Calcium Effect on Arsenic (V) Adsorption 

onto Coal Fly Ash 

Tian Wang1

, Tingzhi Su1, Jianmin Wang1, Ken Ladwig2 

1

 Department of Civil, Architectural & Environmental Engineering, University of 

Missouri – Rolla, Rolla, MO 65409; 2 Electric Power Research Institute (EPRI), 

3420 Hillview Ave., Palo Alto, CA 94304 

KEYWORDS: arsenic, calcium, adsorption, fly ash 

ABSTRACT 

Batch tests indicated that arsenic (As) leaching is significantly affected by the 

calcium concentration in fly ash in the alkaline pH range. Arsenic leaching from 

low calcium fly ash from eastern bituminous coal increased with increase of pH in 

the alkaline pH range (pH 7 – 12). Fly ash from eastern bituminous coal with a 

slightly higher calcium content exhibited an arsenic leaching peak at pH 9, 

followed by decreased arsenic leaching up to pH 12. For alkaline ashes derived 

from subbituminous coal with much higher calcium content, significantly less 

arsenic was leached in the alkaline pH range. To improve understanding of 

arsenic leaching behavior, batch experiments with washed ash were performed 

to evaluate partitioning of As(V) spiked into the system, along with different 

amounts of calcium addition. Results suggested that the addition of calcium 

significantly reduced the soluble arsenic ratio in the alkaline pH range. This 

phenomenon was explained by postulating the formation of two highly 

adsorbable neutral arsenic species, CaHAsO4 and Ca3(AsO4)2, in this pH range. 

An adsorption model was developed to quantify the calcium impact on arsenic 

adsorption. 

INTRODUCTION 

High concentrations of arsenic (As) have long been recognized to be toxic to 

human and animals. Long term exposure to As can cause cancer of skin, liver, 

lung bladder and kidney.1

 Effective January 2006, the Federal Maximum 

Contaminant Level (MCL) for arsenic in drinking water was revised by EPA from 

50 ppb to 10 ppb.2 The stricter regulation may impact alternatives for the disposal 

and use of arsenic containing wastes and products, including coal fly ash. 

Although there have been extensive studies on the general leaching 

characteristics of arsenic from fly ash,3-8 quantification of the calcium effect on 

arsenic leaching has been less well studied. Van der hoek et al. 9 conducted 

batch leaching tests for one acidic and one alkaline fly ash, and observed higher 

arsenic concentrations at higher pH for acidic ashes, but they observed the 

opposite behavior for alkaline ash, indicating different leaching mechanisms. By 

comparing arsenic leaching behavior from fly ash with its adsorption onto the 

2007 World of Coal Ash (WOCA), May 7-10, 2007, Covington, Kentucky, USA Home
major mineral compounds in fly ash, they concluded that arsenic leaching from 

acidic ash was likely to be controlled by surface complexation with iron oxide, 

while a calcium phase was shown to be responsible for alkaline ash. This 

conclusion agrees with that from Zielinski et al., who studied the mode of 

occurrence of arsenic in fly ash with XAFS spectroscopy.10 Results indicated that 

arsenic is associated with some combination of iron oxide, oxyhydroxide or 

sulfate in a highly acidic fly ash, but with a phase similar to calcium arsenate in a 

highly alkaline ash. 

Several hypotheses have been proposed to explain the effect of calcium on 

arsenic leaching and adsorption on fly ash. One hypothesis is that arsenic is 

reacting with calcium and precipitates as calcium arsenate.11 However, Fruchter 

et al.12 and Van der hoek et al.9 showed that As concentrations in leachate could 

not be modeled on the basis of solubility. Another hypothesis is that formation of 

secondary minerals such as ettringite may contribute to arsenic stabilization in fly 

ash.14-16 Since the formation of ettringite only occurs at pH greater than 11, this 

can only explain reduced leaching at very high pH levels. 15 None of these 

hypotheses fully explain the arsenic leaching behavior across the full pH range. 

