# Calcium Effect on Arsenic (V) Adsorption onto Coal Fly Ash

Tian Wang 1 

### , Tingzhi Su 1 

### , Jianmin Wang 1 

### , Ken Ladwig 2

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 in creased with increase of pH in the alkaline pH range (pH 7 – 12). Fly ash from eas tern bituminous coal with a slightly higher calcium content exhibited an arseni c 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 co ntent, significantly less arsenic was leached in the alkaline pH range. To im prove understanding of arsenic leaching behavior, batch experiments with w ashed ash were performed to evaluate partitioning of As(V) spiked into the s ystem, 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 formati on of two highly adsorbable neutral arsenic species, CaHAsO 4 and Ca 3 (AsO4) 2, in this pH range. An adsorption model was developed to quantify the c alcium 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 cau se cancer of skin, liver, lung bladder and kidney. 1 Effective January 2006, the Federal Maximum Contaminant Level (MCL) for arsenic in drinking wat er was revised by EPA from 50 ppb to 10 ppb. 2 The stricter regulation may impact alternatives fo r the disposal and use of arsenic containing wastes and products, including coal fly ash. Although there have been extensive studies on the g eneral leaching characteristics of arsenic from fly ash, 3-8 quantification of the calcium effect on arsenic leaching has been less well studied. Van de r hoek et al. 9 conducted batch leaching tests for one acidic and one alkalin e fly ash, and observed higher arsenic concentrations at higher pH for acidic ashe s, but they observed the opposite behavior for alkaline ash, indicating diff erent leaching mechanisms. By comparing arsenic leaching behavior from fly ash wi th its adsorption onto themajor mineral compounds in fly ash, they concluded that arsenic leaching from

acidic ash was likely to be controlled by surface c omplexation with iron oxide, while a calcium phase was shown to be responsible f or alkaline ash. This conclusion agrees with that from Zielinski et al., who studied the mode of occurrence of arsenic in fly ash with XAFS spectros copy. 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 phas e similar to calcium arsenate in a highly alkaline ash.

Several hypotheses have been proposed to explain th e effect of calcium on arsenic leaching and adsorption on fly ash. One hyp othesis is that arsenic is reacting with calcium and precipitates as calcium a rsenate. 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 contribut e to arsenic stabilization in fly ash. 14-16 Since the formation of ettringite only occurs at p H greater than 11, this can only explain reduced leaching at very high pH l evels. 15 None of these hypotheses fully explain the arsenic leaching behav ior across the full pH range.

Adsorption models that incorporate surface electros tatic effects have been applied to quantify arsenic adsorption on various m edia, 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 mo del 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 compar e the leaching behavior of arsenic from several acidic and alkaline fly ashes, to explore the mechanism of calcium effect on arsenic leaching and sorption/pre cipitation processes on coal fly ash, and to develop a robust adsorption model t o quantify arsenic partitioning in fly ash.

MATERIALS AND METHODS

Fly Ash Samples

A total of seven ash samples were used in this stud y, collected from three different power plants. Ashes #1004, #1005, #1008 a nd #1009 were all collected from one pulverized coal power plant (Plant ID 3310 6) burning eastern bituminous coal. The plant uses cold-side electrost atic precipitators (ESPs) to capture fly ash. Ashes #1004 and #1009 were collec ted from the same unit but at different times, when different eastern bitumino us 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 s ame coal burns as ashes #1004 and 1009, respectively, but from a separate unit with an ammonia-basedselective non-catalytic reduction (SNCR) system for NOx control. Ashes #1015,

#1018, and #7 were collected from power plants burn ing primarily subbituminous coal. Ashes #1015 and #1018 came from a cyclone boi ler power plant (Plant ID 25410) with cold-side ESPs and burning a blend of 8 0% 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 5 0213) with hot-side ESPs and burning 100% subbituminous coal.

In this paper, the four bituminous coal fly ashes w ith natural pH less than 7 are defined as acidic ashes, and the three subbituminou s 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 p H, BET surface area (analyzed using Quantachrome Autosorb-1-C high performance su rface area and pore size analyzer, Quantachrome Instruments, FL, USA), pH pzc (analyzed using Zetasizer 3000, Malvern Instruments, Worcestershire, UK), los s-on-ignition (LOI) (determined using gravimetric methods), and total a rsenic concentration are shown in Table 1. The total As in fly ash was deter mined using microwaveassisted acid digestion (0.4 g fly ash + 10 mL HNO 3 + 5 mL HF + 5 mL HCl) followed by graphite furnace atomic absorption (GFA A) measurement. The accuracy of the As determination was demonstrated b y 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 Fluorescen ce Spectroscopy (X-LAB 2000, SPECTRO Analytical Instruments GmbH & Co. KG)

