Arsenic immobilization by calcium arsenic precipitates in lime treated soils

Arsenic immobilization by calcium arsenic precipitates in lime treated soils

ARTICLE IN PRESS

Science of the Total Environment xx (2004) xxx–xxx

0048-9697/03/$ - see front matter
2004 Elsevier B.V All rights reserved.

doi:10.1016/j.scitotenv.2004.03.016

Arsenic immobilization by calcium–arsenic precipitates in lime

treated soils

W.M Keck Geoenvironmental Laboratory, Center for Environmental Systems, Stevens Institute of Technology,

Castle Point on Hudson, Hoboken, NJ 07030, USA

Accepted 18 March 2004

Abstract

Lime-based stabilizationysolidification(SyS) can be an effective remediation alternative for the immobilization of

arsenic (As) in contaminated soils and sludges.However, the exact immobilization mechanism has not been well

established.Based on previous research, As immobilization could be attributed to sorption andyor inclusion in

pozzolanic reaction products andyor the formation of calcium–arsenic(Ca–As) precipitates.In this study, suspensions

of lime–As and lime–As–kaolinite were studied in an attempt to elucidate the controlling mechanism of As immobilization in lime treated soils.Aqueous lime–As suspensions (slurries) with varying CayAs molar ratios (1:1,

1.5:1, 2:1, 2.5:1 and 4:1) were prepared and soluble As concentrations were determined.X-Ray diffraction (XRD)

analyses were used to establish the resulting mineralogy of crystalline precipitate formation.Depending on the redox state of the As source, different As precipitates were identified.When As (III) was used, the main precipitate

formation was Ca–As–O.With As(V) as the source, Ca (OH) (AsO ) •4H O formed at CayAs molar ratios greater4 2 4 2 2

than 1:1.A significant increase in As (III) immobilization was observed at CayAs molar ratios greater than 1:1

Similarly, a substantial increase in As (V) immobilization was noted at CayAs molar ratios greater than or equal to

2.5:1 This observation was also confirmed by XRD Lime–As–kaolinite slurries were also prepared at different Cay

As molar ratios.These slurries were used to specifically investigate the possibility of forming pozzolanic reaction products.Such products would immobilize As by sorption andyor inclusion along with the formations of different As

precipitates.Toxicity Characteristic Leaching Procedure (TCLP) tests were used to evaluate As leachability in these

slurries.XRD analyses revealed no pozzolanic reaction product formation.Instead, As immobilization was found to

be precipitation controlled.The same Ca–As precipitate, Ca–As–O, identified in the lime–As slurries, was also identified when As (III) was used as the As source, at CayAs molar ratios greater than or equal to 2.5:1 When As (V) was used as the contamination source in the lime–As–kaolinite slurries, the formation of NaCaAsO •7.5H O was4 2

observed.The effectiveness of both As (III) and As (V) immobilization in these slurries appeared to increase with

increasing CayAs molar ratios

2004 Elsevier B.V All rights reserved

Keywords: X-Ray diffraction(XRD); Arsenite; Arsenate; Lime; Ca–As–O; Ca (OH) (AsO ) •4H O; NaCaAsO •7.5H O 4 2 4 2 2 4 2

*Corresponding author.Tel.: q1-201-216-8097; fax: q1-201-216-8212.

E-mail address: dmoon@stevens-tech.edu(D.H Moon).

1 Introduction

Arsenic (As) is known to be a very toxic

element and a carcinogen to humans (Mollah et

al., 1998).Even trace amounts of As can be

harmful to human health(Karim, 2000).In nature,

As is released in the environment through

weath-ering and volcanism (Juillot et al., 1999).Arsenic

is also released by anthropogenic activities.It was

used extensively for agricultural applications such

as herbicides and insecticides (Leist et al., 2000)

and has thus created problems through leaching

and infiltration to subsurface soils and ground

water(Murphy and Aucott, 1998).Arsenic is also

produced as a waste by-product from the mineral

processing and smelting industries.As (III) and

As (V) are the most widespread forms in nature

(Boyle and Jonasson, 1973; Cherry et al., 1979),

with As (III) being both more mobile and toxic

(Boyle and Jonasson, 1973; Pantsar-Kallio and

Manninen, 1997).More specifically, As (III) is

25–60 times more toxic than As (V) (Dutre and´

Vandecasteele, 1995; Corwin et al., 1999)