Adsorption models that incorporate surface electrostatic effects have been 

applied to quantify arsenic adsorption on various media, including soil, iron 

hydroxide and ferric sludge, 17, 18, 13 and, in most cases, this approach works well 

for fitting experimental data. However, it is very complicated in terms of the 

number of parameters to be predicted or calibrated, and results depend heavily 

on the initial assumptions.9 It is even more difficult to apply this type of model to a 

system with multiple constituents interacting with each other. 

Overall, calcium is known to play an important role in the release of arsenic from 

fly ash. The objectives of this study are to compare the leaching behavior of 

arsenic from several acidic and alkaline fly ashes, to explore the mechanism of 

calcium effect on arsenic leaching and sorption/precipitation processes on coal 

fly ash, and to develop a robust adsorption model to quantify arsenic partitioning 

in fly ash. 

MATERIALS AND METHODS 

Fly Ash Samples 

A total of seven ash samples were used in this study, collected from three 

different power plants. Ashes #1004, #1005, #1008 and #1009 were all collected 

from one pulverized coal power plant (Plant ID 33106) burning eastern 

bituminous coal. The plant uses cold-side electrostatic precipitators (ESPs) to 

capture fly ash. Ashes #1004 and #1009 were collected from the same unit but 

at different times, when different eastern bituminous coals were being burned; 

the coal for Ash #1009 had higher calcium content. Ashes #1005 and #1008 

were collected from the same plant and during the same coal burns as ashes 

#1004 and 1009, respectively, but from a separate unit with an ammonia-based 
selective non-catalytic reduction (SNCR) system for NOx control. Ashes #1015, 

#1018, and #7 were collected from power plants burning primarily subbituminous 

coal. Ashes #1015 and #1018 came from a cyclone boiler power plant (Plant ID 

25410) with cold-side ESPs and burning a blend of 80% subbituminous and 20% 

bituminous coal. Ash #1018 was sampled from a unit with SNCR. Sample #7 

came from a pulverized coal power plant (Plant ID 50213) with hot-side ESPs 

and burning 100% subbituminous coal. 

In this paper, the four bituminous coal fly ashes with natural pH less than 7 are 

defined as acidic ashes, and the three subbituminous coal fly ash with natural pH 

greater than 7 were defined as alkaline ashes. The basic physical and chemical 

characteristics of these ashes, including natural pH, BET surface area (analyzed 

using Quantachrome Autosorb-1-C high performance surface area and pore size 

analyzer, Quantachrome Instruments, FL, USA), pHpzc (analyzed using Zetasizer 

3000, Malvern Instruments, Worcestershire, UK), loss-on-ignition (LOI) 

(determined using gravimetric methods), and total arsenic concentration are 

shown in Table 1. The total As in fly ash was determined using microwaveassisted acid digestion (0.4 g fly ash + 10 mL HNO3 + 5 mL HF + 5 mL HCl) 

followed by graphite furnace atomic absorption (GFAA) measurement. The 

accuracy of the As determination was demonstrated by using a certified 

reference material, NIST-1633a (National Institute of Standards and Technology, 

USA; certified As = 145±15 mg/kg, measured As = 156.3±1.3 mg/kg). Total Ca 

concentration was determined using X-Ray Fluorescence Spectroscopy (X-LAB 

2000, SPECTRO Analytical Instruments GmbH & Co. KG). 

Batch Leaching and Batch Titration experiments

Batch leaching experiments were performed to determine the leaching behavior 

of arsenic from raw fly ash under different pH conditions; and batch titration 

experiments were employed to determine the surface site acidity and density. 

Detailed procedures for these two experiments are available in other papers.19-20

Raw ash was dried and used for batch leaching experiments, while DI water 

washed ash was used for the titration experiments under the ionic strength of 

0.01 M (NaNO3). A solid/solution ratio (S/L) of 1:10 was used and pH was 

adjusted to the range of 2-12 for each group of samples. The mixture was 

shaken on an EBERBACH 6010 shaker for 24 hours, then allowed to settle 

overnight. The supernatant was collected and acidified using HNO3 before 

arsenic and calcium analysis. 