.
Batch Leaching and Batch Titration experiments

Batch leaching experiments were performed to determ ine the leaching behavior of arsenic from raw fly ash under different pH cond itions; 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 exper iments, while DI water washed ash was used for the titration experiments u nder the ionic strength of 0.01 M (NaNO 3 ). A solid/solution ratio (S/L) of 1:10 was used an d pH was adjusted to the range of 2-12 for each group of sam ples. The mixture was shaken on an EBERBACH 6010 shaker for 24 hours, the n allowed to settle overnight. The supernatant was collected and acidif ied using HNO 3 before arsenic and calcium analysis.

As(V) Partitioning with Different Calcium Additions

Batch partitioning experiments were performed to ev aluate the effect of calcium on arsenic adsorption onto fly ash. In this experim ent, fly ash samples were washed five times with DI water, and dried before u se. The detailed washing procedure is described elsewhere. 19 The solid/liquid ratio was 1:10. Ionic strength was adjusted with 0.01M NaNO 3 solution. For this study, samples were dividedinto several groups, 5 mg/L of As(V) was added to a ll samples as adsorbate, and

a series of Ca concentrations were added to the dif ferent groups. After mixing on the shaker for 24 hours, all samples were allowed t o settle overnight. The supernatant was then collected for arsenic and calc ium analysis. The final pH was measured using the remaining mixture in the bot tle.

Analytical Method

A GFAA spectrometer (AAnalyst 600, Perkin-Elmer Cor p., Norwalk, Connecticut, USA) and a Flame Atomic Absorption Spectroscopy (FL AA; Model 3110, PerkinElmer Corp., Norwalk, Connecticut, USA) were used t o determine arsenic and calcium concentrations in solution, respectively. A n Orion PerpHecT Triode pH electrode (model 9207BN) and a pH meter (perpHecT L oR model 370) were used for pH measurement.

Data Analysis

The non-linear regression program Kaleidagraph TM (Synergy Software, 2002) was used for titration modeling to determine the su rface site density and acidity constants. SigmaPlot (SPSS Inc., 2001) was used as a multi-variable nonlinear regression program to determine the adsorption cons tants 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 si gnificantly 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 H 3 AsO 4 species dominates when pH is less than 2. Therefore, the neutral arsenic mole cule is considered not adsorbable by ash surface. The dissolution of ash p articles under very acidic conditions might also contribute to the higher solu ble arsenic concentration. When pH increases to above 2, the total concentrati ons of anionic arsenic species (H 2 AsO 4 - and HAsO 4 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 gre ater than 9, the soluble arsenic concentration decreased with the increase o f 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 th e 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 time s greater than those in ash #1005 and #1004; acidic ash #1009 has a calcium con tent about twice as high as the other two acidic ashes. Correspondingly, the so luble calcium concentrations in leachates from the alkaline ashes were significa ntly greater than for the acidic ashes below pH 11 (Figure 1b), but were more sensit ive to pH change and decreased faster with increase of pH. These results indicate that calcium leaching from alkaline ashes was a dissolution/pre cipitation controlled process, and the fairly low leachability of arsenic from the alkaline ashes across a broad pH range was likely associated with the precipitati on or coprecipitation of arsenic with calcium phases. The situation is quite differe nt for acidic ashes, where the soluble calcium concentration decreased smoothly wi th 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 th e alkaline pH range. The leaching mechanism of arsenic from acidic ashes wil l be discussed more fully in the following section.

Arsenic Partitioning in Washed Fly Ash under Differ ent 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 experi ments with different calcium additions. For all partitioning experiments, the as h 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 additio ns 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 syst em, the maximum release reached 12 mg/L at pH 12, indicating that a signifi cant amount of arsenic was released from washed fly ash. Addition of calcium s ignificantly enhanced arsenic adsorption in pH range of 7-12, and a larger adsorp tion 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 adsor ption was increased by 75%. This effect became less significant with calcium ad dition above 50 mg/L. Thearsenic partitioning curve in the presence of calci um is analogous to the leaching

curve of raw ash #1009 (Figure 1a), suggesting a si milar mechanism applies in both circumstances.