Stabilizationysolidification (SyS) is one of the

most effective methods to reduce the mobility of

heavy metals(Yukselen and Alpaslan, 2001

).Var-ious combinations of type I portland cement

(OPC), lime, type F fly ash, silica fumes, iron (II)

or (III), silicates and blast furnace slag have been

used in the treatment of soils contaminated with

As (Akhter et al., 1997; Leist et al., 2000)

Several researchers have shown that As

immo-bilization is mainly controlled by the formation of

Ca–As precipitates Dutre and Vandecasteele´

teele et al.(2002)demonstrated that the formation

of Ca3(AsO ) and CaHAsO precipitates controls4 2 3

the immobilization of As in contaminated soils,

which have been treated with cement, lime and

pozzolanic material.At the high pH levels (12–

13) induced by lime treatment, where a large

fraction of As(III) occurs as HAsO , the precip-2y

3

itation of CaHAsO will take place.Within the3

same pH range, the formation of Ca3(AsO )4 2

occurs in the presence of As(V) ions.These

precipitates were found to be responsible for the

observed reduction in As leachability.Also,

research byBothe and Brown(1999)has

suggest-ed that lime addition rsuggest-educes As mobility in contaminated slurries due to the formation of low solubility Ca–As precipitates such as

Ca4(OH) (AsO ) •4H O2 4 2 2 and johnbaumite,

Ca5(AsO ) (OH).4 3

Moreover, the reaction of alumino-silicious material, lime and water results in the formation

of concrete-like products described as pozzolanic (LaGrega et al., 1994) Dermatas and Meng

applications, the formation of pozzolanic reaction products may be associated with heavy metal immobilization by sorption and inclusion in poz-zolanic reaction products.Therefore, there seems

to be three possible As immobilization mecha-nisms to be considered.These are Ca–As precip-itation, sorption or inclusion in pozzolanic reaction products.In this study, the prepared lime–As and lime–As–clay slurries were tested by X-ray dif-fraction(XRD) analyses and analyzed for soluble

As in order to evaluate these mechanisms.Kaolin-ite was chosen as the clay that will provide the available alumina and silica for the possible for-mation of pozzolanic reaction products

The objectives of this study are:(1) to investi-gate the formation of Ca–As precipitates in lime–

As and lime–As–kaolinite slurries prepared at different CayAs molar ratios; (2) to investigate pozzolanic reaction product formation in lime– As–kaolinite slurries as a function of CayAs molar ratios; (3) to then correlate soluble As concentra-tions with the type of crystalline phases (precipi-tation vs.pozzolanic reaction products) as identified by XRD analyses; (4) to examine the possible oxidation of As(III) in contaminated soils

as a result of lime treatment; and(5) to investigate the aging effect on Ca–As precipitates to evaluate whether the various phases formed persist or redis-solve with time

2 Experimental methodology

2.1 Reagents and materials

Three different commercially available As com-pounds were used as As contamination sources: arsenic oxide(As O ), sodium arsenite (NaAsO )2 3 2

and sodium arsenate (Na HAsO •7H O).The last

ARTICLE IN PRESS

3

D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx

two are very soluble and provide two different As

oxidation states, As(III) and As(V).The As (III)

source, As O2 3 was chosen because of its low

solubility (1.2–3.7 gy100 ml at 20 8C) (

Interna-tional Programme on Chemical Safety, 1997)

com-pared to NaAsO , which is highly soluble.This2

was done in order to evaluate the difference in

solubility between various As (III) forms present

in the soils.These chemicals were obtained from

Fisher Scientific Company (Suwanee,

GA).Kao-linite was provided by Dry Branch Company(Dry

Branch, GA).Chemical grade hydrated lime

(Ca(OH) ) powder was obtained from the Belle-2

fonte Lime Company (Bellefonte, PA)

2.2 Slurry preparation and analysis

Lime–As slurries were prepared at five different

CayAs molar ratios(1:1, 1.5:1, 2:1, 2.5:1 and 4:1)