As(V) Partitioning with Different Calcium Additions

Batch partitioning experiments were performed to evaluate the effect of calcium 

on arsenic adsorption onto fly ash. In this experiment, fly ash samples were 

washed five times with DI water, and dried before use. The detailed washing 

procedure is described elsewhere.19 The solid/liquid ratio was 1:10. Ionic strength 

was adjusted with 0.01M NaNO3 solution. For this study, samples were divided 
into several groups, 5 mg/L of As(V) was added to all samples as adsorbate, and 

a series of Ca concentrations were added to the different groups. After mixing on 

the shaker for 24 hours, all samples were allowed to settle overnight. The 

supernatant was then collected for arsenic and calcium analysis. The final pH 

was measured using the remaining mixture in the bottle. 

Analytical Method

A GFAA spectrometer (AAnalyst 600, Perkin-Elmer Corp., Norwalk, Connecticut, 

USA) and a Flame Atomic Absorption Spectroscopy (FLAA; Model 3110, PerkinElmer Corp., Norwalk, Connecticut, USA) were used to determine arsenic and 

calcium concentrations in solution, respectively. An Orion PerpHecT Triode pH 

electrode (model 9207BN) and a pH meter (perpHecT LoR model 370) were 

used for pH measurement. 

Data Analysis

The non-linear regression program KaleidagraphTM (Synergy Software, 2002)

was used for titration modeling to determine the surface site density and acidity 

constants. SigmaPlot (SPSS Inc., 2001) was used as a multi-variable nonlinear 

regression program to determine the adsorption constants of each arsenic 

species on fly ash. 

RESULTS AND DISCUSSION 

Arsenic and Calcium Leaching From Raw Fly Ash

Batch leaching experiments were performed with six fly ash samples. Both 

arsenic and calcium concentrations in the leachate were analyzed and plotted in 

Figure 1a and 1b respectively. 

Arsenic leaching from all three acidic ashes was significantly affected by the pH 

as observed in Figure 1a. For ashes #1005 and #1004, arsenic release was 

minimal in their natural pH range, between 3 and 7. When pH is below 3, arsenic 

release increased significantly. On the other hand, when pH is above 7, soluble 

arsenic concentration was also increased. The major arsenic species in fly ash 

was reported to be As(V) in previous research. 21, 22 The As(V) speciation 

diagram (Figure 2) indicated that the neutral H3AsO4 species dominates when pH 

is less than 2. Therefore, the neutral arsenic molecule is considered not 

adsorbable by ash surface. The dissolution of ash particles under very acidic 

conditions might also contribute to the higher soluble arsenic concentration. 

When pH increases to above 2, the total concentrations of anionic arsenic 

species (H2AsO4

-

 and HAsO4

2-) also increase. These anions can be adsorbed by 

protonated ash surface site α. When pH is greater than 7, the protonated surface

site α is no longer available, resulting in less arsenic adsorption on the ash 

surface. 
Ash #1009 performed differently from the other two acidic ashes in Figure 1a. 

When pH was less than 9, it had a similar leaching pattern as the other two, with 

minimal release at pH 3-4. However, when pH was greater than 9, the soluble 

arsenic concentration decreased with the increase of pH, and when pH was 

greater than 11, the arsenic concentration began to increase again. 

Unlike acidic ashes, alkaline ashes displayed very low leachability for arsenic 

under neutral and alkaline pH conditions (Figure 1a). One high arsenic 

concentration at pH 3 for ash #1018 was most likely caused by ash dissolution. 

Comparing the leaching results from different ashes with their physical –chemical 

characteristics listed in Table 1 indicates that the general arsenic leaching 

behavior is correlated with the calcium content in fly ash matrix. The calcium 

contents in the three alkaline ashes are 20-30 times greater than those in ash 

#1005 and #1004; acidic ash #1009 has a calcium content about twice as high as 

the other two acidic ashes. Correspondingly, the soluble calcium concentrations 

in leachates from the alkaline ashes were significantly greater than for the acidic 

ashes below pH 11 (Figure 1b), but were more sensitive to pH change and 

decreased faster with increase of pH. These results indicate that calcium 

leaching from alkaline ashes was a dissolution/precipitation controlled process, 

and the fairly low leachability of arsenic from the alkaline ashes across a broad 

