratios of approximately 1.6 and 4.0 for the first leaching stage (2 days) in Figure 5a and Figure 5b, respectively. The equivalent ratios decreased markedly to approximately 1.2 after 2 days in both column and sequential batch leaching systems. Since for every 48-h extraction stage fresh DI water was added to the same ash sample during batch sequential leaching, a new equilibrium 2210



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Figure 7. Scanning electron micrograph and SEM/XEDS elemental analysis of ash sample PZ8-3 (24-32 ft).

was established every 48 h which resulted in continued leaching of calcium species from ash into fresh DI water. Tessier Extractions. Sequential extraction data for arsenic are shown in Table 5. For ash sample PZ8-3 (24-32 ft), most of the arsenic content of ash (47.6%) was bound to the aluminosilicate fraction of ash (fraction V). The percentage of arsenic bound to the iron oxide-manganese oxide fraction (fraction III) was 28.6%. The form in which iron oxides were present in ash could be crystalline or amorphous. Arsenic can be associated with iron oxides as well as with iron oxyhydroxides. Arsenic can also be associated with iron as iron(III) arsenate (FeAsO4). Complexes

of iron can be crystalline as well as amorphous. Most of the arsenic was bound to the iron oxide fraction. Arsenic is adsorbed to iron oxides at very high temperatures; therefore, it is difficult to leach at the pH range observed in the leachate. The exchangeable and organic fractions of ash (fractions I and IV) had low arsenic content of 2.6 and 1.9%, respectively. The carbonate fraction of ash (fraction II) contained 19.4% of the arsenic in ash. In the carbonate fraction of ash, carbonate could be present as calcium, magnesium, manganese, or iron carbonate. For ash sample PZ8-3 (24-32 ft), most of the calcium (66%) was present in the aluminosilicate fraction of ash. Calcium was 2211


Industrial & Engineering Chemistry Research strongly bound in the aluminosilicate fraction and was therefore hard to leach. The carbonate fraction of ash contained 21% of the calcium. The exchangeable and the iron-manganese oxide fractions of ash contained 8.5 and 4.8% of the calcium content of ash, respectively. The organic fraction of ash contained only 0.14% of the calcium content of ash. The most likely fraction of ash for leaching of arsenic was the carbonate fraction (fraction II), which contained 19.4% of the arsenic in ash. Calcium sulfate present in ash could be present in the exchangeable fraction of ash. The pH of the ash samples ranged from neutral to alkaline; since arsenic leaching is pH dependent, the leaching of arsenic was likely to be controlled by leaching of Ca(HCO3)2 and CaCO3. Arsenic present in the carbonate fraction of ash decreased with increasing sand fraction of ash, but increased with increasing silt and clay fractions. Effect of Depth and Particle Size of Ash on Leaching of Arsenic. The levels of arsenic and calcium that leached from ash samples at different depths are shown in Table 6 for sequential batch leaching. The results show that the leaching of calcium increased with depth. Since the arsenic and calcium contents of ash increased with depth, higher leaching of calcium led to an overall increase in leaching of arsenic from deeper sections of ash. More arsenic and calcium leached from the deepest ash sample with values of 19 and 350 mg 3 kg-1, respectively. Arsenic in the exchangeable and carbonate fractions of ash (Table 5) increased with depth. Since leaching occurred primarily from these two ash fractions, arsenic leaching increased with depth. Calcium in the iron-manganese oxides and the aluminosilicate fractions of ash (fractions III and V) increased with depth. Leaching of arsenic decreased with increasing fraction of sand but increased with increasing silt and clay fractions. The arsenic in ash particles was shown to be associated with particle surface area,18 which was larger in smaller particles. Smaller particles of ash (silt and clay fractions) had higher arsenic content. Similar results were obtained for the leaching of calcium as a function of ash particle size. The leaching of calcium increased with increasing silt and clay fractions. The silt and clay fractions of ash increased with depth. The higher calcium content of deeper ash samples rendered the deeper ash more alkaline. The standard Eh-pH stability diagram32 shows that, at the Eh-pH conditions of the ash samples from the PZ8-3 location (Table 2), most of the arsenic should be present as hydrogen arsenate. The arsenic compound present in leachate could be calcium hydrogen arsenate. The similarity between leaching parameters for column and batch sequential leaching shows that, in the absence of column leaching data for other ash depths and ash locations, the batch sequential leaching data would be comparable with column leaching data for other ash depths and locations in the ash impoundment. Surface Analysis of Ash Particles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) with X-ray electron diffraction spectroscopy (XEDS), Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were performed on the PZ8-3 (24-32 ft) ash sample. TEM analysis concluded that the ash sample was heterogeneous (Figure 6) and amorphous alumina was present in ash. SEM analysis showed that the ash particles were heterogeneous, and XEDS showed that the ash surface contained mostly oxygen and carbon and appreciable amounts of aluminum and silicon, in addition to calcium and iron (Figure 7). Raman spectroscopy revealed the presence of amorphous inorganic carbon in the ash. XRD results confirmed the presence of

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crystalline iron(III) oxide and silica in the ash and confirmed the absence of crystalline CaCO3 in the ash, whereas XPS indicated the presence of CaCO3 in the ash. The surface analysis results from XRD, XPS, XEDS, and Raman spectroscopy indicate that mostly amorphous CaCO3 was present in the ash sample PZ8-3 (24-32 ft).

’ CONCLUSIONS The long-term leaching of arsenic was limited by leaching of calcium bicarbonate from the ash in both column and batch sequential leaching. The leachate pH was determined by the calcium bicarbonate in the leachate. Arsenic leached out mainly as calcium hydrogen arsenate from the calcium arsenate present in ash. The comparison of batch sequential and column leaching results shows that they were similar; therefore, column leaching of ash samples from other depths at the same location is expected to be similar to the sequential batch leaching of those samples. More arsenic leached from the deeper ash samples, and arsenic leached mainly from the silt fraction of ash.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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(33) Frankenthal, R. P. Equilibrium Constants. In Handbook of Analytical Chemistry, 1st ed.; Meites, L., Ed.; McGraw-Hill: Toronto, 1963; pp 1-13-1-19. (34) Solubility Data Series, 53 vols.; International Union of Pure and Applied Chemistry, Pergamon Press: Oxford, 1979-1992.

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Leaching Characteristics of Arsenic from Aged Alkaline Coal Fly Ash

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