This was done by using a liquid to solid (L:S)

ratio of 10:1, by weight.Three separate sets of

lime–As slurries were prepared by using three

different As compounds as previously discussed

Likewise, Bothe and Brown (1999) evaluated the

formation of Ca–As precipitates at CayAs molar

ratios that ranged from 1.5:1 to 2.5:1 The prepared

slurry samples were then aged and periodically

shaken at 20 8C.After 4 days of continuous mixing

using an Orbital incubator (Gallenkamp), a

sub-sample was taken with a 5 ml pipette and filtered

through a 47 mm polycarbonate filter (pore size:

0.4 mm).The residue retained on the filter was

air-dried and characterized by XRD analysis.The

filtrate was analyzed for soluble As concentration

All the experiments focused on 4-day test

results, but the effect of aging was also considered

Time allowed for the aging experiments was 4

months for the lime–As O2 3 (lime–As(III))

slur-ries.Initial results indicated no significant change

in As concentrations between 4-month and 4-day

samples.Thus, equilibrium was attained within 4

days.As a result, the time allowed for aging was

shortened to 2 months for the lime–NaAsO2

(lime–As(III)) and 40 days for the lime–

Na HAsO2 4•7H O (lime–As(V)) slurries.At the2

end of the aging experiments, the same procedure

followed in the initial 4-day experiments was used

with regards to XRD and soluble As concentration analyses

Lime–As–kaolinite slurries were prepared at CayAs molar ratios (1:1, 2:1, 2.5:1 and 4:1) similar to those used in lime–As slurries.The amount of kaolinite used for the preparation of the slurry was 30 g.The As source was NaAsO2 (10 wt.%) and Na HAsO •7H O (10 wt.%).The sam-2 4 2

ples were adequately mixed with water to enhance the hydration process.All the prepared samples were cured in plastic bottles at 20 8C for 1 month

in order to enable comparison of data with other studies.A small portion, approximately 10 g, was removed and air-dried.A fraction of this portion was used for XRD analysis

The effectiveness of lime at immobilizing As in lime–As–kaolinite slurries was evaluated using TCLP tests.A fraction of the portion removed and air-dried from lime–As–kaolinite slurries was used for this purpose (L:S ratio was 20:1).More spe-cifically, 60 ml of the extraction fluid(pH 3) was added to 3 g of the air-dried sample.After 18 h

of tumbling, the leachate was filtered through a 0.4 mm pore-size membrane filter to separate the solids from the leachate solution.The leachate solution was then analyzed and soluble As concen-trations were measured

The concentrations of soluble As and TCLP As were analyzed using an inductively coupled plasma optical emission spectrometer(ICP-OES) (Varian Vista-MPX, Palo Alto, CA).A number of blanks and check standards were prepared with each batch

of samples for quality control purposes

2.3 X-Ray diffraction analyses

A Rigaku DXR 3000 computer-automated dif-fractometer was used.Step-scanned XRD data were collected using Bragg–Brentano geometry Diffractometry was conducted at 40 kV and 30

mA using Cu radiation with a diffracted beam graphite-monochromator.The data were collected between 5 and 658 in 2u with a step size of 0.058 and a count time of 5 s per step.All the samples were pulverized and sieved through a 200-mesh sieve(0.075 mm diameter opening) prior to XRD analyses in order to obtain a uniform particle size distribution

It is important to note that when a phase is not

detected by XRD analyses, this does not mean

that it is not there, but rather that it may be there

only in quantities below the detection limit of

XRD.As a general rule, the detection limit of the

XRD analyses of crystalline phases is considered

to be 5% of the total weight of the mixture

However, several factors do have a significant

effect on the actual detection limit for any

partic-ular crystalline phase.These factors include:

degree of crystallinity, hydration, surface texture

of the sample, sample weight, particle orientation,

mass absorption coefficients of different minerals,

etc (Mitchell, 1993).Any of these factors may

have an influence on the resulting peak height and

broadness, and thus the detection limit Carter et

quartz and a 0.03% limit for cristobalite

Converse-ly, pozzolanic mineral phases may very well be

either poorly crystallized or amorphous, especially

at the early stages of formation, which makes their

detection by XRD more difficult

3 Results and discussion

3.1 Formation of different phases in the lime–As

slurries

Lime–As slurries produced Ca–As precipitates

at all CayAs molar ratios tested.The different

phases identified by XRD and the corresponding

soluble As results at 4-days of mixing and after

aging are summarized in Tables 1 and 2,

respec-tively.XRD patterns are presented in Figs.1–6

for all lime–As slurries

In the lime–As O2 3 (lime–As(III)) slurries,

fol-lowing 4 days of continuous shaking, three major

phases were observed: portlandite(Ca(OH) ), cal-2

cium arsenite (Ca–As–O), and calcite (CaCO )3

(Table 1andFig.1).Arsenolite (As O ) was only2 3

identified at the lowest CayAs molar ratio (1:1),

due to its limited solubility, as shown in Fig.1

and Table 1.However, at CayAs molar ratios

greater than 1:1, the peaks of arsenolite

disap-peared(Fig.1).Also, following 4 months of aging,

no arsenolite peaks could be identified for samples

having a CayAs molar ratio of 1:1 (Table 1 and

Fig.2).Aside from the disappearance of arsenolite

peaks, there were no significant differences in the observed XRD patterns between the sub-samples tested following 4 days and 4 months

For the lime–NaAsO2 (lime–As(III)) slurries, three major phases were identified by XRD:

Ca(OH) , Ca–As–O, and CaCO (2 3 Table 1 and

Fig.3).No NaAsO was identified due to its high2

solubility(Table 1 andFig.3).No obvious differ-ences were observed in the XRD patterns between the sub-samples tested at 4 days and 2 months (Fig.4).Overall, regardless of the arsenite source used, whether readily soluble NaAsO2 or less soluble As O , the same Ca–As precipitate, Ca–2 3

As–O, was identified(Table 1)

In lime–Na HAsO2 4•7H O (lime–As(V)) slur-2

ries, five phases were identified(Table 1andFig

5).CaCO and Ca(OH) were identified at all Cay3 2

As molar ratios.Due to its high solubility, no

Na HAsO2 4•7H O was identified (2 Table 1 and

Fig.5).NaCaAsO •7.5H O was identified at Cay4 2

As molar ratios up to 1.5:1 Johnbaumite,

Ca5(AsO ) OH, was observed only at a CayAs4 3

molar ratio of 1:1.Calcium arsenate hydroxide hydrate, Ca4(OH) (AsO ) •4H O, was observed at2 4 2 2

CayAs molar ratios greater than 1:1 (Table 1 and

Fig.5).Following 40 days of aging, NaCaAsO4•7.5H O was no longer detected by2

XRD at CayAs molar ratios greater than 1:1

Ca4(OH) (AsO ) •4H O and johnbaumite were2 4 2 2

still detected following 40 days of aging(Table 1

andFig.6)

Bothe and Brown (1999) also found

Ca4(OH) (AsO ) •4H O at CayAs molar ratios2 4 2 2

between 2:1 and 2.5:1 as well as minor amounts

of johnbaumite.According to their research, the formation of johnbaumite was clearly observed in lime–As(V) slurries at CayAs molar ratios between 1.7:1 and 1.9:1

3.2 Formation of different phases in lime–As– kaolinite slurries

In lime–NaAsO –kaolinite2 (lime–As(III)–kao-linite) slurries, the formation of Ca–As–O was observed at CayAs molar ratios greater than or equal to 2.5:1(Table 3andFig.7)

Kaolinite is composed of alumina and silica, which become quite soluble at the high pH levels

Table 1

Mineral formations in lime–As O , lime–NaAsO and lime–Na HAsO 2 3 2 2 4 •7H O slurries following 4 days of mixing and after aging 2

Ca yAs Lime–As O wAs 2 3 (III)x Lime–NaAsO wAs 2 (III)x Lime–Na HAsO 2 4 •7H OwAs(V)x 2

ratio

Phases following 4 days of mixing

1 As O , Ca–As–O, CaCO , Ca2 3 3 (OH) 2 Ca–As–O, CaCO , Ca 3 (OH) 2 NaCaAsO 4 •7.5H O, Ca (AsO ) OH, CaCO , Ca(OH) 2 5 4 3 3 2