pH range was likely associated with the precipitation or coprecipitation of arsenic 

with calcium phases. The situation is quite different for acidic ashes, where the 

soluble calcium concentration decreased smoothly with increase of pH, and 

arsenic leachability is more dependant on pH change. The observed correlation 

between arsenic leachability and calcium content in fly ashes is further evidence 

of the finding by Van de hoek et al.9

 that a calcium phase in fly ash is likely to 

control the release and adsorption of arsenic in the alkaline pH range. The 

leaching mechanism of arsenic from acidic ashes will be discussed more fully in 

the following section. 

Arsenic Partitioning in Washed Fly Ash under Different Calcium Additions

To test the effect of calcium on arsenic adsorption on fly ash, and to further 

investigate the leaching mechanism of arsenic from acidic fly ashes, ash #1008 

and ash #1005 were selected for partitioning experiments with different calcium 

additions. For all partitioning experiments, the ash was washed prior to testing 

and 5 mg/L of As were added to the system. For ash #1008, sample bottles 

were divided into four groups, with calcium additions of 0 mg/L, 50 mg/L, 100 

mg/l and 150 mg/L. Results are shown Figure 3. The As(V) partitioning curve 

without calcium addition has a similar trend as the raw ash leaching curve. 

Although only 5 mg/L of As(V) was added to the system, the maximum release 

reached 12 mg/L at pH 12, indicating that a significant amount of arsenic was 

released from washed fly ash. Addition of calcium significantly enhanced arsenic 

adsorption in pH range of 7-12, and a larger adsorption difference was observed 

at higher pH. For example, at pH 8 with 50 mg/L of calcium addition, 25% more 

arsenic was adsorbed, while at pH 11, arsenic adsorption was increased by 75%. 

This effect became less significant with calcium addition above 50 mg/L. The 
arsenic partitioning curve in the presence of calcium is analogous to the leaching 

curve of raw ash #1009 (Figure 1a), suggesting a similar mechanism applies in 

both circumstances. 

Three Ca loadings were applied to As(V) partitioning experiment with ash #1005: 

0 mg/L, 100 mg/L and 500 mg/L. Results shown in Figure 4 were similar with 

those of ash #1008, except that soluble Ca concentration decreased sharply at 

pH greater than 11 with 500 mg/L Ca addition, which may be caused by 

precipitation of Ca(OH)2 at high Ca loading and high pH conditions. 

 

To determine whether the precipitation of Ca3(AsO4)2 occurred at high pH in this 

study, the products of [Ca]3

×[AsO4]2 were calculated based on the group of data 

with 50 mg/L of calcium addition for ash #1008. Results are listed in Table 2. 

Instead of one consistent solubility product (Ksp), the large variation of the 

product, up to four orders of magnitude, indicated that the arsenic release was 

not a calcium arsenate precipitation controlled process. Ettringite formation 

contributes to the stabilization of arsenic in fly ash generally in alkaline ashes, 

and only above pH 11, while experimental data indicated that calcium effect in 

these acidic ashes became observable since pH 7-8,.

The current experimental data suggest that some other mechanism is at least 

partially responsible for controlling arsenic release in the acidic ashes. Calcium 

added into the system can form complexes with arsenic in forms of CaH2AsO4

+

, 

CaHAsO4, CaAsO4

-

, Ca3(AsO4)2.

23 It has been reported that the neutral forms of 

metal complexes, 24, 25 have higher affinity to the sorbent surface. Considering 

the occurrence of the two neutral species CaHAsO4 and Ca3(AsO4)2 in the pH 

range where arsenic adsorption was enhanced, it is assumed that the adsorption 

of these two species contributed to the stabilization of arsenic in fly ash. This 

assumption is verified by the modeling process described below. 