Three Ca loadings were applied to As(V) partitionin g experiment with ash #1005: 0 mg/L, 100 mg/L and 500 mg/L. Results shown in Fig ure 4 were similar with those of ash #1008, except that soluble Ca concentr ation 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 Ca 3 (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. Res ults are listed in Table 2. Instead of one consistent solubility product (K sp ), the large variation of the product, up to four orders of magnitude, indicated that the arsenic release was not a calcium arsenate precipitation controlled pro cess. Ettringite formation contributes to the stabilization of arsenic in fly ash generally in alkaline ashes, and only above pH 11, while experimental data indic ated that calcium effect in these acidic ashes became observable since pH 7-8,.

The current experimental data suggest that some oth er mechanism is at least partially responsible for controlling arsenic relea se in the acidic ashes. Calcium added into the system can form complexes with arsen ic in forms of CaH 2 AsO 4 +

, CaHAsO 4 , CaAsO 4 - , Ca 3 (AsO 4 ) 2

. 23 It has been reported that the neutral forms of metal complexes, 24, 25 have higher affinity to the sorbent surface. Consi dering the occurrence of the two neutral species CaHAsO 4 and Ca 3 (AsO 4 ) 2 in the pH range where arsenic adsorption was enhanced, it is assumed that the adsorption of these two species contributed to the stabilizati on of arsenic in fly ash. This assumption is verified by the modeling process desc ribed below.

Modeling As(V) Adsorption onto Washed Ash

Surface Site Characterization

The surface site density and acidity constant of fl y ash are essential parameters for metal adsorption modeling. A previously develop ed titration model 19 was used to determine these parameters. The model is express ed as: 

# ∑   
  

+

− + = ∆

+ + i Hi 0 Hi Hi Ti 0 SS K ] H [ 1

K ] H [ 1

C K S V V (1)

where ∆ V SS is the net volume of stock acid/base (negative val ue for acid) solution consumed by surface sites (mL); V 0 is total volume of the ash mixture (mL); S Ti is the total acid site concentration of species i (M); K Hi is the acidity constant of the species i (M); C is the concentration of the acid/b ase stock solution (M); and [H + ] 0 is the hydrogen ion concentration of the control un it (without acid or base addition) (M). Note that the total surface site concentration S Ti = Γ i × SS, where Γ i is thesurface site density for species i (mol/g-SS) and S S is the solids concentration

(g/L).

After correction using the titration data for blank s, the net titration data for washed Ash #1008 with S/L ratio of 1:10 were plotte d 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. KaleidaGraph TM 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 (pK H ) for each site, α , β , and
γ . Since the pH pzc of this ash was 6.2 (Table 1), which is between th e pK H s of the site α and site β (3.2 and 7.3, respectively), the protonated surfac e sites α is positively charged, denoted as S 1 OH 2 + , 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 th e system can be calculated based on the following equations: H 3 AsO 4 = H 2 AsO 4 - + H + ; pK a1 = 2.26; 26 (2) H 2 AsO 4 - = HAsO 4 2- + H + ; pK a2 = 6.76; 26 (3) HAsO 4 2- = AsO 4 3- + H + ; pK a3 = 11.29; 26 (4) HAsO 4 2- + Ca 2+ = CaHAsO 4 ; logK a4 = 2.75; 23 (5) AsO 4 3- + 1.5 Ca 2+ = Ca 1.5 AsO 4 ; K a5 (6)

The stoichiometry of Ca 3 (AsO 4 ) 2 was converted to Ca 1.5 AsO 4 , to simplify the adsorption equations. K a4 and K a5 are the formation constants of CaHAsO 4 and Ca 3 (AsO 4 ) 2 , respectively. K a5 needs to be determined by modeling, because there is no published value available from literatu re.

Assuming that only the protonated surface site α (denoted as S 1 ) is responsible for the adsorption of three negatively charged arse nic species, denoted as

+ 1 2 + T [S OH ]= α S , where S 1T 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 selectivit y on the three surface sites. Therefore, all surface sites were normalized as one with a total site density of [S T ] when considering the two neutral species. Adsorptio n of different arsenic species by surface sites can be expressed as:

S 1 OH 2 + + H 2 AsO 4 - = S 1 -H 2 AsO 4 + H 2 O; K S1 ; (7) S 1 OH 2 + + HAsO 4 2- = S 1 -HAsO 4 - + H 2 O; K S2 ; (8) S 1 OH 2 + + AsO 4 3- = S 1 -AsO 4 2- + H 2 O; K S3 ; (9) S + CaHAsO 4 = S -CaHAsO 4 ; K S4 ; (10) S + Ca 1.5 AsO 4 = S -Ca 1.5 AsO 4 ; K S5 ; (11)where K S1 , K S2 , K S3 , K S4 and K S5 are adsorption constants of the five arsenic