1.5 Ca–As–O, CaCO , Ca 3 (OH) 2 4 NaCaAsO 4 •7.5H O, Ca (OH) (AsO ) •4H O, CaCO , Ca(OH) 2 4 2 4 2 2 3 2

2 4 4 Ca 4 (OH) (AsO ) •4H O, CaCO , Ca(OH) 2 4 2 2 3 2

Phases after aging

1 Ca–As–O, CaCO , Ca 3 (OH) 2 Ca–As–O, CaCO , Ca 3 (OH) 2 NaCaAsO 4 •7.5H O, Ca (AsO ) OH, CaCO , Ca(OH) 2 5 4 3 3 2

1.5 4 4 Ca 4 (OH) (AsO ) •4H O, CaCO , Ca(OH) 2 4 2 2 3 2

Table 2

Soluble As concentrations in lime–As O , lime–NaAsO and lime–Na HAsO 2 3 2 2 4 •7H O slurries following 4 days of mixing and after 2

aging

Ca yAs Lime–As O wAs 2 3 (III)x Lime–NaAsO wAs 2 (III)x Lime–Na HAsO 2 4 •7H O wAs(V)x 2

ratio

Soluble As concentrations following 4 days of mixing

Soluble As concentrations after aging

Note-NDsnot detectable.

Fig.1.XRD patterns for lime–As O slurries with different Ca yAs molar ratios following 4 days of continuous mixing.

induced by lime addition (Keller, 1964).Upon

lime treatment a wide variety of calcium aluminate

silicate hydrate pozzolanic phases will form

depending on reaction conditions (Transportation

Research Board, 1976).Several types of calcium aluminate hydrate and calcium silicate hydrate pozzolanic phases have been identified in previous research that focused on lime treatment of kaolinite

ARTICLE IN PRESS

7

D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx

Fig.2.XRD patterns for lime–As O slurries with different Ca 2 3 yAs molar ratios following 4 months of aging.

soils and arsenic immobilization (Mitchell and

Dermatas, 1992; Dermatas and Meng, 1996).In

the present study, however, no pozzolanic reaction

products were identified.It is important to note

that such an observation does not exclude the

possibility pozzolanic phases exist within the

mix-ture.If pozzolanic phases do exist within the

mixture, they may be in quantities that are below

the detection limit of XRD analyses.As previously

discussed in the XRD analyses section, many

factors may affect the detection limit of XRD

analyses for any given crystalline phase.The

degree of crystallinity is probably the most

appli-cable factor in this case, since Ca–As precipitates

are more likely to have a higher degree of

crystal-linity than pozzolanic phases.Overall, the lack of

detectable quantities of pozzolanic phases may

suggest that a significant portion of the available

Ca ions were consumed during Ca–As–O

forma-tion and thus could not participate in pozzolanic

reactions

For the lime–Na HAsO2 4•7H O–kaolinite2

(lime–As(V)–kaolinite) slurries, the formation of

NaCaAsO •7.5H O, Ca(OH) and CaCO was

confirmed for all samples tested(Table 3andFig

8).Similar to the lime–As(III)–kaolinite slurries,

as discussed in the previous paragraph, no pozzo-lanic reaction products were detected

The formation of johnbaumite was only detected

at a CayAs molar ratio of 1:1.No Ca4(OH)2

(AsO ) •4H O was detected, even though its for-4 2 2

mation was confirmed by XRD in lime–As(V) slurries.Ca–As precipitates in the lime–As(V)– kaolinite slurries were somewhat different from those found in the lime–As(V) slurries at the same CayAs molar ratios

For the first time in a SyS study of arsenic contaminated soils, a Ca–As–O precipitate was identified.This formation seems to be closely associated with decreasing As concentrations Some of the Ca–As precipitates observed in this study confirmed previous research findings.Akhter

et al (1997) also identified NaCaAsO4•7.5H O2

formation when cement and cement–fly ash bind-ers were contaminated with sodium arsenate (10 wt.%).NaCaAsO •7.5H O was detected even4 2

when As (III) was used as the As source in cement–fly ash mixes, but not in cement-only

Fig.3.XRD patterns for lime–NaAsO slurries with different Ca2 yAs molar ratios following 4 days of continuous mixing.