Modeling As(V) Adsorption onto Washed Ash

Surface Site Characterization 

The surface site density and acidity constant of fly ash are essential parameters 

for metal adsorption modeling. A previously developed titration model19 was used 

to determine these parameters. The model is expressed as: 

∑













+

−

+

∆ = + +

i Hi 0 Hi

0 Ti Hi

SS [H ] K

1

[H ] K

1

C

V S K

V (1) 

where ∆VSS is the net volume of stock acid/base (negative value for acid) solution 

consumed by surface sites (mL); V0 is total volume of the ash mixture (mL); STi is 

the total acid site concentration of species i (M); KHi is the acidity constant of the 

species i (M); C is the concentration of the acid/base stock solution (M); and [H+

]0

is the hydrogen ion concentration of the control unit (without acid or base addition) 

(M). Note that the total surface site concentration STi = Γi × SS, where Γi is the 
surface site density for species i (mol/g-SS) and SS is the solids concentration 

(g/L). 

After correction using the titration data for blanks, the net titration data for 

washed Ash #1008 with S/L ratio of 1:10 were plotted as the equilibrium pH as a 

function of the volume of acid (negative value) or base consumed by fly ash (mL), 

shown in Figure 5. KaleidaGraphTM was employed for the curve fitting. Results 

showed that using three surface sites can best fit the experimental data. Table 3 

lists the surface site density (Γ) and acidity constant (pKH) for each site, α, β, and 

γ. Since the pHpzc of this ash was 6.2 (Table 1), which is between the pKHs of the 

site α and site β (3.2 and 7.3, respectively), the protonated surface sites α is 

positively charged, denoted as S1OH2

+

, while the protonated species of the other 

two surface sites are in neutral form. 

Modeling As(V) Adsorption 

The concentrations of different As(V) species in the system can be calculated 

based on the following equations: 

H3AsO4 = H2AsO4

-

 + H+; pKa1 = 2.26;26 (2) 

H2AsO4

-

 = HAsO4

2- + H+

; pKa2 = 6.76;26 (3) 

HAsO4

2- = AsO43- + H+

; pKa3 = 11.29;26 (4) 

HAsO4

2- + Ca2+ = CaHAsO4; logKa4 = 2.75;23 (5) 

AsO4

3- + 1.5 Ca2+ = Ca 1.5AsO4; Ka5 (6) 

The stoichiometry of Ca3(AsO4)2 was converted to Ca1.5AsO4, to simplify the 

adsorption equations. Ka4 and Ka5 are the formation constants of CaHAsO4 and 

Ca3(AsO4)2, respectively. Ka5 needs to be determined by modeling, because 

there is no published value available from literature. 

Assuming that only the protonated surface site α (denoted as S1) is responsible 

for the adsorption of three negatively charged arsenic species, denoted as 

+

1 2 + T [S OH ]=α S , where S1T is the total concentration an site α, and α+ is the 

fraction of the protonated surface site, then: 

H K H

H

+

= +

+

+

[ ]

[ ]

α ; The two neutral 

calcium-arsenic species have no specific selectivity on the three surface sites. 

Therefore, all surface sites were normalized as one with a total site density of [ST] 

when considering the two neutral species. Adsorption of different arsenic species 

by surface sites can be expressed as: 

S1OH2

+

 + H2AsO4

-

 = S1-H2AsO4 + H2O; KS1; (7) 

S1OH2

+

 + HAsO4

2- = S1-HAsO4-

 + H2O; KS2; (8) 

S1OH2

+

 + AsO4

3- = S1-AsO42- + H2O; KS3; (9) 

S + CaHAsO4 = S-CaHAsO4; KS4; (10) 

S + Ca 1.5AsO4 = S-Ca 1.5AsO4; KS5; (11) 
where KS1, KS2, KS3, KS4 and KS5 are adsorption constants of the five arsenic 

species, respectively. These are the parameters to determine by modeling. 