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

The total soluble adsorbed arsenic concentration [A s(V)] D and total adsorbed arsenic concentration [As(V)] ads can be expressed with the following equations: [As(V)] D = [H 3 AsO4]+[H 2 AsO 4 - ]+[HAsO 4 2- ]+[AsO 4 3- ]+[CaHAsO 4 ]+[Ca 1.5 AsO 4 ] (12) [As(V)] ads =[S 1 -H 2 AsO 4 ]+[S 1 -HAsO 4 - ]+[S 1 -AsO 4 2- ]+[S -CaHAsO 4 ]+[S -Ca 1.5 AsO 4 ] (13)

According to previous research, if the total adsorb ate concentration (in M) is less than 10% of the surface site concentration, which i s true in this study, then the adsorption is in the linear range of the Langmuir i sotherm 19 , 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 3 5 1 2 4
2 3
0 K K K K [Ca] K K K [Ca] 1 B = + +
α [H] [H] (20)

1.5

+ S1 1 S2 1 2 S4 1 2 4 S5 1 2 3 5 1T T 2 2 3 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 o f 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 K 5 , Ks 1 , Ks 2 , Ks 4 and Ks 5 . Modeling results are listed in Table 4 together with the standard error and R 2 . Soluble arsenic concentrations predicted by the adsorption model ar e also plotted in Figure 3 as solid lines. The good agreement between experimenta l and modeling results suggested the validity of the developed model for t his dataset.CONCLUSIONS

This research demonstrated that different arsenic l eaching behaviors from acidic and alkaline fly ashes were correlated with the cal cium 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 fl y ash was significantly enhanced by added calcium under basic conditions. S olubility product calculation indicated that this process was not controlled by p recipitation of Ca 3 (AsO 4 ) 2

. Instead, it is hypothesized that calcium complexes with arsenic and forms two neutral species CaHAsO 4 and Ca 3 (AsO 4 ) 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 successful ly 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 Resea rch Institute (EPRI) and the Environmental Research Center (ERC) for Emerging Co ntaminants at the University of Missouri-Rolla (UMR). The authors als o gratefully acknowledge Mr. Shi Shu at University of Missouri-Rolla for his con tribution to the leaching experiment and chemical analysis; and Dr. C.P. Huan g and Mr. Minghua Li at the University of Delaware for providing the zeta poten tial measurement. Conclusions and statements made in this paper are t hose of the authors, and in no way reflect the endorsement of the aforementione d funding agencies.

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Sample ID Coal Type Natural pH (S/L = 1:10) As

(mg/kg) Ca

(%) BET Area

(m 2 /g) pH pzc 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 ×[AsO 4 ] 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 constan ts 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

pK H 3.2 ± 0.1 7.3 ± 0.5 11.3 ± 0.9

Table 3. Modeling results for arsenic partitioning with ash #1008. Arsenic Species H 2 AsO 4 HAsO 4 CaHAsO 4 Ca 1.5 (AsO 4 ) 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 0.890 200 400 600 800 1000 1200 1400 1600 1800

0 2 4 6 8 10 12 14 pHAs (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 pHCa (mg/L) #1005

#1004

#1009

#1015

#1018

#7

Figure 1. (a) As, (b) Ca leaching from acidic and a lkaline coal fly ashes. Experimental conditions: S/L = 1:10; temperature = 20 – 25 o C; equilibration time = 24 hours.

a

b0.0

0.2 0.4 0.6 0.8 1.0 1.2

0 2 4 6 8 10 12 14
pHFraction H 3 AsO 4 H 2 AsO 4 - HAsO 4 2- AsO 4 3-

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0 2 4 6 8 10 12 14
pHFraction H 3 AsO 4 H 2 AsO 4 - HAsO 4 2- AsO 4 3-

Figure 2. Speciation of arsenic acid.0 2 4 6 8 10 12 14

0 2 4 6 8 10 12 14 pHAs (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 pHCa (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 wi th and without addition of Ca: (a) Arsenic concentration as a function of pH; (b) Calc ium concentration as a function of pH

a

b0 1 2 3 4 5 6 7 8

0 2 4 6 8 10 12 14 pHAs (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 pHCa 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 wi th and without addition of Ca: (a) Arsenic concentration as a function of pH; (b) Calc ium 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 A sh #1008. Experimental conditions: S/L = 1:10; ionic strength = 0.01 M (NaNO 3 ); temperature = 20 – 25 o C; equilibration time = 24 hours