mixes.This may indicate that As (III) was

oxi-dized during SyS applications in the presence of

fly ash.However, during the present study, the

oxidation of As could not be confirmed in any of

the precipitates identified Mollah et al (1998)

identified a Ca3(AsO ) formation when arsenate4 2

was treated with Portland cement type-V(OPC-V)

Vandecasteele et al.(2002) and Dutre et al (1999)´

also predicted the formation of Ca3(AsO ) and4 2

CaHAsO precipitates by using the speciation pro-3

gram MINTEQA2.However, the formation of

CaHAsO has not been established by XRD and3

no XRD data file describing it currently exists

(JCPDS database).None of these precipitates were

identified during this study Bothe and Brown

(AsO )4 2 either.They observed Ca4(OH)2

(AsO ) •4H O and Ca (AsO ) (OH) instead,4 2 2 5 4 3

which were also observed herein (Tables 1–3)

3.3 Soluble As concentration in lime–As slurries and the aging effect

Soluble As concentration results are summarized

in Table 2and presented inFig.9

In lime–As O2 3(lime–As(III)) slurries, the con-centration of As in solution was 589 mgyl at a CayAs molar ratio of 1:1 (Table 1).However, when the ratio was increased to 4:1, the As concentration decreased to 0.52 mgyl

In lime–NaAsO2 (lime–As(III)) slurries, the

As concentration was high(2783 mgyl) at a Cay

As molar ratio of 1:1.The difference in soluble

As concentrations, between lime–NaAsO2 (lime–

As(III)) and lime–As O (lime–As(III)) slurries,2 3

can be readily explained by the solubility differ-ences between the two As contamination sources When the CayAs molar ratio was increased to 4:1, the As concentration in the lime–NaAsO slurries2

ARTICLE IN PRESS

9

D.H Moon et al / Science of the Total Environment xx (2004) xxx–xxx

Fig.4.XRD patterns for lime–NaAsO slurries with different Ca 2 yAs molar ratios following 2 months of aging.

decreased to 3.6 mgyl.Overall, As (III)

immobi-lization in lime–As O2 3 (lime–As(III)) and lime–

NaAsO2 (lime–As(III)) slurries was more

pronounced at CayAs molar ratios greater than or

equal to 1.5:1(Fig.9)

In lime–Na HAsO2 4•7H O (lime–As(V)) slur-2

ries, the soluble As concentration was very high

(5165 mgyl) at a CayAs molar ratio of 1:1, but

decreased significantly to 1.5 mgyl at a CayAs

molar ratio of 4:1 (Table 2).Arsenic (V)

immo-bilization was more pronounced at CayAs molar

ratios greater than or equal to 2.5:1

Soluble As concentrations following the aging

step are also summarized in Table 2 and plotted

in Fig.9.After 4 months of aging, soluble As

concentrations decreased with increasing CayAs

molar ratios for all slurries tested (Table 2) and

were very low at a CayAs molar ratio of 4:1

The variable reaction time (aging) appeared to

have an important effect on soluble As

concentra-tions only in lime–As(V) slurries.In lime–As(III)

slurries, no significant differences in soluble As concentrations were observed between the 4-day and 4-month results.This indicates that

equilibri-um was probably reached within the first 4 days (Fig.9).Conversely, As concentration in lime–

As(V) slurries after aging, at CayAs molar ratios

up to 2.5:1, was significantly reduced when com-pared to 4-day results (Fig.9).However, no significant As concentration reduction was observed for a CayAs molar ratio of 4:1.These results indicate that equilibrium was probably not reached in lime–As(V) slurries and that reactions were ongoing following 40 days of testing for all CayAs molar ratios, except for a CayAs molar ratio of 4:1.This observation requires further investigation

Even though the formation of Ca–As–O was observed in all lime–NaAsO2 (lime–As(III)) and lime–As O2 3 (lime–As(III)) slurries, significant

As immobilization in the presence of this precipi-tate occurs only when CayAs molar ratios are

Fig.5.XRD patterns for lime–Na HAsO 2 4 •7H O slurries with different CayAs molar ratios following 4 days of continuous mixing 2

Fig.6.XRD patterns for lime–Na HAsO •7H O slurries with different CayAs molar ratios following 40 days of aging.