The total soluble adsorbed arsenic concentration [As(V)]D and total adsorbed 

arsenic concentration [As(V)]ads can be expressed with the following equations: 

[As(V)]D = [H3AsO4]+[H2AsO4

-

]+[HAsO4

2-]+[AsO43-]+[CaHAsO4]+[Ca 1.5AsO4] 

(12) 

[As(V)]ads=[S1-H2AsO4]+[S1-HAsO4

-

]+[S1-AsO4

2-]+[S-CaHAsO4]+[S-Ca1.5AsO4] 

(13) 

According to previous research, if the total adsorbate concentration (in M) is less 

than 10% of the surface site concentration, which is true in this study, then the 

adsorption is in the linear range of the Langmuir isotherm19, and the 

concentration of adsorbed species can be expressed as: 

+ -

1 2 4 S1 1 2 2 4 [S -H AsO ]=K [S OH ][H AsO ] (14) 

- + 2-

1 4 S2 1 2 4

[S -HAsO ]=K [S OH ][HAsO ] (15) 

2- + 3-

1 4 S3 1 2 4 [S -AsO ]=K [S OH ][AsO ] (16) 

4 S4 4 [S-CaHAsO ]=K [S][CaHAsO ] (17) 

1.5 4 S5 1.5 4 [S-Ca AsO ]=K [S][Ca AsO ] (18) 

The total arsenic concentration [As(V)]T in the system is: 

[As(V)]T = [As(V)]D + [As(V)]ads 

Based on equations 2-18, [As(V)]D can be solved and expressed as: 

T

D

[As(V)] B [As(V)] =

B+C

×

 (19) 

where, 

1.5

1 2 4 1 2 3 5

2 3

0

1 K K K [Ca] K K K K [Ca] B = + +

α [H] [H]

 (20) 

1.5

+ S1 1 S2 1 2 S4 1 2 4 S5 1 2 3 5

1T 2 2 3 T

K K K K K K K K K [Ca] K K K K K [Ca] C = α [S ]( + )+[S ]( + )

[H] [H] [H] [H]

 (21) 

3

0 3 2

1 1 2 1 2 3

[H]

α =

[H] +[H] K +[H]K K +K K K

 (22) 

Equation 19 and the nonlinear regression function of SigmaPlot sigma were used 

to model experimental data As(V)D as a function of pH and [Ca], and to 

determine the values of constants K5, Ks1, Ks2, Ks4 and Ks5. Modeling results are 

listed in Table 4 together with the standard error and R2. Soluble arsenic 

concentrations predicted by the adsorption model are also plotted in Figure 3 as 

solid lines. The good agreement between experimental and modeling results 

suggested the validity of the developed model for this dataset. 
CONCLUSIONS 

This research demonstrated that different arsenic leaching behaviors from acidic 

and alkaline fly ashes were correlated with the calcium content in bulk ash, and a 

calcium phase was likely to be responsible for the stabilization of arsenic in the 

alkaline pH range. As(V) adsorption on to acidic fly ash was significantly 

enhanced by added calcium under basic conditions. Solubility product calculation 

indicated that this process was not controlled by precipitation of Ca3(AsO4)2.

Instead, it is hypothesized that calcium complexes with arsenic and forms two 

neutral species CaHAsO4 and Ca3(AsO4)2 which have a relatively high affinity for 

fly ash surfaces at high pH, resulting in increased arsenic adsorption. The 

adsorption model developed in this study successfully predicted As(V) adsorption 

behavior on an acidic ash #1008, and the formation and adsorption constants of 

different arsenic species were determined. 

ACKNOWLEDGEMENTS 

This work was supported by the Electric Power Research Institute (EPRI) and the 

Environmental Research Center (ERC) for Emerging Contaminants at the 

University of Missouri-Rolla (UMR). The authors also gratefully acknowledge Mr. 

Shi Shu at University of Missouri-Rolla for his contribution to the leaching 

experiment and chemical analysis; and Dr. C.P. Huang and Mr. Minghua Li at the 

University of Delaware for providing the zeta potential measurement. 

Conclusions and statements made in this paper are those of the authors, and in 

no way reflect the endorsement of the aforementioned funding agencies. 

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Table 1. Sample characterization. 

Sample ID Coal Type Natural pH 

(S/L = 1:10) 

As 

(mg/kg) 

Ca 

(%) 

BET Area 

(m2/g) 

pHpzc LOI 

(%) 

Ash #1005 Bituminous 5.5 44.9 0.50 18.43 6.2 12.7 

Ash #1004 Bituminous 4.5 49.0 0.59 7.57 6.4 6.7 

Ash #1008 Bituminous 6.5 139.4 1.11 6.48 6.2 8.5 

Ash #1009 Bituminous 6.0 100.9 1.0 8.71 7.4 9.8 

Ash #1015 Subbituminous 10.6 37.2 14.3 25.65 7.6 14.8 

Ash #1018 Subbituminous 10.6 52.1 12.98 15.68 6.8 9.7 

Ash #7 Subbituminous 12.3 29.1 16.15 1.24 6.6 0.2 

Table 2. Ion product of [Ca]3×[AsO4]

2

 (with 50 mg/L of Ca addition). 

pH [Ca]3×[AsO4]2

8.00 3.22E-25 

9.15 8.79E-23 

10.25 9.51E-22 

10.70 1.16E-21 

11.88 1.17E-21 

Table 3. Surface site densities and acidity constants for ash #1008. 

Surface Site Parameters α β γ

Γ (10-5 mol/g) 44 ± 1.4 3.6 ± 1.6 5.4 ± 2.0 

pKH 3.2 ± 0.1 7.3 ± 0.5 11.3 ± 0.9 

Table 3. Modeling results for arsenic partitioning with ash #1008. 

Arsenic Species H2AsO4 HAsO4 CaHAsO4 Ca1.5(AsO4) R

2

Formation Constant 

(logK) 

/ / / 4.7 ± 0.7 

Adsorption Constant 

(logKs) 

2.6 ± 0.1 7.2 ± 0.2 1.9 ± 0.2 2.6 ± 0.5 

0.89 
0

200

400

600

800

1000

1200

1400

1600

1800

0 2 4 6 8 10 12 14

pH

As (ug/L)

#1005

#1004

#1009

#1015

#1018

#7

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 2 4 6 8 10 12 14

pH

Ca (mg/L)

#1005

#1004

#1009

#1015

#1018

#7 

Figure 1. (a) As, (b) Ca leaching from acidic and alkaline coal fly ashes. Experimental 

conditions: S/L = 1:10; temperature = 20 – 25 oC; equilibration time = 24 hours. 

a 

b 
0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fraction

H3AsO4 H2AsO4

- HAsO4

2- AsO4

3-

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fraction

H3AsO4 H2AsO4

- HAsO4

2- AsO4

3-

Figure 2. Speciation of arsenic acid. 
0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

pH

As (mg/L)

As=5mg/L, Ca=0mg/L

As=5mg/L, Ca=50mg/L

As=5mg/L, Ca=100mg/L

As=5mg/L, Ca=150mg/L

Model

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14

pH

Ca (mg/L)

As=5mg/L, Ca=0mg/L

As=5mg/L, Ca=50mg/L

As=5mg/L, Ca=100mg/L

As=5mg/L, Ca=150mg/L

Figure 3. Arsenic partitioning for fly ash #1008 with and without addition of Ca: (a) 

Arsenic concentration as a function of pH; (b) Calcium concentration as a function of pH 

a 

b 
0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14

pH

As (mg/L)

As=5mg/L, Ca=0mg/L

As=5mg/L, Ca=100mg/L

As=5mg/L, Ca=500mg/L

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14

pH

Ca Conc (mg/L)

As=5mg/L, Ca=0mg/L

As=5mg/L, Ca=100mg/L

As=5mg/L, Ca=500mg/L

Figure 4. Arsenic partitioning for fly ash #1005 with and without addition of Ca: (a) 

Arsenic concentration as a function of pH; (b) Calcium concentration as a function of pH. 

a 

b 
-5 -4 -3 -2 -1 0 1

0

2

4

6

8

10

12

14

1N base added (mL)

pH

α

β

γ

-5 -4 -3 -2 -1 0 1

0

2

4

6

8

10

12

14

1N base added (mL)

pH

α

β

γ

Figure 5. Titration and curve fitting results for Ash #1008. Experimental conditions: S/L 

= 1:10; ionic strength = 0.01 M (NaNO3); temperature = 20 – 25 oC; equilibration time = 

24 hours